LITHIUM REABSORPTION IN PERFUSED LOOPS OF HENLE - EFFECTS OF PERFUSION RATE AND BUMETANIDE

The contribution of the loop segments to lithium reabsorption in sodiu m-replete, anesthetized Sprague-Dawley rats was examined by perfusing superficial loops of Henle between late proximal convolutions and earl y distal tubules. Preliminary experiments in which lithium was initial ly present only inside or outside the perfused loop confirmed substant ial permeability of one or more of the loop segments to lithium. In su bsequent experiments, in which lithium was infused intravenously and i ncluded in the perfusate so that the perfusate-to-plasma lithium conce ntration 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 (FR (Li)) at perfusion rates of 10, 20, and 30 nl/min were 58 +/- 3, 17 +/ - 2, and 2 +/- 2%, respectively. Bumetanide (10(-6) M) markedly inhibi ted FR(Li) but also reduced water reabsorption, suggesting an effect o n the pars recta at this dose; 10(-7) M bumetanide, which was without effect on water reabsorption, had only a small effect on FR(Li) at a p erfusion rate of 10 nl/min but reduced FR(Li) by similar to 70% at 20 nl/min. We argue that the remarkable flow dependency of lithium reabso rption, together with its bumetanide sensitivity, provides evidence fo r significant voltage-driven lithium reabsorption in the thick ascendi ng limb of Henle.