THE NATRIURETIC EFFECT OF LITHIUM IN MAN - IS THE PROXIMAL TUBULE INVOLVED

The possible role of the proximal tubule in the natriuresis which foll ows the administration of small doses of lithium, as used in lithium c learance studies, was investigated in 12 healthy males on a fixed sodi um 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-m g dose of lithium carbonate caused a 50-60% increase in sodium excreti on, whereas the 100-mg dose was without effect. Creatinine clearance, used as an index of glomerular filtration rate, was unaffected by eith er dose. Three indices of end-proximal fluid delivery were used simult aneously: urine flow rate during suppression of vasopressin secretion (V-max), phosphate clearance and lithium clearance (the latter only on the days on which lithium was administered). No effect of either dose of lithium on V-max or phosphate clearance was evident; nor was there a difference between values for lithium clearance following the two d oses. We conclude that administration of the standard 600-mg test dose of lithium carbonate does not affect proximal tubular function.