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I .34. :5 a b 1.“ This is to certify that the thesis entitled FACTORS INVOLVED IN THE LOCAL AND REMOTE CONTROL OF THE RIGHT CORONARY CIRCULATION IN THE DOG presented by Stephen Wilson Ely has been accepted towards fulfillment of the requirements for Ph .D. degree in Physiology Date 12/14/79 0-7639 M3 2* a R Y Michigan State University a @mwvmnmmuIacw-tfi - w: 25¢ per day per item RETURNING LIBRARY MATERIALS: Place in book return to remove charge from circulation records FACTORS INVOLVED IN THE LOCAL AND REMOTE CONTROL OF THE RIGHT CORONARY CIRCULATION IN THE DOG By Stephen Wilson Ely A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Physiology 1979 ABSTRACT FACTORS INVOLVED IN THE LOCAL AND REMOTE CONTROL OF THE RIGHT CORONARY CIRCULATION IN THE DOG By Stephen Wilson Ely Characterization of the factors which contribute to the regulation of blood flow through the right coronary vascular bed has not been ade— quately accomplished. This vascular system is of great importance in that the right coronary artery is the dominant coronary vessel in 50% of the human population. Therefore, a better understanding of the con- trol of this vascular bed during both physiological and pathophysio- logical conditions is needed. The purpose of this study is to evaluate several aspects of the local and remote control of the right coronary circulation. These studies were carried out in the anesthetized, Open chest dog during constant flow and constant pressure perfusion of the right coronary vascular bed. In some experiments, an isolated donor lung was inter- posed in the perfusion line in order to selectively alter the coronary arterial blood gas tensions. Finally, experiments were performed on the conscious animal, chronically instrumented for the determination of right coronary hemodynamics in order to evaluate experimental interven- tions without the presence of anesthesia. Stephen Wilson Ely These studies were designed to evaluate the role of autoregulation and prostaglandins in the control of blood flow through this circulation. In addition, the effects of local changes in oxygen and carbon dioxide (pH) tensions, infused catecholamines and sympathetic (baroreflex) nerve stimulation were also determined. Selective adrenergic receptor block- ing agent were also employed to define the mechanism of some of the responses seen. The results indicate that the response of this vascular bed to adrenergic stimulation is similar to the left coronary vascular system in that the net effect is dictated by the competition between alpha receptor vasoconstriction and beta receptor (metabolic) vasodilation. However, since the right ventricle performs one sixth the work of the left ventricle, the metabolic influences attributable to myocardial oxygen consumption are less. Therefore, the response to sympathetic nerve stimulation is dominated by alpha mediated coronary vasoconstric- tion. The degree of coronary vasoconstriction is apparently enhanced when flow to this bed is decreased. Intracoronary infusion of NE pro- duces a substantial coronary vasodilation which can be converted to a vasoconstriction in the presence of the beta receptor blocking agents propranolol or practolol. This suggests that infused norepinephrine produces a substantially greater stimulation of the myocardium, hence the metabolic effects dominate and a fall in coronary resistance is observed. The same basic effect was seen with intracoronary bolus injections of NE in the conscious dog. However, during right coronary ischemia, NE produced a substantial vasoconstriction. It was also Stephen Wilson Ely shown that local hypoxia or hypercapnia produces coronary vasodilation, while local or systemic hypocapnia produces substantial coronary vaso- constriction. The response of this vascular bed to sympathetic stimu- lation or norepinephrine infusion was attenuated when the bed was perfused with hypoxic or hypercapnic blood. This suggests that certain changes in local blood gas tensions may alter the response of this vascular bed to adrenergic stimulation. The prostaglandins seem to play a minor role in the regulation of blood flow in this vascular bed. While the reactive dilation seen with 20 sec. interruptions of flow was slightly but significantly attenuated by the inhibition of prostaglandin synthesis with indomethacin, only the reactive hyperemia response to brief (3 second) occlusions was affected by indomethacin in the conscious dog. The coronary responses to sympa- thetic stimulation of norepinephrine infusion or systemic hypocapnia was unaffected by indomethacin, suggesting that prostaglandins do not play a role in the response of the right coronary circulation to these stimuli. It was also determined in these studies that this circulation autoregulates to a slight extent during constant flow perfusion. However, with constant pressure perfusion or under natural flow condi- tions (in the conscious animal) the bed exhibits a fair degree of auto- regulation, although not to the same extent as that seen in the left coronary vascular bed. In conclusion, these studies suggest that the right coronary circulation differs in some important respects from that of the left coronary bed. Metabolic influences do not appear to play as large a Stephen Wilson Ely role in determining right coronary blood flow. In some instances neural (remote) influences may provide the dominant mechanism of regu— lating right coronary vascular resistance. Dedication To my good friend, Booty. 1'1 ACKNOWLEDGMENTS The author takes this opportunity to express his heartfelt appre- ciation to Professor Jerry B. Scott for his support, criticism, invalu— able advice and continual encouragement throughout the course of this degree program. Appreciation is also extended to the members of the author's Guidance Committee, who each played an important role in the educational process, Drs. Tai Akera, John Chimoskey, Ching-Chung Chou, Thomas Emerson and N. Edward Robinson. Special thanks are extended to Dr. Chimoskey for his invaluable direction in the investigations involving chronically instrumented animals. The author also acknowledges the skillful technical assistance of Mr. William Stoffs, Mr. George Gamble, Mr. Donald Anderson, Mr. Ronald Korthuis, Dr. Donald Sawyer, Ms. Connie Kitazumi, and Mrs. Nan King. TABLE OF CONTENTS Page LIST OF TABLES ................................................. vi LIST OF FIGURES ................................................ viii LITERATURE REVIEW .............................................. l Introduction ............................................. l Coronary Arterial Anatomy ................................ 4 Coronary Vascular Resistance ............................. 6 Regulation of Coronary Blood Flow ........................ 7 I. Physical Factors ................................ 7 Pressure-flow Relationships .................. 7 Extravascular Compression .................... 9 Vascular Waterfall Phenomenon ................ ll Transmural Distribution of Blood Flow ........ 13 II. Neuro-humoral Factors ........................... l5 Coronary Adrenergic Receptors ................ 15 Prinzmetal's Angina and Coronary Vasospasm... 21 Baroreceptor Reflex. ......................... 22 Sympathetic Tone ............................. 23 Carotid Body Chemoreceptors .................. 24 Acetylcholine and Vagus Nerve ................ 25 III. Myogenic Factors ................................ 26 IV. Myocardial Oxygen Consumption ................... 29 V. Metabolically Related Vasoactive Agents ......... 33 Oxygen Tension ............................... 34 Carbon Dioxide ............................... 36 Potassium .................................... 39 Osmolality ................................... 4l Prostaglandins ............................... 42 Adenine Nucleotides and Adenosine ............ 45 VI. Right Coronary Blood Flow ....................... 50 iv Page STATEMENT OF OBJECTIVES ....................................... 53 METHODS ....................................................... 55 Experimental Design ..................................... 56 I. Studies on the Anesthetized Dog ................ 56 Constant Flow Preparation ................... 56 Isolated Lung Preparation ................... 6O Constant Pressure Preparation ............... 6l 11. Studies on the Unanesthetized Dog .............. 64 Series I .................................... 66 Protocol .............................. 67 Series II..( ................................ 67 Protocol .............................. 68 Series III .................................. 68 Protocol .............................. 69 Series IV ................................... 7O Protocol .............................. 70 Series V .................................... 7l Protocol .............................. 72 Series VI ................................... 73 STATISTICAL ANALYSIS .......................................... 74 RESULTS ....................................................... 78 Series I ................................................ 78 Series II ............................................... 86 Series III .............................................. 92 Series IV ............................................... lO6 Series V ................................................ 113 Series VI ............................................... lZl DISCUSSION .................................................... 132 Methodology ............................................. 132 Constant Flow Studies ................................... 134 Constant Pressure Studies ............................... T35 SUMMARY AND CONCLUSIONS ....................................... l72 BIBLIOGRAPHY .................................................. 175 APPENDIX--TABLES .............................................. l9l TABLE 1. LIST OF TABLES Effect of sympathetic stimulation via baroreflex during constant flow perfusion on heart rate, mean arterial blood pressure (MABP), right ventricular systolic pressure (RVSP), right ventricular dP/dT, right coronary perfusion pressure (RCA )and right coronary vascular resistance (CVR) before ng after alpha receptor blockade with 600 ug/min intracoronary infusion of phentolamine ............. . Effect of l ug/min intracoronary infusion of norepine- phrine during constant flow perfusion on heart rate, mean arterial blood pressure (MABP), right ventricular sys- tolic pressure (RVSP), right ventricular dP/dT, right coronary perfusion pressure (RCAp ) and right coronary vascular resistance (CVR) before and after alpha receptor blockade with 600 pg/min intracoronary infusion of phentolamine .............................................. . Effect of local hypoxia, hypocapnia, and the combination of hypoxia and hypocapnia during constant flow perfusion on heart rate, mean arterial blood pressure (MABP), right ventricular systolic pressure (RVSP), right ven- tricular dP/dT, right coronary perfusion pressure (RCA ), right coronary resistance (CVR), and coronary arterial pH, 02 and CO2 ................................................ . Effect of sympathetic stimulation (SS) during local nor- moxia, hypoxia, hypocapnia and the combination of hypoxia and hypocapnia during constant flow perfusion on heart rate, mean arterial blood pressure (MABP), right ventricu- lar systolic pressure (RVSP), right ventricular )dP/dT, right coronary artery perfusion pressure (RCA , coronary vascular resistance (CVR) and coronary arteriaT) pH, P0 and PCO .............................................. 02... 2 vi Page 87 88 107 109 TABLE Page 5. Effect of 0.25 ug/min intracoronary infusion of norepine- phrine (NE) during local normoxia, hypoxia, hypocapnia and the combination of hypoxia and hypocapnia during constant flow perfusion on heart rate, mean arterial blood pressure (MABP), right ventricular systolic pressure (RVSP), right ventricular dP/dT, right coronary artery perfusion pres- sure (RCA ), coronary vascular resistance (CVR) and coro- nary arterial pH, P0 and PCO ............................ 110 2 2 6. Effect of 0.5 pg/min intracoronary infusion of norepine- phrine (NE) during local normoxia, hypoxia, hypocapnia, and the combination of hypoxia and hypocapnia during con- stant flow perfusion on heart rate, mean arterial blood pressure (MABP), right ventricular systolic pressure (RVSP), right ventricular dP/dT, right coronary artery perfusion pressure (RCApp) coronary vascular resistance (CVR) and coronary arterial pH, P0 and PCO .............. ll2 2 2 7. Effect of local hypoxia, hypercapnia and hypocapnia during constant pressure perfusion on heart rate, mean arterial blood pressure (MABP), right ventricular systolic pressure (RVSP), right ventricular dP/dT, right coronary flow, right coronary resistance (CVR), and coronary arterial pH, PO and PCO .............................................. 2 2 8. Effect of sympathetic stimulation (SS) during local nor- moxia, hypoxia, hypercapnia, and hypocapnia during con- stant pressure perfusion on heart rate, mean arterial blood pressure (MABP), right ventricular systolic pressure (RVSP), right ventricular dP/dT, right coronary artery flow, coronary vascular resistance (CVR) and coronary arterial pH, PO and PCO ................................. ll6 2 2 9. Effect of 0.25 pg/min intracoronary infusion of norepine— phrine (NE) during constant pressure perfusion on heart rate, mean arterial blood pressure (MABP), right ventricu- lar systolic pressure (RVSP), right ventricular dP/dT, right coronary artery flow, coronary vascular resistance and coronary arterial pH, PO and PCO .................... ll8 2 2 ll4 vii LIST OF FIGURES FIGURE Page l. Preparation for constant flow or constant pressure perfu— sion of right coronary artery ............................ 58 2. Preparation for constant flow or constant pressure perfu- sion of the right coronary artery with isolated lung interposed in perfusion circuit .......................... 63 3. Linear regression analysis for initial coronary resist- ance (Ri) versus change in resistance (AR) for sympa- thetic stimulation (baroreflex) and intracoronary norepinephrine infusion .................................. 76 4. Effect of sympathetic stimulation (SS) via carotid occlu- sion during constant flow perfusion of the right coronary artery on heart rate, mean arterial blood pressure (MABP) and coronary vascular resistance (CVR) at normal flow (NF), low flow (LF) and high flow (HF) rates, before and after beta blockade with propranolol (3 mg/Kg) ........... 8O 5. Effect of l ug/min intracoronary infusion of norepine- phrine (NE) during constant flow perfusion of the right coronary artery on heart rate, mean arterial blood pres- sure (MABP) and coronary vascular resistance (CVR) at three different flows, normal flow (NF), low flow (LF), and high flow (HF), before and after beta blockade with propranolol (3 mg/Kg) .................................... 83 6. Relationship between pressure and flow, as well as resistance and flow during constant flow perfusion of the right coronary artery .................................... 85 7. Effect of sympathetic stimulation (SS) via carotid occlu- sion during constant flow perfusion of the right coronary artery on mean arterial blood pressure (MABP), coronary perfusion pressure and coronary vascular resistance (CVR) at low flow and high flow rates, before and after alpha receptor blockade with 600 pg/min intracoronary infusion of phentolamine .......................................... 9l viii FIGURE Page 8. Effect of l pg/min intracoronary infusion of norepine- phrine (NE) during constant flow perfusion of the right coronary artery on mean arterial pressure (MABP), coronary perfusion pressure and coronary vascular resistance (CVR) at low flow and high flow rates, before and after alpha receptor blockade with 600 pg/min intracoronary infusion of phentolamine ........................................... 94 9. Effect of sympathetic stimulation (SS) via carotid occlu- sion, 0.25 ug/min intracoronary norepinephrine infusion (NE), systemic hypocapnia (HC) and the interaction of these factors on mean arterial blood pressure (MABP), coronary perfusion pressure and coronary vascular resist- ance (CVR) during constant flow perfusion ................. 96 10. Effect of sympathetic stimulation (SS) via carotid occlu- sion, 0.25 pg/min intracoronary norepinephrine infusion (NE), 5 mg/Kg intracoronary infusion of indomethacin (I) and the interaction of these factors on mean arterial blood pressure (MABP), coronary perfusion pressure and coronary vascular resistance (CVR) during constant flow perfusion of the right coronary artery .................... 100 ll. Interaction of indomethacin (I), systemic hypocapnia (HC), sympathetic stimulation (SS), and 0.25 ug/min intracoro- nary norepinephrine infusion (NE) in relation to their effects on mean arterial blood pressure (MABP), coronary perfusion pressure and coronary vascular resistance (CVR) during constant flow perfusion of the right coronary artery .................................................... 102 l2. Response of the right coronary vascular bed perfused at constant flow to 20 second interruptions of flow before and after prostaglandin synthesis inhibition with 5 mg/Kg indomethacin .............................................. 105 l3. Relationship between pressure and flow, as well as pres- sure and resistance during constant pressure perfusion of the right coronary artery ................................. 120 14. Response of the right coronary circulation of the unanthetized dog to three second occlusions of flow before and after blockade of prostaglandin synthesis with 5 mg/Kg indomethacin ...................................... l23 15. Relationship between pressure and flow, as well as pres- sure and resistance in the right coronary artery of the unanesthetized dog ........................................ 126 ix FIGURE Page 16. Effect of intracoronary bolus injections of norepine- phrine (NE) on right coronary perfusion pressure, coro- nary blood flow, and vascular resistance before and after alpha blockade in the unanesthetized dog ........... l28 l7. Effects of intracoronary bolus injections of adenosine and norepinephrine before and during myocardial ischemia on right coronary perfusion pressure, coronary blood flow and vascular resistance in the unanesthetized dog ........ l3l l8. Effects of intracoronary bolus injections of isopro- terenol and norepinephrine on right coronary vascular resistance before and after selective adrenergic receptor blockade with practolol, propranolol and phentolamine, during constant flow perfusion ........................... I53 LITERATURE REVIEW Introduction The coronary circulation supplies the myocardium with nutrients and removes metabolites in order to maintain cardiac function. This system must rapidly adjust to meet the ever changing demands of the heart. The heart extracts a very large fraction of the oxygen supplied by the coronary circulation. As a result, the coronary venous blood contains little reserve oxygen. The flow of metabolism ratio is the lowest of any organ bed in the body. The control of blood flow to the heart is under extrinsic neural influences, is affected by circulating vasoactive substances, and exhibits a great degree of local regulation. Three types of local control phenomena have been observed for the coro- nary circulation: autoregulation (the ability to maintain a near constant blood flow in the face of large variations in perfusion pres- sure), active hyperemia (the increase in blood flow that occurs in response to an increase in metabolic activity), and reactive hyperemia (the transient increase in blood flow above the control level following an interval of arterial occlusion). Several theories have evolved which attempt to explain the mechan- ism of these local regulatory phenomena. Currently, the theory which has the greatest support is the metabolic theory, as described by Berne (1964) as well as Haddy and Scott (l968,l974,l975). This theory proposes that changes in blood flow or tissue metabolism alter the interstitial concentration of vasoactive chemicals which cause subse- quent changes in vascular smooth muscle tone and hence vascular resist- ance. As an example, an increase in the metabolic rate of a tissue would increase the interstitial concentration of vasodilator metabolites eliciting a metabolically induced vasodilation (active hyperemia). Similarly, if blood flow is suddenly increased to a vascular bed by an increase in perfusion pressure, a washout of these vasodilator substances would occur, thereby lowering the interstitial concentration. This would result in an increase in vascular smooth muscle tone and hence resistance (autoregulation). On the other hand, occlusion of the arterial inflow would result in a build-up of the interstitial concen- tration of vasodilator metabolites such that upon release of the occlu- sion, vascular resistance would fall below control and a transient hyperemia would occur. Resistance would gradually return to control as the interstitial metabolites are removed by the elevated flow (reactive hyperemia). A second explanation which has been proposed to explain these local control phenomena is the myogenic hypothesis. This theory is based on the fact that vascular smooth muscle responds to stretch or increased tension with contraction, and responds to a reduction of stretch or decreased tension with relaxation. Thus, factors which alter transmural pressure across a vessel wall could elicit active changes in blood vessel caliber (Bayliss, T902; Folkow, l964; Johnson, 1977). It is therefore postulated that the myogenic response plays a role in autoregulation, active and reactive hyperemia since such phenomena involve changes in transmural pressure (Folkow, l949; Baez, T968; Burton and Johnson, l972). A third hypothesis is the tissue pressure hypothesis which sug- gests that changes in tissue pressure cause compression of capillaries and venules thereby altering vascular resistance (Beer and Rodbard, l970; Rodbard, l966; Rodbard gt_al., l97l). A decrease in tissue pres- sure would therefore cause capillary and venule diameters to increase, thus decreasing vascular resistance. In support of this theory, it was thought that contraction of a muscle caused a translocation of fluid volume from the extravascular compartment to the intravascular compartment, thereby decreasing tissue pressure and decreasing vascular resistance. However, it has been demonstrated that tissue volume increases during skeletal muscle contraction thereby contradicting the application of this hypothesis (Haddy, Scott, and Grega, I976). An elevation of perfusion pressure is thought to cause a net transfer of fluid to the extravascular space, causing an increased tissue pressure and compression of capillaries and veins. This would increase vascular resistance and limit the increase in blood flow, thereby producing an autoregulatory response (Beer and Rodbard, 1970; Rodbard, l966). However, other investigators have shown that resistance changes in response to an increased perfusion pressure or blood flow occurs at the pre-capillary resistance vessels in skeletal muscle (Haddy and Scott, 1964; Hanson and Johnson, 1962; Mellander and Johansson, 1968; Nagle _etial., l968). Furthermore, Driscoll et a1. (l964) showed that abrupt and sustained increases in left coronary perfusion pressure which elicited autoregulatory flow responses were associated with elevations in intramyocardial (tissue) pressure. Following maximal pharmaco- logically induced coronary vasodilation, the autoregulatory response to increased perfusion pressure was abolished, yet the increment in intra- myocardial pressure due to increased perfusion pressure was unchanged. Therefore, the tissue pressure hypothesis is not currently considered a plausible mechanism for local regulatory phenomena. As a result of the balance between extrinsic neural control, circulating vasoactive agents, and local regulatory phenomena, blood flow to the heart is exquisitely regulated under normal conditions in order to match myocardial oxygen delivery (coronary blood flow) to myo- cardial oxygen demand (oxygen consumption). The literature regarding the regulation of coronary blood flow (CBF) is extensive; however, most of the published observations were derived from experiments performed on the left coronary vascular bed. There is little experimental data available concerning the regulation of blood flow in the right coronary vascular bed. This survey will center on the important factors contributing to the regulation of CBF under both normal and pathophysiological conditions. The literature discussed will pertain to studies performed in the left coronary bed unless other- wise indicated. The limited data for the right coronary bed will also be presented. Coronary Arterial Anatomy There are two coronary arteries, the right and left, which take their origin from the right anterior and left anterior aortic sinuses of Valsalva, respectively. The left coronary artery (c.a.) begins as the left main c.a., coursing anteriorly and to the left in the atrioventricu- lar groove between the pulmonary artery and left atrial appendage. This portion of the left c.a. is l to l.5 cm long in man and 2 to 4 mm long in the dog. The left main c.a. then bifurcates to form the left anterior descending (LAD) and the left circumflex (CRFX) coronary arteries. The LAD follows the anterior interventricular sulcus toward the apex. The CRFX follows the A-V groove to the left, passing under the left atrial appendage and terminating on the posterior aspect of the heart. The LAD supplies the interventricular septum via small septal perforating arteries, the anterior portion of the left ventricular wall, apex, and a portion of the right ventricular free wall adjacent to the interventricular septum. The CRFX supplies the left atrium, posterior and lateral walls of the left ventricle, and in dogs sends a branch to supply the A—V node and bundle of His. The right coronary artery passes anteriorly behind the pulmonary artery and follows the A-V groove to the right margin of the heart and passes beneath the right atrial appendage. In dogs this vessel terminates as the marginal branch, supplying most of the right ventricular free wall and right atrial tissue, including the S-A node. In pig and man, the right c.a. reaches the posterior aspect of the heart to become the posterior descending c.a., supplying the posterior wall of the left ventricle and giving off a branch which supplies the A-V node. In the dog, the left coronary artery is dominant, supplying 85% of the myocardium. Left dominance in man is present only 20% of the time. Pigs are generally right coronary dominant, where as man has this pattern 50% of the time. In man, 30% have equal distribution between right and left coronary arteries (Gregg, l963). Anastomotic communica- tion between coronary arteries (intercoronary collaterals) are usually well developed in the dog between the CRFX and LAD. While the LAD may supply a portion of the right ventricle in the dog, there are few collaterals between the LAD and right coronary artery (Gregg, 1960; Murray gt_al,, l978). In the pig, few if any collaterals can be demon- strated between any of the coronary arteries (Schaper, l97l). In man, the existence of intercoronary collaterals is variable; however, in normal human hearts relatively few collaterals can be demonstrated (Fulton, l965). Coronary Vascular Resistance According to Poiseuille's law, the ratio of the pressure drop or pressure gradient across a vascular bed to the rate of flow is a func— tion of the factors serving to resist flow; namely, viscosity (v), vessel length (l) and radius (r). Resistance to flow as derived from Poiseuille's law is directly pr0portional to viscosity and vessel length and inversely proportional to the radius of the vessel to the fourth power, The law applies to a system in which there is laminar flow of a viscous homogenous fluid through rigid tubes. Therefore, Poiseuille's law cannot be quantitatively applied to the cardiovascular system since l) blood vessels are not rigid but will stretch in response to an increased pressure which is seen during systole, 2) whole blood is not a homogenous fluid, and 3) blood is not truly viscous. While Poiseuille's law is not entirely applicable to the cardiovascular system, it certainly can be applied in a qualitative sense. In a strict hemo- dynamic sense, v, l and r are the factors which determine resistance to flow. However, in the normal animal, coronary vessel length is con- sidered to be constant, as is blood viscosity. Therefore, these factors are not considered important determinants of coronary vascular resist— ance. Since resistance increases inversely to the fourth power of the radius, this aspect of vessel geometry is of paramount importance in the determination of vascular resistance. This is illustrated by the fact that a one-fold increase in vessel radius results in a sixteen-fold decrease in vascular resistance. As will be discussed in the following pages, many factors both active and passive may be involved simultane- ously in the net determination of vessel radius and consequently coro- nary vascular resistance. Regulation of Coronary Blood Flow I. Physical Factors Pressure-flow Relationships The pressure gradient (aortic-right atrial pressure) across the coronary bed provides the driving force for blood flow. This force, although essential for blood flow, may not be of primary importance in determining CBF due to the reported ability of the coronary bed to auto- regulate. Cross gt_al. (l96l) reported the relationship between left coronary perfusion pressure and blood flow in the dog heart to be linear and concluded from their analysis of correlation coefficients that coronary perfusion is the only factor which determines CBF. This group determined the coronary pressure gradient on the basis of the difference between aortic and intraventricular pressure rather than right atrial pressure. This has been shown to be an unreliable method of determining the coronary pressure gradient since intramyocardial pressure varies during the course of the cardiac cycle (Gregg and Eckstein, l94l). Eckel_et_al. (1949) first showed that in a beating heart of an open-chest dog with the left coronary artery perfused at constant pressure, sudden increments in perfusion pressure resulted in immediate increases in CBF which returned toward normal even though perfusion pressure remained elevated. Fishback and collegues (1959) reported that coronary resistance increased over the range of 60 to l30 mmHg during constant pressure perfusion of the dog heart. Scott_gt_al. (I960) demonstrated that resistance decreased as flow was raised from 25 to 75 ml/min, and then increased, decreased or remained constant over the range of 75 to 220 ml/min in fibrillating and beating (non-working) dog hearts in which both right and left coronary arteries were perfused through the aortic root. Subsequent studies by Brandfonbrener (l969) and Driscol (1964) demonstrated that the pressure-flow relationships in the left coronary bed is curvilinear with a convexity toward the flow axis indicating that perfusion pressure increases out of proportion to flow. Therefore, the majority of studies support the concept that the left coronary vascular bed at least is capable of autoregulation. Extravascular Compression The heart contracts and relaxes from one cardiac cycle to the next, and in doing so causes a cyclic compression of the coronary vessels, thus contributing to the resistance of this vascular bed (Gregg, l963). The greatest degree of compression on the coronary vessels occurs during ventricular systole. Intramyocardial pressure is responsible for vessel compression, and is primarily determined by interventricular pressure (Kirk and Honig, l964). As a result, the vessels supplying the left ventricle are affected more by extravascular compression than the vessels supplying the right ventricle (Rubio and Berne, l975). Phasic flow tracings from the left coronary artery demonstrate that at the onset of isovolumetric contraction, left CBF decreases abruptly to the point where zero flow or even back flow may occur (Gregg and Fisher, l963; Folkow, l97l). As left ventricular ejection develops, aortic pressure increases and CBF increases to a systolic maximum shortly before aortic pressure peaks, then slightly declines to the end of systole. At the onset of isovolumetric relaxa- tion, CBF suddenly increases to its peak value and then gradually declines as aortic pressure declines throughout diastole. Therefore, the largest fraction of total left CBF occurs during ventricular diastole. It is during this period of the