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Departments of 1 Clinical Studies and 2 Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada N1G 2W1
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Kingston, Janene K., Raymond J. Geor, and Laura Jill
McCutcheon. Rate and composition of sweat fluid losses are
unaltered by hypohydration during prolonged exercise in horses.
J. Appl. Physiol. 83(4):
1133-1143, 1997.
Rate and ionic composition of sweat fluid losses and partitioning of evaporative heat loss into respiratory and cutaneous components were determined in six horses during three 15-km phases of exercise at ~40% of maximal
O2 uptake. Pattern of change in
sweat rate (SR) and composition was similar during each phase. SR
increased rapidly for the first 20 min of exercise but remained at
~24-28
ml · m
2 · min1
during the remainder of each phase. Similarly, the concentrations of Na
and Cl in sweat increased until 30 min of exercise but were unchanged
thereafter. Sweat osmolality and concentrations of Na and Cl were
positively correlated with SR. Sweat K concentration decreased during
exercise but was not correlated with SR. Fluid losses were 33.8 ± 1.5 liters, resulting in decreases of ~21% in plasma volume and
~11% in total body water. The ~6% hypohydration was not
associated with an alteration in SR, sweat composition, or heat
storage. Respiratory and cutaneous evaporative heat loss represented
~23 and 70%, respectively, of the total heat dissipated, and the
partitioning of heat loss was similar in each exercise phase. We
conclude that SR and the relative proportions of respiratory and
cutaneous evaporative heat loss are unchanged in horses during prolonged low-intensity exercise despite moderate hypohydration.
temperature regulation; sweating rate; ion losses; evaporative heat
loss; equine
INTRODUCTION IN HORSES, as in human subjects, sweating is the
principal means of thermoregulation during exercise. However, during
exercise in moderate environmental conditions, sweating rates
(SR) in the horse (expressed per unit area of skin) have
been reported to be more than threefold greater than values in
heat-acclimatized human subjects exercising at similar work intensities
(11, 28). Whereas cutaneous evaporation represents the primary
mechanism for heat dissipation in the horse, respiratory heat loss
(RHL) can also contribute substantially. In human subjects, most
exercise studies of heat production and dissipation that have
partitioned these two major components of evaporative heat loss have
used a figure of 10% to account for respiratory losses. Estimates of RHL in horses during low-intensity exercise have ranged from 10 to 30%
of the metabolic heat produced (10, 11). This range probably reflects
different methods used to estimate each component of heat loss and
demonstrates the need for an accurate measurement of total body fluid
loss when cutaneous and respiratory evaporative losses are partitioned.
Recently, field investigations that included calculation of body water
losses in horses competing in 48- to 163-km endurance events indicated
that the majority of the losses occurred during the first half of the
event (7, 15). These findings suggest that an alteration in the rate of
sweat fluid loss occurs during prolonged exercise. One might speculate
that mechanisms for body fluid conservation would result in a decrease
in the rate of sweat fluid loss. For instance, in hypohydrated human
subjects, there is an increase in the threshold body temperature for
onset of sweating and a decrease in sensitivity of the sweating
response in direct proportion to the degree of hypohydration (26, 32), suggesting the existence of mechanisms linked to sweating responses that are directed at conservation of body fluid. To date,
reports of results from field investigations have not included direct measurements of the rate of sweat fluid and ion losses in the horse.
Furthermore, the effect of progressive hypohydration on sweating rate
(SR) and on other thermoregulatory responses of horses during prolonged
exercise has not been reported.
The objectives of this study were 1)
to determine the rate and ionic composition of sweat fluid losses in
horses during >3 h of low-intensity exercise in moderate
environmental conditions and 2) to
partition evaporative heat loss into its respiratory and cutaneous
components. We hypothesized that the progressive hypohydration
associated with prolonged exercise would evoke a decrease in the rate
of sweat fluid loss, thereby conserving fluid and ions. For the
purposes of this study, we measured
1) local SR on the lateral thorax;
2) sweat composition;
3) rectal, muscle, and pulmonary
artery blood temperatures (Tre,
Tmu, and
Tpa, respectively) for
determination of heat storage; 4)
body mass before and after exercise as a basis for measuring total
fluid loss; and 5) hematocrit and
plasma total protein and osmolality. From measurements of cutaneous
evaporative heat loss and changes in body mass (loss of body water), we
estimated the partitioning of evaporative heat loss into its
respiratory and cutaneous components.
MATERIALS AND METHODS The care and use of animals followed the Guide to the
Care and Use of Experimental Animals (Canadian Council
on Animal Care, Ottawa, ON, Canada). All animal experiments were
conducted after approval by the Animal Care Committee of the University
of Guelph and were performed in compliance with their recommendations.
