E-C Coupling Alterations and Spontaneous Activity in Mice Carrying Cardiac Troponin T Mutations

Ca2+ handling abnormalities are an early-onset pathogenic element in HCM. Here we characterize pro-arrhythmogenic changes in E-C coupling that occur in intact trabeculae and cardiomyocytes from cTnT mutant mouse models of HCM (R92Q, E163R and Δ160E) and test the effects of specific pharmacological interventions. Compared to WT, R92Q trabeculae ([Ca2+]o 2 mM, 30°C) showed (i) preserved peak isometric twitch tension and prolonged relaxation kinetics associated to decreased SERCA levels, (ii) faster mechanical restitution, further accelerated by isoproterenol (Iso) 100nM, (iii) decreased Ca2+-recirculation fraction markedly increased by Iso (iv) frequent after-contractions or regular spontaneous beats during stimulation pauses that increased in response to Iso. Compared to WT, R92Q cardiomyocytesshowed (i) prolonged action potentials associated with ionic current remodeling, (ii) slower rate of Ca2+transient decay, (iii) elevated diastolic [Ca2+]i, (iv) spontaneous Ca2+ waves during stimulation pauses. In R92Q preparations, the late-Na+ current blocker Ranolazine (Ran 10 μM) (i) reduced the rate of spontaneous beats and spontaneous Ca2+ waves, (ii) hastened Ca2+ transient kinetics and reduced diastolic Ca2+, (iii) reduced and reversed the acceleration of mechanical restitution and the increase in Ca2+ recirculation fraction induced by Iso.Compared to R92Q, occurrence of spontaneous contractions was similar in E163R but less pronounced in Δ160E. Iso and Ran showed similar effects in all three mouse models, in spite of some quantitative differences. The results are consistent with those recently reported in human HCM myocytes (Coppini et al, Circulation 2013) and suggest that remodeling and dysfunction of NCX and RyR2 contribute to the pro-arrhythmogenic E-C coupling abnormalities observed in HCM.