Role of the medullary collecting duct in potassium excretion in potassium-adapted animals.

Previous studies have demonstrated that the rate of urinary potassium excretion is markedly greater in animals fed a high potassium diet than in animals on a control diet before and during the acute infusion of potassium salts. Because the contribution of the medullary collecting duct to urinary potassium under these conditions has not been determined, microcatheterization studies were performed to elucidate the role of that nephron segment in regulating potassium excretion in controls and in animals on a potassium-enriched diet. Although there was no evidence of net transport of potassium by the medullary collecting duct in control animals under basal conditions, net secretion of potassium by that nephron segment accounted for nearly one half of the increased rate of urinary excretion in animals fed a high potassium diet. During acute infusion of potassium salts, potassium secretion by the medullary portion of the collecting duct contributed approximately one half of the potassium in the final urine samples in both controls and animals fed a high potassium diet. Because the rate of urinary excretion was twofold greater in experimental animals during acute potassium loading, absolute potassium secretion in medullary collecting duct was twice the control rate in animals adapted to a high potassium dietary intake. These data demonstrate, therefore, that the medullary collecting duct plays an important role in regulating the excretion of potassium when the excretory load is either acutely or chronically increased.