Sodium appetite in lactating rats

Lactating rats that were given free access to sodium-deficient food, water, and 0.51 M NaCl solution showed no evidence of sodium appetite. The estimated daily loss of 1-2 mEq Na in milk was replaced by basal daily intake of 2-5 ml of saline. Sodium loss in urine was minimal, but milk sodium concentration was unchanged, and pups grew normally. Saline intake was enhanced when lactating rats that had been maintained on standard laboratory chow were injected with 30% polyethylene glycol solution to reduce plasma volume but no more so than when virgin female rats or male rats were similarly colloid-treated. Lactating rats markedly increased their intake of NaCl solution after simply depriving them of dietary sodium for 4 days, whereas male and virgin female rats did not. These findings indicate that pronounced sodium appetite does not invariably accompany lactation in rats, although it can occur whenever such animals become hypovolemic or sodium deficient.