Melatonin attenuates hypertension-related proarrhythmic myocardial maladaptation of connexin-43 and propensity of the heart to lethal arrhythmias.

We hypothesized that the pineal hormone melatonin, which exhibits cardioprotective effects, might affect myocardial expression of cell-to-cell electrical coupling protein connexin-43 (Cx43) and protein kinase C (PKC) signaling, and hence, the propensity of the heart to lethal ventricular fibrillation (VF). Spontaneously hypertensive (SHR) and normotensive Wistar rats fed a standard rat chow received melatonin (40 μg/mL in drinking water during the night) for 5 weeks, and were compared with untreated rats. Melatonin significantly reduced blood pressure and normalized triglycerides in SHR, whereas it decreased body mass and adiposity in Wistar rats. Compared with healthy rats, the threshold to induce sustained VF was significantly lower in SHR (18.3 ± 2.6 compared with 29.2 ± 5 mA; p < 0.05) and increased in melatonin-treated SHR and Wistar rats to 33.0 ± 4 and 32.5 ± 4 mA. Melatonin attenuated abnormal myocardial Cx43 distribution in SHR, and upregulated Cx43 mRNA, total Cx43 protein, and its functional phosphorylated forms in SHR, and to a lesser extent, in Wistar rat hearts. Moreover, melatonin suppressed myocardial proapoptotic PKCδ expression and increased cardioprotective PKCε expression in both SHR and Wistar rats. Our findings indicate that melatonin protects against lethal arrhythmias at least in part via upregulation of myocardial Cx43 and modulation of PKC-related cardioprotective signaling.