Altered connexin43 expression produces arrhythmia substrate in heart failure

Myocardial impulse propagation is impaired in right ventricular tissue of Zucker diabetic fatty (ZDF) rats

Background

Diabetes increases the risk of cardiovascular complications including arrhythmias, but the underlying mechanisms remain to be established. Decreased conduction velocity (CV), which is an independent risk factor for re-entry arrhythmias, is present in models with streptozotocin (STZ) induced type 1 diabetes. Whether CV is also disturbed in models of type 2 diabetes is currently unknown.

Methods

We used Zucker Diabetic Fatty (ZDF) rats, as a model of type 2 diabetes, and their lean controls Zucker Diabetic Lean (ZDL) rats to investigate CV and its response to the anti-arrhythmic peptide analogue AAP10. Gap junction remodeling was examined by immunofluorescence and western blotting. Cardiac histomorphometry was examined by Masson`s Trichrome staining and intracellular lipid accumulation was analyzed by Bodipy staining.

Results

CV was significantly slower in ZDF rats (56±1.9 cm/s) compared to non-diabetic controls (ZDL, 66±1.6 cm/s), but AAP10 did not affect CV in either group. The total amount of Connexin43 (C×43) was identical between ZDF and ZDL rats, but the amount of lateralized C×43 was significantly increased in ZDF rats (42±12 %) compared to ZDL rats (30±8%), p<0.04. Judged by electrophoretic mobility, C×43 phosphorylation was unchanged between ZDF and ZDL rats. Also, no differences in cardiomyocyte size or histomorphometry including fibrosis were observed between groups, but the volume of intracellular lipid droplets was 4.2 times higher in ZDF compared to ZDL rats (p<0.01).

Conclusion

CV is reduced in type 2 diabetic ZDF rats. The CV disturbance may be partly explained by increased lateralization of C×43, but other factors are likely also involved. Our data indicates that lipotoxicity potentially may play a role in development of conduction disturbances and arrhythmias in type 2 diabetes.

Effects of valsartan on ventricular arrhythmia induced by programmed electrical stimulation in rats with myocardial infarction

The impact of angiotensin II receptor blockers (ARBs) on electrical remodelling after myocardial infarction (MI) remains unclear. The purpose of the present study was to evaluate the effect of valsartan on incidence of ventricular arrhythmia induced by programmed electrical stimulation (PES) and potential link to changes of myocardial connexins (Cx) 43 expression and distribution in MI rats. Fifty-nine rats were randomly divided into three groups: Sham (n = 20), MI (n = 20) and MI + Val (20 mg/kg/day per gavage, n = 19). After eight weeks, the incidence of PES-induced ventricular tachycardia (VT) and fibrillation (VF) was compared among groups. mRNA and protein expressions of Cx43, angiotensin II type 1 receptor (AT1R) in the LV border zone (BZ) and non-infarct zone (NIZ) were determined by real-time PCR and Western blot, respectively. Connexins 43 protein and collagen distribution were examined by immunohistochemistry in BZ and NIZ sections from MI hearts. Valsartan effectively improved the cardiac function, reduced the prolonged QTc (163.7 ± 3.7 msec. versus 177.8 ± 4.5 msec., P < 0.05) after MI and the incidence of VT or VF evoked by PES (21.1% versus 55%, P < 0.05). Angiotensin II type 1 receptor expression was significantly increased in BZ and NIZ sections after MI, which was down-regulated by valsartan. The mRNA and protein expressions of Cx43 in BZ were significantly reduced after MI and up-regulated by valsartan. Increased collagen deposition and reduced Cx43 expression in BZ after MI could be partly attenuated by Valsartan. Valsartan reduced the incidence of PES-induced ventricular arrhythmia, this effect was possibly through modulating the myocardial AT1R and Cx43 expression.

A high-throughput assay for connexin 43 (Cx43, GJA1) gap junctions using codon-optimized aequorin

Gap junctions (GJs) are intercellular channels which are composed of the connexin family of proteins that allow electrical and chemical communications and synchronization in tissue ensembles. Evidence suggests that pharmaceutical modulators of these channels may have therapeutic potential or carry undesired liability. In this report, we exogenously expressed human connexin 43 (Cx43, GJA1) and demonstrated functionality in a 96-well flow cytometry assay detecting intercellular transfer of the calcein dye. We have designed a 384-well high-throughput method for detecting the transfer of calcium between HeLa cells expressing Cx43. In this assay, donor cells coexpress Cx43 and the α1A adrenergic Gα-coupled receptor, while recipient cells coexpress Cx43 and the cytoplasmic version of the calcium-sensitive luminescent protein aequorin enhanced by codon optimization (cytoAeq). The two cell populations were mixed, dispensed to 384-well plates, and incubated for 3 h to allow the formation of GJs. Activation of α1A by epinephrine in donor cells led to dose-dependent calcium increases in recipient cells, which were detected by measuring the intensity of aequorin luminescence. The response was dependent on the expression of Cx43 and inhibited by the GJ blocker 18α-glycyrrhetinic acid, suggesting Cx43 GJ-mediated activity. In a parallel experiment with capsaicin and the TrpV1 ion channel in place of phenylephrine and α1A, a similar magnitude of difference in the maximal calcium response was detected in both donor and recipient cells, suggesting that calcium is likely the permeant ion through the GJ. This assay may pave the way for high-throughput screening of GJ modulators for drug discovery.

Hypothermia-induced spatially discordant action potential duration alternans and arrhythmogenesis in nonhibernating versus hibernating mammals

The heart of hibernating species is resistant to lethal ventricular fibrillation (VF) induced by hypothermia. Spatially discordant (SDA) cardiac alternans is a promising predictor of VF, yet its role in the mechanism of hypothermic arrhythmogenesis in both nonhibernating and hibernating mammals remains unclear. We optically mapped the posterior epicardial surface of Langendorff-perfused hearts of winter hibernating (WH, n = 13), interbout arousal (IBA; n = 4), and summer active (SA, n = 6) ground squirrels (GSs; Spermophilus undulatus) and rabbits (n = 10). Action potential duration (APD) and conduction velocity (CV) dynamic restitution and alternans were determined at 37 to 17°C. In all animals, hypothermia induced heterogeneous APD prolongation, enhanced APD dispersion, and slowed CV. In all groups, hypothermia promoted the formation of APD alternans, which was predominantly spatially concordant in GSs and SDA in rabbits (SD of APD dispersion: 4.2 ± 0.4% vs. 2.0 ± 0.3% at 37°C and 7.5 ± 1.1% vs. 3.4 ± 0.5% at 17°C, P < 0.001 for rabbits vs. the WH group, respectively). In rabbits, hypothermia significantly increased the magnitude of SDA, which enhanced the ventricular repolarization gradient, caused conduction delays (CV: 3.2 vs. 8.2 cm/s at 17°C in rabbits vs. the WH group), conduction block, and the onset of VF (0% at 37°C vs. 60% at 17°C, P < 0.01). In contrast, no arrhythmia was observed in GS hearts at any temperature. The amplitude of CV alternans was greater in rabbits (5.2 ± 0.4% versus 4.5 ± 0.3% at 37°C and 35.3 ± 4.2% vs. 14.9 ± 1.5% at 17°C in rabbits vs. the WH group, P < 0.001 at 17°C) and correlated with the amplitude of SDA. In conclusion, the mechanism underlying SDA formation during hypothermia is likely associated with CV alternans conditioned by an enhanced dispersion of repolarization. The factors of hibernating species resistance to SDA and VF seem to be the safe and dynamically stable conduction and the low dispersion of repolarization.

