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THENS
'1

2003

This is to certify that the

dissertation entitled

Thirst and rehydration in dehydrated

horses
presented by

Prawit Butudom

has been accepted towards fulﬁllment
of the requirements for

 

PhD degree in Equine exercise
physiology

”wow?

Harold C. Schott II

 

Major professor

Date [0 «I? “2002

 

MS U i: an Afﬁrmative Action/Equal Opportunity Institution 042771

 

 

LIBRARY
Michigan State
University

 

 

 

PLACE IN RETURN Box to remove this checkout from your record.
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6/01 c-JCIRC/Dateouopss-p. 15

THIRST AND REHYDRATION IN DEHYDRATED HORSES
By

Prawit Butudom

A DISSERTATION
Submitted to
Michigan State University

In partial fulﬁllment of the requirements
For the degree of

DOCTOR OF PHILOSOPHY
Department of Large Animal Clinical Sciences

2002

ABSTRACT
THIRST AND REHYDRATION IN DEHYDRATED HORSES
By

Prawit Butudom

Body ﬂuid homeostasis is crucial for maintaining normal physiologic function. During
exercise, especially in endurance horses, a substantial loss of body ﬂuid by sweating can
occur. Replacement of this ﬂuid deﬁcit by drinking is important to restore body ﬂuid
homeostasis. The primary stimulus for thirst is an increase in plasma osmolality or, more
speciﬁcally, an increase in plasma sodium concentration. However, horses typically do
not drink adequately to replace sweat ﬂuid losses, even when water or other rehydration
ﬂuids are readily available. This phenomenon of incomplete restoration of body ﬂuids,
reﬂected by persisting body weight loss after exercise, has been termed “involuntary
dehydration”. The objective of my research was to investigate involuntary dehydration
and factors affecting thirst and voluntary water intake in horses dehydrated by endurance
exercise. Horses are often transported long distances within a few hours aﬂer completion
of an endurance competition. Therefore, this study focused on manipulating rehydration
during the initial hour following exercise in order to enhance recovery prior to transport.
The following hypotheses were tested: The magnitude of “involuntary dehydration ” in
horses after the ﬁrst hour of recovery from endurance exercise is aﬂected by: 1') volume
of water initially offered after exercise-induced dehydration; ii) type of rehydration

solution (water vs. saline) initially oﬂered; and iii) temperature of rehydration ﬂuid.

Limiting the volume of the initial drink of water (to 4 l, 8 l, or an unlimited amount
during the first 5 minutes of recovery) had no signiﬁcant effect on persisting body weight
loss after the ﬁrst hour of recovery. In contrast, initially offering a saline solution (0.45%
or 0.9% NaCl solution during the ﬁrst 5 minutes of recovery) resulted in greater total
ﬂuid intake and attenuated of the magnitude of involuntary dehydration than when plain
water was initially offered. Next, providing rehydration ﬂuid at near ambient temperature
(20°C) resulted in greater voluntary ﬂuid intake by the end of the initial 60-minute
recovery period, in comparison to offering ﬂuid at cool (10°C) or warm (30°C)

temperatures.

In conclusion, sodium content and temperature of rehydration ﬂuid affected voluntary
ﬂuid intake and involuntary dehydration in horses dehydrated by endurance exercise.
Thus, an initial drink of salt water (0.9% NaCl) at temperature near 20°C immediately
after exercise (or at rest periods during prolonged exercise) appears to be a good strategy
for enhancing rehydration of endurance horses, especially during competition under

conditions of high heat and humidity.

ACKNOWLEDGMENTS

This dissertation would have been impossible to accomplish without the help and
support of many people. First of all, I would like to thank to my advisor Dr. Harold
Schott II for introducing me to the studies of ﬂuid and electrolyte changes in exercising
horses. Similarly, many thanks to the other members of my graduate committee: Dr.
Thomas Adams from the Department of Physiology, Dr. Brian Nielsen from the
Department of Animal Sciences, Dr. Ed Robinson (LCS), and Dr. Tom Herdt (LCS) for
their guidance of, and patience with me. Also, special thanks to Sue Eberhart, LVT,
Shannon Axiak, DVM, Matthew Davis, Christina Kobe, and Donna Barnes for their
support and assistance with exercising horses on the treadmill and laboratory work. The
help of the VTH barn crew was also crucial and to them I also offer my thanks. Last, I
would like to thank the Royal Thai Government who provided funding support during my
graduate studies as well as the Department of Large Animal Clinical Sciences and the
Equine Health Research Fund of the American Horses Show Association for funding for

complete of the experiments.

iv

TABLE OF CONTENTS

LIST OF TABLES .............................................................................. vii
LIST OF FIGURES ............................................................................ ix
LIST OF ABBREVIATIONS ................................................................ xiii
CHAPTER 1: LITERATURE REVIEW

BODY FLUID HOMEOSTASIS IN ATHLETIC HORSES .............................. 1
REFFERENCES ................................................................................. 29

CHAPTER 2: EFFECT OF VARYING INITIAL DRINK VOLUME
ON REHDYRATION OF HORSES

ABSTRACT ...................................................................................... 36
INTRODUCTION .............................................................................. 3 7
MATERIALS AND METHODS .............................................................. 39
RESULTS ......................................................................................... 43
DISCUSSION .................................................................................... 5 O
REFFERENCES ................................................................................. 5 7

CHAPTER 3: DRINKING SALT WATER ENHANCES REHYDRATION
IN HORSES DEHYDRATED BY FUROSEMIDE
ADMINISTRATIONAND ENDURANCE EXERCISE

ABSTRACT ..................................................................................... 61
INTRODUCTION .............................................................................. 62
MATERIALS AND METHODS .............................................................. 64
RESULTS ........................................................................................ 69
DISCUSSION .................................................................................... 77
REFFERENCES ................................................................................. 84

CHAPTER 4: REHYDRATION FLUID TEMPERATURE AF F ECTS VOLUNTARY

DRINKNG BY HORSES
ABSTRACT ..................................................................................... 88
INTRODUCTION ............................................................................... 90
MATERIALS AND METHODS .............................................................. 92
RESULTS ........................................................................................ 98
DISCUSSION .................................................................................... 109
REFFERENCES ................................................................................. 115

CHAPTER 5:

GENERAL DISCUSSIONS AND CONCLUSIONS .................................... 118
REFFERENCES ............................................................................... 124
APPENDICES ................................................................................. 126

vi

LIST OF TABLES

Charm 1

Table 1. Approximate concentrations (mmol/L) of electrolytes in plasma, interstitial ﬂuid
and intracellular ﬂuid (typical muscle cell) ............................................ 6

Chapter 2

Table 1. Body weight, hematocrit (Hct), plasma osmolality (Posm), and plasma protein
(PP), sodium, potassium, and chloride concentrations in six horses before and
aﬁer completion of 45-km of treadmill exercise and after 20 and 60 min of
recovery with rehydration with 4 1, 8 l, or an unlimited amount of water during
the initial 5 min of the recovery period ................................................. 46

C_hapt_6t3

Table 1. Body weight, hematocrit (Hct), and plasma protein (PP), sodium, potassium, and
chloride concentrations in six horses before and 2 hours after (pre-exercise)
administration of furosemide (1 mg/kg IV), after completion of 45 km of
treadmill exercise, and after 20 and 60 min and ~18 h of recovery with
rehydration with water, 0.45% NaCl, or 0.9% NaCl during the initial 5 min of
the recovery period ........................................................................ 73

Chapter 4

Table 1. Body weight, hematocrit (Hct), and plasma protein (PP), sodium, potassium, and
chloride concentrations in six horses before and 2 hours after (pre-exercise)
administration of furosemide (1 mg/kg IV), after completion of 30 km of
treadmill exercise, and after 20 and 60 min and ~18 h of recovery with initial
rehydration (0 to 5 min of recovery) with 0.9% NaCl at 10, 20, or 30°C followed
by rehydration with water at 10, 20, or 30°C from 20 to 60 min of the recovery
period ...................................................................................... 102

Table 2. Metabolic heat load produced during 30-km of treadmill exercise in six horses

and cooling effects of drinking 0.9% NaCl at 10, 20, or 30°C during the initial 5
min of recovery ......................................................................... 103

Appendix
Table 2.1 Mean i SE changes in plasma (Posm) and serum osmolality (Sosm) and mixed
venous serum osmolality (Sosm), and heart rate (HR) in horses at start, and

several time points after a completion of 30 to 45-km simulated endurance ride
on a treadmill. Values for trot and canter are values for the last trot and canter for

vii

the last exercise bout .................................................................. 129

Table 6.1 Mean drinking episodes and drinking duration for rehydration ﬂuids (0.9%
NaCl solution) at 10, 20 and 30° C during 0-5 minutes of postexercise recovery
after induction of dehydration by administration of furosemide (1 mg/kg IV)
followed by 30 km of treadmill exercise for horses provided 10, 20, or 30°C
0.9% NaCl solution as the rehydration ﬂuid during the initial 5 min following
exercise and subsequently provided free access to water (at 10, 20, or 30°C)
from 20 to 60 min following exercise (Chapter 4) .............................. 135

viii

Chapter 1

Figure 1.

Figure 2.

granted

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Chapter 3

Figure 1.

Figure 2.

Figure 3.

LIST OF FIGURES

The distribution of total body water (TBW) in a 450—kg horse presented both
as percent of body mass and as absolute volume (L) ........................... 4
A model illustrating the three major pathways for stimulating of thirst:

osmoreceptors, hypovolemia, and angiotensin II ............................... 12
Experimental protocol .............................................................. 40

Percent body weight loss during and after 45-km of treadmill exercise in

six horses offered 4 I (ﬁlled circles), 8 1 (ﬁlled inverted triangles), or an
unlimited amount (UW, ﬁlled squares) of water during the initial 5 min of
recovery ............................................................................... 47

Total water intake during the 60 min recovery period after induction of
dehydration by 45-km of treadmill exercise in six horses provided 4 l, 8 I, or
an unlimited amount (UW) of water during the initial 5 min of recovery and
subsequently provided free access to water from 20 to 60 min following
exercise ................................................................................ 48

Upper panel: Correlation between body weight loss persisting after 60 min
of recovery and total water intake from 0 to 60 min of the recovery period.
Lower panel: Correlation between plasma sodium concentration after 20 min
of recovery and water intake from 20 to 60 min of the recovery .............. 49

Experimental protocol .............................................................. 67

Percent body weight loss aﬁer furosemide treatment (1 mg/kg IV) and
during and after 45 km of treadmill exercise ride in six horses offered water,
0.45% NaCl, or 0.9% NaCl during the initial 5 min of recovery ............. 74

Total ﬂuid intake during the 60 minute recovery period after induction of
dehydration by furosemide treatment (1 mg/kg IV) followed by 45 km of
treadmill exercise in six horses provided water, 0.45% NaCl, or 0.9% NaCl
ﬂuid during the initial 5 min of recovery and subsequently provided free
access to water from 20 to 60 min following exercise ......................... 75

ix

Figure 4. Correlation between total ﬂuid intake from 0 to 60 minutes of the recovery

period and persisting body weight loss after 60 minutes of recovery ............ 76
Chapter 4
Figure 1. Experimental protocol ............................................................... 95

Figure 2. Percent body weight loss after furosemide treatment (1 mg/kg IV) and
during and after 30-km of treadmill exercise ride in six horses offered 0.9%
NaCl at 10, 20, or 30°C during the initial 5 min of recovery and subsequently
provided free access to water (at 10, 20, or 30°C) from 20 to 60 min
following exercise ................................................................... 104

Figure 3.Total ﬂuid intake during the 60 minute recovery period after induction of
dehydration by furosemide treatment (1 mg/kg IV) followed by 30 km of
treadmill exercise in six horses provided 0.9% NaCl at 10, 20, or 30°C
during the initial 5 min of recovery and subsequently provided free access
to water (at 10, 20, or 30°C) from 20 to 60 min following exercise ......... 105

Figure 4. Correlation between total ﬂuid intake from O to 60 minutes of the recovery
period and persisting body weight loss after 60 minutes of recovery ....... 106

Figure 5. Pulmonary arterial blood temperature (Tcore) during two 15-km bouts of

exercise in six horses provided 0.9% NaCl at 10, 20, or 30°C during the
initial 5 min of recovery ........................................................... 107

Figure 6. Correlation between changes in pulmonary arterial blood temperature
(Tcore) from 0 to 5 minutes of recovery and the estimated amount of heat
transfered to ingested 0.9% NaCl during the initial 5 minutes of recovery
........................................................................................ 108

Appendix

Figure 1.1 Experimental protocol -— Exercise speeds: All horses performed a 45-km
simulated treadmill endurance riding which is similar to the ﬁrst 25 miles
of a typical endurance ride with varying speeds. Each 15 km exercise bout
consisted of two 26 min cycles of treadmill exercise at speed varying from
2 (walk), 4 (trot) to 8 (canter) m/s at 0° treadmill slope ..................... 127

Figure 3.1 Correlation between total water intake (liters) from 0 to 60 minutes of
recovery period and change in plasma osmolality (mOsm/kg) after 60
minutes recovery period after induction of dehydration by 45-km of
treadmill exercise. Horses were provided 4 1 (ﬁlled circles), 8 1 (circles),
or an unlimited amount (U W, ﬁlled inverted triangles) of water during the
initial 5 min following exercise and subsequently provided free access to

water from 20-60 min following exercise (Chapter 2) ....................... 130

Figure 3.2 Correlation between total water intake (liters) from 20 to 60 minutes of

recovery period and change in plasma osmolality (mOsm/kg) after 20
minutes recovery period after induction of dehydration by 45—krn of
treadmill exercise. Horses were provided 4 1 (ﬁlled circles), 8 l (circles),
or an unlimited amount (UW, ﬁlled inverted triangles) of water during

the initial 5 min following exercise (Chapter 2) ............................... 131

Figure 4.1 Correlation between total water intake (liters) from 0 to 60 minutes of

recovery period and change in plasma osmolality (mOsm/kg) after 60
minutes recovery period after induction of dehydration by furosemide

(1 mg/kg IV) treatment followed by 45-km of treadmill exercise for

horses provided water during the initial 5 min following exercise and
subsequently provided free access to water from 20 to 60 min following
exercise (Chapter 3) .............................................................. 132

Figure 4.2 Correlation between total water intake (liters) from 20 to 60 minutes of

recovery period and change in plasma osmolality (mOsm/kg) after 20
minutes recovery period aﬂer induction of dehydration by furosemide

(1 mg/kg IV) treatment followed by 45 km of treadmill exercise for horses
provided water (ﬁlled circles), 0.45% NaCl (open circles), or 0.9% NaCl
(ﬁlled inverted triangles) as the rehydration ﬂuid during the initial 5 min
following exercise and subsequently provided free access to water from

20 to 60 min following exercise (Chapter 3) ................................. 133

Figure 5.1 Correlation between total ﬂuid intake during the 60 minute recovery period

after induction of dehydration by administration of furosemide (1 mg/kg
IV) followed by 30 km of treadmill exercise for horses provided 10 (ﬁlled
circles), 20 (circles), or 30°C (ﬁlled inverted triangles) 0.9% NaCl solution
as the rehydration ﬂuid during the initial 5 min following exercise and
subsequently provided free access to water (at 10, 20, or 30°C) from 20 to
60 min following exercise (Chapter 4) ........................................ 134

Figure 7.1 Correlation between total water intake (liters) from 20 to 60 minutes of

recovery period and change in plasma volume (%) at 20 min recovery
period aﬂer induction of dehydration by 45-km of treadmill exercise.
Horses were provided 4 I (ﬁlled circles), 8 l (circles), or an unlimited
amount (UL), ﬁlled inverted triangles) of water during the initial 5 min
following exercise and subsequently provided free access to water from

20 to 60 min following exercise (Chapter 2) ................................. 136

Figure 7.2 Correlation between total water intake (liters) from 20 to 60 minutes of

recovery period and change in plasma volume (%) aﬁer 20 minutes
recovery period after induction of dehydration by furosemide (1 mg/kg IV)
treatment followed by 45 km of treadmill exercise for horses provided

xi

water (ﬁlled circles), 0.45% NaCl (open circles), or 0.9% NaCl (ﬁlled
inverted triangles) as the rehydration ﬂuid during the initial 5 min following
exercise and subsequently provided free access to water from 20 to 60 min
following exercise (Chapter 3) ................................................... 137

Figure 7.3 Correlation between total ﬂuid intake from 20 to 60 min recovery period
after induction of dehydration by administration of furosemide (1 mg/kg
IV) followed by 30 km of treadmill exercise for horses provided 10 (ﬁlled
circles), 20 (circles), or 30°C (ﬁlled inverted triangles) 0.9% NaCl solution
as the rehydration ﬂuid during the initial 5 min following exercise and
subsequently provided free access to water (at 10, 20, or 30°C) from 20 to
60 min following exercise (Chapter 4) ........................................ 138

xii

ADH =
ALD

ATP

B

BW

BM

CSF

ECF

F1

F1 0’-5’ post
GER =
Hct
hr
HR
ICF
ISF =
IV
min
MH
ORS =
Posm
PP
SDF
Sosm :
TBW =
Tcore =
UW =
VOzmax =
w =
WI =
WI 0’-5’ post =
W120’-60’ post =

LIST OF ABBREVIATIONS

Antidiuretic hormone

Aldosterone

Adenosinetriphosphate

Blood

Body weight

Body mass

Cerebrospinal ﬂuid

Extracellular ﬂuid

Fluid intake

Fluid intake during 0 to 5 minute of recovery
Gastric emptying rate

Hematocrit

Hour

Heart rate

Intracellular ﬂuid

Interstitial ﬂuid

Intravenous

Minute

Metabolic heat

Oral rehydration solution

Plasma osmolality

Plasma protein concentration
Synchronous diaphragmatic ﬂutter
Serum osmolality

Total body water

Pulmonary arterial blood temperature
Unlimited water intake

Maximal oxygen uptake

Water

Water intake

Water intake during 0 to 5 minutes of recovery
Water intake during 20 to 60 minutes of recovery

xiii

Chapter 1

Literature review

Body ﬂuid homeostasis in athletic horses

BODY FLUID HOMEOSTASIS IN ATHLETIC HORSES

Physiology of body ﬂuids

Fluid and electrolyte homeostasis

Body ﬂuids (water and electrolytes) are crucial for maintaining normal physiologic
functions. Water and electrolytes are the Milieu interieur, the internal medium
surrounding and sustaining cells of the body. This concept was formulated by the French
physiologist Claud Bernard in the nineteenth century (Peavy 1995). In order for cells of
any tissue to function optimally, it is important that the composition of body ﬂuids is
maintained and regulated appropriately. Perturbations of body ﬂuids can adversely affect
normal physiologic functions, and when severe, can be life-threatening. Therefore, near
constancy of composition and distribution of body ﬂuids is essential for maintenance of
normal cell functions. Physiologic responses that counteract perturbations of the internal
medium (body ﬂuids) and maintain its constancy are collectively called homeostasis.
Walter B. Cannon, an American physiologist in the 20th century introduced this concept
of physiologic self-regulation (Peavy 1995). Homeostatic responses enable the body to
maintain near constant composition and disposition of body ﬂuids in the face of normal

physiologic events. They include both neural and humoral regulatory mechanisms.

Body water and electrolyte content

Total body water (TBW) accounts for 45 to 50 % of body weight (BW) in women and
55 to 60% of BW in men (Rose 1989). Women have lower body water content, in
comparison to men, due to a higher percentage of adipose tissue (Rose 1989). However, a
sex difference in TBW has not been described in horses. In addition, body water content
also varies with age in that the newborn infant has the highest water content (about 70%
of BW); subsequently, water content decreases with age (Stanton and Koeppen 1998).
Total body water is distributed between several body ﬂuid compartments that can be
expressed as a percentage of BW. The two main compartments are intracellular ﬂuid
(ICF) and extracellular ﬂuid (ECF). In humans, ICF accounts for 36% of BW and ECF
accounts for 25% of BW (Rose 1989). The ECF compartment is subdivided into
interstitial ﬂuid (ISF) and plasma; these compartments are separated by the capillary
wall. Interstitial ﬂuid, which represents ﬂuid surrounding cells in various tissues of the
body, comprises 75% of ECF volume. Plasma volume constitutes the remaining 25% of
ECF volume. Three smaller ECF compartments have also been described: water found in
dense connective tissues, such as cartilage and tendons; water in bone; and transcellular
water, which is composed of cerebrospinal, pleural, peritoneal, synovial, and intraocular

ﬂuid as well as ﬂuid in the lumen of the gastrointestinal tract. (Rose 1989).

In the horse, TBW is about 65 to 70% of total BW (Carlson 1987a; Carlson 1987b).
ICF accounts for approximately 43 to 46% of BW and water in interstitial spaces and in
plasma represents approximately 10 to 12% and 4 to 6% of BW, respectively (Schott and
Hinchcliff 1993). In contrast to man, the contents of the gastrointestinal tract of the horse

contain a substantial amount of water (about 6% of BW) (Meyer and Coenen 1989). This

component of transcellular ﬂuid represents an important reservoir for water and
electrolytes during prolonged exercise. Figure 1 illustrates the distribution of total body

water in a typical 450-kg horse.

 

 

 

 

 

 

ECF ICF
GUT PLASMA INTERSTITIAL CELL
FLUID
+ —:—+ ?
27L ‘ 25L I: 48L p ZOOL
6% 5.6% 10.7% 44.4%

 

 

 

 

 

 

 

 

ﬁgm. Distribution of total body water in a 450-kg horse presented both as percent of
body weight and as absolute volume (L). The four major compartments are plasma,
interstitial ﬂuid, intracellular ﬂuid (ICF), and trancellular ﬂuid (mainly in gut). About
200 L of TBW is ICF, and the remaining 100 L is extracellular ﬂuid (ECF). The arrows

show body water exchanges between the four compartments.

Table 1 summarizes the distribution of cations and anions between the intracellular
and extracellular ﬂuid compartments (Rose 1977). Sodium is the major extracellular
cation and potassium is the major intracellular cation. Similarly, the major intracellular
anions are proteins and phosphate, whereas the major extracellular anions are chloride
and bicarbonate. Because plasma and ISF are separated only by a capillary wall, which is
freely permeable to small ions, the ionic composition of these two compartments of the
ECF is similar. However, a major difference between the composition of ISF and plasma
is that plasma contains a greater amount of protein. The negative charge on plasma
protein holds a greater amount of cations (predominantly sodium) in the plasma water
and, as a consequence of the Gibbs-Donnan equilibrium, the distribution of cations and
anions between ISF and plasma is slighly different (Rose 1977; Kutchai 1998; Stanton
and Koeppen 1998; Guyton and Hall 1996). The Donnan effect is the relation that holds
any univalent cation and anion pair in equilibrium between the two compartments
(Kutchai 1998). Although the Donnan effect can affect the distribution of cations and
anions across capillary walls, this effect is normally quite small, and the ionic
composition of the ISF and plasma can be considered identical (Stanton and Koeppen
1998; Guyton and Hall 1996). The asymmetric distribution of sodium and potassium ions
across the plasma membrane (i.e. between ICF and ECF) is maintained by different
membrane permeabilities for sodium and potassium and activity of the Na+K+-ATPase.
By action of the latter, two sodium ions are pumped out of the cell in exchange for three

potassium ions.

