Ion Channels in the Heart

Cardiac L-type calcium channel regulation by Leucine-Rich Repeat-Containing Protein 10

L-type calcium channels (LTCCs), the major portal for Ca2+ entry into cardiomyocytes, are essential for excitation-contraction coupling and thus play a central role in regulating overall cardiac function. LTCC function is finely tuned by multiple signaling pathways and accessory proteins. Leucine-rich repeat-containing protein 10 (LRRC10) is a little studied cardiomyocyte-specific protein recently identified as a modulator of LTCCs. LRRC10 exerts a remarkable effect on LTCC function, more than doubling L-type Ca2+ current (ICa,L) amplitude in a heterologous expression system by altering the gating of the channels without changing their surface membrane expression. Genetic ablation of LRRC10 expression in mouse and zebrafish hearts leads to a significant reduction in ICa,L density and a slowly progressive dilated cardiomyopathy in mice. Rare sequence variants of LRRC10 have been identified in dilated cardiomyopathy and sudden unexplained nocturnal cardiac death syndrome, but these variants have not been clearly linked to disease. Nevertheless, the DCM-associated variant, I195T, converted LRRC10 from a ICa,L potentiator to a ICa,L suppressor, thus illustrating the wide dynamic range of LRRC10-mediated ICa,L regulation. This review focuses on the contemporary knowledge of LTCC modulation by LRRC10 and discusses potential directions for future investigations.

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.

VOLTAGE-GATED ION CHANNEL'S GENE EXPRESSION IN THE MYOCARDIUM OF EMBRYO AND ADULT CHICKENS

The functioning of the cardiovascular system is critical for embryo survival. Cardiac contractions depend on the sequential activation of different classes of voltage-gated ion channels. Understanding the fundamental features of these interactions is important for identifying the mechanisms of pathologies development in the myocardium. However, at present there is no consensus on which ion channels are involved in the formation of automaticity in the early embryonic stages. The aim of this study was to elucidate the expression of genes encoding various types of ion channels that are involved in the generation of electrical activity chicken heart at different stages of ontogenesis. We analyzed the expression of 14 genes from different families of ion channels. It was revealed that the expression profiles of ion channel genes change depending on the stages of ontogenesis. The HCN4, CACNA1D, SCN1A, SCN5A, KCNA1 genes have maximum expression at the tubular heart stage. In adult, a switch occurs to the higher expression of CACNA1C, KCNH6, RYR and SLC8A1 genes. This data correlated with the results obtained by the microelectrode method. It can be assumed that the automaticity of the tubular heart is mainly due to the mechanism of the «membrane-clock» (hyperpolarization-activated current (If), Ca2+-current L-type (ICaL), Na+-current (INa) and the slow component of the delayed rectifier K+-current (IKs)). Whereas in adult birds, the mechanism for generating electrical impulses is determined by both «membrane- clock» and «Ca2+-clock».

Bionic Potassium Ion Channel in Live Cells Repairs Cardiomyocyte Function

The disturbance of potassium current in cardiac myocytes caused by potassium channel dysfunction can lead to cardiac electrophysiological disorders, resulting in associated cardiovascular diseases. The emergence of artificial potassium ion channels opens up a way to replace dysfunctional natural ion channels and cure related diseases. However, bionic potassium ion channels have not been introduced into living cells to regulate cell function. One of the biggest challenges is that when the bionic channel fuses with the cell, it is difficult to control the inserting angle of the bionic potassium channel to ensure its penetration of the entire cell membrane. In nature, the extracellular vesicles can fuse with living cells with a completely preserved structure of vesicle protein. Inspired by this, we developed a vesicle fusion-based bionic porin (VFBP), which integrates bionic potassium ion channels into cardiomyocytes to replace damaged potassium ion channels. Theoretical and experimental results show that the inserted bionic ion channels have a potassium ion transport rate comparable to that of natural ion channels, which can restore the potassium ion outflow in cardiomyocytes and repair the abnormal action potential and excitation-contraction coupling of cardiomyocytes. Therefore, the bionic potassium ion channel system based on membrane fusion is expected to become the research object in many fields such as ultrafast ion transport, transmembrane delivery, and channelopathies treatment.

Transcriptomic analysis reveals PC4's participation in thermotolerance of scallop Argopecten irradians irradians by regulating myocardial bioelectric activity

Rising ocean temperatures due to global warming pose a significant threat to the bay scallop aquaculture industry. Understanding the mechanisms of thermotolerance in bay scallops is crucial for developing thermotolerant breeds. Our prior research identified Arg0230340.1, part of the positive cofactor 4 (PC4) family, as a key gene associated with the thermotolerance index Arrhenius break temperature (ABT) in bay scallops. Further validation through RNA interference (RNAi) reinforced PC4's role in thermotolerance, offering a solid basis for investigating thermal response mechanisms in these scallops. In this study, we performed a comparative transcriptomic analysis on the temperature-sensitive hearts of bay scallops after siRNA-mediated RNAi targeting Arg0230340.1, to delve into the detailed molecular mechanism of PC4's participation in thermotolerance regulation. The analysis revealed that silencing Arg0230340.1 significantly reduced the expression of mitochondrial tRNA and rRNA, potentially affecting mitochondrial function and the heart's blood supply capacity. Conversely, the up-regulation of genes involved in energy metabolism, RNA polymerase II (RNAPII)-mediated basal transcription, and aminoacyl-tRNA synthesis pathways points to an intrinsic protective response, providing energy and substrates for damage repair and maintenance of essential functions under stress. GO and KEGG enrichment analyses indicated that the up-regulated genes were primarily associated with energy metabolism and spliceosome pathways, likely contributing to myocardial remodeling post-Arg0230340.1 knockdown. Down-regulated genes were enriched in ion channel pathways, particularly those for Na+, K+, and Ca2+ channels, whose dysfunction could disrupt normal myocardial bioelectric activity. The impaired cardiac performance resulting from RNAi targeting Arg0230340.1 reduced the cardiac workload in scallop hearts, thus affecting myocardial oxygen consumption and thermotolerance. We propose a hypothetical mechanism where PC4 down-regulation impairs cardiac bioelectric activity, leading to decreased thermotolerance in bay scallops, providing theoretical guidance for breeding thermotolerant scallop varieties and developing strategies for sustainable aquaculture in the face of long-term environmental changes.

Chronic angiotensin-converting enzyme inhibition attenuates frailty and protects against atrial fibrillation in aging mice

BACKGROUND

Aging is a major risk factor for atrial fibrillation (AF); however, not all individuals age at the same rate. Frailty, which is a measure of susceptibility to adverse health outcomes, can be quantified with a frailty index (FI).

OBJECTIVE

This study aimed to determine the effects of angiotensin-converting enzyme (ACE) inhibition on AF and atrial remodeling in aging and frail mice.

METHODS

Aging mice were treated with the ACE inhibitor enalapril for 6 months beginning at 16.5 months of age and frailty was quantified. AF susceptibility and atrial structure and function were assessed by intracardiac electrophysiology in anesthetized mice, high-resolution optical mapping in intact atrial preparations, patch clamping in isolated atrial myocytes, and histology and molecular biology in atrial tissues.

RESULTS

Enalapril attenuated frailty in aging mice with larger effects in females. AF susceptibility was increased in aging mice but attenuated by enalapril. AF susceptibility and duration also increased as a function of FI score. P-wave duration was increased and atrial conduction velocity was reduced in aging mice and improved after enalapril treatment. Furthermore, P-wave duration and atrial conduction velocity were strongly correlated with FI score. Atrial action potential upstroke velocity (Vmax) and Na+ current (INa) were reduced whereas atrial fibrosis was increased in aging mice. Action potential Vmax, INa, and fibrosis were improved by enalapril and also correlated with FI scores.

CONCLUSION

ACE inhibition with enalapril attenuates frailty and reduces AF susceptibility in aging mice by preventing atrial electrical and structural remodeling.

