Basic mechanisms of cardiac impulse propagation and associated arrhythmias

Beneficial Effect of Calcium Treatment for Hyperkalemia Is Not Due to "Membrane Stabilization"

OBJECTIVES

Hyperkalemia is a common life-threatening condition causing severe electrophysiologic derangements and arrhythmias. The beneficial effects of calcium (Ca2+) treatment for hyperkalemia have been attributed to "membrane stabilization," by restoration of resting membrane potential (RMP). However, the underlying mechanisms remain poorly understood. Our objective was to investigate the mechanisms underlying adverse electrophysiologic effects of hyperkalemia and the therapeutic effects of Ca2+ treatment.

DESIGN

Controlled experimental trial.

SETTING

Laboratory investigation.

SUBJECTS

Canine myocytes and tissue preparations.

INTERVENTIONS AND MEASUREMENTS

Optical action potentials and volume averaged electrocardiograms were recorded from the transmural wall of ventricular wedge preparations (n = 7) at baseline (4 mM potassium), hyperkalemia (8-12 mM), and hyperkalemia + Ca2+ (3.6 mM). Isolated myocytes were studied during hyperkalemia (8 mM) and after Ca2+ treatment (6 mM) to determine cellular RMP.

MAIN RESULTS

Hyperkalemia markedly slowed conduction velocity (CV, by 67% ± 7%; p < 0.001) and homogeneously shortened action potential duration (APD, by 20% ± 10%; p < 0.002). In all preparations, this resulted in QRS widening and the "sine wave" pattern observed in severe hyperkalemia. Ca2+ treatment restored CV (increase by 44% ± 18%; p < 0.02), resulting in narrowing of the QRS and normalization of the electrocardiogram, but did not restore APD. RMP was significantly elevated by hyperkalemia; however, it was not restored with Ca2+ treatment suggesting a mechanism unrelated to "membrane stabilization." In addition, the effect of Ca2+ was attenuated during L-type Ca2+ channel blockade, suggesting a mechanism related to Ca2+-dependent (rather than normally sodium-dependent) conduction.

CONCLUSIONS

These data suggest that Ca2+ treatment for hyperkalemia restores conduction through Ca2+-dependent propagation, rather than restoration of membrane potential or "membrane stabilization." Our findings provide a mechanistic rationale for Ca2+ treatment when hyperkalemia produces abnormalities of conduction (i.e., QRS prolongation).

Publication guidelines for human heart rate and heart rate variability studies in psychophysiology-Part 1: Physiological underpinnings and foundations of measurement

This Committee Report provides methodological, interpretive, and reporting guidance for researchers who use measures of heart rate (HR) and heart rate variability (HRV) in psychophysiological research. We provide brief summaries of best practices in measuring HR and HRV via electrocardiographic and photoplethysmographic signals in laboratory, field (ambulatory), and brain-imaging contexts to address research questions incorporating measures of HR and HRV. The Report emphasizes evidence for the strengths and weaknesses of different recording and derivation methods for measures of HR and HRV. Along with this guidance, the Report reviews what is known about the origin of the heartbeat and its neural control, including factors that produce and influence HRV metrics. The Report concludes with checklists to guide authors in study design and analysis considerations, as well as guidance on the reporting of key methodological details and characteristics of the samples under study. It is expected that rigorous and transparent recording and reporting of HR and HRV measures will strengthen inferences across the many applications of these metrics in psychophysiology. The prior Committee Reports on HR and HRV are several decades old. Since their appearance, technologies for human cardiac and vascular monitoring in laboratory and daily life (i.e., ambulatory) contexts have greatly expanded. This Committee Report was prepared for the Society for Psychophysiological Research to provide updated methodological and interpretive guidance, as well as to summarize best practices for reporting HR and HRV studies in humans.

Wavefront curvature analysis derived from preprocedural imaging can identify the critical isthmus in patients with postinfarcted ventricular tachycardia

BACKGROUND

Where activation wavefront curvature is convexly shaped, functional conduction block can occur.

OBJECTIVE

The purpose of this study was to determine whether left ventricular (LV) wall thickness determined from contrast-enhanced computed tomography (CT) is useful in localizing such areas in clinical postinfarction reentrant ventricular tachycardia (VT).

METHODS

We evaluated data from 6 patients who underwent catheter ablation for postinfarction VT. CT imaging with inHEART processing was conducted 1-3 days before electrophysiological (EP) study to determine LV wall thickness (T). Activation wavefront curvature was approximated as ΔT/T, where ΔT represents wall thickness change. During EP study, bipolar LV VT electrograms were acquired using a high-density mapping catheter, and activation times were determined. Maps of T, ΔT/T, and VT activation were subsequently compared using statistical analyses.

RESULTS

Two of 6 cases exhibited dual circuit morphologies, resulting in a total of 8 VT morphologies analyzed. The LV wall near the VT isthmus location tended to be thin, on the order of a few hundred micrometers. Regions of largest ΔT/T partially coincided with the lateral isthmus boundaries where electrical conduction block occurred during VT. ΔT/T at the boundaries, measured from imaging, was significantly larger compared to values at the isthmus midline and to the global LV mean value (P <.001).

CONCLUSION

Wavefront curvature measured by ΔT/T and caused by source-sink mismatch is dependent on ventricular wall thickness. Areas of high wavefront curvature partly coincide with and may be helpful in locating the VT isthmus in infarct border zones using preprocedural imaging analysis.

Chronic nicotine exposure is associated with electrophysiological and sympathetic remodeling in the intact rabbit heart

Nicotine is the primary addictive component of tobacco products. Through its actions on the heart and autonomic nervous system, nicotine exposure is associated with electrophysiological changes and increased arrhythmia susceptibility. To assess the underlying mechanisms, we treated rabbits with transdermal nicotine (NIC, 21 mg/day) or control (CT) patches for 28 days before performing dual optical mapping of transmembrane potential (RH237) and intracellular Ca2+ (Rhod-2 AM) in isolated hearts with intact sympathetic innervation. Sympathetic nerve stimulation (SNS) was performed at the first to third thoracic vertebrae, and β-adrenergic responsiveness was additionally evaluated following norepinephrine (NE) perfusion. Baseline ex vivo heart rate (HR) and SNS stimulation threshold were higher in NIC versus CT (P = 0.004 and P = 0.003, respectively). Action potential duration alternans emerged at longer pacing cycle lengths (PCL) in NIC versus CT at baseline (P = 0.002) and during SNS (P = 0.0003), with similar results obtained for Ca2+ transient alternans. SNS shortened the PCL at which alternans emerged in CT but not in NIC hearts. NIC-exposed hearts tended to have slower and reduced HR responses to NE perfusion, but ventricular responses to NE were comparable between groups. Although fibrosis was unaltered, NIC hearts had lower sympathetic nerve density (P = 0.03) but no difference in NE content versus CT. These results suggest both sympathetic hypoinnervation of the myocardium and regional differences in β-adrenergic responsiveness with NIC. This autonomic remodeling may contribute to the increased risk of arrhythmias associated with nicotine exposure, which may be further exacerbated with long-term use.NEW & NOTEWORTHY Here, we show that chronic nicotine exposure was associated with increased heart rate, increased susceptibility to alternans, and reduced sympathetic electrophysiological responses in the intact rabbit heart. We suggest that this was due to sympathetic hypoinnervation of the myocardium and diminished β-adrenergic responsiveness of the sinoatrial node following nicotine treatment. Though these differences did not result in increased arrhythmia propensity in our study, we hypothesize that prolonged nicotine exposure may exacerbate this proarrhythmic remodeling.

The Role of TOMM40 in Cardiovascular Mortality and Conduction Disorders: An Observational Study

Aims: TOMM40 single nucleotide polymorphism (SNP) rs2075650 consists of allelic variation c.275-31A > G and it has been linked to Alzheimer disease, apolipoprotein and cholesterol levels and other risk factors. However, data on its role in cardiovascular disorders are lacking. The first aim of the study is to evaluate mortality according to TOMM40 genotype in a cohort of selected patients affected by advanced atherosclerosis. Second aim was to investigate the relationship between Xg and AA alleles and the presence of conduction disorders and implantation of defibrillator (ICD) or pacemaker (PM) in our cohort. Materials and Methods: We enrolled 276 patients (mean age 70.16 ± 7.96 years) affected by hemodynamic significant carotid stenosis and/or ischemia of the lower limbs of II or III stadium Fontaine. We divided the population into two groups according to the genotype (Xg and AA carriers). We evaluated several electrocardiographic and echocardiographic parameters, including heart rate, rhythm, presence of right and left bundle branch block (LBBB and RBBB), PR interval, QRS duration and morphology, QTc interval, and left ventricular ejection fraction (LVEF). We clinically followed these patients for 82.53 ± 30.02 months and we evaluated the incidence of cardiovascular events, number of deaths and PM/ICD implantations. Results: We did not find a difference in total mortality between Xg and AA carriers (16.3 % vs. 19.4%; p = 0.62). However, we found a higher mortality for fatal cardiovascular events in Xg carriers (8.2% vs. 4.4%; HR = 4.53, 95% CI 1.179-17.367; p = 0.04) with respect to AA carriers. We noted a higher percentage of LBBB in Xg carriers (10.2% vs. 3.1%, p = 0.027), which was statistically significant. Presence of right bundle branch block (RBBB) was also higher in Xg (10.2% vs. 4.4%, p = 0.10), but without reaching statistically significant difference compared to AA patients. We did not observe significant differences in heart rate, presence of sinus rhythm, number of device implantations, PR and QTc intervals, QRS duration and LVEF between the two groups. At the time of enrolment, we observed a tendency for device implant in Xg carriers at a younger age compared to AA carriers (58.50 ± 0.71 y vs. 72.14 ± 11.11 y, p = 0.10). During the follow-up, we noted no statistical difference for new device implantations in Xg respect to AA carriers (8.2% vs. 3.5%; HR = 2.384, 95% CI 0.718-7.922; p = 0.156). The tendency to implant Xg at a younger age compared to AA patients was confirmed during follow-up, but without reaching a significant difference(69.50 ± 2.89 y vs. 75.63 ± 8.35 y, p = 0.074). Finally, we pointed out that Xg carriers underwent device implantation 7.27 ± 4.43 years before AA (65.83 ± 6.11 years vs. 73.10 ± 10.39 years) and that difference reached a statistically significant difference (p = 0.049) when we considered all patients, from enrollment to follow-up. Conclusions: In our study we observed that TOMM40 Xg patients affected by advanced atherosclerosis have a higher incidence of developing fatal cardiovascular events, higher incidence of LBBB and an earlier age of PM or ICD implantations, as compared to AA carriers. Further studies will be needed to evaluate the genomic contribution of TOMM40 SNPs to cardiovascular deaths and cardiac conduction diseases.

