Thermoregulatory activity in the rat: effects of hypohydration, hypovolemia and hypertonicity and their interaction with short-term heat acclimation

Hypothalamic temperature thresholds to heat-induced (40 degrees C ambient temperature) tail vasodilation (Vth) and salivation (Sth) as well as salivary flow rate and volume were studied in conscious rats, hypohydrated (24 hr water deprivation), hypovolemic (20% dextran sc), hypertonic (1M NaCL po), hypertonic and hypovolemic and heat-acclimated (5 days at 34 degrees C) before and after hypohydration. Sth was elevated in hypohydrated, hypovolemic, hypertonic and heat-acclimated hypohydrated rats concomitantly with a remarkable decrease in saliva volume, flow rate and heat tolerance. Heat acclimation alone resulted in a reduction in Vth, Sth, salivary flow and volume. Vth was not affected by hypohydration, but was elevated following hypovolemia and combined hypovolemia and hypertonicity. It is concluded that alterations in both plasma volume and osmolarity, which may occur during hypohydration, play a major role in the alteration in thermoregulatory responses during hypohydration. Heat acclimation does not improve tolerance during hypohydration. Thus, during hypohydration, the control of body fluids overrides thermoregulation.     

Influence of hydration and airflow on thermoregulatory control in the heat

Exercise-heat exposure results in significant sweat losses due to large biophysical requirements for evaporative heat loss. Progressive body water losses will increase plasma tonicity and decrease blood volume (hypertonic–hypovolemia). The result is reduced dry and evaporative heat exchange through alterations in the core temperature threshold for initiation of skin blood flow and sweating as well as changes in the sensitivity of these thermo-effectors. Regulation of reduced sweating conserves body water, which reduces heat loss and increases exercise hyperthermia, but the magnitude of this effect is modified by environmental heat transfer capabilities. The focus of this paper is to (1) examine the major mechanisms by which hypohydration alters thermoregulatory responses in the heat, and (2) illustrate how important differences in environmental airflow characteristics between laboratory and field settings may modify these effects.     

Thermoregulatory and blood responses during exercise at graded hypohydration levels

We studied the effects of graded hypohydration levels on thermoregulatory and blood responses during exercise in the heat. Eight heat-acclimated male subjects attempted four heat-stress tests (HSTs). One HST was attempted during euhydration, and three HSTs were attempted while the subjects were hypohydrated by 3, 5, and 7% of their body weight. Hypohydration was achieved by an exercise-heat regimen on the day prior to each HST. After 30 min of rest in a 20 degrees C antechamber the HST consisted of a 140-min exposure (4 repeats of 10 min rest and 25 min treadmill walking) in a hot-dry (49 degrees C, 20% relative humidity) environment. The following observations were made: 1) a low-to-moderate hypohydration level primarily reduced plasma volume with little effect on plasma osmolality, whereas a more severe hypohydration level resulted in no further plasma volume reduction but a large increment in plasma osmolality; 2) core temperature and heart rate responses increased with severity of hypohydration; 3) sweating rate responses for a given rectal temperature were systematically decreased with severity of hypohydration; and 4) the reduction in sweating rate was more strongly associated with plasma hyperosmolality than hypovolemia. In conclusion, an individual's thermal strain increases linearly with the severity of hypohydration during exercise in the heat, and plasma hyperosmolality influences the reduction in sweating more profoundly than hypovolemia.     

The thermoregulation of pregnant women during aerobic exercise in the water: a longitudinal approach

Twelve women early in their pregnancies were recruited to examine thermoregulation during immersion and exercise in the water (30 degrees C). Their responses were compared at 15, 25 and 35 weeks of pregnancy as well as 10-12 weeks post pregnancy to determine whether the responses differ between the gravid and non-gravid woman or were modified during pregnancy. Rectal temperature, mean skin temperature, heat storage, and evaporation were similar during immersion or exercise during the 15th, 25th and 35th weeks of pregnancy. Compared to 10 weeks post partum, pregnancy reduced heat storage, lowered skin temperature and increased evaporative heat loss during immersion and exercise (P less than 0.05). The results suggest that pregnancy causes subtle changes in the mechanism of thermoregulation which tend to increase heat production and improve heat conservation.     

