Molecular basis of functional voltage-gated K+ channel diversity in the mammalian myocardium

The key characteristics of cardiotoxicity for the pervasive pollutant phenanthrene

The key characteristic (KCs) framework has been used previously to assess the carcinogenicity and cardiotoxicity of various chemical and pharmacological agents. Here, the 12 KCs of cardiotoxicity are used to evaluate the previously reported cardiotoxicity of phenanthrene (Phe), a tricyclic polycyclic aromatic hydrocarbon (PAH), and major component of fossil fuel-derived air pollution. Phe is a semi-volatile pollutant existing in both the gas phase and particle phase through adsorption onto or into particulate matter (PM). Phe can translocate across the airways and gastrointestinal tract into the systemic circulation, enabling body-wide effects. Our evaluation based on a comprehensive literature review, indicates Phe exhibits 11 of the 12 KCs for cardiotoxicity. These include adverse effects on cardiac electromechanical performance, the vasculature and endothelium, immunomodulation and oxidative stress, and neuronal and endocrine control. Environmental agents that have similarly damaging effects on the cardiovascular system are heavily regulated and monitored, yet globally there is no air quality regulation specific for PAHs like Phe. Environmental monitoring of Phe is not the international standard with benzo[a]pyrene being frequently used as a proxy despite the two PAH species exhibiting significant differences in sources, concentration variations and toxic effects. The evidence summarised in this evaluation highlights the need to move away from proxied PAH measurements and develop a monitoring network capable of measuring Phe concentration. It also stresses the need to raise awareness amongst the medical community of the potential cardiovascular impact of PAH exposure. This will allow the production of mitigation strategies and possibly the development of new policies for the protection of the societal groups most vulnerable to cardiovascular disease.

Effects of electro-mechanical uncouplers, hormonal stimulation and pacing rate on the stability and function of cultured rabbit myocardial slices

Introduction: Recent advances have enabled organotypic culture of beating human myocardial slices that are stable for weeks. However, human myocardial samples are rare, exhibit high variability and frequently originate from diseased hearts. Thus, there is a need to adapt long-term slice culture for animal myocardium. When applied to animal cardiac slices, studies in healthy or genetically modified myocardium will be possible. We present the culture of slices from rabbit hearts, which resemble the human heart in microstructure, electrophysiology and excitation-contraction coupling. Methods: Left ventricular myocardium from New Zealand White rabbits was cut using a vibratome and cultured in biomimetic chambers for up to 7 days (d). Electro-mechanical uncoupling agents 2,3-butanedione monoxime (BDM) and cytochalasin D (CytoD) were added during initiation of culture and effects on myocyte survival were quantified. We investigated pacing rates (0.5 Hz, 1 Hz, and 2 Hz) and hormonal supplements (cortisol, T3, catecholamines) at physiological plasma concentrations. T3 was buffered using BSA. Contractile force was recorded continuously. Glucose consumption and lactate production were measured. Whole-slice Ca2+ transients and action potentials were recorded. Effects of culture on microstructure were investigated with confocal microscopy and image analysis. Results: Protocols for human myocardial culture resulted in sustained contracture and myocyte death in rabbit slices within 24 h, which could be prevented by transient application of a combination of BDM and CytoD. Cortisol stabilized contraction amplitude and kinetics in culture. T3 and catecholaminergic stimulation did not further improve stability. T3 and higher pacing rates increased metabolic rate and lactate production. T3 stabilized the response to β-adrenergic stimulation over 7 d. Pacing rates above 1 Hz resulted in progredient decline in contraction force. Image analysis revealed no changes in volume fractions of cardiomyocytes or measures of fibrosis over 7 d. Ca2+ transient amplitudes and responsiveness to isoprenaline were comparable after 1 d and 7 d, while Ca2+ transient duration was prolonged after 7 d in culture. Conclusions: A workflow for rabbit myocardial culture has been established, preserving function for up to 7 d. This research underscores the importance of glucocorticoid signaling in maintaining tissue function and extending culture duration. Furthermore, BDM and CytoD appear to protect from tissue damage during the initiation phase of tissue culture.

Potassium channels in behavioral brain disorders. Molecular mechanisms and therapeutic potential: A narrative review

Potassium channels (K+-channels) selectively control the passive flow of potassium ions across biological membranes and thereby also regulate membrane excitability. Genetic variants affecting many of the human K+-channels are well known causes of Mendelian disorders within cardiology, neurology, and endocrinology. K+-channels are also primary targets of many natural toxins from poisonous organisms and drugs used within cardiology and metabolism. As genetic tools are improving and larger clinical samples are being investigated, the spectrum of clinical phenotypes implicated in K+-channels dysfunction is rapidly expanding, notably within immunology, neurosciences, and metabolism. K+-channels that previously were considered to be expressed in only a few organs and to have discrete physiological functions, have recently been found in multiple tissues and with new, unexpected functions. The pleiotropic functions and patterns of expression of K+-channels may provide additional therapeutic opportunities, along with new emerging challenges from off-target effects. Here we review the functions and therapeutic potential of K+-channels, with an emphasis on the nervous system, roles in neuropsychiatric disorders and their involvement in other organ systems and diseases.

Long-QT mutations in KCNE1 modulate the 17β-estradiol response of Kv7.1/KCNE1

Estradiol (17[Formula: see text]-E2) is implicated in higher arrhythmia risk of women with congenital or acquired long-QT syndrome (LQTS) compared to men. However, the underlying mechanisms remain poorly understood, and little is known about the impact of LQTS-associated mutations. We show that 17[Formula: see text]-E2 inhibits the human cardiac Kv7.1/KCNE1 channel expressed in Xenopus oocytes. We find that the 17[Formula: see text]-E2 effect depends on the Kv7.1 to KCNE1 stoichiometry, and we reveal a critical function of the KCNE1 carboxyl terminus for the effect. LQTS-associated mutations in the KCNE1 carboxyl terminus show a range of responses to 17[Formula: see text]-E2, from a wild-type like response to impaired or abolished response. Together, this study increases our understanding of the mechanistic basis for 17[Formula: see text]-E2 inhibition of Kv7.1/KCNE1 and demonstrates mutation-dependent responses to 17[Formula: see text]-E2. These findings suggest that the 17[Formula: see text]-E2 effect on Kv7.1/KCNE1 might contribute to the higher arrhythmia risk of women, particularly in carriers with specific LQTS-associated mutations.

Expression of AKRs superfamily and prognostic in human gastric cancer

The human aldo-keto reductase (AKRs) superfamily is involved in the development of various tumors. However, the different expression patterns of AKRs and their prognostic value in gastric cancer (GC) have not been clarified. In this study, we analyzed the gene expression and gene methylation level of AKRs in GC patients and the survival data and immune infiltration based on AKRs expression, using data from different databases. We found that the expression levels of AKR1B10, AKR1C1, AKR1C2, and AKR7A3 in GC tissues were lower and the expression level of AKR6A5 was higher in GC tissues than in normal tissue. These differentially expressed genes (AKR1B10, AKR1C1, AKR1C2, AKR7A3, and AKR6A5) were significantly correlated with the infiltration level. The expression of SPI1 and AKR6A5 in GC was positively correlated. Survival analysis showed that GC levels of AKR6A5 reduced or increased mRNA levels of AKR7A3, and AKR1B10 was expected to have higher overall survival (OS), first progression (FP) survival, and postprogression survival (PPS) rates and a better prognosis. Moreover, the expression of AKR1B1 was found to be correlated with the staging of GC. The methylation of AKR6A5 (KCNAB2) at cg05307871 and cg01907457 was significantly associated with the classification of GC. Meta-analysis and ROC curve analysis show that the expression level of AKR1B1 and the methylation of cg16156182 (KCNAB1), cg11194299 (KCNAB2), cg16132520 (AKR1B1), and cg13801416 (AKR1B1) had a high hazard ratio and a good prognostic value. These data suggest that the expression and methylation of AKR1B1 and AKR6A5 are significantly related to the prognosis.

Proton-coupled monocarboxylate transporters in cancer: From metabolic crosstalk, immunosuppression and anti-apoptosis to clinical applications

The Warburg effect is known as the hyperactive glycolysis that provides the energy needed for rapid growth and proliferation in most tumor cells even under the condition of sufficient oxygen. This metabolic pattern can lead to a large accumulation of lactic acid and intracellular acidification, which can affect the growth of tumor cells and lead to cell death. Proton-coupled monocarboxylate transporters (MCTs) belong to the SLC16A gene family, which consists of 14 members. MCT1-4 promotes the passive transport of monocarboxylate (e.g., lactate, pyruvate, and ketone bodies) and proton transport across membranes. MCT1-4-mediated lactate shuttling between glycolytic tumor cells or cancer-associated fibroblasts and oxidative tumor cells plays an important role in the metabolic reprogramming of energy, lipids, and amino acids and maintains the survival of tumor cells. In addition, MCT-mediated lactate signaling can promote tumor angiogenesis, immune suppression and multidrug resistance, migration and metastasis, and ferroptosis resistance and autophagy, which is conducive to the development of tumor cells and avoid death. Although there are certain challenges, the study of targeted drugs against these transporters shows great promise and may form new anticancer treatment options.

