The cardiac ryanodine receptor, but not sarcoplasmic reticulum Ca2-ATPase, is a major determinant of Ca2 alternans in intact mouse hearts

Abstract

Sarcoplasmic reticulum (SR) Ca2 cycling is governed by the cardiac ryanodine receptor (RyR2) and SR Ca2-ATPase (SERCA2a). Abnormal SR Ca2 cycling is thought to be the primary cause of Ca2 alternans that can elicit ventricular arrhythmias and sudden cardiac arrest. Although alterations in either RyR2 or SERCA2a function are expected to affect SR Ca2 cycling, whether and to what extent altered RyR2 or SERCA2a function affects Ca2 alternans is unclear. Here, we employed a gain-of-function RyR2 variant (R4496C) and the phospholamban-knockout (PLB-KO) mouse model to assess the effect of genetically enhanced RyR2 or SERCA2a function on Ca2 alternans. Confocal Ca2 imaging revealed that RyR2-R4496C shortened SR Ca2 release refractoriness and markedly suppressed rapid pacing–induced Ca2 alternans. Interestingly, despite enhancing RyR2 function, intact RyR2-R4496C hearts exhibited no detectable spontaneous SR Ca2 release events during pacing. Unlike for RyR2, enhancing SERCA2a function by ablating PLB exerted a relatively minor effect on Ca2 alternans in intact hearts expressing RyR2 WT or a loss-of-function RyR2 variant, E4872Q, that promotes Ca2 alternans. Furthermore, partial SERCA2a inhibition with 3 M 2,5-di-tert-butylhydroquinone (tBHQ) also had little impact on Ca2 alternans, whereas strong SERCA2a inhibition with 10 M tBHQ markedly reduced the amplitude of Ca2 transients and suppressed Ca2 alternans in intact hearts. Our results demonstrate that enhanced RyR2 function suppresses Ca2 alternans in the absence of spontaneous Ca2 release and that RyR2, but not SERCA2a, is a key determinant of Ca2 alternans in intact working hearts, making RyR2 an important therapeutic target for cardiac alternans.