P633Ranolazine reduces arrhythmogeneicity in transgenic mouse models of hypertrophic cardiomyopathy

Introduction: Ca2+ handling abnormalities are a main determinant of electro-mechanical dysfunction in human hypertrophic cardiomyopathy (HCM), leading to a pro-arrhythmogenic cellular phenotype. The late-Na+ current blocker ranolazine reduced the rate of arrhythmogenic events in HCM human myocardium via shortening of action potentials and reduction of intracellular Na+ and Ca2+, with increased forward NCX function (Circulation. 2013 Feb 5;127(5):575-84). Methods: Here we characterize pro-arrhythmogenic changes in E-C coupling that occur in cardiomyocytes and intact trabeculae from cTnT mutant mouse models of HCM (R92Q and E163R) and test the effects of ranolazine. Results: Compared to wild type (WT), R92Q cardiomyocytes showed (i) prolonged action potentials associated with ionic current remodeling; (ii) lower Ca2+transient amplitude with slower rate of decay; (iii) elevated diastolic [Ca2+]i; (iv) spontaneous Ca2+ waves and premature Ca2+ transients during pauses. Compared to WT, R92Q trabeculae showed (i) prolonged relaxation kinetics associated with reduced SERCA function and (ii) Frequent after-contractions or regular spontaneous beats during stimulation pauses. In E163R vs. WT trabeculae and cells, the kinetics of force development and relaxation was prolonged, despite Ca2+ transient kinetics and SERCA function were not slower. E163R vs. WT myocardium showed increased rate of spontaneous arrhythmogenic activity. In R92Q preparations, ranolazine (10μM): (i) hastened Ca2+ transient kinetics and reduced diastolic Ca2+; (ii) increased forward NCX function and (iii) reduced the rate of spontaneous beats and Ca2+ waves. In E163R trabeculae and cells ranolazine did not affect the kinetics of Ca2+ transients. Nonetheless, the drug prolonged RyR2 refractoriness and reduced the occurrence of spontaneous Ca2+ and contractile activity. None of these effects of ranolazine was observed in WT. Discussion: As in human HCM, the beneficial effects of ranolazine on R92Q myocardium are likely mediated by the consequences of late Na+ current inhibition. In E163R preparations, myocardial arrhythmogeneicity is not accompanied by remodeling of ion currents or changes of SERCA/NCX function, and thus appear to be a direct consequence of increased myofilaments Ca2+ sensitivity or anomalies of RyR2 function. Therefore the beneficial effects of ranolazine on Ca2+ mediated spontaneous activity in E163R myocardium supports the hypothesis that the anti-arrhythmic effect of ranolazine can also be mediated by mechanisms other that INaL inhibition (i.e. reduction of myofilaments Ca2+ sensitivity or RyR2 inhibition).