O2 max) of each
horse was determined by use of an open circuit calorimeter (8) on two
occasions after training and before the experiments began. The mean
O2 max was 146 ± 6.5 ml · kg1 · min1.
O2 max
(3.6-3.8 m/s) for 45 km, with a 15-min rest after each 15-km
phase. As a result of the similarity in treadmill speed required for
all subjects, each phase of exercise consisted of ~67 min. Exercise
was discontinued if the horses demonstrated signs of fatigue (inability
to keep pace with the treadmill despite verbal encouragement) or
inadequate recovery (HR >70 beats/minute after 10 min) during the
rest phases.
Each experiment consisted of two parts; the same experimental subjects
completed identical bouts of exercise with at least 7 days between
exercise trials. Duplicate runs were necessary because of the extensive
instrumentation required to collect the data from horses during each
experiment. During the first part of the experiment
(experiment 1), data for measurement
of O2 consumption (O2), change in body mass,
change in total body water, and expired air temperature
(Te) and humidity for estimation
of RHL were collected. During the second part of the experiment
(experiment 2), measurements were
made of temperature at selected sites, SR, sweat osmolality, and sweat
concentrations of sodium, chloride, and potassium
([Na+], [Cl], and
[K+], respectively).
O2 was measured at 10-min
intervals throughout exercise by using an open-flow respiratory-gas
collection system (8), and body weight was measured at the end of each
phase of exercise. Te was measured
during exercise with a copper/constantan thermocouple made from
0.076-mm-diameter insulated wire (TW40; Physitemp, Clifton, NJ) that
was positioned in the false nostril of the horse (40).
Preliminary experiments indicated that the extent of fluid losses
incurred during the exercise protocol would result in a degree of
hypohydration (deficit in total body water) that could preclude the
horse's ability to complete the test. To simulate the opportunity for
rehydration available for horses competing in trail and endurance rides
and to limit the degree of hypohydration to ~6%, the horses were
offered water and feed during each rest phase. The quantity of feed and
water consumed was recorded.
The experiments were conducted in an air-conditioned laboratory, with
the temperature and relative humidity maintained at 24 ± 2°C
and 55 ± 8%, respectively. A fan mounted above and 0.5 m in front
of the treadmill was used to maintain an air velocity of 3.5-4 m/s
over the anterior and dorsal aspects of the horse. Air velocity was
measured with anemometers (Davis Instruments, Hayward, CA) positioned
at three sites: lateral midcervical region, lateral and dorsal thorax,
and dorsal to the gluteal region of the hindquarters.
Sweat was collected from an area of skin on the lateral thorax by a
method previously described for use in the horse (21). This area was
chosen after determination of SR at several sites (midcervical, lateral
thorax, gluteal region of hindquarters). Although there are regional
variations in SR in the horse, previous studies have demonstrated that
the SR measured on the lateral thorax is not significantly different
from the mean whole body SR estimated from changes in total body water
after correction for respiratory water losses (14, 19). A
500-cm2 area on the lateral thorax
was clipped and shaved, washed, and then rinsed with distilled water. A
sealed polyethylene pouch enclosing a
150-cm2 area of skin was attached
to the skin on all edges with an adhesive. The edges of the pouch were
further sealed by dermal tape that covered the pouch/skin margin. A
ventral reservoir, formed by a deep fold in the polyethylene, separated
accumulating sweat from the skin surface and facilitated the removal of
all collected sweat through polyethylene tubing (1.67-mm ID,
Intramedic; Becton Dickinson, Parsippany, NJ) incorporated into the
lateral margin of the pouch. Sweat samples were collected every 10 min
throughout each phase of exercise and at 5 and 15 min in each rest
phase. SR, expressed as milliliters per square meter per minute, was calculated on the basis of the volume of sweat collected at the end of
each time interval from the measured skin area within the pouch.
Therefore, the measured SR represents the average rate of sweat
production over a 10-min period. Extrapolation of the local SR, at each
time point during exercise and the rest phases, to the horse's total
body surface area (SA) was used to
calculate a mean whole body SR. SA was
calculated by using the formula (11)
|
], and [K+] in
sweat samples were determined with an ion-selective analyzer (Statprofile 9 Plus, Nova Biomedical Canada, Mississauga, ON). Plasma
and sweat osmolality were determined by freezing-point depression
(model 3MO Plus, Advanced Instruments, Needham, MA). All analyses were
performed in duplicate.
Estimates of metabolic heat production and total evaporative heat
loss.
