Apelin-13 Inhibits Large-Conductance Ca2+-Activated K+ Channels in Cerebral Artery Smooth Muscle Cells via a PI3-Kinase Dependent Mechanism

Apelin-13 causes vasoconstriction by acting directly on APJ receptors in vascular smooth muscle (VSM) cells; however, the ionic mechanisms underlying this action at the cellular level remain unclear. Large-conductance Ca(2+)-activated K(+) (BKCa) channels in VSM cells are critical regulators of membrane potential and vascular tone. In the present study, we examined the effect of apelin-13 on BK(Ca) channel activity in VSM cells, freshly isolated from rat middle cerebral arteries. In whole-cell patch clamp mode, apelin-13 (0.001-1 μM) caused concentration-dependent inhibition of BK(Ca) in VSM cells. Apelin-13 (0.1 µM) significantly decreased BK(Ca) current density from 71.25 ± 8.14 pA/pF to 44.52 ± 7.10 pA/pF (n=14 cells, P<0.05). This inhibitory effect of apelin-13 was confirmed by single channel recording in cell-attached patches, in which extracellular application of apelin-13 (0.1 µM) decreased the open-state probability (NPo) of BK(Ca) channels in freshly isolated VSM cells. However, in inside-out patches, extracellular application of apelin-13 (0.1 µM) did not alter the NPo of BK(Ca) channels, suggesting that the inhibitory effect of apelin-13 on BKCa is not mediated by a direct action on BK(Ca). In whole cell patches, pretreatment of VSM cells with LY-294002, a PI3-kinase inhibitor, markedly attenuated the apelin-13-induced decrease in BK(Ca current density. In addition, treatment of arteries with apelin-13 (0.1 µM) significantly increased the ratio of phosphorylated-Akt/total Akt, indicating that apelin-13 significantly increases PI3-kinase activity. Taken together, the data suggest that apelin-13 inhibits BK(Ca) channel via a PI3-kinase-dependent signaling pathway in cerebral artery VSM cells, which may contribute to its regulatory action in the control of vascular tone.