Remodeling of gap junctional coupling in hypertrophied right ventricles of rats with monocrotaline-induced pulmonary hypertension

Role of subpulmonary right ventricle in sudden cardiac death in adults with congenital heart disease

Despite improved childhood survival of congenital heart disease (CHD) as a result of advances in management, late-onset sudden cardiac death (SCD) from malignant ventricular arrhythmias remains a leading cause of mortality in adults with CHD. Preventing SCD in these patients requires an understanding of the underlying pathophysiological mechanisms. Many CHD patients experience significant hemodynamic stress on the subpulmonary right ventricle (RV), leading to pathologic remodeling. Unlike acquired heart disease in which left ventricular pathology is prevalent, RV pathologies are crucial in the SCD pathogenesis in CHD patients. This review examines the mechanisms and management of SCD related to subpulmonary RV pathologies in CHD patients.

Hypertension Induces Pro-arrhythmic Cardiac Connexome Disorders: Protective Effects of Treatment

Prolonged population aging and unhealthy lifestyles contribute to the progressive prevalence of arterial hypertension. This is accompanied by low-grade inflammation and over time results in heart dysfunction and failure. Hypertension-induced myocardial structural and ion channel remodeling facilitates the development of both atrial and ventricular fibrillation, and these increase the risk of stroke and sudden death. Herein, we elucidate hypertension-induced impairment of "connexome" cardiomyocyte junctions. This complex ensures cell-to-cell adhesion and coupling for electrical and molecular signal propagation. Connexome dysfunction can be a key factor in promoting the occurrence of both cardiac arrhythmias and heart failure. However, the available literature indicates that arterial hypertension treatment can hamper myocardial structural remodeling, hypertrophy and/or fibrosis, and preserve connexome function. This suggests the pleiotropic effects of antihypertensive agents, including anti-inflammatory. Therefore, further research is required to identify specific molecular targets and pathways that will protect connexomes, and it is also necessary to develop new approaches to maintain heart function in patients suffering from primary or pulmonary arterial hypertension.

A Multi-omic and Multi-Species Analysis of Right Ventricular Failure

Right ventricular failure (RVF) is a leading cause of morbidity and mortality in multiple cardiovascular diseases, but there are no approved treatments for RVF as therapeutic targets are not clearly defined. Contemporary transcriptomic/proteomic evaluations of RVF are predominately conducted in small animal studies, and data from large animal models are sparse. Moreover, a comparison of the molecular mediators of RVF across species is lacking. Here, we used transcriptomics and proteomics analyses to define the molecular pathways associated with cardiac MRI-derived values of RV hypertrophy, dilation, and dysfunction in pulmonary artery banded (PAB) piglets. Publicly available data from rat monocrotaline-induced RVF and pulmonary arterial hypertension patients with preserved or impaired RV function were used to compare the three species. Transcriptomic and proteomic analyses identified multiple pathways that were associated with RV dysfunction and remodeling in PAB pigs. Surprisingly, disruptions in fatty acid oxidation (FAO) and electron transport chain (ETC) proteins were different across the three species. FAO and ETC proteins and transcripts were mostly downregulated in rats, but were predominately upregulated in PAB pigs, which more closely matched the human data. Thus, the pig PAB metabolic molecular signature was more similar to human RVF than rodents. These data suggest there may be divergent molecular responses of RVF across species, and that pigs more accurately recapitulate the metabolic aspects of human RVF.

Dapagliflozin reduces the vulnerability of rats with pulmonary arterial hypertension-induced right heart failure to ventricular arrhythmia by restoring calcium handling

Background

Malignant ventricular arrhythmia (VA) is a major contributor to sudden cardiac death (SCD) in patients with pulmonary arterial hypertension (PAH)-induced right heart failure (RHF). Recently, dapagliflozin (DAPA), a sodium/glucose cotransporter-2 inhibitor (SGLT2i), has been found to exhibit cardioprotective effects in patients with left ventricular systolic dysfunction. In this study, we examined the effects of DAPA on VA vulnerability in a rat model of PAH-induced RHF.

Methods

Rats randomly received monocrotaline (MCT, 60 mg/kg) or vehicle via a single intraperitoneal injection. A day later, MCT-injected rats were randomly treated with placebo, low-dose DAPA (1 mg/kg/day), or high-dose (3 mg/kg/day) DAPA orally for 35 days. Echocardiographic analysis, haemodynamic experiments, and histological assessments were subsequently performed to confirm the presence of PAH-induced RHF. Right ventricle (RV) expression of calcium (Ca²⁺) handling proteins were detected via Western blotting. RV expression of connexin 43 (Cx43) was determined via immunohistochemical staining. An optical mapping study was performed to assess the electrophysiological characteristics in isolated hearts. Cellular Ca²⁺ imaging from RV cardiomyocytes (RVCMs) was recorded using Fura-2 AM or Fluo-4 AM.

