. ,

xvi

MICHIGAN 3711

II

 

I

 

 

I
I

I

I

 

 

 

 

I

 

 

IIII

 

 

I

 

 

IIII

I

TE UN
I
I

I

LIBRARIES

IIIIIIIIIIIII/III

I

 

 

I

3 1293 01591 4587

 

 

91.7”" -‘- “av
§ .-
L‘nad-1I‘Y I t‘. a

3 ‘ w '1' 0 Av

( 4”,.4. ...

- fi".‘.--u 1.0,.“ b “8"" H-

I? 9.0
' YNJJT' ”F" I,
Liv-O v‘ “-'."y

 

 

This is to certify that the
dissertation entitled

Contributions of Endogenous Prostaglandins to

the Postprandial Intestinal Hyperemia
presented by

Martin John Mangino

has been accepted towards fulfillment
ofthe requirements for

Ph.D. Philosophy

degree in

 

 

é2:92#¢fi/~—7 s/éfQanéaq

 

Major professor

 

Date fh/k/ 23/ /?W

MSU is an Affirmative Action /Equal Opportunity Institution 0- 12771

 

 

MSU

LIBRARIES

”—

 

 

RETURNING MATERIALS:
Place in book drop to
remove this checkout from
your record. FINES wil]
be charged if book is
returned after the date
stamped beIow.

 

 

 

‘

 

 

 

 

CONTRIBUTIONS OF ENDOGENOUS
PROSTAGLANDINS TO THE

POSTPRANDIAL INTESTINAL HYPEREMIA

By

MARTIN JOHN MANGINO

A DISSERTATION

Submitted to
Michigan State University
in partial Fulfillment of the requirements
For the degree of

DOCTOR OF PHILOSOPHY

Department of Physiology

1985

u/K

ABSTRACT

CONTRIBUTIONS OF ENDOGENOUS PROSTAGLANDINS
TO THE POSTPRANDIAL INTESTINAL HYPEREMIA

By

Martin John Mangino

The postprandial intestinal hyperemia is characterized
by an increase in blood flow and metabolism in those
portions of the intestine exposed to chyme. This response
is complex with many factors involved such as hormones,
oxidative metabolism. histamine and prostaglandins. Many
prostaglandins are synthesized in the intestine. and
inhibition of prostaglandin synthesis has been shown to
potentiate food-induced increases in intestinal blood flow
and oxygen uptake. The purpose of this study was to
elucidate more closely the role of endogenous
prostaglandins in the vascular response to intestinal
nutrient absorption. Therefore, the effects of
intraluminal placement of food on jejunal blood flow were
studied before and during infusions of arachidonic acid. a
prostaglandin precursor. and selective inhibitors of
thromboxane synthesis.

A dose-response series of experiments indicates that

intra—arterial infusions of arachidonic acid elicits a
biphasic effect on jejunal vascular resistance. At low
doses. arachidonic acid increased jejunal vascular

resistance while at higher doses it decreased vascular

Martin John Mangino
resistance. The increase in jejunal vascular resistance
associated with low doses of arachidonic acid was abolished
when the animals were pretreated with aspirin 48 hours
prior to the experiment and all effects of arachidonic acid
on jejunal vascular resistance were abolished with
mefenamic acid.

Placement of a digested food solution into the jejunum
significantly increased local blood flow and oxygen uptake.
Arachidonic acid infused intravenously, intra—arterially or
placed into the lumen with the food solution significantly
attenuated the hyperemia associated with food placement.
The food-induced increases in oxygen uptake were also
attenuated with luminal placement of arachidonic acid.
These effects were abolished with pretreatment with the
cyclooxygenase inhibitor, mefenamic acid.

The selective thromboxane synthesis inhibitors,
imidazole and Sodium 5- (3'-Pyridinylmethyl) Benzofuran -
2 — Carboxylate (U-63557A). had no effect on the
postprandial intestinal hyperemia but significantly
potentiated the food-induced increases in oxygen uptake.
Furthermore, both imidazole and U-63557A had no effect on
jejunal glucose absorption.

When digested food was placed into the jejunal lumen,
capillary exchange capacity. as indexed by the capillary

filtration coefficient. significantly increased over the l6

Martin John Mangino
minute placement period. and imidazole enhanced these
effects. Mefenamic acid. however. had no effect on
intestinal capillary exchange capacity either before or

during food placement.

This study indicates that endogenous intestinal
prostaglandins may serve to limit the postprandial
intestinal hyperemia. Furthermore. endogenous intestinal

thromboxanes appear to be involved in the food—induced
increases in oxygen uptake as well as the regulation of

intestinal capillary exchange capacity.

To my loving wife Janice
and our little Buddy

ii

ACKNOWLEDGEMENTS

I would like to acknowledge the advice. support. and
scientific expertise of my mentor. Dr. C.C. Chou. and the
other members of my guidance committee. Dr’s. Sparks.
Krier. Pittman. and Roth. Also. special thanks to Denise
lngold-wilcox for her friendship and technical expertise.
my parents and family for their undying love and support.
and my brother. Dr. Michael Mangino. for the life long

encouragement and education.

iii

TABLE

LIST OF TABLES. . . . .
LIST OF FIGURES . . . .
INTRODUCTION. . . . . .
LITERATURE REVIEW . . .
MATERIALS AND METHODS .
RESULTS . . . . . . . .
DISCUSSION. . . . . . .
CONCLUSIONS . . . . . .

BIBLIOGRAPHY. . . . . .

OF CONTENTS

iv

Page

v
vi—viii
1
2-30
31-53
54-95
96-112
113-115

116-130

LIST OF TABLES

Table Page

1. The effects of intra-arterial infusions of arachidonic
acid (AA) on the postprandial intestinal hyperemia.
Blood flow expressed as ml1min l009 . * P<:0.05
relative to the corresponding value before infusion.

n=7. . . . . . . . . . . . . . . . . . . . . . . . 61

2. The effects of intravenous infusions of arachindonic
acid (AA) on the postprandial _{ntestinal hyperemia.

Blood flow expressed as ml-min lOOg . * P< 0.05
relative to the corresponding value before infusion.
n=7. 0 O O O O O O O O O O O O O O O O O O O O O O 63

3. The effects of intra—arterial infusions of arachidonic
acid on the food—induced increases in intestinal blood
flow and oxygen uptake after pretreatment with
mefenamic acid. V027 oxygen uptake. V02 and blood flow

expressed as mlemin - lOOg 1. * P<0.05. n=7. . . . 75

4. The effects of intravenous infusions of U-63557A on the
food-induced increases in intestinal blood flow an
oxygep uptake (V02). Values expressed as ml- min -
lOOg . * P (0.05. 1 = P<0.05 relative to the
corresponding value before U-63557A. n=5. . . . . 85

S. The effects of imidazole (5 mgtmin-l. i.a.) and U-

63SS7A (5 mg-kg- . i.v.) on jejunal glucose absorption
in 7 and 5 dogs. respectively. Values are mean i

standard error and expressed as mg min 1 1009"1 .

O O O O O O O C O I Q C O O O O I O O O O O O O O O 89

LIST OF FIGURES

Figure Page

Biosynthetic pathway of the major bisenoic
prostaglandins and thromboxanes. . . . . . . . . 15

Schematic representation of the preparation used in
most experiments. . . . . . . . . . . . . . . . 34

Polygraph chart recording showing the effect of a
single bolus of arachidonic acid (300 ug.kg- ) given
into the pulmonary artery on aortic pressure. This was
done before and 120 m n after i.v. infusions of
mefenamic acid (10 mg-kg ). . . . . . . . 39

Mean venous outflows from 2 adjacent JeJunal segments
containing normal (NS). NS plus arachidonic acid (AA).
food. or food plus AA. Values shown below or above 2
adjacent bars indicate percent differences in flow
between 2 bars. * P < 0.05 relative to preceding
value. ** P < 0.05 between 2 values denoted by
dotted line. n = l2 . . . . . . . . . . . . . 43

Original polygraph chart recording of the data used to
estimate JeJunal capillary filtration coefficient
(CFC). Solid line (intestinal weight panel) represents
the rate of tissue water accumulation following the
blood volume shift used in estimating CFC. . . . 49

Correlation between changes in venous pressure and
changes in capillary pressure in isolated JeJunal
segments. Dotted line represents the observed
regression line of the data. solid line represents 70%
transmission of the imposed venous pressure elevations
to the exchange vessels. . . . . . . . . . . . . 52

Effects of intra-arterial infusion of arachidonic acid
on JeJunal vascular resistance. Dotted lines indicate
a zone of 95% confidence. n = 47 from 7 dogs. AA.
arachidonic acid. . . . . . . . . . . . . . . . 56

vi

'0.

ll.

l2.

'3.

‘4.

Effects of intra—arterial infusions of various doses of
arachidonic acid on jejunal vascular resistance before
and after mefenamic acid (10 mg/kg. i.v.) in dogs
pretreated with a single oral dose of aspirin (25 mg
kg ) 48 h before the experiment. The dashed line
represents the effects of arachidonic acid on jejunal
vascular resistance before mefenamic acid (triangles
are actual data points) and the solid line indicates
the effects of arachidonic acid on jejunal vascular

resistance after mefenamic acid (circles are actual

data points). The regression coefficients are
significantly different at P < 0.05. n = 51 from 6
dogs. AA = arachidonic acid. . . . . . . . . 59

Mean 0 uptakes from 2 adjacent jejunal segments
containTng normal saline (NS). NS plus arachidonic acid
(AA). food. or food plus AA. Values shown below or
above 2 adjacent bars indicate percent differences in
O uptake between 2 bars. * P < 0.05 relative to
pFeceding value. ** P < 0.05 between 2 values denoted
by dotted line. n = 5. V02 = 02 uptake. . . . 68

Correlation between changes in food-induced increases
in jejunal 0 uptake and blood flow. Closed circles.
food alone; gpen circles. food plus arachidonic acid
(AA). 0 O O O O O O O O I O O I I O O O O O O I O 70

Relation between jejunal blood flow and O uptake.
Blood flow was altered mechanically. N mbers in
parentheses indicate number of measurements. Resting
flow before mechanical alterations was 40.2 i 0.5
ml-min- 11009— . . . . . . . . . . . . . . . . . 72

Vascular response to luminal placement of food before
and during intra-arterial infusions of imidazole. Flow
values are means : SEM in ml-min- -l009- . n = 14.

NS. Normal saline. F. food. P < 0.05 relative to before
infusion. . . . . . . . . . . . . . . . . . . . . 78

Effect of intra-arterial infusions of imidazole on the
food-induced increases in oxygen uptake. V0 , oxygen
uptake. NS. normal saline. F. food. * P < 0.05
relative to NS. ** P < 0.05 relative to the
corresponding value before imidazole,n = 14. . .. 80

Correlation between percent changes in V0 (oxygen
uptake) and blood flow elicited by luminal pigcement of
digested food before and during intra-arterial
infusions of imidazole. * P < 0.05 relative to
control. . . . . . . . . . . . . . . . . . . . . 83

vii

IS.

l6.

Time course of changes in jejunal CFC during luminal
placement of food before and during ilT' infusions _of
imidazole. CFC expressed as ml.min . mm Hg - lOOg .
NS = normal saline. F = food. Squares = P < 0.05
relative to NS before food. * P < 0.05 relative to the
corresponding value before imidazole. n = 7. . . . 91

Time course of changes in jejunal CFC during luminal
placement of food before and after i.v. _ nfusigTs of
mefenamic acid. CFC expressed as ml-min ' lOOg . NS
= normal saline. F = food. ' P < 0.05 relative to NS.

. . . . . 94

n=7. 0 o o o o o o o o o o o o o o 0

viii

INTRODUCTION

The increase in intestinal blood flow associated with
nutrient absorption occurs only in those areas of the
intestine that are exposed to chyme (34. 36. 87).
Furthermore. the stimulus for the postprandial intestinal
hyperemia has been shown to be the products of food
digestion (37. 125). The mechanisms contributing to this
response appear to be multifactorial and complex.

Recently. evidence has been presented that implicate a
role for prostaglandins. Prostaglandins are synthesized by
all layers of the small intestine (1. 15. 25. 55. 85. 91.
92. 93. 126) and have been shown to be released during the
movement of ingestia through the gastro-intestinal tract
(45). Furthermore. inhibition of prostaglandin synthesis
with indomethacin or mefenamic acid significantly
potentiate the food-induced increases in intestinal blood
flow and oxygen uptake (63).

The purpose of this study was to examine the effects
of the bisenoic prostaglandin precursor. arachidonic acid.
on the food—induced increases in intestinal blood flow and
oxygenation. Also. in an attempt to determine which
prostaglandins may be involved. the effects of selective
inhibition of thromboxane synthesis on the postprandial

intestinal hyperemia was determined.

LITERATURE REVIEW

Postprandial Intestinal Hyperemia

In 1910. Brodie gt g1; (27) documented the first
evidence of a postprandial hyperemia in the canine small
intestine by using a plethymograph method. In 1934.
Herrick _3 g1; (77) utilized the thermostromuhr method to
measure blood flow in various arteries of the conscious
dog. Blood flow through the carotid. coronary. and
superior mesenteric arteries increase shortly after a fat.
carbohydrate. and protein meal and the increase lasted from
2-5 h. This study indicated that the increase in flow to
the small intestine during digestion was not obtained at
the expense of blood flow to other organs. These findings
were further substantiated by studies conducted between
1940 and 1966. These investigators reported that heart
rate. blood pressure. and cardiac output (41. 114) as well
as blood flow to almost all organs of the body increased
after a meal (114). The flow to various organs of the body
was measured by the 42K fractional distribution technique
in both fed and unfed rats (114).

More recent studies. however. have yielded different
results. The cardiovascular system appears to respond to
feeding in two distinctly different phases. i.e..

anticipation-ingestion and digestion. During anticipation

and ingestion of food. there is an increase in cardiac

output. heart rate. and aortic pressure (61. 131. 133).
Mesenteric vascular resistance either increases (131. 133).
or does not change (61). Renal resistance increases (131.
133). coronary resistance falls (131. 133). and the
vascular resistance of the limbs either increases (61) or
decreases (61. 131. 133). These changes that occur during
the ingestion phase appear to result from activation of the
sympathetic nervous system and can be attenuated or
modified by adrenergic blocking agents (132).

The cardiovascular responses to anticipation and
ingestion of a meal are brief. Within 5—30 minutes after a
meal. cardiac output. heart rate. aortic pressure. and
coronary and renal blood flows return to control levels.
whereas blood flow through the superior mesenteric artery
and pancreas starts to rise and reaches a maximum in 30-90
minutes in the conscious dog (23. 30. 61. 64. 131. 133).
The postprandial hyperemia through the superior mesenteric
artery lasts for 4-7h and blood flow to the limbs and
skeletal muscle decreases (61. 131. 133) or does not change
(64). This indicates that a redistribution of the cardiac
output in favor of the superior mesenteric artery occurs.

The earlier studies conducted between 1910 and 1966
indicated that feeding results in increases in cardiac
output and blood flow through most of the body organs and
tissues. More recent studies. however. have more carefully

characterized the cardiovascular responses to feeding into

two distinctly different phases. The difference in the
findings may be due to basic differences in techniques and
equipment used to measure cardiac output and regional blood
flow.

The postprandial hyperemia is localized to that
portion of the gastrointestinal tract that is exposed to
chyme. In 1976. Chou gt gig (36) found that placement of a
food solution in the stomach of anesthetized dogs resulted
in a rapid increase in blood flow through the celiac artery
lasting less than one hour. Blood flow through the
superior mesenteric artery increased 30 minutes after the
intragastric placement of food and remained elevated for
over three hours. When they infused food directly into the
duodenum. celiac artery flow did not change but superior
mesenteric artery blood flow increased and remained
elevated for several hours.

The celiac artery perfuses the stomach. pancreas.
liver. spleen and proximal duodenum. The authors concluded
that the increase in celiac artery blood flow during
intragastric placement of food was due to increases in
gastric blood flow since it did not occur when food was
infused directly into the duodenum. Furthermore. the delay
in onset of the increase in blood flow through the superior
mesenteric artery was believed to be due to the time
required for gastric emptying and the sufficient
accumulation of chyme in the duodenum. This study also

found that when the venous outflows of two isolated jejunal

segments were measured simultaneously. only the segment
exposed to chyme exhibited a hyperemia and this was not
sufficient to cause a measurable increase in superior
mesenteric artery blood flow. These findings have also
been supported by other studies (34. 37. 87).

Gallavan gt g1; (64) studied the effects of feeding on
regional blood flow in the conscious dog using radiolabled
microspheres. They found that 30 minutes following
ingestion of a meal. the postprandial hyperemia was limited
to the duodenum and proximal jejunum and extended to the
entire small intestine 90 minutes after feeding. The
authors suggested that this may be due to the time required
for the movement of chyme along the length of the small
intestine.

The postprandial intestinal hyperemia is confined to
the mucosal—submucosal layer of the gut wall with no
significant change in flow to the muscularis (36. 64. 141).
Chou and co-workers have shown this to occur in isolated
jejunal segments of anesthetized dogs (36. 64) as well as
in the conscious dog (64). They employed the radiolabeled
microsphere technique in determining compartmental blood
flow within the wall of the small intestine. Bond at 91;
found that blood flow to all three layers of the small
intestine increase uniformly after feeding in the conscious
dog (23). Likewise. Bohlen (21. 22) has shown a uniform
increase in intestinal vascular diameter in all three

layers of the rat small intestine when the lumen is exposed

to a glucose solution. Since the submucosal-mucosal layer
receives over eighty percent of the total intestinal blood
flow. the contribution of the muscularis layer to the
postprandial intestinal hyperemia appears to be minimal.

Chyme that enters the jejunum has various properties
that may contribute to intestinal hyperemia and many of
these properties have been investigated. Jejunal chyme
consists. to a certain extent. of bulk and roughage capable
of causing mechanical stimulation. Biber gt El; (l6) have
shown that mechanical stimulation of the mucosa of an
isolated jejunal segment. by sliding a vinyl tube over the
mucosal surface. increases local blood flow. Another
study. however. has shown that luminal placement of
undigested food has no effect whereas digested food
(consisting of comparable roughage and bulk) increases
blood flow to isolated jejunal segments (37). It seems
that mechanical stimulation of the mucosal surface by the
propulsion and mixing of intestinal chyme is not a normal
physiological factor in the popstprandial intestinal
hyperemia.