Adipocyte-mediated electrophysiological remodeling of human stem cell - derived cardiomyocytes

Adipocytes normally accumulate in the epicardial and pericardial layers around the human heart, but their infiltration into the myocardium can be proarrhythmic. METHODS AND RESULTS: Human adipose derived stem/stromal cells and human induced pluripotent stem cells (hiPSC) were differentiated, respectively into predominantly white fat-like adipocytes (hAdip) and ventricular cardiomyocytes (CMs). Adipocytes cultured in CM maintenance medium (CM medium) maintained their morphology, continued to express adipogenic markers, and retained clusters of intracellular lipid droplets. In contrast, hiPSC-CMs cultivated in adipogenic growth medium displayed abnormal cell morphologies and more clustering across the monolayer. Pre-plated hiPSC-CMs co-cultured in direct contact with hAdips in CM medium displayed prolonged action potential durations, increased triangulation, slowed conduction velocity, increased conduction velocity heterogeneity, and prolonged calcium transients. When hAdip-conditioned medium was added to monolayer cultures of hiPSC-CMs, results similar to those recorded with direct co-cultures were observed. Both co-culture and conditioned medium experiments resulted in increases in transcript abundance of SCN10A, CACNA1C, SLC8A1, and RYR2, with a decrease in KCNJ2. Human adipokine immunoblots revealed the presence of cytokines that were elevated in adipocyte-conditioned medium, including MCP-1, IL-6, IL-8 and CFD that could induce electrophysiological changes in cultured hiPSC-CMs. CONCLUSIONS: Co-culture of hiPSC-CMs with hAdips reveals a potentially pathogenic role of infiltrating human adipocytes on myocardial tissue. In the absence of structural changes, hAdip paracrine release alone is sufficient to cause CM electrophysiological dysfunction mirroring the co-culture conditions. These effects, mediated largely by paracrine mechanisms, could promote arrhythmias in the heart.

Cardiac arrhythmogenesis: roles of ion channels and their functional modification

Cardiac arrhythmias cause significant morbidity and mortality and pose a major public health problem. They arise from disruptions in the normally orderly propagation of cardiac electrophysiological activation and recovery through successive cardiomyocytes in the heart. They reflect abnormalities in automaticity, initiation, conduction, or recovery in cardiomyocyte excitation. The latter properties are dependent on surface membrane electrophysiological mechanisms underlying the cardiac action potential. Their disruption results from spatial or temporal instabilities and heterogeneities in the generation and propagation of cellular excitation. These arise from abnormal function in their underlying surface membrane, ion channels, and transporters, as well as the interactions between them. The latter, in turn, form common regulatory targets for the hierarchical network of diverse signaling mechanisms reviewed here. In addition to direct molecular-level pharmacological or physiological actions on these surface membrane biomolecules, accessory, adhesion, signal transduction, and cytoskeletal anchoring proteins modify both their properties and localization. At the cellular level of excitation-contraction coupling processes, Ca2+ homeostatic and phosphorylation processes affect channel activity and membrane excitability directly or through intermediate signaling. Systems-level autonomic cellular signaling exerts both acute channel and longer-term actions on channel expression. Further upstream intermediaries from metabolic changes modulate the channels both themselves and through modifying Ca2+ homeostasis. Finally, longer-term organ-level inflammatory and structural changes, such as fibrotic and hypertrophic remodeling, similarly can influence all these physiological processes with potential pro-arrhythmic consequences. These normal physiological processes may target either individual or groups of ionic channel species and alter with particular pathological conditions. They are also potentially alterable by direct pharmacological action, or effects on longer-term targets modifying protein or cofactor structure, expression, or localization. Their participating specific biomolecules, often clarified in experimental genetically modified models, thus constitute potential therapeutic targets. The insights clarified by the physiological and pharmacological framework outlined here provide a basis for a recent modernized drug classification. Together, they offer a translational framework for current drug understanding. This would facilitate future mechanistically directed therapeutic advances, for which a number of examples are considered here. The latter are potentially useful for treating cardiac, in particular arrhythmic, disease.

Heart-on-a-chip systems with tissue-specific functionalities for physiological, pathological, and pharmacological studies

Recent advances in heart-on-a-chip systems hold great promise to facilitate cardiac physiological, pathological, and pharmacological studies. This review focuses on the development of heart-on-a-chip systems with tissue-specific functionalities. For one thing, the strategies for developing cardiac microtissues on heart-on-a-chip systems that closely mimic the structures and behaviors of the native heart are analyzed, including the imitation of cardiac structural and functional characteristics. For another, the development of techniques for real-time monitoring of biophysical and biochemical signals from cardiac microtissues on heart-on-a-chip systems is introduced, incorporating cardiac electrophysiological signals, contractile activity, and biomarkers. Furthermore, the applications of heart-on-a-chip systems in intelligent cardiac studies are discussed regarding physiological/pathological research and pharmacological assessment. Finally, the future development of heart-on-a-chip toward a higher level of systematization, integration, and maturation is proposed.

Human sodium current voltage-dependence at physiological temperature measured by coupling a patch-clamp experiment to a mathematical model

Voltage-gated Na+ channels are crucial to action potential propagation in excitable tissues. Because of the high amplitude and rapid activation of the Na+ current, voltage-clamp measurements are very challenging and are usually performed at room temperature. In this study, we measured Na+ current voltage-dependence in stem cell-derived cardiomyocytes at physiological temperature. While the apparent activation and inactivation curves, measured as the dependence of current amplitude on voltage, fall within the range reported in previous studies, we identified a systematic error in our measurements. This error is caused by the deviation of the membrane potential from the command potential of the amplifier. We demonstrate that it is possible to account for this artifact using computer simulation of the patch-clamp experiment. We obtained surprising results through patch-clamp model optimization: a half-activation of -11.5 mV and a half-inactivation of -87 mV. Although the half-activation deviates from previous research, we demonstrate that this estimate reproduces the conduction velocity dependence on extracellular potassium concentration. KEY POINTS: Voltage-gated Na+ currents play a crucial role in excitable tissues including neurons, cardiac and skeletal muscle. Measurement of Na+ current is challenging because of its high amplitude and rapid kinetics, especially at physiological temperature. We have used the patch-clamp technique to measure human Na+ current voltage-dependence in human induced pluripotent stem cell-derived cardiomyocytes. The patch-clamp data were processed by optimization of the model accounting for voltage-clamp experiment artifacts, revealing a large difference between apparent parameters of Na+ current and the results of the optimization. We conclude that actual Na+ current activation is extremely depolarized in comparison to previous studies. The new Na+ current model provides a better understanding of action potential propagation; we demonstrate that it explains propagation in hyperkalaemic conditions.

Evaluation of Tyrosine Kinase Inhibitors Loaded Injectable Hydrogels for Improving Connexin43 Gap Junction Intercellular Communication

Myocardial infarction (MI) is one of the leading causes of death in the developed world, and the loss of cardiomyocytes plays a critical role in the pathogenesis of heart failure. Implicated in this process is a decrease in gap junction intercellular communication due to remodeling of Connexin43 (Cx43). We previously identified that intraperitoneal injection of the Pyk2 inhibitor PF4618433 reduced infarct size, maintained Cx43 at the intercalated disc in left ventricle hypertrophic myocytes, and improved cardiac function in an MI animal model of heart failure. With the emergence of injectable hydrogels as a therapeutic toward the regeneration of cardiac tissue after MI, here, we provide proof of concept that the release of tyrosine kinase inhibitors from hydrogels could have beneficial effects on cardiomyocytes. We developed an injectable hydrogel consisting of thiolated hyaluronic acid and P123-maleimide micelles that can incorporate PF4618433 as well as the Src inhibitor Saracatinib and achieved sustained release (of note, Src activates Pyk2). Using neonatal rat ventricular myocytes in the presence of a phorbol ester, endothelin-1, or phenylephrine to stimulate cardiac hypertrophy, the release of PF4618433 from the hydrogel had the same ability to decrease Cx43 tyrosine phosphorylation and maintain Cx43 localization at the plasma membrane as when directly added to the growth media. Additional beneficial effects included decreases in apoptosis, the hypertrophic marker atrial natriuretic peptide (ANP), and serine kinases upregulated in hypertrophy. Finally, the presence of both PF4618433 and Saracatinib further decreased the level of ANP and apoptosis than each inhibitor alone, suggesting that a combinatorial approach may be most beneficial. These findings provide the groundwork to test if tyrosine kinase inhibitor release from hydrogels will have a beneficial effect in an animal model of MI-induced heart failure.

