Afterdepolarizations Research Articles

The hERG PAS domain: A major regulator of IKr and a promising anti-arrhythmic drug target.

The hERG1 potassium channel conducts the critical cardiac repolarizing current IKr. Given its role in mediating electrical excitability, several groups have investigated the antiarrhythmic capacity of hERG1 activators in cardiac diseases marked by prolonged action potential duration (APD), such as long QT Syndrome. hERG1 activators have had limited success due to their off-target effects. We recently generated a single chain variable fragment (scFv) antibody, scFv2.10, that selectively binds the hERG1 Per-Arnt-Sim (PAS) domain. The PAS domain binds to the C-terminal cyclic nucleotide binding homology domain (CNBHD) and slows channel activation and promotes inactivation. scFv2.10 disrupts the PAS-CNBHD interaction, which in turn causes a two-fold increase in the time course of hERG1 activation and inactivation recovery, thereby increasing hERG1 current in HEK293 cells. In human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), disrupting the PAS-CNBHD interaction with scFv2.10 increases IKr and shortens APD. We hypothesized that the hERG1 PAS domain could represent a novel therapeutic target for diseases of impaired cardiac repolarization. To test this, we measured the antiarrhythmic capacity of scFv2.10 on a novel hiPSC-CM line derived from a patient with Jervell and Lange Nielsen syndrome (JLN). JLN syndrome is a severe form of long QT Syndrome Type 1 characterized by increased APD and incidence of sudden cardiac death. Compared to hiPSC-CMs derived from a healthy genetic background, JLN hiPSC-CMs display hallmarks of pro-arrhythmia including prolonged APD, increased action potential variability, and early afterdepolarizations. scFv2.10 reduced APD, action potential variability, and the incidence of early afterdepolarizations in JLN hiPSC-CMs, compared to GFP controls. These data demonstrate that the hERG1 PAS domain is a potential therapeutic target to treat disorders of electrical excitability.

Physiological role of SK channels in modulating cardiac repolarization: APD and dispersion at slow heart rate in long QT syndrome.

Long QT syndrome (LQTS) is a congenital disease associated with polymorphic ventricular tachycardia (pVT) and sudden cardiac death. The hallmarks of LQTS are early afterdepolarizations that initiate pVT and dispersion of repolarization allowing reentry formation. Small conductance Ca2+-activated K+ (SK) channels are directly gated by Ca2+ and provide feedback from Ca2+ to membrane voltage to accelerate cardiac repolarization, which may help normalizing LQTS. Using optical mapping, we examined the role of ISK in transgenic rabbit models of LQT1, LQT2, and LQT5 and found that enhancing ISK with NS309 consistently shortened APD, reduced APD dispersion, and suppressed pVT genesis universally in the three LQT rabbit models. Enhancement of ISK has a greater impact at longer cycle length, flattening restitution curves at slow heart rate where most pVTs occur in LQTS. We incorporated a six state ISK gating simulation combined with intrinsic rectification and divalent block into the Mahajan rabbit action potential model to investigate the role of ISK in suppressing LQT-related arrhythmias. Increasing levels of SK channel conductance (gSK) were able to shorten and reduce APD dispersion simulated by conductance gradients of gKs by ∼64% under LQT2 conditions and gtof by ∼70% in LQT1 conditions. Computer modeling indicates that ISK can reduce APD dispersion through two mechanisms; increased Ca2+ duration in the prolonged APD cells augment ISK to shorten long APD and lowering plateau Vm by ISK masks IKs heterogeneity by reducing IKs's role in repolarization. In conclusion, the Ca2+-dependent gating of ISK allows it to directly feedback on the APD especially when other K+ channels are not functional and to be a potential therapeutic target for LQTS.

Sophoridine manifests as a leading compound for anti-arrhythmia with multiple ion-channel blocking effects

