Nycthemeral variation in thermal dehydration-induced thirst

Temporal variation in spontaneous water intake in rats is well established but little is known about temporal variation in water intake following dehydration. In the present study different male Sprague-Dawley strain rats were exposed without water for 3 h to either a 25 degrees C or a 40 degrees C environment every 4 h for 20 h. The rats were then allowed access to water in a 25 degrees C environment for 2 h. Rats exposed to 25 degrees C showed significant temporal variation in evaporative water loss, urine output, urine sodium and potassium excretion, water intake, and percent rehydration with higher values occurring during the night. Rats exposed to 40 degrees C had greater evaporative water loss, urine sodium excretion, feces output and water intake than the rats exposed to 25 degrees C and had temporal variations which were similar to those of the rats exposed to 25 degrees C. The robust effects of thermal-dehydration on water balance in rats are additive to rather than interactive with the effects of time of day.