Gap junctional conductance in ventricular myocyte pairs isolated from postischemic rabbit myocardium

Ephaptic Coupling as a Resolution to the Paradox of Action Potential Wave Speed and Discordant Alternans Spatial Scales in the Heart

Previous computer simulations have suggested that existing models of action potential wave propagation in the heart are not consistent with observed wave propagation behavior. Specifically, computer models cannot simultaneously reproduce the rapid wave speeds and small spatial scales of discordant alternans patterns measured experimentally in the same simulation. The discrepancy is important, because discordant alternans can be a key precursor to the development of abnormal and dangerous rapid rhythms in the heart. In this Letter, we show that this paradox can be resolved by allowing so-called ephaptic coupling to play a primary role in wave front propagation in place of conventional gap-junction coupling. With this modification, physiological wave speeds and small discordant alternans spatial scales both occur with gap-junction resistance values that are more in line with those observed in experiments. Our theory thus also provides support to the hypothesis that ephaptic coupling plays an important role in normal wave propagation.

Role of ephaptic coupling in discordant alternans domain sizes and action potential propagation in the heart

Discordant alternans, the spatially out-of-phase alternation of the durations of propagating action potentials in the heart, has been linked to the onset of fibrillation, a major cardiac rhythm disorder. The sizes of the regions, or domains, within which these alternations are synchronized are critical in this link. However, computer models employing standard gap junction-based coupling between cells have been unable to reproduce simultaneously the small domain sizes and rapid action potential propagation speeds seen in experiments. Here we use computational methods to show that rapid wave speeds and small domain sizes are possible when a more detailed model of intercellular coupling that accounts for so-called ephaptic effects is used. We provide evidence that the smaller domain sizes are possible, because different coupling strengths can exist on the wavefronts, for which both ephaptic and gap-junction coupling are involved, in contrast to the wavebacks, where only gap-junction coupling plays an active role. The differences in coupling strength are due to the high density of fast-inward (sodium) channels known to localize on the ends of cardiac cells, which are only active (and thus engage ephaptic coupling) during wavefront propagation. Thus, our results suggest that this distribution of fast-inward channels, as well as other factors responsible for the critical involvement of ephaptic coupling in wave propagation, including intercellular cleft spacing, play important roles in increasing the vulnerability of the heart to life-threatening tachyarrhythmias. Our results, combined with the absence of short-wavelength discordant alternans domains in standard gap-junction-dominated coupling models, also provide evidence that both gap-junction and ephaptic coupling are critical in wavefront propagation and waveback dynamics.

Confocal microscopy-based estimation of intracellular conductivities in myocardium for modeling of the normal and infarcted heart

Ventricular arrhythmias are the leading cause of mortality in patients with ischemic heart diseases, such as myocardial infarction (MI). Computational simulation of cardiac electrophysiology provides insights into these arrhythmias and their treatment. However, only sparse information is available on crucial model parameters, for instance, the anisotropic intracellular electrical conductivities. Here, we introduced an approach to estimate these conductivities in normal and MI hearts. We processed and analyzed images from confocal microscopy of left ventricular tissue of a rabbit MI model to generate 3D reconstructions. We derived tissue features including the volume fraction of myocytes (V_myo), gap junctions-containing voxels (V_gj), and fibrosis (V_fibrosis). We generated models of the intracellular space and intercellular coupling. Applying numerical methods for solving Poisson's equation for stationary electrical currents, we calculated normal (σ_myo,n), longitudinal (σ_myo,l), and transverse (σ_myo,t) intracellular conductivities. Using linear regression analysis, we assessed relationships of conductivities to tissue features. V_gj and V_myo were reduced in MI vs. control, but V_fibrosis was increased. Both σ_myo,l and σ_myo,n were lower in MI than in control. Differences of σ_myo,t between control and MI were not significant. We found strong positive relationships of σ_myo,l with V_myo and V_gj, and a strong negative relationship with V_fibrosis. The relationships of σ_myo,n with these tissue features were similar but less pronounced. Our study provides quantitative insights into the intracellular conductivities in the normal and MI heart. We suggest that our study establishes a framework for the estimation of intracellular electrical conductivities of myocardium with various pathologies.