BackgroundSophoridine (SR) has shown the potential to be an antiarrhythmic agent. However, SR's electrophysiological properties and druggability research are relatively inadequate, which limits the development of SR as an antiarrhythmic candidate. PurposeTo facilitate the development process of SR as an antiarrhythmic candidate, we performed integrated studies on the electrophysiological properties of SR in vitro and ex vivo to gain more comprehensive insights into the multi-ion channel blocking effects of SR, which provided the foundation for the further drugability studies in antiarrhythmic and safety studies. Firstly, SR's electrophysiological properties and antiarrhythmic potentials were recorded and assessed at the cell and tissue levels by comprehensively integrating the patch clamp with the Electrical and Optical Mapping systems. Subsequently, the antiarrhythmic effects of SR were validated by aconitine and ouabain-induced arrhythmia in vivo. Finally, the safety of SR as an antiarrhythmic candidate compound was evaluated based on the guidelines of the Comprehensive in Vitro Proarrhythmia Assay (CiPA). Study designThe antiarrhythmic effect of SR was evaluated at the in vitro, ex vivo, and in vivo levels. MethodsIsolated primary cardiomyocytes and stable cell lines were prepared to explore the electrophysiologic properties of being a multiple ion-channel blocker in vitro by whole-cell patch clamp. Using electrical and optical mapping, the negative chronotropic effect of SR was determined in langendorff-perfused rat or guinea-pig hearts.The antiarrhythmic activity of SR was assessed by the ex vivo tachyarrhythmia models induced by left coronary artery ligation (LCAL) and isoproterenol (ISO). Canonical models of aconitine and ouabain-induced arrhythmia were used to verify the antiarrhythmic effects in vivo. Finally, the pro-arrhythmic risk of SR was detected in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hSCCMs) using a Microelectrode array (MEA). ResultsSingle-cell patch assay validated the multiple ion-channel blockers of SR in transient outward current potassium currents (Ito), l-type calcium currents (ICa-l), and rapid activation delayed rectifier potassium currents (IKr). SR ex vivo depressed heart rates (HR) and ventricular conduction velocity (CV) and prolonged Q-T intervals in a concentration-dependent manner. Consistent with the changes in HRs, SR extended the active time of hearts and increased the action potential duration measured at 90% repolarization (APD90). SR could also significantly lengthen the onset time and curtail the duration of spontaneous ventricular tachycardia (VT) in the ex vivo arrhythmic model induced by LCAL. Meanwhile, SR could also significantly upregulate the programmed electrical stimulation (PES) frequency after the ISO challenge in forming electrical alternans and re-entrant excitation. Furthermore, SR exerted antiarrhythmic effects in the tachyarrhythmia models induced by aconitine and ouabain in vivo. Notably, the pro-arrhythmic risk of SR was shallow for a moderate inhibition of the human ether-à-go-go-related gene (hERG) channel. Moreover, SR prolonged field potential duration (FPDc) of hSCCMs in a concentration-dependent manner without early after depolarization (EAD) and arrhythmia occurrence. ConclusionOur results indicated that SR manifested as a multiple ion-channel blocker in the electrophysiological properties and exerts antiarrhythmic effects ex vivo and in vivo. Meanwhile, due to the low pro-arrhythmic risk in the hERG inhibition assay and the induction of EAD, SR has great potential as a leading candidate in the treatment of ventricular tachyarrhythmia.

Adverse reactions to antiarrhythmic drugs

There are various types of adverse reactions to antiarrhythmic drugs (AADs). Proarrhythmia, which refers to an exacerbation of the preexisting arrhythmia or occurrence of a new arrhythmia, may occur under the therapeutic concentration of an AAD. Bradyarrhythmia is the most common type of proarrhythmia due to AADs, and prior myocardial infarction and old age are known risk factors. Atrial flutter with 1:1 atrioventricular conduction usually occurs during rhythm control of atrial fibrillation with class IC AADs. QT prolongation due to AADs, mainly class III AADs, elevates the risk of torsade de pointes by triggered activity due to early afterdepolarization. The addition of clinical factors that promote QT prolongation, such as hypokalemia, hypomagnesemia, female sex, and bradycardia, increases the risk of developing torsade de pointes. Proarrhythmic monomorphic ventricular tachycardia usually occurs as a result of slow conduction and disparity of refractoriness due to class IC AADs. In patients with preexisting left ventricular systolic dysfunction or structural heart disease, the risk of hypotension or cardiogenic shock caused by negative inotropic effects due to AADs should be considered. To prevent these major adverse reactions to AADs, we need to understand the electrophysiologic properties of AADs in detail. Furthermore, the risk of proarrhythmia could be heightened by interplay with clinical factors, such as electrolyte unbalances, heart rate, and hepatic/renal or myocardial dysfunction. Sufficient awareness about drug-drug interactions, which may affect the metabolism of AADs, will improve patient safety during the management of arrhythmia.