cardiac cycle (diastole) that the passive changes in vessel caliber offered by extravascular compression is at its nadir. The resistance values calculated from the ratio of diastolic pressure to diastolic flow most accurately reflect the vasomotor state of the coronary vasculature (Rubio and Berne, l975). The throttling effect of extravascular compression on CBF can be 10 demonstrated by suddenly producing ventricular asystole by vagal stimu- lation. This results in a 50% increase in CBF above the control flow, and the extent of this increment in CBF is believed to be representative of the magnitude of the mechanical compressive factors which impede CBF (Sabiston and Gregg, 1957; Berne, T974). Such an impediment of flow increases with elevated heart rate due to a greater time spent in systole. Lewis et_al. (l96l) have shown that the contribution of sys- tolic contraction to overall coronary vascular resistance is little altered following the administration of isoproterenol, norepinephrine or epinephrine when the chronotropic effect of these agents are pre- vented. This is unusual since it is generally accepted that positive inotropic agents increase tension development and intramyocardial pres— sure. In the right coronary bed, an analysis of phasic flow shows that flow follows the aortic pressure tracing, owing to the fact that coro- nary artery pressure far exceeds the intramyocardial tension produced by right ventricular contraction (Folkow, l97l). As a result, systolic flow in the right coronary artery exceeds diastolic flow, and the flow pattern follows the contour of the coronary or aortic pressure curve (Gregg, l937). Therefore, extravascular compression contributes little to passive changes in right coronary vessel caliber in the normal heart. However, Brooksggral. (l97l) have shown that when right ventricular systolic pressure rises above normal levels, right CBF is impeded by systolic compression. ll Vascular Waterfall Phenomenon As previously pointed out, during ventricular systole, blood flow in the left coronary artery decreases, stops or in some cases reverses direction. The mechanism by which this occurs has not been completely delineated. One possible explanation is that during systole, dimen- sional changes in the ventricle occur which could produce pinching or kinking of the myocardial resistance vessels, thereby limiting coronary flow. Such dimensional changes have been shown to have only a minimal effect on the increases in resistance seen during systole (Downey, Downey, Kirk, l974). A second hypothesis that has been put forth to explain this is the vascular waterfall phenomenon. An understanding of this phenomenon is gained by using the concept of the Starling resistor. The Starling resistor employs a thin walled collapsible tube traversing a chamber in which pressure surrounding the tube can be set at any desired level as a means of controlling resistance (Knowlton and Starling, l9l2). In a study which focused on flow through collapsible tubes, Holt (l94l) found that lowering the outflow pressure of the tube did not significantly change flow if the outflow pressure was less than the external pressure surrounding the collapsible segment of the tube. This concept was later applied by Permutt gt a1. (l962) to the regula- tion of blood flow through the pulmonary vascular bed. Under conditions where the downstream pressure is less than the external pressure sur- rounding the collapsible tube, the pressure gradient for flow is the difference between the upstream pressure or inflow pressure and the external pressure~ according to the waterfall theory. Therefore, down- stream or outflow pressure may be raised or lowered and have no effect l2 on flow as long as the outflow pressure remains below external pres- sure. Such a mechanism could play a role in the production of the unique phasic flow pattern seen in the left coronary artery throughout a car- diac cycle. In a recent study by Downey and Kirk (l975), the pressure- flow relationships of the maximally dilated (adenosine infusion) left coronary vascular bed were determined for the beating and arrested heart. A linear pressure-flow relationship was observed over the range of 20 to 200 mm Hg when tissue pressure was minimal (arrested state). In the beating heart, the pressure—flow curve was parallel to that of the arrested state but was shifted to the right indicating a higher per- fusion pressure was needed to deliver the same amount of flow. A mathe- matical model, also developed in this report, predicted that if the pressure-flow relationships for the arrested and beating hearts were linear and parallel, the vascular waterfall was the only mechanism con- tributing to the systolic inhibition of coronary blood flow. If factors other than the vascular waterfall were acting to inhibit coronary flow, the two lines would be linear but not parallel. The vascular waterfall undoubtedly has very little effect in the determination of right coronary blood flow in normal hearts. As previ- ously mentioned, the phasic flow pattern follows that of the aortic pressure curve with the greatest flow occurring during systole. It is also likely that intraluminal coronary pressure at any point along the vascular circuit is equal to if not greater than the intramyocardial pressure generated during right ventricular contraction, although the l3 measurement of this pressure has not been reported. In a recent report, Bellamy and Lowensohn (l979) demonstrated that there was little measur— able difference between the pressure-flow relationships obtained during systole (systolic pressure vs. systolic flow) and diastole (diastolic pressure vs. diastolic flow). The only condition in which a significant waterfall effect was seen was when right ventricular pressures (and muscle mass) were increased in dogs with congenital pulmonic stenosis. In such animals, the diastolic and systolic pressure-flow curves diverged considerably, indicating a significant inhibition of right coronary blood flow during systole. It therefore appears that the vascular waterfall phenomenon may play a role in the limitation of left coronary blood flow during systole, but plays no role in the normal situation in the determination of blood flow in the right coronary artery. Transmural Distribution of Blood Flow Kirk and Honig (l964) measured intramyocardial pressure by using a 26 gauge curved needle with a 2 mm gap in the midsection (where the wall of the needle was filed away) and inserted it into the myocardium such that the open tip on the end of the needle emerged from the epi- cardial surface. The opening in the tissue created by inserting the needle forms a collapsible segment connecting the intact portions of the needle. Fluid is forced through the needle from a pressurized reservoir. Therefore, fluid flow should cease when the external pres- sure on the collapsible segment exceeds the distending pressure. By varying the pressure driving the fluid through the needle, a measure of l4 intramyocardial pressure could be obtained. This measurement was also accomplished with the use of catheters with pressure sensitive tips. They reported that subepicardial pressure is zero and increases linearly across the myocardial wall, with the greatest pressures developed in the subendocardium. Subendocardial pressure was found to be equal to or greater than interventricular pressure during both systole and diastole. Buckberg et a1. (l972,l973) have demonstrated that blood flow is evenly distributed across the myocardial wall even though intramyo- cardial pressure may be different in each of the muscle layers. This is possible if diastolic resistance is relatively less in the subendocardium compared to subepicardium. Therefore, subendocardial layers will receive a greater amount of blood flow during diastole than subepicardial layers, with the next result over the period of a minute being equal distribution of blood flow between subendocardium and subepicardium (Moir, l972). Since the subendocardial vessels have relatively less vascular tone and are therefore more vasodilated than subepicardium, the degree of additional vasodilation obtainable in response to increased myocar- dial oxygen consumption, or decreased CBF as a result of decreased coro- nary perfusion pressure or stenosis, is limited (Rubio and Berne, l974; Neill §t_al,, l975). In recent studies by Neill et a1. (l975) and Griggs_et.al. (l972) in which CBF was decreased by coronary artery stenosis, the subendocardial layers became relatively more ischemic compared to subepicardial layers, and anerobic metabolism was found to increase in the subendocardial layers. These data would suggest that IS the subendocardium is more susceptible to damage induced by hypoxic or ischemic conditions than is the subepicardium. II. Neuro-humoral Factors Coronary Adrenergic Receptors It has been difficult to elucidate the direct effects of sympa- thetic nerve stimulation and subsequent norepinephrine release on the coronary vascular bed, since nerve stimulation results in a variety of other responses which produce secondary effects on the coronary bed, such as increased heart rate, contractility, extravascular compression and myocardial oxygen consumption (Berne, l974; Berne, l964). However, the direct effects of norepinephrine on coronary arteries have been demonstrated in experiments utilizing isolated coronary arterial strips, which removes the vessels from the mechanical and metabolic influence of the myocardium. Zuberbuhler and Bohr (l965) utilized this technique in demonstrating that norepinephrine (which has primarily alpha receptor activity with some beta receptor activity) causes relaxation of small (400 u) left coronary vessels from dogs. Following administration of a beta receptor blocking agent, the relaxation in response to norepine- phrine (NE) is blocked and constriction of the strips occurs. In large coronary artery strips, both NE and epinephrine (EPI) cause constric- tion, a response which can be blocked by the alpha receptor blocking agent dibenzyline. Isoproterenol (a beta agonist) causes relaxation of large coronary artery strips. From this study it appears that left coronary artery strips contain both alpha-vasoconstrictor receptors and beta-vasodilator receptors with large coronary arteries containing more l6 alpha than beta receptors and vice versa for small coronary arteries. Other experimental studies have demonstrated the presence of alpha- vasoconstrictor and beta-vasodilator receptors in man and pig as well (Anderson et_al., l972; Bayer et_al., l974). Population densities of adrenergic receptors may differ from one specific portion of the vascu— lature to another, and therefore the response of a particular agonist may also differ in different vascular segments. This concept is sup- ported by the work of Malindzak et a1. (l978) in which left coronary end- diastolic resistances were determined for the large and small coronary arteries in intact, anesthetized open chest dogs in response to NE, EPI and stellate ganglion (sympathetic) stimulation. This study demon- strated that large coronary artery end-diastolic resistance was increased l90, l95, and l30% by NE, EPI and stellate stimulation, respectively. Small artery resistance was decreased by 53, 49, and 60%, respectively. The increase in large artery resistance was blocked by phenoxybenzamine (an alpha blocker). Following propranolol administration (beta blockade) small artery resistance increased by l70, l80 and l25% in response to NE, EPI and nerve stimulation, respectively. This demonstrates that large and small coronary arteries may respond differently to adrenergic agonists. Other investigators have shown that the response of the total left or right and left coronary vascular bed to sympathetic nerve stimu- lation or intracoronary norepinephrine infusion is of a biphasic nature. Studies by Berne et a1. (l958,l965) and Hardin et_al. (l96l) in which anesthetized dogs with beating working and nonworking hearts respectively, were subjected to stellate ganglion stimulation or NE administration. This resulted in a brief period of vasoconstriction which preceded the 17 increase in heart rate, followed by a prolonged vasodilation associated with a reduction in coronary sinus P02. Feigl (l967) reported that following beta blockade (which prevents the beta mediated increases in heart rate and myocardial contractility) stellate ganglion stimulation produced only coronary vasoconstriction which could be blocked by alpha receptor blockade. These studies suggest that the coronary vasodilation seen with sympathetic nerve stimulation is probably due to the fact that myocardial metabolism is greatly enhanced due to beta stimulation in the myocardium. While alpha receptor activation is most likely occurring simultaneously, metabolic vasodilation is the dominant response. Only when the increase in myocardial metabolism is prevented does alpha mediated coronary vasoconstriction become manifest. Pitt _et.al. (l967) demonstrated that systemic administration of various catecholamines to conscious dogs resulted in coronary constriction when the animals were pretreated with a beta blocking agent. This constric- tion could be prevented by alpha blockade. A transient coronary vasodi- lation was sometimes seen in dogs which were not beta blocked in response to catecholamine administration which preceded changes in myocardial or systemic hemodynamics. This vasodilation could be blocked with propranolol, suggesting that the coronary vascular bed may exhibit direct beta receptor mediated vasodilation. Klocke et a1. (l965) demonstrated the presence of beta receptors in the coronary vasculature in a non—beating, non—working, K+ arrested heart supported by an extracorporeal circulation. Injection of isopro- terenol into the coronary arteries resulted in an increase in CBF which was accompanied by an increase in coronary sinus P02. The effect could be blocked by pretreatment with propranolol. These studies suggest that there are two types of beta receptors, one located in the myocar- dium and the other located in the coronary vasculature. The myocardial beta receptors are called Beta l, and when stimulated by beta agonists such as isoproterenol, NE or EPI mediate the augmentation of heart rate and myocardial contractility. The vascular beta receptors Beta 2, when stimulated by the same agonists, mediate vascular smooth muscle relaxa— tion and hence vasodilation (Braunwald gt_al,, l976). McRaven et__l. (l97l) demonstrated that coronary vasodilation induced by intracoronary isoproterenol was reduced by 30% after pretreatment with practolol, a specific beta l blocking agent. This study demonstrated that a signifi- cant amount of the coronary vasodilation seen with administration of beta agonists is due to direct stimulation of the vascular beta 2 receptors. Hamilton and Feigl (1976) also demonstrated the presence of coronary beta 2 receptors in the dog; however, they reported that the coronary vasodilation seen with intracoronary isoproterenol following beta l and alpha receptor blockade was slight and concluded that there is little functional significance of these vascular receptors. Baron and Bohr (1972) reported that practolol blocked the vasodilator action of isoproterenol in isolated strips of coronary arteries, which suggests that these coronary beta receptors could be of the beta l variety as well. These studies provide evidence for the presence of beta receptors mediating vasodilation in coronary arteries; however, the functional significance is controversial. Vatner et_al, (l974) demonstrated that intravenous norepinephrine infusion in conscious dogs produced a brief fall in left coronary l9 vascular resistance which was followed by a sustained rise in coronary resistance and a reduction in coronary sinus PO . They also showed that the early vasodilation could be prevented by beta blockade and the prolonged vasoconstriction could be prevented by alpha blockade. This study was repeated on the same chronically instrumented animals during sodium pentobarbital anesthesia. Norepinephrine, in this case produced only coronary dilation. This study suggests that the alpha vasoconstric- tor mechanism may be predominant over the vasodilator mechanism in the conscious dog. Pitt et_al. (l967) reported similar findings for the conscious dog. Other studies suggest that the overall response of the left coro- nary vascular bed to neurogenic, humoral and metabolic influences is in part dictated by the competition between alpha mediated vasoconstriction and metabolic or beta mediated vasodilation. Mohrman and Feigl (l978) demonstrated that adrenergic stimulation via intracoronary NE infusion or baroreflex mediated sympathetic stimulation produced a significantly higher oxygen delivery per unit increase in myocardial oxygen consump- tion after alpha receptor blockade than they did before. Prior to alpha block, adrenergic stimulation resulted in a significant increase in myo- cardial oxygen extraction and a decrease in coronary venous oxygen con- tent, presumably as a result of an alpha mediated restriction of blood flow. Following alpha receptor blockade, myocardial oxygen extraction and coronary venous oxygen content changed only slightly. These authors concluded that the net effect of alpha receptor activiation was to restrict the metabolically related flow increase by approximately 30%, thus increasing oxygen extraction. This study demonstrates that alpha 20 mediated coronary constriction is present even in the presence of potent metabolic vasodilator influences. While vasodilator metabolites decrease coronary vascular resistance, they only act on small coronary resistance vessels. These vessels may be maximally dilated during enhanced metabolic activity, ischemia, etc., but concomitant alpha receptor activation may result in a superimposed increase in large . artery resistance (Malindzak at al., l978). This concept is also sup- ported by a clinical investigation by Mudge et a1. (l979) in which the coronary vascular response to adrenergic stimulation was obtained in 2 groups of patients, those with ischemic coronary artery disease (CAD) and those without (control group). Adrenergic stimulation was produced by the cold-pressor test, arterial blood pressure recorded, coronary blood flow determined by the continuous thermodilution method, and heart rate was held constant. In patients with CAD adrenergic stimulation resulted in a decrease in coronary blood flow in seven of thirteen patients, with a mean increase in coronary resistance of 24% for all thirteen patients, In the control group subjected to the cold—pressor test, coronary blood flow increased and resistance remained unchanged. This study suggests that patients with coronary stenosis may have limited coronary vasodilator reserve due to a significant degree of ischemia, with near maximal vasodilation of the (small) coronary resistance vessels. Consequently, adrenergic stimulation and alpha receptor activation results in vasoconstriction, possibly due to increases in large coronary artery resistances. This obviously would result in an even more pronounced ischemia. 21 Prinzmetal's Angina and Coronary_yasospasm This concept of large coronary artery vasoconstriction has gained much attention in recent years as coronary artery vasospasm has been identified clinically, and found to be capable of producing transient myocardial ischemia seen in patients with Prinzmetal's angina (Oliva .gtial,, l973). It is also interesting to note that large coronary artery vasospasm can occur in normal vessels as well as vessels with significant stenotic lesions (Oliva et 31., (l973). It can also occur in the denervated hearts of transplanted patients, and occurs with sur- prising frequency in the right coronary artery (Zacca gt_al., l979). Coronary vasospasm has recently been shown to be intimately involved in the development and pathogenesis of myocardial infarction (Maseri $3.212, l978). While the actual mechanisms involved in this severe form of large coronary artery vasoconstriction have yet to be defined, several physiological factors have been found to potentiate and/or initiate vasopastic activity; namely, systemic alkalosis and hypocapnia (Yasue .et_al., l978), sympathetic nerve activity (Yasue et 11., l974), as well as the release of thromboxaneAQ from platelet aggregations (Ellis_et_al., l976). The fact that alpha blocking agents have been successful in the treatment and prevention of large coronary artery vasospasm suggests that the mechanism may involve catecholamine activation of large coro- nary artery alpha receptors (Braunwald, l978). Other drugs such as nifedipine (a coronary dilator which acts to block slow calcium channels in smooth muscle) and verapimil (another calcium antagonist) have also been successful in the treatment of this condition, suggesting that 22 vasospastic activity may be the result of deranged calcium transport or metabolism in coronary vascular smooth muscle. Baroreceptor Reflex Physiological neural reflexes such as the baroreceptor reflex have been shown to affect coronary vascular resistance. In l963, Szentivanyi and Juhasz-Nagy demonstrated that carotid sinus hypotension produced by bilateral carotid occlusion in anesthetized, vagotomized dogs produced tachycardia, an increase in systemic arterial pressure and a decrease in left coronary vascular resistance. A repeat of this maneuver after beta—blockade with propranolol showed that the tachy- cardia and myocardial inotropy were prevented, and coronary resistance then increased. This increase in resistance in response to baroreflex sympathetic stimulation was prevented by cardiac sympathectomy. This study was later repeated by Feigl (l968) with similar results. Powell and Feigl (l979) subsequently published a report in which anesthetized, closed chest, vagotomized dogs were pretreated with propranolol and subjected to baroreflex stimulation. Myocardial oxygen consumption and heart rate did not change and the rise in arterial pressure was limited to l5 mmHg with the use of a pressure control reservoir. During baro- reflex stimulation, left diastolic coronary resistance increased by 2l%. Following alpha blockade, resistance increased only 5%. It appears that baroreceptor reflex stimulation produces left coronary vasoconstriction only in beta blocked animals, and this vasoconstriction is mediated by alpha receptor activation. 23 Sympathetic Tone There is good experimental evidence to suggest that a degree of tonic coronary vasoconstriction exists which is mediated by outflow from sympathetic efferents. Brachfield gt_al. (l960) demonstrated that acute surgical denervation of the heart in the anesthetized dog produces a decrease in coronary vascular resistance and a fall in coronary arterio—venous oxygen extraction. Vatner_et_al. (l970) supplied further evidence to support the concept of tonic coronary vasoconstrictor tone mediated by sympathetic nerves. In this study on conscious dogs, electrical stimulation of the carotid sinus nerve resulted in a decrease in aortic blood pressure, heart rate and left coronary vascular resist- ance. After atropine (a parasympatholitic agent) and propranolol (a beta blocker) were administered, cartoid sinus nerve stimulation still produced a decrease in aortic pressure and coronary resistance. However, after alpha blockade alone with phentolamine or sympathetic blockade with guanethidine, carotid sinus nerve stimulation produced no change in coronary resistance. Furthermore, sinus nerve stimulation during tread- mill exercise (which in itself decreases coronary resistance) produces a further decrease in coronary resistance. The authors conclude that a degree of resting sympathetic vasoconstrictor tone is present in the conscious dog which is withdrawn during carotid sinus nerve stimulation resulting in coronary vasodilation. They also suggest that resting sympathetic tone persists in the coronary bed of the dog even during exercise when there is concomitant metabolic vasodilation. Vatner _gt_al. (1971) also demonstrated that alpha mediated constrictor tone in the coronary vascular bed may be rapidly withdrawn relexly. 24 This conclusion was drawn from their study in which baboons were chronic- ally instrumented for monitoring left coronary blood flow by radiotelem- etry. They reported that spontaneous increases in CBF and decreases in coronary resistance occurred periodically while the baboons were asleep, and that these changes were independent of changes in heart rate or arterial pressure. Carotid Body Chemoreceptors The effect of carotid body chemoreceptor stimulation on coronary vascular resistance was examined by Ehrhart at al. (l975). In this study, the carotid bodies of dogs were selectively stimulated with hypoxemia and hypercapnic blood. Left coronary resistance did not change under conditions of natural flow or constant flow perfusion, both with and without vagotomy. This study suggests that local stimula- tion of the carotid chemoreceptors has little effect on coronary resist- ance. This study is in apparent contrast to the work of Hackett gt_al. (1972) in which the circumflex coronary artery of dogs was perfused at constant flow, and the animals were treated with practolol (Bl blocker) and electrically paced in order to minimize the indirect effects of chemoreceptor induced myocardial responses on coronary resistance. The aortic or carotid chemoreceptors were stimulated by intra-arterial in- jections of nicotine or cyanide. Such activation of the chemoreceptors produced substantial coronary vasodilation which could be blocked by vagotomy or atropine. These authors suggest that chemoreceptor activa- tion produces a substantial vagally mediated reflex which results in coronary vasodilation. Indeed, this study demonstrates the ability of 25 chemoreceptors to reflexly alter coronary resistance. However, this study was carried out using pharmacological stimulation of chemorecep- tors. Therefore, this reflex may not be of functional significance when elicited by physiological stimuli in the basically intact animal as is suggested by Ehrhart et_al. (l975), and by studies which are summarized below, dealing with the effects of acetylcholine and the vagus nerve. Acetylcholine and Vaggs Nerve While most investigators agree that acetylcholine is a potent coronary vasodilator (Berne, l958; Denison and Green, l958; Gregg, l950) there is less agreement concerning the effects of vagal stimulation on coronary resistance. Schreiner 23.21- (l957) demonstrated that vagus nerve stimulation in the open-chest dog produced no change in coronary inflow, outflow or coronary sinus oxygen saturation when heart rate was held constant. Other investigators using similar experimental tech- niques also failed to obtain any coronary vascular response to vagal nerve stimulation which could not be accounted for by reductions in blood pressure or heart rate (Denison and Green, l958; Szentizanyi and Juhasz- Nagy, l959; Wang et_al , 1960). These studies were preceded by the work of Garcia-Ramos et a1. (l950), in which vagal stimulation to paced or fibrillating dog hearts resulted in an increase in coronary blood flow using constant pressure coronary perfusion. This finding is sup- ported by the work of Feigl (l969) who also demonstrated that electrical vagal stimulation of the heart in anesthetized, open-chest dogs that were beta blocked, paced and sympathectomized, resulted in a decrease in aortic blood pressure and an increase in left coronary blood flow. 26 Late diastolic coronary resistance fell 62% from control with 5 seconds of vagal stimulation. This effect could be blocked by atropine. This study provided evidence that vagal stimulation results in direct para- sympathetic coronary vasodilation which is independent of vagally mediated negative inotropic and chronotropic effects. While this study demonstrates that the coronary bed receives parasympathetic innervation, the functional significance of this innervation may be limited in the intact animal. III. Myogenic Factors One of the possible mechanisms for regulating blood flow to a vascular bed is the direct response of smooth muscle to a change in transmural pressure or tension. This concept originated in I902 with Bayliss who first suggested that an active myogenic relaxation of vascular smooth muscle could be the mechanism of the hyperemia seen following brief (8—15 sec) occlusions of the arterial supply to the dog hindlimb. This study came under criticism by Anrep in l9l2, who concluded from his experiments that the periods of occlusion used by Bayliss were long enough to allow for tissue metabolite accumulation, and that the hyperemia was most likely due to a metabolic vasodilation. Folkow (I949) came to the defense of Bayliss with a study in which reactive hyperemia was produced by partially reducing intravascular pressure. He suggested that since flow was only minimally reduced the resulting hyperemia was most likely due to a myogenic phenomenon. Berne (l964) has criticized the myogenic theory on the following basis: for flow to be maintained constant with an increase in perfusion 27 pressure (autoregulation), the vascular smooth muscle cells must con- tract to reduce the luminal diameter of the vessel. If stretch is the stimulus for smooth muscle contraction, then following an initial stretch, the vessel contracts to its original size and the stimulus for further contraction of the smooth muscle is gone, yet the newly achieved vessel diameter is maintained. Folkow (I960) addressed this problem by providing evidence to suggest that the resistance vessels respond to stretch by increasing their frequency of contraction and therefore these vessels spend a greater amount of time in the contracted state. The overall effect of this response would be an increase in vascular tone. This concept was supported by Johansson and Bohr (I966) who reported that helical strips from canine paw arteries responded to an increased passive stretch by increasing the frequency of spontaneous rhythmic contractions. Another explanation for the myogenic theory is that vascular smooth muscle responds to changes in tension rather than changes in length. As perfusion pressure is increased, tension in the vessel wall is increased which elicits a contraction of the vascular smooth muscle. This reduces the luminal diameter until the total vessel wall tension returns to control levels. Burton and Stinson (I960) as well as Sparks and Bohr (I962) demonstrated that active tension due to vascular smooth muscle is increased in response to passive distension or stretch. Therefore, the increase in total wall tension in response to stretch can be restored only by a reduction in the internal radius of the vessel to a value less than the original control value, according to the law of Laplace (T = PR/W), where T = wall tension, P = transmural 28 pressure, R = vessel radius, W = vessel wall thickness. The mechanism by which this occurs is thought to be related to the cell membrane acting as a type of tension-receptor. Stretch causes changes in mem- brane activity of vascular smooth muscle cells (single unit) which precede the mechanical contractile response (Burnstock et al., l963). Since the generator potential of the invertebrate stretch receptor is dependent on both the rate and amplitude of the applied stretch (Ezyguirre and Kufler, I955), it was suggested that perhaps membrane depolarization of smooth muscle is also dependent on rate and amplitude of stretch. Sparks (l964) demonstrated that active tension development in the human umbilical artery in response to passive stretch is directly related to the resting tension, the rate and increment of stretch. These data provide support for the theory that active responses by vascular smooth muscle could function in the local regulation of blood flow. As mentioned previously, autoregulation could be due to a constriction or relaxation of vascular smooth muscle in response to an increase or decrease in perfusion pressure, respectively. To what ex- tent a direct response of vascular smooth to changes in transmural pres- sure is responsible for autoregulation in the coronary bed is difficult to ascertain. The little amount of evidence to support this is purely conjectural, and can be explained just as easily on the basis of changes in metabolic activity. The myogenic hypothesis has also been applied in theory to the genesis of exercise hyperemia, with the contraction of the exercising muscle passively decreasing the stretch of the vascu- lar smooth muscle causing smooth muscle relaxation and hyperemia. 