Differences between outward currents of human atrial and subepicardial ventricular myocytes

Human induced pluripotent stem cell-derived atrial cardiomyocytes recapitulate contribution of the slowly activating delayed rectifier currents IKs to repolarization in the human atrium

AIMS

Human induced pluripotent stem cell-derived atrial cardiomyocytes (hiPSC-aCM) could be a helpful tool to study the physiology and diseases of the human atrium. To fulfil this expectation, the electrophysiology of hiPSC-aCM should closely resemble the situation in the human atrium. Data on the contribution of the slowly activating delayed rectifier currents (IKs) to repolarization are lacking for both human atrium and hiPSC-aCM.

METHODS AND RESULTS

Human atrial tissues were obtained from patients with sinus rhythm (SR) or atrial fibrillation (AF). Currents were measured in human atrial cardiomyocytes (aCM) and compared with hiPSC-aCM and used to model IKs contribution to action potential (AP) shape. Action potential was recorded by sharp microelectrodes. HMR-1556 (1 µM) was used to identify IKs and to estimate IKs contribution to repolarization. Less than 50% of hiPSC-aCM and aCM possessed IKs. Frequency of occurrence, current densities, activation/deactivation kinetics, and voltage dependency of IKs did not differ significantly between hiPSC-aCM and aCM, neither in SR nor AF. β-Adrenoceptor stimulation with isoprenaline did not increase IKs neither in aCM nor in hiPSC-aCM. In tissue from SR, block of IKs with HMR-1556 did not lengthen the action potential duration, even when repolarization reserve was reduced by block of the ultra-rapid repolarizing current with 4-aminopyridine or the rapidly activating delayed rectifier potassium outward current with E-4031.

CONCLUSION

I Ks exists in hiPSC-aCM with biophysics not different from aCM. As in adult human atrium (SR and AF), IKs does not appear to relevantly contribute to repolarization in hiPSC-aCM.

Biophysical properties of NaV1.5 channels from atrial-like and ventricular-like cardiomyocytes derived from human induced pluripotent stem cells

Generating atrial-like cardiomyocytes derived from human induced pluripotent stem cells (hiPSCs) is crucial for modeling and treating atrial-related diseases, such as atrial arrythmias including atrial fibrillations. However, it is essential to obtain a comprehensive understanding of the electrophysiological properties of these cells. The objective of the present study was to investigate the molecular, electrical, and biophysical properties of several ion channels, especially NaV1.5 channels, in atrial hiPSC cardiomyocytes. Atrial cardiomyocytes were obtained by the differentiation of hiPSCs treated with retinoic acid (RA). The quality of the atrial specification was assessed by qPCR, immunocytofluorescence, and western blotting. The electrophysiological properties of action potentials (APs), Ca2+ dynamics, K+ and Na+ currents were investigated using patch-clamp and optical mapping approaches. We evaluated mRNA transcript and protein expressions to show that atrial cardiomyocytes expressed higher atrial- and sinoatrial-specific markers (MYL7, CACNA1D) and lower ventricular-specific markers (MYL2, CACNA1C, GJA1) than ventricular cardiomyocytes. The amplitude, duration, and steady-state phase of APs in atrial cardiomyocytes decreased, and had a shape similar to that of mature atrial cardiomyocytes. Interestingly, NaV1.5 channels in atrial cardiomyocytes exhibited lower mRNA transcripts and protein expression, which could explain the lower current densities recorded by patch-clamp. Moreover, Na+ currents exhibited differences in activation and inactivation parameters. These differences could be explained by an increase in SCN2B regulatory subunit expression and a decrease in SCN1B and SCN4B regulatory subunit expressions. Our results show that a RA treatment made it possible to obtain atrial cardiomyocytes and investigate differences in NaV1.5 channel properties between ventricular- and atrial-like cells.

A critical role of retinoic acid concentration for the induction of a fully human-like atrial action potential phenotype in hiPSC-CM

Retinoic acid (RA) induces an atrial phenotype in human induced pluripotent stem cells (hiPSCs), but expression of atrium-selective currents such as the ultrarapid (IKur) and acetylcholine-stimulated K+ current is variable and less than in the adult human atrium. We suspected methodological issues and systematically investigated the concentration dependency of RA. RA treatment increased IKur concentration dependently from 1.1 ± 0.54 pA/pF (0 RA) to 3.8 ± 1.1, 5.8 ± 2.5, and 12.2 ± 4.3 at 0.01, 0.1, and 1 μM, respectively. Only 1 μM RA induced enough IKur to fully reproduce human atrial action potential (AP) shape and a robust shortening of APs upon carbachol. We found that sterile filtration caused substantial loss of RA. We conclude that 1 μM RA seems to be necessary and sufficient to induce a full atrial AP shape in hiPSC-CM in EHT format. RA concentrations are prone to methodological issues and may profoundly impact the success of atrial differentiation.

Leak current, even with gigaohm seals, can cause misinterpretation of stem cell-derived cardiomyocyte action potential recordings

AIMS

Human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have become an essential tool to study arrhythmia mechanisms. Much of the foundational work on these cells, as well as the computational models built from the resultant data, has overlooked the contribution of seal-leak current on the immature and heterogeneous phenotype that has come to define these cells. The aim of this study is to understand the effect of seal-leak current on recordings of action potential (AP) morphology.

METHODS AND RESULTS

Action potentials were recorded in human iPSC-CMs using patch clamp and simulated using previously published mathematical models. Our in silico and in vitro studies demonstrate how seal-leak current depolarizes APs, substantially affecting their morphology, even with seal resistances (Rseal) above 1 GΩ. We show that compensation of this leak current is difficult due to challenges with obtaining accurate measures of Rseal during an experiment. Using simulation, we show that Rseal measures (i) change during an experiment, invalidating the use of pre-rupture values, and (ii) are polluted by the presence of transmembrane currents at every voltage. Finally, we posit that the background sodium current in baseline iPSC-CM models imitates the effects of seal-leak current and is increased to a level that masks the effects of seal-leak current on iPSC-CMs.

CONCLUSION

Based on these findings, we make recommendations to improve iPSC-CM AP data acquisition, interpretation, and model-building. Taking these recommendations into account will improve our understanding of iPSC-CM physiology and the descriptive ability of models built from such data.

Generation of cardiomyocytes from human-induced pluripotent stem cells resembling atrial cells with ability to respond to adrenoceptor agonists

Atrial fibrillation (AF) is the most common chronic arrhythmia presenting a heavy disease burden. We report a new approach for generating cardiomyocytes (CMs) resembling atrial cells from human-induced pluripotent stem cells (hiPSCs) using a combination of Gremlin 2 and retinoic acid treatment. More than 40% of myocytes showed rod-shaped morphology, expression of CM proteins (including ryanodine receptor 2, α-actinin-2 and F-actin) and striated appearance, all of which were broadly similar to the characteristics of adult atrial myocytes (AMs). Isolated myocytes were electrically quiescent until stimulated to fire action potentials with an AM profile and an amplitude of approximately 100 mV, arising from a resting potential of approximately -70 mV. Single-cell RNA sequence analysis showed a high level of expression of several atrial-specific transcripts including NPPA, MYL7, HOXA3, SLN, KCNJ4, KCNJ5 and KCNA5. Amplitudes of calcium transients recorded from spontaneously beating cultures were increased by the stimulation of α-adrenoceptors (activated by phenylephrine and blocked by prazosin) or β-adrenoceptors (activated by isoproterenol and blocked by CGP20712A). Our new approach provides human AMs with mature characteristics from hiPSCs which will facilitate drug discovery by enabling the study of human atrial cell signalling pathways and AF. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Human induced pluripotent stem cell-derived cardiomyocytes as an electrophysiological model: Opportunities and challenges-The Hamburg perspective

Models based on human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CM) are proposed in almost any field of physiology and pharmacology. The development of human induced pluripotent stem cell-derived cardiomyocytes is expected to become a step forward to increase the translational power of cardiovascular research. Importantly they should allow to study genetic effects on an electrophysiological background close to the human situation. However, biological and methodological issues revealed when human induced pluripotent stem cell-derived cardiomyocytes were used in experimental electrophysiology. We will discuss some of the challenges that should be considered when human induced pluripotent stem cell-derived cardiomyocytes will be used as a physiological model.

