Ca2+ dependence of flow-stimulated K secretion in the mammalian cortical collecting duct

Apical low-conductance SK and high-conductance Ca2+-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 [Ca2+](i), leading us to conclude that BK channels mediate flow-stimulated J(K). To examine the Ca2+ dependence and sources of Ca2+ contributing to flow-stimulated JK, J(K) and net Na absorption (J(Na)) were measured at slow (similar to 1) and fast (similar to 5 nl (.) min(-1) (.) mm(-1)) flow rates in rabbit CCDs microperfused in the absence of luminal Ca2+ or after pretreatment with BAPTA-AM to chelate intracellular Ca2+, 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 [Ca2+](i) stimulate exocytosis. To test whether flow induces exocytic insertion of preformed BK channels into the apical membrane, CCDs were pretreated with 10 mu M colchicine (COL) to disrupt microtubule function or 5 mu 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 [Ca2+] i transient. We thus speculate that BK channel-mediated, flow-stimulated JK requires an increase in [Ca2+](i) due, in part, to luminal Ca2+ entry and ER Ca2+ release, microtubule integrity, and exocytic insertion of preformed channels into the apical membrane.