Ca2+-activated K+ channels in murine endothelial cells:: Block by intracellular calcium and magnesium

The intermediate (IKCa) and small (SKCa) conductance Ca2+-sensitive K+ channels in endothelial cells (ECs) modulate vascular diameter through regulation of EC membrane potential. However, contribution of IKCa and SKCa channels to membrane current and potential in native endothelial cells remains unclear. In freshly isolated endothelial cells from mouse aorta dialyzed with 3 mu M free [Ca2+](i) and 1 mM free [Mg2+](i), membrane currents reversed at the potassium equilibrium potential and exhibited an inward rectification at positive membrane potentials. Blockers of large-conductance, Ca2+-sensitive potassium (BKCa) and strong inward rectifier potassium (K-ir) channels did not affect the membrane current. However, blockers of IKCa channels, charybdotoxin (ChTX), and of SKCa channels, apamin (Ap), significantly reduced the whole-cell current. Although IKCa and SKCa channels are intrinsically voltage independent, ChTX- and Ap-sensitive currents decreased steeply with membrane potential depolarization. Removal of intracellular Mg2+ significantly increased these currents. Moreover, concomitant reduction of the [Ca2+](i) to 1 mu M caused an additional increase in ChTX- and Ap-sensitive currents so that the currents exhibited theoretical outward rectification. Block of IK Ca and SK Ca channels caused a significant endothelial membrane potential depolarization (approximate to 11 mV) and decrease in [Ca2+] i in mesenteric arteries in the absence of an agonist. These results indicate that [Ca2+] i can both activate and block IK Ca and SK Ca channels in endothelial cells, and that these channels regulate the resting membrane potential and intracellular calcium in native endothelium.