Chyme consists of digested food. undigested food.
pancreatic secretions and gallbladder bile. Of these. only
digested food illicits an intestinal hyperemia (37).
Studies by Siragar and Chou (125) determined the relative
contribution of digested fat. protein. and carbohydrate to
the intestinal hyperemia in isolated jejunal segments of

the dog. They found that fat produces the greatest

'I-‘Tt

.W'

l

hyperemia on a weight basis but protein and carbohydrate
also produced increases in flow. Furthermore. all three
solutions produced increases in intestinal oxygen uptake.
The authors concluded that the intestinal hyperemia
probably results from a synergistic effect of all three
dietary components.

Although bile itself is not vasoactive when placed
into the jejunal lumen. it markedly potentiates the
postprandial intestinal hyperemia (37). Kvietys gt g1; (87)
examined the effects of various test meals on jejunal
blood flow with and without addition of bile. The authors
concluded that bile plays an important role in the
postprandial intestinal hyperemia because it potentiates
the glucose-induced hyperemia and only in its presence can
oleic acid. amino acids. caproic acid. and digested
triolein increase intestinal blood flow. The mechanisms of
bile-induced potentiation of the postprandial jejunal
hyperemia have not been elucidated to date. Bile increases
blood flow in the ileum and this may be due to the active
absorption of bile salts (38).

The pH of chyme changes drastically from acidic to
alkaline during gastric emptying. Intestinal chyme has a
pH range of 3.5-7.4 (24. 59). But. intraduodenal perfusion
of Tyrode's solutions having pH values between 2.5 and 11

does not alter local intestinal blood flow (35. 80).

Therefore. it seems that changes in the pH of luminal
contents associated with gastric emptying does not play a
direct role in the postprandial intestinal hyperemia.

The dumping syndrome is a pathological condition
characterized by a large increase in blood flow to the
liver and upper intestinal tract. These symptoms have been
associated with the rapid arrival of hypertonic solutions
into the jejunum. It seems at least possible. therefore.
that lumen osmolality may play a role in the postprandial
intestinal hyperemia. Indeed. it has been shown that
placement of solutions having osmolalities above 1500
mOSm/kg increase local blood flow (34). But since lumen
contents after a meal have osmolalities betwen 220-320
mOSm/kg (59). it seems unlikely that lumen osmolality plays
a role in food-induced increases in intestinal blood flow
under normal physiological conditions.

The small intestine is richly inervated from both
extrinisic and intrinsic nerves. The role of nerves in the
postprandial intestinal hyperemia has been examined in many
different preparations (22. 34. 52. 53. 89. 109. 128. 131).
In the anesthetized cat. vagotomy has been shown to reduce
the hyperemia associated with the placement of fat in the
duodenum (53). On the other hand. vagotomy has been shown
to have no effect on the postprandial intestinal hyperemia
in dogs (128. 131). Blockade of the muscarinic receptors
with atropine also seems to generate conflicting results.

In the intact gastroinestinal tract of cats (52) and dogs

(131). atropine has been shown to block the nutrient-
induced increase in blood flow through the superior
mesenteric artery. Atropine. however. had no effect on the
hypermia produced in vascularly isolated intestinal
preparations of the dog (89. 108) and rat (21. 22). These
differences may be due to the relative importance of
gastrointestinal motility in intact versus isolated
intestinal preparations. Atropine decreases
gastrointestinal motility and may decrease the absorptive
surface area in contact with chyme. This would be a factor
in intact gastrointestinal preparations but of little
importance in isolated systems.

The enteric nervous system is located within the wall
of the intestine and has been shown to play a role in
motility (66) and the hyperemia induced by mechanically
stroking the intestinal mucosa (17). A possible role of
the enteric nervous system in the postprandial intestinal
hyperemia was examined by Nyhof and Chou (109). They
reported that methysergide (S—HT blocker). hexamethonium
(ganglion blocker) and tetrodotoxin (Na channel blocker)
all failed to alter the vascular or metabolic response to
placement of glucose or oleic acid in the jejunum of
anesthetized dogs. The increases in blood flow and oxygen
uptake produced by glucose or oleic acid. however. were
blocked or attenuated after exposing the jejunal mucosa to
dibucane. a local anesthetic. Furthermore. dibucane

decreased active glucose transport across rat intestinal

t:
5"

4'.

10

sacs and decreased oxygen consumption of canine jejunal

mucosa i vitro. The authors concluded that nutrient-

 

induced hyperemia appears not to be neurally mediated but
may be influenced by neurally mediated alterations in
parenchymal metabolism and glucose absorption.

Decentralization of isolated intestinal segments in
the dog has been shown to have no effect on the
postprandial intestinal hyperemia (108). Furthermore.
alpha or beta adrenergic blockade does not influence food-
induced hyperemia in dogs (131). Thus. it seems that the
role of nerves in the postprandial intestinal hyperemia is
very minor at best.

Approximately 13 hormones and paracrine substances
have been identified in the gastrointestinal tract. many of
which are extremely vasoactive. A recent review by Chou
_t git (38) looked at the effects of gastrointestinal
hormones on intestinal blood flow. The authors concluded
that only the vasodilator effect of CCK in the duodenum and
jejunum and that of neurotensin in the ileum could be
considered physiological since only these two hormones
elicited vasodilatation at plasma concentrations observed
following a meal. It should also be noted that some
hormones such as VIP. substance-P. and enkephalin may act
as neurotransmitters or local tissue hormones exerting

paracrine effects rather than acting as circulating

ll

hormones. Currently. no evidence is available to verify
this possibility as a contributer to the postprandial
intestinal hyperemia.

Recently. histamine has been shown to be involved in
the postprandial intestinal hyperemia. Chou and Siregar
(39) have reported that histamine H-l receptor blockade
significantly attenuates both the vascular and metabolic
response to nutrient absorption in isolated canine jejunal
preparations. This effect appears to be due only to the H—
1 receptor since H-2 receptor blockade with metiamide has
no effect on the food-induced increases in intestinal blood
flow and oxygen uptake.

Prostaglandins have also been implicated in the
postprandial intestinal hyperemia. Gallavan and Chou (63)
have shown that inhibition of prostaglandin synthesis with
either mefenamic acid or indomethacin significantly
potentiates food-induced increases in both intestinal blood
flow and oxygen uptake in the canine jejunum.
Furthermore. changes in intestinal blood flow and oxygen
uptake were significantly correlated both before and after
prostaglandin synthesis inhibition. The same correlations
have also been observed both before and after histamine H—l
receptor blockade (39). The authors concluded that
endogenous prostaglandins serve to limit the postprandial
intestinal hyperemia. and this may be due to changes in

intestinal oxidative metabolism.

12

Based on the literature characterizing the
postprandial intestinal hyperemia and the possible
mechanisms involved. it seems clear that this physiological
phenomenon is extremely complex and the mechanisms are
multifactorial. An intricate network of physiological
feedback loops is beginning to emerge in which

prostaglandins appear to play a significant role.

Prostanoids

The prostaglandin story began in 1930 when Kurzrok and
Lieb (86) noticed that fresh human semen caused rhythmic
contractions and relaxations of human myometrlal strips.
In 1935 Goldblatt and VonEuler (49. 67. 135) confirmed
these observations and believed the active compound(s)
originated from the prostate gland and thus coined the term
"prostaglandin". In 1959. Eliasson (48) found that
VonEuler’s "prostaglandin" actually originated from the
seminal vesicles which still today remain a major
laboratory machine for the production of the cyclooxygenase
enzyme system. It wasn’t until 1960. however. that
Bergstrom and Sjovall (l3. l4) determined that
prostaglandin was a mixture of biologically active
compounds by finally isolating and crystalizing

prostaglandin E1 and prostaglandin F . Studies by

Ix
Samuelsson (11) and VanDorp (129) in l964 determined that
prostaglandins are derived from the transformation of fatty

acids. especially arachidonic acid which is predominantly

13

found in phospholipid components of animal cell membranes.
In 1973. Hamberg and Samuelsson (73) discovered two labile
arachidonic acid metabolites (prostaglandin G2 and
prostaglandin H2) that could be transformed into other
prostaglandins. One of these cyclic endoperoxide
metabolites was described in 1975 by Hamberg gt 21;. (74)
and was called thromboxane A2. a potent vasoconstrictor and
aggregator of platelets (74). In 1976. Vane g: EL; (130)
isolated prostaglandin I2 (prostacyclin). a
prostaglandin formed mainly by endothelial cells and
capable of preventing platelet aggregation as well as
producing vasodilatation.

Prostaglandins and prostanoids are derived from
essential fatty acids. The most common prostaglandin. the
bisenoic (prostaglandin E2. prostaglandin F2“ .
prostaglandin 02' prostaglandin 12 and TXAZ) are derived
from arachidonic acid or from linoleic acid after
elongation of the carbon chain to 20 units. The monoenoic
prostalandins (prostaglandin E1' etc) are derived from
dihomo-x-linolenic acid and the trienoic prostaglandins
(prostaglandin E3. etc.) are derived from 5. 8. ll. l4. l7
— eicosapentaenoic acid (130). The production of the most
common bisenoic prostaglandins from arachidonic acid are
depicted in Figure 1.

Thromboxanes and most prostaglandins that reach the
systemic circulation are rapidly inactivated in the lungs

(10. 56. 137) and gastrointestinal prostaglandins that

 

14

Figure l. Biosynthetic pathway of the major bisenoic
prostaglandins and thromboxanes.

(I

I

”1‘

pl!

It!

15
Fig. 1
r} F

<§

Z»

an
Q?

.777

Ali “I

[le

‘1

(IA

1%“

DISRUPTED

,9?

M;
1256131

1va

QM)

.465

If”?

¢ § - - - - — mass
thromooune synthetase

fl

l6

reach the hepatic portal circulation can be inactivated by
the liver (56). Prostaglandin 12 appears not to be
metabolized by the lungs (4. 76) but can be converted to
biologically inactive metabolites by the liver and kidneys
(127. 138).

Prostaglandin synthesis occurs throughout the small
intestine (l. 15. 25. 55. 85. 91. 92. 93. 126) and in both
muscle and epithelial layers (93). In 1979 LeDuc and
Needleman (93) characterized prostaglandin production in
the canine stomach and intestinal tract. In the gastric
mucosa. the major arachidonic acid metabolites were
prostaglandin E2. prostaglandin I2 and TXAZ while the
gastric muscularis also produced prostaglandin F2°< . 1n
the small intestine. the mucosa contained relatively low
cyclooxygenase activity but was found to be rich in both
prostacyclin and thromboxane synthetase. This was
determined by incubating mucosal microsomes with
prostaglandin H2 which resulted in substantial production
of both prostaglandin 12 and TXAZ while arachidonic acid
produced relatively low levels of these two prostaglandins.
The major prostaglandins produced by the intestinal
muscularis were found to be prostaglandin 12. prostaglandin
E2. and prostaglandin F2“ . Finally. the colonic
muscularis produces prostaglandin I2 and prostaglandin E2
while the mucosa produced large quantities of TXAZ and some

prostaglandin 12. Prostaglandin 02 was also produced by

all tissue preparations but in minute quantities.

17

Lg, 31x9 studies indicated that prostaglandin E and
prostaglandin F are released from the "resting" intestinal
tract (134) and in response to various stimuli such as
acetylcholine (119). mechanical stimulation (15). luminal
distention (140). and feeding (45). Dupont gt El; (45)
found that concentrations of prostaglandin E1’
prostaglandin E2. and prostaglandin F2“ increase in the

gastrointestinal tract of miniature swine following
ingestion of a liquid nutrient meal. Furthermore.
prostaglandin concentrations in various anatomical
locations of the gastrointestinal tract correlated

strongly with the time required for the movement of
ingestia through the respective regions of the
gastrointestinal tract. For example. gastric prostaglandin
production increased 30 minutes after feeding and fell 60
minutes after feeding while jejunal prostaglandin
production remained unchanged for sixty minutes but
subsequently rose thereafter. The authors concluded that
feeding stimulates gastrointestinal prostaglandin
production and this may be a local phenomenon.

The various effects of prostaglandins on intestinal
motor activity has been extensively characterized.

especially for prostaglandins E and F. Sanders gt, a1.

determined prostaglandin synthesizing capabilities by
microsomes of both circular and longitudinal
gastrointestinal muscle (118). Prostaglandin 12,

prostaglandin F2“ . prostaglandin E2 . prostaglandin D2.

18

prostaglandin A2. and thromboxane A2 levels were determined
by thin layer chromatography following incubation of canine
microsomal preparations with radiolabeled arachidonic acid.

The major prostaglandins formed were prostaglandin I

2'
prostaglandin E2. and prostaglandin F2“ . Furthermore.
all areas of the GI tract except the jejunum produced

prostaglandin levels in both longitudinal and circular
muscle layers that were the same. in the jejunum.
prostaglandin production was greater in the longitudinal
muscle. All areas of the gastrointestinal tract also
produced low levels of prostaglandin D2 and prostaglandin
A2 in both muscle layers. The results of this study
reinforce earlier works by LeDuc and Needleman (93).

Lg XLEEQ studies by Bergstrom (12) indicate
prostaglandins E and F causes contractions of jejunal
segments in the rat. chicken. rabbit and guinea pig.
Furthermore. prostaglandin F2“ has been shown to contract
rabbit and rat duodenal segments (2). stimulate circular
muscle contractions in the canine duodenum and ileum (42).
and stimulate intestinal motility in the intact canine
jejunum (121).

Prostaglandin E1’ and E2 contract human longitudinal
muscle strips but relax or inhibit acetylcholine-induced
contractions in the same preparation (9, 10). Furthermore.

in whole muscle preparations. prostaglandin E either

1!
contracts (79) or relaxes (83) the rat duodenum. It

appears that two of the major prostalgandins produced by

‘1

i

1

19

intestinal muscle (prostaglandin E and prostaglandin F)
are strong modulators of intestinal motor activity.
Prostaglandin F stimulates circular muscle. longitudinal
muscle. and whole muscle preparations while prostaglandin E
stimulates or inhibits motor activity depending on the
muscle layer. species. and investigator.

Sanders (117) found that inhibition of endogenous
prostaglandin production with indomethacin enhanced the
amplitude of spontaneous and acetylcholine-induced
contractions of isolated canine intestinal muscle. Other
studies have also demonstrated this effect (119).
Furthermore. the enhanced mechanical activity was
associated with enhanced electrical slow wave amplitude
and generation of action potentials. The increase in
mechanical activity caused by indomethacin was also
accompanied by a decrease in prostaglandin 12 production.

the major prostaglandin produced by intestinal smooth

muscle (93. 118). The author concluded that
prostaglandin 12 inhibits both spontaneous and
acetylcholine-induced contractions of intestinal smooth

muscle at physiological production levels and this may be
due to an electrical mechanism.

Prostaglandin synthesis inhibition with indomethacin
has also been shown to significantly enhance the inhibitory
effects of either sympathetic nerve stimulation or
exogenous norepinephrine on ileal motility in the guinea

pig (7). Furthermore. this effect is reversed by giving

20

prostaglandins E1. and E2 (7. 120): that is. both
prostaglandins inhibit the effects of sympathetic
stimulation. In addition to the inhibition of the
inhibitory effects of sympathetic stimulation on motility.
prostaglandin E has also been shown to inhibit both
spontaneous and acetylcholine-induced contractions of
canine circular muscle (116) and acetylcholine-induced
contraction of human longitudinal muscle strips (9. 10).
Sanders (116) also reported that indomethacin
increased the magnitude of either acetylcholine or
potassium-induced canine ileal contractions. However.
Bartho (7) reported that indomethacin inhibited
cholinergically initiated ileal contractions in whole
guinea pig ileal preparations. It is not clear why this
discrepancy exists. however. Sanders primarily used only
circular muscle preparations whereas other investigators
use whole muscle preparations. It seems that circular
muscle from most species reacts differently to some
prostaglandins. Undoubtedly. the data regarding the
effects of various prostaglandins or prostaglandin
synthesis inhibitors on intestinal motor activity are
confusing. contradictory. and arcane. These discrepancies
may be due to differences in the prostaglandins used (for
example Type 1 vs. Type II or PGE vs. PGF). specific areas

of the small intestine. species. or methodology.

21

in addition to the effects of prostaglandins on
autonomic modulations of intestinal motor activity.
prostaglandins have also been shown to inhibit serotonin
(50) and cholecystokinin (142) induced contractions of
isolated guinea pig ileal smooth muscle. It seems that
prostaglandins are capable of regulating intestinal
motility via either direct actions on intestinal smooth
muscle or indirectly by altering neural and humoral inputs
to the gut.

Prostaglandins have also been shown to alter
gastrointestinal absorption and secretion (5. 29. 60. 70.
84. 100. 101. 102. 104. 111. 112). Prostaglandin E2. has been
shown to increase water. Na+ . Cl“ . and HCO3- secretion
in the human jejunum (29. 100. 104. 112) and to decrease
glucose absorption in the human (101. 102) and dog jejunum
(84). Granger gtt g1 (70) studied the effects of
prostaglandin E. on water absorption. lymph flow and
transcapillary protein flux in the cat ileum. They found
that Prostaglandin E. increased secretion in the ileum as
well as lymph flow and protein transport across the
capillary endothelial lining. Their studies also indicate
gross damage to the intestinal microvilli membrane

following prostaglandin EI administration.

22

Prostaglandin F2“ has also been shown to increase
water and protein secretion (111) and to decrease glucose
absorption (5) in the canine jejunum. The previous study
(5) also showed decreases in glucose absorption by
prostaglandin E2.

Endogenous prostaglandin synthesis has also been
implicated in the control of gastric secretion.
Arachidonic acid. the precursor of type II prostaglandins.
has been shown to inhibit both pentagastrin and histamine
stimulated HCl secretion in the rat stomach (60).
Furthermore. these effects were blocked by indomethacin
indicating that endogenous prostaglandins were involved.
Prostaglandin E1 is most likely involved since it has been
shown to be a powerful inhibitor of gastric acid secretion
(95).