Results

High-dose DAPA treatment attenuated RV structural remodelling, improved RV function, alleviated Cx43 remodelling, increased the conduction velocity, restored the expression of key Ca²⁺ handling proteins, increased the threshold for Ca²⁺ and action potential duration (APD) alternans, decreased susceptibility to spatially discordant APD alternans and spontaneous Ca²⁺ events, promoted cellular Ca²⁺ handling, and reduced VA vulnerability in PAH-induced RHF rats. Low-dose DAPA treatment also showed antiarrhythmic effects in hearts with PAH-induced RHF, although with a lower level of efficacy.

Conclusion

DAPA administration reduced VA vulnerability in rats with PAH-induced RHF by improving RVCM Ca²⁺ handling.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12933-022-01614-5.

1,8-cineole Ameliorates Right Ventricle Dysfunction Associated With Pulmonary Arterial Hypertension by Restoring Connexin 43 and Mitochondrial Homeostasis

For the first time, the present study unravels a cardiospecific therapeutic approach for Pulmonary Arterial Hypertension (PAH), a disease with a very poor prognosis and high mortality rates due to right ventricle dysfunction. We first established a new in vitro model of high-pressure-induced hypertrophy that closely resembles heart defects associated with PAH and validated our in vitro findings on a preclinical in vivo model of monocrotaline (MCT)-induced PAH. Our results showed the in vitro antihypertrophic effect of 1,8-cineole, a monoterpene widely found in several essential oils. Also, a decrease in RV hypertrophy and fibrosis, and an improvement in heart function in vivo was observed, when 1,8-cineole was applied topically. Furthermore, 1,8-cineole restored gap junction protein connexin43 distribution at the intercalated discs and mitochondrial functionality, suggesting it may act by preserving cardiac cell-to-cell communication and bioenergetics. Overall, our results point out a promising therapeutic compound that can be easily applied topically, thus paving the way for the development of effective cardiac-specific therapies to greatly improve PAH outcomes.

Importance of Cx43 for Right Ventricular Function

In the heart, connexins form gap junctions, hemichannels, and are also present within mitochondria, with connexin 43 (Cx43) being the most prominent connexin in the ventricles. Whereas the role of Cx43 is well established for the healthy and diseased left ventricle, less is known about the importance of Cx43 for the development of right ventricular (RV) dysfunction. The present article focusses on the importance of Cx43 for the developing heart. Furthermore, we discuss the expression and localization of Cx43 in the diseased RV, i.e., in the tetralogy of Fallot and in pulmonary hypertension, in which the RV is affected, and RV hypertrophy and failure occur. We will also introduce other Cx molecules that are expressed in RV and surrounding tissues and have been reported to be involved in RV pathophysiology. Finally, we highlight therapeutic strategies aiming to improve RV function in pulmonary hypertension that are associated with alterations of Cx43 expression and function.

Nanoscale Study of Calcium Handling Remodeling in Right Ventricular Cardiomyocytes Following Pulmonary Hypertension

Pulmonary hypertension is a complex disorder characterized by pulmonary vascular remodeling and right ventricular hypertrophy, leading to right heart failure. The mechanisms underlying this process are not well understood. We hypothesize that the structural remodeling occurring in the cardiomyocytes of the right ventricle affects the cytosolic Ca²⁺ handling leading to arrhythmias. After 12 days of monocrotaline-induced pulmonary hypertension in rats, epicardial mapping showed electrical remodeling in both ventricles. In myocytes isolated from the hypertensive rats, a combination of high-speed camera and confocal line-scan documented a prolongation of Ca²⁺ transients along with a higher local Ca²⁺-release activity. These Ca²⁺ transients were less synchronous than in controls, likely due to disorganized transverse-axial tubular system. In fact, following pulmonary hypertension, hypertrophied right ventricular myocytes showed significantly reduced number of transverse tubules and increased number of axial tubules; however, Stimulation Emission Depletion microscopy demonstrated that the colocalization of L-type Ca²⁺ channels and RyR2 (ryanodine receptor 2) remained unchanged. Finally, Stimulation Emission Depletion microscopy and super-resolution scanning patch-clamp analysis uncovered a decrease in the density of active L-type Ca²⁺ channels in right ventricular myocytes with an elevated open probability of the T-tubule anchored channels. This may represent a general mechanism of how nanoscale structural changes at the early stage of pulmonary hypertension impact on the development of the end stage failing phenotype in the right ventricle.