Metabolic heat load (MHL) was estimated from the rate of metabolic
O2 measured at 10-min
intervals during exercise. It was assumed that 80% of the calorific
value of O2 consumed during work
was released as heat and that each liter of
O2 consumed had an energy
equivalent of 21 kJ (11, 16). Therefore, the MHL was calculated by MHL = O2 (l/min) × 0.8 × exercise duration (min) × 21 kJ/l
O2 (i.e., this equation assumes
that 20% of the metabolic free energy is transformed to positive
work). The total O2 for the
duration of the experiment was calculated by integration of the
individual O2 over time.
Estimates of heat dissipation and storage (S) were made at the end of
each phase of exercise by using the following equation
|
1 · °C1)
was used.
RHL was calculated by using the formula of Hanson (9)
|
E is minute
ventilation in liters per minute at standard temperature and pressure,
cp is the volumetric specific heat capacity of air (J/l), Te 
Ti is the difference
between expired and inspired air temperature (°C), La is the latent
heat of vaporization of water (J/g), and
We Wi is the difference between
expired and inspired water vapor (g/min).
E
was not measured in this study. However, in a previous study of horses of equivalent mass exercising at a workload equivalent to 40% of
O2 max,
E was
linearly related to Tpa (2).
Therefore, we estimated
E by using
the following formula (2)
|
RESULTS
O2 increased
from 4.9 ± 1.0 ml · kg1 · min1
at rest to 52 ± 4 ml · kg1 · min1
(range 46-57
ml · kg1 · min1)
during exercise.
Changes in body mass and hydration in response to exercise.
After consumption of feed and water were accounted for, and including
fecal and urinary water losses, body mass was decreased by 33.8 ± 1.5 kg after exercise (7.3% of preexercise body mass; range
5.1-10.1%). When expressed as a percentage of preexercise total
body water (0.66 × body mass), this decrease represented an
11.1% (range 7.8-15.3%) reduction in total body water. The decrease in body mass in phases II
(12.1 ± 0.5 kg) and III (11.8 ± 0.4 kg) was significantly (P < 0.01) higher than during phase I (9.8 ± 0.3 kg).
The mean quantity of water consumed during the rest period after each
phase of exercise was 6.4 ± 1.1, 3.4 ± 0.7, and 8.1 ± 0.8 liters for phases I, II, and
III, respectively (total water consumption was 17.9 ± 1.1 liters). Therefore, total mean volume consumed before the end of exercise was 9.8 liters, with an additional 8.1 liters consumed after the exercise test and before final
measurement of change in body mass. Assuming complete intestinal
absorption of the water consumed, the net deficits in body mass and
total body water at the beginning of phase
III were 2.6% (range 1.7-4.1%) and 3.8% (range
2.5-6.5%), respectively. At the end of the exercise protocol,
there was a net 5.9% (range 4.1-8.6%) deficit in body mass and
an 8.5% (range 6.0-12.6%) deficit in total body water.
Values for plasma total solids, hematocrit, and plasma osmolality
during each phase of exercise and in recovery are shown in Table
1. During
phase I, there was a rapid and
significant (P < 0.001) increase in
hematocrit from 39 to 46% within the first 2 min of exercise,
consistent with splenic contraction. For the remainder of
phase I and the subsequent phases of
exercise, the extent of changes in hematocrit were smaller and
paralleled alterations in plasma total solids (hemoconcentration).
There were significant (P < 0.001)
and progressive increases in plasma total solids during the three
phases of exercise. At the end of phases I,
II, and III, values
for plasma total solids were 7.0, 14.1, and 21.1% higher,
respectively, when compared with preexercise values. Plasma volume, as
determined by the change in plasma total solids, was decreased by
~21% at the end of the third phase of exercise. Despite the
hemoconcentration, there was no significant
(P > 0.05) change in plasma
osmolality throughout the exercise protocol.
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2 · min1
during the remainder of each phase. Compared with the corresponding time points in the first exercise phase, SR was significantly (P < 0.001) higher at 10 and 20 min
of exercise in phases II and III (Fig.
1A). After each phase of exercise
was completed, SR declined rapidly. By 10 min of rest, the hair coat
was dry, and no sweat accumulated in the sealed pouch between
minutes 5 and 15 of the rest period.
(B) and K+
(C) in 6 horses during 3 phases of
exercise at 40% of maximal O2
uptake (O2 max).
Values are means ± SE.
Mean values for whole body SR in experiment 1, calculated from the total body water losses (from change in body mass in each phase) after correction for estimated respiratory water losses, were 23.0 ± 2.2, 26.3 ± 3.1, and 26.1 ± 2.0 ml · m
2 · min1
for phases I, II, and
III, respectively. These values
were not significantly
(P > 0.05) different from whole body
SR measured by use of direct sweat collection in each exercise phase in
experiment 2 (24.8 ± 2.5, 27.8 ± 3.0, and 27.4 ± 2.4 ml · m2 · min1
for phases I, II, and
III, respectively).