Insights from an electro-mechanical heart failure cell model: Role of SERCA enhancement on arrhythmogenesis and myocyte contraction

Background and objectiveStructural and electrical remodeling in heart failure predisposes the heart to ventricular arrhythmias. Computer modeling approaches, used to complement experimental results, can provide a more mechanistic knowledge of the biophysical phenomena underlying cardiac pathologies. Indeed, previous in-silico studies have improved the understanding of the electrical correlates of heart failure involved in arrhythmogenesis; however, information on the crosstalk between electrical activity, intracellular Ca2+ and contraction is still incomplete. This study aims to investigate the electro-mechanical behavior of virtual failing human ventricular myocytes to help in the development of therapies, which should ideally target pump failure and arrhythmias at the same time. MethodsWe implemented characteristic remodeling of heart failure with reduced ejection fraction by including reported changes in ionic conductances, sarcomere function and cell structure (e.g. T-tubules disarray). Model parametrization was based on published experimental data and the outcome of simulations was validated against experimentally observed patterns. We focused on two aspects of myocardial dysfunction central in heart failure: altered force-frequency relationship and susceptibility to arrhythmogenic early afterdepolarizations. Because biological variability is a major problem in the generalization of in-silico findings based on a unique set of model parameters, we generated and evaluated a population of models. ResultsThe population-based approach is crucial in robust identification of parameters at the core of abnormalities and in generalizing the outcome of their correction. As compared to non-failing ones, failing myocytes had prolonged repolarization, a higher incidence of early afterdepolarizations, reduced contraction and a shallower force-frequency relationship, all features peculiar of heart failure. Component analysis applied to the model population identified reduced SERCA function as a relevant contributor to most of these derangements, which were largely reverted or diminished by restoration of SERCA function alone. ConclusionsThese simulated results encourage the development of strategies comprising SERCA stimulation and highlight the need to evaluate both electrical and mechanical outcomes.

Spatiotemporal patterns of early afterdepolarizations underlying abnormal T-wave morphologies in a tissue model of the Purkinje-ventricular system.

Sudden cardiac death (SCD) is a leading cause of death worldwide, and the majority of SCDs are caused by acute ventricular arrhythmias (VAs). Early afterdepolarizations (EADs) are an important trigger of VA under pathological conditions, e.g., inherited or acquired long QT syndrome (LQTS). However, it remains unclear how EAD events at the cellular level are spatially organized at the tissue level to induce and maintain ventricular arrhythmias and whether the spatial-temporal patterns of EADs at the tissue level are associated with abnormal T-wave morphologies that are often observed in LQTS, such as broad-based, notched or bifid; late appearance; and pointed T-waves. Here, a tissue model of the Purkinje-ventricular system (PVS) was developed to quantitatively investigate the complex spatial-temporal dynamics of EADs during T-wave abnormalities. We found that (1) while major inhibition of ICaL can substantially reduce the excitability of the PVS leading to conduction failures, moderate ICaL inhibition can promote occurrences of AP alternans at short cycle lengths (CLs), and EAD events preferentially occur with a major reduction of IKr (>50%) at long CLs; (2) with a minor reduction of ICaL, spatially synchronized steady-state EAD events with inverted and biphasic T-waves can be "weakened" into beat-to-beat concurrences of spatially synchronized EADs and T-wave alternans, and as pacing CLs increase, beat-to-beat concurrences of localized EADs with late-appearing and pointed T-wave morphologies can be observed; (3) under certain conditions, localized EAD events in the midmyocardium may trigger slow uni-directional electric propagation with inverted (antegrade) or upright (retrograde) broad-based T-waves; (4) spatially discordant EADs were typically characterized by desynchronized spontaneous onsets of EAD events between two groups of PVS tissues with biphasic T-wave morphologies, and they can evolve into spatially discordant oscillating EAD patterns with sustained or self-terminated alternating EAD and electrocardiogram (ECG) patterns. Our results provide new insights into the spatiotemporal aspects of the onset and development of EADs and suggest possible mechanistic links between the complex spatial dynamics of EADs and T-wave morphologies.

Alternating Early Afterdepolarizations Underlying Bradycardia-Dependent Macroscopic T Wave and Discordant Mechanical Alternans.

