Reduced sarcoplasmic reticulum Ca2+ pump activity is antiarrhythmic in ischemic cardiomyopathy

Abstract

We have described an arrhythmic mechanism seen only in cardiomyopathy that involves increased mitochondrial Ca2+ handling and selective transfer of Ca2+ to the sarcoplasmic reticulum (SR). Modeling suggested that mitochondrial Ca2+ transfer to the SR via type 2a sarco/endoplasmic reticulum Ca2+-ATPase (SERCA2a) is a crucial element of this arrhythmic mechanism. We tested the role of SERCA2a in arrhythmias during ischemic cardiomyopathy. Myocardial infarction (MI) was induced in wild-type (Wt) and SERCA2a heterozygous knockdown (SERCA+/-) mice. Compared with Wt MI mice, SERCA2a heterozygous knockdown (SERCA+/-) MI mice had a substantially lower mortality after 3 weeks of MI without a significant change in MI area. Aside from a significant delay of the cytoplasmic Ca2+ transient decay existed in SERCA+/- compared with Wt, SERCA+/- did not affect cardiac systolic and diastolic function at the whole organ or single cell levels either before or after MI. After MI, SERCA+/- mice had reduced SERCA2a expression in the MI border zone compared with Wt MI mice. SERCA+/- mice had significantly decreased corrected QT intervals and less ventricular tachycardia compared with Wt MI mice. SERCA+/- cardiomyocytes from MI mice showed a reduced action potential duration and reduced triggered activity compared with Wt MI cardiomyocytes. Reduction in arrhythmic risk was accompanied by reduced diastolic SR Ca2+ sparks, reduced SR Ca2+ content, reduced oxidized ryanodine receptor, and increased calsequestrin 2 in SERCA+/- MI mice. SERCA2a knockdown was antiarrhythmic after MI without affecting overall systolic performance. Possible antiarrhythmic mechanisms included reduced SR free Ca2+ and reduced diastolic SR Ca2+ release.