Impact of R4496C RyR2 mutation on myocardial contractility

Cardiac Ryanodine Receptor (RyR2) is a crucial determinant of Ca2+ availability in myofilament activation and force generation. To investigate the impact of RyR2 arrhythmia-related mutations on contractile function, atrial and ventricular trabeculae and ventricular myocytes from Wild Type (WT) and heterozygous R4496C RyR2 mutant (RyRR4496C+/-) mice were isolated and studied. Although under basal conditions contractility and Ca2+-transient amplitude are normal, RyRR4496C+/- preparations show reduced inotropic responses to high stimulation frequency, isoproterenol and other experimental interventions that potentiate contraction. The diminished inotropic response results from a smaller Sarcoplasmic Reticulum (SR) calcium load after inotropic stimulation than that of WT preparations. Moreover, RyRR4496C+/- muscles have faster mechanical restitution and shorter recovery of the amplitude of Ca2+release. In RyRR4496C+/- vs. WT myocytes sarcolemmal Ca2+-fluxes (ICa-L and INCX) and SR Ca2+-uptake are unchanged. We suggest that changes in gating of the mutant RyR2 Ca2+ channel are directly responsible for these observations. Fitting our experimental data for the R4496C mutation into a cardiomyocyte model, where the RyR2 gating properties are represented by a four state equilibrium and are modulated by luminal [Ca2+] ([Ca2+]SR), we predict an increase in sensitivity to [Ca2+]SR of both opening and closing channel transitions rates of mutant channels. Specifically, increasing [Ca2+]SR-sensitivity of closed-to-open transitions leads to SR-calcium depletion, blunted simulated inotropic responses and faster mechanical restitution. However, for the model to fully emulate our experimental results, [Ca2+]SR-sensitivity of open-to-closed transition also has to be increased, suggesting an effect of the R4496C mutation on RyR2-deactivation and Ca2+-release termination.