A Working Hypothesis Regarding Identical Pathomechanisms between Clinical Efficacy and Adverse Reaction of Clozapine via the Activation of Connexin43

Clozapine (CLZ) is an approved antipsychotic agent for the medication of treatment-resistant schizophrenia but is also well known as one of the most toxic antipsychotics. Recently, the World Health Organization’s (WHO) global database (VigiBase) reported the relative lethality of severe adverse reactions of CLZ. Agranulocytosis is the most famous adverse CLZ reaction but is of lesser lethality compared with the other adverse drug reactions of CLZ. Unexpectedly, VigiBase indicated that the prevalence and relative lethality of pneumonia, cardiotoxicity, and seizures associated with CLZ were more serious than that of agranulocytosis. Therefore, haematological monitoring in CLZ patients monitoring system provided success in the prevention of lethal adverse events from CLZ-induced agranulocytosis. Hereafter, psychiatrists must amend the CLZ patients monitoring system to protect patients with treatment-resistant schizophrenia from severe adverse CLZ reactions, such as pneumonia, cardiotoxicity, and seizures, according to the clinical evidence and pathophysiology. In this review, we discuss the mechanisms of clinical efficacy and the adverse reactions of CLZ based on the accumulating pharmacodynamic findings of CLZ, including tripartite synaptic transmission, and we propose suggestions for amending the monitoring and medication of adverse CLZ reactions associated with pneumonia, cardiotoxicity, and seizures.

Norepinephrine Leads to More Cardiopulmonary Toxicities than Epinephrine by Catecholamine Overdose in Rats

While catecholamines like epinephrine (E) and norepinephrine (NE) are commonly used in emergency medicine, limited studies have discussed the harm of exogenously induced catecholamine overdose. We investigated the possible toxic effects of excessive catecholamine administration on cardiopulmonary function and structure via continuous 6 h intravenous injection of E and/or NE in rats. Heart rate, echocardiography, and ventricular pressure were measured throughout administration. Cardiopulmonary structure was also assessed by examining heart and lung tissue. Consecutive catecholamine injections induced severe tachycardia. Echocardiography results showed NE caused worse dysfunction than E. Simultaneously, both E and NE led to higher expression of Troponin T and connexin43 in the whole ventricles, which increased further with E+NE administration. The NE and E+NE groups showed severe pulmonary edema while all catecholamine-administering groups demonstrated reduced expression of receptor for advanced glycation end products and increased connexin43 levels in lung tissue. The right ventricle was more vulnerable to catecholamine overdose than the left. Rats injected with NE had a lower survival rate than those injected with E within 6 h. Catecholamine overdose induces acute lung injuries and ventricular cardiomyopathy, and E+NE is associated with a more severe outcome. The similarities of the results between the NE and E+NE groups may indicate a predominant role of NE in determining the overall cardiopulmonary damage. The results provide important clinical insights into the pathogenesis of catecholamine storm.

Spiral Wave Propagation in Communities with Spatially Correlated Heterogeneity

Many multicellular communities propagate signals in a directed manner via excitable waves. Cell-to-cell heterogeneity is a ubiquitous feature of multicellular communities, but the effects of heterogeneity on wave propagation are still unclear. Here, we use a minimal FitzHugh-Nagumo-type model to investigate excitable wave propagation in a two-dimensional heterogeneous community. The model shows three dynamic regimes in which waves either propagate directionally, die out, or spiral indefinitely, and we characterize how these regimes depend on the heterogeneity parameters. We find that in some parameter regimes, spatial correlations in the heterogeneity enhance directional propagation and suppress spiraling. However, in other regimes, spatial correlations promote spiraling, a surprising feature that we explain by demonstrating that these spirals form by a second, distinct mechanism. Finally, we characterize the dynamics using techniques from percolation theory. Despite the fact that percolation theory does not completely describe the dynamics quantitatively because it neglects the details of the excitable propagation, we find that it accounts for the transitions between the dynamic regimes and the general dependency of the spiral period on the heterogeneity and thus provides important insights. Our results reveal that the spatial structure of cell-to-cell heterogeneity can have important consequences for signal propagation in cellular communities.

