Action Potential Duration Alternans Research Articles

Activation of PKC affects the ventricular restitution properties and arrhythmogenesis through L-type Ca+ current.

To investigate the role of protein kinase C (PKC) in action potential duration (APD) restitution and ventricular tachyarrhythmias (VAs). Rabbits hearts were isolated and prepared for Langendorff perfusion technique. The stimuli-extra-stimulus (S1-S2) method and dynamic S1 pacing protocol were performed to construct APD restitution and to induce APD alternans or VA, respectively, at 10 sites throughout the ventricular chamber. Administration of phorbol-12-myristate-13-acetate (PMA) (100nM) (n=15) greatly steepened the restitution curves (Smax>1) (p<.01) at each site compared to the control group (n=15). Furthermore, treatment with PMA also induced larger spatial dispersions of Smax (p<.05) and decreased the thresholds of the VA and APD alternans (p<.01). However, perfused with the PKC inhibitor, bisindolylmaleimide (BIM) (500nM) (n=10), reversibly flattened the APD restitution curves at each site (Smax<1), decreased the spatial dispersions of Smax, and increased the thresholds of APD alternans and VA. According to the results of patch-clamp, peak amplitude of L-type Ca2+ current was significantly increased by addition of PMA compared with control (CTL) group (p<.05). Antagonize this current with verapamil (n=10) can fully inhibited the PMA induced increasing of Smax and inducibility of VA and alternans. PKC activation increased the dispersion of APD restitution and thus led to occurrence of VA, which possibly related to the increased Ca2+ influx.

RyR2 inhibition with dantrolene is antiarrhythmic, prevents further pathological remodeling, and improves cardiac function in chronic ischemic heart disease

Diastolic Ca2+ leak due to cardiac ryanodine receptor (RyR2) hyperactivity has been widely documented in chronic ischemic heart disease (CIHD) and may contribute to ventricular tachycardia (VT) risk and progressive left-ventricular (LV) remodeling. Here we test the hypothesis that targeting RyR2 hyperactivity can suppress VT inducibility and progressive heart failure in CIHD by the RyR2 inhibitor dantrolene. Methods and resultsCIHD was induced in C57BL/6 J mice by left coronary artery ligation. Four weeks later, mice were randomized to either acute or chronic (6 weeks via implanted osmotic pump) treatment with dantrolene or vehicle. VT inducibility was assessed by programmed stimulation in vivo and in isolated hearts. Electrical substrate remodeling was assessed by optical mapping. Ca2+ sparks and spontaneous Ca2+ releases were measured in isolated cardiomyocytes. Cardiac remodeling was quantified by histology and qRT-PCR. Cardiac function and contractility were measured using echocardiography. Compared to vehicle, acute dantrolene treatment reduced VT inducibility. Optical mapping demonstrated reentrant VT prevention by dantrolene, which normalized the shortened refractory period (VERP) and prolonged action potential duration (APD), preventing APD alternans. In single CIHD cardiomyocytes, dantrolene normalized RyR2 hyperactivity and prevented spontaneous intracellular Ca2+ release. Chronic dantrolene treatment not only reduced VT inducibility but also reduced peri-infarct fibrosis and prevented further progression of LV dysfunction in CIHD mice. ConclusionsRyR2 hyperactivity plays a mechanistic role for VT risk, post-infarct remodeling, and contractile dysfunction in CIHD mice. Our data provide proof of concept for the anti-arrhythmic and anti-remodeling efficacy of dantrolene in CIHD.

Calcium- and voltage-driven atrial alternans: Insight from [Ca]i and Vm asynchrony.

Cardiac alternans is defined as beat-to-beat alternations in contraction strength, action potential duration (APD), and Ca transient (CaT) amplitude. Cardiac excitation-contraction coupling relies on the activity of two bidirectionally coupled excitable systems, membrane voltage (Vm ) and Ca release. Alternans has been classified as Vm - or Ca-driven, depending whether a disturbance of Vm or [Ca]i regulation drives the alternans. We determined the primary driver of pacing induced alternans in rabbit atrial myocytes, using combined patch clamp and fluorescence [Ca]i and Vm measurements. APD and CaT alternans are typically synchronized; however, uncoupling between APD and CaT regulation can lead to CaT alternans in the absence of APD alternans, and APD alternans can fail to precipitate CaT alternans, suggesting a considerable degree of independence of CaT and APD alternans. Using alternans AP voltage clamp protocols with extra APs showed that most frequently the pre-existing CaT alternans pattern prevailed after the extra-beat, indicating that alternans is Ca-driven. In electrically coupled cell pairs, dyssynchrony of APD and CaT alternans points to autonomous regulation of CaT alternans. Thus, with three novel experimental protocols, we collected evidence for Ca-driven alternans; however, the intimately intertwined regulation of Vm and [Ca]i precludes entirely independent development of CaT and APD alternans.

