A novel electrophysiologic effect of melatonin on ischemia/reperfusion-induced arrhythmias in isolated rat hearts

Reperfusion after a short period of cardiac ischemia triggers ventricular arrhythmias attributable to ionic imbalance and oxidative stress. Melatonin offers some degree of protection, but its effects on the cardiac action potentials are unknown. We evaluated the effects of 5, 10, 20 and 50 mu m melatonin in isolated perfused rat hearts subjected to 10 min of regional ischemia. ECG and membrane potentials were synchronously displayed. After 15 min of reperfusion, total antioxidant capacity (TAC) was determined. Melatonin did not change the ischemic depolarization nor the action potential amplitude depression, but at the end of ischemia the action potential duration (APD) decreased in control and 5 mu m melatonin-treated hearts. By contrast, it returned to preischemic levels in hearts given 20 and 50 mu m melatonin. Melatonin reduced the incidence of reperfusion arrhythmias from 100% in control to 50% in 5 and 10 mu m, to 40% in 20 mu m and 30% in 50 mu m hearts. TAC values were higher at all melatonin concentrations. We conclude that melatonin reduced the incidence of reperfusion arrhythmias because of its antioxidant effects. In addition, at 20 and 50 mu m lengthened APD and promoted an improved protection. This latter effect should be considered when in vivo applications of melatonin are considered.