Effect of heterogeneities in the cellular microstructure on propagation of the cardiac action potential

Cardiac arrhythmias are initiated in regions that undergo cellular remodeling as a result of disease. Using a sub-cellular model of myocardium, we studied the mechanism of block caused by tissue microstructure remodeling: cell geometry [quantified as length/width (L/W) cell ratio] and cell-to-cell coupling (G(j)). Heterogeneities in cell L/W ratio and G ( j ) lead to block when excitability is reduced and the corresponding space constant λ (in the direction of propagation) increases by >40 %. Tissue architectures with elongated cells (i.e. large cell L/W ratios) that are better coupled (i.e. large G(j)) are less prone to block at sites of regional heterogeneities in cell geometry and/or cell coupling than tissue architectures consisting of cells with smaller L/W ratios and/or poorer coupling. Whether an increase in tissue anisotropic ratio (ANR) is arrhythmogenic or not depends on the cellular mechanism of the increase: ANR leads to an increased risk of block when G(j) decreases, but to a decreased risk of block when cell L/W ratio increases. Our findings are useful to understand the mechanisms of block in cardiac pathologies that result in tissue architecture remodeling.

Mechanisms of ventricular arrhythmias: a dynamical systems-based perspective

Defining the cellular electrophysiological mechanisms for ventricular tachyarrhythmias is difficult, given the wide array of potential mechanisms, ranging from abnormal automaticity to various types of reentry and kk activity. The degree of difficulty is increased further by the fact that any particular mechanism may be influenced by the evolving ionic and anatomic environments associated with many forms of heart disease. Consequently, static measures of a single electrophysiological characteristic are unlikely to be useful in establishing mechanisms. Rather, the dynamics of the electrophysiological triggers and substrates that predispose to arrhythmia development need to be considered. Moreover, the dynamics need to be considered in the context of a system, one that displays certain predictable behaviors, but also one that may contain seemingly stochastic elements. It also is essential to recognize that even the predictable behaviors of this complex nonlinear system are subject to small changes in the state of the system at any given time. Here we briefly review some of the short-, medium-, and long-term alterations of the electrophysiological substrate that accompany myocardial disease and their potential impact on the initiation and maintenance of ventricular arrhythmias. We also provide examples of cases in which small changes in the electrophysiological substrate can result in rather large differences in arrhythmia outcome. These results suggest that an interrogation of cardiac electrical dynamics is required to provide a meaningful assessment of the immediate risk for arrhythmia development and for evaluating the effects of putative antiarrhythmic interventions.

Improvement of cardiac function and reversal of gap junction remodeling by Neuregulin-1β in volume-overloaded rats with heart failure

OBJECTIVE

We performed experiments using Neuregulin-1β (NRG-1β) treatment to determine a mechanism for the protective role derived from its beneficial effects by remodeling gap junctions (GJs) during heart failure (HF).

METHODS

Rat models of HF were established by aortocaval fistula. Forty-eight rats were divided randomly into the HF (HF, n = 16), NRG-1β treatment (NRG, n = 16), and sham operation (S, n = 16) group. The rats in the NRG group were administered NRG-1β (10 µg/kg per day) for 7 days via the tail vein, whereas the other groups were injected with the same doses of saline. Twelve weeks after operation, Connexin 43 (Cx43) expression in single myocytes obtained from the left ventricle was determined by immunocytochemistry. Total protein was extracted from frozen left ventricular tissues for immunoblotting assay, and the ultrastructure of myocytes was observed by transmission electron microscopy.

RESULTS

Compared with the HF group, the cardiac function of rats in the NRG group was markedly improved, irregular distribution and deceased Cx43 expression were relieved. The ultrastructure of myocytes was seriously damaged in HF rats, and NRG-1β reduced these pathological damages.

CONCLUSIONS

Short-term NRG-1β treatment can rescue pump failure in experimental models of volume overload-induced HF, which is related to the recovery of GJs structure and the improvement of Cx43 expression.

Sex differences in cardiomyocyte connexin43 expression

Decreases in cardiac connexin43 (Cx43) play a critical role in abnormal cell-to-cell communication and have been linked to the resistance of the female heart to arrhythmias. We therefore hypothesized that Cx43 expression would be greater in female cardiomyocytes than in male cardiomyocytes under pathologic conditions. Adult ventricular myocytes were isolated from male and female rats and treated with phenylephrine (PE), a well-established pathologic stimulus. Cx43 gene and protein expression was determined. The expression of micro-RNA-1 (miR-1), a micro-RNA known to control Cx43 protein expression in cardiomyocytes, was also determined. Cx43 mRNA and protein levels were significantly higher in the female cardiomyocytes than in the male cardiomyocytes (mRNA: 1.4-fold; Protein: 5-fold, both P < 0.05) under both basal and pathologic conditions. PE treatment increased Cx43 expression only in female cardiomyocytes. Cx43 phosphorylation, a marker of preserved Cx43 function, was also higher (P < 0.05), and The expression of miR-1 was lower (P < 0.05) in the female cardiomyocytes after PE treatment. The expression of miR-1 was unchanged by PE treatment in male cardiomyocytes. Thus, a sex difference in miR-1 may be responsible for the sex difference in Cx43 expression in cardiomyocytes under pathologic conditions. Taken together, our results demonstrate a sex difference in Cx43 expression and site-specific phosphorylation that favors cardioprotection in female cardiomyocytes.

Altered spatial calcium regulation enhances electrical heterogeneity in the failing canine left ventricle: implications for electrical instability

Myocytes across the left ventricular (LV) wall of the mammalian heart are known to exhibit heterogeneity of electrophysiological properties; however, the transmural variation of cellular electrophysiology and Ca2+ homeostasis in the failing LV is incompletely understood. We studied action potentials (APs), the L-type calcium (Ca2+) current ( ICa,L), and intracellular Ca2+ transients ([Ca2+]i) of subendocardial (Endo), midmyocardial (Mid), and subepicardial (Epi) tissue layers in the canine normal and tachycardia pacing-induced failing left ventricles. Heart failure (HF) was associated with significant prolongation of the AP duration in Mid myocytes. There were no differences in ICa,L density in normal Endo, Mid, and Epi myocytes, whereas in the failing heart, ICa,L density was downregulated by 45% and 26% (at +10 mV) in Endo and Mid myocytes, respectively. The rates of sarcoplasmic reticulum (SR) Ca2+ release and decay of the [Ca2+]i were slowed, and the amplitude of the [Ca2+]i was depressed in Endo and Epi myocytes isolated from failing, compared with normal, hearts. Experiments in sodium (Na+)-free solutions showed that Epi and Mid myocytes of the failing ventricle exhibit a greater reliance on the Na+-Ca2+ exchanger to remove cytosolic Ca2+ than myocytes isolated from normal hearts. Simulation studies in Endo, Mid, and Epi canine myocytes demonstrate the importance of L-type current density and SR Ca2+ uptake in modulating the potentially arrhythmogenic repolarization in HF. In conclusion, these results demonstrate that spatially heterogeneous decreases in ICa,L and defective cytosolic Ca2+ removal contribute to the altered [Ca2+]i and AP profiles across the canine failing LV. These distinct electrophysiological features in myocytes from a failing heart contribute to a characteristic electrogram arising from increased dispersion of refractoriness across the LV, which may result in significant arrhythmogenic sequellae.

