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⁺ (BK_Ca) 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 BK_Ca currents in HUVECs and MASMCs. The EC₅₀ of KB-R7943 for BK_Ca current activation in HUVECs was determined to be 6.78 ± 0.7 μM. In inside-out and outside-out patches, KB-R7943 markedly increased BK_Ca 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 μM 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 μM KB-R7943). In conclusion, KB-R7943 was found to act as a potent BK_Ca channel activator, which increases the sensitivity of BK_Ca channels to cytosolic free Ca²⁺ and membrane potential, and thereby BK_Ca channel activity. These results should be considered when KB-R7943 is used as NCX blocker.