Mechanistic investigation of Ca2+ alternans in human heart failure and its modulation by fibroblasts

Background

Heart failure (HF) is characterized, among other factors, by a progressive loss of contractile function and by the formation of an arrhythmogenic substrate, both aspects partially related to intracellular Ca²⁺ cycling disorders. In failing hearts both electrophysiological and structural remodeling, including fibroblast proliferation, contribute to changes in Ca²⁺ handling which promote the appearance of Ca²⁺ alternans (Ca-alt). Ca-alt in turn give rise to repolarization alternans, which promote dispersion of repolarization and contribute to reentrant activity. The computational analysis of the incidence of Ca²⁺ and/or repolarization alternans under HF conditions in the presence of fibroblasts could provide a better understanding of the mechanisms leading to HF arrhythmias and contractile function disorders.

Methods and findings

The goal of the present study was to investigate in silico the mechanisms leading to the formation of Ca-alt in failing human ventricular myocytes and tissues with disperse fibroblast distributions. The contribution of ionic currents variability to alternans formation at the cellular level was analyzed and the results show that in normal ventricular tissue, altered Ca²⁺ dynamics lead to Ca-alt, which precede APD alternans and can be aggravated by the presence of fibroblasts. Electrophysiological remodeling of failing tissue alone is sufficient to develop alternans. The incidence of alternans is reduced when fibroblasts are present in failing tissue due to significantly depressed Ca²⁺ transients. The analysis of the underlying ionic mechanisms suggests that Ca-alt are driven by Ca²⁺-handling protein and Ca²⁺ cycling dysfunctions in the junctional sarcoplasmic reticulum and that their contribution to alternans occurrence depends on the cardiac remodeling conditions and on myocyte-fibroblast interactions.

Conclusion

It can thus be concluded that fibroblasts modulate the formation of Ca-alt in human ventricular tissue subjected to heart failure-related electrophysiological remodeling. Pharmacological therapies should thus consider the extent of both the electrophysiological and structural remodeling present in the failing heart.

Extended Bidomain Modeling of Defibrillation: Quantifying Virtual Electrode Strengths in Fibrotic Myocardium

Defibrillation is a well-established therapy for atrial and ventricular arrhythmia. Here, we shed light on defibrillation in the fibrotic heart. Using the extended bidomain model of electrical conduction in cardiac tissue, we assessed the influence of fibrosis on the strength of virtual electrodes caused by extracellular electrical current. We created one-dimensional models of rabbit ventricular tissue with a central patch of fibrosis. The fibrosis was incorporated by altering volume fractions for extracellular, myocyte and fibroblast domains. In our prior work, we calculated these volume fractions from microscopic images at the infarct border zone of rabbit hearts. An average and a large degree of fibrosis were modeled. We simulated defibrillation by application of an extracellular current for a short duration (5 ms). We explored the effects of myocyte-fibroblast coupling, intra-fibroblast conductivity and patch length on the strength of the virtual electrodes present at the borders of the normal and fibrotic tissue. We discriminated between effects on myocyte and fibroblast membranes at both borders of the patch. Similarly, we studied defibrillation in two-dimensional models of fibrotic tissue. Square and disk-like patches of fibrotic tissue were embedded in control tissue. We quantified the influence of the geometry and fibrosis composition on virtual electrode strength. We compared the results obtained with a square and disk shape of the fibrotic patch with results from the one-dimensional simulations. Both, one- and two-dimensional simulations indicate that extracellular current application causes virtual electrodes at boundaries of fibrotic patches. A higher degree of fibrosis and larger patch size were associated with an increased strength of the virtual electrodes. Also, patch geometry affected the strength of the virtual electrodes. Our simulations suggest that increased fibroblast-myocyte coupling and intra-fibroblast conductivity reduce virtual electrode strength. However, experimental data to constrain these modeling parameters are limited and thus pinpointing the magnitude of the reduction will require further understanding of electrical coupling of fibroblasts in native cardiac tissues. We propose that the findings from our computational studies are important for development of patient-specific protocols for internal defibrillators.

