Lithium clearance in mineralocorticoid escape in humans

Lithium clearance (C/sub Li/) has been advanced as an indicator of Na delivery from the proximal tubules. The authors studied C/sub Li/ in eight healthy males before and after mineralocorticoid escape, a maneuver that may induce suppression of fractional proximal Na reabsorption (FPR/sub Na/). FPR/sub Na/ was also estimated from changes in maximal free water clearance (C/sub H2O/). Plasma volume was measured as the 131I-labeled albumin distribution space. Extracellular fluid volume was estimated as the 82Br vector distribution volume. According to the latter method, FPR/sub Na/ dropped whereas inulin clearance rose. The changes in C/sub Li/ were surprisingly large. If lithium is a valid marker of Na handling in the proximal tubule in humans, this change would imply a fall in FPR/sub Na/, suggesting a much larger shift in tubular Na reabsorption in escape than hitherto suspected. In addition, it would suggest that the inevitable back diffusion of a part of the solute-free water in the distal nephron, and thus overestimation of FPR/sub Na/ by the C/sub H2O/ method, increases importantly during escape. Alternately, lithium may not be a good marker of proximal tubular Na handling. For instance, both lithium reabsorption and escape may take place beyond the proximal tubule, or lithium may be excreted in the distal nephron in certain conditions. Present methods do not permit further analysis of these options in the human model