Similar enhancement of BK(Ca) channel function despite different aerobic exercise frequency in aging cerebrovascular myocytes

Aging-induced down-regulation of Pka/Bkca pathway in rat cerebral arteries

The incidence of cerebrovascular diseases increases significantly with aging. This study aimed to test the hypothesis that aging may influence the protein kinase A (PKA)-dependent vasodilation via RyR/BKCa pathway in the middle cerebral arteries (MCA). Male Sprague-Dawley rats were randomly divided into control (4-6 month-old) and aged (24-month-old) groups. The functions of MCA and ion channel activities in smooth muscle cells were examined using myograph system and patch-clamp. Aging decreased the isoproterenol/forskolin-induced relaxation in the MCA. Large-conductance Ca2+-activated-K+ (BKCa) channel inhibitor, iberiotoxin, significantly attenuated the forskolin-induced vasodilatation and hyperpolarization in the young group, but not in the aged group. The amplitude and frequency of spontaneous transient outward currents (STOCs) were significantly decreased in the aged group. Single channel recording revealed that the mean open time of BKCa channels were decreased, while an increased mean closed time of BKCa channels were found in the aged group. The Ca2+/voltage sensitivity of the channels was decreased accompanied by reduced BKCa α and β1-subunit, the expression of RyR2, PKA-Cα and PKA-Cβ subunits were also declined in the aged group. Aging induced down-regulation of PKA/BKCa pathway in cerebral artery in rats. The results provides new information on further understanding in cerebrovascular diseases resulted from age-related cerebral vascular dysfunction.

Melatonin activates BKCa 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 Ca²⁺-activated K⁺ (BK_Ca) 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_Ca 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^ω-nitro-L-arginine methyl ester, melatonin-elicited relaxation was significantly inhibited by iberiotoxin (BK_Ca channel blocker). Melatonin significantly increased BK_Ca currents but not voltage-gated K⁺ (K_V) currents in whole-cell recordings. Melatonin decreased the amplitude of Ca²⁺ sparks and spontaneous transient outward currents (STOCs), however, a significant increase in open probability of BK_Ca channels was observed in both inside-out and cell-attached patch-clamp recordings. This melatonin-induced enhancement of BK_Ca channel activity was significantly suppressed by luzindole (melatonin MT₁/MT₂ 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 Ca²⁺. These findings indicate that melatonin-induced vasorelaxation of CAs is partially attributable to direct (passing through the cell membrane) and indirect (via melatonin MT₁/MT₂ receptors-PLC-PKC pathway) activation of BK_Ca channels on CA myocytes.