 

 

 

 

 

 

 

Plasma Interstitial Skeletal
Ions Plasma Watera Fluid muscle cell
Cations
Na+ 142.0 152.7 145.1 12.0
K+ 4.3 4.6 4.4 150.0
Ca2+ 2.5 2.7 2.4 4.0
Mg2+ 1.1 1.2 1.1 34.0
Total 149.9 161.2 153.0 200.0
Anions
Cl" 104.0 111.9 117.4 4.0
HCO3' 24.0 25.8 27.1 12.0
Phosphate 2.0 2.2 2.3 40.0
Protein’ 14.0 15.0 0.0 54.0
Othersb 5.9 6.3 6.2 90.0
Total 149.9 161.2 153.0 200.0

 

 

Table 1. Approximate concentrations (mEq/L) of ions in plasma, interstitial fluid and
intracellular ﬂuid (typical muscle cell) (Adapted from Rose 1977). a = Plasma water

content assumed to be 93%, b = This largely represents organic phosphates such as ATP.

Regulation of water balance

A dynamic balance between water loss and gain normally maintains TBW within a
narrow range. As reviewed by Anderson (1978), water is lost through urine, feces, sweat,
and humidiﬁcation of inspired air. Water is gained by intake of food, water drinking, and
water from metabolism. F inc-tuning of water balance is achieved by action of antidiuretic
hormone (ADH) on the collecting ducts in the kidneys. A decrease in TBW initially
induces secretion of ADH and subsequently stimulates thirst. Thus, the first response to a
decrease in TBW is an increase in ADH secretion to enhance water reabsorption by the
kidneys and, with more severe water deﬁcits, drinking is stimulated by thirst. Two
regulatory mechanisms work in parallel to correct decreases in TBW: 1) osmotic
regulation, which acts in response to reduced volume of cerebral osmoreceptors (cell
dehydration); and 2) volume regulation which acts in response to decreases in blood

volume (hypovolemia).

1. Osmotic regulation

An increase in the osmolality of body ﬂuids, consequent toa decrease in TBW, is the
most important stimulus for water gain. In 1937, Gilman observed that administration of
a hyperosmolar NaCl solution resulted in water drinking by euhydrated subjects. In
addition, much greater water intake was observed with administration of the
hyperosmolar NaCl solution in comparison to administration of an equally hyperosmolar
solution of urea (a solute that freely crosses cell membranes). Although both solutes

produced similar increase in plasma osmolality (Posm), only the NaCl solution caused a

fall in plasma protein concentration, indicating movement of water from ICF to ECF.
Thus, cellular dehydration, not simply an increase in Posm, appears to be important for
osmotic regulation of water balance. Speciﬁcally, a change in volume of osmoreceptors
located in the anterior hypothalamus is necessary for osmotic stimulation of ADH release

and thirst.

Initially, rather modest increases in plasma tonicity (e. g., a 3 to 5 mOsm/kg increase in
Posm or a 1 to 2 mmol/L increase in plasma sodium concentration lead to secretion of
ADH. Again, ADH acts by inserting more water channels in the collecting duct epithelial
cells to increase renal water conservation. With greater increase in Posm (e.g., 3%, 3 %,
or 3-6 % in humans, ponies and dogs, respectively) thirst is also induced (Wood et al.

1977).

2. Volume regulation

Another stimulus for ADH secretion and thirst is a reduction in plasma volume (i.e.,
hypovolemia). Hypovolemia can induce ADH release and thirst by either cardiovascular
reﬂexes or activation of the renin-angiotensin system (Anderson 1978; Fitzsimmons
1998). Hypovolemia is detected by stretch receptors in the right atrium of the heart that
may directly stimulate ADH release and thirst via neural pathways. Hypovolemia, which
decreases renal perfusion, also activates the renin-angiotensin system, which provides a

further stimulus for ADH secretion and thirst (Fregly 1982; Phillips et al. 1982).

The renin-angiotensin system can be activated by a decrease in blood pressure, a
decrease in plasma sodium concentration ([Na+]), an increase in plasma potassium
concentration ([K+]), or B-adrenergic receptor stimulation via adrenergic nerves or
circulating catecholamines. These stimuli increase renin release from the kidneys. After
cleavage from angotensinogen by renin, angiotensin I is converted to angiotensin II by
converting enzyme in the lungs. Angiotensin II may affect water gain directly by
stimulating supraoptic neurons to induce ADH secretion and hypothalamic centers
associated with drinking and indirectly by stimulating aldosterone secretion and
increasing tubular reabsorption of sodium. A somewhat separate renin-angiotensin
system located within the central nervous system (anterior hypothalamus and peoptic
region of the brain) also appears to be involved in thirst stimulation. The most sensitive
structures to the dipsogenic action of the angiotensin II are the subfomical organ and the

organ vasculosum of the lamina terminalis (Fitzsimmons 1998).

Horses drink in response to an isosmotic loss of blood volume and a decrease of
plasma volume to 6% is sufficient to stimulate drinking (Hinton 1977). Reduced plasma
volume and increased water intake has also been demonstrated in sheep (Zimmerman and
Stricker 1978) and rats (Rabe 1975) after furosemide administration. In horses, the dose
of furosemide necessary to stimulate signiﬁcant intake is on the high end (2 mg/kg) of the

recommended clinical dose (Suﬁt et al. 1985).

With hypovolemia, activation of the renin-angiotensin system has been suggested to

be more important than cardiovascular reﬂexes in volume regulation of water gain.

Maximal drinking was preceded by an elevation of plasma angiotensin 11 concentration,
suggesting that activation of the renin-angiotensin system, rather than diminished
stimulation of cardiovascular distension receptors, was the crucial thirst-eliciting factor.
Finally, hypovolemia appears to be a less potent stimulus for water gain than activation
of osmoreceptors. For example, experiments in man, monkeys, rats and dogs have
demonstrated that at least 10-15% of blood volume has to be lost before an increase in
ADH secretion or drinking is observed (reviewed by Greenleaf (1992) and Anderson
(1978). In contrast, as mentioned above, an increase of plasma tonicity by as little as 1 to

2% can produce an increase in ADH secretion.

In the equine species, most drinking occurs rapidly (within the first couple of minutes)
after water is provided to dehydrated su jects (Suﬁt et al. 1985; Jones et al. 1989;
Dﬁsterdieck et al. 1999). In euhydrated animals most water drinking accompanies eating
(peripandial drinking) (Kissileff 1969; Houpt et al. 1983; Suﬁt et al. 1985; Jones et al.
1989). Management practices should accommodate the horse’s drinking patterns by
providing water ad libitum in association with feeding. An important difference between
humans and horses is the ability of the gastrointestinal tract to function as a reservoir for
water and electrolytes, containing up to 6% of BW, that can be utilized by dehydrated

horses (e. g., during prolonged exercise or after furosemide administration).

A model of thirst and drinking induction

Although the physiological mechanisms that induce thirst and drinking are not fully
understood, a model illustrating the major regulatory mechanisms: osmoreceptors
(cellular dehydration), hypovolemia (extracellular dehydration), and activation of the
renin-angiotensin system is demonstrated in Figure 2 (Greenleaf 1992). At least six
distinct stimuli have been used experimentally for studying the mechanisms of water
intake (reviewed by Greenleaf and Fregly 1982): 1) hypertonic saline; 2) B-adrenergic
agonist (isoproterenol); 3) angiotensin II; 4) polyethylene glycol; 5)
parasympathomimetic agents; and 6) dehydration. The results of these stimuli suggest
that multiple factors affect thirst. According to the proposed model, the mechanisms of
cellular and extracellular dehydration can act either synchronously or independently to
induce ﬂuid intake. In addition to these major pathways, dry mouth and oropharyngeal

mechanisms also play a role in ﬂuid intake.

11

 

Dry mouth/Oropharyngeal
mechanisms

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ICF T Plasma tonicity
4 Cellular dehydration } Osmorecentors

T
Stimuli j 4

V 113?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Polyethylene Glycol + Thirst/
3:052:55. 18100451] ’ 0......
Hypertonic Saline TB-Adrenergic AA
Rznin Hypovolemia
Angithensin I i ,
CE Ca d1ac #
A Y receptors

 

Angiotensin II

 

 

 

 

 

 

Receptors in CircumY/entricular organs

Figure 2. A model illustrating major pathways-osmoreceptors, hypovolemia, angiotensin
II-and associated factors for induction of ﬂuid intake. J GA = J uxtaglomerular Apparatus;

ACE = angiotensin converting enzyme Adapted from Greenleaf (1992).

12

Body fluid shifts during exercise

During exercise, metabolism in muscle converts potential chemical energy into kinetic
mechanical energy. In addition to work, by-products of increased muscle metabolism
include heat, acid, C02 and H20 release from muscle glycogen breakdown (McMiken
1983)

Fuel +oxygen -—> work + acid + heat + C02 + H20

A wide range of physiological changes in ﬂuid, electrolyte, and acid-base balance

occurs in the exercising horse. These changes depend on both the intensity and duration

of exercise.

High intensity exercise

Fluid and electrolyte losses by sweat are of minimal importance during short term
high-intensity exercise (reviewed by Schott and Hinchcliff 1993). However, ﬂuid and
electrolytes shift between ﬂuid compartments during strenuous exercise. The major ﬂuid
shiﬂ is water movement from vascular and ISF spaces into active muscle cells. This
occurs in response to an increase in intracellular osmotic forces due to by products of the
increased metabolic rate (i.e., lactate, phosphate and creatine). Some of these metabolic
by products, most notably lactate, also leave the cell leading to an increase in ISF
osmolality and osmotic pressure. The net effect of this water shift is a reduction in plasma
volume and increase in Posm and plasma protein concentration (PP). During high-
intensity exercise in horses, Posm may increase from z280 mOsm/kg at rest to :315

mOsm/kg and PP may increase from 6.0 to 6.5 g/dl to 7.5 to 8.0 g/dl from rest to the end

13

of exercise (reviewed by Schott and Hinchcliff 1993). This dramatic ﬂuid shift results in
substantial hemoconcentration (i.e., packed cell volume >60%) but there is no evidence

that it adversely affects performance.

Of perhaps greater interest are the electrolyte changes accompanying high-intensity
exercise. Speciﬁcally, ([K+]) may approach 10 mmol/l during strenuous exercise in
horses (Harris and Snow 1988; Harris and Snow 1992). This marked increase in [K+] is
due to loss of potassium from active muscle cells (Schott et al. 2002). Hyperkalemia, in
combination with a decrease in intracellular [K+], may lead to a decrease in contractile
force by blocking depolarization (Carlson 1987a; Coffman et al. 1978; Schott et a1.
2002). In summary, during high-intensity exercise, a rapidly increasing body temperature,
accumulation of acid, and electrolyte changes, speciﬁcally hyperkalemia, may be more

important factors contributing to fatigue than body ﬂuid shifts.

Prolonged, low-intensity exercise

During low-intensity exercise, ﬂuid and electrolyte shifts are similar in direction to
those occurring during high-intensity exercise, but the magnitude of change is generally
smaller (reviewed by Schott and Hinchcliff 1993). In contrast to high-intensity exercise,
progressive depletion of ﬂuid and electrolyte stores is a greater challenge to maintenance
of homeostasis than is any shift of ﬂuid or electrolyte (Schott and Hinchcliff 1993). Fluid
and electrolyte depletion is a consequence of loss of large amounts of ﬂuid and

electrolytes by sweating. Equine sweat contains high concentrations of sodium (143.0 i

14

9.0 mmol/l), potassium (28.2 i 2.1 mmol/l), and chloride (158.0 i 7.1 mmol/l)
(McCutcheon et al. 1995). Based on loss of body weight, sweating rates in horses during
low-intensity exercise have been estimated to approach 10 to 15 liters per hour (Carlson
1985; Carlson 1987a; Carlson 1987b). Depending on environmental conditions,
endurance horses may lose 30 to 45 kg (as sweat) over the course of a ride (Carlson 1983;
Carlson 1987a; Carlson 1987b). In addition to water loss in sweat, horses also lose large
amounts of electrolytes during prolonged exercise. In the approximately 2 hours during
which horses completed a treadmill speed and endurance exercise test which simulated
running speeds and distances required for this phase of an Olympic level (CCI****) 3-
day-event under hot ambient conditions (temperature = 33 to 35°C, relative humidity =
45 to 55%), sweat ion loss (mainly sodium and chloride) was ~6500 mmol (as sweat ﬂuid
loss was ~19 liters) (McCutcheon and Geor 1996). Prolonged sweating results in
signiﬁcant chloride loss and a reduction in plasma chloride concentration and endurance
horses typically develop a hypochloremic metabolic alkalosis during the later stages of
endurance exercise (Schott and Hinchcliff 1993). Endurance horses may also develop
respiratory alkalosis during exercise as hyperpnea, hypocapnea, and an increased blood
pH were reported in horses exercised on a treadmill at 40% V02 max for 60 minutes
(Bayly et al. 1995). In summary, during prolonged, low-intensity exercise, ﬂuid loss in

sweat is likely an important factors limiting performance (i.e., contributing to fatigue).

Effects of body ﬂuid loss on performance

Hi gh-intensity exercise

Changes in body ﬂuid balance prior to exercise may affect performance. In human
athletes, hyperhydration prior to exercise can improve performance, whereas
hypohydration may decrease performance (Sawka and Pandolf 1990; Sawka 1992).
However, mild dehydration appears to have little effect on performance of short bouts of
high-intensity exercise. In racing horses, hypohydration is commonly induced by
furosemide administration (0.5 to 1.0 mg/kg, IV) prior to the race and this practice could
be expected to affect performance. Brieﬂy, furosemide-induced dehydration results in an
isotonic loss of 5 to 10% of ECF water and electrolyte content (Hinchcliff and Muir
1991). This reduction in ECF also produces a 2 to 3% loss in body weight. A loss of body
weight has recently been speculated to actually enhance performance in racing horses

(Gross et al. 1999).

Prolonged, low-intensity exercise

Metabolic heat production by an endurance horse (speed ~8 m/s) could increase
pulmonary arterial blood temperature (Tcore) by ~21°C/hr if no heat were dissipated
(Guthrie and Lund 1998). However, more than 90% of metabolic heat generated during
prolonged endurance exercise is dissipated (Kingston et al. 1997). As previously
mentioned, ﬂuid loss via sweat can be substantial and may approach 10 to 15 liters per
hour in endurance horses (Carlson 1983). This ﬂuid loss can lead to a decrease in
circulating volume and competition between tissues (e.g., active muscle and skin) for
blood ﬂow. Furthermore, under hot ambient condition (32° to 34°C), a decreased

sweating rate was observed in horses exercising at 50% Vozmax for 90 minutes

16

(McCutcheon and Geor 1998). Thus, progressive dehydration may limit continued
performance during prolonged exercise by impairing therrnoregulation (Schott and
Hinchcliff 1993; Schott et al. 1997). Initially, mild dehydration (~3% of BW loss) may
reduce performance (Dahlbom et al. 1995) but as dehydration becomes more severe (7
tolO% BW loss), serious medical problems consequent to ﬂuid and electrolyte depletion,
hypertherrnia, and heat exhaustion can develop (Carlson 1985; Goer and McCutcheon
1996). Characteristic clinical signs exhibited by horses approaching fatigue during
prolonged exercise include: reduced effort and responsiveness; reluctance or inability to
continue; slow capillary reﬁll time and jugular vein distensability (hypovolemia); lack of
gastro-intestinal sounds (poor motility); thermoregulatory failure (elevated rectal
temperature); and a persistently elevated heart rate. Further, due to loss of large amounts
of electrolytes in sweat, plasma tonicity is decreased. As discussed later, hypo-osmotic
dehydration can diminish thirst despite marked dehydration. If these signs are not
recognized or appropriate treatment (oral or intravenous ﬂuid) is not instituted, various
medical problems may develop. These can include postexertional ileus (Schott and
Charlton 1996), synchronous diaphragmatic ﬂutter (SDF) (Schott et al. 1997), or a
combination of medical problems that have been collectively grouped with the term

exhaustive horse syndrome (Carlson 1983; Carlson 1985).

Involuntary dehydration

Involuntary dehydration, also called “voluntary dehydration”, is a term used to

describe persistent body ﬂuid depletion and a persisting loss in BW resulting from

17

incomplete voluntary replacement of body ﬂuid losses (Adolph et a1. 1947). That is, the
subject drinks to satiety, but a water deﬁcit remains. Although “involuntary dehydration”
is a well-recognized phenomenon, little is known about its physiological mechanism. In
humans, involuntary dehydration has been observed following endurance exercise or
exposure to hot or cold ambient conditions (Greenleaf 1992). During exercise and heat
exposure, ﬂuid loss through sweating appears to be the major factor contributing to
involuntary dehydration, while cold exposure appears to lead to peripheral
vasoconstriction and increased ﬂuid loss in urine. Greenleaf and co-workers found that
the magnitude of involuntary dehydration was directly proportional to the level of
exercise or thermal stress on the subject and the greater the body water deﬁcit, the longer
it took for complete restoration of that deﬁcit (reviewed by Greenleaf 1992). Although
unproven, “involuntary dehydration” may be a protective mechanism to prevent
hyponatremia that could develop consequent to full replacement of body ﬂuid losses by

water drinking alone (Greenleaf 1992).

Along with the amount of exercise or thermal stress, studies in humans have shown
that additional factors can inﬂuence the magnitude of involuntary dehydration. An
obvious factor is availability of water or other rehydration ﬂuids. Other more subtle
factors include type, temperature, and ﬂavoring of ﬂuid available for rehydration (Boulze
et al. 1983; Sandick et al. 1984; Barr et al. 1991; Mitchell et al. 1994). Further,
considerable interindividual variability in voluntary drinking has been reported in human
subjects performing exercise or exposed to thermal stress. In fact, human athletes have

been characterized as either “better drinkers” (that maintain less than a 2% BW loss) or

18

“poorer drinkers” (that maintain greater than 2% BW loss) (Szlyk et al. 1989). Finally,
involuntary dehydration also appears to be affected by social customs that inﬂuence what
is consumed as well as the capacity and rate of ﬂuid absorption from the gastrointestinal

system (reviewed by Greenleaf, 1992).

Mechanisms of ﬂuid intake related to “involuntary dehydration”

According to the proposed model of ﬂuid intake (Figure 2), increases in Posm or
[Na+], hypovolemia, and activation of the renin-angiotensin system are the stimuli for
thirst and drinking. Stimulation of thirst is clearly an important factor affecting voluntary
rehydration and recovery of water losses. Because sweat contains substantial amounts of
Na+, the increases in Posm and [Na+] during endurance exercise are less than would be
produced by loss of similar volume of pure water. Although small increases are likely
corrected by drinking various rehydration beverages, attenuation of these increases
appears to be an important factor for development of involuntary dehydration. Next,
despite ﬂuid shifts from ISF and ICF into plasma, in attempt to maintain plasma volume,
mild hypovolemia may develop during exercise leading to both direct stimulation of thirst
and indirect stimulation of thirst by activation of the rennin-angiotensin system. As
discussed earlier, “involuntary dehydration” may act as a protective mechanism to guard

against excessive plasma dilution leading to hyponatremia.

19

Involuntary dehydration and equine athletes

Although development of involuntary dehydration in horses performing prolonged
exercise has not been speciﬁcally described, a number of investigators have reported BW
losses of 3 to 7% in horses competing in 3-day events and endurance rides (Lawrence et
al. 1992; Ecker and Lindinger 1992; Andrews et al. 1994; Schott et a1. 1996; Schott et al.
1997). As recently conﬁrmed by Kingston et al. (1997), weight loss by horses performing
prolonged exercise is an accurate estimate of ﬂuid loss as sweat. Furthermore, a 3 to 4%
BW loss can persist after an overnight recovery period (Schott et al. 1996; Schott et al.
1997), suggesting that substantial time may be required for complete restoration of sweat
ﬂuid losses. Persisting BW loss occurred in endurance horses even though horses had free
access to water during the ride and recovery period. During a simulated ﬁeld endurance
ride (Nyman et al. 1996) voluntarily drinking during the ride replaced only 38% to 45%
of BW loss and BW did not return to pre-ride values in any of the horses after 3 hours of
recovery despite continuous access to water. Further, after an overnight recovery period
(16 hours after the ride) BW deﬁcits were still 0.5 to 1.5% (Nyman et al. 1996). All in

all, these ﬁndings support that involuntary dehydration occurs in endurance horses.

The BW losses discussed above are greater than those reported in most studies of
human endurance athletes and suggest that horses may experience a comparatively
greater magnitude of involuntary dehydration. In contrast to human sweat, which is
typically hypotonic (sodium concentration of 50 to 70 mmol/l) to plasma (Convertino et

al. 1996), equine sweat is nearly isotonic to plasma due to a higher sodium concentration

20

(120 to 140 mmol/l) (McCutcheon and Geor 1996). Consequently, horses lose a
comparatively greater amount of electrolytes with each liter of sweat produced. As a
result, increases in Posm and [Na‘] are even less than in human athletes, leading to even
greater blunting of thirst. Thus, it would seem reasonable to speculate that athletic horses
may experience a relatively greater magnitude of involuntary dehydration (perhaps a

threshold of 3 to 4% BW loss) than their human counterparts.

Problems associated with “involuntary dehydration” in equine athletes

It has been well documented that sweating during exercise is a critical
thermoregulatory response for dissipation of metabolic heat (Hodgson et al. 1993). As
stated earlier, mild dehydration (3% BW loss) may reduce exercise performance
(Dahlbom et al. 1995), but as dehydration becomes more severe (7 to 10% BW loss),
fatigue and exhaustion may ensue (Carlson 1985; Geor and McCutcheon 1996).
Therefore, “excessive” involuntary dehydration appears to be an important risk factor for
the development of exhaustion and associated medical problems during and after
prolonged exercise (Carlson 1985; Schott and Charlton 1996; McCutcheon and Geor
1996). Although discipline speciﬁc data are limited, the number of horses that experience
medical problems as a consequence of exercise-induced dehydration is substantial. For
example, “failure to complete” data for all 1996 endurance rides provided by the
American Endurance Ride Conference (AERC) revealed that about 15% and 30% of the
nearly 11,000 horses that competed in 50 and 100-mile endurance rides in 1996,
respectively, failed to compete the ride. Approximately 20% of the horses that failed to

ﬁnish were eliminated or “pulled” due to development of “metabolic problems” and 44%

21

(about 850 horses) required immediate veterinary treatment (Cassotis and Schott 1997).
This number did not include horses that may have required treatment several hours later
or after a trailer ride home. Similarly, data for 1997 3-day-events provided by the United
States Combined Training Association (USCTA) revealed that 25 to 30% of all horses
entered in 1 to 4 star competitions failed to complete the event (L Cozzi, USCTA,
personal communication 1997). Although the percentage of horses that failed to
complete as a consequence of development of dehydration/medical problems was not
speciﬁcally recorded, an estimate as low as 10% (of the horses that did not ﬁnish)
coupled with extension to the more than 30,000 horses that compete annually in USCTA
events (at all levels) could yield an estimate as high as 750 horses that suffer from heat-
related exhaustion and/or illness. Clearly, these data demonstrate that involuntary

dehydration is an important problem in equine athletes.

