Acute Genetic Ablation of Cardiac Sodium/Calcium Exchange in Adult Mice: Implications for Cardiomyocyte Calcium Regulation, Cardioprotection, and Arrhythmia

Abstract

Background Sodium‐calcium (Ca 2+ ) exchanger isoform 1 (NCX1) is the dominant Ca 2+ efflux mechanism in cardiomyocytes and is critical to maintaining Ca 2+ homeostasis during excitation‐contraction coupling. NCX1 activity has been implicated in the pathogenesis of cardiovascular diseases, but a lack of specific NCX1 blockers complicates experimental interpretation. Our aim was to develop a tamoxifen‐inducible NCX1 knockout (KO) mouse to investigate compensatory adaptations of acute ablation of NCX1 on excitation‐contraction coupling and intracellular Ca 2+ regulation, and to examine whether acute KO of NCX1 confers resistance to triggered arrhythmia and ischemia/reperfusion injury. Methods and Results We used the α‐myosin heavy chain promoter (Myh6)‐MerCreMer promoter to create a tamoxifen‐inducible cardiac‐specific NCX1 KO mouse. Within 1 week of tamoxifen injection, NCX1 protein expression and current were dramatically reduced. Diastolic Ca 2+ increased despite adaptive reductions in Ca 2+ current and action potential duration and compensatory increases in excitation‐contraction coupling gain, sarcoplasmic reticulum Ca 2+ ATPase 2 and plasma membrane Ca2+ ATPase. As these adaptations progressed over 4 weeks, diastolic Ca 2+ normalized and SR Ca 2+ load increased. Left ventricular function remained normal, but mild fibrosis and hypertrophy developed. Transcriptomics revealed modification of cardiovascular‐related gene networks including cell growth and fibrosis. NCX1 KO reduced spontaneous action potentials triggered by delayed afterdepolarizations and reduced scar size in response to ischemia/reperfusion. Conclusions Tamoxifen‐inducible NCX1 KO mice adapt to acute genetic ablation of NCX1 by reducing Ca 2+ influx, increasing alternative Ca 2+ efflux pathways, and increasing excitation‐contraction coupling gain to maintain contractility at the cost of mild Ca 2+ ‐activated hypertrophy and fibrosis and decreased survival. Nevertheless, KO myocytes are protected against spontaneous action potentials and ischemia/reperfusion injury.