Management of arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a disease characterised by fibrofatty replacement of the ventricular myocardium due to specific mutations, leading to ventricular arrhythmias and sudden cardiac death. Treating this condition can be challenging due to progressive fibrosis, phenotypic variations and small patient cohorts limiting the feasibility of conducting meaningful clinical trials. Although widely used, the evidence base for anti-arrhythmic drugs is limited. Beta-blockers are theoretically sound, yet their efficacy in reducing arrhythmic risk is not robust. Additionally, the impact of sotalol and amiodarone is inconsistent with studies reporting contradictory results. Emerging evidence suggests that combining flecainide and bisoprolol may be efficacious.Radiofrequency ablation has shown some potential in disrupting ventricular tachycardia circuits, with combined endo and epicardial ablation yielding better results which could be considered at the index procedure. In addition, stereotactic radiotherapy may be a future option that can decrease arrhythmias beyond simple scar formation by altering levels of Nav1.5 channels, Connexin 43 and Wnt signalling, potentially modifying myocardial fibrosis.Future therapies, such as adenoviruses and GSk3b modulation, are still in early-stage research. While implantable cardioverter-defibrillator implantation is a key intervention for reducing arrhythmic death, the risks of inappropriate shocks and device complications must be carefully considered.

Determinants of electrical propagation and propagation block in Arrhythmogenic Cardiomyopathy

Gap junction and ion channel remodeling occur early in Arrhythmogenic Cardiomyopathy (ACM), but their pathogenic consequences have not been elucidated. Here, we identified the arrhythmogenic substrate, consisting of propagation slowing and conduction block, in ACM models expressing two different desmosomal gene variants. Neonatal rat ventricular myocytes were transduced to express variants in genes encoding desmosomal proteins plakoglobin or plakophilin-2. Studies were performed in engineered cells and anisotropic tissues to quantify changes in conduction velocity, formation of unidirectional propagation, cell-cell electrical coupling, and ion currents. Conduction velocity decreased by 71% and 63% in the two ACM models. SB216763, an inhibitor of glycogen synthase kinase-3 beta, restored conduction velocity to near normal levels. Compared to control, both ACM models showed greater propensity for unidirectional conduction block, which increased further at greater stimulation frequencies. Cell-cell electrical conductance measured in cell pairs was reduced by 86% and 87% in the two ACM models. Computer modeling showed close correspondence between simulated and experimentally determined changes in conduction velocity. The simulation identified that reduced cell-cell electrical coupling was the dominant factor leading to slow conduction, while the combination of reduced cell-cell electrical coupling, reduced sodium current and inward rectifier potassium current explained the development of unidirectional block. Expression of two different ACM variants markedly reduced cell-cell electrical coupling and conduction velocity, and greatly increased the likelihood of developing unidirectional block - both key features of arrhythmogenesis. This study provides the first quantitative analysis of cellular electrophysiological changes leading to the substrate of reentrant arrhythmias in early stage ACM.

Absence of Arrhythmogenicity with Biphasic Pulsed Electric Fields Delivered to Porcine Airways

Pulsed electric field (PEF) technologies treat many types of tissue. Many systems mandate synchronization to the cardiac cycle to avoid the induction of cardiac arrhythmias. Significant differences between PEF systems make the assessment of cardiac safety from one technology to another challenging. A growing body of evidence suggests that shorter duration biphasic pulses obviate the need for cardiac synchronization, even when delivered in a monopolar fashion. This study theoretically evaluates the risk profile of different PEF parameters. It then tests a monopolar, biphasic, microsecond-scale PEF technology for arrhythmogenic potential. PEF applications of increasing likelihood to induce an arrhythmia were delivered. The energy was delivered throughout the cardiac cycle, including both single and multiple packets, and then with concentrated delivery on the t-wave. There were no sustained changes to the electrocardiogram waveform or to the cardiac rhythm, despite delivering energy during the most vulnerable phase of the cardiac cycle, and delivery of multiple packets of PEF energy across the cardiac cycle. Only isolated premature-atrial contractions (PAC) were observed. This study provides evidence that certain varieties of biphasic, monopolar PEF delivery do not require synchronized energy delivery to prevent harmful arrhythmias.

Role of Genetic Variation in Transcriptional Regulatory Elements in Heart Rhythm

Genetic predisposition to cardiac arrhythmias has been a field of intense investigation. Research initially focused on rare hereditary arrhythmias, but over the last two decades, the role of genetic variation (single nucleotide polymorphisms) in heart rate, rhythm, and arrhythmias has been taken into consideration as well. In particular, genome-wide association studies have identified hundreds of genomic loci associated with quantitative electrocardiographic traits, atrial fibrillation, and less common arrhythmias such as Brugada syndrome. A significant number of associated variants have been found to systematically localize in non-coding regulatory elements that control the tissue-specific and temporal transcription of genes encoding transcription factors, ion channels, and other proteins. However, the identification of causal variants and the mechanism underlying their impact on phenotype has proven difficult due to the complex tissue-specific, time-resolved, condition-dependent, and combinatorial function of regulatory elements, as well as their modest conservation across different model species. In this review, we discuss research efforts aimed at identifying and characterizing-trait-associated variant regulatory elements and the molecular mechanisms underlying their impact on heart rate or rhythm.

Non-Invasive Electroanatomical Mapping: A State-Space Approach for Myocardial Current Density Estimation

Electroanatomical mapping is a method for creating a model of the electrophysiology of the human heart. Medical professionals routinely locate and ablate the site of origin of cardiac arrhythmias with invasive catheterization. Non-invasive localization takes the form of electrocardiographic (ECG) or magnetocardiographic (MCG) imaging, where the goal is to reconstruct the electrical activity of the human heart. Non-invasive alternatives to catheter electroanatomical mapping would reduce patients' risks and open new venues for treatment planning and prevention. This work introduces a new system state-based method for estimating the electrical activity of the human heart from MCG measurements. Our model enables arbitrary propagation paths and velocities. A Kalman filter optimally estimates the current densities under the given measurements and model parameters. In an outer optimization loop, these model parameters are then optimized via gradient descent. This paper aims to establish the foundation for future research by providing a detailed mathematical explanation of the algorithm. We demonstrate the feasibility of our method through a simplified one-layer simulation. Our results show that the algorithm can learn the propagation paths from the magnetic measurements. A threshold-based segmentation into healthy and pathological tissue yields a DICE score of 0.84, a recall of 0.77, and a precision of 0.93.

Unexpected impairment of INa underpins reentrant arrhythmias in a knock-in swine model of Timothy syndrome

Timothy syndrome 1 (TS1) is a multi-organ form of long QT syndrome associated with life-threatening cardiac arrhythmias, the organ-level dynamics of which remain unclear. In this study, we developed and characterized a novel porcine model of TS1 carrying the causative p.Gly406Arg mutation in CACNA1C, known to impair CaV1.2 channel inactivation. Our model fully recapitulated the human disease with prolonged QT interval and arrhythmic mortality. Electroanatomical mapping revealed the presence of a functional substrate vulnerable to reentry, stemming from an unforeseen constitutional slowing of cardiac activation. This signature substrate of TS1 was reliably identified using the reentry vulnerability index, which, we further demonstrate, can be used as a benchmark for assessing treatment efficacy, as shown by testing of multiple clinical and preclinical anti-arrhythmic compounds. Notably, in vitro experiments showed that TS1 cardiomyocytes display Ca2+ overload and decreased peak INa current, providing a rationale for the arrhythmogenic slowing of impulse propagation in vivo.

Technical development and feasibility of a reusable vest to integrate cardiovascular magnetic resonance with electrocardiographic imaging

BACKGROUND

Electrocardiographic imaging (ECGI) generates electrophysiological (EP) biomarkers while cardiovascular magnetic resonance (CMR) imaging provides data about myocardial structure, function and tissue substrate. Combining this information in one examination is desirable but requires an affordable, reusable, and high-throughput solution. We therefore developed the CMR-ECGI vest and carried out this technical development study to assess its feasibility and repeatability in vivo.

METHODS

CMR was prospectively performed at 3T on participants after collecting surface potentials using the locally designed and fabricated 256-lead ECGI vest. Epicardial maps were reconstructed to generate local EP parameters such as activation time (AT), repolarization time (RT) and activation recovery intervals (ARI). 20 intra- and inter-observer and 8 scan re-scan repeatability tests.

RESULTS

77 participants were recruited: 27 young healthy volunteers (HV, 38.9 ± 8.5 years, 35% male) and 50 older persons (77.0 ± 0.1 years, 52% male). CMR-ECGI was achieved in all participants using the same reusable, washable vest without complications. Intra- and inter-observer variability was low (correlation coefficients [rs] across unipolar electrograms = 0.99 and 0.98 respectively) and scan re-scan repeatability was high (rs between 0.81 and 0.93). Compared to young HV, older persons had significantly longer RT (296.8 vs 289.3 ms, p = 0.002), ARI (249.8 vs 235.1 ms, p = 0.002) and local gradients of AT, RT and ARI (0.40 vs 0.34 ms/mm, p = 0,01; 0.92 vs 0.77 ms/mm, p = 0.03; and 1.12 vs 0.92 ms/mm, p = 0.01 respectively).

CONCLUSION

Our high-throughput CMR-ECGI solution is feasible and shows good reproducibility in younger and older participants. This new technology is now scalable for high throughput research to provide novel insights into arrhythmogenesis and potentially pave the way for more personalised risk stratification.

CLINICAL TRIAL REGISTRATION

Title: Multimorbidity Life-Course Approach to Myocardial Health-A Cardiac Sub-Study of the MRC National Survey of Health and Development (NSHD) (MyoFit46). National Clinical Trials (NCT) number: NCT05455125. URL: https://clinicaltrials.gov/ct2/show/NCT05455125?term=MyoFit&draw=2&rank=1.