Vasopressin in thermoregulation--competitive demands: experimental evidence and theoretical considerations.

Previous studies have substantiated the antipyretic role played by extrahypothalamic limbic system (EXHY-LS) AVP during fever. Repeated attempts to elucidate other thermoregulatory functions of this hormone have failed. Circumstantial evidence, however, suggest central role for this hormone in thermoregulation under hypohydration. Hypohydration, hyperosmolarity and hypovolaemia induced upward shifts in temperature thresholds for activation of heat dissipating mechanisms. When hypovolaemia is superimposed on hyperosmolarity these shifts are additive. Analogously, these two stressors when combined, decrease the osmotic threshold for AVP release. In rats, the elevated temperature thresholds for evaporative cooling and peripheral vasodilation occurring with hypohydration are positively correlated with lower Hypothalamic/EXHY-LS AVP ratio. Reciprocal relations between limbic system and blood AVP contents suggest competitive interaction between central and peripheral demands. Hypothesis for the possible mode of action of central AVP in thermoregulation under hypohydration is discussed.     

Changes in plasma volume during hypohydration and rehydration in subjects from the tropics

Plasma volume (PV) at different levels of hypohydration was determined using radio-iodinated serum albumin-125 in 28 heat acclimated male volunteers in hot dry condition in a climatic chamber. The heat acclimated subjects were hypohydrated to varying degrees i.e. 1%, 2%, 3% and 4% body mass deficit by moderate work in hot conditions in a climatic chamber maintained at 45 degrees C dry bulb temperature and 30% relative humidity. A rehydration study was carried out in only those subjects who were hypohydrated to 3% and 4% body mass and they were brought back to a 2% level of hypohydration by giving a calculated amount of water. A significant decrease in PV was observed at 3% and 4% hypohydration only. The magnitude of the decrease was the same in both the groups and not related to the level of hypohydration. With partial rehydration in the 3% hypohydrated group PV was restored fully, while in the 4% hypohydrated group restoration was incomplete, indicating that at this hypohydration level some of the replenished water that entered in plasma may have moved to the intracellular compartment which may have contributed more at 4% hypohydration. It is suggested that with higher levels of thermal hypohydration significant reduction in the intracellular compartment may result in accentuated physiological strain during work in the heat.     

Effect of primary hypohydration on physical work capacity

Physical work capacity (PWC₁₈₀) was assessed with different levels of hypohydration in 25 heat-acclimatized male volunteers in hot dry (45°C DB, 30% RH) and hot humid (39°C DB, 60% RH) conditions equated to a heat stress level of 34°C on the WBGT scale. Heat acclimatization was carried out by exposing the subjects for 8 consecutive days in a climatic chamber with moderate work for two 50 min work cycles and 10 min intervening rest pauses. Acclimatization resulted in significant decreases in heart rate (27 bpm), oral temperature (0.8°C), mean skin temperature (1.2°C) and a significant increase in sweating rate (120 g h^–1 m^–2). Day-to-day variations in body hypohydration levels during heat acclimatization were not significantly different, although water intake was found to increase significantly from day 3 onwards when the subjects were in ad lib water intake state. The heat acclimatized subjects were then hypohydrated to varying degrees, viz. 1%, 2% and 3% body weight deficit, with moderate work in heat in the climatic chamber and after successful recovery from the effects of thermal stress and exercise; their physical work capacity was assessed individually. Physical work capacity was found to decrease significantly with hypohydration as compared to controls. The decrease was of the order of 9%, 11% and 22% in the hot dry condition and 6%, 8% and 20% in the hot humid condition with hypohydration levels of 1%, 2% and 3% respectively. The decrease was more pronounced during 3% hypohydration level under both heat stress conditions. This decrease was in spite of significant increases in maximal ventilation. However, the PWC₁₈₀ under the two heat stress conditions, when compared, did not reveal any significant difference. It was concluded that the heat stress vehicle did not adversely affect the physical work capacity. On the other hand, the decreases in physical work capacity were found to be closely related to the primary hypohydration level in heat-acclimatized tropical subjects.Abbreviations WBGT wet bulb globe temperature - bam beats per minute - YSI Yellow Springs Instrument - EKG electrocardiogram     