Computational modeling of cardiac electrophysiology and arrhythmogenesis: toward clinical translation

The complexity of cardiac electrophysiology, involving dynamic changes in numerous components across multiple spatial (from ion channel to organ) and temporal (from milliseconds to days) scales, makes an intuitive or empirical analysis of cardiac arrhythmogenesis challenging. Multiscale mechanistic computational models of cardiac electrophysiology provide precise control over individual parameters, and their reproducibility enables a thorough assessment of arrhythmia mechanisms. This review provides a comprehensive analysis of models of cardiac electrophysiology and arrhythmias, from the single cell to the organ level, and how they can be leveraged to better understand rhythm disorders in cardiac disease and to improve heart patient care. Key issues related to model development based on experimental data are discussed, and major families of human cardiomyocyte models and their applications are highlighted. An overview of organ-level computational modeling of cardiac electrophysiology and its clinical applications in personalized arrhythmia risk assessment and patient-specific therapy of atrial and ventricular arrhythmias is provided. The advancements presented here highlight how patient-specific computational models of the heart reconstructed from patient data have achieved success in predicting risk of sudden cardiac death and guiding optimal treatments of heart rhythm disorders. Finally, an outlook toward potential future advances, including the combination of mechanistic modeling and machine learning/artificial intelligence, is provided. As the field of cardiology is embarking on a journey toward precision medicine, personalized modeling of the heart is expected to become a key technology to guide pharmaceutical therapy, deployment of devices, and surgical interventions.

Digital twinning of cardiac electrophysiology for congenital heart disease

In recent years, blending mechanistic knowledge with machine learning has had a major impact in digital healthcare. In this work, we introduce a computational pipeline to build certified digital replicas of cardiac electrophysiology in paediatric patients with congenital heart disease. We construct the patient-specific geometry by means of semi-automatic segmentation and meshing tools. We generate a dataset of electrophysiology simulations covering cell-to-organ level model parameters and using rigorous mathematical models based on differential equations. We previously proposed Branched Latent Neural Maps (BLNMs) as an accurate and efficient means to recapitulate complex physical processes in a neural network. Here, we employ BLNMs to encode the parametrized temporal dynamics of in silico 12-lead electrocardiograms (ECGs). BLNMs act as a geometry-specific surrogate model of cardiac function for fast and robust parameter estimation to match clinical ECGs in paediatric patients. Identifiability and trustworthiness of calibrated model parameters are assessed by sensitivity analysis and uncertainty quantification.

Inhibition of the P2X7 receptor prevents atrial proarrhythmic remodeling in experimental post-operative atrial fibrillation

BACKGROUND

Post-operative atrial fibrillation (POAF) is a common complication in patients undergoing cardiac surgery. The purinergic receptor P2X7 (P2X7R) is involved in some cardiovascular diseases, whereas its effects on atrial fibrillation (AF) are unclear.

OBJECTIVE

This study was to assess the effect of P2X7R on atrial arrhythmogenic remodeling in the rat model of sterile pericarditis (SP).

METHODS

Male Sprague-Dawley (SD) rats were used to induce the SP model. Electrocardiogram, atrial electrophysiological protocol, histology, mRNA sequencing, real-time quantitative PCR, western blot, and Elisa assay were performed.

RESULTS

SP significantly up-regulated P2X7R expression; increased AF susceptibility; reduced the protein expression of ion channels including Nav1.5, Cav1.2, Kv4.2, Kv4.3, and Kv1.5; caused atrial fibrosis; increased norepinephrine (NE) level in plasma; promoted the production of inflammatory cytokines such as TNF-α, IL-1β, and IL-6; increased the accumulation of immune cells (CD68- and MPO- positive cells); and activated NLRP3 inflammasome signaling pathway. P2X7R antagonist Brilliant Blue G (BBG) mitigated SP-induced alterations. The mRNA sequencing demonstrated that BBG prevented POAF mainly by regulating the immune system. In addition, another selective P2X7R antagonist A740003, and IL-1R antagonist anakinra also reduced AF inducibility in the SP model.

CONCLUSIONS

P2X7R inhibition prevents SP-induced atrial proarrhythmic remodeling, which is closely associated with the improvement of inflammatory changes, ion channel expression, atrial fibrosis, and sympathetic activation. The findings point to P2X7R inhibition as a promising target for AF (particularly POAF) and perhaps other conditions.

POPDC1 Variants Cause Atrioventricular Node Dysfunction and Arrhythmogenic Changes in Cardiac Electrophysiology and Intracellular Calcium Handling in Zebrafish

Popeye domain-containing (POPDC) proteins selectively bind cAMP and mediate cellular responses to sympathetic nervous system (SNS) stimulation. The first discovered human genetic variant (POPDC1S201F) is associated with atrioventricular (AV) block, which is exacerbated by increased SNS activity. Zebrafish carrying the homologous mutation (popdc1S191F) display a similar phenotype to humans. To investigate the impact of POPDC1 dysfunction on cardiac electrophysiology and intracellular calcium handling, homozygous popdc1S191F and popdc1 knock-out (popdc1KO) zebrafish larvae and adult isolated popdc1S191F hearts were studied by functional fluorescent analysis. It was found that in popdc1S191F and popdc1KO larvae, heart rate (HR), AV delay, action potential (AP) and calcium transient (CaT) upstroke speed, and AP duration were less than in wild-type larvae, whereas CaT duration was greater. SNS stress by β-adrenergic receptor stimulation with isoproterenol increased HR, lengthened AV delay, slowed AP and CaT upstroke speed, and shortened AP and CaT duration, yet did not result in arrhythmias. In adult popdc1S191F zebrafish hearts, there was a higher incidence of AV block, slower AP upstroke speed, and longer AP duration compared to wild-type hearts, with no differences in CaT. SNS stress increased AV delay and led to further AV block in popdc1S191F hearts while decreasing AP and CaT duration. Overall, we have revealed that arrhythmogenic effects of POPDC1 dysfunction on cardiac electrophysiology and intracellular calcium handling in zebrafish are varied, but already present in early development, and that AV node dysfunction may underlie SNS-induced arrhythmogenesis associated with popdc1 mutation in adults.

Digital twinning of cardiac electrophysiology for congenital heart disease

In recent years, blending mechanistic knowledge with machine learning has had a major impact in digital healthcare. In this work, we introduce a computational pipeline to build certified digital replicas of cardiac electrophysiology in pediatric patients with congenital heart disease. We construct the patient-specific geometry by means of semi-automatic segmentation and meshing tools. We generate a dataset of electrophysiology simulations covering cell-to-organ level model parameters and utilizing rigorous mathematical models based on differential equations. We previously proposed Branched Latent Neural Maps (BLNMs) as an accurate and efficient means to recapitulate complex physical processes in a neural network. Here, we employ BLNMs to encode the parametrized temporal dynamics of in silico 12-lead electrocardiograms (ECGs). BLNMs act as a geometry-specific surrogate model of cardiac function for fast and robust parameter estimation to match clinical ECGs in pediatric patients. Identifiability and trustworthiness of calibrated model parameters are assessed by sensitivity analysis and uncertainty quantification.

Natriuretic Peptide Receptor B Protects Against Atrial Fibrillation by Controlling Atrial cAMP Via Phosphodiesterase 2

BACKGROUND

β-AR (β-adrenergic receptor) stimulation regulates atrial electrophysiology and Ca2+ homeostasis via cAMP-dependent mechanisms; however, enhanced β-AR signaling can promote atrial fibrillation (AF). CNP (C-type natriuretic peptide) can also regulate atrial electrophysiology through the activation of NPR-B (natriuretic peptide receptor B) and cGMP-dependent signaling. Nevertheless, the role of NPR-B in regulating atrial electrophysiology, Ca2+ homeostasis, and atrial arrhythmogenesis is incompletely understood.

METHODS

Studies were performed using atrial samples from human patients with AF or sinus rhythm and in wild-type and NPR-B-deficient (NPR-B+/-) mice. Studies were conducted in anesthetized mice by intracardiac electrophysiology, in isolated mouse atrial preparations using high-resolution optical mapping, in isolated mouse and human atrial myocytes using patch-clamping and Ca2+ imaging, and in mouse and human atrial tissues using molecular biology.

RESULTS

Atrial NPR-B protein levels were reduced in patients with AF, and NPR-B+/- mice were more susceptible to AF. Atrial cGMP levels and PDE2 (phosphodiesterase 2) activity were reduced in NPR-B+/- mice leading to larger increases in atrial cAMP in the presence of the β-AR agonist isoproterenol. NPR-B+/- mice displayed larger increases in action potential duration and L-type Ca2+ current in the presence of isoproterenol. This resulted in the occurrence of spontaneous sarcoplasmic reticulum Ca2+ release events and delayed afterdepolarizations in NPR-B+/- atrial myocytes. Phosphorylation of the RyR2 (ryanodine receptor) and phospholamban was increased in NPR-B+/- atria in the presence of isoproterenol compared with the wildtypes. C-type natriuretic peptide inhibited isoproterenol-stimulated L-type Ca2+ current through PDE2 in mouse and human atrial myocytes.

CONCLUSIONS

NPR-B protects against AF by preventing enhanced atrial responses to β-adrenergic receptor agonists.

