The natriuretic effect of lithium in man: is the proximal tubule involved?

The possible role of the proximal tubule in the natriuresis which follows the administration of small doses of lithium, as used in lithium clearance studies, was investigated in 12 healthy males on a fixed sodium intake. Subjects were given placebo tablets, or 100 mg or 600 mg of lithium carbonate; renal function was assessed 3-6 h later. The 600-mg dose of lithium carbonate caused a 50-60% increase in sodium excretion, whereas the 100-mg dose was without effect. Creatinine clearance, used as an index of glomerular filtration rate, was unaffected by either dose. Three indices of end-proximal fluid delivery were used simultaneously: urine flow rate during suppression of vasopressin secretion (Vmax), phosphate clearance and lithium clearance (the latter only on the days on which lithium was administered). No effect of either dose of lithium on Vmax or phosphate clearance was evident; nor was there a difference between values for lithium clearance following the two doses. We conclude that administration of the standard 600-mg test dose of lithium carbonate does not affect proximal tubular function.