Modulation of KV4.3-KChIP2 Channels by IQM-266: Role of DPP6 and KCNE2

The transient outward potassium current (I_tof) is generated by the activation of K_V4 channels assembled with KChIP2 and other accessory subunits (DPP6 and KCNE2). To test the hypothesis that these subunits modify the channel pharmacology, we analyzed the electrophysiological effects of (3-(2-(3-phenoxyphenyl)acetamido)-2-naphthoic acid) (IQM-266), a new KChIP2 ligand, on the currents generated by K_V4.3/KChIP2, K_V4.3/KChIP2/DPP6 and K_V4.3/KChIP2/KCNE2 channels. CHO cells were transiently transfected with cDNAs codifying for different proteins (K_V4.3/KChIP2, K_V4.3/KChIP2/DPP6 or K_V4.3/KChIP2/KCNE2), and the potassium currents were recorded using the whole-cell patch-clamp technique. IQM-266 decreased the maximum peak of K_V4.3/KChIP2, K_V4.3/KChIP2/DPP6 and K_V4.3/KChIP2/KCNE2 currents, slowing their time course of inactivation in a concentration-, voltage-, time- and use-dependent manner. IQM-266 produced an increase in the charge in K_V4.3/KChIP2 channels that was intensified when DPP6 was present and abolished in the presence of KCNE2. IQM-266 induced an activation unblocking effect during the application of trains of pulses to cells expressing K_V4.3/KChIP2 and K_V4.3/KChIP2/KCNE2, but not in K_V4.3/KChIP2/DPP6 channels. Overall, all these results are consistent with a preferential IQM-266 binding to an active closed state of Kv4.3/KChIP2 and Kv4.3/KChIP2/KCNE2 channels, whereas in the presence of DPP6, IQM-266 binds preferentially to an inactivated state. In conclusion, DPP6 and KCNE2 modify the pharmacological response of K_V4.3/KChIP2 channels to IQM-266.

A computational model of rabbit geometry and ECG: Optimizing ventricular activation sequence and APD distribution

Computational modeling of electrophysiological properties of the rabbit heart is a commonly used way to enhance and/or complement findings from classic lab work on single cell or tissue levels. Yet, thus far, there was no possibility to extend the scope to include the resulting body surface potentials as a way of validation or to investigate the effect of certain pathologies. Based on CT imaging, we developed the first openly available computational geometrical model not only of the whole heart but also the complete torso of the rabbit. Additionally, we fabricated a 32-lead ECG-vest to record body surface potential signals of the aforementioned rabbit. Based on the developed geometrical model and the measured signals, we then optimized the activation sequence of the ventricles, recreating the functionality of the Purkinje network, and we investigated different apico-basal and transmural gradients in action potential duration. Optimization of the activation sequence resulted in an average root mean square error between measured and simulated signal of 0.074 mV/ms for all leads. The best-fit T-Wave, compared to measured data (0.038 mV/ms), resulted from incorporating an action potential duration gradient from base to apex with a respective shortening of 20 ms and a transmural gradient with a shortening of 15 ms from endocardium to epicardium. By making our model and measured data openly available, we hope to give other researchers the opportunity to verify their research, as well as to create the possibility to investigate the impact of electrophysiological alterations on body surface signals for translational research.

Oxytocin exerts harmful cardiac repolarization prolonging effects in drug-induced LQTS

Background

Oxytocin is used therapeutically in psychiatric patients. Many of these also receive anti-depressant or anti-psychotic drugs causing acquired long-QT-syndrome (LQTS) by blocking HERG/I_Kr. We previously identified an oxytocin-induced QT-prolongation in LQT2 rabbits, indicating potential harmful effects of combined therapy. We thus aimed to analyze the effects of dual therapy with oxytocin and fluoxetine/risperidone on cardiac repolarization.

Methods

Effects of risperidone, fluoxetine and oxytocin on QT/QTc, short-term variability (STV) of QT, and APD were assessed in rabbits using in vivo ECG and ex vivo monophasic AP recordings in Langendorff-perfused hearts. Underlying mechanisms were assessed using patch clamp in isolated cardiomyocytes.

Results

Oxytocin, fluoxetine and risperidone prolonged QTc and APD in whole hearts. The combination of fluoxetine + oxytocin resulted in further QTc- and APD-prolongation, risperidone + oxytocin tended to increase QTc and APD compared to monotherapy. Temporal QT instability, STV_QTc was increased by oxytocin, fluoxetine / fluoxetine + oxytocin and risperidone / risperidone + oxytocin. Similar APD-prolonging effects were confirmed in isolated cardiomyocytes due to differential effects of the compounds on repolarizing ion currents: Oxytocin reduced I_Ks, fluoxetine and risperidone reduced I_Kr, resulting in additive effects on I_Ktotal-tail. In addition, oxytocin reduced I_K1, further reducing the repolarization reserve.

Conclusion

Oxytocin, risperidone and fluoxetine prolong QTc / APD. Combined treatment further prolongs QTc/APD due to differential effects on I_Ks and I_K1 (block by oxytocin) and I_Kr (block by risperidone and fluoxetine), leading to pronounced impairment of repolarization reserve. Oxytocin should be used with caution in patients in the context of acquired LQTS.

Ionic currents underlying different patterns of electrical activity in working cardiac myocytes of mammals and non-mammalian vertebrates

The orderly contraction of the vertebrate heart is determined by generation and propagation of cardiac action potentials (APs). APs are generated by the integrated activity of time- and voltage-dependent ionic channels which carry inward Na⁺ and Ca²⁺ currents, and outward K⁺ currents. This review compares atrial and ventricular APs and underlying ion currents between different taxa of vertebrates. We have collected literature data and attempted to find common electrophysiological features for two or more vertebrate groups, show differences between taxa and cardiac chambers, and indicate gaps in the existing data. Although electrical excitability of the heart in all vertebrates is based on the same superfamily of channels, there is a vast variability of AP waveforms between atrial and ventricular myocytes, between different species of the same vertebrate class and between endothermic and ectothermic animals. The wide variability of AP shapes is related to species-specific differences in animal size, heart rate, stage of ontogenetic development, excitation-contraction coupling, temperature and oxygen availability. Some of the differences between taxa are related to evolutionary development of genomes, which appear e.g. in the expression of different Na⁺ and K⁺ channel orthologues in cardiomyocytes of vertebrates. There is a wonderful variability of AP shapes and underlying ion currents with which electrical excitability of vertebrate heart can be generated depending on the intrinsic and extrinsic conditions of animal body. This multitude of ionic mechanisms provides excellent material for studying how the function of the vertebrate heart can adapt or acclimate to prevailing physiological and environmental conditions.

Pharmacological characterisation of electrocardiogram J-Tpeak interval in conscious Guinea pigs

Drug-induced human ether-à-go-go-related gene (hERG) channel block and QT interval prolongation increase torsade de pointes (TdP) risk. However, some drugs block hERG channels and prolong QT interval with low TdP risk, likely because they block additional inward currents. We investigated the utility of J-T_peak interval, a novel biomarker of inward current block and TdP risk, in conscious telemetered guinea pigs. Electrocardiogram parameters were analysed in Hartley guinea pigs orally administered one of eight test compounds (dofetilide, flecainide, nifedipine, quinidine, quinine, ranolazine, sotalol, verapamil) or vehicle alone as controls. Heart rate-corrected QT (QTcX) and J-T_peak (J-T_peakcX) were calculated to evaluate the relations of QT-RR and J-T_peak-RR. Dofetilide and sotalol significantly increased ΔQTcX and ΔJ-T_peakcX intervals to similar degrees. Quinidine, quinine and flecainide also increased ΔQTcX and ΔJ-T_peakcX intervals, but the degrees of ΔJ-T_peakcX interval prolongation were shorter than those of ΔQTcX interval prolongation. Ranolazine showed slight increasing trends in ΔQTcX and ΔJ-T_peakcX intervals, but the differences were not significant. Verapamil and nifedipine did not increase the ΔQTcX or ΔJ-T_peakcX intervals. Based on the relations of ΔΔJ-T_peakcX and ΔΔQTcX intervals, dofetilide, sotalol and quinidine were classified as high risk for TdP, quinine, flecainide and ranolazine were classified as intermediate risk and verapamil and nifedipine were classified as low risk. These results supported the usefulness of J-T_peak interval assessment in conscious guinea pigs for predicting drug-induced balanced block of inward currents and TdP risk in early-stage preclinical studies.