Cellular and electrophysiological characterization of triadin knockout syndrome using induced pluripotent stem cell-derived cardiomyocytes.

Triadin knockout syndrome (TKOS) is a malignant arrhythmia disorder caused by recessive null variants in TRDN-encoded cardiac triadin. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from two unrelated TKOS patients and an unrelated control. CRISPR-Cas9 gene editing was used to insert homozygous TRDN-p.D18fs∗13 into a control line to generate a TKOS model (TRDN-/-). Western blot confirmed total knockout of triadin in patient-specific and TRDN-/- iPSC-CMs. iPSC-CMs from both patients revealed a prolonged action potential duration (APD) at 90% repolarization, and this was normalized by protein replacement of triadin. APD prolongation was confirmed in TRDN-/- iPSC-CMs. TRDN-/- iPSC-CMs revealed that loss of triadin underlies decreased expression and co-localization of key calcium handling proteins, slow and decreased calcium release from the sarcoplasmic reticulum, and slow inactivation of the L-type calcium channel leading to frequent cellular arrhythmias, including early and delayed afterdepolarizations and APD alternans.

Mechanistic insights into spontaneous transition from cellular alternans to ventricular fibrillation.

T-wave alternans (TWA) has been used for predicting the risk of malignant cardiac arrhythmias and sudden cardiac death (SCD) in multiple clinical settings; however, possible mechanism(s) underlying the spontaneous transition from cellular alternans reflected by TWA to arrhythmias in impaired repolarization remains unclear. The healthy guinea pig ventricular myocytes under E-4031 blocking IKr (0.1μM, N=12; 0.3μM, N=10; 1μM, N=10) were evaluated using whole-cell patch-clamp. The electrophysiological properties of isolated perfused guinea pig hearts under E-4031 (0.1μM, N=5; 0.3μM, N=5; 1μM, N=5) were evaluated using dual- optical mapping. The amplitude/threshold/restitution curves of action potential duration (APD) alternans and potential mechanism(s) underlying the spontaneous transition of cellular alternans to ventricular fibrillation (VF) were examined. There were longer APD80 and increased amplitude and threshold of APD alternans in E-4031 group compared with baseline group, which was reflected by more pronounced arrhythmogenesis at the tissue level, and were associated with steep restitution curves of the APD and the conduction velocity (CV). Conduction of AP alternans augmented tissue's functional spatiotemporal heterogeneity of regional AP/Ca alternans, as well as the AP/Ca dispersion, leading to localized uni-directional conduction block that spontaneous facilitated the formation of reentrant excitation waves without the need for additional premature stimulus. Our results provide a possible mechanism for the spontaneous transition from cardiac electrical alternans in cellular action potentials and intercellular conduction without the involvement of premature excitations, and explain the increased susceptibility to ventricular arrhythmias in impaired repolarization. In this study, we implemented voltage-clamp and dual-optical mapping approaches to investigate the underlying mechanism(s) for the arrhythmogenesis of cardiac alternans in the guinea pig heart at cellular and tissue levels. Our results demonstrated a spontaneous development of reentry from cellular alternans, arising from a combined actions of restitution properties of action potential duration, conduction velocity of excitation wave and interplay between alternants of action potential and the intracellular Ca handling. We believe this study provides new insights into underlying the mechanism, by which cellular cardiac alternans spontaneously evolves into cardiac arrhythmias.

Effects of hydroxychloroquine on atrial electrophysiology in in silico wild-type and PITX2+/- atrial cardiomyocytes.