These studies indicate that intestinal prostaglandins
are capable of inhibiting intestinal glucose absorption and
stimulating secretion. The biological diversity of
prostaglandins is exemplified by their ability to inhibit
secretions in the stomach (60. 95) while stimulating
secretions in the small intestine (29. 70. 100. 104. 112.
111).

Prostaglandins are powerful vasoactive agents in the
intestinal vascular bed. The role of prostaglandins in
regulating resting intestinal blood flow appears to favor
the vasodilators since prostaglandin synthesis inhibitors

decrease intestinal blood flow in rabbits (18). cats (119).

23

humans (107) and dogs (33. 63. 65). It is interesting to
note. however. that indomethacin inhibits the contractile
effects of norepinephrine on rat mesenteric artery strips
(96).

Individual prostaglandins have variable effects on
intestinal blood flow both in direction and magnitude.
Chapnick §£.§l; (33) determined the effects of a variety of
prostaglandins on intestinal blood flow in the dog. Over
the dose range used. prostaglandin 02 caused dose dependant
decreases in blood flow through the superior mesenteric
artery. Decreases in blood flow ranged from 24-64% and
were rapid in onset and short in duration. In addition.
prostaglandin F2“ also produced dose dependant decreases in
intestinal blood flow ranging from 30-80%. These effects
were also short in duration and rapid in onset.
Prostaglandin 12 increased intestinal blood flow in a dose
dependant fashion which ranged from 33-1331. The stable
metabolite of prostaglandin 12. 6—keto-prostaglandin Flu .
had no significant affect on blood flow thus indicating
that the effects of prostaglandin 12 were not the result of
metabolites. Prostaglandin E2 was also a powerful
vasodilator increasing intestinal blood flow 43-1121.

Dusting gt gig (47) also studied the effects of
various prostaglandins on mesenteric perfusion pressure in
constant flow preparations in the dog. They found

prostaglandin 12. prostaglandin E2. and the cyclic

endoperoxide prostaglandin Hz were vasodilators and TXA2

5
hi
I
.
v
1.

174

24

was a powerful vasoconstrictor. similar in potency to
norepinephrine. They also found prostaglandin F20< to be a
very weak vasoconstrictor which is different from that
observed by other investigators (5. 33. 51. 110) who
generally found prostaglandin F2“_ to be a moderate or
powerful vasoconstrictor. Prostaglandin FZN» has also been
shown to decrease intestinal oxygen consumption in the dog
(110) and this may be a possible mechanism of the most
commonly observed vascular effects (5. 33. 51. 110).
Prostaglandin D2 appears to exert variable effects on
the intestinal circulation. Dose dependant decreases in
intestinal blood flow have been observed when
prostaglandin D2 is given by single bolus injections into
the superior mesenteric artery of the canine (33).
Constant infusions. however. have been shown to produce
biphasic responses (58). Blood flow initially decreased.
returned to control. and then rose substantially above
preinfusion values. Furthermore. Fondacaro gt git (58)
found that during the hyperemic phase of prostaglandin 02
administration. blood flow was redistriubted away from the
mucosa and toward the muscularis. intestinal oxygen uptake
was also reported to increase during prostaglandin D

2
infusions (58).

Prostaglandin I2 is a vasodilator in the small
intestine (33. 47. 58. 110). its stable metabolite. 6-
keto-prostaglandin Flu . is not vasoactive when bolus

injections are given into the superior mesenteric artery

25

(33). It has also been shown that the hyperemia caused by

prostaglandin I2 is directed mainly to the mucosal bed (58.

110). Furthermore. prostaglandin 12 has been shown to
increase intestinal oxygen uptake (58. 110).

Prostaglandins. therefore. appear to be powerful
modulators of intestinal blood flow. It is not clear.
however. if such observed vascular effects are
physiological or merely pharmacological. Furthermore.

prostaglandins may exert their vascular effects by direct
actions on vascular smooth muscle. indirect actions such as
metabolism or influences on the release of vasoactive
substances. or both.

Recent evidence suggests that prostaglandins may act
via their own receptors (54. 75. 97. 103). Greenberg gt
git (71) have attempted to characterize the nature of
prostaglandin receptors in both rabbit and canine vascular
smooth muscle. The contractile responses of vascular
smooth muscle strips to various prostaglandins was
attenuated or abolished when dithiothreitol was added.
Dithiothreitol is a disulfide-reducing agent. Since this
effect was specific only for prostaglandin-induced muscle
contraction. the authors concludedd that a disulfide-
protein complex was a necessary component for the
expression of the vascular effects of prostaglandins in

vascular smooth muscle.

*1

26

Arachidonic Acid

There is little or no free arachidonic acid in cells.
therefore. synthesis of prostaglandins and thromboxanes
require arachidonate release which is mainly esterified to
cell phospholipids (96). The major phospholipids involved
in arachidonate release by phospholipases appear to be
phosphatidylcholine (18) and phosphatidylinositol (7. 27).
Furthermore. natural release or addition of exogenous
arachidonic acid to cells with prostaglandin synthesis
enzymes results in rapid production of prostaglandins (3.
111. 133).

Arachidonic acid could become available for
prostaglandin synthesis in the small intestine either by
”natural" release from membrane bound phospholipid stores
(8. I9. 28. 99) or by absorption from the intestinal lumen
(40). Chow and Hollander (40) looked at the mechanisms and
factors influencing luminal absorption of arachidonic acid
in rat intestinal preparations. This study indicates that
arachidonic acid absorption is concentration dependant and
does not require metabolic energy since metabolic
uncouplers have no effect on absorption rates.
Furthermore. absorption was enhanced by disrupting the
unstirred water layer. by decreasing the luminal pH and by
substituting bile salts for Tween 80. Arachidonic acid
absorption decreased when oleic or linoleic acid was added
indicating competition with other fatty acids.

Finally. arachidonate absorption was not affected by the

27

addition of amino acids or sugars to the luminal contents.

The authors concluded that arachidonic acid was readily

absorbed from the small intestine in a dose dependant
manner. They also concluded that absorption was via
carrier mediated diffusion since Michaelis-Menton

saturation Kinetics were observed in the presence of
metabolic uncouplers. and various alterations in the
luminal environment can alter arachidonic acid absorption.
Arachidonic acid causes a large increase in blood flow
through the canine superior mesenteric artery when large
bolus injections are given (33). Furthermore. indomethacin
abolished this hyperemia. Dusting gt git (47) have
observed the same effects of arachidonic acid on perfusion
pressure in constant flow preparations in the dog intestine
and hindlimb preparation. Dusting gt Q1; (46) also found
that arachidonic acid relaxed vascular smooth muscle strips
from rabbit and rat mesenteric arteries using the blood
bathed organ technique. In addition. they also reported
that the same dosages of arachidonic acid contracted these
preparations when the prostaglandin substrate was allowed
to incubate with blood for 0.1-4 minutes. They attributed
the latter response to preferential formation of platelet
thromboxanes. Similar results were reported by Kauffman
and Whittle (82) using a constant flow gastric preparation
in the dog. Bolus injections of arachidonic acid (25-200
mg) into a proximal port (3 seconds transit time) produced

a dose dependant decrease in perfusion pressure. The same

28

dosages. however. given into a distal port (30 seconds
transit time) produced dose-dependant increases in
perfusion pressure. These studies indicate that
arachidonic acid is a vasodilator in the intestinal and
gastric vascular beds. These effects are probably due to
prostaglandin production and not due to the substrate ggt
gg since arachidonic acid-induced vasodilatation can be
abolished by indomethacin. Finally. extraneous
prostaglandin production by non—intestinal tissues (e.g.
platelets) during arachidonic acid administration is

capable of altering these effects.

Thromboxane Synthesis and Inhibition

In 1975. Hamberg gt_gtt (74) first described the
conversion of prostaglandin G2 to an unstable prostanoid
capable of contracting vascular smooth muscle and
aggregating platelets. This prostanoid was originally
called rabbit aorta contracting substance (74) but today
is better known as thromboxane A2.

Le Duc and Needleman (93) found that the canine
jejunum is capable of producing thromboxane A2 and only
the mucosal layer of the gut wall has the necessary enzyme
for thromboxane production. They also found that the
jejunal mucosa produces prostaglandin I2 and imidazole

inhibited thromboxane production but did not affect

29

prostaglandin 12 production. These investigators
determined thromboxane levels in intestinal tissue
incubation media by radioimmunoassay.

Moncada gt gt; (105) examined the effects of imidazole
and 21 imidazole derivatives on thromboxane synthesis
inhibition in platelets. They found that the concentration
of imidazole required to produce 50% inhibition of
thromboxane production by platelets (icgo) was 22 ug/ml.
All of the imidazole derivatives tested required at least 8
times that concentration to produce 50% inhibition. The
authors concluded that the inhibition of thromboxane
synthesis by imidazole was more powerful than imidazole
derivatives. Furthermore. imidazole selectively inhibited
thromboxane synthesis since synthesis of other
cyclooxygenase and lipoxygenase products were unaffected.

Another potent and highly selective inhibitor of
thromboxane synthesis is Sodium — 5 — (3'-Pyridinylmethyl)
Benzofuran - 2 - Carboxylate (U—63557A) recently developed
by the Upjohn Company (139). Wynalda gt_glt (139) examined
the effects of U-63557A on thromboxane synthesis inhibition
as well as its pharmacokinetics in dogs. Single doses of

l i.v. resulted in a 901

U-63557A given at 5 mgckg-
inhibition of plasma thromboxane levels that lasted for

over two hours.

30

Summary

The postprandial intestinal hyperemia is a complex
physiological phenomenon and many factors are involved.
Such factors include gastrointestinal hormones. the
products of food digestion. histamine. and possibly
endogenous prostaglandins.

Prostaglandins are capable of modulating many
intestinal functions such as absorption. secretion. motor
activity. metabolism. and blood flow. Given that
prostaglandins are capable of such alterations in the
physiology of the intestine and given that prostaglandin
synthesis inhibition has significant effects on the food—
induced increases in intestinal blood flow and oxygen
uptake (63). it seems likely that endogenous intestinal
prostaglandins play a significant role in the postprandial
intestinal hyperemia. Therefore. this possibility warrants
further investigation. The purpose of this study was to
determine the effects of the prostaglandin precursor.
arachidonic acid. on the postprandial intestinal hyperemia.
In addition. the effects of selective inhibition of
thrombane synthesis on the postprandial intestinal

hyperemia was also examined.

METHODS AND MATERIALS

Surgjcal Preparation

All experiments were conducted on mongrel dogs (14-31
kg) of both sexes which were deprived of food for 24 hours.
The animals were anesthetized by intravenous injections of
pentobarbital sodium at an initial dosage of 30 mg/kg and
supplemented as needed to maintain a surgical plane of
anesthesia. All animals were ventillated with a positive-
pressure respirator (Harvard Apparatus. Millis. MA) that
was adjusted to achieve normal arterial blood pH (7.39 i
0.02). 02 tension (87.9 16.6). and CO2 tension (37.5 I
2.4) before each experiment. Systemic arterial blood
pressure was continuously monitored through a cannula in
the femoral artery.

The abdominal cavity was exposed via a midline
incision and either one or two segments of the jejunum
about 25 cm aboral to the ligament of Treitz were
exteriorized. Care was taken to ensure that the autonomic
nerves were intact. After a rubber tube was placed into
the lumen of each segment for placement and withdrawal of
solutions. both ends of the segment(s) were tied and cut
away from the adjacent jejunum to exclude collateral flow.
Luminal pressure was recorded by connecting the rubber tube
to a pressure transducer (Statham PZBGb) when a solution
was present in the lumen. Before cannulation of blood

I

vessels. heparin sodium (500 Ui-kg— ) was administered

31

32

intravenously. The exteriorized jejunal segments were kept
moist with saline soaked gauze and a clear plastic sheet
and maintained at 370 C with a heat lamp and temperature
probe. At the end of all experiments. the animals were
euthanized by an overdose of pentobarbital sodium. Figure
2 schematically represents the methodological and surgical
interventions in most of these experiments. Details of

individual series of experiments are described later.

Preparation of Solutions

The food solution used contained equal parts by weight
of fat. carbohydrate. and protein. It was prepared by
adding 30 g high-fat test diet. 15 g high-protein test
diet. and 5 g high-carbohydrate test diet (US Biochemical
Cleveland. OH) to 400 ml of 0.1 N NaHCO3 containing 750 mg
of a pancreatic enzyme preparation (Viokase. VioBin.
Monticello. IL). The mixture was then mixed gently with a
magnetic stirrer at room temperature for 5 h to permit
digestion. Before the experiment. nine parts of digested
food were mixed with one part of gallbladder bile. and the
pH and osmolality of the mixture were adjusted with either
NaHCO3 or HCl and NaCl. respectively [pH 7.1 1 0.1:
osmolality 316 i 5 mosmol/kg (n = 35)]. Both the digested
food plus bile and the normal saline were kept at 370 C

during the experiment.

33

Figure 2. Schematic representation of the preparation used
in most experiments.

 

3‘1

Pig. 2

 

 

 

 

 

 

 

 

torce
transducer
1
- -1. ,,
F . VA 7
/ . - Intestinal
cannulg [ segment .

 

 

 

 

é ' Plattorrn

 

 

 

 

 

 

 

I-u‘Intuslon I collateral A

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Z‘
2 .5
B 3'
tlowmeter
Pressure I
transducer
_ , j
rteriaT soul le A-VOZ sample
analyzer
0
a outflows
:
a
reservoir

 

 

 

to tern

 

l vein

‘7

35

Arachidonic acid. sodium salt (Sigma Chemical Co.. St.
Louis. MO) was prepared immediately before use. Solutions
were prepared by dissolving appropriate quantities in a
0.9% NaCl solution and subjecting the solution to sonic
disruption to aid the dissolution process. Mefenamic acid
(Park Davis. Detroit. MI) was prepared by adding
appropriate quantities to a 0.9% NaCl solution and the pH
was raised with 1.0 M NaOH until the drug went into
solution. (pH 8.4 t 0.2. n = 10). Imidazole (Sigma
Chemical Co.. St. Louis. MO) and U—63557A (Sodium 5- (3’-
Pyridinylmethyl) Benzofuran - 2 - Carboxylate. a gift
from the Upjohn Co.. Kalamazoo. M1) were prepared by

dissolving appropriate amounts in 0.9% NaCl solutions.

Experimental Protocol

Eleven series of experiments were performed (n = 89).
In Series I (n = 14). the effects of various doses of
arachidonic acid on jejunal vascular resistance were
determined in constant flow preparations before and after
mefenamic acid administration (a cyclooxygenase inhibitor).
A single jejunal segment was exteriorized and the single
artery perfusing the segment was cannulated. The segment
was perfused at a constant rate with the aortic blood by
interposing a Masterflex pump (Cole-Parmer Instruments.
Chicago. IL) between the local intestinal artery and a
femoral artery. The perfusion pressure was monitored via a

catheter in the perfusion line attached to a pressure

36

transducer (Statham P236b). Pump flow rate was adjusted
until perfusion pressure was 10 mmHg below the systemic
arterial pressure and then maintained constant throughout
the experiment. Once a steady state had been reached.
arachidonic acid (AA). sodium salt (Sigma Chemical. St.
Louis. MO). or its carrier solution. 0.9% NaCl. was infused
upstream from the pump with a Harvard infusion pump set at
various infusion rates ranging from 1.0 to 267 pg. min-l.
The AA concentration of the infusate ranged from 0.5 to 2.3
mM. and the volume rate of infusion never exceeded
0.75 mlomin_l. The transit time for AA between the site of
infusion and the gut loop was less than 5 3. Each
infusion lasted until perfusion pressure reached a steady
level. At the end of each experiment. the rate of pump
blood flow was measured. and local blood concentration of
AA achieved at each infusion rate was calculated from the
pump blood flow rate and infusion rate.

in seven dogs in this series. cyclooxygenase
inhibitors were also used to determine whether the effects
of AA were due to AA 2g; gg or its metabolites. i.e..
prostaglandins. The animals were pretreated 48 hours
before the experiment with acetylsalicilic acid (oral. 25
mg/kg. Sigma Chemical. St. Louis. MO) and the vascular
response to AA was tested by infusing solutions of various
concentrations of arachidonic acid into the arterial inflow

line as previously described. Acetylsalicylic acid was

used to preferentially inhibit cyclooxygenase in blood

37

tissues. A Swan-Gantz catheter was then advanced into the
pulmonary artery via a femoral vein. A single i.v. bolus
injection of arachidonic acid (300 ug/kg) was given and the
resulting drop in aortic pressure was noted. Mefenamic
acid was then infused intravenously at a dosage of 10
mg/kg for 10 min. Two hours later. another i.v. bolus
injection of arachidonic acid (300 ug/kg) was given. If
aortic pressure was unaffected by the arachidonic acid
injection. then the vascular response to arachidonic acid
was tested as described above. Figure 3 represents ~the
effects of intra—pulmonary injections of arachidonic acid
on aortic pressure before and two hours after mefenamic
acid administration.

In Series II (n = 7). the effect of i.v. infusions of
arachidonic acid (40 pgokg—l- min—l) on the postprandial
jejunal hyperemia was examined. The single vein draining
the single segment preparation was cannulated for
measurement of venous outflow by timed collection with a
stopwatch and graduated cylinder. Blood flow was also
continuously monitored by an extracorporeal flow transducer
(BL 2048-E04. Biotronex Laboratories. Silver Spring. MD)
placed in the venous outflow line and connected to an
electromagnetic flowmeter (BL610. Biotromex Laboratories).
The venous outflow was directed into a reservoir held at
37° c. and the blood returned to the animal via a catheter
in the femoral vein at a rate equal to the venous outflow.

Ten milliliters of normal saline was placed into the lumen

38

Figure 3. Polygraph chart recording showing_the effect of
a single bolus of arachidonic acid (300 ug.kg ) given into

the pulmonary artery on aortic pressure. This was done
before agd 120 min after i.v. infusion of mefenamic acid
(10 mg~kg ).