Macroscopic T wave alternans (macro-TWA) often heralds the onset of Torsades de Pointes in patients with QT prolongation. However, the mechanisms underlying macro-TWA remain unclear. We examined the cellular and ionic basis for macro-TWA in rabbits with left ventricular hypertrophy (LVH). The renovascular hypertension model was used to induce LVH in rabbits. Action potentials were simultaneously recorded from epicardium and endocardium together with a transmural ECG and isometric contractility in arterially perfused left ventricular wedges. Late sodium current (INa-L) was recorded in single-isolated left ventricular myocytes with the whole cell patch-clamp technique. Macro-TWA and accompanied mechanical alternans occurred spontaneously in 8 of 33 LVH rabbits (P<0.05, versus 0/15 in controls) and were induced by an INa-L enhancer ATX-II at 1 to 3 nM in additional 7. Macro-TWA and mechanical alternans occurred discordantly, that is, that longer QT interval and larger T wave were associated with weaker isometric contvractility. Alternating early afterdepolarizations in the endocardium caused macro-TWA in 12 of 15 LVH rabbits and, therefore, early afterdepolarization-dependent R-from-T extrasystoles and Torsades de Pointes always originated from the beats with longer QT and larger T wave during macro-TWA. INa-L density was significantly larger in LVH myocytes than that of control myocytes. Macro-TWA, mechanical alternans, R-from-T extrasystoles, and Torsades de Pointes were all abolished by INa-L blocker ranolazine or mexiletine. LVH enhances INa-L density and promotes alternating early afterdepolarizations in the left ventricular endocardium that manifest as macro-TWA with discordant mechanical alternans. INa-L blockade abolishes macro-TWA, mechanical alternans, early afterdepolarization-dependent R-from-T extrasystoles, and Torsades de Pointes.

Cardiac Piezo1 Exacerbates Lethal Ventricular Arrhythmogenesis by Linking Mechanical Stress with Ca2+ Handling After Myocardial Infarction.

Ventricular arrhythmogenesis is a key cause of sudden cardiac death following myocardial infarction (MI). Accumulating data show that ischemia, sympathetic activation, and inflammation contribute to arrhythmogenesis. However, the role and mechanisms of abnormal mechanical stress in ventricular arrhythmia following MI remain undefined. We aimed to examine the impact of increased mechanical stress and identify the role of the key sensor Piezo1 in ventricular arrhythmogenesis in MI. Concomitant with increased ventricular pressure, Piezo1, as a newly recognized mechano-sensitive cation channel, was the most up-regulated mechanosensor in the myocardium of patients with advanced heart failure. Piezo1 was mainly located at the intercalated discs and T-tubules of cardiomyocytes, which are responsible for intracellular calcium homeostasis and intercellular communication. Cardiomyocyte-conditional Piezo1 knockout mice (Piezo1Cko) exhibited preserved cardiac function after MI. Piezo1Cko mice also displayed a dramatically decreased mortality in response to the programmed electrical stimulation after MI with a markedly reduced incidence of ventricular tachycardia. In contrast, activation of Piezo1 in mouse myocardium increased the electrical instability as indicated by prolonged QT interval and sagging ST segment. Mechanistically, Piezo1 impaired intracellular calcium cycling dynamics by mediating the intracellular Ca2+ overload and increasing the activation of Ca2+-modulated signaling, CaMKII, and calpain, which led to the enhancement of phosphorylation of RyR2 and further increment of Ca2+ leaking, finally provoking cardiac arrhythmias. Furthermore, in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), Piezo1 activation remarkably triggered cellular arrhythmogenic remodeling by significantly shortening the duration of the action potential, inducing early afterdepolarization, and enhancing triggered activity.This study uncovered a proarrhythmic role of Piezo1 during cardiac remodeling, which is achieved by regulating Ca2+ handling, implying a promising therapeutic target in sudden cardiac death and heart failure.

Pacing Dynamics Determines the Arrhythmogenic Mechanism of the CPVT2-Causing CASQ2G112+5X Mutation in a Guinea Pig Ventricular Myocyte Computational Model.