KairoSight-3.0: A validated optical mapping software to characterize cardiac electrophysiology, excitation-contraction coupling, and alternans

Background

Cardiac optical mapping is an imaging technique that measures fluorescent signals across a cardiac preparation. Dual optical imaging of voltage-sensitive and calcium-sensitive probes allows for simultaneous recordings of cardiac action potentials and intracellular calcium transients with high spatiotemporal resolution. The analysis of these complex optical datasets is both time intensive and technically challenging; as such, we have developed a software package for semi-automated image processing and analysis. Herein, we report an updated version of our software package (KairoSight-3.0) with features to enhance the characterization of cardiac parameters using optical signals.

Methods

To test software validity and applicability, we used Langendorff-perfused heart preparations to record transmembrane voltage and intracellular calcium signals from the epicardial surface. Isolated hearts from guinea pigs and rats were loaded with a potentiometric dye (RH237) and/or calcium indicator dye (Rhod-2AM) and fluorescent signals were acquired. We used Python 3.8.5 programming language to develop the KairoSight-3.0 software. Cardiac maps were validated with a user-specified manual mapping approach.

Results

Manual maps of action potential duration (30 or 80 % repolarization), calcium transient duration (30 or 80 % reuptake), action potential and calcium transient alternans were constituted to validate the accuracy of software-generated maps. Manual and software maps had high accuracy, with >97 % of manual and software values falling within 10 ms of each other and >75 % within 5 ms for action potential duration and calcium transient duration measurements (n = 1000-2000 pixels). Further, our software package includes additional measurement tools to analyze signal-to-noise ratio, conduction velocity, action potential and calcium transient alternans, and action potential-calcium transient coupling time to produce physiologically meaningful optical maps.

Conclusions

KairoSight-3.0 has enhanced capabilities to perform measurements of cardiac electrophysiology, calcium handling, alternans, and the excitation-contraction coupling with satisfactory accuracy.

Dual-modality imaging system for monitoring human heart organoids beating in vitro

To reveal the three-dimensional microstructure and calcium dynamics of human heart organoids (hHOs), we developed a dual-modality imaging system combining the advantages of optical coherence tomography (OCT) and fluorescence microscopy. OCT provides high-resolution volumetric structural information, while fluorescence imaging indicates the electrophysiology of the hHOs' beating behavior. We verified that concurrent OCT motion mode (M-mode) and calcium imaging retrieved the same beating pattern from the heart organoids. We further applied dynamic contrast OCT (DyC-OCT) analysis to strengthen the verification and localize the beating clusters inside the hHOs. This imaging platform provides a powerful tool for studying and assessing hHOs in vitro, with potential applications in disease modeling and drug screening.

Glucagon-Like Peptide-1 Protects Against Atrial Fibrillation and Atrial Remodeling in Type 2 Diabetic Mice

Atrial fibrillation (AF) is highly prevalent in type 2 diabetes where it increases morbidity and mortality. Glucagon-like peptide (GLP)-1 receptor agonists are used in the treatment of type 2 diabetes (T2DM), but their effects on AF in T2DM are poorly understood. The present study demonstrates type 2 diabetic db/db mice are highly susceptible to AF in association with atrial electrical and structural remodeling. GLP-1, as well as the long-acting GLP-1 analogue liraglutide, reduced AF and prevented atrial remodeling in db/db mice. These data suggest that GLP-1 and related analogues could protect against AF in patients with T2DM.

Building blocks of microphysiological system to model physiology and pathophysiology of human heart

Microphysiological systems (MPS) are drawing increasing interest from academia and from biomedical industry due to their improved capability to capture human physiology. MPS offer an advanced in vitro platform that can be used to study human organ and tissue level functions in health and in diseased states more accurately than traditional single cell cultures or even animal models. Key features in MPS include microenvironmental control and monitoring as well as high biological complexity of the target tissue. To reach these qualities, cross-disciplinary collaboration from multiple fields of science is required to build MPS. Here, we review different areas of expertise and describe essential building blocks of heart MPS including relevant cardiac cell types, supporting matrix, mechanical stimulation, functional measurements, and computational modelling. The review presents current methods in cardiac MPS and provides insights for future MPS development with improved recapitulation of human physiology.

Adapting to a new environment: postnatal maturation of the human cardiomyocyte

The postnatal mammalian heart undergoes remarkable developmental changes, which are stimulated by the transition from the intrauterine to extrauterine environment. With birth, increased oxygen levels promote metabolic, structural and biophysical maturation of cardiomyocytes, resulting in mature muscle with increased efficiency, contractility and electrical conduction. In this Topical Review article, we highlight key studies that inform our current understanding of human cardiomyocyte maturation. Collectively, these studies suggest that human atrial and ventricular myocytes evolve quickly within the first year but might not reach a fully mature adult phenotype until nearly the first decade of life. However, it is important to note that fetal, neonatal and paediatric cardiac physiology studies are hindered by a number of limitations, including the scarcity of human tissue, small sample size and a heavy reliance on diseased tissue samples, often without age-matched healthy controls. Future developmental studies are warranted to expand our understanding of normal cardiac physiology/pathophysiology and inform age-appropriate treatment strategies for cardiac disease.

Human atrial fibrillation and genetic defects in transient outward currents: mechanistic insights from multi-scale computational models

Previous studies have linked dysfunctional I_to arising from mutations to KCND3-encoded Kv4.3 and KCND2-encoded Kv4.2 to atrial fibrillation. Using computational models, this study aimed to investigate the mechanisms underlying pro-arrhythmic effects of the gain-of-function Kv4.3 (T361S, A545P) and Kv4.2 (S447R) mutations. Wild-type and mutant I_to formulations were developed from and validated against experimental data and incorporated into the Colman et al. model of human atrial cells. Single-cell models were incorporated into one- (1D) and two-dimensional (2D) models of atrial tissue, and a three-dimensional (3D) realistic model of the human atria. The three gain-of-function mutations had similar, albeit quantitatively different, effects: shortening of the action potential duration; lowering the plateau membrane potential, abbreviating the effective refractory period (ERP) and the wavelength (WL) of atrial excitation at the tissue level. Restitution curves for the WL, the ERP and the conduction velocity were leftward shifted, facilitating the conduction of atrial excitation waves at high excitation rates. The mutations also increased lifespan and stationarity of re-entry in both 2D and 3D simulations, which further highlighted a mutation-induced increase in spatial dispersion of repolarization. Collectively, these changes account for pro-arrhythmic effects of these Kv4.3 and Kv4.2 mutations in facilitating AF. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Empagliflozin reduces arrhythmogenic effects in rat neonatal and human iPSC-derived cardiomyocytes and improves cytosolic calcium handling at least partially independent of NHE1

The antidiabetic agent class of sodium-glucose cotransporter 2 (SGLT2) inhibitors confer unprecedented cardiovascular benefits beyond glycemic control, including reducing the risk of fatal ventricular arrhythmias. However, the impact of SGLT2 inhibitors on the electrophysiological properties of cardiomyocytes exposed to stimuli other than hyperglycemia remains elusive. This investigation tested the hypothesis that the SGLT2 inhibitor empagliflozin (EMPA) affects cardiomyocyte electrical activity under hypoxic conditions. Rat neonatal and human induced pluripotent stem cell (iPSC)-derived cardiomyocytes incubated or not with the hypoxia-mimetic agent CoCl₂ were treated with EMPA (1 μM) or vehicle for 24 h. Action potential records obtained using intracellular microelectrodes demonstrated that EMPA reduced the action potential duration at 30%, 50%, and 90% repolarization and arrhythmogenic events in rat and human cardiomyocytes under normoxia and hypoxia. Analysis of Ca²⁺ transients using Fura-2-AM and contractility kinetics showed that EMPA increased Ca²⁺ transient amplitude and decreased the half-time to recover Ca²⁺ transients and relaxation time in rat neonatal cardiomyocytes. We also observed that the combination of EMPA with the Na⁺/H⁺ exchanger isoform 1 (NHE1) inhibitor cariporide (10 µM) exerted a more pronounced effect on Ca²⁺ transients and contractility than either EMPA or cariporide alone. Besides, EMPA, but not cariporide, increased phospholamban phosphorylation at serine 16. Collectively, our data reveal that EMPA reduces arrhythmogenic events, decreases the action potential duration in rat neonatal and human cardiomyocytes under normoxic or hypoxic conditions, and improves cytosolic calcium handling at least partially independent of NHE1. Moreover, we provided further evidence that SGLT2 inhibitor-mediated cardioprotection may be partly attributed to its cardiomyocyte electrophysiological effects.

