Mitochondrial Ca2+ Influx Contributes to Early After Depolarizations in Nonischemic Cardiomyopathic Mice

Introduction: Heart failure (HF) is associated with increased arrhythmic risk and triggered activity. Abnormal Ca 2+ handling is thought to underlie triggered activity, and mitochondria participate in Ca 2+ homeostasis. Hypothesis: Therefore, we tested whether mitochondrial Ca 2+ flux contributed to early afterdepolarizations (EADs) in HF. Methods: A model of nonischemic HF (NI-HF) was induced in C57BL/6 mice by hypertension. Tail-cuff plethysmography, echocardiography, ECG telemetry, patch-clamp, cytoplasmic (and mitochondrial) Ca 2+ transient measurement, real-time PCR and Western blotting techniques were used. Results: The left ventricular end-diastolic volume was significantly enlarged and the ejection fraction was substantially reduced in cardiomyopathic mice. Meanwhile, NI-HF mice developed a significant augmentation of both systolic and diastolic blood pressure. Isoproterenol-induced premature ventricular contractions and ventricular fibrillation were more prevalent in NI-HF mice than sham controls. Isolated myopathic myocytes showed a substantial decreased cytoplasmic Ca 2+ transients, a prominent increased mitochondrial Ca 2+ transients and a significant increase in the action potential duration at 90% repolarization (APD90). The alteration of APD90 was consistent with in vivo QTc elongation and could be explained by augmented L-type Ca 2+ currents, increased Na + -Ca 2+ exchange currents (NCX) and decreased total K + currents. Sixty-six percent of myopathic ventricular myocytes showed EADs compared with 17% of sham myocytes ( p u0026lt0.05). Intracellular application of 1 μM Ru360, a mitochondrial Ca 2+ uniporter (MCU) specific antagonist, could reduce mitochondrial Ca 2+ transients, decrease APD90 and ameliorate EADs. Furthermore, knock down MCU could inhibit mitochondrial Ca 2+ uptake, reduce NCX, decrease APD90 and depress EADs in NI-HF mice ventricular cells which led to less ventricular fibrillation in NI-HF mice, implicating mitochondrial Ca 2+ handling in modulating the arrhythmic risk through NCX induced EADs. Conclusions: Mitochondrial Ca 2+ handling plays an important role in EADs seen with nonischemic cardiomyopathy and may represent a novel therapeutic target to reduce arrhythmic risk in this condition.