Differential conditions for early after-depolarizations and triggered activity in cardiomyocytes derived from transgenic LQT1 and LQT2 rabbits

Management of patients with an electrical storm or clustered ventricular arrhythmias: a clinical consensus statement of the European Heart Rhythm Association of the ESC-endorsed by the Asia-Pacific Heart Rhythm Society, Heart Rhythm Society, and Latin-American Heart Rhythm Society

Electrical storm (ES) is a state of electrical instability, manifesting as recurrent ventricular arrhythmias (VAs) over a short period of time (three or more episodes of sustained VA within 24 h, separated by at least 5 min, requiring termination by an intervention). The clinical presentation can vary, but ES is usually a cardiac emergency. Electrical storm mainly affects patients with structural or primary electrical heart disease, often with an implantable cardioverter-defibrillator (ICD). Management of ES requires a multi-faceted approach and the involvement of multi-disciplinary teams, but despite advanced treatment and often invasive procedures, it is associated with high morbidity and mortality. With an ageing population, longer survival of heart failure patients, and an increasing number of patients with ICD, the incidence of ES is expected to increase. This European Heart Rhythm Association clinical consensus statement focuses on pathophysiology, clinical presentation, diagnostic evaluation, and acute and long-term management of patients presenting with ES or clustered VA.

E3 ubiquitin ligase rififylin has yin and yang effects on rabbit cardiac transient outward potassium currents (Ito) and corresponding channel proteins

Genome-wide association studies have reported a correlation between a SNP of the RING finger E3 ubiquitin protein ligase rififylin (RFFL) and QT interval variability in humans (Newton-Cheh et al., 2009). Previously, we have shown that RFFL downregulates expression and function of the human-like ether-a-go-go-related gene potassium channel and corresponding rapidly activating delayed rectifier potassium current (IKr) in adult rabbit ventricular cardiomyocytes. Here, we report that RFFL also affects the transient outward current (Ito), but in a peculiar way. RFFL overexpression in adult rabbit ventricular cardiomyocytes significantly decreases the contribution of its fast component (Ito,f) from 35% to 21% and increases the contribution of its slow component (Ito,s) from 65% to 79%. Since Ito,f in rabbits is mainly conducted by Kv4.3, we investigated the effect of RFFL on Kv4.3 expressed in HEK293A cells. We found that RFFL overexpression reduced Kv4.3 expression and corresponding Ito,f in a RING domain-dependent manner in the presence or absence of its accessory subunit Kv channel-interacting protein 2. On the other hand, RFFL overexpression in Kv1.4-expressing HEK cells leads to an increase in both Kv1.4 expression level and Ito,s, similarly in a RING domain-dependent manner. Our physiologically detailed rabbit ventricular myocyte computational model shows that these yin and yang effects of RFFL overexpression on Ito,f, and Ito,s affect phase 1 of the action potential waveform and slightly decrease its duration in addition to suppressing IKr. Thus, RFFL modifies cardiac repolarization reserve via ubiquitination of multiple proteins that differently affect various potassium channels and cardiac action potential duration.

Ventricular Tachycardia Due to Triggered Activity: Role of Early and Delayed Afterdepolarizations

Most forms of sustained ventricular tachycardia (VT) are caused by re-entry, resulting from altered myocardial conduction and refractoriness secondary to underlying structural heart disease. In contrast, VT caused by triggered activity (TA) is unrelated to an abnormal structural substrate and is often caused by molecular defects affecting ion channel function or regulation of intracellular calcium cycling. This review summarizes the cellular and molecular bases underlying TA and exemplifies their clinical relevance with selective representative scenarios. The underlying basis of TA caused by delayed afterdepolarizations is related to sarcoplasmic reticulum calcium overload, calcium waves, and diastolic sarcoplasmic reticulum calcium leak. Clinical examples of TA caused by delayed afterdepolarizations include sustained right and left ventricular outflow tract tachycardia and catecholaminergic polymorphic VT. The other form of afterpotentials, early afterdepolarizations, are systolic events and inscribe early afterdepolarizations during phase 2 or phase 3 of the action potential. The fundamental defect is a decrease in repolarization reserve with associated increases in late plateau inward currents. Malignant ventricular arrhythmias in the long QT syndromes are initiated by early afterdepolarization-mediated TA. An understanding of the molecular and cellular bases of these arrhythmias has resulted in generally effective pharmacologic-based therapies, but these are nonspecific agents that have off-target effects. Therapeutic efficacy may need to be augmented with an implantable defibrillator. Next-generation therapies will include novel agents that rescue arrhythmogenic abnormalities in cellular signaling pathways and gene therapy approaches that transfer or edit pathogenic gene variants or silence mutant messenger ribonucleic acid.

Deciphering the roles of triiodothyronine (T3) and thyroid-stimulating hormone (TSH) on cardiac electrical remodeling in clinical and experimental hypothyroidism

Hypothyroidism is the most frequent endocrine pathology. Although clinical or overt hypothyroidism has been traditionally associated to low T3 / T4 and high thyrotropin (TSH) circulating levels, other forms exist such as subclinical hypothyroidism, characterized by normal blood T3 / T4 and high TSH. In its different forms is estimated to affect approximately 10% of the population, especially women, in a 5:1 ratio with respect to men. Among its consequences are alterations in cardiac electrical activity, especially in the repolarization phase, which is accompanied by an increased susceptibility to cardiac arrhythmias. Although these alterations have traditionally been attributed to thyroid hormone deficiency, recent studies, both clinical trials and experimental models, demonstrate a fundamental role of TSH in cardiac electrical remodeling. Thus, both metabolic thyroid hormones and TSH regulate cardiac ion channel expression in many and varied ways. This means that the different combinations of hormones that predominate in different types of hypothyroidism (overt, subclinic, primary, central) can generate different forms of cardiac electrical remodeling. These new findings are raising the relevant question of whether serum TSH reference ranges should be redefined.

Dissecting the roles of calcium cycling and its coupling with voltage in the genesis of early afterdepolarizations in cardiac myocyte models

Early afterdepolarizations (EADs) are abnormal depolarizations during the plateau phase of the action potential, which are known to be associated with lethal arrhythmias in the heart. There are two major hypotheses for EAD genesis based on experimental observations, i.e., the voltage (Vm)-driven and intracellular calcium (Ca)-driven mechanisms. In ventricular myocytes, Ca and Vm are bidirectionally coupled, which can affect each other's dynamics and result in new dynamics, however, the roles of Ca cycling and its coupling with Vm in the genesis of EADs have not been well understood. In this study, we use an action potential model that is capable of independent Vm and Ca oscillations to investigate the roles of Vm and Ca coupling in EAD genesis. Four different mechanisms of EADs are identified, which are either driven by Vm oscillations or Ca oscillations alone, or oscillations caused by their interactions. We also use 5 other ventricular action potential models to assess these EAD mechanisms and show that EADs in these models are mainly Vm-driven. These mechanistic insights from our simulations provide a theoretical base for understanding experimentally observed EADs and EAD-related arrhythmogenesis.

Phase 2 Re-Entry Without Ito: Role of Sodium Channel Kinetics in Brugada Syndrome Arrhythmias

BACKGROUND

In Brugada syndrome (BrS), phase 2 re-excitation/re-entry (P2R) induced by the transient outward potassium current (Ito) is a proposed arrhythmia mechanism; yet, the most common genetic defects are loss-of-function sodium channel mutations.

OBJECTIVES

The authors used computer simulations to investigate how sodium channel dysfunction affects P2R-mediated arrhythmogenesis in the presence and absence of Ito.

METHODS

Computer simulations were carried out in 1-dimensional cables and 2-dimensional tissue using guinea pig and human ventricular action potential models.

RESULTS

In the presence of Ito sufficient to generate robust P2R, reducing sodium current (INa) peak amplitude alone only slightly potentiated P2R. When INa inactivation kinetics were also altered to simulate reported effects of BrS mutations and sodium channel blockers, however, P2R occurred even in the absence of Ito. These effects could be potentiated by delaying L-type calcium channel activation or increasing ATP-sensitive potassium current, consistent with experimental and clinical findings. INa-mediated P2R also accounted for sex-related, day and night-related, and fever-related differences in arrhythmia risk in BrS patients.

CONCLUSIONS

Altered INa kinetics synergize powerfully with reduced INa amplitude to promote P2R-induced arrhythmias in BrS in the absence of Ito, establishing a robust mechanistic link between altered INa kinetics and the P2R-mediated arrhythmia mechanism.

Theoretical prediction of early afterdepolarization-evoked triggered activity formation initiating ventricular reentrant arrhythmias

BACKGROUND AND OBJECTIVE

Excessive prolongation of QT interval on ECGs in patients with congenital/acquired long QT syndrome and heart failure is a sign suggesting the development of early afterdepolarization (EAD), an abnormal repolarization in the action potential of ventricular cardiomyocytes. The development of EAD has been believed to be a trigger for fatal tachyarrhythmia, which can be a risk for sudden cardiac death. The role of EAD in triggering ventricular tachycardia (VT) remains unclear. The aim of this study was to elucidate the mechanism of EAD-induced triggered activity formation that leads to the VT such as Torsades de Pointes.

METHODS

We investigated the relationship between EAD and tachyarrhythmia initiation by constructing homogeneous myocardial sheet models consisting of the mid-myocardial cell version of a human ventricular myocyte model and performing simulations of excitation propagation.