Effects of JZTX-V on the wild type Kv4.3 Expressed in HEK293T and Molecular Determinants in the Voltage-sensing Domains of Kv4.3 Interacting with JZTX-V

JZTX-V is a toxin isolated from the venom of the Chinese spider Chilobrachys jingzhao. Previous studies had shown that JZTX-V could inhibit the transient outward potassium current of Kv4.2 and Kv4.3 expressed in Xenopus oocytes but had no effects on Kv1.2–1.4. However, the underlying action mechanism of JZTX-V on Kv4.3 remains unclear. In our study, JZTX-V could inhibit not only transient outward potassium currents evoked in small-sized DRG neurons but also Kv4.3-encoded currents expressed in HEK293T cells in the concentration and voltage dependence. The half maximal inhibitory concentration of JZTX-V on Kv4.3 was 9.6 ± 1.2 nM. In addition, the time course for JZTX-V inhibition and release of inhibition after washout were 15.8 ± 1.54 s and 58.8 ± 4.35 s. Electrophysiological assays indicated that 25 nM JZTX-V could shift significantly the voltage dependence of steady-state activation and steady-state inactivation to depolarization. Meanwhile, 25 nM JZTX-V decreased markedly the time constant of activation and inactivation but had no effect on the time constant of recovery from inactivation. To study the molecular determinants of Kv4.3, we performed alanine scanning on a conserved motif of Kv4.3 and assayed the affinity between mutants and JZTX-V. The results not only showed that I273, L275, V283, and F287 were molecular determinants in the conserved motif of Kv4.3 for interacting with JZTX-V but also speculated the underlying action mechanism that the hydrophobic interaction and steric effects played key roles in the binding of JZTX-V with Kv4.3. In summary, our studies have laid a scientific theoretical foundation for further research on the interaction mechanism between JZTX-V and Kv4.3.

Glucocorticoid Induces Atrial Arrhythmogenesis via Modification of Ion Channel Gene Expression in Rats

Excess psychological stress is one of the precipitating factors for paroxysmal atrial fibrillation (AF), although the involved mechanisms are still uncertain. To test a hypothesis that one of the stress-induced hormones, glucocorticoid, is involved in the pathogenesis of stress-induced AF, we investigated whether the glucocorticoid could alter the temporal profile of cardiac ion channels gene expression, thereby leading to atrial arrhythmogenesis.Dexamethasone (DEX, 1.0 mg/kg) was injected subcutaneously in Sprague-Dawley rats. At predetermined times after DEX injection (0, 1, 3, 6, 12, and 24 hours), the mRNA levels of cardiac ion channel genes (erg, KvLQT1, Kv4.3, Kv4.2, Kv2.1, Kv1.5, Kv1.4, Kv1.2, SUR2A, Kir6.2, Kir3.4, Kir3.1 Kir2.2, Kir2.1, SCN5A, and α1C) were determined using RNase protection assay. DEX induced immediate and transient increase in the mRNA level of Kv1.5 and Kir2.2 with peaks at 6 (5.0 fold) and 3 hours (3.3 fold) after DEX injection, respectively. Patch-clamp studies revealed a significantly increased current density of the corresponding current, I_Kur and I_K1 at 6 hours after DEX injection. Simultaneously, electrophysiological study in isolated perfused hearts showed significantly increased number of repetitive atrial responses induced by single atrial extrastimulus (3.2 ± 2.4 to 26.7 ± 16.4, P = 0.004) with shorting of the refractory period (36.4 ± 4.6 to 27.4 ± 5.5 ms, P = 0.049) after DEX injection.Glucocorticoid immediately modified Kv1.5 and Kir2.2 gene expression at pretranslational levels, thus leading to effective refractory period shortening that could be arrhythmogenic. These results implied that transient glucocorticoid-induced biochemical modification of cardiac ion channels might be one of the mechanisms underlying the stress-induced paroxysmal AF.

Genetically modified rabbit models for cardiovascular medicine

Genetically modified (GM) rabbits are outstanding animal models for studying human genetic and acquired diseases. As such, GM rabbits that express human genes have been extensively used as models of cardiovascular disease. Rabbits are genetically modified via prokaryotic microinjection. Through this process, genes are randomly integrated into the rabbit genome. Moreover, gene targeting in embryonic stem (ES) cells is a powerful tool for understanding gene function. However, rabbits lack stable ES cell lines. Therefore, ES-dependent gene targeting is not possible in rabbits. Nevertheless, the RNA interference technique is rapidly becoming a useful experimental tool that enables researchers to knock down specific gene expression, which leads to the genetic modification of rabbits. Recently, with the emergence of new genetic technology, such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR), and CRISPR-associated protein 9 (CRISPR/Cas9), major breakthroughs have been made in rabbit gene targeting. Using these novel genetic techniques, researchers have successfully modified knockout (KO) rabbit models. In this paper, we aimed to review the recent advances in GM technology in rabbits and highlight their application as models for cardiovascular medicine.

Celebrities in the heart, strangers in the pancreatic beta cell: Voltage-gated potassium channels Kv 7.1 and Kv 11.1 bridge long QT syndrome with hyperinsulinaemia as well as type 2 diabetes

Voltage‐gated potassium (K_v) channels play an important role in the repolarization of a variety of excitable tissues, including in the cardiomyocyte and the pancreatic beta cell. Recently, individuals carrying loss‐of‐function (LoF) mutations in KCNQ1, encoding K_v7.1, and KCNH2 (hERG), encoding K_v11.1, were found to exhibit post‐prandial hyperinsulinaemia and episodes of hypoglycaemia. These LoF mutations also cause the cardiac disorder long QT syndrome (LQTS), which can be aggravated by hypoglycaemia. Interestingly, patients with LQTS also have a higher burden of diabetes compared to the background population, an apparent paradox in relation to the hyperinsulinaemic phenotype, and KCNQ1 has been identified as a type 2 diabetes risk gene. This review article summarizes the involvement of delayed rectifier K⁺ channels in pancreatic beta cell function, with emphasis on K_v7.1 and K_v11.1, using the cardiomyocyte for context. The functional and clinical consequences of LoF mutations and polymorphisms in these channels on blood glucose homeostasis are explored using evidence from pre‐clinical, clinical and genome‐wide association studies, thereby evaluating the link between LQTS, hyperinsulinaemia and type 2 diabetes.

Renal artery denervation prevents ventricular arrhythmias in long QT rabbit models

Long QT syndrome (LQTS) is commonly presented with life-threatening ventricular arrhythmias (VA). Renal artery denervation (RDN) is an alternative antiadrenergic treatment that attenuates sympathetic activity. We aimed to evaluate the efficacy of RDN on preventing VAs in LQTS rabbits induced by drugs. The subtypes of LQTS were induced by infusion of HMR-1556 for LQTS type 1 (LQT1), erythromycin for LQTS type 2 (LQT2), and veratridine for LQTS type 3 (LQT3). Forty-four rabbits were randomized into the LQT1, LQT2, LQT3, LQT1-RDN, LQT2-RDN, and LQT3-RDN groups. All rabbits underwent cardiac electrophysiology studies. The QTc interval of the LQT2-RDN group was significantly shorter than those in the LQT2 group (650.08 ± 472.67 vs. 401.78 ± 42.91 ms, p = 0.011). The QTc interval of the LQT3-RDN group was significantly shorter than those in the LQT3 group (372.00 ± 22.41 vs. 335.70 ± 28.21 ms, p = 0.035). The VA inducibility in all subtypes of the LQT-RDN groups was significantly lower than those in the LQT-RDN groups, respectively (LQT1: 9.00 ± 3.30 vs. 47.44 ± 4.21%, p < 0.001; LQT2: 11.43 ± 6.37 vs. 45.38 ± 5.29%, p = 0.026; LQT3: 10.00 ± 6.32 vs. 32.40 ± 7.19%, p = 0.006). This study demonstrated the neuromodulation of RDN leading to electrical remodeling and reduced VA inducibility of the ventricular substrate in LQT models.

Docosahexaenoic acid normalizes QT interval in long QT type 2 transgenic rabbit models in a genotype-specific fashion

AIM

Long QT syndrome (LQTS) is a cardiac channelopathy predisposing to ventricular arrhythmias and sudden cardiac death. Since current therapies often fail to prevent arrhythmic events in certain LQTS subtypes, new therapeutic strategies are needed. Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid, which enhances the repolarizing IKs current.

METHODS AND RESULTS

We investigated the effects of DHA in wild type (WT) and transgenic long QT Type 1 (LQT1; loss of IKs), LQT2 (loss of IKr), LQT5 (reduction of IKs), and LQT2-5 (loss of IKr and reduction of IKs) rabbits. In vivo ECGs were recorded at baseline and after 10 µM/kg DHA to assess changes in heart-rate corrected QT (QTc) and short-term variability of QT (STVQT). Ex vivo monophasic action potentials were recorded in Langendorff-perfused rabbit hearts, and action potential duration (APD75) and triangulation were assessed. Docosahexaenoic acid significantly shortened QTc in vivo only in WT and LQT2 rabbits, in which both α- and β-subunits of IKs-conducting channels are functionally intact. In LQT2, this led to a normalization of QTc and of its short-term variability. Docosahexaenoic acid had no effect on QTc in LQT1, LQT5, and LQT2-5. Similarly, ex vivo, DHA shortened APD75 in WT and normalized it in LQT2, and additionally decreased AP triangulation in LQT2.