KairoSight-3.0 : A Validated Optical Mapping Software to Characterize Cardiac Electrophysiology, Excitation-Contraction Coupling, and Alternans

Background

Cardiac optical mapping is an imaging technique that measures fluorescent signals across a cardiac preparation. Dual optical mapping of voltage-sensitive and calcium-sensitive probes allow for simultaneous recordings of cardiac action potentials and intracellular calcium transients with high spatiotemporal resolution. The analysis of these complex optical datasets is both time intensive and technically challenging; as such, we have developed a software package for semi-automated image processing and analysis. Herein, we report an updated version of our software package ( KairoSight-3 . 0 ) with features to enhance characterization of cardiac parameters using optical signals.

Methods

To test software validity and applicability, we used Langendorff-perfused heart preparations to record transmembrane voltage and intracellular calcium signals from the epicardial surface. Isolated hearts from guinea pigs and rats were loaded with a potentiometric dye (RH237) and/or calcium indicator dye (Rhod-2AM) and fluorescent signals were acquired. We used Python 3.8.5 programming language to develop the KairoSight-3 . 0 software. Cardiac maps were validated with a user-specified manual mapping approach.

Results

Manual maps of action potential duration (30 or 80% repolarization), calcium transient duration (30 or 80% reuptake), action potential and calcium transient alternans were constituted to validate the accuracy of software-generated maps. Manual and software maps had high accuracy, with >97% of manual and software values falling within 10 ms of each other and >75% within 5 ms for action potential duration and calcium transient duration measurements (n=1000-2000 pixels). Further, our software package includes additional cardiac metric measurement tools to analyze signal-to-noise ratio, conduction velocity, action potential and calcium transient alternans, and action potential-calcium transient coupling time to produce physiologically meaningful optical maps.

Conclusions

KairoSight-3 . 0 has enhanced capabilities to perform measurements of cardiac electrophysiology, calcium handling, and the excitation-contraction coupling with satisfactory accuracy.

Graphical Abstract Demonstrating Experimental and Data Analysis Workflow

Created with Biorender.com.

HiPSC-derived cardiomyocyte to model Brugada syndrome: both asymptomatic and symptomatic mutation carriers reveal increased arrhythmogenicity

Brugada syndrome is an inherited cardiac arrhythmia disorder that is mainly associated with mutations of the cardiac voltage-gated sodium channel alpha subunit 5 (SCN5A) gene. The clinical symptoms include ventricular fibrillation and an increased risk of sudden cardiac death. Human-induced pluripotent stem cell (hiPSC) lines were derived from symptomatic and asymptomatic individuals carrying the R1913C mutation in the SCN5A gene. The present work aimed to observe the phenotype-specific differences in hiPSC-derived cardiomyocytes (CMs) obtained from symptomatic and asymptomatic mutation carriers. In this study, CM electrophysiological properties, beating abilities and calcium parameters were measured. Mutant CMs exhibited higher average sodium current densities than healthy CMs, but the differences were not statistically significant. Action potential durations were significantly shorter in CMs from the symptomatic individual, and a spike-and-dome morphology of action potential was exclusively observed in CMs from the symptomatic individual. More arrhythmias occurred in mutant CMs at single cell and cell aggregate levels compared with those observed in wild-type CMs. Moreover, there were no major differences in ionic currents or intracellular calcium dynamics between the CMs of asymptomatic and symptomatic individuals after the administration of adrenaline and flecainide.In conclusion, mutant CMs were more prone to arrhythmia than healthy CMs but did not explain why only one of the mutation carriers was symptomatic.

Application of machine learning to improve the efficiency of electrophysiological simulations used for the prediction of drug-induced ventricular arrhythmia

BACKGROUND AND OBJECTIVE

In silico prediction of drug-induced ventricular arrhythmia often requires computationally intensive simulations, making its application tedious and non-interactive. This inconvenience can be mitigated using matrices of precomputed simulation results, allowing instantaneous computation of biomarkers such as action potential duration at 90% of the repolarisation (APD₉₀). However, preparing such matrices can be computationally intensive for the method developers, limiting the range of simulated conditions. In this work, we aim to optimise the generation of these matrices so that they can be obtained with less effort and for a broader range of input values.

METHODS

Machine learning methods were applied, building models trained with only a small fraction of the originally simulated results. The predictive performances of the models were assessed by comparing their predicted values with the actual simulation results, using percentual mean absolute error and mean relative error, as well as the percentage of data with a relative error below 5%.

RESULTS

Our method obtained highly accurate estimations of the original values, leading to a nearly one hundred-fold decrease in computation time. This method also allows precomputing more complex matrices, describing the effect of more ion channels on the APD_90. The best results were obtained by applying Support Vector Machine models, which yielded errors below 1% in most cases. This approach was further validated by predicting the APD₉₀ of a set of 12 CiPA compounds and exporting the optimal settings for predicting APD₉₀ using a different set of ion channels, always with satisfactory results.

CONCLUSIONS

The proposed method effectively reduces the computational effort required to generate matrices of precomputed electrophysiological simulation values. The same approach can be applied in other fields where computationally costly simulations are applied repeatedly using slightly different input values.

The Application of High-Throughput Approaches in Identifying Novel Therapeutic Targets and Agents to Treat Diabetes

During the past decades, unprecedented progress in technologies has revolutionized traditional research methodologies. Among these, advances in high-throughput drug screening approaches have permitted the rapid identification of potential therapeutic agents from drug libraries that contain thousands or millions of molecules. Moreover, high-throughput-based therapeutic target discovery strategies can comprehensively interrogate relationships between biomolecules (e.g., gene, RNA, and protein) and diseases and significantly increase the authors' knowledge of disease mechanisms. Diabetes is a chronic disease primarily characterized by the incapacity of the body to maintain normoglycemia. The prevalence of diabetes in modern society has become a severe public health issue that threatens the well-being of millions of patients. Although a number of pharmacological treatments are available, there is no permanent cure for diabetes, and discovering novel therapeutic targets and agents continues to be an urgent need. The present review discusses the technical details of high-throughput screening approaches in drug discovery, followed by introducing the applications of such approaches to diabetes research. This review aims to provide an example of the applicability of high-throughput technologies in facilitating different aspects of disease research.

Whole-heart multiparametric optical imaging reveals sex-dependent heterogeneity in cAMP signaling and repolarization kinetics

Cyclic adenosine 3',5'-monophosphate (cAMP) is a key second messenger in cardiomyocytes responsible for transducing autonomic signals into downstream electrophysiological responses. Previous studies have shown intracellular heterogeneity and compartmentalization of cAMP signaling. However, whether cAMP signaling occurs heterogeneously throughout the intact heart and how this drives sex-dependent functional responses are unknown. Here, we developed and validated a novel cardiac-specific fluorescence resonance energy transfer-based cAMP reporter mouse and a combined voltage-cAMP whole-heart imaging system. We showed that in male hearts, cAMP was uniformly activated in response to pharmacological β-adrenergic stimulation. In contrast, female hearts showed that cAMP levels decayed faster in apical versus basal regions, which was associated with nonuniform action potential changes and notable changes in the direction of repolarization. Apical phosphodiesterase (PDE) activity was higher in female versus male hearts, and PDE inhibition prevented repolarization changes in female hearts. Thus, our imaging approach revealed sex-dependent regional breakdown of cAMP and associated electrophysiological differences.

Sex differences in heart rate and heart rate variability in rats: Implications for translational research

The present study aimed to investigate sex differences in measures of cardiac chronotropy and heart rate variability (HRV) in 132 young adult wild-type Groningen rats (n = 45 females). Electrocardiographic signals were recorded for 48 h in freely moving rats to quantify heart rate (HR) and inter-beat interval (IBI) as measures of cardiac chronotropy, and time- and frequency-domain HRV parameters as physiological readouts of cardiac vagal modulation. Females showed greater vagally-mediated HRV despite having higher HR and shorter IBI than males during undisturbed conditions. Such differences were evident i) at any given level of HRV, and ii) both during the 12-h light/inactive and 12-h dark/active phase of the daily cycle. These findings replicate the paradoxical cardiac chronotropic control reported by human meta-analytic findings, since one would expect greater vagally-mediated HRV to be associated with lower HR and longer IBI. Lastly, the association between some HRV measures and HR was stronger in female than male rats. Overall, the current study in young adult rats provides data illustrating a sex-dependent association between vagally-mediated HRV and indexes of cardiac chronotropy. The current results i) are in line with human findings, ii) suggest to always consider biological sex in the analysis and interpretation of HRV data in rats, and iii) warrant the use of rats for investigating the neuro-hormonal basis and temporal evolution of the impact of sex on the association between vagally-mediated HRV and cardiac chronotropy, which could inform the human condition.

