The Cardiac Ryanodine Receptor, but Not Sarcoplasmic Reticulum Ca-ATPase, is a Major Determinant of Ca Alternans in Intact Mouse Hearts

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