Calsequestrin 2 deletion shortens the refractoriness of Ca2+ release and reduces rate-dependent Ca2+-alternans in intact mouse hearts

Abstract

Calsequestrin (Casq2) is a low affinity Ca2+-binding protein located in sarcoplasmic reticulum (SR) of cardiac myocytes. Casq2 acts as a Ca2+ buffer regulating free Ca2+ concentration in the SR lumen and plays a significant role in the regulation of Ca2+ release from this intracellular organelle. In addition, there is experimental evidence supporting the hypothesis that Casq2 also modulates the activity of the cardiac Ca2+ release channels, ryanodine receptors (RyR2). In this study, Casq2 knockout mice (Casq2−/−) were used as a model to evaluate the effects of the Casq2 on the cytosolic and intra-SR Ca2+ dynamics, and the electrical activity in the ventricular epicardial layer of intact beating hearts. Casq2−/− mice have accelerated intra-SR Ca2+ refilling kinetics (76±22 vs. 136.5±15ms) and a reduced refractoriness of Ca2+ release (182±32ms Casq2+/+ and 111±22ms Casq2−/− ). In addition, mice display reduced Ca2+ alternans (67% decline in the amplitude of Ca2+ alternans at 7 Hz, 21oC) and less T-wave alternans at the electrocardiographic level. The results presented in this paper support the idea of Casq2 acting both as a buffer and a direct regulator of the Ca2+ release process. Finally, we propose that alterations in Ca2+ release refractoriness shown here could explain the relationship between Casq2 function and an increase in the risk for ventricular arrhythmias.