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.

Roles of subcellular Na+ channel distributions in the mechanism of cardiac conduction

The gap junction and voltage-gated Na(+) channel play an important role in the action potential propagation. The purpose of this study was to elucidate the roles of subcellular Na(+) channel distribution in action potential propagation. To achieve this, we constructed the myocardial strand model, which can calculate the current via intercellular cleft (electric-field mechanism) together with gap-junctional current (gap-junctional mechanism). We conducted simulations of action potential propagation in a myofiber model where cardiomyocytes were electrically coupled with gap junctions alone or with both the gap junctions and the electric field mechanism. Then we found that the action potential propagation was greatly affected by the subcellular distribution of Na(+) channels in the presence of the electric field mechanism. The presence of Na(+) channels in the lateral membrane was important to ensure the stability of propagation under conditions of reduced gap-junctional coupling. In the poorly coupled tissue with sufficient Na(+) channels in the lateral membrane, the slowing of action potential propagation resulted from the periodic and intermittent dysfunction of the electric field mechanism. The changes in the subcellular Na(+) channel distribution might be in part responsible for the homeostatic excitation propagation in the diseased heart.

Cell-to-cell electrical interactions during early and late repolarization

Cardiac electrical activity is significantly affected by variations in the conductance of gap junctions that connect myocytes to one another. To better understand how intrinsic (single cell) electrical activity is modulated by junctional conductance, we used a two-myocyte coupling system in which physically separate cells were electrically coupled via a variable resistance set by the investigator. This brief review summarizes our findings regarding: (1) the effect of the early phase of action potential repolarization (phase 1) and transient outward current (I(to)) on action potential conduction, and (2) the effect of coupling on the action potential plateau (late repolarization). We found that inhibition of I(to) markedly increased the ability of action potentials to propagate from cell-to-cell when junctional conductance was low. Electrically coupling two myocytes together also suppressed their beat-to-beat variability in action potential duration and contraction. Similarly, early afterdepolarizations (EADS) were readily suppressed by connecting a normal myocyte to one generating EADs. This high sensitivity of the plateau to variations in junctional interactions arises from the large increase in membrane resistance that occurs during this phase of the action potential.

Cell size and communication: role in structural and electrical development and remodeling of the heart

With the advent of new information about alterations of cardiac gap junctions in disease conditions associated with arrhythmias, there have been major advances in the genetic and metabolic manipulation of gap junctions. In contrast, in naturally occurring cardiac preparations, little is known about cell-to-cell transmission and the subcellular events of propagation or about structural mechanisms that may affect conduction events at this small size scale. Therefore, the aim of this article is to review results that produce the following unifying picture: changes in cardiac conduction due to remodeling cardiac morphology ultimately are limited to changes in three morphologic parameters: (1) cell geometry (size and shape), (2) gap junctions (distribution and conductivity), and (3) interstitial space (size and distribution). In this article, we consider changes in conduction that result from the remodeling of cell size and gap junction distribution that occurs with developmental ventricular hypertrophy from birth to maturity. We then go on to changes in longitudinal and transverse propagation in aging human atrial bundles that are produced by remodeling interstitial space due to deposition of collagenous septa. At present, experimental limitations in naturally occurring preparations prevent measurement of the conductance of individual gap junctional plaques, as well as the delays in conduction associated with cell-to-cell transmission. Therefore, we consider the development of mathematical electrical models based on documented cardiac microstructure to gain insight into the role of specific morphologic parameters in generating the changes in anisotropic propagation that we measured in the tissue preparations. A major antiarrhythmic implication of the results is that an "indirect" therapeutic target is interstitial collagen, because regulation of its deposition and turnover to prevent or alter microfibrosis can enhance side-to-side electrical coupling between small groups of cells in aging atrial bundles.