Expression of connexin-43 in the cardiac muscle of children diagnosed with hypoplastic left heart syndrome: a Western blot and confocal laser scanning microscopy study

Hypoplastic left heart syndrome consists of several structural abnormalities in the left side of the heart and may be associated with a hereditary genetic cause, possibly related to the connexin gene GJA1; however, only a few studies have investigated it. The present study aimed to analyse the expression of connexin-43 in the cardiac muscle of hypoplastic left heart syndrome children by Western blot method and confocal laser scanning microscopy. For that, tissue samples were taken during corrective surgery to treat heart defects. Patients of control group (8) presented any type of heart defect not related to hypoplastic left heart syndrome, connexin-43, or its gene and those of hypoplastic left heart syndrome group (9) presented this disease singly, without any other associated congenital diseases. By means of confocal laser scanning microscopy, it was noticed no connexin-43 qualitative differences in positioning and location pattern between both groups. From Western blot analysis, the connexin-43 expression did not show a statistically significant difference (p = 0.0571) as well. Within the limits of this study, it is suggested that cardiomyocytes of hypoplastic left heart syndrome children are similar in connexin-43 location, distribution, and structural and conformational patterns to those of children with heart defects not related to this protein and its genes.

Intra-tracheal gene delivery of aerosolized SERCA2a to the lung suppresses ventricular arrhythmias in a model of pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) results in right ventricular (RV) failure, electro-mechanical dysfunction and heightened risk of sudden cardiac death (SCD), although exact mechanisms and predisposing factors remain unclear. Because impaired chronotropic response to exercise is a strong predictor of early mortality in patients with PAH, we hypothesized that progressive elevation in heart rate can unmask ventricular tachyarrhythmias (VT) in a rodent model of monocrotaline (MCT)-induced PAH. We further hypothesized that intra-tracheal gene delivery of aerosolized AAV1.SERCA2a (AAV1.S2a), an approach which improves pulmonary vascular remodeling in PAH, can suppress VT in this model.

OBJECTIVE

To determine the efficacy of pulmonary AAV1.S2a in reversing electrophysiological (EP) remodeling and suppressing VT in PAH.

METHODS

Male rats received subcutaneous injection of MCT (60 mg/kg) leading to advanced PAH. Three weeks following MCT, rats underwent intra-tracheal delivery of aerosolized AAV1.S2a (MCT + S2a, N = 8) or saline (MCT, N = 9). Age-matched rats served as controls (CTRL, N = 7). The EP substrate and risk of VT were determined using high-resolution optical action potential (AP) mapping ex vivo. The expression levels of key ion channel subunits, fibrosis markers and hypertrophy indices were measured by RT-PCR and histochemical analyses.

RESULTS

Over 80% of MCT but none of the CTRL hearts were prone to sustained VT by rapid pacing (P < .01). Aerosolized gene delivery of AAV1.S2a to the lung suppressed the incidence of VT to <15% (P < .05). Investigation of the EP substrate revealed marked prolongation of AP duration (APD), increased APD heterogeneity, a reversal in the trans-epicardial APD gradient, and marked conduction slowing in untreated MCT compared to CTRL hearts. These myocardial EP changes coincided with major remodeling in the expression of K and Ca channel subunits, decreased expression of Cx43 and increased expression of pro-fibrotic and pro-hypertrophic markers. Intra-tracheal gene delivery of aerosolized AAV1 carrying S2a but not luciferase resulted in selective upregulation of the human isoform of SERCA2a in the lung but not the heart. This pulmonary intervention, in turn, ameliorated MCT-induced APD prolongation, reversed spatial APD heterogeneity, normalized myocardial conduction, and suppressed the incidence of pacing-induced VT. Comparison of the minimal conduction velocity (CV) generated at the fastest pacing rate before onset of VT or at the end of the protocol revealed significantly lower values in untreated compared to AAV1.S2a treated PAH and CTRL hearts. Reversal of EP remodeling by pulmonary AAV1.S2a gene delivery was accompanied by restored expression of key ion channel transcripts. Restored expression of Cx43 and collagen but not the pore-forming Na channel subunit Nav1.5 likely ameliorated VT by improving CV at rapid rates in PAH.

CONCLUSION

Aerosolized AAV1.S2a gene delivery selectively to the lungs ameliorates myocardial EP remodeling and VT susceptibility at rapid heart rates. Our findings highlight for the first time the utility of a non-cardiac gene therapy approach for arrhythmia suppression.