Rehydration following exercise and involuntary dehydration: strategies for fluid

rehydration in exercising horses

Although there has been a fairly large recent research effort to study the effects of
dehydration on equine endurance performance and treatments to attenuate dehydration
(Equine Veterinary Journal Supplements 20, 1995 & 22, 1996, reviewed in Schott and
Hinchcliff 1998), almost all recent work has involved “forced” hyperhydration or
rehydration by administering solutions via a nasogastric tube. Although studies have been
limited, another manipulation to enhance rehydration during and following exercise has

been administration of electrolytes in feed or as oral pastes in an attempt to increase

22

voluntary water drinking (Coenen et a1. 1995; Nyman et al. 1996; Dﬁsterdieck et al.

1999).

Coenen et al. (1995) investigated the effects of pre-exercise electrolyte
supplementation by feeding ponies a supplement that contained approximately 300 mmol
of sodium (in addition to a dietary intake of 18 g per day) at l or 4 hours prior to
exercise. Voluntary water intake during the 4 hours before exercise and during recovery
was signiﬁcantly greater for both supplemented groups in comparison to non-
supplemented controls. Supplementation of electrolytes was concluded to improve

maintenance of body water due to the greater pre-exercise and post-exercise water intake.

In a ﬁeld study of endurance-trained horses by Nyman et al. (1996), [Na+] increased
in horses administered an oral salt paste or offered saline to drink in comparison to horses
offered plain water. In addition, ﬂuid intake and BW recovery were greater in the salt
paste and saline groups, compared to the plain water group, and at the end of a 3-hour
recovery period plasma aldosterone (ALD) was increased in the water group only. These
ﬁndings suggested that provision of additional Na+ as a salt paste or salt water during

endurance exercise enhanced restoration of body ﬂuid and electrolyte stores.

In another attempt to stimulate greater voluntary water intake during endurance
exercise, Dilsterdieck et al. (1999) demonstrated that supplementation of electrolytes as
an oral paste attenuated BW loss and enhanced voluntary water intake by horses

performing a simulated endurance ride on a treadmill. Plasma osmolality increased to a

23

greater extent in horses administered electrolyte pastes in comparison to non-
supplemented horses. These investigators also demonstrated that the greater increases in
Posm and [Na+] in the electrolyte supplemented horses were correlated with greater
voluntary water intakes, supporting that an increase in plasma tonicity was a stimulus of

thirst.

Oral rehydration solutions (ORS) have also been used to restore ﬂuid and electrolyte
balance and thereby improve recovery from exercise during acclimatization to hot and
humid conditions. Hyyppa et al. (1996) demonstrated that rehydration of horses with an
ORS (isotonic 0.5% glucose solution containing about 40 mmol/l of sodium)
administered via a nasogastric tube 30 minutes after completion of a treadmill exercise
bout simulating the endurance phase of a 3-day event resulted in greater voluntary water
intake during overnight recovery than in horses administered plain water. Persistent BW

loss was also less after overnight recovery in horses given the ORS.

In summary, recent studies in both human and equine subjects have clearly
demonstrated that restoration of body ﬂuid stores is more rapid and complete when
rehydration solutions containing electrolytes (primarily Na+ containing solutions) are
used in place of water (Maughan and Shirreffs 1994; Shirreffs et al. 1996; Marlin et al.
1998a, 1998b; Hyyppéi et al. 1996). In fact, in a 1996 Position Stand entitled Exercise
and Fluid Replacement the American College of Sports Medicine recommended

including Na" in rehydration solutions at an amount estimated to replace sweat losses,

24

during exercise lasting longer than 1 hour (Convertino et al. 1996). Addition of Na+

appears to have a further advantage of enhancing palatability of the rehydration solution.

However, a major difference between human and equine athletes is that humans can
force themselves to drink rehydration solutions in the absence of thirst while the old
saying “You can lead a horse to water, but can’t make it drink” is as true today as
when the saying was originated. To date, there has been limited study of factors affecting
voluntary ﬂuid replacement (by drinking) in horses dehydrated by endurance exercise.
Thus, a practical limitation of using ORSs in exercising horses is that solutions would
ideally be consumed voluntarily, rather than require “forced” rehydration by

administering with nasogastric tube or as an oral electrolyte paste.

Factors affecting the magnitude of “involuntary dehydration” in exercising horses

Factors that affect the ability to drink during or after endurance exercise or exposure
to stressful enviromnents can contribute to involuntary dehydration. In horses, there has
been little study of factors that may affect involuntary dehydration. Over the past few
years, studies by Schott and colleagues (Schott et al. 1999; Dﬁsterdieck et al. 1999;
Schott et al. 2002a) have demonstrated that the following factors may affect drinking and
involuntary dehydration, i) the mechanism by which dehydration is induced (i.e.,
endurance exercise or furosemide administration); ii) amount and composition of
electrolytes administered for rehydration; and iii) variability between horses. To further
understand factors affecting involuntary dehydration in exercising horses, other potential

factors need to be investigated.

25

Summary and hypothesis

This chapter reviewed current knowledge about thirst and voluntary drinking in
relation to involuntary dehydration in horses. Involuntary dehydration is a perplexing
situation that occurs when obviously dehydrated horses drink less or refuse to drink when
water is offered. Thus, a body water deﬁcit persists despite free access to water. This
phenomenon occurs in humans and horses when they are exposed to exercise or thermal
stress, and the magnitude of involuntary dehydration is related proportional to the degree

of total stress imposed on the body.

Thirst is stimulated as water is lost from blood and the remaining minerals, especially
sodium, become more concentrated in blood, causing an increase in plasma tonicity.
Coupled with activation of renin-angiotensin system by hypovolemia, a combination of
these physiological changes signals the thirst center in the hypothalamus to induce
drinking. Consumption of water will return plasma tonicity to the normal range. Although
the physiological mechanisms responsible for involuntary dehydration are not completely
understood, a logical explanation is that thirst and drinking are limited due to the isotonic
nature of equine sweat. Loss of nearly isotonic ﬂuid as sweat limits the increase in
plasma tonicity leaving hypovolemia and activation of renin-angiotensin sytem as
perhaps more important, yet less potent, thirst stimuli during and after endurance
exercise. Therefore, attenuation of thirst appears to play a role in involuntary
dehydration. Human athletes have learned the importance of maintaining body ﬂuid

homeostasis during prolonged exercise and will force themselves to drink rehydration

26

solutions in the absence of thirst. In contrast, modiﬁcation of drinking behavior is not
feasible in equine athletes. The end result is that equine endurance athletes may often be

placed at greater risk of developing heat stroke/exhaustion than human athletes.

Many factors can affect voluntary drinking and the degree of involuntary dehydration
in human athletes including volume of ﬂuid imbibed and the temperature, ﬂavoring, and
electrolyte content of ﬂuid available for rehydration. Further, considerable individual
variability in drinking has also been observed. In contrast, factors affecting development
of involuntary dehydration in horses have not been well studied. Most prior studies
focused on enhancing rehydration during and following exercise, and thereby
counteracting involuntary dehydration, have involved “forced” rehydration rather than
manipulation to enhance voluntary rehydration. Thus, the objective of my studies was to
investigate involuntary dehydration and factors affecting thirst and voluntary water intake
in exercising horses. The following hypotheses were tested: The magnitude of
“involuntary dehydration " in horses is affected by: 1) volume of water initially made
available after dehydration is induced; it) type of rehydration solution offered (water vs.

saline); and iii) temperature of ﬂuid available for rehydration.

Speciﬁc aims:
1. Demonstrate that the magnitude of “involuntary dehydration” (or volume of water
imbibed) is affected by the volume of water (41 v5.81 vs. unlimited water intake)

initially made available for rehydration following exercise-induced dehydration.

27

2. Demonstrate that the magnitude of “involuntary dehydration” (or volume of
rehydration solution imbibed) is affected by the type of solution (water vs. 0.45%
NaCl solution vs. 0.9% NaCl solution) initially made available for rehydration
following furosemide-and exercise-induced dehydration.

3. Demonstrate that the magnitude of “involuntary dehydration” (or volume of water
imbibed) is affected by temperature of rehydration ﬂuid initially made available

for rehydration following furosemide-and exercise-induced dehydration (10°C

vs.20°C vs. 30°C).

Of interest, improving the hydration status of equine endurance athletes during the
ﬁrst hour of recovery could be of great beneﬁt by decreasing the magnitude of
dehydration prior to further stresses including further exercise or prolonged transport.
Initially providing the most appropriate rehydration ﬂuid could enhance total water intake
during and aﬁer completion of exercise and improved recovery would be expected to
decrease the risk of developing medical problems after exercise and transport. If the
hypotheses noted above can be substantiated, the results should provide important and
practical recommendations for management of equine athletes competing in a number of

athletic endeavors.

28

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33

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34

Chapter 2

Effect of varying initial drink volume
on rehydration of horses

Prawit Butudom, Harold C. Schott 11, Shannon M. Axiak,
Brian D. Nielsen, and Susan W. Eberhart

Department of Large Animal Clinical Sciences, College of Veterinary Medicine, and
Department of Animal Sciences, College of Agriculture and Natural Sciences, Michigan
State University, East Lansing, MI 48824, USA.

Acknowledgements: This study was supported by the Department of Large Animal
Clinical Sciences, College of Veterinary Medicine, Michigan State University.

35

ABSTRACT

Body weight (BW), water intake (WI), and plasma osmolality (Posm) and electrolyte
concentrations were measured in six 2-year-old Arabian horses provided either 4 l, 8 l, or
an unlimited (UW) amount of water for drinking during the initial 5 min of recovery from
45 km of treadmill exercise. After weighing, horses were placed in a stall and further WI
between 20 and 60 min of recovery was measured. During exercise, horses lost 3.3 i 0.3,
3.2 i- 0.1, and 3.3 i 0.2% (p>0.05) ofBW and P05m increased by 7.2 i 0.5, 7.9 i 0.8, and
7.7 i 0.5 mOsm/kg (p>0.05) for 4 l, 8 l, and UW, respectively. WI during the ﬁrst 5 min
of recovery was 4.0 i 0.0, 8.0 i 0.0, and 9.0 i 1.3 l and was accompanied by 2.4 i 0.4,
5.8 i 0.9, and 6.1 i 0.7 mOsm/kg decreases (p<0.05) in Posm for 4 l, 8 l, and UW,
respectively. Between 20 and 60 min of recovery, WI was 6.2 i 1.5, 1.2 i 0.6, and 1.0 i
0.7 l (p<0.05) for 4 l, 8 l, and UW, respectively. Thus, total WI was 10.2 i 1.5, 9.2 i 0.6,
and 10.0 i 1.1 L (p>0.05) for 4 l, 8 l, and UW, respectively. After 60 min of recovery,
persisting BW loss was 1.3 i 0.5, 1.1 i 0.2, and 1.0 i 0.2% (p>0.05) for 4 l, 8 l, and UW,
respectively (p>0.05) and P05m had returned to pre-exercise values for all treatments. In
conclusion, limiting the volume of water initially provided to horses dehydrated by
endurance exercise had no signiﬁcant effect on total WI during the initial 60 min of
recovery; however, persisting BW loss was observed with all treatments. Further,
following exercise-induced dehydration, the primary stimulus of thirst was an increase in

plasma tonicity rather than hypovolemia.

36

Introduction

During exercise, maintenance of total body water requires matching water intake (WI)
with water loss (via respiration and sweat). However, such a balance rarely occurs such
that prolonged exercise usually leads to development of dehydration. Dehydration is
often the combined result of lack of intake (when water is not available) and a mismatch
between thirst and the magnitude of the body water deﬁcit. However, even when water or
other rehydration solutions are readily available, body water losses are only partially
replaced by drinking during and shortly after the exercise bout. This incomplete
restoration of body ﬂuid, usually measured as persisting body weight (BW) loss, has been
termed both voluntary (Hubbard et a]. 1984) and involuntary dehydration (Greenleaf

1992)

The primary stimulus for voluntary water intake is an increase in plasma tonicity
(Fitzsimons 1998). Since sweating results in loss of both water and electrolytes, the
increases in plasma osmolality (Posm) and sodium concentration ([Na+]) during exercise
are less than would be produced by loss of a similar volume of pure water. Consequently,
involuntary dehydration appears to develop because the loss of electrolytes in sweat
attenuates development of plasma hypertonicity. The magnitude of involuntary
dehydration observed in human endurance athletes is often around a 2% BW loss,
although considerable variability has been observed (Hubbard et a]. 1984; Greenleaf
1992; Szlyk et a]. 1989). In contrast to human sweat that is hypotonic relative to plasma
(Convertino et a]. 1996), equine sweat is nearly isotonic to plasma (McCutcheon and

Geor 1996). As a result, exercising horses lose comparatively greater amounts of

37

electrolytes with each liter of sweat produced. Horses also have a greater reserve of water
and electrolytes in ingesta in the lumen of the gastrointestinal tract (12 to 13% of BW,
Meyer 1989a; Meyer 1989b), in comparison to human athletes (1 to 2% of BW,
Greenleaf 1992), and this reservoir can be used to partially replace sweat ﬂuid losses
during exercise. Due to these species differences, it would be reasonable to expect equine
athletes to develop a greater magnitude of involuntary dehydration (BW loss) following
endurance exercise. In fact, ﬁeld studies have demonstrated that endurance horses
experience BW losses of 3 to 10% during 80- and 160-km competitions (Ecker and
Lindinger 1995; Schott et a]. 1997; Schott et a]. 1996), and a 3 to 4% BW loss may still
persist after an overnight recovery period (Schott et a]. 1997; Schott et a]. 1996). Further,
in both human and equine athletes, dehydration consequent to prolonged exercise is a
well-recognized risk factor for failure to complete the event and development of medical

problems in the recovery period (Sawka 1992; Geor and McCutcheon 1996).

After many competitive events, horses are often limited to a small drink of water
immediately after completion. Free access to water is subsequently provided 15 to 60 min
later after the horse has been washed off and cooled out. This traditional practice has no
basis in science but has been propagated over the centuries because providing an
unlimited amount of water to hot horses has been suggested to cause acute abdominal
pain or laminitis, a painful condition affecting the hooves (Hinton 1987). We suspected
that an initial small drink of water could actually be detrimental for rehydration after
exercise as it could produce a rapid decline in plasma tonicity and attenuate thirst when

water was again provided later. Thus, we hypothesized that limiting immediate water

38

intake by horses dehydrated by endurance exercise would decrease total W1 during the
initial hour of recovery and, thereby, potentiate the magnitude of involuntary

dehydration.

Materials and Methods
Horses and conditioning program

Six 2-year-old Arabian horses (three geldings and three ﬁllies) were studied. They
were conditioned by performing three 15-km exercise bouts weekly on a treadmill
(Mustang 2000, Kagra AG, Fahrwangen, Switzerland) and all horses completed two 30-
km exercise bouts before the ﬁrst experimental run. After each training run, horses were
immediately offered water (18 to 22°C) in a hand-held bucket while standing on the
treadmill to accustom them to post-exercise water availability. Throughout the training
and study period, horses were maintained at pasture with free access to water and no
supplemental feed or salt was provided. The training program and experimental protocol
were performed in accordance with the guidelines of the Institutional Animal Care and

Use Committee of Michigan State University.

Experimental protocol

All horses performed three 45-km experimental runs and experiments on each horse
were separated by a minimum of 10 days. The experimental run (Figure 1) consisted of
three 15-km treadmill exercise bouts (0° slope) separated by 15 min rest periods. Each
lS-km exercise bout lasted 52 min during which horses worked at speeds varying

between 1.6 m/s (walk) and 8 m/s (canter) to simulate a typical 45-km endurance ride.

39

Experimental Protocol

start ﬁnish

instrumentation ‘ 4. a: a. a: a a:

90’ pre

 

   

60’ post

 

 

B/BW B/BW/B B/BW/B B B/BW B/BW
initial drink H h

(41,81, or UW) water
in stall

HR

 

Figure 1. Experimental protocol. Horses were instrumented during the 90 min before
exercise. The experimental run consisted of three lS-km exercise bouts separated by two
15 min rest periods. Immediately (within 10 s) after completion of exercise, either 4 l, 8 l,
or an unlimited amount (UW) was offered to the horses, in a randomized order, for
voluntary drinking (shorter double-headed arrow). After 5 min of recovery, horses were
walked off the treadmill and washed down with water. After 20 min of recovery, horses
were placed free in a stall (without feed) and further voluntary water intake from 20 to 60
min of recovery was measured (longer double-headed arrow). Blood (B) was collected
and body weight (BW) and heart rate (HR) were recorded at various times point during

the experiment.

40

BW was measured on a digital scale (3: 0.5 kg) 90 min before the start of exercise (prior
to instrumentation), 5 min after completion of each 15-km exercise bout, and 20 and 60
min after completion of the experimental run. All feces produced after initial weighing
were collected, and although samples were not collected, urination was also recorded. No
water was offered during the experimental run but immediately (within 15 seconds) after
completion of the experimental run, water was offered to the horses in a hand-held bucket
while they were standing on the treadmill. Horses were offered, in a randomized order, 4
l (~1/2 of a 10 l bucket), 8 I (nearly a full bucket), or an unlimited amount (UW, 2 full
buckets were available) of water for voluntary drinking. After 5 min, the bucket was
taken away and horses were walked off the treadmill. A nasogastric tube was used to
administer any volume of water that was not voluntarily drank when offered 4 or 8 1 of
water and horses were subsequently washed down with water and weighed. At 20 min
post-exercise, horses were placed into a stall without feed but with free access to water in

a large bucket (garbage can) and further WI from 20 to 60 min was measured.

Instrumentation, sample collection, and analysis

At 90 min before and at 20 and 60 min after completion of the 45-km run, blood
samples were collected by jugular venipuncture and heart rate (HR) was determined by
auscultation. During the remaining pre-exercise period, surface electrocardiographic
electrodes were applied (for telemetric recording of HR) and a 7 French, 110 cm catheter
(Swan Ganz ﬂow directed thermodilution catheter, American Edwards Laboratories,
Aﬁasco, Puerto Rico) was aseptically passed into the pulmonary artery via an 8 French

introducer inserted into the right jugular vein (for collection of mixed venous blood

41

samples). HR was recorded at the start, during the 4 m/s (trot) and 8 m/s (canter) phases,
and 1 and 5 min after completion of each lS-km exercise bout. Mixed venous blood was
collected into heparinized and dry plastic syringes at the start and ﬁnish of each 15 km
exercise bout and after the initial 5 min of recovery. The heparinized samples were
placed on ice until analyzed for plasma electrolyte (Na+, K+, and Cl') concentrations
within 30 min of collection (Stat Proﬁle 9 Analyzer, Nova Biomedical, Waltham, MA).
The remainder of each sample was used for measurement of hematocrit (microhematocrit
method), plasma protein concentration (PP, by refractometry), and Posm by freezing point
depression (Model 3M0 plus Advanced Micro-Osmometer, Advanced Instruments,

Norwood, MA).

Data analysis

All values provided in the text are presented as means :t SE. Data were initially
analyzed by a two way repeated measures ANOVA (SigmaStat, Jandel Scientiﬁc, St.
Paul, MN) to assess main effects of time and treatment (amount of water offered) and,
when F ratios were signiﬁcant (p<0.05), a Student-Newman-Keuls post-hoc test was
performed to detect speciﬁc differences. One way repeated measures ANOVA was also
performed on selected data (total WI and BW loss at the end of the experiment) and,
when F ratios were signiﬁcant (p<0.05), a Student-Newman-Keuls post-hoc test was
again used. Correlation analyses were also performed to examine the relationships
between persisting BW loss and total WI during the 60 min recovery period and between

plasma [Na+] after 20 min of recovery and further WI from 20 to 60 min of recovery.

42

Results

All six horses completed the three experimental runs. During the experimental runs,
ambient temperature ranged between 18.0 and 215°C (mean i SE = 19.5 i 0.2 °C) and
relative humidity ranged between 53 and 92% (mean 1- SE = 76.7 i 2.6%) and there were
no differences between treatments. Temperature of the water provided ranged between
18.5 °C and 22.0 °C (mean t SE = 19.3 i 0.3 °C). Mean HR ranged from 106.7 i 1.4 to
110.7 i 2.0, from 138.6 i 1.9 to 143.3 i 1.9, and from 67.8 i 2.6 to 70.8 i 2.6 beats/min
during trot (4 m/s) and canter (8 m/s) phases of each exercise bout and after 1 min of
recovery, respectively. No differences in HR over time (ﬁrst through third 15-km

exercise bout) or between treatments were observed.

By the end of the 45-km run, horses had lost 3.3 i 0.3, 3.2 i 0.1, and 3.3 i 0.2%
(p<0.05 compared to pre-exercise values and p>0.05 for treatment) of their pre-exercise
BW for the 4 l, 8 l, and UW treatments, respectively (Table 1 and Figure 2). Aﬁer 20 min
of recovery, persisting BW loss was greater (p<0.05) for the 4 1 treatment than for the 8 1
or UW treatments (Fig. 2) but this was an expected consequence of the experimental
design. After 60 min of recovery, persisting BW loss was 1.3 i 0.5, 1.1 i 0.2, and 1.0 i
0.2% (p<0.05 compared to pre-exercise values and p>0.05 for treatment) for 4 l, 8 l, and
UW, respectively. Mean fecal production was 1.0 i 0.4, 0.9 i 0.2, and 0.8 i 0.1 kg
(p>0.05) for 4 l, 8 1, and UW, respectively. Two horses passed urine during one of the
rest breaks or recovery period with the 4 1 treatment while three horses passed urine with

each of the other treatments.

43

At the end of exercise, horses drank water immediately after it was offered and the
majority of WI was observed within the ﬁrst minute of recovery. All horses consumed 4 l
of water but three horses initially offered 8 1 had to have remaining amounts (0.5, 1.5, and
7.5 1) administered by a nasogastric tube. Initial WI for the UW treatment ranged from 4
to 12.5 1 (mean 9.0 i 1.3 l) and no adverse effects of unlimited drinking were apparent.
From 20 to 60 min of recovery, horses initially offered 4 l consumed a greater (p<0.05)
amount of water (6.2 i 1.5 l) in comparison to horses initially offered 8 l (1.2 i 0.6 l) or
UW (1.0 i 0.7 1). When combined, total WI during the 60 min recovery period was 10.2
i 1.5, 9.2 i 0.6, and 10.0 i 1.1 l (p>0.05) for 4 l, 8 l, and UW, respectively (Figure 3). In
addition, there was a signiﬁcant negative correlation (r = -0.53, p<0.02) between total WI

and persisting BW loss at the end of the 60 min recovery period (Figure 4).