Calsequestrin Type 2 (CASQ2) is a high-capacity, low-affinity, Ca2+-binding protein expressed in the sarcoplasmic reticulum (SR) of the cardiac myocyte. Mutations in CASQ2 have been linked to the arrhythmia catecholaminergic polymorphic ventricular tachycardia (CPVT2) that occurs with acute emotional stress or exercise can result in sudden cardiac death (SCD). CASQ2G112+5X is a 16 bp (339-354) deletion CASQ2 mutation that prevents the protein expression due to premature stop codon. Understanding the subcellular mechanisms of CPVT2 is experimentally challenging because the occurrence of arrhythmia is rare. To obtain an insight into the characteristics of this rare disease, simulation studies using a local control stochastic computational model of the Guinea pig ventricular myocyte investigated how the mutant CASQ2s may be responsible for the development of an arrhythmogenic episode under the condition of β-adrenergic stimulation or in the slowing of heart rate afterward once β-adrenergic stimulation ceases. Adjustment of the computational model parameters based upon recent experiments explore the functional changes caused by the CASQ2 mutation. In the simulation studies under rapid pacing (6 Hz), electromechanically concordant cellular alternans appeared under β-adrenergic stimulation in the CPVT mutant but not in the wild-type nor in the non-β-stimulated mutant. Similarly, the simulations of accelerating pacing from slow to rapid and back to the slow pacing did not display alternans but did generate early afterdepolarizations (EADs) during the period of second slow pacing subsequent acceleration of rapid pacing.

Modeling incomplete penetrance in long QT syndrome type 3 through ion channel heterogeneity: an in silico population study.

Many cardiac diseases are characterized by an increased late sodium current, including heart failure, hypertrophic cardiomyopathy, and inherited long QT syndrome type 3 (LQT3). The late sodium current in LQT3 is caused by a gain-of-function mutation in the voltage-gated sodium channel Nav1.5. Despite a well-defined genetic cause of LQT3, treatment remains inconsistent because of incomplete penetrance of the mutation and variability of antiarrhythmic efficacy. Here, we investigate the relationship between LQT3-associated mutation incomplete penetrance and variability in ion channel expression, simulating a population of 1,000 individuals with the O'Hara-Rudy model of the human ventricular myocyte. We first simulate healthy electrical activity (i.e., in the absence of a mutation) and then incorporate heterozygous expression for three LQT3-associated mutations (Y1795C, I1768V, and ΔKPQ), to directly compare the effects of each mutation on individuals across a diverse population. For all mutations, we find that susceptibility, defined by either the presence of an early afterdepolarization (EAD) or prolonged action potential duration (APD), primarily depends on the balance between the conductance of IKr and INa, for which individuals with a higher IKr-to-INa ratio are less susceptible. Furthermore, we find distinct differences across the population, observing individuals susceptible to zero, one, two, or all three mutations. Individuals tend to be less susceptible with an appropriate balance of repolarizing currents, typically via increased IKs or IK1. Interestingly, the more critical repolarizing current is mutation specific. We conclude that the balance between key currents plays a significant role in mutant-specific presentation of the disease phenotype in LQT3.NEW & NOTEWORTHY An in silico population approach investigates the relationship between variability in ion channel expression and gain-of-function mutations in the voltage-gated sodium channel associated with the congenital disorder long QT syndrome type 3 (LQT3). We find that ion channel variability can contribute to incomplete penetrance of the mutation, with mutant-specific differences in ion channel conductances leading to susceptibility to proarrhythmic action potential duration prolongation or early afterdepolarizations.

Sphingosine as a New Antifungal Agent against Candida and Aspergillus spp.

This study investigated whether sphingosine is effective as prophylaxis against Aspergillus spp. and Candida spp. In vitro experiments showed that sphingosine is very efficacious against A. fumigatus and Nakeomyces glabrataa (formerly named C. glabrata). A mouse model of invasive aspergillosis showed that sphingosine exerts a prophylactic effect and that sphingosine-treated animals exhibit a strong survival advantage after infection. Furthermore, mechanistic studies showed that treatment with sphingosine leads to the early depolarization of the mitochondrial membrane potential (Δψm) and the generation of mitochondrial reactive oxygen species and to a release of cytochrome C within minutes, thereby presumably initiating apoptosis. Because of its very good tolerability and ease of application, inhaled sphingosine should be further developed as a possible prophylactic agent against pulmonary aspergillosis among severely immunocompromised patients.

Association between Sugar Intake and an ECG Parameter: A Case Study on Young Athletes

In many recent studies, high sugar intake has been shown to have harmful effects on the cardiovascular system, especially in children. This study aimed to check the association between sugar intake and T-peak to T-end interval, an electrocardiogram (ECG) parameter associated with a higher risk of early afterdepolarizations and ventricular arrhythmias in young athletes. The study participants were 96 young athletes aged 7–15, 34 girls and 62 boys trained in various sports. ECGs were performed on all participants and analyzed for the duration of the period from the peak of the T-wave to the end of the T-wave. The data on years of training and training hours per week, type of sport, age, height, and weight were taken, as well as the child’s sugar intake per day. The cut-off was a minimum of 30 g of concentrated sugar in sweets or drinks. The group of children that consumed sugar had, on average, a significantly longer duration of the T-peak to T-end (TpTe) interval. There is an association between sugar intake and the duration of the T-peak to T-end (TpTe) interval. In our case study, it was demonstrated that the benefit of playing sports cannot compensate for the harm of overconsumption of sugar, so an important public health target would be to lower sugar intake in children’s diets (particularly in the diets of young athletes). It would be necessary to determine interventions and activities that would affect the reduction in sugar intake from early childhood. Aside from the fact that overconsumption of sugar is associated with human health issues, there are many environmental benefits of reducing sugar production and consumption.

