Heat acclimation and thirst in rats

Abstract

The effects of heat acclimation on water intake and urine output responses to thermal dehydration and other thirst stimuli were studied in male Sprague-Dawley rats. Rats were heat acclimated by continuous exposure to a 34°C environment for at least 6 weeks. Thermal dehydration-induced thirst was brought about by exposing the heat-acclimated rats and control rats housed at 24°C to a 37.5°C environment for 4 h without access to food or water. Heat acclimation reduced evaporative and urinary water losses and the increases in plasma sodium and osmolality during thermal dehydration, which led to a reduction in thermal dehydration-induced thirst. Heat acclimation reduced the rate of rehydration following thermal dehydration but did not alter the final rehydration level, indicating that heat acclimation does not alter the primary control of thermal dehydration-induced thirst. Heat acclimation did not alter water intake or urine output following administration of hypertonic saline, which selectively stimulates intracellular thirst, but led to greater water intake following administration of angiotensin II, which plays an important role in extracellular/volemic thirst, and following water deprivation, which activates both thirst pathways. Cardiovascular responses to angiotensin II were not altered by heat acclimation. Heat acclimation thus reduces water loss during heat exposure in rats, but does not have major effects on thermal dehydration-induced or extracellular thirst but does appear to alter volemic thirst.

Keywords: Angiotensin II; intracellular thirst; kidney; rehydration; thermal dehydration; volemic thirst.

Figure 1

Mean ± SE body weight of one set of control and heat‐acclimated rats for 9 weeks of exposure. Control rats were kept at 24 ± 2°C and the heat‐acclimated rats were initially placed in an environmental chamber at 31 ± 0.5°C. The temperature of the environmental chamber was increased by 0.5°C each day for 6 days and then the temperature was kept at 34 ± 0.5°C for the remainder of the experiment. N = 15 for both groups. ANOVA indicated significant effects of group and time and a significant interaction between group and time on body weight.

Figure 2

Mean evaporative water loss (top), urine output (middle) and feces output (bottom) in control and heat‐acclimated rats for 4 h of exposure to either a 25°C or 37.5°C environment. Error bars show SE. n = 8 for the heat acclimated 37.5°C group and 9 for all other groups. ANOVA indicated significant effects of group and temperature and a significant interaction between group and temperature on evaporative water loss. ANOVA indicated a significant effect of temperature on urine output and feces output and a significant interaction between group and temperature on feces output.

Figure 3

Mean ± SE cumulative water intake (top), urine output (middle), and percent rehydration (bottom) of control and heat‐acclimated rats for 4 h of access to water at an environmental temperature of 25°C following 4 h of exposure to either a 25°C or 37.5°C environment. n = 8 for the heat acclimated 37.5°C group and 9 for all other groups. ANOVA indicated significant effects of group, temperature, and time and significant interactions between group and time, group and temperature, and temperature and time on water intake. ANOVA indicated significant effects of group, temperature, and time and a significant interaction between group and temperature on urine output. ANOVA indicated a significant effect of time and significant interactions between group and time and temperature and time on percent rehydration.

Figure 4

Mean ± SE cumulative water intake (top) and urine output (bottom) of control and heat‐acclimated rats for 4 h of access to water at an environmental temperature of 25°C following 24 h of water deprivation at an environmental temperature of 25°C. n = 12 per group. ANOVA indicated significant effects of group and time on water intake and a significant effect of time on urine output.

Figure 5

Mean ± SE cumulative water intake (top) and urine output (bottom) of control and heat‐acclimated rats for 3 h of access to water at an environmental temperature of 25°C following administration of 10 mL 0.75 mol/L NaCl/kg i.p. n = 6 per group. ANOVA indicated only significant effects of time on water intake and urine output.

Figure 6

Mean ± SE cumulative water intake (top) and urine output (bottom) of control and heat‐acclimated rats for 3 h of access to water at an environmental temperature of 25°C following administration of 200 μg angiotensin II/kg i.p. n = 12 for the control group and 11 for the heat‐acclimated group. ANOVA indicated significant effects of group and time on water intake and a significant effect of time on urine output.

Figure 7

Mean ± SE cumulative water intake (top) and urine output (bottom) of control and heat‐acclimated rats for 90 min of access to water at an environmental temperature of 25°C and i.v. infusion of 0.25 μg angiotensin II/kg‐min. n = 8 per group. ANOVA indicated significant effects of group and time on water intake and a significant effect of time on urine output.

Figure 8

Mean ± SE mean arterial blood pressure (top) and heart rate (bottom) of anesthetized control and heat‐acclimated rats during i.v. infusion of various doses of angiotensin II. n = 8 per group. ANOVA indicated significant effects of dose of angiotensin II on arterial blood pressure and significant effects of group and dose of angiotensin II on heart rate.