Mapping of epicardial activation in a rabbit model of chronic myocardial infarction:

The effect of non-local coupling of fibroblasts on pacing dynamics in a 2D tissue: a simulation study

Although myocytes in healthy hearts are usually coupled to nearest neighbours via gap junctions, under conditions such as fibrosis, in scar tissue, or across ablation lines, myocytes can uncouple from their neighbours. However it has been experimentally observed that electrical conduction can still occur across these uncoupled regions via fibroblasts. In this paper we propose a novel model of non-local coupling between myocytes and fibroblasts in a 2D tissue, and hypothesise that such long-range coupling can give rise to pro-arrhythmic re-entrant wave dynamics. We have simulated the scar and the surrounding border zone via simultaneous coupling of fibroblasts with both proximal and distal regions of myocardium. We find that in this setup the border zone itself is a dynamical outcome of the coupling between cells within and outside the scar. We have determined the effect of the border zone on the stability of waves generated by rapid pacing. Furthermore we have identified key parameters that determine wave dynamics in this geometry, and have also described the mechanism underlying the complex wave dynamics. These findings are of significance for our understanding of cardiac arrhythmias associated with regions of myocardial scar.

A novel method for the percutaneous induction of myocardial infarction by occlusion of small coronary arteries in the rabbit

Arrhythmic sudden cardiac death (SCD) is an important cause of mortality following myocardial infarction (MI). The rabbit has similar cardiac electrophysiology to humans and is therefore an important small animal model to study post-MI arrhythmias. The established approach of surgical coronary ligation results in thoracic adhesions that impede epicardial electrophysiological studies. Adhesions are absent following a percutaneously induced MI, which is also associated with reduced surgical morbidity and so represents a clear refinement of the approach. Percutaneous procedures have previously been described in large rabbits (3.5-5.5 kg). Here, we describe a novel method of percutaneous MI induction in smaller rabbits (2.5-3.5 kg) that are readily available commercially. New Zealand White rabbits (n = 51 males, 3.1 ± 0.3 kg) were anesthetized using isoflurane (1.5-3%) and underwent either a percutaneous MI procedure involving microcatheter tip deployment (≤1.5 Fr, 5 mm), coronary ligation surgery, or a sham procedure. Electrocardiography (ECG) recordings were used to confirm ST-segment elevation indicating coronary occlusion. Blood samples (1 and 24 h) were taken for cardiac troponin I (cTnI) levels. Ejection fraction (EF) was measured at 6-8 wk. Rabbits were then euthanized (Euthatal) and hearts were processed for magnetic resonance imaging and histology. Mortality rates were similar in both groups. Scar volume, cTnI, and EF were similar between both MI groups and significantly different from their respective sham controls. Thus, percutaneous coronary occlusion by microcatheter tip deployment is feasible in rabbits (2.5-3.5 kg) and produces an MI with similar characteristics to surgical ligation with lower procedural trauma and without epicardial adhesions.NEW & NOTEWORTHY Surgical coronary ligation is the standard technique to induce myocardial infarction (MI) in rabbits but is associated with procedural trauma and the generation of thoracic adhesions. Percutaneous coronary occlusion avoids these shortcomings and is established in pigs but has only been applicable to large rabbits because of a mismatch between the equipment used and target vessel size. Here, we describe a new scalable approach to percutaneous MI induction that is safe and effective in 2.5-3.5-kg rabbits.

Mannitol and hyponatremia regulate cardiac ventricular conduction in the context of sodium channel loss of function

Scn5a heterozygous null (Scn5a+/-) mice have historically been used to investigate arrhythmogenic mechanisms of diseases such as Brugada syndrome (BrS) and Lev's disease. Previously, we demonstrated that reducing ephaptic coupling (EpC) in ex vivo hearts exacerbates pharmacological voltage-gated sodium channel (Nav)1.5 loss of function (LOF). Whether this effect is consistent in a genetic Nav1.5 LOF model is yet to be determined. We hypothesized that loss of EpC would result in greater reduction in conduction velocity (CV) for the Scn5a+/- mouse relative to wild type (WT). In vivo ECGs and ex vivo optical maps were recorded from Langendorff-perfused Scn5a+/- and WT mouse hearts. EpC was reduced with perfusion of a hyponatremic solution, the clinically relevant osmotic agent mannitol, or a combination of the two. Neither in vivo QRS duration nor ex vivo CV during normonatremia was significantly different between the two genotypes. In agreement with our hypothesis, we found that hyponatremia severely slowed CV and disrupted conduction for 4/5 Scn5a+/- mice, but 0/6 WT mice. In addition, treatment with mannitol slowed CV to a greater extent in Scn5a+/- relative to WT hearts. Unexpectedly, treatment with mannitol during hyponatremia did not further slow CV in either genotype, but resolved the disrupted conduction observed in Scn5a+/- hearts. Similar results in guinea pig hearts suggest the effects of mannitol and hyponatremia are not species specific. In conclusion, loss of EpC through either hyponatremia or mannitol alone results in slowed or disrupted conduction in a genetic model of Nav1.5 LOF. However, the combination of these interventions attenuates conduction slowing.NEW & NOTEWORTHY Cardiac sodium channel loss of function (LOF) diseases such as Brugada syndrome (BrS) are often concealed. We optically mapped mouse hearts with reduced sodium channel expression (Scn5a+/-) to evaluate whether reduced ephaptic coupling (EpC) can unmask conduction deficits. Data suggest that conduction deficits in the Scn5a+/- mouse may be unmasked by treatment with hyponatremia and perinexal widening via mannitol. These data support further investigation of hyponatremia and mannitol as novel diagnostics for sodium channel loss of function diseases.

Fibroblast mediated dynamics in diffusively uncoupled myocytes: a simulation study using 2-cell motifs

In healthy hearts myocytes are typically coupled to nearest neighbours through gap junctions. Under pathological conditions such as fibrosis, or in scar tissue, or across ablation lines myocytes can uncouple from their neighbours. Electrical conduction may still occur via fibroblasts that not only couple proximal myocytes but can also couple otherwise unconnected regions. We hypothesise that such coupling can alter conduction between myocytes via introduction of delays or by initiation of premature stimuli that can potentially result in reentry or conduction blocks. To test this hypothesis we have developed several 2-cell motifs and investigated the effect of fibroblast mediated electrical coupling between uncoupled myocytes. We have identified various regimes of myocyte behaviour that depend on the strength of gap-junctional conductance, connection topology, and parameters of the myocyte and fibroblast models. These motifs are useful in developing a mechanistic understanding of long-distance coupling on myocyte dynamics and enable the characterisation of interaction between different features such as myocyte and fibroblast properties, coupling strengths and pacing period. They are computationally inexpensive and allow for incorporation of spatial effects such as conduction velocity. They provide a framework for constructing scar tissue boundaries and enable linking of cellular level interactions with scar induced arrhythmia.