Sweat composition.
Values for sweat osmolality and [Na+],
[Cl], and [K+] are
shown in Fig. 1, A-C. During each
phase of exercise, the [Na+] and
[Cl] in sweat were lowest in samples
collected after the first 10 min of exercise, were significantly
(P < 0.001) increased in the samples
collected at 20 and 30 min of exercise, and then did not change
significantly (P > 0.05) in samples
collected at 40, 50, and 60 min and at the end of exercise (Fig.
1B). The highest value for sweat
[Na+] (131.2 ± 4.2 mM) was attained in the
last 10 min of phase III. There was a
similar pattern of change in sweat osmolality. Values were lowest in
samples collected at 10 min of exercise and highest in samples
collected at 20 or 30 min of exercise in each phase (Fig.
1A). Although mean values for
sweat [Na+] were higher in
phases II and
III compared with the corresponding time points in phase I, these
differences were not statistically significant
(P > 0.05; Fig.
1B). In contrast to initial
increases in the [Na+] and
[Cl] in sweat, [K+]
(Fig. 1C) was highest in the first
20 min of exercise in phase I, then
decreased gradually throughout the remaining phases of exercise, so
that the values for [K+] were significantly
(P < 0.001) lower by the end of
phase III compared with values
measured in phase I.
Effect of SR on sweat composition.
Linear regression analysis demonstrated a positive correlation between
SR and sweat osmolality
(r2 = 0.70, P < 0.0001), SR and sweat
[Na+]
(r2 = 0.62, P < 0.0001), and SR and sweat
[Cl]
(r2 = 0.63, P < 0.0001). The decline in sweat
[K+] was independent of alterations in SR
(r2 = 0.11, P = 0.2).
Temperature responses to exercise.
Values for these measurements during exercise and recovery are shown in
Fig. 2, A
and B. There was no significant
(P > 0.05) difference between
temperature measured at the left and right skin sites and, therefore,
these data were pooled. Throughout each phase of exercise,
Tpa,
Tre, and
Tsk progressively increased, and
mean values were significantly (P < 0.0001) greater than preexercise values after 10 min of exercise in
each phase. In particular, the rate of change in
Tpa and
Tsk was greatest during the first 10 min in each phase of exercise, whereas
Tre increased more gradually throughout the entire period of exercise. Compared with the mean value
for Tmu measured before exercise
in phase I (37.3 ± 0.1°C), Tmu was significantly
(P < 0.0001) increased at the end of
exercise in each phase, with no significant
(P > 0.05) difference between end-exercise values for all three phases of exercise
(phase I, 39.7 ± 0.1°C;
phase II, 39.9 ± 0.1°C;
phase III, 40.0 ± 0.1°C). In
contrast, Tmu before the onset of
exercise in phases II and III (39.0 ± 0.1 and 39.2 ± 0.1°C, respectively) was significantly (P < 0.0001) increased compared with
preexercise Tmu in
phase I (37.3 ± 0.1°C) and
increased <1°C during each of the last two phases
of exercise. After each phase of exercise, the
Tpa and Tsk returned to preexercise values
within the first 10 min of each rest phase. In contrast,
Tre remained significantly
(P < 0.001) higher
(~0.5°C) at the end of each rest phase compared with preexercise (phase I) values,
although preexercise Tre for phases II and
III was not significantly different
(P > 0.05).
O2 max. Values are
means ± SE.
Estimates of metabolic heat production and total evaporative heat loss. The estimated total heat production from the start of exercise in phase I to the completion of phase III was 80,998 ± 1,105 kJ, representing heat production of ~27,000 kJ during each phase of exercise. Estimated values for heat production, dissipation, and storage for each phase of exercise are presented in Fig. 3, A and B. Of the total heat produced, heat storage at the end of exercise in phase III was estimated as 2,386 ± 355 kJ and as 1,316 ± 165 kJ by 15 min of recovery. Total RHL was 18,023 ± 876 kJ (6,214 + 5,827 + 5,982 kJ) or ~23% of dissipated heat. Of the remaining heat loss, 94% was assumed to be evaporative heat loss from the skin (55,209 ± 1,850 kJ), and 6% was assumed to include heat loss by radiation and convection (4,780 ± 150 kJ).
DISCUSSION
The major question addressed in this study was whether progressive hypohydration and the associated loss of plasma volume would alter the rate of sweat production or the ionic composition of sweat fluid produced during prolonged, low-intensity exercise in horses. To answer this question, we have measured the rate and composition of sweat fluid losses during >3 h of moderate-intensity exercise in horses. Furthermore, we have measured the changes in body temperature and body mass and, by utilizing the values for cutaneous evaporative heat loss, estimated the partitioning of evaporative heat loss into its respiratory and cutaneous components.