Hydroxychloroquine (HCQ) is commonly used in the treatment of autoimmune diseases and increases the risk of QT interval prolongation. However, it is unclear how HCQ affects atrial electrophysiology and the risk of atrial fibrillation (AF). We quantitatively examined the potential atrial arrhythmogenic effects of HCQ on AF using acomputational model of human atrial cardiomyocytes. We measured atrial electrophysiological markers after systematically varying HCQ concentrations. The HCQ concentrations were positively correlated with the action potential duration (APD), resting membrane potential, refractory period, APD alternans threshold, and calcium transient alternans threshold (p < 0.05). By contrast, HCQ concentrations were inversely correlated with the maximum upstroke velocity and calcium transient amplitude (p < 0.05). When the therapeutic concentration (Cmax) of HCQ was applied, HCQ increased APD90 by 1.4% in normal sinus rhythm, 1.8% in wild-type AF, and 2.6% in paired-like homeodomain transcription factor2 (PITX2)+/- AF, but did not affect the alternans thresholds. The overall in silico results suggest no significant atrial arrhythmogenic effects of HCQ at Cmax, instead implying apotential antiarrhythmic role of low-dose HCQ in AF. However, at an HCQ concentration of fourfold Cmax, arapid pacing rate of 4 Hz induced prominent APD alternans, particularly in the PITX2+/- AF model. Our in silico analysis suggests a potential antiarrhythmic role of low-dose HCQ in AF. Concomitant PITX2 mutations and high-dose HCQ treatments may increase the risk of AF, and this potential genotype/dose-dependent arrhythmogenic effect of HCQ should be investigated further.

Data-driven, patient-specific simulations of long-term heart rate variability and cardiac alternans formation.

Individuals suffering from chronic heart failure (CHF) have a greater risk for death mediated by ventricular arrhythmias, which may be related to changes in heart rate. Heart rate constantly fluctuates from beat-to-beat on a timescale ranging from seconds to hours under physiological conditions, termed heart rate variability (HRV). Individuals with CHF have reduced HRV and faster heart rates, both of which influence the formation of pro-arrhythmic alternans, a beat-to-beat alternation in the action potential duration (APD) and intracellular calcium (Ca) levels. It is unclear how long-term changes in heart rate influence alternans and arrhythmia formation. We obtained sequences of RR intervals (time between successive R waves in an electrocardiogram) from individuals with normal sinus rhythm (NSR) and CHF from a Physionet database. These RR-sequences are used to define the pacing period for a discrete-time coupled map model of APD and intracellular Ca cycling for a single cardiac myocyte, modified to account for human heart rates and pathological Ca-remodeling in CHF. These modifications include fitting human ventricular myocyte APD restitution parameters, including a new term for Ca leak from the sarcoplasmic reticulum (SR), and scaling SR Ca-related parameters. Importantly, this minimal model enables simulation of long-term trends, on the order of 24 hours, in large populations. We find that the alternations in APD and Ca vary over time, such that there are periods with both small and large beat-to-beat differences in both populations. However, the CHF population tends to have a larger alternations magnitude, demonstrating that CHF-associated heart rates promotes more pro-arrhythmic phenomena. Future work will extend this methodology to determine what statistical properties govern the CHF RR-sequences, and further to test to what extent pro-arrhythmic alternans depends on CHF-related changes in heart rate or remodeling.

Activation of small conductance Ca2+ -activated K+ channels suppresses Ca2+ transient and action potential alternans in ventricular myocytes.

At the cellular level, cardiac alternans is observed as beat-to-beat alternations in contraction strength, action potential (AP) morphology and Ca2+ transient (CaT) amplitude, and is a risk factor for cardiac arrhythmia. The (patho)physiological roles of small conductance Ca2+ -activated K+ (SK) channels in ventricles are poorly understood. We tested the hypothesis that in single rabbit ventricular myocytes pharmacological modulation of SK channels plays a causative role for the development of pacing-induced CaT and AP duration (APD) alternans. SK channel blockers (apamin, UCL1684) had only a minor effect on AP repolarization. However, SK channel activation by NS309 resulted in significant APD shortening, demonstrating that functional SK channels are well expressed in ventricular myocytes. The effects of NS309 were prevented or reversed by apamin and UCL1684, indicating that NS309 acted on SK channels. SK channel activation abolished or reduced the degree of pacing-induced CaT and APD alternans. Inhibition of KV 7.1 (with HMR1556) and KV 11.1 (with E4031) channels was used to mimic conditions of long QT syndromes type-1 and type-2, respectively. Both HMR1556 and E4031 enhanced CaT alternans that was prevented by SK channel activation. In AP voltage-clamped cells the SK channel activator had no effect on CaT alternans, confirming that suppression of CaT alternans was caused by APD shortening. APD shortening contributed to protection from alternans by lowering sarcoplasmic reticulum Ca2+ content and curtailing Ca2+ release. The data suggest that SK activation could be a potential intervention to avert development of alternans with important ramifications for arrhythmia prevention and therapy for patients with long QT syndrome. KEY POINTS: At the cellular level, cardiac alternans is observed as beat-to-beat alternations in contraction strength, action potential (AP) morphology and intracellular Ca2+ release amplitude, and is a risk factor for cardiac arrhythmia. The (patho)physiological roles of small conductance Ca2+ -activated K+ (SK) channels in ventricles are poorly understood. We investigated whether pharmacological modulation of SK channels affects the development of cardiac alternans in normal ventricular cells and in cells with drug-induced long QT syndrome (LQTS). While SK channel blockers have only a minor effect on AP morphology, their activation leads to AP shortening and abolishes or reduces the degree of pacing-induced Ca2+ and AP alternans. AP shortening contributed to protection against alternans by lowering sarcoplasmic reticulum Ca2+ content and curtailing Ca2+ release. The data suggest SK activation as a potential intervention to avert the development of alternans with important ramifications for arrhythmia prevention for patients with LQTS.