Plasma hormonal responses at graded hypohydration levels during exercise-heat stress

The effects of graded levels of hypohydration (3, 5, and 7% of body weight) on hormonal responses to exercise in the heat were examined in six heat-acclimated male volunteers. On the day following dehydration, subjects performed light (approximately 25% maximal O2 consumption, 1.03 1 X min-1) exercise in a hot (49 degrees C, 20% relative humidity) environment for four consecutive 25-min intervals interspaced by 10-min rests; blood was obtained before exercise and at approximately 10 min before completion of each exercise period. During euhydration, plasma cortisol (PC) levels manifested significant decrements over time (e.g., time 0, 14.2 micrograms X 100 ml-1 vs. time 2, 8.9 micrograms X 100 ml-1), probably related to its diurnal periodicity. However, during hypohydration, levels of PC were increased and correlated with hypohydration intensity (e.g., time 0, 0, 3, 5, and 7% hypohydration, 14.2, 16.5, 19.8, and 36.2 micrograms X 100 ml-1, respectively). Plasma renin activity (PRA) was increased significantly by hypohydration (e.g., time 0, euhydrated vs. 3%, 3.7 vs. 6.2 units) but was unaffected by exercise in the heat. Plasma aldosterone (ALD) levels were generally increased by exercise in the heat (e.g., time 0 vs. time 4, 3% hypohydration, 12.1 vs. 18.7 ng X 100 ml-1). Regression analysis illustrated that graded intensities of hypohydration were correlated with incremented PRA and ALD through 5% hypohydration. Conversely, PC was incrementally elevated through 7% hypohydration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypohydration and exercise: effects of heat acclimation, gender, and environment

This study examined the effects of heat acclimation and subject gender on treadmill exercise in comfortable (20 degrees C, 40% rh), hot-dry (49 degrees C, 20% rh), and hot-wet (35 degrees C, 79% rh) environments while subjects were hypo- or euhydrated. Six male and six female subjects, matched for maximal aerobic power and percent body fat, completed two exercise tests in each environment both before and after a 10-day heat acclimation program. One exercise test was completed during euhydration and one during hypohydration (-5.0% from baseline body weight). In general, no significant (P greater than 0.05) differences were noted between men and women at the completion of exercise for rectal temperature (Tre), mean skin temperature (Tsk), or heat rate (HR) during any of the experimental conditions. Hypohydration generally increased Tre and HR values and decreased sweat rate values while not altering Tsk values. In the hypohydration experiments, heat acclimation significantly reduced Tre (0.19 degrees C) and HR (13 beats X min-1) values in the comfortable environment, but only HR values were reduced in hot-dry (21 beats X min-1) and hot-wet (21 beats X min-1) environments. The present findings indicated that men and women respond in a physiologically similar manner to hypohydration during exercise. They also indicated that for hypohydrated subjects heat acclimation decreased thermoregulatory and cardiovascular strain in a comfortable environment, but only cardiovascular strain decreased in hot environments.     

Thermal and circulatory responses during prolonged exercise at different levels of hydration

After a control experiment under initial normal hydration (N), five healthy unacclimated subjects were studied to investigate the effects of initial hypo- and hyperhydration on cardiovascular and thermo-regulatory responses to prolonged intermittent exercise in the heat (To = 36 degrees C; Tdp = 10 degrees C; Va = 0.6 m.s-1). Prior hydrohydration (O) was obtained by diuretics and prior hyperhydration (R) by ingestion of 0.5 L of isotonic (ISO) electrolyte sucrose solution 30 min before the experiments (4 h) started. Exercise (70 W) lasted 3 hours, and was periodically interrupted by resting periods (5-10 min). Three dehydration (D) runs were thus performed under the three initial hydration states (O,N,R) without fluid replacement during the exercise period. Four additional rehydration runs were carried out: 2 in each initial hydration level (O, R) included ingestion (at 36 degrees C) of water or ISO-solution during the first 3 hours. Physiological measurements were continuously recorded and hourly blood samples (15 ml) were obtained. Results showed that dehydration increased core temperature and heart rate and provoked blood hypovolemia and hyperosmolarity, the latter being somewhat prevented by prior ISO-ingestion. Dehydration reduced significantly the overall sweat rate only in hypohydrated subjects and the large hyperosmolarity seemed to be responsible for this. The significant Tcore rise during dehydration is unlikely to be the result of a decrease in evaporative heat transfer, which was found only in the case of initial hypohydration. Rehydration during exercise with water or ISO-solution induced different dynamic responses depending on the initial hydration level, but it never restored plasma volume.(ABSTRACT TRUNCATED AT 250 WORDS)     

