Change in salt appetite due to rehydration level in rats.

Thermally dehydrated rats were given a choice of tap water and saline (0.9% or 1.8% NaCl), and the change in the salinity of their choice during rehydration was measured up to 15 hr. The rats consumed more water than saline for about 2 hr after the start of fluid replacement (about 55 mEq/l), while they consumed more saline than tap water (about 120 mEq/l) thereafter. Urine output and urinary Na output were only about 20% of their intake during the initial 4 hr of rehydration, while after 4 hr the output became almost equal with the intake. The change in salt intake occurred when about 90% of Na loss and 60% of fluid loss was regained. The results indicate that rats choose dilute salt solution to lower plasma osmolality during the initial period of the rehydration and then regain body fluid isotonically. Urine volume and urinary Na output increased only after volume repletion. Thus, osmoregulation with salt appetite has priority over fluid volume regulation in restitution from thermal dehydration.