Chronic catestatin treatment reduces atrial fibrillation susceptibility via improving calcium handling in post-infarction heart failure rats

BackgroundAbnormal Ca2+ handling is a pivotal element of atrial fibrillation (AF) substrates. Catestatin (CST) modulates intracellular Ca2+ handling in cardiomyocytes (CMs). We investigated the effects of CST administration on atrial Ca2+ handling and AF susceptibility in rats with post-infarction heart failure (HF). MethodsMyocardial infarction (MI) was established by ligation of the left anterior descending coronary artery in rats. Two-week later, rats with post-infarction HF were randomly treated with saline (MI group) or CST (MI + CST group) for 4-week. Cellular Ca2+ imaging was performed by incubating atrial CMs with Fura-2 AM. An in vitro electrophysiological study was performed to assess the vulnerability to action potential duration (APD) alternans and AF. Ca2+ handling proteins expression was determined using western blotting. ResultsIn atrial CMs, compared with the sham group, the sarcoplasmic reticulum (SR) Ca2+ load, Ca2+ transient (CaT) amplitude, and threshold for Ca2+ alternans were significantly decreased, but the diastolic intracellular Ca2+ level, SR Ca2+ leakage, and spontaneous Ca2+ events were markedly increased in the MI group. However, CST attenuated these Ca2+-handling abnormalities induced by post-infarction HF. Moreover, vulnerability to atrial APD alternans and AF was significantly increased in isolated hearts from the MI group compared to the sham group, whereas all effects were prevented by CST. CST treatment also preserved SR Ca2+-ATPase protein expression but decreased the protein levels of phosphorylated-ryanodine receptor 2 and phosphorylated-Ca2+/calmodulin-dependent protein kinase II in atria from post-infarction HF rats. ConclusionChronic CST treatment reduces AF vulnerability in rats with MI-induced HF by improving Ca2+ handling.

Sensitivity Analysis of Cardiac Alternans and Tachyarrhythmia to Ion Channel Conductance Using Population Modeling.

Action potential duration (APD) alternans, an alternating phenomenon between action potentials in cardiomyocytes, causes heart arrhythmia when the heart rate is high. However, some of the APD alternans observed in clinical trials occurs under slow heart rate conditions of 100 to 120 bpm, increasing the likelihood of heart arrhythmias such as atrial fibrillation. Advanced studies have identified the occurrence of this type of APD alternans in terms of electrophysiological ion channel currents in cells. However, they only identified physiological phenomena, such as action potential due to random changes in a particular ion channel's conductivity through ion models specializing in specific ion channel currents. In this study, we performed parameter sensitivity analysis via population modeling using a validated human ventricular physiology model to check the sensitivity of APD alternans to ion channel conductances. Through population modeling, we expressed the changes in alternans onset cycle length (AOCL) and mean APD in AOCL (AO meanAPD) according to the variations in ion channel conductance. Finally, we identified the ion channel that maximally affected the occurrence of APD alternans. AOCL and AO meanAPD were sensitive to changes in the plateau Ca2+ current. Accordingly, it was expected that APD alternans would be vulnerable to changes in intracellular calcium concentration.

Protein Kinase D1 Regulates Cardiac Hypertrophy, Potassium Channel Remodeling, and Arrhythmias in Heart Failure.

