Osmotic factors in restitution from thermal dehydration in rats.

The degree of voluntary dehydration after thermal dehydration was assessed while supplying drinking water of varying NaCl concentrations. Adult male albino rats were exposed to a hot-dry environment (dry bulb temp 36 degrees C; relative humidity 20%) for 6-8 h, and dehydration of 7% body wt was induced. After dehydration the rats were divided into three groups, and distilled water and 0.45 or 0.9% NaCl solution was given ad libitum. The amount of fluid intake reached an equilibrium in 6-12 h; the 0.45 and 0.9% NaCl groups regained almost all lost water, whereas the distilled water group regained only half. During rehydration the 0.45 and 0.9% NaCl groups gained Na by approximately 430-650 mueq and lost K by 90-130 mueq, whereas the distilled water group lost Na slightly and K by 80 mueq/100 g body wt. As for the electrolyte balance during thermal dehydration, rats excreted Na and K into urine and saliva. Na loss was 200 mueq/100 g, almost all of which was derived from the interstitial space of skin and skeletal muscle, and K loss was 230 mueq/100 g, almost all of which was derived from intracellular space of skeletal muscle. Total cation loss (Na, K) during thermal dehydration, including K excretion during rehydration, was 510-560 mueq/100 g, which was almost identical to the Na gained by rats given 0.45 or 0.9% NaCl solution. These results suggest that voluntary dehydration is caused by the dilutional inhibition of drinking due to loss of electrolytes during thermal dehydration.