Aldosterone increases K(Ca)1.1 (BK) channel-mediated colonic K+ secretion

Mammalian K+ homeostasis results from highly regulated renal and intestinal absorption and secretion, which balances the unregulated K+ intake. Aldosterone is known to enhance both renal and colonic K+ secretion. In mouse distal colon K+ secretion occurs exclusively via luminal K(Ca)1.1 (BK) channels. Here we investigate if aldosterone stimulates colonic K+ secretion via BK channels. Luminal Ba2+ and iberiotoxin (IBTX)-sensitive electrogenic K+ secretion was measured in Ussing chambers. In vivo aldosterone was augmented via a high K+ diet. High K+ diet led to a 2-fold increase of luminal Ba2+ and IBTX-sensitive short-circuit current in distal mouse colonic mucosa. This effect was absent in BK alpha-subunit-deficient (BK-/-) mice. The resting and diet-induced K+ secretion was stimulated by luminal ionomycin. In BK-/- mice luminal ionomycin did not stimulate K+ secretion. In vitro addition of aldosterone likewise triggered a 2-fold increase in K+ secretion, which was inhibited by the mineralocorticoid receptor antagonist spironolactone and the BK channel blocker IBTX. Semi-quantification of mRNA from colonic crypts showed up-regulation of BK alpha- and beta(2)-subunits in high K+ diet mice. The BK channel could be detected luminally in colonic crypt cells by immunohistochemistry. The expression level of the channel in the luminal membrane was strongly up-regulated in K+-loaded animals. Taken together, these data strongly suggest that aldosterone-induced K+ secretion occurs via increased expression of luminal BK channels.