Myofibroblasts impair myocardial impulse propagation by heterocellular connexin43 gap-junctional coupling through micropores

Aim: Composite population of myofibroblasts (MFs) within myocardial tissue is known to alter impulse propagation, leading to arrhythmias. However, it remains unclear whether and how MFs alter their propagation patterns when contacting cardiomyocytes (CMs) without complex structural insertions in the myocardium. We attempted to unveil the effects of the one-sided, heterocellular CM-MF connection on the impulse propagation of CM monolayers without the spatial insertion of MFs as an electrical or mechanical obstacle. Methods and results: We evaluated fluo8-based spatiotemporal patterns in impulse propagation of neonatal rat CM monolayers cultured on the microporous membrane having 8-μm diameter pores with co-culture of MFs or CMs on the reverse membrane side (CM-MF model or CM-CM model, respectively). During consecutive pacing at 1 or 2 Hz, the CM monolayers exhibited forward impulse propagation from the pacing site with a slower conduction velocity (θ) and a larger coefficient of directional θ variation in the CM-MF model than that in the CM-CM model in a frequency-dependent manner (2 Hz >1 Hz). The localized placement of an MF cluster on the reverse side resulted in an abrupt segmental depression of the impulse propagation of the upper CM layer, causing a spatiotemporally non-uniform pattern. Dye transfer of the calcein loaded in the upper CM layer to the lower MF layer was attenuated by the gap-junction inhibitor heptanol. Immunocytochemistry identified definitive connexin 43 (Cx43) between the CMs and MFs in the membrane pores. MF-selective Cx43 knockdown in the MF layer improved both the velocity and uniformity of propagation in the CM monolayer. Conclusion: Heterocellular Cx43 gap junction coupling of CMs with MFs alters the spatiotemporal patterns of myocardial impulse propagation, even in the absence of spatially interjacent and mechanosensitive modulations by MFs. Moreover, MFs can promote pro-arrhythmogenic impulse propagation when in face-to-face contact with the myocardium that arises in the healing infarct border zone.

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.

Nonmyocytes as electrophysiological contributors to cardiac excitation and conduction

Although cardiac action potential (AP) generation and propagation have traditionally been attributed exclusively to cardiomyocytes (CM), other cell types in the heart are also capable of forming electrically conducting junctions. Interactions between CM and nonmyocytes (NM) enable and modulate each other's activity. This review provides an overview of the current understanding of heterocellular electrical communication in the heart. Although cardiac fibroblasts were initially thought to be electrical insulators, recent studies have demonstrated that they form functional electrical connections with CM in situ. Other NM, such as macrophages, have also been recognized as contributing to cardiac electrophysiology and arrhythmogenesis. Novel experimental tools have enabled the investigation of cell-specific activity patterns in native cardiac tissue, which is expected to yield exciting new insights into the development of novel or improved diagnostic and therapeutic strategies.

Apelin-13 Increases Functional Connexin-43 through Autophagy Inhibition via AKT/mTOR Pathway in the Non-Myocytic Cell Population of the Heart

Studies have shown a link between the downregulation of connexin 43 (Cx43), the predominant isoform in cardiac gap junctions, and high susceptibility to cardiac arrhythmias and cardiomyocyte death. Non-myocytic cells (NMCs), the most abundant component of the heart, exert multiple cardiac functions and represent an important therapeutic target for diseased cardiac tissue. A few studies have investigated the effect of Apelin-13, an endogenous peptide with a key role in various cardiovascular functions, on Cx43 expression in cardiomyocytes. However, it remained unknown whether Apelin-13 influences Cx43 expression in NMCs. Here, we found that in NMCs, Cx43 protein expression increased after Apelin-13 treatment (100 nM for 48 h). Furthermore, dye transfer assays proved that Apelin-13-treated NMCs had a greater ability to communicate with surrounding cardiomyocytes, and this effect was abrogated by carbenoxolone, a gap junction inhibitor. Interestingly, we showed that Apelin-13 increased Cx43 through autophagy inhibition, as proved by the upregulation of p62 and LC3I, acting as 3-MA, a well-known autophagy inhibitor. In addition, Apelin-13-induced AKT and mTOR phosphorylation was abolished by LY294002 and rapamycin inhibitors resulting in Cx43 increased suppression. These results open the possibility of targeting gap junctions in NMCs with Apelin-13 as an exciting therapeutic approach with great potential.

Effects of fibroblast on electromechanical dynamics of human atrial tissue-insights from a 2D discrete element model

Roughly 75% of normal myocardial tissue volume is comprised of myocytes, however, fibroblasts by number are the most predominant cells in cardiac tissue. Previous studies have shown distinctive differences in cellular electrophysiology and excitability between myocytes and fibroblasts. However, it is still unclear how the electrical coupling between the two and the increased population of fibroblasts affects the electromechanical dynamics of cardiac tissue. This paper focuses on investigating effects of fibroblast-myocyte electrical coupling (FMEC) and fibroblast population on atrial electrical conduction and mechanical contractility by using a two-dimensional Discrete Element Method (DEM) model of cardiac tissue that is different to finite element method (FEM). In the model, the electro-mechanics of atrial cells are modelled by a biophysically detailed model for atrial electrical action potentials and myofilament kinetics, and the atrial fibroblasts are modelled by an active model that considers four active membrane ionic channel currents. Our simulation results show that the FMEC impairs myocytes' electrical action potential and mechanical contractibility, manifested by reduced upstroke velocity, amplitude and duration of action potentials, as well as cell length shortening. At the tissue level, the FMEC slows down the conduction of excitation waves, and reduces strain of the tissue produced during a contraction course. These findings provide new insights into understandings of how FMEC impairs cardiac electrical and mechanical dynamics of the heart.

Novel Optics-Based Approaches for Cardiac Electrophysiology: A Review

Optical techniques for recording and manipulating cellular electrophysiology have advanced rapidly in just a few decades. These developments allow for the analysis of cardiac cellular dynamics at multiple scales while largely overcoming the drawbacks associated with the use of electrodes. The recent advent of optogenetics opens up new possibilities for regional and tissue-level electrophysiological control and hold promise for future novel clinical applications. This article, which emerged from the international NOTICE workshop in 2018, reviews the state-of-the-art optical techniques used for cardiac electrophysiological research and the underlying biophysics. The design and performance of optical reporters and optogenetic actuators are reviewed along with limitations of current probes. The physics of light interaction with cardiac tissue is detailed and associated challenges with the use of optical sensors and actuators are presented. Case studies include the use of fluorescence recovery after photobleaching and super-resolution microscopy to explore the micro-structure of cardiac cells and a review of two photon and light sheet technologies applied to cardiac tissue. The emergence of cardiac optogenetics is reviewed and the current work exploring the potential clinical use of optogenetics is also described. Approaches which combine optogenetic manipulation and optical voltage measurement are discussed, in terms of platforms that allow real-time manipulation of whole heart electrophysiology in open and closed-loop systems to study optimal ways to terminate spiral arrhythmias. The design and operation of optics-based approaches that allow high-throughput cardiac electrophysiological assays is presented. Finally, emerging techniques of photo-acoustic imaging and stress sensors are described along with strategies for future development and establishment of these techniques in mainstream electrophysiological research.

