2+ ihandling proteins changes identified, an enhanced I NaL seems to be a major contributor to the electrophysiological and Ca

The plasma membrane of cardiac myocytes presents complex invaginations known as the transverse-axial tubular system (TATS). Despite TATS's crucial role in excitation-contraction coupling and morphological alterations found in pathological settings, TATS's electrical activity has never been directly investigated in remodeled tubular networks. Here we develop an ultrafast random access multiphoton microscope that, in combination with a customly synthesized voltage-sensitive dye, is used to simultaneously measure action potentials (APs) at multiple sites within the sarcolemma with submillisecond temporal and submicrometer spatial resolution in real time. We find that the tight electrical coupling between different sarcolemmal domains is guaranteed only within an intact tubular system. In fact, acute detachment by osmotic shock of most tubules from the surface sarcolemma prevents AP propagation not only in the disconnected tubules, but also in some of the tubules that remain connected with the surface. This indicates that a structural disorganization of the tubular system worsens the electrical coupling between the TATS and the surface. The pathological implications of this finding are investigated in failing hearts. We find that AP propagation into the pathologically remodeled TATS frequently fails and may be followed by local spontaneous electrical activity. Our findings provide insight on the relationship between abnormal TATS and asynchronous calcium release, a major determinant of cardiac contractile dysfunction and arrhythmias.cardiac disease | nonlinear microscopy | voltage imaging | t tubules T he transverse-axial tubular system (TATS) is a complex network characterized by transverse (t tubules, TT) and longitudinal (axial tubules, AT) components running from one transverse tubule to the next (1-3). The TATS of a myocyte rapidly conducts depolarization of the surface sarcolemma (SS) to the core of the cardiomyocyte (4). The coupling between sarcolemmal Ca 2+ entry during an action potential (AP) and Ca 2+ release from sarcoplasmic reticulum (SR) promotes synchronous myofibril activation throughout the myocyte (5). Recent studies highlight that AP-relevant ion channels and transporters are expressed in TATS membrane with different densities and isoforms from those in SS (6, 7). These findings, in combination with diffusional limitations in TATS's lumen (3,8), raise the possibility that AP may differ among membrane domains. Further interest in the TATS AP stems from the recent finding of loss and disorganization of tubules in several pathological conditions, including heart failure (9-14). Because correlation between morphological TATS alterations and Ca 2+ -release asynchronicity has been observed (14, 15), recording AP propagation in TATS can elucidate the fundamental electrophysiological mechanism linking structural and functional anomalies.Although the uniformity of the AP across the whole sarcolemma has been mathematically (16) and experimentally (17) proved, a potential alteration of the AP propagation i...

Action potentials (APs), via the transverse axial tubular system (TATS), synchronously trigger uniform Ca 2+ release throughout the cardiomyocyte. In heart failure (HF), TATS structural remodeling occurs, leading to asynchronous Ca 2+ release across the myocyte and contributing to contractile dysfunction. In cardiomyocytes from failing rat hearts, we previously documented the presence of TATS elements which failed to propagate AP and displayed spontaneous electrical activity; the consequence for Ca 2+ release remained, however, unsolved. Here, we develop an imaging method to simultaneously assess TATS electrical activity and local Ca 2+ release. In HF cardiomyocytes, sites where T-tubules fail to conduct AP show a slower and reduced local Ca 2+ transient compared with regions with electrically coupled elements. It is concluded that TATS electrical remodeling is a major determinant of altered kinetics, amplitude, and homogeneity of Ca 2+ release in HF. Moreover, spontaneous depolarization events occurring in failing T-tubules can trigger local Ca 2+ release, resulting in Ca 2+ sparks. The occurrence of tubuledriven depolarizations and Ca 2+ sparks may contribute to the arrhythmic burden in heart failure.cardiac disease | voltage imaging | calcium imaging | nonlinear microscopy

T-tubule uncoupling from the plasma membrane leads to myocardial contractile abnormalities.