Background Structural and electrophysiological remodeling characterize heart failure (HF) enhancing arrhythmias. PKD1 (protein kinase D1) is upregulated in HF and mediates pathological hypertrophic signaling, but its role in K+ channel remodeling and arrhythmogenesis in HF is unknown. Methods and Results We performed echocardiography, electrophysiology, and expression analysis in wild-type and PKD1 cardiomyocyte-specific knockout (cKO) mice following transverse aortic constriction (TAC). PKD1-cKO mice exhibited significantly less cardiac hypertrophy post-TAC and were protected from early decline in cardiac contractile function (3 weeks post-TAC) but not the progression to HF at 7 weeks post-TAC. Wild-type mice exhibited ventricular action potential duration prolongation at 8 weeks post-TAC, which was attenuated in PKD1-cKO, consistent with larger K+ currents via the transient outward current, sustained current, inward rectifier K+ current, and rapid delayed rectifier K+ current and increased expression of corresponding K+ channels. Conversely, reduction of slowly inactivating K+ current was independent of PKD1 in HF. Acute PKD inhibition slightly increased transient outward current in TAC and sham wild-type myocytes but did not alter other K+ currents. Sham PKD1-cKO versus wild-type also exhibited larger transient outward current and faster early action potential repolarization. Tachypacing-induced action potential duration alternans in TAC animals was increased and independent of PKD1, but diastolic arrhythmogenic activities were reduced in PKD1-cKO. Conclusions Our data indicate an important role for PKD1 in the HF-related hypertrophic response and K+ channel downregulation. Therefore, PKD1 inhibition may represent a therapeutic strategy to reduce hypertrophy and arrhythmias; however, PKD1 inhibition may not prevent disease progression and reduced contractility in HF.

Dapagliflozin reduces the vulnerability of rats with pulmonary arterial hypertension-induced right heart failure to ventricular arrhythmia by restoring calcium handling

BackgroundMalignant ventricular arrhythmia (VA) is a major contributor to sudden cardiac death (SCD) in patients with pulmonary arterial hypertension (PAH)-induced right heart failure (RHF). Recently, dapagliflozin (DAPA), a sodium/glucose cotransporter-2 inhibitor (SGLT2i), has been found to exhibit cardioprotective effects in patients with left ventricular systolic dysfunction. In this study, we examined the effects of DAPA on VA vulnerability in a rat model of PAH-induced RHF.MethodsRats randomly received monocrotaline (MCT, 60 mg/kg) or vehicle via a single intraperitoneal injection. A day later, MCT-injected rats were randomly treated with placebo, low-dose DAPA (1 mg/kg/day), or high-dose (3 mg/kg/day) DAPA orally for 35 days. Echocardiographic analysis, haemodynamic experiments, and histological assessments were subsequently performed to confirm the presence of PAH-induced RHF. Right ventricle (RV) expression of calcium (Ca2+) handling proteins were detected via Western blotting. RV expression of connexin 43 (Cx43) was determined via immunohistochemical staining. An optical mapping study was performed to assess the electrophysiological characteristics in isolated hearts. Cellular Ca2+ imaging from RV cardiomyocytes (RVCMs) was recorded using Fura-2 AM or Fluo-4 AM.ResultsHigh-dose DAPA treatment attenuated RV structural remodelling, improved RV function, alleviated Cx43 remodelling, increased the conduction velocity, restored the expression of key Ca2+ handling proteins, increased the threshold for Ca2+ and action potential duration (APD) alternans, decreased susceptibility to spatially discordant APD alternans and spontaneous Ca2+ events, promoted cellular Ca2+ handling, and reduced VA vulnerability in PAH-induced RHF rats. Low-dose DAPA treatment also showed antiarrhythmic effects in hearts with PAH-induced RHF, although with a lower level of efficacy.ConclusionDAPA administration reduced VA vulnerability in rats with PAH-induced RHF by improving RVCM Ca2+ handling.

In-silico drug trials for precision medicine in atrial fibrillation: From ionic mechanisms to electrocardiogram-based predictions in structurally-healthy human atria.

Atrial fibrillation (AF) inducibility, sustainability and response to pharmacological treatment of individual patients are expected to be determined by their ionic current properties, especially in structurally-healthy atria. Mechanisms underlying AF and optimal cardioversion are however still unclear. In this study, in-silico drug trials were conducted using a population of human structurally-healthy atria models to 1) identify key ionic current properties determining AF inducibility, maintenance and pharmacological cardioversion, and 2) compare the prognostic value for predicting individual AF cardioversion of ionic current properties and electrocardiogram (ECG) metrics. In the population of structurally-healthy atria, 477 AF episodes were induced in ionic current profiles with both steep action potential duration (APD) restitution (eliciting APD alternans), and high excitability (enabling propagation at fast rates that transformed alternans into discordant). High excitability also favored 211 sustained AF episodes, so its decrease, through prolonged refractoriness, explained pharmacological cardioversion. In-silico trials over 200 AF episodes, 100 ionic profiles and 10 antiarrhythmic compounds were consistent with previous clinical trials, and identified optimal treatments for individual electrophysiological properties of the atria. Algorithms trained on 211 simulated AF episodes exhibited >70% accuracy in predictions of cardioversion for individual treatments using either ionic current profiles or ECG metrics. In structurally-healthy atria, AF inducibility and sustainability are enabled by discordant alternans, under high excitability and steep restitution conditions. Successful pharmacological cardioversion is predicted with 70% accuracy from either ionic or ECG properties, and it is optimal for treatments maximizing refractoriness (thus reducing excitability) for the given ionic current profile of the atria.