The 'Reverse FDUF' Mechanism of Atrial Excitation-Contraction Coupling Sustains Calcium Alternans-A Hypothesis

Cardiac calcium alternans is defined as beat-to-beat alternations of Ca transient (CaT) amplitude and has been linked to cardiac arrhythmia, including atrial fibrillation. We investigated the mechanism of atrial alternans in isolated rabbit atrial myocytes using high-resolution line scan confocal Ca imaging. Alternans was induced by increasing the pacing frequency until stable alternans was observed (1.6-2.5 Hz at room temperature). In atrial myocytes, action potential-induced Ca release is initiated in the cell periphery and subsequently propagates towards the cell center by Ca-induced Ca release (CICR) in a Ca wave-like fashion, driven by the newly identified 'fire-diffuse-uptake-fire' (FDUF) mechanism. The development of CaT alternans was accompanied by characteristic changes of the spatio-temporal organization of the CaT. During the later phase of the CaT, central [Ca]_i exceeded peripheral [Ca]_i that was indicative of a reversal of the subcellular [Ca]_i gradient from centripetal to centrifugal. This gradient reversal resulted in a reversal of CICR propagation, causing a secondary Ca release during the large-amplitude alternans CaT, thereby prolonging the CaT, enhancing Ca-release refractoriness and reducing Ca release on the subsequent beat, thus enhancing the degree of CaT alternans. Here, we propose the 'reverse FDUF' mechanism as a novel cellular mechanism of atrial CaT alternans, which explains how the uncoupling of central from peripheral Ca release leads to the reversal of propagating CICR and to alternans.

Guidelines for assessment of cardiac electrophysiology and arrhythmias in small animals

Cardiac arrhythmias are a major cause of morbidity and mortality worldwide. Although recent advances in cell-based models, including human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM), are contributing to our understanding of electrophysiology and arrhythmia mechanisms, preclinical animal studies of cardiovascular disease remain a mainstay. Over the past several decades, animal models of cardiovascular disease have advanced our understanding of pathological remodeling, arrhythmia mechanisms, and drug effects and have led to major improvements in pacing and defibrillation therapies. There exist a variety of methodological approaches for the assessment of cardiac electrophysiology and a plethora of parameters may be assessed with each approach. This guidelines article will provide an overview of the strengths and limitations of several common techniques used to assess electrophysiology and arrhythmia mechanisms at the whole animal, whole heart, and tissue level with a focus on small animal models. We also define key electrophysiological parameters that should be assessed, along with their physiological underpinnings, and the best methods with which to assess these parameters.

Relationship among surface electric double layer of cardiomyocyte membrane and toxicology of digoxin and opening of ion channels

We applied a new idea that the potential effect can change the ion adsorption structure on the cell surface to explore the mechanism of digoxin poisoning and the regulation of ion channels. The effects of digoxin on the electrophoretic mobility and behaviors (non-contraction or contraction or autorhythmicity) of cardiomyocytes were observed by single-cell electrophoresis technique (imitate the opening method of in vivo channel) and the method of decomposing surface potential components on the cells. As well as affect the association with electrical activity. The results suggested that the increase of cardiomyocytes transmembrane potential and the Na⁺-K⁺ exchange on the cell surface of the action potential phase 4 caused by the poisoning dose of digoxin, leading to the oscillation of adsorbed ions on the cell surface and the incomplete channel structure, which were the mechanism of cardiac ectopic beats. The results revealed that the opening of ion channels is regulated by the surface electric double layer of the cell membrane.

Regional and temporal progression of atrial remodeling in angiotensin II mediated atrial fibrillation

Atrial fibrillation (AF) is associated with electrical and structural remodeling in the atria; however, the regional and temporal progression of atrial remodeling is incompletely understood. The objective of this study was to investigate the regional and temporal progression of atrial remodeling leading to changes in AF susceptibility in angiotensin II (Ang II) mediated hypertension. Mice were infused with Ang II for 3, 10 or 21 days. AF susceptibility and atrial electrophysiology were studied in vivo using intracardiac electrophysiology. Right and left atrial myocyte electrophysiology was studied using patch-clamping. Atrial fibrosis was assessed histologically. P wave duration and atrial effective refractory period increased progressively from 3 to 21 days of Ang II. AF susceptibility tended to be increased at 10 days of Ang II and was elevated at 21 days of Ang II. Left, but not right, atrial AP upstroke velocity and Na+ current were reduced at 10 and 21 days of Ang II. Left atrial action potential (AP) duration increased progressively from 3 to 21 days of Ang II due to reductions in repolarizing K+ current. Right atrial AP prolongation was increased only after 21 days of Ang II. Left and right atrial fibrosis developed progressively from 3 to 21 days, but increases were larger in the left atrium. In conclusion, Ang II mediated atrial electrical and structural remodeling develop earlier and more extensively in the left atrium compared to the right atrium, providing insight into how atrial remodeling leads to enhanced AF susceptibility in Ang II mediated hypertension.

Cardiac ion channel expression in the equine model - In-silico prediction utilising RNA sequencing data from mixed tissue samples

Understanding cardiomyocyte ion channel expression is crucial to understanding normal cardiac electrophysiology and underlying mechanisms of cardiac pathologies particularly arrhythmias. Hitherto, equine cardiac ion channel expression has rarely been investigated. Therefore, we aim to predict equine cardiac ion channel gene expression. Raw RNAseq data from normal horses from 9 datasets was retrieved from ArrayExpress and European Nucleotide Archive and reanalysed. The normalised (FPKM) read counts for a gene in a mix of tissue were hypothesised to be the average of the expected expression in each tissue weighted by the proportion of the tissue in the mix. The cardiac-specific expression was predicted by estimating the mean expression in each other tissues. To evaluate the performance of the model, predicted gene expression values were compared to the human cardiac gene expression. Cardiac-specific expression could be predicted for 91 ion channels including most expressed Na+ channels, K+ channels and Ca2+ -handling proteins. These revealed interesting differences from what would be expected based on human studies. These differences included predominance of NaV 1.4 rather than NaV 1.5 channel, and RYR1, SERCA1 and CASQ1 rather than RYR2, SERCA2, CASQ2 Ca2+ -handling proteins. Differences in channel expression not only implicate potentially different regulatory mechanisms but also pathological mechanisms of arrhythmogenesis.

Re-evaluating methods reporting practices to improve reproducibility: an analysis of methodological rigor for the Langendorff whole-heart technique

In recent decades, the scientific community has seen an increased interest in rigor and reproducibility. In 2017, concerns of methodological thoroughness and reporting practices were implicated as significant barriers to reproducibility within the preclinical cardiovascular literature, particularly in studies employing animal research. The Langendorff, whole-heart technique has proven to be an invaluable research tool, being modified in a myriad of ways to probe questions across the spectrum of physio- and pathophysiologic function of the heart. As a result, significant variability in the application of the Langendorff technique exists. This literature review quantifies the different methods employed in the implementation of the Langendorff technique and provides brief examples of how individual parametric differences can impact the outcomes and interpretation of studies. From 2017-2020, significant variability of animal models, anesthesia, cannulation time, and perfusate composition, pH, and temperature demonstrate that the technique has diversified to meet new challenges and answer different scientific questions. The review also reveals which individual methods are most frequently reported, even if there is no explicit agreement upon which parameters should be reported. The analysis of methods related to the Langendorff technique suggests a framework for considering methodological approach when interpreting seemingly contradictory results, rather than concluding that results are irreproducible.

Cardiac Macrophages and Their Effects on Arrhythmogenesis

Cardiac electrophysiology is a complex system established by a plethora of inward and outward ion currents in cardiomyocytes generating and conducting electrical signals in the heart. However, not only cardiomyocytes but also other cell types can modulate the heart rhythm. Recently, cardiac macrophages were demonstrated as important players in both electrophysiology and arrhythmogenesis. Cardiac macrophages are a heterogeneous group of immune cells including resident macrophages derived from embryonic and fetal precursors and recruited macrophages derived from circulating monocytes from the bone marrow. Recent studies suggest antiarrhythmic as well as proarrhythmic effects of cardiac macrophages. The proposed mechanisms of how cardiac macrophages affect electrophysiology vary and include both direct and indirect interactions with other cardiac cells. In this review, we provide an overview of the different subsets of macrophages in the heart and their possible interactions with cardiomyocytes under both physiologic conditions and heart disease. Furthermore, we elucidate similarities and differences between human, murine and porcine cardiac macrophages, thus providing detailed information for researchers investigating cardiac macrophages in important animal species for electrophysiologic research. Finally, we discuss the pros and cons of mice and pigs to investigate the role of cardiac macrophages in arrhythmogenesis from a translational perspective.

