A MICROPUNCTURE STUDY OF RENAL TUBULAR LITHIUM REABSORPTION IN SODIUM-DEPLETED RATS

1. The marked reduction in fractional lithium excretion (FE(Li)) which accompanies chronic sodium depletion was investigated using free-flow micropuncture in anaesthetized rats which had been maintained in a so dium-depleted state for 8-10 days. 2. Compared with previous values in sodium-replete rats, sodium depletion was associated with small reduc tions in total and superficial nephron glomerular filtration rate and enhanced fractional reabsorption of water, sodium and lithium in the p roximal convoluted tubule. 3. In untreated (sodium-depleted) rats, fra ctional deliveries of lithium (FDLi) to the late proximal convoluted t ubule, early distal tubule and late distal tubule mere 0.41 +/- 0.02, 0.20 +/- 0.01 and 0.18 +/- 0.02 (means +/- S.E.M), respectively. Fract ional lithium excretion (0.08 +/- 0.01) was significantly lower than l ate distal FDLi (P < 0.001). 4. Treatment with amiloride did not affec t segmental lithium handling up to the late distal tubule. Frusemide h ad no effect on lithium reabsorption in the proximal convoluted tubule , but early distal FDLi (0.30 +/- 0.01) was raised compared with the u ntreated group (P < 0.001). Both diuretics eliminated the difference b etween late distal FDLi and FE(Li), respective values being 0.17 +/- 0 .02 and 01.5 +/- 0.01. (amiloride-treated rats) and 0.31 +/- 0.02 and 0.34 +/- 0.02 (husemide-treated rats). 5. These data indicate that par t of the reduction in FE(Li) in chronic sodium depletion is due to enh anced fractional fluid (and lithium) reabsorption in the proximal conv oluted tubule. In addition, however, they provide direct evidence for amiloride-sensitive lithium reabsorption in the collecting ducts. Alth ough frusemide may also inhibit lithium reabsorption at this site, a d isproportionate increase in lithium delivery from deep nephrons could partly explain the findings with this diuretic.