Mechanism of carvedilol induced action potential and calcium alternans

ABSTRACT Carvedilol is a nonspecific β-blocker clinically used for the treatment of cardiovascular diseases but has also been shown to have profound effects on excitation-contraction coupling and Ca signaling at the cellular level. We investigate the mechanism by which carvedilol facilitates Ca transient (CaT) and action potential duration (APD) alternans in rabbit atrial myocytes. Carvedilol lowered the frequency threshold for pacing-induced CaT alternans and facilitated alternans in a concentration-dependent manner. Carvedilol prolonged the sarcoplasmic reticulum (SR) Ca release refractoriness by significantly increasing the time constant τ of recovery of SR Ca release; however, no changes in L-type calcium current recovery from inactivation or SR Ca load were found after carvedilol treatment. Carvedilol enhanced the degree of APD alternans nearly two-fold. Carvedilol slowed the APD restitution kinetics and steepened the APD restitution curve at the pacing frequency (2 Hz) where alternans were elicited. No effect on the CaT or APD alternans ratios was observed in experiments with a different β-blocker (metoprolol), excluding the possibility that the carvedilol effect on CaT and APD alternans was determined by its β-blocking properties. These data suggest that carvedilol contributes to the generation of CaT and APD alternans in atrial myocytes by modulating the restitution of CaT and APD.

BS-516-02 OPTICAL MAPPING OF EXPLANTED HUMAN HEARTS ENABLES REFINED IONIC MODELS OF ACTION POTENTIAL AND CONDUCTION VELOCITY RESTITUTION CURVES FOR ARRHYTHMIA SIMULATION

Dynamics and stability of electrical waves in the heart has been attributed to the action potential duration (APD) restitution curve, which is a measure of how the tissue responds to a given cycle length (CL) and the conduction velocity (CV). The restitution curves have been used to investigate how changes in stimulation CL can lead to the induction of alternans and arrhythmias. Only few restitution curves are available from clinical measurements in human hearts, and they were generated using monophasic action potential measurements or from single human cardiomyocytes. To obtain APD and CV restitution curves from explanted human hearts with optical mapping, and take advantage of the experimental data to refine the dynamics of an ionic model, which can be valuable for arrhythmia simulation studies. Optical mapping of membrane potentials was performed on explanted human hearts (n=2 from recipient patients and n=1 from a rejected donor) at high resolutions (256x256 pixels, at 500 Hz) and a large field of view of 8 x 8 cm2. Action potential was measured across the anterior epicardial RV and LV and the endocardium LV, paced at CLs from 3 sec and shorter, until conduction block or VF induction. APD and CV restitution were obtained across the mapped tissue. Optical voltage signals were obtained by pacing the heart at different CLs (Panel A). APDs and CVs were calculated, and restitution curves for APD and CV were calculated for different regions in the hearts. APD alternans, between even and odd beats, were observed and quantified (panels B and C). A human ventricular cardiomyocyte model was fitted to the experimentally obtained APD and CV restitution curves to reproduce AP duration (APD restitution) and conduction velocity (CV restitution) at all CLs. The resultant ionic model recapitulates the experimental data including the bifurcation period and magnitude of alternans as well as the wave propagations. Optical mapping of action potential from explanted human hearts were used to refine the dynamics of an ionic model of human ventricular myocytes. The refined model reproduced the experimental wave propagations at all CLs, including alternans. Our study presents a clinically relevant in silico model to study the induction and dynamics of arrhythmias in whole heart simulations.

