Role of sarcoplasmic reticulum calcium in development of secondary calcium rise and early afterdepolarizations in long QT syndrome rabbit model

Sodium–Glucose cotransporter 2 inhibitor empagliflozin decreases ventricular arrhythmia susceptibility by alleviating electrophysiological remodeling post-myocardial-infarction in mice

Background: Recent clinical trials indicate that sodium–glucose cotransporter 2 (SGLT2) inhibitors improve cardiovascular outcomes in myocardial infarction (MI) patients, but the underlying mechanisms remain unknown. As arrhythmia often occurs during myocardial infarction, it is the main cause of death.

Objective: The purpose of this study was to investigate the influence of empagliflozin (EMPA), an SGLT2 inhibitor, on cardiac electrophysiological remodeling and arrhythmia susceptibility of myocardial infarction mice.

Methods: ECG was obtained from mice 1 week after MI to determine the QT interval. In an electrophysiological study and optical mapping was performed to evaluate the function of EMPA and underlying mechanisms of post-myocardial-infarction in mice.

Results: EMPA treatment significantly reduced the QT interval of MI mice (MI + EMPA 50.24 ms vs. MI 64.68 ms). The membrane potential and intracellular Ca [Ca_i] were mapped from 13 MI hearts and five normal hearts using an optical mapping technique. A dynamic pacing protocol was used to determine action potential duration and [Ca_i] at baseline and after EMPA (10 umol/L) infusion. EMPA perfusion did not change the APD₈₀ and CaT₈₀ in normal ventricles while shortening them in an infarct zone, bordering zone, and remote zone of MI hearts at 200 ms, 150 ms, 120 ms, and 100 ms pacing cycle length. The conduction velocity of infarcted ventricles was 0.278 m/s and 0.533 m/s in normal ventricles at baseline (p < 0.05). After EMPA administration, the conduction velocity of infarcted ventricles increased to 0.363 m/s, whereas no significant changes were observed in normal ventricles. The action potential rise time, CaT rise time, and CaT tau time were improved after EMPA perfusion in infarcted ventricles, whereas no significant changes were observed in normal ventricles. EMPA decreases early afterdepolarizations premature ventricular beats, and ventricular fibrillation (VF) in infarcted ventricles. The number of phase singularities (baseline versus EMPA, 6.26 versus 3.25), dominant frequency (20.52 versus 10.675 Hz), and ventricular fibrillation duration (1.072 versus 0.361 s) during ventricular fibrillation in infarcted ventricles were all significantly decreased by EMPA.

Conclusion: Treatment with EMPA improved post-MI electrophysiological remodeling and decreased substrate for VF of MI mice. The inhibitors of SGLT2 may be a new class of agents for the prevention of ventricle arrhythmia after chronic MI.

Bifurcations and Proarrhythmic Behaviors in Cardiac Electrical Excitations

The heart is a hierarchical dynamic system consisting of molecules, cells, and tissues, and acts as a pump for blood circulation. The pumping function depends critically on the preceding electrical activity, and disturbances in the pattern of excitation propagation lead to cardiac arrhythmia and pump failure. Excitation phenomena in cardiomyocytes have been modeled as a nonlinear dynamical system. Because of the nonlinearity of excitation phenomena, the system dynamics could be complex, and various analyses have been performed to understand the complex dynamics. Understanding the mechanisms underlying proarrhythmic responses in the heart is crucial for developing new ways to prevent and control cardiac arrhythmias and resulting contractile dysfunction. When the heart changes to a pathological state over time, the action potential (AP) in cardiomyocytes may also change to a different state in shape and duration, often undergoing a qualitative change in behavior. Such a dynamic change is called bifurcation. In this review, we first summarize the contribution of ion channels and transporters to AP formation and our knowledge of ion-transport molecules, then briefly describe bifurcation theory for nonlinear dynamical systems, and finally detail its recent progress, focusing on the research that attempts to understand the developing mechanisms of abnormal excitations in cardiomyocytes from the perspective of bifurcation phenomena.