Unraveling Impacts of Chamber-Specific Differences in Intercalated Disc Ultrastructure and Molecular Organization on Cardiac Conduction

BACKGROUND

Propagation of action potentials through the heart coordinates the heartbeat. Thus, intercalated discs, specialized cell-cell contact sites that provide electrical and mechanical coupling between cardiomyocytes, are an important target for study. Impaired propagation leads to arrhythmias in many pathologies, where intercalated disc remodeling is a common finding, hence the importance and urgency of understanding propagation dependence on intercalated disc structure. Conventional modeling approaches cannot predict changes in propagation elicited by perturbations that alter intercalated disc ultrastructure or molecular organization, because of lack of quantitative structural data at subcellular through nano scales.

OBJECTIVES

This study sought to quantify intercalated disc structure at these spatial scales in the healthy adult mouse heart and relate them to chamber-specific properties of propagation as a precursor to understanding the effects of pathological intercalated disc remodeling.

METHODS

Using super-resolution light microscopy, electron microscopy, and computational image analysis, we provide here the first ever systematic, multiscale quantification of intercalated disc ultrastructure and molecular organization.

RESULTS

By incorporating these data into a rule-based model of cardiac tissue with realistic intercalated disc structure, and comparing model predictions of electrical propagation with experimental measures of conduction velocity, we reveal that atrial intercalated discs can support faster conduction than their ventricular counterparts, which is normally masked by interchamber differences in myocyte geometry. Further, we identify key ultrastructural and molecular organization features underpinning the ability of atrial intercalated discs to support faster conduction.

CONCLUSIONS

These data provide the first stepping stone to elucidating chamber-specific effects of pathological intercalated disc remodeling, as occurs in many arrhythmic diseases.

Chronic nicotine exposure is associated with electrophysiological and sympathetic remodeling in the intact rabbit heart

Nicotine is the primary addictive component in tobacco products. Through its actions on the heart and autonomic nervous system, nicotine exposure is associated with electrophysiological changes and increased arrhythmia susceptibility. However, the underlying mechanisms are unclear. To address this, we treated rabbits with transdermal nicotine (NIC, 21 mg/day) or control (CT) patches for 28 days prior to performing dual optical mapping of transmembrane potential (RH237) and intracellular Ca 2+ (Rhod-2 AM) in isolated hearts with intact sympathetic innervation. Sympathetic nerve stimulation (SNS) was performed at the 1 st - 3 rd thoracic vertebrae, and β-adrenergic responsiveness was additionally evaluated as changes in heart rate (HR) following norepinephrine (NE) perfusion. Baseline ex vivo HR and SNS stimulation threshold were increased in NIC vs. CT ( P = 0.004 and P = 0.003 respectively). Action potential duration alternans emerged at longer pacing cycle lengths (PCL) in NIC vs. CT at baseline ( P = 0.002) and during SNS ( P = 0.0003), with similar results obtained for Ca 2+ transient alternans. SNS reduced the PCL at which alternans emerged in CT but not NIC hearts. NIC exposed hearts also tended to have slower and reduced HR responses to NE perfusion. While fibrosis was unaltered, NIC hearts had lower sympathetic nerve density ( P = 0.03) but no difference in NE content vs. CT. These results suggest both sympathetic hypo-innervation of the myocardium and diminished β-adrenergic responsiveness with NIC. This autonomic remodeling may underlie the increased risk of arrhythmias associated with nicotine exposure, which may be further exacerbated with continued long-term usage.

NEW & NOTEWORTHY

Here we show that chronic nicotine exposure was associated with increased heart rate, lower threshold for alternans and reduced sympathetic electrophysiological responses in the intact rabbit heart. We suggest that this was due to the sympathetic hypo-innervation of the myocardium and diminished β- adrenergic responsiveness observed following nicotine treatment. Though these differences did not result in increased arrhythmia propensity in our study, we hypothesize that prolonged nicotine exposure may exacerbate this pro-arrhythmic remodeling.

Muscone attenuates susceptibility to ventricular arrhythmia by inhibiting NLRP3 inflammasome activation in rats after myocardial infarction

Fibrosis and abnormal expression of connexin 43 (Cx43) in the ventricle play vital roles in ventricular arrhythmias (VAs) after myocardial infarction (MI). Muscone, an active monomer of heart-protecting musk pill, has various biological activities, but its effect on susceptibility to VAs in rats with MI has not been determined. In the present study, we investigated the effects of muscone on ventricular inflammation, fibrosis, Cx43 expression, and the occurrence of VAs after MI. An MI model was established by ligating the proximal left anterior descending coronary artery. Then, the MI model rats were administered muscone (2 mg/kg/day) or vehicle (saline)via intragastric injection for 14 days. Cardiac function was evaluated by echocardiography, and an in vivo electrophysiological study was performed on Day 14. Cardiac inflammation, fibrosis, and Cx43 expression were determined by histochemical analysis and western blot analysis. Our results indicated that muscone treatment significantly improved cardiac function and inhibited ventricular inflammation, fibrosis, and nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing 3 (NLRP3) inflammasome activation. Electrocardiogrphy and electrophysiology studies showed that muscone shortened the QRS interval, QT interval, QTc interval, and action potential duration; prolonged the effective refractory period; and reduced susceptibility to VAs in rats after MI. Furthermore, Cx43 expression in the BZ was increased by muscone treatment, and this change was coupled by inhibition of the NLRP3/IL-1β/p38 MAPK pathway. Taken together, our results demonstrated that muscone reduces susceptibility to VA, mainly by decreasing ventricular inflammation and fibrosis, and attenuates abnormal Cx43 expression by inhibiting NLRP3 inflammasome activation after myocardial infarction in rats.

The fungicide tebuconazole modulates the sodium current of human NaV1.5 channels expressed in HEK293 cells

The fungicide Tebuconazole is a widely used pesticide in agriculture and may cause cardiotoxicity. In our present investigation the effect of Tebuconazole on the sodium current (INa) of human cardiac sodium channels (NaV1.5) was studied using a heterologous expression system and whole-cell patch-clamp techniques. Tebuconazole reduced the amplitude of the peak INa in a concentration- and voltage-dependent manner. At the holding potential of -120 mV the IC50 was estimated at 204.1 ± 34.3 μM, while at -80 mV the IC50 was 0.3 ± 0.1 μM. The effect of the fungicide is more pronounced at more depolarized potentials, indicating a state-dependent interaction. Tebuconazole caused a negative shift in the half-maximal inactivation voltage and delayed recovery from fast inactivation of INa. Also, it enhanced closed-state inactivation, exhibited use-dependent block in a voltage-dependent manner. Furthermore, Tebuconazole reduced the increase in late sodium current induced by the pyrethroid insecticide β-Cyfluthrin. These results suggest that Tebuconazole can interact with NaV1.5 channels and modulate INa. The observed effects may lead to decreased cardiac excitability through reduced INa availability, which could be a new mechanism of cardiotoxicity to be attributed to the fungicide.

Elevated high-sensitivity C-reactive protein levels increase the risk of new-onset cardiac conduction disorders

BACKGROUND

Previous studies have reported that inflammatory responses can promote the onset of cardiovascular diseases; however, its association with cardiac conduction disorders remains unclear. The present community-based cohort study aimed to elucidate the effects of inflammatory responses on the risk of developing cardiac conduction disorders.

METHODS

After the exclusion of participants failing to meet the inclusion criteria, 86,234 eligible participants (mean age: 50.57 ± 11.88 years) were included. The participants were divided into high-sensitivity C-reactive protein (hsCRP)  ≤  3 mg/L, and hsCRP > 3 mg/L groups based on hsCRP values. Multivariate Cox proportional hazard model was used to analyze the relationship between inflammatory responses and various cardiac conduction disorders.

RESULTS

After adjusting for confounding factors, we observed that compared with the hsCRP ≤ 3 mg/L group, the hsCRP > 3 mg/L group exhibited increased risks of atrioventricular block (hazard ratio [HR]:1.64, 95%confidence interval [CI] 1.44-1.87) and left (HR:1.25, 95% CI 1.07-1.45) and right bundle branch block (HR:1.31, 95% CI 1.17-1.47). Moreover, the risk of various cardiac conduction disorders increased for every 1 standard deviation increase in log (hsCRP). The restricted cubic spline function confirmed a linear relationship between log (hsCRP) and the risk of developing cardiac conduction disorders (All nonlinearity P > 0.05).

CONCLUSIONS

High hsCRP levels are an independent risk factor for cardiac conduction disorders, and hsCRP levels are dose-dependently associated with the risk of conduction disorders. Our study results may provide new strategies for preventing cardiac conduction disorders.

Insights Into the Effects of Low-Level Vagus Nerve Stimulation on Atrial Electrophysiology: Towards Patient-Tailored Cardiac Neuromodulation

BACKGROUND

Low-level vagus nerve stimulation through the tragus (tLLVNS) is increasingly acknowledged as a therapeutic strategy to prevent and treat atrial fibrillation. However, a lack in understanding of the exact antiarrhythmic properties of tLLVNS has hampered clinical implementation.

OBJECTIVES

In this study, the authors aimed to study the effects of tLLVNS on atrial electrophysiology by performing intraoperative epicardial mapping during acute and chronic tLLVNS.

METHODS

Epicardial mapping of the superior right atrium was performed before and after arterial graft harvesting in patients undergoing coronary artery bypass grafting without a history of atrial fibrillation. The time needed for arterial graft harvesting was used to perform chronic tLLVNS. Electrophysiological properties were compared before and during chronic tLLVNS.

RESULTS

A total of 10 patients (median age 74 years [IQR: 69-78 years]) underwent tLLVNS for a duration of 56 minutes (IQR: 43-73 minutes). During acute and chronic tLLVNS, a shift of the sinoatrial node exit site toward a more cranial direction was observed in 5 (50%) patients. Unipolar potential voltage increased significantly during acute and chronic tLLVNS (3.9 mV [IQR: 3.1-4.8 mV] vs 4.7 mV [IQR: 4.0-5.3 mV] vs 5.2 mV [IQR: 4.8-7.0 mV]; P = 0.027, P = 0.02, respectively). Total activation time, slope of unipolar potentials, amount of fractionation, low-voltage areas and conduction velocity did not differ significantly between baseline measurements and tLLVNS. Two patients showed consistent "improvement" of all electrophysiological properties during tLLVNS, while 1 patient appeared to have no beneficial effect.

