AJP - Renal Physiology, Vol 266, Issue 5 806-F812, Copyright © 1994 by American Physiological Society
Lithium reabsorption in perfused loops of Henle: effects of perfusion rate and bumetanide
R. J. Unwin, S. J. Walter and D. G. Shirley
Department of Nephrology, University College London Medical School, United Kingdom.
The contribution of the loop segments to lithium reabsorption in
sodium-replete, anesthetized Sprague-Dawley rats was examined by perfusing
superficial loops of Henle between late proximal convolutions and early
distal tubules. Preliminary experiments in which lithium was initially
present only inside or outside the perfused loop confirmed substantial
permeability of one or more of the loop segments to lithium. In subsequent
experiments, in which lithium was infused intravenously and included in the
perfusate so that the perfusate-to-plasma lithium concentration ratio was
close to that found in late proximal tubules during lithium clearance
studies, lithium reabsorption was inversely related to the perfusion rate:
values for fractional lithium reabsorption (FRLi) at perfusion rates of 10,
20, and 30 nl/min were 58 +/- 3, 17 +/- 2, and 2 +/- 2%, respectively.
Bumetanide (10(-6) M) markedly inhibited FRLi but also reduced water
reabsorption, suggesting an effect on the pars recta at this dose; 10(-7) M
bumetanide, which was without effect on water reabsorption, had only a
small effect on FRLi at a perfusion rate of 10 nl/min but reduced FRLi by
approximately 70% at 20 nl/min. We argue that the remarkable flow
dependency of lithium reabsorption, together with its bumetanide
sensitivity, provides evidence for significant voltage-driven lithium
reabsorption in the thick ascending limb of Henle.
