The cardiac ryanodine receptor, but not sarcoplasmic reticulum Ca

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