Lithium clearance as an indicator of proximal tubular sodium handling during furosemide diuresis.

To evaluate the use of the renal Li clearance as an indicator of proximal tubular Na handling during furosemide diuresis, clearance experiments were performed in conscious catheterized rats. Maximal depression of proximal Na reabsorption was induced by isotonic saline expansion (40 ml/hr) and under these conditions furosemide (7.5 mg/kg/hr) did not increase absolute or fractional clearance of Li. These results provide indirect evidence against the existence of a furosemide-sensitive Li reabsorption in the ascending limb of Henle. The increase of absolute and fractional Li clearance observed in nonexpanded animals given furosemide therefore most likely reflects inhibition of electrolyte reabsorption in the proximal tubules. Using Li clearance as a measure for Na output from the proximal tubules, the study provided information about the contribution of proximal and distal nephron segments to saline expansion natriuresis. The results indicate that the natriuresis is caused by enhanced delivery of Na from the proximal tubules, which is only partially compensated for by a load-dependent increase of absolute distal Na reabsorption, most likely occurring in the thick ascending limb of Henle.