Amiloride-sensitive lithium reabsorption in rats: a micropuncture study.

It has been suggested that in sodium-restricted rats lithium is being reabsorbed in the collecting tubule, and that such reabsorption can be prevented by amiloride. We studied these options in rats by means of cortical micropuncture. Indeed, an important fraction of the filtered lithium (10.5%) was reabsorbed beyond the early distal tubule in sodium-restricted rats. Amiloride infusion abolished this reabsorption. The dosage used did not affect the reabsorption of water, sodium and lithium in the proximal tubule, and also did not change the reabsorption of water and sodium in the loop segment. However, the reabsorption of lithium in this segment tended to be higher during amiloride (P = .06), and the delivery of lithium to the early distal puncture site was significantly less (26.1 +/- 1.2%) than in the control rats (31.4 +/- 1.8%, P < .05). The reason for this apparent increase in lithium reabsorption in the loop segment is not clear. Interestingly, amiloride increased urine flow and decreased urine osmolality. The mechanism underlying this effect needs further exploration. We conclude that in sodium-restricted rats lithium reabsorption beyond the early distal tubule level occurs. Lithium clearance data therefore underestimate end-proximal delivery under these conditions. Lithium reabsorption beyond the early distal tubule can be prevented by amiloride treatment. However, because amiloride may stimulate lithium reabsorption further upstream, lithium clearance data obtained during amiloride infusion may still underestimate end-proximal delivery of water and sodium.