29 This theory is not substantiated by a recent study by Bacchus (l978) in Which intramuscular pressure was passively increased to levels seen during contraction, and no evidence of myogenically mediated vasodila- tion was seen. The reduction in intravascular pressure as a result of coronary artery occlusion could theoretically cause relaxation of the vascular smooth muscle and vasodilation. Release of the occlusion would then result in a reactive hyperemia. One must bear in mind that coro- nary occlusion has profound effects on myocardial metabolism, and it is difficult to separate the myogenic and metabolic components of the post- occlusion hyperemia. However, Eikens and Wilcken (l974) have observed reactive hyperemia following coronary occlusions as short in duration as one to two beats, and they point out that occlusions this brief are un— likely to result in a significant degree of myocardial ischemia, there- fore the hyperemic response following brief occlusions is probably myogenic in nature. Olsson (l975) supports the myogenic theory as the mechanism of the hyperemia following extremely brief coronary occlusions, but suggests that any contributions it might make to hyperemia following longer occlusions are obscured by dominant metabolic factors. IV._Myocardial Oxygen Consumption The heart relies almost exclusively on the (aerobic) oxidation of substrates for the generation of energy. Therefore it can only tolerate a small oxygen debt. In the steady state, myocardial oxygen consumption (MVOZ) provides a relatively precise measurement of the hearts metabolic status. MVO2 is determined by the product of coronary blood flow and the coronary arterial-venous oxygen content. The normal left ventricle 30 in intact dog or man consumes 8-10 ml/min/IOO gm (Folkow and Neil, 1971). There are three major determinants of MVOZ: myocardial wall tension, contractility and heart rate. Sarnoff_et_al. (1958) reported in a study which utilized the isolated perfused canine heart that total wall tension as estimated by the tension-time index is the primary determinant of MVO Tension time index is determined by area under the 2. aortic systolic pressure curve or the mean systolic pressure x the duration of systole. This study also demonstrated the relative contribu- tion of preload (flow work) and afterload (pressure work) to MVOZ. To study pressure work, left ventricular work was increased by elevating aortic pressure (afterload) while cardiac output and heart rate were held constant. The increase in work (175%) was paralleled by an in- crease in MVO2 (178%), indicating that afterload is an important deter- minant of MVOZ. To study flow work, left ventricular work was elevated by augmenting cardiac output (preload) while mean aortic pressure and heart rate were held constant. In this case, the increase in work (696%) was accompanied by only a 53% increase in MVOZ, indicating that flow work is relatively unimportant in determining MVOZ. The importance of heart rate as a determinant of MVO2 was also reported by Sarnoff _et.al. (1958). Using the sanm~ aforementioned preparation, flow work was increased by elevating cardiac output and holding aortic pressure constant at 120 mmHg and heart rate at 120 b/min. This type of flow run was again repeated at constant heart rates of 160 b/min, and 200 b/min. It was found that a higher heart rate at any given work level was accompanied by an increased MVOZ, demonstrating the importance of 31 heart rate on MVOZ. Klocke E£.El- (1966) demonstrated that the oxygen requirements of myocardial depolarization and repolarization constitute only 0.5% of the total oxygen consumed by the normal working heart. Therefore, the increase in MVO2 with increased heart rate is not due to the cost of electrical activity but due to the cost of contractile activity. Braunwald et_al, (1958) demonstrated that oxygen consumption and left coronary blood flow are well correlated, as had been previously shown by other investigations (Eckenhoff et al., 1947; Foltz et_al., 1950; Alella gt _l., 1955). However, the study by Braunwaldl_t__l. (1958) demonstrated that coronary blood flow is significantly augmented by increased pressure work (afterload), and is slightly increased by flow work (preload). The same was also true for oxygen consumption, providing further evidence to support the concept that one of the pri- mary determinants of coronary blood flow is myocardial oxygen consump- tion. The importance of contractility as a determinant of MVO2 was demonstrated by Graham et_al. (1968). In this study, which utilized the isovolumetrically contracting left ventricle, peak developed tension was increased at a constant calculated Vmax by increasing ventricular volume, and the effect on MVO was determined. Following this, calcu- 2 lated Vmax was increased at a constant peak developed tension by infus- ing norepinephrine and decreasing ventricular volume to match the tension existing before norepinephrine infusion, and MVO2 was redeter- mined. MVO2 consistently increased either with increased Vmax or increased developed tension, and were related by the following equation: 32 M1102 ml/beat/lOO gm LV = K + 0.25 peak developed tension (g/cmz) + 1.43 Vmax (cm/sec). This study demonstrated that the rate of MVO2 is sensitive to both tension and contractility, and is 6 times more sensitive to changes in the maximum velocity of contraction (index of contractility). The studies by Eckenhoff et_al. (1947) demonstrated a correlation between the magnitude of coronary blood flow at any given level of myocardial oxygen consumption, suggesting a coupling mechanism between myocardial metabolic activity and blood flow. This study, which was later confirmed by other investigators, gave rise to the hypothesis that myocardial oxygen consumption is the primary determinant of coronary blood flow. This relationship shows a positive correlation even at very high heart rates where left CBF is impeded by a greater amount extravascular compression. The ratio of myocardial oxygen consumption to oxygen delivery determines myocardial tissue P02. Tissue P02 is normally maintained at a very low level and is probably less than the P seen in coronary venous blood (25 mmHg). The coronary venous O oxggen content is normally about 5 ml 02/100 ml blood. Increasing myo- cardial metabolic activity (and hence MVOZ) not only results in an increased CBF but an increased oxygen extraction such that venous oxy- gen content may fall to less than 1 m1 02/100 ml blood (Berne gt_gl., 1957; Scott, 1961). An elevation of tissue P02, as judged by an ele- vated coronary venous oxygen content of > 6 ml 02/100 ml blood, disrupts the correlation seen between oxygen consumption and CBF. During such conditions, CBF becomes primarily dependent on perfusion pressure 33 (Scott, 1961). This situation rarely occurs under physiological condi- tions however. The precise mechanism responsible for maintaining CBF at a level appropriate for the existing metabolic needs of the myocardium is not known. One possible mechanism is a direct effect of tissue P02 or intraluminal P02 on vascular smooth muscle, thereby regulating coronary resistance and hence blood flow. A second possible mechanism is re— lated to the release of vasoactive metabolites by cardiac cells in response to increased metabolic activity and subsequent lack of oxygen. There are several such vasoactive agents currently under investigation as potential mediators in the local metabolic regulation of blood flow. Among those that will be discussed in this review are oxygen, carbon dioxide, hydrogen ion, potassium ion, adenosine and adenine nucleotides, osmolality and the prostaglandins. V. Metabolically Related Vasoactive Agents For a chemical to be considered as an important factor in the local regulation of blood flow, it must satisfy all of the following criteria (Haddy and Scott, 1968; Haddy and Scott, 1975; Rubio and Berne, 1974): a) It should be a naturally occurring substance that is present within the tissue, and its breakdown products should be detectable in the tissue or venous effluent. b) Intra-arterial administration of the substance, in concentra- tions similar to those determined for a particular vascular bed should produce a response in the resting organ that is 34 similar to that seen during the conditions in which it is naturally produced. c) It should not disrupt the normal function of the organ to which it is administered. d) The time course of its appearance and disappearance should correspond to the time course of the local regulatory response. Oxygen Tension It is welléknown that hypoxia is considered to be the most potent physiological stimulus in producing coronary vasodilation. But the mechanism by which low oxygen tension produces a reduction in coronary resistance is controversial. Such controversy began in 1913 when Maukwalder and Starling suggested that coronary vasodilation seen in the heart lung preparation in response to hypoxia or epinephrine administra- tion was due to the production and release of vasodilator metabolites. This idea was challenged in 1925 by Hilton and Eichholtz who suggested that with hypoxemia, the reduced oxygen tension acts directly on the blood vessel wall to produce coronary vasodilation. Their assumption was based largely on the fact that MVO2 was constant during the hypoxemia and therefore no tissue oxygen deficit occurred. It has been difficult to distinguish between the direct and indirect effects of low oxygen tensions during hypoxic vasodilation. Duling and Berne (1970) reported that in the rat cremaster muscle and the hamster cheek pouch, the arteriolar wall is freely permeable to oxygen, and that the P in the resistance vessel wall is largely 02 35 determined by the luminal P02. Furthermore, there is a substantial loss of oxygen along the length of the arterial tree. However, it is reason- able to assume that the arteriolar P02 closely resembles arterial P02. Low P02, similar to that seen in coronary venous blood has had much attention as a possible mediator of vascular smooth muscle relaxation. Detar and Bohr (1968), using isolated helical strips of rabbit aorta, demonstrated that high oxygen tensions in the bathing solution produces contraction and low oxygen tension produces relaxation, indicating a direct effect of oxygen on the contractile activity of vascular smooth muscle. However, Gellai 23.11- (1973) demonstrated that resting tension of helical coronary artery strips was unaffected or only minimally decreased when the P02 of the bathing solution was decreased to 5-10 mmHg. Only at a P02 of 0 mmHg was contractile tension markedly de- pressed. Furthermore, Duling (1974) demonstrated that arteriolar diam- eters were not significantly different when a nitrogen containing bath- ing solution (P02 = 0 mmHg) or an oxygen rich bathing solution (P02 200 mmHg) were injected by micropipette around arterioles under direct observation jg_yiyg. Therefore, there still is no definitive answer as to the direct role of oxygen in the regulation of resistance vessels. Studies on the intact heart by Sobol et a1. (1962) demonstrated that an increase in P02 induced by ventilation with 100% O2 resulted in a decreased coronary blood flow as measured by coronary sinus effluent. Daugherty gt_gl, (1967) demonstrated that a local decrease in left coro- nary artery PO to < 40 mmHg resulted in a decrease in coronary resist- 2 ance and left ventricular contractile force. Berne t al. (1957) studied the effect of local left coronary hypoxemia in the open chest 36 dog. At normal perfusion pressure, hypoxemia resulted in a large increase in CBF. After a return to normoxemia, CBF was increased by increasing perfusion pressure. Hypoxemia then produced a greater extraction of oxygen from coronary blood, a high coronary sinus P02 and no change in CBF. It was determined that if the hypoxemia produced a coronary sinus oxygen content above 5 vol%, CBF was unchanged, whereas it increased proportionately with a reduction of sinus 02 content below this level. Therefore, they concluded that arterial P02 was not a critical factor in the production of hypoxic vasodilation, but that myocardial P (as estimated by venous P may be the important factor 02 02) through a mechanism by which low 02 elicits the production of vasodilator metabolites. Finally the fact that the duration of coronary occlusion is approximately proportional to the duration of the hyperemia further supports the concept of vasodilation mediated by metabolite release. If PO had a direct effect on the contractile state of vascular smooth 2 muscle in resistance vessels, then the tone of this coronary smooth muscle should return upon the release of occlusion and immediate restor- ation of oxygen rich blood. This evidence is against the concept that oxygen plays a major role in the direct control of coronary resistance vessels. Carbon Dioxide There is evidence that CO2 ([H+]) is vasoactive in the left coro- nary circulation. Case and Greenberg (1976) reported that hypocapnia (P = 23 mmHg), produced locally in the coronary arterial perfusate CO 2 of open-chest dogs resulted in an increased coronary resistance of 84% 37 with myocardial metabolism held constant. Daugherty gt_gl. (1967) also demonstrated that local coronary hypocapnia in open-chest dogs produced vasoconstriction, while hypercapnia produced vasodilation. Since CO2 and H+ ion are interrelated via the bicarbonate-buffer system, it is difficult to determine whether or not CO2 is the locally vasoactive agent or whether the vasoactivity is mediated through the effect on pH. Hydrogen ions are known to exert a highly potent antagonistic action on calcium ions since these two ions compete for the same active sites, both on the transmembrane calcium transport system and at the myo- fibrillar ATPase. Thus, vasoconstriction occurs if the calcium concen- tration is increased or if the hydrogen ion concentration is decreased (Mrwa 22.21:, 1974). The effect of CO2 on coronary resistance may be a direct effect on the vascular wall, or an indirect effect through H+ ion as mentioned previously, or by affecting the production or action of vasodilator metabolites. Alella gt a1. (1955) demonstrated that the coronary sinus PCO may not always rise to vasoactive levels during 2 enhanced cardiac activity. Furthermore, Case t l. (1978) demonstrated that CO2 is a less potent coronary vasodilator than is hypoxia since coronary sinus PCO had to be increased twice as much as coronary sinus 2 P0 was decreased in order to achieve the same degree of vasodilation 2 under constant flow conditions. Feinberg et a1. (1960) also reported that CO2 was a poor coronary vasodilator during natural flow, constant pressure perfusion. In 1973, Duling reported on experiments using the hamster cheek pouch preparation that elevation of the PC02 of the bath- ing solution from O to 32 mmHg increased arteriolar diameter by 18%. In any experiment in which blood PCO is altered, there may be 2 38 concomitant changes in other parameters as well, such as serum ionized calcium, serum potassium, oxygen tension and sympatho—adrenal activity. Therefore, such experiments must be carefully interpreted. There have been several studies which have examined the effects of hypo and hypercapnia on coronary vascular resistance. Such studies have also provided evidence that excess C02 produces coronary vasodila- tion and a lack of C0 produces coronary vasoconstriction (Kittle gt al. 2 1965; Ledingham et al., 1970; Neill and Hattehauer, 1975; Vance et al., 1973). In a recent report by Rooke and Sparks (1978) the effects of systemic hyper and hypocapnia on coronary conductance during isoproter- enol induced enhancement of cardiac activity were studied. They re- ported that large changes in arterial PCO2 caused only minimal differ- ences in CBF at any given level of myocardial oxygen consumption, which suggests that CO2 plays a minor role, if any, in the regulation of CBF. This study as well as others in which systemic changes in CO2 were elicited cannot be readily applied to the role of CO2 in the local regulation of CBF since the results of such studies may easily be influ- enced by extrinsic neural (reflex) activity or changes in myocardial function other than that represented by oxygen consumption. Another possible role of CO2 in the local regulation of CBF may be that ofonctionally'interacting with adenosine in modulating coronary vascular resistance. Degenring (1976) has shown that hypercapnia or aCidosis is capable of increasing adenosine levels in the isolated perfused guinea pig heart. Furthermore, Raberger gt g1. (1975) have shown that elevated CO2 can potentiate the coronary vasodilator-activity of adenosine. This finding is also supported by the work of Merrill 39 __t_al. (1978) in the isolated perfused guinea pig heart. While there is no other information available on this point, it certainly repre- sents an interesting topic worthy of further investigation. Potassium Katz and Linder (1938) first reported the effects of K+ on the coronary circulation. It was demonstrated that in concentrations slightly above normal, K+ was a vasodilator, and at high concentrations (above normal) or concentrations below normal it acted as a vasocon- strictor. These effects are opposite to those predicted by the Nernst or Goldman constant field equations. These equations predict that a reduction of [K+]O should result in hyperpolarization of the vascular smooth muscle and hence decrease resistance and vice versa for an increase in [K+]O. This problem was addressed by Brace gt_al. (1974) who determined that when K+ was removed by dialysis from blood perfus- ing the left common coronary artery of the dog at constant flow or constant pressure, a substantial increase in coronary resistance occurred, associated with an increase in left ventricular contractile force. They postulated that the increase in coronary resistance seen with local hypokalemia was the result of inhibition of the membrane NA+-K+-ATPase which cause depolarization of the smooth muscle cells and contraction, since ouabain infusion blocked most of the vasoconstrictor response to the hypokalemia. Driscol and Berne (1957) also demon- strated the vasoactivity of K+ in the left coronary bed of the open— chest dog. However, they reported that the magnitude of increased flow with K+ concentrations ranging from 4 to 12 meq/L was far less than 40 those observed in response to physiological stimuli. They also reported that increasing cardiac activity produced large increases in CBF but no significant changes in K+ release into coronary sinus blood. Jelliffe et_gl, (1957) also demonstrated that coronary-sinus blood samples col- lected from a heart experiencing increased work or decreased 02 supply, when reoxygenated and infused into a bioassay coronary vessel, failed to elicit any vasoactive response. Such data provided evidence against K+ as an important factor in the metabolic adjustment of CBF. There is some evidence to suggest that K+ acts as an initiating factor in the vascular response to enhanced myocardial activity. Gellai and Detar (1974) reported that isolated coronary artery strips of rabbits responded to elevated potassium concentrations by a relaxation of resting tension, but that this response was only transient, lasting only 5-6 minutes. Furthermore, the release of K+ into coronary venous blood is only seen transiently when heart rate or contractility are elevated (Gilmore_et_gl., 1971; Sybers_et_al., 1971). Murray gt_al. (l979) utilized a compartmental mathematical model of the heart and its circulation which took into account vascular transit time effects to determine the magnitude of interstitial [K+] which accompanied stepwise increases in heart rate. In six of nine dogs coronary sinus [K+] was transiently elevated, and in three dogs it was sustained. The change in [K+] preceded the coronary vasodilation seen with increased heart rate, and the calculated rise in interstitial [K+] was suggested to be sufficient in magnitude to account for approximately half of the 75% decrease in coronary resistance seen. Therefore, this study suggests that under constant flow conditions, K+ seems to be involved in the 41 initial coronary vasodilation seen with increased cardiac activity. However, under natural flow conditions, stellate ganglion stimulation produced no change in coronary sinus [K+] (Scott and Radawski, l97l). Sybers et_al. (1971) has provided evidence to suggest that the release of K+ from the myocardium is prolonged and not transient when enhanced cardiac activity is produced during hypoxia. The role of K+ in the control of coronary resistance may therefore be enhanced under such pathophysiological conditions. Osmolality Gellai and Detar (1971) using isolated rabbit coronary artery strips reported that a 30 milliosmole/liter increase in the osmolality of a solution bathing the strips produced a 30% relaxation which was only of a transient nature. While this study suggests that the coronary arteries are slightly sensitive to changes in osmolality, Scott and Radawski (1971) reported that increased cardiac activity produced by left stellate ganglion stimulation in anesthetized dogs was associated with substantial coronary vasodilation yet no change in coronary sinus plasma osmolality occurred. Furthermore, Brace et_al. (1975) showed that by inducing hyposmolality in the blood perfusing the left coronary artery of anesthetized dogs inconsistently produced slight coronary vasoconstriction. Gazitua gt a1. (1971) demonstrated that infusions of hypertonic (350 mosm) sodium chloride into the left coronary artery produced a substantial fall in coronary resistance which was accompanied by an initial fall and subsequent rise in contractile force. Infusions of hypertonic dextrose or urea also produced a decrease in coronary 42 resistance which was accompanied by an increased contractile force. This study suggests that the coronary vascular bed may be slightly sensitive to changes in osmolality; however, it is not always clear whether the resistance changes seen when osmolality is altered are mediated directly through the effects of osmolality or indirectly through the effects on contractile performance. While it is apparent that osmolality does not measurably change in coronary venous blood during cardiac stimulation, it still is not certain whether changes in osmolality are involved in the local regulation of blood flow through the heart. _Ig_yitrg evidence would suggest that changes in osmolality has a direct effect on the coronary vasculature. Krishnamurty et_al. (1978) reported that small and medium coronary arteries perfused in a bath with a physiological salt solution showed relaxation when exposed to a 50 mosm increase in osmolality above normal with mannitol. With- drawal of the mannitol from the perfusate produced contraction of the vessels which was not prevented by alpha or beta blockade, nitroglycerin or norepinephrine. Furthermore, the responses to nitroglycerin, nor- epinephrine and papaverine were attenuated in the presence of hypertonic mannitol. This suggests that local changes in osmolality may attenuate the vasodilator responses to various pharmacological agents, at least in the jg_yitrg situation. The role of local changes in osmolality, if it even occurs, is yet to be determined. Prostaglandins The products of arachidonic acid metabolism (the prostaglandins) have been implicated as endogenous mediators of local blood flow 43 regulation in the heart. However, the evidence for this role is not generally agreed upon by most investigators. In regard to the role of prostaglandins in the genesis of reactive hyperemia, Alexander et_al. (1975) reported that indomethacin (prosta- glandin synthesis inhibition) significantly attenuated the reactive hyperemia seen in response to 10, 15, and 20 second occlusions of the left coronary artery in the open-chest dog. Furthermore, a radio- immunassay for PGE detected a basal level of release from the heart which was increased by coronary occlusions. This effect was also blocked by indomethacin. Owen et al. (1975) provided evidence to suggest the opposite role of prostaglandin in reactive hyperemia. They reported that indomethacin had no effect on the reactive hyperemia seen follow- ing 5 and 15 second occlusions of the left coronary artery in closed- chest dogs. This result was also confirmed by Needleman (1975) and Hintze and Kaley (1977). All but a few investigators feel that the prostaglandins play a minor role if any in the hyperemic response seen following coronary occlusions; however, there still exists a great deal of controversy concerning the role of prostaglandins in the overall control of coronary flow. There is experimental evidence, also controversial, to suggest that prostaglandins play a role in the coronary vascular response to hypoxia. Afonso gt_gl, (1974) found that hypoxic coronary vasodilation was significantly attenuated following the administration of indometha- cin in closed—chest dogs. Needleman §£.§l- (1975) reported that hypoxia caused a transient release of prostaglandins from the isolated rabbit heart preparation. Wenmalm gt a1. (1974) demonstrated that the release 44 of prostaglandins occurred only upon a return to normal oxygen delivery following a hypoxic episode. Yet, Needleman et_al. (1975) using the isolated perfused rabbit heart, and Hintze and Kaley (1977) using the open-chest dog demonstrated that the coronary vasodilation in response to hypoxia was not attenuated by prostaglandin synthesis inhibition. .Ig_!itrg studies by Kalsner (1975,1976) demonstrated that isolated bovine coronary artery strips released baseline levels of prostaglandins, and upon decreasing the P02 of the bath from 515 to 38 mmHg, the rate of prostaglandin release increased and relaxation of the strips also occurred. Both of these responses to hypoxia were significantly reduced by the administration of prostaglandin synthesis inhibitors (aspirin and indomethacin). Other_ifl vitro studies by Alexander and Gimbrone (1976) and Gimbrone and Alexander (1975) demonstrated the release of vasodilator (E type) prostaglandins from cultured human umbilical vein smooth muscle cells and from human vascular endothelial cells, respec- tively. While such_iglyitrg evidence favors the involvement of prosta- glandins in local blood flow regulation, its applicability to the role of prostaglandins in the intact animal is undetermined. The role of prostaglandins in the regulation of coronary blood flow during enhanced cardiac activity was investigated by Sunahara and Talesnik (1973). They reported that norepinephrine, when given to isolated, perfused rat hearts, resulted in an increased contractile force and coronary blood flow. Following the blockage of prostaglandin synthesis with aspirin or indomethacin, the same dose of norepinephrine enhanced the coronary flow response but did not change the effect on contractile force. Furthermore, Talesnik and Sunahara (1974) 45 demonstrated that the administration of prostaglandins E1 also attenu- ated the coronary flow response (compared to control) as a result of the administration of norepinephrine or isoproterenol. 0n the basis of these data, these investigators suggested that endogenously released prostaglandins may act as a brake on coronary metabolic vasodilation. In order to test the validity of this hypothesis in a more intact preparation, Harlan_et_gl. (1978) studied the effects of isoproterenol on coronary blood flow and myocardial oxygen consumption before and after the blockade of prostaglandin synthesis with indomethacin. The results of this study demonstrated that indomethacin had no effect on the relationship between left coronary blood flow and myocardial oxygen consumption, or the degree of coronary vasodilation or myocardial oxygen consumption at any given dose of isoproterenol. This study suggests that prostaglandins do not play a role in the regulation of coronary blood flow during enhanced metabolic activity. Adenine Nucleotides and Adenosine The adenosine hypothesis for the regulation of coronary blood flow proposes that sincethere~is rapid metabolism of the adenine nucleotides (ATP, ADP, AMP) in cardiac tissue, when any factor such as hypoxia, ischemia, or increased oxygen consumption contributes to an imbalance between oxygen delivery and oxygen utilization, net nucleotide degrada- tion occurs resulting in increased levels of AMP in the myocardial cells. At the outer cell margin, the enzyme 51-nucleotidase catalyzes the hydrolysis of AMP to the nucleoside adenosine (Rubio_et_gl., 1973). Adenosine, which can readily pass through cell membranes (Whittam, 1960), 46 enters the interstitial fluid and dilates the resistance vessels so that coronary flow increases and a new steady state is reached. Since adenosine is known as a potent vasodilator (Winburert_al., 1953), the role for adenosine in the metabolic regulation of coronary blood flow is attractive since the adenine nucleotides are so important in energy metabolism, and since such a high degree of correlation exists between coronary flow and oxygen consumption. Adenosine, released from myocardial cells can either re-enter the myocardial cell where it can be rephosphorylated by adenosine kinase to AMP (Mustata et_al., 1975) (Jacob and Berne, 1960), or enters the inter- stitium where it can act on the resistance vessels to cause vasodilation. Adenosine that is lost to the vascular system is acted upon by the degradative enzymes adenosine deaminase and nucleoside phosphorylase which rapidly break down adenosine to inosine and hypoxanthine respec- tively, as adenosine crosses the capillary endothelium (Rubio gt gl., l972). Inosine and hypoxanthine are not vasoactive. Just as there is good biochemical evidence to support the adeno- sine hypothesis, good experimental evidence exists as well. Rubio and Berne (1967) reported that adenosine is continuously produced by the normal heart in amounts sufficient to qualify it as a candidate for a role in local blood flow regulation. Furthermore, Berne and Rubio (1974) reported that brief (5 sec) coronary artery occlusions increase the adenosine concentration in the ischemic tissue, and also in 1974 demonstrated that adenosine is increased in the coronary venous blood during hypoxic perfusion or epinephrine stimulation of isolated per- fused hearts. Fox_et.al. (1974) also reported that adenosine was 47 released from human hearts during angina pectoris induced by rapid atrial pacing in patients with ischemic coronary heart disease. Scott et a1. (1965) reported that perfusion of the dog forelimb or kidney with venous blood from the active or hypoxic heart produces dilation and constriction, respectively, which also supports the adenosine hypothesis since the only known endogenous substances which elicit such responses in these vascular beds are adenosine and AMP. This study was followed by a similar report (Scott gt__l., 1979) in which coronary venous blood from the open-chest dog heart was perfused into an autologous (bioassay) kidney. During reactive dilation, the kidney responded with a large increase in resistance which was blocked by theophylline (a competitive inhibitor of adenosine and AMP) adenosine autoblockade and adenosine deaminase. Hypoxic dilation of the coronary also produced an increase in renal resistance. This response was also blocked by theophylline and adenosine autoblockade; yet, following the administration of adenosine deaminase the renal response was only reduced by 40%. This study suggests that adenosine is the vasoactive substance which appears in coronary venous blood during brief coronary occlusions and both adenosine and AMP appear during local cardiac hypoxic dilation. Rubio et_gl. (1974) demonstrated in the isolated perfused guinea pig heart preparation that a gradual decrease in the coronary perfusate 02 content resulted in a continuous increase in coronary blood flow which is paralleled by an increase in the rate of adenosine release and tissue adenosine levels. While the aforementioned evidence implicates a role for adenosine in the coronary vascular response to brief occlusions and hypoxia, 48 there exists good evidence to the contrary. Bittar and Pauly (1971) demonstrated in the open-chest dog that aminophylline produced a signi- ficant diminution of the coronary response to injected adenosine, and lidoflazine produced a significant enhancement of the coronary response to injected adenosine. However, they showed that the coronary flow response to 30, 60 and 120 second left coronary occlusions were unaffec- ted by pretreatment with aminophylline or lidoflazine, indicating that adenosine is not a mediator of myocardial reactive hyperemia. Giles and Wilcken (1977) also reported that in open-chest dogs, aminophylline reduced the coronary blood flow response to adenosine by 80% yet attenu- ated the flow response to an 8 second coronary artery occlusion by only 20%, also indicating that adenosine does not play a