Renal Tubular Transport of 3 H-Digoxin in Saline Diuresis in Rats Evaluation by Micropuncture

3H-digoxin by three different methods in anesthetized rats made diuretic by infusion of 2.5% saline. In one group small volumes of 3 H-digoxin and "C-inulin were injected simultaneously into surface proximal convolutions, and urine was collected serially from both ureters. Digoxin recovery was lower after early (62.1 ± 5.3%) than after late (86.9 ± 7.7%) proximal administration but inulin recovery was complete (99.6 ± 2.7%) after all injections. Most of the digoxin was excreted simultaneously with inulin. Delayed recovery was low. In another group of rats digoxin and inulin were applied directly to the capsule of the left kidney. Two-thirds of the recovered digoxin appeared from the left ureter and one-third from the right. The difference (41.9 ± 7.4%) is an estimate of transtubular digoxin influx. Digoxin excretion preceded inulin only on the left. Digoxin to inulin concentration ratios were 6 times higher from the left than the right, whereas inulin recoveries from the two sides were similar. In a third group of rats tubular fluid was collected from surface convolutions of proximal and distal tubule. In the accessible segment of the proximal tubule 35.9% of the filtered digoxin was reabsorbed. In the more distal nephron, drug was added into the lumen; this resulted in a net urinary excretion of 80.2 ± 18.2%. These findings are compatible with free filtration of digoxin at the glomerulus followed by passive proximal tubular reabsorption and an influx against a concentration gradient in the distal nephron.