RESULTS

A solitary island-like (clustering) occurrence of EADs in the homogeneous myocardial sheet could induce a focal excitation wave. However, reentrant excitation, an entity of tachyarrhythmia, was not able to be triggered regardless of the EAD cluster size when the focal excitation wave formed a repolarization potential difference boundary consisting of only a convex surface. The discontinuous distribution of multiple EAD clusters in the ventricular tissue formed a specific repolarization heterogeneity due to the repolarization potential difference, the shape of which depended on EAD cluster size and placed intervals. We found that the triggered activity was formed in such a manner that the repolarization potential difference boundary included a concave surface.

CONCLUSIONS

The formation of triggered activity that led to tachyarrhythmia required not only the occurrence of EAD onset-mediated focal excitation wave but also a repolarization heterogeneity-based specific repolarization potential difference boundary shape formed within the tissue.

Steady-state and transient effects of SK channel block and adrenergic stimulation to counteract acetylcholine-induced arrhythmogenic effects in the human atria: A computational study

Hyperactivity of the parasympathetic nervous system has been linked to the development of paroxysmal atrial fibrillation (AF). The parasympathetic neurotransmitter acetylcholine (ACh) causes a reduction in action potential (AP) duration (APD) and an increase in resting membrane potential (RMP), both of which contribute to enhance the risk for reentry. Research suggests that small-conductance calcium activated potassium (SK) channels may be an effective target for treating AF. Therapies targeting the autonomic nervous system, either alone or in combination with other drugs, have been explored and have been shown to decrease the incidence of atrial arrhythmias. This study uses computational modeling and simulation to examine the impact of SK channel block (SKb) and β-adrenergic stimulation through Isoproterenol (Iso) on countering the negative effects of cholinergic activity in human atrial cell and 2D tissue models. The steady-state effects of Iso and/or SKb on AP shape, APD at 90% repolarization (APD₉₀) and RMP were evaluated. The ability to terminate stable rotational activity in cholinergically-stimulated 2D tissue models of AF was also investigated. A range of SKb and Iso application kinetics, which reflect varying drug binding rates, were taken into consideration. The results showed that SKb alone prolonged APD₉₀ and was able to stop sustained rotors in the presence of ACh concentrations up to 0.01 μM. Iso terminated rotors under all tested ACh concentrations, but resulted in highly-variable steady-state outcomes depending on baseline AP morphology. Importantly, the combination of SKb and Iso resulted in greater APD₉₀ prolongation and showed promising anti-arrhythmic potential by stopping stable rotors and preventing re-inducibility.

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 (0.1 μM, N = 12; 0.3 μM, N = 10; 1 μM, N = 10) were evaluated using whole-cell patch-clamp. The electrophysiological properties of isolated perfused guinea pig hearts under E-4031 (0.1 μM, N = 5; 0.3 μM, N = 5; 1 μM, 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.

Subcellular Propagation of Cardiomyocyte β-Adrenergic Activation of Calcium Uptake Involves Internal β-Receptors and AKAP7

β-adrenergic receptor (β-AR) signaling in cardiac myocytes is central to cardiac function, but spatiotemporal activation within myocytes is unresolved. In rabbit ventricular myocytes, β-AR agonists or high extracellular [Ca] were applied locally at one end, to measure β-AR signal propagation as Ca-transient (CaT) amplitude and sarcoplasmic reticulum (SR) Ca uptake. High local [Ca]_o, increased CaT amplitude under the pipette faster than did ISO, but was also more spatially restricted. Local isoproterenol (ISO) or norepinephrine (NE) increased CaT amplitude and SR Ca uptake, that spread along the myocyte to the unexposed end. Thus, local [Ca]_i decline kinetics reflect spatio-temporal progression of β-AR end-effects in myocytes. To test whether intracellular β-ARs contribute to this response, we used β-AR-blockers that are membrane permeant (propranolol) or not (sotalol). Propranolol completely blocked NE-dependent CaT effects. However, blocking surface β-ARs only (sotalol) suppressed only ∼50% of the NE-induced increase in CaT peak and rate of [Ca]_i decline, but these changes spread more gradually than NE alone. We also tested whether A-kinase anchoring protein 7γ (AKAP7γ; that interacts with phospholamban) is mobile, such that it might contribute to intracellular spatial propagation of β-AR signaling. We found AKAP7γ to be highly mobile using fluorescence recovery after photobleach of GFP tagged AKAP7γ, and that PKA activation accelerated AKAP7γ-GFP wash-out upon myocyte saponin-permeabilization, suggesting increased AKAP7γ mobility. We conclude that local β-AR activation can activate SR Ca uptake at remote myocyte sites, and that intracellular β-AR and AKAP7γ mobility may play a role in this spread of activation.

Rate dependence of early afterdepolarizations in the His-Purkinje system: A simulation study

BACKGROUND AND OBJECTIVE

Early afterdepolarizations (EADs) are associated with a variety of arrhythmias and have the property of rate dependence. EADs can occur in Purkinje cells while the effect of rate dependence of EADs in the His-Purkinje system has not been fully investigated. In order to reveal the rate dependence of EADs in the His-Purkinje system and its effect on ventricular electrical activities, the simulation research was carried out in this manuscript.

METHODS

This manuscript first studied the relationship between the occurrence of EADs and stimulation cycle length on the DiFranNoble cell model. Then, the relationship between the rate dependence of EADs and the conduction block of the His-Purkinje system at slow heart rates was studied on the rabbit whole ventricular model including the His-Purkinje system, and its mechanism was analyzed from multiple angles.

RESULTS

① The rate dependence of EADs is related to the inconsistency of EADs occurrence in the His-Purkinje system. When the stimulation cycle length is long or short enough, EADs either occur or not occur stably in the His-Purkinje system, while in a certain stimulation cycle length window, the chaotic state of EADs will be observed. ② The key subcellular factors x-gate is an important mechanism involved to the rate dependence of EADs in the His-Purkinje system. ③ The discrete distribution of x-gate values and the "source-sink" mechanism lead to the inconsistency of EADs in the His-Purkinje system. The prolonged action potential duration caused by EADs can lead to conduction block at slow heart rates.

CONCLUSION

The rate dependence of EADs in Purkinje system can lead to disordered ventricular electrical activity.

Emerging evidence for a mechanistic link between low-frequency oscillation of ventricular repolarization measured from the electrocardiogram T-wave vector and arrhythmia

Strong recent clinical evidence links the presence of prominent oscillations of ventricular repolarization in the low-frequency range (0.04-0.15 Hz) to the incidence of ventricular arrhythmia and sudden death in post-MI patients and patients with ischaemic and non-ischaemic cardiomyopathy. It has been proposed that these oscillations reflect oscillations of ventricular action potential duration at the sympathetic nerve frequency. Here we review emerging evidence to support that contention and provide insight into possible underlying mechanisms for this association.

Inhibition of the unfolded protein response reduces arrhythmic risk after myocardial infarction

Ischemic cardiomyopathy is associated with an increased risk of sudden death, activation of the unfolded protein response (UPR), and reductions in multiple cardiac ion channels. When activated, the protein kinase-like ER kinase (PERK) branch of the UPR reduces protein translation and abundance. We hypothesized that PERK inhibition could prevent ion channel downregulation and reduce arrhythmic risk after myocardial infarct (MI). MI induced by coronary artery ligation resulted in mice exhibited reduced ion channel levels, ventricular tachycardia (VT), and prolonged corrected intervals between the Q and T waves of the ECGs (QTc). Protein levels of major cardiac ion channels were decreased. MI cardiomyocytes showed significantly prolonged action potential duration and decreased maximum upstroke velocity. Cardiac-specific PERK knockout (PERKKO) reduced electrical remodeling in response to MI with shortened QTc intervals, less VT episodes, and higher survival rates (P<0.05 vs. MI). Pharmacological PERK inhibition had similar effects. In conclusion, activated PERK during MI contributed to arrhythmic risk by downregulation of select cardiac ion channels. PERK inhibition prevented these changes and reduced arrhythmic risk. These results suggest that ion channel downregulation during MI is a fundamental arrhythmic mechanism and maintaining ion channel levels is antiarrhythmic.

Mechanisms of phase-3 early afterdepolarizations and triggered activities in ventricular myocyte models

Early afterdepolarizations (EADs) are abnormal depolarizations during the repolarizing phase of the action potential, which are associated with cardiac arrhythmogenesis. EADs are classified into phase-2 and phase-3 EADs. Phase-2 EADs occur during phase 2 of the action potential, with takeoff potentials typically above -40 mV. Phase-3 EADs occur during phase 3 of the action potential, with takeoff potential between -70 and -50 mV. Since the amplitude of phase-3 EADs can be as large as that of a regular action potential, they are also called triggered activities (TAs). This also makes phase-3 EADs and TAs much more arrhythmogenic than phase-2 EADs since they can propagate easily in tissue. Although phase-2 EADs have been widely observed, phase-3 EADs and TAs have been rarely demonstrated in isolated ventricular myocytes. Here we carry out computer simulations of three widely used ventricular action potential models to investigate the mechanisms of phase-3 EADs and TAs. We show that when the T-type Ca2+ current (ICa,T ) is absent (e.g., in normal ventricular myocytes), besides the requirement of increasing inward currents and reducing outward currents as for phase-2 EADs, the occurrence of phase-3 EADs and TAs requires a substantially large increase of the L-type Ca2+ current and the slow component of the delayed rectifier K+ current. The presence of ICa,T (e.g., in neonatal and failing ventricular myocytes) can greatly reduce the thresholds of these two currents for phase-3 EADs and TAs. This implies that ICa,T may play an important role in arrhythmogenesis in cardiac diseases.