Plasma protein (PP) concentration increased (p<0.05) by the end of exercise with all
treatments, reﬂecting a 3 to 5% decrease in plasma volume during exercise, but values
were not different from the pre-exercise values at any recovery time (Table 1). By the end
of the experimental run, P05m had increased by 7.2 i 0.5, 7.9 i 0.8, and 7.7 i 0.5
mOsm/kg (p>0.05) for 4 1, 8 1, and UW, respectively (Table 1). During the ﬁrst 5 min of
recovery, WI was accompanied by decreases in P05m of 2.4 i 0.4, 5.8 i 0.9, and 6.1 i 0.7
mOsm/kg (p<0.05) for 4 l, 8 1, and UW, respectively. After 20 min of recovery, P05m had
decreased (from end-exercise values) by 3.9 i 0.9, 7.8 i 1.4, and 8.2 i 0.5 mOsm/kg
(p<0.05) for 4 l, 8 l, and UW, respectively. Although differences between treatments
were not observed, P08m remained increased (p<0.05) from the pre-exercise value after 20

min of recovery for the 4 1 treatment alone. From 20 to 60 min of recovery further WI

44

was accompanied by decreases in P08m of 3.1 i 1.1, 0.2 i 1.1, and 1.5 i 0.8 mOsm/kg

(p>0.05) for 4 l, 8 l, and UW, respectively.

Similar to Posm, plasma [Na+] increased during the experimental run by 3.4 i 0.6, 3.3 i
0.4, and 3.1 i 0.2 mmol/l (p>0.05) for 4 l, 8 l, and UW, respectively (Table 1). During
the ﬁrst 5 min of recovery, WI was accompanied by decreases in plasma [Na+] of 1.4 i
0.6, 1.8 i 0.3, and 1.8 i 0.5 mmol/l (p>0.05) for 4 l, 8 l, and UW, respectively. After 20
min of recovery, plasma [Na+] had decreased (from end-exercise values) by 2.0 i 0.8, 2.7
i 0.5, and 3.7 i 0.6 mmol/l (p>0.05) for 4 l, 8 l, and UW, respectively. From 20-60 min
of recovery, further WI was accompanied by decreases in plasma [Na+] of 1.8 i 0.5, 0.8 i
0.3, and 0.2 i 0.5 mmol/l (p>0.05) for 4 l, 8 l, and UW, respectively. Further, there was a
signiﬁcant positive correlation (r = 0.53, p<0.02) between plasma [Na+] after 20 min of
recovery and additional Wl from 20 to 60 min of the recovery period (Figure 4). Unlike

plasma [Na+], signiﬁcant changes in plasma [KL] and [Cl'] were not observed during

exercise or recovery (Table l).

45

 

 

pre- end exercise 20 min 60 min
exercise recovery recovery
RW 4 liters 373.1 d: 13.3 360.7 :t 133* 364.6i 13.3* 368.3 :t 142*
(kg) 8 liters 373.3 a: 13.3 361.4 3: 129* 369.4 :1: 128* 369.0 i 133*
unlimited 370.8 d: 14.6 358.8 :t 14.2“ 367.8 i 14.7* 367.0 i 146*
Hot 4 liters 34.4 i 1.4 38.9 i 1.1* 37.1 i 0.9 34.4 i 2.1
(%) 8 liters 34.3 i 1.0 39.0 it 1.2* 35.7 i 1.1 34.0 i 2.0
unlimited 34.0 :1: 0.8 39.3 i 1.1* 37.0 i 1.0 36.7 i 1.0
PF 4 liters 6.2 :t 0.2 6.5 :1: 0.2* 6.2 :L- 0.3 6.2 i 0.3
(g/dl) 8 liters 6.3 :1: 0.2 6.6 i 0.3* 6.4 d: 0.3 6.4 3: 0.3
unlimited 6.3 d: 0.3 6.5 i 0.3* 6.3 i 0.3 6.2 i: 0.2
Posm 4 liters 283.7 i 1.8 290.8 d: 1.6* 287.0 i 1.0* 283.9 t 1.9
(mOsm/kg) 8 liters 284.9 :1: 1.2 292.8 i 1.6* 285.0 :1: 1.8 284.8 :t 1.5
unlimited 287.2 i 1.6 294.9 i 1.4”“ 286.7 i 1.1 285.2 :1: 1.6
[Na*] 4 liters 141.2 :1: 0.9 144.6 i 0.4* 142.6 i 0.8 141.0 s 0.5
(mmol/L) 8 liters 141.4 :t 0.5 144.8 i 0.6* 142.1 i 0.5 141.3 :1: 0.7
unlimited 141.6 :1: 0.7 144.7 3: 0.6* 141.0 i 0.6 140.8 :t 0.3
[K+] 4liters 3340.1 3210.1 3.1401 2.830.]
(mol/L) 8 liters 3.3 21:01 3.1i0.1 3.0:t0.1 3.0:t0.l
unlimited 3.6i0.l 3.4i0.l 3.13:0.1 3.13:0.1
[Cl'] 4 liters 105.0 i 0.8 105.9 3: 0.9 105.9 :1: 0.9 105.5 :t 0.9
(mmol/L) 8 liters 105.6 1 1.0 106.6 :t 1.2 105.9 i 0.9 105.4 :1: 0.7
unlimited 106.0 :t 1.0 107.0 d: 1.0 104.5 1 0.7 105.0 i 1.0

 

 

 

* signiﬁcantly different (p<0.05) from the pre-exercise value (within each row)

Dye—1. Body weight (BW), hematocrit (Hct), plasma protein (PP), plasma osmolality
(Posm), sodium, potassium, and chloride concentrations in six horses before and after
completion of 45-km of treadmill exercise and after 20 and 60 min of recovery with
rehydration with 4 l, 8 l, or an unlimited amount of water during the initial 5 min of the

recovery period.

46

 

II." _' ‘..

 

 

 

 

 

4 ‘ *
+ 41
A /
8° 3 ' ' 8' \ *
V —l— UW a
91 */
—°- 4
H 2 *
.1:
DD
'5 *
3 1 ‘ /
>5 ,
'U
.3
0
R1 R2 End 20'POSt 60'Post
' .- I - I i i
-1 I I V t ' 1
bout 1 bout 2 bout 3 recovery

Figure 2. Percent body weight loss during and after 45-km of treadmill exercise for
horses offered 4 1 (ﬁlled circles), 8 1 (ﬁlled inverted triangles), or an unlimited amount
(UW, ﬁlled squares) during the initial 5 min of recovery. * = signiﬁcantly different
(p<0.05) from the pre-exercise value; a = 4 l signiﬁcantly different (p<0.05) from 8 l and
UW; see Figure 1 legend for description of the experimental protocol. R1 = rest 1 (15
min) after the ﬁrst bout of exercise. R2 = rest 2 (15 min) after the second bout of
exercise. 20’Post = 20 min after the end of exercise. 60’Post = 60 min after the end of

exercise.

47

[:1 WI 0'-5' post
WI 20'-60' post

 

 

 

 

 

 

 

 

 

12 r
r: 9 '
E“
:4
S
.E 6 '
a
E
3 .
0 . T .
41 81 UW
treatment

Figure 3. Total water intake during the 60 min recovery period after induction of
dehydration by 45-km of treadmill exercise for horses provided 4 l, 8 l, or an unlimited
amount (UW) of water during the initial 5 min following exercise and subsequently
provided free access to water from 20 to 60 min following exercise. WI 0-5’ = water
intake during 0 to 5 minute post-exercise; WI 20-60 = water intake during 20 to 60

minute of recovery.

48

 

 

 

 

,.. E ° r = -0532, p<0.023
.3. 35 v v 0
w o 60
'50.: 1 u 8 V
.5 v
3 c O O
>,\O
'8 '5: 0 ‘ '
a 4::
N
O
-1 . . .
5 10 15 20
146 1 total water Intake (liters) .
’s‘ E
E § 144 <3 0
v "" v o
+—-"5 v o
a ,1: 142 - o o o
a E
a O .
a N O
a 5 140
=- “a v r = 0.529, p<0.024
138 . . . 1
0 5 10 15

mm. Upper panel. Correlation between body weight loss persisting after 60 min of
recovery (magnitude of involuntary dehydration) and total water intake from 0 to 60 min
of the recovery period after induction of dehydration by 45-km of treadmill exercise for
horses provided 4 1 (ﬁlled circles), 8 1 (open circles), or an unlimited amount (UW, ﬁlled

inverted triangles) of water during the initial 5 min following exercise and subsequently

provided free access

Correlation between plasma [Na+] after 20 min of recovery and water intake from 20 to

60 min of the recovery period for horses initially provided 4 1 (ﬁlled circles), 8 1 (open

water intake (liters)
from 20-60 min of recovery

to water from 20 to 60 min following exercise. Lower panel.

circles), or UW (ﬁlled inverted triangles) of water.

49

Discussion

The results of this study demonstrate that offering horses a small drink of water
immediately after exercise did not substantially blunt the thirst response when water was
again provided after a short cooling down period. Thus, our hypothesis that limiting
immediate WI by horses dehydrated by endurance exercise would decrease total WI
during the initial hour of recovery, and thereby increase the magnitude of involuntary
dehydration, was not supported by these data. However, involuntary dehydration was
observed (~1% persisting BW loss [range of 0 to 2.8%] with a mean loss of 4.3 i 0.7 kg)
at the end of the recovery period with all treatments despite free access to water from 20
to 60 min of recovery. The fact that the magnitude of the persisting BW loss was
inversely correlated to total WI during the recovery period supports inadequate thirst as

the primary mechanism responsible for involuntary dehydration.

Although feces and urine were produced during and after the experimental runs and
contributed to BW loss, these losses were only partly responsible for the persisting BW
loss after 60 min of recovery. Mean values for fecal production during exercise and
recovery were less than 1 kg for all treatments, in comparison to a mean BW loss
exceeding 4 kg. Urine was also voided by about half of the horses and explains the
tendency for BW loss to increase from 20 to 60 min of recovery with the 8 1 and UW
treatments (Figure 2) despite further intake of a small amount of water. Although not
quantiﬁed in this study, mares produced urine at a rate of ~5 ml/min during a similar
experimental run in our laboratory (Dusterdieck et al. 1999). As a result, it can be

estimated that these horses produced 1.5 to 2.0 l of urine from the start to the ﬁnish of the

50

experiment (~5‘/2 h in duration). Assuming that horses passed ~3 1 during voiding
(including residual urine in the bladder at the start of the experiment), urine losses could
be responsible for a mean BW loss of ~1.5 kg (because only half of the horses voided
urine). Thus, passage of feces and urine may explain about half, but not all, of the

persisting BW loss at the end of the recovery period in these horses.

In contrast to actual competitive events during which horses are sometimes
encouraged to drink by frequent offering of small amounts of water, horses in this study
were not allowed to drink until the end of the 45-km run. As a consequence, P05m and
plasma [Na+] increased by 2 to 3% during the experimental run indicating that water was
lost to a greater extent than Na+ during exercise. Assuming that equine sweat [Na+] is
similar to that of plasma (McCutcheon and Geor 1996), water would have to be lost via
other routes (respiration and urine production) in exercising horses. For these ~370 kg
Arabian horses with ~80 l of extracellular ﬂuid (ECF, Lindinger and Ecker, 1995), a 3
mmol/l increase in plasma [Na+] would represent a loss of ~2 1 of free water from ECF.
This would equate to 15 to 20% of the total ﬂuid loss during exercise (10 to 12 l), a value
that could easily be attributed to respiratory water loss (Hodgson et a]. 1993). In contrast
to [Na+], plasma [Cl’] remained unchanged during exercise and this lack of change can be

attributed to a greater [C1] in sweat than plasma (McCutcheon and Geor 1996).
Unlike the ﬁndings in these horses, increases in plasma [Na+] from the start to ﬁnish

of endurance events have not usually been observed in ﬁeld studies (Ecker and Lindinger

1995; Schott et a]. 1997; Lindinger and Ecker 1995; Carlson and Mansmann 1974; Lucke

51

and Hall 1980; Rose et a]. 1983; Rose et a]. 1980). However, when blood samples have
also been collected near the mid-point of competition (Schott et a]. 1997; Rose et a].
1983) or when the event included faster speed phases (e.g., the steeplechase portion of 3-
day-events, Andrews et a]. 1995), mild increases in plasma [Na+] have been found. The
lack of a persisting increase in plasma [Na+] at the end of competition can be attributed to
WI at rest breaks during the events. Although both an increase in plasma tonicity and a
decrease in blood (plasma) volume are well-documented stimuli for thirst (Fitzsimons
1998), an increase in plasma tonicity appears to be the more important stimulus during
and after endurance exercise. This statement is supported by the signiﬁcant positive
correlation found between plasma [Na+] after 20 min of recovery (that remained
increased from the pre-exercise value for horses initially offered 4 l) and subsequent WI
from 20 to 60 min of the recovery period. Further, PP (and presumably plasma volume)
had returned to pre-exercise values after 20 min for all treatments and PP at 20 min of
recovery was not correlated (r=-0.05, p>0.83) with WI from 20 to 60 min of the recovery
period. In a previous experiment in which horses completed a similar experimental run,
WI was also signiﬁcantly correlated with the increase in plasma [Na+] induced by
administration of oral electrolyte pastes (Dusterdieck et a]. 1999). In that experiment
electrolyte administration was actually accompanied by a decrease in PP and, presumably

plasma volume expansion, during the later stages of the exercise bout.

Consistent with previous studies of dehydrated horses (Houpt et al. 1991; Sneddon et

a]. 1993) and ponies (Suﬁt et a]. 1985), drinking occurred immediately after water was

provided. In our horses essentially all WI occurred within 1 to 2 minutes after stopping

52

exercise and little further WI was observed during the remainder of the initial 5 min
recovery period. When water deprivation was used to induce dehydration, horses and
ponies completely recovered the BW loss with initial drinking and the increases in Posm
and plasma [Nai] were corrected within 15 to 30 min of drinking (Sneddon et a]. 1993;
Suﬁt et a]. 1985). In contrast, when horses and ponies were dehydrated by furosemide
administration (resulting in sodium losses in urine similar to losses in sweat during
exercise), initial WI replaced only about two-thirds of the ﬂuid lost in urine (Schott et a].
1996; Suﬁt et a]. 1985; Schott et a]. 2002). Thus, similar to endurance exercise,
involuntary dehydration was produced when dehydration was induced by loss of urine
with a [Na+] similar to that in plasma. However, regardless of the method used to induce
dehydration (water deprivation, furosemide administration, or exercise), rapid declines in
P05m and plasma [Na+] after W1 indicate that intestinal function (at least for water
absorption) is well maintained in dehydrated horses and ponies (Schott et a]. 1996;
Sneddon et al. 1993; Suﬁt et a]. 1985). This observation remained true even when the
magnitude of induced dehydration (12% BW loss after 72 h of water deprivation in desert
dwelling horses) was substantial (Sneddon et a]. 1993). The fact that more than 50% of
the decreases in P05m and plasma [Na+] observed between the cessation of exercise to 20
min of recovery were observed within the initial 5 min of recovery in our horses further

supports that absorption of imbibed water is rapid in dehydrated horses.

Not surprisingly, values for Posm and plasma [Na+] below the starting values have been

measured in horses performing prolonged (160-km) endurance events, especially late in

the event or during the recovery period (Schott eta]. 1997; Carlson and Mansmann 1974;

53

Rose et a]. 1980). During and after exercise, WI leads to replacement of ECF losses with
a hypotonic ﬂuid that dilutes Na+ remaining in ECF. In human athletes, development of
hyponatremia during prolonged exercise has actually been described as water intoxication
(Noakes et a]. 1985), especially when the competitors drank water frequently during the
exercise bout. In addition to ongoing water and Na+ loss in sweat, a shift of sodium-poor
intracellular ﬂuid (ICF) to the ECF during the later stages of prolonged exercise is an
additional contributor to dilution of Na+ remaining in ECF (Lindinger and Ecker 1995).
This latter mechanism may be of greater importance in development of hyponatremia in
exhausted endurance horses (Schott et a]. 1997), rather than excessive water drinking.
However, prior to development of exhaustion, both mechanisms act to limit increases in

Posm and [Na‘], leading to lack of thirst despite a deﬁcit of total body ﬂuid.

In both human and equine athletes, partial replacement of electrolytes lost in sweat is
achieved by drinking beverages containing electrolytes or (in horses) oral administration
of concentrated electrolyte pastes. In both species, intake of electrolytes may attenuate,
but rarely prevents, development of involuntary dehydration following exercise
(Greenleaf 1992; Dt'rsterdieck et a]. 1999). In human athletes, complete recovery of body
ﬂuid losses incurred by sweating during endurance exercise actually requires drinking a
volume of an electrolyte-containing beverage that exceeds the volume of sweat produced
in order to compensate for ongoing urine production (Shirreffs eta]. 1996). Since athletes
would rarely drink such large volumes of a sports drink during or following exercise, full
restoration of electrolyte deﬁcits usually accompanies meal ingestion following exercise

that is accompanied by further drinking to replace the water deﬁcit (Maughan et a].

54

1996). Although the effects of post-exercise meal consumption on restoration of
electrolyte and water deﬁcits in horses have been little studied, it is likely of similar, if
not greater, importance to equine athletes that have a large reservoir of water and

electrolytes in ingesta in the intestinal tract (Meyer 1989a; Meyer 1989b).

In conclusion, we hypothesized that a small initial drink of water could produce rapid
decreases in P05m and plasma [Na+], thereby satiating thirst. However, total WI during the
60 min recovery period was not different with the three volumes of water initially offered
to these horses. Although our hypothesis was refuted, the persisting BW loss at the end of
the 60 min recovery period provides further support that horses performing endurance
exercise develop involuntary dehydration despite free access to water. It is important to
mention that the run distance performed by these horses was relatively short (45-km) in
comparison to competitive endurance rides that are typically 80 tol60 km. The shorter
distance (and duration) of exercise is the most likely explanation for the lesser degree of
involuntary dehydration observed in these horses, in comparison to a persisting 3 to 4%
BW loss after overnight recovery reported in ﬁeld studies (Schott et a]. 1997; Schott et
a]. 1996). To limit or prevent involuntary dehydration, human endurance athletes have
learned to force themselves to drink throughout the competition, even when thirst is
absent. Unfortunately, “forced” drinking is not an option for horses and when combined
with species differences in sweat composition, it is not without merit to suggest that
involuntary dehydration may be a greater problem for equine endurance athletes than
their human counterparts. In addition, horses are often transported long distances within a

few hours after completion of an endurance competition. The muscular work performed

55

to maintain balance during transport is essentially a continuation of the exercise bout and
involuntary dehydration prior to transport could increase the risk of developing
exhaustion and associated medical disorders in this species (Hinton 1977; Schott and
Charlton 1996). Thus, a practical recommendation would be to offer all horses frequent
opportunities to drink during endurance competition and to provide water ad libitum
immediately after completion of exercise. Finally, our data can also be added to the pool
of knowledge used to educate horse owners that providing unlimited water to horses that

have just ﬁnished exercise is not only a safe practice but is preferable to limiting WI.

56

References

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Williamson, L.and Provenza, M. (1995) Weight loss, water loss and cation balance
during the endurance test of a 3-day-event. Equine vet. J. Suppl. 18, 294-297.

Carlson, G. P. and Mansmann, R. A. (1974) Serum electrolyte and plasma protein
alterations in horses used in endurance rides. J. Am. vet. med. Assoc. 165, 262-264.

Convertino, V. A., Armstrong, L. E., Coyle, E. F ., Mack, G. W., Sawka, M. N., Senay, L.
C. and Sherman, W. M. (1996) Position stand: exercise and ﬂuid replacement. Med.
Sci. Sports Exer. 28, i-vii.

Dtisterdieck, K. F ., Schott, H. C., Eberhart, S. W., Woody, K. A.and Coenen, M. (1999)
Electrolyte and glycerol supplementation improve water intake by horses performing
simulated 60 km endurance ride. Equine vet. J. Suppl. 30, 418-424.

Ecker, G. L. and Lindinger, M. I. (1995) Effects of terrain, speed, temperature and distance
on water and ion losses. Equine vet. J. Suppl. 18, 298-305.

F itzsimons, J. T. (1998) Angiotensin, thirst, and sodium appetite. Physiol. Rev. 78, 583-686.
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associated with exercise and heat stress. Compend. Cont. Educ. Pract. Vet. 18, 436-

444.

Greenleaf, J. E. (1992) Problem: thirst, drinking behavior, and involuntary dehydration.
Med. Sci. Sports Exer. 24, 645-656.

Hinton, M. (1987) On the watering of horses: a review. Equine vet. J. 10, 27-31.

Hinton, M. (1977) Long distance horse riding and the problem of dehydration and
rhabdomyolosis. Vet. Annual. 17, 136-141.

Hodgson, D. R., McCutcheon L. J ., Byrd, S. K., Brown, W. S., Bayly, W. M.,
Brengelrnann, G. L. and Golnick, P. D. (1993) Dissipation of metabolic heat in the horse
during exercise. J. Appl. Physiol. 74, 1161-1170.

Houpt, K. A., Northrup, N., Wheatley, T. and Houpt, T. R. (1991) Thirst and salt appetite
in horses treated with furosemide. J. app]. Physiol. 71, 2380-2386.

Hubbard, R. W., Sandick, B. L., Matthew, W. T., Francesconi, R. P., Sampson, J. B.,
Durkot, M. J ., Maller, O. and Engell, D. (1984) Voluntary dehydration and alliesthesia
for water. J. app]. Physio]. 57, 868-875.

Lindinger, M. I. and Ecker, G. L. (1995) Ion and water losses from body ﬂuids during a

57

163 km endurance ride. Equine vet. J. Suppl. 18, 298-305.

Lucke, J. N. and Hall, G. M. (1980) A biochemical study of the Arab Horse Society’s
marathon race. Vet. Rec. 107, 523-525.

Maughan, R. J ., Leiper, J. B. and Shirreffs, S. M. (1996) Restoration of ﬂuid balance
after exercise-induced dehydration: effects of food and ﬂuid intake. Eur. J. app].
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McCutcheon, L. J. and Geor, R. J. (1996) Sweat ﬂuid and ion losses in horses during
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supplementation. Equine vet. J. Suppl. 22, 54-62.

Meyer, H. (1989a) Inﬂuence of feed intake and composition, feed and water restriction,
and exercise on gastrointestinal ﬁll in horses. Part 1. Equine Pract. 18, 26-29.

Meyer, H. (1989b) Inﬂuence of feed intake and composition, feed and water restriction,
and exercise on gastrointestinal ﬁll in horses. Part 3. Equine Pract. 18, 25-28.

Noakes, T. D., Goodwin, N., Rayner, B. L., Branken, T. and Taylor, R. K. (1985) Water
intoxication: a possible complication during endurance exercise. Med. Sci. Sports
Exer. 17, 370-375.

Rose, R. J ., Arnold, K. S., Church, S. and Paris, R. (1980) Plasma and sweat electrolyte
concentrations in the horse during long distance exercise. Equine vet. J. 12, 19-22.

Rose, R. J ., Hodgson, D. R., Sampson, D. and Champ, W. (1983) Changes in plasma
biochemistry in horses competing in a 160 km endurance ride. Aust. vet. J. 60, 101-
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Sawka, M. N. (1992) Physiologic consequences of hypohydration: exercise performance
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administration of electrolyte pastes on rehydration of horses. Am. J. vet. Res. 63, 19-27.

Schott, H. C., McGlade, K. S., Molander, H. A., Leroux, A. J. and Hines, M. T. (1997)
Body weight, ﬂuid, electrolyte, and hormonal changes in horses during and after
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58

Shirreffs, S. M., Taylor, A. J ., Leiper, J. B. and Maughan, R. J. (1996) Post-exercise
rehydration in man: effects of volume consumed and drink sodium content. Med. Sci.
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Effects of dehydration and rehydration on plasma vasopressin and aldosterone in
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Behav. 45, 639-647.