Cardiac Conduction Velocity, Remodeling and Arrhythmogenesis

Cardiac electrophysiological disorders, in particular arrhythmias, are a key cause of morbidity and mortality throughout the world. There are two basic requirements for arrhythmogenesis: an underlying substrate and a trigger. Altered conduction velocity (CV) provides a key substrate for arrhythmogenesis, with slowed CV increasing the probability of re-entrant arrhythmias by reducing the length scale over which re-entry can occur. In this review, we examine methods to measure cardiac CV in vivo and ex vivo, discuss underlying determinants of CV, and address how pathological variations alter CV, potentially increasing arrhythmogenic risk. Finally, we will highlight future directions both for methodologies to measure CV and for possible treatments to restore normal CV.

Ventricular Arrhythmias in Ischemic Cardiomyopathy-New Avenues for Mechanism-Guided Treatment

Ischemic heart disease is the most common cause of lethal ventricular arrhythmias and sudden cardiac death (SCD). In patients who are at high risk after myocardial infarction, implantable cardioverter defibrillators are the most effective treatment to reduce incidence of SCD and ablation therapy can be effective for ventricular arrhythmias with identifiable culprit lesions. Yet, these approaches are not always successful and come with a considerable cost, while pharmacological management is often poor and ineffective, and occasionally proarrhythmic. Advances in mechanistic insights of arrhythmias and technological innovation have led to improved interventional approaches that are being evaluated clinically, yet pharmacological advancement has remained behind. We review the mechanistic basis for current management and provide a perspective for gaining new insights that centre on the complex tissue architecture of the arrhythmogenic infarct and border zone with surviving cardiac myocytes as the source of triggers and central players in re-entry circuits. Identification of the arrhythmia critical sites and characterisation of the molecular signature unique to these sites can open avenues for targeted therapy and reduce off-target effects that have hampered systemic pharmacotherapy. Such advances are in line with precision medicine and a patient-tailored therapy.

Are Interactions between Epicardial Adipose Tissue, Cardiac Fibroblasts and Cardiac Myocytes Instrumental in Atrial Fibrosis and Atrial Fibrillation?

Atrial fibrillation is very common among the elderly and/or obese. While myocardial fibrosis is associated with atrial fibrillation, the exact mechanisms within atrial myocytes and surrounding non-myocytes are not fully understood. This review considers the potential roles of myocardial fibroblasts and myofibroblasts in fibrosis and modulating myocyte electrophysiology through electrotonic interactions. Coupling with (myo)fibroblasts in vitro and in silico prolonged myocyte action potential duration and caused resting depolarization; an optogenetic study has verified in vivo that fibroblasts depolarized when coupled myocytes produced action potentials. This review also introduces another non-myocyte which may modulate both myocardial (myo)fibroblasts and myocytes: epicardial adipose tissue. Epicardial adipocytes are in intimate contact with myocytes and (myo)fibroblasts and may infiltrate the myocardium. Adipocytes secrete numerous adipokines which modulate (myo)fibroblast and myocyte physiology. These adipokines are protective in healthy hearts, preventing inflammation and fibrosis. However, adipokines secreted from adipocytes may switch to pro-inflammatory and pro-fibrotic, associated with reactive oxygen species generation. Pro-fibrotic adipokines stimulate myofibroblast differentiation, causing pronounced fibrosis in the epicardial adipose tissue and the myocardium. Adipose tissue also influences myocyte electrophysiology, via the adipokines and/or through electrotonic interactions. Deeper understanding of the interactions between myocytes and non-myocytes is important to understand and manage atrial fibrillation.

In vivo grafting of large engineered heart tissue patches for cardiac repair

Engineered heart tissue (EHT) strategies, by combining cells within a hydrogel matrix, may be a novel therapy for heart failure. EHTs restore cardiac function in rodent injury models, but more data are needed in clinically relevant settings. Accordingly, an upscaled EHT patch (2.5 cm × 1.5 cm × 1.5 mm) consisting of up to 20 million human induced pluripotent stem cell–derived cardiomyocytes (hPSC-CMs) embedded in a fibrin-based hydrogel was developed. A rabbit myocardial infarction model was then established to test for feasibility and efficacy. Our data showed that hPSC-CMs in EHTs became more aligned over 28 days and had improved contraction kinetics and faster calcium transients. Blinded echocardiographic analysis revealed a significant improvement in function in infarcted hearts that received EHTs, along with reduction in infarct scar size by 35%. Vascularization from the host to the patch was observed at week 1 and stable to week 4, but electrical coupling between patch and host heart was not observed. In vivo telemetry recordings and ex vivo arrhythmia provocation protocols showed that the patch was not pro-arrhythmic. In summary, EHTs improved function and reduced scar size without causing arrhythmia, which may be due to the lack of electrical coupling between patch and host heart.

Electroimmunology and cardiac arrhythmia

Conduction disorders and arrhythmias remain difficult to treat and are increasingly prevalent owing to the increasing age and body mass of the general population, because both are risk factors for arrhythmia. Many of the underlying conditions that give rise to arrhythmia - including atrial fibrillation and ventricular arrhythmia, which frequently occur in patients with acute myocardial ischaemia or heart failure - can have an inflammatory component. In the past, inflammation was viewed mostly as an epiphenomenon associated with arrhythmia; however, the recently discovered inflammatory and non-canonical functions of cardiac immune cells indicate that leukocytes can be arrhythmogenic either by altering tissue composition or by interacting with cardiomyocytes; for example, by changing their phenotype or perhaps even by directly interfering with conduction. In this Review, we discuss the electrophysiological properties of leukocytes and how these cells relate to conduction in the heart. Given the thematic parallels, we also summarize the interactions between immune cells and neural systems that influence information transfer, extrapolating findings from the field of neuroscience to the heart and defining common themes. We aim to bridge the knowledge gap between electrophysiology and immunology, to promote conceptual connections between these two fields and to explore promising opportunities for future research.