Adenosine and Adenosine Receptors: Advances in Atrial Fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in the world. Because the key to developing innovative therapies that limit the onset and the progression of AF is to fully understand the underlying molecular mechanisms of AF, the aim of the present narrative review is to report the most recent advances in the potential role of the adenosinergic system in the pathophysiology of AF. After a comprehensive approach describing adenosinergic system signaling and the mechanisms of the initiation and maintenance of AF, we address the interactions of the adenosinergic system's signaling with AF. Indeed, adenosine release can activate four G-coupled membrane receptors, named A₁, A_2A, A_2B and A₃. Activation of the A_2A receptors can promote the occurrence of delayed depolarization, while activation of the A₁ receptors can shorten the action potential's duration and induce the resting membrane's potential hyperpolarization, which promote pulmonary vein firing, stabilize the AF rotors and allow for functional reentry. Moreover, the A_2B receptors have been associated with atrial fibrosis homeostasis. Finally, the adenosinergic system can modulate the autonomous nervous system and is associated with AF risk factors. A question remains regarding adenosine release and the adenosine receptors' activation and whether this would be a cause or consequence of AF.

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.

Human Atrial Cardiac Microtissues for Chamber-Specific Arrhythmic Risk Assessment

Introduction

Although atrial fibrillation is the most prevalent disorder of electrical conduction, the mechanisms behind atrial arrhythmias remain elusive. To address this challenge, we developed a robust in vitro model of 3D atrial microtissue from human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and evaluated chamber-specific chemical responses experimentally and computationally.

Methods

We differentiated atrial and ventricular cardiomyocytes (aCMs/vCMs) from GCaMP6f-expressing hiPSCs and assessed spontaneous AP activity using fluorescence imaging. Self-assembling 3D microtissues were formed with lactate purified CMs and 5% human cardiac fibroblasts and electrically stimulated for one week before high resolution action potential (AP) optical mapping. AP responses to the atrial-specific potassium repolarizing current I _Kur-blocker 4-Aminopyridine (4-AP) and funny current I _f-blocker Ivabradine were characterized within their therapeutic window. Finally, we expanded upon a published hiPSC-CM computational model by incorporating the atrial-specific I _Kur current, modifying ion channel conductances to match the AP waveforms of our microtissues, and employing the updated model to reinforce our experimental findings.

Results

High purity CMs (> 75% cTnT⁺) demonstrated subtype specification by MLC2v expression. Spontaneous beating rates significantly decreased following 3D microtissue formation, with atrial microtissues characterized by their faster spontaneous beating rate, slower AP rise time, and shorter AP duration (APD) compared to ventricular microtissues. We measured atrial-specific responses, including dose-dependent APD prolongation with 4-AP treatment and dose-dependent reduction in spontaneous activity post-Ivabradine treatment.

Conclusion

The presented in vitro platform for screening atrial-specific responses is both robust and sensitive, with high throughput, enabling studies focused at elucidating the mechanisms underlying atrial arrhythmias.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00703-x.

New Strategies for the Treatment of Atrial Fibrillation

Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia in the clinical practice. It significantly contributes to the morbidity and mortality of the elderly population. Over the past 25-30 years intense effort in basic research has advanced the understanding of the relationship between the pathophysiology of AF and atrial remodelling. Nowadays it is clear that the various forms of atrial remodelling (electrical, contractile and structural) play crucial role in initiating and maintaining the persistent and permanent types of AF. Unlike in ventricular fibrillation, in AF rapid ectopic firing originating from pulmonary veins and re-entry mechanism may induce and maintain (due to atrial remodelling) this complex cardiac arrhythmia. The present review presents and discusses in detail the latest knowledge on the role of remodelling in AF. Special attention is paid to novel concepts and pharmacological targets presumably relevant to the drug treatment of atrial fibrillation.

Quinazolinone dimers as a potential new class of safer Kv1 inhibitors: Overcoming hERG, sodium and calcium channel affinities

The discovery of more selective and safer voltage-gated potassium channel blockers is an extremely demanding approach. Designing selective Kv1.5 inhibitors is very challenging as only limited data is available on this target due to a lacking crystal structure for this ion channel receptor. Herein, we synthesized a series of 21 novel quinazolinone dimers 3a-i, 5a-i and 10a-c. We tried to avoid structural features responsible for non-selectivity and for most potassium channel blockers' side effects in our design. In contrast to other works, which lack investigation over wide ranges of potassium and sodium channels, we screened the inhibitory activity of our synthesized compounds over multiple voltage-gated potassium channels, including six different human Kv1 channel subtypes Kv1.1, Kv1.2, Kv1.3, Kv1.4, Kv1.5 and Kv1.6 channels as well as Kv2.1, Kv3.1, Kv4.3, Kv7.2, Kv7.3, Kv10.1, hERG, and Shaker IR. Moreover, these compounds' selectivity was investigated on sodium channels Nav1.2, Nav1.4 and Nav1.5 and calcium channels Cav3.1-Cav3.3. The results revealed two compounds (3a and 3e) with low micromolar Kv1.5 inhibition activity with EC₅₀ values of 5.1 ± 0.9 µM and 12.5 ± 1.1 µM, respectively. However, at higher concentrations, they also showed inhibitory activity on Kv1.3 and Kv1.1 channels. This might be due to structural similarities between these three Kv1 channel isoforms. Compound 3a shows a slight preference for Kv1.5. Interestingly, they lack any activity on other potassium channels (including hERG), sodium channels, and calcium channels. Our findings recommend quinazolinone dimers with ethylene linker as a potential new class of safer Kv1 inhibitors and a good start for designing more selective and potent Kv1.5 inhibitors.

Patch-Clamp Recordings of Action Potentials From Human Atrial Myocytes: Optimization Through Dynamic Clamp

Introduction: Atrial fibrillation (AF) is the most common cardiac arrhythmia. Consequently, novel therapies are being developed. Ultimately, the impact of compounds on the action potential (AP) needs to be tested in freshly isolated human atrial myocytes. However, the frequent depolarized state of these cells upon isolation seriously hampers reliable AP recordings.

Purpose: We assessed whether AP recordings from single human atrial myocytes could be improved by providing these cells with a proper inward rectifier K⁺ current (I_K1), and consequently with a regular, non-depolarized resting membrane potential (RMP), through “dynamic clamp”.

Methods: Single myocytes were enzymatically isolated from left atrial appendage tissue obtained from patients with paroxysmal AF undergoing minimally invasive surgical ablation. APs were elicited at 1 Hz and measured using perforated patch-clamp methodology, injecting a synthetic I_K1 to generate a regular RMP. The injected I_K1 had strong or moderate rectification. For comparison, a regular RMP was forced through injection of a constant outward current. A wide variety of ion channel blockers was tested to assess their modulatory effects on AP characteristics.

Results: Without any current injection, RMPs ranged from −9.6 to −86.2 mV in 58 cells. In depolarized cells (RMP positive to −60 mV), RMP could be set at −80 mV using I_K1 or constant current injection and APs could be evoked upon stimulation. AP duration differed significantly between current injection methods (p < 0.05) and was shortest with constant current injection and longest with injection of I_K1 with strong rectification. With moderate rectification, AP duration at 90% repolarization (APD₉₀) was similar to myocytes with regular non-depolarized RMP, suggesting that a synthetic I_K1 with moderate rectification is the most appropriate for human atrial myocytes. Importantly, APs evoked using each injection method were still sensitive to all drugs tested (lidocaine, nifedipine, E-4031, low dose 4-aminopyridine, barium, and apamin), suggesting that the major ionic currents of the atrial cells remained functional. However, certain drug effects were quantitatively dependent on the current injection approach used.

Conclusion: Injection of a synthetic I_K1 with moderate rectification facilitates detailed AP measurements in human atrial myocytes. Therefore, dynamic clamp represents a promising tool for testing novel antiarrhythmic drugs.