Gap junctions, homeostasis, and injury

Gap junctions (Gj) play an important role in the communication between cells of many tissues. They are composed of channels that permit the passage of ions and low molecular weight metabolites between adjacent cells, without exposure to the extracellular environment. These pathways are formed by the interaction between two hemichannels on the surface of opposing cells. These hemichannels are formed by the association of six identical subunits, named connexins (Cx), which are integral membrane proteins. Cell coupling via Gj is dependent on the specific pattern of Cx gene expression. This pattern of gene expression is altered during several pathological conditions resulting in changes of cell coupling. The regulation of Cx gene expression is affected at different levels from transcription to post translational processes during injury. In addition, Gj cellular communication is regulated by gating mechanisms. The alteration of Gj communication during injury could be rationalized by two opposite theories. One hypothesis proposes that the alteration of Gj communication attenuates the spread of toxic metabolites from the injured area to healthy organ regions. The alternative proposition is that a reduction of cellular communication reduces the loss of important cellular metabolisms, such as ATP and glucose.

Electrophysiologic properties and ventricular fibrillation in normal and myopathic hearts

This study tests the hypothesis that moderate myocardial dysfunction is associated with altered myocardial anisotropic properties and structurally altered ventricular fibrillation (VF). Mongrel dogs were randomized to either a control group or a group that was rapidly paced at 250 beats/min until the left ventricular ejection fraction was [Formula: see text] 40%. Changes in anisotropic properties and the electrical characteristics of VF associated with the development of moderate myocardial dysfunction were assessed by microminiature epicardial mapping studies. In vivo conduction, refractory periods, and repolarization times were prolonged in both longitudinal and transverse directions in myopathic animals versus controls. VF was different in myopathic versus control animals. There were significantly more conducted deflections during VF in normal hearts compared with myopathic hearts. Propagated deflection-to-deflection intervals during VF were significantly longer in myopathic hearts compared with controls (125.5 ± 49.06 versus 103.4 ± 32.9 ms, p = 0.009). There were no abnormalities in cell size, cell shape, or the number of intercellular gap junctions and there was no detectable change in the expression of the gap junction proteins Cx43 and Cx45. Moderate myocardial dysfunction is associated with significant electrophysiological abnormalities in the absence of changes in myocardial cell morphology or intercellular connections, suggesting a functional abnormality in cell-to-cell communication.Key words: cardiomyopathy, anisotropy, fibrillation, defibrillation.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Myocardial connexin43 expression in left ventricular hypertrophy resulting from aortic regurgitation

Intercellular conduction in the working myocardium of the mammalian heart is mediated by gap junctions composed of connexin43 or 45. Recently, it has been shown that myocardial connexin expression is malleable and may be altered with disease. To better understand myocardial conduction in left ventricular hypertrophy resulting from volume overload, we used indirect immunofluorescence microscopy to examine cardiac connexin43 expression in 10 New Zealand white rabbits with surgically induced aortic regurgitation (AR) and in 10 age-matched sham-operated controls. Animals were sacrificed at approximately 1 month or > or =2.5 years after operation. All AR animals developed eccentric hypertrophy; none evidenced heart failure. The heart-to-body weight ratios for the 1 month AR and control groups were 2.9+/-0.8 vs 1.8+/-0.2 g/kg (p < or = 0.01) while ratios for the > or =2.5 year AR and control groups were 2.4+/-0.3 vs 1.9+/-0.3 (p < or = 0.05). No significant differences in posterior wall thickness were found among any of the groups. Although the overall pattern of connexin43-like immunoreactivity was similar for all four groups, staining in the I month AR animals tended to be less than that of age-matched controls; staining was increased in the > or =2.5 year AR animals and was greater than control (p < 0.05), in which staining did not change with animal age. This disease duration-related increase differs from the long-term decrease in connexin43 expression associated with other forms of heart disease and suggests that alterations in connexin expression may play a role in the rhythm abnormalities commonly seen in AR.