Transgenic rabbit models for cardiac disease research

To study the pathophysiology of human cardiac diseases and to develop novel treatment strategies, complex interactions of cardiac cells on cellular, tissue and on level of the whole heart need to be considered. As in vitro cell-based models do not depict the complexity of the human heart, animal models are used to obtain insights that can be translated to human diseases. Mice are the most commonly used animals in cardiac research. However, differences in electrophysiological and mechanical cardiac function and a different composition of electrical and contractile proteins limit the transferability of the knowledge gained. Moreover, the small heart size and fast heart rate are major disadvantages. In contrast to rodents, electrophysiological, mechanical and structural cardiac characteristics of rabbits resemble the human heart more closely, making them particularly suitable as an animal model for cardiac disease research. In this review, various methodological approaches for the generation of transgenic rabbits for cardiac disease research, such as pronuclear microinjection, the sleeping beauty transposon system and novel genome-editing methods (ZFN and CRISPR/Cas9)will be discussed. In the second section, we will introduce the different currently available transgenic rabbit models for monogenic cardiac diseases (such as long QT syndrome, short-QT syndrome and hypertrophic cardiomyopathy) in detail, especially in regard to their utility to increase the understanding of pathophysiological disease mechanisms and novel treatment options.

Long-QT founder variant T309I-Kv7.1 with dominant negative pattern may predispose delayed afterdepolarizations under β-adrenergic stimulation

The variant c.926C > T (p.T309I) in KCNQ1 gene was identified in 10 putatively unrelated Czech families with long QT syndrome (LQTS). Mutation carriers (24 heterozygous individuals) were more symptomatic compared to their non-affected relatives (17 individuals). The carriers showed a mild LQTS phenotype including a longer QTc interval at rest (466 ± 24 ms vs. 418 ± 20 ms) and after exercise (508 ± 32 ms vs. 417 ± 24 ms), 4 syncopes and 2 aborted cardiac arrests. The same haplotype associated with the c.926C > T variant was identified in all probands. Using the whole cell patch clamp technique and confocal microscopy, a complete loss of channel function was revealed in the homozygous setting, caused by an impaired channel trafficking. Dominant negativity with preserved reactivity to β-adrenergic stimulation was apparent in the heterozygous setting. In simulations on a human ventricular cell model, the dysfunction resulted in delayed afterdepolarizations (DADs) and premature action potentials under β-adrenergic stimulation that could be prevented by a slight inhibition of calcium current. We conclude that the KCNQ1 variant c.926C > T is the first identified LQTS-related founder mutation in Central Europe. The dominant negative channel dysfunction may lead to DADs under β-adrenergic stimulation. Inhibition of calcium current could be possible therapeutic strategy in LQTS1 patients refractory to β-blocker therapy.

Kv1.1 potassium channel subunit deficiency alters ventricular arrhythmia susceptibility, contractility, and repolarization

Epilepsy-associated Kv1.1 voltage-gated potassium channel subunits encoded by the Kcna1 gene have traditionally been considered absent in heart, but recent studies reveal they are expressed in cardiomyocytes where they could regulate intrinsic cardiac electrophysiology. Although Kv1.1 now has a demonstrated functional role in atria, its role in the ventricles has never been investigated. In this work, electrophysiological, histological, and gene expression approaches were used to explore the consequences of Kv1.1 deficiency in the ventricles of Kcna1 knockout (KO) mice at the organ, cellular, and molecular levels to determine whether the absence of Kv1.1 leads to ventricular dysfunction that increases the risk of premature or sudden death. When subjected to intracardiac pacing, KO mice showed normal baseline susceptibility to inducible ventricular arrhythmias (VA) but resistance to VA under conditions of sympathetic challenge with isoproterenol. Echocardiography revealed cardiac contractile dysfunction manifesting as decreased ejection fraction and fractional shortening. In whole-cell patch-clamp recordings, KO ventricular cardiomyocytes exhibited action potential prolongation indicative of impaired repolarization. Imaging, histological, and transcript analyses showed no evidence of structural or channel gene expression remodeling, suggesting that the observed deficits are likely electrogenic due to Kv1.1 deficiency. Immunoblots of patient heart samples detected the presence of Kv1.1 at relatively high levels, implying that Kv1.1 contributes to human cardiac electrophysiology. Taken together, this work describes an important functional role for Kv1.1 in ventricles where its absence causes repolarization and contractility deficits but reduced susceptibility to arrhythmia under conditions of sympathetic drive.

Mechanisms of Premature Ventricular Complexes Caused by QT Prolongation

QT prolongation, due to lengthening of the action potential duration in the ventricles, is a major risk factor of lethal ventricular arrhythmias. A widely known consequence of QT prolongation is the genesis of early afterdepolarizations (EADs), which are associated with arrhythmias through the generation of premature ventricular complexes (PVCs). However, the vast majority of the EADs observed experimentally in isolated ventricular myocytes are phase-2 EADs, and whether phase-2 EADs are mechanistically linked to PVCs in cardiac tissue remains an unanswered question. In this study, we investigate the genesis of PVCs using computer simulations with eight different ventricular action potential models of various species. Based on our results, we classify PVCs as arising from two distinct mechanisms: repolarization gradient (RG)-induced PVCs and phase-2 EAD-induced PVCs. The RG-induced PVCs are promoted by increasing RG and L-type calcium current and are insensitive to gap junction coupling. EADs are not required for this PVC mechanism. In a paced beat, a single or multiple PVCs can occur depending on the properties of the RG. In contrast, phase-2 EAD-induced PVCs occur only when the RG is small and are suppressed by increasing RG and more sensitive to gap junction coupling. Unlike with RG-induced PVCs, in each paced beat, only a single EAD-induced PVC can occur no matter how many EADs in an action potential. In the wide parameter ranges we explore, RG-induced PVCs can be observed in all models, but the EAD-induced PVCs can only be observed in five of the eight models. The links between these two distinct PVC mechanisms and arrhythmogenesis in animal experiments and clinical settings are discussed.

Inhibition of cardiac potassium currents by oxidation-activated protein kinase A contributes to early afterdepolarizations in the heart

Reactive oxygen species (ROS) have been shown to prolong cardiac action potential duration resulting in afterdepolarizations, the cellular basis of triggered arrhythmias. As previously shown, protein kinase A type I (PKA I) is readily activated by oxidation of its regulatory subunits. However, the relevance of this mechanism of activation for cardiac pathophysiology is still elusive. In this study, we investigated the effects of oxidation-activated PKA I on cardiac electrophysiology. Ventricular cardiomyocytes were isolated from redox-dead PKA-RI Cys17Ser knock-in (KI) and wild-type (WT) mice and exposed to H₂O₂ (200 µmol/L) or vehicle (Veh) solution. In WT myocytes, exposure to H₂O₂ significantly increased oxidation of the regulatory subunit I (RI) and thus its dimerization (threefold increase in PKA RI dimer). Whole cell current clamp and voltage clamp were used to measure cardiac action potentials (APs), transient outward potassium current (I_to) and inward rectifying potassium current (I_K1), respectively. In WT myocytes, H₂O₂ exposure significantly prolonged AP duration due to significantly decreased I_to and I_K1 resulting in frequent early afterdepolarizations (EADs). Preincubation with the PKA-specific inhibitor Rp-8-Br-cAMPS (10 µmol/L) completely abolished the H₂O₂-dependent decrease in I_to and I_K1 in WT myocytes. Intriguingly, H₂O₂ exposure did not prolong AP duration, nor did it decrease I_to, and only slightly enhanced EAD frequency in KI myocytes. Treatment of WT and KI cardiomyocytes with the late I_Na inhibitor TTX (1 µmol/L) completely abolished EAD formation. Our results suggest that redox-activated PKA may be important for H₂O₂-dependent arrhythmias and could be important for the development of specific antiarrhythmic drugs.NEW & NOTEWORTHY Oxidation-activated PKA type I inhibits transient outward potassium current (I_to) and inward rectifying potassium current (I_K1) and contributes to ROS-induced APD prolongation as well as generation of early afterdepolarizations in murine ventricular cardiomyocytes.

Canard analysis reveals why a large Ca2+ window current promotes early afterdepolarizations in cardiac myocytes

The pumping of blood through the heart is due to a wave of muscle contractions that are in turn due to a wave of electrical activity initiated at the sinoatrial node. At the cellular level, this wave of electrical activity corresponds to the sequential excitation of electrically coupled cardiac cells. Under some conditions, the normally-long action potentials of cardiac cells are extended even further by small oscillations called early afterdepolarizations (EADs) that can occur either during the plateau phase or repolarizing phase of the action potential. Hence, cellular EADs have been implicated as a driver of potentially lethal cardiac arrhythmias. One of the major determinants of cellular EAD production and repolarization failure is the size of the overlap region between Ca2+ channel activation and inactivation, called the window region. In this article, we interpret the role of the window region in terms of the fast-slow structure of a low-dimensional model for ventricular action potential generation. We demonstrate that the effects of manipulation of the size of the window region can be understood from the point of view of canard theory. We use canard theory to explain why enlarging the size of the window region elicits EADs and why shrinking the window region can eliminate them. We also use the canard mechanism to explain why some manipulations in the size of the window region have a stronger influence on cellular electrical behavior than others. This dynamical viewpoint gives predictive power that is beyond that of the biophysical explanation alone while also uncovering a common mechanism for phenomena observed in experiments on both atrial and ventricular cardiac cells.