Characterization of Electrical Activity in Post-myocardial Infarction Scar Tissue in Rat Hearts Using Multiphoton Microscopy

Background: The origin of electrical behavior in post-myocardial infarction scar tissue is still under debate. This study aims to examine the extent and nature of the residual electrical activity within a stabilized ventricular infarct scar. Methods and Results: An apical infarct was induced in the left ventricle of Wistar rats by coronary artery occlusion. Five weeks post-procedure, hearts were Langendorff-perfused, and optically mapped using di-4-ANEPPS. Widefield imaging of optical action potentials (APs) on the left ventricular epicardial surface revealed uniform areas of electrical activity in both normal zone (NZ) and infarct border zone (BZ), but only limited areas of low-amplitude signals in the infarct zone (IZ). 2-photon (2P) excitation of di-4-ANEPPS and Fura-2/AM at discrete layers in the NZ revealed APs and Ca²⁺ transients (CaTs) to 500-600 μm below the epicardial surface. 2P imaging in the BZ revealed superficial connective tissue structures lacking APs or CaTs. At depths greater than approximately 300 μm, myocardial structures were evident that supported normal APs and CaTs. In the IZ, although 2P imaging did not reveal clear myocardial structures, low-amplitude AP signals were recorded at discrete layers. No discernible Ca²⁺ signals could be detected in the IZ. AP rise times in BZ were slower than NZ (3.50 ± 0.50 ms vs. 2.23 ± 0.28 ms) and further slowed in IZ (9.13 ± 0.56 ms). Widefield measurements of activation delay between NZ and BZ showed negligible difference (3.37 ± 1.55 ms), while delay values in IZ showed large variation (11.88 ± 9.43 ms). Conclusion: These AP measurements indicate that BZ consists of an electrically inert scar above relatively normal myocardium. Discrete areas/layers of IZ displayed entrained APs with altered electrophysiology, but the structure of this tissue remains to be elucidated.

Overexpression of Cx43 in cells of the myocardial scar: Correction of post-infarct arrhythmias through heterotypic cell-cell coupling

Ventricular tachycardia (VT) is the most common and potentially lethal complication following myocardial infarction (MI). Biological correction of the conduction inhomogeneity that underlies re-entry could be a major advance in infarction therapy. As minimal increases in conduction of infarcted tissue markedly influence VT susceptibility, we reasoned that enhanced propagation of the electrical signal between non-excitable cells within a resolving infarct might comprise a simple means to decrease post-infarction arrhythmia risk. We therefore tested lentivirus-mediated delivery of the gap-junction protein Connexin 43 (Cx43) into acute myocardial lesions. Cx43 was expressed in (myo)fibroblasts and CD45⁺ cells within the scar and provided prominent and long lasting arrhythmia protection in vivo. Optical mapping of Cx43 injected hearts revealed enhanced conduction velocity within the scar, indicating Cx43-mediated electrical coupling between myocytes and (myo)fibroblasts. Thus, Cx43 gene therapy, by direct in vivo transduction of non-cardiomyocytes, comprises a simple and clinically applicable biological therapy that markedly reduces post-infarction VT.

Directed fusion of cardiac spheroids into larger heterocellular microtissues enables investigation of cardiac action potential propagation via cardiac fibroblasts

Multicellular spheroids generated through cellular self-assembly provide cytoarchitectural complexities of native tissue including three-dimensionality, extensive cell-cell contacts, and appropriate cell-extracellular matrix interactions. They are increasingly suggested as building blocks for larger engineered tissues to achieve shapes, organization, heterogeneity, and other biomimetic complexities. Application of these tissue culture platforms is of particular importance in cardiac research as the myocardium is comprised of distinct but intermingled cell types. Here, we generated scaffold-free 3D cardiac microtissue spheroids comprised of cardiac myocytes (CMs) and/or cardiac fibroblasts (CFs) and used them as building blocks to form larger microtissues with different spatial distributions of CMs and CFs. Characterization of fusing homotypic and heterotypic spheroid pairs revealed an important influence of CFs on fusion kinetics, but most strikingly showed rapid fusion kinetics between heterotypic pairs consisting of one CF and one CM spheroid, indicating that CMs and CFs self-sort in vitro into the intermixed morphology found in the healthy myocardium. We then examined electrophysiological integration of fused homotypic and heterotypic microtissues by mapping action potential propagation. Heterocellular elongated microtissues which recapitulate the disproportionate CF spatial distribution seen in the infarcted myocardium showed that action potentials propagate through CF volumes albeit with significant delay. Complementary computational modeling revealed an important role of CF sodium currents and the spatial distribution of the CM-CF boundary in action potential conduction through CF volumes. Taken together, this study provides useful insights for the development of complex, heterocellular engineered 3D tissue constructs and their engraftment via tissue fusion and has implications for arrhythmogenesis in cardiac disease and repair.

Modeling the Electrophysiological Properties of the Infarct Border Zone

Ventricular arrhythmias (VA) in patients with myocardial infarction (MI) are thought to be associated with structural and electrophysiological remodeling within the infarct border zone (BZ). Personalized computational models have been used to investigate the potential role of the infarct BZ in arrhythmogenesis, which still remains incompletely understood. Most recent models have relied on experimental data to assign BZ properties. However, experimental measurements vary significantly resulting in different computational representations of this region. Here, we review experimental data available in the literature to determine the most prominent properties of the infarct BZ. Computational models are then used to investigate the effect of different representations of the BZ on activation and repolarization properties, which may be associated with VA. Experimental data obtained from several animal species and patients with infarct show that BZ properties vary significantly depending on disease's stage, with the early disease stage dominated by ionic remodeling and the chronic stage by structural remodeling. In addition, our simulations show that ionic remodeling in the BZ leads to large repolarization gradients in the vicinity of the scar, which may have a significant impact on arrhythmia simulations, while structural remodeling plays a secondary role. We conclude that it is imperative to faithfully represent the properties of regions of infarction within computational models specific to the disease stage under investigation in order to conduct in silico mechanistic investigations.

The effect of left ventricular pacing on transmural activation delay in myopathic human hearts

Aims

Left ventricular (LV) epicardial pacing (LVEpiP) in human myopathic hearts does not decrease global epicardial activation delay compared with right ventricular (RV) endocardial pacing (RVEndoP); however, the effect on transmural activation delay has not been evaluated. To characterize the transmural electrical activation delay in human myopathic hearts during RVEndoP and LVEpiP compared with global epicardial activation delay.

Methods and results

Explanted hearts from seven patients (5 male, 46 ± 10 years) undergoing cardiac transplantation were Langendorff-perfused and mapped using an epicardial sock electrode array (112 electrodes) and 25 transmural plunge needles (four electrodes, 2 mm spacing), for a total of 100 unipolar transmural electrodes. Electrograms were recorded during LVEpiP and RVEndoP, and epicardial (sock) and transmural (needle) activation times, along with patterns of activation, were compared. There was no difference between the global epicardial activation times (LVEpiP 147 ± 8 ms vs. RVEndoP 156 ± 17 ms, P = 0.46). The mean LV transmural activation time during LVEpiP was significantly shorter than that during RVEndoP (125 ± 44 vs. 172 ± 43 ms, P < 0.001). During LVEpiP, of the transmural layers endo-, mid-myocardium and epicardium, LV endocardial layer was often the earliest compared with other transmural layers.

Conclusion

In myopathic human hearts, LVEpiP did not decrease global epicardial activation delays compared with RVEndoP. LV epicardial pacing led to early activation of the LV endocardium, revealing the importance of the LV endocardium even when pacing from the LV epicardium.