Golden Standard or Obsolete Method? Review of ECG Applications in Clinical and Experimental Context

Cardiovascular system and its functions under both physiological and pathophysiological conditions have been studied for centuries. One of the most important steps in the cardiovascular research was the possibility to record cardiac electrical activity. Since then, numerous modifications and improvements have been introduced; however, an electrocardiogram still represents a golden standard in this field. This paper overviews possibilities of ECG recordings in research and clinical practice, deals with advantages and disadvantages of various approaches, and summarizes possibilities of advanced data analysis. Special emphasis is given to state-of-the-art deep learning techniques intensely expanded in a wide range of clinical applications and offering promising prospects in experimental branches. Since, according to the World Health Organization, cardiovascular diseases are the main cause of death worldwide, studying electrical activity of the heart is still of high importance for both experimental and clinical cardiology.

Engineering of thick human functional myocardium via static stretching and electrical stimulation

Human cardiac-muscle patches (hCMPs) constructed from induced pluripotent stem cells derived cardiomyocytes (iCMs) can replicate the genetics of individual patients, and consequently be used for drug testing, disease modeling, and therapeutic applications. However, conventional hCMPs are relatively thin and contain iCMs with fetal cardiomyocyte structure and function. Here, we used our layer-by-layer (lbl) fabrication to construct thicker (>2.1 mm), triple-layered hCMPs, and then evaluated iCM maturity after ten days of standard culture (Control), static stretching (Stretched), or stretching with electrical stimulation at 15 or 22 V (Stretched+15V or Stretched+22V). Assessments of stained hCMPs suggested that expression and alignment of contractile proteins was greater in Stretched+22V, whereas quantification of mRNA abundance and protein expression indicated the Stretched+22V enhanced biomolecular maturation. Transmission electron microscope images indicated that stretching and electrical stimulation were associated with increases in development of Z-lines and gap junctions, and sarcomeres were significantly longer following any of the maturation protocols.

L-type Ca2+ channel recovery from inactivation in rabbit atrial myocytes

Adaptation of the myocardium to varying workloads critically depends on the recovery from inactivation (RFI) of L-type Ca²⁺ channels (LCCs) which provide the trigger for cardiac contraction. The goal of the present study was a comprehensive investigation of LCC RFI in atrial myocytes. The study was performed on voltage-clamped rabbit atrial myocytes using a double pulse protocol with variable diastolic intervals in cells held at physiological holding potentials, with intact intracellular Ca²⁺ release, and preserved Na⁺ current and Na⁺ /Ca²⁺ exchanger (NCX) activity. We demonstrate that the kinetics of RFI of LCCs are co-regulated by several factors including resting membrane potential, [Ca²⁺ ]_i , Na⁺ influx, and activity of CaMKII. In addition, activation of CaMKII resulted in increased I_Ca amplitude at higher pacing rates. Pharmacological inhibition of NCX failed to have any significant effect on RFI, indicating that impaired removal of Ca²⁺ by NCX has little effect on LCC recovery. Finally, RFI of intracellular Ca²⁺ release was substantially slower than LCC RFI, suggesting that inactivation kinetics of LCC do not significantly contribute to the beat-to-beat refractoriness of SR Ca²⁺ release. The study demonstrates that CaMKII and intracellular Ca²⁺ dynamics play a central role in modulation of LCC activity in atrial myocytes during increased workloads that could have important consequences under pathological conditions such as atrial fibrillations, where Ca²⁺ cycling and CaMKII activity are altered.

Multilayer control of cardiac electrophysiology by microRNAs

The electrophysiological properties of the heart include cardiac automaticity, excitation (i.e., depolarization and repolarization of action potential) of individual cardiomyocytes, and highly coordinated electrical propagation through the whole heart. An abnormality in any of these properties can cause arrhythmias. MicroRNAs (miRs) have been recognized as essential regulators of gene expression through the conventional RNA interference (RNAi) mechanism and are involved in a variety of biological events. Recent evidence has demonstrated that miRs regulate the electrophysiology of the heart through fine regulation by the conventional RNAi mechanism of the expression of ion channels, transporters, intracellular Ca²⁺-handling proteins, and other relevant factors. Recently, a direct interaction between miRs and ion channels has also been reported in the heart, revealing a biophysical modulation by miRs of cardiac electrophysiology. These advanced discoveries suggest that miR controls cardiac electrophysiology through two distinct mechanisms: immediate action through biophysical modulation and long-term conventional RNAi regulation. Here, we review the recent research progress and summarize the current understanding of how miR manipulates the function of ion channels to maintain the homeostasis of cardiac electrophysiology.

Drivers of Sinoatrial Node Automaticity in Zebrafish: Comparison With Mechanisms of Mammalian Pacemaker Function

Cardiac excitation originates in the sinoatrial node (SAN), due to the automaticity of this distinct region of the heart. SAN automaticity is the result of a gradual depolarisation of the membrane potential in diastole, driven by a coupled system of transarcolemmal ion currents and intracellular Ca²⁺ cycling. The frequency of SAN excitation determines heart rate and is under the control of extra- and intracardiac (extrinsic and intrinsic) factors, including neural inputs and responses to tissue stretch. While the structure, function, and control of the SAN have been extensively studied in mammals, and some critical aspects have been shown to be similar in zebrafish, the specific drivers of zebrafish SAN automaticity and the response of its excitation to vagal nerve stimulation and mechanical preload remain incompletely understood. As the zebrafish represents an important alternative experimental model for the study of cardiac (patho-) physiology, we sought to determine its drivers of SAN automaticity and the response to nerve stimulation and baseline stretch. Using a pharmacological approach mirroring classic mammalian experiments, along with electrical stimulation of intact cardiac vagal nerves and the application of mechanical preload to the SAN, we demonstrate that the principal components of the coupled membrane- Ca²⁺ pacemaker system that drives automaticity in mammals are also active in the zebrafish, and that the effects of extra- and intracardiac control of heart rate seen in mammals are also present. Overall, these results, combined with previously published work, support the utility of the zebrafish as a novel experimental model for studies of SAN (patho-) physiological function.

SGLT2 Inhibitors and Their Antiarrhythmic Properties

Sodium-glucose cotransporter 2 (SGLT2) inhibitors are gaining ground as standard therapy for heart failure with a class-I recommendation in the recently updated heart failure guidelines from the European Society of Cardiology. Different gliflozins have shown impressive beneficial effects in patients with and without diabetes mellitus type 2, especially in reducing the rates for hospitalization for heart failure, yet little is known on their antiarrhythmic properties. Atrial and ventricular arrhythmias were reported by clinical outcome trials with SGLT2 inhibitors as adverse events, and SGLT2 inhibitors seemed to reduce the rate of arrhythmias compared to placebo treatment in those trials. Mechanistical links are mainly unrevealed, since hardly any experiments investigated their impact on arrhythmias. Prospective trials are currently ongoing, but no results have been published so far. Arrhythmias are common in the heart failure population, therefore the understanding of possible interactions with SGLT2 inhibitors is crucial. This review summarizes evidence from clinical data as well as the sparse experimental data of SGLT2 inhibitors and their effects on arrhythmias.

Mettl3 deficiency leads to the upregulation of Cav1.2 and increases arrhythmia susceptibility in mice

Methyltransferase-like 3 (Mettl3) is a component of methyltransferase complex that mediates mA modification of RNAs, and participates in multiple biological processes. However, the role of Mettl3 in cardiac electrophysiology remains unknown. This study aims to explore the ventricular arrhythmia susceptibility of Mettl3 mice and the underlying mechanisms. Mice were anesthetized with 2% avertin (0.1 mL/ body weight) for echocardiography and programmed electrical pacing. Whole-cell patch clamp technique was used to examine the electrophysiological property of cardiomyocytes. The expression of Cav1.2 was determined by qRT-PCR and western blot analysis. The mA medication of mRNA was examined by MeRIP-Seq and MeRIP-qPCR. No differences are found in the morphology and function of the hearts between Mettl3 mice and wild-type (WT) controls. The QT and QTc intervals of Mettl3 mice are significantly longer. High-frequency electrical stimulation showed that heterozygous knockout of Mettl3 increases ventricular arrhythmia susceptibility. The whole-cell patch-clamp recordings showed that the APD is prolonged in Mettl3 ventricular myocytes and more EADs were observed. The density of is substantially increased in ventricular myocytes of Mettl3 mice. The pore-forming subunit of L-type calcium channel Cav1.2 is upregulated in Mettl3 mice, while the mRNA of its coding gene does not change. MeRIP-Seq and MeRIP-qPCR showed that the mA methylation of mRNA is decreased in cultured Mettl3-knockdown cardiomyocytes and Mettl3 hearts. Collectively, deficiency of Mettl3 increases ventricular arrhythmia susceptibility due to the upregulation of Cav1.2 by reducing mA modification onmRNA in mice. This study highlights the role of mA modification in the regulation of cardiac electrophysiology.