Prediction of drug pro-arrhythmic cardiotoxicity using a semi-physiologically based pharmacokinetic model linked to cardiac ionic currents inhibition

Drug-induced torsades de pointes (TdP) risks are responsible for the withdrawal of many drugs from the market. Nowadays, assessments of drug-induced TdP risks are mainly based on maximum effective free therapeutic plasma concentration (EFTPCmax) and cardiac ionic current inhibitions using the human ventricular myocytes model (Tor-ORd model). Myocytes are targets of drug-induced TdP. The TdP risks may be directly linked to myocyte drug concentrations. We aimed to develop a semi-physiologically based pharmacokinetic (Semi-PBPK) model linked to cardiac ionic current inhibition (pharmacodynamics, PD) (Semi-PBPK-PD) to simultaneously predict myocyte drug concentrations and their TdP risks in humans. Alterations in action potential duration (ΔAPD90) were simulated using the Tor-ORd model and ionic current inhibition parameters based on myocyte or plasma drug concentrations. The predicted ΔAPD90 values were translated into in vivo alterations in QT interval(ΔQTc) induced by moxifloxacin, dofetilide, or sotalol. Myocyte drug concentrations of moxifloxacin, dofetilide, and sotalol gave better predictions of ΔQTc than plasma. Following validating the developed semi-PBPK-PD model, TdP risks of 37 drugs were assessed using ΔAPD90 and early afterdepolarization occurrence, which were estimated based on 10 × EFTPCmax and 10 × EFTMCmax (maximum effective free therapeutic myocyte concentration). 10 × EFTMCmax gave more sensitive and accurate predictions of pro-arrhythmic cardiotoxicity and the predicted TdP risks were also closer to clinic practice than 10 × EFTPCmax. In conclusion, pharmacokinetics and TdP risks of 37 drugs were successfully predicted using the semi-PBPK-PD model. Myocyte drug concentrations gave better predictions of ΔQTc and TdP risks than plasma.

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 [Cai] 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 [Cai] 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.

Spiral- and scroll-wave dynamics in mathematical models for canine and human ventricular tissue with varying Potassium and Calcium currents

We conduct a systematic, direct-numerical-simulation (DNS) study, in mathematical models for ventricular tissue, of the dependence of spiral- and scroll-wave dynamics on GKr, the maximal conductance of the delayed rectifier Potassium current (IKr) channel, and the parameter γCao, which determines the magnitude and shape of the current ICaL for the L-type calcium-current channel, in both square and anatomically realistic, whole-ventricle simulation domains. We study canine and human models. In the former, we use a canine-ventricular geometry, with fiber-orientation details, obtained from diffusion-tensor-magnetic-resonance-imaging (DTMRI) data; and we employ the physiologically realistic Hund-Rudy-Dynamic (HRD) model for a canine ventricular myocyte. To focus on the dependence of spiral- and scroll-wave dynamics on GKr and γCao, we restrict ourselves to an HRD-model parameter regime, which does not produce spiral- and scroll-wave instabilities because of other, well-studied causes like a very sharp action-potential-duration-restitution (APDR) curve or early after depolarizations (EADs) at the single-cell level. We find that spiral- or scroll-wave dynamics are affected predominantly by a simultaneous change in ICaL and IKr from their original values in the model, rather than by a change in any one of these currents; other currents do not have such a large effect on these wave dynamics in this parameter regime of the HRD model. In particular, we examine spiral-wave dynamics for ten different values of GKr and ten different values of γCao in our 2D DNSs. For our 3D DNSs in an anatomically realistic domain, we chose 16 parameter sets. In the parameter regime we begin with, the system displays broken spiral or scroll states with S1–S2 initial conditions (see below). We show that, by simultaneously increasing GKr and reducing γCao, we can get to a parameter regime in which the system displays single, stable rotating spirals or scroll waves. We obtain stability diagrams (or phase diagrams) in the GKr−γCao plane; and we find that these diagrams are significantly different in our 2D and 3D studies. In the 3D case, the geometry of the domain itself supports the confinement of the scroll waves and makes them more stable compared to their spiral-wave counterparts in our flat, 2D simulation domain. Thus, a combination of functional and geometrical mechanisms produce different dynamics for 3D scroll waves and their 2D spiral-wave counterparts. In particular, the former do not break easily because, in an anatomically realistic ventricular geometry, they are not easily absorbed at boundaries, nor do they break near boundaries. We have also carried out a comparison of our HRD results with their counterparts for the human-ventricular TP06 model; and we have found important differences between wave dynamics in these two models. The region in parameter space, where we obtain broken spiral or scroll waves in the HRD model is the region of stable rotating waves in the TP06 model; the default parameter values produce broken waves in the HRD model, but stable scrolls in the TP06 model. In both these models, to make a transition, (most simply, from broken-wave to stable-scroll states) we must simultaneously increase IKr and decrease ICaL; a modification of only one of these currents is not enough to effect this transition. Furthermore, the converse, i.e., an increase in ICaL along with a decrease in IKr does not yield any interesting dynamical transitions in the HRD model, for, in this range of currents, this model does not sustain spiral or scroll waves or broken waves.

