In vivo and ex vivo electrophysiological study of the mouse heart to characterize the cardiac conduction system, including atrial and ventricular vulnerability

A GABAergic system in atrioventricular node pacemaker cells controls electrical conduction between the atria and ventricles

Physiologically, the atria contract first, followed by the ventricles, which is the prerequisite for normal blood circulation. The above phenomenon of atrioventricular sequential contraction results from the characteristically slow conduction of electrical excitation of the atrioventricular node (AVN) between the atria and the ventricles. However, it is not clear what controls the conduction of electrical excitation within AVNs. Here, we find that AVN pacemaker cells (AVNPCs) possess an intact intrinsic GABAergic system, which plays a key role in electrical conduction from the atria to the ventricles. First, along with the discovery of abundant GABA-containing vesicles under the surface membranes of AVNPCs, key elements of the GABAergic system, including GABA metabolic enzymes, GABA receptors, and GABA transporters, were identified in AVNPCs. Second, GABA synchronously elicited GABA-gated currents in AVNPCs, which significantly weakened the excitability of AVNPCs. Third, the key molecular elements of the GABAergic system markedly modulated the conductivity of electrical excitation in the AVN. Fourth, GABAA receptor deficiency in AVNPCs accelerated atrioventricular conduction, which impaired the AVN's protective potential against rapid ventricular frequency responses, increased susceptibility to lethal ventricular arrhythmias, and decreased the cardiac contractile function. Finally, interventions targeting the GABAergic system effectively prevented the occurrence and development of atrioventricular block. In summary, the endogenous GABAergic system in AVNPCs determines the slow conduction of electrical excitation within AVNs, thereby ensuring sequential atrioventricular contraction. The endogenous GABAergic system shows promise as a novel intervention target for cardiac arrhythmias.

Biocompatible Electronic Skins for Cardiovascular Health Monitoring

Cardiovascular diseases represent a significant threat to the overall well-being of the global population. Continuous monitoring of vital signs related to cardiovascular health is essential for improving daily health management. Currently, there has been remarkable proliferation of technology focused on collecting data related to cardiovascular diseases through daily electronic skin monitoring. However, concerns have arisen regarding potential skin irritation and inflammation due to the necessity for prolonged wear of wearable devices. To ensure comfortable and uninterrupted cardiovascular health monitoring, the concept of biocompatible electronic skin has gained substantial attention. In this review, biocompatible electronic skins for cardiovascular health monitoring are comprehensively summarized and discussed. The recent achievements of biocompatible electronic skin in cardiovascular health monitoring are introduced. Their working principles, fabrication processes, and performances in sensing technologies, materials, and integration systems are highlighted, and comparisons are made with other electronic skins used for cardiovascular monitoring. In addition, the significance of integrating sensing systems and the updating wireless communication for the development of the smart medical field is explored. Finally, the opportunities and challenges for wearable electronic skin are also examined.

Intracardiac electrophysiology to characterize susceptibility to ventricular arrhythmias in murine models

Introduction: Sudden cardiac death (SCD) and ventricular fibrillation are rare but severe complications of many cardiovascular diseases and represent a major health issue worldwide. Although the primary causes are often acute or chronic coronary diseases, genetic conditions, such as inherited channelopathies or non-ischemic cardiomyopathies are leading causes of SCD among the young. However, relevant experimental models to study the underlying mechanisms of arrhythmias and develop new therapies are still needed. The number of genetically engineered mouse models with cardiac phenotype is growing, making electrophysiological studies in mice essential tools to study arrhythmogenicity and arrhythmia mechanisms and to test novel treatments. Recently, intracardiac catheterization via the jugular vein was described to induce and record ventricular arrhythmias in living anesthetized mice. Several strategies have been reported, developed in healthy wild-type animals and based on aggressive right ventricular stimulation. Methods: Here, we report a protocol based on programmed electrical stimulation (PES) performed in clinical practice in patients with cardiac rhythm disorders, adapted to two transgenic mice models of arrhythmia - Brugada syndrome and cardiolaminopathy. Results: We show that this progressive protocol, based on a limited number of right ventricular extrastimuli, enables to reveal different rhythmic phenotypes between control and diseased mice. In this study, we provide detailed information on PES in mice, including catheter positioning, stimulation protocols, intracardiac and surface ECG interpretation and we reveal a higher susceptibility of two mouse lines to experience triggered ventricular arrhythmias, when compared to control mice. Discussion: Overall, this technique allows to characterize arrhythmias and provides results in phenotyping 2 arrhythmogenic-disease murine models.

A comprehensive protocol combining in vivo and ex vivo electrophysiological experiments in an arrhythmogenic animal model