59

Chapter 3

Drinking salt water enhances rehydration in horses
dehydrated by furosemide administration and
endurance exercise

Prawit Butudom, Harold C. Schott 11, Matthew W. Davis, Christina A. Kobe,
Brian D. Nielsen, and Susan W. Eberhart

Department of Large Animal Clinical Sciences, College of Veterinary Medicine, and
Department of Animal Sciences, College of Agriculture and Natural Sciences, Michigan
State University, East Lansing, MI 48824, USA.

Acknowledgements: Supported by the Equine Health Research Fund of the American
Horse Shows Association and the Department of Large Animal Clinical Sciences,
College of Veterinary Medicine, Michigan State University. The authors thank Dr. J.
Shelle, Department of Animal Sciences, Michigan State University for providing the
Arabian horses used in the study.

60

ABSTRACT

Because the primary stimulus for thirst is an increase in plasma tonicity, we
hypothesized that dehydrated horses would voluntarily drink a greater total volume of

ﬂuid during the ﬁrst hour of recovery when they were initially offered salt water. To test

this hypothesis, body weight (BW), ﬂuid intake (FI), and [Na+] were measured in six 2-
year-old Arabian horses offered three rehydration solutions. After dehydration was
induced by furosemide administration (1 mg/kg, IV) followed by 45 km of treadmill
exercise, water (W), 0.45% NaCl, or 0.9% NaCl was offered, in a randomized order,
during the initial 5 min after completing exercise. Horses were subsequently placed in a
stall and ﬁirther intake of plain water during the ﬁrst hour of recovery was measured. By
the end of exercise, horses lost 5.2 :1: 0.2, 5.6 i 0.3, and 5.7 i 0.2 % (p>0.05) of BW and

F1 during the ﬁrst 5 min of recovery was 10.5 d: 0.7, 11.6 i 0.8, and 11.6 i 1.5 l (p>0.05)

for W, 0.45% NaCl, and 0.9% NaCl, respectively. After 20 min of recovery, [Na+] had
decreased with W but remained unchanged from the end exercise values for both saline
solutions. During the initial hour of recovery, further water intake was 0.9 i 0.4, 5.0 i
0.5, and 6.9 i 0.7 l (p<0.05) for W, 0.45% NaCl, and 0.9% NaCl, respectively. Thus total
F1 was 11.4 i 0.5, 16.6 i 0.7, and 18.5 i 1.7 l (p<0.05) for W, 0.45% NaCl, and 0.9%
NaCl, respectively, and persisting BW loss after 60 min of recovery was greater (p<0.05)
for W (3.5%) than for the two saline solutions (2.4% for 0.45% NaCl and 1.9% for 0.9%

NaCl). In conclusion, providing salt water as the initial rehydration ﬂuid maintained an

elevated [Na+] and resulted in greater total FI and recovery of BW loss during the ﬁrst

hour of recovery, in comparison to offering only plain water.

61

Introduction

During and after endurance exercise, replacement of water and electrolytes lost in
sweat is important for prevention of medical problems that may develop consequent to
dehydration (Geor and McCutcheon 1996). However, concurrent loss of water and
electrolytes in sweat leads to a condition that has been termed both “voluntary
dehydration” and “involuntary dehydration”: incomplete voluntary replacement of body
ﬂuid losses due to a lack of thirst (Hubbard et a]. 1984; Greenleaf 1992). The primary
stimulus for thirst is an increase in plasma osmolality or, more speciﬁcally, an increase in
plasma sodium concentration ([Na+]) (Andersson 1978; Fitzsimons 1998). Because
sweating results in loss of both body water and electrolytes, the increases in plasma
osmolality and [Na+] concentration during exercise are less than would be produced by
loss of a similar volume of pure water. Attenuation of these increases appears to be an

important factor for limiting thirst and maintenance of dehydration.

The magnitude of involuntary dehydration in human athletes following endurance
exercise is usually about 2% of body weight (BW), although considerable interindividual
variability has been observed (Szlyk et a]. 1989; Greenleaf 1992). As in human athletes,
BW loss by horses during prolonged exercise has recently been shown to be an accurate
estimate of ﬂuid loss by sweating (Kingston et a]. 1997). In endurance horses competing
in 50 and 100 mile endurance rides (80 and 160 km rides), BW losses of 3 to 4% are
common and these losses can persist after an overnight recovery period (Schott et a].
1997). Similarly, horses performing a simulated 60 km endurance ride on a treadmill also

experienced a ~3% BW loss despite frequent access to water during the exercise bout

62

(Dusterdieck et a]. 1999). In both of these studies, ambient conditions were mild but the
comparatively greater loss of BW in horses suggests that the magnitude of involuntary
dehydration may be greater in this species. This could be a consequence of the higher
[Na+] in equine sweat and a comparatively greater loss electrolyte with each liter of sweat

produced (Convertino et a]. 1996; McCutcheon and Geor 1996).

Recent studies in both human and equine subjects have clearly demonstrated that post-
exercise restoration of body ﬂuids and recovery of BW is more rapid and complete when
rehydration solutions containing electrolytes (primarily Na+) are used in place of water
(Hypppa et a]. 1996; Nyman et a]. 1996; Shirreffs et a]. 1996; Maughan and Shirreffs
1997; Marlin et al. 1998a; Marlin et al. 1998b; Monreal et a]. 1999). Although there have
been a number of studies designed to assess treatments used to attenuate or correct
dehydration in horses (work up to 1997 reviewed in Schott and Hinchcliff 1998; Marlin
et al. 1998a; Marlin et al. 1998b; Sosa Leon et a]. 1998; Dusterdieck et a]. 1999;
Monreal et a]. 1999; Schott et a]. 2001; Schott et a]. 2002), most investigations have
studied “forced” hyperhydration or rehydration by administering solutions via a
nasogastric tube or use of oral electrolyte pastes to stimulate drinking. In contrast, there
has been little study of voluntary rehydration in which horses are allowed to drink

solutions of varying composition.

An early investigation by Randall et a]. (1978) found that euhydrated weanling foals

would readily drink saline solutions as long as the concentration did not exceed 0.6% NaCl.

More recently, other investigators have shown that euhydrated horses can also be trained to

63

drink 0.9% NaCl solutions (Houpt et a]. 1995). Anecdotally, competitors in endurance
events have also trained their horses to drink “salt water” but further data is needed to
determine if this practice is truly effective before recommendations can be made to
competitors. Thus, we tested the hypothesis that horses dehydrated by a combination of
furosemide administration and prolonged exercise would voluntarily drink a greater total
volume of ﬂuid during the first hour of recovery when they were ﬁrst offered salt water, in

comparison to water, during the initial 5 min of recovery.

Materials and Methods
Horses and conditioning program

Six 2-year-old Arabian geldings ranging in weight from 362 to 449 kg (mean i SE =
384.4 i 13.7 kg) were studied. The horses were trained on a treadmill1 three times a week
for a period of 5 weeks and all horses completed two 30 km exercise bouts before the
ﬁrst experimental run. Saline solutions (either 0.45% NaCl or 0.9% NaCl) were offered
after each conditioning run to train horse to drink salt water. Throughout the study period
(May through August, 2000), horses were maintained at pasture with free access to water
and no supplemental feed or salt was provided. The training program and all
experimental protocols were performed in accordance with the guidelines of the

Institutional Animal Care and Use Committee of Michigan State University.

ExperimentalJprotocol

Horses were brought in from pasture on the afternoon before each experiment and

housed overnight in a stall with free access to a grass/alfalfa mixed hay and water. On the

64

morning of each experimental run, initial dehydration was induced by furosemide2
administration (1.0 mg/kg IV) 120 min before the start of the 45 km exercise bout.
Because the goal was to produce dehydration in excess of a 4% BW loss, furosemide
treatment was added to the experimental protocol. Otherwise, 75 to 90 km of treadmill
exercise would have been required to produce this degree of dehydration under the mild
ambient conditions in our laboratory and a longer exercise_bout would have increased the
risk of musculoskeletal injury. During the remaining pre-exercise period, surface
electrocardiographic electrodes were applied (for telemetric recording of heart rate) and a
7 French, 110 cm Swan Ganz catheter3 was aseptically passed into the pulmonary artery
via an 8 French introducer inserted into the right jugular vein (for collection of mixed
venous blood samples). Each horse was studied three times when offered, in a
randomized order, either water, 0.45% NaCl, or 0.9% NaCl during the initial recovery
period and experiments on each horse were separated by a minimum of 10 days during

which further exercise was not performed.

During each experiment BW was measured on a digital scale (nearest 0.5 kg), heart
rate was recorded, and a blood sample (~10 ml) was collected before and 120 min after
(pre-exercise sample) furosemide administration. Horses then performed 45 km of
treadmill exercise at varying speeds to simulate a 25 mile endurance ride (Figure 1). The
experimental run consisted of three 15 km exercise bouts (lasting 56 min; speeds between
1.6 [walk] and 8 m/s [canter]; 0o slope) separated by 15 min rest periods. Horses did not
have access to water or any other rehydration solution during the run. Immediately

(within 10 s) after completion of exercise, one of the three rehydration solutions was

65

offered for voluntarily drinking from a hand-held bucket. Aﬁer the initial 5 min of
recovery, the rehydration solution was taken away and horses were walked off the
treadmill, the pulmonary arterial catheter was removed, and horses were washed with
water. The horses had no access to rehydration ﬂuid from 5 to 20 min after the end of
exercise. At 20 min of the recovery period, horses were weighed and placed free in a stall
(without feed) and_further voluntarily water intake and number of drinking episodes from
20 to 60 minutes of recovery was measured. After 60 min of recovery, horses were
weighed again and returned to the stall where they remained overnight with free access to
hay and water. A ﬁnal weight was recorded after overnight recovery (~18 h after

completion of the 45 km run).

66

Experimental Protocol
Finish

    
  

 

 

 

 

 

Start
1 ”6° End
i
1:. . .. €60? Post Eire-mm,
5‘ i 5' 207 160'
1 TT 111 "T 1 1
B B/BW B/BW
B/BW BBWB BBWB . ..~ . h
Furosemide initial drink
ling/kg IV (0.45% N aCl, 0.9% NaCl, water)
HR
D

F_igur£_l. Experimental protocol. Furosemide (1.0 mg/kg IV) was given 2 h prior to and
horses were instrumented during the 60 min before exercise. The experimental run
consisted of three 15 km exercise bouts separated by two 15 minutes rest periods.
Immediately (within 10 seconds) after completion of exercise, one of three rehydration
ﬂuids (water, a 0.45% NaCl solution, or a 0.9% NaCl solution) was offered to the horses,
in a randomized order, for voluntary drinking. After 5 minutes of recovery, horses were
walked off the treadmill and washed down with water. After 20 minutes of recovery,
horses were placed free in a stall (without feed) and further voluntary water intake from
20 to 60 minutes of recovery was measured. Blood (B) was collected and body weight

(BW) and heart rate (HR) were recorded at various times point during the experiment.

67

Sample collection and analysis

At 120 minutes before and at 20 min and 60 min and ~18 h after completion of the 45
km run, blood samples were collected by jugular venipuncture and heart rate was
determined by auscultation. During exercise heart rate was recorded at the start, during
the 4 m/s (trot) and 8 m/s (canter) phases, and 1 and 5 min after completion of each 15
km exercise bout. Mixed venous blood was collected at the start and finish of each 15 km
of exercise bout and after 1, 3, and 5 min after completion of the 45 km exercise test.
Blood was collected anerobically into heparinised plastic syringes that were placed on ice
until analysed for plasma electrolyte (Na+, K+, and Cl") concentrations within 30 min of
collection (Nova Stat Proﬁle 9 Analyzer)4. The remainder of each sample was used for
measurement of haematocrit (microhaematocrit method) and plasma protein
concentration (PP, by refractometry). All feces and urine produced from the time of
furosemide administration to the end of the 60 min recovery period were collected and
weighed with the exception of urine produced prior to exercise in response to furosemide
administration. BW loss (with the exception of ~18 h recovery) was corrected for fecal
losses but not for urine losses because the latter was an important factor in the pre-

exercise BW loss.

Data analysis

All values provided in the text are presented as means :1: SE. Total ﬂuid intake during
the recovery period (magnitude of involuntary dehydration) were analyzed using
commercial software5 by a one way repeated measures analysis of variance ANOVA to

assess differences between rehydration solutions. When F ratios were significant

68

(p<0.05), a Student-Newman-Keuls test was performed to detect speciﬁc differences.
Changes in parameters measured at multiple times (BW, heart rate, and blood values)
were analyzed by a two way repeated measures analysis of variance (main effects of time
and treatment) and, when F ratios were signiﬁcant (p<0.05), a Student-Newman-Keuls
post-hoe was again performed to detect specific differences. Selected relationships
between parameters measured were also examined by Pearson product moment

correlation analysis.

Results

All six horses completed the three experimental runs. Ambient temperature in the
laboratory ranged between 20 and 27°C (mean 24.6 i 0.40C) and relative humidity ranged
between 48 and 72% (mean 59.9 i 1.8%) during exercise and signiﬁcant differences
between the runs for each rehydration solution were not observed. Heart rate during the
trot and canter portions of the exercise bouts had mean values of 108 i l and 136 i l

beats/min and differences between each 15 km exercise bout or between treatments were

not observed.

Furosemide administration produced a BW loss prior to exercise of 1.7 i 0.3, 1.8 i
0.2, and 2.1 i 0.3% (p>0.05) for water (control), 0.45% NaCl, and 0.9% NaCl
treatments, respectively. The combined effects of furosemide administration and 45 km
of treadmill exercise resulted in BW losses of 5.2 i 0.2, 5.6 i 0.3, and 5.7 i 0.2%

(p<0.05 in comparison to starting values for all treatments; p>0.05 for treatment

differences) for water, 0.45% NaCl, and 0.9% NaCl treatments, respectively (Table 1 and

69

Figure 2). After 60 min of recovery, BW remained decreased (p<0.05) from starting
values for all treatments (3.4 i 0.1, 2.3 i 0.2, and 1.9 i 0.3% for water, 0.45% NaCl, and
0.9% NaCl treatments, respectively). However, BW loss was greater (p<0.05) for water
in comparison to both saline solutions. None of the horses produced more than 2 kg of

feces during the entire experiment and the volume of urine produced from the start of
exercise to the end of the 60 min recovery period was not different (3.0 i 0.7, 2.5 i 0.7,
and 2.4 i 0.6 l for water, 0.45% NaCl, and 0.9% NaCl treatments, respectively). The loss
of ~1% of BW as urine explains the further BW loss observed with the control treatment
from 20 to 60 min of recovery (Figure 2). After overnight recovery, BW remained
decreased (p<0.05) from starting values for all treatments (Table l) and the magnitude
was signiﬁcantly greater (p<0.05) for the water treatment (2.3 i 0.2%) in comparison to
both saline treatments (1.6 i 0.2 and 1.2 i 0.2% for 0.45% NaCl and 0.9% NaCl

treatments, respectively).

The amount of ﬂuid imbibed during the initial 5 min of recovery was not different
among treatments (10.5 i 0.7, 11.6 i 0.8, and 11.6 i 1.5 l for water, 0.45% NaCl, and
0.9% NaCl groups, respectively) and replaced ~50% of lost body fluid (Figure 3). With
all rehydration solutions, horses drank immediately when the bucket was offered and the
majority of drinking occurred within the ﬁrst minute of recovery. From 20 to 60 min of
recovery, voluntary water intake was greater (p<0.05) for both saline treatments (5.0 i
0.5 and 6.9 i 0.7 l for 0.45% NaCl and 0.9% NaCl, respectively) than for the control
treatment (0.9 i 0.4 1). When combined, these values resulted in a total voluntary ﬂuid

intake during the 60 min recovery period of 11.4 i 0.5, 16.6 i 0.7, and 18.5 i 1.7 l for

70

water, 0.45% NaCl, and 0.9% NaCl treatments, respectively (p<0.05, water less than the
two saline solutions, Figure 3). The number of drinking episodes from 20 to 60 min of
recovery was also greater (p<0.05) for both saline treatments (4.0 i 0.6 and 4.2 i 0.7 for
0.45% NaCl and 0.9% NaCl, respectively) in comparison to the control treatment (1.2 i
0.3). When total ﬂuid intake was compared to persisting BW loss (magnitude of

involuntary dehydration) at the end of the 60 min recovery period, a highly signiﬁcant

negative correlation was found (Figure 4).

Haematocrit decreased (p<0.05) after furosemide administration and subsequently
increased (p<0.05) during the exercise bout (Table 1). Plasma protein and Na+
concentrations increased (p<0.05) from starting values during the 45 km exercise bout
(Table 1). PP remained elevated for all treatments after 20 min of recovery but values
were not different from the starting values by 60 min of recovery. In contrast, plasma
[Na+] after 20 min of recovery decreased to the starting value with water treatment but
remained increased for both the 0.45% NaCl and 0.9% NaCl treatments. In fact, after 20
min of recovery, plasma [Na+] was greater for the 0.9% NaCl treatment, in comparison to
both the water and 0.45% NaCl treatments, and after 60 min of recovery, plasma [Na+]
remained increased from the starting value for the 0.9% NaCl treatment. After overnight
recovery, plasma [Na+] was decreased (p<0.05) from the starting value for the water
treatment but this value was not different from plasma [Na+] for the two saline treatments

at this time point.

71

Plasma [K+] decreased (p<0.05) after furosemide administration and remained
decreased from the starting values for all treatments throughout the 60 min recovery
period (Table 1). Plasma [Cl‘] tended to decrease aﬁer furosemide administration but this
was not a signiﬁcant ﬁnding. In contrast, plasma [Cl’] was decreased (p<0.05) from the
starting value after 20 min of recovery with water treatment and remained decreased after

60 min of recovery with this treatment (Table l).

72

 

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73

7 “ + Water

+ 0.45% NaCl *
6 * —-— 0.9% NaCl

  

+

a: \ /
/

Body weight loss (%)
u

 

 

 

 

 

 

 

2 '* a

1 . *

0 Start R1 R2 End 20’P 1hrP 18hrP
Furo ' ' * ~ “rmxlwwl.m l 1 l
120'Pre Boutl Bout2 Bout3 Recovery

ﬁgure; Percent body weight loss after furosemide treatment (1 mg/kg IV) and during
and aﬂer 45 km of treadmill exercise ride for horses offered water (ﬁlled circles), 0.45%

NaCl (ﬁlled rectangles), or 0.9% NaCl (ﬁlled triangles) during the initial 5 min of
recovery. See Figure 1 legend for explanation of the experimental protocol. 120’Pre =
120 min pre-exercise. Furo = Furosemide. R1 = rest 1 (15 min. after bout 1). R2 = rest 2
(15 min, after bout 2). 20’P = 20 min after the end of exercise. 1hrP = 1 hour after the end
of exercise. 18hrP = 18 hours after the end of exercise. * = signiﬁcantly different

(P<0.05) from 120’Pre. + = signiﬁcantly different (P<0.05) from 0.45% and 0.9% NaCl.

74

 

 

 

 

 

 

 

 

 

20 -
16.6
.1
E“ 11.4
g W 1:: WI 0-5'
:10 - W120-60'
_
l-
3
a
3 5 -
o . . .
w 0.45% 0.90%

Figure 3. Total ﬂuid intake during the 60 minute recovery period after induction of
dehydration by furosemide (1 mg/kg IV) treatment followed by 45 km of treadmill
exercise for horses provided water, 0.45% NaCl, or 0.9% NaCl as the rehydration ﬂuid
during the initial 5 min following exercise and subsequently provided free access to water
from 20 to 60 min following exercise. See Figure 1 legend for explanation of the
experimental protocol. WI 0-5’ = ﬂuid intake during 0-5 minute post-exercise; WI 20-60
= ﬂuid intake during 20 to 60 minute of recovery; * = Signiﬁcantly different (P<0.05)

from 0.45% NaCl and 0.9% NaCl.

75

 

 

5 q
A
c\° 0 Water
:4 " . O 0.45%) NaCl
.5” . V 0.90/0 NaCl

0

'8 o . .
3 3 ‘
>~. o O
'3 ' o '
.a 2 . v
.E o
o v
3:”
a 1 ‘
ﬂ
0 r = 41.30, p<0.001 V

0 I I I I I I

0 5 10 15 20 25 30

Voluntary water intake (L)

ﬂgureﬁ. Correlation between total ﬂuid intake from O to 60 minutes of the recovery
period and persisting body weight loss (magnitude of involuntary dehydration) after 60
minutes of recovery after induction of dehydration by furosemide (1 mg/kg IV) treatment
followed by 45 km of treadmill exercise for horses provided water (filled circles), 0.45%
NaCl (open circles), or 0.9% NaCl (filled inverted triangles) as the rehydration ﬂuid
during the initial 5 min following exercise and subsequently provided free access to water

from 20 to 60 min following exercise.

76

Discussion

The results of this study demonstrate that offering salt water as the initial rehydration
solution to horses dehydrated by furosemide administration and endurance exercise can
be an effective strategy to increase total ﬂuid intake during the early recovery period.
Although initial rehydration with either 0.45% NaCl or 0.9% NaCl did not result in full
replacement of body ﬂuid losses, it decreased the magnitude of dehydration to ~2% (for
0.9% NaCl) after the initial hour of recovery, a value comparable to that observed in
human endurance athletes after a similar recovery period (Hubbard et al. 1984; Szlyk et
al. 1989; Greenleaf 1992). Although provision of salt water in this study was short in
duration and dehydration was induced, in part, by furosemide administration, the data are
relevant to horses competing in endurance events that may have limited time to recover
before being subjected to a long trailer ride. Even though BW remained decreased from
the starting value for all treatments after overnight recovery, initial rehydration with
either saline solution attenuated the magnitude of BW loss at this time point as well. The
latter ﬁnding suggested that most of the electrolytes ingested in the saline rehydration
solutions during the initial 5 min of recovery were likely retained in the body ﬂuids of
these horses. Retention of the electrolytes imbibed in an initial drink of salt water would

clearly be beneﬁcial to horses competing again on the following day.

Of the previous studies investigating rehydration during and after endurance exercise
in horses, only one has evaluated voluntary drinking of different rehydration ﬂuids. In
that 62 km ﬁeld study under mild ambient conditions, Nyman et al. (1996) offered horses

frequent access to either plain water or a 0.9% NaCl solution during the exercise bout and

77

the initial hour of recovery. At the end of exercise, horses drinking plain water had
replaced 38% of their BW loss while those drinking a 0.9% NaCl solution had replaced
of 45% of their estimated losses. Although this difference was small, horses that had been
offered water alone only drank a further 2 1 while those provided the 0.9% NaCl solution
drank 14 l of water from 1 to 3 h of recovery and had much greater recovery of BW loss.
In our study, horses were not offered any rehydration ﬂuid during the exercise bout but
initial drinking of a saline solution also resulted in greater subsequent water intake and
recovery of BW from 20 to 60 min of recovery. Taken together, the results of both
studies demonstrate that saline solutions can be effectively used, in a variety of manners,

to improve rehydration and recovery after endurance exercise.