Late INa Blocker GS967 Supresses Polymorphic Ventricular Tachycardia in a Transgenic Rabbit Model of Long QT Type 2

BACKGROUND

Long QT syndrome has been associated with sudden cardiac death likely caused by early afterdepolarizations (EADs) and polymorphic ventricular tachycardias (PVTs). Suppressing the late sodium current (INaL) may counterbalance the reduced repolarization reserve in long QT syndrome and prevent EADs and PVTs.

METHODS

We tested the effects of the selective INaL blocker GS967 on PVT induction in a transgenic rabbit model of long QT syndrome type 2 using intact heart optical mapping, cellular electrophysiology and confocal Ca2+ imaging, and computer modeling.

RESULTS

GS967 reduced ventricular fibrillation induction under a rapid pacing protocol (n=7/14 hearts in control versus 1/14 hearts at 100 nmol/L) without altering action potential duration or restitution and dispersion. GS967 suppressed PVT incidences by reducing Ca2+-mediated EADs and focal activity during isoproterenol perfusion (at 30 nmol/L, n=7/12 and 100 nmol/L n=8/12 hearts without EADs and PVTs). Confocal Ca2+ imaging of long QT syndrome type 2 myocytes revealed that GS967 shortened Ca2+ transient duration via accelerating Na+/Ca2+ exchanger (INCX)-mediated Ca2+ efflux from cytosol, thereby reducing EADs. Computer modeling revealed that INaL potentiates EADs in the long QT syndrome type 2 setting through (1) providing additional depolarizing currents during action potential plateau phase, (2) increasing intracellular Na+ (Nai) that decreases the depolarizing INCX thereby suppressing the action potential plateau and delaying the activation of slowly activating delayed rectifier K+ channels (IKs), suggesting important roles of INaL in regulating Nai.

CONCLUSIONS

Selective INaL blockade by GS967 prevents EADs and abolishes PVT in long QT syndrome type 2 rabbits by counterbalancing the reduced repolarization reserve and normalizing Nai. Graphic Abstract: A graphic abstract is available for this article.

Transgenic LQT2, LQT5, and LQT2-5 rabbit models with decreased repolarisation reserve for prediction of drug-induced ventricular arrhythmias

Background and Purpose

Reliable prediction of pro‐arrhythmic side effects of novel drug candidates is still a major challenge. Although drug‐induced pro‐arrhythmia occurs primarily in patients with pre‐existing repolarisation disturbances, healthy animals are employed for pro‐arrhythmia testing. To improve current safety screening, transgenic long QT (LQTS) rabbit models with impaired repolarisation reserve were generated by overexpressing loss‐of‐function mutations of human HERG (HERG‐G628S, loss of I_Kr; LQT2), KCNE1 (KCNE1‐G52R, decreased I_Ks; LQT5), or both transgenes (LQT2‐5) in the heart.

Experimental Approach

Effects of K⁺ channel blockers on cardiac repolarisation and arrhythmia susceptibility were assessed in healthy wild‐type (WT) and LQTS rabbits using in vivo ECG and ex vivo monophasic action potential and ECG recordings in Langendorff‐perfused hearts.

Key Results

LQTS models reflect patients with clinically “silent” (LQT5) or “manifest” (LQT2 and LQT2‐5) impairment in cardiac repolarisation reserve: they were more sensitive in detecting I_Kr‐blocking (LQT5) or I_K1/I_Ks‐blocking (LQT2 and LQT2‐5) properties of drugs compared to healthy WT animals. Impaired QT‐shortening capacity at fast heart rates was observed due to disturbed I_Ks function in LQT5 and LQT2‐5. Importantly, LQTS models exhibited higher incidence, longer duration, and more malignant types of ex vivo arrhythmias than WT.

Conclusion and Implications

LQTS models represent patients with reduced repolarisation reserve due to different pathomechanisms. As they demonstrate increased sensitivity to different specific ion channel blockers (I_Kr blockade in LQT5 and I_K1 and I_Ks blockade in LQT2 and LQT2‐5), their combined use could provide more reliable and more thorough prediction of (multichannel‐based) pro‐arrhythmic potential of novel drug candidates.

Transgenic Rabbit Models in Proarrhythmia Research

Drug-induced proarrhythmia constitutes a potentially lethal side effect of various drugs. Most often, this proarrhythmia is mechanistically linked to the drug's potential to interact with repolarizing cardiac ion channels causing a prolongation of the QT interval in the ECG. Despite sophisticated screening approaches during drug development, reliable prediction of proarrhythmia remains very challenging. Although drug-induced long-QT-related proarrhythmia is often favored by conditions or diseases that impair the individual's repolarization reserve, most cellular, tissue, and whole animal model systems used for drug safety screening are based on normal, healthy models. In recent years, several transgenic rabbit models for different types of long QT syndromes (LQTS) with differences in the extent of impairment in repolarization reserve have been generated. These might be useful for screening/prediction of a drug's potential for long-QT-related proarrhythmia, particularly as different repolarizing cardiac ion channels are impaired in the different models. In this review, we summarize the electrophysiological characteristics of the available transgenic LQTS rabbit models, and the pharmacological proof-of-principle studies that have been performed with these models—highlighting the advantages and disadvantages of LQTS models for proarrhythmia research. In the end, we give an outlook on potential future directions and novel models.

Conditions for the genesis of early afterdepolarization in a model of a ventricular myocyte

Early afterdepolarization (EAD) is a major arrhythmogenic factor in the long QT syndrome (LQTS), whose conditions for genesis have puzzled people for several decades. Here, we employ the phase I Luo-Rudy ventricular myocyte model to investigate EAD using methods from nonlinear dynamics and provide valuable insights into EAD genesis from a physical perspective. Two major results are obtained: (i) Sufficient parametric conditions for EAD are analytically determined and then used to analyze in detail the effects of the physiological parameters. (ii) The normal form of the Hopf bifurcation that leads to EAD is derived and then used to determine whether the Hopf bifurcation is subcritical or supercritical for EAD genesis and the corresponding amplitude and period of the EAD oscillation. Our work here paves the way for further studies of more complicated multi-scale dynamics of EAD and may lead to effective treatments for LQTS arrhythmias.

Whole-Cell cAMP and PKA Activity are Epiphenomena, Nanodomain Signaling Matters

Novel targeted fluorescent biosensors provide key insights into very local nanodomains of cAMP and PKA activity, and how they respond differently to β-adrenergic activation in cardiac myocytes. This unique spatiotemporal detail in living cells is not available with biochemical measurements of total cellular cAMP and PKA, and provides unique physiological insights.

Time Course of Low-Frequency Oscillatory Behavior in Human Ventricular Repolarization Following Enhanced Sympathetic Activity and Relation to Arrhythmogenesis

Background and Objectives: Recent studies in humans and dogs have shown that ventricular repolarization exhibits a low-frequency (LF) oscillatory pattern following enhanced sympathetic activity, which has been related to arrhythmic risk. The appearance of LF oscillations in ventricular repolarization is, however, not immediate, but it may take up to some minutes. This study seeks to characterize the time course of the action potential (AP) duration (APD) oscillatory behavior in response to sympathetic provocations, unveil its underlying mechanisms and establish a potential link to arrhythmogenesis under disease conditions.

Materials and Methods: A representative set of human ventricular computational models coupling cellular electrophysiology, calcium dynamics, β-adrenergic signaling, and mechanics was built. Sympathetic provocation was modeled via phasic changes in β-adrenergic stimulation (β-AS) and mechanical stretch at Mayer wave frequencies within the 0.03–0.15 Hz band.

Results: Our results show that there are large inter-individual differences in the time lapse for the development of LF oscillations in APD following sympathetic provocation, with some cells requiring just a few seconds and other cells needing more than 3 min. Whereas, the oscillatory response to phasic mechanical stretch is almost immediate, the response to β-AS is much more prolonged, in line with experimentally reported evidences, thus being this component the one driving the slow development of APD oscillations following enhanced sympathetic activity. If β-adrenoceptors are priorly stimulated, the time for APD oscillations to become apparent is remarkably reduced, with the oscillation time lapse being an exponential function of the pre-stimulation level. The major mechanism underlying the delay in APD oscillations appearance is related to the slow I_Ks phosphorylation kinetics, with its relevance being modulated by the I_Ks conductance of each individual cell. Cells presenting short oscillation time lapses are commonly associated with large APD oscillation magnitudes, which facilitate the occurrence of pro-arrhythmic events under disease conditions involving calcium overload and reduced repolarization reserve.

Conclusions: The time course of LF oscillatory behavior of APD in response to increased sympathetic activity presents high inter-individual variability, which is associated with different expression and PKA phosphorylation kinetics of the I_Ks current. Short time lapses in the development of APD oscillations are associated with large oscillatory magnitudes and pro-arrhythmic risk under disease conditions.

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.

Stellate ganglion stimulation causes spatiotemporal changes in ventricular repolarization in pig

BACKGROUND

Dispersion in ventricular repolarization is relevant for arrhythmogenesis.