.............

AA

 

mefenamate 120 ml

.....

AA

1- 1-
(6” mm) ornssord aguoo

40

for 15 min. This procedure was repeated 4—5 times until
blood flow reached a steady state. at which time the
luminal contents were changed to digested food. The change
in blood flow due to food was determined as the difference
in steady-state flows during luminal placement of normal
saline and digested food. After determining the
response to food. AA was infused intravenously at
a rate of 40 ug-kg-l-min-l with a Harvard dual-infusion
pump (Harvard Apparatus). When blood flow reached a steady
state. the response due to food was again measured as
described above during the AA infusion. The infusion of AA
lasted 30 min and the total dose of AA given was 1200 ug
kg-l. The carrier solution. 0.9% NaCl. was infused
intravenously at 0.76 ml/min as a control. and the response
due to food was measured during the infusion.

In Series 111 (n = 8). the effect of i.a. infusions of
arachidonic acid (4-8 ug~m1 blood-l) on the postprandial
jejunal hyperemia was examined. The same protocol was used
as described in Series II. but AA was infused into a side
branch of the single artery of the segment at 0.2 ml/min.
The carrier solution infused at the same rate served as a
control. The vascular response to luminal placement of
digested food was determined before and after AA infusion
as in Series 11.

In Series IV (n = 12). the effects of luminal
placement of arachidonic acid on the food-induced increases

in jejunal blood flow and oxygen uptake were determined.

41

Two jejunal segments were isolated. the vein draining each
segment was cannulated. and the venous outflows were
measured as described above. When blood flows reached a
steady state after repeated placements of normal saline to
both segments. the vascular response to food was tested in
both segments. The protocol for this series of experiments
is shown in Fig. 4. At different 15-min periods. the lumen
of each segment contained normal saline. normal saline plus
AA. food. and food plus AA. The concentration of AA in the

mixtures was 6.5 x 10-4

M (200 ug/ml). All solutions were
subjected to sonic disruption for at least 5 min to ensure
homogenous dissolution before placement into the jejunal
lumen. Before placement of the next solution. the lumens
were thoroughly and gently rinsed with normal saline and
flows to both segments were allowed to attain a steady
state. In five dogs. the arterial and venous outflow blood
samples were obtained for the measurement of blood 02
content by a Lex-OZ-Con-TL 02 content analyzer (Lexington
Instruments. Waltham. MO). Jejunal 02 uptake was
calculated as the product of venous outflow and
arteriovenous 02 content difference.

Series V (n = 3) was conducted to determine if changes
in jejunal blood flow by mechanical means would change
jejunal oxygen uptake. The purpose was to determine
whether changes in 02 uptake observed in other experiments

are blood flow dependent. A loop of the jejunum was

exteriorized as above. The single artery supplying the

42

Figure 4. Mean venous outflows from 2 adjacent jejunal
segments containing normal (NS). NS plus arachidonic acid
(AA). food. or food plus AA. Values shown below or above 2
adjacent bars indicate percent differences in flow between
2 bars. * P < 0.05 relative to preceding value. ** P <
0.05 between 2 values denoted by dotted line. n = 12.

‘13

Fig. 4

=00..— mz w:
v. . . {an «.8

i i
_. a e.
ado ado

a :35
oofimdo ....... so ..... powwow
s... a”... x a
m an v.3 ....... s s iiiiiii 3H O we
<.< <.<
_..u m:
Qua 0.8

< :38on

00

M01! (10018

‘-

|‘_111300|~llllll-|lil|

44

segment was cannulated. and a perfusion circuit with an
interposed pump (Masterflex) was established between a
femoral artery and the single segmental artery. The
circuit was designed so that the jejunal segment could be
alternately perfused by the pump set at various flow rates
or naturally by systemic arterial pressure. Under natural
perfusion conditions. blood flow was altered by adjusting a
variable clamp on the inflow line. Perfusion pressure was
monitored through a cannula inserted into the arterial
circuit. The single vein draining the segment was
cannulated. Arteriovenous 02 content difference was
determined continuously by perfusing femoral arterial blood
and a portion of the venous outflow at 6 ml/min through
separate cuvettes of an arteriovenous 02 content difference
analyzer (A—VOX Systems. San Antonio. TX) with a Gilson
pump (Minipuls 2. Gilson Medical Electronics. Middleton.
WI). The arteriovenous 02 content difference analyzer had
been calibrated with a Lex-OZ-Con TL 02 content analyzer
(Lexington Instruments. Waltham. M0). The venous outflow
and outflows from the cuvettes were allowed to drain into a
reservoir. The blood in the reservoir was pumped back to
the animal via a femoral vein at a rate equal to the total
outflows. Venous outflow was measured by timed collection
of the outflow. Intestinal 02 uptake was calculated as the
product of venous outflow and arteriovenous 02 difference,

The time required for venous blood to reach the

arteriovenous 02 analyzer was 35 s and was compensated for

45

by the CSICUIBtIOD Of 02 uptake. After all measured
variables had stabilized for 20-30 min. the effects of
reductions in blood flow under natural flow conditions on
02 uptake were determined by stepwise increases in arterial
inflow occlusion. The perfusion circuit was then switched
to pump perfusion. and the effects of pump—induced
alterations in blood flow on 02 uptake were determined.
The effects of changes in flow on 02 uptake were studied
twice under natural and pump perfusion in each dog. After
each experiment. the jejunal segments were weighed and all
data were normalized to 100 g of tissue.

In Series VI (n = 7). the effects of i.a. infusions of

arachidonic acid (4-8 ugoml blood-l

) on the food-induced
increases in jejunal blood flow and oxygen uptake were
determined after administration of a cyclooxygenase
inhibitor. Mefenamic acid was infused intravenously over a
10 min period at 10 mg-kg—l. Two hours after the end of
the infusion. the cyclooxygenase block was tested as
previously described in Series I (Figure 3). The effects
of i.a. infusions of arachidonic acid on the food—induced
increases in jejunal blood flow and oxygen uptake were then
determined. Jejunal 02 uptake was determined as previously
described in Series V.

In Series VII (n = 14). the effects of imidazole
(Sigma Chemical Co.. St. Louis. M0) on the food-induced

increases in jejunal blood flow and oxygen uptake were

determined. Imidazole was infused intra-arterially at a

46

rate of 5 mg/min designed to achieve a blood concentration
of approximately 100 ug/ml. which has been shown to
effectively block thromboxane production (45). The food-
induced increases in jejunal blood flow and oxygen uptake
were determined before and during imidazole infusions. The
imidazole infusion lasted for 30 min. and the total dose of
imidazole was 150 mg.

In Series VIII (n = 5). another inhibitor of
thromboxane synthesis was used. The protocol was identical
to Series VII except the selective thromboxane synthetase
inhibitor U-63557A (Upjohn. Kalamazoo. MI) was used in
place of imidazole. U-63557A was given intravenously at a
dosage of 5.0 mg/kg.

In Series IX. the effects of imidazole and U-63557A on
glucose absorption were determined in seven and five dogs.
respectively. A jejunal segment was isolated. the vein
draining the segment was cannulated. and the venous
outflows were measured as described above. Arterial and
venous blood samples were obtained before and after luminal
placement of food for determination of plasma glucose
concentration with a glucose oxidase glucose analyzer
(Yellow Springs Instrument Company. Yellow Springs. OH).
This was repeated after i.a. infusions of imidazole
(5 mg. min“) and after i.v. infusions of U-63557A
(5 mg-kg"). Glucose absorption was calculated as the
product of blood flow and the venous-arterial glucose

content difference and expressed as mg - min’l- IOOg-l.

47

Glucose absorption during food placement was expressed as
the difference in glucose absorption before and during
luminal placement of digested food.

In Series X. the capillary filtration coefficient
(CFC) was determined gravimetrically in 6 dogs. A jejunal
segment was isolated as before and placed on a platform
vertically suspended from a force displacement transducer
(FT 038. Grass Instruments). The venous outflow was
continuously monitored until a steady state was achieved.
CFC was determined after a sudden elevation of venous
pressure (to approximately 10 mm Hg lasting approximately
10-15 seconds) which precipitated a change in intestinal
weight. The weight gain due to venous pressure elevation
was characterized by two distinct components: an initial
rapid increase due to venous capacitance filling and a more
prolonged phase of weight gain assumed to represent
capillary filtration. The point of demarcation between
these two phases was obtained by observing where venous
flow equilibration occurs. Venous outflow falls initially
upon raising venous pressure but subsequently attains a
plateau lower than the control value. This flow
equilibration point represents the end of the blood volume
shift. and measuring the rate of intestinal weight gain
from this point provides an estimate of capillary
filtration rate (Figure 5). The slope of the filtration

component of the jejunal weight gain was divided by the

48

Figure 5. Original polygraph chart recording of the data
used to estimate jejunal capillary filtration coefficient
(CFC). Solid line (intestinal weight panel) represents the
rate of tissue water accumulation following the blood
volume shift used in estimating CFC.

49
Pig. 5

2:32.. 2.2.2,

25: :83

ea;
eases

L—
O

I
10
6L! Ill 11/

 

E

 

 

50

change in capillary pressure. In all experiments. the
increase in venous pressure had no affect on the intestinal
fluid volume.

In estimating capillary pressure. we assumed a 70%
transmission of the imposed venous pressure change to the
exchange vessles. To validate this assumption. four
additional experiments were performed in which capillary
pressures were measured during a wide range of venous
pressure changes. Capillary pressures were estimated using
the venous occlusion technique as previously described by
Granger gt Q1; (68). As can be seen in Figure 6. the
percent transmission of the imposed venous pressure change
to the capillaries is tightly adherent to the line assuming
70 percent transmission. This is especially evident
between venous pressure changes of 10 and 15 mm Hg. which
is the range in which our experiments were conducted.

Jejunal capillary filtration coefficients were
determined both before and during intra-arterial imidazole
(5 mg/min) infusions with either saline or digested food in
the jejunal lumen. Furthermore. multiple CFC readings were
obtained throughout the food placement period (16 min) both
before and during imidazole infusions in order to profile
time dependent changes in jejunal CFC during nutrient

absorption.

51

Figure 6. Correlation between changes in venous pressure
and changes in capillary pressure in isolated jejunal
segments. Dotted line represents the observed regression

line of the data. solid line represents 70% transmission of
the imposed venous pressure elevations to the exchange
vessels.

52

Fig. 6

mm

3: .55 £335... moozm> 4

ON
ee\
eK
\

\
e\
\
\\ mg
VN-Xm

m—

OP

m

or

mp

ON

in
N

(511 mm) aanssaad Auvnidva V

53

In Series XI (n = 6). the jejunal CFC was determined
before and after food placement as previously described in
Series X. Then mefenamic acid (10 mg/kg IV) was given. and

after two hours. the above procedure was repeated.

Treatment of Data

All blood flow. oxygen uptake. and glucose absorption
data were normalized to 1009 tissue weight. Some data were
expressed as percent change from control or absolute change
between treated and control groups. Most of the data were
expressed as the mean : S.E.M.

The statistical procedures used in analyzing the data
were the Students' t test modified for comparison of two
sample means. analysis of variance. linear-regression
analysis. the ordinary least-squares technique. polynomial
analysis for multiple correlation. the variance ratio test.
and the chi-square test for normality of frequency
distribution. Statistical significance was set at

P < 0.05.

RESULTS
Systemic arterial pressure 123 i 7 mm Hg (n = 84) was
not significantly altered by intravenous or intra-arterial

infusions of arachidonic acid. imidazole. U—63557A. or

their carrier solutions. Furthermore. systemic arterial
pressure was not altered by luminal placement of
arachidonic acid or digested food. Systemic arterial

pressure was reduced by bolus injections of arachidonic
acid into the pulmonary artery at 300 ug/kg but returned
toward control within approximately 6 minutes (Figure 3).
Intravenous infusions of mefenamic acid in Series 6 and 11
significantly increased aortic blood pressure from
129 1 5.0 mm Hg to 150 t 9.0 mm Hg. All experiments.
however. were conducted under steady state conditions of
aortic pressure.

Figure 7 shows the effects of local intra—arterial
infusions of AA on jejunal vascular resistance when the
local arterial AA concentration was raised from 0 to 13 pg
ml blood’1
1). The control jejunal vascular resistance was 2.13 i
0.18 mmHgoml-l min3100 g: Infusion of the drug carrier.
0.9% NaCl. at the same rates of infusion as AA did not
significantly alter jejunal vascular resistance. An
increase in local arterial AA concentrations to a range

between 1.0 and 5.7 pgaiml blood--l

significantly
increased vascular resistance whereas an increase in the

concentration between 8.0 and 13.0 pgi- ml blood-1

54

from circulating levels already present (Series

55

Figure 7. Effects of intra-arterial infusion of
arachidonic acid on jejunal vascular resistance. Dotted
lines indicate a zone of 95% confidence. n = 47 from 7
dogs. AA. arachidonic acid.

0.. NP 3 0.. m m h 0 m
t: ,
ones... , . .
x8.» P .Nxoaiaxk P ..>

Arooosm 3.03 5

v

('IOHLNOO W083 96V) EONVLSISBU

57

significantly decreased vascular resistance. The data
were analyzed by a digital computer utilizing 47 data
points from seven animals using the ordinary least-squares
technique. which resulted in a third-order polynomial
equation that was 70% correlated with the data points.
Upper and lower boundaries for each dosage were computed
using the parameter covariance matrix and t values such
that P < 0.05. Lumen pressure of the jejunal segments was
not significantly altered at all infusion rates.

Figure 8 shows the effects of local intra-arterial
infusions of AA on jejunal vascular resistance before and
after mefenamic acid in the 7 dogs pretreated with aspirin
48 hours prior to the experiment. Before mefenamic acid.
i.a. infusions of arachidonic acid produced dose dependant
decreases in jejunal vascular resistance. After mefenamic
acid (10 mgokg-l). however. the same dogs were completely
refractory to the effects of i.a. infusions of arachidonic
acid on jejunal vascular resistance (solid regression
line). Aspirin pretreatment. therefore. abolished the
vasoconstrictor effect of AA at low doses (Figure 7) and
aspirin pretreatment and mefenamic acid prevented all
vascular effects of AA on jejunal vascular resistance
(Figure 8).

Table 1 and Table 2 show the effects of intra—
arterial (5-8 pg-ml blood’l) and intravenous (40 pg'vkg—J
min-l) infusion of AA on food-induced jejunal hyperemia.

respectively. Intra-arterial infusion of AA did not

58

Figure 8. Effects of intra-arterial infusions of various
doses of arachidonic acid on jejunal vascular resistance
before and after mefenamic acid (10 mg/kg. 1. v. ) in dogs
pretreated with a single oral dose of aspirin (25 mg- kg

48 h before the experiemnt. The dashed line represents the
effects of arachidonic acid on jejunal vascular resistance
before mefenammic acid (triangles are actual data points)
and the solid line indicates the effects of arachidonic
acid on jejunal vascular resistance after mefenamic acid
(circles are actual data ponts). The regression
coefficients are significantly different at P < 0.05. n =
51 from 6 dogs. AA = arachidonic acid.

59

too... E: u: <<

 

ON or o— 3 up or m a o N
’1 e
111
It
II
II
II
II
11
e ,1
C ,1
II .
11
NB. n a III 1
t ‘
5.- .2. i 1/
II
t 4
It
8 II .
I
. It
s . It
.1 . ll .
F II
It
. JII.
O ’l
I
ee e [I
1 .
I . .III
N o. u a . . .
Vs . + xmpr u >
e e
e e e e e

OP-

aanssalia NOlSl'lzlilid 010v

60

Table 1. The effects of intra-arterial infusions of
arachidonic acid (AA) on the postprandial intestinal
hyperemia. Blood flow expressed as ml~min ~ 1009 . ' P<

0.05 relative to the corresponding value before infusion.
n=7.

61

 

 

 

TABLE 1
Lumen Content Flow A Flow A 7. Flow
Before AA infusion
Saline 36.8:2.5
1! 1!
20.1:3.9 54.3:9.2
Food 56.9:5.5
During AA infusion
Saline 36.8:5.4
I Q
8.4:2.l 23.5:5.3

Food 43.2:7.l

62

Table 2. The effects of intravenous infusions of
arachindonic acid (AA) on the postprandial intestinal
hyperemia. Blood flow expressed as mlomin - 1009 . ' P<

0.05 relative to the corresponding value before infusion.
n=7.

63

 

 

 

TABLE 2
Lumen Content Flow A Flow A '/,Flow
Before AA infusion
Saline 37.8:4.l
l6.8:3.0 44.0:6.6
Food 54.6:6.5
During AA infusion
*
Saline 29.2:1.8
* R
7.58:1.3 26.8:S.2

Food 36.8:2.l

64

significantly alter control blood flow (Table 1). but
intravenous infusion significantly decreased control blood
flow from 37.8 to 29.2 ml-min-lsloo g“l (Table 2). The
dosages for intra-arterial infusion were chosen based on
the data from Figure 7 to maintain local arterial
concentration within the range of 5-8 ug/ml. whereas the
dosage of intravenous infusion was chosen because no
systemic or local side effects were observed (46).
Regardless of the route of infusion. AA produced the same
results. Before intra-arterial or intravenous infusion.
luminal placement of digested food significantly increased
blood flow by 54.3 or 44.0%. After intra—arterial or
intravenous infusion. luminal placement of digested food
still significantly increased blood flow. but the hyperemia
produced was significantly less than before infusion. This
is true whether the data are expressed as absolute change
in blood flow (+20.1 1 3.9 before vs. +8.4 1 2.1

l 1

mlo min‘ - 100 9‘ after i.a. infusion and +16.8 1 3.0

l- 100-1 9 after i.v.

before vs. +7.58 1 1.3 ml-min'
infusion) or as the percent change from control (+54.3 1
9.2 before vs. +23.5 1 5.3% after i.a. infusion or +44.0 +

6.6 before vs. +26.8 1 5.21 after i.v. infusion). In both
the intra-arterial and intravenous series. the vascular
response to digested food was not significantly affected by
infusion of the carrier solution. Food produced a 49.2 1

7.9 and 43.2 1 8.1% increase in flow before and after the

infusion of the carrier.