Recognition, Prevention, and Management of Arrhythmias and Autonomic Disorders in Cardio-Oncology: A Scientific Statement From the American Heart Association

With the advent of novel cancer therapeutics and improved screening, more patients are surviving a cancer diagnosis or living longer with advanced disease. Many of these treatments have associated cardiovascular toxicities that can manifest in both an acute and a delayed fashion. Arrhythmias are an increasingly identified complication with unique management challenges in the cancer population. The purpose of this scientific statement is to summarize the current state of knowledge regarding arrhythmia identification and treatment in patients with cancer. Atrial tachyarrhythmias, particularly atrial fibrillation, are most common, but ventricular arrhythmias, including those related to treatment-induced QT prolongation, and bradyarrhythmias can also occur. Despite increased recognition, dedicated prospective studies evaluating true incidence are lacking. Moreover, few studies have addressed appropriate prevention and treatment strategies. As such, this scientific statement serves to mobilize the cardio-oncology, electrophysiology, and oncology communities to develop clinical and scientific collaborations that will improve the care of patients with cancer who have arrhythmias.

Electrocardiographic Changes Associated With Ibrutinib Exposure

Although ibrutinib-associated atrial and ventricular arrhythmias have been well described, there is little information about ibrutinib's effects on other electrocardiographic parameters, particularly the QT interval. Using our database of 137 patients treated with ibrutinib, we retrospectively identified 21 patients in whom an electrocardiogram (ECG) was obtained both prior to and after ibrutinib exposure. All traditional ECG parameters as well as QT dispersion were manually measured by an electrophysiologist. Compared to baseline ECGs, post ibrutinib ECGs demonstrated QT interval shortening from 386 ms to 356 ms (P = .007), corrected QT interval shortening using Bazett's formula from 446 ms to 437 ms (P = .04), and corrected QT interval shortening using Fridericia's formula from 425 ms to 407 ms (P = .003). QT dispersion also increased post ibrutinib exposure compared to baseline (39.8 ms vs 57.3 ms, P = .002). There was no significant change in other ECG parameters. In conclusion, both the absolute and corrected QT intervals significantly shortened after ibrutinib exposure, while there was a significant increase in QT dispersion. These findings may point to a common underlying electrophysiologic mechanism of ibrutinib-associated arrhythmias.

A greedy classifier optimization strategy to assess ion channel blocking activity and pro-arrhythmia in hiPSC-cardiomyocytes

Novel studies conducting cardiac safety assessment using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are promising but might be limited by their specificity and predictivity. It is often challenging to correctly classify ion channel blockers or to sufficiently predict the risk for Torsade de Pointes (TdP). In this study, we developed a method combining in vitro and in silico experiments to improve machine learning approaches in delivering fast and reliable prediction of drug-induced ion-channel blockade and proarrhythmic behaviour. The algorithm is based on the construction of a dictionary and a greedy optimization, leading to the definition of optimal classifiers. Finally, we present a numerical tool that can accurately predict compound-induced pro-arrhythmic risk and involvement of sodium, calcium and potassium channels, based on hiPSC-CM field potential data.

Multiple Dynamical Mechanisms of Phase-2 Early Afterdepolarizations in a Human Ventricular Myocyte Model: Involvement of Spontaneous SR Ca 2+ Release