Small Conductance Ca2 +-Activated K+ (SK) Channel mRNA Expression in Human Atrial and Ventricular Tissue: Comparison Between Donor, Atrial Fibrillation and Heart Failure Tissue

In search of more efficacious and safe pharmacological treatments for atrial fibrillation (AF), atria-selective antiarrhythmic agents have been promoted that target ion channels principally expressed in the atria. This concept allows one to engage antiarrhythmic effects in atria, but spares the ventricles from potentially proarrhythmic side effects. It has been suggested that cardiac small conductance Ca²⁺-activated K⁺ (SK) channels may represent an atria-selective target in mammals including humans. However, there are conflicting data concerning the expression of SK channels in different stages of AF, and recent findings suggest that SK channels are upregulated in ventricular myocardium when patients develop heart failure. To address this issue, RNA-sequencing was performed to compare expression levels of three SK channels (KCNN1, KCNN2, and KCNN3) in human atrial and ventricular tissue samples from transplant donor hearts (no cardiac disease), and patients with cardiac disease in sinus rhythm or with AF. In addition, for control purposes expression levels of several genes known to be either chamber-selective or differentially expressed in AF and heart failure were determined. In atria, as compared to ventricle from transplant donor hearts, we confirmed higher expression of KCNN1 and KCNA5, and lower expression of KCNJ2, whereas KCNN2 and KCNN3 were statistically not differentially expressed. Overall expression of KCNN1 was low compared to KCNN2 and KCNN3. Comparing atrial tissue from patients with AF to sinus rhythm samples we saw downregulation of KCNN2 in AF, as previously reported. When comparing ventricular tissue from heart failure patients to non-diseased samples, we found significantly increased ventricular expression of KCNN3 in heart failure, as previously published. The other channels showed no significant difference in expression in either disease. Our results add weight to the view that SK channels are not likely to be an atria-selective target, especially in failing human hearts, and modulators of these channels may prove to have less utility in treating AF than hoped. Whether targeting SK1 holds potential remains to be elucidated.

Bioengineering Clinically Relevant Cardiomyocytes and Cardiac Tissues from Pluripotent Stem Cells

The regenerative capacity of cardiomyocytes is insufficient to functionally recover damaged tissue, and as such, ischaemic heart disease forms the largest proportion of cardiovascular associated deaths. Human-induced pluripotent stem cells (hiPSCs) have enormous potential for developing patient specific cardiomyocytes for modelling heart disease, patient-based cardiac toxicity testing and potentially replacement therapy. However, traditional protocols for hiPSC-derived cardiomyocytes yield mixed populations of atrial, ventricular and nodal-like cells with immature cardiac properties. New insights gleaned from embryonic heart development have progressed the precise production of subtype-specific hiPSC-derived cardiomyocytes; however, their physiological immaturity severely limits their utility as model systems and their use for drug screening and cell therapy. The long-entrenched challenges in this field are being addressed by innovative bioengingeering technologies that incorporate biophysical, biochemical and more recently biomimetic electrical cues, with the latter having the potential to be used to both direct hiPSC differentiation and augment maturation and the function of derived cardiomyocytes and cardiac tissues by mimicking endogenous electric fields.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Ultrarapid Delayed Rectifier K+ Channelopathies in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Atrial fibrillation (AF) is the most common cardiac arrhythmia. About 5-15% of AF patients have a mutation in a cardiac gene, including mutations in KCNA5, encoding the K_v1.5 α-subunit of the ion channel carrying the atrial-specific ultrarapid delayed rectifier K⁺ current (I_Kur). Both loss-of-function and gain-of-function AF-related mutations in KCNA5 are known, but their effects on action potentials (APs) of human cardiomyocytes have been poorly studied. Here, we assessed the effects of wild-type and mutant I_Kur on APs of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). We found that atrial-like hiPSC-CMs, generated by a retinoic acid-based differentiation protocol, have APs with faster repolarization compared to ventricular-like hiPSC-CMs, resulting in shorter APs with a lower AP plateau. Native I_Kur, measured as current sensitive to 50 μM 4-aminopyridine, was 1.88 ± 0.49 (mean ± SEM, n = 17) and 0.26 ± 0.26 pA/pF (n = 17) in atrial- and ventricular-like hiPSC-CMs, respectively. In both atrial- and ventricular-like hiPSC-CMs, I_Kur blockade had minimal effects on AP parameters. Next, we used dynamic clamp to inject various amounts of a virtual I_Kur, with characteristics as in freshly isolated human atrial myocytes, into 11 atrial-like and 10 ventricular-like hiPSC-CMs, in which native I_Kur was blocked. Injection of I_Kur with 100% density shortened the APs, with its effect being strongest on the AP duration at 20% repolarization (APD₂₀) of atrial-like hiPSC-CMs. At I_Kur densities < 100% (compared to 100%), simulating loss-of-function mutations, significant AP prolongation and raise of plateau were observed. At I_Kur densities > 100%, simulating gain-of-function mutations, APD₂₀ was decreased in both atrial- and ventricular-like hiPSC-CMs, but only upon a strong increase in I_Kur. In ventricular-like hiPSC-CMs, lowering of the plateau resulted in AP shortening. We conclude that a decrease in I_Kur, mimicking loss-of-function mutations, has a stronger effect on the AP of hiPSC-CMs than an increase, mimicking gain-of-function mutations, whereas in ventricular-like hiPSC-CMs such increase results in AP shortening, causing their AP morphology to become more atrial-like. Effects of native I_Kur modulation on atrial-like hiPSC-CMs are less pronounced than effects of virtual I_Kur injection because I_Kur density of atrial-like hiPSC-CMs is substantially smaller than that of freshly isolated human atrial myocytes.

Accuracy considerations for capacitance estimation by voltage steps in cardiomyocytes

Electrophysiologists routinely use simple voltage steps to evaluate cell membrane capacitance derived from corresponding current responses. Frequently, the resting membrane voltage V_rest is employed as holding potential for the subsequent command voltage step and more or less accurate methods are utilised to analyse the transient current. Another choice as holding potential is the peak of the "quasi steady-state" current to voltage relationship, V_peak. The aim of this study is the systematic evaluation of capacitance estimation accuracy from voltage step experiments depending on the choice of holding potential and analysis method. In this paper, a simulation approach is employed to analyse the current response of a model patch-clamp circuit. Four commonly accepted methods are implemented, utilizing different aspects of the transient current (charge, membrane time constant, and influence of the series resistance) in various combinations and with various degrees of refinement. This simulation study indicates an acceptable accuracy of the elaborated methods for capacitance estimation at holding potentials V_rest and V_peak over a broad range of capacitance as well as series resistance values. Simple integration of the current transient provides sufficient accuracy at holding potentials, which effectively minimizes changes in resistive membrane current flow during command voltage steps (particularly around V_peak). However, biphasic command protocols performed at V_peak activate voltage dependent sodium channels, thereby possibly leading to the threshold voltage for an action potential. Compared to V_rest, all methods utilizing monophasic step protocols, gain additional accuracy, when applied at V_peak as holding potential.

Hypokalemia Promotes Arrhythmia by Distinct Mechanisms in Atrial and Ventricular Myocytes

RATIONALE

Hypokalemia occurs in up to 20% of hospitalized patients and is associated with increased incidence of ventricular and atrial fibrillation. It is unclear whether these differing types of arrhythmia result from direct and perhaps distinct effects of hypokalemia on cardiomyocytes.

OBJECTIVE

To investigate proarrhythmic mechanisms of hypokalemia in ventricular and atrial myocytes.

METHODS AND RESULTS

Experiments were performed in isolated rat myocytes exposed to simulated hypokalemia conditions (reduction of extracellular [K⁺] from 5.0 to 2.7 mmol/L) and supported by mathematical modeling studies. Ventricular cells subjected to hypokalemia exhibited Ca²⁺ overload and increased generation of both spontaneous Ca²⁺ waves and delayed afterdepolarizations. However, similar Ca²⁺-dependent spontaneous activity during hypokalemia was only observed in a minority of atrial cells that were observed to contain t-tubules. This effect was attributed to close functional pairing of the Na⁺-K⁺ ATPase and Na⁺-Ca²⁺ exchanger proteins within these structures, as reduction in Na⁺ pump activity locally inhibited Ca²⁺ extrusion. Ventricular myocytes and tubulated atrial myocytes additionally exhibited early afterdepolarizations during hypokalemia, associated with Ca²⁺ overload. However, early afterdepolarizations also occurred in untubulated atrial cells, despite Ca²⁺ quiescence. These phase-3 early afterdepolarizations were rather linked to reactivation of nonequilibrium Na⁺ current, as they were rapidly blocked by tetrodotoxin. Na⁺ current-driven early afterdepolarizations in untubulated atrial cells were enabled by membrane hyperpolarization during hypokalemia and short action potential configurations. Brief action potentials were in turn maintained by ultra-rapid K⁺ current (I_Kur); a current which was found to be absent in tubulated atrial myocytes and ventricular myocytes.

