Suppression of feeding by sodium depletion in food-deprived rats

Food-deprived rats were depleted of sodium peritoneal dialysis (pd) against 5.0% glucose solution and offered food 24 h later. Food intake was suppressed compared with fasting sham-dialyzed controls, or controls dialyzed against Ringer-Tyrode. Sodium depletion by subcutaneous formalin also suppressed feeding. Measurements of osmolality and sodium chloride (NaCl) concentrations of the ascitic fluid showed that osmolality increased 19% mainly due to NaCl diffused 96.2 +/- 7.23 mEq/l. Consequently serum sodium concentration fell significantly (5.5% below control value). Serum potassium concentration did not change. Measurements of blood glucose levels showed that 1.87 g/l of glucose had been absorbed while it was in the peritoneum. Twenty four hours after sodium depletion, blood glucose levels of fasting sodium-depleted animals did not differ from nondepleted controls. Stomach load of glucose, in the same amount given for pd, did not suppress feeding in fasting rats 24 h later. Fasting sodium-depleted rats, offered 0.3 M NaCl to drink instead of food, drank as much as non-fasting sodium-depleted controls, ruling out the possibility of impaired behavior. Suppression of feeding was reversed in fasting sodium-depleted animals which were allowed to replace their sodium deficit by free-drinking or by stomach loading of sodium solutions. These results provide the first demonstration that depletion of sodium suppresses feeding in food-deprived rats.