EFFECT OF VASOPRESSIN ON RENAL LITHIUM REABSORPTION - A MICROPUNCTUREAND MICROPERFUSION STUDY

Micropuncture techniques were used to investigate the nephron site(s) responsible for the vasopressin-induced reductions in lithium clearanc e and fractional lithium excretion (FE(Li)) in anesthetized Brattlebor o rats lacking endogenous vasopressin. In rats treated intravenously w ith the vasopressin analogue 1-desamino-8-D-arginine vasopressin (DDAV P; 40 pg/min), FE(Li) was significantly lower than in untreated animal s (0.23 +/- 0.01 vs. 0.28 +/- 0.02, P < 0.05). Free-flow micropuncture showed that fractional lithium delivery (FDLi) to late proximal convo lutions was identical in the two groups, whereas at the early distal t ubule both FDLi (0.28 +/- 0.02 vs. 0.33 +/- 0.01, P < 0.05) and the tu bular fluid-to-plasma lithium concentration ratio (1.18 +/- 0.04 vs. 1 .36 +/- 0.06, P < 0.05) were lower in the DDAVP-treated group. No diff erences between the groups with respect to lithium handling beyond the early distal tubule were observed. In rats subjected to in vivo micro perfusion of loops of Henle, intravenous infusion of DDAVP (40 pg/min) increased fractional lithium reabsorption in the loop from 0.56 +/- 0 .03 to 0.66 +/- 0.04 (P < 0.05) and from 0.39 +/- 0.02 to 0.45 +/- 0.0 2 (P < 0.05) at perfusion rates of 10 and 15 nl/min, respectively. We conclude that DDAVP stimulates lithium reabsorption in the loop of Hen le and suggest that this results from an increased transepithelial pot ential difference in the thick ascending limb.