CONCLUSIONS

Docosahexaenoic acid exerts a genotype-specific beneficial shortening/normalizing effect on QTc and APD75 and reduces pro-arrhythmia markers STVQT and AP triangulation through activation of IKs in LQT2 rabbits but has no effects if either α- or β-subunits to IKs are functionally impaired. Docosahexaenoic acid could represent a new genotype-specific therapy in LQT2.

A comparative review on heart ion channels, action potentials and electrocardiogram in rodents and human: extrapolation of experimental insights to clinic

Electrocardiogram (ECG) is a non-invasive valuable diagnostic tool that is used in clinics for investigation and monitoring of heart electrical rhythm/conduction, ischemia/injury of heart, electrolyte disturbances and agents/drugs induced cardiac toxicity. Nowadays using animal models to study heart diseases such as electrical and mechanical disturbance is common. In addition, given to ethical consideration and availability, the use of small rodents has been a top priority for cardiovascular researchers. However, extrapolation of experimental findings from the lab to the clinic needs sufficient basic knowledge of similarities and differences between heart action potential and ECG of rodents and humans in normal and disease conditions. This review compares types of human action potentials, the dominant ion currents during action potential phases, alteration in ion channels activities in channelopathies-induced arrhythmias and the ECG appearance of mouse, rat, guinea pig, rabbit and human. Also, it briefly discusses the responsiveness and alterations in ECG following some interventions such as cardiac injury and arrhythmia induction. Overall, it provides a roadmap for researchers in selecting the best animal model/species whose studies results can be translated into clinical practice. In addition, this study will also be useful to biologists, physiologists, pharmacologists, veterinarians and physicians working in the fields of comparative physiology, pharmacology, toxicology and diseases.

Heritable arrhythmias associated with abnormal function of cardiac potassium channels

Cardiomyocytes express a surprisingly large number of potassium channel types. The primary physiological functions of the currents conducted by these channels are to maintain the resting membrane potential and mediate action potential repolarization under basal conditions and in response to changes in the concentrations of intracellular sodium, calcium, and ATP/ADP. Here, we review the diversity and functional roles of cardiac potassium channels under normal conditions and how heritable mutations in the genes encoding these channels can lead to distinct arrhythmias. We briefly review atrial fibrillation and J-wave syndromes. For long and short QT syndromes, we describe their genetic basis, clinical manifestation, risk stratification, traditional and novel therapeutic approaches, as well as insights into disease mechanisms provided by animal and cellular models.

ESC working group on cardiac cellular electrophysiology position paper: relevance, opportunities, and limitations of experimental models for cardiac electrophysiology research

Cardiac arrhythmias are a major cause of death and disability. A large number of experimental cell and animal models have been developed to study arrhythmogenic diseases. These models have provided important insights into the underlying arrhythmia mechanisms and translational options for their therapeutic management. This position paper from the ESC Working Group on Cardiac Cellular Electrophysiology provides an overview of (i) currently available in vitro, ex vivo, and in vivo electrophysiological research methodologies, (ii) the most commonly used experimental (cellular and animal) models for cardiac arrhythmias including relevant species differences, (iii) the use of human cardiac tissue, induced pluripotent stem cell (hiPSC)-derived and in silico models to study cardiac arrhythmias, and (iv) the availability, relevance, limitations, and opportunities of these cellular and animal models to recapitulate specific acquired and inherited arrhythmogenic diseases, including atrial fibrillation, heart failure, cardiomyopathy, myocarditis, sinus node, and conduction disorders and channelopathies. By promoting a better understanding of these models and their limitations, this position paper aims to improve the quality of basic research in cardiac electrophysiology, with the ultimate goal to facilitate the clinical translation and application of basic electrophysiological research findings on arrhythmia mechanisms and therapies.

Alamandine significantly reduces doxorubicin-induced cardiotoxicity in rats

Doxorubicin (DOX) is an anthracycline antibiotic. Despite its unwanted side effects, it has been successfully used in tumor therapy. Given that oxidative stress and inflammatory factors are essential to cardiotoxicity caused by DOX, we assumed that alamandine, which enhances endogenous antioxidants and has anti-inflammatory effects, may prevent DOX-induced cardiotoxicity. Rats received DOX (3.75 mg/kg) i.p on days 14, 21, 28, and 35 (total cumulative dose = 15 mg/kg) and alamandine (50 μg/kg/day) via mini-osmotic pumps for 42 days. At the end of the 42-day period, we evaluated hemodynamic parameters, electrocardiogram, cardiac troponin I (cTnI), superoxidase dismutase (SOD), total antioxidant capacity (TAC), malondialdehyde (MDA), inflammatory cytokines (tumor necrosis factor-α (TNF-α), IL-1β, NF-κB), apoptosis markers (caspase 3), and histopathology of haemotoxylin- and eosin-stained cardiac muscle fibers were evaluated. DOX significantly increased QT, corrected QT (QTc), and RR intervals. Alamandine co-therapy prevented ECG changes. Alamandine administration restored DOX-induced disruptions in the cardiac muscle architecture and vascular congestion. Alamandine co-therapy also alleviated other effects of DOX, including cardiac contractility, decreased systolic and diastolic blood pressure, and increased left ventricular end-diastolic pressure. Moreover, alamandine co-therapy substantially decreased the elevation of oxidative stress markers, inflammatory cytokines, and caspase 3 in DOX-treated rats. The results suggest that alamandine reduced DOX-induced cardiotoxicity via antioxidant, anti-inflammatory, and anti-apoptotic activities.

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.

Riluzole inhibits Kv4.2 channels acting on the closed and closed inactivated states

Riluzole is an anticonvulsant drug also used to treat the amyotrophic lateral sclerosis and major depressive disorder. This compound has antiglutamatergic activity and is an important multichannel blocker. However, little is known about its actions on the Kv4.2 channels, the molecular correlate of the A-type K⁺ current (I_A) and the fast transient outward current (I_tof). Here, we investigated the effects of riluzole on Kv4.2 channels transiently expressed in HEK-293 cells. Riluzole inhibited Kv4.2 channels with an IC₅₀ of 190 ± 14 μM and the effect was voltage- and frequency-independent. The activation rate of the current (at +50 mV) was not affected by the drug, nor the voltage dependence of channel activation, but the inactivation rate was accelerated by 100 and 300 μM riluzole. When Kv4.2 channels were maintained at the closed state, riluzole incubation induced a tonic current inhibition. In addition, riluzole significantly shifted the voltage dependence of inactivation to hyperpolarized potentials without affecting the recovery from inactivation. In the presence of the drug, the closed-state inactivation was significantly accelerated, and the percentage of inactivated channels was increased. Altogether, our findings indicate that riluzole inhibits Kv4.2 channels mainly affecting the closed and closed-inactivated states.

ERG1 plays an essential role in rat cardiomyocyte fate decision by mediating AKT signaling

ERG1, a potassium ion channel, is essential for cardiac action potential repolarization phase. However, the role of ERG1 for normal development of the heart is poorly understood. Using the rat embryonic stem cells (rESCs) model, we show that ERG1 is crucial in cardiomyocyte lineage commitment via interactions with Integrin β1. In the mesoderm phase of rESCs, the interaction of ERG1 with Integrin β1 can activate the AKT pathway by recruiting and phosphorylating PI3K p85 and focal adhesion kinase (FAK) to further phosphorylate AKT. Activation of AKT pathway promotes cardiomyocyte differentiation through two different mechanisms, (a) through phosphorylation of GSK3β to upregulate the expression levels of β-catenin and Gata4; (b) through promotion of nuclear translocation of nuclear factor-κB by phosphorylating IKKβ to inhibit cell apoptosis, which occurs due to increased Bcl2 expression. Our study provides solid evidence for a novel role of ERG1 on differentiation of rESCs into cardiomyocytes.

Pharmacological Approaches for the Modulation of the Potassium Channel KV4.x and KChIPs

Ion channels are macromolecular complexes present in the plasma membrane and intracellular organelles of cells. Dysfunction of ion channels results in a group of disorders named channelopathies, which represent an extraordinary challenge for study and treatment. In this review, we will focus on voltage-gated potassium channels (K_V), specifically on the K_V4-family. The activation of these channels generates outward currents operating at subthreshold membrane potentials as recorded from myocardial cells (I_TO, transient outward current) and from the somata of hippocampal neurons (I_SA). In the heart, K_V4 dysfunctions are related to Brugada syndrome, atrial fibrillation, hypertrophy, and heart failure. In hippocampus, K_V4.x channelopathies are linked to schizophrenia, epilepsy, and Alzheimer's disease. K_V4.x channels need to assemble with other accessory subunits (β) to fully reproduce the I_TO and I_SA currents. β Subunits affect channel gating and/or the traffic to the plasma membrane, and their dysfunctions may influence channel pharmacology. Among K_V4 regulatory subunits, this review aims to analyze the K_V4/KChIPs interaction and the effect of small molecule KChIP ligands in the A-type currents generated by the modulation of the K_V4/KChIP channel complex. Knowledge gained from structural and functional studies using activators or inhibitors of the potassium current mediated by K_V4/KChIPs will better help understand the underlying mechanism involving K_V4-mediated-channelopathies, establishing the foundations for drug discovery, and hence their treatments.