Regulation of cardiac Ca2+ and ion channels by shear mechanotransduction

Cardiac contraction is controlled by a Ca²⁺ signaling sequence that includes L-type Ca²⁺ current-gated opening of Ca²⁺ release channels (ryanodine receptors) in the sarcoplasmic reticulum (SR). Local Ca²⁺ signaling in the atrium differs from that in the ventricle because atrial myocytes lack transverse tubules and have more abundant corbular SR. Myocardium is subjected to a variety of forces with each contraction, such as stretch, shear stress, and afterload, and adapts to those mechanical stresses. These mechanical stimuli increase in heart failure, hypertension, and valvular heart diseases that are clinically implicated in atrial fibrillation and stroke. In the present review, we describe distinct responses of atrial and ventricular myocytes to shear stress and compare them with other mechanical responses in the context of local and global Ca²⁺ signaling and ion channel regulation. Recent evidence suggests that shear mechanotransduction in cardiac myocytes involves activation of gap junction hemichannels, purinergic signaling, and generation of mitochondrial reactive oxygen species. Significant alterations in Ca²⁺ signaling and ionic currents by shear stress may be implicated in the pathogenesis of cardiac arrhythmia and failure.

Atrioventricular Node Dysfunction and Ion Channel Transcriptome in Pulmonary Hypertension

BACKGROUND

Heart block is associated with pulmonary hypertension, and the aim of the study was to test the hypothesis that the heart block is the result of a change in the ion channel transcriptome of the atrioventricular (AV) node.

METHODS AND RESULTS

The most commonly used animal model of pulmonary hypertension, the monocrotaline-injected rat, was used. The functional consequences of monocrotaline injection were determined by echocardiography, ECG recording, and electrophysiological experiments on the Langendorff-perfused heart and isolated AV node. The ion channel transcriptome was measured by quantitative PCR, and biophysically detailed computer modeling was used to explore the changes observed. After monocrotaline injection, echocardiography revealed the pattern of pulmonary artery blood flow characteristic of pulmonary hypertension and right-sided hypertrophy and failure; the Langendorff-perfused heart and isolated AV node revealed dysfunction of the AV node (eg, 50% incidence of heart block in isolated AV node); and quantitative PCR revealed a widespread downregulation of ion channel and related genes in the AV node (eg, >50% downregulation of Ca_v1.2/3 and HCN1/2/4 channels). Computer modeling predicted that the changes in the transcriptome if translated into protein and function would result in heart block.

CONCLUSIONS

Pulmonary hypertension results in a derangement of the ion channel transcriptome in the AV node, and this is the likely cause of AV node dysfunction in this disease.

Pinocembrin ex vivo preconditioning improves the therapeutic efficacy of endothelial progenitor cells in monocrotaline-induced pulmonary hypertension in rats

Pulmonary hypertension is still not curable and the available current therapies can only alleviate symptoms without hindering the progression of disease. The present study was directed to investigate the possible modulatory effect of pinocembrin on endothelial progenitor cells transplanted in monocrotaline-induced pulmonary hypertension in rats. Pulmonary hypertension was induced by a single subcutaneous injection of monocrotaline (60mg/kg). Endothelial progenitor cells were in vitro preconditioned with pinocembrin (25mg/L) for 30min before being i.v. injected into rats 2weeks after monocrotaline administration. Four weeks after monocrotaline administration, blood pressure, electrocardiography and right ventricular systolic pressure were recorded. Rats were sacrificed and serum was separated for determination of endothelin-1 and asymmetric dimethylarginine levels. Right ventricles and lungs were isolated for estimation of tumor necrosis factor-alpha and transforming growth factor-beta contents as well as caspase-3 activity. Moreover, protein expression of matrix metalloproteinase-9 and endothelial nitric oxide synthase in addition to myocardial connexin-43 was assessed. Finally, histological analysis of pulmonary arteries, cardiomyocyte cross-sectional area and right ventricular hypertrophy was performed and cryosections were done for estimation of cell homing. Preconditioning with pinocembrin provided a significant improvement in endothelial progenitor cells' effect towards reducing monocrotaline-induced elevation of inflammatory, fibrogenic and apoptotic markers. Furthermore, preconditioned cells induced a significant amelioration of endothelial markers and cell homing and prevented monocrotaline-induced changes in right ventricular function and histological analysis compared with native cells alone. In conclusion, pinocembrin significantly improves the therapeutic efficacy of endothelial progenitor cells in monocrotaline-induced pulmonary hypertension in rats.