CONCLUSIONS

We demonstrated that tLLVNS resulted in a significant increase in unipolar potential voltage. In addition, we observed the following in selective patients: 1) reduction in total activation time; 2) steeper slope of unipolar potentials; 3) decrease in the amount of fractionation; and 4) change in sinoatrial node exit sites.

A detailed analysis of single-channel Nav 1.5 recordings does not reveal any cooperative gating

Cardiac voltage-gated sodium (Na+ ) channels (Nav 1.5) are crucial for myocardial electrical excitation. Recent studies based on single-channel recordings have suggested that Na+ channels interact functionally and exhibit coupled gating. However, the analysis of such recordings frequently relies on manual interventions, which can lead to bias. Here, we developed an automated pipeline to de-trend and idealize single-channel currents, and assessed possible functional interactions in cell-attached patch clamp experiments in HEK293 cells expressing human Nav 1.5 channels as well as in adult mouse and rabbit ventricular cardiomyocytes. Our pipeline involved de-trending individual sweeps by linear optimization using a library of predefined functions, followed by digital filtering and baseline offset. Subsequently, the processed sweeps were idealized based on the idea that the ensemble average of the idealized current identified by thresholds between current levels reconstructs at best the ensemble average current from the de-trended sweeps. This reconstruction was achieved by non-linear optimization. To ascertain functional interactions, we examined the distribution of the numbers of open channels at every time point during the activation protocol and compared it to the distribution expected for independent channels. We also examined whether the channels tended to synchronize their openings and closings. However, we did not uncover any solid evidence of such interactions in our recordings. Rather, our results indicate that wild-type Nav 1.5 channels are independent entities or exhibit only very weak functional interactions that are probably irrelevant under physiological conditions. Nevertheless, our unbiased analysis will be important for further studies examining whether auxiliary proteins potentiate functional Na+ channel interactions. KEY POINTS: Nav 1.5 channels are critical for cardiac excitation. They are part of macromolecular interacting complexes, and it was previously suggested that two neighbouring channels may functionally interact and exhibit coupled gating. Manual interventions when processing single-channel recordings can lead to bias and inaccurate data interpretation. We developed an automated pipeline to de-trend and idealize single-channel currents and assessed possible functional interactions between Nav 1.5 channels in HEK293 cells and cardiomyocytes during activation protocols using the cell-attached patch clamp technique. In recordings consisting of up to 1000 sweeps from the same patch, our analysis did not reveal any evidence of functional interactions or coupled gating between wild-type Nav 1.5 channels. Our unbiased analysis may be useful in further studies examining how Na+ channel interactions are affected by mutations and auxiliary proteins.

Catheter ablation of ventricular tachycardia: strategies to improve outcomes

Catheter ablation of ventricular arrhythmias has evolved considerably since it was first described more than 3 decades ago. Advancements in understanding the underlying substrate, utilizing pre-procedural imaging, and evolving ablation techniques have improved the outcomes of catheter ablation. Ensuring safety and efficacy during catheter ablation requires adequate planning, including analysis of the 12 lead ECG and appropriate pre-procedural imaging. Defining the underlying arrhythmogenic substrate and disease eitology allow for the developed of tailored ablation strategies, especially for patients with non-ischemic cardiomyopathies. During ablation, the type of anesthesia can affect VT induction, the quality of the electro-anatomic map, and the stability of the catheter during ablation. For high risk patients, appropriate selection of hemodynamic support can increase the success of VT ablation. For patients in whom VT is hemodynamically unstable or difficult to induce, substrate modification strategies can aid in safe and successful ablation. Recently, there has been an several advancements in substrate mapping strategies that can be used to identify and differentiate local late potentials. The incorporation of high-definition mapping and contact-sense technologies have both had incremental benefits on the success of ablation procedures. It is crucial to harness newer technology and ablation strategies with the highest level of peri-procedural safety to achieve optimal long-term outcomes in patients undergoing VT ablation.

Atrial extrasystoles enhance low-voltage fractionation electrograms in patients with atrial fibrillation

BACKGROUND AND AIMS

Atrial extrasystoles (AES) provoke conduction disorders and may trigger episodes of atrial fibrillation (AF). However, the direction- and rate-dependency of electrophysiological tissue properties on epicardial unipolar electrogram (EGM) morphology is unknown. Therefore, this study examined the impact of spontaneous AES on potential amplitude, -fractionation, -duration, and low-voltage areas (LVAs), and correlated these differences with various degrees of prematurity and aberrancy.

METHODS AND RESULTS

Intra-operative high-resolution epicardial mapping of the right and left atrium, Bachmann's Bundle, and pulmonary vein area was performed during sinus rhythm (SR) in 287 patients (60 with AF). AES were categorized according to their prematurity index (>25% shortening) and degree of aberrancy (none, mild/opposite, moderate and severe). In total, 837 unique AES (457 premature; 58 mild/opposite, 355 moderate, and 154 severe aberrant) were included. The average prematurity index was 28% [12-45]. Comparing SR and AES, average voltage decreased (-1.1 [-1.2, -0.9] mV, P < 0.001) at all atrial regions, whereas the amount of LVAs and fractionation increased (respectively, +3.4 [2.7, 4.1] % and +3.2 [2.6, 3.7] %, P < 0.001). Only weak or moderate correlations were found between EGM morphology parameters and prematurity indices (R2 < 0.299, P < 0.001). All parameters were, however, most severely affected by either mild/opposite or severely aberrant AES, in which the effect was more pronounced in AF patients. Also, there were considerable regional differences in effects provoked by AES.

CONCLUSION

Unipolar EGM characteristics during spontaneous AES are mainly directional-dependent and not rate-dependent. AF patients have more direction-dependent conduction disorders, indicating enhanced non-uniform anisotropy that is uncovered by spontaneous AES.

Conduction Velocity Dispersion Predicts Postinfarct Ventricular Tachycardia Circuit Sites and Associates With Lipomatous Metaplasia

BACKGROUND

Regional myocardial conduction velocity (CV) dispersion has not been studied in postinfarct patients with ventricular tachycardia (VT).

OBJECTIVES

This study sought to compare the following: 1) the association of CV dispersion vs repolarization dispersion with VT circuit sites; and 2) myocardial lipomatous metaplasia (LM) vs fibrosis as the anatomic substrate for CV dispersion.

METHODS

Among 33 postinfarct patients with VT, we characterized dense and border zone infarct tissue by late gadolinium enhancement cardiac magnetic resonance, and LM by computed tomography, with both images registered with electroanatomic maps. Activation recovery interval (ARI) was the time interval from the minimum derivative within the QRS complex to the maximum derivative within the T-wave on unipolar electrograms. CV at each EAM point was the mean CV between that point and 5 adjacent points along the activation wave front. CV and ARI dispersion were the coefficient of variation (CoV) of CV and ARI per American Heart Association (AHA) segment, respectively.

RESULTS

Regional CV dispersion exhibited a much larger range than ARI dispersion, with median 0.65 vs 0.24; P < 0.001. CV dispersion was a more robust predictor of the number of critical VT sites per AHA segment than ARI dispersion. Regional LM area was more strongly associated with CV dispersion than fibrosis area. LM area was larger (median 0.44 vs 0.20 cm2; P < 0.001) in AHA segments with mean CV <36 cm/s and CoV_CV >0.65 than those with mean CV <36 cm/s and CoV_CV <0.65.

CONCLUSIONS

Regional CV dispersion more strongly predicts VT circuit sites than repolarization dispersion, and LM is a critical substrate for CV dispersion.

Lipomatous Metaplasia Facilitates Slow Conduction in Critical Ventricular Tachycardia Corridors Within Postinfarct Myocardium

BACKGROUND

Myocardial lipomatous metaplasia (LM) has been reported to be associated with post-infarct ventricular tachycardia (VT) circuitry.

OBJECTIVES

This study examined the association of scar versus LM composition with impulse conduction velocity (CV) in putative VT corridors that traverse the infarct zone in post-infarct patients.

METHODS

The cohort included 31 post-infarct patients from the prospective INFINITY (Intra-Myocardial Fat Deposition and Ventricular Tachycardia in Cardiomyopathy) study. Myocardial scar, border zone, and potential viable corridors were defined by late gadolinium enhancement cardiac magnetic resonance (LGE-CMR), and LM was defined by computed tomography. Images were registered to electroanatomic maps, and the CV at each electroanatomic map point was calculated as the mean CV between that point and 5 adjacent points along the activation wave front.

RESULTS

Regions with LM exhibited lower CV than scar (median = 11.9 vs 13.5 cm/s; P < 0.001). Of 94 corridors computed from LGE-CMR and electrophysiologically confirmed to participate in VT circuitry, 93 traversed through or near LM. These critical corridors displayed slower CV (median 8.8 [IQR: 5.9-15.7] cm/s vs 39.2 [IQR: 28.1-58.5]) cm/s; P < 0.001) than 115 noncritical corridors distant from LM. Additionally, critical corridors demonstrated low-peripheral, high-center (mountain shaped, 23.3%) or mean low-level (46.7%) CV patterns compared with 115 noncritical corridors distant from LM that displayed high-peripheral, low-center (valley shaped, 19.1%) or mean high-level (60.9%) CV patterns.

CONCLUSIONS

The association of myocardial LM with VT circuitry is at least partially mediated by slowing nearby corridor CV thus facilitating an excitable gap that enables circuit re-entry.