OBJECTIVE

The purpose of this study was to determine the spatiotemporal effects of sympathetic stimulation on ventricular repolarization.

METHODS

In 5 anesthetized female open-chest pigs, ventricular repolarization was measured from the anterior, lateral, and posterior walls of the left ventricle (LV) and right ventricle using up to 40 transmural plunge needles (4 electrodes each) before and after left stellate ganglion stimulation (LSGS) and right stellate ganglion stimulation. In addition, LSGS was performed in 3 pigs (2 male, 1 female) before and after verapamil (5-10 mg/h) administration.

RESULTS

LSGS yielded a biphasic response in repolarization in the lateral and posterior walls of the LV, with prolongation at ∼5 seconds (10 ± 1.5 ms) and shortening at 20-30 seconds of stimulation (-28.9 ± 4.4 ms) during a monotonic pressure increase. While the initial prolongation was abolished by verapamil, late shortening was augmented. Sequential transections of the vagal nerve and stellate ganglia augmented repolarization dispersion responses to LSGS in 2 of 5 hearts. An equal pressure increase by aortic occlusion resulted in a homogeneous shortening of repolarization in the LV, and the effects were smaller than those during LSGS. Right stellate stimulation shortened repolarization mainly in the anterior LV wall, but the effects were smaller than those of LSGS.

CONCLUSION

LSGS first prolongs (through the L-type calcium current) and then shortens repolarization. The effect of LSGS was prominent in the posterior and lateral, not the anterior, LV walls.

R-From-T as a Common Mechanism of Arrhythmia Initiation in Long QT Syndromes

BACKGROUND

Long QT syndromes (LQTS) arise from many genetic and nongenetic causes with certain characteristic ECG features preceding polymorphic ventricular tachyarrhythmias (PVTs). However, how the many molecular causes result in these characteristic ECG patterns and how these patterns are mechanistically linked to the spontaneous initiation of PVT remain poorly understood.

METHODS

Anatomic human ventricle and simplified tissue models were used to investigate the mechanisms of spontaneous initiation of PVT in LQTS.

RESULTS

Spontaneous initiation of PVT was elicited by gradually ramping up ICa,L to simulate the initial phase of a sympathetic surge or by changing the heart rate, reproducing the different genotype-dependent clinical ECG features. In LQTS type 2 (LQT2) and LQTS type 3 (LQT3), T-wave alternans was observed followed by premature ventricular complexes (PVCs). Compensatory pauses occurred resulting in short-long-short sequences. As ICa,L increased further, PVT episodes occurred, always preceded by a short-long-short sequence. However, in LQTS type 1 (LQT1), once a PVC occurred, it always immediately led to an episode of PVT. Arrhythmias in LQT2 and LQT3 were bradycardia dependent, whereas those in LQT1 were not. In all 3 genotypes, PVCs always originated spontaneously from the steep repolarization gradient region and manifested on ECG as R-on-T. We call this mechanism R-from-T, to distinguish it from the classic explanation of R-on-T arrhythmogenesis in which an exogenous PVC coincidentally encounters a repolarizing region. In R-from-T, the PVC and the T wave are causally related, where steep repolarization gradients combined with enhanced ICa,L lead to PVCs emerging from the T wave. Since enhanced ICa,L was required for R-from-T to occur, suppressing window ICa,L effectively prevented arrhythmias in all 3 genotypes.

CONCLUSIONS

Despite the complex molecular causes, these results suggest that R-from-T is likely a common mechanism for PVT initiation in LQTS. Targeting ICa,L properties, such as suppressing window ICa,L or preventing excessive ICa,L increase, could be an effective unified therapy for arrhythmia prevention in LQTS.

Short-Long Heart Rate Variation Increases Dispersion of Action Potential Duration in Long QT Type 2 Transgenic Rabbit Model

The initiation of polymorphic ventricular tachycardia in long QT syndrome type 2 (LQT2) has been associated with a characteristic ECG pattern of short-long RR intervals. We hypothesize that this characteristic pattern increases APD dispersion in LQT2, thereby promoting arrhythmia. We investigated APD dispersion and its dependence on two previous cycle lengths (CLs) in transgenic rabbit models of LQT2, LQT1, and their littermate controls (LMC) using random stimulation protocols. The results show that the short-long RR pattern was associated with a larger APD dispersion in LQT2 but not in LQT1 rabbits. The multivariate analyses of APD as a function of two previous CLs (APD_n = C + α₁CL_n−1 + α₂CL_n−2) showed that α₁ (APD restitution slope) is largest and heterogeneous in LQT2 but uniform in LQT1, enhancing APD dispersion under long CL_n−1 in LQT2. The α₂ (short-term memory) was negative in LQT2 while positive in LQT1, and the spatial pattern of α₁ was inversely correlated to α₂ in LQT2, which explains why a short-long combination causes a larger APD dispersion in LQT2 but not in LQT1 rabbits. In conclusion, short-long RR pattern increased APD dispersion only in LQT2 rabbits through heterogeneous APD restitution and the short-term memory, underscoring the genotype-specific triggering of arrhythmias in LQT syndrome.

Different paths, same destination: divergent action potential responses produce conserved cardiac fight-or-flight response in mouse and rabbit hearts

KEY POINTS

Cardiac electrophysiology and Ca²⁺ handling change rapidly during the fight-or-flight response to meet physiological demands. Despite dramatic differences in cardiac electrophysiology, the cardiac fight-or-flight response is highly conserved across species. In this study, we performed physiological sympathetic nerve stimulation (SNS) while optically mapping cardiac action potentials and intracellular Ca²⁺ transients in innervated mouse and rabbit hearts. Despite similar heart rate and Ca²⁺ handling responses between mouse and rabbit hearts, we found notable species differences in spatio-temporal repolarization dynamics during SNS. Species-specific computational models revealed that these electrophysiological differences allowed for enhanced Ca²⁺ handling (i.e. enhanced inotropy) in each species, suggesting that electrophysiological responses are fine-tuned across species to produce optimal cardiac fight-or-flight responses.

ABSTRACT

Sympathetic activation of the heart results in positive chronotropy and inotropy, which together rapidly increase cardiac output. The precise mechanisms that produce the electrophysiological and Ca²⁺ handling changes underlying chronotropic and inotropic responses have been studied in detail in isolated cardiac myocytes. However, few studies have examined the dynamic effects of physiological sympathetic nerve activation on cardiac action potentials (APs) and intracellular Ca²⁺ transients (CaTs) in the intact heart. Here, we performed bilateral sympathetic nerve stimulation (SNS) in fully innervated, Langendorff-perfused rabbit and mouse hearts. Dual optical mapping with voltage- and Ca²⁺ -sensitive dyes allowed for analysis of spatio-temporal AP and CaT dynamics. The rabbit heart responded to SNS with a monotonic increase in heart rate (HR), monotonic decreases in AP and CaT duration (APD, CaTD), and a monotonic increase in CaT amplitude. The mouse heart had similar HR and CaT responses; however, a pronounced biphasic APD response occurred, with initial prolongation (50.9 ± 5.1 ms at t = 0 s vs. 60.6 ± 4.1 ms at t = 15 s, P < 0.05) followed by shortening (46.5 ± 9.1 ms at t = 60 s, P = NS vs. t = 0). We determined the biphasic APD response in mouse was partly due to dynamic changes in HR during SNS and was exacerbated by β-adrenergic activation. Simulations with species-specific cardiac models revealed that transient APD prolongation in mouse allowed for greater and more rapid CaT responses, suggesting more rapid increases in contractility; conversely, the rabbit heart requires APD shortening to produce optimal inotropic responses. Thus, while the cardiac fight-or-flight response is highly conserved between species, the underlying mechanisms orchestrating these effects differ significantly.

Antiarrhythmic mechanisms of beta blocker therapy

Sympathetic activity plays an important role in modulation of cardiac rhythm. Indeed, while exerting positive tropic effects in response to physiologic and pathologic stressors, β-adrenergic stimulation influences cardiac electrophysiology and can lead to disturbances of the heart rhythm and potentially lethal arrhythmias, particularly in pathological settings. For this reason, β-blockers are widely utilized clinically as antiarrhythmics. In this review, the molecular mechanisms of β-adrenergic action in the heart, the cellular and tissue level cardiac responses to β-adrenergic stimulation, and the clinical use of β-blockers as antiarrhythmic agents are reviewed. We emphasize the complex interaction between cardiomyocyte signaling, contraction, and electrophysiology occurring over multiple time- and spatial-scales during pathophysiological responses to β-adrenergic stimulation. An integrated understanding of this complex system is essential for optimizing therapies aimed at preventing arrhythmias.

