EFFECTS OF FUROSEMIDE OR ACETAZOLAMIDE INFUSION ON RENAL HANDLING OF LITHIUM - A MICROPUNCTURE STUDY IN RATS

Renal lithium (Li) handling was studied by micropuncture at the late proximal (LPT) and early distal (EDT) tubules in control rats and rats infused with furosemide (FUR) or acetazolamide (ACTZ). In control rats, the tubular fluid-to-plasma Li concentration ratio [(T/P)Li] at the LPT exceeded unity (1.05 +/- 0.02, P < 0.05). Some 25% of the filtered load (FL) of Li and water was reabsorbed in proportion between the LPT and the EDT, and consequently the (T/P)Li at the EDT (1.03 +/- 0.03) did not change. FUR inhibited Li reabsorption in the proximal convoluted tubules (PCT), by approximately 7% of the FL. Reabsorption of Li and water in the loop segment was also inhibited, virtually in proportion, by approximately 10% of the FL. These data suggest that FUR-sensitive Li reabsorption in the loop mainly takes place in the pars recta. However, a small increase in the (T/P)Li at the EDT (to 1.10 +/- 0.01) suggested inhibition of some Li transport (approximately 2% of the filtered load of Li) without water, most likely in the thick ascending limb (TAL). In the PCT, ACTZ reduced Li reabsorption by approximately 16% of its FL. Although it is likely that ACTZ also inhibited the pars recta, net Li reabsorption in the loop was not reduced. This suggests that TAL Li reabsorption can compensate for increased delivery. We conclude that 1) Li delivery to the LPT slightly overestimates water delivery; 2) a small amount of Li appears to be reabsorbed in the TAL; and 3) most of the effect of FUR on the fractional excretion of Li appears to be effected in the proximal tubules.