major role in myo- cardial reactive hyperemia. Afonso__t.al. (1972) studied the effects of systemic hypoxia on coronary blood flow before and after the adminis- tration of aminophylline in the closed-chest dog. Coronary blood flow as determined by coronary sinus thermodilution technique, was signifi- cantly increased by 83% before aminophylline, and 74% after amino- phylline. While the same degree of hypoxia was produced in each case, the slightly decreased coronary flow response after aminophylline could be explained on the basis of a greater heart rate and left ventricular work which occurred during hypoxia before the administration of amino- phylline. These results suggest that adenosine is not involved in the coronary dilation seen during hypoxia. In an effort to answer this apparent discrepancy, Curnish et_al. (1972) reported that aminophylline decreased the volume and duration of the coronary hyperemia following adenosine administration by 41 and 11% respectively. Following 5 to 60 49 second coronary occlusions, aminophylline decreased the volume and dura- tion of the hyperemic flow by 42 and 31%, respectively, suggesting a role for adenosine in this response. These authors attribute the dis— crepancy between their results and the results of the aforementioned investigation as being due to 1) the duration of ischemia or hypoxia, since long periods of either condition could result in endogenous adenosine produced in such great concentrations as to negate the pharmacological antagonistic action of aminophylline, and 2) the analy- sis of data; they suggest that the reactive hyperemic response should be analyzed in terms of the volume of flow rather than the peak hyperemic response. Therefore, while there is a great deal of indirect evidence to support the adenosine hypothesis, several aspects of this theory are yet to be resolved, and are the subject of current investigation. The role of ATP in the local regulation of coronary blood flow is a relatively unexplored topic. Chen et_al. (1972) found evidence for ATP participating in the active hyperemia seen with stellate ganglion stimu- lation of the dog heart. ATP was assayed from the coronary sinus efflu- ent and was found to increase by approximately 150% during active hyperemia. This corresponded with a similar increase in coronary blood flow. No evidence of ATP could be found in the coronary effluent during the reactive hyperemia following a 20 second occlusion. In a follow-up study, Stowe et a1. (1974) reported that isolated guinea pig hearts perfused with a Krebs-Ringer solution showed no release of ATP in response to coronary occlusion and subsequent reactive hyperemia or to anoxia. They concluded that the ATP release in the study by Chen 50 and co-workers was probably not from the myocardium but could have come from nerves and/or formed elements in the blood. VI. Right Coronary Blood Flow The studies concerning the regulation of blood flow in the right coronary circulation are few; therefore, we lack a basic understanding of this vascular bed. However, from the few studies published on this topic, some information can be gleaned to establish a starting point for further investigation. As mentioned previously, Gregg (1937) determined that due to the low intramyocardial tension produced in the right ven— tricular wall, phasic flow in the right coronary artery (RCA) has the greatest magnitude during systole and the flow pattern follows the con- tour of the aortic pressure curve. Therefore, extravascular compression is not a significant factor in determining the passive changes in vessel 1. (1976) using the caliber for this vascular bed. Lowensohn_et conscious dog preparation, corroborated the early findings of Gregg. They also noted that dogs with congenital pulmonic stenosis, right ventricular hypertrophy and elevated right ventricular pressures demon- strated a reduction of systolic flow or sometimes a reversal of flow similar to that which occurs in the left coronary system. This throttling of right coronary blood flow was directly related to right ventricular systolic pressure. Lowensohn gt_al. (1978) also demonstrated that 10 second occlusions of the RCA produced a peak hyperemic response 300% above resting control values which is less than that reported (300-700%) by Olsson and Gregg (1975) for the left circumflex coronary artery. Since the peak hyperemic response to occlusion was less for the 5l RCA than that reported for the left coronary system, it could be con- cluded that the energy (oxygen) demands of this tissue are also signifi- cantly less than those of the left ventricle. This point is further supported by the fact that the right ventricle does much less pressure work than the left, and therefore creates less wall tension. In a recent publication by Manohar_et.gl. (1979) right coronary and right ventricular hemodynamics were measured during normal resting conditions, systemic hypoxia and increased right ventricular afterload in the awake calf. Resting right coronary blood flow determined by the microsphere technique, was 73 ml/min/lOO gm tissue, a value similar to that reported for the left ventricle. During increased right ventricular afterload, right coronary blood flow increased slightly even though coronary driv- ing pressure decreased indicating a degree of coronary vasodilation. Right coronary blood flow increased 100% during systemic hypoxia (P02 43 mmHg), and the combination of hypoxia and increased afterload in- creased blood flow 400% above control values. These data for the right ventricle indicate that even though the oxygen consumption is in all likelihood relatively lower at the same rate of oxygen delivery as compared to the left (a state of hyperperfusion), the right coronary vascular bed is still sensitive to factors which alter the metabolic state of the tissue. That is, this vascular bed is capable of signifi- cant metabolic vasodilation. Brooke gt g1. (1971) demonstrated that acute occlusion of the right coronary artery in the anesthetized open-chest dog caused no change in cardiac output or right ventricular pressure, although right ven- tricular contractile force was significantly reduced. This study 52 suggests that a normally contracting right ventricular free wall is not necessary for maintenance of normal cardiac output or right ventricular pressure. This finding could be due to the fact that adequate ventricu- lar hemodynamic function can be maintained by septal wall contraction when the right ventricular free wall is removed and replaced by a Gortex patch graft (Peterson gt 31., 1978), or could be due to the fact that the right coronary artery of the dog does not supply the total right ventricular tissue mass as some of the arterial blood is supplied by the LAD branch of the left coronary artery (Murray gt al., 1979). The effects of right and left cardiac sympathetic nerve stimula- tion on left and right coronary blood flow in the anesthetized open— chest dog was reported by Ross and Mulder (1969). Both right and left nerve stimulation (10 V, 5 msec, lO/sec) apparently produced an initial rise in right coronary blood flow followed by right coronary vasocon- striction as demonstrated by a reduction of mean flow at an unchanged or increased aortic pressure. This response was slightly enhanced fol— lowing beta blockade except that the initial vasodilation was blocked. Flow through the left coronary artery increased during stimulation, and following beta blockade, the dilation was converted to constriction. This study suggests that the right coronary circulation is more sensi- tive to the direct effects of sympathetic stimulation than is the left coronary circulation. STATEMENT OF OBJECTIVES A review of the literature discloses that the characterization of the regulation of blood flow through the right coronary vascular bed has never been adequately accomplished. The metabolic environment of the myocardial tissue supplied by the right coronary artery is much different from that of the left, and the balance of factors contributing to regulation of blood flow through this bed may also be quite different. This vascular system is of great importance since 50% of the human population is right coronary dominant. Therefore, it was felt that this circulation warranted further basic physiological investigation. The studies described in this dissertation were designed to evaluate several aspects of local and remote control of the right coro- nary circulation during constant pressure and constant flow perfusion. This report attempts to define the role of autoregulation, infused catecholamines, sympathetic nerve stimulation, adrenergic receptors, prostaglandins, oxygen and carbon dioxide in the control of blood flow through this bed, as well as the interaction of several of these various factors. Since this vascular bed has never been systematically investi- gated, these studies were conducted in order to determine if the response of this circulation to the aforementioned experimental inter- ventions was similar or different from that reported for the left 53 54 coronary vascular bed. The right ventricular myocardium possesses some unique features that make it different in many respects when com- pared to the left ventricle. The work performed by the right ventricle is approximately six times less than that performed by the left ventri- cle. The wall tension generated by the right ventricle is similarly less. Therefore, the balance of forces acting to control blood flow in the right coronary circulation may be somewhat different than those reported for the left coronary vascular bed. The studies reported here were designed to test this hypothesis. Furthermore, because of the recent interest in the mechanisms responsible for coronary artery vasospasm, and the high frequency with which it is reported to occur in the right coronary artery, this study attempts to examine the effects of various vasoconstrictor influences and their interactions in order to determine if a maximal constriction of the right coronary artery could be induced. METHODS All experiments were performed on mongrel dogs of both sexes, weighting 25-35 kg. Anesthesia was achieved by initial induction with thiamylal (5 mg/lb) and maintenance with 100 mg/kg alpha chloralose and 500 mg/kg urethane, administered intravenously. The animals were intubated with a cuffed endotracheal tube and ventilated by a positive pressure respirator (Harvard Apparatus Company, Model 613, Millis, Mass.). Volume and rate of ventilation were adjusted to maintain arterial PCO2 within the physiological range. If needed pH was adjusted by intravenous infusions of an isotonic NaHCO3 solution. Arterial P02, CO and pH were measured by radiometer blood gas analyzer (Radiometer- 2 Copenhagen, blood micro system, acid base analyzer, Copenhagen, Denmark). P Following surgical preparation, anticoagulation was achieved by the intravenous administration of sodium heparin in an initial dose of 600 USP units/kg followed by hourly supplements of 250 USP units/kg. Blood volume was maintained with a 6% solution of dextran (average molecular weight = 75,000) in saline. All blood pressures were continuously monitored with pressure transducers (Statham Laboratory, low volume displacement model P23 Gb, Hato Rey, Puerto Rico) and recorded via inputs into a direct writing oscillograph (Hewlett-Packard, Model 77964, Boston, Mass.). 55 56 Experimental Design These experiments were designed such that paired comparisons could be made for each experimental maneuver. Steady state control conditions were reached before and during each experimental interven- tion. Where possible, the order of experimental maneuvers was randomized. I. Studies on the Anesthetized Dog Constant Flow Preparation In Series I, II, III, and IV, a constant flow perfusion prepara- tion was utilized. Following anesthesia, the animals were placed in a dorsal recumbancy and standard limb leads were attached to provide input into a bioelectric amplifier. Lead II of the electrocardiogram was monitored for detection of arrythmias and determination of heart rate. This preparation is schematically represented in Figure l. The left femoral artery and vein were cannulated with polyethylene tubing, P.E. 240 (Intramedic Tubing, Clay Adams, Parsippany, N.J.), for the monitoring of arterial blood pressure and administration of intravenous fluids, respectively. The neck was opened in the midline and the common carotids and vagi isolated bilaterally. The vagi were cut and ligatures placed around each of the common carotid arteries. By apply- ing a tourniquet to these ligatures, carotid sinus hypotension could be produced in most animals, thus eliciting a systemic sympathetic dis- charge via the baroreceptor reflex. The chest was opened by median sternotomy and the pericardium incised and sutured to the chest wall to form a cradle. Adequate lung inflation and deflation was obtained in 57 Figure 1. Preparation for constant flow or constant pressure perfusion of the right coronary artery 58 RESPI RATOR FLOW CONTROL 1 PUMP I m Vagi severed Carotid snare ’1“ V‘ \f/ BATH ' \\\—————~*CORONARY PRESSURE‘—————~// ’//////) ARTERIAL PRESSURE L dP R. VENTRICULAR PRESSURE 8 /dT‘ (RECORDER) Figure I 59 the open chest by applying a 2 cm H20 positive end expiratory pressure to the outflow tubing of the Harvard respirator. The right atrial appendage was retracted and the right coronary artery was isolated 1-3 cm from its origin, and two silk ligatures were placed loosely around it. Following heparinization, blood withdrawn from the cannulated right femoral artery was pumped (Sigmamotor Inc., Model T-6SH, Middleport, N.Y.) through a cannula which was placed in the isolated segment of the right coronary artery and secured by the silk ligatures. Perfusion pressure was monitored from the perfusion circuit just proximal to the cannula's entry into the vessel. Intracoronary drug infusions were achieved by a Harvard infusion pump (Harvard Apparatus, C0., Millis, Mass.) delivering the drug into the coronary perfusion circuit proximal to the Sigma-motor pump. To demonstrate that this vascular bed was free of collateral vessels from left coronary sources the perfusion pump was turned off briefly, and coronary perfusion pressure was found to fall to less than 20 mmHg. At the end of each experiment (with the exception of Series I) 5 ml of crystal violet dye, dissolved in ethanol and saline (Sigma Chemical C0., St. Louis, MO.) was injected into the perfusion circuit to stain the area of the myocardium perfused. This tissue was then excised and the wet weight determined. The Sigmamotor pump was calibrated for flow at the end of each experiment using timed collections of blood in a graduated cylinder. The flow measurements were multiplied by 100 and then divided by the weight of the tissue perfused in grams to give normalized blood flow in ml/min/IOO gm tissue. Right coronary resistance was then calculated by the ratio of the perfusion pressure (mmHg) to flow (ml/min/IOO gm) to yield resistance 60 in peripheral resistance units (mmHg/ml/min/IOO gm or PRU lOO). Isolated Lung Preparation Series IV (constant flow) and Series V (constant pressure) employed the use of an isolated perfused donor lung interposed in the coronary perfusion circuit in order to study the local effects of blood gas tension alterations on coronary resistance. To achieve this, a donor dog (IO-12 Kg) was given intravenous heparin and dose of sodium pentobarbitol sufficient to produce euthanasia. A thoracotomy was performed in the fourth left intercostal space and the left lung and heart were removed by dividing the trachea, pulmonary artery, aorta and vena cavae. The lobes of the right lung were ligated at the hilus and cut off. The heart was cut in a transverse section just below the A-V groove. Blood withdrawn from the cannulated right femoral vein of the experimental animal was pumped (Masterflex pump, model 7564, Cole-Parmer, Chicago, Ill.) into the left pulmonary artery of the isolated lung. The trachea was connected to a Harvard positive pressure respirator and ventilated at the necessary rate and volume. Pulmonary venous blood flowed into a large bore cannula tied into the preserved left atrium, and was delivered at constant flow (via Sigmamotor pump) or constant pressure (Holter roller pump) to the right coronary artery. Pulmonary venous pressure was monitored from a catheter (PE60) advanced from the left atrial cannula into the left atrium and maintained con- stant at a pressure of 5 mmHg with the use of a feedback controller system (Leeds-Northrup Century CAT Controller, Oak Park, MI.) which varied the speed of the masterflex pump (and hence the pulmonary artery 6l inflow). In these experiments various gas mixtures were used to alter the blood gas tensions of the coronary perfusate. Hypoxia was produced by ventilating the isolated lung with 0% 02, 5% C02, 95% N2. Normoxia was produced with 20% 02, 5% C02, and 75% N2. Hypocapnia was produced by hyperventilation of the lung on room air. The combination of hypoxia and hypocapnia was produced by hyperventilating the lung on 100% N2. Hypercapnia was produced by ventilating the lung with 20% 02, 15% CO and 65% N2. This preparation is illustrated in Figure 2. 2 The use of the extracorporeal lung permitted rapid changes in local blood gas tensions without producing detectable changes in sys- temic blood gas tensions. Daugherty ep_pl. (1967) have demonstrated that samplings of systemic arterial blood during ventilation of the isolated lung with hypoxic or hypercapnic gas mixtures showed no altera- tion of systemic blood P , P or pH. 02 C02 Constant Pressure Preparation In this preparation, the surgical procedures and instrumentation were the same as the constant flow preparation except that an isolated lung was interposed in the perfusion circuit, and a Holter roller pump was used to deliver the coronary perfusate. Right coronary perfusion pressure was monitored and held constant by a second feedback control system similar to that described for the isolated lung preparation. Coronary flow was determined by delivering the coronary pump speed signal as an input into the Hewlett-Packard oscillograph. At the end of each experiment, the pump was calibrated by timed collection of blood in a graduated cylinder, and was found to be linear over the Figure 2. 62 Preparation for constant flow or constant pressure perfusion of the right coronary artery with isolated lung interposed in perfusion circuit. FLOW F—CONTROL J 63 RESPIRATOR PUMP RESPIRATOR 0.0—1 1 ' J; FLOW CONTROL Vagi severed Carotid snare \J‘~ K\\\\\\\______ R\\ \\\\\\\\______:‘PULMONARY venous PRESSURE ) -J/::::ii[///// dP R. VENTRICULAR PRESSURE 81 /dT‘ "CORONARY PRESSURE ARTERIAL PRESSURE ‘ (RECO RDER) Figure 2 64 entire range of flows used. This monitoring of the feedback system provided constant pressure perfusion and a permanent recording of instantaneous changes in coronary flow. II. Studies on the Unanesthetized Dog In order to determine if the data obtained in the anesthetized, open-chest dog are comparable to the responses found in the intact conscious animal, a second experimental method was employed. Male mongrel dogs (25-35 Kg) were conditioned for one month prior to instrumentation. Conditioning included examination of the stool for parasites, examination of the blood for microfilaria, and vaccination against rabies, distemper, leptospirosis and hepatitis. During the course of the study, the dogs were maintained on a diet of standard dog chow (Wayne Dog Food, Allied Mills, Inc., Chicago, Ill.) and water_ag libitum. Procaine pencillin (1 million units) and streptomycin (0.5 gm) were given as single intramuscular injection as a prophylactic measure against wound infection on the day of surgery. This treatment was continued once a day for three days postoperatively. Following a fast- ing period of 24 hours, the animals were anesthetized with thiamylal (5 mg/lb) and maintained on methoxyflurane and oxygen delivered through a cuffed endothracheal tube. The dogs were positioned in a dorsal recumbency and the ventral surface of the chest prepped and draped in the usual manner. Under sterile conditions, the chest was opened by median sternotomy, and the pericardium opened to form a cradle. The proximal 1-3 cm of the right coronary artery was isolated in preparation 65 for instrumentation. Using the technique of Herd and Barger (1964), a small heparin-filled Teflon catheter was placed in the distal portion of the isolated segment for the measurement of right coronary perfusion pressure, and sutured to the vessel wall with 4-0 polyethylene. Proximal to the catheter, a 4 mm balloon-type occluder (Rhodes Medical Instruments Inc., Woodland Hills, CA.) was placed around the vessel to provide occlusive zero flow determinations, produce reactive hyperemic responses and to produce ischemic flow conditions. Proximal to the occluder, a 2.5 mm electromagnetic flow probe (Zepeda Instruments, Seattle, Wash.) with cables axial to the vessel was placed around the artery. Care was taken to assure that no side branches of the vessel existed between the flow probe and occluder. Electrocardiographic leads were sutured to the epicardial surface of the right ventricle in a region perfused solely by the right coronary artery and placed in the subcutaneous tissue of the back for ECG recording. The cables and catheters were exteriorized through the chest wall and tunneled sub- cutaneously to exit the skin on the right side approximately 10 cm lateral to the spine. The sternum was then reapproximated with 2.0 stainless steel sutures. The muscle and subcutaneous tissue sutured with 1-0 surgical silk, and the skin incision closed with 1-0 vetafil suture. A chest tube was used to evacuate air from the chest. The animals were allowed to recover for a period of l-2 weeks before begin- ning data acquisition. Blood flow was measured with the use of a square wave electromagnetic flowmeter (Zepeda Instruments, Seattle, Wash.) which was demonstrated to be linear over the range of flows measured. Occlusive zero flow determinations were made immediately 66 before and after each experimental intervention. Right coronary vascu- lar resistance could be calculated from the ratio of right coronary perfusion pressure and right coronary flow. All recording was made with a Grass Model 7B direct writing poly- graph via inputs from Stratham low volume displacement pressure trans- ducers and the Zepeda flowmeter. The dogs were trained to lie quietly on their left side on a table during the experimental protocols. Because of technical problems, we were unable to calibrate the flow probe_i_ situ as was previously planned. Therefore, since the flow range measured was linear, flow was measured as an artitrary unit using mm divisions on the recorder strip chart. The rationale for each series of experiments and their respective protocols are described below. Series I Using the constant flow perfusion technique, we examined the rela- tionships between pressure and flow, and resistance and flow over the range of 35-175 mmHg in order to determine the autoregulatory character- istics of this bed. This series also examined the effects of adrenergic stimulation (intracoronary norepinephrine infusion and baroreflex sympa- thetic stimulation) on right coronary resistance at different flow rates both before and after beta blockade with propranolol (3 mg/Kg, Sigma Chemical C0., St. Louis, MO.) in order to evaluate the interaction between flow, neuro-humoral factors and beta receptor activity in the regulation of the right coronary circulation. 67 Protocol: 1. The pressure-flow relationships were determined over the range of 25-180 mmHg with flow being altered to produce changes in pressure in 25 mmHg steps. . Sympathetic stimulation (baroreflex) was studied during perfu— sion of the coronary at 30 mmHg (low flow conditions). . Norepinephrine was infused at a rate of l ug/min into the coro- nary perfusion circuit behind the perfusion pump during the low flow conditions. . Sympathetic stimulation (baroreflex) was studied during perfu- sion of the coronary at 100 mmHg (normal flow conditions). . Norepinephrine was infused at a rate of l ug/min into the coro- nary perfusion circuit during normal flow conditions. . Sympathetic stimulation (baroreflex) was studied during perfu- sion of the coronary at 170 mmHg (high flow conditions). . Norepinephrine was infused at a rate of 1 ug/min into the coro— nary perfusion circuit during high flow conditions. . Propranolol (3 mg/Kg) was given intravenously and a period of one hour was allowed for the drug to take effect. . Repeat steps 2-7. Series II The purpose of this series was to evaluate the effect of adrener- gic stimulation at various flow rates before and after alpha-blockade with phentolamine (Ciba, Summit, N.J.) in order to determine the role of alpha receptor activation in these responses. The constant flow 68 preparation was again utilized with the addition of the measurement of right ventricular pressure and its first derivative (dP/dT) from a 6F USCI cardiac catheter advanced from the external jugular vein in the right ventricular chamber, and connected to a Statham pressure trans- ducer, and derivative computer for dP/dT. Protocol: 1. Sympathetic stimulation (baroreflex) was studied during perfu- sion of the coronary at 30 mmHg (low flow conditions). 2. Norepinephrine was infused at a rate of l pg/min into the coro— nary perfusion circuit behind the perfusion pump during low flow conditions. 3. Sympathetic stimulation (baroreflex) was studied during perfu- sion of the coronary at 100 mmHg (normal flow conditions). 4. Norepinephrine was infused at a rate of l ug/min into the coro- nary perfusion circuit during normal flow conditions. 5. Sympathetic stimulation (baroreflex) was studied during perfu- sion of the coronary at 170 mmHg (high flow conditions). 6. Norepinephrine was infused at a rate of 1 ug/min into the coro- nary perfusion circuit during high flow conditions. 7. Phentolamine was infused at a rate of 600 ug/min into the coro— nary perfusion citcuit. 8. Steps 1-6 were repeated during phentolamine infusion. Series III The purpose of this series of experiments was 1) to determine what role if any the prostaglandins play in the reactive dilation seen 69 in response to 20 second interruptions of right coronary flow, and 2) to determine the response of the vascular bed supplied by the right coronary to adrenergic stimulation and systemic hypocapnia (produced by hyperventilation of the animal to yield PCO2 = 18 mmHg) following prostaglandin synthesis blockade by indomethacin. The indomethacin (Sigma Chemical Co.) was prepared by stirring the chemical in a solution of saline (90 ml) and 100 mg.HCO3 until dissolved. The indomethacin was then infused into the coronary perfusion line at a rate of 2 ml/min. Experimental maneuvers were performed one hour after the administration of the indomethacin to assure its effectiveness. The constant flow perfusion preparation was again used in this series. Protocol: All meneuvers in this and subsequent series were performed during normal flow perfusion conditions with initial perfusion pressures set at approximately 100 mmHg. 1. Coronary flow was interrupted for a period of 20 seconds and the degree of reactive dilation was determined following the re-institution of flow. 2. The response to sympathetic stimulation (baroreflex) was determined. 3. The response to 0.25 ug/min intracoronary norepinephrine was obtained. 4. The animal was hyperventilated on room air to reduce the sys- temic arterial PCO and the coronary response was determined. 2 5. The response to sympathetic stimulation was again studied while 70 the animal was systemically hypocapnic. 6. The response to 0.25 pg/min norepinephrine was obtained during systemic hypocapnia. 7. Prostaglandin synthesis was blocked by a intravenous 5 mg/Kg dose of indomethacin. One hour was allowed for the drug to take effect and the coronary response to indomethacin was recorded. 8. Steps 1-6 were repeated to observe the responses with prosta- glandin synthesis blocked. Series IV This series employed the constant flow preparation with an iso- lated perfused donor lung interposed in the coronary perfusion circuit. The purpose of this series was to determine the local vascular effects of hypoxia (coronary arterial P0 = 12 mmHg), hypocapnia (PCO = 6 mmHg) 2 2 and the combination of hypoxia and hypocapnia (P0 = 13, PCO = 7 mmHg). 2 2 Moreover, we determined if these conditions alter the response of the right coronary to adrenergic stimulation. Protocol: 1. The responses to sympathetic (baroreflex) stimulation were obtained, followed by 0.25 ug/min and 0.50 pg/min intracoronary norepinephrine infusions during perfusion with normoxic, normo- capnic blood. 2. The response to local hypocapnia was obtained by hyperventi- lating the isolated lung on room air. 71 3. During hypocapnic perfusion, the responses to sympathetic stimulation, 0.25 pg/min and 0.50 ug/min norepinephrine were again obtained. 4. After reaching a control condition by switching back to normoxic normocapnic perfusion, the response to local hypoxia was obtained by ventilating the isolated lung with 5% C02. 95% N2. 5. During hypoxic perfusion, the responses to sympathetic stimula- tion, 0.25 ug/min and 0.50 ug/min norepinephrine were again obtained. 6. After reaching a control condition by switching back to nor— moxic, normocapnic perfusion, the response to local hypoxia and hypocapnia was obtained by ventilating the isolated lung with 100% N2. 7. With hypoxic and hypocapnic perfusion combined, the responses to sympathetic stimulation, 0.25 ug/min and 0.50 ug/min norepinephrine were again obtained. Series V In order to simulate a more physiologically normal situation, a constant pressure perfusion preparation was employed for this series experiments which included an isolated perfused donor lung interposed in the coronary perfusion circuit to locally alter coronary blood gas tensions. Indomethacin 5 mg/Kg was infused into the donor lung and experimental animal prior to performing the experimental maneuvers in order to preclude the involvement of the prostaglandins in any of the 72 responses observed. The purpose of this series was I) to determine the pressure-flow relationships under these conditions for comparison to the constant flow preparation, 2) to determine the vasoactivity of local hypoxia, hypocapnia and hypercapnia during constant pressure perfusion, and 3) to determine if the effects of adrenergic stimulation are altered when local coronary blood gas tensions are altered. Protocol: 1. The pressure-flow relationship during constant pressure perfu- sion were obtained over the range of 50—175 mmHg. Perfusion pressure was increased or decreased in steps of 25 mmHg and the steady state flow responses recorded. . The responses to sympathetic (baroreflex) stimulation and 0.25 ug/min intracoronary norepinephrine infusion were obtained. Perfusion pressure was held constant at 100 mmHg for all inter- ventions in this series. . The response to local hypocapnia was obtained by hyperventilat- ing the isolated lung on room air. . During hypocapnic perfusion, the responses to sympathetic stimulation and 0.25 ug/min norepinephrine infusion were obtained. . After reaching a control condition by switching back to nor- moxic, normocapnic perfusion, the response to local hypercapnia was obtained by ventilating the isolated lung with 15% C02. 20% 02, 65% N2. 73 6. During hypercapnic perfusion, the responses to sympathetic stimulation and 0.25 pg/min norepinephrine were again obtained. 7. After reaching a control condition by switching back to normoxic, normocapnic perfusion, the response to local hypoxia was obtained by ventilating the lung with 5% C02, 95% N2. 