The involvement of gap junctions in the delayed phase of the protection induced by cardiac pacing in dogs

The present study has examined the role of GJ (gap junctions) in the delayed anti-arrhythmic effect of cardiac pacing, with particular reference to the time-course changes in Cx43 (connexin43) expression both after pacing (4×5 min, at a rate of 240 beats/min) and 24 h later, when the dogs were subjected to a 25 min occlusion and reperfusion of the LAD (left anterior descending coronary artery). Compared with the SP (sham-paced) controls (n=20), in dogs paced 24 h previously (n=16) there were reductions in arrhythmia severity [e.g. number of VPB (ventricular premature beats) during occlusion 294±78 compared with 63±25; survival from the combined ischaemia/reperfusion insult 20% compared with 78%], and in other ischaemic changes [epicardial ST-segment, TAT (total activation time) and tissue impedance]. Pacing also prevented the ischaemia-induced structural impairment of the intercalated discs, and preserved GJ permeability and Cx43 phosphorylation, without modifying Cx43 protein content. Following cardiac pacing the membrane and total Cx43 protein contents were unchanged up to 6 h, but were significantly reduced 12 h later (preceded by a down-regulation of Cx43 mRNA at 6 h), and returned to normal by 24 h. Interestingly, dogs that were subjected to ischaemia 12 h after cardiac pacing showed increased arrhythmia generation. We conclude that cardiac pacing results in time-dependent changes in Cx43 expression, which may alter GJ function and influence arrhythmia generation during a subsequent ischaemia/reperfusion insult. This effect is manifested in protection 24 h after pacing, but of potential clinical interest is the finding that there is a time interval after pacing during which an ischaemic event may generate severe ventricular arrhythmias.

Conduction remodeling in human end-stage nonischemic left ventricular cardiomyopathy

BACKGROUND

Several arrhythmogenic mechanisms have been inferred from animal heart failure models. However, the translation of these hypotheses is difficult because of the lack of functional human data. We aimed to investigate the electrophysiological substrate for arrhythmia in human end-stage nonischemic cardiomyopathy.

METHODS AND RESULTS

We optically mapped the coronary-perfused left ventricular wedge preparations from human hearts with end-stage nonischemic cardiomyopathy (heart failure, n=10) and nonfailing hearts (NF, n=10). Molecular remodeling was studied with immunostaining, Western blotting, and histological analyses. Heart failure produced heterogeneous prolongation of action potential duration resulting in the decrease of transmural action potential duration dispersion (64 ± 12 ms versus 129 ± 15 ms in NF, P<0.005). In the failing hearts, transmural activation was significantly slowed from the endocardium (39 ± 3 cm/s versus 49 ± 2 cm/s in NF, P=0.008) to the epicardium (28 ± 3 cm/s versus 40 ± 2 cm/s in NF, P=0.008). Conduction slowing was likely due to connexin 43 (Cx43) downregulation, decreased colocalization of Cx43 with N-cadherin (40 ± 2% versus 52 ± 5% in NF, P=0.02), and an altered distribution of phosphorylated Cx43 isoforms by the upregulation of the dephosphorylated Cx43 in both the subendocardium and subepicardium layers. Failing hearts further demonstrated spatially discordant conduction velocity alternans which resulted in nonuniform propagation discontinuities and wave breaks conditioned by strands of increased interstitial fibrosis (fibrous tissue content in heart failure 16.4 ± 7.7 versus 9.9 ± 1.4% in NF, P=0.02).

CONCLUSIONS

Conduction disorder resulting from the anisotropic downregulation of Cx43 expression, the reduction of Cx43 phosphorylation, and increased fibrosis is likely to be a critical component of arrhythmogenic substrate in patients with nonischemic cardiomyopathy.

Genetic testing of candidate genes in arrhythmogenic right ventricular cardiomyopathy/dysplasia

Arrhythmogenic right ventricular cardiomyopathy/dysplasia (ARVC/D) is a rare cardiac genetic disease characterized by the presence of structural alterations in the right ventricle which may cause ventricular arrhythmias and may induce sudden cardiac death. ARVC/D has been associated with mutations in genes encoding myocyte adhesion proteins. However, only 30%-50% of patients have mutations in these genes. Genetic testing is useful in obtaining a diagnosis, particularly in individuals who do not completely fulfill clinical criteria, thereby also enabling the undertaking of preventive strategies in family members. The main goal of this study was to identify mutations in candidate genes associated with intercalate disks that could be potentially involved in ARVC/D pathogenesis. We analyze a cohort of 14 Spanish unrelated patients clinically diagnosed with ARVC/D without any genetic alteration in all previously known responsible genes. Thus, a genetic screening has been performed in 7 additional potential candidate genes (ACTC1 -actin alpha cardiac muscle 1-, CDHN -cadherin 2 type 1 or N-cadherin-, CTNNA1 -catenin alpha 1-, Cx43 or GJA1 -gap junction protein alpha 1-, MVCL -Metavinculin-, MYL2 -myosin light chain 2- and MYL3 -myosin light chain 3-) by direct sequencing analysis. Our genetic analysis did not identify any disease-causing mutation. Thirty single nucleotides polymorphisms were found, six of them novel. In conclusion, our ARVC/D Spanish cohort has not shown any mutations in the analyzed candidate genes despite their involvement in formation and maintenance of the intercalated disk.

Myofibroblasts cause heterogeneous Cx43 reduction and are unlikely to be coupled to myocytes in the healing canine infarct

Following myocardial infarction (MI) inflammatory responses transform cardiac fibroblasts to myofibroblasts, which in vitro studies show form heterocellular gap junctions with cardiac myocytes via Connexin43 (Cx43). The ability to form heterocellular junctions in the intact heart and the impact of these junctions on propagation is unclear. We used a canine model of MI and characterized the distribution and quantity of myofibroblasts in surviving epicardial cells [epicardial border zone (EBZ)]. We found a significant increase in myofibroblasts within the EBZ and no gap junction plaques between myofibroblasts and myocytes. Because myofibroblasts produce IL-1β, which downregulates Cx43, we asked whether myofibroblast proliferation causes loss of Cx43 near myofibroblast clusters. In vitro studies showed that IL-1β caused loss of Cx43 and reduced coupling. Western blot showed a significant increase of IL-1β in the EBZ, and immunohistochemistry showed a loss of Cx43 in regions of myofibroblasts in the intact heart. Additionally, dye studies in intact heart showed no coupling between myocytes and myofibroblasts. To quantify the effect of myofibroblasts on propagation we used a two-dimensional subcellular computer model of the EBZ, which showed that heterogeneities in myofibroblast density lead to conduction abnormalities. In conclusion, an increase of myofibroblasts in the infarcted heart causes heterogeneous Cx43 levels, possibly as a result of the release of IL-1β and decreased cell-cell communication, which leads to conduction abnormalities following MI.

