Mitochondrial Ca2+ Influx Contributes to Arrhythmic Risk in Nonischemic Cardiomyopathy

Abstract

Introduction: It had been reported that mitochondrial Ca2+ was overloaded in ischemia-reperfusion injury. Inhibition of mitochondrial Ca2+ uniporter (MCU) prevented cell death. In this study, pharmacological block or genetic knockdown MCU inhibited mitochondrial Ca2+ uptake, depressed early afterdepolarizations (EADs) and reduced ventricular fibrillation in nonischemic heart failure (NI-HF) mice. Methods: A model of NI-HF was induced in CD1 mice by hypertension. Echocardiography, patch-clamp (and Rhod-2 fluorescence density recorded synchronously), telemetry monitoring, real-time PCR and Western blotting were employed in this study. Computer simulations were carried out. Results: Echocardiography showed an impairment of systolic function as the average EF value decreased from 63 ± 2% to 51 ± 3% in NI-HF mice (p<0.05). Isolated myopathic myocytes showed decreased cytoplasmic Ca2+ transients, increased mitochondrial Ca2+ transients, and increased action potential duration at 90% (APD90) repolarization. The alteration of APD90 was consistent with in vivo QTc prolongation and could be explained by augmented L-type Ca2+ currents, increased NCX and decreased total K+ currents. Sixty-six percent of myopathic ventricular myocytes showed EADs compared with 17% of sham myocytes (p<0.05). Intracellular application of 1 µM Ru360 could reduce mitochondrial Ca2+ transients, decrease APD90 and ameliorate EADs. Furthermore, genetic knockdown of MCU inhibited mitochondrial Ca2+ uptake, reduced NCX, decreased APD90, depressed EADs, and reduced ventricular fibrillation in NI-HF mice. Inhibiting mitochondrial NCX slowed mitochondrial Ca2+ uptake, decreased APD90, and depressed EADs. Computer simulations showed that EADs were promoted by heart failure remodeling, which were abolished by blocking either the MCU or the L-type Ca2+ current. Simulations revealed that EADs were promoted via a positive feedback loop among APD, SR Ca2+, and mitochondrial Ca2+, resulting in an all-or-none behavior. Conclusions: Mitochondrial Ca2+ handling plays an important role in EADs seen with chronic and moderate nonischemic cardiomyopathy.