Redistribution of blood within the body is important for thermoregulation in an ectothermic vertebrate (<Emphasis Type="Italic">Crocodylus porosus</Emphasis>)

Changes in blood flow are a principal mechanism of thermoregulation in vertebrates. Changes in heart rate will alter blood flow, although multiple demands for limited cardiac output may compromise effective thermoregulation. We tested the hypothesis that regional differences in blood flow during heating and cooling can occur independently from changes in heart rate. We measured heart rate and blood pressure concurrently with blood flow in the crocodile, Crocodylus porosus. We measured changes in blood flow by laser Doppler flowmetry, and by injecting coloured microspheres. All measurements were made under different heat loads, with and without blocking cholinergic and β-adrenergic receptors (autonomic blockade). Heart rates were significantly faster during heating than cooling in the control animals, but not when autonomic receptors were blocked. There were no significant differences in blood flow distribution between the control and autonomic blockade treatments. In both treatments, blood flow was directed to the dorsal skin and muscle and away from the tail and duodenum during heating. When the heat source was switched off, there was a redistribution of blood from the dorsal surface to the duodenum. Blood flow to the leg skin and muscle, and to the liver did not change significantly with thermal state. Blood pressure was significantly higher during the autonomic blockade than during the control. Thermal time constants of heating and cooling were unaffected by the blockade of autonomic receptors. We concluded that animals partially compensated for a lack of differential heart rates during heating and cooling by redistributing blood within the body, and by increasing blood pressure to increase flow. Hence, measures of heart rate alone are insufficient to assess physiological thermoregulation in reptiles.     

Effect of blood volume on forearm venous and cardiac stroke volume during exercise

Five healthy men exercised at 65-70% of maximum O2 uptake (VO2 max) for 30 min in an ambient temperature of 30 degrees C. Duplicate experiments were conducted at three levels of plasma volume:control, hypovolemia, in which blood volume (BV) was reduced an average of 490 ml (9.7%) with diuretics, and hypervolemia, in which BV was increased an average of 440 ml (7.8%) by infusing an isotonic solution containing 5% human serum albumin. Marked venoconstriction occurred during exercise in all conditions and persisted despite large increases in deep body temperature. The degree of venoconstriction was similar during control and hypervolemic conditions, but was potentiated during hypovolemia. The observed venoconstriction appeared to consist of two components: an early one related to autonomic adjustments at the onset of exercise, and a later one possibly related to progressive decreases in cardiac filling. Heart rate, cardiac stroke volume (SV), and cardiac output during exercise were significantly affected by changes in BV. During hypovolemia the average differences from control values were 10 beats X min-1, -14 ml, and -2.2 l X min-1, respectively; during hypervolemia the differences from control were -7 X min-1, 10 ml, and 1.0 l X min-1, respectively. The pattern of SV over the course of exercise indicates that pooling of blood in veins may be quantitatively more important than plasma water loss in reducing cardiac filling pressure in the heat.     

Physiological and metabolic responses to work in heat with graded hypohydration in tropical subjects