8. During hypoxic perfusion, the responses to sympathetic stimu- lation and 0.25 ug/min norepinephrine infusion were again obtained. Series VI The purpose of this series was to determine the response of the right coronary circulation of the chronically instrumented unanesthe- tized dog to various physiological and pharmacological stimuli in order to compare these results with those obtained in the anesthetized prep- arations. This series presents the vascular responses of the right coronary to brief occlusions of flow of 3 seconds (reactive hyperemia) before and after inhibition of prostaglandin synthesis with 5 mg/Kg indomethacin. It also demonstrates the response to intracoronary bolus injections of NE during control and ischemic conditions, and the response to NE before and after alpha blockade with l mg/Kg phentolamine. The relationships between pressure and flow are presented for stepwise changes in perfusion pressure below 100 mmHg in order to determine the autoregulatory response of this vascular bed. STATISTICAL ANALYSIS The data presented in the figures and tables of this dissertation were analyzed using the student's t test modified for paired replicates. The experimental design of these experiments was such that a paired analysis was possible for each experimental intervention with initial non-experimental values serving as statistical controls. In the data presented, only the mean and standard error of the mean are depicted. A "p“ value of less than 0.05 was taken as the level of statistical significance. In some cases, it was important to know if the response to a particular stimulus was altered when the baseline conditions were altered. In most cases, altering the baseline conditions altered the initial resistance of the vascular bed. In order to determine if initial resistance was an important factor in the determination of the response to a stimulus, a linear regression analysis was performed for sympathetic stimulation and norepinephrine infusion for 23 and 13 animals respectively. This data is graphically represented in Figure 3. The regression analysis showed a correlation coefficient of 41% and 32% for sympathetic stimulation and norepinephrine infusion, respectively. An analysis of variance for the regression showed no significant corre- lation between the initial resistance and the change in resistance in response to stimulation or norepinephrine. The poor correlation between these factors is also borne out by the low coefficient of correlation. 74 75 Em oo_\:wE\FE\u:EE u mocepmwmwe pcovoewwooo COEUGFmLLoo n L .cowm345w ocwccn -mcwamco: new coEUMFJEEpm AwaCQLogmmv owpmgumasam Low Am A.mv mocmpmwmmg acmcoeoo meuwcw Low meAchm coemmmcmwc Lemcw4 .m ogzmvd 76 m mesmwd ion. 8.: E . one own om . on .8. ice. 18. .8. 2n 2 :_E\mzmwd octzaocfiocoz 3.". E one ohm ow om row. row. m< ion. mm N Z roe. cozmsszw 2.65355 m4 77 This analysis suggests that over the range of initial resistances studied, the response to a stimulus was not dependent on the initial resistance of the vascular bed. With this in mind, in order to deter- mine if a response was altered when baseline or background conditions were changed, the absolute change in resistance (AR) for the control response was compared (using Student's t test, or paired t test) to the absolute change in resistance (AR') for the experimental response (that obtained during altered background conditions). It was felt that this type of analysis was preferential to a comparison of the percent changes from one group to that of another since percent change normal- izes the data for initial resistance, a factor shown not to be important by the regression analysis. However, for each case in which it was desirable to determine whether or not a response was altered, both an analysis of AR as well as percent change was performed. Both of these analyses provided the same results in all cases. RESULTS Series I Figure 4 presents average data from nine experiments in which the steady state effects of sympathetic (baroreflex) stimulation on heart rate, arterial blood pressure and right coronary vascular resistance were examined at normal flow rates (16 ml/min), low flow rates (4.5 ml/min) and high flow rates (27 ml/min) before and after beta blockade in vagotomized dogs. These flows produced coronary perfusion pressures of 106, 42 and 168 mmHg before beta blockade and 129, 55, and 188 mmHg after beta blockade, respectively. At each flow rate before beta block, sympathetic stimulation produced a significant increase in heart rate, arterial pressure and coronary resistance. By analyzing the change in resistance (AR) it was found that the vasoconstriction seen at normal flow was greater than that seen at high flow. The vasoconstriction seen at low flow was also greater than that seen at high flow, suggest- ing that the degree of sympathetic vasoconstriction is increased as flow is decreased. Following beta blockade with 3 mg/Kg d-l propranolol, resting coronary vascular resistance increased significantly by 38%. Sympathetic stimulation during these conditions resulted in a slight yet significant increase in heart rate, and increase in arterial pres- sure and coronary resistance at each of the three flow rates. The coronary response to sympathetic stimulation after beta blockade was 78 Figure 4. 79 Effects of sympathetic stimulation (SS) via corotid occlusion during constant flow perfusion of the right coronary artery on heart rate, mean arterial blood pressure (MABP) and coronary vascular resistance (CVR) at normal flow (NF), low flow (LF) and high flow (HF) rates, before and after beta blockade with propranolol (3 mg/Kg). 9 P < 0.05 compared to control bars represent mean and standard error of the mean C = control N * II II 80 hh=9 *pc05 CONTROLS BETA-BLOCKED * 200- * * r : t t t HEART l F14 ’3‘ —z— ! Fi—F'T RATE + "5" : b . (“”0100- : I I I O l C SS C 55 C 55 C 85 C 55 C 55 NF LF HF NF LF HF MABQOO- I * * I p 1 (mmHg) T "I‘1 l ri— * 'k I 1 . 1 T 100- F? i J I I I 0 I C 55 C 55 C 55 C 55 C 55 C NF LF HF NF LF HF55 20- ' I 187: , ~* I ‘61 . T-I 14- a: I . 1t CVR 12- : mmHg/mI/min - I | , t 10- * ' - + * = F .1 + + ' ‘ ‘ I 6: 1 I 4: , l 2: . O l C 55 C 55 C 55 C 55 C 55 C 55 NF LF HF NF LF HF Figure 4 81 not different from that before beta blockade for any of the three flow rates studied. Figure 5 presents data obtained in Series I in which the steady state effects of intracoronary norepinephrine infusion(l ug/min) on heart rate, arterial blood pressure and right coronary vascular resist- ance were examined at the same three flow rates described previously, both before and after beta blockade. During control conditions norepine- phrine produced a significant increase in heart rate, but had no effect on arterial pressure or coronary resistance at any of the three flow rates studied. Following beta blockade, norepinephrine had no effect on heart rate or blood pressure, yet produced a significant increase in coronary resistance at each of the three flow rates studied. The degree of coronary constriction produced by norepinephrine was not different between each of the three flow rates studied. Figure 6 depicts the relationships between pressure and flow as well as resistance and flow through the right coronary artery of eight animals during constant flow perfusion. Flow was varied in a stepwise manner over a range of 3 to 60 ml/min., and the steady state perfusion pressures recorded at each step. The pressures ranged from 25—180 mmHg. Resistances were also calculated and related to coronary blood flow. The upper panel of Figure 3 demonstrates that as flow is decreased, resistance remains relatively constant until pressure is decreased below 30 mmHg at which point resistance increases substantially. The bottom panel depicts the pressure-flow relationships, and demonstrates that this relationship is virtually linear over the range studied. Figure 5. 82 Effect of l pg/min intracoronary infusion of norepinephrine 1 (NE) during constant flow perfusion of the right coronary artery on heart rate, mean arterial blood pressure (MABP) and coronary vascular resistance (CVR) at three different flows, normal flow (NF), low flow (LF) and high flow (HF), before and after beta blockade with propranolol (3 mg/Kg). 9 P < 0.05 compared to control 5 represent mean and standard error of the mean control "'5 II II N * Ba C 83 N:9 *thO5 CONTROL BETA - BLOCKED 200. 1% _§_ .3. HEART ‘ RATE + + +—I— 4“?” (II/min) 100 . O CNE CNE CNE CNE CNE CNE NP LP HF NF LP HF 200. MABP (mmHg) ; 4—1—+ 100' 1l++ 1- T O CNE CNE CNE CNE CNE CNE NF LF HF NF LF HF t 20" 18- I..- 16- I 14- * CVR 1 12- . , * mmHg/ml/min . I + 10- l' 8- T 4 .. J. l 4- 1 2- O CNE CNE CNE CNE CNE CNE NP LP HF NP LP HF Figure 5 84 w n z .xLome xcwcocoo pngL esp we cowmswcoq EOFC pcwpmcoo mzvcsu zo_$ wee wocmpmwmoe mm _Fm3 mm .3o_+ use wgzmmmca cmozpmn Q_;m:owpmrmm .o we:m_m 85 12 . 8 Resrstance mmHg/mein Perfusion Pressure 12 (MMHgI CD 60 0 0 Right Coronary Blood Flow (ml/min) Figure 6 86 Mean arterial pressure, heart rate and right atrial pressure were not changed over the range of perfusion pressures studied. Series II Table 1 presents data demonstrating the effect of sympathetic stimulation, intracoronary phentolamine infusion and stimulation after alpha blockade with phentolamine infusion on heart rate, arterial blood pressure, right ventricular systolic pressure and its first derivative dP/dT, right coronary perfusion pressure and coronary resistance. Right ventricular diastolic pressure is not presented in this or any subse- quent data analyses as it always fell within normal limits and was never significantly altered by experimental intervention. Coronary flow was held constant, and averaged 62 ml/min/IOO g. In these experiments, sympathetic stimulation produced no change in heart rate or right ventricular systolic pressure, but increased arterial blood pressure, dP/dT, coronary pressure and resistance. Intracoronary phentolamine infusion produced no significant change in any of the measured variables. Sympathetic stimulation during alpha blockade produced increases in arterial blood pressure, right ventricular systolic pressure and dP/dT. However, no change was seen in heart rate, coronary perfusion pressure or coronary resistance. Table 2 depicts the effects of intracoronary norepinephrine infusion before and after alpha blockade with an intracoronary infusion of phentolamine. Norepinephrine produced a slight increase in arterial blood pressure, a decrease in coronary perfusion pressure and coronary 87 dpSS wmnm UL.wd 1U”? momm m. m.n on on emwe mmme en en en NR ape CPR m mum. a; 8; we ES Ex 88 EN EN 1S 8 EN EN w on )d 9U: 09 on an men on 8 Em... 8m... m... 3 No. NA SA SA Wm. mm; mg E: E2 $8 $8 mm a 8 2: SN 0: mm (3 “VS mm ”D. mm...” NRA mp. m... NRA SN». NA NA me. E :4, :4, mm EMF.N No.m kmr_ FFF knmpm mmmP RN em *eVF wo_ ow_ om? m.mu m u m u m u m u m u m o moo_\c?E\PE\wIEE mIEE oom\mIEE @125 area :PE\Q an m>u m am>m Qmov mocmwmwmmc Empzome> zgmcogoo Unmet wee A psmre .Amm>mv mezmmmca owFopmxm cm—zowcpcm> agave .Aam cowpm_:Ewpm oeumzpmasxm mo powmwm ._ mpamp 88 dGN ”no BJJ Ulra 1Ud 05.... m. w mm.“ omR NR we. I? «own me. 3. en me 07a 2n m. .m. rmm._ mm._ ANN FFF NFmN ooom Rom mm mm _m opm opm w w. 3 IN . 0 NJ 53 Wm. mauo m_.n mm.n me en Rem“ more en NR we we __R __R MW *0N.F eo.m Rex N__ mmmm Noc_ mm mm kn—F mo_ omm ow— m.w. . a m o m u m o m o m u m o moop\c?E\PEVmIEE ares omm\m:EE area @153 cws\n an m mm>m am MFochoo op uogquoo mo.ouva u * mFMUCoEPLmaxm ”Focucoo .mCCEmFopcmgq mo cowm:+:w xgwcoeoomcpcw cwe\m1 oom spwz mumxoopn novawomg mga_m Lopem nzm ocowon Am>ov museumwmog LmF:Omm> xcmcocoo gnaw; use AQQ gnaw; .Aam>mv mgzmmmga OVFOpmxm Lepaowgpco> “cmCL .A¢mmh\ool %.mem . Dev 2339.. 106 resistance flow relationships, and the results were similar to those seen in Figure 6. Series IV Table 3 presents data obtained in six experiments which examined the effects of local hypoxia, hypocapnia and the combination of hypoxia and hypocapnia on heart rate, mean arterial blood pressure right ven- tricular systolic pressure and dP/dT, coronary perfusion pressure, coro- nary resistance and coronary blood gas tensions. Coronary flow was held constant, and averaged 70 ml/min/IOO 9. With perfusion of the coronary bed with hypoxic blood (pH = 7.34, P02 = 12 mmHg, PCO2 = 40 mmHg), heart rate, arterial pressure, ventricular pressure and dP/dT are unaf- fected, but coronary perfusion pressure and coronary resistance are decreased substantially. When the coronary perfusate is made hypocapnic (pH = 7.77; P02 = 138, PCO2 = 6) no effect is seen on heart rate, arteri- al pressure, ventricular pressure or dP/dT. However, a significant coronary vasoconstriction is seen as indicated by the increase in coro- nary pressure and resistance. When the coronary is perfused with blood that is both hypoxic and hypocapnic (pH = 7.80, P02 = 13, PCO2 = 7), again no effect is seen on heart rate, arterial pressure, ventricular pressure or dP/dT, but a significant coronary vasodilation is seen as indicated by a fall in coronary pressure and resistance. The magnitude of the decrease in resistance seen during these conditions was not sig- nificantly different from that which occurred during hypoxic perfusion alone. 107 HH .A.A dd 00 an. NA NA m.p NA po.n po.n m_.p pp.p me we omNR SNNR mp mp me we HNH NNR .w 9 Amp eqp «N me Row.N NN.N knw.o pm._ Pmm oN_ onp ommp mm mm QNP mpp Nmp emp m.m. m d w Np NR w.“ NR mo.p po.n me.p pm.p ppp NH Nmmp ewmp mp NH mp me NNH SN“ .w mmp SSF am we ANN.N NN.N ¥m¢.N om.P emmp Npp oemp camp mm mm m—_ app Nmp omp m. 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In the pressure-flow diagram it is apparent that as pressure is increased from 50-75 mmHg flow increased in a relatively proportionate fashion. From 75—150 mmHg, flow remains relatively constant until pressure exceeds 150 mmHg, at which point flow increases again. The pressure-resistance diagram indicates that calculated resistance generally increases over the range in which flow is seen to remain relatively constant. Series VI Data presented for this series was obtained from two animals which were chronically instrumented for the determination of right coronary blood flow in the unanesthetized state in response to a variety of stimuli. While it is recognized that this small N number may not pro- vide information that is applicable to the rest of the population, and the data is not analyzed by statistical methods, it is still of inter- est and provides some information that can be used to compare to the results obtained in the acute, anesthetized preparations. Figure 14 depicts phasic coronary blood flow and coronary pres- sure tracings for one animal in which blood flow was stopped for three seconds by inflating a balloon cuff which was implanted around the proximal right coronary artery. Following release of the occlusion, a reactive hyperemic response is seen. This maneuver was repeated after a systemic blocking dose of the prostaglandin synthesis inhibitor indomethacin had been given. A comparison of the control and experi- mental responses to a three second occlusion reveals that the duration Figure 14. 122 Response of the right coronary circulation of the unanesthetized dog to three second occlusion of flow before and after blockade of prostaglandin synthesis with 5 mg/kg indomethacin. N = 1 123 CONTROL 3 sec occlusion 50- RIGHT CORONARY‘O' BLOOD 30-1 FLOW 20‘ (mm) 10- 0: DURATION =10 sec 200. PEAK now INCREASE=150% RIGHT 160. CORONARY 12° PRESSURE ' (mmHg) 80' 40- 0d INDOMETHACIN 3 sec occlusion RIGHT 50' CORONARY 40- BLOOD 30- FLIJVV (mm) 20- 10- O: DURATION=8 sec 200 PEAK now INCREASE=1102 RIGHT 16° CORONARY PRESSURE 12 (mmHg) 80 40 0 Figure 14 124 of the hyperemia was shorter (10 vs 8 seconds) after indomethacin, as was the peak flow (150% vs 110% above control flow). This indicates that indomethacin had a slight affect on the reactive hyperemic response. Responses were also obtained for 5 second, 10 second and 30 second occlusions before and after indomethacin. No differences were seen between these two conditions for these longer occlusion periods. The hyperemic response was noted to increase as the duration of occlusion increased. Figure 15 depicts the pressure—flow relationships and the pressure resistance relationships (solid line and dashed line, respectively) for one animal in which coronary pressure was decreased in a stepwise fashion by inflation of the occluder cuff. At each new pressure level, the steady state blood flow was recorded and an arbitrary resistance unit calculated. From the diagram it can be demonstrated that as pres— sure is decreased, flow is maintained at a relatively constant level over the range of 90-30 mmHg. Over this range, calculated resistance is seen to fall until a pressure of 30 mmHg is reached. At this point a further decrease in pressure appears to be associated with a propor- tionate fall in blood flow with no change in resistance. This data supports the findings of Series V in which the pressure flow relation- ships were determined during constant pressure perfusion. Figure 16 demonstrates the effects of intracoronary bolus injec- tions of norepinephrine before and after alpha blockade in one unanesthetized dog. At a dose of 0.05 ug norepinephrine, a biphasic response is seen. There is an early decrease in blood flow associated with an increase in calculated resistance, followed by a prolonged F n z .moo ooNppogpmococo ogp po mgoppo mpococoo pcmpp ocp op oocopmpmoc oco ocommopo mo ppm; mo .zopp oco ocommopo ooozpoo mopgmcoppopom .m— ocompm 125 126 ?:\o: is; 32:23: COP m om mp oasmpo AOXIIV 2:535. 23:: 5222.3 :52 Ow CV ON N o 9 S; g: 3 as: pgpzazco 9 an 127 omcoomom opopm mooopm opop u omcoomom poopmcocm mpcom n poppcoo n F“ ZULU—I .moo ooNppomeococ: ogp :p oooxoopo ogopo Loppo oco opowoo oocopmpmop Lo—oowo> oco .3opp coo—o apococoo .opsmmopo copmoppoo myocopoo pgmp; co Amzv ocppsoocpooeo: mo mcoppoomcp mopon mpocopooogpcp So pooppm .mp oesopo 128 mp opsopa ozoaou. wzonoo. A m ”o ,4 m ”o mZoaou. .— m 0 m2 mane. .— m U filo In 15551.55 woz1420m00 - HIOE r2. 10 I00 AuIEEv mmDmmmmn— .- >m<20m00 FIG—m l GNP 129 increase in blood flow and decrease in calculated resistance. The same response is seen when the dose of norepinephrine is increased to 0.2 pg. Phentolamine was then given at a dose of 1 mg/kg intravenously and the responses to norepinephrine again obtained. Norepinephrine (0.05 pg) again resulted in a biphasic response except that the initial increase in resistance was limited to 15% where it had been 78% in the control situation. This was followed by a mild decrease in resistance in the steady state. At 0.2 pg injection of norepinephrine, the initial rise in resistance was also limited to only 10% (vs 45% at control) and was followed by a 30% fall in resistance in the steady state. Figure 17 depicts the effects of intracoronary bolus injections of adenosine and norepinephrine on coronary pressure, blood flow and resistance in a second unanesthetized animal. A 1 pg injection of aden- osine produced a substantial increase in coronary blood flow, and a decrease in coronary resistance of approximately 66%. A 0.3 pg nor- epinephrine injection produced an increased coronary blood flow and a decreased coronary resistance of approximately 57% in the steady state. No transients were noted in these recordings. Coronary pressure was then decreased from 80 to 50 mmHg and norepinephrine was again injected. The response seen was a decrease in coronary blood flow and a substan- tial increase in coronary resistance (100%). Figure 17. 130 Effects of intracoronary bolus injections of adenosine and injections of norepinephrine before and during myocardial ischemia on right coronary perfusion pres— sure, coronary blood flow and vascular resistance in the unanesthetized dog. 1 Control N C E Experimental 12o. RIGHT CORONARY " PRESSURE (mmHg) 60 -. 131 OJ 40! RIG HT _ CORONARY BLOOD FLOW 20- Ohm) 0- 10 " RESISTANCE _. (mmHg/mm) 5 - OJ c e c E ADENOSINE NE 1M9 Q3ug Figure 17 c E NE during Ischemia 0.3 ug DISCUSSION Methodology It was important to determine at the onset of this study whether or not significant collateral vessels exist between the right coronary artery and branches of the left coronary artery. To test this possi- bility, the pump perfusing the right coronary artery was stopped. This yielded a non-pulsatile baseline perfusion pressure of approximately 15 mmHg. A tourniquet was then applied to the descending thoracic aorta which increased pressure in the ascending aorta to approximately 170 mmHg. It was assumed that if any significant collaterals were present between the right and left coronary arteries, this maneuver would result in an increase in the baseline (pump off) perfusion pres- sure in the right coronary artery. However, no such effect was seen. Furthermore, infusion of crystal violet dye into the right coronary artery at perfusion pressures up to 180 mmHg in both the beating and fibrillating heart produced staining of a discrete portion of the right ventricular free wall, with no staining occurring in the interventricu- lar septum or left ventricular free wall. This evidence indicated that the right coronary circulation was essentially free of any significant collateral communication with the left coronary system. Microsphere studies in the dog have shown that a portion of the right ventricular free wall and the interventricular septum are 132 133 perfused by branches of the left coronary artery (Murray pp_pl., 1979), although as previously mentioned, these vessels apparently do not anastomose with those from the right coronary artery. However, the fact that the right ventricle is supplied by both right and left coronary arteries in the dog is important when attempting to interpret the ven— tricular hemodynamic data obtained in the present study. Therefore, the effect of experimental interventions which only affect the portion of the ventricle perfused by the right coronary artery may not be clearly seen when overall right ventricular hemodynamic data is analyzed. Is it also important to note that all anesthetized animals in this study were vagotomized. This was performed in order to eliminate the buffering effect of the baroreceptors located in the aortic arch from the baroreflex obtained by the production of carotid sinus hypo- tension. Vagotomy was associated with a high resting heart rate in nearly every animal. As a result, heart rates were not consistently seen to rise in response to stimuli which have known positive chrono- tropic effects. Adequate Characterization of the factors which contribute to the regulation of blood flow through the right coronary vascular bed has never been accomplished. Since this vascular system is of great importance in the majority of the human population, further physiologi- cal investigation was warranted. The studies described herein have attempted to better define the role of autoregulation, circulating catecholamines, adrenergic receptor activity, prostaglandins, oxygen and carbon dioxide and sympathetic nerve stimulation in both the local and remote control of blood flow through this vascular bed. 134 Briefly, the results of this study have shown that in anestetized dogs, the pressure/flow relationships are virtually linear over the physiological range of pressures during constant flow perfusion of the right coronary artery. During constant pressure perfusion, flow is maintained relatively constant over the range of 75-150 mmHg, indicating that this bed exhibits autoregulation to a greater extent during con- stant pressure perfusion than during constant flow perfusion. Constant Flow Studies In the constant flow studies it was demonstrated that sympathetic stimulation increased heart rate, mean arterial blood pressure, right ventricular systolic pressure, and right ventricular dP/dT, yet caused an increased coronary vascular resistance. This increased resistance is undoubtedly related to active coronary vasoconstriction since there was no change in hematocrit, hence, no change in viscosity. The coro- nary vasoconstriction was enhanced as flow to the bed was decreased. Infusion of norepinephrine on the other hand usually increased heart rate, right ventricular systolic pressure, dP/dT, and produced a sig- nificant reduction in coronary resistance. In the presence of propranolol (beta blockade), no greater vasoconstriction was seen in response to sympathetic stimulation, but the vasodilation seen with norepinephrine was converted to a vasoconstriction. The vasoconstric- tion seen with sympathetic stimulation could be blocked with phentol- amine, indicating that it is an alpha adrenergically mediated phenomenon. 135 During constant flow perfusion, both systemic and local hypo- Capnia significantly increased coronary vascular resistance. Subsequent sympathetic stimulation produced an additive vasoconstriction, however, the response was not enhanced during these conditions. lluevasodilatory response to norepinephrine was either attenuated (high dose) or blocked (low dose) in the presence of systemic or local hypocapnia. Inhibition of prostaglandin synthesis by indomethacin had no significant effect on resting coronary resistance and had no apparent effect on the response to sympathetic stimulation, norepinephrine infu- sion or hypocapnia. The reactive dilation seen with 20 second inter- ruptions of coronary flow was significantly decreased by indomethacin. Local hypoxia or the combination of hypoxia and hypocapnia resulted in a profound coronary vasodilation. The response to sympa- thetic stimulation was not changed by the combination of hypoxia and hypocapnia, but the response was absent in the presence of hypoxia alone. Norepinephrine (low dose) produced the same degree of dilation during hypoxia but had no significant effect during the combination of hypoxia and hypocapnia. At the higher dose, norepinephrine produced a coronary vasodilation that was significantly less in magnitude during hypoxia and the combination of hypoxia and hypocapnia. Constant Pressure Studies In other studies, a constant pressure perfusion technique was used to determine if the responses seen previously were dependent on constant flow perfusion conditions. Under these conditions, hypoxia produced a profound decrease in coronary resistance with no measurable change in 136 ventricular function. Hypercapnia was found to produce a significant decrease in coronary resistance as well, even in the face of a decrease in dP/dT. Hypocapnia was found to increase arterial blood pressure, heart rate, and ventricular pressure, but had no consistent effect on coronary resistance. Sympathetic stimulation had no effect on coronary resistance or ventricular performance when the bed was rendered hypoxic, but increased resistance and dP/dT during local hypercapnia. During hypocapnic perfusion, sympathetic stimulation had no consistent effect on coronary resistance or ventricular performance; however, resistance was seen to increase in four out of seven animals. Infusion of nor- epinephrine during hypoxic or hypercapnia perfusion had no effect on coronary resistance; whereas, ventricular function was increased during hypercapnia with norepinephrine. During hypocapnia perfusion, norepine- phrine increased ventricular function and decreased coronary resistance to a similar degree as during normoxic, normocapnic perfusion. In order to relate some of the findings in the anesthetized, open- chest preparation to those that might occur in the conscious animal, two dogs were chronically instrumented for the determination of right coronary hemodynamics. Following recovery from the surgery required for instrumentation, data was obtained from these animals while awake and resting quietly. These results showed that as coronary pressure was decreased from 90—30 mmHg, a substantial degree of autoregulation was seen. Blood flow was fairly well-maintained over this range and calcu- lated coronary resistance fell. The reactive hyperemic responses before and after prostaglandin synthesis inhibition with indomethacin to 3, 5, IO, and 30 second coronary occlusions were obtained. 137 Indomethacin had no effect on the responses to 5, 10, or 30 second occlusions. However, it appeared that the hyperemic response to 3 second occlusions was slightly attentuated following indomethacin. Intracoronary bolus injections of norepinephrine resulted in a biphasic response in one animal. Coronary resistance increased transiently, followed by a prolonged decrease in resistance. The initial increase in resistance could be prevented with alpha receptor blockade. In a second animal, norepinephrine produced only a prolonged decrease in resistance, except when the bed was rendered ischemic, in which case norepinephrine increased resistance substantially. Early work by several investigators has shown that sympathetic stimulation via stellate ganglion stimulation or mediated through the baroreceptor mechanism produces an increase in heart rate and systemic arterial pressure and a fall in left coronary vascular resistance (Berne pp _l., 1958; Szentivany and Juhasz-Nagy, 1963; Feigl, 1968; DiSalvo pp al., 1971). The decline in resistance was sometimes pre- ceded by a transient rise in resistance. Furthermore, other studies have attempted to delineate the direct and indirect effects of the release of norepinephrine from sympathetic nerves on left coronary vascular resistance. Following beta receptor blockade, stellate stimu- lation produces only coronary vasoconstriction which, in turn, can be blocked by alpha receptor blockade (Feigl, 1967). It therefore appears that with sympathetic stimulation, alpha (coronary vasocon- strictor) and beta (myocardial and vascular) receptors are activated simultaneously, and coronary vasodilation is the dominant response in the left coronary circulation. This was confirmed by a study in which 138 the net effect of adrenergic stimulation through the sympathetic nerves was an increase in myocardial oxygen extraction, a decrease in coronary venous oxygen content coupled with a rise in coronary blood flow. After alpha receptor blockade, stimulation produced only slight changes in oxygen extraction and coronary venous oxygen content and a 30% greater increase in coronary blood flow (Mohrman and Feigl, 1978). These studies suggested that the overall response of the left coronary vascular bed to sympathetic nerve stimulation is in part dictated by the competition between alpha mediated vasoconstriction and beta 1 or 2 receptor mediated vasodilation. In a more recent report, Powell and Feigl (1979) demonstrated that baroreflex stimulation produced a 21% increase in left coronary disatolic resistance when the metabolic fac- tors are eliminated by beta blockade and maintenance of a constant afterload. The rise in resistance was shown to be mediated by alpha receptor activation. In the present study, the effects of baroreflex sympathetic stimu- lation on right coronary hemodynamics and right ventricular performance were determined. It was hypothesized that the metabolic influences in the right ventricular myocardium would be considerably less than in the left ventricular myocardium since myocardial wall tension and cardiac work are much less. This point cannot be documented since it is not technically feasible to directly measure myocardial oxygen consumption for the right ventricle because it is impossible to obtain right coro- nary venous blood in the dog. However, because the predicted meta- bolically related influences in the right ventricle would be less during sympathetic stimulation, then perhaps the response of this vascular 139 bed would be different from that observed for the left coronary vascular bed. The results of the present study demonstrate that under constant flow or constant pressure perfusion, sympathetic stimulation via the baroreflex mechanism produces an increase in heart rate and mean arteri- al pressure which indicates an intact reflex. During stimulation, right ventricular performance was enhanced, and was accompanied by a para— doxical increase in right coronary vascular resistance. During such conditions, coronary resistance would be expected to decrease in order to accommodate an increase in coronary oxygen delivery at a time when oxygen demand is apparently increased. Following beta blockade with 3 mg/Kg propranolol, sympathetic stimulation produced an increase in coronary resistance that was not different in magnitude from that seen prior to beta blockade. It should be noted that beta blockade did not completely prevent the increase in heart rate seen with sympathetic stimulation. This could be inter- preted as an incomplete degree of beta blockade which could explain why the coronary vasoconstriction in response to nerve stimulation was not enhanced during these conditions. However, Donald pp pl. (1968) demon- strated that propranolol does not totally prevent the increase in heart rate in conscious dogs in response to exercise or reflex sympathetic activity, but it does prevent the increase in heart rate during exercise or reflex nerve activity when the hearts are totally denervated. Unblocked denervated hearts show an increase in heart rate during exer- cise due to elevated levels of circulating catecholamines. These authors suggested that propranolol does not completely block the heart rate 140 response to exercise or reflex sympathetic activity but does block the response to circulating catecholamines. This report aids in the explana- tion of the results obtained in the present study. It appears that propranolol is incapable of totally blocking the heart rate response to sympathetic nerve activity. The administration of propranolol resulted in an increase in rest- ing coronary resistance by approximately 38% in the current study. Whitsitt pp_gl. (1967) demonstrated that the administration of propran- olol produced a 37% increase in resting coronary resistance in the left coronary artery. These investigators attributed the increase in resist- ance to a decrease in heart rate and contractile force which would tend to decrease oxygen consumption and hence oxygen demand. While contrac- tile force was not measured in the present study, heart rate was seen to decrease by approximately 7% in response to beta blockade. It is assumed that the rise in resistance seen with beta blockade is the result of a decrease in the oxygen demand of this vascular bed. Because the increase in right coronary resistance seen with sym- pathetic stimulation is not any greater in magnitude following beta blockade, it is conceivable that the vasodilator influence from beta 1 myocardial and/or beta 2 vascular receptor activation are minimal, and only a dominant alpha mediated vasoconstriction is seen in both the normal and beta blocked condition. This point is supported by the work of Murray and Vatner (1979) who recently reported that baroreflex stimulation in the conscious dog produced that same degree of right coronary vasoconstriction after beta blockade as before. 