Role of Non-Myocyte Gap Junctions and Connexin Hemichannels in Cardiovascular Health and Disease: Novel Therapeutic Targets?

The heart is a complex organ composed of multiple cell types, including cardiomyocytes and different non-myocyte populations, all working closely together to determine the hearts properties and maintain normal cardiac function. Connexins are abundantly expressed proteins that form plasma membrane hemichannels and gap junctions between cells. Gap junctions are intracellular channels that allow for communication between cells, and in the heart they play a crucial role in cardiac conduction by coupling adjacent cardiomyocytes. Connexins are expressed in both cardiomyocytes and non-myocytes, including cardiac fibroblasts, endothelial cells, and macrophages. Non-myocytes are the largest population of cells in the heart, and therefore it is important to consider what roles connexins, hemichannels, and gap junctions play in these cell types. The aim of this review is to provide insight into connexin-based signalling in non-myocytes during health and disease, and highlight how targeting these proteins could lead to the development of novel therapies. We conclude that connexins in non-myocytes contribute to arrhythmias and adverse ventricular remodelling following myocardial infarction, and are associated with the initiation and development of atherosclerosis. Therefore, therapeutic interventions targeting these connexins represent an exciting new research avenue with great potential.

Cardiac repair in guinea pigs with human engineered heart tissue from induced pluripotent stem cells

Myocardial injury results in a loss of contractile tissue mass that, in the absence of efficient regeneration, is essentially irreversible. Transplantation of human pluripotent stem cell-derived cardiomyocytes has beneficial but variable effects. We created human engineered heart tissue (hEHT) strips from human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes and hiPSC-derived endothelial cells. The hEHTs were transplanted onto large defects (22% of the left ventricular wall, 35% decline in left ventricular function) of guinea pig hearts 7 days after cryoinjury, and the results were compared with those obtained with human endothelial cell patches (hEETs) or cell-free patches. Twenty-eight days after transplantation, the hearts repaired with hEHT strips exhibited, within the scar, human heart muscle grafts, which had remuscularized 12% of the infarct area. These grafts showed cardiomyocyte proliferation, vascularization, and evidence for electrical coupling to the intact heart tissue in a subset of engrafted hearts. hEHT strips improved left ventricular function by 31% compared to that before implantation, whereas the hEET or cell-free patches had no effect. Together, our study demonstrates that three-dimensional human heart muscle constructs can repair the injured heart.

A novel method of standardized myocardial infarction in aged rabbits

The incidence of both myocardial infarction (MI) and sudden cardiac death increases with age. Here, we describe the development of a minimally invasive large animal model of MI that can be applied to young or aged animals. We demonstrate that rabbit coronary anatomy is highly variable, more so than described in previous literature. In this work, we categorize the coronary pattern of 37 young rabbits and 64 aged rabbits. Aged rabbits had a higher degree of branching from the left main coronary artery. Standardizing the model across age cohorts required a new approach, targeting an area of myocardium rather than a specific vessel. Here, we present a method for achieving a reproducible infarct size, one that yielded a consistent scar encompassing ~30% of the apical left ventricular free wall. The model's consistency allowed for more valid comparisons of MI sequelae between age cohorts.NEW & NOTEWORTHY This study describes the coronary angiographic imaging of young and aged rabbits. We developed and improved a novel minimally invasive approach for coil embolization that targets a specific area of myocardium and yielded a consistent scar encompassing ~30% of the left ventricular free wall of young and aged rabbit hearts.

Electrotonic coupling of excitable and nonexcitable cells in the heart revealed by optogenetics

Electrophysiological studies of excitable organs usually focus on action potential (AP)-generating cells, whereas nonexcitable cells are generally considered as barriers to electrical conduction. Whether nonexcitable cells may modulate excitable cell function or even contribute to AP conduction via direct electrotonic coupling to AP-generating cells is unresolved in the heart: such coupling is present in vitro, but conclusive evidence in situ is lacking. We used genetically encoded voltage-sensitive fluorescent protein 2.3 (VSFP2.3) to monitor transmembrane potential in either myocytes or nonmyocytes of murine hearts. We confirm that VSFP2.3 allows measurement of cell type-specific electrical activity. We show that VSFP2.3, expressed solely in nonmyocytes, can report cardiomyocyte AP-like signals at the border of healed cryoinjuries. Using EM-based tomographic reconstruction, we further discovered tunneling nanotube connections between myocytes and nonmyocytes in cardiac scar border tissue. Our results provide direct electrophysiological evidence of heterocellular electrotonic coupling in native myocardium and identify tunneling nanotubes as a possible substrate for electrical cell coupling that may be in addition to previously discovered connexins at sites of myocyte-nonmyocyte contact in the heart. These findings call for reevaluation of cardiac nonmyocyte roles in electrical connectivity of the heterocellular heart.

Computational Representations of Myocardial Infarct Scars and Implications for Arrhythmogenesis

Image-based computational modeling is becoming an increasingly used clinical tool to provide insight into the mechanisms of reentrant arrhythmias. In the context of ischemic heart disease, faithful representation of the electrophysiological properties of the infarct region within models is essential, due to the scars known for arrhythmic properties. Here, we review the different computational representations of the infarcted region, summarizing the experimental measurements upon which they are based. We then focus on the two most common representations of the scar core (complete insulator or electrically passive tissue) and perform simulations of electrical propagation around idealized infarct geometries. Our simulations highlight significant differences in action potential duration and focal effective refractory period (ERP) around the scar, driven by differences in electrotonic loading, depending on the choice of scar representation. Finally, a novel mechanism for arrhythmia induction, following a focal ectopic beat, is demonstrated, which relies on localized gradients in ERP directly caused by the electrotonic sink effects of the neighboring passive scar.

Novel therapeutic strategies targeting fibroblasts and fibrosis in heart disease

Our understanding of the functions of cardiac fibroblasts has moved beyond their roles in heart structure and extracellular matrix generation and now includes their contributions to paracrine, mechanical and electrical signalling during ontogenesis and normal cardiac activity. Fibroblasts also have central roles in pathogenic remodelling during myocardial ischaemia, hypertension and heart failure. As key contributors to scar formation, they are crucial for tissue repair after interventions including surgery and ablation. Novel experimental approaches targeting cardiac fibroblasts are promising potential therapies for heart disease. Indeed, several existing drugs act, at least partially, through effects on cardiac connective tissue. This Review outlines the origins and roles of fibroblasts in cardiac development, homeostasis and disease; illustrates the involvement of fibroblasts in current and emerging clinical interventions; and identifies future targets for research and development.

Can heart function lost to disease be regenerated by therapeutic targeting of cardiac scar tissue?