Stochastic mixed-mode oscillations in the canards region of a cardiac action potential model

Mixed-mode oscillations (MMOs) and canards play important role in the dynamics of many mathematical models in the life sciences. In particular, pathological early afterdepolarizations in heart cells behavior can be associated with MMOs. The present paper aims to study a stochastic version of the reduced Luo–Rudy model of cardiac action potential. We show that in the parameter zone where the dynamics of the original deterministic system is characterized by canard-type limit cycles, weak additive noise can induce MMOs. Using stochastic sensitivity functions, Mahalanobis metrics, and confidence domains, we analyze the probabilistic mechanism of this stochastic phenomenon. Moreover, we find a critical value of the parameter corresponding to the “super-sensitive” cycle and show that for this value, stochastic splitting of oscillations is observed. Furthermore, we show that in the canard zone of the model, noise-driven transition to chaos occurs. To conclude, this paper reveals and analyzes new stochastic phenomena in the model of cardiac activity, moreover, the presented results can be helpful for studies of other complex models.

Effects of β-blockers on ventricular repolarization documented by 24-hour electrocardiography in long QT syndrome type 2

Long QT syndrome (LQTS) is an inherited arrhythmia disorder characterized by ventricular repolarization abnormalities and a risk of sudden cardiac death. The electrophysiological components generating the high risk of arrhythmias in LQTS are prolonged repolarization, increased dispersion of repolarization, and early afterdepolarizations, which are clinically estimated as QT interval, T-wave peak to T-wave end (TPE) interval, and T2/T1-wave amplitude ratio, respectively. In experimental LQTS type 2 (LQT2) models, β-blockers decrease dispersion of repolarization and prevent early afterdepolarizations. In clinical studies in patients with LQT2 , β-blockers are more effective against exercise-induced than arousal-induced cardiac events. The aim of the study was to investigate the effects of β-blocker therapy on repolarization properties in LQT2. QT and TPE intervals and maximal T2/T1-wave amplitude ratios recorded by 24-hour electrocardiograms before and during β-blocker therapy were evaluated in 25 patients with LQT2. β-Blocker therapy decreased the maximal T2/T1-wave amplitude ratio from 2.9 ± 1.1 to 1.8 ± 0.7 (P < .001), but did not change the pause-induced T2/T1-wave amplitude ratio. Under medication, abrupt maximal TPE intervals were shorter at heart rates of ≥75 beats/min and maximal QT intervals were shorter at a heart rate of 100 beats/min. β-Blockers stabilize ventricular repolarization in LQT2 by reducing electrocardiographic early afterdepolarizations and by reducing abrupt prolongation of electrocardiographic dispersion of repolarization and ventricular repolarization duration at elevated heart rates. The effect of β-blockers on pause-induced electrocardiographic early afterdepolarizations is weak. The findings provide electrocardiographic explanation for the protective effects of β-blockers against exercise-induced cardiac events in LQT2.

Aldehyde dehydrogenase 2 and arrhythmogenesis.