Degradation of T-Tubular Microdomains and Altered cAMP Compartmentation Lead to Emergence of Arrhythmogenic Triggers in Heart Failure Myocytes: An in silico Study

Heart failure (HF) is one of the most common causes of morbidity and mortality worldwide. Although many patients suffering from HF die from sudden cardiac death caused by arrhythmias, the mechanism linking HF remodeling to an increased arrhythmogenic propensity remains incomplete. HF is typically characterized by a progressive loss of transverse tubule (T-tubule) domains, which leads to an altered distribution of L-type calcium channels (LTCCs). Microdomain degradation also causes the disruption of the β₂ adrenergic receptor (β₂AR) and phosphodiesterase (PDE) signaling localization, normally confined to the dyadic space. The goal of this study was to analyze how these subcellular changes affect the function of LTCCs and lead to the emergence of ventricular cell-level triggers of arrhythmias. To accomplish this, we developed a novel computational model of a human ventricular HF myocyte in which LTCCs were divided into six different populations, based on their location and signaling environment they experience. To do so, we included T-tubular microdomain remodeling which led to a subset of LTCCs to be redistributed from the T-tubular to the surface membrane and allowed for different levels of phosphorylation of LTCCs by PKA, based on the presence of β₂ARs and PDEs. The model was used to study the behavior of the LTCC current (I_CaL) under basal and sympathetic stimulation and its effect on cellular action potential. Our results showed that channels redistributed from the T-tubular membrane to the bulk of the sarcolemma displayed an altered function in their new, non-native signaling domain. Incomplete calcium dependent inactivation, which resulted in a longer-lasting and larger-in-magnitude LTCC current, was observed when we decoupled LTCCs from ryanodine receptors and removed them from the dyadic space. The magnitude of the LTCC current, especially in the surface sarcolemma, was also increased via phosphorylation by the redistributed β₂ARs and PDEs. These changes in LTCC current led to the development of early afterdepolarizations. Thus, our study shows that altered LTCC function is a potential cause for the emergence of cell-level triggers of arrhythmia, and that β₂ARs and PDEs present useful therapeutic targets for treatment of HF and prevention of sudden cardiac death.

The cardiac electrophysiology effects of higenamine in guinea pig heart

BACKGROUND

Higenamine (HG) is an active compound derived from Aconiti root with a cardiotonic effect. It has been approved by the Chinese SFDA for clinical trials due to its effect as a potent inotropic and chronotropic agent in the heart. However, the direct mode of action of HG on cardiac electrophysiology is unclear.

METHODS

The experiments were performed at both cell levels and the isolated organ. The major cardiac ion currents and the action potential duration (APD) were measured using patch-clamps in single guinea-pig left ventricular myocytes. ECG was recorded in isolated guinea pig hearts.

RESULTS

In the left ventricular myocytes, HG increased I_Ca-L and I_Ks in concentration- and voltage-dependent manners in the left ventricular myocytes. It potentiated the I_Ca-L and I_Ks simultaneously for synchronization. The EC₅₀ values were 0.27 μM and 0.64 μM for the I_Ca-L and I_Ks, respectively. HG (0.1 μM, 0.5 μM and 1 μM) had no effect on the I_Kr and I_Na. HG slightly prolonged APD at lower concentrations, and shortened the APD at higher concentrations. HG can induce the delayed after depolarization (DAD), which showed some pro-arrhythmic effect. In the isolated perfused heart, HG increased the heart rate via an action on the sinoatrial node cells, but did not induce cardiac arrhythmias, even at high concentrations. The EC₅₀ value for the sinoatrial node that controls the heart rate was 0.13 μM. The sinoatrial node cells appeared to be more sensitive than ventricular myocytes to HG. The effects of HG on ventricular cells and sinoatrial node cells were both mediated through stimulation of β1-AR.

CONCLUSION

We show for the first time that HG produced a predominant action on the sinoatrial node. HG appears to control the cardiac electrophysiology through its predominant effect on the sinoarial node cells, without induction of the ectopic activity that causes cardiac arrhythmias. Thus, HG might be useful for the treatment of bradycardia.

Determinants of early afterdepolarization properties in ventricular myocyte models

Early afterdepolarizations (EADs) are spontaneous depolarizations during the repolarization phase of an action potential in cardiac myocytes. It is widely known that EADs are promoted by increasing inward currents and/or decreasing outward currents, a condition called reduced repolarization reserve. Recent studies based on bifurcation theories show that EADs are caused by a dual Hopf-homoclinic bifurcation, bringing in further mechanistic insights into the genesis and dynamics of EADs. In this study, we investigated the EAD properties, such as the EAD amplitude, the inter-EAD interval, and the latency of the first EAD, and their major determinants. We first made predictions based on the bifurcation theory and then validated them in physiologically more detailed action potential models. These properties were investigated by varying one parameter at a time or using parameter sets randomly drawn from assigned intervals. The theoretical and simulation results were compared with experimental data from the literature. Our major findings are that the EAD amplitude and takeoff potential exhibit a negative linear correlation; the inter-EAD interval is insensitive to the maximum ionic current conductance but mainly determined by the kinetics of I_Ca,L and the dual Hopf-homoclinic bifurcation; and both inter-EAD interval and latency vary largely from model to model. Most of the model results generally agree with experimental observations in isolated ventricular myocytes. However, a major discrepancy between modeling results and experimental observations is that the inter-EAD intervals observed in experiments are mainly between 200 and 500 ms, irrespective of species, while those of the mathematical models exhibit a much wider range with some models exhibiting inter-EAD intervals less than 100 ms. Our simulations show that the cause of this discrepancy is likely due to the difference in I_Ca,L recovery properties in different mathematical models, which needs to be addressed in future action potential model development.

Early afterdepolarizations (EADs) are abnormal depolarizations during the plateau phase of action potential in cardiac myocytes, arising from a dual Hopf-homoclinic bifurcation. The same bifurcations are also responsible for certain types of bursting behaviors in other cell types, such as beta cells and neuronal cells. EADs are known to play important role in the genesis of lethal arrhythmias and have been widely studied in both experiments and computer models. However, a detailed comparison between the properties of EADs observed in experiments and those from mathematical models have not been carried out. In this study, we performed theoretical analyses and computer simulations of different ventricular action potential models as well as different species to investigate the properties of EADs and compared these properties to those observed in experiments. While the EAD properties in the action potential models capture many of the EAD properties seen in experiments, the inter-EAD intervals in the computer models differ a lot from model to model, and some of them show very large discrepancy with those observed in experiments. This discrepancy needs to be addressed in future cardiac action potential model development.

German Cardiac Society Working Group on Cellular Electrophysiology state-of-the-art paper: impact of molecular mechanisms on clinical arrhythmia management

Cardiac arrhythmias remain a common challenge and are associated with significant morbidity and mortality. Effective and safe rhythm control strategies are a primary, yet unmet need in everyday clinical practice. Despite significant pharmacological and technological advances, including catheter ablation and device-based therapies, the development of more effective alternatives is of significant interest to increase quality of life and to reduce symptom burden, hospitalizations and mortality. The mechanistic understanding of pathophysiological pathways underlying cardiac arrhythmias has advanced profoundly, opening up novel avenues for mechanism-based therapeutic approaches. Current management of arrhythmias, however, is primarily guided by clinical and demographic characteristics of patient groups as opposed to individual, patient-specific mechanisms and pheno-/genotyping. With this state-of-the-art paper, the Working Group on Cellular Electrophysiology of the German Cardiac Society aims to close the gap between advanced molecular understanding and clinical decision-making in cardiac electrophysiology. The significance of cellular electrophysiological findings for clinical arrhythmia management constitutes the main focus of this document. Clinically relevant knowledge of pathophysiological pathways of arrhythmias and cellular mechanisms of antiarrhythmic interventions are summarized. Furthermore, the specific molecular background for the initiation and perpetuation of atrial and ventricular arrhythmias and mechanism-based strategies for therapeutic interventions are highlighted. Current "hot topics" in atrial fibrillation are critically appraised. Finally, the establishment and support of cellular and translational electrophysiology programs in clinical rhythmology departments is called for to improve basic-science-guided patient management.

NCX-Mediated Subcellular Ca2+ Dynamics Underlying Early Afterdepolarizations in LQT2 Cardiomyocytes

Long QT syndrome type 2 (LQT2) is a congenital disease characterized by loss of function mutations in hERG potassium channels (I_Kr). LQT2 is associated with fatal ventricular arrhythmias promoted by triggered activity in the form of early afterdepolarizations (EADs). We previously demonstrated that intracellular Ca²⁺ handling is remodeled in LQT2 myocytes. Remodeling leads to aberrant late RyR-mediated Ca²⁺ releases that drive forward-mode Na⁺-Ca²⁺ exchanger (NCX) current and slow repolarization to promote reopening of L-type calcium channels and EADs. Forward-mode NCX was found to be enhanced despite the fact that these late releases do not significantly alter the whole-cell cytosolic calcium concentration during a vulnerable period of phase 2 of the action potential corresponding to the onset of EADs. Here, we use a multiscale ventricular myocyte model to explain this finding. We show that because the local NCX current is a saturating nonlinear function of the local submembrane calcium concentration, a larger number of smaller-amplitude discrete Ca²⁺ release events can produce a large increase in whole-cell forward-mode NCX current without increasing significantly the whole-cell cytosolic calcium concentration. Furthermore, we develop novel insights, to our knowledge, into how alterations of stochastic RyR activity at the single-channel level cause late aberrant Ca²⁺ release events. Experimental measurements in transgenic LTQ2 rabbits confirm the critical arrhythmogenic role of NCX and identify this current as a potential target for antiarrhythmic therapies in LQT2.

Transient Outward K+ Current (Ito) Underlies the Right Ventricular Initiation of Polymorphic Ventricular Tachycardia in a Transgenic Rabbit Model of Long-QT Syndrome Type 1

BACKGROUND

Sudden death in long-QT syndrome type 1 (LQT1), an inherited disease caused by loss-of-function mutations in KCNQ1, is triggered by early afterdepolarizations (EADs) that initiate polymorphic ventricular tachycardia (pVT). We investigated ionic mechanisms that underlie pVT in LQT1 using a transgenic rabbit model of LQT1.