In the present study, we have shown that after an initial rise in the
rate of sweat production at the beginning of each phase of exercise,
the rate of sweat production did not change significantly throughout
the remainder of each phase of exercise. Furthermore, the rate of sweat
production at the end of the third phase of exercise was not different
compared with the SR measured during the first phase of exercise.
Similarly, sweat [Na+] and
[Cl] did not change significantly after the
first 20 min of exercise in each phase. This constant rate of sweat
production and sweat [Na+] and
[Cl] existed despite a slight but constant
rise in Tpa and
Tre throughout the latter portion
of each exercise phase.
Tpa, Tre, and Tmu at the end of phases II and III were not different compared with values measured at the end of phase I. The similarity in the values for temperatures measured at the end of each phase of exercise, assuming a constant rate of heat production, would indicate that there was no change in the rate of heat loss. Because calculated cutaneous evaporative heat losses were unchanged in each phase of exercise, these findings demonstrate that there was also no significant change in the respiratory component of evaporative heat loss under the conditions of this experimental protocol.
We hypothesized that SR would decrease as a result of progressive hypohydration during prolonged exercise. Furthermore, a decrement in SR would be associated with an increased rate of rise in body temperature (25). Because the rate of sweat production did not change, the contribution of cutaneous evaporative heat loss was assumed to have been unaltered. Moreover, there was no significant change in the osmolality or composition of sweat fluid despite progressive hypohydration (8.5% net decrease in total body water); neither was the hypohydration associated with an increase in plasma osmolality, although plasma volume was estimated to have decreased by ~21% at the end of the third phase of exercise.
The experimental protocol used in the present study was designed to provide low-intensity exercise of sufficient duration to allow comparison of SR and sweat ion composition over time. However, the exercise duration was also long enough and fluid losses were of sufficient magnitude that some of the horses would not have completed the test without some fluid replacement. By providing access to fluid at specified rest stops, the protocol simulated conditions in competitive events. However, allowing voluntary fluid consumption introduced some variability with respect to replacement of fluid losses (15 to 35% of total fluid losses). Furthermore, this fluid intake reduced the degree of hypohydration incurred by the subjects.
Hypohydration of 3% or greater has been associated with significant increases in plasma osmolality in human subjects undergoing moderate exercise (26, 32). In addition, studies of human subjects have demonstrated that hypohydration increases the threshold temperature for sweating and decreases sweating sensitivity in a graded manner, with changes in the sweating response evident at a hypohydration level of 3% (32). This contrasts with the findings of the present study in which the rate of sweat production was unaltered at hypohydration equivalent to a 5.9% decrease in body mass or 8.5% loss of total body water by the end of exercise. This level of hypohydration reflects the net loss of body water after accounting for water consumed (9.8 liters) before the third phase of exercise. Given the uncertainty regarding the rate of gastric emptying and intestinal uptake of the consumed water, our estimate of the actual extent of hypohydration may be conservative.
Sawka and colleagues (32) demonstrated an inverse relationship between
the degree of plasma hyperosmolality and SR
(r = 0.76;
P < 0.05), whereas the relationship
between the reduction in plasma volume and SR was less clear. The
results of the present study also fail to demonstrate any relationship
between plasma volume and SR. Additionally, the rate of sweat fluid
loss imposed by the intensity of the exercise and the environmental
conditions used in this experiment did not result in an alteration in
plasma osmolality. A similar rate of sweat fluid loss has been
demonstrated by others during shorter periods of low-intensity
exercise. Hodgson et al. (11) measured SR of 24 ± 4 ml · m2 · min1
in horses that exercised for ~38 min at 40%
O2 max. Naylor and
co-workers (28) also measured SR similar to those reported in this
study in euhydrated horses and in horses dehydrated before exercise by
administration of furosemide or by water deprivation. Despite a
significant degree of hypohydration before exercise (3.2-3.9%
decrease in total body water), these researchers were unable to detect
a decrease in SR during 40 min of exercise (4.6-5.5% decrease in
total body water by the end of exercise) at the same work intensity
undertaken by the horses in the present study. However, Naylor et al.
reported that hypohydration decreased the dissipation of heat, as
reflected in a reduction in internal transfer of heat from core to
periphery. Also, this impairment of thermoregulation, as demonstrated
by the increases in temperature of the carotid artery,
Tpa,
Tre, gluteal
Tmu, and
Tsk, was more pronounced after hypertonic vs. isotonic dehydration.