Early afterdepolarization (EAD) is known to cause lethal ventricular arrhythmias in long QT syndrome (LQTS). In this study, dynamical mechanisms of EAD formation in human ventricular myocytes (HVMs) were investigated using the mathematical model developed by ten Tusscher and Panfilov (Am J Physiol Heart Circ Physiol 291, 2006). We explored how the rapid (IKr) and slow (IKs) components of delayed-rectifier K+ channel currents, L-type Ca2+ channel current (ICa L), Na+/Ca2+ exchanger current (INCX), and intracellular Ca2+ handling via the sarcoplasmic reticulum (SR) contribute to initiation, termination and modulation of phase-2 EADs during pacing in relation to bifurcation phenomena in non-paced model cells. Parameter-dependent dynamical behaviors of the non-paced model cell were determined by calculating stabilities of equilibrium points (EPs) and limit cycles, and bifurcation points to construct bifurcation diagrams. Action potentials (APs) and EADs during pacing were reproduced by numerical simulations for constructing phase diagrams of the paced model cell dynamics. Results are summarized as follows: (1) A modified version of the ten Tusscher-Panfilov model with accelerated ICaL inactivation could reproduce bradycardia-related EADs in LQTS type 2 and β-adrenergic stimulation-induced EADs in LQTS type 1. (2) Two types of EADs with different initiation mechanisms, ICaL reactivation-dependent and spontaneous SR Ca2+ release-mediated EADs, were detected. (3) Termination of EADs (AP repolarization) during pacing depended on the slow activation of IKs. (4) Spontaneous SR Ca2+ releases occurred at higher Ca2+ uptake rates, attributable to the instability of steady-state intracellular Ca2+ concentrations. Dynamical mechanisms of EAD formation and termination in the paced model cell are closely related to stability changes (bifurcations) in dynamical behaviors of the non-paced model cell, but they are model-dependent. Nevertheless, the modified ten Tusscher-Panfilov model would be useful for systematically investigating possible dynamical mechanisms of EAD-related arrhythmias in LQTS.

Paradoxical Effects of Sodium-Calcium Exchanger Inhibition on Torsade de Pointes and Early Afterdepolarization in a Heart Failure Rabbit Model

Calcium homeostasis plays an important role in development of early afterdepolarizations (EADs) and torsade de pointes (TdP). The role of sodium-calcium exchanger (NCX) inhibition in genesis of secondary Ca rise and EAD-TdP is still debated. Dual voltage and intracellular Ca optical mapping were conducted in 6 control and 9 failing rabbit hearts. After baseline electrophysiological and optical mapping studies, E4031 was given to simulate long QT syndrome. ORM-10103 was then administrated to examine the electrophysiological effects on EAD-TdP development. E4031 enhanced secondary Ca rise, EADs development, and TdP inducibility in both control and failing hearts. The results showed that ORM-10103 reduced premature ventricular beats but was unable to suppress the inducibility of TdP or EADs. The electrophysiological effects of ORM-10103 included prolongation of action potential duration (APD) and increased APD heterogeneity in failing hearts. ORM-10103 had a neutral effect on the amplitude of secondary Cai rise in control and heart failure groups. In this model, most EADs generated from long-short APD junction area. In conclusion, highly selective NCX inhibition with ORM-10103 reduced premature ventricular beat burden but was unable to suppress secondary Ca rise, EADs development, or inducibility of TdP. The possible electrophysiological mechanisms include APD prolongation and increased APD heterogeneity.

Simulation study of the ionic mechanisms underlying Torsade de Pointes in a 2D cardiac tissue

BACKGROUND

To understand the ionic mechanism behind the genesis of Torsade de Pointes (TdP) occurring with long QT syndrome 2 (LQTS2) in a remodelled transmural tissue.

METHODS

The TP06 model is used to simulate the electrical activity of cells in a 2D transmural ventricular model. LQTS2 is realised by reducing the potassium current (I_Kr) to 0.5 in each cell. Each cell of the tissue is remodelled by increasing the conductance of calcium current (G_CaL). The above two factors make the cells prone to early after depolarizations (EADs) development. The rise in G_CaL that can develop a sustained TdP at normal pacing rate is determined from this study. A look at the calcium dynamics, sodium-calcium exchanger current (I_NaCa) and slow delayed rectifier potassium current (I_Ks) distribution maps of the tissue helps us in analysing the mechanism of TdP generation.

