Control of water intake in thermally dehydrated rats

Male rats were thermally dehydrated by exposure without water to an environmental temperature of 40 degrees C for 0-4 hr or to environmental temperatures of 25-40 degrees C for 4 hr. Water intake was then measured for 2 hr or a blood sample was taken to determine the effect of heat exposure on body water status. Evaporative water loss and water intake increased with increased duration and severity of heat exposure. Heat exposure significantly increased plasma osmolality and plasma sodium concentration and significantly decreased plasma potassium concentration. Hematocrit and plasma protein concentration increased slightly but not significantly with heat exposure. The increases in water intake in association with increases in evaporative water loss, plasma osmolality and plasma sodium concentration with no significant increases in hematocrit or plasma protein concentration indicates that the thirst induced by thermal dehydration is primarily osmotic in nature. Water intake equal to about 50% of the evaporative plus urinary water loss reduced plasma osmolality and sodium concentration to control levels, removing the stimulus to drink before the water loss was replaced.