65

In Series 4. the effects of luminal placement of AA on
both the vascular and metabolic responses to luminal
placement of digested food were examined. The vascular
responses of the two adjacent jejunal segments to luminal

placement of normal saline or digested food with and

without AA are depicted in Fig. 4. The concentration of
AA. 0.65 mM. was chosen after preliminary studies. The
studies indicated that at higher concentrations AA

increased intestinal secretion and motility and produced
fluctuations in jejunal blood flow. The concentration
used. 0.65 mM. was the highest concentration at which the
above effects were not observed. As can be seen in Fig. 4.
the design of the experiment was to use one segment as the
control for the other segment. i.e.. in a particular
period. one segment contained only normal saline or food
alone while the other segment contained one of these
solutions plus AA.

Digested food significantly increased blood flow
whether or not AA was added to digested food. Although
addition of AA to normal saline did not significantly alter
blood flow. addition of AA to digested food significantly
attenuated the food-induced hyperemia. This is true
whether the effects of food alone and food plus AA were
compared within the same segment (Figure 4. page 43) (+48
vs. +24.4% in segment A and +38.5 vs. +28.6% in segment 8)
or between the two segments when each solution was placed

simultaneously in each of the two segments (+48 vs. +28.6%

66

in the third period and +38.5 vs. +24.4% in the last
period). Thus AA significantly attenuated the food-induced
hyperemia whether AA was administered intra-arterially
(Table I). intravenously (Table 2). or placed into the
lumen with food (Fig. 4).

AA also attenuated the food-induced increase in 02
uptake. As shown in Fig. 9. a 26% increase in 02 uptake
produced by food alone in segment A was significantly
greater than that produced by food plus AA in segment A
(+9.7%) or segment 8 (+15.3%). Likewise. a 28.8% increase
in 02 uptake produced by food alone in segment 8 was
significantly greater than that produced by food plus AA in
segment A (+9.7%) or segment 8 (+15.3%). As shown in Fig.
10. there was a significant correlation between the food-
induced increases in 02 uptake and blood flow. This
correlation holds for both food alone and food plus AA.

Series 5 determined whether or not the attenuation of
the food-induced increases in intestinal oxygen uptake by
arachidonic acid resulted from the attenuation of the food-
induced increases in blood flow. Figure 11 shows the
relation between changes in blood flow and changes in 02
uptake obtained from three jejunal segments in which flow
was altered by a pump. Although a decrease in blood

1 1

flow from about 34 mlomin- -100 g- significantly decreased

02 uptake (P < 0.05. analysis of variance). a stepwise

l 1

increase in blood flow from 34 to 105 ml-min-c 100 9. did

not significantly alter 02 uptake. Thus it appears that 02

67

Figure 9. Mean 0 uptakes from 2 adjacent jejunal segments
containing normaT saline (NS). NS plus arachidonic acid
(AA). food. or food plus AA. Values shown below or above 2
adjacent bars indicate percent differences in O uptake
between 2 bars. * P < 0.05 relative to precedin value.
** P < 0.05 between 2 values denoted by dotted line. n = 5.

V02 = 02 uptake.

'-100 g")

V02 (Ml-M

2.5

2.0

1.0

.5

2.5

1.5

1.0

.50

SEGMENT A

2.07 I 2.56
NS FOOD

SEGMENT B

I

2.09 2.27
NS rooo
+ +

AA AA

26.0 116.8 ---------- I 0 ---------- 9.70 t 3.6

6%

at}:
l
l

l
I
1
111's

15.3: 7.8 ------------ I It ----------- 28.8 211.1

I»

i»
I
2.07 1.98 2.47
I
NS ‘ NS FOOD

69

Figure 10. Correlation between changes in food—induced
increases in jejunal O uptake and blood flow. Closed
circles. food alone; 0 en circles. food plus arachidonic

acid (AA).

70
Fig. 10

Ca oo...-___e.__e zeazazs seas
on, ad we no! ad md so no No 13

hwouz o o o
Nvd+xo§u> . s

1.0

O
1-

1.0
F

(.1113 00111111111") M011 111101811

0
N

71

Figure 11. Relation between jejunal blood flow and O
uptake. Blood flow was altered mechanigally.
Numbers in parentheses indicate number of measurements.

Resting flow before mechanical alterations was 40.2 1 0.5
m1~min- ~1009- .

72
Fig. 11

(4)

(7)

5)

(5)

(4)

(3)

 

 

3.0-

1.0.. ‘}
(3

2.041

(Goouuuu/lw)
BMVLdn NSOAXO

BLOOD FLOW

(ml/minliOOQ)

73

uptake is independent of blood flow at flow rates above 35

1 I 1

ml- min- .100 g- . but at flow rates below 35 mlomin--

g.1 02 uptake is flow dependent. Resting jejunal blood

100

flows in all experiments performed. except series 2 (Table
2. after i.v. infusions of arachidonic acid). were above
the critical value of 35 mlvmin—L 1009—1. This indicates
that any changes in oxygen uptake due to experimental
perturbations were not due to changes in blood flow.

Series 6 was designed to test whether the effects of
arachidonic acid on food-induced changes in intestinal
blood flow and oxygen uptake were due to the action of
arachidonic acid or to its metabolites. The effects of
intra—arterial infusions of arachidonic acid on both the
metabolic and vascular responses to luminal placement of
food were examined after pretreatment with the
prostaglandin synthesis inhibitor mefenamic acid. The
effectiveness of the cyclooxygenase blockade was tested as
previously described (Figure 3). Table 3 illustrates these
results. Before arachidonic acid infusions. luminal
placement of digested food increased blood flow by 21.3 ml‘

1 -I -1

min- ° 1009 and oxygen uptake by 0.68 ml- min ~ 1009—1.

During arachidonic acid infusions. luminal placement of

digested food increased blood flow by 18.6 ml'min—l~ 1009--1

1- 1009-1. The food-

and oxygen uptake by 0.60 mlimin-
induced increases in both intestinal blood flow and oxygen
uptake before arachidonic acid administration were not

significantly different from those induced after the

 

74

Table 3. The effects of intra-arterial infusions of
arachidonic acid on the food—induced increases in
intestinal blood flow and oxygen uptake after pretreatment
with mefenamic acid. VO =_oxygen gptake. V0 and blood
flow expressed as ml-mifi - lOOg . * P<0.05. n=7.

75

 

I. N

 

8. +8. 2. +om.~ S. + a; o >
.mauoas aaumém Smut? sons soc;
"aqua. 3835a: wage
ofiHoo. ataxia. stag; Nos
saeuméa sausém Rmumém .8: e003
"ease asaooabuaus mucosa
mocmpommaa poom ocaaom

 

HZMHZOU ZMZDA
m mum:

76

administration. This indicates that the inhibitory effects
of arachidonic acid on the food-induced increases in blood
flow (Table 1) were due to the products of cyclooxygenase
metabolism and not to the arachidonic acid ggt gg.

In Series 7. the effects of imidazole. a selective
inhibitor of thromboxane synthesis. on food-induced
increases in intestinal blood flow and oxygen uptake were
examined in 14 dogs. Figure 12 shows the effects of i.a.

infusions of imidazole (5 mg-min—l

) on the food-induced
jejunal hyperemia. Imidazole Egg gg had no effect on
resting jejunal blood flow (48.1 1 3.1 before imidazole
vs. 50.7 1 .3 ml-min“-ioo 9‘1 during imidazole). The
food-induced increase in flow was also not affected. This
is true whether the data are expressed as absolute change
in blood flow (23.0 1 3.5 before. vs. 20.4 1 2.9

1- 1009"1

ml- min- during imidazole infusion) or as the
percent change from control (49.8 1 7.1 before. vs. 42.0 1
6.5% during imidazole infusion). Luminal placement of
digested food produced significant increases in jejunal
blood flow both before and during imidazole infusions.
Figure 13 shows the effects of i.a. infusions of
imidazole on food-induced increases in jejunal oxygen
uptake (V02). Imidazole had no effect on resting V02 (2.08

1 0.07 before. vs. 2.04 1 .10 ml-min-l- 1009-.1

during
imidazole infusion) but significantly potentiated the
food-induced increase in V02. This is true whether the

data are expressed as absolute change (+0.406 1 .06 before.

77

Figure 12. Vascular response to luminal placement of food

before and during intra-arterial infusions of imidazole.

Flow values are means 1 SEM in ml-min- -lOOg— . n = 14.

NS. Normal Saline. F. Food.=kP < 0.05 relative to before
infusion.

78
Fig. 12

N.:.

«.0 n QNQ
QN u WON

.i.

o_0uav_E_

m2

5.0m

mats—u

 

5. M adv
md n CAN

m2

fima

O

flow—up}; 30.0..

2:Uf5&€as

30.: <
a
1.
O
O
0
Cu
B
O
M
co 1
m.
0
cm hi
6 c
on (

cop

79

Figure 13. Effect of intra-arterial infusions of imidazole
on the food-induced increases in oxygen uptake. V0 .
oxxgen uptake. NS. normal saline. F. food. * P < 0.65
relative to NS. ** P < 0.05 relative to the corresponding
value before imidazole. n = 14.

80
Fig. 13

eeosd fl 06¢

00

OON

9.0 H 00.0

 

I.

e_oua_u_E_ octet

mad

CO.” H Nap
06.0w 3.0

m2

mags.

e_owo1_E_ e532.

«09 $4

«2:
no
3 A.
O
7v
8.. M
uh
o.~ m
0
or
ad
on

81

vs. +0.858 : .15 ml- min—1 - 100g_l during imidazole
infusion) or as percent change from control (+19.7 1 3.0
before vs. +44.9 1 8.7% during imidazole infusion).
Luminal placement of digested food produced significant
increases in V02 both before and during imidazole
infusions.

Figure 14 shows the correlations between food-induced
increases in jejunal oxygen uptake (V02) and jejunal blood
flow before and during imidazole infusion. Changes in the
two variables are significantly correlated both before and
during imidazole infusions. However. the slopes of the two
regression lines are significantly different (2.60 before
vs. 0.96 during imidazole infusions). This indicates that
for a given food—induced increase in jejunal blood flow.
the food-induced increase in jejunal oxygen uptake is
significantly greater during imidazole administration than
before the administration.

In order to assess whether the effects observed in the
above series were a result of thromboxane synthesis
inhibition or the action of imidazole ggt gg. experiments
were repeated in which U-63557A was substituted for
imidazole. This compound is also a selective inhibitor of
thromboxane synthesis and has been shown to selectively
inhibit thromboxane synthesis at the dosage used (5.0 mg-
kg'l. i.v.) (139). Table 4 shows the effects of U—63557A

on the food-induced increases in both intestinal blood

flow and oxygen uptake. U-63557A ger gg had no effect on

82

Figure 14. Correlation between percent changes in V0
(oygen uptake) and blood flow elicited by luminal placemen
of digested food before and during intra-arterial infusions
of imidazole. * P < 0.05 relative to control.

63
Fig. 14

00w ms

mm. an
ode + x00. u>

on ma

J
O
U
I
eOJ
I

mh.u~.
adds. x@.Nu>

mu v
7.
9
1|
0
0
0

on u
0
M

as.

oo.

84

Table 4. The effects of intravenous infusions of U-63557A

on the food-induced increases in intestinal blood flg
and _oxygen uptake (V0 ). Values expressed as ml- min‘
100g . * P (0.05. 1 = P<0.05 relative to the corresponding

value before U-63557A. n=5.

85

k.m .H 0.ss

+1

m.NH m.-

m.m.u N.0H

0.0.“ 0.s~

No.0 .H 0k.0

m
.8
+1

N.¢H

00.0 .H mm.0

0.N .H 0.Hs

 

o.o

00.0

a.o

+ mm.~

+|

H.Nm

N.mo

‘

 

 

 

0.0 .H k.~s sass ooosm
"akmmmaia soue<
00.0 .H 00.H N0 0
0.0 .H 0.00 sass oases
”akmmmaio museum
seesaw

 

HZMHZOU szDA

o mum<p

86

the hyperemia. This is true whether the data are expressed
as absolute change in blood flow (+11.9 1 2.9 mlamin_1~ min
1009‘1 before U-63557A vs. +14.2 1 5.3 ml- min". 1009’1
after U-63557A) or as percent change from control (+21.9 1
4.4 before U-63557A vs. +27.5 1 12.5 after U-63557A).
Table 4 also shows that U-63557A significantly potentiated
the food-induced increase in intestinal oxygen uptake.
This is true whether the data are expressed as absolute
change in oxygen uptake (+0.35 1 0.08 ml- min—l~ 1009-1
before U—63557A vs. +0.78 1 0.07 mlvmin-l~1009-l after u-
63557A) or as percent change from control (+19.2 1 3.5
before U-63557A vs. +4l.6 1 5.7 after U-63557A).
Furthermore. there was a significant correlation between
the food-induced increases in intestinal oxygen uptake and
blood flow before and after administration of U-63557A.
Like the imidazole data (Figure 14. page 83 ). the slope
of the linear regression of the food-induced changes in
intestinal oxygen uptake and blood flow before U—63557A

treatment was significantly greater (Y 0.90 X + 2.5. r =

0.95) than that after the treatment (Y = 0.46 X - 0.67. r
0.95). This is also true if the data are expressed as
absolute change from control; Y = 31.8 X + 0.10. r = 0.98
before U-63557A vs. Y = 9.2 X + 1.27. r = 0.73 after U-
63557A. This indicates that for a given food-induced
increase in jejunal oxygen uptake (X). the corresponding
increase in jejunal blood flow (Y) is significantly less

after U-63557A treatment.

87

Table 5 illustrates the effects of imidazole and U-
63557A on jejunal glucose absorption. Before imidazole.
the rate of jejunal glucose absorption was 9.6 1 1.4 mg
min-l- 1009-1 and during imidazole infusion the jejunal

glucose absorption rate was 12.3 1 3.2 mg~min-L lOOg-l.

Furthermore. before U—63557A. the rate of glucose

1 1

absorption was 11.7 1 2.6 mg-min-~ 1009- and after U—

63557A. the rate of glucose absorption by the jejunum was
7.64 1 3.3 mg.min-l 1009-1. Table 5 also shows that
jejunal glucose absorption was negative when normal saline
was in the lumen. This indicates that the intestine was
removing glucose from the arterial blood.

Figure 15 shows the time course of changes in the
jejunal capillary filtration coefficient (CFC) during
luminal placement of food before and during i.a. infusions
of imidazole. Before imidazole and with saline in the

lumen. CFC (ml~ min-i mm Hg-l

.1009") was 0.335 1 0.02.
During food placement. CFC rose to 0.401 1 0.04. 0.428 1
0.04. 0.410 1 0.04. and 0.492 1 0.02. 4. 8. 12. and 16
minutes following food placement. respectively. CFC
significantly rose above control after 8 and 16 minutes of
food placement. Imidazole ggt gg significantly increased
CFC from 0.335 1 0.02 to 0.446 1 0.01. Placement
of food increased CFC to 0.469 1 0.03. 0.471 1 0.04. 0.564
1 0.07. and 0.595 1 0.02. 4. 8. 12. and 16 minutes

following food placement. respectively. The increased CFC

observed 8-16 minutes following food placement were

88

Table 5. The effects of imidazole (5 mg~min-l. i.a.) and
U-63557A (5 mg:kg . i.v.) on jejunal glucose absorption in

7 and 5 dogs. respectively. _ Valueslare mean 1 standard
error and expressed as mg-min - lOOg .

89

 

04..“
3.7

~.HH cowuopomp< uoz
CNN
na.n coon
To.“
mm.ol oaaamm Hmeaoz

 

<hmmmca pmum<

<nmmm©lb opowom

 

 

N.m+ a.H+
m.~H oo.a GOHOQuomA< uoz
0.0“ 0.11.
om.m mm.n coon
00...“ N0.“
wa.Ni ¢H.~i oaaamm Hoahoz
maonmcaam poow< maonmoHaH anamom uqauaoo posed

m m4m<H

90

Figure 15. Time course of changes in jejunal CFC during
luminal placement of food before and during i.a._linfusigns
of imidazole. CFC expressed as ml'min ~ mm Hg - 1009 .

NS — normal saline. F = food. Squares P < 0.05 relative to

NS. * P (0.05 relative to the corresponding value before
imidazole. n = 7.

‘11
Fig. 15

OD

m2

9in

OD

N70

OD

mé

 

v.0 AEEV e8:
u 02 .uufifl
N.
o_ow01_E_ m.
20an
a.
o_owa_u_E_
m... :
I+\\.. _. _.
m.
m.

3:13

92

significantly higher than that measured during normal
saline placement. Furthermore. the CFC measured during
the last 8 minutes of food placement during imidazole
infusions were significantly greater than those measured
before imidazole. This study indicates that endogenous
TXA2 acts to inhibit capillary exchange capacity at resting
conditions as well as during digestion. Elevation of
venous pressure had no significant affect on either the
volume of fluid absorbed or motility before or during
imidazole treatment.

Figure 16 shows the effect of mefenamic acid
(cyclooxygenase inhibitor) on the jejunal CFC before food
placement and 4. 8. 12. and 16 minutes following placement
of food into the jejunal lumen. Before mefenamic acid and
with saline in the lumen. CFC was 0.371 1 0.01. During
food placement. CFC rose to 0.558 1 0.06. 0.524 1 0.03.
0.453 1 0.02. and 0.562 1 0.01. 4. 8. 12. and 16 minutes
following food placement. respectively. All of these
values were significantly greater than the control value
(0.371 1 0.01). After mefenamic acid and with saline in
the lumen. CFC was 0.375 1 0.02 which was not significantly
different from that observed before mefenamic acid (0.371 1
0.01). During food placement. CFC rose to 0.533 1 0.03.
0.517 1 0.02. 0.445 1 0.02. and 0.566 1 0.04. 4. 8. 12. and
16 minutes following food placement. respectively. All
these values were significantly greater than control (0.375

1 0.02). Mefenamic acid treatment. therefore. had no

93

Figure 16. Time course of changes in jejunal CFC during
luminal placement of food before and after i.leinfusion_of
mefegamic acid. CFC expressed as mlsmin ' mm Hg

1009 . NS = normal saline. F = food. * P (0.05 relative
to NS. n = 7.

m2

Eva £509.39: actot lllll

“:90 Ottawa—0‘05 ObOmOJ

 

AEEV OE:

.co.c0u
coEo.