The lack of change in SR in response to the degree of hypohydration demonstrated in the horses in the present study probably represents at least two differences between human and equine subjects. First, the ionic composition of equine sweat during exercise is normally slightly hypertonic, with an osmolality ranging from ~290 to 320 mosmol/kgH2O, as reported in the present and previous studies (18, 21). Human sweat is hypotonic relative to extracellular fluid, and the protocol used in this study would have elicited a hyperosmotic hypovolemia in human subjects. In contrast, despite the extensive sweat fluid losses during the prolonged exercise, the fluid loss incurred by the horses in this study would more closely approximate an isotonic loss and would therefore result in an isosmotic hypohydration. Other studies have demonstrated that the horse is capable of maintaining serum osmolality close to, or even below, resting values during prolonged exercise (31). Because hyperosmolality has been demonstrated to impair thermoregulation (26, 32), an isotonic fluid loss may assist the equine athlete in maintaining SR and adequate heat dissipation in the face of comparatively larger fluid losses.
Second, the interstitial and intestinal fluid volume of the horse represents a significantly larger reserve of fluid and ions potentially available for reabsorption during low-intensity exercise. Webb and Weaver (38) reported that the content of the intestinal tract is ~6% of body mass, or 27 kg for a 450-kg horse. Because >75% of the intestinal content is water (36), the gastrointestinal tract represents a substantial fluid reservoir. Meyer and Coenen (24) demonstrated significant decreases in the water, Na, and Cl content of the gastrointestinal tract of ponies after 1 h of low-intensity exercise. These researchers suggested that, during low-intensity exercise, the fluid reservoir present in the intestinal tract may assist in maintaining plasma volume and the rate of sweat fluid losses.
A decline in SR over time has been reported in horses when sweating is
induced by adrenaline infusion (13, 18). Kerr and Snow (13) determined
that SR decreased after 1-3 h despite increasing concentrations of
adrenaline in an infusion, whereas McConaghy et al. (18) found that
some subjects stopped sweating after <30 min of an adrenaline
infusion. In the horse, the production of sweat is under sympathetic
nervous control (12) and involves 
2-adrenoreceptors (33).
Although sweating can be induced solely by adrenaline (13, 18),
exercise-induced sweating will involve the contribution of circulating
adrenaline and sympathetic nervous stimulation (30). The SR and fluid
losses incurred by adrenaline infusion are substantially less than
those produced in response to prolonged exercise in this and other
studies of endurance exercise (5, 15, 31, 34). The responsiveness of
the sweat gland may be altered by prolonged continuous exposure to high
blood adrenaline concentrations, resulting in a decline in sweat
production. Whereas the sweating response during exercise at >60%
O2 max may reflect a
substantial contribution of elevated adrenaline concentration in the
blood, the role of adrenaline is probably relatively minor in the
formation of sweat produced during prolonged low-intensity exercise.
Differences in [Na+] in equine sweat and plasma
[~100-125 mM in sweat (Fig.
1A) vs. ~139-145 mM in
plasma] suggest there is modification of the ionic composition of
extracellular fluid before sweat secretion. Wilson et al. (39)
demonstrated reabsorption of ions by the cells in the equine sweat duct
during thermal stimulation, suggesting that some modification of the
ionic composition of sweat occurs during its passage through the sweat
gland duct. Higher [Na+] and
[Cl] are present in sweat produced in
response to moderate to high-intensity exercise compared with
adrenaline infusion. These higher ion concentrations probably reflect
the higher SR induced by exercise (13, 18-21, 31). The
change in composition of sweat relative to the increase in SR during
the initial portion of each exercise phase in the present study adds
further support to the probability of some modification of equine
sweat.
In contrast to [Na+] and
[Cl], the [K+] in
sweat declined by ~12-14% by the end of each phase of exercise
when compared with sweat samples collected after the first 10 min of
exercise (Fig. 1C). Equine and human
sweat glands have been shown to differ from sweat glands in several
other domestic species in that there is evidence for K efflux from the
gland during secretory activity, and this loss of K ions assists in
maintaining secretory drive (12). However, the factors that contribute
to sweat [K+] that decline with time during
exercise are not clear. The lack of a linear relationship between SR
and sweat [K+] has been demonstrated in studies
of human subjects (6, 37). Adrenergic stimulation of the sweat
gland can contribute to secretory drive. However, equine
sweat produced in response to epinephrine infusion has a lower
[K+] (18) and, as stated earlier, it is
unlikely that there was substantial elevation of blood adrenalin
concentrations during exercise in the present study. It has been
suggested that changes in [K+] of interstitial
fluid associated with K efflux from working skeletal muscle during
exercise may be reflected in sweat [K+] (35).