The connexin43 carboxyl terminus and cardiac gap junction organization

The precise spatial order of gap junctions at intercalated disks in adult ventricular myocardium is thought vital for maintaining cardiac synchrony. Breakdown or remodeling of this order is a hallmark of arrhythmic disease of the heart. The principal component of gap junction channels between ventricular cardiomyocytes is connexin43 (Cx43). Protein-protein interactions and modifications of the carboxyl-terminus of Cx43 are key determinants of gap junction function, size, distribution and organization during normal development and in disease processes. Here, we review data on the role of proteins interacting with the Cx43 carboxyl-terminus in the regulation of cardiac gap junction organization, with particular emphasis on Zonula Occludens-1. The rapid progress in this area suggests that in coming years we are likely to develop a fuller understanding of the molecular mechanisms causing pathologic remodeling of gap junctions. With these advances come the promise of novel approach to the treatment of arrhythmia and the prevention of sudden cardiac death. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Functional consequences of abnormal Cx43 expression in the heart

The major gap junction protein expressed in the heart, connexin43 (Cx43), is highly remodeled in the diseased heart. Usually, Cx43 is down-regulated and heterogeneously redistributed to the lateral sides of cardiomyocytes. Reverse remodeling of the impaired Cx43 expression could restore normal cardiac function and normalize electrical stability. In this review, the reduced and heterogeneous Cx43 expression in the heart will be addressed in hypertrophic, dilated and ischemic cardiomyopathy together with its functional consequences of conduction velocity slowing, dispersed impulse conduction, its interaction with fibrosis and propensity to generate arrhythmias. Finally, different therapies are discussed. Treatments aimed to improve the Cx43 expression levels show new potentially anti-arrhythmic therapies during heart failure, but those in the context of acute ischemia can be anti-arrhythmogenic at the cost of larger infarct sizes. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Intramural activation during early human ventricular fibrillation

BACKGROUND

We investigated patterns of intramural activation in early human ventricular fibrillation (VF) and hypothesized that intramural reentry colocalizes to sites with increased intramural fibrosis.

METHODS AND RESULTS

Thirteen human Langendorff hearts were used for this study. Twenty-five plunge needles (4 unipoles/needle) were used to map 100 intramural sites. For the global mapping component, 11 20-s episodes of early VF were studied in 6 hearts. Simultaneous activation of all 4 electrodes was the most common pattern observed in 48.7% of needles, followed by an endocardial-to-epicardial activation pattern (9.8% of needles) and epicardial-to-endocardial activation pattern (5.5% of needles); 19.3% of needles had nonuniform multidirectional patterns. In 2 orthogonal planes, 1 parallel and 1 perpendicular to the epicardium and endocardium, reentry was detected in 14.3% of beats at any 1 level, and 5.8% of these were transmural. Simultaneous mapping of the epicardium and endocardium in 5 hearts detected concurrently rotating rotors with similar chirality and cycle length, suggesting the presence of transmural scroll waves (n=6), which was confirmed by high-resolution fixed-space mapping in 2 of those hearts plus 1 additional heart. Transmural optical mapping in 1 additional heart confirmed simultaneous epicardial and endocardial activation. Histopathology revealed greater fibrosis at sites of reentry compared to areas without (53.3±11.9% versus 27.5±2.4%, P=0.02).

CONCLUSIONS

Intramural activation patterns suggest that early human VF does not organize as multiple reentrant wavefronts but is best explained by transmural scroll wave activation. Intramural reentry localizes to regions of greater intramural fibrosis.

Enhanced activation of p21-activated kinase 1 in heart failure contributes to dephosphorylation of connexin 43

AIMS

We previously showed decreased cellular coupling and dephosphorylation of the gap junctional protein connexin 43 (Cx43) in left ventricular (LV) myocytes from an arrhythmogenic rabbit model of non-ischaemic heart failure (HF) that was associated with a 2.5-fold increase in the amount of protein phosphatase type 2A (PP2A) co-localized with Cx43. Here, we further explore the molecular mechanisms of enhanced dephosphorylation of Cx43 in HF. p21-activated kinase 1 (PAK1) is a serine-threonine protein kinase that has been shown to activate PP2A.

METHODS AND RESULTS

We found that total PAK1 and activated PAK1 (PAK1-P(Thr423)) were both increased in HF rabbit LV (vs. controls). PAK1 co-immunoprecipitated (co-IP'd) with Cx43 protein and, with HF, co-IP'd PAK1 and PAK1-P(Thr423) were increased. With failing human LV, PAK1 total protein and PAK1-P(Thr423) were also increased globally and locally (co-IP'd with Cx43). To further explore the role of PAK1 in modulating Cx43 dephosphorylation and intercellular coupling, we overexpressed active PAK1 in isolated LV myocytes from control rabbits and in HEK293 cells with genetically modified overexpression of Cx43 (HEK293-Cx43). PAK1 overexpression in both rabbit myocytes and HEK293-Cx43 cells significantly increased PP2A activity (globally and at the level of Cx43), increased dephosphorylated Cx43, and markedly reduced intercellular dye coupling. These effects were attenuated with PP2A inhibition using okadaic acid (10 nM).

CONCLUSIONS

PAK1 and PP2A are integral components of a macromolecular complex with cardiac Cx43, and increased activation of associated PAK1 can contribute to enhanced Cx43 dephosphorylation and impaired intercellular coupling that may underlie slow conduction in HF.

Cardiac electrical remodeling in health and disease

Electrical remodeling of the heart takes place in response to both functional (altered electrical activation) and structural (including heart failure and myocardial infarction) stressors. These electrophysiological changes produce a substrate that is prone to malignant ventricular arrhythmias. Understanding the cellular and molecular mechanisms of electrical remodeling is important in elucidating potential therapeutic targets designed to alter maladaptive electrical remodeling. For example, altered patterns of electrical activation lead primarily to electrical remodeling, without significant structural remodeling. By contrast, secondary remodeling arises in response to a structural insult. In this article we review cardiac electrical remodeling (predominantly in the ventricle) with an emphasis on the mechanisms causing these adaptations. These mechanisms suggest novel therapeutic targets for the management or prevention of the most devastating manifestation of heart disease, sudden cardiac death (SCD).

Propofol and arrhythmias: two sides of the coin

The hypnotic agent propofol is effective for the induction and maintenance of anesthesia. However, recent studies have shown that propofol administration is related to arrhythmias. Propofol displays both pro- and anti-arrhythmic effects in a concentration-dependent manner. Data indicate that propofol can convert supraventricular tachycardia and ventricular tachycardia and may inhibit the conduction system of the heart. The mechanism of the cardiac effects remains poorly defined and may involve ion channels, the autonomic nervous system and cardiac gap junctions. Specifically, sodium, calcium and potassium currents in cardiac cells are suppressed by clinically relevant concentrations of propofol. Propofol shortens the action potential duration (APD) but lessens the ischemia-induced decrease in the APD. Furthermore, propofol suppresses both sympathetic and parasympathetic tone and preserves gap junctions during ischemia. All of these effects cumulatively contribute to the antiarrhythmic and proarrhythmic properties of propofol.

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.

Enhanced PKCε mediated phosphorylation of connexin43 at serine 368 by a carboxyl-terminal mimetic peptide is dependent on injury

The gap junction (GJ) protein connexin (Cx43) is important for organized action potential propagation between mammalian cardiomyocytes. Disruption of the highly ordered distribution of Cx43 GJs is characteristic of cardiac tissue after ischemic injury. We recently demonstrated that epicardial administration of a peptide mimetic of the Cx43 carboxyl-terminus reduced pathologic remodeling of Cx43 GJs and protected against induced arrhythmias following ventricular injury. Treatment of injuries with the carboxyl-terminal peptide was associated with an increase in phosphorylation at serine 368 of the Cx43 carboxyl-terminus. Here, we report that Cx43 peptide treatment of uninjured hearts does not prompt a similar increase in phosphorylation. Moreover, we show that peptide treatment of undisturbed cultured HeLa cells expressing a Cx43 construct also exhibit no changes in Cx43 phosphorylation at serine 368. However, in parallel with the results in vivo, a trend of increasing phosphorylation at serine 368 was observed in Cx43-expressing HeLa cells following scratch wounding of cultured monolayers. These results suggest that peptide-enhanced phosphorylation of the Cx43 carboxyl-terminus is dependent on injury-mediated cellular responses.

