Melatonin activates BK channels in cerebral artery myocytes via both direct and MT receptor/PKC-mediated pathway.

The pineal hormone melatonin is a neuroendocrine hormone with high membrane permeability that is involved in regulation of circadian rhythm of several biological functions. Large-conductance Ca-activated K (BK) channels are abundantly expressed in vascular smooth muscle cells and play an important role in vascular tone regulation. We investigated the mechanisms through which myocyte BK channels mediate effects of melatonin on cerebral arteries (CAs). Arterial contractility measurements showed that melatonin alone did not change vascular tone in CAs; however, it induced concentration-dependent vasodilation of phenylephrine-induced contraction in CAs. In the presence of the potent endothelial oxide synthase inhibitor, N-nitro-L-arginine methyl ester, melatonin-elicited relaxation was significantly inhibited by iberiotoxin (BK channel blocker). Melatonin significantly increased BK currents but not voltage-gated K (K) currents in whole-cell recordings. Melatonin decreased the amplitude of Ca sparks and spontaneous transient outward currents (STOCs), however, a significant increase in open probability of BK channels was observed in both inside-out and cell-attached patch-clamp recordings. This melatonin-induced enhancement of BK channel activity was significantly suppressed by luzindole (melatonin MT/MT receptor inhibitor), U73122 (phospholipase C (PLC) inhibitor), and Ro31-8220 (protein kinase C (PKC) inhibitor). Melatonin had no significant effects on sarcoplasmic reticulum release of Ca. These findings indicate that melatonin-induced vasorelaxation of CAs is partially attributable to direct (passing through the cell membrane) and indirect (via melatonin MT/MT receptors-PLC-PKC pathway) activation of BK channels on CA myocytes.