Impairment of Excitation-Contraction Coupling in Right Ventricular Hypertrophied Muscle with Fibrosis Induced by Pulmonary Artery Banding

Interstitial myocardial fibrosis is one of the factors responsible for dysfunction of the heart. However, how interstitial fibrosis affects cardiac function and excitation-contraction coupling (E-C coupling) has not yet been clarified. We developed an animal model of right ventricular (RV) hypertrophy with fibrosis by pulmonary artery (PA) banding in rats. Two, four, and six weeks after the PA-banding operation, the tension and intracellular Ca²⁺ concentration of RV papillary muscles were simultaneously measured (n = 33). The PA-banding rats were clearly divided into two groups by the presence or absence of apparent interstitial fibrosis in the papillary muscles: F+ or F- group, respectively. The papillary muscle diameter and size of myocytes were almost identical between F+ and F-, although the RV free wall weight was heavier in F+ than in F-. F+ papillary muscles exhibited higher stiffness, lower active tension, and lower Ca²⁺ responsiveness compared with Sham and F- papillary muscles. In addition, we found that the time to peak Ca²⁺ had the highest correlation coefficient to percent of fibrosis among other parameters, such as RV weight and active tension of papillary muscles. The phosphorylation level of troponin I in F+ was significantly higher than that in Sham and F-, which supports the idea of lower Ca²⁺ responsiveness in F+. We also found that connexin 43 in F+ was sparse and disorganized in the intercalated disk area where interstitial fibrosis strongly developed. In the present study, the RV papillary muscles obtained from the PA-banding rats enabled us to directly investigate the relationship between fibrosis and cardiac dysfunction, the impairment of E-C coupling in particular. Our results suggest that interstitial fibrosis worsens cardiac function due to 1) the decrease in Ca²⁺ responsiveness and 2) the asynchronous activation of each cardiac myocyte in the fibrotic preparation due to sparse cell-to-cell communication.

Spatial Heterogeneity of Cx43 is an Arrhythmogenic Substrate of Polymorphic Ventricular Tachycardias during Compensated Cardiac Hypertrophy in Rats

BACKGROUND

Ventricular remodeling increases the propensity of ventricular tachyarrhythmias and sudden death in patients. We studied the mechanism underlying these fatal arrhythmias, electrical and structural cardiac remodeling, as well as arrhythmogeneity during early, compensated hypertrophy in a rat model of chronic pressure overload.

METHODS

Twenty-six Wistar rats were subjected to transverse aortic constriction (TAC) (n = 13) or sham operation (n = 13). Four weeks postoperative, echo- and electrocardiography was performed. Epicardial (208 or 455 sites) and transmural (30 sites) ventricular activation mapping was performed on Langendorff perfused hearts. Subsequently, hearts were processed for (immuno)histological and molecular analyses.

RESULTS

TAC rats showed significant hypertrophy with preserved left ventricular (LV) function. Epicardial conduction velocity (CV) was similar, but more dispersed in TAC. Transmural CV was slowed in TAC (37.6 ± 2.9 cm s(-1)) compared to sham (58.5 ± 3.9 cm s(-1); P < 0.01). Sustained polymorphic ventricular tachycardias were induced from LV in 8/13 TAC and in 0/13 sham rats. During VT, electrical activation patterns showed variable sites of earliest epicardial activation and altering sites of functional conduction block. Wandering epicardial reentrant activation was sporadically observed. Collagen deposition was significantly higher in TAC compared to sham, but not different between arrhythmogenic and non-arrhythmogenic TAC animals. Connexin43 (Cx43) expression was heterogeneous with a higher prevalence of non-phosphorylated Cx43 in arrhythmogenic TAC animals.

CONCLUSION

In TAC rats with compensated cardiac hypertrophy, dispersion of conduction correlated to arrhythmogenesis, an increased heterogeneity of Cx43, and a partial substitution with non-phosphorylated Cx43. These alterations may result in the increased vulnerability to polymorphic VTs.

Evaluation of cardiac electrophysiological properties in an experimental model of right ventricular hypertrophy and failure

BACKGROUND

Malignant arrhythmias are a major cause of sudden cardiac death in adults with congenital heart disease. We developed a model to serially investigate electrophysiological properties in an animal model of right ventricular hypertrophy and failure.

METHOD

We created models of compensated (cHF; n=11) and decompensated (dHF; n=11) right ventricular failure in Wistar rats by pulmonary trunk banding. Healthy controls underwent sham operation (Control; n=13). Surface electrocardiography was recorded from extremities, and inducibility of ventricular tachycardia was evaluated in vivo by programmed stimulation. Isolated right ventricular myocardium was analysed for mRNA expression of selected genes.