Although reuptake of K occurs very rapidly, it is possible that initial
increases in plasma K could be reflected in the
[K+] in sweat secretions.
To the authors' knowledge, the present study is the first report of
simultaneous measurement in the horse of SR and sweat composition
during prolonged exercise. The protocol utilized in this study provided
the advantage of greater sampling frequency and controlled exercise
conditions compared with previous field studies. Carlson and Ocen (5),
Rose et al. (31), and Snow et al. (34) measured ion composition in the
sweat of horses during or after prolonged exercise. However, field
conditions limited the number of samples that they could obtain and
also their choice of collection techniques. Carlson and Ocen (5) and
Rose et al. (31) collected samples after completion of the exercise,
whereas Snow and colleagues (34) obtained samples after 16, 64, and 80 km of exercise. The researchers were therefore only able to obtain
samples at intervals of 1 h or more. In each study, absorbent pads were
used to collect sweat, and, although the skin beneath the pad was
protected from direct contact with air, some evaporation of sweat fluid
occurred, altering the sweat ion concentrations measured. More
recently, McConaghy et al. (18) used a revision of this collection
technique during low-intensity treadmill exercise and obtained
sufficient sweat for ion analysis with samples collected at 15-min
intervals. However, sweat ion concentrations measured by McConaghy and
colleagues (18) were still slightly higher than values measured by
McCutcheon et al. (21) utilizing a sealed pouch, and the former group
of researchers attributed these differences to a small degree of
evaporative loss. McConaghy et al. (18) also reported variation in
sweat [Na+] and [Cl]
of as much as 200 mM during 30 min of low-intensity exercise. In the
present study, this degree of variation in sweat
[Na+] and [Cl] was
not evident between individuals or over time. By comparison, there was
a greater degree of individual variation in sweat
[K+], and this variation may have precluded the
detection of a correlation between sweat [K+]
and SR.
The results of the present study also demonstrated no change in sweat
ion concentration with progressive hypohydration during exercise.
Although numerous studies of human subjects describe changes in SR and
sweat sensitivity associated with exercise-induced hypohydration
and/or hyperthermia, there is relatively little information
with regard to human sweat ion composition under these circumstances.
Most reports of variation in sweat ion concentration relate to repeated
exercise during training or heat acclimation (1, 29). During heat
acclimation in human subjects, there is a progressive decrease in sweat
[Na+] during the first 8-10 days of heat
exposure. These changes are noted in sweat samples collected from
exercise performed on successive days, rather than within a single bout
of exercise. Changes in sweat ion composition have also been reported
in response to incremental exercise of short duration (35). However,
there are few instances of multiple sampling over a prolonged period of
low- to moderate-intensity exercise. In the present study, because SR
was unaltered, it is perhaps not surprising that the
[Na+] and [Cl] in
sweat remained unchanged; there appears to be little information that
would suggest that the concentrations of these ions would change in a
manner other than that associated with an alteration in SR.
There were parallel increases in
Tpa,
Tre, and Tsk in
response to exercise (Fig. 2, A and
B). However, the rate and extent of
the increase in temperature were ~50% lower compared with previous reports of horses exercising at a similar work intensity (11, 28). The
difference in the rate of rise in body temperature could be accounted
for by lower heat production during exercise or by greater efficiency
of heat dissipation. Lower heat production is unlikely, because the
metabolic rate (O2) of our
horses during exercise was similar to values measured in horses in the
studies reported by Hodgson et al. (11) and Naylor et al. (28). Values for HR during exercise were comparable to values reported for the
horses in the aforementioned previous studies, providing additional verification of the similarity in the workload (data not shown). A more
likely explanation for the difference in rate and extent of the rise in
body temperature is greater efficiency of heat dissipation during
exercise. In particular, the close matching of treadmill and fan speeds
would promote both evaporative and convective heat loss. In the studies
by Hodgson et al. and Naylor et al., fan speed was ~50-55% of
the horse's running speed, and most of the exposed skin was covered by
unevaporated sweat during exercise. Thus the environmental conditions
limited the rate of heat dissipation. In the present study, the
moderate room temperature and the close approximation of treadmill
speed and wind speed created by the fan resulted in most of the sweat
evaporating from the body surface during and after exercise. The
greater efficacy of cutaneous heat loss is further supported by the
lower Tsk and blood temperatures
maintained during each phase of exercise in this study compared with
the findings of Naylor et al. and Hodgson et al. Also, in the present
study, the greater differential between Tre and
Tpa maintained throughout exercise
may have reflected a sustained and more substantial effect of the
transfer of cooled blood from the skin to the central circulation.