141 The results presented in Table l reaffirm this hypothesis. Sympathetic stimulation produces a significant increase in right coro- nary resistance, and this increase in resistance can be completely abolished by the infusion of the competitive alpha receptor blocking agent phentolamine. This demonstrates that the mechanism of the neural- ly induced coronary vasoconstriction involves alpha receptor activation. These results also demonstrate that following alpha blockade, sympathetic stimulation produces no change in coronary resistance. This indicates that in the absence of alpha receptor activity, activation of myocardial and vascular beta receptors through sympathetic nerve activity has no vasodilator effect on the right coronary vascular bed. This is a situa- tion that is quite different from that reported for the leftcoronary system (Feigl, 1967; Mohrman and Feigl, 1978; Powell and Feigl, 1979; Szentivanyi and Juhasz, 1963) in which sympathetic stimulation produced a substantial decrease in coronary resistance. It seems unlikely that such stimulation would not increase myocardial oxygen consumption of the bed supplied by the right coronary artery to some extent. Manohar .pt_pl. (1979) and Murray pp_pl. (1979) have recently shown that blood flow to the right ventricular myocardium is on the order of 70-80 ml/min/lOO gm tissue using the microsphere technique. This value is very close to that reported for the left ventricular myocardium. It seems that the right ventricular myocardium may be hyperperfused such that the oxygen delivery to this tissue may actually exceed oxygen demand. If this were the case, it is possible that with enhanced metabolic activity as should be seen with sympathetic stimulation, the increased oxygen demand could be met simply with an increased oxygen 142 extraction, without a need for an increased coronary flow. A resistance change would not necessarily occur in this situation. It was also important to determine if the coronary vasoconstric- tion seen with sympathetic stimulation was altered during constant flow conditions when flow rate was set at levels above and below normal val- ues. These results are depicted in Figure 4. These data show that as flow is decreased from a state of hyperperfusion (perfusion pressure = 170 mmHg) to a state of hypoperfusion (perfusion pressure = 30 mmHg), the degree of vasoconstriction associated with sympathetic stimulation is enhanced. It therefore appears that flow may influence the degree of sympathetic coronary vasoconstriction. This may be the result of low flow producing a higher local concentration of norepinephrine due to a decreased rate of washout, enzymatic degradation or neuronal reuptake. A further objective of the present study was to determine if the response of the right coronary circulation to adrenergic stimuli is altered when local blood gas tensions are altered. However, as a pre— lude to this study, it was important to determine the right coronary response to alterations in local blood gas tensions. It has been well-established that hypoxia is a potent coronary vasodilator. As early as 1913, Markwalder and Starling demonstrated that hypoxia produces coronary vasodilation in the heart-lung prepara- tion. The controversy which has ensued since that time has been whether the vasoactivity associated with low oxygen tensions is a direct effect on the vascular smooth muscle or an indirect effect mediated through the release of vasodilator metabolites. While the work of Berne pp g1. (1958,1964,1974,l975) supports the metabolite theory, other 143 workers have shown that in isolated strips, the contraction and relaxa- tion of vascular smooth muscle can be induced directly by raising and lowering the oxygen tension of the bathing solution (Detar and Bohr, 1968). The local vasoactivity of oxygen in the intact animal has been demonstrated for the left coronary bed by Daugherty pp pl. (1967). These investigators reported that oxygen tension had to be decreased below 40 mmHg before a fall in coronary resistance could be seen. Manohar gp_pl. (1979) demonstrated that systemic hypoxia (P02 = 43 mmHg) produced a 100% increase in right coronary blood flow as determined in the calf using the microsphere technique. However, systemic hypoxia also produces chemoreceptor activation and catecholamine release. Therefore, it is impossible to relate the coronary response seen to the direct effect of hypoxia. The results from the current study support the pre-existing evidence that hypoxia is a potent coronary vasodilator. The data pre- sented here demonstrates that local hypoxia is associated with a 50-75% decrease in right coronary resistance during both constant flow and constant pressure perfusion. The mechanism by which hypoxia produces coronary vasodilation (direct or indirect) can not be elucidated from this study. It is noted that hypoxia had no effect on right ventricular pressure or dP/dT. However, as previously mentioned, the right ventricle gets a substantial amount of blood supply from the left coronary artery which probably serves to maintain right ventricular function. The local effects of changes in carbon dioxide tensions (and/or hydrogen ion concentration) on coronary vascular resistance is somewhat controversial. First, it is unclear whether carbon dioxide acts 144 directly on vascular smooth muscle or whether it acts indirectly through the bicarbonate-buffer system to effect changes in hydrogen ion and consequently calcium ion activity (Mrwa pp p1,, 1974). Reference to carbon dioxide in this discussion will also imply the involvement of the hydrogen ion as well. Second, it appears that the degree of vaso- activity associated with changes in carbon dioxide tension is also related to the experimental conditions under which the studies are made. Daugherty pp pl. (1967) demonstrated that a decrease in local carbon dioxide tension produced an increase in left coronary vascular resist— ance when the bed was perfused at constant flow. Similarly, Case and Greenberg (1976) reported that hypocapnia, produced locally in the left coronary bed, produced a substantial increase in coronary resistance when perfused at constant flow. However, Feinberg_pp_gl. (1960) reported that systemically administered carbon dioxide was a poor vaso- dilator in the left coronary artery when perfused at natural flow. This position was also supported by the work of Rooke and Sparks (1978), while Alella pt_gl, (1955) had previously demonstrated that coronary sinus carbon dioxide tension does not rise to vasoactive levels during enhanced cardiac activity. Data from the present study demonstrates that under constant flow conditions, systemic or local hypocapnia results in a significant increase in right coronary resistance without a measureable change in ventricular performance. During constant pressure perfusion, hypocapnia had a variable effect, producing coronary constriction in four out of seven animals studied. With hypocapnia, ventricular pressure was slightly but significantly decreased, which could, in part, explain the 145 rise in coronary resistance. However, no change in ventricular func- tion was noted with hypocapnia during constant flow perfusion. Threfore, it seems more likely that changes in coronary resistance seen are related to the direct influences of changes in carbon dioxide tension. Hypercapnia during constant pressure perfusion was associated with a significant degree of right coronary vasodilation. This coronary vasodilation was also associated with a significant decrease in dP/dT. It is more apparent in this case that the metabolic factors are not mediating the vascular response since a fall in dP/dT should cause a fall in myocardial oxygen consumption and a subsequent increase in coronary resistance. The fact that resistance decreased in response to hypercapnia again suggests that the vascular response is the result of direct effects of carbon dioxide tensions (or H+ ion) on the coronary vasculature. These results support the proposal that the vasoactivity associ- ated with local changes in carbon dioxide are related to the direct effects of CO on the right coronary vasculature. These results do not 2 necessarily suggest that CO2 is involved in the local regulation of blood flow. They do, however, support the work of Daugherty pp pl. (1967) and Case and Greenberg (1976) in which locally produced changes in carbon dioxide tensions were shown to be vasoactive in the left coronary vascular bed. The work of Feinberg (1960) and Rooke and Sparks (1978) would support the opposite view. However, in both of these studies (Feinberg, Rooke), CO2 was administered systemically. This methodology greatly complicates the picture since CO2 has many 146 systemic effects; such as, chemoreceptor reflex activation, catechol- amine release, etc. Therefore, it is impossible to compare these studies with those in which carbon dioxide is introduced locally. The response to the combination of local coronary hypoxia and hypocapnia was also determined for the right coronary vascular bed. Such a combination produced a substantial coronary vasodilation during constant flow perfusion. The degree of vasodilation seen during these conditions is not different from that which occurs during hypoxia alone, which further supports the concept that when hypoxia and hypocapnia are combined, the effects from hypoxia predominate and coronary vasodilation is seen. It was also important to determine if changes in blood gas ten- sions altered the response of the right coronary circulation to adren— ergic stimuli. It was hypothesized that perhaps the coronary vasocon- striction seen with sympathetic stimulation would be enhanced during hypocapnia conditions since Yasue pp pl. (1978) had shown that coronary vasospasm could be induced in patients with Prinzmetal's angina by hyper- ventilation and systemic alkalosis. It is also generally believed that the effect of catecholamines is enhanced in an alkalotic medium. Hence, the present study investigated the response to sympathetic stimulation with coronary hypoxia, hypocapnia, the combination of the two, as well as hypercapnia. During systemic or local hypocapnia, with constant flow perfusion, the effects of sympathetic stimulation were the same as those seen with normocapnic perfusion. Therefore, it appears that the degree of coronary vasoconstriction is not changed when the bed is rendered hypocapnic. However, the vasoconstrictor effects of hypocapnia and 147 sympathetic stimulation are additive such that the combination of the two produces a substantial degree of coronary vasoconstriction. The results obtained with constant pressure perfusion are not as clear. As a group it appears that the sympathetic vasoconstriction on the coronary bed was prevented during hypocapnia. A closer examination of the results reveals that three out of the seven animals studied did not respond to stimulation during hypocapnic perfusion. 0f the four that did respond, the coronary vasoconstriction seen with stimulation was not different in magnitude from that which occurred during control conditions. This suggests that the results from the constant pressure group may not be much different from those obtained with constant flow perfusion. Sympathetic stimulation during hypoxic conditions with either constant flow or constant pressure perfusion had no effect on ventricu- lar function or coronary resistance. Detar and Bohr (1972) demonstrated that isolated aortic strips showed a profound decrease in contractile response to epinephrine when exposed to a bath with a low oxygen ten- sion. This suggests that the contractile machinery in vascular smooth muscle is substantially depressed by an oxygen lack. This finding is supported by the present study in that the ventricular and coronary response to stimulation were completely blocked during hypoxic perfusion. As previously mentioned, when hypoxia and hypocapnia are combined dur- ing constant flow perfusion, a coronary vasodilation is seen. With subsequent sympathetic stimulation, the enhanced ventricular performance and coronary vasoconstriction occur to a similar degree as seen during normoxic perfusion. The fact that sympathetic stimulation produces 148 coronary vasoconstriction when the coronary is perfused with hypoxic and hypocapnic blood does not support the previously mentioned hypothe- sis that the ability of a blood vessel to actively develop tension dur- ing hypoxic conditions is greatly attenuated. The fact that the ability of the vessel to constrict during hypoxic and hypocapnic conditions is preserved suggests that the interactions between sympathetic nerve activ- ity and alterations in local blood gas tensions (in relation to their effects on coronary resistance) may occur on levels other than that simply predicted by the direct effects of blood gas tensions alone. For instance, changes in pH may be partially responsible for the modula— tion of the sympathetic response. During constant pressure perfusion, hypercapnia produced a decrease in coronary resistance and dP/dT. Subsequent sympathetic stimulation resulted in an increase in ventricular performance as well as coronary resistance. This increase in resistance was slightly but significantly decreased in magnitude when compared to that which occurred during normoxic, normocapnic perfusion. From these studies it appears that the response of the right ventricle and right coronary circulation to sympathetic stimulation is unaffected by local or systemic hypocapnia, or by the combination of hypoxia and hypocapnia. The response is slightly diminished by local hypercapnia and is completely abolished by local hypoxia. It can be concluded from these experiments that local changes in blood gas tensions may alter the response of the coronary circulation to nerve stimulation. 149 Norepinephrine (NE) is a mixed adrenergic agonist, exhibiting primarily alpha receptor affinity with some degree of beta receptor affinity (Goodman and Gilman, 1970). The effect of exogenously admin- istered NE on coronary blood flow has been reported by several investi- gators. Hardin gp_pl. (1961) demonstrated that infusion of NE resulted in a transient increase in total coronary resistance which precedes the increase in heart rate. This is followed by a prolonged decrease in coronary resistance. This observation was also made in an earlier report by Berne p;_pl, (1958) in which NE was administered to the left coronary artery of the beating, intact dog heart. In a more recent study, Malinzak p§_pl, (1978) demonstrated that intravenous injections of NE has a differential response, depending on the portion of the vascular segment in question. The large artery segment of the left coronary artery responds to NE with an increase in resistance on the order of 190%. This rise in resistance could be prevented with alpha receptor blockade. The vascular segment distal to the large artery segment responds to NE with a 53% decrease in resistance. Coronary blood flow increases 250% above control with aortic pressure increasing to a lesser extent indicating that total coronary resistance has also decreased. Following beta blockade with propranolol, NE causes small artery resistance to increase while large artery resistance is rela- tively unchanged. Large artery resistance is unchanged during these conditions because the administration of propranolol results in a near maximal coronary vasoconstriction in the large artery segment. Zuberbuhler and Bohr (1965) demonstrated that large vascular strips taken from the left coronary arteries in dogs responded to NE with an 150 increase in tension while strips taken from small coronary arteries responded to NE with a decrease in tension. These studies suggest that NE acts directly on coronary vessels to produce alpha mediated vasoconstriction which is primarily found in the large artery segment. The small coronary arteries apparently have a population of beta recep- tors which mediate coronary vasodilation. NE also has indirect vaso— dilatory effects which are primarily mediated through the enhanced metabolism seen with stimulation of the myocardial beta 1 receptor. The beta 2 receptors located in the small coronary vessels seem to directly mediate coronary vasodilation when stimulated by beta agonists (Klocke 3; pl., I965; Braunwald pt pl., 1976; McRaven pp 21., 1971). However, Hamilton and Feigl (1976) observed only slight coronary vascu- lar responses in the left coronary bed which were attributable to vascular beta 2 receptors, and concluded that they are of little func- tional significance. To complicate the issue, Baron and Bohr (1972) using coronary strips reported that practolol (beta 1 blocker) abolished the coronary vascular response to a pure beta agonist (isoproterenol), suggesting that the vascular beta receptors may be of the beta 1 variety, the same as those found in the myocardium. The current study presents data pertaining to the steady state response of the right coronary circulation to intracoronary infusions of NE. In virtually every animal (Tables 5, 6 and 9), NE increased heart rate, ventricular pressure during both constant pressure and con- stant flow perfusion. The infusion of NE was also associated in most instances with a decrease in right coronary vascular resistance. This coronary vascular response is different from that seen with sympathetic 151 stimulation. It is hypothesized that the reason nerve stimulation produces coronary constriction and NE produces coronary dilation may be the result of a greater stimulation of the myocardium with NE producing a significant metabolic influence on coronary resistance. The results in Figure 5 demonstrate that following beta-blockade with propranolol, the heart rate response to NE was completely blocked and NE produced a significant increase in coronary resistance during constant flow perfu- sion. This suggests that norepinephrine infusion results in coronary vasodilation in the control state due to an increased myocardial metab- olism. However, it is uncertain in this series of experiments to what extent the vascular beta 2 receptors play in the vasodilatory response. While it is generally believed that vascular beta receptors do not play a large role in regulating vascular resistance, McRaven pt pl. (1971) suggested that as much as 70% of the left coronary vasodilation seen with isoproterenol infusion was due to stimulation of the vascular beta 2 receptors. In this regard, we have begun a study to try and under— stand the precise mechanism of the vasodilation seen with NE challenge in the present study. The responses to intracoronary bolus injections of NE and isoproterenol were obtained for the right coronary artery perfused at constant flow, before and after selective beta 1 receptor blockade with practolol (10 mg/kg), alpha receptor blockade with phentolamine (600 pg/min, intracoronary infusion) and beta 1 and 2 receptor blockade with propranolol (3 mg/kg). The results of this experiment for four animals is represented in Figure 18. Only the coronary resistance values are shown. Figure 18. 152 Effects of intracoronary bolus injections of isoproterenol (0.5 pg) and norepinephrine (1 pg) on right coronary vascular resistance before and after selective adrenergic receptor blockade with practolol (10 mg/Kg), propranolol (3 mg/Kg) and phentolamine (600 pg/min infusion) during constant flow perfusion. 4 Significantly different from control at P < 0.05. N at CVR (PRU) CVR (PRU) + 4.. 153 ISOPROTERENOL FI:44 0.5119 30% 3% 2% 'A' ._,__h '1“ C E (I E (I E CONTROL PRACTOIOL PHENTOLAMINE PRAC‘IOLOL NOREPINEPHRINE N=4 1P9 1% C E PRO'RANOLOL PHENTOLAMINE PRAC‘IOLOL C E c. E (I E CONTROL PIAC IOLOL PHENIOLAMINE PIAC IOIOL Figure 18 Cr E PIOPIANOLOI PHENIOLANINE PIACIO lOl 154 Assuming that practo101 b10cks on1y the beta 1 receptor, these resu1ts demonstrate that fo11owing beta 1 b1ockade with practo1o1, isoprotereno1 causes a significant but very s1ight coronary vasodi1a- tion compared to the contro1 response. This suggests that the vaso- di1ation attributab1e to vascu1ar beta 2 receptors is minima1. As for the resu1ts with NE injection, after beta 1 b10ckade with practo1o1, the vasodi1ation seen during contr01 conditions is converted to a vaso- constriction. This finding supports the hypothesis that the vasodi1a- tion seen with intracoronary infusion of NE is indirect and due prim— ari1y to the fact that myocardia1 metabo1ism is increased through beta 1 receptor activation. The coronary vasoconstriction seen with NE and practo101 is b10cked when phentoa1mine is infused into the coronary artery. These resu1ts attest to the fact that the response of the right coronary circu1ation to exogenous NE is dictated by the competi- tion between a1pha vasoconstrictor receptors and beta 1 myocardia1 receptors. The net effect of intracoronary NE infusion or injection is vasodi1ation primari1y due to a greater inf1uence of metabo1ic vaso- di1ators. Vascu1ar beta 2 receptors have 1itt1e functiona1 significance in this vascu1ar bed. The response of the coronary circu1ation to NE in conscious dogs is apparent1y somewhat different than that seen when the anima1s are anesthetized. Vatner gt a1. (1974) demonstrated that intravenous NE in the awake anima1 produced a brief fa11 in 1eft coronary resistance fo110wed by a sustained increase in coronary resistance. These respon- ses were prevented by proprano1o1 (ear1y di1ation) and phentoTamine (1ate vasoconstriction), respective1y. When the study was repeated 155 with the same anima1$ anesthetized with sodium pentobarbitaT, on1y coronary di1ation was seen. This phenomenon was a1so reported by Pitt 3; 31, (1967). In order to try and re1ate the data obtained in the present study to the unanesthetized situation, two dogs were chronica1— 1y instrumented for determination of right coronary b1ood f1ow and resistance in the awake state. In one anima1, boTus injections of intra- coronary NE resu1ted in a transient increase in coronary resistance f011owed by a pro1onged decrease. The initia1 rise in resistance cou1d be prevented to a great extent by a1pha b10ckade (Figure 16). In the second anima1, no transients were seen as NE resu1ted on1y in coronary vasodi1ation. These resu1ts support the data obtained in the present study with anesthetized anima1$ except that no transient responses were seen. It is not entireTy c1ear how this data re1ates to that pub1ished by other investigators since these responses were obtained in different vascu1ar beds using different techniques. The present study examined the effect of various f1ow rates on the right coronary response to NE. In the anesthetized dog, the response to NE was not different at any of the three f1ow rates studied. This resu1t is different from that seen with sympathetic stimu1ation, in which the response was enhanced at 1ow f10w rates. With NE infusion, a greater re1ease of vasodi1ator substances compared to nerve stimu1a- tion wou1d be predicted. Hence, as f10w is decreased, the washout of these vasodi1ators wou1d be diminished. Therefore, it is not surprising that the response to NE at different f1ow rates is unaffected. This resu1t is supported by the work of Hardin §t_§l, (1961) in which f1ow 156 did not affect the response of the 1eft coronary circu1ation to NE infusion. In the anesthetized dog, the response to NE was not different at any of the three f1ow rates studied. However, in the unanesthetized preparation, NE was administered during myocardia1 ischemia, which was produced by inf1ation of a ba11oon cuff around the coronary artery. Coronary perfusion pressure was decreased by approximate1y 50% and coro- nary b100d f10w by approximate1y 20% by cuff inf1ation. Subsequent intracoronary bo1us injection of NE produced a substantia1 (100%) increase in coronary resistance. This finding suggests that as f10w is 1owered, the response to NE is a1tered. This is supported by a pre1imi- nary report by wa1insky et a1. (1978) who showed that NE caused an increase in 1eft coronary b1ood f1ow during norma1 perfusion, but caused a significant decrease in b100d f10w when the coronary artery was stenotic. There is a paucity of evidence in the Titerature re1ating the effect of changing b1ood gas tensions 10ca11y in the coronary bed on the response to catecho1amines. As previousTy mentioned, Detar and Bohr (1972) showed that the contracti1e responses of aortic strips to epinephrine was drastica11y reduced when the oxygen tension of the bath was 1owered to a 1eve1 of 1mmHg. Since NE has been shown to be a good vasodi1ator through indirect mechanisms, during hypoxia (a condition which decreases resistance substantia11y) it might be expected that 1itt1e further vasodi1ation wou1d occur in response to NE. It is a1so known that catecho1amines exert a greater effect in an a1ka1otic medium as seen with hypocapnia b100d, and 1ess of an effect 157 with an acidotic medium as seen with hypercapnic b1ood. Therefore, NE may exert a greater vasodi1ator inf1uence as the carbon dioxide tension of the coronary b1ood is Towered. The present study attempted to define the effects of NE during a1terations in coronary b100d gas tensions. The resu1ts suggest that coronary hypoxia diminishes the response to NE during constant f10w perfusion, and comp1ete1y prevents the response during constant pressure perfusion. No vasoconstrictor activity was seen with NE during hypoxia which suggests that the vascu- 1ar bed is near1y maxima11y di1ated, and is for the most part incapab1e of constricting in this situation. This resu1t then gives jfl_yivg support to the jn_yjtrg_work of Detar and Bohr (1972) performed on 1eft coronary vascu1ar strips. At 1ow doses (0.25 ug/min) of NE, systemic or 10ca1 hypocapnia prevented the di1ation seen in response to the NE during constant f1ow perfusion. However, if the dose was doub1ed, the response was not pre- vented by hypocapnia. If the bed was perfused at constant pressure, 10ca1 hypocapnia again did not a1ter the response to the 1ow dose of NE, and the same degree of vasodi1ation was again seen. During constant f1ow hypocapnia, statistica1 ana1ysis showed that there was no signifi- cant response to NE, however, four out of the six anima1s studied showed the same degree of vasodi1ation as seen with NE during contro1 perfusion. Using the more physio1ogica1 of the perfusion techniques (constant pres- sure perfusion), it appears that hypocapnia has 1itt1e, if any, effect on the response of the right coronary circu1ation to NE infusion. The data obtained from the constant f1ow studies wou1d a1so support this view. 158 The effects of NE during hypercapnic conditions were observed dur- ing constant pressure perfusion. Hypercapnia a1one produced a signifi— cant coronary vasodi1ation, a1though not to the same extent as that seen with hypoxia. The infusion of NE during hypercapnia had no effect on coronary resistance; however, it did produce an increase in ventricu1ar function as judged by dP/dT. Whi1e it was shown (Figure 1) that there is no corre1ation between initia1 resistance and the change in resist— ance in response to NE in our preparation, the initia1 resistances obtained with hypercapnia fe11 out of the range ana1yzed in Figure 1. At extreme1y 1ow va1ues of initia1 resistance, it is on1y reasonab1e to assume that a vasodi1ator wou1d have 1ess of an effect. This seems to be the case in the present study. The same resu1t occurs when hypoxia and hypocapnia are combined during constant pressure perfusion. Such a combination resu1ts in a profound decrease in resistance, and subsequent NE infusion resu1ts in a diminished vasodi1atory response at the high dose and no response at the 10w dose. As one progresses from a hypercapnia to a hypocapnia medium, the concentration of hydrogen ions decreases. This causes increased binding of ca1cium ions to sites on contracti1e proteins in cardiac and vascu1ar smooth musc1e, which increases contracti1ity or smooth musc1e tension (Katz and Hecht, 1969). This may a1so resu1t in a transient re1ease of histamine and/or changes in hematocrit via changes in the size of red b1ood ce11s (Kontos gt al., 1971). These effects cou1d be responsib1e for enhanced cardiac contracti1ity (histamine) and changes in vascu1ar resistance through changes in viscosity (RBC size). In the present study, no increases in right ventricu1ar contracti1ity were appreciated 159 with hypocapnia; however, a substantia1 increase in coronary resistance was noted. Since it is genera11y be1ieved that the effect of catecho1- amines is enhanced in an a1ka1otic medium, it was surprising that no greater response to NE infusion was seen in the presence of 1ow 1eve1s of carbon dioxide tension. The same was true for sympathetic stimu1a- tion. However, the response to these stimu1i were attenuated when the carbon dioxide tension of the b1ood was increased (hypercapnia). Therefore, these resu1ts support the concept that changes in car- bon dioxide tension may a1ter the vascu1ar response to endogenous or exogenous NE in this vascu1ar bed. Whi1e the response to catecho1amines was not enhanced during hypocapnia, it was depressed by hypercapnia. The ro1e of the endogenous prostag1andins in the regu1ation of either 1eft or right coronary b1ood f1ow is not we11-defined. Investiga- tors have proposed severa1 hypotheses for participation of the prosta- g1andins in the regu1ation of coronary b100d f1ow; however, due to differing experimenta1 resu1ts, a genera1 agreement among investigators has not been reached. Part of the disparity of be1iefs may be due to different experimenta1 mode1s and techniques. Severa1 investigators be1ieve that the prostag1andins participate in the coronary vascu1ar response to hypoxia. Need1eman et a1. (1975) showed that hypoxia caused a transient re1ease of prostag1andins from iso1ated perfused rabbit heart. Afonso gt al. (1974) demonstrated that hypoxic coronary vasodi1ation was attenuated fo11owing b1ockade of prostag1andin synthesis with indomethacin in the c1osed-chest dog. However, Need1eman et a1. (1975) in the iso1ated perfused rabbit heart and Hintze and Ka1ey (1977) in the open-chest dog showed that 160 indomethacin had no effect on hypoxic coronary vasodi1ation. ln_vitro studies by Ka1sner (1975,1976) demonstrated that hypoxia caused re1ease of vasodi1ator prostag1andins from iso1ated bovine coronary artery 1. (1975) proposed a ro1e for prostag1andins in strips. A1exander gt the genesis of coronary reactive hyperemia in a report that demonstrated the attenuation of the reactive hyperemia and prostag1andin (PGE) re1ease from the 1eft coronary bed of the dog fo11owing administration of indomethacin. In a report that contradicted these findings, Owen _et_al. (1975) reported that indomethacin had no effect of 1eft coronary artery reactive hyperemia in the c1osed-chest dog. This was confirmed by Need1eman (1975) and Hintze and Ka1ey (1977). In the present study, indomethacin had no significant effect on resting right coronary resistance. However, resistance was increased in eight of nine anima1s by approximate1y 10%. It is possib1e that a base1ine 1eve1 of vasodi1ator prostag1andins is being re1eased by these vesse1s; however, the data are not definitive on this point. The effect of indomethacin on the response of the right coronary circu1ation to interruptions of f1ow was a1so determined. During con- stant f1ow perfusion, a 20 second interruption of f1ow produced a reactive di1ation as seen in Figure 12. Fo11owing administration of indomethacin, the reactive di1ation was significant1y decreased in magnitude (area) and duration by 24% and 6%, respective1y. The hypoxia which resu1ts from the f1ow