Individual Contributions of Cardiac Ion Channels on Atrial Repolarization and Reentrant Waves: A Multiscale In-Silico Study

The excitation, contraction, and relaxation of an atrial cardiomyocyte are maintained by the activation and inactivation of numerous cardiac ion channels. Their collaborative efforts cause time-dependent changes of membrane potential, generating an action potential (AP), which is a surrogate marker of atrial arrhythmias. Recently, computational models of atrial electrophysiology emerged as a modality to investigate arrhythmia mechanisms and to predict the outcome of antiarrhythmic therapies. However, the individual contribution of atrial ion channels on atrial action potential and reentrant arrhythmia is not yet fully understood. Thus, in this multiscale in-silico study, perturbations of individual atrial ionic currents (I_Na, I_to, I_CaL, I_Kur, I_Kr, I_Ks, I_K1, I_NCX and I_NaK) in two in-silico models of human atrial cardiomyocyte (i.e., Courtemanche-1998 and Grandi-2011) were performed at both cellular and tissue levels. The results show that the inhibition of I_CaL and I_NCX resulted in AP shortening, while the inhibition of I_Kur, I_Kr, I_Ks, I_K1 and I_NaK prolonged AP duration (APD). Particularly, in-silico perturbations (inhibition and upregulation) of I_Kr and I_Ks only minorly affected atrial repolarization in the Grandi model. In contrast, in the Courtemanche model, the inhibition of I_Kr and I_Ks significantly prolonged APD and vice versa. Additionally, a 50% reduction of I_to density abbreviated APD in the Courtemanche model, while the same perturbation prolonged APD in the Grandi model. Similarly, a strong model dependence was also observed at tissue scale, with an observable I_K1-mediated reentry stabilizing effect in the Courtemanche model but not in the Grandi atrial model. Moreover, the Grandi model was highly sensitive to a change on intracellular Ca²⁺ concentration, promoting a repolarization failure in I_CaL upregulation above 150% and facilitating reentrant spiral waves stabilization by I_CaL inhibition. Finally, by incorporating the previously published atrial fibrillation (AF)-associated ionic remodeling in the Courtemanche atrial model, in-silico modeling revealed the antiarrhythmic effect of I_Kr inhibition in both acute and chronic settings. Overall, our multiscale computational study highlights the strong model-dependent effects of ionic perturbations which could affect the model’s accuracy, interpretability, and prediction. This observation also suggests the need for a careful selection of in-silico models of atrial electrophysiology to achieve specific research aims.

Structural and Electrical Remodeling of the Sinoatrial Node in Diabetes: New Dimensions and Perspectives

The sinoatrial node (SAN) is composed of highly specialized cells that mandate the spontaneous beating of the heart through self-generation of an action potential (AP). Despite this automaticity, the SAN is under the modulation of the autonomic nervous system (ANS). In diabetes mellitus (DM), heart rate variability (HRV) manifests as a hallmark of diabetic cardiomyopathy. This is paralleled by an impaired regulation of the ANS, and by a pathological remodeling of the pacemaker structure and function. The direct effect of diabetes on the molecular signatures underscoring this pathology remains ill-defined. The recent focus on the electrical currents of the SAN in diabetes revealed a repressed firing rate of the AP and an elongation of its tracing, along with conduction abnormalities and contractile failure. These changes are blamed on the decreased expression of ion transporters and cell-cell communication ports at the SAN (i.e., HCN4, calcium and potassium channels, connexins 40, 45, and 46) which further promotes arrhythmias. Molecular analysis crystallized the RGS4 (regulator of potassium currents), mitochondrial thioredoxin-2 (reactive oxygen species; ROS scavenger), and the calcium-dependent calmodulin kinase II (CaMKII) as metabolic culprits of relaying the pathological remodeling of the SAN cells (SANCs) structure and function. A special attention is given to the oxidation of CaMKII and the generation of ROS that induce cell damage and apoptosis of diabetic SANCs. Consequently, the diabetic SAN contains a reduced number of cells with significant infiltration of fibrotic tissues that further delay the conduction of the AP between the SANCs. Failure of a genuine generation of AP and conduction of their derivative waves to the neighboring atrial myocardium may also occur as a result of the anti-diabetic regiment (both acute and/or chronic treatments). All together, these changes pose a challenge in the field of cardiology and call for further investigations to understand the etiology of the structural/functional remodeling of the SANCs in diabetes. Such an understanding may lead to more adequate therapies that can optimize glycemic control and improve health-related outcomes in patients with diabetes.

Arrhythmogenic Hearts in PKD2 Mutant Mice Are Characterized by Cardiac Fibrosis, Systolic, and Diastolic Dysfunctions

Autosomal dominant polycystic kidney disease (PKD) is a hereditary disorder affecting multiple organs, including the heart. PKD has been associated with many cardiac abnormalities including the arrhythmogenic remodeling in clinical evaluations. In our current study, we hypothesized that Pkd2 gene mutation results in structural and functional defects in the myocardium. The structural and functional changes of Pkd2 mutant hearts were analyzed in the myocardial-specific Pkd2 knockout (KO) mouse. We further assessed a potential role of TGF-b₁ signaling in the pathology of Pkd2-KO hearts. Hearts from age-matched 6-month-old MyH6•Pkd2 ^wt/wt (control or wild-type) and MyH6•Pkd2 ^flox/flox (mutant or Pkd2-KO) mice were used to study differential heart structure and function. Cardiac histology was used to study structure, and the "isolated working heart" system was adapted to mount and perfuse mouse heart to measure different cardiac parameters. We found that macrophage1 (M1) and macrophage 2 (M2) infiltration, transforming growth factor (TGF-b₁) and TGF-b₁ receptor expressions were significantly higher in Pkd2-KO, compared to wild-type hearts. The increase in the extracellular matrix in Pkd2-KO myocardium led to cardiac hypertrophy, interstitial and conduction system fibrosis, causing cardiac dysfunction with a predisposition to arrhythmia. Left ventricular (LV) expansion or compliance and LV filling were impaired in fibrotic Pkd2-KO hearts, resulted in diastolic dysfunction. LV systolic contractility and elastance decreased in fibrotic Pkd2-KO hearts, resulted in systolic dysfunction. Compared to wild-type hearts, Pkd2-KO hearts were less responsive to the pharmacological stress-test and changes in preload. In conclusion, Pkd2-KO mice had systolic and diastolic dysfunction with arrhythmogenic hearts.

Autonomic modulation of ventricular electrical activity: recent developments and clinical implications

PURPOSE

This review aimed to provide a complete overview of the current stance and recent developments in antiarrhythmic neuromodulatory interventions, focusing on lifethreatening vetricular arrhythmias.

METHODS

Both preclinical studies and clinical studies were assessed to highlight the gaps in knowledge that remain to be answered and the necessary steps required to properly translate these strategies to the clinical setting.

RESULTS

Cardiac autonomic imbalance, characterized by chronic sympathoexcitation and parasympathetic withdrawal, destabilizes cardiac electrophysiology and promotes ventricular arrhythmogenesis. Therefore, neuromodulatory interventions that target the sympatho-vagal imbalance have emerged as promising antiarrhythmic strategies. These strategies are aimed at different parts of the cardiac neuraxis and directly or indirectly restore cardiac autonomic tone. These interventions include pharmacological blockade of sympathetic neurotransmitters and neuropeptides, cardiac sympathetic denervation, thoracic epidural anesthesia, and spinal cord and vagal nerve stimulation.

CONCLUSION

Neuromodulatory strategies have repeatedly been demonstrated to be highly effective and very promising anti-arrhythmic therapies. Nevertheless, there is still much room to gain in our understanding of neurocardiac physiology, refining the current neuromodulatory strategic options and elucidating the chronic effects of many of these strategic options.