Fibre-infused gel scaffolds guide cardiomyocyte alignment in 3D-printed ventricles

Hydrogels are attractive materials for tissue engineering, but efforts to date have shown limited ability to produce the microstructural features necessary to promote cellular self-organization into hierarchical three-dimensional (3D) organ models. Here we develop a hydrogel ink containing prefabricated gelatin fibres to print 3D organ-level scaffolds that recapitulate the intra- and intercellular organization of the heart. The addition of prefabricated gelatin fibres to hydrogels enables the tailoring of the ink rheology, allowing for a controlled sol-gel transition to achieve precise printing of free-standing 3D structures without additional supporting materials. Shear-induced alignment of fibres during ink extrusion provides microscale geometric cues that promote the self-organization of cultured human cardiomyocytes into anisotropic muscular tissues in vitro. The resulting 3D-printed ventricle in vitro model exhibited biomimetic anisotropic electrophysiological and contractile properties.

Cardiomyocyte electrophysiology and its modulation: current views and future prospects

Normal and abnormal cardiac rhythms are of key physiological and clinical interest. This introductory article begins from Sylvio Weidmann's key historic 1950s microelectrode measurements of cardiac electrophysiological activity and Singh & Vaughan Williams's classification of cardiotropic targets. It then proceeds to introduce the insights into cardiomyocyte function and its regulation that subsequently emerged and their therapeutic implications. We recapitulate the resulting view that surface membrane electrophysiological events underlying cardiac excitation and its initiation, conduction and recovery constitute the final common path for the cellular mechanisms that impinge upon this normal or abnormal cardiac electrophysiological activity. We then consider progress in the more recently characterized successive regulatory hierarchies involving Ca2+ homeostasis, excitation-contraction coupling and autonomic G-protein signalling and their often reciprocal interactions with the surface membrane events, and their circadian rhythms. Then follow accounts of longer-term upstream modulation processes involving altered channel expression, cardiomyocyte energetics and hypertrophic and fibrotic cardiac remodelling. Consideration of these developments introduces each of the articles in this Phil. Trans. B theme issue. The findings contained in these articles translate naturally into recent classifications of cardiac electrophysiological targets and drug actions, thereby encouraging future iterations of experimental cardiac electrophysiological discovery, and testing directed towards clinical management. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Treatment of pathophysiologic propagation outside of the pulmonary veins in retreatment of atrial fibrillation patients: RECOVER AF study

AIMS

RECOVER AF evaluated the performance of whole-chamber non-contact charge-density mapping to guide the ablation of non-pulmonary vein (PV) targets in persistent atrial fibrillation (AF) patients following either a first or second failed procedure.

METHODS AND RESULTS

RECOVER AF was a prospective, non-randomized trial that enrolled patients scheduled for a first or second ablation retreatment for recurrent AF. The PVs were assessed and re-isolated if necessary. The AF maps were used to guide the ablation of non-PV targets through elimination of pathologic conduction patterns (PCPs). Primary endpoint was freedom from AF on or off antiarrhythmic drugs (AADs) at 12 months. Patients undergoing retreatment with the AcQMap System (n = 103) were 76% AF-free at 12 months [67% after single procedure (SP)] on or off AADs (80% free from AF on AADs). Patients who had only received a pulmonary vein isolation (PVI) prior to study treatment of non-PV targets with the AcQMap System were 91% AF-free at 12 months (83% SP). No major adverse events were reported.

CONCLUSION

Non-contact mapping can be used to target and guide the ablation of PCPs beyond the PVs in persistent AF patients returning for a first or second retreatment with 76% freedom from AF at 12 months. The AF freedom was particularly high, 91% (43/47), for patients enrolled having only a prior de novo PVI, and freedom from all atrial arrhythmias for this cohort was 74% (35/47). These early results are encouraging and suggest that guiding individualized targeted ablation of PCPs may therefore be advantageous to target at the earliest opportunity in patients with persistent AF.

A Review on the Applications of Natural Biodegradable Nano Polymers in Cardiac Tissue Engineering

As cardiac diseases, which mostly result in heart failure, are increasing rapidly worldwide, heart transplantation seems the only solution for saving lives. However, this practice is not always possible due to several reasons, such as scarcity of donors, rejection of organs from recipient bodies, or costly medical procedures. In the framework of nanotechnology, nanomaterials greatly contribute to the development of these cardiovascular scaffolds as they provide an easy regeneration of the tissues. Currently, functional nanofibers can be used in the production of stem cells and in the regeneration of cells and tissues. The small size of nanomaterials, however, leads to changes in their chemical and physical characteristics that could alter their interaction and exposure to stem cells with cells and tissues. This article aims to review the naturally occurring biodegradable nanomaterials that are used in cardiovascular tissue engineering for the development of cardiac patches, vessels, and tissues. Moreover, this article also provides an overview of cell sources used for cardiac tissue engineering, explains the anatomy and physiology of the human heart, and explores the regeneration of cardiac cells and the nanofabrication approaches used in cardiac tissue engineering as well as scaffolds.

Leaf-venation-directed cellular alignment for macroscale cardiac constructs with tissue-like functionalities

Recapitulating the complex structural, mechanical, and electrophysiological properties of native myocardium is crucial to engineering functional cardiac tissues. Here, we report a leaf-venation-directed strategy that enables the compaction and remodeling of cell-hydrogel hybrids into highly aligned and densely packed organizations in predetermined patterns. This strategy contributes to interconnected tubular structures with cell alignment along the hierarchical channels. Compared to randomly-distributed cells, the engineered leaf-venation-directed-cardiac tissues from neonatal rat cardiomyocytes manifest advanced maturation and functionality as evidenced by detectable electrophysiological activity, macroscopically synchronous contractions, and upregulated maturation genes. As a demonstration, human induced pluripotent stem cell-derived leaf-venation-directed-cardiac tissues are engineered with evident structural and functional improvement over time. With the elastic scaffolds, leaf-venation-directed tissues are assembled into 3D centimeter-scale cardiac constructs with programmed mechanical properties, which can be delivered through tubing without affecting cell viability. The present strategy may generate cardiac constructs with multifaceted functionalities to meet clinical demands.

Unraveling Chamber-specific Differences in Intercalated Disc Ultrastructure and Molecular Organization and Their Impact on Cardiac Conduction

During each heartbeat, the propagation of action potentials through the heart coordinates the contraction of billions of individual cardiomyocytes and is thus, a critical life process. Unsurprisingly, intercalated discs, which are cell-cell contact sites specialized to provide electrical and mechanical coupling between adjacent cardiomyocytes, have been the focus of much investigation. Slowed or disrupted propagation leads to potentially life-threatening arrhythmias in a wide range of pathologies, where intercalated disc remodeling is a common finding. Hence, the importance and urgency of understanding intercalated disc structure and its influence on action potential propagation. Surprisingly, however, conventional modeling approaches cannot predict changes in propagation elicited by perturbations that alter intercalated disc ultrastructure or molecular organization, owing to lack of quantitative structural data at subcellular through nano scales. In order to address this critical gap in knowledge, we sought to quantify intercalated disc structure at these finer spatial scales in the healthy adult mouse heart and relate them to function in a chamber-specific manner as a precursor to understanding the impacts of pathological intercalated disc remodeling. Using super-resolution light microscopy, electron microscopy, and computational image analysis, we provide here the first ever systematic, multiscale quantification of intercalated disc ultrastructure and molecular organization. By incorporating these data into a rule-based model of cardiac tissue with realistic intercalated disc structure, and comparing model predictions of electrical propagation with experimental measures of conduction velocity, we reveal that atrial intercalated discs can support faster conduction than their ventricular counterparts, which is normally masked by inter-chamber differences in myocyte geometry. Further, we identify key ultrastructural and molecular organization features underpinning the ability of atrial intercalated discs to support faster conduction. These data provide the first stepping stone to elucidating chamber-specific impacts of pathological intercalated disc remodeling, as occurs in many arrhythmic diseases.

Integrated technologies for continuous monitoring of organs-on-chips: Current challenges and potential solutions

Organs-on-chips (OoCs) are biomimetic in vitro systems based on microfluidic cell cultures that recapitulate the in vivo physicochemical microenvironments and the physiologies and key functional units of specific human organs. These systems are versatile and can be customized to investigate organ-specific physiology, pathology, or pharmacology. They are more physiologically relevant than traditional two-dimensional cultures, can potentially replace the animal models or reduce the use of these models, and represent a unique opportunity for the development of personalized medicine when combined with human induced pluripotent stem cells. Continuous monitoring of important quality parameters of OoCs via a label-free, non-destructive, reliable, high-throughput, and multiplex method is critical for assessing the conditions of these systems and generating relevant analytical data; moreover, elaboration of quality predictive models is required for clinical trials of OoCs. Presently, these analytical data are obtained by manual or automatic sampling and analyzed using single-point, off-chip traditional methods. In this review, we describe recent efforts to integrate biosensing technologies into OoCs for monitoring the physiologies, functions, and physicochemical microenvironments of OoCs. Furthermore, we present potential alternative solutions to current challenges and future directions for the application of artificial intelligence in the development of OoCs and cyber-physical systems. These "smart" OoCs can learn and make autonomous decisions for process optimization, self-regulation, and data analysis.