CONCLUSIONS

Distinct mechanisms underlie hypokalemia-induced arrhythmia in the ventricle and atrium but also vary between atrial myocytes depending on subcellular structure and electrophysiology.

Challenges Faced with Small Molecular Modulators of Potassium Current Channel Isoform Kv1.5

The voltage-gated potassium channel Kv1.5, which mediates the cardiac ultra-rapid delayed-rectifier (IKur) current in human cells, has a crucial role in atrial fibrillation. Therefore, the design of selective Kv1.5 modulators is essential for the treatment of pathophysiological conditions involving Kv1.5 activity. This review summarizes the progress of molecular structures and the functionality of different types of Kv1.5 modulators, with a focus on clinical cardiovascular drugs and a number of active natural products, through a summarization of 96 compounds currently widely used. Furthermore, we also discuss the contributions of Kv1.5 and the regulation of the structure-activity relationship (SAR) of synthetic Kv1.5 inhibitors in human pathophysiology. SAR analysis is regarded as a useful strategy in structural elucidation, as it relates to the characteristics that improve compounds targeting Kv1.5. Herein, we present previous studies regarding the structural, pharmacological, and SAR information of the Kv1.5 modulator, through which we can assist in identifying and designing potent and specific Kv1.5 inhibitors in the treatment of diseases involving Kv1.5 activity.

Acetylcholine Delays Atrial Activation to Facilitate Atrial Fibrillation

Background

Acetylcholine (ACh) shortens action potential duration (APD) in human atria. APD shortening facilitates atrial fibrillation (AF) by reducing the wavelength for reentry. However, the influence of ACh on electrical conduction in human atria and its contribution to AF are unclear, particularly when combined with impaired conduction from interstitial fibrosis.

Objective

To investigate the effect of ACh on human atrial conduction and its role in AF with computational, experimental, and clinical approaches.

Methods

S1S2 pacing (S1 = 600 ms and S2 = variable cycle lengths) was applied to the following human AF computer models: a left atrial appendage (LAA) myocyte to quantify the effects of ACh on APD, maximum upstroke velocity (V _max ), and resting membrane potential (RMP); a monolayer of LAA myocytes to quantify the effects of ACh on conduction; and 3) an intact left atrium (LA) to determine the effects of ACh on arrhythmogenicity. Heterogeneous ACh and interstitial fibrosis were applied to the monolayer and LA models. To corroborate the simulations, APD and RMP from isolated human atrial myocytes were recorded before and after 0.1 μM ACh. At the tissue level, LAAs from AF patients were optically mapped ex vivo using Di-4-ANEPPS. The difference in total activation time (AT) was determined between AT initially recorded with S1 pacing, and AT recorded during subsequent S1 pacing without (n = 6) or with (n = 7) 100 μM ACh.

Results

In LAA myocyte simulations, S1 pacing with 0.1 μM ACh shortened APD by 41 ms, hyperpolarized RMP by 7 mV, and increased V _max by 27 mV/ms. In human atrial myocytes, 0.1 μM ACh shortened APD by 48 ms, hyperpolarized RMP by 3 mV, and increased V _max by 6 mV/ms. In LAA monolayer simulations, S1 pacing with ACh hyperpolarized RMP to delay total AT by 32 ms without and 35 ms with fibrosis. This led to unidirectional conduction block and sustained reentry in fibrotic LA with heterogeneous ACh during S2 pacing. In AF patient LAAs, S1 pacing with ACh increased total AT from 39.3 ± 26 ms to 71.4 ± 31.2 ms (p = 0.036) compared to no change without ACh (56.7 ± 29.3 ms to 50.0 ± 21.9 ms, p = 0.140).

Conclusion

In fibrotic atria with heterogeneous parasympathetic activation, ACh facilitates AF by shortening APD and slowing conduction to promote unidirectional conduction block and reentry.

Differential Expression and Remodeling of Transient Outward Potassium Currents in Human Left Ventricles

BACKGROUND

Myocardial, transient, outward currents, I_to, have been shown to play pivotal roles in action potential (AP) repolarization and remodeling in animal models. The properties and contribution of I_to to left ventricular (LV) repolarization in the human heart, however, are poorly defined.

METHODS AND RESULTS

Whole-cell, voltage-clamp recordings, acquired at physiological (35°C to 37°C) temperatures, from myocytes isolated from the LV of nonfailing human hearts identified 2 distinct transient currents, I_to,fast (I_to,f) and I_to,slow (I_to,s), with significantly (P<0.0001) different rates of recovery from inactivation and pharmacological sensitives: I_to,f recovers in ≈10 ms, 100× faster than I_to,s, and is selectively blocked by the Kv4 channel toxin, SNX-482. Current-clamp experiments revealed regional differences in AP waveforms, notably a phase 1 notch in LV subepicardial myocytes. Dynamic clamp-mediated addition/removal of modeled human ventricular I_to,f, resulted in hyperpolarization or depolarization, respectively, of the notch potential, whereas slowing the rate of I_to,f inactivation resulted in AP collapse. AP-clamp experiments demonstrated that changes in notch potentials modified the time course and amplitudes of voltage-gated Ca²⁺ currents, I_Ca. In failing LV subepicardial myocytes, I_to,f was reduced and I_to,s was increased, notch and plateau potentials were depolarized (P<0.0001) and AP durations were prolonged (P<0.001).

CONCLUSIONS

I_to,f and I_to,s are differentially expressed in nonfailing human LV, contributing to regional heterogeneities in AP waveforms. I_to,f regulates notch and plateau potentials and modulates the time course and amplitude of I_Ca. Slowing I_to,f inactivation results in dramatic AP shortening. Remodeling of I_to,f in failing human LV subepicardial myocytes attenuates transmural differences in AP waveforms.

Long-term 4-AP treatment facilitates functional expression of human Kv1.5 channel

The human Kv1.5 channel (hKv1.5) produces the ultrarapid delayed rectifier potassium current (I_Kur), which is important for determining the repolarization of action potential in the cardiac atrium. However, the expression of I_Kur is reduced in patients with chronic atrial fibrillation. 4-Aminopyridine (4-AP) can specifically suppress I_Kur, suggesting that it modifies hKv1.5 as a chaperone molecule. Herein, the effects of long-term 4-AP treatment on hKv1.5 protein expression and function were investigated in HEK cells. 4-AP treatment (24 h) improved hKv1.5 protein levels, promoted hKv1.5 glycosylation, and facilitated the hKv1.5 current in a time-dependent manner. Long-term 4-AP treatment also markedly enhanced hKv1.5 localization in the cell membrane, endoplasmic reticulum, and Golgi. Importantly, the Ile508 residue located in the hKv1.5 channel pore was found to be important for 4-AP inhibitory activity. These results provide insight into developing hKv1.5 channel blocker that can functionally rescue I_Kur in patients with chronic atrial fibrillation.

Ionic and cellular mechanisms underlying TBX5/PITX2 insufficiency-induced atrial fibrillation: Insights from mathematical models of human atrial cells

Transcription factors TBX5 and PITX2 involve in the regulation of gene expression of ion channels and are closely associated with atrial fibrillation (AF), the most common cardiac arrhythmia in developed countries. The exact cellular and molecular mechanisms underlying the increased susceptibility to AF in patients with TBX5/PITX2 insufficiency remain unclear. In this study, we have developed and validated a novel human left atrial cellular model (TPA) based on the ten Tusscher-Panfilov ventricular cell model to systematically investigate how electrical remodeling induced by TBX5/PITX2 insufficiency leads to AF. Using our TPA model, we have demonstrated that spontaneous diastolic depolarization observed in atrial myocytes with TBX5-deletion can be explained by altered intracellular calcium handling and suppression of inward-rectifier potassium current (IK1). Additionally, our computer simulation results shed new light on the novel cellular mechanism underlying AF by indicating that the imbalance between suppressed outward current IK1 and increased inward sodium-calcium exchanger current (INCX) resulted from SR calcium leak leads to spontaneous depolarizations. Furthermore, our simulation results suggest that these arrhythmogenic triggers can be potentially suppressed by inhibiting sarcoplasmic reticulum (SR) calcium leak and reversing remodeled IK1. More importantly, this study has clinically significant implications on the drugs used for maintaining SR calcium homeostasis, whereby drugs such as dantrolene may confer significant improvement for the treatment of AF patients with TBX5/PITX2 insufficiency.

