Effect of aldosterone on BK channel expression in mammalian cortical collecting duct

Apical large-conductance [Ca.sup.2+]-activated [K.sup.+] (BK) channels in the cortical collecting duct (CCD) mediate flow-stimulated [K.sup.+] secretion. Dietary [K.sup.+] loading for 10-14 days leads to an increase in BK channel mRNA abundance, enhanced flow-stimulated [K.sup.+] secretion in microperfused CCDs, and a redistribution of immunodetectable channels from an intracellular pool to the apical membrane (Najjar F, Zhou H, Morimoto T, Bruns JB, Li HS, Liu W, Kleyman TR, Satlin LM. Am J Physiol Renal Physiol 289: F922-F932, 2005). To test whether this adaptation was mediated by a [K.sup.+]-induced increase in aldosterone, New Zealand White rabbits were fed a low-[Na.sup.+] (LS) or high-[Na.sup.+] (HS) diet for 7-10 days to alter circulating levels of aldosterone but not serum [K.sup.+] concentration. Single CCDs were isolated for quantitation of BK channel subunit (total, s-splice variants, [beta]-isoforms) mRNA abundance by real-time PCR and measurement of net transepithelial [Na.sup.+] (JNa) and [K.sup.+] ([J.sub.K]) transport by microperfusion; kidneys were processed for immunolocalization of BK [alpha]-subunit by immunofluorescence microscopy. At the time of death, LS rabbits excreted no urinary [Na.sup.+] and had higher circulating levels of aldosterone than HS animals. The relative abundance of BK [alpha]-, [[beta].sub.2]-, and [[beta].sub.4]-subunit mRNA and localization of immunodetectable [alpha]-subunit were similar in CCDs from LS and HS animals. In response to an increase in tubular flow rate from ~1 to 5 nl x [min.sup.-1] x [mm.sup.-1], the increase in [J.sub.Na] was greater in LS vs. HS rabbits, yet the flow-stimulated increase in [J.sub.K] was similar in both groups. These data suggest that aldosterone does not contribute to the regulation of BK channel expression/activity in response to dietary [K.sup.+] loading. ROMK; epithelial sodium channel; mechanoregulation; laminar shear stress; epithelial cell