Myocardial infarction results in scar tissue that cannot actively contribute to heart mechanical function and frequently causes lethal arrhythmias. The healing response after infarction involves inflammation, biochemical signaling, changes in cellular phenotype, activity, and organization, and alterations in electrical conduction due to variations in cell and tissue geometry and alterations in protein expression, organization, and function - particularly in membrane channels. The intensive research focus on regeneration of myocardial tissues has, as of yet, only met with modest success, with no near-term prospect of improving standard-of-care for patients with heart disease. An alternative concept for novel therapeutic approach is the rejuvenation of cardiac electrical and mechanical properties through the modification of scar tissue. Several peptide therapeutics, locally applied genetic therapies, or delivery of genetically modified cells have shown promise in improving the characteristics of the fibrous scar and post-myocardial infarction prognosis in experimental models. This review highlights several factors that contribute to arrhythmogenesis in scar formation and how these might be targeted to regenerate some of the electrical and mechanical function of the post-MI scar.

Prolongation of atrio-ventricular node conduction in a rabbit model of ischaemic cardiomyopathy: Role of fibrosis and connexin remodelling

Conduction abnormalities are frequently associated with cardiac disease, though the mechanisms underlying the commonly associated increases in PQ interval are not known. This study uses a chronic left ventricular (LV) apex myocardial infarction (MI) model in the rabbit to create significant left ventricular dysfunction (LVD) 8 weeks post-MI. In vivo studies established that the PQ interval increases by approximately 7 ms (10%) with no significant change in average heart rate. Optical mapping of isolated Langendorff perfused rabbit hearts recapitulated this result: time to earliest activation of the LV was increased by 14 ms (16%) in the LVD group. Intra-atrial and LV transmural conduction times were not altered in the LVD group. Isolated AVN preparations from the LVD group demonstrated a significantly longer conduction time (by approximately 20 ms) between atrial and His electrograms than sham controls across a range of pacing cycle lengths. This difference was accompanied by increased effective refractory period and Wenckebach cycle length, suggesting significantly altered AVN electrophysiology post-MI. The AVN origin of abnormality was further highlighted by optical mapping of the isolated AVN. Immunohistochemistry of AVN preparations revealed increased fibrosis and gap junction protein (connexin43 and 40) remodelling in the AVN of LVD animals compared to sham. A significant increase in myocyte–non-myocyte connexin co-localization was also observed after LVD. These changes may increase the electrotonic load experienced by AVN muscle cells and contribute to slowed conduction velocity within the AVN.

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Chronic myocardial infarction (MI) causes changes in atrio-ventricular node (AVN) function.

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Isolated hearts post-MI show delays in ventricular activation due to slowed conduction via the AVN.

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Isolated AVN preparations demonstrated AVN electrical remodelling post-MI.

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Electrical remodelling is associated with fibrosis and altered expression of connexins in the AVN.

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AVN dysfunction post-MI is caused by localized functional and structural remodelling.

Electrical consequences of cardiac myocyte: fibroblast coupling

Gap junctions are channels which allow electrical signals to propagate through the heart from the sinoatrial node and through the atria, conduction system and onwards to the ventricles, and hence are essential for co-ordinated cardiac contraction. Twelve connexin (Cx) proteins make up one gap junction channel, of which there are three main subtypes in the heart; Cx40, Cx43 and Cx45. In the cardiac myocyte, gap junctions are present mainly at the intercalated discs between neighbouring myocytes, and assist in rapid electrical conduction throughout the ventricular myocardium. Fibroblasts provide the structural skeleton of the myocardium and fibroblast numbers significantly increase in heart disease. Fibroblasts also express connexins and this may facilitate heterocellular electrical coupling between myocytes and fibroblasts in the setting of cardiac disease. Interestingly, cardiac fibroblasts have been demonstrated to increase Cx43 expression in experimental models of myocardial infarction and functional gap junctions between myocytes and fibroblasts have been reported. Therefore, in the setting of heart disease enhanced cardiac myocyte: fibroblast coupling may influence the electrical activity of the myocyte and contribute to arrhythmias.

The Living Scar--Cardiac Fibroblasts and the Injured Heart

Cardiac scars, often dubbed 'dead tissue', are very much alive, with heterocellular activity contributing to the maintenance of structural and mechanical integrity following heart injury. To form a scar, non-myocytes such as fibroblasts are recruited from intra- and extra-cardiac sources. Fibroblasts perform important autocrine and paracrine signaling functions. They also establish mechanical and, as is increasingly evident, electrical junctions with other cells. While fibroblasts were previously thought to act simply as electrical insulators, they may be electrically connected among themselves and, under some circumstances, to other cells including cardiomyocytes. A better understanding of these biophysical interactions will help to target scar structure and function, and will facilitate the development of novel therapies aimed at modifying scar properties for patient benefit.

Fibroblast-myocyte coupling in the heart: Potential relevance for therapeutic interventions

Cardiac myocyte-fibroblast electrotonic coupling is a well-established fact in vitro. Indirect evidence of its presence in vivo exists, but few functional studies have been published. This review describes the current knowledge of fibroblast-myocyte electrical signaling in the heart. Further research is needed to understand the frequency and extent of heterocellular interactions in vivo in order to gain a better understanding of their relevance in healthy and diseased myocardium. It is hoped that associated insight into myocyte-fibroblast coupling in the heart may lead to the discovery of novel therapeutic targets and the development of agents for improving outcomes of myocardial scarring and fibrosis.

The Interaction between Adult Cardiac Fibroblasts and Embryonic Stem Cell-Derived Cardiomyocytes Leads to Proarrhythmic Changes in In Vitro Cocultures

Transplantation of stem cell-derived cardiomyocytes is one of the most promising therapeutic approaches after myocardial infarction, as loss of cardiomyocytes is virtually irreversible by endogenous repair mechanisms. In myocardial scars, transplanted cardiomyocytes will be in immediate contact with cardiac fibroblasts. While it is well documented how the electrophysiology of neonatal cardiomyocytes is modulated by cardiac fibroblasts of the same developmental stage, it is unknown how adult cardiac fibroblasts (aCFs) affect the function of embryonic stem cell-derived cardiomyocytes (ESC-CMs). To investigate the effects of aCFs on ESC-CM electrophysiology, we performed extra- and intracellular recordings of murine aCF-ESC-CM cocultures. We observed that spontaneous beating behaviour was highly irregular in aCF-ESC-CM cocultures compared to cocultures with mesenchymal stem cells (coefficient of variation of the interspike interval: 40.5 ± 15.2% versus 9.3 ± 2.0%, p = 0.008) and that action potential amplitude and maximal upstroke velocity (V max) were reduced (amplitude: 52.3 ± 1.7 mV versus 65.1 ± 1.5 mV, V max: 7.0 ± 1.0 V/s versus 36.5 ± 5.3 V/s), while action potential duration (APD) was prolonged (APD50: 25.6 ± 1.0 ms versus 16.8 ± 1.9 ms, p < 0.001; APD90: 52.2 ± 1.5 ms versus 43.3 ± 3.3 ms, p < 0.01) compared to controls. Similar changes could be induced by aCF-conditioned medium. We conclude that the presence of aCFs changes automaticity and induces potentially proarrhythmic changes of ESC-CM electrophysiology.