Calcium in the Pathophysiology of Atrial Fibrillation and Heart Failure

Atrial fibrillation (AF) is commonly associated with heart failure. A bidirectional relationship exists between the two-AF exacerbates heart failure causing a significant increase in heart failure symptoms, admissions to hospital and cardiovascular death, while pathological remodeling of the atria as a result of heart failure increases the risk of AF. A comprehensive understanding of the pathophysiology of AF is essential if we are to break this vicious circle. In this review, the latest evidence will be presented showing a fundamental role for calcium in both the induction and maintenance of AF. After outlining atrial electrophysiology and calcium handling, the role of calcium-dependent afterdepolarizations and atrial repolarization alternans in triggering AF will be considered. The atrial response to rapid stimulation will be discussed, including the short-term protection from calcium overload in the form of calcium signaling silencing and the eventual progression to diastolic calcium leak causing afterdepolarizations and the development of an electrical substrate that perpetuates AF. The role of calcium in the bidirectional relationship between heart failure and AF will then be covered. The effects of heart failure on atrial calcium handling that promote AF will be reviewed, including effects on both atrial myocytes and the pulmonary veins, before the aspects of AF which exacerbate heart failure are discussed. Finally, the limitations of human and animal studies will be explored allowing contextualization of what are sometimes discordant results.

Description of the Human Atrial Action Potential Derived From a Single, Congruent Data Source: Novel Computational Models for Integrated Experimental-Numerical Study of Atrial Arrhythmia Mechanisms

Introduction: The development of improved diagnosis, management, and treatment strategies for human atrial fibrillation (AF) is a significant and important challenge in order to improve quality of life for millions and reduce the substantial social-economic costs of the condition. As a complex condition demonstrating high variability and relation to other cardiac conditions, the study of AF requires approaches from multiple disciplines including single-cell experimental electrophysiology and computational modeling. Models of human atrial cells are less well parameterized than those of the human ventricle or other mammal species, largely due to the inherent challenges in patch clamping human atrial cells. Such challenges include, frequently, unphysiologically depolarized resting potentials and thus injection of a compensatory hyperpolarizing current, as well as detecting certain ion currents which may be disrupted by the cell isolation process. The aim of this study was to develop a laboratory specific model of human atrial electrophysiology which reproduces exactly the conditions of isolated-cell experiments, including testing of multiple experimental interventions. Methods: Formulations for the primary ion currents characterized by isolated-cell experiments in the Workman laboratory were fit directly to voltage-clamp data; the fast sodium-current was parameterized based on experiments relating resting membrane potential to maximal action potential upstroke velocity; compensatory hyperpolarizing current was included as a constant applied current. These formulations were integrated with three independent human atrial cell models to provide a family of novel models. Extrapolated intact-cell models were developed through removal of the hyperpolarizing current and introduction of terminal repolarization potassium currents. Results: The isolated-cell models quantitatively reproduced experimentally measured properties of excitation in both control and pharmacological and dynamic-clamp interventions. Comparison of isolated and intact-cell models highlighted the importance of reproducing this cellular environment when comparing experimental and simulation data. Conclusion: We have developed a laboratory specific model of the human atrial cell which directly reproduces the experimental isolated-cell conditions and captures human atrial excitation properties. The model may be particularly useful for directly relating model to experiment, and offers a complementary tool to the available set of human atrial cell models with specific advantages resulting from the congruent input data source.

Identification of an INa-dependent and Ito-mediated proarrhythmic mechanism in cardiomyocytes derived from pluripotent stem cells of a Brugada syndrome patient

Brugada syndrome (BrS) is an inherited cardiac arrhythmia commonly associated with SCN5A mutations, yet its ionic mechanisms remain unclear due to a lack of cellular models. Here, we used human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from a BrS patient (BrS1) to evaluate the roles of Na⁺ currents (I_Na) and transient outward K⁺ currents (I_to) in BrS induced action potential (AP) changes. To understand the role of these current changes in repolarization we employed dynamic clamp to “electronically express” I_K1 and restore normal resting membrane potentials and allow normal recovery of the inactivating currents, I_Na, I_Ca and I_to. HiPSC-CMs were generated from BrS1 with a compound SCN5A mutation (p. A226V & p. R1629X) and a healthy sibling control (CON1). Genome edited hiPSC-CMs (BrS2) with a milder p. T1620M mutation and a commercial control (CON2) were also studied. CON1, CON2 and BrS2, had unaltered peak I_Na amplitudes, and normal APs whereas BrS1, with over 75% loss of I_Na, displayed a loss-of-I_Na basal AP morphology (at 1.0 Hz) manifested by a reduced maximum upstroke velocity (by ~80%, p < 0.001) and AP amplitude (p < 0.001), and an increased phase-1 repolarization pro-arrhythmic AP morphology (at 0.1 Hz) in ~25% of cells characterized by marked APD shortening (~65% shortening, p < 0.001). Moreover, I_to densities of BrS1 and CON1 were comparable and increased from 1.0 Hz to 0.1 Hz by ~ 100%. These data indicate that a repolarization deficit could be a mechanism underlying BrS.

Deep phenotyping of human induced pluripotent stem cell-derived atrial and ventricular cardiomyocytes

Generation of homogeneous populations of subtype-specific cardiomyocytes (CMs) derived from human induced pluripotent stem cells (iPSCs) and their comprehensive phenotyping is crucial for a better understanding of the subtype-related disease mechanisms and as tools for the development of chamber-specific drugs. The goals of this study were to apply a simple and efficient method for differentiation of iPSCs into defined functional CM subtypes in feeder-free conditions and to obtain a comprehensive understanding of the molecular, cell biological, and functional properties of atrial and ventricular iPSC-CMs on both the single-cell and engineered heart muscle (EHM) level. By a stage-specific activation of retinoic acid signaling in monolayer-based and well-defined culture, we showed that cardiac progenitors can be directed towards a highly homogeneous population of atrial CMs. By combining the transcriptome and proteome profiling of the iPSC-CM subtypes with functional characterizations via optical action potential and calcium imaging, and with contractile analyses in EHM, we demonstrated that atrial and ventricular iPSC-CMs and -EHM highly correspond to the atrial and ventricular heart muscle, respectively. This study provides a comprehensive understanding of the molecular and functional identities characteristic of atrial and ventricular iPSC-CMs and -EHM and supports their suitability in disease modeling and chamber-specific drug screening.

Low Resting Membrane Potential and Low Inward Rectifier Potassium Currents Are Not Inherent Features of hiPSC-Derived Cardiomyocytes

Human induced pluripotent stem cell (hiPSC) cardiomyocytes (CMs) show less negative resting membrane potential (RMP), which is attributed to small inward rectifier currents (IK1). Here, IK1 was measured in hiPSC-CMs (proprietary and commercial cell line) cultured as monolayer (ML) or 3D engineered heart tissue (EHT) and, for direct comparison, in CMs from human right atrial (RA) and left ventricular (LV) tissue. RMP was measured in isolated cells and intact tissues. IK1 density in ML- and EHT-CMs from the proprietary line was similar to LV and RA, respectively. IK1 density in EHT-CMs from the commercial line was 2-fold smaller than in the proprietary line. RMP in EHT of both lines was similar to RA and LV. Repolarization fraction and IK,ACh response discriminated best between RA and LV and indicated predominantly ventricular phenotype in hiPSC-CMs/EHT. The data indicate that IK1 is not necessarily low in hiPSC-CMs, and technical issues may underlie low RMP in hiPSC-CMs.

Elevated potassium outward currents in hyperoxia treated atrial cardiomyocytes

Supplementation of 100% oxygen is a very common intervention in intensive care units (ICU) and critical care centers for patients with dysfunctional lung and lung disorders. Although there is advantage in delivering sufficient levels of oxygen, hyperoxia is reported to be directly associated with increasing in-hospital deaths. Our previous studies reported ventricular and electrical remodeling in hyperoxia treated mouse hearts, and in this article, for the first time, we are investigating the effects of hyperoxia on atrial electrophysiology using whole-cell patch-clamp electrophysiology experiments along with assessment of Kv1.5, Kv4.2, and KChIP2 transcripts and protein profiles using real-time quantitative RT-PCR and Western blotting. Our data showed that induction of hyperoxia for 3 days in mice showed larger outward potassium currents with shorter action potential durations (APD). This increase in current densities is due to significant increase in ultrarapid delayed rectifier outward K⁺ currents (I_Kur ) and rapidly activating, rapidly inactivating transient outward K+ current (I_to ) densities. We also observed a significant increase in both transcripts and protein levels of Kv1.5 and KChIP2 in hyperoxia treated atrial cardiomyocytes, whereas no significant change was observed in Kv4.2 transcripts or protein. The data presented here further support our previous findings that hyperoxia induces not only ventricular remodeling, but also atrial electrical remodeling.