333

95

significant effect on the jejunal CFC either before food
placement or at any time during food placement. Elevation
of venous pressure had no significant affect on either the
volume of fluid absorbed or motility before or after

mefenamic acid treatment.

DISCUSSION

The stimuli for the postprandial intestinal hyperemia
are the products of food digestion such as fatty acids.
sugars. and amino acids (37. 87). Many factors are
involved in this response such as villus hyperosmolality
(34). and gastrointestinal hormones (38. 52). Recently.
locally released compounds have also been shown to be
involved in this complex metabolic and vascular response to
nutrient absorption. Chou and Siregar (39) have proposed
that histamine plays an important role in the metabolic and
vascular response to feeding. They found that
tripelenamine. an H-l histamine receptor antagonist.
significantly attenuated the food-induced increases in
intestinal blood flow and oxygen uptake. Another class of
compounds which are present throughout the gastrointestinal
tract that have been implicated in the postprandial
intestinal hyperemia are the prostaglandins. Gallavan and
Chou (63) have shown that inhibition of prostaglandin
synthesis with indomethacin or mefenamic acid.
significantly potentiated the food-induced increases in
intestinal blood flow and oxygen uptake. Furthermore.
there was a significant correlation between food-induced
changes in blood flow and oxygen uptake before and after
inhibition of prostaglandin synthesis. The authors

concluded that endogenous intestinal prostaglandins serve

96

97

to limit the postprandial intestinal hyperemia and this may
be secondary to prostaglandin-induced reductions in
oxidative metabolism during nutrient absorption.

The postprandial intestinal hyperemia is confined
mainly to the mucosal layer of the gut wall (36. 64. 141).
The jejunal mucosa produces prostaglandins and the
prostaglandins produced by the jejunal mucosa are
prostaglandin I2 and thromboxane A2 (92. 93).

The purpose of the present study was to examine the
contributions of endogenous intestinal prostaglandins to
the postprandial intestinal hyperemia from the other
direction. namely. to stimulate endogenous intestinal
prostaglandin production with arachidonic acid. Also. in
an attempt to identify the prostaglandins involved. the
effect of selective inhibition of thromboxane synthesis on
food-induced increases in jejunal blood flow and 02 uptake
was examined.

Arachidonic acid is the precursor of the bisenoic
prostaglandins and has been shown to stimulate intestinal
prostaglandin synthesis (3. 92. 115. 136). Since
arachidonic acid has been reported to exert powerful
vascular effects in the intestinal vascular bed (33. 46.
47). a dose response relationship of arachidonic acid on
intestinal vascular resistance was determined. As shown in

Figure 7. arachidonic acid exerts a biphasic effect on

98

jejunal vascular resistance in constant flow jejunal
preparation. Low doses of arachidonic acid increase
resistance while higher doses decrease resistance.

Most investigators have reported that arachidonic acid
is a vasodilator in the intestinal vascular bed (33. 46.
47). but some have observed a biphasic response (33. 46.
82). In the gastric vascular bed (82) and in 1Q 21222
studies using intestinal vascular smooth muscle strips
(46). the same dose (82) or concentration (46) of
arachidonic acid responsible for eliciting vasodilitation
caused vasoconstriction if the amount of time that
arachidonic acid was in contact with blood cells was
increased. The authors (46. 82) suggested that the
conversion from vasodilatory to vasoconstrictor action may
be due to a preferential formation of platelet born
vasoconstrictor thromboxanes. In order to test this
possibility. the effects of i.a. infusions of arachidonic
acid on jejunal vascular resistance were determined after
the animals were pretreated with a single dose of aspirin
(25 m9~ik9-!. oral) 48 hours prior to the experiment.
Aspirin blocks cyclooxygenase and platelets are 20-40 fold
more sensitive to aspirin-induced inhibition of
cyclooxygenase than other tissues (6). Furthermore.
aspirin irreversibly acetylates platelet cyclooxygenase
rendering them permanently incapable of prostaglandin
production because they cannot synthesize protein (98).

Nucleated cells. however. can completely replace their

99

acetylated cyclooxygenase in as little as 36 hours (81).
On this basis. single low doses of aspirin have been used
to preferentially inhibit the formation of cyclooxygenase
products from platelets (81. 98).

As shown in Figure 8. increasing concentrations of
arachidonic acid produced a dose dependent decrease in
jejunal vascular resistance when the animals were
pretreated with a single dose of aspirin 48 hours before
the experiment (regression coefficient significant at P <
0.05). Furthermore. the increase in jejunal vascular
resistance observed with arachidonic acid concentrations
between 1.0 and 5.0 mg-ml blood"l (Figure 7) was abolished.
This indicates that the increase in jejunal vascular
resistance observed with low concentrations of arachidonic
acid (Figure 7) may have been due to the preferential
formation of platelet prostaglandins. specifically
thromboxane A2.

The effects of i.a. infusions of arachidonic acid on
jejunal vascular resistance in aspirin untreated or treated
group (Figure 7 and Figure 8). may have been due to the
products of arachidonic acid metabolism by cyclooxygenase.
lipoxygenase. monooxygenase. or arachidonic acid ggt gg.
In order to determine if the vascular actions of
arachidonic acid were due to only the products of
cyclooxygenase metabolism. the effects of i.a. infusions of

arachidonic acid on jejunal vascular resistance were

determined after treatment with mefenamic acid. a selective

100

cyclooxygenase inhibitor. As shown in Figure 8. infusions
of arachidonic acid at various concentrations had no effect
on jejunal vascular resistance after mefenamic acid
treatment (the regression coefficient is not significant at
P < 0.05). This indicates that the effects of arachidonic
acid on jejunal vascular resistance (Figure 7 and Figure 8)
were due to the products of cyclooxygenase metabolism.

I.V. and i.a. infusions of either mefenamic acid or
indomethacin (prostaglandin synthesis inhibitors)
significantly potentiate the postprandial jejunal hyperemia
(63). Therefore. we determined the effects of i.v. and
i.a. infusions of arachidonic acid. a prostaglandin
precursor. on the postprandial intestinal hyperemia in
natural flow jejunal preparations. Table 2 shows that i.v.
infusions of arachidonic acid (40 pg - kg"1 - minTl)
significantly attenuate the postprandial jejunal hyperemia.
Furthermore. Table I shows that i.a. infusions of

arachidonic acid (4-8 ugcml blood-l

) also significantly
attenuated the postprandial jejunal hyperemia. The
attenuation of the postprandial jejunal hyperemia by
arachidonic acid (Table 1 and 2) suggest that endogenous
intestinal prostaglandins serve to limit the vascular
response to luminal flood placement. a hypothesis put forth

by Gallavan and Chou who used cyclooxygenase inhibitors

(63).

101

Because local i.a. infusions of indomethacin
or mefenamic acid potentiated the postprandial intestinal
hyperemia (63). Gallavan and Chou proposed that the effects
of prostaglandin synthesis inhibition on the postprandial
intestinal hyperemia were due to the inhibition of local
jejunal prostaglandins and not due to an unidentified
central mechanism or due to inhibition of prostaglandin
synthesis by other somatic tissues. The present study
supports that conclusion since local i.a. infusions of
arachidonic acid attenuated the postprandial intestinal
hyperemia (Table 1) as well as i.v. infusions (Table 2).

Both i.v. and i.a. infusions of arachidonic acid
attenuate the postprandial jejunal hyperemia (Table 2 and
1). Unlike i.a. infusions of arachidonic acid. however.
i.v. infusions significantly reduced jejunal blood flow
when saline was in the lumen (Table 2). It is not clear
from this study why this occurred. This effect may be due
to unidentified central mechanisms or possibly
vasoconstrictor prostaglandins produced by tissues other
than the jejunum. For example. blood concentrations of
arachidonic acid between 1.0 and 5.0 pg-ml blood"l increase
jejunal vascular resistance (Figure 7) and since the
vasoconstrictor effect is abolished by single low doses of
aspirin (Figure 8). the blood concentration of arachidonic
acid reaching the intestine in the i.v. series might result
in preferential production of platelet born vasoconstrictor

prostaglandins.

102

The jejunal vascular and metabolic changes associated
with luminal placement of digested food are confined mainly
to the mucosal layer of the gut wall (36. 64. 141).
Gallavan and Chou (63) have shown that i.a. administration
of cyclooxygenase inhibitors potentiate the food-induced
increases in jejunal blood flow and oxygen uptake.
Conversly. i.a. infusions of arachidonic acid attenuate the
postprandial intestinal hyperemia (Table 1). These data
indicate that these effects may be due to changes in local
intestinal prostagalndin production. more specifically.
mucosal prostaglandins. In an attempt to test this
possibility. the effects of luminal placement of
arachidonic acid (0.65 mM) on the food-induced increases in
intestinal blood flow and oxygen uptake in natural flow
jejunal preparations was examined. Arachidonic acid is
readily absorbed from the jejunal lumen (40) and would be
expected to be available for cyclooxygenase metabolism only
in the mucosal epithelial cells because fatty acids (like
arachidonic acid) are transported through the lymphatics
following chylomicron formation in the mucosal epithelium.
As shown in Figure 4. luminal placement of arachidonic acid
significantly attenuated the postprandial intestinal
hyperemia. Likewise. Figure 9 shows that luminal placement
of arachidonic acid attenuated the food-induced increases
in jejunal oxygen uptake. Furthermore. the increases in
jejunal blood flow and oxygen uptake following luminal

placement of food are significantly correlated both before

103

and during arachidonic acid administration (Figure 10).
The same correlations were also observed before and after
prostaglandin synthesis inhibition by Gallavan and Chou
(63). These authors (63) concluded that the effects of
endogenous jejunal prostaglandins on the food-induced
increases in jejunal blood flow may be the result of
primary alterations in the food—induced increases in
jejunal oxidative metabolism. The strong correlation
between the food-induced increases in intestinal blood flow
and oxygen uptake both before and during arachidonic acid
administration (Figure 10) supports this hypothesis. It is
also possible that the attenuation of the food-induced
increases in jejunal blood flow by arachidonic acid were
due to changes in mucosal prostaglandin production since
luminal placement of arachidonic acid attenuated the food-
induced increases in both intestinal blood flow and oxygen
uptake.

Arachidonic acid attenuates the food-induced increases
in intestinal oxygen uptake (Figure 9) as well as blood
flow (Table 1. Table 2. and Figure 4). The attenuation of
the food-induced increases in oxygen uptake might be
secondary to the attenuation of the hyperemia. To test
this possibility. the effects of mechanical alterations in
intestinal blood flow on intestinal oxygen uptake were
examined. Figure 11 shows that oxygen uptake remains
unchanged over the blood flow range of 34-110 ml« min’--1

1009-1. Below 34 mlominTI-loog‘l. oxygen uptake was flow

104

dependant. Other investigators have also shown this
relationship in ileal preparations (88). Since the blood
flow values in our preparations were above 34 mlrvmin—l~
1009-1. any alterations in intestinal oxygen uptake appear
not to be the result of alterations in intestinal blood
flow. In fact. the converse may be argued. that is. the
attenuation of the food-induced increases in intestinal
blood flow may be secondary to the attenuation of the food-
induced increases in intestinal oxygen uptake.

The attenuation of the postprandial intestinal
hyperemia by arachidonic acid (Figure 4. Table l. and Table
2) as well as the attenuation of the food-induced increases
in intestinal oxygen uptake (Figure 9) may have been due to
the action of either prostaglandins or arachidonic acid egg
gg. In order to assess this possibility. the effects of
arachidonic acid on the food-induced increases in
intestinal blood flow and oxygen uptake were determined
after blocking cyclooxygenase with mefenamic acid. Table 3
shows that the attenuation of the food-induced increases in
intestinal blood flow and oxygen uptake by intra-arterial
infusions of arachidonic acid were abolished by mefenamic
acid. a prostaglandin synthesis inhibitor. This indicates
that the effects of arachidonic acid were the result of
stimulation of prostaglandin synthesis. The use of
mefenamic acid in this series also rules out the possible
involvement of other arachidonic acid pathways since

mefenamic acid only inhibits cyclooxygenase. the enzyme

105

responsible for production of bisenoic prostaglandins and
prostanoids. More specifically. the results of this series
of experiments rules out the involvement of the
lipoxygenase or monooxygenase products in the attenuation
of the metabolic and vascular responses to food placement
induced by arachidonic acid.

The prostaglandins produced by the jejunal mucosa are
prostaglandin 12 and thromboxane A2 (92. 93). Furthermore.
thromboxane A2 is only produced in the mucosal layer of the
gut wall (93). Selective inhibition of thromboxane
synthesis has been shown to inhibit the production of
thromboxane A2 and increase the production of prostaglandin
I2 by reorienting endoperoxide metabolism toward
prostaglandin [2 synthesis (20. 43. 105). Therefore.
selective inhibitors of thromboxane synthesis were used to
determine if a change in the thromboxane to prostacyclin
production ratio would have an effect on either the food-
induced increases in jejunal blood flow or oxygen uptake.

Imidazole has been shown to be one of the most
efficacious inhibitors of thromboxane synthesis 13 1119

(44) and i vitro (43. 57. 105) and was one of the two

 

thromboxane synthesis inhibitiors used in this study. As
shown in Figure 12. i.a. infusions of imidazole had no
effect on the food—induced increases in jejunal blood flow

but significantly potentiated the food-induced increases in

106

jejunal oxygen uptake (Figure 13). This means that the
increased oxygen uptake by imidazole was met by increases
in oxygen extraction and not blood flow.

Furthermore. the slope of the correlation between
food-induced increases in intestinal blood flow and oxygen
uptake of the treated group (imidazole) is significantly
less than that of the untreated group (control) (Figure
14). In Figure 14. the data are expressed as percent
change from control but the same correlation also holds if
absolute flow and oxygen uptake values are used. More
specifically. the equation for food-induced increases in
oxygen uptake (X) vs. food-induced increases in blood flow
(y) was y = 67.3 x + 7.0. r = 0.70 (linear regression
analysis) and after imidazole the equation was y = 20.9 x +
1.5. R = 0.91 (linear regression anaylysis). The slopes
are significantly different at P < 0.05. In previous
studies using total prostaglandin synthesis inhibition
(mefenamic acid) (63) or total prostaglandin synthesis
stimulation (arachidonic acid) (Figure 10). the
correlations between flow and oxygen uptake of the treated
and untreated groups share the same regression line. i.e.
for a given increase in oxygen uptake. the corresponding
increase in blood flow in the treated and untreated groups
are the same. This is different from that shown in Figure
14. i.e. for a given increase in oxygen uptake. the
corresponding increase in blood flow during imidazole

infusions is significantly less than that observed before

1.1!

lb)"

1 W

107

the infusion (control. Figure 14). The difference may be
that imidazole enhances oxygen uptake by enhancing
intestinal metabolism during nutrient absorption without
affecting the intestinal hyperemia.

In order to determine whether imidazole increases
glucose absorption. thereby increasing intestinal
metabolism and oxygen uptake. the effect of imidazole on
glucose absorption was studied. As shown in Table 5.
glucose absorption during food placement was unaffected by
imidazole treatment. This would suggest that the increase
in metabolic rate that accompanied active glucose
absorption is not affected by imidazole. Furthermore.
imidazole 9g; gg did not alter resting intestinal oxygen
uptake (Figure 13). It appears that the potentiation of
the food-induced increases in intestinal oxygen uptake by
imidazole is not the result of enhanced glucose absorption.

The effects of imidazole on the food-induced increases
in blood flow (Figure 12). oxygen uptake (Figure 13). and
glucose absorption (Table 5) are believed to be due to
inhibition of thromboxane synthesis. To further strengthen
this argument. the effects of Sodium 5- (3'-
Pyridinylmethyl) Benzofuran - 2 - Carboxylate (U-63557A) on
food-induced increases in blood flow. oxygen uptake. and
glucose absorption were studied. U—63557A is a selective
inhibitor of thromboxane synthesis and does not contain an
imidazole moity in its molecular structure (139). Table 4

shows that U-63557A had no effect on the food-induced

,.‘.

108

increases in intestinal blood flow. but like imidazole
(Figure 13). significantly potentiated the food-induced
increases in intestinal oxygen uptake. Furthermore. U-
63557A had no significant effect on intestinal glucose
absorption (Table 5). The glucose absorption data shown
in Table 5 have large standard errors. The reason for this
diversity is unclear. All glucose absorption data were
normalized to 1009 tissue weight. but it seems possible
that the absorptive surface area may be different for
intestinal segments of the same weight. If this were the
case. glucose absorption rates may also be different. and
this may account for the large standard errors (Table 5).
Imidazole appeared to enhance glucose absorption. while U—
63557A decreased glucose absorption. Therefore. we believe
that thromboxane synthesis inhibition has no significant
effect on intestinal glucose absorption. Since we were
able to reproduce the findings from the imidazole studies
with a different thromboxane synthesis inhibitor. it seems
very possible that intestinal thromboxanes are involved in
regulating oxygen uptake during nutrient absorption.

It has been shown that the increase in intestinal
oxygen uptake during nutrient absorption is not necessarily
accompanied by an increase in blood flow (122). The
increased oxygen uptake in this situation was primarily met
by an increase in tissue oxygen extraction. Although the
underlying mechanisms are unclear. enhanced glucose

absorption has been implicated to be a possible mechanism

109

whereby oxygen uptake increases via enhanced oxygen
extraction and not blood flow (122). However. our data
indicate that the rate of intestinal glucose absorption is
unaffected by thromboxane synthesis inhibitors (Table 5).
Although glucose absorption is not affected by imidazole or
U-63557A. it may be that the potentiation of the food
induced increase in oxygen uptake by these two inhibitors
were the result of increased metabolism due to enhanced
peptide. animo acid. fat or electrolyte absorption. This
possibility was not examined in this study.