A review of the literature on cardiac electrical activity between fibroblasts and myocytes

Myocardial injuries often lead to fibrotic deposition. This review presents evidence supporting the concept that fibroblasts in the heart electrically couple to myocytes.

Physiological Implications of Myocardial Scar Structure

Once myocardium dies during a heart attack, it is replaced by scar tissue over the course of several weeks. The size, location, composition, structure, and mechanical properties of the healing scar are all critical determinants of the fate of patients who survive the initial infarction. While the central importance of scar structure in determining pump function and remodeling has long been recognized, it has proven remarkably difficult to design therapies that improve heart function or limit remodeling by modifying scar structure. Many exciting new therapies are under development, but predicting their long-term effects requires a detailed understanding of how infarct scar forms, how its properties impact left ventricular function and remodeling, and how changes in scar structure and properties feed back to affect not only heart mechanics but also electrical conduction, reflex hemodynamic compensations, and the ongoing process of scar formation itself. In this article, we outline the scar formation process following a myocardial infarction, discuss interpretation of standard measures of heart function in the setting of a healing infarct, then present implications of infarct scar geometry and structure for both mechanical and electrical function of the heart and summarize experiences to date with therapeutic interventions that aim to modify scar geometry and structure. One important conclusion that emerges from the studies reviewed here is that computational modeling is an essential tool for integrating the wealth of information required to understand this complex system and predict the impact of novel therapies on scar healing, heart function, and remodeling following myocardial infarction.

A mathematical model of the carp heart ventricle during the cardiac cycle

The poikilothermic heart has been suggested as a model for studying some of the mechanisms of early postnatal mammalian heart adaptations. We assessed morphological parameters of the carp heart (Cyprinus carpio L.) with diastolic dimensions: heart radius (5.73mm), thickness of the compact (0.50mm) and spongy myocardium (4.34mm), in two conditions (systole, diastole): volume fraction of the compact myocardium (20.7% systole, 19.6% diastole), spongy myocardium (58.9% systole, 62.8% diastole), trabeculae (37.8% systole, 28.6% diastole), and cavities (41.5% systole, 51.9% diastole) within the ventricle; volume fraction of the trabeculae (64.1% systole, 45.5% diastole) and sinuses (35.9% systole, 54.5% diastole) within the spongy myocardium; ratio between the volume of compact and spongy myocardium (0.35 systole, 0.31 diastole); ratio between compact myocardium and trabeculae (0.55 systole, 0.69 diastole); and surface density of the trabeculae (0.095μm(-1) systole, 0.147μm(-1) diastole). We created a mathematical model of the carp heart based on actual morphometric data to simulate how the compact/spongy myocardium ratio, the permeability of the spongy myocardium, and sinus-trabeculae volume fractions within the spongy myocardium influence stroke volume, stroke work, ejection fraction and p-V diagram. Increasing permeability led to increasing and then decreasing stroke volume and work, and increasing ejection fraction. An increased amount of spongy myocardium led to an increased stroke volume, work, and ejection fraction. Varying sinus-trabeculae volume fractions within the spongy myocardium showed that an increased sinus volume fraction led to an increased stroke volume and work, and a decreased ejection fraction.

Acidosis slows electrical conduction through the atrio-ventricular node

Acidosis affects the mechanical and electrical activity of mammalian hearts but comparatively little is known about its effects on the function of the atrio-ventricular node (AVN). In this study, the electrical activity of the epicardial surface of the left ventricle of isolated Langendorff-perfused rabbit hearts was examined using optical methods. Perfusion with hypercapnic Tyrode's solution (20% CO₂, pH 6.7) increased the time of earliest activation (T_act) from 100.5 ± 7.9 to 166.1 ± 7.2 ms (n = 8) at a pacing cycle length (PCL) of 300 ms (37°C). T_act increased at shorter PCL, and the hypercapnic solution prolonged T_act further: at 150 ms PCL, T_act was prolonged from 131.0 ± 5.2 to 174.9 ± 16.3 ms. 2:1 AVN block was common at shorter cycle lengths. Atrial and ventricular conduction times were not significantly affected by the hypercapnic solution suggesting that the increased delay originated in the AVN. Isolated right atrial preparations were superfused with Tyrode's solutions at pH 7.4 (control), 6.8 and 6.3. Low pH prolonged the atrial-Hisian (AH) interval, the AVN effective and functional refractory periods and Wenckebach cycle length significantly. Complete AVN block occurred in 6 out of 9 preparations. Optical imaging of conduction at the AV junction revealed increased conduction delay in the region of the AVN, with less marked effects in atrial and ventricular tissue. Thus acidosis can dramatically prolong the AVN delay, and in combination with short cycle lengths, this can cause partial or complete AVN block and is therefore implicated in the development of brady-arrhythmias in conditions of local or systemic acidosis.

Fibroblast-myocyte electrotonic coupling: does it occur in native cardiac tissue?

Heterocellular electrotonic coupling between cardiac myocytes and non-excitable connective tissue cells has been a long-established and well-researched fact in vitro. Whether or not such coupling exists in vivo has been a matter of considerable debate. This paper reviews the development of experimental insight and conceptual views on this topic, describes evidence in favour of and against the presence of such coupling in native myocardium, and identifies directions for further study needed to resolve the riddle, perhaps less so in terms of principal presence which has been demonstrated, but undoubtedly in terms of extent, regulation, patho-physiological context, and actual relevance of cardiac myocyte–non-myocyte coupling in vivo. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

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Electrical coupling of cardiomyocytes and fibroblasts is well-established in vitro

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Whether such hetero-cellular coupling exists in vivo has been a matter of debate

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We review the development of experimental and conceptual insight into the topic

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Conclusion 1: hetero-cellular coupling in heart tissue has been shown in principle

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Conclusion 2: extent, regulation, context, and relevance remain to be established

Myofilament calcium de-sensitization and contractile uncoupling prevent pause-triggered ventricular tachycardia in mouse hearts with chronic myocardial infarction