Synergistic Anti-arrhythmic Effects in Human Atria with Combined Use of Sodium Blockers and Acacetin

Atrial fibrillation (AF) is the most common cardiac arrhythmia. Developing effective and safe anti-AF drugs remains an unmet challenge. Simultaneous block of both atrial-specific ultra-rapid delayed rectifier potassium (K⁺) current (I_Kur) and the Na⁺ current (I_Na) has been hypothesized to be anti-AF, without inducing significant QT prolongation and ventricular side effects. However, the antiarrhythmic advantage of simultaneously blocking these two channels vs. individual block in the setting of AF-induced electrical remodeling remains to be documented. Furthermore, many I_Kur blockers such as acacetin and AVE0118, partially inhibit other K⁺ currents in the atria. Whether this multi-K⁺-block produces greater anti-AF effects compared with selective I_Kur-block has not been fully understood. The aim of this study was to use computer models to (i) assess the impact of multi-K⁺-block as exhibited by many I_Kur blokers, and (ii) evaluate the antiarrhythmic effect of blocking I_Kur and I_Na, either alone or in combination, on atrial and ventricular electrical excitation and recovery in the setting of AF-induced electrical-remodeling. Contemporary mathematical models of human atrial and ventricular cells were modified to incorporate dose-dependent actions of acacetin (a multichannel blocker primarily inhibiting I_Kur while less potently blocking I_to, I_Kr, and I_Ks). Rate- and atrial-selective inhibition of I_Na was also incorporated into the models. These single myocyte models were then incorporated into multicellular two-dimensional (2D) and three-dimensional (3D) anatomical models of the human atria. As expected, application of I_Kur blocker produced pronounced action potential duration (APD) prolongation in atrial myocytes. Furthermore, combined multiple K⁺-channel block that mimicked the effects of acacetin exhibited synergistic APD prolongations. Synergistically anti-AF effects following inhibition of I_Na and combined I_Kur/K⁺-channels were also observed. The attainable maximal AF-selectivity of I_Na inhibition was greatly augmented by blocking I_Kur or multiple K⁺-currents in the atrial myocytes. This enhanced anti-arrhythmic effects of combined block of Na⁺- and K⁺-channels were also seen in 2D and 3D simulations; specially, there was an enhanced efficacy in terminating re-entrant excitation waves, exerting improved antiarrhythmic effects in the human atria as compared to a single-channel block. However, in the human ventricular myocytes and tissue, cellular repolarization and computed QT intervals were modestly affected in the presence of actions of acacetin and I_Na blockers (either alone or in combination). In conclusion, this study demonstrates synergistic antiarrhythmic benefits of combined block of I_Kur and I_Na, as well as those of I_Na and combined multi K⁺-current block of acacetin, without significant alterations of ventricular repolarization and QT intervals. This approach may be a valuable strategy for the treatment of AF.

In Silico Assessment of Efficacy and Safety of IKur Inhibitors in Chronic Atrial Fibrillation: Role of Kinetics and State-Dependence of Drug Binding

Current pharmacological therapy against atrial fibrillation (AF), the most common cardiac arrhythmia, is limited by moderate efficacy and adverse side effects including ventricular proarrhythmia and organ toxicity. One way to circumvent the former is to target ion channels that are predominantly expressed in atria vs. ventricles, such as K_V1.5, carrying the ultra-rapid delayed-rectifier K⁺ current (I_Kur). Recently, we used an in silico strategy to define optimal K_V1.5-targeting drug characteristics, including kinetics and state-dependent binding, that maximize AF-selectivity in human atrial cardiomyocytes in normal sinus rhythm (nSR). However, because of evidence for I_Kur being strongly diminished in long-standing persistent (chronic) AF (cAF), the therapeutic potential of drugs targeting I_Kur may be limited in cAF patients. Here, we sought to simulate the efficacy (and safety) of I_Kur inhibitors in cAF conditions. To this end, we utilized sensitivity analysis of our human atrial cardiomyocyte model to assess the importance of I_Kur for atrial cardiomyocyte electrophysiological properties, simulated hundreds of theoretical drugs to reveal those exhibiting anti-AF selectivity, and compared the results obtained in cAF with those in nSR. We found that despite being downregulated, I_Kur contributes more prominently to action potential (AP) and effective refractory period (ERP) duration in cAF vs. nSR, with ideal drugs improving atrial electrophysiology (e.g., ERP prolongation) more in cAF than in nSR. Notably, the trajectory of the AP during cAF is such that more I_Kur is available during the more depolarized plateau potential. Furthermore, I_Kur block in cAF has less cardiotoxic effects (e.g., AP duration not exceeding nSR values) and can increase Ca²⁺ transient amplitude thereby enhancing atrial contractility. We propose that in silico strategies such as that presented here should be combined with in vitro and in vivo assays to validate model predictions and facilitate the ongoing search for novel agents against AF.

Cardiac Potassium Channels: Physiological Insights for Targeted Therapy

The development of novel drugs specifically directed at the ion channels underlying particular features of cardiac action potential (AP) initiation, recovery, and refractoriness would contribute to an optimized approach to antiarrhythmic therapy that minimizes potential cardiac and extracardiac toxicity. Of these, K⁺ channels contribute numerous and diverse currents with specific actions on different phases in the time course of AP repolarization. These features and their site-specific distribution make particular K⁺ channel types attractive therapeutic targets for the development of pharmacological agents attempting antiarrhythmic therapy in conditions such as atrial fibrillation. However, progress in the development of such temporally and spatially selective antiarrhythmic drugs against particular ion channels has been relatively limited, particularly in view of our incomplete understanding of the complex physiological roles and interactions of the various ionic currents. This review summarizes the physiological properties of the main cardiac potassium channels and the way in which they modulate cardiac electrical activity and then critiques a number of available potential antiarrhythmic drugs directed at them.

Comparison of the effects of IK,ACh, IKr, and INa block in conscious dogs with atrial fibrillation and on action potentials in remodeled atrial trabeculae

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major cause of morbidity and mortality. Traditional antiarrhythmic agents used for restoration of sinus rhythm have limited efficacy in long-term AF and they may possess ventricular proarrhythmic adverse effects, especially in patients with structural heart disease. The acetylcholine receptor-activated potassium channel (I_K,ACh) represents an atrial selective target for future AF management. We investigated the effects of the I_K,ACh blocker tertiapin-Q (TQ), a derivative of the honeybee toxin tertiapin, on chronic atrial tachypacing-induced AF in conscious dogs, without the influence of anesthetics that modulate a number of cardiac ion channels. Action potentials (APs) were recorded from right atrial trabeculae isolated from dogs with AF. TQ significantly and dose-dependently reduced AF incidence and AF episode duration, prolonged atrial effective refractory period, and prolonged AP duration. The reference drugs propafenone and dofetilide, both used in the clinical management of AF, exerted similar effects against AF in vivo. Dofetilide prolonged atrial AP duration, whereas propafenone increased atrial conduction time. TQ and propafenone did not affect the QT interval, whereas dofetilide prolonged the QT interval. Our results show that inhibition of I_K,ACh may represent a novel, atrial-specific target for the management of AF in chronic AF.