METHODS

Optical mapping, cellular patch clamping, and computer modeling were used to elucidate the mechanisms of EADs in transgenic LQT1 rabbits.

RESULTS

The results showed that shorter action potential duration in the right ventricle (RV) was associated with focal activity during pVT initiation. RV cardiomyocytes demonstrated higher incidence of EADs under 50 nmol/L isoproterenol. Voltage-clamp studies revealed that the transient outward potassium current (Ito) magnitude was 28% greater in RV associated with KChiP2 but with no differences in terms of calcium-cycling kinetics and other sarcolemmal currents. Perfusing with the Ito blocker 4-aminopyridine changed the initial focal sites of pVT from the RV to the left ventricle, corroborating the role of Ito in pVT initiation. Computer modeling showed that EADs occur preferentially in the RV because of the larger conductance of the slow-inactivating component of Ito, which repolarizes the membrane potential sufficiently rapidly to allow reactivation of ICa,L before IKr has had sufficient time to activate.

CONCLUSIONS

Ito heterogeneity creates both triggers and an arrhythmogenic substrate in LQT1. In the absence of IKs, Ito interactions with ICa,L and IKr promote EADs in the RV while prolonging action potential duration in the left ventricle. This heterogeneity of action potential enhances dispersion of refractoriness and facilitates conduction blocks that initiate pVTs.

Beat-to-Beat Variability of Ventricular Action Potential Duration Oscillates at Low Frequency During Sympathetic Provocation in Humans

Background: The temporal pattern of ventricular repolarization is of critical importance in arrhythmogenesis. Enhanced beat-to-beat variability (BBV) of ventricular action potential duration (APD) is pro-arrhythmic and is increased during sympathetic provocation. Since sympathetic nerve activity characteristically exhibits burst patterning in the low frequency range, we hypothesized that physiologically enhanced sympathetic activity may not only increase BBV of left ventricular APD but also impose a low frequency oscillation which further increases repolarization instability in humans. Methods and Results: Heart failure patients with cardiac resynchronization therapy defibrillator devices (n = 11) had activation recovery intervals (ARI, surrogate for APD) recorded from left ventricular epicardial electrodes alongside simultaneous non-invasive blood pressure and respiratory recordings. Fixed cycle length was achieved by right ventricular pacing. Recordings took place during resting conditions and following an autonomic stimulus (Valsalva). The variability of ARI and the normalized variability of ARI showed significant increases post Valsalva when compared to control (p = 0.019 and p = 0.032, respectively). The oscillatory behavior was quantified by spectral analysis. Significant increases in low frequency (LF) power (p = 0.002) and normalized LF power (p = 0.019) of ARI were seen following Valsalva. The Valsalva did not induce changes in conduction variability nor the LF oscillatory behavior of conduction. However, increases in the LF power of ARI were accompanied by increases in the LF power of systolic blood pressure (SBP) and the rate of systolic pressure increase (dP/dtmax). Positive correlations were found between LF-SBP and LF-dP/dtmax (rs = 0.933, p < 0.001), LF-ARI and LF-SBP (rs = 0.681, p = 0.001) and between LF-ARI and LF-dP/dtmax (rs = 0.623, p = 0.004). There was a strong positive correlation between the variability of ARI and LF power of ARI (rs = 0.679, p < 0.001). Conclusions: In heart failure patients, physiological sympathetic provocation induced low frequency oscillation (~0.1 Hz) of left ventricular APD with a strong positive correlation between the LF power of APD and the BBV of APD. These findings may be of importance in mechanisms underlying stability/instability of repolarization and arrhythmogenesis in humans.

Mechanisms linking T-wave alternans to spontaneous initiation of ventricular arrhythmias in rabbit models of long QT syndrome

KEY POINTS

T-wave alternans (TWA) and T-wave lability (TWL) are precursors of ventricular arrhythmias in long QT syndrome; however, the mechanistic link remains to be clarified. Computer simulations show that action potential duration (APD) prolongation and slowed heart rates promote APD alternans and chaos, manifesting as TWA and TWL, respectively. Regional APD alternans and chaos can exacerbate pre-existing or induce de novo APD dispersion, which combines with enhanced I_Ca,L to result in premature ventricular complexes (PVCs) originating from the APD gradient region. These PVCs can directly degenerate into re-entrant arrhythmias without the need for an additional tissue substrate or further exacerbate the APD dispersion to cause spontaneous initiation of ventricular arrhythmias. Experiments conducted in transgenic long QT rabbits show that PVC alternans occurs at slow heart rates, preceding spontaneous intuition of ventricular arrhythmias.

ABSTRACT

T-wave alternans (TWA) and irregular beat-to-beat T-wave variability or T-wave lability (TWL), the ECG manifestations of action potential duration (APD) alternans and variability, are precursors of ventricular arrhythmias in long QT syndromes. TWA and TWL in patients tend to occur at normal heart rates and are usually potentiated by bradycardia. Whether or how TWA and TWL at normal or slow heart rates are causally linked to arrhythmogenesis remains unknown. In the present study, we used computer simulations and experiments of a transgenic rabbit model of long QT syndrome to investigate the underlying mechanisms. Computer simulations showed that APD prolongation and slowed heart rates caused early afterdepolarization-mediated APD alternans and chaos, manifesting as TWA and TWL, respectively. Regional APD alternans and chaos exacerbated pre-existing APD dispersion and, in addition, APD chaos could also induce APD dispersion de novo via chaos desynchronization. Increased APD dispersion, combined with substantially enhanced I_Ca,L , resulted in a tissue-scale dynamical instability that gave rise to the spontaneous occurrence of unidirectionally propagating premature ventricular complexes (PVCs) originating from the APD gradient region. These PVCs could directly degenerate into re-entrant arrhythmias without the need for an additional tissue substrate or could block the following sinus beat to result in a longer RR interval, which further exacerbated the APD dispersion giving rise to the spontaneous occurrence of ventricular arrhythmias. Slow heart rate-induced PVC alternans was observed in experiments of transgenic LQT2 rabbits under isoproterenol, which was associated with increased APD dispersion and spontaneous occurrence of ventricular arrhythmias, in agreement with the theoretical predictions.

Activation of the unfolded protein response downregulates cardiac ion channels in human induced pluripotent stem cell-derived cardiomyocytes

RATIONALE

Heart failure is characterized by electrical remodeling that contributes to arrhythmic risk. The unfolded protein response (UPR) is active in heart failure and can decrease protein levels by increasing mRNA decay, accelerating protein degradation, and inhibiting protein translation.

OBJECTIVE

Therefore, we investigated whether the UPR downregulated cardiac ion channels that may contribute to arrhythmogenic electrical remodeling.

METHODS

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were used to study cardiac ion channels. Action potentials (APs) and ion channel currents were measured by patch clamp recording. The mRNA and protein levels of channels and the UPR effectors were determined by quantitative RT-PCR and Western blotting. Tunicamycin (TM, 50 ng/mL and 5 μg/mL), GSK2606414 (GSK, 300 nmol/L), and 4μ8C (5 μmol/L) were utilized to activate the UPR, inhibit protein kinase-like ER kinase (PERK) and inositol-requiring protein-1 (IRE1), respectively.

RESULTS

TM-induced activation of the UPR caused significant prolongation of the AP duration (APD) and a reduction of the maximum upstroke velocity (dV/dt_max) of the AP phase 0 in both acute (20-24 h) and chronic treatment (6 days). These changes were explained by reductions in the sodium, L-type calcium, the transient outward and rapidly/slowly activating delayed rectifier potassium currents. Na_v1.5, Ca_v1.2, K_v4.3, and K_vLQT1 channels showed concomitant reductions in mRNA and protein levels under activated UPR. Inhibition of PERK or IRE1 shortened the APD and reinstated dV/dt_max. The PERK branch regulated Na_v1.5, K_v4.3, hERG, and K_vLQT1. The IRE1 branch regulated Na_v1.5, hERG, K_vLQT1, and Ca_v1.2.

CONCLUSIONS

Activated UPR downregulates all major cardiac ion currents and results in electrical remodeling in hiPSC-CMs. Both PERK and IRE1 branches downregulate Na_v1.5, hERG, and K_vLQT1. The PERK branch specifically downregulates K_v4.3, while the IRE1 branch downregulates Ca_v1.2. Therefore, the UPR contributed to electrical remodeling, and targeting the UPR might be anti-arrhythmic.