A Mathematical Model of the Mouse Atrial Myocyte With Inter-Atrial Electrophysiological Heterogeneity

Biophysically detailed mathematical models of cardiac electrophysiology provide an alternative to experimental approaches for investigating possible ionic mechanisms underlying the genesis of electrical action potentials and their propagation through the heart. The aim of this study was to develop a biophysically detailed mathematical model of the action potentials of mouse atrial myocytes, a popular experimental model for elucidating molecular and cellular mechanisms of arrhythmogenesis. Based on experimental data from isolated mouse atrial cardiomyocytes, a set of mathematical equations for describing the biophysical properties of membrane ion channel currents, intracellular Ca²⁺ handling, and Ca²⁺-calmodulin activated protein kinase II and β-adrenergic signaling pathways were developed. Wherever possible, membrane ion channel currents were modeled using Markov chain formalisms, allowing detailed representation of channel kinetics. The model also considered heterogeneous electrophysiological properties between the left and the right atrial cardiomyocytes. The developed model was validated by its ability to reproduce the characteristics of action potentials and Ca²⁺ transients, matching quantitatively to experimental data. Using the model, the functional roles of four K⁺ channel currents in atrial action potential were evaluated by channel block simulations, results of which were quantitatively in agreement with existent experimental data. To conclude, this newly developed model of mouse atrial cardiomyocytes provides a powerful tool for investigating possible ion channel mechanisms of atrial electrical activity at the cellular level and can be further used to investigate mechanisms underlying atrial arrhythmogenesis.

Sex-specific IKAS activation in rabbit ventricles with drug-induced QT prolongation

BACKGROUND

Female sex is a known risk factor for drug-induced long QT syndrome (diLQTS). We recently demonstrated a sex difference in apamin-sensitive small-conductance Ca²⁺-activated K⁺ current (I_KAS) activation during β-adrenergic stimulation.

OBJECTIVE

The purpose of this study was to test the hypothesis that there is a sex difference in I_KAS in the rabbit models of diLQTS.

METHODS

We evaluated the sex difference in ventricular repolarization in 15 male and 22 female Langendorff-perfused rabbit hearts with optical mapping techniques during atrial pacing. HMR1556 (slowly activating delayed rectifier K⁺ current [I_Ks] blocker), E4031 (rapidly activating delayed rectifier K⁺ current [I_Kr] blocker) and sea anemone toxin (ATX-II, late Na⁺ current [I_NaL] activator) were used to simulate types 1-3 long QT syndrome, respectively. Apamin, an I_KAS blocker, was then added to determine the magnitude of further QT prolongation.

RESULTS

HMR1556, E4031, and ATX-II led to the prolongation of action potential duration at 80% repolarization (APD₈₀) in both male and female ventricles at pacing cycle lengths of 300-400 ms. Apamin further prolonged APD₈₀ (pacing cycle length 350 ms) from 187.8±4.3 to 206.9±7.1 (P=.014) in HMR1556-treated, from 209.9±7.8 to 224.9±7.8 (P=.003) in E4031-treated, and from 174.3±3.3 to 188.1±3.0 (P=.0002) in ATX-II-treated female hearts. Apamin did not further prolong the APD₈₀ in male hearts. The Ca_i transient duration (Ca_iTD) was significantly longer in diLQTS than baseline but without sex differences. Apamin did not change Ca_iTD.

CONCLUSION

We conclude that I_KAS is abundantly increased in female but not in male ventricles with diLQTS. Increased I_KAS helps preserve the repolarization reserve in female ventricles treated with I_Ks and I_Kr blockers or I_NaL activators.

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.

MG53, A Novel Regulator of KChIP2 and Ito,f, Plays a Critical Role in Electrophysiological Remodeling in Cardiac Hypertrophy

BACKGROUND

KChIP2 (K⁺ channel interacting protein) is the auxiliary subunit of the fast transient outward K⁺ current ( I_to,f) in the heart, and insufficient KChIP2 expression induces I_to,f downregulation and arrhythmogenesis in cardiac hypertrophy. Studies have shown muscle-specific mitsugumin 53 (MG53) has promiscuity of function in the context of normal and diseased heart. This study investigates the possible roles of cardiac MG53 in regulation of KChIP2 expression and I_to,f, and the arrhythmogenic potential in hypertrophy.

METHODS

MG53 expression is manipulated by genetic ablation of MG53 in mice and adenoviral overexpression or knockdown of MG53 by RNA interference in cultured neonatal rat ventricular myocytes. Cardiomyocyte hypertrophy is produced by phenylephrine stimulation in neonatal rat ventricular myocytes, and pressure overload-induced mouse cardiac hypertrophy is produced by transverse aortic constriction.

RESULTS

KChIP2 expression and I_to,f density are downregulated in hearts from MG53-knockout mice and MG53-knockdown neonatal rat ventricular myocytes, but upregulated in MG53-overexpressing cells. In phenylephrine-induced cardiomyocyte hypertrophy, MG53 expression is reduced with concomitant downregulation of KChIP2 and I_to,f, which can be reversed by MG53 overexpression, but exaggerated by MG53 knockdown. MG53 knockout enhances I_to,f remodeling and action potential duration prolongation and increases susceptibility to ventricular arrhythmia in mouse cardiac hypertrophy. Mechanistically, MG53 regulates NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) activity and subsequently controls KChIP2 transcription. Chromatin immunoprecipitation demonstrates NF-κB protein has interaction with KChIP2 gene. MG53 overexpression decreases, whereas MG53 knockdown increases NF-κB enrichment at the 5' regulatory region of KChIP2 gene. Normalizing NF-κB activity reverses the alterations in KChIP2 in MG53-overexpressing or knockdown cells. Coimmunoprecipitation and Western blotting assays demonstrate MG53 has physical interaction with TAK1 (transforming growth factor-b [TGFb]-activated kinase 1) and IκBα (nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha), critical components of the NF-κB pathway.

CONCLUSIONS

These findings establish MG53 as a novel regulator of KChIP2 and I_to,f by modulating NF-κB activity and reveal its critical role in electrophysiological remodeling in cardiac hypertrophy.

Murine model of left ventricular diastolic dysfunction and electro-mechanical uncoupling following high-fat diet

BACKGROUND/OBJECTIVES

It is well established that obesity is an independent risk factor for cardiac death. In particular various cardiac alterations have been described in obese patients such as long QT on ECG, impaired diastolic filling of the left ventricle (LV), and all-type arrhythmias. In the present study, the above alterations were all reproduced in a mouse model of fat diet-induced obesity.

ANIMALS/METHODS

In C57BL6 mice fed on a high fat (n = 20, HF-group) or standard diet (n = 20, C-group) for 13 weeks, balanced by sex and age, we examined heart morphology and function by high-frequency ultrasounds and electric activity by surface ECG. Besides, the autonomic sympathovagal balance (heart-rate variability) and the arrhythmogenic susceptibility to adrenergic challenge (i.p. isoproterenol) were compared in the two groups, as well as glucose tolerance (i.p. glucose test) and liver steatosis (ultrasounds).

RESULTS

Body weight in HF-group exceeded C-group at the end of the experiment (+28% p < 0.01). An abnormal ventricular repolarization (long QTc on ECG) together with impaired LV filling rate and increased LV mass was found in HF-group as compared to C. Moreover, HF-group showed higher heart rate, unbalanced autonomic control with adrenergic prevalence and a greater susceptibility to develop rhythm disturbances under adrenergic challenge (i.p. isoprenaline). Impaired glucose tolerance and higher liver fat accumulation were also found in HF mice compared to C.

CONCLUSIONS

The described murine model of 13 weeks on HF diet, well reproduced the cardiovascular and metabolic disorders reported in clinical obesity, suggesting its potential utility as translational mean suitable for testing new pharmaco-therapeutic approaches to the treatment of obesity and its comorbidity.