Revealing kinetics and state-dependent binding properties of IKur-targeting drugs that maximize atrial fibrillation selectivity

The K_V1.5 potassium channel, which underlies the ultra-rapid delayed-rectifier current (I_Kur) and is predominantly expressed in atria vs. ventricles, has emerged as a promising target to treat atrial fibrillation (AF). However, while numerous K_V1.5-selective compounds have been screened, characterized, and tested in various animal models of AF, evidence of antiarrhythmic efficacy in humans is still lacking. Moreover, current guidelines for pre-clinical assessment of candidate drugs heavily rely on steady-state concentration-response curves or IC₅₀ values, which can overlook adverse cardiotoxic effects. We sought to investigate the effects of kinetics and state-dependent binding of I_Kur-targeting drugs on atrial electrophysiology in silico and reveal the ideal properties of I_Kur blockers that maximize anti-AF efficacy and minimize pro-arrhythmic risk. To this aim, we developed a new Markov model of I_Kur that describes K_V1.5 gating based on experimental voltage-clamp data in atrial myocytes from patient right-atrial samples in normal sinus rhythm. We extended the I_Kur formulation to account for state-specificity and kinetics of K_V1.5-drug interactions and incorporated it into our human atrial cell model. We simulated 1- and 3-Hz pacing protocols in drug-free conditions and with a [drug] equal to the IC₅₀ value. The effects of binding and unbinding kinetics were determined by examining permutations of the forward (k_on) and reverse (k_off) binding rates to the closed, open, and inactivated states of the K_V1.5 channel. We identified a subset of ideal drugs exhibiting anti-AF electrophysiological parameter changes at fast pacing rates (effective refractory period prolongation), while having little effect on normal sinus rhythm (limited action potential prolongation). Our results highlight that accurately accounting for channel interactions with drugs, including kinetics and state-dependent binding, is critical for developing safer and more effective pharmacological anti-AF options.

Patch-Clamp Recording from Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes: Improving Action Potential Characteristics through Dynamic Clamp

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) hold great promise for studying inherited cardiac arrhythmias and developing drug therapies to treat such arrhythmias. Unfortunately, until now, action potential (AP) measurements in hiPSC-CMs have been hampered by the virtual absence of the inward rectifier potassium current (I_K1) in hiPSC-CMs, resulting in spontaneous activity and altered function of various depolarising and repolarising membrane currents. We assessed whether AP measurements in "ventricular-like" and "atrial-like" hiPSC-CMs could be improved through a simple, highly reproducible dynamic clamp approach to provide these cells with a substantial I_K1 (computed in real time according to the actual membrane potential and injected through the patch-clamp pipette). APs were measured at 1 Hz using perforated patch-clamp methodology, both in control cells and in cells treated with all-trans retinoic acid (RA) during the differentiation process to increase the number of cells with atrial-like APs. RA-treated hiPSC-CMs displayed shorter APs than control hiPSC-CMs and this phenotype became more prominent upon addition of synthetic I_K1 through dynamic clamp. Furthermore, the variability of several AP parameters decreased upon I_K1 injection. Computer simulations with models of ventricular-like and atrial-like hiPSC-CMs demonstrated the importance of selecting an appropriate synthetic I_K1. In conclusion, the dynamic clamp-based approach of I_K1 injection has broad applicability for detailed AP measurements in hiPSC-CMs.

Rate-Dependent Role of IKur in Human Atrial Repolarization and Atrial Fibrillation Maintenance

The atrial-specific ultrarapid delayed rectifier K⁺ current (I_Kur) inactivates slowly but completely at depolarized voltages. The consequences for I_Kur rate-dependence have not been analyzed in detail and currently available mathematical action-potential (AP) models do not take into account experimentally observed I_Kur inactivation dynamics. Here, we developed an updated formulation of I_Kur inactivation that accurately reproduces time-, voltage-, and frequency-dependent inactivation. We then modified the human atrial cardiomyocyte Courtemanche AP model to incorporate realistic I_Kur inactivation properties. Despite markedly different inactivation dynamics, there was no difference in AP parameters across a wide range of stimulation frequencies between the original and updated models. Using the updated model, we showed that, under physiological stimulation conditions, I_Kur does not inactivate significantly even at high atrial rates because the transmembrane potential spends little time at voltages associated with inactivation. Thus, channel dynamics are determined principally by activation kinetics. I_Kur magnitude decreases at higher rates because of AP changes that reduce I_Kur activation. Nevertheless, the relative contribution of I_Kur to AP repolarization increases at higher frequencies because of reduced activation of the rapid delayed-rectifier current I_Kr. Consequently, I_Kur block produces dose-dependent termination of simulated atrial fibrillation (AF) in the absence of AF-induced electrical remodeling. The inclusion of AF-related ionic remodeling stabilizes simulated AF and greatly reduces the predicted antiarrhythmic efficacy of I_Kur block. Our results explain a range of experimental observations, including recently reported positive rate-dependent I_Kur-blocking effects on human atrial APs, and provide insights relevant to the potential value of I_Kur as an antiarrhythmic target for the treatment of AF.

Selective IKur Inhibitors for the Potential Treatment of Atrial Fibrillation: Optimization of the Phenyl Quinazoline Series Leading to Clinical Candidate 5-[5-Phenyl-4-(pyridin-2-ylmethylamino)quinazolin-2-yl]pyridine-3-sulfonamide

We have recently disclosed 5-phenyl-N-(pyridin-2-ylmethyl)-2-(pyrimidin-5-yl)quinazolin-4-amine 1 as a potent I_Kur current blocker with selectivity versus hERG, Na and Ca channels, and an acceptable preclinical PK profile. Upon further characterization in vivo, compound 1 demonstrated an unacceptable level of brain penetration. In an effort to reduce the level of brain penetration while maintaining the overall profile, SAR was developed at the C2' position for a series of close analogues by employing hydrogen bond donors. As a result, 5-[5-phenyl-4-(pyridin-2-ylmethylamino)quinazolin-2-yl]pyridine-3-sulfonamide (25) was identified as the lead compound in this series. Compound 25 showed robust effects in rabbit and canine pharmacodynamic models and an acceptable cross-species pharmacokinetic profile and was advanced as the clinical candidate. Further optimization of 25 to mitigate pH-dependent absorption resulted in identification of the corresponding phosphoramide prodrug (29) with an improved solubility and pharmacokinetic profile.

Electrophysiological mechanisms of long and short QT syndromes

The QT interval on the human electrocardiogram is normally in the order of 450 ms, and reflects the summated durations of action potential (AP) depolarization and repolarization of ventricular myocytes. Both prolongation and shortening in the QT interval have been associated with ventricular tachy-arrhythmias, which predispose affected individuals to sudden cardiac death. In this article, the molecular determinants of the AP duration and the causes of long and short QT syndromes (LQTS and SQTS) are explored. This is followed by a review of the recent advances on their arrhythmogenic mechanisms involving reentry and/or triggered activity based on experiments conducted in mouse models. Established and novel clinical risk markers based on the QT interval for the prediction of arrhythmic risk and cardiovascular mortality are presented here. It is concluded by a discussion on strategies for the future rational design of anti-arrhythmic agents.

Sumo Modification of Ion Channels

Recently, a role for SUMO modification outside of the nucleus has emerged. Although the number of extranuclear proteins known to be sumoylated is comparatively small, ion channels represent one important new class of these proteins. Ion channels are responsible for the control of membrane excitability and therefore are critical for fundamental physiological processes such as muscle contraction, neuronal firing, and cellular homeostasis. As such, these ion-conducting proteins are subject to precise regulation. Recently, several studies have identified sumoylation as a novel mechanism of modulating ion channel function. These studies expand the list of known functions of sumoylation and reveal that, in addition to its more established role in the regulation of nuclear proteins, this modification plays important roles at the cytoplasmic face of membranes.

Functional Characterization of Human Stem Cell-Derived Cardiomyocytes

Cardiac toxicity is a leading contributor to late-stage attrition in the drug discovery process and to withdrawal of approved from the market. In vitro assays that enable earlier and more accurate testing for cardiac risk provide early stage predictive indicators that aid in mitigating risk. Human cardiomyocytes, the most relevant subjects for early stage testing, are severely limited in supply. But human stem cell-derived cardiomyocytes (SC-hCM) are readily available from commercial sources and are increasingly used in academic research, drug discovery and safety pharmacology. As a result, SC-hCM electrophysiology has become a valuable tool to assess cardiac risk associated with drugs. This unit describes techniques for recording individual currents carried by sodium, calcium and potassium ions, as well as single cell action potentials, and impedance recordings from contracting syncytia of thousands of interconnected cells.