RESULTS

Banding caused an increased mRNA expression of both connexin 43 and the voltage-gated sodium channel 1.5, as well as a prolongation of PQ, QRS and QTc intervals. Ventricular tachycardia was induced in the majority of banded animals compared with none in the healthy control group. No differences were found between the two degrees of failure in neither the electrophysiological parameters nor inducibility.

CONCLUSIONS

The electrophysiological properties of rat hearts subjected to pulmonary trunk banding were significantly changed with increased susceptibility to ventricular tachycardia, but no differences were found between compensated and decompensated right ventricular failure. Furthermore, we demonstrate that in vivo electrophysiological evaluation is a sensitive method to characterise the cardiac electric phenotype in an experimental rat model.

Shear stress induces a longitudinal Ca(2+) wave via autocrine activation of P2Y1 purinergic signalling in rat atrial myocytes

Atrial myocytes are exposed to shear stress during the cardiac cycle and haemodynamic disturbance. In response, they generate a longitudinally propagating global Ca(2+) wave. Here, we investigated the cellular mechanisms underlying the shear stress-mediated Ca(2+) wave, using two-dimensional confocal Ca(2+) imaging combined with a pressurized microflow system in single rat atrial myocytes. Shear stress of ∼16 dyn cm(-2) for 8 s induced ∼1.2 aperiodic longitudinal Ca(2+) waves (∼79 μm s(-1)) with a delay of 0.2-3 s. Pharmacological blockade of ryanodine receptors (RyRs) or inositol 1,4,5-trisphosphate receptors (IP3 Rs) abolished shear stress-induced Ca(2+) wave generation. Furthermore, in atrial myocytes from type 2 IP3R (IP3R2) knock-out mice, shear stress failed to induce longitudinal Ca(2+) waves. The phospholipase C (PLC) inhibitor U73122, but not its inactive analogue U73343, abolished the shear-induced longitudinal Ca(2+) wave. However, pretreating atrial cells with blockers for stretch-activated channels, Na(+)-Ca(2+) exchanger, transient receptor potential melastatin subfamily 4, or nicotinamide adenine dinucleotide phosphate oxidase did not suppress wave generation under shear stress. The P2 purinoceptor inhibitor suramin, and the potent P2Y1 receptor antagonist MRS 2179, both suppressed the Ca(2+) wave, whereas the P2X receptor antagonist, iso-PPADS, did not alter it. Suppression of gap junction hemichannels permeable to ATP or extracellular application of ATP-metabolizing apyrase inhibited the wave. Removal of external Ca(2+) to enhance hemichannel opening facilitated the wave generation. Our data suggest that longitudinally propagating, regenerative Ca(2+) release through RyRs is triggered by P2Y1-PLC-IP3R2 signalling that is activated by gap junction hemichannel-mediated ATP release in atrial myocytes under shear stress.

Role of connexin 43 in cardiovascular diseases

Gap junctions (GJs) channels provide the basis for intercellular communication in the cardiovascular system for maintenance of the normal cardiac rhythm, regulation of vascular tone and endothelial function as well as metabolic interchange between the cells. They allow the transfer of small molecules and may enable slow calcium wave spreading, transfer of "death" or of "survival" signals. In the cardiomyocytes the most abundant isoform is Connexin 43 (Cx43). Alterations in Cx43 expression and distribution were observed in myocardium disease; i.e. in hypertrophic cardiomyopathy, heart failure and ischemia. Recent reports suggest the presence of Cx43 in the mitochondria as well, at least in the inner mitochondrial membrane, where it plays a central role in ischemic preconditioning. In this review, the current knowledge on the relationship between the remodeling of cardiac gap junctions and cardiac diseases are summarized.

Biorealistic cardiac cell culture platforms with integrated monitoring of extracellular action potentials

Current platforms for in vitro drug development utilize confluent, unorganized monolayers of heart cells to study the effect on action potential propagation. However, standard cell cultures are of limited use in cardiac research, as they do not preserve important structural and functional properties of the myocardium. Here we present a method to integrate a scaffolding technology with multi-electrode arrays and deliver a compact, off-the-shelf monitoring platform for growing biomimetic cardiac tissue. Our approach produces anisotropic cultures with conduction velocity (CV) profiles that closer resemble native heart tissue; the fastest impulse propagation is along the long axis of the aligned cardiomyocytes (CVL) and the slowest propagation is perpendicular (CVT), in contrast to standard cultures where action potential propagates isotropically (CVL ≈ CVT). The corresponding anisotropy velocity ratios (CVL/CVT = 1.38 - 2.22) are comparable with values for healthy adult rat ventricles (1.98 - 3.63). The main advantages of this approach are that (i) it provides ultimate pattern control, (ii) it is compatible with automated manufacturing steps and (iii) it is utilized through standard cell culturing protocols. Our platform is compatible with existing read-out equipment and comprises a prompt method for more reliable CV studies.

