Potassium recycling in the renal medulla: effects of acute potassium chloride administration to rats fed a potassium-free diet.

According to the hypothesis of potassium recycling in the renal medulla, a portion of potassium in fluid in the medullary collecting duct is reabsorbed, trapped in the medullary interstitium by countercurrent exchange, and secreted in either the pars recta or descending limb of the juxtamedullary nephron. To examine the effects of an acute change in potassium balance on recycling, we performed a micropuncture study on the exposed papilla of 8 chloride. A second group of 6 rats was studied under identical conditions and infused with potassium chloride and amiloride. In the first group, the fraction of filtered potassium remaining at the end of the juxtamedullary descending limb increased with time to values over 100% concomitantly with the rise in urinary excretion of potassium. A strong association was found between those two variables (P less than 0.025). In the second group, in which the increase in urinary fractional excretion of potassium was prevented by amiloride, the rise in fractional of filtered potassium remaining at the end of the juxtamedullary nephron was abolished. These findings are interpreted as providing further support for the hypothesis of medullary recycling of potassium.