Spontaneous high-frequency action potential

Action potential, which is the foundation of physiology and electrophysiology, is most vital in physiological research. This work starts by detecting cardiac electrophysiology (tachyarrhythmias), combined with all spontaneous discharge phenomena in vivo such as wound currents and spontaneous neuropathic pain, elaborates from generation, induction, initiation, to all of the features of spontaneous high-frequency action potential-SSL action potential mechanism, i.e., connecting-end hyperpolarization initiates spontaneous depolarization and action potential in somatic membrane. This work resolves the conundrums of in vivo spontaneous discharge in tachyarrhythmias, wounds, denervation supersensitivity, neurogenic pain (hyperalgesia and allodynia), epileptic discharge and diabetic pain in pathophysiological and clinical researches that have puzzled people for a hundred years.

Effect of resistive barrier location on the relationship between T-wave alternans and cellular repolarization alternans: a 1-D modeling study

Structural inhomogeneities in cardiac tissue have been associated with increased cellular repolarization alternans in animal experiments and increased T-wave alternans (TWA) in clinical studies. However, the effect of structural inhomogeneities on the relationship between cellular alternans and TWA has not been thoroughly investigated. We created 1-dimensional multicellular fiber models with and without a resistive barrier in various fiber regions and paced each model to induce cellular alternans. The models demonstrate that a resistive barrier in one fiber region substantially alters cellular repolarization alternans throughout the fiber. A midmyocardial or subepicardial barrier increase both TWA amplitude and maximum cellular alternans magnitude, relative to a fiber without a barrier. In addition, a direct relationship exists between TWA amplitude and maximum cellular alternans magnitude, which was highly dependent on barrier location. These results suggest that the position of a structural inhomogeneity within the myocardium may have substantial effects on dynamic repolarization instability and arrhythmogenicity.

The Renin-Angiotensin system mediates the effects of stretch on conduction velocity, connexin43 expression, and redistribution in intact ventricle

Effect of Stretch on Conduction and Cx43.

INTRODUCTION

In disease states such as heart failure, myocardial infarction, and hypertrophy, changes in the expression and location of Connexin43 (Cx43) occur (Cx43 remodeling), and may predispose to arrhythmias. Stretch may be an important stimulus to Cx43 remodeling; however, it has only been investigated in neonatal cell cultures, which have different physiological properties than adult myocytes. We hypothesized that localized stretch in vivo causes Cx43 remodeling, with associated changes in conduction, mediated by the renin-angiotensin system (RAS).

METHODS AND RESULTS

In an open-chest canine model, a device was used to stretch part of the right ventricle (RV) by 22% for 6 hours. Activation mapping using a 312-electrode array was performed before and after stretch. Regional stretch did not change longitudinal conduction velocity (post-stretch vs baseline: 51.5 ± 5.2 vs 55.3 ± 8.1 cm/s, P = 0.24, n = 11), but significantly reduced transverse conduction velocity (28.7 ± 2.5 vs 35.4 ± 5.4 cm/s, P < 0.01). It also reduced total Cx43 expression, by Western blotting, compared with nonstretched RV of the same animal (86.1 ± 32.2 vs 100 ± 19.4%, P < 0.02, n = 11). Cx43 labeling redistributed to the lateral cell borders. Stretch caused a small but significant increase in the proportion of the dephosphorylated form of Cx43 (stretch 9.95 ± 1.4% vs control 8.74 ± 1.2%, P < 0.05). Olmesartan, an angiotensin II blocker, prevented the stretch-induced changes in Cx43 levels, localization, and conduction.

CONCLUSION

Myocardial stretch in vivo has opposite effects to that in neonatal myocytes in vitro. Stretch in vivo causes conduction changes associated with Cx43 remodeling that are likely caused by local stretch-induced activation of the RAS.

Deciphering Arrhythmia Mechanisms - Tools of the Trade

Pathophysiological remodeling of cardiac function occurs at multiple levels, spanning the spectrum from molecular and sub-cellular changes to those occurring at the organ-system levels. Of key importance to arrhythmias are changes in electrophysiological and calcium handling properties at the tissue level. In this review, we discuss how high-resolution optical action potential and calcium transient imaging has advanced our understanding of basic arrhythmia mechanisms associated with multiple cardiovascular disorders, including the long QT syndrome, heart failure, and ischemia-reperfusion injury. We focus on the role of repolarization gradients (section 1) and calcium mediated triggers (section 2) in the initiation and maintenance of complex arrhythmias in these settings.

Increased Cell-Cell Coupling Increases Infarct Size and Does not Decrease Incidence of Ventricular Tachycardia in Mice

Increasing connexin43 (Cx43) gap junctional conductance as a means to improve cardiac conduction has been proposed as a novel antiarrhythmic modality. Yet, transmission of molecules via gap junctions may be associated with increased infarct size. To determine whether maintaining open gap junction channels impacts on infarct size and induction of ventricular tachycardia (VT) following coronary occlusion, we expressed the pH- and voltage-independent connexin isoform connexin32 (Cx32) in ventricle and confirmed Cx32 expression. Wild-type (WT) mice injected with adenovirus-Cx32 (Cx32inj) were examined following coronary occlusion to determine infarct size and inducibility of VT. There was an increased infarct size in Cx32inj hearts as compared to WT (WT 22.9 ± 4%; Cx32inj 44.3 ± 5%; p < 0.05). Programmed electrical stimulation showed no difference in VT inducibility in WT and Cx32inj mice (VT was reproducibly inducible in 55% of shams and 50% of Cx32inj mice (p > 0.05). Following coronary occlusion, improving cell–cell communication increased infarct size, and conferred no antiarrhythmic benefit.

A peptide mimetic of the connexin43 carboxyl terminus reduces gap junction remodeling and induced arrhythmia following ventricular injury

RATIONALE

Remodeling of connexin (Cx)43 gap junctions (GJs) is linked to ventricular arrhythmia.

OBJECTIVES

A peptide mimetic of the carboxyl terminal (CT) of Cx43, incorporating a postsynaptic density-95/disks-large/ZO-1 (PDZ)-binding domain, reduces Cx43/ZO-1 interaction and GJ size remodeling in vitro. Here, we determined: (1) whether the Cx43-CT mimetic αCT1 altered GJ remodeling following left ventricular (LV) injury in vivo; (2) whether αCT1 affected arrhythmic propensity; and (3) the mechanism of αCT1 effects on arrhythmogenicity and GJ remodeling.

