Effect of acute potassium load on reabsorption in Henle's loop in chronic renal failure in the rat.

To determine the effect of an acute load of potassium on potassium reabsorption by the loop of Henle in chronic renal failure, the right kidney was removed and branches of the left renal artery were ligated in 17 rats. One week later and after 2 days of a potassium-free diet, rats were studied before (period 1) and after (period 2) acute loads of potassium chloride (KCl group), equimolar sodium chloride (NaCl group) or no solute (time control). The KCl load increased urinary potassium excretion to a greater extent (from 5 to 50%, P less than 0.005) than in NaCl (14 to 27%) or time control (9 to 14%), and caused as great a diuresis and natriuresis as did NaCl. Fractional delivery of water, sodium, and potassium to the end-proximal tubule increased similarly in the NaCl and KCl groups and slightly less so in the time control group in period 2. The major finding was a striking increase in potassium delivery to the beginning of the distal tubule (from 17 to 37%) in period 2 which was substantially greater than in the combined control groups (13 to 19%, P less than 0.025) and was equivalent to three-quarters of urinary potassium excretion. This was the consequence of an increase in the filtered load of potassium, an increase in absolute delivery of potassium from the proximal tubule (P less than 0.005), and a decrease in fractional potassium reabsorption by the loop of Henle from 64 to 48%, versus 72 to 69% in the control groups (P less than 0.01). These results suggest that the proximal tubule and, in particular, Henle's loop play a role in excreting an acute potassium load in chronic renal failure.