Calcineurin-dependent regulation of gap junction conductance and connexin phosphorylation in guinea pig left atrium

Atrial fibrillation (AF) occurs from disordered atrial action potential conduction and is associated with reduced gap junction electrical conductance (G_j). The Ca²⁺ and calmodulin-dependent phosphatase, calcineurin, reduces G_j in ventricular myocardium via a protein phosphatase-1 (PP1)-dependent pathway culminating in phosphorylation of serine368 on connexin43 (pSer368-Cx43). However, characterisation of corresponding pathways in left atrial myocardium, which have a more complex connexin subtype profile, is undefined and was the aim of this study. G_j was measured in guinea-pig left atrium from the frequency-dependent variation of intracellular impedance; intracellular [Ca²⁺], ([Ca²⁺]_i) in low-Na solution was measured by Fura-2 fluorescence. Phosphorylation of guinea-pig Ser368-Cx43 residues was measured by Western blot; Cx40 was immunoprecipitated and probed for serine/threonine residue phosphorylation. Low-Na solution reversibly reduced G_j, in turn attenuated or prevented by calcineurin inhibitors cyclosporin-A or CAIP, respectively. Moreover, Ser368-Cx43 phosphorylation in low-Na solution was also prevented by CAIP. Changes were partially prevented by fostreicin (FST), a protein phosphatase-2A (PP2A) inhibitor; but not by tautomycin, a PP1 inhibitor. Serine/threonine residues on Cx40 were also phosphorylated in low-Na solution; prevented by CAIP and attenuated by FST. Reduced G_j with raised [Ca²⁺]_i is paralleled by a changed Cx43/Cx40 phosphorylation status; changes mediated by calcineurin and PP2A-dependent pathways, but not PP1. The pharmacological profile underlying changes to guinea-pig atrial gap junction electrical conductance with raised intracellular [Ca²⁺]_i is fundamentally different from that in ventricular myocardium. This provides a targeted drug model whereby atrial and ventricular myocardium can be selectively targeted to correct conduction defects.

A compact multi-functional model of the rabbit atrioventricular node with dual pathways

The atrioventricular node (AVN) is considered a “black box”, and the functioning of its dual pathways remains controversial and not fully understood. In contrast to numerous clinical studies, there are only a few mathematical models of the node. In this paper, we present a compact, computationally lightweight multi-functional rabbit AVN model based on the Aliev-Panfilov two-variable cardiac cell model. The one-dimensional AVN model includes fast (FP) and slow (SP) pathways, primary pacemaking in the sinoatrial node, and subsidiary pacemaking in the SP. To obtain the direction-dependent conduction properties of the AVN, together with gradients of intercellular coupling and cell refractoriness, we implemented the asymmetry of coupling between model cells. We hypothesized that the asymmetry can reflect some effects related to the complexity of the real 3D structure of AVN. In addition, the model is accompanied by a visualization of electrical conduction in the AVN, revealing the interaction between SP and FP in the form of ladder diagrams. The AVN model demonstrates broad functionality, including normal sinus rhythm, AVN automaticity, filtering of high-rate atrial rhythms during atrial fibrillation and atrial flutter with Wenckebach periodicity, direction-dependent properties, and realistic anterograde and retrograde conduction curves in the control case and the cases of FP and SP ablation. To show the validity of the proposed model, we compare the simulation results with the available experimental data. Despite its simplicity, the proposed model can be used both as a stand-alone module and as a part of complex three-dimensional atrial or whole heart simulation systems, and can help to understand some puzzling functions of AVN.

Cellular electrophysiological effects of botulinum toxin A on neonatal rat cardiomyocytes and on cardiomyocytes derived from human-induced pluripotent stem cells

Botulinum toxin A is a well-known neurotransmitter inhibitor with a wide range of applications in modern medicine. Recently, botulinum toxin A preparations have been used in clinical trials to suppress cardiac arrhythmias, especially in the postoperative period. Its antiarrhythmic action is associated with inhibition of the nervous system of the heart, but its direct effect on heart tissue remains unclear. Accordingly, we investigate the effect of botulinum toxin A on isolated cardiac cells and on layers of cardiac cells capable of conducting excitation. Cardiomyocytes of neonatal rat pups and human cardiomyocytes obtained through cell reprogramming were used. A patch-clamp study showed that botulinum toxin A inhibited fast sodium currents and L-type calcium currents in a dose-dependent manner, with no apparent effect on potassium currents. Optical mapping showed that in the presence of botulinum toxin A, the propagation of the excitation wave in the layer of cardiac cells slows down sharply, conduction at high concentrations becomes chaotic, but reentry waves do not form. The combination of botulinum toxin A with a preparation of chitosan showed a stronger inhibitory effect by an order of magnitude. Further, the inhibitory effect of botulinum toxin A is not permanent and disappears after 12 days of cell culture in a botulinum toxin A-free medium. The main conclusion of the work is that the antiarrhythmic effect of botulinum toxin A found in clinical studies is associated not only with depression of the nervous system but also with a direct effect on heart tissue. Moreover, the combination of botulinum toxin A and chitosan reduces the effective dose of botulinum toxin A.

Optimization of anti-tachycardia pacing efficacy through scar-specific delivery and minimization of re-initiation: a virtual study on a cohort of infarcted porcine hearts

AIMS

Anti-tachycardia pacing (ATP) is a reliable electrotherapy to painlessly terminate ventricular tachycardia (VT). However, ATP is often ineffective, particularly for fast VTs. The efficacy may be enhanced by optimized delivery closer to the re-entrant circuit driving the VT. This study aims to compare ATP efficacy for different delivery locations with respect to the re-entrant circuit, and further optimize ATP by minimizing failure through re-initiation.

METHODS AND RESULTS

Seventy-three sustained VTs were induced in a cohort of seven infarcted porcine ventricular computational models, largely dominated by a single re-entrant pathway. The efficacy of burst ATP delivered from three locations proximal to the re-entrant circuit (septum) and three distal locations (lateral/posterior left ventricle) was compared. Re-initiation episodes were used to develop an algorithm utilizing correlations between successive sensed electrogram morphologies to automatically truncate ATP pulse delivery. Anti-tachycardia pacing was more efficacious at terminating slow compared with fast VTs (65 vs. 46%, P = 0.000039). A separate analysis of slow VTs showed that the efficacy was significantly higher when delivered from distal compared with proximal locations (distal 72%, proximal 59%), being reversed for fast VTs (distal 41%, proximal 51%). Application of our early termination detection algorithm (ETDA) accurately detected VT termination in 79% of re-initiated cases, improving the overall efficacy for proximal delivery with delivery inside the critical isthmus (CI) itself being overall most effective.

CONCLUSION

Anti-tachycardia pacing delivery proximal to the re-entrant circuit is more effective at terminating fast VTs, but less so slow VTs, due to frequent re-initiation. Attenuating re-initiation, through ETDA, increases the efficacy of delivery within the CI for all VTs.

Nano-scale solution of the Poisson-Nernst-Planck (PNP) equations in a fraction of two neighboring cells reveals the magnitude of intercellular electrochemical waves

The basic building blocks of the electrophysiology of cardiomyocytes are ion channels integrated in the cell membranes. Close to the ion channels there are very strong electrical and chemical gradients. However, these gradients extend for only a few nano-meters and are therefore commonly ignored in mathematical models. The full complexity of the dynamics is modelled by the Poisson-Nernst-Planck (PNP) equations but these equations must be solved using temporal and spatial scales of nano-seconds and nano-meters. Here we report solutions of the PNP equations in a fraction of two abuttal cells separated by a tiny extracellular space. We show that when only the potassium channels of the two cells are open, a stationary solution is reached with the well-known Debye layer close to the membranes. When the sodium channels of one of the cells are opened, a very strong and brief electrochemical wave emanates from the channels. If the extracellular space is sufficiently small and the number of sodium channels is sufficiently high, the wave extends all the way over to the neighboring cell and may therefore explain cardiac conduction even at very low levels of gap junctional coupling.

The Anti-Epileptic Drugs Lamotrigine and Valproic Acid Reduce the Cardiac Sodium Current

Anti-epileptic drugs (AEDs) are associated with increased risk of sudden cardiac death. To establish whether gabapentin, lamotrigine, levetiracetam, pregabalin, and valproic acid reduce the Na_v1.5 current, we conducted whole-cell patch-clamp studies to study the effects of the five AEDs on currents of human cardiac Na_v1.5 channels stably expressed in HEK293 cells, and on action potential (AP) properties of freshly isolated rabbit ventricular cardiomyocytes. Lamotrigine and valproic acid exhibited inhibitory effects on the Na_v1.5 current in a concentration-dependent manner with an IC₅₀ of 142 ± 36 and 2022 ± 25 µM for lamotrigine and valproic acid, respectively. In addition, these drugs caused a hyperpolarizing shift of steady-state inactivation and a delay in recovery from inactivation. The changes on the Na_v1.5 properties were reflected by a reduction in AP upstroke velocity (43.0 ± 6.8% (lamotrigine) and 23.7 ± 10.6% (valproic acid) at 1 Hz) and AP amplitude; in contrast, AP duration was not changed. Gabapentin, levetiracetam, and pregabalin had no effect on the Na_v1.5 current. Lamotrigine and valproic acid reduce the Na_v1.5 current density and affect its gating properties, resulting in a decrease of the AP upstroke velocity. Gabapentin, levetiracetam, and pregabalin have no effects on the Na_v1.5 current.

Forced activation of dystrophin transcription by CRISPR/dCas9 reduced arrhythmia susceptibility via restoring membrane Nav1.5 distribution

Dystrophin deficiency due to genetic mutations causes cardiac abnormalities in Duchenne's muscular dystrophy. Dystrophin is also shown to be downregulated in conventional failing hearts. Whether restoration of dystrophin expression possesses any therapeutic potential for conventional heart failure (HF) remains to be examined. HF mouse model was generated by transverse aortic constriction (TAC). In vivo activation of dystrophin transcription was achieved by tail-vein injection of adeno-associated virus 9 carrying CRISPR/dCas system for dystrophin. We found that activation of dystrophin expression in TAC mice significantly reduced the susceptibility to arrhythmia of TAC mice and the mortality rate. We further demonstrated that over-expression of dystrophin increased cardiac conduction of hearts in TAC mice by optical mapping evaluation. Activation of dystrophin expression also increased peak sodium current in isolated ventricular myocytes from hearts of TAC mice as recorded by the patch-clamp technique. Immunoblotting and immunofluorescence showed that increased dystrophin transcription restored the membrane distribution of Nav1.5 in the hearts of TAC mice. In summary, correction of dystrophin downregulation by the CRISPR-dCas9 system reduced the susceptibility to arrhythmia of conventional HF mice through restoring Nav1.5 membrane distribution. This study paved the way to develop a new therapeutic strategy for HF-related ventricular arrhythmia.