METHODS AND RESULTS

A cryoinjury model generating a reproducible wound and injury border zone (IBZ) in the LV was used. Adherent methylcellulose patches formulated to locally release αCT1 (< 48 hours) were placed on cryoinjuries. Relative to controls, Cx43/ZO-1 colocalization in the IBZ was reduced by αCT1 by 24 hours after injury. Programmed electric stimulation ex vivo and optical mapping of voltage transients indicated that peptide-treated hearts showed reduced inducible arrhythmias and increased ventricular depolarization rates 7 to 9 days after injury. At 24 hours and 1 week after injury, αCT1-treated hearts maintained Cx43 in intercalated disks (IDs) in the IBZ, whereas by 1 week after injury, controls demonstrated Cx43 remodeling from IDs to lateralized distributions. Over a postinjury time course of 1 week, αCT1-treated IBZs showed increased Cx43 phosphorylation at serine368 (Cx43-pS368) relative to control tissues. In biochemical assays, αCT1 promoted phosphorylation of serine368 by protein kinase (PK)C-ε in a dose-dependent manner that was modulated by, but did not require ZO-1 PDZ2.

CONCLUSIONS

αCT1 increases Cx43-pS368 in vitro in a PKC-ε-dependent manner and in the IBZ in vivo acutely following ventricular injury. αCT1-mediated increase in Cx43-pS368 phosphorylation may contribute to reductions in inducible-arrhythmia following injury.

WITHDRAWN: Inhibition of NADPH oxidase up-regulates connexin 43 and ameliorates electrical remodeling in rabbits with heart failure

The Publisher regrets that this article is an accidental duplication of an article that has already been published, http://dx.doi.org/10.1016/j.biomag.2010.08.001. The duplicate article has therefore been withdrawn.

Mesenchymal stem cell injection ameliorates the inducibility of ventricular arrhythmias after myocardial infarction in rats

BACKGROUND

Mesenchymal stem cell transplantation is a promising new therapy to improve cardiac function after myocardial infarction (MI). The electrophysiological consequences of MSC implantation has not been systematically studied.

METHODS

We investigated the electrophysiological and arrhythmogenic effects of mesenchymal stem cells (MSCs) therapy in experimental infarction model. Rats were subjected to MI operation by LAD ligation and randomly allocated to receive intramyocardially injection PBS (MI-PBS) or 5 × 10(5) EGFP labeled MSCs (MI-MSCs). Electrophysiological study, histological examination, and western blotting were performed 2 weeks after cell transplantation.

RESULTS

Programmed electrical stimulation (PES) showed a significant reduced inducible ventricular tachycardias (VTs), raised ventricular fibrillation threshold (VFT) and prolonged ventricular effective refractory period (VERP) in MSC-treated rats compared to PBS-treated animals. MSC implantation led to markedly longer action potential duration (APD) and shorter activation time (AT) in infarcted border zone (IBZ) of left ventricular epicardium compared with PBS-treated hearts. Histological study revealed that fibrotic area and collagen deposition in infarcted region were significantly lower in MI-MSC group than in MI-PBS group. Abnormal alterations of Connexin 43 including reduction and lateralization were significantly attenuated by MSC treatment.

CONCLUSIONS

This study provide strong evidence that MSC implantation ameliorates interstitial fibrosis and the remodeling of gap junction, attenuates focal heterogeneity of reporlarization and conduction and reduces vulnerability to VTs. The results suggest that MSC transplantation might emerge as a new preventive strategy against VAs besides improving cardiac performance in ischemic heart disease.

Spatiotemporal induction of matrix metalloproteinase-9 transcription after discrete myocardial injury

Radiofrequency (RF) ablation of the myocardium causes discrete sites of injury. RF scars can expand, altering the extracellular matrix (ECM) structure and the continuity of the electrical syncytium of the adjacent myocardium. Matrix metalloproteinases (MMPs), such as MMP-9, contribute to ECM remodeling. However, whether and to what degree transcriptional induction of MMP-9 occurs after myocardial RF injury and the association with electrical conduction patterns after RF injury remains unexplored. This study examined MMP-9 gene promoter (M9PROM) activation after myocardial RF injury using mice in which the M9PROM was fused to a β-galactosidase (β-gal) reporter. RF lesions (0.5-mm probe, 80°C, 30 s) were created on the left ventricular (LV) epicardium of M9PROM mice (n=62) and terminally studied at 1 h, 1 d, 3 d, 7 d, 14 d, and 28 d after RF injury. M9PROM activation was localized through β-gal staining. The RF scar area and the area of β-gal staining were measured and normalized to LV area (planimetry). RF scar size increased from 1 h post-RF-injury values by 7 d and remained higher at 28 d. M9PROM activation became evident at 3 d and peaked at 7 d. Electrical conduction was measured (potentiometric dye mapping) at 7 d after RF injury. Heterogeneities in action potentials and electrical impulse propagation coincident with M9PROM activation were observed after RF injury. For example, conduction proximal to the RF site was slower than that in the remote myocardium (0.15±0.02 vs. 0.83±0.08 mm/ms, P<0.05). Thus, a unique spatiotemporal pattern of MMP-9 transcriptional activation occurred after discrete myocardial injury, which was associated with the development of electrical heterogeneity. Therefore, these findings suggest that changes in a key determinant of extracellular matrix remodeling, in addition to changes in myocardial structure, can contribute to arrhythmogenesis around the region of myocardial injury.

Effect of cell geometry on conduction velocity in a subcellular model of myocardium

We have studied the effect of cell geometry on propagation velocity of the cardiac impulse using a subcellular computer model of myocardium. Variation of cell size has only small effects on longitudinal and transverse conduction velocities, when the ratio of cell length/width is constant, for cell sizes (length x width) between (60 microm x 20 microm) and (120 microm x 40 microm). The results were not dependent on gap-junction conductance (range 0.25-1 microS), gap-junction distribution, or the specific tissue architecture. Longitudinal conduction velocity increased with the cell length/width ratio and transverse velocity decreased. The cell length/width ratio was a good estimator of the anisotropic ratio. In conclusion, cell length/width ratio is more important than cell size in determining conduction velocity.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Connexin-43 redistribution and gap junction activation during forced restraint protects against sudden arrhythmic death in rats

BACKGROUND

Connexin-43 (Cx43) expression is reduced or redistributed in heart disease. Restraint or other emotional stressors might cause sudden death in persons with such diseases, but the mechanism of death and its connection to Cx43 during restraint remain unknown. Whether Cx43 distribution or gap junction (GJ) function during restraint is involved in sudden arrhythmic death in rats is addressed in this study.

METHODS AND RESULTS

Male Sprague-Dawley rats underwent immobilization (IMO), and individual electrocardiographic responses were monitored by telemetry. Heart sections were used to examine ventricular Cx43 distribution, and GJ intercellular communication (GJIC) activity was assessed using a dye-transfer assay. IMO induced the translocation of Cx43 into to the GJ-rich fraction, with a peak at 60 min. During IMO, Cx43 immunofluorescence was enhanced at intercalated discs, in association with GJIC activation, and premature ventricular contractions (PVCs) increased. In the presence of the GJ inhibitor, carbenoxolone (0.25 mg.kg(-1).h(-1)), IMO induced lethal ventricular tachycardia or fibrillation in 21.7% of rats, in association with QRS prolongation and increased PVCs.

CONCLUSIONS

IMO causes Cx43 translocation to intercalated discs, thereby reducing vulnerability to lethal arrhythmias via enhancing GJ coupling.

Mechanisms of gap junction traffic in health and disease

Gap junctions (GJs) allow direct communication between cells. In the heart, GJs mediate the electrical coupling of cardiomyocytes and as such dictate the speed and direction of cardiac conduction. A prominent feature of acquired structural heart disease is remodeling of GJ protein expression and localization concomitant with increased susceptibility to lethal arrhythmias, leading many to hypothesize that the two are causally linked. Detailed understanding of the cellular mechanisms that regulate GJ localization and function within cardiomyocytes may therefore uncover potential therapeutic strategies for a significant clinical problem. This review will outline our current understanding of GJ cell biology with the intent of highlighting cellular mechanisms responsible for GJ remodeling associated with cardiac disease.