During the initial 5 min of recovery, the volume of rehydration solution imbibed was
not different for any of the treatments (range 8 to 15.5 1). This suggested that composition
of the rehydration solution was not an important factor in determining initial ﬂuid intake.
Because the volumes drank were similar to the reported capacity of the equine stomach
(Pfeiffer and MacPherson 1990), satiation or cessation of drinking was likely a
consequence of gastric ﬁlling as has been described in rats (Engstrom and Deaux 1974).
This suggestion is supported by the observation that the majority of initial drinking also

occurred within the ﬁrst minute after rehydration ﬂuid was offered.

From studies of the diuretic response of horses to furosemide (Freestone et a1. 1989;

Sosa Leon et al. 1998) and sweat electrolyte concentrations in exercising horses

(McCutcheon and Geor 1996), the amount of Na+ lost by these horses likely approached

78

3000 to 3500 mmol (~1500 mmol after furosemide administration and ~ 1500 to 2000
mmol in sweat [12 to 15 l sweat x 130 mmol/1]). The initial drink of 0.9% NaCl would
have provided only 1700 to 1800 mmol of both ions and replaced less than approximately
half of the sodium deﬁcit. Nevertheless, when coupled with increased absorption of
electrolytes and water from the gastrointestinal tract, the 0.9% NaCl solution may have
provided sufﬁcient Na+ to replace the majority of the deﬁcit. In previous studies of
treadmill exercise (Dﬁsterdieck et al. 1999) and furosemide-induced dehydration (Schott
et al. 2002) in our laboratory, supplementation of NaCl as an oral paste at doses
estimated to fully replace the electrolytes lost in sweat or urine was accompanied by
increased urinary Na+ losses following electrolyte administration. These results suggested
that the doses administered were either greater than the losses (unlikely based on
estimated losses) or were greater than needed because a substantial portion of the deﬁcits

may have been replaced by intestinal stores.

In horses fed a hay diet, ingesta in the large intestine has been estimated to contain a
reservoir of ~15 l of water, containing ~2000 to 2500 mmol each of Na: K+, and Cl'
(Meyer 1989a; Meyer 1989b). This reservoir would provide ~125 to 150 g of NaCl or
~15 l of 0.9% NaCl. Absorption of these intestinal reserves is likely an important reason
that horses performing endurance exercise rarely develop signiﬁcant hyponatremia
despite substantial Na+ losses in sweat and typical replacement with a hypotonic ﬂuid
(water). Although not measured in these horses, increases in aldosterone concentration
(ALD) have been observed in horses performing endurance exercise under both ﬁeld and

laboratory conditions (Nyman er al. 1996; Schott et al. 1997; Schott er al. 1999). The

79

 

ALD response has been attributed to Na+ losses in sweat and it was either abolished or
attenuated in horses administered oral electrolyte pastes or saline water, respectively
(Nyman et al. 1996; Schott et al. 1999). Attenuation of the ALD response when horses in
this study drank 0.45% NaCl or 0.9% NaCl would have been expected and would have
provided further information as to whether initial drinking of salt water was partly or
completely effective in replacing the Na+ deﬁcit induced by furosemide administration

and endurance exercise.

In the horses that initially drank salt water, plasma [Na+] remained increased from the
starting values at 20 min of recovery while rapid intestinal absorption of ﬂuid by the
horses that drank water led to a decrease in plasma [Na+]. Similarly, PP decreased to a
greater extent after an initial drink of water while PP remained relatively unchanged after
drinking saline. Taken together, these observations could support more rapid intestinal
absorption of water than saline. More importantly, the fact that plasma [Na+] remained
elevated was likely an important stimulus for greater water drinking by both saline
groups once water was again offered from 20 to 60 min of recovery (Schott et al. 1999;
Schott et al. 2002). Although the composition of the initial rehydration ﬂuid could have
also affected gastric emptying rate (GER), it is unlikely that saline solutions slowed GER
as Sosa-Leon et al. (1998) found no decrease in GER with saline solutions with
osmolalities up to twice that of 0.9% NaCl. The fact that horses that initially drank salt
water also readily drank greater amounts of water from 20 to 60 min of recovery further
supports that GER was not slowed by the saline solutions offered. Although it remained

well within the reference range, plasma [Na+] actually decreased below the starting value

80

 

after overnight recovery in horses that were initially offered water. Thus, it is likely that
the greater magnitude of persisting BW loss in the horses offered only water was related
to depletion of body electrolyte stores despite the fact that horses had free access to hay
during the overnight recovery period. This observation further supports that continued
salt supplementation in the form of either salt water, oral pastes, or electrolytes added to
concentrate feed, would be an appropriate management recommendation for horses
recovering from endurance exercise even when supplementation was provided before and

during the competition.

Palatability of the rehydration ﬂuid selected is another important factor that can affect
voluntary drinking and post-exercise rehydration in human athletes (Szlyk et al. 1989).
Although an early study by Randall et al. (1978) showed that the upper limit of NaCl
concentration that horses would readily drink was 0.6%, that study was performed in
weanling foals that were not dehydrated. Subsequent study (Houpt et al. 1995) found that
horses would willingly drink 0.9% NaCl once they had become accustomed to offering of
salt water. Further, adverse effects of drinking salt water at this concentration were not
observed in our horses or in these prior studies. Finally, although not statistically
signiﬁcant, there was a tendency for greater water drinking after an initial drink of 0.9%
NaCl in comparison to an initial drink of 0.45% NaCl. Thus, in horses that would be
provided salt water as a rehydration strategy, a concentration of 0.9% would seem to be a

reasonable concentration to employ.

81

 

In conclusion, our results and those of Nyman et al. (1996) clearly demonstrate that
provision of salt water (0.9% NaCl solution) during and following endurance exercise is a
better rehydration strategy than providing only water. Drinking salt water can attenuate
loss of body ﬂuid stores by stimulating greater subsequent water intake and it appears
that the electrolytes imbibed in salt water are largely retained. Rehydration with plain
water alone is not recommended because ingestion of this hypotonic ﬂuid can lead to
thirst inhibition and could delay overall recovery by an additional day or longer. The
recommendation to provide salt water would be of greatest beneﬁt to horses competing in
endurance events lasting several days but should only be pursued after horses had been

previously trained to drink salt water during and after exercise.

82

 

Footnotes

‘ Mustang 2000, Kagra AG, Fahrwangen, Switzerland.

2 Lasix® (furosemide) injection 5%, Hoechst Roussel Vet, Warren, NJ.

3 Swan Ganz ﬂow directed thermodilution catheter, American Edwards Laboratories,
Aﬁasco, Puerto Rico.

4 Stat 7 Proﬁle Analyzer, Nova Biomedical, Waltham, MA.

5SigmaStat, Jandel Scientiﬁc, St. Paul, MN.

83

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Convertino, V. A., Armstrong, L. E., Coyle, E. F., Mack, G. W., Sawka, M. N., Senay, L. C.
and Sherman, W. M. (1996) Position stand: exercise and ﬂuid replacement. Med. Sci.
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Dtisterdieck, K. F ., Schott, H. C., Eberhart, S. W., Woody, K. A. and Coenen, M. (1999)
Electrolyte and glycerol supplementation improve water intake by horses performing a
simulated 60 km endurance ride. Equine vet. J. Suppl. 30, 418-424.

Engstrom, R. and Deaux E. (1974) Stomach distension as a regulation of ﬂuid intake.
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F itzsimons, J. T. (1998) Angiotensin, thirst, and sodium appetite. Physio]. Rev. 78, 583-686.
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sodium bicarbonate-induced alkalosis in the horse and response to oral KCl or NaCl 5:
therapy. Am. J. vet. Res. 50, 1334-1339.

 

Geor, R. J. and McCutcheon, L. J. (1996) Thermoregulation and clinical disorders
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Greenleaf, J. E. (1992) Problem: thirst, drinking behavior, and involuntary dehydration.
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Houpt, K. A., Northrup, N., Wheatley, T. and Houpt, T. R. (1995) Thirst and salt appetite
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Hubbard, R. W., Sandick, B. L., Matthew, W. T., Francesconi, R. P., Sampson, J. B.,
Durkot, M. J ., Maller, O. and Engell, D. (1984) Voluntary dehydration and alliesthesia
for water. J. appl. Physio]. 57, 868-875.

Hyyppa, S., Saastamoinen, M. and P636, A. R. (1996) Restoration of water and electrolyte
balance in horses after repeated exercise in hot and humid conditions. Equine vet. J.
Suppl. 22, 108-113.

Kingston, J. K., Goer, R. J. and McCutcheon, L. J. (1997) Use of dew-point hygrometry,
direct sweat collection, and measurement of body water losses to determine sweating
rates in exercising horses. Am. J. vet. Res. 58, 175-181.

Marlin, D. J ., Scott, C. M., Mills, P. C., Louwes, H. and Vaarten, J. (19983) Effects of

administration of water versus an isotonic oral rehydration (ORS) at rest and changes
during exercise and recovery. Vet. J. 155, 69-78.

84

 

 

Marlin, D. J ., Scott, C. M., Mills, P. C., Louwes, H. and Vaarten, J. ( 1998b) Rehydration
following exercise: Effects of administration of water versus an isotonic oral rehydration
solution (ORS). Vet. J. 156, 41-49.

Maughan, R. J. and Shirreffs, S. M. (1994) Recovery from prolonged exercise: restoration
of water and electrolyte balance. J. Sports Sci. 15, 297-303.

McCutcheon, L. J. and Geor, R. J. (1996) Sweat ﬂuid and ion losses in horses during
training and competition in cool vs. hot ambient conditions: implications for ions
supplementation. Equine vet. J., Suppl. 22, 54-62.

Meyer, H. (1989a) Inﬂuence of feed intake and composition, feed and water restriction,
and exercise on gastrointestinal ﬁll in horses, Part 1. Equine Pract. 18(7), 26-29.

Meyer, H. (1989b) Inﬂuence of feed intake and composition, feed and water restriction,
and exercise on gastrointestinal ﬁll in horses, Part 3. Equine Pract.18(10), 25-28.

Monreal, L., Garzon, N., Espada, Y., Ruiz-Gopegui, R. and Homedes, J. (1999) Electrolyte
vs. glucose-electrolyte isotonic solutions for oral rehydration therapy in horses. Equine
vet. J. Suppl. 30, 425-429.

Nyman, S., J ansson, A., Dahlbom, K. and Lindholrn, A. (1996) Strategies for voluntary
dehydration in horses during endurance exercise. Equine vet. J., Suppl. 22, 99-106.

Pﬁeffer, C J. and MacPherson, B. R. (1990) Anatomy of the gastrointestinal tract and
peritoneal cavity. In: The Equine Acute Abdomen. Ed: N. A. White. Lea & F ebiger,
Philadelphia. pp 2-24.

Randall, R. P., Schurg, W. A. and Church, D. C. (1978) Response of horses to sweet,
salty, sour, and bitter solutions. J. Anim. Sci. 47, 51-57.

Schott, H. C., Axiak, S. M., Woody, K. A. and Eberhart, S. W. (2002) Effect of oral
electrolyte pastes on rehydration of horses. Am. J. vet. Res. 63, 19-27.

Schott, H. C., Patterson, K. S. and Eberhart, S. W. (2001) Glycerol hyperhydration in
resting horses. Vet. J. 161, 194-204.

Schott, H. C., Diisterdieck, K. F., Eberhart, S. W., Woody, K. A., Refsal, K. R. and
Coenen, M. (1999) Effects of electrolyte and glycerol supplementation on recovery

from endurance exercise. Equine vet. J. Suppl. 30, 384-393.

Schott, H. C. and Hinchcliff, K. W. (1998) Treatments affecting ﬂuid and electrolyte status
during exercise. Vet. C ]in. North Amer. .' Equine Pract. 14, 175-204.

Schott, H. C., McGlade, K. S., Molander, H. A., Leroux, A. J. and Hines, M. T. (1997)
Bodyweight, ﬂuid and electrolyte, and hormonal changes in response to 50 and 100 mile

85

 

endurance rides. Am. J. vet. Res. 58, 303-309.

Shirreffs, S. M., Taylor, A. J ., Leiper, J. B. and Maughan, R. J. (1996) Post-exercise
rehydration in man: effects of volume consumed and drink sodium content. Med. Sci.
Sports Exer. 28, 1260-1271.

Sosa Leon, L. A., Hodgson, D. R., Carlson, G. P. and Rose, R. J. (1998) Effects of
concentrated electrolytes administered via a paste on ﬂuid, electrolyte, and acid base
balance in horses. Am. J. vet. Res. 59, 898-903.

Sosa Leon, L. A., Hodgson, D. R. and Rose, R. J. (1997) Gastric emptying of oral
rehydration solutions at rest and after exercise in horses. Res. vet. Sci. 63, 183-187.

Szlyk, P., Sils, I. V., Francesconi, R. P., Hubbard, R. W. and Armstrong, L. E. (1989)

Effects of water temperature and ﬂavoring on voluntary dehydration in men. Physiol.
Behav. 45, 639-647.

86

Chapter 4

Rehydration ﬂuid temperature affects voluntary
drinking in horses
dehydrated by furosemide administration and
endurance exercise

Prawit Butudom, Donna J. Barnes, Matthew W. Davis,
Brian D. Nielsen, Susan W. Eberhart, and Harold C. Schott II

Department of Large Animal Clinical Sciences, College of Veterinary Medicine
(Butudom, Davis, Barnes, Eberhart, and Schott), and Department of Animal Sciences,
College of Agriculture and Natural Sciences (Nielsen), Michigan State University, East
Lansing, MI 48824, USA.

87

 

ABSTRACT

There has been little study of the effect of rehydration ﬂuid temperature on voluntary
intake by dehydrated horses. In human athletes, temperature of the rehydration solution
has been demonstrated to affect voluntary intake during the initial recovery period and
the magnitude of dehydration. A recent study has been shown that offering a saline
solution (0.9% NaCl) to dehydrated horses as the initial rehydration ﬂuid after exercise
results in greater total ﬂuid intake (and attenuation of the magnitude of dehydration) than
when plain water is offered. We hypothesized that dehydrated horses would voluntarily
drink a greater total volume of cool ﬂuid than warm ﬂuid during the ﬁrst hour of

recovery. To test this hypothesis, we measured body weight (BW), ﬂuid intake (FI),

serum osmolality (Osm) and [Na+] in six horses offered three different temperatures of
0.9% NaCl. After dehydration was induced by furosemide administration (1 mg/kg, IV)
followed by 30 km of treadmill exercise; 10°C, 20°C and 30°C 0.9% NaCl was offered,
in a randomized order, immediately after completing exercise. The initial rehydration
ﬂuid was removed after 5 minutes and horses were washed off and cooled out. After 20
min of recovery, horses were placed in a stall (without feed) and the volume of further
voluntary water (at 10, 20, or 30°C) intake from 20 to 60 minutes of recovery were
measured and the number of drink episodes was recorded. By the end of exercise, horses
lost 4.2 i 0.3, 4.5 i 0.3, and 4.5 i 0.2% of BW (p<0.05 in comparison to pre-furosemide
values for all treatments; p>0.05 for treatment differences) and F1 during the ﬁrst 5

minutes of recovery was 9.8 i 2.5, 12.3 i 2.1, and 9.7 i 2.0 L for 10, 20, and 30°C

treatments, respectively. After 20 min of recovery, [Na+] increased by 3.6 i 0.4, 2.6 i

88

 

0.5, and 3.7 i 0.5 (P>0.05) mmol/L. Between 20 and 60 min of recovery, the numbers of
drinking episodes was not different among treatments (P>0.05), but water intake for 20°C
(7.7 i 0.8 L) and 30°C 0.9% NaCl (6.6 :t 1.2 L) was greater than for 10°C 0.9% NaCl
(4.9 i 0.5 L; P<0.05). When total ﬂuid intake was compared to persisting BW loss at the
end of the 60 min recovery period, a highly signiﬁcant negative correlation (r= -0.91,
P<0.01) was found. Thus, total FI was 14.7 :1: 2.5 L, 19.9 i 2.5 L (P<0.05, in comparison
to 10°C 0.9% NaCl), and 16.3 i 2.4 L for 10°C, 20°C and 30°C 0.9% NaCl, respectively.
At 60 min of recovery, persisting BW loss was not different among treatments. In
conclusion, providing rehydration ﬂuid at near ambient (20°C) temperature resulted in
greater voluntary ﬂuid intake by the end of the initial 60-minute recovery period, in

comparison to offering cool (10°C) or warm (30°C) temperatures.

89

v 8- U‘ ~r

 

Introduction

Replacement of water and electrolytes lost in sweat during endurance exercise is
important for continued work as well as for prevention of medical problems that may
develop during the recovery period (Geor and McCutcheon, 1996). However, both human
and equine athletes fail to completely replace body ﬂuid losses by voluntary drinking
during the ﬁrst few hours of the recovery period, despite free access to various
rehydration solutions (Engell et al., 1987; Schott et al., 1997). This condition, measured
as persisting body weight (BW) loss, has been termed both “voluntary dehydration” and
“involuntary dehydration” and has been attributed to blunted thirst (Hubbard et al., 1984;
Greenleaf, 1992). Because sweating results in loss of both water and electrolytes,
exercise-induced dehydration results in lesser increases in plasma osmolality (Posm) and
sodium concentration ([Na+]) than dehydration induced by water deprivation. Since the
primary stimulus for thirst is an increase in plasma tonicity (Andersson, 1978; Fitzsimons,
1998), attenuation of these increases with sweating appears to be an important factor for

limiting thirst and rehydration during and after exercise.

Restoration of body ﬂuids and recovery of BW following exercise, in both human and
equine athletes, is more rapid and complete when rehydration solutions containing
electrolytes are used in place of plain water (Maughan and Shirreffs, 1994; Hyyppéi et al.,
1996; Nyman et al., 1996; Shirreffs et al., 1996; Marlin et al., 1998). However, most
previous studies have investigated the effects of “forced” hydration by instructing
subjects to drink speciﬁc amounts or, in horses, by administering solutions via a

nasogastric tube. In contrast, there has been less investigation of voluntary drinking

90

h 31“» ' s r

 

during or after exercise in either species. In human subjects with varying degrees of
hypohydration, ﬂuid intake during a one hour recovery period, although insufﬁcient to
replace BW losses, was correlated with the increase in P0sm during exercise (Engell et al.,
1987). Recent studies in our laboratory have shown a similar relationship between the
increase in plasma tonicity and voluntary ﬂuid intake by horses dehydrated by endurance

exercise (Dr'isterdieck et al., 1999; Butudom et al., 2002).

In addition to electrolyte content, more subtle factors including temperature and
ﬂavoring of rehydration solutions also affect voluntary intake by human athletes (Adolph
and Wills, 1947; Boulze et al., 1983; Sandick et al., 1984; Szlyk et al., 1989). The inﬂuence
of water temperature has been fairly well documented in human athletes: following
exercise, they both prefer and drink greater volumes of cool water, in comparison to
water at temperatures at or above ambient temperature (Boulze et al., 1983; Sandick et al.,
1984; Szlyk et al., 1989). In addition, cool or cold water ingested during and after exercise
can act as a heat sink and lower core temperature (Wimer et al., 1997). In horses, there
has been little study of the effects of water temperature on voluntary drinking. In the only
study we could ﬁnd, ponies stabled in a cold environment (near freezing) consumed a
greater volume when warm (31-48°C) water was offered, in comparison to when cold (0-

4°C) water was offered (Kristula and McDonnell, 1994).

There have been no studies in horses examining the effects of water temperature on
drinking responses or core temperature changes during or after endurance exercise. Thus,

similar to observations in human athletes, we hypothesized that horses dehydrated by a

91

 

combination of furosemide administration and prolonged exercise would drink a greater
total volume of a cool (10°C) rehydration ﬂuid than ﬂuid at ambient (20°C) or near body
(30°C) temperatures during the ﬁrst hour of recovery. In addition, drinking cool ﬂuid was
hypothesized to produce a more rapid decrease in core temperature. Based on prior work
in our laboratory (Butudom et al., 2002), we elected to provide 0.9% NaCl at various
temperatures as the initial rehydration solution (0-5 minutes post-exercise) followed by

water at various temperatures for the remainder of the 60 minute recovery period.

Materials and Methods

Horses and conditioning program

 

Six 2- to 3-year-old mixed breed horses (three males and three females) ranging in
weight from 287-460 kg (mean t SE = 360 i 14 kg) were studied. The horses had
recently completed a 60 day training study during which they exercised 6 days per week
in a free-ﬂow horse walker (Centaur Horse Walker, Inc.). During the ﬁnal week of that
training study they exercised 70 min/day alternating between walking (1.7 m/sec for 40
min) and trotting (3.4 m/sec for 30 min). Between the end of the training study and the
start of this study (a 3 week period), horses were accustomed to treadmill exercise
(Mustang 2000, Kagra AG) three times per week and all horses completed two 30 km
exercise bouts before the ﬁrst experimental run. Salt water (0.9% NaCl) at different
temperatures (10, 20, or 30°C) was offered after each conditioning run to introduce
horses to the different treatments. Throughout the study period (May-August, 2001),
horses were maintained at pasture with free access to water and no supplemental feed or

salt was provided. The training program and all experimental protocols were performed

92

 

in accordance with the guidelines of the Institutional Animal Care and Use Committee of

Michigan State University.

ExperimentaL protocol

 

Horses were brought in from pasture on the afternoon before each experiment and
housed overnight in a stall with free access to a grass/alfalfa mixed hay and water. On the
morning of each experimental run, initial dehydration was induced by furosemide
administration (Lasix® [furosemide] injection 5%, Hoechst Roussel Vet, 1.0 mg/kg IV)
120 min before the start of the 30 km exercise bout. Because our goal was to produce
dehydration in excess of a 4% BW loss, furosemide treatment was added to the
experimental protocol. Otherwise, 60-90 km of treadmill exercise would have been
required to produce this degree of dehydration under the mild ambient conditions in our
laboratory and a longer exercise bout would have increased the risk of musculoskeletal
injury. During the remaining pre-exercise period, surface electrocardiographic electrodes
were applied (for telemetric recording of heart rate [HR]) and a 7 French, 110 cm catheter
(Swan Ganz ﬂow directed thermodilution catheter, American Edwards Laboratories) was
aseptically passed into the pulmonary artery (conﬁrmed by pressure waveform) via an 8
French introducer inserted into the right jugular vein (for collection of mixed venous
blood samples and measurement of core temperature). Each horse was studied three times
when offered, in a randomized order, 0.9% NaCl at 10, 20, or 30°C during the initial 5
min of recovery and water at 10, 20, or 30°C (same temperature as the initial saline drink)
during the remainder of the 60 min recovery period. Experiments on each horse were

separated by a minimum of 7 days during which further exercise was not performed.

