Mitochondrial dysfunction causing cardiac sodium channel downregulation in cardiomyopathy

Cardiomyopathy is associated with cardiac Na+ channel downregulation that may contribute to arrhythmias. Previously, we have shown that elevated intracellular NADH causes a decrease in cardiac Na+ current (INa) signaled by an increase in mitochondrial reactive oxygen species (ROS). In this study, we tested whether the NADH–mitochondria ROS pathway was involved in the reduction of INa in a nonischemic cardiomyopathic model and correlated the findings with myopathic human hearts. Nonischemic cardiomyopathy was induced in C57BL/6 mice by hypertension after unilateral nephrectomy, deoxycorticosterone acetate (DOCA) pellet implantation, and salt water substitution. Sham operated mice were used as controls. After six weeks, heart tissue and ventricular myocytes isolated from mice were utilized for whole cell patch clamp recording, NADH/NAD+ level measurements, and mitochondrial ROS monitoring with confocal microscopy. Human explanted hearts were studied using optical mapping. Compared to the sham mice, the arterial blood pressure was higher, the left ventricular volume was significantly enlarged (104.7 ± 3.9 vs. 87.9 ± 6.1 μL, P < 0.05), and the ejection fraction was reduced (37.1 ± 1.8% vs. 49.4 ± 3.7%, P < 0.05) in DOCA mice. Both the whole cell and cytosolic NADH level were increased (279 ± 70% and 123 ± 2% of sham, respectively, P < 0.01), INa was decreased (60 ± 10% of sham, P < 0.01), and mitochondrial ROS overproduction was observed (2.9 ± 0.3-fold of sham, P < 0.01) in heart tissue and myocytes of myopathic mice vs. sham. Treatment of myocytes with NAD+ (500 μM), mitoTEMPO (10 μM), chelerythrine (50 μM), or forskolin (5 μM) restored INa back to the level of sham. Injection of NAD+ (100 mg/kg) or mitoTEMPO (0.7 mg/kg) twice (at 24 h and 1 h before myocyte isolation) to animals also restored INa. All treatments simultaneously reduced mitochondrial ROS levels to that of controls. CD38 was found to transduce the extracellular NAD+ signal. Correlating with the mouse model, failing human hearts showed a reduction in conduction velocity that improved with NAD+. Nonischemic cardiomyopathy was associated with elevated NADH level, PKC activation, mitochondrial ROS overproduction, and a concomitant decrease in INa. Reducing mitochondrial ROS by application of NAD+, mitoTEMPO, PKC inhibitors, or PKA activators, restored INa. NAD+ improved conduction velocity in human myopathic hearts.

► We tested how nonischemic cardiomyopathy causes reduced cardiac sodium current. ► Cardiomyopathy was associated with elevated NADH, PKC activation, and mitochondrial ROS. ► Reducing mitochondrial ROS (e.g. with NAD+) restored cardiac sodium current. ► NAD+ restored conduction velocity in human hearts.