deprivation may enhance the synthesis of vasodi1ator prostag1andins. This is supported by the observation that the attenuation in the magnitude of the response (area) is greater than that predicted by the effects of indomethacin a1one on resting coronary 161 resistance. Indomethacin increased resting coronary resistance by approximate1y 10% in most anima1s, but decreased the magnitude of the reactive di1ation by 24%. This suggests that the prostag1andins may be invo1ved in the response to interruptions of f1ow in this preparation. In this series of experiments, the methodo1ogy invo1ved perfusion of the coronary circu1ation by iso1ating the vesse1, cutting through the wa11 of the vesse1 and inserting a cannu1a into the 1umen. Damage to the wa11 of b1ood vesse1s is thought to enhance the synthesis of vaso- di1ator prostag1andins (Sivakoff et al., 1979). This factor may be responsib1e for an enhanced base1ine synthesis of vasodi1ator prosta- g1andins; however, it does not account for the enhanced magnitude of reactive di1ation fo11owing administration of indomethacin seen in the present study. In an additiona1 study, the effect of brief occ1usions of the right coronary artery in the conscious, intact anima1 instrumented for the measurement of right coronary b1ood f1ow was determined before and after administration of indomethacin. The resu1ts of this study are i11ustrated in Figure 14. The reactive hyperemic response to a 3 sec- ond occ1usion was decreased in terms of peak f1ow and duration of the response fo11owing indomethacin. Indomethacin had no effect on the response to 5, 10, or 30 second occ1usions. The conditions under which these observations were made were far more physio1ogica1 than those with the anesthetized open-chest anima1 perfused at constant f1ow. However, this data represents on1y one anima1, and therefore it is dif- ficu1t to re1ate this resu1t to the group of anesthetized anima1s. Whi1e these resu1ts are far from definitive, it appears that the 162 prostag1andins may p1ay a minor ro1e in the response of the right coro- nary circu1ation to periods of brief occ1usions. The issue of the invo1vement of prostag1andins in the regu1ation of coronary b100d f1ow during enhanced cardiac activity is a1so contro- versia1. Sunahara and Ta1esnik (1973) reported that the coronary f1ow response in iso1ated rat hearts to NE was enhanced fo11owing indometha— cin administration whi1e the contracti1e force response was unchanged. Ta1esnik and Sunahara (1974) 1ater showed that prostag1andin E1 infu- sion (in doses which did not affect coronary resistance) a1so attenuated the coronary f1ow response to NE or isoprotereno1. The coronary re- sponse to other direct vasodi1ators was unaffected. These investigators suggested that the prostag1andins may act as a brake on coronary meta- bo1ic vasodi1ation. Har1an gt a1. (1978) emp1oyed the use of an intact dog mode1 to show that indomethacin had no effect on the re1ationship between 1eft coronary b100d f1ow and myocardia1 oxygen consumption in response to isoprotereno1 infusions. This study provides evidence to support the concept that the endogenous prostag1andins do not p1ay a ro1e in the regu1ation of coronary b1ood f1ow during enhanced metabo1ic activity, at 1east in the intact anesthetized dog. In the present study, the effects of sympathetic stimu1ation and NE infusion were determined before and after the b1ockade of prosta- g1andin synthesis with indomethacin. These resu1ts (depicted in Figure 10) demonstrate that the coronary constriction seen with sympa- thetic stimu1ation fo11owing indomethacin is not different from that seen prior to indomethacin. NE prior to indomethacin produced a sig- nificant coronary vasodi1ation. Fo11owing indomethacin NE had no 163 no statistica11y significant effect on coronary resistance. However, six out of seven anima1s demonstrated a degree of coronary vasodi1ation in response to NE fo11owing indomethacin that was not different from that seen prior to indomethacin. One anima1 out of the group showed an increase in resistance with NE infusion fo11owing indomethacin adminis- tration. It is a1so noted that the coronary vascu1ar response to systemic hypocapnia is not different fo11owing indomethacin administra- tion as compared to before. Therefore, it appears that the response to the right coronary circu1ation to adrenergic stimu1ation is not affected by the b1ockade of prostag1andin synthesis. These findings are in contrast to those reported for the 1eft coronary circu1ation by Ta1esnik and Sunahara (1974) and Sunahara and Ta1esnik (1973) which provide good evidence for the participation of the prostag1andins in the 1eft coronary f1ow response to norepinephrine or isoprotereno1. It has been reported by many investigators that the 1eft coronary circu1ation demonstrates the abi1ity to autoregu1ate its b1ood f1ow (Ecke1_et_al., 1949; Fishback_gt_al., 1959; Scott gt al., 1960; Brandfonbrener, 1969; Drisco11, 1964). However, the autoregu1atory abi1ity of the right coronary circu1ation has never been described. In the present study, autoregu1ation was assessed by pump perfusing the right coronary artery and making stepwise changes in f1ow (constant f1ow) or pressure (constant pressure) and observing the resu1tant responses. The resu1ts of these maneuvers during constant f1ow perfu- sion are depicted in Figure 6. These data demonstrate that the pressure/f1ow re1ationships are virtua11y 1inear over the range of f1ow 164 studied. The re1ationship between f1ow and ca1cu1ated resistance is such that resistance remains re1ative1y constant as f1ow is decreased over the range of 60-10 m1/min. If this bed were autoregu1ating to any great extent, resistance wou1d decrease over this range. At a f1ow of approximate1y 1O m1/min. resistance dramatica11y increases. This is probab1y due to the passive co11apse of the vascu1ature. These data wou1d suggest that the right coronary circu1ation does not demonstrate a great abi1ity to autoregu1ate. In order to support this resu1t, the abi1ity of the right coronary circu1ation to autoregu1ate was assessed during constant pressure perfusion. Perfusion pressure was varied in a stepwise manner over the range of 50-175 mmHg and the f1ow response at each new 1eve1 of pressure was recorded. These resu1ts are depicted in Figure 13. These data demonstrate that as pressure is increased over the range of 75-150 mmHg, f1ow is maintained re1ative1y constant. The re1ationship between pressure and ca1cu1ated resistance shows that resistance genera11y increases over the range of pressures studied. The data suggest that during constant pressure conditions, this vascu1ar bed autoregu1ates to a much greater extent than suggested from the constant f1ow studies. The pressure/f1ow re1ationships were a1so determined in one un- anesthetized dog chronica11y instrumented for the determination of right coronary hemodynamics. A hydrau1ic occ1uder was inf1ated in a stepwise manner to 1ower pressure over a range of 90-20 mmHg, and the coronary f1ow response recorded. Over this range, pressure decreases out of proportion to f1ow such that f1ow appeared to be re1ative1y we11- maintained. As pressure fa11s, ca1cu1ated resistance a1so fa11s in 165 order to maintain f1ow. This data obtained during natura1 f1ow condi- tions supports the resu1ts obtained during constant pressure perfusion in the anesthetized dog. It therefore appears that this vascu1ar bed demonstrates the abi1ity to autoregu1ate much better during constant pressure, natura1 f1ow conditions, than during constant f1ow conditions. This conf1ict- ing resu1t can be hypothetica11y exp1ained on the basis of myocardia1 oxygen consumption. During constant pressure perfusion, f1ow is a11owed to vary according to the needs of the myocardium. As pressure is in— creased, a myogenic response coup1ed with a change in the concentration of vasodi1ator metabo1ites may occur which 1imit the rise in b1ood f1ow through a rise in resistance. During constant f1ow perfusion, f1ow is mechanica11y increased and he1d constant at the new 1eve1. Perfusion pressure increases which wou1d again e1icit a myogenic response. However, this wou1d not 1imit the increase in f1ow since it is maintained at a constant 1eve1. This sustained higher f1ow rate wi11 increase 02 de1ivery and may raise oxy- gen consumption and ho1d it at higher steady state va1ue. The increased oxygen consumption may resu1t in e1evated 1eve1s of vasodi1ator metab- o1ites and thereby decrease resistance. The resu1t is that as f1ow is increased in a stepwise manner, resistance does not increase, as seen during constant pressure perfusion. Instead, resistance is maintained at a re1ative1y 1ower 1eve1, thereby producing the f1at f1ow/resistance re1ationship as depicted in Figure 6. Scott gt a1. (1969) reported a virtua11y identica1 pressure/f1ow and f1ow/resistance re1ationship when 166 the tota1 coronary bed was perfused at constant f1ow in the beating non-working dog heart. The author is current1y conducting experiments on the right coro- nary circu1ation of the pig in which pressure/f1ow/oxygen consumption determinations are made during constant f1ow and constant pressure perfusion in the same anima1. The pre1iminary data support the hypothe- sis that for the same increase in pressure, a re1ative1y greater increase in oxygen consumption is seen with constant f1ow perfusion compared to constant pressure perfusion. The pressure/f1ow curves are a1so simi1ar for the pig and the dog with the two perfusion techniques. It, therefore, appears that for the norma1 heart under constant pressure, natura1 f1ow conditions, the right coronary circu1ation does exhibit the abi1ity to autoregu1ate its b1ood f10w. It is a1so possib1e that an experimenta1 artifact inf1uenced the resu1ts obtained during con- stant f10w conditions. Perfusion during these studies at constant f1ow was accomp1ished with a Sigmamotor pump, an apparatus known to cause some hemoTysis re1ated to pump speed. Moreover, hemo1yzed b1ood is known to cause coronary di1ation. Therefore, as pump f1ow was increased, hemo1ysis increased and the concentration of di1ator substance in the coronary b1ood increased which offset the autoregu1atory response. This wou1d not be as 1ike1y to occur during constant pressure perfusion as a ro11er pump was used, a device which is far 1ess traumatic to the b1ood. The metabo1ic hypothesis for the regu1ation of 1eft coronary b1ood f1ow is current1y the subject of intense investigation. It is a1so genera11y fe1t by most investigators to be the dominant mechanism 167 by which coronary b100d f1ow is a1tered to meet the moment to moment oxygen demands of the tissue. In recent years, however, work by severa1 investigators has shown that there is a direct neura1 component which antagonizes the metabo1ica11y mediate changes in coronary vascu1ar resistance. Such work has 1ed to the concept of competition between neura11y mediated coronary vasoconstriction and metabo1ica11y mediated coronary vasodi1ation. The vast bu1k of the evidence to support these concepts has been derived from experiments performed on the 1eft coro- nary circu1ation. It is obvious that in this vascu1ar bed, the meta- bo1ic component is norma11y the dominant factor in the moment to moment regu1ation of coronary b1ood f10w. The current study as we11 as the work of other investigators 1., 1978; Murray and Vatner, 1979) indicate that the (Lowensohn-gt metabo1ic requirements of the myocardium supp1ied by the right coronary artery are much 1ess with respect to the 1eft ventric1e. This is based on the observation that the reactive hyperemic responses to the same duration of coronary occ1usions are 1ess in the right coronary bed (Lowensohn_et_al., 1978) than for those reported for the 1eft (O1sson and Gregg, 1975). Furthermore, the response of the right coronary cir- cu1ation to baroref1ex sympathetic activation produces a coronary con- striction, which is not a1tered by beta b1ockade (Murray and Vatner, 1979). The present study confirmed this and a1so demonstrated that sympathetic stimu1ation fo11owing a1pha receptor b1ockade produced no significant effect on right coronary resistance. This suggests that under these conditions, right ventricu1ar metabo1ism is minima11y affected. Simi1ar stimu1ation produces a substantia1 decrease in 1eft 168 coronary resistance which is prevented when the metabo1ic effects are b10cked with proprano1o1 (DiSa1vo gt_al,, 1971; Feig1, 1968). Determinations of regiona1 coronary b1ood f1ow for the 1eft and right ventric1es have been reported using the microsphere technique. Cobb et_al. (1974) demonstrated that coronary b1ood f1ow in the 1eft ventric1e of the awake dog averaged 70—80 m1/min/1OO g and increased to 100-110 m1/min/1OO 9 under conditions of anesthesia. Using simi1ar techniques, Murray et_al. (1979) reported coronary b1ood f1ow in the right ventric1e of the awake dog to be 63 m1/min/1OO g, a va1ue some- what 1ower than that reported for the 1eft. Manohar_et_al. (1979) reported va1ues of 73 m1/min/1OO 9 right ventric1e in the awake ca1f a1so using the microsphere technique. This va1ue c1ose1y approximates the reported f1ows for the 1eft ventric1e. The fact that this va1ue is s1ight1y higher than that reported by Murray cou1d be a species dif- ference. These resu1ts suggest that the coronary f1ow/gm of tissue may be somewhat 1ower for the right ventricu1ar myocardium as compared to the 1eft. This concept is a1so supported by the va1ues reported for coronary resistances in the 1eft versus the right coronary circu1ations. Murray_etial. (1979) reported va1ues for mean 1eft coronary vascu1ar resistances in the anesthetized dog perfused at constant f1ow to be on the order of 1.4 mmHg/m1/min/1OO 9 LV. Using a simi1ar preparation, Case and Greenberg (1976) obtained a va1ue of 1.27 mmHg/m1/min/1OO 9 LV. Mean va1ues for 1eft coronary resistance may give an inaccurate1y high estimate due to the substantia1 extravascu1ar compressive forces which act on the 1eft coronary vascu1ature during systo1e. Therefore, 1eft coronary resistance obtained during the 1ate disasto1ic phase of the 169 cardiac cyc1e gives a more accurate assessment of the resistance attributab1e to vascu1ar smooth musc1e activity of the 1eft coronary vascu1ar bed. Murray and Vatner (1979) reported 1eft coronary diasto1ic resistance in resting, conscious dogs to be on the order of 0.8 mmHg/ m1/min. In contrast, Lowensohn_gt_al. (1976) demonstrated that mean right coronary resistance in the awake dog was 2.1—2.9 mmHg/m1/min/TOO g RV, with diasto1ic resistance being on1y s1ight1y 1ess. These data a1so suggest that f1ow per gram of tissue is 1ess in the right ventric1e than the 1eft. The present study emp1oyed two perfusion techniques. Constant f1ow perfusion uti1ized a finger-type Sigmamotor pump. Initia11y, f1ow was set to produce a perfusion pressure of approximate1y 110 mmHg. F1ow per 100 9 right ventric1e ranged from 60-85 m1/min/1OO g and averaged 72 m1/min/1OO g for a11 anima1s studied. These va1ues are simi1ar to those reported for the 1eft ventricu1ar myocardium. However, during constant pressure perfusion, va1ues averaged 53 m1/min/ 100 g at a perfusion pressure of 100 mmHg. An exp1anation for the dif- ference in va1ues for right coronary b100d f1ow using these two perfu- sion techniques may be found in the type of pumps used in each perfusion system. The Sigmamotor pump, used to provide perfusion at constant f1ow, is known to produce higher b1ood f1ows for the same perfusion pressure in ske1eta1 musc1e when compared to the f1ows seen under natur- a1 f1ow conditions. This may be accounted for by the hemo1ysis of red b100d ce11s produced by the Sigmamotor pump, and subsequent re1ease of vasodi1ator substances, such as adenine nuc1eotides. In the constant pressure system, a Ho1ter ro11er pump was used to de1iver b1ood to the '— 170 coronary artery. This type of pump is known to be 1ess traumatic for red b100d ce11s and hence hemo1ysis is not as big a factor. This cou1d then account for the higher resting right coronary b100d f1ows seen with constant f1ow perfusion. It is therefore assumed, that the f1ows obtained with the constant pressure perfusion apparatus are more repre- sentative of the coronary f1ows that are actua11y experienced by the norma1 right ventric1e. This wou1d suggest that the f1ow in the right ventricu1ar myocardium may be 1ess than that appreciated by the 1eft ventric1e. Certain1y, the oxygen consumption of the right ventric1e must be 1ess than that of the 1eft ventric1e since the work performed by the right ventric1e is 1/6 that of the 1eft, and wa11 tension as predicted from the 1aw of Lap1ace wou1d a1so be 1ess. These two factors are primary determinants of myocardia1 oxygen consumption. It wou1d, therefore, stand to reason that whi1e the f1ow to the right ventric1e is somewhat 1ess than to the 1eft, the oxygen consumption of the right ventric1e may be far 1ess than that seen in the 1eft. This wou1d pro- duce a greater f1ow/metabo1ic ratio that is much greater for the right ventric1e. In this situation, oxygen de1ivery to the right ventricu1ar myocardium may great1y exceed the oxygen demand re1ative to the 1eft. The demands of increasing oxygen consumption, within certain 1imits, cou1d therefore be theoretica11y met by increasing oxygen extraction. It is unfortunate that right coronary venous b100d cannot be obtained in the dog. However, pre1iminary studies by the author in the right coronary circu1ation of the pig have provided severa1 interesting obser— vations that may support the aforementioned theories. First, the dif- ference in right coronary arterio—venous oxygen content under resting 171 conditions is approximate1y 6 vo1s. percent, compared to the 15 vo1s. percent reported for the 1eft ventric1e of the dog and the pig. This indicates that the oxygen extraction by the right ventric1e is far 1ess than that of the 1eft. Second, the oxygen consumption for the tissue supp1ied by the right coronary artery ranges from 3-5 m1 OZ/min/1OO 9 RV., compared to 8-10 m1 OZ/min/1OO 9 reported for the 1eft ventric1e. These pre1iminary resu1ts obtained in the pig support the data presented in the current study. It is apparent that oxygen extraction in the 1eft coronary circu1ation is near1y maxima1. Therefore, in- creased oxygen demand through increased oxygen consumption must be met main1y by increases in 1eft coronary b1ood f1ow. This, coup1ed with the fact that the 1eft ventric1e has a high rate of oxygen consumption supports the hypothesis that metabo1ic factors may serve to provide the dominant inf1uence in the moment to moment regu1ation of 1eft coronary b1ood f1ow. Theoretica11y, the f1ow to metabo1ism ratio in the right coronary circu1ation shou1d be greater than in the 1eft coronary circu- 1ation. Therefore, within 1imits, metabo1ic factors wou1d not be expected to p1ay as 1arge a ro1e in the contro1 of right coronary b1ood f1ow. The current studies support this hypothesis in that they demon- strate that the autoregu1atory response in the right coronary circu1a- tion is 1ess effective than in the 1eft, and that the right coronary vascu1ar bed responds different1y in the steady state to sympathetic stimu1ation. Moreover, sympathetic stimu1ation fo11owing a1pha b1ockade does not decrease right coronary vascu1ar resistance. SUMMARY AND CONCLUSIONS 1. Loca1 coronary hypoxia, hypercapnia, and the combination of hypoxia and hypocapnia produce significant right coronary vasodi1ation without an associated change in ventricu1ar function as judged by right ventricu1ar systo1ic pressure and dP/dT. Loca1 or systemic hypocapnia produce a substantia1 increase in right coronary resistance. From these studies it appears that a1terations in 10ca1 oxygen or carbon dioxide tensions are capab1e of producing changes in right coronary resistance. The effects of carbon dioxide (hydrogen ion concentration) are most 1ike1y mediated through direct effects on the coronary vascu1ature whi1e it is unc1ear whether the vasoactivity associated with 1ow oxygen ten- sions are mediated through direct or indirect mechanisms. 2. Sympathetic (baroref1ex) stimu1ation produces an e1evated right ventricu1ar pressure, dP/dT, and right coronary vascu1ar resistance. This effect is not enhanced by beta b1ockade, but is prevented by a1pha b1ockade. Stimu1ation after a1pha b1ockade produces no significant effect on right coronary resistance. These data suggest that the response of the right coronary circu1ation to sympathetic stimu1ation is dominated by a1pha mediated coronary vasoconstriction. Apparent1y, stimu1ation has 1itt1e inf1uence on metabo1ic vasodi1ator production. 3. The response of the right coronary circu1ation to sympathetic stimu1ation was determined under a variety of background conditions 172 173 to attempt to determine if there are conditions which a1ter the response of this bed to stimu1ation. The present study demonstrates that decreasing f1ow to this vascu1ar bed resu1ts in an enhanced coro- nary vasoconstriction when the sympathetic nervous system is activated. The response to stimu1ation is not a1tered during systemic or 10ca1 hypocapnia; however, the vasoconstrictor effects of these two stimu1i are additive. Loca1 hypercapnia attenuates the coronary vasoconstric- tion seen with stimu1ation and 10ca1 hypoxia prevents the response a1together. Therefore, decreases in f1ow, oxygen or carbon dioxide tensions may serve to modu1ate the response of this vascu1ar bed to sympathetic nerve activity. 4. Intracoronary infusion of NE produced an increase in right ventricu1ar systo1ic pressure, dP/dT, and a pronounced fa11 in right coronary vascu1ar resistance. Fo11owing beta receptor b1ockade with proprano1o1 or practo1o1, NE produced a significant increase in right coronary resistance. This rise in resistance cou1d be prevented with a1pha receptor b1ockade. These data suggest that NE produces di1ation of the right coronary vascu1ar bed primari1y through a stimu1ation of myocardia1 metabo1ism through beta 1 receptor activation. B1ockade of the beta receptors cause an unmasking of the a1pha receptor mediated coronary vasoconstriction. 5. It was a1so determined whether or not the response of the right coronary circu1ation to NE was a1tered by changes in f1ow, oxygen or carbon dioxide tensions in the coronary b1ood. These data suggest that the response of this vascu1ar bed to NE infusion was una1tered with the changes in f1ow. Furthermore, the response to NE was una1tered 174 during 10ca1 or systemic hypocapnia, but was attenuated during 10ca1 hypoxia or hypercapnia. 6. In order to determine if 10ca11y synthesized prostag1andins were invo1ved in the regu1ation of b1ood f1ow in the right coronary vascu1ar bed, the responses to brief interruptions of coronary b100d f1ow, sympathetic stimu1ation and NE infusion were obtained before and after the administration of indomethacin. These data suggest that the prostag1andins may be invo1ved in the reactive di1ation associated with 20 second interruptions of f1ow. The coronary vascu1ar response to sympathetic stimu1ation or NE infusion was apparent1y unaffected by the b1ockade of prostag1andin synthesis. Therefore, the prostag1andins do not appear to be invo1ved in the response of the right coronary circu1ation to adrenergic stimu1ation. 7. The abi1ity of this vascu1ar bed to autoregu1ate was assessed using constant f1ow and constant pressure perfusion techniques. During constant f1ow perfusion stepwise changes in right coronary f1ow produced a virtua11y 1inear pressure/f1ow re1ationship over the pressure range of 25-180 mmHg. As f1ow was decreased, ca1cu1ated resistance remained constant unti1 pressure fa11s be1ow 30 mmHg at which point resistance increases substantia11y. This suggests that the bed is autoregu1ating but on1y to a minor extent. During constant pressure perfusion, pres- sure was varied in a stepwise fashion over this range of 50-175 mmHg. As pressure increases over the range of 75-150 mmHg, f1ow remains re1ative1y constant. Ca1cu1ated resistance genera11y increases over this same range of pressures. These data suggest that the right coro- nary bed demonstrates much better autoregu1ation with constant pressure 175 perfusion, a1though not to the same extent as that reported for the 1eft coronary vascu1ar bed. In conc1usion, this study suggests that the right coronary vascu- 1ar bed is capab1e of exhibiting some degree of 10ca1 regu1ation. However, within 1imits, the inf1uence of metabo1ic factors do not appear to p1ay as 1arge a ro1e in regu1ation of b1ood f1ow through this vascu1ar bed as compared to the 1eft coronary circu1ation. The regu1a- tion of the right coronary circu1ation is apparent1y more substantia11y inf1uenced by neura1 and/or myogenic factors. BIBLIOGRAPHY BIBLIOGRAPHY Afonso, S.; G. T. Bandow; G. C. Rowe. Indomethacin and the prostag1an- din hypothesis of coronary b1ood f1ow regu1ation. J. Physio1. 241 299, 1974. Afonso, S.; T. J. Ansfie1d; T. B. Berndt; G. C. Rose. Coronary vaso- di1ator responses to hypoxia before and after aminophy11ine. J. Physio1. (London). 221:589, 1972. A1e11a, A.; F. L. Wi11iams; C. Bo1ene Ni11iams; L. N. Katz. Interre1a- tion between cardiac oxygen consumption and coronary b1ood f1ow. Am. J. Physio1. 183:570, 1955. A1exander, R. w.; K. M. Kent; J. J. Pisano; H. R. Keiser; T. Cooper. Regu1ation of post-occ1usive hyperemia by endogenous1y synthe- sized prostag1andins in the dog heart. Acta Med. Scand. 191:241, 1972. A1exander, R. w.; M. A. Gimbrone. Stimu1ation of prostag1andin E syn- thesis in cu1tured human umbi1ica1 vein smooth musc1e ce11s. Proc. Nat. Acad. Sci. 73:1617, 1976. Anderson, R; S. Ho1mberg; N. Suedmyr; G. Abert. Adrenergic a1pha and beta receptors in coronary vesse1s in man. Acta Med. Scand. 191:241, 1972. Anrep, G. V. On 10ca1 vascu1ar reactions and their interpretations. J. Physio1. (London). 45:318, 1912. Bacchus, A. Loca1 contro1 of b1ood f1ow in canine ske1eta1 musc1e: II. Effects of vascu1ar transmura1 pressure changes in an iso1ated musc1e. Mich. St. Univ. Ph.D. dissertation, 1979. Baez, S. Bay1iss response in the microcircu1ation. Fed. Proc. 27:1410, 1968. Baron, G. D.; R. N. Speden; D. F. Bohr. Beta-adrenergic receptors in coronary and ske1eta1 arteries. Am. J. Physio1. 223:878, 1972. Bayer, B. L.; P. Mentz; w. Forster. Characterization of the adreno- receptors in coronary arteries of pigs. Eur. J. Pharm. 29:58, 1974. Bay1iss w. M. 0n the 10ca1 reactions of the arteria1 wa11 to changes of interna1 pressure. J. Physio1. (London). 28:220, 1902. 176 177 Beer. G.; S. Rodbard. Infusion rate effects on arteria1 pressure, vascu1ar conductance and musc1e weight. Proc. Soc. Exp. Bio1. .Mgd. 134:1055, 1970. Be11amy, R. F.; H. S. Lowensohn. Pressure—f1ow re1ationships in the canine right coronary circu1ation. Physio1ogist. 22:9, 1979. Berne, R. M. Coronary circu1ation. In: The Mamma1ian Myocardium. G. A. Langer and A. Brady (eds.). New York, Ni1ey, 1974. Berne, R. M. Effect of epinephrine and norepinephrine on the coronary circu1ation. Circ. Res. 6:644, 1958. Berne, R. M.; H. DeGeest; M. N. Levy. Inf1uence of cardiac nerves on coronary resistance. Am. J. Physio1. 208:763, 1965. Berne, R. M.; J. R. B1ackman; T. H. Gardner. Hypoxia and coronary f1ow. J. C1in. Invest. 36:1101, 1957. Berne, R. M. Metabo1ic regu1ation of b1ood f1ow. Circ. Res. 15 (Supp1 I): 261, 1964. Berne, R. M. Myocardia1 b1ood f1ow: metabo1ic determinants. In: Periphera1 Circu1ations. R. Ze1is (ed.). New York, Grune and Stratton, 1975. Berne, R. M. Regu1ation of coronary b100d f1ow. Physio1. Rev. 44:1, 1964. Berne, R. M.; R. Rubio. Adenine nuc1eotide metabo1ism in the heart. Circ. Res. (Supp1 III) 35:109, 1974. Berne, R. M.; R. Rubio. Regu1ation of coronary b100d f1ow. .Adv. Cardio1. 13:303. Base1, Karger, 1974. Bittar, N.; T. J. Pau1y. Myocardia1 reactive hyperemia responses in the dog after aminophy11ine and 1idof1azine. Am. J. Physio1. 220:812, 1971. Brace, R. A.; D. K. Anderson; w. T. Chen; J. B. Scott; F. J. Haddy. Loca1 effects of hypoka1emia on coronary resistance and myocardia1 contracti1e force. Am. J. Physio1. 227:590, 1974. Brace, R. A.; J. B. Scott; w. T. Chen; D. K. Anderson; F. J. Haddy. Direct effects of hypo-osmo1a1ity on coronary vascu1ar resistance and myocardia1 contracti1e force. Proc. Soc. Exp. Bio1. Med. 148:578, 1975. Brachfie1d, N.; R. G. Monroe; R. Gor1in. Effects of pericoronary dener- vation on coronary hemodynamics. Am. J. Physio1. 199:174, 1960. 178 Brandfonbrener, M.; D. Gracey; R. Nice. Coronary pressure—b100d f1ow re1ations. Am. J. Cardio1. 23:417, 1969. Braunwa1d, E. Contro1 of myocardia1 oxygen consumption. Am. J. Cardio1. 27:416, 1971. Braunwa1d, E. Coronary spasm and acute myocardia1 infarction; new possibi1ity for treatment and prevention. New Eng. J. Med. 299:1301, 1978. Braunwa1d, E; J. Ross; E. H. Sonnenb1ick. Regu1ation of coronary b1ood f1ow. In: Mechanisms of Contraction of the Norma1 and Fai1ing Heart. Boston, Litt1e, Brown and C0., 1976. Braunwa1d, E.; S. J. Sarnoff; R. 8. Case; w. N. Stainsby; B. H. We1ch. Hemodynamic determinants of coronary f1ow: Effect of changes in aortic pressure and cardiac output on the re1ationship between myocardia1 oxygen consumption and coronary f1ow. Am. J. Physio1. 192:157, 1958. Brooks, H.; E. S. Kirk; P. S. Vokonas, C. N. Ursche1; E. H. Sonnenb1ick. Performance of the right ventric1e under stress: Re1ation to right coronary f1ow. J. C1in. Invest. 50:2176, 1971. Buckberg, G. 0.; A. A. Kattus. Factors determining the distribution and adequacy of 1eft ventricu1ar myocardia1 b1ood f1ow. In: Current Topics in Coronarngesearch. C. M. B1oor, and R. A. 01sson (eds.). New York, P1enum Press, 1973. Buckberg, G. D.; K. F. Fix1er; J. P. Archie. Experimenta1 subendo- cardia1 ischemia in dogs with norma1 coronary arteries. Circ. Res, 30:67, 1972. Burnstock, G.; M. E. Ho1man; C. L. Prosser. E1ectrophysio1ogy of smooth musc1e. Physio1. Rev. 43:482, 1963. Burton, A. C.; R. H. Stinson. The measurement of tension in vascu1ar smooth musc1e. J. Physio1. (London). 153:290, 1960. Burton, K. M.; P. C. Johnson. Reactive hyperemia in sing1e capi11aries of mamma1ian ske1eta1 musc1e. Am. J. Physio1. 223:517, 1972. Case, R. B.; A. Fe1ix; M. Wachter; G. Kyrigkidis; F. Caste11ana. Re1ative effect of CO on canine coronary vascu1ar resistance. Circ. Res. 42:410, 1978. Case, R. B.; H. Greenberg. The response of canine coronary resistance to 10ca1-a1terations in coronary arteria1 pCOZ. Circ. Res. 39:558, 1976. 179 Chen, w. T.; C. C. Chou; J. B. Scott; F. J. Haddy. Evidence for parti- cipation of ATP in active hyperemia of heart during ste11ate gang1ion stimu1ation. Physio1ogist. 15:104, 1972. Cobb, F. R.; R. J. Bache; J. C. Greenfie1d, Jr. Regiona1 myocardia1 b1ood f1ow in awake dogs. J. C1in. Invest. 53:1618, 1974. Cross, C. E.; P. A. Rieben; R. F. Sa1isbury. Coronary driving pressure and vasomotor tonus as determinants of coronary b1ood f1ow. Circ. Res. 9:589, 1961. Curnish, R. R.; R. M. Berne; R. Rubio. Effect of aminophy11ine on myocardia1 reactive hyperemia. Proc. Soc. Exp, Bio. Med. 141:593, 1972. Daugherty, R. M.; J. B. Scott; J. M. Dabney, F. J. Haddy. Loca1 effects of 02 and 002 on 1imb, rena1 and coronary vascu1ar resistances. Am. J. Physio1. 213:1102, 1967. Degenring, F. H. The effect of acidosis and a1ka1osis on coronary f1ow and cardiac nuc1eotide metabo1ism. Basic Res. Cardio1. 71:291, 1976. Denison, A. B.; H. D. Green. Effects of autonomic nerves and their mediators on the coronary circu1ation and myocardia1 contraction. Circ. Res. 6:633, 1958. Detar, R.; D. F. Bohr. Oxygen and vascu1ar smooth musc1e contraction. Am. J. Physio1. 214:241, 1968. Detar, R.; D. F. Bohr. Contracti1e responses of iso1ated vascu1ar smooth musc1e during pro1onged exposure to anoxia. Am. J. Physio1. 222:2169, 1972. Di Sa1vo, J.; P. E. Parker; J. B. Scott; F. J. Haddy. Carotid baro- receptor inf1uence on coronary vascu1ar resistance in the anesthetized dog. Am. J. Physio1. 221:156, 1971. Dona1d, D. E.; D. A. Ferguson; S. E. Mi1burn. Effect of beta adrenergic receptor b1ockade on racing performance of greyhounds with norma1 and denervated hearts. Circ. Res. 22:127, 1968. Downey, J. M.; H. F. Downey; E. S. Kirk. Effects of myocardia1 strain on coronary b1ood f1ow. Circ. Res. 34:286, 1974. Downey, J. M.; E. S. Kirk. Inhibition of coronary b1ood f1ow by a vascu1ar waterfa11 mechanism. Circ. Res. 36:753, 1975. Drisco1, T. E.; R. M. Berne. Ro1e of potassium in regu1ation of coro- nary b1ood f1ow. Proc. Soc. Exp. Bio1. Med. 95:505, 1957. 180 Drisco1, T. E.; T. w. Moir; R. w. Eckstein. Vascu1ar effects of per- fusion pressure changes in the non-ischemic and ischemic heart. Circ. Res. (Supp1 I) 24-25z94, 1964. Du1ing, B. Oxygen sensitivity of vascu1ar smooth musc1e. II. I__vivo studies. Am. J. Physio1. 