Determinants of Abnormal Excitation-Contraction Coupling in Cardiomyocytes from Patients with Hypertrophic Cardiomyopathy

Abstract

Diastolic dysfunction and ventricular arrhythmias are the clinical hallmarks of hypertrophic cardiomyopathy (HCM). Studies on animal models of HCM highlighted the leading role of excitation-contraction coupling (ECC) abnormalities as cellular determinants of increased arrhythmogenicity and abnormal contraction. However, mechanisms of ECC anomalies have never been studied on human HCM samples, thereby limiting the development of novel therapies.We aim to characterize the HCM-related ECC changes in cardiomyocytes isolated from the interventricular septum of HCM patients who underwent surgical myectomy for symptomatic obstruction, compared to non-failing non-hypertrophic surgical patients (controls).Compared to controls, HCM cardiomyocytes displayed slower kinetics of Ca2+-transients (duration at 90% decay= 719±50ms vs. 419±55ms at 1Hz) and higher diastolic Ca2+ (at 1Hz, 226±16nmol vs. 139±7nmol), especially at high stimulation rates. Moreover, the rate of spontaneous Ca2+ releases leading to delayed after depolarizations (DADs) was 5-fold higher.Several mechanisms underlie such changes in HCM cardiomyocytes:1)severe reduction of t-tubular density, leading to prolonged Ca2+-transients raising phase;2)reduced expression of SERCA, contributing to slower Ca2+-transient decay;3)increased Ca2+ current amplitude with prolonged inactivation, associated with 2-fold increased Calmodulin kinase II (CaMKII)-dependent phosphorylation of Ca2+ channel;4)3-fold increase of CaMKII-dependent phosphorylation of ryanodine receptors (RyR2) and 15% higher SR Ca2+-load, leading to increased RyR2 spontaneous activity and DADs;5)enhanced late Na+ current (INaL) and the resulting increased intracellular Na+, leading to altered function of Na+/Ca2+ exchanger and reduced NCX-mediated Ca2+ extrusion;The latter was tested by inhibiting INaL with ranolazine: via reduction of intracellular Na+, ranolazine partially counteracted ECC abnormalities, leading to decreased diastolic Ca2+, faster Ca2+-transients, lower SR Ca2+-load and rate of DADs.Albeit multiple abnormalities contribute to ECC changes in human HCM, INaL inhibition with ranolazine represents a possible therapeutic option to reduce arrhythmias and diastolic dysfunction.