Aldosterone up-regulates basolateral Na -K -2Cl cotransporter-1 to support enhanced large-conductance K channel-mediated K secretion in rat distal colon.

Na -K -2Cl cotransporter-1 (NKCC1) facilitates basolateral K and Cl uptake, supporting their efflux across mucosal membranes of colonic epithelial cells. NKCC1 activity has also been shown to be critical for electrogenic K secretion induced by aldosterone, which is known to stimulate large-conductance K (BK) channel expression in mucosal membranes. This study was aimed to (1) identify whether aldosterone enhances NKCC1 expression specifically to support BK-mediated K secretion and (2) to determine whether increased NKCC1 supports electrogenic Cl secretion in parallel to K secretion. Dietary Na depletion was used to induce secondary hyperaldosteronism in rats, or aldosterone was administered ex vivo to rat distal colonic mucosae. NKCC1-dependent electrogenic K or Cl secretion was measured as a function of short circuit current (I ). qRT-PCR, western blot, and immunofluorescence analyses were performed using standard techniques. Aldosterone enhanced NKCC1 and BKα expression and electrogenic K secretion in the distal colon, which was inhibited by either serosal bumetanide (NKCC1 inhibitor) or mucosal iberiotoxin (IbTX; BK channel blocker), but not TRAM-34 (IK channel blocker). Expression of NKCC1 and BKα proteins was enhanced in crypt cells of hyper-aldosterone rats. However, neither NKCC1-dependent Cl secretion nor CFTR (apical Cl channel) expression was enhanced by aldosterone. We conclude that aldosterone enhances NKCC1 to support BK-mediated K secretion independently of Cl secretion in the distal colon. The regulation of NKCC1 expression/K secretion by aldosterone may be a therapeutic target in treating gastrointestinal disorders associated with alterations in colonic K transport, such as colonic pseudo-obstruction, and hyperkalemia associated with renal disease.