Suppression of potassium-recycling in the renal medulla by short-term potassium deprivation.

Recently we proposed that potassium, like urea, normally undergoes medullary recycling from collecting tubule to the pars recta or descending limb of the juxtamedullary nephron and suggested that the extent of recycling is a function of the concentration of potassium in collecting tubule fluid. To test this hypothesis further, we fed young rats a potassium-free diet for 3 days and then prepared them for micropuncture of the left renal papilla. Compared to findings in normally fed animals, potassium deprivation caused a significant fall in plasma potassium and urinary excretion of potassium. There was a striking decrease in the fraction of filtered potassium remaining at the end of the justamedullary descending limb for 94 +/- 11% to 38 +/- 3% (P less than 0.001). The latter value is not significantly different from the fraction of filtered sodium remaining (36 +/- 4%) and suggests that net addition of potassium to the pars recta or descending limb was completely abolished. A correlation was observed between the fraction of filtered potassium remaining at the end of the descending limb and either urinary potassium excretion (P less than 0.001) or urinary potassium concentration(P less than 0.001) in the contralateral unexposed kidney. These results lend further support to the hypothesis of medullary recycling of potassium.