Carvedilol blockage of delayed rectifier Kv2.1 channels and its molecular basis

Previous studies indicated that one of the action targets of carvedilol is the voltage-gated potassium (Kv) channel, which has a fundamental role in the control of electrical properties in excitable cells. It is not clear whether this compound exerts any actions specifically on delayed rectifier Kv2.1 channels. The present study is conducted on Kv2.1 currents heterologously expressed in HEK293 cells to characterize the block by carvedilol in detail, identifying molecular determinants and providing biophysical insights of the block. Macroscopic Kv2.1 currents obtained by whole-cell recording were substantially inhibited after addition of carvedilol with an IC₅₀ value of 5.1 μM. This drug also led to a largely hyperpolarizing shift (30 mV) of the inactivation curve, which may contribute to the blocking action due to more inactivation of Kv2.1 currents occurred in depolarization potentials. Mutations at Y380 (a putative TEA binding site) and K356 (a K⁺ binding site) in the outer vestibule of Kv2.1 channels significantly eliminated the inhibitory effects of carvedilol and prevented the leftward shift of inactivation. Moreover, mutations at above positions modulated the effects of carvedilol on the deactivation but not activation kinetics of Kv2.1 channels. Collected data indicate that carvedilol can exert a blocking effect on the closed-state of Kv2.1 channels, and specific residues within the S5-P and P-S6 linkers in the outer vestibule may play crucial roles in carvedilol-induced blocking for Kv2.1 channels.

Inhibitory effects of class I antiarrhythmic agents on Na+ and Ca2+ currents of human iPS cell-derived cardiomyocytes

INTRODUCTION

Human induced pluripotent stem cells (hiPSCs) harboring cardiac myosin heavy chain 6 promoter can differentiate into functional cardiomyocytes called "iCell cardiomyocytes" under blasticidin treatment condition. While iCell cardiomyocytes are expected to be used for predicting cardiotoxicity of drugs, their responses to antiarrhythmic agents remain to be elucidated. We first examined electrophysiological properties of iCell cardiomyocytes and mRNA levels of ion channels and Ca handling proteins, and then evaluated effects of class I antiarrhythmic agents on their Na+ and Ca2+ currents.

METHODS

iCell cardiomyocytes were cultured for 8-14 days (38-44 days after inducing their differentiation), according to the manufacturer's protocol. We determined their action potentials (APs) and sarcolemmal ionic currents using whole-cell patch clamp techniques, and also mRNA levels of ion channels and Ca handling proteins by RT-PCR. Effects of three class I antiarrhythmic agents, pirmenol, pilsicainide and mexiletine, on Na+ channel current (INa) and L-type Ca2+ channel current (ICaL) were evaluated by the whole-cell patch clamp.

RESULTS

iCell cardiomyocytes revealed sinoatrial node-type (18%), atrial-type (18%) and ventricular-type (64%) spontaneous APs. The maximum peak amplitudes of INa, ICaL, and rapidly-activating delayed-rectifier K+ channel current were -62.7 ± 13.7, -8.1 ± 0.7, and 3.0 ± 1.0 pA/pF, respectively. The hyperpolarization-activated cation channel and inward-rectifier K+ channel currents were observed, whereas the T-type Ca2+ channel or slowly-activating delayed-rectifier K+ channel current was not detectable. mRNAs of Nav1.5, Cav1.2, Kir2.1, HCN4, KvLQT1, hERG and SERCA2 were detected, while that of HCN1, minK or MiRP was not. The class Ia antiarrhythmic agent pirmenol and class Ic agent pilsicainide blocked INa in a concentration-dependent manner with IC50 of 0.87 ± 0.37 and 0.88 ± 0.16 μM, respectively; the class Ib agent mexiletine revealed weak INa block with a higher IC50 of 30.0 ± 3.0 μM. Pirmenol, pilsicainide and mexiletine blocked ICaL with IC50 of 2.00 ± 0.39, 7.7 ± 2.5 and 5.0 ± 0.1 μM, respectively.

CONCLUSIONS

In iCell cardiomyocytes, INa was blocked by the class Ia and Ic antiarrhythmic agents and ICaL was blocked by all the class I agents within the ranges of clinical concentrations, suggesting their cardiotoxicity.

Biochemical and physiological properties of K+ channel-associated AKR6A (Kvβ) proteins

Voltage-gated potassium (Kv) channels play an essential role in the regulation of membrane excitability and thereby control physiological processes such as cardiac excitability, neural communication, muscle contraction, and hormone secretion. Members of the Kv1 and Kv4 families are known to associate with auxiliary intracellular Kvβ subunits, which belong to the aldo-keto reductase superfamily. Electrophysiological studies have shown that these proteins regulate the gating properties of Kv channels. Although the three gene products encoding Kvβ proteins are functional enzymes in that they catalyze the nicotinamide adenine dinucleotide phosphate (NAD[P]H)-dependent reduction of a wide range of aldehyde and ketone substrates, the physiological role for these proteins and how each subtype may perform unique roles in coupling membrane excitability with cellular metabolic processes remains unclear. Here, we discuss current knowledge of the enzymatic properties of Kvβ proteins from biochemical studies with their described and purported physiological and pathophysiological influences.

From Bernstein's rheotome to Neher-Sakmann's patch electrode. The action potential

The aim of this review was to provide an overview of the most important stages in the development of cellular electrophysiology. The period covered starts with Bernstein's formulation of the membrane hypothesis and the measurement of the nerve and muscle action potential. Technical innovations make discoveries possible. This was the case with the use of the squid giant axon, allowing the insertion of "large" intracellular electrodes and derivation of transmembrane potentials. Application of the newly developed voltage clamp method for measuring ionic currents, resulted in the formulation of the ionic theory. At the same time transmembrane measurements were made possible in smaller cells by the introduction of the microelectrode. An improvement of this electrode was the next major (r)evolution. The patch electrode made it possible to descend to the molecular level and record single ionic channel activity. The patch technique has been proven to be exceptionally versatile. In its whole-cell configuration it was the solution to measure voltage clamp currents in small cells. See also: https://doi.org/10.14814/phy2.13860 & https://doi.org/10.14814/phy2.13862.

Haloperidol affects coupling between QT and RR intervals in guinea pig isolated heart

Prolonged QT interval is an independent risk factor for development of ventricular arrhythmias. Haloperidol is one of the drugs inducing QT prolongation. Previous studies showed that haloperidol affects not only QT duration but also heart rate (RR interval). The present work focused on relationship between QT and RR and its changes under acute and chronic haloperidol administration. The study included 14 male guinea pigs divided into control and haloperidol-treated group. After 21-days administration of haloperidol or vehiculum, electrograms in isolated hearts were recorded. QT/RR and dQT/dRR coupling were calculated. Chronic haloperidol administration significantly decreases the coupling between QT and RR. Acute haloperidol exposure significantly decreases the dQT/dRR coupling in both treated and untreated guinea pig hearts. Flatter QT/RR relationship reveals a lack of QT adaptation to increased heart rate. It should be emphasized that in such situation ECG recording will not show significant QT prolongation evaluated according to clinical rules. However, if QT interval does not adapt to increased heart rate sufficiently, the risk of ventricular arrhythmias may be increased despite practically normal QT interval length. The results are supported by findings in biochemical analyses, which proved eligibility of the used model.

Cholesterol Protects Against Acute Stress-Induced T-Tubule Remodeling in Mouse Ventricular Myocytes

Efficient excitation-contraction coupling in ventricular myocytes depends critically on the presence of the t-tubular network. It has been recently demonstrated that cholesterol, a major component of the lipid bilayer, plays an important role in long-term maintenance of the integrity of t-tubular system although mechanistic understanding of underlying processes is essentially lacking. Accordingly, in this study we investigated the contribution of membrane cholesterol to t-tubule remodeling in response to acute hyposmotic stress. Experiments were performed using isolated left ventricular cardiomyocytes from adult mice. Depletion and restoration of membrane cholesterol was achieved by applying methyl-β-cyclodextrin (MβCD) and water soluble cholesterol (WSC), respectively, and t-tubule remodeling in response to acute hyposmotic stress was assessed using fluorescent dextran trapping assay and by measuring t-tubule dependent I_K1 tail current (I_K1,tail). The amount of dextran trapped in t-tubules sealed in response to stress was significantly increased when compared to control cells, and reintroduction of cholesterol to cells treated with MβCD restored the amount of trapped dextran to control values. Alternatively, application of WSC to normal cells significantly reduced the amount of trapped dextran further suggesting the protective effect of cholesterol. Importantly, modulation of membrane cholesterol (without osmotic stress) led to significant changes in various parameters of I_K1, _tail strongly suggesting significant but essentially hidden remodeling of t-tubules prior to osmotic stress. Results of this study demonstrate that modulation of the level of membrane cholesterol has significant effects on the susceptibility of cardiac t-tubules to acute hyposmotic stress.

Remote-Controlling Potassium Channels in Living Cells through Photothermal Inactivation of Calmodulin

Spatiotemporal regulation of cellular functions provides a powerful strategy for understanding underlying mechanisms of cellular bioprocesses. Here, a strategy is reported to realize the remote control of the activities of potassium channels via photothermal inactivation of calmodulin (CaM) by using reduced graphene oxide decorated with calmodulin binding peptide (rGO-P) as the transducer with near-infrared light (NIR) irradiation. Upon NIR light irradiation, the CaM/Ca²⁺ bound to rGO-P is inactivated by the photothermal effect of rGO-P, resulting in the incapability of binding with Ca²⁺ . Hence, the closed Kv10.1 channel is converted to be open in the presence of calcium in living cells. Meanwhile, the SK2 channel is induced to be closed from the open state and the Kir2.1 channel is unaffected by the intracellular inactivation of CaM. This strategy gives a noninvasive and effective approach to remotely control the activities of potassium channels, offering an alternative for the development of optogenetics.

