Ca dependence of flow-stimulated K secretion in the mammalian cortical collecting duct

Liu W, Morimoto T, Woda C, Kleyman TR, Satlin LM. Ca dependence of flow-stimulated K secretion in the mammalian cortical collecting duct. Am J Physiol Renal Physiol 293: F227–F235, 2007. First published March 27, 2007; doi:10.1152/ajprenal.00057.2007.—Apical low-conductance SK and high-conductance Ca -activated BK channels are present in distal nephron, including the cortical collecting duct (CCD). Flow-stimulated net K secretion (JK) in the CCD is 1) blocked by iberiotoxin, an inhibitor of BK but not SK channels, and 2) associated with an increase in [Ca ]i, leading us to conclude that BK channels mediate flow-stimulated JK. To examine the Ca dependence and sources of Ca contributing to flow-stimulated JK, JK and net Na absorption (JNa) were measured at slow ( 1) and fast ( 5 nl min 1 mm ) flow rates in rabbit CCDs microperfused in the absence of luminal Ca or after pretreatment with BAPTA-AM to chelate intracellular Ca , 2-aminoethoxydiphenyl borate (2-APB), to inhibit the inositol 1,4,5-trisphosphate (IP3) receptor or thapsigargin to deplete internal stores. These treatments, which do not affect flow-stimulated JNa (Morimoto et al. Am J Physiol Renal Physiol 291: F663–F669, 2006), inhibited flow-stimulated JK. Increases in [Ca ]i stimulate exocytosis. To test whether flow induces exocytic insertion of preformed BK channels into the apical membrane, CCDs were pretreated with 10 M colchicine (COL) to disrupt microtubule function or 5 g/ml brefeldin-A (BFA) to inhibit delivery of channels from the intracellular pool to the plasma membrane. Both agents inhibited flow-stimulated JK but not JNa (Morimoto et al. Am J Physiol Renal Physiol 291: F663–F669, 2006), although COL but not BFA also blocked the flow-induced [Ca ]i transient. We thus speculate that BK channel-mediated, flow-stimulated JK requires an increase in [Ca ]i due, in part, to luminal Ca entry and ER Ca release, microtubule integrity, and exocytic insertion of preformed channels into the apical membrane.