Transmural dispersion of repolarization in failing and nonfailing human ventricle

RATIONALE

Transmural dispersion of repolarization has been shown to play a role in the genesis of ventricular tachycardia and fibrillation in different animal models of heart failure (HF). Heterogeneous changes of repolarization within the midmyocardial population of ventricular cells have been considered an important contributor to the HF phenotype. However, there is limited electrophysiological data from the human heart.

OBJECTIVE

To study electrophysiological remodeling of transmural repolarization in the failing and nonfailing human hearts.

METHODS AND RESULTS

We optically mapped the action potential duration (APD) in the coronary-perfused scar-free posterior-lateral left ventricular free wall wedge preparations from failing (n=5) and nonfailing (n=5) human hearts. During slow pacing (S1S1=2000 ms), in the nonfailing hearts we observed significant transmural APD gradient: subepicardial, midmyocardial, and subendocardial APD80 were 383+/-21, 455+/-20, and 494+/-22 ms, respectively. In 60% of nonfailing hearts (3 of 5), we found midmyocardial islands of cells that presented a distinctly long APD (537+/-40 ms) and a steep local APD gradient (27+/-7 ms/mm) compared with the neighboring myocardium. HF resulted in prolongation of APD80: 477+/-22 ms, 495+/-29 ms, and 506+/-35 ms for the subepi-, mid-, and subendocardium, respectively, while reducing transmural APD80 difference from 111+/-13 to 29+/-6 ms (P<0.005) and presence of any prominent local APD gradient. In HF, immunostaining revealed a significant reduction of connexin43 expression on the subepicardium.

CONCLUSIONS

We present for the first time direct experimental evidence of a transmural APD gradient in the human heart. HF results in the heterogeneous prolongation of APD, which significantly reduces the transmural and local APD gradients.

Electrophysiological remodeling in heart failure

Heart failure affects nearly 6 million Americans, with a half-million new cases emerging each year. Whereas up to 50% of heart failure patients die of arrhythmia, the diverse mechanisms underlying heart failure-associated arrhythmia are poorly understood. As a consequence, effectiveness of antiarrhythmic pharmacotherapy remains elusive. Here, we review recent advances in our understanding of heart failure-associated molecular events impacting the electrical function of the myocardium. We approach this from an anatomical standpoint, summarizing recent insights gleaned from pre-clinical models and discussing their relevance to human heart failure.

Limited forward trafficking of connexin 43 reduces cell-cell coupling in stressed human and mouse myocardium

Gap junctions form electrical conduits between adjacent myocardial cells, permitting rapid spatial passage of the excitation current essential to each heartbeat. Arrhythmogenic decreases in gap junction coupling are a characteristic of stressed, failing, and aging myocardium, but the mechanisms of decreased coupling are poorly understood. We previously found that microtubules bearing gap junction hemichannels (connexons) can deliver their cargo directly to adherens junctions. The specificity of this delivery requires the microtubule plus-end tracking protein EB1. We performed this study to investigate the hypothesis that the oxidative stress that accompanies acute and chronic ischemic disease perturbs connexon forward trafficking. We found that EB1 was displaced in ischemic human hearts, stressed mouse hearts, and isolated cells subjected to oxidative stress. As a result, we observed limited microtubule interaction with adherens junctions at intercalated discs and reduced connexon delivery and gap junction coupling. A point mutation within the tubulin-binding domain of EB1 reproduced EB1 displacement and diminished connexon delivery, confirming that EB1 displacement can limit gap junction coupling. In zebrafish hearts, oxidative stress also reduced the membrane localization of connexin and slowed the spatial spread of excitation. We anticipate that protecting the microtubule-based forward delivery apparatus of connexons could improve cell-cell coupling and reduce ischemia-related cardiac arrhythmias.

Gap junction heterogeneity as mechanism for electrophysiologically distinct properties across the ventricular wall

Gap junctions are critical to maintaining synchronized impulse propagation and repolarization. Heterogeneous expression of the principal ventricular gap junction protein connexin43 (Cx43) is associated with action potential duration (APD) dispersion across the anterior ventricular wall. Little is known about Cx43 expression patterns and their disparate impact on regional electrophysiology throughout the heart. We aimed to determine whether the anterior and posterior regions of the heart are electrophysiologically distinct. Multisegment, high-resolution optical mapping was performed in canine wedge preparations harvested separately from the anterior left ventricle (aLV; n = 8) and posterior left ventricle (pLV; n = 8). Transmural APD dispersion was significantly greater on the aLV than the pLV (45 +/- 13 vs. 26 +/- 8.0 ms; P < 0.05). Conduction velocity dispersion was also significantly higher (P < 0.05) across the aLV (39 +/- 7%) than the pLV (16 +/- 3%). Carbenoxolone perfusion significantly enhanced APD and conduction velocity dispersion on the aLV (by 1.53-fold and 1.36-fold, respectively), but not the pLV (by 1.27-fold and 1.2-fold, respectively), and produced a 4.2-fold increase in susceptibility to inducible arrhythmias in the aLV. Confocal immunofluorescence microscopy revealed significantly (P < 0.05) greater transmural dispersion of Cx43 expression on the aLV (44 +/- 10%) compared with the pLV wall (8.3 +/- 0.7%), suggesting that regional expression of Cx43 expression patterns may account for regional electrophysiological differences. Computer simulations affirmed that localized uncoupling at the epicardial-midmyocardial interface is sufficient to produce APD gradients observed on the aLV. These data demonstrate that the aLV and pLV differ importantly with respect to their electrophysiological properties and Cx43 expression patterns. Furthermore, local underexpression of Cx43 is closely associated with transmural electrophysiological heterogeneity on the aLV. Therefore, regional and transmural heterogeneous Cx43 expression patterns may be an important mechanism underlying arrhythmia susceptibility, particularly in disease states where gap junction expression is altered.

Pathophysiology and clinical implications of cardiac memory

Altering the pattern of activation of the ventricle causes remodeling of the mechanical and electrical properties of the myocardium. The electrical remodeling is evident on the surface electrocardiogram as significant change in T-wave polarity following altered activation; this phenomenon is ascribed to as "T-wave memory" or "cardiac memory." The electrophysiological remodeling following altered activation is characterized by distinct changes in regions proximal (early-activated) versus distal (late-activated) to the site of altered activation. The early-activated region exhibits marked attenuation of epicardial phase 1 notch due to reduced expression of the transient outward potassium current (I(to)). This is attributed to electrotonic changes during altered activation, and angiotensin-mediated regulation of Kv4.3 (the pore-forming alpha subunit responsible for I(to)). The late-activated region exhibits the most significant action potential prolongation due to markedly increased mechanical strain through a mechano-electrical feedback mechanism. Consequently, regionally heterogeneous action potential remodeling occurs following altered activation. This enhances regional repolarization gradients that underlie the electrophysiological basis for T-wave memory. Further, recent clinical studies highlight detrimental consequences of altered activation including worsening mechanical function and increased susceptibility to arrhythmias. Future studies to identify molecular mechanisms that link electrotonic and mechanical strain-induced changes to cellular electrophysiology will provide important insights into the role of altered activation in regulating cardiac repolarization and arrhythmogenesis.