The Antidepressant Paroxetine Reduces the Cardiac Sodium Current

A considerable amount of literature has been published on antidepressants and cardiac ion channel dysfunction. The antidepressant paroxetine has been associated with Brugada syndrome and long QT syndrome, albeit on the basis of conflicting findings. The cardiac voltage-gated sodium channel (Na_V1.5) is related to both of these syndromes, suggesting that paroxetine may have an effect on this channel. In the present study, we therefore carried out patch clamp experiments to examine the effect of paroxetine on human Na_V1.5 channels stably expressed in human embryonic kidney 293 (HEK-293) cells as well as on action potentials of isolated rabbit left ventricular cardiomyocytes. Additionally, computer simulations were conducted to test the functional effects of the experimentally observed paroxetine-induced changes in the Na_V1.5 current. We found that paroxetine led to a decrease in peak Na_V1.5 current in a concentration-dependent manner with an IC₅₀ of 6.8 ± 1.1 µM. In addition, paroxetine caused a significant hyperpolarizing shift in the steady-state inactivation of the Na_V1.5 current as well as a significant increase in its rate of inactivation. Paroxetine (3 µM) affected the action potential of the left ventricular cardiomyocytes, significantly decreasing its maximum upstroke velocity and amplitude, both of which are mainly regulated by the Na_V1.5 current. Our computer simulations demonstrated that paroxetine substantially reduces the fast sodium current of human left ventricular cardiomyocytes, thereby slowing conduction and reducing excitability in strands of cells, in particular if conduction and excitability are already inhibited by a loss-of-function mutation in the Na_V1.5 encoding SCN5A gene. In conclusion, paroxetine acts as an inhibitor of Na_V1.5 channels, which may enhance the effects of loss-of-function mutations in SCN5A.

Premature atrial contractions promote local directional heterogeneities in conduction velocity vectors

AIMS

Loss of cell-to-cell communication results in local conduction disorders and directional heterogeneity (LDH) in conduction velocity (CV) vectors, which may be unmasked by premature atrial contractions (PACs). We quantified LDH and examined differences between sinus rhythm (SR) and spontaneous PACs in patients with and without atrial fibrillation (AF).

METHODS AND RESULTS

Intra-operative epicardial mapping of the right and left atrium (RA, LA), Bachmann's bundle (BB) and pulmonary vein area (PVA) was performed in 228 patients (54 with AF). Conduction velocity vectors were computed at each electrode using discrete velocity vectors. Directions and magnitudes of individual vectors were compared with surrounding vectors to identify LDH. Five hundred and three PACs [2 (1-3) per patient; prematurity index of 45 ± 12%] were included. During SR, most LDH were found at BB and LA [11.9 (8.3-14.9) % and 11.3 (8.0-15.2) %] and CV was lowest at BB [83.5 (72.4-94.3) cm/s, all P < 0.05]. Compared with SR, the largest increase in LDH during PAC was found at BB and PVA [+13.0 (7.7, 18.3) % and +12.5 (10.8, 14.2) %, P < 0.001]; CV decreased particularly at BB, PVA and LA [-10.0 (-13.2, -6.9) cm/s, -9.3 (-12.5, -6.2) cm/s and -9.1 (-11.7, -6.6) cm/s, P < 0.001]. Comparing patients with and without AF, more LDH were found during SR in AF patients at PVA and BB, although the increase in LDH during PACs was similar for all sites.

CONCLUSION

Local directional heterogeneity is a novel methodology to quantify local heterogeneity in CV as a possible indicator of electropathology. Intra-operative high-resolution mapping indeed revealed that LDH increased during PACs particularly at BB and PVA. Also, patients with AF already have more LDH during SR, which becomes more pronounced during PACs.

New insights into atrioventricular nodal anatomy, physiology, and immunochemistry: A comprehensive review and a proposed model of the slow-fast atrioventricular nodal reentrant tachycardia circuit in agreement with direct potential recordings in the Koch's triangle area

Atrioventricular nodal reentrant tachycardia (AVNRT) is the most frequent regular tachycardia in humans. In this review, we describe the most recent discoveries regarding the anatomical, physiological, and molecular biological features of the atrioventricular junction that could underlie the typical slow-fast AVNRT mechanisms, as these insights could lead to the proposal of a new theory concerning the circuit of this arrhythmia. Despite several models have been proposed over the years, the precise anatomical site of the reentrant circuit and the pathway involved in the slow-fast AVNRT have not been conclusively defined. One possible way to evaluate all the hypotheses regarding the nodal tachycardia circuit in humans is to map this circuit. Thus, we tried to identify the slow potential of nodal and inferior extension structures by using automated mapping of atrial activation during both sinus rhythm and typical slow-fast AVNRT. This constitutes a first step toward the definition of nodal area activation in sinus rhythm and during slow-fast AVNRT. Further studies and technical improvements in recording the potentials of the atrioventricular node structures are necessary to confirm our initial results.

The scar: the wind in the perfect storm-insights into the mysterious living tissue originating ventricular arrhythmias

BACKGROUND

Arrhythmic death is very common among patients with structural heart disease, and it is estimated that in European countries, 1 per 1000 inhabitants yearly dies for sudden cardiac death (SCD), mainly as a result of ventricular arrhythmias (VA). The scar is the result of cardiac remodelling process that occurs in several cardiomyopathies, both ischemic and non-ischemic, and is considered the perfect substrate for re-entrant and non-re-entrant arrhythmias.

METHODS

Our aim was to review published evidence on the histological and electrophysiological properties of myocardial scar and to review the central role of cardiac magnetic resonance (CMR) in assessing ventricular arrhythmias substrate and its potential implication in risk stratification of SCD.

RESULTS

Scarring process affects both structural and electrical myocardial properties and paves the background for enhanced arrhythmogenicity. Non-uniform anisotropic conduction, gap junctions remodelling, source to sink mismatch and refractoriness dispersion are some of the underlining mechanisms contributing to arrhythmic potential of the scar. All these mechanisms lead to the initiation and maintenance of VA. CMR has a crucial role in the evaluation of patients suffering from VA, as it is considered the gold standard imaging test for scar characterization. Mounting evidences support the use of CMR not only for the definition of gross scar features, as size, localization and transmurality, but also for the identification of possible conducting channels suitable of discrete ablation. Moreover, several studies call out the CMR-based scar characterization as a stratification tool useful in selecting patients at risk of SCD and amenable to implantable cardioverter-defibrillator (ICD) implantation.

CONCLUSIONS

Scar represents the substrate of ventricular arrhythmias. CMR, defining scar presence and its features, may be a useful tool for guiding ablation procedures and for identifying patients at risk of SCD amenable to ICD therapy.

Polymer Kernels as Compact Carriers for Suspended Cardiomyocytes

Induced pluripotent stem cells (iPSCs) constitute a potential source of patient-specific human cardiomyocytes for a cardiac cell replacement therapy via intramyocardial injections, providing a major benefit over other cell sources in terms of immune rejection. However, intramyocardial injection of the cardiomyocytes has substantial challenges related to cell survival and electrophysiological coupling with recipient tissue. Current methods of manipulating cell suspensions do not allow one to control the processes of adhesion of injected cells to the tissue and electrophysiological coupling with surrounding cells. In this article, we documented the possibility of influencing these processes using polymer kernels: biocompatible fiber fragments of subcellular size that can be adsorbed to a cell, thereby creating the minimum necessary adhesion foci to shape the cell and provide support for the organization of the cytoskeleton and the contractile apparatus prior to adhesion to the recipient tissue. Using optical excitation markers, the restoration of the excitability of cardiomyocytes in suspension upon adsorption of polymer kernels was shown. It increased the likelihood of the formation of a stable electrophysiological coupling in vitro. The obtained results may be considered as a proof of concept that the stochastic engraftment process of injected suspension cells can be controlled by smart biomaterials.

Role of cardiac mitofusins in cardiac conduction following simulated ischemia-reperfusion

Mitochondrial dysfunction induced by acute cardiac ischemia-reperfusion (IR), may increase susceptibility to arrhythmias by perturbing energetics, oxidative stress production and calcium homeostasis. Although changes in mitochondrial morphology are known to impact on mitochondrial function, their role in cardiac arrhythmogenesis is not known. To assess action potential duration (APD) in cardiomyocytes from the Mitofusins-1/2 (Mfn1/Mfn2)-double-knockout (Mfn-DKO) compared to wild-type (WT) mice, optical-electrophysiology was conducted. To measure conduction velocity (CV) in atrial and ventricular tissue from the Mfn-DKO and WT mice, at both baseline and following simulated acute IR, multi-electrode array (MEA) was employed. Intracellular localization of connexin-43 (Cx43) at baseline was evaluated by immunohistochemistry, while Cx-43 phosphorylation was assessed by Western-blotting. Mfn-DKO cardiomyocytes demonstrated an increased APD. At baseline, CV was significantly lower in the left ventricle of the Mfn-DKO mice. CV decreased with simulated-ischemia and returned to baseline levels during simulated-reperfusion in WT but not in atria of Mfn-DKO mice. Mfn-DKO hearts displayed increased Cx43 lateralization, although phosphorylation of Cx43 at Ser-368 did not differ. In summary, Mfn-DKO mice have increased APD and reduced CV at baseline and impaired alterations in CV following cardiac IR. These findings were associated with increased Cx43 lateralization, suggesting that the mitofusins may impact on post-MI cardiac-arrhythmogenesis.