Cardiac arrhythmia is a common cardiovascular disease that leads to considerable economic burdens and significant global public health challenges. Despite the remarkable progress made in recent decades, antiarrhythmic therapy remains suboptimal. Aldehyde dehydrogenase 2 (ALDH2), a critical detoxifying enzyme, catalyzes toxic aldehydes and protects individuals from damage caused by oxidative stress. Accumulating evidence has demonstrated that ALDH2 activation has potential antiarrhythmic benefits. The correlation between ALDH2 deficiency and arrhythmogenesis has been widely recognized. In this review, we summarize recent research on the potential role of ALDH2 activation and antiarrhythmic protection, as well as the role played by the ALDH2∗2 polymorphism (rs671) in promoting arrhythmic risk. Additionally, we discuss important new findings illustrating the use of ALDH2 activators, which may prove to be promising antiarrhythmic therapy agents.

Disrupted CaV1.2 selectivity causes overlapping long QT and Brugada syndrome phenotypes in the CACNA1C-E1115K iPS cell model

A missense mutation in the α1c subunit of voltage-gated L-type Ca2+ channel-coding CACNA1C-E1115K, located in the Ca2+ selectivity site, causes a variety of arrhythmogenic phenotypes. We aimed to investigate the electrophysiological features and pathophysiological mechanisms of CACNA1C-E1115K in patient-specific induced pluripotent stem cell (iPSC)-derived cardiomyocytes (CMs). We generated iPSCs from a patient carrying heterozygous CACNA1C-E1115K with overlapping phenotypes of long QT syndrome, Brugada syndrome, and mild cardiac dysfunction. Electrophysiological properties were investigated using iPSC-CMs. We used iPSCs from a healthy individual and an isogenic iPSC line corrected using CRISPR-Cas9-mediated gene editing as controls. A mathematical E1115K-CM model was developed using a human ventricular cell model. Patch-clamp analysis revealed that E1115K-iPSC-CMs exhibited reduced peak Ca2+ current density and impaired Ca2+ selectivity with an increased permeability to monovalent cations. Consequently, E1115K-iPSC-CMs showed decreased action potential plateau amplitude, longer action potential duration (APD), and a higher frequency of early afterdepolarization compared with controls. In optical recordings examining the antiarrhythmic drug effect, late Na+ channel current (INaL) inhibitors (mexiletine and GS-458967) shortened APDs specifically in E1115K-iPSC-CMs. The AP-clamp using a voltage command obtained from E1115K-iPSC-CMs with lower action potential plateau amplitude and longer APD confirmed the upregulation of INaL. An in silico study recapitulated the invitro electrophysiological properties. Our iPSC-based analysis in CACNA1C-E1115K with disrupted CaV1.2 selectivity demonstrated that the aberrant currents through the mutant channels carried by monovalent cations resulted in specific action potential changes, which increased endogenous INaL, thereby synergistically contributing to the arrhythmogenic phenotype.

Changes in cellular Ca2+ and Na+ regulation during the progression towards heart failure.

In adapting to disease and loss of tissue, the heart shows great phenotypic plasticity that involves changes to its structure, composition and electrophysiology. Together with parallel whole body cardiovascular adaptations, the initial decline in cardiac function resulting from the insult is compensated. However, in the long term, the heart muscle begins to fail and patients with this condition have a very poor prognosis, with many dying from disturbances of rhythm. The surviving myocytes of these hearts gain Na+ , which is positively inotropic because of alterations to Ca2+ fluxes mediated by the Na+ /Ca2+ exchange, but compromises Ca2+ -dependent energy metabolism in mitochondria. Uptake of Ca2+ into the sarcoplasmic reticulum (SR) is reduced because of diminished function of SR Ca2+ ATPases. The result of increased Ca2+ influx and reduced SR Ca2+ uptake is an increase in the diastolic cytosolic Ca2+ concentration, which promotes spontaneous SR Ca2+ release and induces delayed afterdepolarisations. Action potential duration prolongs because of increased late Na+ current and changes in expression and function of other ion channels and transporters increasing the probability of the formation of early afterdepolarisations. There is a reduction in T-tubule density and so the normal spatial arrangements required for efficient excitation-contraction coupling are compromised and lead to temporal delays in Ca2+ release from the SR. Therefore, the structural and electrophysiological responses that occur to provide compensation do so at the expense of (1) increasing the likelihood of arrhythmogenesis; (2) activating hypertrophic, apoptotic and Ca2+ signalling pathways; and (3) decreasing the efficiency of SR Ca2+ release.