ACTIVE LITHIUM TRANSPORT BY RAT RENAL PROXIMAL TUBULE - A MICROPUNCTURE STUDY

We tested the hypothesis that proximal tubular Li+ reabsorption is due to passive transport. Clearances of [C-14]inulin (C-In) and Li+ (C-Li), proximal transepithelial electrical potential difference (PD), and tubular fluid-to-plasma L(i)(+) concentration ratios [(TF/P)(Li)] were measured in anesthetized rats before and after induction of osmotic mannitol diuresis. Late proximal (TF/P)(Li) was measured after acute intravenous LICl administration and after addition of LiCl to the diet for 2 days. Glomerular filtration rate (C-In) decreased, whereas C-Na and C-Li increased during osmotic diuresis. Control early proximal PD was -0.6 mV (lumen negative); late proximal PD (PDLP) was 1.1 mV (lumen positive). PDLP decreased by 1.5 mV to -0.4 mV (lumen negative) after mannitol infusion. Late proximal (TF/P)(Li) was 1.01 after oral Li+, 1.16 after intravenous Li+ (P < 0.01), and 1.00 during osmotic diuresis. It is concluded that proximal Li+ transfer is distinct from that of Na+, closely parallels proximal water transfer, and involves an active transport mechanism independent of the PD. The data suggest that acute elevation of plasma Li+ concentration may activate a delayed Li+ transport pathway in the proximal convoluted tubule.