Effects of dynamic change of action potential on evolution behavior of spiral wave

It is observed in cardiac patients that the steepnesses of action potential duration (APD) restitution curve of cardiomyocytes in different regions of the ventricle are significantly different from region to region. However, the steep APD restitution curve can either lead the spiral wave to break up and set up the ventricular fibrillation in certain conditions or result in no breakup of spiral wave in other conditions. The relationship between the dynamic behavior of spiral wave and steep APD restitution curve is still not completely clear. Therefore, further research is needed. In this paper, a two-dimensional excitable medium cellular automata model is used to study the influences of the APD restitution curves with different steepnesses on the dynamic behavior of spiral wave. Numerical simulation results show that the steep APD restitution curve can stabilize the meandering spiral wave, causing the stable spiral wave to roam or break, and even to disappear. When the total average slope of APD restitution curve is greater than 1, it is observed that spiral wave may be still stable or meandering. When the total average slope of APD restitution curve is much smaller than 1, the breakup of spiral waves may occur. Three types of spiral wave breakups are observed. They are the Doppler instability, Eckhaus instability, and APD alternation. The Doppler instability and Eckhaus instability are related to the total average slope of APD restitution curve greater than 1, and the spiral wave breakup caused by APD alternans may occur when the total average slope of APD restitution curve is much smaller than 1. When the total average slope of APD restitution curve is greater than 1, the phenomena that spiral waves disappear by meandering out of the system boundary and conduction barriers are observed. In addition, we also find that increasing cellular APD is beneficial to preventing spiral wave from breaking up. The physical mechanisms behind those phenomena are heuristically analyzed.

Voltage-mediated mechanism for calcium wave synchronization and arrhythmogenesis in atrial tissue

A wide range of atrial arrythmias are caused by molecular defects in proteins that regulate calcium (Ca) cycling. In many cases, these defects promote the propagation of subcellular Ca waves in the cell, which can perturb the voltage time course and induce dangerous perturbations of the action potential (AP). However, subcellular Ca waves occur randomly in cells and, therefore, electrical coupling between cells substantially decreases their effect on the AP. In this study, we present evidence that Ca waves in atrial tissue can synchronize in-phase owing to an order-disorder phase transition. In particular, we show that, below a critical pacing rate, Ca waves are desynchronized and therefore do not induce substantial AP fluctuations in tissue. However, above this critical pacing rate, Ca waves gradually synchronize over millions of cells, which leads to a dramatic amplification of AP fluctuations. We exploit an underlying Ising symmetry of paced cardiac tissue to show that this transition exhibits universal properties common to a wide range of physical systems in nature. Finally, we show that in the heart, phase synchronization induces spatially out-of-phase AP duration alternans which drives wave break and reentry. These results suggest that cardiac tissue exhibits a phase transition that is required for subcellular Ca cycling defects to induce a life-threatening arrhythmia.

Istaroxime treatment ameliorates calcium dysregulation in a zebrafish model of phospholamban R14del cardiomyopathy

The heterozygous Phospholamban p.Arg14del mutation is found in patients with dilated or arrhythmogenic cardiomyopathy. This mutation triggers cardiac contractile dysfunction and arrhythmogenesis by affecting intracellular Ca2+ dynamics. Little is known about the physiological processes preceding induced cardiomyopathy, which is characterized by sub-epicardial accumulation of fibrofatty tissue, and a specific drug treatment is currently lacking. Here, we address these issues using a knock-in Phospholamban p.Arg14del zebrafish model. Hearts from adult zebrafish with this mutation display age-related remodeling with sub-epicardial inflammation and fibrosis. Echocardiography reveals contractile variations before overt structural changes occur, which correlates at the cellular level with action potential duration alternans. These functional alterations are preceded by diminished Ca2+ transient amplitudes in embryonic hearts as well as an increase in diastolic Ca2+ level, slower Ca2+ transient decay and longer Ca2+ transients in cells of adult hearts. We find that istaroxime treatment ameliorates the in vivo Ca2+ dysregulation, rescues the cellular action potential duration alternans, while it improves cardiac relaxation. Thus, we present insight into the pathophysiology of Phospholamban p.Arg14del cardiomyopathy.

