Role of the medullary collecting duct in potassium conservation.

A capacity for both net potassium absorption and net potassium secretion has been demonstrated in the inner medullary collecting duct. The quantitative importance, however, of the inner medullary collecting duct in the regulation of urinary potassium in potassium deficiency, however has not been established. To assess the contribution of this segment to potassium conservation, microcatherization studies were performed in male Sprague-Dawley rats maintained either on a control diet or on a potassium free diet for 72 h. In control animals approximately 15% of the filtered load of potassium was excreted. Analysis of tubule fluid along the inner medullary collecting duct failed to demonstrate evidence of net potassium movement. Administration of a potassium free diet resulted in a marked reduction in potassium excretion to 0.3% of the filtered load. In contrast to control the inner medullary collecting duct of experimental animals absorbed nearly 90% of the amount of potassium entering this segment, since fractional delivery to the terminal portion of the nephron was about 2%. These data indicate that the inner medullary collecting duct makes a significant contribution to maximal renal conservation of potassium, since previous studies have shown that only 5 to 10% of filtered potassium is present in the late distal tubule of surface nephrons in animals on a low potassium intake.