Characteristics and a functional implication of Ca²⁺-activated K⁺ current in mouse aortic endothelial cells

We employed the patch-clamp technique to investigate a Ca ²⁺-activated K⁺ (K_Ca) current in cultured mouse aortic endothelial cells (MAECs). In the whole-cell mode, an increase in cytosolic [Ca²⁺] ([Ca²⁺]_i) to 2 μM activated an outwards current. The [K⁺]_o-dependent change of the reversal potentials agreed well with the predicted Nernstian relation, suggesting that it was a K_Ca current. The Hill coefficient (4) and EC₅₀ (740 nM) were obtained from the current/[Ca²⁺] _i relationship. Iberiotoxin (50 nM) or apamin (200 nM) failed to inhibit the current, whereas TEA (10 mM) suppressed the current to 73.6 ±1.6% of control (n=9). The intermediate-conductance, Ca ²⁺-activated K⁺ (IK_Ca) channel blockers charybdotoxin (50 nM), clotrimazole (10 μM) and econazole (10 μM) inhibited the K_Ca current to 10.5±1.3% (n=6), 16.6±3. 1% (n=6), and 19.3±2.5% (n=5) of control, respectively. The EK _Ca channel openers chlorzoxazone, zoxazolamine and 1-ethyl-2-benz-imidazolinone and the Ca²⁺-activated Cl^- channel blocker niflumic acid activated the K_Ca current. In inside-out patches, the single-channel conductance was 17.7 pS in symmetrical K⁺ solutions. RT-PCR analysis showed transcripts of the murine IK1 channel (mIK1) in MAECs. The IK_Ca channel blockers inhibited the ATP-induced [Ca²⁺]_i increase in MAECs and the endothelium-dependent relaxation of mouse aortic rings. In addition, the IK _Ca channel openers augmented ATP-induced [Ca²⁺] _i increase in MAECs and evoked endothelium-dependent relaxation of mouse aorta. These results suggest that an mIK1-like channel mediates the native IK_Ca current in MAECs and may contribute to endothelium-dependent relaxation by modulating MAEC [Ca²⁺] _i.