Cellular and pathophysiological mechanisms of ventricular arrhythmias in acute ischemia and infarction

Ventricular arrhythmias in the setting of acute myocardial ischemia and infarction remain a serious health problem because of their sudden and unpredictable nature and their potentially grave results. Electrophysiological changes that may be responsible for these arrhythmias have been described in cardiac cells and in ischemic tissue. Experimental models have played a major role in elucidating the diversity of potential mechanisms for these arrhythmias. Increases in extracellular K+, the presence of toxic metabolites, and the accumulation of catecholamines in ischemic tissue all appear to have a role in arrhythmogenesis. The autonomic nervous system also appears to play a major role in these arrhythmias. With increased understanding of the pathophysiology underlying these arrhythmias, prevention can be enhanced and therapy can be better targeted.

Modulation of procainamide's effect on conduction by cellular uncoupling in perfused rabbit hearts

INTRODUCTION

How cell-to-cell electrical coupling influences an antiarrhythmic agent's effect on conduction is largely unknown. To investigate this, we evaluated the effects of procainamide on myocardial conduction at decreasing degrees of cell-to-cell electrical coupling induced by graded doses of heptanol.

METHODS AND RESULTS

Electrograms were recorded from 50 ventricular epicardial sites in a 1 cm x 0.5 cm area during pacing to produce conduction longitudinal or transverse to myocardial fiber orientation in Langendorff-perfused rabbit hearts. The effects of procainamide (15 mg/L) on conduction velocity were determined in the presence of increasing doses of heptanol (0.2, 0.5, and 1.0 mM). In addition, using standard microelectrode techniques in isolated superfused rabbit myocardium, intracellular potentials were recorded in the presence of 15 mg/L procainamide and heptanol (1.0 mM). In the absence of heptanol, procainamide slowed conduction velocity. In the presence of increasing doses of heptanol, procainamide's contribution to the depressant effect on conduction velocity was attenuated and reversed at the highest dose. The latter effect was preferentially seen for conduction longitudinal to myocardial fiber orientation. Heptanol had no effect on action potential amplitude or maximum rate of depolarization in the presence of procainamide.

CONCLUSIONS

Procainamide's effect on conduction velocity is influenced by the underlying degree of cell-to-cell electrical coupling. The present model should be useful in evaluating the relative ability of other pharmacologic agents to modulate conduction under conditions of changing cell coupling.

Myocardial gap junction organization in ischemia and infarction

Ischemia causes an increase in myocardial resistivity and a decrease in conduction velocity, thereby enhancing cardiac contractile dysfunction and arrhythmic tendency. Myocardial gap junctions, as principal determinants of conduction velocity, may, therefore, be expected to be deranged in ischemia. Despite a lack of consensus, attempts at correlating gap junction ultrastructural morphology with functional state have revealed the component connexons of gap junctions in freeze-fractured myocardium to be in multiple small hexagonal arrays, tending to become randomly distributed and compacted under uncoupling conditions. Further hypoxic uncoupling causes ultrastructural damage and a reduction in gap-junctional surface area. Immunohistochemical detection of connexin43 gap junctions in chronically ischemic non-infarcted human myocardium demonstrates a reduction in junctional surface area within a normal number of intercalated disks per myocyte, and with a normal distribution of junction sizes. In healed canine infarction there are smaller and fewer gap junctions in the fibrotic myocardium adjacent to infarcts, with reductions in overall gap-junctional content and the proportion of side-to-side vs. end-to-end intercellular connections. Immunohistochemical examination of intact human ventricular myocardium shows the myocytes immediately abutting healed infarcts to have connexin43 gap junctions spread longitudinally over the cell surfaces, and not in discrete transversely orientated intercalated disks as in normal myocardium. Early after canine infarction, and before fibrotic healing, the connexin43 gap junctions in myocytes abutting the infarct show disorganization similar to that described in healed human infarcts, suggesting that this disturbance is an early pathophysiological cellular response, and not simply due to later fibrotic distortion. Such changes in gap-junctional organization in myocardial ischemia and infarction may be implicated in the elusive link between subcellular structure, contractile dysfunction and arrhythmogenesis.