Cardiovascular Action of Insulin in Health and Disease: Endothelial L-Arginine Transport and Cardiac Voltage-Dependent Potassium Channels

Impairment of insulin signaling on diabetes mellitus has been related to cardiovascular dysfunction, heart failure, and sudden death. In human endothelium, cationic amino acid transporter 1 (hCAT-1) is related to the synthesis of nitric oxide (NO) and insulin has a vascular effect in endothelial cells through a signaling pathway that involves increases in hCAT-1 expression and L-arginine transport. This mechanism is disrupted in diabetes, a phenomenon potentiated by excessive accumulation of reactive oxygen species (ROS), which contribute to lower availability of NO and endothelial dysfunction. On the other hand, electrical remodeling in cardiomyocytes is considered a key factor in heart failure progression associated to diabetes mellitus. This generates a challenge to understand the specific role of insulin and the pathways involved in cardiac function. Studies on isolated mammalian cardiomyocytes have shown prolongated action potential in ventricular repolarization phase that produces a long QT interval, which is well explained by attenuation in the repolarizing potassium currents in cardiac ventricles. Impaired insulin signaling causes specific changes in these currents, such a decrease amplitude of the transient outward K(+) (Ito) and the ultra-rapid delayed rectifier (IKur) currents where, together, a reduction of mRNA and protein expression levels of α-subunits (Ito, fast; Kv 4.2 and IKs; Kv 1.5) or β-subunits (KChIP2 and MiRP) of K(+) channels involved in these currents in a MAPK mediated pathway process have been described. These results support the hypothesis that lack of insulin signaling can produce an abnormal repolarization in cardiomyocytes. Furthermore, the arrhythmogenic potential due to reduced Ito current can contribute to an increase in the incidence of sudden death in heart failure. This review aims to show, based on pathophysiological models, the regulatory function that would have insulin in vascular system and in cardiac electrophysiology.

Dynamics and modulation studies of human voltage gated Kv1.5 channel

The voltage gated Kv1.5 channels conduct the ultrarapid delayed rectifier current (IK_ur) and play critical role in repolarization of action potential duration. It is the most rapidly activated channel and has very little or no inactivated states. In human cardiac cells, these channels are expressed more extensively in atrial myocytes than ventricle. From the evidences of its localization and functions, Kv1.5 has been declared a selective drug target for the treatment of atrial fibrillation (AF). In this present study, we have tried to identify the rapidly activating property of Kv1.5 and studied its mode of inhibition using molecular modeling, docking, and simulation techniques. Channel in open conformation is found to be stabilized quickly within the dipalmitoylphosphatidylcholine membrane, whereas most of the secondary structure elements were lost in closed state conformation. The obvious reason behind its ultra-rapid property is possibly due to the amino acid alteration in S4-S5 linker; the replacement of Lysine by Glutamine and vice versa. The popular published drugs as well as newly identified lead molecules were able to inhibit the Kv1.5 in a very similar pattern, mainly through the nonpolar interactions, and formed sable complexes. V512 is found as the main contributor for the interaction along with the other important residues such as V505, I508, A509, V512, P513, and V516. Furthermore, two screened novel compounds show surprisingly better inhibitory potency and can be considered for the future perspective of antiarrhythmic survey.

Mechanisms of cardiac arrhythmias

Blood circulation is the result of the beating of the heart, which provides the mechanical force to pump oxygenated blood to, and deoxygenated blood away from, the peripheral tissues. This depends critically on the preceding electrical activation. Disruptions in the orderly pattern of this propagating cardiac excitation wave can lead to arrhythmias. Understanding of the mechanisms underlying their generation and maintenance requires knowledge of the ionic contributions to the cardiac action potential, which is discussed in the first part of this review. A brief outline of the different classification systems for arrhythmogenesis is then provided, followed by a detailed discussion for each mechanism in turn, highlighting recent advances in this area.

Multiscale Interactions in a 3D Model of the Contracting Ventricle

A biophysical detailed multiscale model of the myocardium is presented. The model was used to study the contribution of interrelated cellular mechanisms to global myocardial function. The multiscale model integrates cellular electrophysiology, excitation propagation dynamics and force development models into a geometrical fiber based model of the ventricle. The description of the cellular electrophysiology in this study was based on the Ten Tusscher-Noble-Noble-Panfilov heterogeneous model for human ventricular myocytes. A four-state model of the sarcomeric control of contraction developed by Negroni and Lascano was employed to model the intracellular mechanism of force generation. The propagation of electrical excitation was described by a reaction-diffusion equation. The 3D geometrical model of the ventricle, based on single fiber contraction was used as a platform for the evaluation of proposed models. The model represents the myocardium as an anatomically oriented array of contracting fibers with individual fiber parameters such as size, spatial location, orientation and mechanical properties. Moreover, the contracting ventricle model interacts with intraventricular blood elements linking the contractile elements to the heart's preload and afterload, thereby producing the corresponding pressure-volume loop. The results show that the multiscale ventricle model is capable of simulating mechanical contraction, pressure generation and load interactions as well as demonstrating the individual contribution of each ion current.

The positive frequency-dependent electrophysiological effects of the IKur inhibitor XEN-D0103 are desirable for the treatment of atrial fibrillation

Background

Selective inhibitors of K_v1.5 channels are being developed for the treatment of atrial fibrillation (AF).

Objectives

The purpose of this study was to investigate the effects of the highly selective K_v1.5 inhibitor XEN-D0103 on human atrial action potentials (APs) at high excitation rates and to assess safety.

Methods

Intracellular APs (stimulation rates 1–5 Hz) were measured in right atrial trabeculae from patients in sinus rhythm (SR), chronic AF (cAF; AF of >6 months duration), and paroxysmal AF (pAF). The safety and tolerability of XEN-D0103 were tested in a double-blind, randomized, placebo-controlled phase 1 study.

Results

Depending on its concentration, XEN-D0103 elevated the plateau potential. At 1 Hz, XEN-D0103 (3 µM) shortened action potential duration at 90% repolarization (APD₉₀) and effective refractory period (ERP) in SR preparations, but prolonged these parameters in cAF preparations. In SR and pAF preparations, the shortening effects on APD₉₀ and ERP turned into prolongation at high rates. In cAF trabeculae, XEN-D0103 prolonged APD₉₀ and ERP at 2 and 3 Hz. At high rates, more SR and pAF preparations failed to capture excitation in the presence of the drug than in its absence. XEN-D0103 (10 µM) did not significantly affect human ventricular APs. Even with plasma concentrations reaching 7000 ng/mL, XEN-D0103 did not increase ∆∆QTcF (QT interval corrected by the Fridericia formula) in the analysis of electrocardiograms of healthy volunteers, and no subjects receiving an active treatment had a QT or QTcF interval >450 ms, or increase in QTcF from baseline >30 ms.

Conclusion

APD prolongation and suppression of APs by XEN-D0103 at high stimulation rates in SR and pAF tissue, but not cAF, could be of therapeutic benefit for reducing AF burden. This concept needs to be confirmed in clinical trials.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Interaction of DPP10a with Kv4.3 channel complex results in a sustained current component of human transient outward current Ito

The sustained component of the K(+) outward current in human atrial myocytes is believed to be due to the slowly inactivating ultra-rapid potassium current I Kur and not to the fast inactivating transient outward current Ito. Here we provide evidence for contribution of Ito to this late current due to the effects of dipeptidyl peptidase-like protein (DPP) 10 (DPP10a) interacting with Kv4.3 channels. We studied the late current component of Ito in human atrial myocytes and CHO cells co-expressing Kv4.3 or Kv4.3/KChIP2 (control) and DPP proteins using voltage-clamp technique and a pharmacological approach. A voltage dependent and slowly inactivating late current (43% of peak amplitude) could be observed in atrial myocytes. We found a similar current in CHO cells expressing Kv4.3/KChIP2 + DPP10a, but not in cells co-expressing Kv4.3 + DPP or Kv4.3/KChIP2 + DPP6-S. Assuming that DPP10a influences atrial Ito, we detected DPP10 expression of three alternatively spliced mRNAs, DPP10 protein and colocalization of Kv4.3 and DPP10 proteins in human atrial myocytes. DPP10a did not affect properties of expressed Kv1.5 excluding a contribution to the sustained IKur in atrial cells. To test for the contribution of Kv4-based Ito on sustained K(+) outward currents in human atrial myocytes, we used 4-AP to block IKur, in combination with Heteropoda toxin 2 to block Kv4 channels. We could clearly separate an Ito fraction of about 19% contributing to the late current in atrial myocytes. Thus, the interaction of DPP10a, expressed in human atrium, with Kv4.3 channels generates a sustained current component of Ito, which may affect late repolarization phase of atrial action potentials.