The estimated evaporative heat loss by cutaneous and respiratory routes was unchanged in the three phases of exercise. When estimates of cutaneous evaporative heat loss are based solely on total sweat fluid losses, there is a tendency to overestimate heat loss. This discrepancy largely represents the difficulty in ascertaining the extent of sweat drippage. Total evaporation of the estimated volume of sweat fluid losses would have accounted for heat losses of ~19,400, 21,750, and 21,300 kJ during phases I, II, and III, respectively, and would represent 80% of the total heat dissipated during the experimental period. After calculation of RHL, and assuming that radiation and convection accounted for 6% of the total heat dissipated, ~70% of the MHL was dissipated by evaporation of sweat. This difference in estimates reflects sweat drippage during the experimental protocol.
Of the small number of studies that provide estimates of RHL in
exercising horses, few reports include accurate measurement of total
fluid losses or SR. In the present study, RHL averaged ~90-100
kJ/min throughout the three phases of exercise. Heilemann et al. (10)
reported values of ~50 kJ/min in horses during trotting exercise in
moderate environmental conditions, whereas Hodgson et al. (11) reported
values of ~80 and ~180 kJ/min at the onset and after 35 min of
exercise eliciting 40%
O2 max, respectively. Lund and colleagues (16) have recently calculated RHL as a much larger
component of total heat loss (~200-250 kJ/min) when exercise is
brief and maximal and the SR is low. These disparities in estimates of
the quantity of heat dissipated via the respiratory tract reflect differences in work intensity,
E,
environmental conditions, and the methods used for calculation of RHL.
Heat loss via the respiratory tract was calculated on the basis of the
difference in the temperature and water vapor content of inspired and
expired air and an estimate of
E. Total RHL was calculated from the sum of the convective and evaporative components by using an equation similar to that used by Hanson (9).
Given the low specific-heat capacity of air, the majority of the heat
was probably lost by evaporation of water from the mucous membranes of
the respiratory tract, because the inspired air was saturated with
water vapor (16, 27). In this study, we had no direct measure of
E. However,
Bayly et al. (2) demonstrated a linear relationship between
Tpa and tidal volume and
E for horses
exercising at 40%
O2 max. On the basis of this relationship, we estimated
E to be in the
range of 800-1,100 l/min during exercise. These values are
comparable to those obtained from direct measurements of
E in horses
exercising at similar workloads (2, 3). Thus, although we did not
directly measure RHL, the results presented here provide some estimate
of the relative contribution of respiratory and cutaneous heat loss
during low-intensity exercise in moderate conditions. However, it is
clear that these proportions could be altered considerably when ambient
conditions and/or workloads are changed.
There are few published estimates of heat production, dissipation, and storage in the horse during exercise or of the relative proportions of heat dissipated via the respiratory and cutaneous routes. As previously suggested, such estimates will vary considerably depending on methodology, environmental conditions, and exercise intensity. The advantage of the more prolonged exercise undertaken in this study was that there were periods of ~40 min in each exercise phase during which SR did not change significantly. In other studies in which heat production and dissipation have been estimated (11, 16, 17), a larger proportion of the overall exercise bout represents a period of adjustment to the required workload. As a result, calculations of the quantity of heat dissipated and of the estimated respiratory evaporative component of heat loss vary from 79 to 93% and from 22 to 49%, respectively, of the total MHL. In the present study, ~95% of the MHL was dissipated by the end of exercise, with RHL estimated to represent ~23% of total heat losses.
In summary, during >3 h of low-intensity exercise
1) hypohydration of ~6%
was incurred by horses, despite voluntary intake of water during two
rest phases during the exercise protocol; 2) SR and the ionic composition of
sweat did not change during three phases of exercise;
3) sweat
[Na+] and [Cl] were
positively correlated with SR; 4)
plasma osmolality was unchanged despite an ~21% decrease in plasma
volume and an ~8.5% reduction in total body water; and
5) ~95% of the MHL was dissipated during exercise, of which ~70% was lost by evaporation of sweat and
~23% was dissipated via the respiratory tract. We conclude that SR
and the relative proportions of respiratory and cutaneous evaporative
heat loss can be maintained unchanged during prolonged low-intensity
exercise in horses in thermoneutral conditions despite moderate
hypohydration.
ACKNOWLEDGEMENTS
We gratefully acknowledge the technical assistance of M. J. Hare, K. Gowdy, J. Byrne, T. Leslie, L. Curle, and Hua Shen.
FOOTNOTES
Address for reprint requests: R. J. Geor, Dept. of Veterinary Clinical Sciences, College of Veterinary Medicine, Ohio State Univ., 601 Vernon L. Tharp St., Columbus, OH 43210 (E-mail: geor.1{at}osu.edu).
Received 12 April 1996; accepted in final form 6 June 1997.
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