Autonomic conflict exacerbates long QT associated ventricular arrhythmias

This study tested the hypothesis that concomitant sympathetic and parasympathetic stimulation (“autonomic conflict”) may act as a trigger for arrhythmias in long QT syndrome (LQTS). Studies were performed in isolated innervated rabbit hearts treated with clofilium (100 nmol/L); a potassium channel blocker. The influence of vagus nerve stimulation (VNS) on spontaneous ventricular arrhythmias was assessed in the absence/presence of sustained noradrenaline perfusion (100 nmol/L) and with sudden adrenergic stress (injections of noradrenaline into the perfusion line). Hearts were instrumented for a pseudo-electrocardiogram and monophasic action potential recordings. VNS, which slows heart rate, was associated with a stimulation frequency-dependent incidence of spontaneous early after-depolarisations (EADs) and ventricular tachycardia (VT), best predicted by the duration of the electrocardiographic T-wave and by triangulation of the ventricular action potential. In the presence of sustained (steady-state) noradrenaline perfusion, the incidence of EADs and VT with VNS was decreased from 73/55% to 45/27%, respectively. However, sudden adrenergic stress, imposed during periods of sustained VNS, was associated with a transient increase in the incidence of severity of observed arrhythmias, as indicated by an increase in the average arrhythmias score (1.6 ± 0.4 vs. 2.1 ± 0.7, p = .01). Analysis of electrophysiological parameters suggests that sudden adrenergic stress is associated with a transient prolongation, and increased triangulation, of the ventricular action potential, which may predispose to triggered activity. This study demonstrates that autonomic conflict is a pro-arrhythmic stimulus in LQTS. However, combined adrenergic and parasympathetic stimulation has a complex relationship with arrhythmogenicity, with differences in the effects of steady-state adrenergic activation vs. sudden adrenergic stress.

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Long QT syndrome associated arrhythmias were studied with dual autonomic stimulation.

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EADs and TdP were triggered by increased vagal tone.

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Sudden adrenergic stress increased the severity of observed arrhythmias.

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Arrhythmias were prevented by sustained adrenergic stimulation.

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The sequence of autonomic stimulation influences the severity of long QT arrhythmias.

Arrhythmias and Heart Rate: Mechanisms and Significance of a Relationship

The occurrence of arrhythmia is often related to basic heart rate. Prognostic significance is associated with such a relationship; furthermore, heart rate modulation may result as an ancillary effect of therapy, or be considered as a therapeutic tool. This review discusses the cellular mechanisms underlying arrhythmia occurrence during tachycardia or bradycardia, considering rate changes per se or as a mirror of autonomic modulation. Besides the influence of steady-state heart rate, dynamic aspects of changes in rate and autonomic balance are considered. The discussion leads to the conclusion that the prognostic significance of arrhythmia relationship with heart rate, and the consequence of heart rate on arrhythmogenesis, may vary according to the substrate present in the specific case and should be considered accordingly.

Effects of early afterdepolarizations on excitation patterns in an accurate model of the human ventricles

Early Afterdepolarizations, EADs, are defined as the reversal of the action potential before completion of the repolarization phase, which can result in ectopic beats. However, the series of mechanisms of EADs leading to these ectopic beats and related cardiac arrhythmias are not well understood. Therefore, we aimed to investigate the influence of this single cell behavior on the whole heart level. For this study we used a modified version of the Ten Tusscher-Panfilov model of human ventricular cells (TP06) which we implemented in a 3D ventricle model including realistic fiber orientations. To increase the likelihood of EAD formation at the single cell level, we reduced the repolarization reserve (RR) by reducing the rapid delayed rectifier Potassium current and raising the L-type Calcium current. Varying these parameters defined a 2D parametric space where different excitation patterns could be classified. Depending on the initial conditions, by either exciting the ventricles with a spiral formation or burst pacing protocol, we found multiple different spatio-temporal excitation patterns. The spiral formation protocol resulted in the categorization of a stable spiral (S), a meandering spiral (MS), a spiral break-up regime (SB), spiral fibrillation type B (B), spiral fibrillation type A (A) and an oscillatory excitation type (O). The last three patterns are a 3D generalization of previously found patterns in 2D. First, the spiral fibrillation type B showed waves determined by a chaotic bi-excitable regime, i.e. mediated by both Sodium and Calcium waves at the same time and in same tissue settings. In the parameter region governed by the B pattern, single cells were able to repolarize completely and different (spiral) waves chaotically burst into each other without finishing a 360 degree rotation. Second, spiral fibrillation type A patterns consisted of multiple small rotating spirals. Single cells failed to repolarize to the resting membrane potential hence prohibiting the Sodium channel gates to recover. Accordingly, we found that Calcium waves mediated these patterns. Third, a further reduction of the RR resulted in a more exotic parameter regime whereby the individual cells behaved independently as oscillators. The patterns arose due to a phase-shift of different oscillators as disconnection of the cells resulted in continuation of the patterns. For all patterns, we computed realistic 9 lead ECGs by including a torso model. The B and A type pattern exposed the behavior of Ventricular Tachycardia (VT). We conclude that EADs at the single cell level can result in different types of cardiac fibrillation at the tissue and 3D ventricle level.

Pro-arrhythmic effects of low plasma [K+] in human ventricle: An illustrated review

Potassium levels in the plasma, [K⁺]_o, are regulated precisely under physiological conditions. However, increases (from approx. 4.5 to 8.0mM) can occur as a consequence of, e.g., endurance exercise, ischemic insult or kidney failure. This hyperkalemic modulation of ventricular electrophysiology has been studied extensively. Hypokalemia is also common. It can occur in response to diuretic therapy, following renal dialysis, or during recovery from endurance exercise. In the human ventricle, clinical hypokalemia (e.g., [K⁺]_o levels of approx. 3.0mM) can cause marked changes in both the resting potential and the action potential waveform, and these may promote arrhythmias. Here, we provide essential background information concerning the main K⁺-sensitive ion channel mechanisms that act in concert to produce prominent short-term ventricular electrophysiological changes, and illustrate these by implementing recent mathematical models of the human ventricular action potential. Even small changes (~1mM) in [K⁺]_o result in significant alterations in two different K⁺ currents, I_K1 and HERG. These changes can markedly alter in resting membrane potential and/or action potential waveform in human ventricle. Specifically, a reduction in net outward transmembrane K⁺ currents (repolarization reserve) and an increased substrate input resistance contribute to electrophysiological instability during the plateau of the action potential and may promote pro-arrhythmic early after-depolarizations (EADs). Translational settings where these insights apply include: optimal diuretic therapy, and the interpretation of data from Phase II and III trials for anti-arrhythmic drug candidates.

Hysteretic Dynamics of Multi-Stable Early Afterdepolarisations with Repolarisation Reserve Attenuation: A Potential Dynamical Mechanism for Cardiac Arrhythmias

Some cardiovascular and non-cardiovascular drugs frequently cause excessive prolongation of the cardiac action potential (AP) and lead to the development of early afterdepolarisations (EADs), which trigger lethal ventricular arrhythmias. Combining computer simulations in APs with numerical calculations based on dynamical system theory, we investigated stability changes of APs observed in a paced human ventricular myocyte model by decreasing and/or increasing the rapid (I_Kr) and slow (I_Ks) components of delayed rectifying K⁺ current. Upon reducing I_Kr, the APs without EADs (no-EAD response) showed gradual prolongation of AP duration (APD), and were annihilated without AP configuration changes due to the occurrence of saddle-node bifurcations. This annihilation caused a transition to an AP with EADs as a new stable steady state. Furthermore, reducing repolarisation currents (repolarisation reserve attenuation) evoked multi-stable states consisting of APs with different APDs, and caused multiple hysteretic dynamics. Depending on initial ion circumstances within ventricular myocytes, these multi-stable AP states might increase the local/global heterogeneity of AP repolarisations in the ventricle. Thus, the EAD-induced arrhythmias with repolarisation reserve attenuation might be attributed to the APD variability caused by multi-stability in cardiac AP dynamics.

Correlation between human ether-a-go-go-related gene channel inhibition and action potential prolongation

BACKGROUND AND PURPOSE

Human ether-a-go-go-related gene (hERG; Kv 11.1) channel inhibition is a widely accepted predictor of cardiac arrhythmia. hERG channel inhibition alone is often insufficient to predict pro-arrhythmic drug effects. This study used a library of dofetilide derivatives to investigate the relationship between standard measures of hERG current block in an expression system and changes in action potential duration (APD) in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). The interference from accompanying block of Cav 1.2 and Nav 1.5 channels was investigated along with an in silico AP model.

EXPERIMENTAL APPROACH

Drug-induced changes in APD were assessed in hiPSC-CMs using voltage-sensitive dyes. The IC50 values for dofetilide and 13 derivatives on hERG current were estimated in an HEK293 expression system. The relative potency of each drug on APD was estimated by calculating the dose (D150 ) required to prolong the APD at 90% (APD90 ) repolarization by 50%.

KEY RESULTS

The D150 in hiPSC-CMs was linearly correlated with IC50 of hERG current. In silico simulations supported this finding. Three derivatives inhibited hERG without prolonging APD, and these compounds also inhibited Cav 1.2 and/or Nav 1.5 in a channel state-dependent manner. Adding Cav 1.2 and Nav 1.2 block to the in silico model recapitulated the direction but not the extent of the APD change.

CONCLUSIONS AND IMPLICATIONS

Potency of hERG current inhibition correlates linearly with an index of APD in hiPSC-CMs. The compounds that do not correlate have additional effects including concomitant block of Cav 1.2 and/or Nav 1.5 channels. In silico simulations of hiPSC-CMs APs confirm the principle of the multiple ion channel effects.

Spatial Patterns of Excitation at Tissue and Whole Organ Level Due to Early Afterdepolarizations

Early after depolarizations (EAD) occur in many pathological conditions, such as congenital or acquired channelopathies, drug induced arrhythmias, and several other situations that are associated with increased arrhythmogenicity. In this paper we present an overview of the relevant computational studies on spatial EAD dynamics in 1D, 2D, and in 3D anatomical models and discuss the relation of EADs to cardiac arrhythmias. We also discuss unsolved problems and highlight new lines of research in this area.