The Beneficial Effects of Electroacupuncture at PC6 Acupoints (Neiguan) on Myocardial Ischemia in ASIC3 -/- mice

This study aims to investigate the possible mechanisms of electroacupuncture (EA) at PC6 to improve myocardial ischemia (MI) by regulating the cardiac transient outward potassium current channel (Ito). According to the random number table, the mice were divided into six groups of six mice each: control group, MI group, PC6, LU7 (Lieque-point), ST36 (Zusanli-point), and nonacupoint group. Mice in the control group were injected with saline (20 mg/kg, 24 hours interval), and the other ASIC3 -/- mice were injected subcutaneously twice with isoproterenol (ISO) (20 mg/kg, 24 hours interval). In the preexperiment, 5 mg/kg, 10 mg/kg, 20 mg/kg, and 30 mg/kg of ISO were used, and the results showed that 5 mg/kg and 10 mg/kg of ISO both could induce acute MI, but shorter duration of sustained MI. On the other hand, an injection of 30 mg/kg can make the mice experience arrhythmia or die immediately, and EA was operated at PC6, LU7, ST36 acupoints, and nonacupoint in the mice of PC6, LU7, ST36, and nonacupoint groups, respectively, after injecting twice. Then Western blotting techniques (Western Blot) were used to analyze the protein expressions of Kv1.4, Kv4.2, Kv4.3, and KchIP2. The results of this experiment showed that the protein expressions of Kv1.4, Kv4.2, Kv4.3, and KChIP2 in MI group were significantly lower than those in the control group (p < 0.01). Compared with MI group, the results of PC6, LU7, and ST36 groups obviously increased (p < 0.05). Furthermore, the expressions of PC6 group were higher than LU7 group and ST36 group (p < 0.05). And electrocardiogram's T-waves showed obvious pathological changes in the MI group compared to the control group (p < 0.01). After EA, the abnormal T-waves voltage of ECG in PC6, LU7, and ST36 groups was improved (p < 0.05). In addition, the rate change of PC6 group was larger than that of both LU7 and ST36 groups (p < 0.05). But the T-waves voltage of the nonacupoint group was not significantly different than that of the MI group (p > 0.05).

Isolated heart models for studying cardiac electrophysiology: a historical perspective and recent advances

Experimental models used in cardiovascular research range from cellular to whole heart preparations. Isolated whole hearts show higher levels of structural and functional integration than lower level models such as tissues or cellular fragments. Cardiovascular diseases are multi-factorial problems that are dependent on highly organized structures rather than on molecular or cellular components alone. This article first provides a general introduction on the animal models of cardiovascular diseases. It is followed by a detailed overview and a historical perspective of the different isolated heart systems with a particular focus on the Langendorff perfusion method for the study of cardiac arrhythmias. The choice of species, perfusion method, and perfusate composition are discussed in further detail with particular considerations of the theoretical and practical aspects of experimental settings.

Effects of Nitric Oxide on Voltage-Gated K⁺ Currents in Human Cardiac Fibroblasts through the Protein Kinase G and Protein Kinase A Pathways but Not through S-Nitrosylation

This study investigated the expression of voltage-gated K⁺ (K_V) channels in human cardiac fibroblasts (HCFs), and the effect of nitric oxide (NO) on the K_V currents, and the underlying phosphorylation mechanisms. In reverse transcription polymerase chain reaction, two types of K_V channels were detected in HCFs: delayed rectifier K⁺ channel and transient outward K⁺ channel. In whole-cell patch-clamp technique, delayed rectifier K⁺ current (I_K) exhibited fast activation and slow inactivation, while transient outward K⁺ current (I_to) showed fast activation and inactivation kinetics. Both currents were blocked by 4-aminopyridine. An NO donor, S-nitroso-N-acetylpenicillamine (SNAP), increased the amplitude of I_K in a concentration-dependent manner with an EC₅₀ value of 26.4 µM, but did not affect I_to. The stimulating effect of SNAP on I_K was blocked by pretreatment with 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or by KT5823. 8-bromo-cyclic GMP stimulated the I_K. The stimulating effect of SNAP on I_K was also blocked by pretreatment with KT5720 or by SQ22536. Forskolin and 8-bromo-cyclic AMP each stimulated I_K. On the other hand, the stimulating effect of SNAP on I_K was not blocked by pretreatment of N-ethylmaleimide or by DL-dithiothreitol. Our data suggest that NO enhances I_K, but not I_to, among K_V currents of HCFs, and the stimulating effect of NO on I_K is through the PKG and PKA pathways, not through S-nitrosylation.

Differential responses of rabbit ventricular and atrial transient outward current (Ito) to the Ito modulator NS5806

Transient outward potassium current (I_to) in the heart underlies phase 1 repolarization of cardiac action potentials and thereby affects excitation–contraction coupling. Small molecule activators of I_to may therefore offer novel treatments for cardiac dysfunction, including heart failure and atrial fibrillation. NS5806 has been identified as a prototypic activator of canine I_to. This study investigated, for the first time, actions of NS5806 on rabbit atrial and ventricular I_to. Whole cell patch‐clamp recordings of I_to and action potentials were made at physiological temperature from rabbit ventricular and atrial myocytes. 10 μmol/L NS5806 increased ventricular I_to with a leftward shift in I_to activation and accelerated restitution. At higher concentrations, stimulation of I_to was followed by inhibition. The EC₅₀ for stimulation was 1.6 μmol/L and inhibition had an IC₅₀ of 40.7 μmol/L. NS5806 only inhibited atrial I_to (IC₅₀ of 18 μmol/L) and produced a modest leftward shifts in I_to activation and inactivation, without an effect on restitution. 10 μmol/L NS5806 shortened ventricular action potential duration (APD) at APD₂₀‐APD₉₀ but prolonged atrial APD. NS5806 also reduced atrial AP upstroke and amplitude, consistent with an additional atrio‐selective effect on Na⁺ channels. In contrast to NS5806, flecainide, which discriminates between Kv1.4 and 4.x channels, produced similar levels of inhibition of ventricular and atrial I_to. NS5806 discriminates between rabbit ventricular and atrial I_to, with mixed activator and inhibitor actions on the former and inhibitor actions against the later. NS5806 may be of significant value for pharmacological interrogation of regional differences in native cardiac I_to.

Caveolae in Rabbit Ventricular Myocytes: Distribution and Dynamic Diminution after Cell Isolation

Caveolae are signal transduction centers, yet their subcellular distribution and preservation in cardiac myocytes after cell isolation are not well documented. Here, we quantify caveolae located within 100 nm of the outer cell surface membrane in rabbit single-ventricular cardiomyocytes over 8 h post-isolation and relate this to the presence of caveolae in intact tissue. Hearts from New Zealand white rabbits were either chemically fixed by coronary perfusion or enzymatically digested to isolate ventricular myocytes, which were subsequently fixed at 0, 3, and 8 h post-isolation. In live cells, the patch-clamp technique was used to measure whole-cell plasma membrane capacitance, and in fixed cells, caveolae were quantified by transmission electron microscopy. Changes in cell-surface topology were assessed using scanning electron microscopy. In fixed ventricular myocardium, dual-axis electron tomography was used for three-dimensional reconstruction and analysis of caveolae in situ. The presence and distribution of surface-sarcolemmal caveolae in freshly isolated cells matches that of intact myocardium. With time, the number of surface-sarcolemmal caveolae decreases in isolated cardiomyocytes. This is associated with a gradual increase in whole-cell membrane capacitance. Concurrently, there is a significant increase in area, diameter, and circularity of sub-sarcolemmal mitochondria, indicative of swelling. In addition, electron tomography data from intact heart illustrate the regular presence of caveolae not only at the surface sarcolemma, but also on transverse-tubular membranes in ventricular myocardium. Thus, caveolae are dynamic structures, present both at surface-sarcolemmal and transverse-tubular membranes. After cell isolation, the number of surface-sarcolemmal caveolae decreases significantly within a time frame relevant for single-cell research. The concurrent increase in cell capacitance suggests that membrane incorporation of surface-sarcolemmal caveolae underlies this, but internalization and/or micro-vesicle loss to the extracellular space may also contribute. Given that much of the research into cardiac caveolae-dependent signaling utilizes isolated cells, and since caveolae-dependent pathways matter for a wide range of other study targets, analysis of isolated cell data should take the time post-isolation into account.

Molecular cloning and functional expression of the K+ channel KV7.1 and the regulatory subunit KCNE1 from equine myocardium

BACKGROUND

The voltage-gated K⁺-channel K_V7.1 and the subunit KCNE1, encoded by the KCNQ1 and KCNE1 genes, respectively, are responsible for termination of the cardiac action potential. In humans, mutations in these genes can predispose patients to arrhythmias and sudden cardiac death (SCD).