Ventricular arrhythmias contribute significantly to cardiovascular mortality, with coronary artery disease as the predominant underlying cause. Understanding the mechanisms of arrhythmogenesis is essential to identify proarrhythmic factors and develop novel approaches for antiarrhythmic prophylaxis and treatment. Animal models are vital in basic research on cardiac arrhythmias, encompassing molecular, cellular, ex vivo whole heart, and in vivo models. Most studies use either in vivo protocols lacking important information on clinical relevance or exclusively ex vivo protocols, thereby missing the opportunity to explore underlying mechanisms. Consequently, interpretation may be difficult due to dissimilarities in animal models, interventions, and individual properties across animals. Moreover, proarrhythmic effects observed in vivo are often not replicated in corresponding ex vivo preparations during mechanistic studies. We have established a protocol to perform both an in vivo and ex vivo electrophysiological characterization in an arrhythmogenic rat model with heart failure following myocardial infarction. The same animal is followed throughout the experiment. In vivo methods involve intracardiac programmed electrical stimulation and external defibrillation to terminate sustained ventricular arrhythmia. Ex vivo methods conducted on the Langendorff-perfused heart include an electrophysiological study with optical mapping of regional action potentials, conduction velocities, and dispersion of electrophysiological properties. By exploring the retention of the in vivo proarrhythmic phenotype ex vivo, we aim to examine whether the subsequent ex vivo detailed measurements are relevant to in vivo pathological behavior. This protocol can enhance greater understanding of cardiac arrhythmias by providing a standardized, yet adaptable model for evaluating arrhythmogenicity or antiarrhythmic interventions in cardiac diseases.NEW & NOTEWORTHY Rodent models are widely used in arrhythmia research. However, most studies do not standardize clinically relevant in vivo and ex vivo techniques to support their conclusions. Here, we present a comprehensive electrophysiological protocol in an arrhythmogenic rat model, connecting in vivo and ex vivo programmed electrical stimulation with optical mapping. By establishing this protocol, we aim to facilitate the adoption of a standardized model for investigating arrhythmias, enhancing research rigor and comparability in this field.

Organ-on-chip systems as a model for nanomedicine

Nanomedicine is giving rise to increasing numbers of successful drugs, including cancer treatments, molecular imaging agents, and novel vaccine formulations. However, traditionally available model systems offer limited clinical translation and, compared to the number of preclinical studies, the approval rate of nanoparticles (NPs) for clinical use remains disappointingly low. A new paradigm of modeling biological systems on microfluidic chips has emerged in the last decade and is being gradually adopted by the nanomedicine community. These systems mimic tissues, organs, and diseases like cancer, on devices with small physical footprints and complex geometries. In this review, we report studies that used organ-on-chip approaches to study the interactions of NPs with biological systems. We present examples of NP toxicity studies, studies using biological NPs such as viruses, as well as modeling biological barriers and cancer on chip. Organ-on-chip systems present an exciting opportunity and can provide a renewed direction for the nanomedicine community.

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

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Overview of programmed electrical stimulation to assess atrial fibrillation susceptibility in mice

Atrial fibrillation (AF) is the most common human arrhythmia and is associated with increased risk of stroke, dementia, heart failure, and death. Among several animal models that have been used to investigate the molecular determinants of AF, mouse models have become the most prevalent due to low cost, ease of genetic manipulation, and similarity to human disease. Programmed electrical stimulation (PES) using intracardiac or transesophageal atrial pacing is used to induce AF as most mouse models do not develop spontaneous AF. However, there is a lack of standardized methodology resulting in numerous PES protocols in the literature that differ with respect to multiple parameters, including pacing protocol and duration, stimulus amplitude, pulse width, and even the definition of AF. Given this complexity, the selection of the appropriate atrial pacing protocol for a specific model has been arbitrary. Herein we review the development of intracardiac and transesophageal PES, including commonly used protocols, selected experimental models, and advantages and disadvantages of both techniques. We also emphasize detection of artifactual AF induction due to unintended parasympathetic stimulation, which should be excluded from results. We recommend that the optimal pacing protocol to elicit an AF phenotype should be individualized to the specific model of genetic or acquired risk factors, with an analysis using several definitions of AF as an endpoint.

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.

Demographic and Methodological Heterogeneity in Electrocardiogram Signals From Guinea Pigs

Electrocardiograms (ECG) are universally used to measure the electrical activity of the heart; however, variations in recording techniques and/or subject demographics can affect ECG interpretation. In this study, we investigated variables that are likely to influence ECG metric measurements in cardiovascular research, including recording technique, use of anesthesia, and animal model characteristics. Awake limb lead ECG recordings were collected in vivo from adult guinea pigs using a platform ECG system, while recordings in anesthetized animals were performed using both a platform and needle ECG system. We report significant heterogeneities in ECG metric values that are attributed to methodological differences (e.g., ECG lead configuration, ECG recording platform, presence or absence of anesthesia) that persist even within the same cohort of animals. Further, we report that variability in animal demographics is preserved in vivo ECG recordings—with animal age serving as a significant contributor, while sex-specific influences were less pronounced. Methodological approaches and subject demographics should be fully considered when interpreting ECG values in animal models, comparing datasets between studies, or developing artificial intelligence algorithms that utilize an ECG database.

Paradigm shift: new concepts for HCN4 function in cardiac pacemaking

Hyperpolarization-activated cyclic nucleotide–gated (HCN) channels are the molecular correlate of the I_f current and are critically involved in controlling neuronal excitability and the autonomous rhythm of the heart. The HCN4 isoform is the main HCN channel subtype expressed in the sinoatrial node (SAN), a tissue composed of specialized pacemaker cells responsible for generating the intrinsic heartbeat. More than 40 years ago, the I_f current was first discovered in rabbit SAN tissue. Along with this discovery, a theory was proposed that cyclic adenosine monophosphate–dependent modulation of I_f mediates heart rate regulation by the autonomic nervous system—a process called chronotropic effect. However, up to the present day, this classical theory could not be reliably validated. Recently, new concepts emerged confirming that HCN4 channels indeed play an important role in heart rate regulation. However, the cellular mechanism by which HCN4 controls heart rate turned out to be completely different than originally postulated. Here, we review the latest findings regarding the physiological role of HCN4 in the SAN. We describe a newly discovered mechanism underlying heart rate regulation by HCN4 at the tissue and single cell levels, and we discuss these observations in the context of results from previously studied HCN4 mouse models.