Changes in connexin expression and the atrial fibrillation substrate in congestive heart failure

RATIONALE

Although connexin changes are important for the ventricular arrhythmic substrate in congestive heart failure (CHF), connexin alterations during CHF-related atrial arrhythmogenic remodeling have received limited attention.

OBJECTIVE

To analyze connexin changes and their potential contribution to the atrial fibrillation (AF) substrate during the development and reversal of CHF.

METHODS AND RESULTS

Three groups of dogs were studied: CHF induced by 2-week ventricular tachypacing (240 bpm, n=15); CHF dogs allowed a 4-week nonpaced recovery interval after 2-week tachypacing (n=16); and nonpaced sham controls (n=19). Left ventricular (LV) end-diastolic pressure and atrial refractory periods increased with CHF and normalized on CHF recovery. CHF caused abnormalities in atrial conduction indexes and increased the duration of burst pacing-induced AF (DAF, from 22+/-7 seconds in control to 1100+/-171 seconds, P<0.001). CHF did not significantly alter overall atrial connexin (Cx)40 and Cx43 mRNA and protein expression levels, but produced Cx43 dephosphorylation, increased Cx40/Cx43 protein expression ratio and caused Cx43 redistribution toward transverse cell-boundaries. All of the connexin-alterations reversed on CHF recovery, but CHF-induced conduction abnormalities and increased DAF (884+/-220 seconds, P<0.001 versus control) remained. The atrial fibrous tissue content increased from 3.6+/-0.7% in control to 14.7+/-1.5% and 13.3+/-2.3% in CHF and CHF recovery, respectively (both P<0.01 versus control), with transversely running zones of fibrosis physically separating longitudinally directed muscle bundles. In an ionically based action potential/tissue model, fibrosis was able to account for conduction abnormalities associated with CHF and recovery.

CONCLUSIONS

CHF causes atrial connexin changes, but these are not essential for CHF-related conduction disturbances and AF promotion, which are rather related primarily to fibrotic interruption of muscle bundle continuity.

Spontaneous calcium release in tissue from the failing canine heart

Abnormalities in calcium handling have been implicated as a significant source of electrical instability in heart failure (HF). While these abnormalities have been investigated extensively in isolated myocytes, how they manifest at the tissue level and trigger arrhythmias is not clear. We hypothesize that in HF, triggered activity (TA) is due to spontaneous calcium release from the sarcoplasmic reticulum that occurs in an aggregate of myocardial cells (an SRC) and that peak SCR amplitude is what determines whether TA will occur. Calcium and voltage optical mapping was performed in ventricular wedge preparations from canines with and without tachycardia-induced HF. In HF, steady-state calcium transients have reduced amplitude [135 vs. 170 ratiometric units (RU), P < 0.05] and increased duration (252 vs. 229 s, P < 0.05) compared with those of normal. Under control conditions and during beta-adrenergic stimulation, TA was more frequent in HF (53% and 93%, respectively) compared with normal (0% and 55%, respectively, P < 0.025). The mechanism of arrhythmias was SCRs, leading to delayed afterdepolarization-mediated triggered beats. Interestingly, the rate of SCR rise was greater for events that triggered a beat (0.41 RU/ms) compared with those that did not (0.18 RU/ms, P < 0.001). In contrast, there was no difference in SCR amplitude between the two groups. In conclusion, TA in HF tissue is associated with abnormal calcium regulation and mediated by the spontaneous release of calcium from the sarcoplasmic reticulum in aggregates of myocardial cells (i.e., an SCR), but importantly, it is the rate of SCR rise rather than amplitude that was associated with TA.

Pathological remodeling of cardiac gap junction connexin 43-With special reference to arrhythmogenesis

A dysfunction of the cardiac gap junction, which contributes to electrical cell-to-cell coupling is one of essential factors known to generate arrhythmias. The function of the gap junction depends on the regulation of connexin which composes the gap junction channel. A dysfunction of the gap junction is possibly caused by the down-regulation of connexin. In this review, the relationship between pathological remodeling of connexin 43 (Cx43) and susceptibility of the heart to the ventricular fibrillation, which is a lethal ventricular tachyarrhythmia, is addressed. A suppression of the PKA-mediated phosphorylation or an augmentation of the PKC-mediated phosphorylation of Cx43 induces the downward remodeling of Cx43. Factors regarding downward remodeling of Cx43, such as hypoxia (including intracellular Ca overload and intracellular acidosis), angiotensin II or an activation of PKCvarepsilon make the heart more susceptible to the ventricular fibrillation, while factors regarding upward remodeling of Cx43, such as cyclic AMP or an activation of PKA, lower susceptibility. As a result, from a clinical point of view, angiotensin II antagonists (synthesis inhibitors or receptor blockades), PKC inhibitors or PKA activators are thus considered to provide a therapeutic approach for the treatment of the initiation or advancement of the ventricular fibrillation.

Generation of histo-anatomically representative models of the individual heart: tools and application

This paper presents methods to build histo-anatomically detailed individualized cardiac models. The models are based on high-resolution three-dimensional anatomical and/or diffusion tensor magnetic resonance images, combined with serial histological sectioning data, and are used to investigate individualized cardiac function. The current state of the art is reviewed, and its limitations are discussed. We assess the challenges associated with the generation of histo-anatomically representative individualized in silico models of the heart. The entire processing pipeline including image acquisition, image processing, mesh generation, model set-up and execution of computer simulations, and the underlying methods are described. The multifaceted challenges associated with these goals are highlighted, suitable solutions are proposed, and an important application of developed high-resolution structure-function models in elucidating the effect of individual structural heterogeneity upon wavefront dynamics is demonstrated.

Ventricular arrhythmias after left ventricular assist device implantation

BACKGROUND

Left ventricular assist devices (LVADs) have been used as a bridge to cardiac transplantation and as destination therapy in patients with advanced heart failure. The period after LVAD support is associated with ventricular arrhythmias (VAs) despite ventricular unloading and such VAs can have a detrimental effect on survival. Despite the increasing use of LVAD, little is known regarding post-LVAD VAs at the molecular level and in vivo.

METHODS

Forty-two patients who received LVAD over a 24-month period were evaluated and grouped on the basis of the presence or absence of VAs during LVAD support. We completed a comparative microarray analyses between six patients who developed ventricular tachycardia (VT) or ventricular fibrillation (VF) after LVAD support and six patients who did not develop VAs after LVAD.

RESULTS

VAs occurred in 15 patients (35.7%) during LVAD support at a median post-LVAD day of 25.2. VAs were strongly associated with nonusage of a beta-blocker post-LVAD (odds ratio of 7.04, P-value = 0.001). Analysis of a subset of patients who had VT or VF after LVAD placement showed a decrease in the expression of connexin 43 (0.48 +/- 0.07), Na+/K+-ATPase (0.60 +/- 0.05), and voltage-gated K+ channel Kv4.3 (0.42 +/- 0.04), and an increase in Na+/Ca2+ exchanger (2.2 +/- 0.4) and the structural genes: Titin (2.1 +/- 0.2), laminin (1.7 +/- 0.4), calsequestrin (1.8 +/- 0.5), skeletal muscle isoform of troponin T (5.1 +/- 0.9), and skeletal muscle isoform of troponin I (3.9 +/- 0.7).

CONCLUSION

After LVAD, the increased risk of VAs is strongly associated with nonusage of beta-blocker postoperatively.