Mitochondrial Ca²⁺ influx contributes to arrhythmic risk in nonischemic cardiomyopathy

Background--Heart failure (HF) is associated with increased arrhythmia risk and triggered activity. Abnormal Ca²⁺ handling is thought to underlie triggered activity, and mitochondria participate in Ca²⁺ homeostasis. Methods and Results--A model of nonischemic HF was induced in C57BL/6 mice by hypertension. Computer simulations were performed using a mouse ventricular myocyte model of HF. Isoproterenol-induced premature ventricular contractions and ventricular fibrillation were more prevalent in nonischemic HF mice than sham controls. Isolated myopathic myocytes showed decreased cytoplasmic Ca²⁺ transients, increased mitochondrial Ca²⁺ transients, and increased action potential duration at 90% repolarization. The alteration of action potential duration at 90% repolarization was consistent with in vivo corrected QT prolongation and could be explained by augmented L-type Ca²⁺ currents, increased Na⁺-Ca²⁺ exchange currents, and decreased total K⁺ currents. Of myopathic ventricular myocytes, 66% showed early afterdepolarizations (EADs) compared with 17% of sham myocytes (P < 0.05). Intracellular application of 1 lmol/L Ru360, a mitochondrial Ca²⁺ uniporter-specific antagonist, could reduce mitochondrial Ca²⁺ transients, decrease action potential duration at 90% repolarization, and ameliorate EADs. Furthermore, genetic knockdown of mitochondrial Ca²⁺ uniporters inhibited mitochondrial Ca²⁺ uptake, reduced Na⁺-Ca²⁺ exchange currents, decreased action potential duration at 90% repolarization, suppressed EADs, and reduced ventricular fibrillation in nonischemic HF mice. Computer simulations showed that EADs promoted by HF remodeling could be abolished by blocking either the mitochondrial Ca²⁺ uniporter or the L-type Ca²⁺ current, consistent with the experimental observations. Conclusions--Mitochondrial Ca²⁺ handling plays an important role in EADs seen with nonischemic cardiomyopathy and may represent a therapeutic target to reduce arrhythmic risk in this condition.