Quinidine pharmacodynamics in patients with arrhythmia: effects of left ventricular function

OBJECTIVES

This study was undertaken to determine whether quinidine pharmacodynamics are altered in the presence of left ventricular dysfunction.

BACKGROUND

Left ventricular function is an independent predictor of antiarrhythmic drug efficacy. However, the effects of left ventricular dysfunction on the pharmacodynamics of antiarrhythmic drugs have not been studied extensively.

METHODS

Signal-averaged electrocardiograms were obtained and quinidine plasma concentrations measured during 24-h quinidine washout in 22 patients.

RESULTS

Linear quinidine concentration-effect relations were observed for QRS and QT intervals corrected for heart rate. The slopes of the concentration-effect relation describing changes in the corrected QT (QTc) interval were significantly higher in the group with left ventricular ejection fraction > or = 0.35 ([mean +/- SD] 29.5 +/- 11.2 ms/micrograms per ml) than in the group with a low left ventricular ejection fraction (15.7 +/- 9.7 ms/micrograms per ml, p = 0.001). The QRS concentration-effect relations were not different in the two groups. A significant linear correlation was observed between the slopes of the concentration-effect relations describing changes in QTc intervals and left ventricular ejection fraction (r = 0.7, p < 0.001). Nineteen patients with inducible ventricular tachycardia underwent serial electrophysiologic studies for evaluation of quinidine efficacy. Ventricular tachycardia could not be induced during quinidine therapy in eight patients. The slopes of the quinidine concentration-effect relations for QTc intervals were significantly higher in quinidine responders than in nonresponders (p < 0.05).

CONCLUSIONS

The effects of quinidine on ventricular repolarization are linearly related to left ventricular ejection fraction. Quinidine concentration-effect relations describing ventricular repolarization are associated with antiarrhythmic efficacy in patients with ventricular tachycardia.

Pathophysiology of gap junctions in heart disease

Electrical coupling between cardiac muscle cells is mediated by specialized sites of plasma membrane interaction termed gap junctions. These junctions consist of clusters of membrane channels that directly link the cytoplasmic compartments of neighboring cells. Each gap-junctional channel consists of two connexons, one from each of the interacting plasma membranes, extending across the narrow extracellular gap. Connexons are constructed from connexins, a multigene family of conserved proteins. Different connexins confer specific electrophysiologic characteristics on the assembled channel protein. The major connexin of the mammalian heart is connexin43, although other types of connexins are also expressed, notably connexin40 in myocytes of the atrioventricular conduction system. Confocal laser scanning microscopy of anti-connexin43 immunolabeled samples reveals two major abnormalities in myocardial gap junctions in ischemic heart disease: loss of the usual ordered distribution of gap junctions at border zones adjacent to infarct scars, and reduction in the quantity of connexin43 gap junctions in myocardium distant from the infarct. These and other changes reported in myocardial gap-junctional communication pathways following infarction may result in heterogeneous anisotropic conduction and reduced conduction velocity, thereby forming a proarrhythmic substrate. Current evidence suggests that reduction in connexin43 content is a general pathogenetic feature of cardiac disease, and that changes in the expression levels of other connexin types may contribute to altered electrophysiologic function in the diseased heart.