B-YIA2-02 CELLULAR AND ELECTROPHYSIOLOGICAL CHARACTERIZATION OF TRIADIN KNOCKOUT SYNDROME USING HUMAN INDUCED PLURIPOTENT STEM CELL-DERIVED CARDIOMYOCYTES

Triadin knockout syndrome (TKOS) is a malignant arrhythmia disorder caused by recessive null variants in TRDN-encoded cardiac triadin. TKOS is characterized by QT prolongation, T-wave inversions in the precordial leads, ectopy upon stress testing, and a severe disease expression of cardiac arrest in childhood. To characterize the cellular and electrophysiological phenotype of TKOS using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). iPSC-CMs were generated from two unrelated patients with TKOS and an unrelated control (TKOS Patient 1: p.D18fs*13/D18fs*13; TKOS Patient 2: p.N9fs*5/K147fs*0). CRISPR/Cas9 was used to insert homozygous p.D18fs*13 into a different healthy control line thereby generating a patient-independent TKOS model (TRDN-/-) and isogenic control iPSC-CMs. Protein expression was measured using immunofluorescence. Action potential duration (APD) and L-type calcium channel (LTCC) properties were measured by whole cell patch-clamp. Calcium handling was assessed using Fluo-4 calcium indicator. APD90 was prolonged significantly from 507±20 ms (n=9) in the unrelated control to 662±62 ms (n=9; P<0.05) in TKOS Patient 1 and 653±39 ms (n=9; P<0.05) in TKOS Patient 2. This was confirmed in TRDN-/- iPSC-CMs compared to isogenic control (518±27 ms, n=17 vs. 312±20 ms, n=16; P<0.0001). Additional work in TRDN-/- iPSC-CMs revealed that loss of triadin underlies decreased expression and co-localization of RyR2 and Casq2 leading to slow and decreased calcium release from the sarcoplasmic reticulum. These abnormal calcium transients lead to slow inactivation of the LTCC which contributes to the observed APD prolongation. Finally, these changes underlie frequent cellular arrhythmias including early- and delayed afterdepolarizations and APD alternans. Here, we characterized the first set of iPSC-CM models of TKOS and provide further evidence for recessive null variants in TRDN as a self-sufficient monogenetic substrate for potentially lethal genetic heart disease. These cells display APD prolongation, calcium handling abnormalities, and arrhythmias which likely underlie the unique clinical and arrhythmogenic phenotype observed in patients with TKOS.

Role of Reduced Sarco-Endoplasmic Reticulum Ca2+-ATPase Function on Sarcoplasmic Reticulum Ca2+ Alternans in the Intact Rabbit Heart.

Sarcoplasmic reticulum (SR) Ca2+ cycling is tightly regulated by ryanodine receptor (RyR) Ca2+ release and sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) Ca2+ uptake during each excitation–contraction coupling cycle. We previously showed that RyR refractoriness plays a key role in the onset of SR Ca2+ alternans in the intact rabbit heart, which contributes to arrhythmogenic action potential duration (APD) alternans. Recent studies have also implicated impaired SERCA function, a key feature of heart failure, in cardiac alternans and arrhythmias. However, the relationship between reduced SERCA function and SR Ca2+ alternans is not well understood. Simultaneous optical mapping of transmembrane potential (Vm) and SR Ca2+ was performed in isolated rabbit hearts (n = 10) using the voltage-sensitive dye RH237 and the low-affinity Ca2+ indicator Fluo-5N-AM. Alternans was induced by rapid ventricular pacing. SERCA was inhibited with cyclopiazonic acid (CPA; 1–10 μM). SERCA inhibition (1, 5, and 10 μM of CPA) resulted in dose-dependent slowing of SR Ca2+ reuptake, with the time constant (tau) increasing from 70.8 ± 3.5 ms at baseline to 85.5 ± 6.6, 129.9 ± 20.7, and 271.3 ± 37.6 ms, respectively (p < 0.05 vs. baseline for all doses). At fast pacing frequencies, CPA significantly increased the magnitude of SR Ca2+ and APD alternans, most strongly at 10 μM (pacing cycle length = 220 ms: SR Ca2+ alternans magnitude: 57.1 ± 4.7 vs. 13.4 ± 8.9 AU; APD alternans magnitude 3.8 ± 1.9 vs. 0.2 ± 0.19 AU; p < 0.05 10 μM of CPA vs. baseline for both). SERCA inhibition also promoted the emergence of spatially discordant alternans. Notably, at all CPA doses, alternation of SR Ca2+ release occurred prior to alternation of diastolic SR Ca2+ load as pacing frequency increased. Simultaneous optical mapping of SR Ca2+ and Vm in the intact rabbit heart revealed that SERCA inhibition exacerbates pacing-induced SR Ca2+ and APD alternans magnitude, particularly at fast pacing frequencies. Importantly, SR Ca2+ release alternans always occurred before the onset of SR Ca2+ load alternans. These findings suggest that even in settings of diminished SERCA function, relative refractoriness of RyR Ca2+ release governs the onset of intracellular Ca2+ alternans.