Studies were conducted on 25 healthy male volunteers aged 20-25 years drawn randomly from the tropical regions of India. The subjects initially underwent an 8 day heat acclimatization schedule with 2 hours moderate work in a climatic chamber at 45 degrees C DB and 30% RH. These heat acclimatized subjects were then hypohydrated to varying levels of body weight deficits, i.e. 1.3 +/- 0.03, 2.3 +/- 0.04 and 3.3 +/- 0.04%, by a combination of water restriction and moderate exercise inside the hot chamber. After 2 hours rest in a thermoneutral room (25 +/- 1 degree C) the hypohydrated subjects were tested on a bicycle ergometer at a fixed submaximal work rate (40 W, 40 min) in a hot dry condition (45 degrees C DB, 30% RH, 34 degrees C WBGT). Significant increases in exercise heart rate and oral temperature were observed in hypohydrated subjects as compared to euhydration. Sweat rate increased with 1% and 2% hypohydration as compared to euhydration, but a significant decrease was observed with 3% hypohydration. Na+ & K+ concentrations in arm sweat increased with increase in the level of hypohydration. Oxygen consumption increased significantly only when hypohydration was about 2% or more. It appears that the increased physiological strain observed in tropical subjects working in heat with graded hypohydration is not solely due to reduced sweat rates.     

Non-thermal influences on the control of skin blood flow have minimal effects on heat transfer during exercise

During exercise, circulatory reflexes ensure that the cardiac output is sufficiently elevated to meet the oxygen delivery requirements of the contracting skeletal muscles and the heat delivery requirements of the body to the skin. The latter requirements are met by increasing skin blood flow. These increases are largely driven by elevations in the body temperatures, although non-thermal effects on the control of skin blood flow occur in certain conditions. These effects are largely the consequence of high and/or low baroreflex stimulation. Even in the face of such non-thermal effects, which occur during exercise in the heat, the body's requirements for heat transfer from core to skin are largely met by the increased skin blood flow. Thus, non-thermal effects on the control of skin blood flow are relatively unimportant in the body's overall regulatory response to exercise.     

Hypohydration increases the plasma catecholamine response to moderate exercise in the dog (Canis)

1. The effects of hypohydration produced by 48 hr water deprivation were examined in dogs during moderate treadmill exercise at an ambient temperature (T_a) of 21°C.2. Hypohydration caused a significant elevation in plasma levels of adrenaline (A), proteins (pp) and osmolality (pOsm).3. During 1 hr of running, plasma concentrations of adrenaline (A) and noradrenaline (NA) rose significantly, whilst no change in these hormones occurred in dogs hydrated ad libitum.4. The results suggest that hypovolemia in the dog may be a sufficient stimulus to intensify the sympatho-adrenal response to moderate exercise performed at a room T_a.     

Environmental stress after atropine treatment

1. 1.|Atropine administration resulted in higher skin temperatures in both sensible and insensible environments and a higher core temperature in the hot environment, due to the reduction in whole body sweating. Exercise time was reduced 28 min following atropine in the hot environment, but was not affected in the humid environment.

2. 2.|The effect of heat storage (significantly higher after atropine) was shown to be greater in the hot environment due to inadequate sweat secretion for subsequent evaporative cooling. In the warm environment, enhanced sensible heat loss resulted in more effective thermoregulation.

3. 3.|Based on the effective temperature (ET^*) it is suggested that exercise in the heat can be accomplished during environmental stress at warm temperatures after atropine treatment.

Environmental impact on crew of armoured vehicles: Effects of 24 h combat exercise in a hot desert

A field study was undertaken to investigate the effects of combined noise, vibration and heat stress on the physiological functions of the crew of armoured vehicles during prolonged combat exercise in a desert. The sound pressure level of noise was measured with a sound level meter and accelerations by vibration analyser. The thermal load on the crew was evaluated by calculating the wet bulb globe temperature index. The physiological responses of the subjects (n=9), included significant increases in the heart rate, 24 h water intake and urinary catecholamine concentration. A significant decrease was recorded in body mass, peak expiratory flow rate and 24 h urinary output. The high heat load on the crew resulted in a hypohydration of 3% body mass and appeared to be the dominant factor in producing the physiological strain.     

