Abstract 16932: Melatonin Inhibits Large Conductance, Calcium-Activated K Channel Activity in Porcine Coronary Artery Smooth Muscle Cells by Activating MT 2 -Receptors

Abstract

Introduction: Increasing evidence indicates that melatonin plays a role in cardiovascular homeostasis by regulating blood vessel diameter, thereby impacting arterial blood pressure and local blood flow to organs and tissues. Alterations in circulating melatonin levels are associated with several cardiovascular disorders, including hypertension, ischemic heart disease, myocardial infarction, and heart failure. We have shown previously that melatonin impairs nitric oxide (NO)-induced relaxations in porcine coronary arteries by activating melatonin type-2 (MT 2 )-receptors (JPET 336: 127-33, 2011). Hypothesis: Melatonin impairs NO-induced relaxation of coronary arteries by inhibiting BK Ca channel activity in coronary artery smooth muscle cells (SMC). Methods and Results: Sodium nitroprusside (SNP), an exogenous NO-donor, caused relaxation of isolated porcine coronary arteries (-logEC 50 = 7.49 + 0.1; n=5). In the presence of either melatonin (10 -7 M) or iberiotoxin (IbTx; 10 -7 M), a selective BK Ca -channel blocker, the SNP dose-response was shifted to the right by approximately 6-fold (p<0.05). The presence of BK Ca α- and β-subunits was identified by immunoblot analysis in intact porcine coronary arteries. Immunocytochemistry studies demonstrated that both subunits co-localized with α-actin in isolated single coronary SMC. Whole-cell K currents (10-mV steps from -70mV to +60mV, 200 ms duration) were obtained in freshly isolated coronary SMC in the absence or presence of IbTx (10 -7 M) or SNP (10 -5 M) (n=5-6). IbTx markedly inhibited (p<0.05) both the voltage-dependent activation of whole-cell K currents and the SNP-induced increase in BK Ca currents in coronary SMC. Treatment of coronary SMC with melatonin (10 -7 M) inhibited the activation of BK Ca currents (56.8 ± 2.9% inhibition of peak current density at +60 mV; n=12; p<0.05) and shifted the I-V curve to the right, as compared to control. The inhibitory effect of melatonin (10 -7 M) on BK Ca currents was abolished in the presence of the selective MT 2 -receptor antagonist, 4P-PDOT (10 -7 M). Conclusions: Melatonin acts via MT 2 -receptors to inhibit BK Ca activity in porcine coronary SMC, suggesting a mechanistic basis for the inhibitory effect of melatonin on NO-induced relaxation.