The Na+/Ca2+ exchanger inhibitor KB-R7943 activates large-conductance Ca2+-activated K+ channels in endothelial and vascular smooth muscle cells

The effect of the selective inhibitor of Na+/Ca2+ exchanger (NCX), KB-R7943, on large-conductance Ca2+-activated K+ (BKCa) channels was examined in cultured human umbilical vein endothelial cells (HUVECs) and freshly isolated mouse aortic smooth muscle cells (MASMCs). In voltage-clamped cells, KB-R7943 reversibly activated BKCa currents in HUVECs and MASMCs. The EC50 of KB-R7943 for BKCa current activation in HUVECs was determined to be 6.78 +/- 0.7 mu M. In inside-out and outside-out patches, KB-R7943 markedly increased BKC channel activity and slightly decreased single channel current amplitudes. In inside-out patches, KB-R7943 shifted the relationship between [Ca2+](i) and open probability (P-o) to the left-, the [Ca2+](i) required to evoke half-maximal activation changed from 1220 +/- 68 nM (in the absence of KB-R7943) to 620 +/- 199 nM (in the presence of 10 mu M KB-R7943). In addition, KB-R7943 shifted the relationship between membrane potential and P-o, to the left; the membrane potential to evoke half-maximal activation changed from 76.86 +/- 1.09 mV (in the absence of KB-R7943) to 49.62 +/- 2.55 mv (in the presence of 10 mu M KB-R7943). In conclusion, KB-R7943 was found to act as a potent BKCa channel activator, which increases the sensitivity of BKCa channels to cytosolic free Ca2+ and membrane potential, and thereby BKCa channel activity. These results should be considered when KB-R7943 is used as NCX blocker. (c) 2008 Elsevier B.V. All rights reserved.