Exercise-induced blood prolactin variations in trained adult males: a thermic stress more than an osmotic stress

Blood prolactin (PRL) variations have been linked to temperature and osmotic changes in several species. The latter factors are here explored to better understand blood PRL responses frequently induced during physical exercise. Since body heat generated by exercise can lead to marked body fluid shifts, it was postulated that PRL changes observed during exercise could be associated with variations in body temperature and/or blood osmolality (OSM). A wide range (38.5-40.5 degrees C) of rectal temperatures (Tr; used here to appreciate core temperatures) were theoretically selected and randomly assigned as targets to male runners. Measured by thermistor probe, target Tr were obtained by a combination of factors: (a) increases heat production by treadmill running, and (b) decreases heat losses by appropriate clothing (decreases evaporation) in warmed (decreases radiation) and hypoventilated (decreases convection) laboratory conditions. For each subject, target Tr was attained not prior to 30 min after initiation of running, and had to be maintained for at least 10 min, for a mean (+/- SD) running time of 52.6 +/- 10.0 min. In a first protocol, hypohydration was provoked in 26 runners (23.9 +/- 4.7 years) by total restriction of water intake. In a second protocol (10 different runners: 22.3 +/- 3.3 years), euhydration was maintained by water intake (20 ml/kg body weight). Venous blood was sampled at rest before and immediately after the run. PRL was assayed by RIA; OSM was measured by freezing point depression; sodium was analyzed by flame photometry. At rest, before the heat-producing exercise, mean PRL values were 9.4 +/- 3.4 ng/ml for both eu/hypohydrated groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Human thermoregulatory responses during prolonged walking in water at 25, 30 and 35°C

Eight healthy and physically well-trained male students exercised on a treadmill for 60 min while being immersed in water to the middle of the chest in a laboratory flowmill. The water velocity was adjusted so that the intensity of exercise correspond to 50% maximal oxygen uptake of each subject, and experiments were performed once at each of three water temperatures: 25, 30, 35°C, following a 30-min control period in air at 25°C, and on a treadmill in air at an ambient temperature of 25°C. Thermal states during rest and exercise were determined by measuring rectal and skin temperatures at various points, and mean skin temperatures were calculated. The intensity of exercise was monitored by measuring oxygen consumption, and heart rate was monitored as an indicator for cardiovascular function. At each water temperature, identical oxygen consumption levels were attained during exercise, indicating that no extra heat was produced by shivering at the lowest water temperature. The slight rise in rectal temperature during exercise was not influenced by the water temperature. The temperatures of skin exposed to air rose slightly during exercise at 25°C and 30°C water temperature and markedly at 35°C. The loss of body mass increased with water temperature indicating that both skin blood flow and sweating during exercise increased with the rise in water temperature. The rise in body temperature provided the thermoregulatory drive for the loss of the heat generated during exercise. Heart rate increased most during exercise in water at 35°C, most likely due to enhanced requirements for skin blood flow. Although such requirements were certainly smallest at 25°C water temperature, heart rate at this temperature was slightly higher than at 30°C suggesting reflex activation of sympathetic control by cold signals from the skin. There was a significantly greater increase in mean skin and rectal temperatures in subjects exercising on the treadmill in air, compared to those exercising in water at 25°C. Accepted: 22 May 1998     

Heat stress and age: Skin blood flow and body temperature

1. 1. The ability to increase skin blood flow is an important mechanism for transferring heat from the body core to the skin for dissipation.

2. 2. During exercise, skin blood flow is typically 20–40% lower in men and women aged 55 and over (compared with 20–30 years old) at a given body core temperature. Yet criterion measures of heat tolerance (changes in core temperature, heat storage) often show minimal or no age-related alterations. From a series of studies conducted in our laboratory over the past 5 years, the following conclusions can be drawn.

3. 3. When fit healthy older subjects are matched with younger subjects of the same gender, size and body composition, V_O2max, acclimation state, and hydration level, age-related differences in skin blood flow are evident. However, these differences often do not translate into “poorer” heat tolerance or higher core temperatures.

4. 4. The larger core-to-skin thermal gradient maintained by the older individuals allows for effective heat transfer at lower skin blood flows.

5. 5. Furthermore, there is an increased coefficient of variation for thermoregulatory response variables with increasing age.

6. 6. Despite differences in the mechanisms underlying thermoregulation, true thermal tolerance is less a function of chronological age than of functional capacity and physiological health status.

7. 7. While this conclusion is based primarily on cross-sectional studies, it is supported by the results of more recent studies using multiple regression analyses.

8. 8. Implicit in this conclusion is the notion that thermal tolerance, at any age, is a modifiable individual characteristic.