BackgroundMild hypertrophy but increased arrhythmic risk characterizes the stereotypic phenotype proposed for hypertrophic cardiomyopathy (HCM) caused by thin-filament mutations. However, whether such clinical profile is different from more prevalent thick-filament–associated disease is unresolved.ObjectivesThis study aimed to assess clinical features and outcomes in a large cohort of patients with HCM associated with thin-filament mutations compared with thick-filament HCM.MethodsAdult HCM patients (age >18 years), 80 with thin-filament and 150 with thick-filament mutations, were followed for an average of 4.5 years.ResultsCompared with thick-filament HCM, patients with thin-filament mutations showed: 1) milder and atypically distributed left ventricular (LV) hypertrophy (maximal wall thickness 18 ± 5 mm vs. 24 ± 6 mm; p < 0.001) and less prevalent outflow tract obstruction (19% vs. 34%; p = 0.015); 2) higher rate of progression to New York Heart Association functional class III or IV (15% vs. 5%; p = 0.013); 3) higher prevalence of systolic dysfunction or restrictive LV filling at last evaluation (20% vs. 9%; p = 0.038); 4) 2.4-fold increase in prevalence of triphasic LV filling pattern (26% vs. 11%; p = 0.002); and 5) similar rates of malignant ventricular arrhythmias and sudden cardiac death (p = 0.593).ConclusionsIn adult HCM patients, thin-filament mutations are associated with increased likelihood of advanced LV dysfunction and heart failure compared with thick-filament disease, whereas arrhythmic risk in both subsets is comparable. Triphasic LV filling is particularly common in thin-filament HCM, reflecting profound diastolic dysfunction.

Heart failure invariably affects patients with various forms of muscular dystrophy (MD), but the onset and molecular sequelae of altered structure and function resulting from full-length dystrophin (Dp427) deficiency in MD heart tissue are poorly understood. To better understand the role of dystrophin in cardiomyocyte development and the earliest phase of Duchenne muscular dystrophy (DMD) cardiomyopathy, we studied human cardiomyocytes differentiated from induced pluripotent stem cells (hiPSC-CMs) obtained from the urine of a DMD patient.

Current rescue therapies for life-threatening arrhythmias ignore the pathological electro-anatomical substrate and base their efficacy on a generalized electrical discharge. Here, we developed an all-optical platform to examine less invasive defibrillation strategies. An ultrafast wide-field macroscope was developed to optically map action potential propagation with a red-shifted voltage sensitive dye in whole mouse hearts. The macroscope was implemented with a random-access scanning head capable of drawing arbitrarily-chosen stimulation patterns with sub-millisecond temporal resolution allowing precise epicardial activation of Channelrhodopsin2 (ChR2). We employed this optical system in the setting of ventricular tachycardia to optimize mechanistic, multi-barrier cardioversion/defibrillation patterns. Multiple regions of conduction block were created with a very high cardioversion efficiency but with lower energy requirements as compared to whole ventricle interventions to interrupt arrhythmias. This work demonstrates that defibrillation energies can be substantially reduced by applying discrete stimulation patterns and promotes the progress of current anti-arrhythmic strategies.

Background and PurposeIn 30–40% of hypertrophic cardiomyopathy (HCM) patients, symptomatic left ventricular (LV) outflow gradients develop only during exercise due to catecholamine‐induced LV hypercontractility (inducible obstruction). Negative inotropic pharmacological options are limited to β‐blockers or disopyramide, with low efficacy and tolerability. We assessed the potential of late sodium current (INaL)‐inhibitors to treat inducible obstruction in HCM.Experimental ApproachThe electrophysiological and mechanical responses to β‐adrenoceptor stimulation were studied in human myocardium from HCM and control patients. Effects of INaL‐inhibitors (ranolazine and GS‐967) in HCM samples were investigated under conditions simulating rest and exercise.Key ResultsIn cardiomyocytes and trabeculae from 18 surgical septal samples of patients with obstruction, the selective INaL‐inhibitor GS‐967 (0.5 μM) hastened twitch kinetics, decreased diastolic [Ca2+] and shortened action potentials, matching the effects of ranolazine (10μM). Mechanical responses to isoprenaline (inotropic and lusitropic) were comparable in HCM and control myocardium. However, isoprenaline prolonged action potentials in HCM myocardium, while it shortened them in controls. Unlike disopyramide, neither GS‐967 nor ranolazine reduced force at rest. However, in the presence of isoprenaline, they reduced Ca2+‐transient amplitude and twitch tension, while the acceleration of relaxation was maintained. INaL‐inhibitors were more effective than disopyramide in reducing contractility during exercise. Finally, INaL‐inhibitors abolished arrhythmias induced by isoprenaline.Conclusions and ImplicationsRanolazine and GS‐967 reduced septal myocardium tension during simulated exercise in vitro and therefore have the potential to ameliorate symptoms caused by inducible obstruction in HCM patients, with some advantages over disopyramide and β‐blockers.