227:42, 1974. Du1ing, B. R.; R. M. Berne. Longitudina1 gradients in periarterio1ar oxygen tension--A possib1e mechanism for the participation of oxygen in 10ca1 regu1ation of b100d f1ow. Circ. Res. 27:669, 1970. Ecke1, R.; R. w. Eckstein; M. Stroud; w. H. Pritchard. Effect of over and under perfusion upon coronary arteria1 b1ood f1ow. Fed. Proc. 8:38, 1949. Eckenhoff, J. E.; J. H. Hafkenschie1; C. M. Landmesser; M. Harme1. Cardiac oxygen metabo1ism and contro1 of the coronary circu1ation. Am. J. Physio1. 149:634, 1947. Ehrhart, I. C.; P. E. Parker; w. J. Heidner; J. M. Dabney; J. B. Scott; F. J. Haddy. Coronary vascu1ar and myocardia1 responses to carotid body stimu1ation. Am. J. Physio1. 229:754, 1975. Eikens, E.; D. E. L. Nikken. Reactive hyperemia in the dog heart: Effect of temporari1y restricting arteria1 inf1ow and of coronary occ1usions 1asting one and two cardiac cyc1es. Circ. Res. 35:702, 1974. E11is, E. F.; O. Oe1z; L. J. Roberts. Coronary arteria1 smooth musc1e contraction mediated by a substance re1eased by p1ate1ets: evidence that it is thromboxane A2. Science. 193:1135, 1976. Eyzaquirre, C.; S. w. Kuff1er. Processes of excitation in the dendrites and in the soma of sing1e iso1ated sensory nerve ce11s of the 1obster and crayfish. J. Gen. Physio1. 39:87, 1955. Feig1, E. 0. Carotid sinus ref1ex contro1 of coronary b1ood f1ow. Circ. Res. 23:223, 1968. Feig1, E. 0. Parasympathetic contro1 of coronary b1ood f10w in dogs. Circ. Res. 25:509, 1969. Feig1, E. O. Sympathetic contro1 of the coronary circu1ation. Circ. Res. 20:262, 1967. Feinberg, H.; A. Gero1a, L. N. Katz. Effect of changes in b100d CO 1eve1s on coronary f1ow and myocardia1 oxygen consumption. Am. J. Physio1. 199:349, 1960. 2 181 Fishback, M. E.; L. Burnett; A. M. Scher. Autoregu1ation of coronary b1ood f1ow in the dog heart. C1in. Res. 1:60, 1959. Fo1kow, B. Intravascu1ar pressure as a factor regu1ating the tone of the sma11 vesse1s. Acta Physio1. Scand. 17:289, 1949. Fo1kow, B.; J. Frost; B. Uvnas. Action of acety1cho1ine, adrena1ine and nor-adrena1ine on the coronary b1ood f1ow of the dog. Acta Physio1. Scand. 17:201, 1949. Fo1kow, B. Ro1e of the nervous system in the contro1 of vascu1ar tone. Circ. 21:760, 1960. Fo1kow, 8. Description of the myogenic hypothesis. Circ. Res. (Supp1 I) 14:47, 1964. Fo1kow, B.; E. Nei1. Coronary circu1ation. In: Circu1ation. London, Oxford University Press, 1971. Foth, E. L.; R. G. Page; W. F. She1don; S. K. Wong; W. J. Tuddenham; A. J. Weiss. Factors in variation and regu1ation of coronary b1ood f1ow in intact anesthetized dogs. Am. J. Physio1. 162:521, 1950. Fox, A. C.; G. E. Reed; E. G1assman. Re1ease of adenosine from human hearts during angina induced by rapid atria1 pacing. J. C1in. Invest. 53:1447, 1974. Fu1ton, W. F. M. The Coronary Arteries. Springfie1d, 111., Thomas, 1965. Gazitua, S.; J. B. Scott; B. Swinda11; F. J. Haddy. Resistance re- sponses to 10ca1 changes in p1asma osmo1a1ity in three vascu1ar beds. Am. J. Physio1. 220:384, 1971. Ge11ai, M.; J. M. Norton; R. Detar. Evidence for a direct contro1 of coronary vascu1ar tone by oxygen. Circ. Res. 32:279, 1973. Ge11ai, M.; R. Detar. Evidence in suppott of hypoxia but against high potassium and hyperosmo1arity as possib1e mediators of sustained vasodi1ation in rabbit cardiac and ske1eta1 musc1e. Circ. Res. 35:681, 1974. Gi1es, R. H.; E. L. Wi1cken. Reactive hyperaemia in the dog heart: interre1ations between adenosine, ATP and aminophy11ine and the effect of indomethacin. Cardiovasc. Res. 11:113, 1977. Gi1more, J. P; J. A. Nizo1ek; R. J. Jacob. Further characterization of myocardia1 K+ Toss induced by changing contraction frequency. Am. J. Physio1. 221:465, 1975. 182 Gimbrone, M. A.; R. W. A1exander. Angiotensin II stimu1ation of pros- tag1andin production in cu1tured human vascu1ar endothe1ium. Science. 1892219, 1975. Goodman, L. S., A. Gi1man. The Pharmaco1ogica1 Basis of Therapeutics. New York, Macmi11an, 1970. Graham, T. P.; J. W. Cove11; E. H. Sonnenb1ick; J. Ross; E. Braunwa1d. Contro1 of myocardia1 oxygen consumption: Re1ative inf1uence of contracti1e state and tension deve1opment. J. C1in. Invest. 47:375, 1968. Gregg, D.; L. C. Fischer. B1ood supp1y to the heart. In: Handbook of Physio1ggy, V01. 11. Washington, D. C., American Physio1ogica1 Society, 1963. Gregg, D. E. The Coronary Circu1ation in Hea1th and Disease. Phi1ade1phia, Lea and Febiger, 1960. Gregg, D. E. Phasic b1ood f1ow and its determinants in the right coro— nary artery. Am. J. Physio1. 119:580, 1937. Gregg, D. E.; R. W. Eckstein. Measurements of intramyocardia1 pressure. Am. J. Physio1. 132:781, 1941. Griggs, D. M.; V. V. Tchokoev; C. C. Chen. Transmura1 differences in ventricu1ar tissue substrate 1eve1s due to coronary constriction. Am. J. Physio1. 2222705, 1972. Hackett, J. G.; F. M. Abboud; A. L. Mark; P. G. Schmid; D. D. Heistad. Coronary vascu1ar responses to stimu1ation of chemoreceptors and baroreceptors. Circ. Res. 21:8, 1972. Haddy, F. J.; J. B. Scott. Effects of f1ow rate, venous pressure, metabo1ites and oxygen upon resistance to b1ood f1ow through the dog fore1imb. Circ. Res. (Supp1 XIV-XV) 1:1, 1964. Haddy, F. J.; J. B. Scott; G. J. Grega. Periphera1 circu1ation: F1uid transfer across the microvascu1ar membrane. In: Internationa1 Review of Physio1ogy, Cardiovascu1ar Physio1ogy II. A. C. Guyton, A. W. Cow1ey (eds.), V01. 9. Ba1timore, University Park Press, 1976. Haddy, F. J.; J. B. Scott. Active hyperemia, reactive hyperemia, and autoregu1ation of b1ood f1ow. In: Microcircu1ation, G. Ka1ey and B. M. A1tura (eds.). Ba1timore, University Park Press, 1974. Haddy, F. J.; J. B. Scott. Metabo1ic factors in periphera1 circu1atory regu1ations. Fed. Proc. 34:2006, 1975. 183 Haddy, F. J.; J. B. Scott. Metabo1ica11y 1inked vasoactive chemica1s in 10ca1 regu1ation of b1ood f1ow. Physio1. Rev. 48:688, 1968. Hami1ton, F. N.; E. O. Feig1. Coronary vascu1ar sympathetic beta- receptor innervation. Am. J. Physio1. 230:1569, 1976. Hanson, K. M.; P. C. Johnson. Vascu1ar resistance and arteria1 pressure in autoperfused dog hind 1imb. Am. J. Physio1. 203:615, 1962. Hardin, R. A.; J. B. Scott; F. J. Haddy. Effect of epinephrine and norepinephrine on coronary vascu1ar resistance in the dog. Am. J. Physio1. 201:276, 1961. Har1an, D. M.; T. W. Rooke; F. L. Be11oni; H. V. Sparks. Effect of indomethacin on coronary vascu1ar response to increased myo- cardia1 oxygen consumption. Am. J. Physio1. 235:372, 1978. Herd, J. A.; A. C. Barger. Simp1ified technique for chronic catheteri- zation of b1ood vesse1s. J. App1. Physio1. 19:791, 1964. Hi1ton, R.; F. Eichho1tz. The inf1uence of chemica1 factors on the coronary circu1ation. J. Physio1.(London) 59:413, 1925. Hintze, T. H.; G. Ka1ey. Prostag1andins and the contro1 of b1ood f1ow in the canine myocardium. Circ. Res. 40:313, 1977. Ho1t, J. P. The co11apse factor in the measurement of venous pressure: The f1ow—through co11apsib1e tubes. Am. J. Physio1. 134:292, 1941. Jacob, M. 1.; R. M. Berne. Metabo1ism of purine derivatives by the iso1ated cat heart. Am. J. Physio1. 193:322, 1960. Je11iffe, R. W.; C. R. Wo1f; R. M. Berne; R. W. Eckstein. Absence of vasoactive and cardiotropic substances in coronary sinus b1ood of dogs. Circ. Res. 5:382, 1957. Johannson, B; D. F. Bohr. Rhythmic activity in smooth musc1e from sma11 cutaneous arteries. Am. J. Physio1. 210:801, 1966. Johnson, P. C. The myogenic response and the microcu1ation. Microvas. .Rgs. 13:1, 1977. Ka1sner, S. Endogenous prostag1andin re1ease contributes direct1y to coronary artery tone. Can. J. Physio1. Pharm. 53:560, 1975. Ka1sner, S. Intrinsic prostag1andin re1ease: a mediator of anoxia- induced re1axation in an iso1ated coronary artery preparation. B1ood Vesse1s. 13:155, 1976. 184 Katz, A. M.; H. H. Hecht. Ear1y "pump" fai1ure of the ischemic myo- cardium. Am. J. Med. 47:497, 1969. Katz, L. N.; E. Linder. The action of excess Na, Ca and K on the coro- nary vesse1s. Am. J. Physio1. 124:155, 1938. Kirk, E. S.; C. R. Honig. An experimenta1 and theoretica1 ana1ysis of myocardia1 tissue pressure. Am. J. Physio1. 207:361, 1964. Kirk, E. S.; C. R. Honig. Non-uniform distribution of b1ood f1ow and gradients of oxygen tension within the heart. Am. J. Physio1. 207;661, 1964. Kitt1e, E. F.; H. Aoki; E. Brown. The ro1e of pH and CO2 in the dis- tribution of b1ood f1ow. Surgery 57:129, 1965. K1ocke, F. J.; E. Braunwa1d; J. Ross. Oxygen cost of e1ectrica1 activ- ity of the heart. Circ. Res. 18:357, 1966. K1ocke, F. J.; G. A. Kaiser; J. Ross; E. Braunwa1d. An intrinsic adrenergic vasodi1ator mechanism in the coronary vascu1ar bed of the dog. Circ. Res. 16:376, 1965. Know1ton, F. P.; E. H. Star1ing. The inf1uences of variations in temperature and b1ooc pressure on the performance of the iso1ated mamma1ian heart. J. Physio1. (London). 44:206, 1912. Kontos, H. A.; M. D. Thames; A. Lombana; C. O. Wat1ington; F. Jesse. Vasodi1ator effects of 1oca1 hypercapnic acidosis in dog ske1eta1 musc1e. Am. J. Physio1. 220:1569, 1971. Krishnamurty, V. S. R.; H. R. Adams; G. H. Temp1eton; J. T. Wi11erson. Inhibitory effects of hypertonic mannito1 on vasoconstrictor and vasodi1ator responses of iso1ated coronary arteries. Am. J. Physio1. 235:728, 1978. Ledingham, I. M.; T. I. McBride; J. R. Parratt; J. P. Vance. The effect of hypercapnia on myocardia1 b1ood f1ow and metabo1ism. J. Physio1. 210:87, 1970. Lewis, B. F.; J. D. Coffman; D. E. Gregg. Effect of heart rate and intracoronary isoprotereno1, 1evartereno1 and epinephrine on coro— nary b1ood f1ow and resistance. Circ. Res. 9:89, 1961. Lowensohn, H. S.; E. M. Khouri; D. E. Gregg; R. L. Py1e; R. E. Patterson. Phasic right coronary artery b1ood f1ow in conscious dogs with norma1 and e1evated right ventricu1ar pressures. Circ. Res. 39:760, 1976. 185 Lundva11, J.; S. Me11ander; H. V. Sparks. Myogenic response of resist- ance vesse1s and pre-capi11ary sphincters in ske1eta1 musc1e dur- ing exercise. Acta Physio1. Scand. 70:257, 1967. Ma1indzak, G. S.; E. J. Kosinski; A. D. Green; G. W. Yarborough. The effects of adrenergic stimu1ation on conductive and resistive segments of the coronary vascu1ar bed. J. Pharm. Exp. Ther. 206:248, 1978. Manohar, M.; G. E. Bisgard; V. Bu11ard; J. A. Wi11; D. Anderson; J. H. G. Rankin. Regiona1 myocardia1 b1ood f1ow and myocardia1 function during acute right ventricu1ar pressure over1oad in ca1ves. Circ. Res. 44:531, 1979. Markwa1der, J.; E. H. Star1ing. A note on some factors which determine the b100d f1ow through the coronary cricu1ation. J. Physio1. (London). 47:275, 1913. Maseri, A. Coronary vasospasm as a possib1e cause of myocardia1 infarction. New Engng. Med. 299:1271, 1978. McRaven, D. R.; A. 0. Mark; F. M. Abboud; H. E. Mayer. Responses of coronary vesse1s to adrenergic stimu1i. J. C1in. Invest. 50:773, 1971. Me11ander, S.; B. Johansson. Contro1 of resistance, exchange, and capacitance functions in the periphera1 circu1ation. Pharm. Rev. 20:117, 1968. Merri11, G. F.; F. J. Haddy; J. M. Dabney. Adenosine, theophy11ine, and perfusate pH in the iso1ated, perfused guinea pig heart. Circ. Res. 42:225, 1978. Mohrman, D. E.; E. O. Feig1. Competition between sympathetic vasocon- striction and metabo1ic vasodi1ation in the canine coronary circu- 1ation. Circ. Res. 42:79, 1978. Moir, T. W. Brief Reviews. Subendocardia1 distribution of coronary b1ood f1ow and the effects of antiangina1 drugs. Circ. Res. 30:621, 1972. Mo1nar, J. I.; J. B. Scott; E. D. Froh1ich; F. J. Haddy. Loca1 effects of various anions and H+ on dog 1imb and coronary vascu1ar resistances. Am. J. Physio1. 203:125, 1962. Mrwa, V.; I. Achtig; J. C. Ruegg. Inf1uences of ca1cium concentration and pH on the tension deve1opment and ATPase activity of the arteria1 actomyosin contracti1e system. B1ood Vesse1s 11:277, 1974. 186 Mudge, G. H.; S. Go1dberg; S. Gunthar; T. Mann; W. Grossman. Comparison of metabo1ic and vasoconstrictor stimu1i on coronary vascu1ar resistance in man. Circu1ation 59:544, 1979. Murray, P. A.; H. Baig; S. F. Vatner. Assessment of co11atera1ization of severe1y hypertrophied right ventric1e in conscious dogs. Fed. Proc. 38:955, 1979. Murray, P. A.; F. L. Be11oni; H. V. Sparks. The ro1e of potassium in the metabo1ic contro1 of coronary vascu1ar resistance of the dog. Circ. Res. 44:767, 1979. Murray, P. A.; S. F. Vatner. A1pha-adrenceptor attenuation of the coro- nary vascu1ar response to severe exercise in the conscious dog. Circ. Res. 45:654, 1979. Murray, P. A.; S. F. Vatner. Reduced coronary vasoconstriction with baroreceptor un1oading in severe1y hypertrophied right ventric1es of conscious dogs. Physio1ogist. 22:92, 1979. Mustafa, S. J.; R. Rubio; R. M. Berne. Uptake of adenosine by dis— persed chick embryonic cardiac ce11s. Am. J. Physio1. 228:62, 1975. Nag1e, F. 5.; J. B. Scott; B. Swinda11; F. J. Haddy. Venous resistances in ske1eta1 musc1e and skin during 1oca1 b1ood f1ow regu1ation. Am. J. Physio1. 2142885, 1968. Need1eman, P.; S. L. Key; P. C. Isakson; P. S. Ku1karni. Re1ationship between oxygen tension, coronary vasodi1ation and prostag1andin biosynthesis in the iso1ated rabbit heart. Prostag1andins 9:123, 1975. Nei11, W. A.; J. Oxedine; N. Phe1ps; R. P. Anderson. Subendocardia1 ischemia provoked by tachycardia in conscious dogs with coronary stenosis. Am. J. Cardio1. 35:30, 1975. Nei11, W. A.; M. Hattehauer. Impairment of myocardia1 02 supp1y due to hyperventi1ation. Circu1ation 52:854, 1975. 01iva, P. B.; D. E. Potts; R. G. P1uss. Coronary arteria1 spasm in Prinzmeta1's angina. New Eng. J. Med. 2882745, 1973. O1sson, R. A. Brief reviews: Myocardia1 reactive hyperemia. Circ. Rev. 37:263, 1975. 01sson, R. A.; D. E. Gregg. Myocardia1 reactive hyperemia in the unanesthetized dog. Circ. Res. 208:224, 1965. 187 Owen, T. L.; I. C. Ehrhart; W. J. Weidner; J. B. Scott; E. J. Haddy. Effects of indomethacin on 1oca1 b1ood f1ow regu1ation in canine heart and kidney. Proc. Soc. Exp. Bio1. Med. 149:871, 1975. Permutt, S; B. Bromberger—Barned; H. N. Bane. A1veo1ar pressure, pu1monary venous pressure, and vascu1ar waterfa11. Med. Thorac. 19:239, 1962. Peterson, R. J.; J. B. Seward; A. J. Tajik; E. L. Ritman; M. P. Kaye. Eva1uation of right ventricu1ar free wa11 rep1acement in dogs. Circ. 58:174, 1978. Pitt, 8.; E. C. E11iot; D. E. Gregg. Adrenergic receptor activity in the coronary arteries of the unanesthetized dog. Circ. Res. 21275, 1967. Powe11, J. R.; E. O. Feig1. Carotid sinus ref1ex coronary vasoconstric— tion during contro11ed myocardia1 oxygen metabo1ism in the dog. Circ. Res. 44:44, 1979. Raberger, G.; W. Schutz; D. Kraupp. Coronary reactive hyperemia and coronary di1ator action of adenosine during norma1 respiration and hypercapnic acidosis in the dog. C1in. Exp. Pharm. and Physio1. 2:373, 1975. Rodbard, S.; N. Hande1; L. Sadja. F1ow patterns in a mode1 of a con— tracting musc1e. Cardiovasc. Res. 5:396, 1971. Rodbard, S. Evidence that vascu1ar conductance is regu1ated at the capi11ary. Angio1ogy. 17:549, 1966. Rooke, T.; H. V. Sparks. Effect of arteria1 CO changes on de1ivery and myocardia1 consumption of O (abstracE). Physio1ogist. 21:101. 1978. 2 Ross, G.; D. G. Mu1der. Effects of right and 1eft cardiosympathetic nerve stimu1ation on b100d f1ow in the major coronary arteries of the anesthetized dog. Cardiovasc. Res. 3:22, 1969. Rubio, R; R. M. Berne; J. G. Dobson. Sites of adenosine production in cardiac and ske1eta1 musc1e. Am. J. Physio1. 225:938, 1973. Rubio, R.; R. M. Berne. Re1ease of adenosine by the norma1 myocardium and its re1ationship to the regu1ation of coronary resistance. Circ. Res. 25:407, 1969. Rubio, R.; R. M. Berne. Regu1ation of coronary b1ood f1ow. Prog. in Cardiovasc. Dis. 18:105, 1975. 188 Rubio, R.; V. T. Weidmeier; R. M. Berne. Re1ationship between coronary f1ow and adenosine production and re1ease. J. M01. Ce11. Cardio1. 6:561, 1974. Rubio, R.; V. T. Wiedmeier; R. M. Berne. Nuc1eoside phosphory1ase: 1oca1ization and ro1e in the distribution of purines. Am. J. Physio1. 222:550, 1972. Sabiston, D. D.; D. E. Gregg. Effects of cardiac contraction on coro- nary b1ood f1ow. Circu1ation. 15:14, 1957. Sarnoff, S. J.; E. Braunwa1d; G. H. We1ch; R. B. Case; W. N. Stainsby, R. Macruz. Hemodynamic determinants of oxygen consumption of the heart with specia1 reference to the tension-time index. Am. J. Physio1. 192:148, 1958. Schaper, W. The Co11atera1 Circu1ation of the Heart. Amsterdam, North-Ho11and, 1971. Schreiner, G. L.; E. Berg1und; H. G. Borst; R. G. Monroe. Effects of vagus stimu1ation and of acety1cho1ine on myocardia1 contracti1— ity, O2 consumption and coronary f1ow in dogs. Circ. Res. 5:502, 1957. Scott, J. B.; D. Radawski. Ro1e of hyperosmo1a1ity in the genesis of active and reactive hyperemia. Circ. Res. (Supp1 I) 28:26, 1971. Scott, J. B.; E. D. Froh1ich; R. A. Hardin; F. J. Haddy. Na+, K+, Ca+ and Mg + action on coronary vascu1ar resistance in dog heart. Am. J. Physio1. 201:1095, 1961. Scott, J. B.; R. A. Hardin; F. J. Haddy. Pressure f1ow re1ationships in the coronary vascu1ar bed of the dog. Am. J. Physio1. 199:765, 1960. Scott, J. B.; R. M. Daugherty; J. M. Dabney; F. J. Haddy. Ro1e of chemica1 factors in regu1ation of f1ow through kidney, hind1imb and heart. Am. J. Physio1. 208:813, 1965. Scott, J. B.; W. T. Chen; B. T. Swinda11; J. M. Dabney; F. J. Haddy. Bioassay evidence indicating a ro1e for adenosine in cardiac ischemic di1ation and for AMP and adenosine in hypoxic di1ation. Circ. Res. In press. 1979. Scott, J. C. Myocardia1 coefficient of oxygen uti1ization. Circ. Res. 9:906, 1961. Sivakoff, M.; E. Pure; W. Hsueh; P. Need1eman. Prostag1andins and the heart. Fed. Proc. 38:78, 1979. 189 Sobo1, B. J.; S. A. Wan1ass; E. 8. Joseph; I. Azarshahy. A1teration of coronary b1ood f1ow in the dog by 100% oxygen inha1ation. Circ. Res. 11:797, 1962. Sparks, H. V.; D. F. Bohr. Effect of stretch on passive tension and contracti1ity of iso1ated vascu1ar smooth musc1e. Am. J. Physio1. 2022835, 1962. Sparks, H. V. Effect of quick stretch on iso1ated vascu1ar smooth musc1e. Circ. Res. (Supp1 I) 15:254, 1964. Stowe, D. F.; T. E. Su11ivan; J. M. Dabney; J. B. Scott; F. J. Haddy. Ro1e of ATP in coronary f1ow regu1ation in the iso1ated perfused guinea pig heart. Physio1ogist. 17:339, 1974. Sunahara, F. A.; J. Ta1esnik. Prostag1andin inhibition of metabo1ica11y induced coronary vasodi1ation. J. Pharm. Exp. Ther. 188:135, 1974. Sybers, H. D.; P. R. He1mer; Q. R. Murphy. Effects of hypoxia on myo- cardia1 potassium ba1ance. Am. J. Physio1. 220:1047, 1971. Szentizanyi, M.; A. Juhasz-Nagy. A new aspect of the nervous contro1 of the coronary b1ood vesse1s. Q. J. Expt. Physio1. 44:67, 1959. Szentivanyi, M.; N. Juhasz-Nagy. Physio1ogica1 ro1e of the coronary constrictor fibers. Q. J. Expt. Physio1. 48:93, 1963. Ta1esnik, J.; F. A. Sunahara. Enhancement of metabo1ic coronary di1a- tion by aspirin-1ike substances by suppression of prostag1andin feedback contro1. Nature. 244:351, 1973. Vance, J. P.; D. M. Brown; G. Smith. The effects of hypocapnia on myocardia1 b1ood f1ow and metabo1ism. Brit. J. Anesth. 45:455, 1973. Vatner, S. F.; C. B. Higgins; E. Braunwa1d. Effects of norepinephrine on the coronary circu1ation and 1eft ventricu1ar dynamics in the conscious dog. Circ. Res. 342812, 1974. Vatner, S. F.; [L Frank1in; C. B. Higgins, T. Patrick, S. White; R. I. Van Citters. Coronary dynamics in unrestrained conscious baboons. Am. J. Physio1. 221:1396, 1971. Vatner, S. F.; D. Frank1in; R. L. Van Citters; E. Braunwa1d. Effects of carotid sinus nerve stimu1ation on the coronary circu1ation of the conscious dog. Circ. Res. 27:11, 1970. Wa1insky, P.; L. Wiener; A. N. Brest; W. Santamore. A1tered response to norepinephrine in stenosed coronary arteries. Fed. Proc. 372417, 1978. 190 Wang, H. H.; M. R. B1umentha1, S. C. Wang. Effect of efferent vaga1 stimu1ation on coronary sinus out f1ow and cardiac work in the anesthetized dog. Circ. Res. 8:271, 1960. Wennma1m, A.; P. H. Chanh; M. Junstad. Hypoxia causes prostag1andin re1ease from perfused rabbit hearts. Acta. Physio1. Scand. 91:133, 1974. Whitsitt, L. S.; B. R. Lucchesi. Effects of proprano1o1 and its stereoisomers upon coronary vascu1ar resistance. Circ. Res. 212305, 1967. Whittan, R. The high permeabi1ity of human red ce11s to adenine and hypoxanthine and their ribosides. J. Physio1. (London). 154:614, 1960. Winbury, M. M. ,D. H. Papierski; M. L. Hemmer; W. E. Hambourger. Coronary di1ator action of the adenine- ATP series. J. Pharm. Exp. Ther. 109:255, 1953. Yasue, H. Coronary arteria1 spasm and Prinzmeta1's variant form of angina induced by hyperventi1ation and tris-buffer infusion. Circu1ation 58:56, 1978. Yasue, H. Ro1e of autonomic nervous system in the pathogenesis of Prinzmeta1's variant form of angina. Circu1ation 50:534, 1974. Zacca, N.; R. A. Chahine; A. E. Raizner; T. Ishimor; R. J. Luchi; R. R. Mi11er. The angiographic spectrum of coronary artery spasm. C1in. Res. 27:218A, 1979. Zuberbuh1er, R. C.; D. F. Bohr. Responses of coronary vascu1ar smooth musc1e to catecho1amines. Circ. Res. 16:431, 1965. APPENDIX APPENDIX The tab1es represented on the fo11owing pages contain raw resist- ance va1ues for each series of experiments conducted in this study. This data is presented as a representative examp1e of the statistica1 method emp1oyed (Student's t test modified for paired rep1icates) to determine significance of the effect seen with experimenta1 interven— tion on each measured variab1e. The resistance va1ues are presented as they are the foca1 point of this study. The difference (AR) between contro1 (C) and experimenta1 (E) va1ues are presented, as are the means (i) and standard error of the means (i SEM) for each respective group. The paired t statistic is a1so reported for each ana1ysis. PRU = periphera1 resistance unit mmHg/m1/min PRU100 = periphera1 resistance unit mmHg/m1/min/1OO g * = significant1y different from contro1 at P < 0.05 191 192 SERIES I PRU C E AR Sympathetic stimu1ation 5.90 6.36 -.46 at norma1 f1ow 4.21 4.84 —.63 6.38 6.94 -.56 5.75 8.0 -2.25 12.5 14.3 -1.80 11.66 13.33 -1.67 6.25 7.25 -1.00 4.54 4.68 - 14 _ 9.09 9.36 -.27 x i SEM 7.35i1.12 8.34i1.14 -0.97:.25 t=-3.88* Sympathetic stimu1ation 11.66 11.66 0 at 10w f1ow 7.14 9.28 -2.14 6.0 7.6 -1.6 8.75 9.50 -.75 8.75 10.5 -1.75 18.33 21.66 -3.33 11.0 13.6 -2.6 13 46 15.38 -1.92 _ 7.77 8.44 -.67 x i SEM 1O 31i1.27 11.95:1.46 -1.64i.34 t=-4.76* Sympathetic stimu1ation 8.43 8.75 —.32 at high f1ow 4.16 4.71 -.55 5.31 5.40 -.15 5.66 6.40 -.74 11.0 12.5 -1.5 12.33 13.33 -1.0 5.78 5.93 -.15 5.00 5.23 -.23 _ 5.48 5.87 -.39 x i SEM 7.01:.96 7.57:1.07 -.55i1.5 t=-3.70* 1 pg NE infusion at 5.45 5.90 -.45 norma1 f1ow 3.90 3.12 .78 6.38 3.05 3.33 5.50 6.25 -.75 13.75 8.75 5.0 12.22 13.88 -1.66 7.00 6.75 .25 4.54 3 40 1.14 9.09 5 45 3.64 x i SEM \l 01 (A) H- .4 continued SERIES I--continued 193 PRU C E AR 1 pg NE infusion at 11.66 10.0 1.66 1ow f1ow 7.85 5.71 2.14 6.00 4.66 1.34 9.75 9.25 .50 8.75 9.00 -.25 18.33 23.33 -5.0 11.0 12.0 -1.0 13 46 13.84 -.38 _ 7.77 7.55 .22 x i SEM 10 50i1.23 10 59i1.85 -0.08i.7 t=-0.12 1 pg NE infusion at 7.81 5.62 2.19 high f1ow 4.28 3.45 .83 5.06 2.96 .10 6.33 7.00 -.67 12.5 9.28 3.22 13.33 14.66 -1.33 6.18 6.09 .09 5 14 3.08 2.06 _ 5.64 5.32 .32 x i SEM 7 36:1.1 6.38:1.23 0.97:.5 t=1.94 Sympathetic stimu1ation 9.09 10.45 -1.36 at norma1 f1ow after 8.05 8.33 -.28 beta-b1ockade 7.22 7.77 -.55 7.25 7.65 -.40 20.00 20.00 .60 11.66 13.33 -1.67 8.75 9.00 -.25 6.25 6.56 - 31 _ 14.00 15.20 -1.20 x i SEM 10.25i1.46 10.9811 52 - 73:.17 t=-4.14* Sympathetic stimu1ation 11.66 12.33 -.67 at 10w f1ow after 8.57 9.28 -.71 beta-b1ockade 8.00 8.93 -.93 22.50 23.75 —1.25 12.50 14.25 -1.75 18.33 21.66 -3.33 15.00 15.20 -.20 13.46 15.38 -1 92 _ 17.50 19.00 -1.50 x i SEM 14.16i1 54 15.53i1 77 -1 361 3 t=-4 43* continued SERIES I--continued 194 PRU C E AR Sympathetic stimu1ation 7.81 8.43 -.62 at high f1ow after 5.93 6.25 —.32 beta-b1ockade 5.31 5.59 -.28 8.16 8.66 -.50 14.28 15.00 -.72 12.33 13.33 -1.00 7.50 7.65 -.15 5.66 5.83 -.17 _ 15.90 16.72 -.82 i SEM 9.20i1.31 9.71i1.4 -.50i.1 t=-5.06* 1 pg NE infusion at 10.45 11.36 —.91 norma1 f1ow after 7.22 8.88 -1.66 beta-b1ockade 7.50 8.33 -.83 7.65 8.15 -.50 20.00 21.25 -1.25 17.77 18.33 -.56 8.75 9.25 -.50 6.56 6.62 -.06 _ 13.75 3714. —.62 i SEM 11.07+1. 65 83+1.11. 69 -.76:.15 t--4 89* 1 pg NE infusion at 11.66 13 33 -1 67 10w f1ow after 8.57 9.00 - 43 beta-b1ockade 9.33 11 33 -2 00 23.75 27 50 -3 75 12.50 13 75 -1 25 31.66 35 OO —3 34 15.00 17 40 -2 40 13.46 14 61 —1 15 _ 17.50 20.00 -2.50 i SEM 15.93:2.48 17.99+2. 79 -2. 05+. 35 t=—5. 77* 1 pg NE infusion at 9.06 10.00 -.94 high f1ow after 6.56 6.87 -.31 beta-b1ockade 5.46 5.93 -.47 8.66 9.00 -.34 14.28 15.35 -1.07 14.66 14.66 0 7.50 7.81 -.31 5.73 6.06 - 33 _ 15.90 16.36 -.46 x i SEM 9.75i1.36 10.22i1 36 - 47i 11 t—-4 22* 195 SERIES II PRU100 C E AR Sympathetic stimu1ation 1.28 1.41 -.13 at norma1 f1ow 1.36 1.43 -.07 1.47 1.54 -.07 1.39 1.51 -.12 1.79 1.87 -.08 2.00 2.08 -.08 3 67 3.82 -.15 _ 3.23 3.38 -.15 x i SEM 2 02i.32 2.13:.33 -.10i.01 t=-8.58* Phento1amine infusion 1.34 1.28 .06 at norma1 f1ow 1.36 1.36 O 1.47 1.44 .03 1.36 1.30 .06 2.10 1.95 .15 2.33 1.01 .42 3.90 2.20 1.70 _ 4.41 3.97 .44 x i SEM 2.28:.43 1.92i.31 0.35:.2 t=1.77 Sympathetic stimu1ation 1.28 1.30 —.02 during phento1amine in- 1.36 1.36 0 fusion at norma1 f1ow 1.47 1.47 0 1.30 1.31 -.01 1.95 1.95 O 1.91 1.91 O 2.20 2.23 -.03 x i SEM 1.93:.3 1.91:.2 —.008:.OO4 t=-1.86 1 pg NE infusion at 1.30 0.96 .34 norma1 f1ow 1.39 1.02 .37 1.47 1.13 .34 1.42 1.19 .23 1.79 1.01 .78 2.08 1.16 .92 3.67 2.05 1.62 _ 3 25 1.61 1.64 x i SEM 2.04i.32 1.26:.13 .78i.2 t=3.83* continued 196 SERIES II--continued PRU100 C E AR 1 pg NE infusion with 1.28 0.89 .39 phento1amine at 1.36 0.95 .41 norma1 f1ow 1.47 1.02 .45 1.30 1.07 .23 1.87 1.32 .55 1.91 1.08 .83 2.35 1.47 .88 _ 4.41 3.38 1.03 x i SEM 1.99i.36 1.39:.29 .59i.09 t=5.97* Sympathetic stimu1ation 1.02 1.08 -.06 at high f1ow 0.82 0.88 -.06 1 60 1.67 — 07 _ 0.91 0.95 -.04 x i SEM 1.08i.17 1.14:.17 -.05i.006 t=-9.13* Sympathetic stimu1ation 1.01 1.01 0 with phento1amine at 0.82 0.83 -.01 high f1ow 1.60 1.61 —.01 _ 0.91 0.91 0 x i SEM 1.08i.17 1.09i.17 -.005i.002 t=-1.73 Sympathetic stimu1ation 0.97 1.11 -.14 at 10w f1ow 1.16 1.26 -.10 3 88 4.44 - 56 _ 0.94 1.02 :;Q§ x i SEM 1.73i.71 1.95:.80 -.22:.11 t=-1.92 Sympathetic stimu1ation 0 97 0.97 0 with phento1amine at 1.16 1.16 O 1ow f1ow 3.88 3.88 0 _ 0.94 0.94 _11 x i SEM 1.73i.71 1.73i.71 OiO t=0 1 pg NE inf1usion at 1.04 0.83 .21 high f1ow 0.84 0.77 .07 1.65 1.36 29 _ 0.92 0.81 _;11 x i SEM 1.11:.18 0.94:.13 . 7i.04 t=3.42* continued SERIES II--continued 100 C E 4R 1 mg NE infusion with 1.01 0.80 .21 phento1amine at 0.82 0.75 .07 high f1ow 1.60 1.32 .28 _ 0.91 0.75 lplg x i SEM 1.08:.17 0.90:.13 .18i.04 t=4.07* 1 Pg NE infusion at 1.00 0.83 .17 10w f1ow 1.16 1.13 .03 3.88 3.77 .11 _ 0.94 0.91 _;93 x i SEM 1.74:,71 1.66i.70 .08i.03 t=2.49 1 mg NE infusion with 0.97 0.83 .14 phento1amine at 1ow 1.16 1.10 .06 f1ow 3.88 3.77 .11 _ 0.94 0.81 .;l§ x i SEM 1.73:.71 1.62i.71 11i.01 t=6.18* 198 SERIES III PRU100 C E AR Sympathetic stimu1ation 1.11 1.30 -.19 1.03 1.11 -.08 1.21 1.30 -.09 0.89 0.98 -.09 1.30 1.42 -.12 2.18 2.45 -.27 _ 2.87 3.19 -.32 x i SEM 1.51i.27 1.72:.09 -.16i.03 t=-4.53* Systemic hypocapnia 1.33 1.46 -.13 1.03 1.08 -.05 1.16 1.16 0 0.82 0.89 -.07 1.30 1.48 -.18 2 09 2 36 -.27 _ 2.44 2.65 -.21 x : SEM 1 45i.15 1 58i.25 -.13i.03 t=-3.57* Sympathetic stimu1ation 1.46 1.61 -.15 during hypocapnia 1.08 1.11 -.03 1.16 1.19 -.03 0.89 1.07 -.18 1.48 1.54 -.06 2.36 2 54 -.18 _ 2.65 2.87 -.22 x i SEM 1.58i.25 1 701.27 -.12i.03 t=-4.04* 0.25 Mg NE infusion 1.27 1.33 -.06 1.08 0.82 .26 1.21 0.75 .46 0.93 0.66 .27 1.36 1.25 .11 2 27 2.18 .09 _ 2.87 2.76 .11 x i SEM 1 57i.27 1.39:.30 O.17i.06 t=2.79* 0.25 ug NE infusion 1.55 1.66 -.11 during hypocapnia 1.03 0.82 .21 1.16 0.87 .29 1.07 0.89 .18 1.42 1.30 .12 2.36 2.27 .09 _ 2.65 2.02 .63 x i SEM 1.60i.24 1.401.22 0.20:.08 t=2.35 continued 199 SERIES III--continued PRU100 C E A1? 0.25 09 NE after 2.00 2.00 0 indomethacin 0.67 0.61 .06 1.39 1.45 -.06 1.25 1.11 .14 1.42 1.19 .23 2.36 2.32 .04 _ 2.97 1.70 1.27 x i SEM 1 72i.29 1.48i.21 0.24i.17 t-1.36 Indomethacin 0.95 1.17 -.22 0.88 1.00 -.12 1.72 1.91 -.19 1.03 0.67 .36 1.16 1.51 -.35 0.89 1.02 -.13 1.30 1.48 -.18 1.81 2.27 -.46 _ 2.44 2.65 -.21 x i SEM 1.35i.17 1.52i.21 -.16:.07 t=-2.21 Sympathetic stimu1ation 1.17 1.22 -.05 after indomethacin 1.00 1.22 -.22 1.91 2.00 -.09 0.67 0.69 -.02 1.27 1.39 -.12 1.02 1.25 —.23 1.42 1.52 -.10 2.27 2.45 -.18 _ 2.97 3.08 —.11 x i SEM 1.52i.24 1.64:.24 -.12:.02 t=-5.16* Hypocapnia after 0.67 0.72 .05 indomethacin 1.29 1.56 -.27 1.42 1.54 -.12 2.09 2 36 — 27 2.97 3.29 0.32 x i SEM 1.68i.39 1.89i.43 - 20i.05 t=-4.00* Sympathetic stimu1ation 0.72 0.71 .01 during hypocapnia and 1.56 1.62 -.06 after indomethacin 1.54 1.57 -.03 2.36 2.69 -.33 _ 2.97 3.08 -.11 x i SEM 1.83:.38 1.93i.42 -.10i.05 t=-1.73 continued SERIES III--continued 200 PRU 100 C E AR 0.25 pg NE infusion 0.72 0.67 .05 during hypocapnia and 1.56 1.33 .23 after indomethacin 1.54 1.19 .35 2.69 2.60 .09 _ 2.97 2.55 .42 x i SEM 1.89i.41 1.66:.38 .22:.07 t=3.18* SERIES IV ('3 Sympathetic stimu1ation during normoxia O O O O O O —I O 0.25 pg NE infusion during normoxia N—J-d—‘NOON Sympathetic stimu1ation during hypoxia ' OOOOO 0.25 pg NE infusion during hypoxia 00000—1 #mhookow hO—‘VN-A '0 O \l m 1+ t=-3.95* t=2.60* t=4.72* t=2.95* continued 202 SERIES IV--continued PRU100 C E AR Hypocapnia 2.04 2.45 —.41 2.60 3.91 -1.31 1.92 3.50 -1.58 1.13 1.34 —.21 1 74 2.12 -.38 _ 1.38 1.44 —.06 x i SEM 1.80i.21 2.46i.43 -.65i 25 t=-2 56* Sympathetic stimu1ation 2.45 2 54 -.09 during hypocapnia 3 91 5 13 —.22 3 50 3 59 -.09 1 34 1 42 —.08 2 12 2 27 -.15 _ 1.44 1.50 -.06 x i SEM 2.45:.43 2 57:.44 -.11i.02 t=-.472* 0.25 pg NE infusion 2.45 2.45 0 during hypocapnia 4.13 4.19 —.06 3.50 3.24 .26 1.28 0.97 .31 2 12 1.59 .53 _ 1.38 0.66 .72 x i SEM 2 47+.46 2.18i.55 .29i.12 t=2.39 Hypoxia and 2.45 1.31 1.14 hypocapnia 2.71 1.30 1.41 1.75 1.05 .70 1.39 0.46 .93 1 59 0.60 .99 _ 1.61 0.55 1.06 x i SEM 1.91i.21 0.87: 15 1.031.09 t=10.79* Sympathetic stimu1ation 1.31 1.31 0 during hypoxia and 1.30 1.30 0 hypocapnia 1 05 1.14 -.09 0 46 0.51 -.05 0 60 0.68 -.08 _ 0 55 0.66 -.11 x i SEM 0 87i.15 0.93i 14 -.05:.01 t=—2.87* continued 203 SERIES IV-—continued PRU100 C E AR 0.25 pg NE infusion 1.22 1.22 0 during hypoxia and 1.30 1.41 -.11 hypocapnia 1.14 1.05 .09 0.46 0.41 .05 0.60 0.57 .03 _ 0.55 0.47 .08 x i SEM 0.87i.15 0.85:.17 .02i.02 t=0.78 0.50 pg NE infusion 2.13 2.04 .09 during normoxia 3.21 2.93 .28 2.19 1.05 1.14 1.28 0.41 .87 _ 1.96 1.06 .90 x i SEM 2.15i.30 1.49i.44 .65i.20 t=3.27* 0.5 pg NE infusion 1.31 1.22 .09 during hypoxia 0.97 0.97 0 0.87 0.78 .09 0 41 0.36 .05 _ .60 0.53 .07 x i SEM 0.83i 15 O.77i.15 .06i.01 t=3.58* 0.5 pg NE infusion 2.45 2.29 .16 during hypocapnia 4.13 3.80 .33 3.50 3.07 .43 1.28 0.77 .51 2.12 1.51 .61 2 i SEM 2.69i.50 2.28i.52 .40:.07 t=5.28* 0.5 pg NE infusion 1.22 1.14 .08 during hypoxia and 1.41 1.30 .11 hypocapnia 1.14 0.87 .27 0.46 0 41 .05 _ 0.60 0.53 .07 x i SEM 0.96:.18 0 85:.17 . 1i.03 t=2.92* 204 SERIES V PRU100 C E AR Sympathetic 2.77 2.93 .16 2.85 3.12 -.27 1.81 2.12 -.31 1.66 1.71 -.05 1.42 1.61 -.19 2.38 2.56 -.18 1.66 1.81 -.15 _ 1.53 1.69 -.16 x i SEM 2.03i.21 2.21i.22 -.18:.02 t=-6.58* 0.25 pg NE 2.77 2.21 .56 2.38 1.90 .48 1.81 1.17 .64 1.78 1.13 .65 1.42 0.95 .47 2.38 1.42 .96 1.66 1.42 .24 _ 1.53 0.98 .55 x i SEM 1.98i.18 1.41i.19 .59i.07 t=7.89* Hypoxia 2.90 0.58 2.32 2.05 0.43 1.62 1.81 0.50 1.31 1.78 0.48 1.28 1.07 0.53 0.54 2.38 0.39 1.99 1 66 0.60 1 06 _ 1.53 0.39 1.14 x i SEM 1 89i.19 0.48i.02 1.40_.19 t=7.12* Sympathetic stimu1ation during hypoxia 0.58 0.58 0 0.43 0.44 -.01 0.53 0.53 0 _ 0.39 0.42 -.03 x i SEM 0 48 0.49 -.01i.007 t=-1.41 0.25 09 NE during 1.10 0.77 .33 hypoxia 0.42 0.45 -.03 0.53 0.53 0 _ 0.35 0.38 -.03 x i SEM 0 60¢ 17 0 53i 08 .06: 08 t= 76 continued 205 SERIES V--continued PRU100 C E AR Hypercapnia 1.23 0.73 .50 1.58 2.54 1.09 1.31 1.00 .31 1.42 0.78 .64 2.04 1.02 1.02 1.66 0.95 .71 _ 1.53 0.63 .90 x i SEM 1.53i.09 0.81i.07 .73i.10 t=7.09* Sympathetic stimu1ation 0.71 0.73 -.02 during hypercapnia 0.78 0.95 -.17 1.02 1.07 -.05 0.86 0.90 -.04 _ 0.63 0.67 -.04 x i SEM 0.80i.06 0.86i.07 -.06:.02 t=-2.37* 0.25 pg/min NE during 0.73 0.75 -.02 hypercapnia 0.86 0.90 -.04 1.02 0.95 .07 0.95 0 90 .05 _ 0.63 0.67 -.04 x i SEM 0.83i.07 0 84:.06 .004i.02 t=0.17 Hypocapnia 2.32 3.32 -1.0 1.51 1.72 -O.21 1.47 0.96 0.51 0.81 0.88 -0.07 2.04 2.85 —0.81 1 66 1.75 -0.09 _ 1.53 2.27 -O.74 x i SEM 1 62i.18 1.96i.34 -O.34i.20 t=-1.71 Sympathetic stimu1ation 3.32 3.88 -0.56 during hypocapnia 1.72 1.81 -0.09 0.96 1.00 -0.04 1.88 1.88 0 2.85 2.85 0 1.66 1.66 0 _ 2.27 2.38 -0.11 x i SEM 2.09i.29 2.20:.35 - 11i 07 t=-1.49 continued 206 SERIES V--continued 100 C E AR 0.25 Mg NE during 3.32 1.94 1.38 hypocapnia 1.72 1.58 0.14 1.00 0.80 0.20 1.88 0.86 1.02 2.85 1.29 1.56 1.66 1 53 0 13 _ 2.27 1.78 0.49 x i SEM 2.10i.29 .39i.16 O. 0i.23 t=3.04* ”111111111111111’1111111111111111111ES 3 333333333333