Myocardial infarction (MI) is a major risk for ventricular arrhythmia. Pause-triggered ventricular arrhythmia can be caused by increased myofilament Ca binding due to sarcomeric mutations or Ca-sensitizing compounds. Myofilament Ca sensitivity is also increased after MI. Here we hypothesize that MI increases risk for pause-triggered ventricular arrhythmias, which can be prevented by myofilament Ca-desensitization and contractile uncoupling. To test this hypothesis, we generated a murine chronic MI model using male B6SJLF1/J mice (n=40) that underwent permanent ligation of the left anterior descending coronary artery. 4 weeks post MI, cardiac structure, function and myofilament Ca sensitivity were evaluated. Pause-dependent arrhythmia susceptibility was quantified in isolated hearts with pacing trains of increasing frequency, followed by a pause and an extra stimulus. Coronary ligation resulted in a mean infarct size of 39.6±5.7% LV and fractional shortening on echocardiography was reduced by 40% compared to non-infarcted controls. Myofilament Ca sensitivity was significantly increased in post MI hearts (pCa50: Control=5.66±0.03; MI=5.84±0.05; P<0.01). Exposure to the Ca desensitizer/contractile uncoupler blebbistatin (BLEB, 3 μM) reduced myofilament Ca sensitivity of MI hearts to that of control hearts and selectively reduced the frequency of post-pause ectopic beats (MI 0.12±0.04 vs MI+BLEB 0.01±0.005 PVC/pause; P=0.02). BLEB also reduced the incidence of ventricular tachycardia in chronic MI hearts from 59% to 10% (P<0.05). We conclude that chronic MI hearts exhibit increased myofilament Ca sensitivity and pause-triggered ventricular arrhythmias, which can be prevented by blebbistatin. Decreasing myofilament Ca sensitivity may be a strategy to reduce arrhythmia burden after MI.

Subepicardial action potential characteristics are a function of depth and activation sequence in isolated rabbit hearts

BACKGROUND

Electric excitability in the ventricular wall is influenced by cellular electrophysiology and passive electric properties of the myocardium. Action potential (AP) rise time, an indicator of myocardial excitability, is influenced by conduction pattern and distance from the epicardial surface. This study examined AP rise times and conduction velocity as the depolarizing wavefront approaches the epicardial surface.

METHODS AND RESULTS

Two-photon excitation of di-4-aminonaphthenyl-pyridinum-propylsulfonate was used to measure electric activity at discrete epicardial layers of isolated Langendorff-perfused rabbit hearts to a depth of 500 μm. Endo-to-epicardial wavefronts were studied during right atrial or ventricular endocardial pacing. Similar measurements were made with epi-to-endocardial, transverse, and longitudinal pacing protocols. Results were compared with data from a bidomain model of 3-dimensional (3D) electric propagation within ventricular myocardium. During right atrial and endocardial pacing, AP rise time (10%-90% of upstroke) decreased by ≈50% between 500 and 50 μm from the epicardial surface, whereas conduction velocity increased and AP duration was only slightly shorter (≈4%). These differences were not observed with other conduction patterns. The depth-dependent changes in rise time were larger at higher pacing rates. Modeling data qualitatively reproduced the behavior seen experimentally and demonstrated a parallel reduction in peak I(Na) and electrotonic load as the wavefront approaches the epicardial surface.

CONCLUSIONS

Decreased electrotonic load at the epicardial surface results in more rapid AP upstrokes and higher conduction velocities compared with the bulk myocardium. Combined effects of tissue depth and pacing rate on AP rise time reduce conduction safety and myocardial excitability within the ventricular wall.

Quantitative analysis of cardiac tissue including fibroblasts using three-dimensional confocal microscopy and image reconstruction: towards a basis for electrophysiological modeling

Electrophysiological modeling of cardiac tissue is commonly based on functional and structural properties measured in experiments. Our knowledge of these properties is incomplete, in particular their remodeling in disease. Here, we introduce a methodology for quantitative tissue characterization based on fluorescent labeling, 3-D scanning confocal microscopy, image processing and reconstruction of tissue micro-structure at sub-micrometer resolution. We applied this methodology to normal rabbit ventricular tissue and tissue from hearts with myocardial infarction. Our analysis revealed that the volume fraction of fibroblasts increased from 4.83±0.42% (mean ± standard deviation) in normal tissue up to 6.51±0.38% in myocardium from infarcted hearts. The myocyte volume fraction decreased from 76.20±9.89% in normal to 73.48±8.02% adjacent to the infarct. Numerical field calculations on 3-D reconstructions of the extracellular space yielded an extracellular longitudinal conductivity of 0.264±0.082 S/m with an anisotropy ratio of 2.095±1.11 in normal tissue. Adjacent to the infarct, the longitudinal conductivity increased up to 0.400±0.051 S/m, but the anisotropy ratio decreased to 1.295±0.09. Our study indicates an increased density of gap junctions proximal to both fibroblasts and myocytes in infarcted versus normal tissue, supporting previous hypotheses of electrical coupling of fibroblasts and myocytes in infarcted hearts. We suggest that the presented methodology provides an important contribution to modeling normal and diseased tissue. Applications of the methodology include the clinical characterization of disease-associated remodeling.

Microscopic magnetic resonance imaging reveals high prevalence of third coronary artery in human and rabbit heart

AIM

The human coronary tree is commonly assumed to have two roots: the left and right coronary arteries (LCA and RCA, respectively). However, a third coronary artery (TCA) has been observed in humans and animals, usually arising from the right anterior aortic sinus near the RCA. Using high-resolution magnetic resonance imaging, we identified TCA prevalence and characteristics in rabbit and human hearts.

METHODS AND RESULTS

Third coronary artery presence was analysed in hearts from 11 New Zealand white rabbits and 7 human cadavers, using excised tissue that was fixed, gadolinium-treated, and agar-embedded for imaging-based reconstruction. A TCA was identified in all rabbit hearts and six of seven human hearts, originating either from an independent ostium (7 of 11 rabbits, 2 of 7 humans) or an ostium shared with the RCA (4 of 11 rabbits, 4 of 7 humans). Proximal TCA cross-sectional area in rabbits was 15.3 ± 6.0% of RCA area (mean ± SD, based on n = 9 rabbit hearts in which reliable measurements could be taken for both vessels), and 26.7 ± 10.1% in humans (n = 4). In all-but-one case where a TCA was observed, it originated ventral to the RCA, progressing towards the right ventricular outflow tract. In one rabbit, the TCA originated dorsal to the RCA and progressed towards the Crista terminalis in the right atrium. A fourth vessel, forming a separate aortic Vas vasorum was occasionally seen, originating from the right anterior aortic sinus either from an ostium common with (1 of 11 rabbits, 0 of 7 humans) or independent of (1 of 11 rabbits, 1 of 7 humans) the TCA. Pilot optical mapping experiments showed that TCA occlusion had variable acute effects on rabbit cardiac electrophysiology.

CONCLUSION

Third coronary artery presence is common in rabbit and human hearts. Functional effects of disrupted TCA blood supply are ill-investigated, and the rabbit may be a suitable species for such research.

Optical mapping in the developing zebrafish heart

Understanding the developmental basis of cardiac electrical activity has proven technically challenging, largely as a result of the inaccessible nature of the heart during cardiogenesis in many organisms. The emergence of the zebrafish as a model organism has availed the very earliest stages of heart formation to experimental exploration. The zebrafish also offers a robust platform for genetic and chemical screening. These tools have been exploited in screens for modifiers of cardiac electrophysiologic phenotypes and in screens for novel drugs.