AIM

To characterize equine K_V7.1/KCNE1 currents and compare them to human K_V7.1/KCNE1 currents to determine whether K_V7.1/KCNE1 plays a similar role in equine and human hearts.

METHODS

mRNA encoding K_V7.1 and KCNE1 was isolated from equine hearts, sequenced, and cloned into expression vectors. The channel subunits were heterologously expressed in Xenopus laevis oocytes or CHO-K1 cells and characterized using voltage-clamp techniques.

RESULTS

Equine K_V7.1/KCNE1 expressed in CHO-K1 cells exhibited electrophysiological properties that are overall similar to the human orthologs; however, a slower deactivation was found which could result in more open channels at fast rates.

CONCLUSION

The results suggest that the equine K_V7.1/KCNE1 channel may be important for cardiac repolarization and this could indicate that horses are susceptible to SCD caused by mutations in KCNQ1 and KCNE1.

Adrenergic Stress Protection of Human iPS Cell-Derived Cardiomyocytes by Fast Kv7.1 Recycling

The fight-or-flight response (FFR), a physiological acute stress reaction, involves positive chronotropic and inotropic effects on heart muscle cells mediated through β-adrenoceptor activation. Increased systolic calcium is required to enable stronger heart contractions whereas elevated potassium currents are to limit the duration of the action potentials and prevent arrhythmia. The latter effect is accomplished by an increased functional activity of the K_v7.1 channel encoded by KCNQ1. Current knowledge, however, does not sufficiently explain the full extent of rapid K_v7.1 activation and may hence be incomplete. Using inducible genetic KCNQ1 complementation in KCNQ1-deficient human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), we here reinvestigate the functional role of K_v7.1 in adapting human CMs to adrenergic stress. Under baseline conditions, K_v7.1 was barely detectable at the plasma membrane of hiPSC-CMs, yet it fully protected these from adrenergic stress-induced beat-to-beat variability of repolarization and torsade des pointes-like arrhythmia. Furthermore, isoprenaline treatment increased field potential durations specifically in KCNQ1-deficient CMs to cause these adverse macroscopic effects. Mechanistically, we find that the protective action by K_v7.1 resides in a rapid translocation of channel proteins from intracellular stores to the plasma membrane, induced by adrenergic signaling. Gene silencing experiments targeting RAB GTPases, mediators of intracellular vesicle trafficking, showed that fast K_v7.1 recycling under acute stress conditions is RAB4A-dependent.Our data reveal a key mechanism underlying the rapid adaptation of human cardiomyocytes to adrenergic stress. These findings moreover aid to the understanding of disease pathology in long QT syndrome and bear important implications for safety pharmacological screening.

Electrophysiological characteristics of pressure overload-induced cardiac hypertrophy and its influence on ventricular arrhythmias

OBJECTIVE

To explore the cardiac electrophysiological characteristics of cardiac hypertrophy and its influence on the occurrence of ventricular tachyarrhythmias.

METHODS

Adult C57BL6 mice were randomly divided into a surgery group and a control group. Thoracic aortic constriction was performed on mice in the surgery group, and cardiac anatomical and ultrasonic evaluations were performed to confirm the success of the cardiac hypertrophy model 4 weeks after the operation. Using the Langendorff method of isolated heart perfusion, monophasic action potentials (MAPs) and the effective refractory period (ERP) at different parts of the heart (including the epi- and endo-myocardium of the left and right ventricles) were measured, and the induction rate of ventricular tachyarrhythmias was observed under programmed electrical stimulus (PES) and burst stimulus. Whole-cell patch-clamp was used to obtain the I-V characteristics of voltage-gated potassium channels in cardiomyocytes of different parts of the heart (including the epi- and endo-myocardium of the left and right ventricles) as well as the channels' properties of steady-state inactivation and recovery from inactivation.

RESULTS

The ratio of heart weight to body weight and the ratio of left ventricular weight to body weight in the surgery group were significantly higher than those in the control group (P < 0.05). Ultrasonic evaluation revealed that both interventricular septal diameter (IVSD) and left ventricle posterior wall diameter (LVPWD) in the surgery group were significantly larger than those in the control group (P < 0.05). Under PES and burst stimuli, the induction rates of arrhythmias in the surgery group significantly increased, reaching 41.2% and 23.5%, respectively. Both the QT interval and action potential duration (APD) in the surgery group were significantly longer than in the control group (P<0.01), and the changes showed obvious spatial heterogeneity. Whole-cell patch-clamp recordings demonstrated that the surgery group had significantly lower potassium current densities (IPeak, Ito, IKur, Iss, and IK1) at different parts of the heart than the control group (P < 0.01), and there were significant differences in the half-inactivation voltage (V1/2) and the inactivation-recovery time constant (τ) of Ito and IKur at different parts of the heart (P < 0.01) between the surgery group and the control group. In addition, the surgery group had significantly lower densities of IPeak, Ito, and IKur in cells of the endo-myocardium (P < 0.05), and the changes showed obvious spatial heterogeneity.

CONCLUSION

Changes in the current density and function of potassium channels contributed to irregular repolarization in cardiac hypertrophy, and the spatially heterogeneous changes of the channels may increase the occurrence of ventricular arrhythmias that accompany cardiac hypertrophy.

Interregional electro-mechanical heterogeneity in the rabbit myocardium

BACKGROUND

Increased electrical heterogeneity has been causatively linked to arrhythmic disorders, yet the knowledge about physiological heterogeneity remains incomplete. This study investigates regional electro-mechanical heterogeneities in rabbits, one of the key animal models for arrhythmic disorders.

METHODS AND FINDINGS

7 wild-type rabbits were examined by phase-contrast magnetic resonance imaging in vivo to assess cardiac wall movement velocities. Using a novel data-processing algorithm regional contraction-like profiles were calculated. Contraction started earlier and was longer in left ventricular (LV) apex than base. Patch clamp recordings showed longer action potentials (AP) in LV apex compared to the base of LV, septum, and right ventricle. Western blots of cardiac ion channels and calcium handling proteins showed lower expression of Cav1.2, KvLQT1, Kv1.4, NCX and Phospholamban in LV apex vs. base. A single-cell in silico model integrating the quantitative regional differences in ion channels reproduced a longer contraction and longer AP in apex vs. base.

CONCLUSIONS

Apico-basal electro-mechanical heterogeneity is physiologically present in the healthy rabbit heart. An apico-basal electro-mechanical gradient exists with longer APD and contraction duration in the apex and associated regionally heterogeneous expression of five key proteins. This pattern of apical mechanical dominance probably serves to increase pumping efficiency.

The IK1/Kir2.1 channel agonist zacopride prevents and cures acute ischemic arrhythmias in the rat

Arrhythmogenesis in acute myocardial infarction (MI) is associated with depolarization of resting membraine potential (RMP) and decrease of inward rectifier potassium current (I_K1) in cardiomyocytes. However, clinical anti-arrhythmic agents that primarily act on RMP by enhancing the I_K1 channel are not currently available. We hypothesized that zacopride, a selective and moderate agonist of the I_K1/Kir2.1 channels, prevents and cures acute ischemic arrhythmias. To test this viewpoint, adult Sprague-Dawley (SD) rats were subjected to MI by ligating the left main coronary artery. The antiarrhythmic effects of zacopride (i.v. infusion) were observed in the settings of pre-treatment (zacopride given 3 min prior to coronary occlusion), post-treatment (zacopride given 3 min after coronary occlusion) and therapeutic treatment (zacopride given 30 s after the onset of the first sustained ventricular tachycardia (VT)/ventricular fibrillation (VF) post MI). In all the three treatment modes, zacopride (15 μg/kg) inhibited MI-induced ventricular tachyarrhythmias, as shown by significant decreases in the premature ventricular contraction (PVC) and the duration and incidence of VT or VF. In Langendorff perfused rat hearts, the antiarrhythmic effect of 1 μmol/L zacopride were reversed by 1 μmol/L BaCl₂, a blocker of I_K1 channel. Patch clamp results in freshly isolated rat ventricular myocytes indicated that zacopride activated the I_K1 channel and thereby reversed hypoxia-induced RMP depolarization and action potential duration (APD) prolongation. In addition, zacopride (1 μmol/L) suppressed hypoxia- or isoproterenol- induced delayed afterdepolarizations (DADs). In Kir2.x transfected Chinese hamster ovary (CHO) cells, zacopride activated the Kir2.1 homomeric channel but not the Kir2.2 or Kir2.3 channels. These results support our hypothesis that moderately enhancing I_K1/Kir2.1 currents as by zacopride rescues ischemia- and hypoxia- induced RMP depolarization, and thereby prevents and cures acute ischemic arrhythmias. This study brings a new viewpoint to antiarrhythmic theories and provides a promising target for the treatment of acute ischemic arrhythmias.