Luminal placement of food solutions into the small
intestine increase intestinal capillary exchange capacity
as indexed by the capillary filtration coefficient (122).
the capillary permeability—surface area product from Rb
clearance (122). and by the osmotic reflection coefficient
(69). The increase in the intestinal PS product is
believed to be due to capillary recruitment which enhances
the delivery of oxygen to the mucosal cells during the
metabolic stress of nutrient absorption (122). In order to
assess whether thromboxanes would increase jejunal
capillary exchange capacity. the jejunal capillary
filtration coefficient was determined before and during
luminal food placement and the same procedures repeated
following imidazole treatment.

Figures 15 and 16 clearly show that luminal placement
of a digested food solution into the jejunum increases the

capillary filtration coefficient. Furthermore. Figure 15

110

shows that imidazole enhanced the jejunal capillary
filtration coefficient both before and during luminal food
placement. Imidazole inhibits the production of
vasoconstrictor thromboxane and increases production of
vasodilator prostaglandin I2 (43, 44, 105), If this were
to occur at the level of the precapillary sphincters. the
logical consequence would be capillary recruitment and an
increase in the capillary filtration coefficient. An
increase in capillary recruitment by imidazole during the
metabolic stress of nutrient absorption could lead to an
enhanced flux of oxygen into the metabolically active
intestinal parenchamal cells by decreasing the capillary-
to-cell diffusion distance. Before food placement.
however. an increase in the jejunal capillary exchange
capacity would not result in enhanced oxygen flux. Oxygen
delivery may still be enhanced but a large gradient for
oxygen diffusion is not present in the nonabsorbing
intestine. Indeed. Figure 15 shows that imidazole enhanced
jejunal capillary exchange capacity before the metabolic
stress of nutrient absoprtion but imidazole had no effect
on resting oxygen uptake (Figure 13).

Mefenamic acid inhibits all prostaglandin production
by preventing endoperoxide formation. Mefenamic acid also
potentiates the food-induced increases in intestinal blood
flow and oxygen uptake (63). in our studies. mefenamic
acid did not alter the time dependant changes in jejunal

CFC when food was placed into the lumen (Figure 16). This

111

may be because only the relative amounts of prostaglandin
I2 and thromboxane were altered and not their ratios. The
potentiation of the food-induced increases in jejunal
oxygen uptake by mefenamic acid (63) may be due to direct
actions on tissue metabolism and not alterations in
capillary exchange capacity.

Selective inhibition of intestinal thromboxane
production increases oxygen uptake during nutrient
absorption. and this is accomplished by an increase in
oxygen extraction and not blood flow. Furthermore. this
appears not to be due to enhanced mucosal metabolism
associated with enhanced glucose absorption. Inhibition of
thromboxane synthesis also enhances intestinal capillary
exchange capacity (CFC) before and during nutrient
absorption. It may be that the potentiation of the food—
induced increase in intestinal oxygen uptake associated
with inhibition of thromboxane synthesis is the result of
concommitant microcirculatory adjustments resulting in
increases in the capillary-to—cell flux of oxygen in the
mucosa during the metabolic stress of nutrient absorption.

In conclusion. endogenous intestinal prostaglandins
serve to limit the postprandial intestinal hyperemia.
Endogenous prostaglandins appear to act as a "break" in

controlling intestinal blood flow during nutrient

‘4

. .

,-

112

absorption. Furthermore. the prostaglandins involved may
be those produced by the mucosa and their effects may be
due to alterations in the metabolic or microcirculatory

adjustments associated with nutrient absorption.

CONCLUSIONS

1. Intra-arterial infusions of arachidonic acid
produce a biphasic effect on intestinal vascular
resistance: low doses increase whereas higher doses
decrease resistance. These effects are due to

prostaglandin production and the vasoconstrictor effects

may be due to platelet born thromboxanes.

2. Arachidonic acid significantly attenuates the
food-induced increases in jejunal blood flow when
administered i.a.. i.v.. or placed into the jejunal lumen.
Luminal placement of arachidonic acid attenuates the food-
induced increases in intestinal oxygen uptake. These
effects are due to prostaglandin synthesis and not
arachidonic acid egg gg. The prostaglandins involved are

locally produced and may be those produced by the mucosa.

3. Mechanical alterations of jejunal blood flow have
no effect on jejunal oxygen uptake. This indicates that
the attenuation of the food-induced increases in jejunal
oxygen uptake by arachidonic acid was not the result of

primary changes in jejunal blood flow.

113

114

4. Food-induced changes in jejunal oxygen uptake and
blood flow are significantly correlated before (control
group) and during arachidonic acid administration (treated
group). Furthermore. the control and treated groups share

the same regression line.

5. Thromboxane synthesis inhibitors (imidazole and U—
63557A) have no effect on food induced increases in jejunal
blood flow but significantly potentiate food—induced
increases in jejunal oxygen uptake. Thromboxane synthesis
inhibitors have no effect on the rate of jejunal glucose
absorption. This indicates that the potentiation of the
food-induced increases in jejunal oxygen uptake by
thromboxane synthesis inhibitors is not due to enhanced

metabolism associated with enhanced glucose absorption.

6. Food-induced changes in jejunal oxygen uptake and
blood flow are significantly correlated before (control
group) and after thromboxane synthesis inhibitors (treated
group). Furthermore. the regression line of the treated

group is significantly different from the control group.

115

7. Luminal placement of food significantly increases

the jejunal capillary filtration coefficient (CFC).

Imidazole significantly increased jejunal CFC before and

during food placement. Mefenamic acid. a cyclooxygenase

inhibitor. had no effect on jejunal CFC before or during

food placement.

BIBLIOGRAPHY

BIBLIOGRAPHY

Ambache. N.. H.C. Brummer and J.G. Rose. Thin layer
chromatography of spasmogenic unsaturated hydroxy
acids from various tissues. J. Physiol (Lon.)

 

185: 77-78 P. 1966.

Anggard. E. and S. Bergstrom. Biological effects of

 

an unsaturated Trihydroxy acid (PGF ) from
normal swine lung. Acta. Physiol. Sca d. 58: l-
12. 1963.

Anggard E. and B. Samuelsson. Biosynthesis of

prostaglandins from arachidonic acid in guinea
pig lung. J Biol Chem 240: 3518-3521. 1965.

Armstrong. J.M.. N. Lattimer. S. Moncada and
J.R. Vane. Comparison of the vasodepressor
effects of prostacyclin and 6-oxoprostaglandin
F with those of prostaglandin E2 in rats
a d rabbits. Br. J. Pharmacol. 62. 105-129. 1978.

Balent. G.A.. Z.F. Riss. T. Varkonyi. T. Whittmann
and V. Vargo. Effects of prostaglandin E and
F on the absorption and portal transporE of
s gar on the local intestine circulation.
Prostaglandins 18: 265-268. 1979.

Baenziger. N.L.. P.R. Beckerer. and P.W. Majerus.
Characteristics of prostaglandin synthesis in
cultured human arterial smooth muscle cells.
venous endothelial cells. and skin fibroblasts.
Cell 16: 967-974. 1979.

Bartho. L. Adrenergic responses of the guinea pig
ileum: inhibition by prostaglandins and
potentiation by non-steroid anti-inflammatory
drugs. Am._Jt_Physiol. 235: 238-247. 1978.

116

10.

ll.

12.

l3.

I4.

15.

l6.

l7.

l8.

19.

117

Bell. R. L. and P. W. Majerus. Thrombin—induced
hydrolysis of phosphatidylinositol in human
platelets. J Biol Chem 255: 1790-1792. 1980.

Bennett. A.. K.G. Eley and 6.8. Scholes. Effects of
prostaglandins 5} and E2 on human. guinea pig.
and rat isolated small intestine.
Br. J. Pharmacol. 34: 630-638. 1968.

Bennett. A.. K.G. Eley and G.B. Scholes. Effects of
prostaglandins E and E on intestinal motility

in the guinea pig and rat. Br. J. Pharmacol.
34: 639-647. 1968.

Bergstrom. S.H.. H. Danielsson and B. Samuelsson.
The enzymatic formation of prostaglandin F from
arachidonic acid. Biochim Biophys Acta 96:207.
1964.

Bergstrom. S.. R. Eliasson. U.S. v. Euler. and J.
Sjovall. Some biological effects of two
crystalline prostaglandin factors.
Acta. Physiol. Scand. 45: 133-144. 1959.

Bergstrom. S. and J. Sjovall. The isolation of
prostaglandin. Acta Chem Scand 11: 1086. 1957.

Bergstrom. S. and J. Sjovall. The isolation of
prostaglandin F from sheep prostate glands.
Acta Chem Scand 14: 1701. 1960.

Beubler. E. and H. Juan. PGE release. blood flow and
transmucosal water movement after mechanical
stimulation of the rat jejunal mucosa.
Naun Schmeig:Arch. Pharmacol. 305: 91-95. 1978.

Biber. 8.. O. Lundgren and J. Svanoik. Studies on
the intestinal vasodilation observed after
mechanical stimulation of the mucosa of the gut.
Acta. Physiol. Scand. 82: 177-190. 1971.

Biber. B. Vasodilator mechanisms in the small
intestine. Acta Physiol. Scand. Suppl. 401. 1973

Bill. A. Effects of indomethacin on regional blood
flow in conscious rabbits — a microsphere study.
Acta Physiol. Scand. 105: 437-442. 1979.

81113. T.K.. J.B. Smith and M.J. Silver. Selective
release of arachidonic acid from the
phospholipids of human platelets in response to
thrombin. J Clin Invest 60: 1-6. 1977.

20.

21.

22.

23.

24.

25.

26.

27.

28.

118

Blackwell. G.J.. R.J. Flower. N. Russel-Smith.
J.A. Salmon. R.B. Thorogood and J.R. Vane.
l—n-Butylimidazole: A potent and selective
inhibitor of thromboxane synthetase.
Proceedings_of_the_B.P.S. p. 435. 1978.

Bohlen. H.G. Intestinal mucosal oxygenation
influences absorptive hyperemia. Am. J. Physiol.
239 (Heart Circ. Physiol. 8): H489-H493. 1980b.

 

Bohlen. H.G. Intestinal tissue PO and microvascular

responses during glugose exposure.
Am. J. Physiol. 238 (Heart Circ. Physiol. 7):
H164—H17l. 1980a.

Bond. J.H.. R.A. Prentiss and M.D. Levitt. The
effects of feeding on blood flow to the stomach.
small bowel. and colon of the conscious dog.

J. Lab Clin. Med. 93: 594-599. 1979.

Borgstrom. 8.. A. Dahlquist. G. Lundh and J.
Sjovall. Studies of intestinal digestion and
absorption in the human. J. Clin. Invest. 36:
1521-1536. 1957.

Botting. J.H. The effect of indomethacin on the
release of prostaglandin E and acetylcholine
from guinea pig isolated ileué at rest and during

 

field stimulation. Br. J. Pharmacol. 50: 119-
124. 1974.
Brandt. J.L.. L. Castleman. H.D. Ruskin.

J.J. Greenwald and J. Kelly. The effect of oral
protein and glucose feeding on splanchnic blood

 

flow and oxygen utilization in normal and
cirrhotic subjects. J. Clin. Invest. 34: 1017-
1025. 1955.

Brodie. T.G.. W.C. Cullis and W.D. Halliburton. The
gaseous metabolism of the small intestine. II.
The gaseous exchanges during the absorption of
Witte’s peptone. J. Physiol. 40: 173-189. 1910.

Broekman. M.J.. J.W. Ward and A.J. Marcus. Phosopho-
lipid metabolism in stimulated human platelets.
Changes in phosphatidylinositol. phosphatidic
acid. and lysophospholipids. J Clin Invest 66:
275—283. 1980.

29.

30.

31.

32.

33.

34.

35.

36.

37.

119

Bukhave. K. and J. Rask-Madsen. Saturation kinetics
applied to 1g_vitro effects of low prostaglandin
52 and F concentrations on ion trans ort across
human jgjunal mucosa. Gastroent. 8: 32-42.
1980.

 

Burns. G.P. and W.G. Schenk. Effect of digestion
and exercise on intestinal blood flow and cardiac
output. Arch._Surg. 98: 790-794. 1969.

 

Burns. G.P. and W.G. Schenk. Intestinal blood flow
in the conscious dog. Surg. Forum 18: 313-315.
1967.

 

Castenfors. H.. H. Elliasch and E. Hultman. Effects
of ingestion of hyperosmotic glucose solution on
the splanchnic circulation in normal subjects and
in partially gastrectomized patients reacting with
circulatory collapse. Scan. J. Clin. Lab.Invest.
13: 512-524. 1961.

Chapnick. B.M.. L.P. Feigen. A.L. Hyman and
P.J. Kadowitz. Differential effects of
prostaglandins in the mesenteric vascular bed.
Am. J. Physiol. 235: H326-H332. 1978.

Chou. C.C.. T.D. Burns. C.P. Hsiehm and J.M. Dabney.
Mechanisms of local vasodilation with hypertonic
glucose in the jejunum. Surgery 71: 380—387.
1972.

Chou. C.C.. C.P. Hsieh. T.D. Burns and J.M. Dabney.
Effects of lumen pH and osmolarity on duodenal
blood flow and motility. Gastoenterology 60:

648. 1971.
Chou. C.C.. C.P. Hsieh. Y.M. Yu. P. Kvietys.
L.C. Yu. R. Pittman and J.M. Dabney.

Localization of mesenteric hyperemia during
digestion in dogs. Am. J. Physiol. 230: 583-589.

1976.

Chou. C.C.. P. Kvietys. J. Post and S.P. Sit.
Constituents of chyme responsible for
postprandial intestinal hyperemia.

Am. J. Physiol. 235 (Heart Circ. Physiol. 4):
H667-H682. 1978.

38.

39.

40.

41.

42.

43.

44.

45.

46.

120

Chou. C.C.. M.J. Mangino and D.R. Sawmiller.
Gastrointestinal hormones and intestinal blood

 

 

 

 

flow. in: Physiology of the Intestinal
Circulation. edited by A.P. Shepherd and D.N.
Granger. New York. New York: Raven Press. 1984.
p. 121-130.

Chou. C.C. and H. Siregar. Role of histamine H —
80d H -receptors in postprandial intestinal
hyperem a. Am. J. Physiol. 243
(Gastrointest. Liver Physiol. 6): G248-G252.
1982.

Chow. S.L. and D. Hollander. Arachidonic acid

intestinal absorption: mechanism of transport and
influence of luminal factors on absorption in
vitro. Lipids 13: 768-776. 1978.

 

 

Dagenais. G.R.. A. Oriol and M. HcGregor.
Hemodynavic effects of carbohydrate and protein
meals in man: rest and exercise.
J. Appl. Physiol. 21: 1157-1162. 1966.

DaJani. E.Z.. R.E. Bertermann. E.. A.N. Roge.
F.L. Schweingruber and E.H. Hoods. Canine
gastrointestinal motility effects of
PGE in vivo. Am. J. Physiol. 237:

16—54. 1979.

DeFreyn. G. A TXA synthetase inhibitor reorients

endoperoxide metgbolism in whole blood towards

PGI2 and PGE2. Thromboxane Res. 26:
389-400. 1982.

Dunham. B.M.. G.A. Grindlinger. T. Utsonomiya.
H.H. Krausz. H.B. Hechtman and D. Shepro. Role
of prostaglandins in end-expiratory pressure-
1nduced negative inotropism. Am. J. Physiol.
241: H783-H788. 1981.

Dupont. J.. S. Heydani. G.L. Case. R.V. Phillips

and L.D. Lewis. Prostaglandins and their
metabolites in the gastrointestinal tract of
Yucatan miniature swine. Am. J. Clin. Nutr. 34:

 

2048-2053. 1981.

Dusting. G.J.. S. Moncada. K.H. Mullane and
J.R. Vane. Biotransformation of arachidonic acid
in the circulation of the dog. 8r. J. Pharmacol.
63: 359P. 1978.

121

47. Dusting. G.J.. S. Moncada and J.R. Vane. Vascular
actions of arachadonic acid and its metabolites
in perfused mesenteric and femoral beds of the
dog. Eur. J. Pharmacol. 49: 65-72. 1978.

 

48. Eliasson. R. Studies on prostaglandins --
occurence. formation. and biological actions.
Acta_Physiol_Scand 46:1 (suppl 158). 1959.

 

49. Euler. U.S. von. Uber die spezifische
blutdrecksenkende substanz des menslicken
prostataund samenblasensektetes. Klin. Wschr.

 

14: 1182-1183. 1935.

50. Famaey. J.P.. J. Fontaine. I. Seaman and J. Reuse.
A possible role of prostaglandins in guinea pig
isolated ileum contractions to serotonin.
Prostaglandins 14: 119-124. 1977.

 

51. Fara. J.H.. K.H. Barth. R.I. White and T.E. Bynum.
Mesenteric vascular effects of prostaglandin F

and 32. Radiol, 133: 317-320. 1979. 2
52. Fara. J.H.. E.H. Rubinstein and R.R. Sonnenshein.
Intestinal hormones in the mesenteric
vasodilation after intraduodenal agents.

Am. J. Physiol. 223: 1058-1067. 1972.

53. Fara. J.W.. E.H. Rubinstein and R.R. Sonnenshein.

 

Visceral and behavioral responses to
intraduodenal fat. Science 166: 110-111. 1969.

54. Feigen. L. and 8. Chapnick. Evidence for separate
PUG and PGF receptors in the canine mesenteric
vaséular bed. Adv. Prost. Thromb. Res. 6: 385-
387. 1980.

55. Ferreira. S.H.. A.G. Herman and J.R. Vane.
Prostaglandin production by rabbit isolated
Jejunum and its relationship to the inherent tone
of the preparation. Br. J. Pharmacol. 56: 469-
477. 1976.

56. Ferreira. S.H. and J.R. Vane. Prostaglandins: Their
disappearance from and release into the
circulation. Nature 216: 868-873. 1967.

57. Fitzgerald. G.A.. and J.A. Oates. Selective and
nonselective inhibition of thromboxane formation.
Clin. Pharmacol. Ther. 35:633-640. 1984.

