Aldosterone and potassium secretion by the cortical collecting duct

BACKGROUND.: Aldosterone has been implicated in the regulation of both Na and K concentrations in the plasma. Release of the hormone is known to be stimulated by high plasma K, and infusion of aldosterone lowers plasma K. However, the correlation between changes in mineralocorticoid levels and rates of K secretion is not perfect, suggesting that other factors may be involved. METHODS.: Patch-clamp recordings were made of K-channel activity in the split-open cortical collecting tubule of the rat. Estimates of channel density were made in cell-attached patches on the luminal membrane of principal cells of this segment. RESULTS.: Most of the K conductance of the apical membrane is mediated through low-conductance "SK" channels. The number of conducting SK channels is increased when animals are placed on a high-K diet. However, increasing plasma aldosterone levels by infusion of the hormone or by sodium restriction failed to change the number of active channels. CONCLUSIONS.: At least two circulating factors are required for the regulation of renal K secretion by the cortical collecting tubule. Aldosterone mainly stimulates secretion by increasing the driving force for K movement through apical channels. A second, as yet unidentified, factor increases the number of conducting K channels.