93

. . .I1

 

During each experiment BW was measured on a digital scale (nearest 0.5 kg) before
and 120 min after (pre-exercise) furosemide administration. Horses then performed 30
km of treadmill exercise at varying speeds to simulate an endurance ride (Fig. 1). The
experimental run consisted of two 15 km exercise bouts (lasting 56 min; speeds between
1.6 [walk] and 8 m/s [cantar]; 0o slope) separated by a 15 min rest period. Horses did not
have access to water or any other rehydration solution during the run. Immediately
(within 10 s) after completion of exercise, 0.9% NaCl at one of the three temperatures
was offered for voluntarily drinking from a hand-held bucket. In addition to documenting
total volume imbibed, the number of drinks and total duration of drinking was determined
by subsequent review of a continuous video recording collected during the initial 5 min
recovery period. The time from which the horse ﬁrst began sipping water until it raised
its lips away from contact with water was recorded as a single drinking episode. After the
initial 5 min of recovery, the rehydration solution was taken away and horses were
walked off the treadmill, the pulmonary arterial catheter was removed, and horses were
washed with water. The horses had no access to rehydration ﬂuid from 5-20 min after the
end of exercise. At 20 min of the recovery period, horses were weighed and placed free in
a stall (without feed) and further voluntarily water intake (at 10, 20, or 30°C) was
measured. In addition, the number of drinking episodes from 20-60 minutes of recovery
was recorded by continuous observation during this 40 min period. After 60 min of
recovery horses were weighed again and returned to the stall where they remained
overnight with free access to hay and water. A ﬁnal weight was recorded after overnight

recovery (~18 h after completion of the 45 km run).

94

 

Experimental Protocol

Finish

 

   

 

 

 

 

609
M 0 0 59 0 5, 20, 60’
i i l i B B/BW B/BW
B/BW B BW B *- <—>
initial drink water
Furosemide (0.9% NaCl at (10, 20, or 30%;)
llng/kg IV 10, 20, orSU’C) In stall
HR

ﬁgm. Experimental protocol. Furosemide (1.0 mg/kg IV) was given 2 h prior to and
horses were instrumented during the 60 min before exercise. The experimental run
consisted of two 15-km exercise bouts separated by one 15-min rest periods. Immediately
(within 10 seconds) after completion of exercise, one of three rehydration ﬂuids (10, 20,
or 30°C 0.9% NaCl solution) was offered to the horses, in a randomized order, for
voluntary drinking (shorter double-headed arrow). Aﬁer 5 minutes of recovery, horses
were walked off the treadmill and washed down with water. After 20 min of recovery,
horses were placed free in a stall (without feed) and further voluntary water intake (at 10,
20, or 30°C) from 20-60 minutes of recovery was measured (longer double-headed
arrow). Blood (B) was collected and body weight (BW) and heart rate (HR) were

recorded at various times point during the experiment.

95

 

Sample collection and anaQsis

At 120 minutes before and at 20 and 60 min and ~18 h after completion of the 30 km
run, blood samples (~10 ml) were collected by jugular venipuncture and HR was
determined by auscultation. During exercise, HR and pulmonary arterial blood
temperature (Tcore, 9520A Cardiac Output Computer, Baxter Healthcare Corp., Edwards
Critical-Care Division) were recorded at the start, during the 4 m/s (trot) and 8 m/s
(canter) phases, and 1 and 5 min after completion of each 15 km exercise bout. Blood
samples were collected at the start and ﬁnish of each 15 km of exercise bout and after 1,
3, and 5 min after completion of the 45 km exercise test. Blood was collected
anaerobically into heparinized plastic syringes that were placed on ice until analyzed for
plasma electrolyte (Na+, KL, and Cl') concentrations within 30 min of collection (Stat 7
Proﬁle Analyzer, Nova Biomedical). The remainder of each sample was used for
measurement of hematocrit (microhematocrit method) and plasma protein concentration
(PP, by refractometry). All feces and urine produced from the time of furosemide
administration to the end of the 60 min recovery period were collected and weighed with
the exception of urine produced prior to exercise in response to furosemide
administration. BW loss (with the exception of ~18 h recovery) was corrected for fecal
losses but not for urine losses because the latter was an important factor in the pre-

exercise BW loss.

Dissipation of metabolic heat by drinking

In an attempt to determine the heat dissipating effects of the initial period of drinking

0.9% NaCl at 10, 20, or 30°C, the gain in heat content of the ingested ﬂuid was estimated

96

 

by assuming that the speciﬁc heat of 0.9% NaCl was the same as that of water (1.0
kcal/l/°C) and that all ﬂuid drank was warmed to 38°C after 5 min of recovery. This heat
gain by ingested ﬂuid would be equal to the loss of metabolic heat (MH) by the horse.
Next, the total MH produced during the two 15 km exercise bouts was estimated to
determine the fraction that could be dissipated into the ingested salt water. MH was
estimated by the following formula:

MH = oxygen consumption (ml/min/kg) x BW (kg) x k (kcal/l 02) x duration
(min)
where k, a constant for the amount of heat liberated per liter of Oz consumed, was
assumed to be 4 kcal/l Oz (Hodgson et al., 1993). Horses were assumed to have a
maximal oxygen consumption of 120 ml/kg/min and to work at an average load of 30%
of maximal oxygen consumption during the two 52 min exercise bouts. Finally, the role
of heat transfer in cooling was evaluated by comparing the decline in Tcore from the end

of exercise to 5 min of recovery to the gain in heat content of the ingested ﬂuid.

Data analysis

All values provided in the text are presented as means :1: SE. Total ﬂuid intake during
the recovery period was analyzed using commercial software (SigmaStat, Jandel
Scientiﬁc) by a one way repeated measures ANOVA to assess differences between
temperatures of the rehydration solutions. When F ratios were signiﬁcant (p<0.05), a
Student-Newman-Keuls test was performed to detect speciﬁc differences. Changes in
parameters measured at multiple times (BW, HR, Tcore, and blood values) were analyzed

by a two way repeated measures ANOVA (main effects of time and treatment) and, when

97

 

F ratios were signiﬁcant (p<0.05), a Student-Newman-Keuls post-hoc was again
performed to detect speciﬁc differences. Selected relationships between parameters

measured were also examined by Pearson product moment correlation analysis.

Results

Five of six horses completed all three experimental runs while only ﬁve horses
completed experiments in which they were offered rehydration solutions at 10°C.
Ambient temperature in the treadmill laboratory ranged between 18 and 28°C (mean 25.0
at. 07°C) and relative humidity ranged between 46 and 75% (mean 59.9 i 1.8%) during
exercise and signiﬁcant differences between the runs for each temperature of rehydration
solution were not observed. Mean HR ranged from 110.2 i 3.6 to 117.4 i 3.1, from 142.8
i 3.0 to 153.4 i 4.3, and from 79.0 i 7.7 to 87.5 i 5.3 beats/min during the trot (4 m/s)

and canter (8 m/s) phases of exercise and after 1 min of recovery, respectively. No
differences over time (ﬁrst and second 15 km exercise bout) or between treatments were

observed.

Furosemide administration produced a BW loss prior to exercise of 2.1 i 0.3, 2.2 i
0.2, and 2.0 i 0.1% (p>0.05) for 10, 20, and 30°C treatments, respectively. The
combined effects of furosemide administration and 30 km of treadmill exercise resulted
in BW losses of 4.2 i 0.3, 4.5 i 0.3, and 4.5 i 0.2% (p<0.05 in comparison to pre-
furosemide values for all treatments; p>0.05 for treatment differences) for 10, 20, and

30°C treatments, respectively (Table I and Fig. 2). After 60 min of recovery, BW was not

different from the pre-furosemide values for any of the treatments but BW loss for 10 and

98

 

30°C treatments (0.8 i 0.5 and 0.5 i 0.8%, respectively) tended (p=0.15) to be greater
than the value for the 20°C treatment (-0.3 i 0.8) for which a mild gain in BW was
actually observed. Similarly, after 18 h of recovery, BW was not different from the pre-
furosemide values for any of the treatments and differences between treatments were not
detected (decrease of 0.9 i 0.7, 0.3 i 0.5, and 0.3 i 0.5% for 10, 20, and 30°C treatments,

respectively).

The amount of ﬂuid imbibed during the initial 5 min of recovery was not different
among treatments (9.8 i 2.5, 12.3 i 2.1, and 9.7 i 2.0 1 for 10, 20, and 30°C treatments,
respectively) and replaced approximately two-thirds of lost body ﬂuid (Fig. 3). With all
rehydration solution temperatures, horses drank immediately when the bucket was
offered and the majority of drinking occurred within the ﬁrst 1 to 2 min of recovery.
However, there was a tendency for horses to take fewer (p=0.07), longer drinks when
offered the 20°C saline (6.0 i 1.7 drinks lasting 20.3 i 5.3 s) in comparison to saline at
10°C (10.3 i 2.0 drinks lasting 11.6 i 3.8 s) or 30°C (11.0 i 2.0 drinks lasting 14.6 i 2.7
s). From 20 to 60 min of recovery, voluntary water intake was greater (p<0.05) for both
20 and 30°C saline (7.7 i 0.8 and 6.6 i 1.2 1, respectively) than for saline at 10°C (4.9 i
0.5 1). When combined, these values resulted in a total voluntary ﬂuid intake during the
60 min recovery period of 14.7 i 2.5, 19.9 i 2.5, and 16.3 i 2.4 l for 10, 20, and 30°C
treatments, respectively (p<0.05, 20°C greater than 10°C, Fig. 3). The number of drinking
episodes from 20-60 min of recovery was not different for water at any temperature (4.0

i 0.8, 4.6 i 0.8, and 4.3 i 1.0 for 10, 20, and 30°C treatments, respectively). When total

99

-. .FD‘ \ 3.3—1

 

ﬂuid intake was compared to persisting BW loss at the end of the 60 min recovery period,

a highly signiﬁcant negative correlation (r= -0.91, P<0.01) was found (Fig. 4).

Hematocrit tended to decrease after furosemide administration but the only signiﬁcant
change was an increase (p<0.05) by the end of exercise (Table 1). PP and plasma [Na+]
increased (p<0.05) after furosemide administration and remained increased through the
end of exercise (Table 1). PP tended to decrease within 5 min of recovery and was not
different than the pre-furosemide values after 20 min of recovery. In contrast, plasma
[Na+] reached the highest values after 20 min of recovery and remained greater than the
pre-furosemide value after 60 min of recovery for the 10°C treatment. After overnight
recovery plasma [Na+] was not different from the pre-furosemide values for any of the
treatments. Plasma [K+] decreased (p<0.05) after furosemide administration and
remained decreased from the pre-furosemide values for all treatments throughout the 60
min recovery period (Table 1). After overnight recovery plasma [K+] was not different
from the pre-furosemide value for the 10°C treatment but remained decreased (p<0.05)
for the 20 and 30°C treatments. Plasma [Cl'] decreased (p<0.05) after furosemide

administration and remained decreased until 20 min of recovery for all treatments (Table

1).

Mean Tcore ranged from 38.8 i 0.2 to 39.0 i 0.2, from 39.1 i 0.2 to 39.5 i 0.2, and
from 38.2 i 0.2 to 38.3 i 0.2°C during the last canter phases of the ﬁrst and second

exercise bouts and after 1 min of recovery, respectively. Although Tcore increased

(p<0.05) from the pre-exercise values during the canter phases of both exercise bouts,

100

 

values after 1 min of recovery had returned to the pre-exercise values and no differences
between treatments were observed (Fig. 5). Depending on both temperature and volume
consumed, estimated MH transferred to the ﬂuid imbibed during the initial 5 min of
recovery ranged from 48 to 532 kcal (Table 2). In addition, there was a signiﬁcant
positive linear correlation between the change in Tcore from 0 to 5 min of recovery and the
amount of MH transferred to the ingested ﬂuid (Fig. 6). However, exercise at an intensity r“
of 30% of maximal 0; consumption for 104 minutes produced an estimated MH load ,-

ranging from ~4300 to ~6900 kcal, depending on the BW of the horse (Table 2). Thus,

dissipation of heat by warming of ﬂuid imbibed from 0-5 min of recovery accounted for a

 

maximum of ~8% of the total MH generated in one horse while in most horses it was

responsible for dissipation of less than 5% of the total MH generated during exercise.

101

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+ 10 *

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\
c I . *
E: 3 . 30° / / (10 and 30°C
29 only)
IE» 2' \
g 1 -
>3 \
'8
.c: 0

-1 Start R End

|

bout 2 ' 0-20' ' 20—60' '
recovery recovery

'2 1 furosemide I boutl

W. Percent body weight loss after furosemide treatment (1 mg/kg IV) and during
and after 45 km of treadmill exercise ride for horses offered 10°C (ﬁlled circles), 20°C
(ﬁlled inverted triangles), or 30°C 0.9% NaCl (ﬁlled squares) during the initial 5 min of
recovery. "‘ = signiﬁcantly different (p<0.05) from the pre-furosemide value; see Figure l

legend for description of the experimental protocol.

104

 

I: FI 0'-5' post
WI 20'-60' post

N
U]

b
19.9

N
O

ab
16.3

l—t
UI

 

ﬂui intake (1)
E3

 

 

 

 

 

 

 

 

 

0 - . .
10 20 30

rehydration ﬂuid temperature (°C)

Figure 3. Total ﬂuid intake during the 60 minute recovery period after induction of
dehydration by administration of furosemide (1 mg/kg IV) followed by 30 km of
treadmill exercise for horses provided 10, 20, or 30°C 0.9% NaCl solution as the
rehydration ﬂuid during the initial 5 min following exercise and subsequently provided
free access to water (at 10, 20, or 30°C) from 20 to 60 min following exercise. FI O’-5’ =
ﬂuid intake during 0 to 5 minute post-exercise; FI 20’-60’ = ﬂuid intake during 20 to 60

minute of recovery.

105

 

 

 

6i
’2
c\ ' o 0 10C NaCl
:1 4 - 0 20C NaCl
4::
.9.” . v 30C NaCl
‘9 o
B 2 -
g} V
'8 O . ‘v 0 v o
.E o o
q, o o
E”
aux-2 *
a
U r= -o.ss, P<0.001 V
-4 U I I I I I
0 5 10 15 20 25 30

Voluntary ﬂuid intake (liters)

Figure 4. Correlation between total ﬂuid intake from 0 to 60 minutes of the recovery
period and persisting body weight loss (magnitude of involuntary dehydration) after 60
minutes of recovery after induction of dehydration by administration of furosemide (1
mg/kg IV) followed by 30 km of treadmill exercise for horses provided 10, 20, or 30°C
0.9% NaCl solution as the rehydration ﬂuid during the initial 5 min following exercise
and subsequently provided free access to water (at 10, 20, or 30°C) from 20-60 min

following exercise.

106

 

 

 

 

 

 

 

40 . A
*
3‘ / + 10C
'53 \ +3oc
U
V
0
§ 38 ./ ,/
P" -
i. R
37 2 52 min 15 min 52 min 0-5 min recovery

 

Start 1' Canter 2' Start 2' Canter End 5' post

Exercise bout

Figure 5. Pulmonary artery blood (Tcore) changes in 6 horses during 2 phases of
exercise until 5 minutes of postexercise recovery after induction of dehydration by
administration of furosemide (1 mg/kg IV) followed by 30 km of treadmill exercise for

horses provided 10, 20, or 30°C 0.9% NaCl solution as the rehydration ﬂuid during the

initial 5 min following exercise. * = signiﬁcant diference from pre-exercise values (all 3)

107

-. _ ‘._. “5’ H45;-
5

 

lu‘l' C [Ii-”ll

   
 

 

 

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.E 1.2 -
.5...’
0 1.0 «
U
V
a >} 0.8 'l v .
c 1;; v
0 0.6 -
H 2
3 3 0.4 -
0.0 h
g :1 0.2 - 10 C NaCl
.:: E 0 0 o O 0 20 C NaCl
0 II.) ' i v' v 30 C NaCl
c -o.2 « 0
E -o.4 - ' r=0.60, P<0.01
J:
.006 I I r U U I
o 100 200 300 400 500 600

Heat transfer into ingested water (kcal)

Figure 6. Correlation between changes in Tcore from O to 5 minutes of the recovery
period and the amount of heat transfer to ingested water (0.9% NaCl) during 0-5 minutes
of recovery after induction of dehydration by administration of furosemide (1 mg/kg IV)

followed by 30 km of treadmill exercise for horses provided provided 10, 20, or 30°C

0.9% NaCl solution as the rehydration ﬂuid during the initial 5 min following exercise.

108

 

Discussion

The results of this study demonstrate that temperature of rehydration ﬂuid inﬂuences
voluntary drinking in horses recovering from furosemide- and exercise-induced
dehydration. Compared to either cooler (10°C) or warmer (30°C) temperatures, offering
ﬂuid at near ambient temperature (20°C) resulted in the greatest voluntary ﬂuid intake by
the end of the initial 60 min recovery period. Thus, our hypothesis that horses dehydrated
by a combination of furosemide administration and prolonged exercise would drink a
greater total volume of a cool (10°C) rehydration ﬂuid, than ﬂuid at ambient (20°C) or
near body (30°C) temperatures, during the ﬁrst hour of recovery was refuted. Even
though there was a difference in ﬂuid intake between different ﬂuid temperatures during
the ﬁrst hour of recovery, BW loss after 60 minutes of recovery did not differ between
treatments. However, BW loss for the 10 and 30°C treatments tended to be greater than

the value for the 20°C treatment for which a mild gain in BW was actually observed.

As in a previous study (Butudom et al. 2002), initial rehydration with a saline solution
attenuated the magnitude of BW loss after the ﬁrst hour of recovery. Indeed, “involuntary
dehydration” did not occur in horses in this study. In addition, this ﬁnding suggested that
the saline solution drank during the initial 5 minutes of recovery was largely retained in
the body ﬂuids of these horses. Drinking salt water (0.9% NaCl) can attenuate loss of
body ﬂuid stores by stimulating greater subsequent water intake (Nyman et al. 1996;
Butudom et al. 2002). Adding Na+ to the rehydration ﬂuid increases Posm, which
enhances renal water conservation (Maughan and Shirreffs 1994; Shirreffs et al. 1996). In

contrast, increased production of dilute urine is observed in human subjects after

109

 

consumption of plain water or hypotonic oral rehydration solutions (Mitchell et al. 1994;
Mitchell et al. 2000). In horses, rehydration with water alone is less effective because
ingestion of this hypotonic ﬂuid can lead to thirst inhibition and can delay recovery from
exercise (Nyman et al. 1996; Butudom et al. 2002). Thus, initial intake of salt water
appears to be beneﬁcial to horses that may have limited time to recover from endurance
events before being subjected to further exercise, including transport after ﬁnishing the

competition.

Although limited to 6 horses, the results of this study demonstrate that horses
dehydrated by endurance exercise may prefer rehydration ﬂuids at near ambient
temperatures (in a temperate climate). In addition to the volume imbibed, the pattern of
drinking may also be used to assess ﬂuid preference. Although the number of drinking
episodes during the initial 5 min of recovery was not different for ﬂuid at any temperature,
there was a tendency for horses to take fewer, longer drinks when offered the NaCl solution
at 20°C, in comparison to 0.9% NaCl at 10°C or 30°C during the ﬁrst 5 minutes of recovery.
In addition to supporting a preference for ﬂuid at 20°C, these results could also suggest a
temperature dependence of taste of saline solutions in horses. Studies in humans and
laboratory animals have demonstrated that saline solutions taste different at various
temperatures (McBumey et al. 1973; Lundy and Contreras 1997). For example, human
subjects reported that saline tasted salty when the temperature of the solution was below
22°C but that the solution was tasteless when the temperature was above 22°C (McBumey

et al. 1973).

110

 

As in our previous experiment in which the composition of the initial rehydration
solution was varied (Butudom et al. 2002), horses in this study drank a similar mean
volume of 0.9%NaCl at all temperatures during the ﬁrst 5 minutes of recovery. This
particularly striking ﬁnding suggests that neither composition nor temperature of the
rehydration solution may be an important factor in determining initial ﬂuid intake.
Rather, dehydrated horses appeared to drink until satiated by a mechanism independent
of ﬂuid composition or temperature. Gastric ﬁlling has been suggested to be an important
factor in satiation of thirst and cessation of drinking (Towbin 1949; Engstrom and Deaux
1974). Indeed, the volumes drank by these horses were similar to the reported capacity of
the equine stomach (Pfeiffer and MacPherson 1990). The suggestion that gastric ﬁlling
contributes to initial satiation is further supported by the observation that the majority of
initial ﬂuid consumption occurred within the ﬁrst two minutes after completing the

exercise bout.

In contrast to laboratory animals that tend to prefer and drink the greatest amount of
water at 25-35°C, in a thermoneutral environment (Carlisle 1977), resting humans appear
to prefer water at 20°C (Boulze et al. 1983). This preference for cooler water by adult
humans may be a learned behavior because it is not observed in newborns (Boulze et al.
1983). Next, human athletes that become both dehydrated and hyperthermic show a
preference for water at 5-15°C (Boulze et al. 1983; Sandick et al. 1984; Szlyk et al.,
1989). Although colder water is preferred following exercise, offering a progressively
colder drink (<10°C) can actual decrease volume consumed (Boulze et al., 1983). As a

result, hyperthermia, rather than dehydration, has been suggested to be a more important

11]

 

mechanism for the preference for colder water (Boulze et al. 1983) while oropharyngeal
cooling has been advanced as a mechanism of greater thirst satiation and decreased intake
of colder water (Gold et al. 1973; Ramsauer et al. 1974; Carlisle 1977). Further, gastric
emptying is slower for cold solutions than for warm solutions (Deaux 1973). Thus, intake
of cold water could also prolong satiation by slowing gastric emptying rate. Core
temperature of horses in the present study at the end of exercise was not increased from
pre-exercise values; thus, hypertherrnia was not a stimulus for drinking. Lack of
hyperthermia, in addition to greater oropharyngeal cooling, could potentially explain why
our horses drank the least amount of ﬂuid at 10°C. Although horses cannot rate a ﬂuid
preference, our data suggest that, like dehydrated-normothermic human athletes,
dehydrated-normothermic equine athletes exercising under moderate environmental

conditions may prefer to drink rehydration ﬂuids at near ambient temperature (20°C).

Horses in this study were offered 0.9% NaCl as the initial rehydration ﬂuid during the
ﬁrst 5 min of the recovery period and plasma [Na+] remained increased from the pre-
exercise values after 20 min of recovery. In contrast, drinking during the initial 5 min of
recovery caused a prompt return of PP (and presumably plasma volume) to pre-exercise
values by 20 min of recovery (except for the 10°C treatment). Although both an increase in
plasma tonicity and a decrease in blood Qalasma) volume are well-documented stimuli for
thirst (Fitzsimons 1998), the increase in plasma tonicity appeared to be the more
important stimulus for further drinking from 20 to 60 min of recovery. In another study in
which horses maintained plasma volume during a similar experimental run, water intake

was signiﬁcantly correlated with the increase in plasma [Na+] induced by administration

112

 

of oral electrolyte pastes (Diisterdieck et al. 1999). Of interest, both plasma [Na+] and PP
were greatest after 20 min of recovery in horses initially offered 0.9% NaCl at 10°C.
However, horses drank the least amount of water at 10°C from 20 to 60 min of recovery.
This ﬁnding could support that oropharyngeal cooling may also have played a role in

satiation of thirst with this treatment.