RESULTS

A TdP type pattern at normal pacing rate is generated when G_CaL is more than 3.5 times the control parameter. From the M-cell island, an adequate number of cells spontaneously release calcium from their sarcoplasmic reticulum leading to increased intracellular calcium and inward sodium current through the sodium calcium exchanger current (I_NaCa). These contribute to the development of EADs which create a depolarising wavefront that triggers TdP in the tissue. When G_CaL is less than 3.5 times the control value, premature ventricular complexes (PVC) occur interspersed between normal beats.

CONCLUSION

Normal pacing rates can induce a multi focal TdP when sufficient number of M-cells simultaneously undergo spontaneous calcium release (SCR) events.

A model of cardiac ryanodine receptor gating predicts experimental Ca2+-dynamics and Ca2+-triggered arrhythmia in the long QT syndrome

Abnormal Ca²⁺ handling is well-established as the trigger of cardiac arrhythmia in catecholaminergic polymorphic ventricular tachycardia and digoxin toxicity, but its role remains controversial in Torsade de Pointes (TdP), the arrhythmia associated with the long QT syndrome (LQTS). Recent experimental results show that early afterdepolarizations (EADs) that initiate TdP are caused by spontaneous (non-voltage-triggered) Ca²⁺ release from Ca²⁺-overloaded sarcoplasmic reticulum (SR) rather than the activation of the L-type Ca²⁺-channel window current. In bradycardia and long QT type 2 (LQT2), a second, non-voltage triggered cytosolic Ca²⁺ elevation increases gradually in amplitude, occurs before overt voltage instability, and then precedes the rise of EADs. Here, we used a modified Shannon-Puglisi-Bers model of rabbit ventricular myocytes to reproduce experimental Ca²⁺ dynamics in bradycardia and LQT2. Abnormal systolic Ca²⁺-oscillations and EADs caused by SR Ca²⁺-release are reproduced in a modified 0-dimensional model, where 3 gates in series control the ryanodine receptor (RyR2) conductance. Two gates control RyR2 activation and inactivation and sense cytosolic Ca²⁺ while a third gate senses luminal junctional SR Ca²⁺. The model predicts EADs in bradycardia and low extracellular [K⁺] and cessation of SR Ca²⁺-release terminate salvos of EADs. Ca²⁺-waves, systolic cell-synchronous Ca²⁺-release, and multifocal diastolic Ca²⁺ release seen in subcellular Ca²⁺-mapping experiments are observed in the 2-dimensional version of the model. These results support the role of SR Ca²⁺-overload, abnormal SR Ca²⁺-release, and the subsequent activation of the electrogenic Na⁺/Ca²⁺-exchanger as the mechanism of TdP. The model offers new insights into the genesis of cardiac arrhythmia and new therapeutic strategies.

Study of factors affecting the progression and termination of drug induced Torsade de pointes in two dimensional cardiac tissue

INTRODUCTION

To study the conditions leading to the initiation and termination of drug induced Torsade de pointes (TdP) along with QT prolongation.

METHODS

A 2D anisotropic transmural section of the ventricular myocardium is modeled using the TP06 equations and the cells are interconnected with gap junction conductances (GJC). The tissue is remodeled by reducing the repolarization reserve (by increasing calcium current (I_CaL)) of all cells thus making them vulnerable to development of early after depolarizations (EADs).

RESULTS

Clinical risk conditions like decreased potassium current (I_Kr), bradycardia, hypokalemia and short-long-short (SLS) triggering sequences are included in the tissue. A pseudo-electrocardiogram is created to realize the intensity of remodeling required in presence of risk factors to initiate TdP. On initiating TdP, the effect of increasing GJC and decreasing I_CaL is shown to terminate a non-self-limiting TdP.

CONCLUSION

Without the inclusion of underlying increase in I_CaL along with risk factors, TdP cannot be initiated.