Effect of a sodium-poor diet on lithium clearance at different dietary potassium contents in rats.

In rats on a diet with a sodium content of 300 mmol/kg, lithium is reabsorbed exclusively in the proximal tubules, and lithium clearance (CLi) equals the proximal tubular fluid output (Vprox). In rats on a sodium-poor diet (5 mmol/kg), lithium is also reabsorbed in the distal nephron and CLi is therefore lower than Vprox. The present paper examines the reduction of CLi in response to a low sodium diet in Brattleboro rats with hereditary diabetes insipidus maintained on different dietary potassium contents. CLi was reduced by about 60 microliter/min./100 g body weight in response to a low sodium diet. The absolute reduction in CLi caused by low sodium diet was unaffected by an increase of CLi provoked by administration of potassium with the diet, and it was unaffected by variations of CLi which occurred spontaneously within each group. CLi was accordingly reduced by a constant absolute value rather than by a constant percentage. The reduction of CLi (60 microliter/min./100 g body weight) was equal to CLi in the group given a potassium-poor diet with a normal sodium content. In the low-sodium and low-potassium group CLi was reduced to almost zero. Using CLi as a measure of Vprox in the rats on a 300 mmol/kg sodium diet it is concluded that the absolute reduction of CLi in response to a sodium-poor diet is 1) unaffected by increase of Vprox produced by administration of potassium with the food,2) unaffected by spontaneous variations of Vprox, and 3) equal to Vprox in rats given a potassium-poor diet.