The potential cooling effect of ingesting saline at different temperatures during the
initial 5 min of recovery was evaluated by comparing the decline in Tcore from 0-5 min of
recovery to the estimated amount of MH transferred to ingested ﬂuid. Although a
signiﬁcant positive linear correlation between these measures was found, supporting
cooling by the ingested ﬂuid, less than 5% of the estimated MH load generated during
exercise was likely transferred to ingested ﬂuid. These results indicate that any direct
cooling effect of ingested ﬂuid in horses performing endurance exercise is minimal and

conﬁrm that the major route of heat dissipation is by evaporation of sweat.

In conclusion, the results of this study clearly show that rehydration ﬂuid temperature
inﬂuences intake by dehydrated-nonnothermic horses. Although a signiﬁcant difference
in the volume of saline drank during the initial 5 min of recovery was not found, horses
offered saline at 20°C tended to take fewer, longer drinks and consumed 2-2.5 l more
during this initial recovery period. Further, due to the low number of experimental
subjects, our ability to detect signiﬁcance was limited by a lack of power. From 20-60

min of the recovery period, horses did drink the greatest amount of water at near ambient

temperature (20°C), in comparison to water at either cooler (10°C) or warmer (30°C)

113

 

temperatures. These ﬁndings support that attention should be paid to rehydration ﬂuid
temperature when attempting to maximize short-term water intake during and aﬁer
endurance exercise. Further, future studies of the effects of oropharyngeal cooling on
ﬂuid intake by horses are warranted. When combined with our previous ﬁndings
(Butudom et al., 2002), these data support that horses exercising for longer than an hour
may beneﬁt from initial provision of salt water (0.9% NaCl) at a temperature near 20°C
followed by frequent opportunities to drink plain water at temperature near 20°C.
Enhancing short-term rehydration should improve recovery and would be expected to

decrease the risk of developing medical problems after exercise and transport.

114

 

 

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and effects for humans after exercise. Physio]. Behav. 32, 851-855.

116

 

Schott, H.C., McGlade, K.S., Molander, H.A., Leroux, A.J. and Hines, MT. (1997)
Bodyweight, ﬂuid and electrolyte, and hormonal changes in response to 50 and 100 mile
endurance rides. Am. J. vet. Res. 58, 303-309.

Shirreffs, S.M., Taylor, A J., Leiper, J .B. and Maughan, R.J. (1996) Post-exercise
Rehydration in man: effects of volume consumed and drink sodium content. Med. Sci.
Sports Exer. 28, 1260-1271.

Szlyk, P., Sils, I.V., Francesconi, R.P., Hubbard, R.W. and Armstrong, LE. (1989) Effects
of water temperature and ﬂavoring on voluntary dehydration in men. Physiol. Behav. 45, 'r- ..
639-647.

Towbin, E. (1949) Gastric distension as a factor in the sensation of thirst in
esophagotomized dogs. Am. J. Physio]. 159, 533-541.

Wimer, G.S., Lamb, D.R., Sherman, WM. and Swanson, SC. (1997) Temperature of
ingested water and therrnoregulation during moderate intensity exercise. Can. J. Appl.
Physio]. 22, 479-493.

 

117

 

 

Chapter 5

General Discussion and conclusions

118

 

 

 

DISCUSSION AND CONCLUSIONS

Incomplete voluntary replacement of body ﬂuid losses sustained by sweating during
endurance exercise, despite free access to water or other rehydration solutions during and
after the exercise bout, is well-recognized in both human and equine athletes. This
phenomenon, which has been termed both voluntary and involuntary dehydration, is
recognized as a persisting body weight loss and has been attributed to blunted thirst.
Thus, factors affecting stimulation of thirst and voluntary drinking must contribute to
development of involuntary dehydration. The primary stimulus for thirst appears to be an
increase in plasma osmolality or, more speciﬁcally, an increase in plasma Na+
concentration coupled with activation of the renin-angiotensin system (Andersson 1978;

F itzsimmons 1998).

In human athletes, a number of factors including volume imbibed, sodium content,
temperature, and ﬂavoring of the rehydration solution have been demonstrated to affect
the magnitude of involuntary dehydration during the initial recovery period (Boulze eta].
1983; Sandick et a]. 1984; Szlyk et al. 1989; Barr et a]. 1991; Shirreffs et a]. 1996). In
order to prevent involuntary dehydration, the volume of ﬂuid imbibed must actually
exceed the volume of ﬂuid lost as sweat to account for ongoing water loss in urine
(Shirreffs et a]. 1996). Rarely are such volumes of rehydration ﬂuid imbibed by athletes
recovering from endurance exercise. In addition to adequate volume, a number of studies
(Barr et a]. 1991; Shirreffs er a]. 1996) have clearly demonstrated that inclusion of
electrolytes, particularly sodium, in the rehydration ﬂuid is critical for retention of

imbibed ﬂuid. Although consumption of plain water is better than no ﬂuid ingestion, it is

119

 

accompanied by a large increase in urine ﬂow leading to poor ﬂuid retention. The ﬁnding
of numerous studies in the past couple of decades led the American College of Sports
Medicine to publish a 1996 Position stand entitled Exercise and ﬂuid replacement in
which sodium is recommended to be included in rehydration ﬂuid at an amount estimated
to replace sweat losses for exercise lasting longer than 1 hour (Convertino eta]. 1996). In
addition to electrolyte content, more subtle factors including temperature and ﬂavoring of
the rehydration ﬂuid also affect voluntary intake by human athletes (Boulze et a]. 1983;
Sandick et a]. 1984; Szlyk et a]. 1989). Finally, considerable interindividual variability in
voluntary drinking has also been observed in human athletes and this can be another

factor affecting the magnitude of involuntary dehydration (Szlyk eta]. 1989).

Over the past ten years, there have been a number of studies designed to assess
treatments used to attenuate or correct dehydration in equine endurance athletes (work up
to 1997 reviewed in Schott and Hinchcliff 1998; Marlin eta]. 1998a; Marlin eta]. 1998b;
Sosa Leén et a]. 1998; Diisterdieck et a]. 1999; Monreal et a]. 1999). Although a few of
these studies demonstrated that administration of electrolytes as oral pastes can attenuate
loss of body ﬂuid stores, primarily by stimulating water intake, most studies involved
“forced” hyperhydration prior to exercise or rehydration during and after exercise by
administering solutions via a nasogastric tube. In the study by Diisterdieck et a]. 1999 in
which horses completed a 60-km simulated endurance ride on a treadmill,
supplementation of electrolytes as an oral paste before and during exercise enhanced
voluntary water intake and, thereby, attenuated body weight loss. Electrolyte

supplementation also stimulated horses to drink earlier during the course of the exercise

120

 

test and greater water intake was demonstrated to be correlated to an increase in plasma

osmolality and sodium concentration.

In horses, there has been little study of drinking and strategies to enhance voluntary
ﬂuid replacement during and after endurance exercise. In the only study to date to
address this question, Nyman et a] (1996) reported that horses completing a 62-km
simulated ﬁeld endurance ride voluntary drank more water from 1 to 3 hours of recovery
when offered a saline solution (0.9% NaCl) during the ride and the ﬁrst hour of recovery
than when offered plain water. Thus, studies of factors affecting voluntary rehydration by
horses performing endurance exercise were clearly needed. We investigated the effects of
volume (experiment 1, chapter 2), sodium content (experiment 2, chapter 3), and
temperature of rehydration ﬂuid (experiment 3, chapter 4) on voluntary rehydration of

horses dehydrated by endurance exercise.

In experiment 1 (Chapter 2), limiting the volume of water (to 4 l, 8 l, or an unlimited
amount during the ﬁrst 5 minutes of recovery) initially provided to horses dehydrated by
endurance exercise had no signiﬁcant effects on total ﬂuid intake, but all horses had
involuntary dehydration (persisting body weight loss) at 60 minutes of recovery.
Experiment 2 (Chapter 3) demonstrated that offering a saline solution (0.45% or 0.9%
NaCl) as the initial rehydration ﬂuid after exercise maintained an elevated [Na+] and
resulted in greater total ﬂuid intake (and attenuation of the magnitude of involuntary
dehydration) than when plain water was offered. In experiment 3 (Chapter 4), we found

that providing rehydration ﬂuid (0.9% NaCl) at near ambient (20°C) temperature resulted

121

 

in the greatest voluntary ﬂuid intake by the end of the initial 60-minute recovery period,

in comparison to offering cool (10°C) or warm (30°C) temperatures.

Our studies provide further support that an increase in plasma tonicity is a stimulus of
thirst in dehydrated horses. More importantly, plasma osmolality declined rapidly (within
5 minutes) after water drinking, supporting rapid absorption of imbibed water. Further,
regardless of the rehydration strategiy used, most drinking occurred within the ﬁrst
couple of minutes after the rehydration solution was provided at the end of exercise. This
observed of pattern of drinking was similar to that previously described for horses and
ponies (Suﬁt et a]. 1985; Jones et a]. 1989; Dﬁsterdieck et a]. 1999). Next, the
observation that the mean volume ﬂuid imbibed during the initial 5 minuites of recovery
was typically between 10 and 12 liters suggested that gastric ﬁlling may be a limiting
factor for rapid rehydration. However, the pattern of drinking was somewhat different
when the temperature of rehydration ﬂuid was manipulated (Chapter 4). The number of
drinking episodes and duration of drinking was less when the rehydration ﬂuid
temperature was below (10°C) or above (30°C) ambient temperature (20°C). As for
human athletes, this ﬁnding indicated that there is an optimal drink temperature for
rehydration ﬂuid for horses. In contrast to human athletes that can provide verbal
feedback, it is more difﬁcult to assess ﬂuid preference in horses. In addition to volume
imbibed, the difference in drinking patterns observed in experiment 3 should also be

assessed in future studies when investigating factors that affect the rehydration process.

122

 

In conclusion, even though the exact physiological mechanisms of “involuntary
dehdyration” in dehydrated horses remain unclear, we documented several factors that
affect voluntary drinking and rehydration of horses dehydrated by endurance exercise.
Further, our ﬁndings can have immediate practical application to limit or prevent
involuntary dehydration following endurance exercise. A strategy to enhance voluntary
ﬂuid intake would be to offer all horses an initial drink of salt water at near 20°C during
the ﬁrst few minutes after completion of exercise as well as offering horses frequent
opportunities to drink salt water during an endurance ride (i.e., at all rest stops). Before
this recommendation is put into practice during competition, training horses to drink salt
water should be pursued. Finally, further research needs to be performed to better
understand the physiological mechanisms of involuntary dehydration in exercising

horses and to design additional strategies to prevent or, at least, attenuate its occurrence.

123

 

 

 

References
Andersson, B. (1978) Regulation of water intake. Physio]. Rev. 58, 582-603

Barr, S. 1., Costill, D. L. and F ink, W. J. (1991) Fluid replacement during prolonged
exercise: effects of water, saline, or no ﬂuid. Med. Sci. Sports Exer. 23 (7), 811-817.

Boulze, D., Montastruc, P. and Cabanac, M. (1983) Water intake, pleasure and water
temperature in humans. Physio]. Behav. 30, 97-102.

Convertino, V. A., Armstrong, L. A., Coyle, E. R, Mack, G. W., Sawka, M. ., Senay, L. C.
and Sherman, W. M. (1996) Position Stand: exercise and ﬂuid replacement. Med. Sci.
Sports Exer. 28, i-vii.

Diisterdieck, K. F ., Schott, H. C., Eberhart, S. W., Woody, K. A. and Coenen, M. (1999)
Electrolyte and glycerol supplementation improve water intake by horses performing a
simulated 60 km endurance ride. Equine vet. J. Suppl. 30, 418-424.

Greenleaf, J. E. (1992) Problem: thirst, drinking behavior, and involuntary dehydration.
Med. Sci. Sports Exer. 24, 645-656.

Jones, N. L., Houpt, K. A. and Houpt, T. R. (1989) Stimuli of thirst in donkeys (Equus
asinus). Physiol. Behav. 46, 661 -665.

Fitzsimmons, J. T. (1998) Angiotensin, thirst, and sodium appetite. Physio. Rev. 78, 583-
686.

Marlin, D. J ., Scott, C. M., Mills, P. C., Louwes, H. and Vaarten, J. (1998a) Effects of
administration of water versus an isotonic oral rehydration (ORS) at rest and
changes during exercise and recovery. Vet. J. 155, 69-78.

Marlin, D. J., Scott, C. M., Mills, P. C., Louwes, H. and Vaarten, J. (1998b)
Rehydration following exercise: Effects of administration of water versus an
isotonic oral rehydration solution (ORS). Vet. J. 156, 41-49.

Monreal, L., Garzén, N., Espada, Y., Ruiz-Gopegui, R. and Homedes, J. (1999)
Electrolyte vs. glucose-electrolyte isotonic solutions for oral rehydration therapy
in horses. Equine vet. J. Suppl. 30, 425-429.

Nyman, S., Jansson, A., Dahlbom, K. and Lindhohn, A. (1996) Strategies for
voluntary dehydration in horses during endurance exercise. Equine Vet. J.,

Suppl. 22, 99-106.

Sandick, B. L., Engell, D. B. and Maller, O. (1984) Perception of drinking water
temperature and effects for humans after exercise. Physio]. Behav. 32, 851-855.

124

 

Schott, H. C. and Hinchcliff, K. W. (1998) Treatments affecting ﬂuid and electrolyte
status during exercise. Vet. Clin. North Amer. : Equine Pract. 14, 175-204.

Shirreffs, S. M., Taylor, A. J., Leiper, J. B. and Maughan, R. J. (1996) Post-exercise
rehydration in man: effects of volume consumed and drink soditun content. Med. Sci.
Sports Exer. 28 (10), 1260-1271.

Sosa Leén, L. A., Hodgson, D. R., Carlson, G. P. and Rose, R. J. (1998) Effects of
concentrated electrolytes administered via a paste on ﬂuid, electrolyte, and acid base
balance in horses. Am. J. vet. Res. 59, 898-903.

Suﬁt, E., Houpt, K. A. and Sweeting, M. (1985) Physiological stimuli of thirst and drinking
patterns in ponies. Equine vet. J. 17, 12-16.

Szlyk, P., Sils, I. V., Francesconi, R. P., Hubbard, R. W. and Armstrong, L. E. (1989)

Effects of water temperature and ﬂavoring on voluntary dehydration in men. Physio].
Behav. 45, 639-647.

125

 

Appendices

126

 

Appendix 1

Experimental protocol-Exercise speeds

Exercise Protocol

Start R1 R2 End
60’ Pre Bout 1 Bout 2 Bout 3 60’ Post

1

exercise bout

 

 

 

 

 

 

 

 

 

 

 

speed (m/s)
4 N vb ea or:

 

o 5 1015 20 25 30 35 4o 45 5052
time (minutes)

Figure 1.1 Experimental protocol — Exercise speeds: All horses performed a 45 km
simulated treadmill endurance riding which is similar to the ﬁrst 25 miles of a typical
endurance ride with varying speeds. Each 15 km exercise bout consisted of two 26 min
cycles of treadmill exercise at speed varying from 2 (walk), 4 (trot) to 8 (canter) m/s at
0° treadmill slope.

127

 

Appendix 2

Mean i SE changes in plasma (Posm) and serum osmolality (Sosm) and mixed venous
serum osmolality (Sosm), and heart rate (HR) in horses at start, and several time points
after a completion of 30 to 45-km simulated endurance ride on a treadmill. Values for trot
and canter are values for the last trot and canter for the last exercise bout.

* = values signiﬁcant from 120 Pre-exercise, a = Values of body temperature recorded per
rectum.

128

 

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129

Appendix 3

 

 

 

 

18-
o
16-

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{3312‘ 1?
a: v o o o ‘
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3... o o .

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0 I I 1 I l I t I

-16 -14 -12 -10 -8 -6 -4 -2 0
Change in Plasma osmolality (mOsm/kg) after 60 min of recovery

 

F_igur_e 3.1 Correlation between total water intake (liters) from O to 60 minutes of
recovery period and change in plasma osmolality (mOsm/kg) after 60 minutes recovery
period aﬁer induction of dehydration by 45-km of treadmill exercise. Horses were
provided 4 1 (filled circles), 8 l (circles), or an unlimited amount (UW, ﬁlled inverted
triangles) of water during the initial 5 min following exercise and subsequently provided

free access to water from 20-60 min following exercise (Chapter 2).

130

t—s
uh
1

put
N
n

r = 0.64, p = 0.04

u—t
e
1

m
n

&
1
Q
0

Water intake (liters)
from 20-60 min of recovery

N O\

4

O
l
O
O
4
4
4
4
O

 

 

-12 -10 -8 -6 -4 -2 0

Change in plasma osmolality (mOsm/kg) after 20 min of recover

Egg; 3.2 Correlation between total water intake (liters) from 20 to 60 minutes of
recovery period and change in plasma osmolality (mOsm/kg) after 20 minutes recovery
period after induction of dehydration by 45-km of treadmill exercise. Horses were
provided 4 1 (ﬁlled circles), 8 l (circles), or an unlimited amount (UW, ﬁlled inverted

triangles) of water during the initial 5 min following exercise (Chapter 2).

131

 

 

Appendix 4

 

 

14*
13- 0
Z‘
"83 o
§§ 12‘
:1...
Vt... . .
3:
c3..- 114
EE
:3 '
3:510-
35 .
“-1
9.,
r=-0.2,p=0.7
8 I r I I I
-12 -11 -10 -9 -8 -7

Change in plasma osmolality (mOsm/kg) after 60 min of recovery

Elgar; 4.1 Correlation between total water intake (liters) from O to 60 minutes of
recovery period and change in plasma osmolality (mOsm/kg) after 60 minutes recovery
period after induction of dehydration by furosemide (1 mg/kg IV) treatment followed by
45-km of treadmill exercise for horses provided water during the initial 5 min following
exercise and subsequently provided free access to water from 20-60 min following

exercise (Chapter 3).

132

 

ll

u—s
N
n

in}. v
A?

3° 8*

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.22 o
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I I U 1 I T r I I

-12 -10 -8 -6 -4 -2 0 2 4 6
Change in plasma osmolality (mOsm/kg) after 20 min of recovery

 

 

Eiggg 4.2 Correlation between total water intake (liters) from 20 to 60 minutes of
recovery period and change in plasma osmolality (mOsm/kg) after 20 minutes recovery
period after induction of dehydration by furosemide (1 mg/kg IV) treatment followed by
45 km of treadmill exercise for horses provided water (ﬁlled circles), 0.45% NaCl (open
circles), or 0.9% NaCl (ﬁlled inverted triangles) as the rehydration ﬂuid during the initial
5 min following exercise and subsequently provided free access to water from 20 to 60

min following exercise (Chapter 3).

133

Appendix 5

 

 

30-
25- O O
E‘ ' 0
Q)
’35 v
‘5‘8‘ v
15.515- '
‘58 0
an: V
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5°10-
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0 T U I I U U I I

-12 -10 -8 -6 -4 -2 0 2 4
Change in serum osmolality (mOsm/kg) after 60 min of recovery

F_igu_r§ 5.1 Correlation between total ﬂuid intake during the 60 minute recovery period
after induction of dehydration by administration of furosemide (1 mg/kg IV) followed by
30 km of treadmill exercise for horses provided 10 (ﬁlled circles), 20 (circles), or 30°C
(ﬁlled inverted triangles) 0.9% NaCl solution as the rehydration ﬂuid during the initial 5
min following exercise and subsequently provided free access to water (at 10, 20, or

30°C) from 20 to 60 min following exercise (Chapter 4).

134

Appendix 6

 

 

Rehydration ﬂuid Fluid intake 0-5 min Drink episodes Drink duration
of recovery (mean, L) (mean) (mean, second)
10°C 0.9% NaCl 9.8 10.3 11.6
20°C 0.9% NaCl 12.3 6 20.3
30°C 0.9% NaCl 9.7 11 11.6

 

 

 

 

 

 

Ta_ble 6.1 Mean drinking episodes and drinking duration for rehydration ﬂuids (0.9%
NaCl solution) at 10, 20 and 30° C during 0 to 5 minutes of postexercise recovery after
induction of dehydration by administration of furosemide (1 mg/kg IV) followed by 30
km of treadmill exercise for horses provided 10, 20, or 30°C 0.9% NaCl solution as the
rehydration ﬂuid during the initial 5 min following exercise and subsequently provided
free access to water (at 10, 20, or 30°C) from 20 to 60 min following exercise (Chapter

4). No signiﬁcant differences between treatments.

135

 

Appendix 7

 

 

 

101
8' 0 4L
“A
E: 6‘ 0 8L
3"“ o . O V UL
ct‘
>4; 4‘
a
° v
E8 2‘ o
5:...
94‘s 0 o o
.55 it 0
as -2-
:6
«N _4..
A“ V
U“ 0
-6 ' r=0.009, p=0.97
v
'8' T r I U I r I

O 2 4 6 8 10 12 14

Water intake from 20-60 min of recovery (L)

F_igpr_e 7.1 Correlation between total water intake (liters) from 20 to 60 minutes of
recovery period and change in plasma volume (%) at 20 min recovery period after
induction of dehydration by 45-km of treadmill exercise. Horses were provided 4 1 (ﬁlled
circles), 8 l (circles), or an unlimited amount (UL), ﬁlled inverted triangles) of water
during the initial 5 min following exercise and subsequently provided free access to water

from 20-60 min following exercise (Chapter 2).

136

 

 

 

4 . 0 Water
a; A ’ O 0.45% NaCl
5 § -6 a o O v 0.9% NaCl
g g '8 ' . 0
V
g 3'10 ' . . . '
3 E v
Eh-lz " O
s: a
o- =—l4 'l V 6
m .-
élo E-l6 «
g G
a N
5 *5-18 4
-20 . r=—0.44, p=0.08 v
'22 I I I I 1 I I
0 2 4 6 8 10 12

water intake from 20-60 min of recovery (L)

F_igu_r_e 7.2 Correlation between total water intake (liters) from 20 to 60 minutes of
recovery period and change in plasma volume (%) after 20 minutes recovery period aﬁer
induction of dehydration by furosemide (1 mg/kg IV) treatment followed by 45 km of
treadmill exercise for horses provided water (ﬁlled circles), 0.45% NaCl (open circles),
or 0.9% NaCl (ﬁlled inverted triangles) as the rehydration ﬂuid during the initial 5 min
following exercise and subsequently provided free access to water from 20 to 60 min

following exercise (Chapter 3).

137

 

0 10C NaCl

 

 

10q O 20CNaCl
V 30C NaCl
0 A 5 ‘ . O
E ¢\° O
3 V 0
g E 0 o v v '
a z .
m 8 '5 . 8 V
.5 ’- v
Qa—
5 2"“‘ v
m 0-
o E
on -15.
5 3
"= *5
U -20 ‘
r=-0.07, p=0.77 . O
-25 1 ﬁr I I U I
0 2 4 6 8 10 12

Water intake from 20-60 min of recovery (L)

F_ig1_ig§ 7.3 Correlation between total ﬂuid intake from 20 to 60 min recovery period aﬁer
induction of dehydration by administration of furosemide (1 mg/kg IV) followed by 30
km of treadmill exercise for horses provided 10 (ﬁlled circles), 20 (circles), or 30°C
(ﬁlled inverted triangles) 0.9% NaCl solution as the rehydration ﬂuid during the initial 5
min following exercise and subsequently provided free access to water (at 10, 20, or

30°C) from 20 to 60 min following exercise (Chapter 4).

138