58.

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

122

Fondacaro. J.D.. K.H. Walus. N. Schwaiger and
E.D. Jacobson. Vasodilation of the normal and
ischemic canine mesenteric circulation.

Gastroent. 80: 1542-1549. 1981.

Fordtran. J.S.. and T.w. Locklear. Ionic
constituents and osmolality of gastric and small-
intestinal fluids after eating. Am. J. Dig. Dis.
11: 503-521. 1966.

Frame. H.H. and I.H.M. Main. Effects of Arachidonic
acid on rat gastric acid secretion in response to
different secretagogues: Inhibition of these
effects by indomethacin. Br. J. Pharmacol. 69:
171-178. 1980.

 

Fronek. K. and A. Fronek. Combined effect of

 

exercise and digestion on hemodynamics in
conscious dogs. Am. J. Physiol. 218: 555-559.
1970.

Fronek. K. and L.H. Stahlgren. Systemic and
regional hemodynamic changes during food intake
and digestion in nonanesthetized dogs.

Circ. Res. 23: 687-692. 1968.

 

 

Gallavan. R.H.. Jr. and C.C. Chou. Prostaglandin
synthesis inhibition and postprandial intestinal
hyperemia. Am. J._Physiol. 242
(Gastrointest. Liver Physiol. 5): Gl40-G146.
1982.

Gallavan. R.H.. Jr.. C.C. Chou. P.R. Kvietys and
S.P. Sit. Regional blood flow during digestion
in the conscious dog. Am. J. Physiol. 238:
H220-H225. 1980.

Gallavan. R.H.. Jr.. and E.D. Jacobson. Pros-
taglandins and the splanchnic circulation.
Proc. Soc. Exp. Biol. Med. 170: 391-397. 1982.

Gershon. ".0. and S.H. Erde. The nervous system of
the gut. Gastroenterology 80: 1571-1594. 1981.

 

Goldblatt. A.W. A depressor substance in seminal
fluid. J. Soc. Chem. 1nd. (Lond) 52: 1056.
1933.

Granger. D.N.. P.R. Kvietys and H.A. Taylor. Role
of exchange vessels in the regulation of
intestinal oxygenation. Am. J. Physiol. 242:
G570-GS74. 1982.

69.

70.

71.

72.

73.

74.

75.

76.

77.

123

Granger. D.N.. M.A. Perry. P.R. Kvietys. and

A.E. Taylor. Permeability of intestinal
capillaries: Efect of fat absorption and
gastrointestinal hormones. Am. J. Physiol. 242:

 

8194-6202. 1982.

Granger. D.N.. J.S. Scackelford and A.E. Taylor.
PGE -induced intestinal secretion - mechanism of
enhanced transmucosal protein efflux.
Am. J. Physiol. 236: E788-E796. 1979.

Greenberg. S.P.. J. Kadowitz. J.P. Long and
w.R. Wilson. Studies on the nature of a
prostaglandin receptor in canine and rabbit
vascular smooth muscle. Circ. Res. 39: 66-76.
1976.

 

Grollman. A. Physiological variations in the cardiac
output of man. 111. The effect of the ingestion
of food on the cardiac output. pulse rate. blood
pressure and oxygen consumption of man.
Am. J. Physiol. 89: 366-370. 1929.

Hamberg. M. and B. Samuelsson. Detection and isola-
tion of an endoperoxide intermediate in
prostaglandin biosynthesis. Proc. Natl. Acad.
Sci. USA 70: 899. 1973.

Hamberg. N.. Svensson. J. and B. Samuelsson. Throm-
boxanes: A new group of biologically active
compounds derived from prostaglandin
endoperoxides. Proc. Natl. Acad Sci. USA 72:
2994. 1975.

Hatano. Y.. J.D. Kohli. L.I. Goldberg and J. Freid.
Interactions between different prostaglandins and
other relaxing agents on isolated vascular smooth
muscle. Adv. Prost. Thromb. Res. 7: 683-686.
1980.

Hawkins. H.. J. Smith. K. Nicoloau and T. Eling.
Studies of the mechanisms involved in the fate of
prostacyclin and 6-keto—PGF in the pulmonary

circulation. Prostaglandinsll6: 871-884. 1978.

Herrick. J. F.. H. E. Essex. R. C. Mann and
E.J. Baldes. The effect of digestion on blood
flow in certain blood vessels of the dog.
Am. J. Physiol. 108: 621-268. 1934.

78.

79.

80.

81.

82.

83.

84.

85.

86.

87.

124

Hopkinson. B. R. and W. G. Schenk. The
electromagnetic measurement of liver blood flow
and cardiac output in conscious dogs during
feeding and exercise. Surgery 63" 970-975.

1968.

Horton 9 E . W . ACtiOl’l Of: PGE on tissues wh i ch
respond to bradykinin. Nature 200: 892-893.
1963.

Hsieh. C. P.. J. M. Dabney. W. T. Chen. and
C. C. Chou. Effect of lumen acidity and
osmolarity on duodenal bloodflow and motility.
Physiologist 13: 227. 1970.

Jaffe. E.A.. and 8.8. Weksler. Recovery of
endothelial cell prostacylin production after
inhibition of law doses of aspirin.

J. Clin. Invest. 63: 532-535. 1979.

Kauffman. G. L.. Jr.. and J. R. Whittle. Gastric
vascular actions of prostanoids and the dual
effect of arachidonic acid. Am. J.Physiol.
(Gastrointest. Liver Physiol. 5): G582-G587.
1982.

Khalrallah. P. A.. I. H. Page and R. K. Tucker.
Some properties of prostaglandin E action on
muscle. Arch. int. Pharm. 169: 328—341. 1967.

Kimberg. D. V.. M. Field. J. Johnson. A. Henderson
and E. Gershow. Stimulation of intestinal
mucosal adenyl cyclase by cholera endotoxin and
prostaglandins. J. Clin. Inves. 50: 1218-1230.
1971.

Knapp. H. R.. O. Oelz. B. J. Sweetman and J. A. Oates
Synthesis and metabolism of prostaglandins E2.

F2 . and D2 by the rat gastrointestinal tract.
Stimulation by a hypertonic environment 1Q vitro.

Prostaglandins 15: 751-575. 1978.

Kurzrok. R. and C. C. Lieb. Biochemical studies of

 

human semen: II. The action of semen on the
human uterus. Proc. Soc. Exp. Biol. Med. 28:
268. 1930.

Kvietys. P.. R. Gallavan. R. Nyhof and C. C. Chou.
Contribution of bile to the prostprandial
intestinal hyperemia. Am. J. Physiol. 238:
G284-6288. 1980.

88.

89.

90.

91.

92.

93.

94.

95.

96.

97.

125

Kvietys. P. R.. and D. N. Granger. Relation between
intestinal blood flow and oxygen uptake.
Am._J._Physiol. 242: G202-G208. 1982.

Kvietys. P. R.. D. N. Granger. N. A. Mortillaro
and A. E. Taylor. Effect of atropine on fatty
acid induced changes in blood flow. oxygen
consumption. and water transport. Physiologist
22: 74. 1979.

 

Kvietys. P. R.. M. A. Perry and D. N. Granger.
Intestinal capillary exchange capacity and oxygen
delivery—to-demand ratio. Am. J. Physiol. 245:
G635-G640. 1983.

Lackeman. R. and L. Herman. Release of prosta-
glandin like material by various agonists from
guinea pig lung and intestine. Arch. Int. Pharm.
236: 307-309. 1978.

LeDuc. L. E. and P. Needleman. Prostaglandin
synthesis by dog gastrointestinal tract.
Adv. Prost. Thromb.:_Res. 8: 1515-1517. 1980.

LeDuc. L. E. and P. Needleman. Regional local-
ization of prostacyclin and thromboxane synthesis
in dog stomach and intestinal tract.
J. Pharm. Exp. Ther. 211: 181-188. 1979.

Lunderquist. A. Angiographic changes during diges-
tion. Am. J. Roetgenol. 107: 191-197. 1969.

Main. I. H. M. and B. J. R. Whittle. The effects of
E and A prostaglandins on gastric mucosal blood
flow and acid secretion in the rat.
Br. J. Pharmacol. 49: 428-436. 1973.

Malik. K. U.. P. Ryan and . C. HcEiff. Modifica-
tion by prostaglandins E and E . indomethacin
and arachadonic acid of the vasoconstrictor
responses of the isolated perfused rabbit and rat
mesenteric arteries to adrenergic blockade.
Circ. Res. 39: 163-168. 1976.

Manku. H. S.. D. F. Horrobin. S. C. Cunnane.
A. I. Ally. H. Karmazyn. R. A. Karmali.
R. 0. Morgan. K.1C. Nicglaou aad W. E. Barnette.
Prostaglandins E and E and I : Evidence for
three distinct actions in vascular smooth muscle.

Biochem. Biophys. Res. Comm. 83: 295-299. 1978.

98.

99.

100.

101.

102.

103.

104.

105.

'06.

107.

108.

126

Marcus. A. J. The role of prostaglandins in platelet
function. In: Progress in Hemotology. ed.. E.B.
Brown. New York: Grune and Stratton. Vol. 2. pp.
147-171. 1979.

Marcus. A. J.. H. L. Ullman and L. B. Safier.
Lipid composition of subcellular particles of
human blood platelets. J. Lipid Res. 10: 108-

 

117. 1969.

Matuchansky. C. and J. J. Brenier. Effects of PGE1
on unidirectional movement of water and
electrolytes across Jejunal mucosa in man. Gut

12: 854-855. 1971.

Matuchansky. C. andI J. J. Brenier. Effects of
prostaglandin E on Jejunal absorption in man.
Gastroent. 64: 1111-1118. 1973.

 

Matuchansky. C.. J. Y. Mary and J. J. Brenier.
Effects de la prostaglandin E sur l’absorption
du glucose et 1es mouvements transintestinaux de
l'eau et des electrolytes dans 1e Jejunum humain.
Biol. Gastroent 5: 636C. 1972.

Mikkelessen. E. and K. E. AndErsson. Contractile
effects of prostaglandin F on isolated human
peripheral arteries and veins.

Acta. Pharmacol_§t Toxicol 43: 398-404. 1978.

Miy. B. F.. W. C. Wu and G. E. Whalen. The effect
of prostaglandin E on net Jejunal transport and
mean transit time. Gastorent 66: 747-750. 1973.

Moncada. 5.. S. Bunting. K. Mullane. P. Thorogood.
and J. R. Vane. Imidazole: A selective inhibitor
of thromboxane synthetase. Prostaglandins 13:
611-618. 1977.

Norryd. C.. H. Denker. A. Lunderquist. T. Olin and
U. Tylen. Superior mesenteric blood flow during
digestion in man. Acta Chir. Scand. 141: 197-

 

202. 1975.

Nowak. J. and A. Wennonalm. Influence of indo-
methacin and of prostaglandin E on total and
regional blood flow in man.

Acta. Physiol. Scand. 102: 484-491. 1978.

Nyhof. R. A. The role of mucosal nerves in the
nutrient-induced intestinal hyperemia. Doctoral
Dissertation. Michigan State University. 1981.

109.

110.

112.

113.

114.

115.

116.

117.

118.

119.

120.

127

Nyhof. R. A. and C. C. Chou. Evidence against local
neural mechanisms for intestinal postprandial
hyperemia. Am. J. Physiol. 245: H437-H446. 1983.

Pawlik. W.. A. P. Shepherd and E. D. Joacobson.
Effects of vasoactive agents on intestinal oxygen
consumption and blood flow in dogs.
J. Clin. Inv. 56: 484-490. 1975.

Pierce. N. F.. C. C. J. Carpenter. H. L. Elliott
and W. B. Grennough. Effects of prostaglandin.
theophylline and cholera endotoxin upon
transmucosal water and electrolyte movement in
the canine jejunum. Gastroent 60: 22-32. 1971.

Rask-Madsen. J. and K. Bukhave. Saturation kinetics
applied to the inhibition by PGE2 of ion
transport in the isolated human jejunum.
Adv. Prost. Thromb. Res. 8: 1595-1601. 1980.

Renninger. E. J. and S. Nutik. Determination of car-
diac output following a meal in unanesthetized
dogs. Fed. Proc. 19: 118. 1960.

Renninger. E.J. and L. A. Sapirstein. Effect of
digestion on distribution of blood flow in the
rat. Science 126: 1176. 1957. '

Samuelsson. B. Biosynthesis of prostaglandins.
Prog. Biochem. Phamrmacol 5: 109-128. 1969.

Sanders. K. M. Endogenous prostaglandin E and
contractile activity of isolated ileal smooth
muscle. Am. J. Physiol. 234: E208-E212. 1978.

Sanders. K. M. Evidence that prostaglandins are
local regulatory agents in canine ileal circular
muscle. Am. J. Physiol. 246: 6361-6371. 1984.

Sanders. K. M. and T. E. Northrup. Prostaglandin
synthesis by microsomes of circular and
longitudinal gastrointestinal muscle.
Am. J. Physiol. 244: 6442-6448. 1983.

Sanders. K. M. and 6. Ross. Effects of endogenous
prostaglandin E on intestinal motility.
Am. J. Physiol. 234: E204-E208. 1978.

Sanger. 6. J. and A. J. Watt. The effect of PGE2 on
peristalsis and on pervivascular nerve inhibition
of peristaltic activity in guinea pig isolated
ileum. J. Pharm. Pharmacol. 30: 762-765. 1978.

 

121.

122.

123.

124.

125.

126.

127.

128.

129.

130.

131.

128

Shehadeh. Z.. W. E. Price and E. D. Jacobson.
Effects of vasoactive agents on intestinal blood
flow and motility in the dog. Am. J. Physiol.
216: 386-392. 1969.

Shepherd. A. P. Intestinal capillary blood flow dur-
ing metabolic hyperemia. Am. J. Physiol. 237:
E548-E554. 1979.

Shepherd. A. P. Metabolic control of intestinal oxy—
genation and blood flow. Fed. Proc. 41: 2084-
2089. 1982.

Shepherd. A. P. and C. 6. Burger. A solid-state
arteriovenous oxygen difference analyzer for
flowing whole blood. Am. J. Physiol. 232: H437-
H440. 1977.

 

Siragar. H. and C. C. Chou. Relative contributions
of fat. protein. carbohydrate. and ethanol to
intestinal hyperemia. Am. J. Physiol. 242: 627-
631. 1982.

Subramanian. N.. W. L. Whitmore. F. J. Seidler and
T. A. Slotkin. Histamine stimulates brain
phospholipid turnover through a direct H-l
receptor-mediated mechanism. Life Sciences 27:
1315-1319. 1980.

Sun. F. and B. Taylor. Metabolism of prostacyclin in
rat. Biochem. 17: 4096-4101. 1978.

Tibblin. S.. 6. Burns. P. B. Hahnloser and W. 6.
Schenk. Jr. The influence of vagotomy on the
superior mesenteric artery blood flow.
Surgg. Gyn. and Obstet. 129: 1231-1234. 1969.

VanDorp. D. A.. R. K. Beerthuis. D. H. Nutgeren

 

and H. Vonkeman. The biosynthesis of
prostaglandins. Biochem Biophys Acta 90: 204.
1964.

Vane. J. R. and S. Moncada. Polyunsaturated fatty
acids as precursors of prostaglandins.
Acta_Cardiol._(Brux) 23: 21-37. 1979.

Vatner. S. F.. D. L. Franklin and R. L. Van
Critters. Coronary and visceral vasoactivity
assocatied with eating and digestion in the
conscious dog. Am. J. Physiol. 219: 1380-1385.
1970.

132.

133.

134.

135.

136.

137.

138.

139.

140

141.

129

Vatner. S. F.. D. L. Franklin and R. L. Van
Critters. Mesenteric vasoactivity associated
with eating and digestion in the conscious dog.
Am. J. Physiol. 219: 170-174. 1970.

Vatner. S. F.. T. A. Patrick. C. B. Higgins and
0. Franklin. Regional circulatory adjustments to
eating and digestion in conscious unrestrained
primates. J. Appl. Physiol. 36: 524-529. 1974.

Vogt. W.. T. Suzuki and S. Bubilli. Prostaglandins
in SR S-C and in darmstoff preparation from frog

intestinal dialyates. Mem. Soc. Endocrinol. 14:
137-141. 1966.

Von Euler. U.S. Arch Exp Pathol Pharmacol 175:78.
1934.

Wallach. D. P. and E. G. Daniels. Properties of a
novel preparation of prostaglandin synthetase
from sheep seminal vesicles.
Biochim Biophys. Acta. 231: 445-457. 1971.

Wong. P.. J. McGiff. F. Sun and K. Malik. Pulmonary
metabolism of prostacyclin in the rabbit.
Biochem. Biophys. Res. Comm. 83: 731-738. 1978.

Wong. P.. F. Sun. K. Malik. L. Cagen and J. McGiff.
Metabolism of prostacyclin in the isolated kidney
and lung of the rabbit. in Prostacyclin (Eds.
Vane and Bergstrom) Raven Press. N.Y.. 1979.

Wynalda. M.A.. W. F. Jigett. and F.A. Fitzpatrick.
Sodium 5-(3'-Pyridinylmethy1) Beonzofuran - 2 -
Carboxylate (U-63557A). a new. selective
thromboxane synthase inhibitor: intravenous and
oral pharmacokinetics in dogs an correlations
with g; situ thromboxane B - production.
Porstaglandlns 26: 311-324. 1983.

' Yagasaki. 0.. M. Takai. and l. Yanagiya. Release

of prostaglandins from the passively distended
wall of the guinea pig small intestine.
Japan. J. Pharmacol. 30: 853-860. 1980.

Yu. Y.. C. Luke and C. C. Chou. Distribution of
blood flow in the intestine with hypertonic
glucose in the lumen. Surgery 78: 520-525. 1975.

130

142. Zseli. J.. T. T0pok. lS. V121 and J. Knoll. Effect
of prostaglandin E and indomethacin on responses
of longitudinal muscle of guinea pig ileum to

cholecystokinin. Eur. J. Pharm. 56: 139-144.
1979.