Ion Channel Expression and Electrophysiology of Singular Human (Primary and Induced Pluripotent Stem Cell-Derived) Cardiomyocytes

Subtype-specific human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are promising tools, e.g., to assess the potential of drugs to cause chronotropic effects (nodal hiPSC-CMs), atrial fibrillation (atrial hiPSC-CMs), or ventricular arrhythmias (ventricular hiPSC-CMs). We used single-cell patch-clamp reverse transcriptase-quantitative polymerase chain reaction to clarify the composition of the iCell cardiomyocyte population (Fujifilm Cellular Dynamics, Madison, WI, USA) and to compare it with atrial and ventricular Pluricytes (Ncardia, Charleroi, Belgium) and primary human atrial and ventricular cardiomyocytes. The comparison of beating and non-beating iCell cardiomyocytes did not support the presence of true nodal, atrial, and ventricular cells in this hiPSC-CM population. The comparison of atrial and ventricular Pluricytes with primary human cardiomyocytes showed trends, indicating the potential to derive more subtype-specific hiPSC-CM models using appropriate differentiation protocols. Nevertheless, the single-cell phenotypes of the majority of the hiPSC-CMs showed a combination of attributes which may be interpreted as a mixture of traits of adult cardiomyocyte subtypes: (i) nodal: spontaneous action potentials and high HCN4 expression and (ii) non-nodal: prominent INa-driven fast inward current and high expression of SCN5A. This may hamper the interpretation of the drug effects on parameters depending on a combination of ionic currents, such as beat rate. However, the proven expression of specific ion channels supports the evaluation of the drug effects on ionic currents in a more realistic cardiomyocyte environment than in recombinant non-cardiomyocyte systems.

Gender Differences in Cardiac Chronotropic Control: Implications for Heart Rate Variability Research

There is a continuing debate concerning “adjustments” to heart period variability [i.e., heart rate variability (HRV)] for the heart period [i.e., increases inter-beat-intervals (IBI)]. To date, such arguments have not seriously considered the impact a demographic variable, such as gender, can have on the association between HRV and the heart period. A prior meta-analysis showed women to have greater HRV compared to men despite having shorter IBI and higher heart rate (HR). Thus, it is plausible that men and women differ in the association between HRV and HR/IBI. Thus, the present study investigates the potential moderating effect of gender on the association between HRV and indices of cardiac chronotropy, including both HR and IBI. Data from 633 participants (339 women) were available for analysis. Cardiac measures were assessed during a 5-min baseline-resting period. HRV measures included the standard deviation of inter-beat-intervals, root mean square of successive differences, and autoregressive high frequency power. Moderation analyses showed gender significantly moderated the association between all HRV variables and both HR and IBI (each p < 0.05). However, results were not consistent when using recently recommended HRV variables “adjusted” for IBI. Overall, the current investigation provides data illustrating a differential association between HRV and the heart period based on gender. Substantial neurophysiological evidence support the current findings; women show greater sensitivity to acetylcholine compared to men. If women show greater sensitivity to acetylcholine, and acetylcholine increases HRV and the heart period, then the association between HRV and the heart period indeed should be stronger in women compared to men. Taken together, these data suggest that routine “adjustments” to HRV for the heart period are unjustified and problematic at best. As it relates to the application of future HRV research, it is imperative that researchers continue to consider the potential impact of gender.

Quantitative cross-species translators of cardiac myocyte electrophysiology: Model training, experimental validation, and applications

Animal experimentation is key in the evaluation of cardiac efficacy and safety of novel therapeutic compounds. However, interspecies differences in the mechanisms regulating excitation-contraction coupling can limit the translation of experimental findings from animal models to human physiology and undermine the assessment of drugs’ efficacy and safety. Here, we built a suite of translators for quantitatively mapping electrophysiological responses in ventricular myocytes across species. We trained these statistical operators using a broad dataset obtained by simulating populations of our biophysically detailed computational models of action potential and Ca2+ transient in mouse, rabbit, and human. We then tested our translators against experimental data describing the response to stimuli, such as ion channel block, change in beating rate, and β-adrenergic challenge. We demonstrate that this approach is well suited to predicting the effects of perturbations across different species or experimental conditions and suggest its integration into mechanistic studies and drug development pipelines.

Distinct Effects of Ibrutinib and Acalabrutinib on Mouse Atrial and Sinoatrial Node Electrophysiology and Arrhythmogenesis

Background

Ibrutinib and acalabrutinib are Bruton tyrosine kinase inhibitors used in the treatment of B‐cell lymphoproliferative disorders. Ibrutinib is associated with new‐onset atrial fibrillation. Cases of sinus bradycardia and sinus arrest have also been reported following ibrutinib treatment. Conversely, acalabrutinib is less arrhythmogenic. The basis for these different effects is unclear.

Methods and Results

The effects of ibrutinib and acalabrutinib on atrial electrophysiology were investigated in anesthetized mice using intracardiac electrophysiology, in isolated atrial preparations using high‐resolution optical mapping, and in isolated atrial and sinoatrial node (SAN) myocytes using patch‐clamping. Acute delivery of acalabrutinib did not affect atrial fibrillation susceptibility or other measures of atrial electrophysiology in mice in vivo. Optical mapping demonstrates that ibrutinib dose‐dependently impaired atrial and SAN conduction and slowed beating rate. Acalabrutinib had no effect on atrial and SAN conduction or beating rate. In isolated atrial myocytes, ibrutinib reduced action potential upstroke velocity and Na⁺ current. In contrast, acalabrutinib had no effects on atrial myocyte upstroke velocity or Na⁺ current. Both drugs increased action potential duration, but these effects were smaller for acalabrutinib compared with ibrutinib and occurred by different mechanisms. In SAN myocytes, ibrutinib impaired spontaneous action potential firing by inhibiting the delayed rectifier K⁺ current, while acalabrutinib had no effects on SAN myocyte action potential firing.

Conclusions

Ibrutinib and acalabrutinib have distinct effects on atrial electrophysiology and ion channel function that provide insight into the basis for increased atrial fibrillation susceptibility and SAN dysfunction with ibrutinib, but not with acalabrutinib.

What keeps us ticking? Sinoatrial node mechano-sensitivity: the grandfather clock of cardiac rhythm

The rhythmic and spontaneously generated electrical excitation that triggers the heartbeat originates in the sinoatrial node (SAN). SAN automaticity has been thoroughly investigated, which has uncovered fundamental mechanisms involved in cardiac pacemaking that are generally categorised into two interacting and overlapping systems: the 'membrane' and 'Ca²⁺ clock'. The principal focus of research has been on these two systems of oscillators, which have been studied primarily in single cells and isolated tissue, experimental preparations that do not consider mechanical factors present in the whole heart. SAN mechano-sensitivity has long been known to be a contributor to SAN pacemaking-both as a driver and regulator of automaticity-but its essential nature has been underappreciated. In this review, following a description of the traditional 'clocks' of SAN automaticity, we describe mechanisms of SAN mechano-sensitivity and its vital role for SAN function, making the argument that the 'mechanics oscillator' is, in fact, the 'grandfather clock' of cardiac rhythm.

Unravelling Atrioventricular Block Risk in Inflammatory Diseases: Systemic Inflammation Acutely Delays Atrioventricular Conduction via a Cytokine-Mediated Inhibition of Connexin43 Expression

Background

Recent data suggest that systemic inflammation can negatively affect atrioventricular conduction, regardless of acute cardiac injury. Indeed, gap‐junctions containing connexin43 coupling cardiomyocytes and inflammation‐related cells (macrophages) are increasingly recognized as important factors regulating the conduction in the atrioventricular node. The aim of this study was to evaluate the acute impact of systemic inflammatory activation on atrioventricular conduction, and elucidate underlying mechanisms.

Methods and Results

We analyzed: (1) the PR‐interval in patients with inflammatory diseases of different origins during active phase and recovery, and its association with inflammatory markers; (2) the existing correlation between connexin43 expression in the cardiac tissue and peripheral blood mononuclear cells (PBMC), and the changes occurring in patients with inflammatory diseases over time; (3) the acute effects of interleukin(IL)‐6 on atrioventricular conduction in an in vivo animal model, and on connexin43 expression in vitro. In patients with elevated C‐reactive protein levels, atrioventricular conduction indices are increased, but promptly normalized in association with inflammatory markers reduction, particularly IL‐6. In these subjects, connexin43 expression in PBMC, which is correlative of that measured in the cardiac tissue, inversely associated with IL‐6 changes. Moreover, direct IL‐6 administration increased atrioventricular conduction indices in vivo in a guinea pig model, and IL‐6 incubation in both cardiomyocytes and macrophages in culture, significantly reduced connexin43 proteins expression.

Conclusions

The data evidence that systemic inflammation can acutely worsen atrioventricular conduction, and that IL‐6‐induced down‐regulation of cardiac connexin43 is a mechanistic pathway putatively involved in the process. Though reversible, these alterations could significantly increase the risk of severe atrioventricular blocks during active inflammatory processes.