Renal lithium handling during water loading and subsequent d-DAVP-induced anti-diuresis

Assuming that lithium is exclusively reabsorbed in the proximal tubules pari passu with sodium (Na), the lithium clearance (CLi) has been advanced as an index of filtrate delivery from the proximal tubules. In order to determine whether water loads in a range commonly given during clearance studies affect CLi, we studied nine normal subjects (Na intake 150 mmol day-1) on three water loads resulting in stable urine flow rates of 2.2 +/- 0.9, 6.1 +/- 0.7 and 11.7 +/- 3.0 ml min-1, respectively. We also studied the effect of acute anti-diuresis (urine flow less than 1 ml min-1) induced by d-DAVP given i.v. at the end of all studies. Water loading up to induction of maximum water diuresis did not significantly affect absolute or fractional CLi. Sodium and chloride excretion were reduced at the highest water intake level. Administration of d-DAVP caused a 50% reduction in fractional Na excretion and a small but significant rise in fractional lithium reabsorption from 75.5 +/- 3.9 to 77.5 +/- 3.6%. We conclude that CLi is hardly affected over a wide range of water intakes and urine flow rates, despite concomitant changes in Na excretion. The finding that d-DAVP, which probably enhances Na reabsorption in the thick ascending limb of Henle's loop, also enhanced lithium reabsorption, suggests that lithium may be partly reabsorbed in this nephron segment. Thus, lithium may not be the exact, quantitative marker of Na reabsorption in the proximal tubules that it is purported to be.