1. The influences of lithium dosage, urine flow rate and acute administration of amiloride on the renal handling of lithium in normal conscious dogs were investigated.
2. Lithium was administered in the diet at daily doses of 100 mg or 2 mg of lithium carbonate for the 2 days preceding the investigation. Urine flow rate was altered by water loading with and without arginine vasopressin infusion (5 pg min−1 kg−1). Amiloride was administered as an intravenous bolus (130 μg/kg) followed by a continuous infusion (1.22 μg h−1 kg−1).
3. Glomerular filtration rate (exogenous creatinine clearance) did not change within series and was not different between series; it averaged 3.27 ml min−1 kg−1. Control levels of fractional lithium excretion (12.4 ± 1.2%, mean ± sem) were not influenced by hydration, hydration plus arginine vasopressin administration or the lithium dosage. However, in hydrated dogs having a plasma lithium concentration of 130–140 μmol/l, amiloride administration was associated with a 5% increase in fractional lithium excretion (P 0.01).
4. It is concluded that distal tubular lithium reabsorption may take place in sodium-replete conscious dogs undergoing water diuresis. The low fractional lithium excretion even during amiloride infusion (14.1–16.8%) may well be due to a high fractional reabsorption of lithium in the proximal tubules; however, a significant reabsorption of lithium distal to the proximal straight tubules by amiloride-insensitive pathways cannot be excluded.