Stem and progenitor cell therapy for pulmonary arterial hypertension: effects on the right ventricle (2013 Grover Conference Series)

In experimental animals and in patients with pulmonary arterial hypertension (PAH), a wide spectrum of structural and functional conditions is known that may be responsible for the switch of a state of "compensated" right ventricular (RV) hypertrophy to a state of RV failure. In recent years, therapy with differentiated cells, endothelial progenitor cells, and mesenchymal stem cells has been shown to cause partial or complete reversal of pathological characteristics of PAH. The therapeutic effects of stem or progenitor cell therapy are considered to be (1) paracrine effects from stem or progenitor cells that had engrafted in the myocardium (or elsewhere), by compounds that have anti-inflammatory, antiapoptotic, and proangiogenic actions and (2) unloading effects on the right ventricle due to stem or progenitor cell-induced decrease in pulmonary vascular resistance and decrease in pulmonary artery pressure.

Passive ventricular remodeling in cardiac disease: focus on heterogeneity

Passive ventricular remodeling is defined by the process of molecular ventricular adaptation to different forms of cardiac pathophysiology. It includes changes in tissue architecture, such as hypertrophy, fiber disarray, alterations in cell size and fibrosis. Besides that, it also includes molecular remodeling of gap junctions, especially those composed by Connexin43 proteins (Cx43) in the ventricles that affect cell-to-cell propagation of the electrical impulse, and changes in the sodium channels that modify excitability. All those alterations appear mainly in a heterogeneous manner, creating irregular and inhomogeneous electrical and mechanical coupling throughout the heart. This can predispose to reentry arrhythmias and adds to a further deterioration into heart failure. In this review, passive ventricular remodeling is described in Hypertrophic Cardiomyopathy (HCM), Dilated Cardiomyopathy (DCM), Ischemic Cardiomyopathy (ICM), and Arrhythmogenic Cardiomyopathy (ACM), with a main focus on the heterogeneity of those alterations mentioned above.

Trafficking highways to the intercalated disc: new insights unlocking the specificity of connexin 43 localization

With each heartbeat, billions of cardiomyocytes work in concert to propagate the electrical excitation needed to effectively circulate blood. Regulated expression and timely delivery of connexin proteins to form gap junctions at the specialized cell-cell contact region, known as the intercalated disc, is essential to ventricular cardiomyocyte coupling. We focus this review on several regulatory mechanisms that have been recently found to govern the lifecycle of connexin 43 (Cx43), the short-lived and most abundantly expressed connexin in cardiac ventricular muscle. The Cx43 lifecycle begins with gene expression, followed by oligomerization into hexameric channels, and then cytoskeletal-based transport toward the disc region. Once delivered, hemichannels interact with resident disc proteins and are organized to effect intercellular coupling. We highlight recent studies exploring regulation of Cx43 localization to the intercalated disc, with emphasis on alternatively translated Cx43 isoforms and cytoskeletal transport machinery that together regulate Cx43 gap junction coupling between cardiomyocytes.

Oral administration of eicosapentaenoic acid or docosahexaenoic acid modifies cardiac function and ameliorates congestive heart failure in male rats

This study assessed the effects of eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) on normal cardiac function (part 1) and congestive heart failure (CHF) (part 2) through electrocardiogram analysis and determination of EPA, DHA, and arachidonic acid (AA) concentrations in rat hearts. In part 2, pathologic assessments were also performed. For part 1 of this study, 4-wk-old male rats were divided into a control group and 2 experimental groups. The rats daily were orally administered (1 g/kg body weight) saline, EPA-ethyl ester (EPA-Et; E group), or DHA-ethyl ester (DHA-Et; D group), respectively, for 28 d. ECGs revealed that QT intervals were significantly shorter for groups E and D compared with the control group (P ≤ 0.05). Relative to the control group, the concentration of EPA was higher in the E group and concentrations of EPA and DHA were higher in the D group, although AA concentrations were lower (P ≤ 0.05). In part 2, CHF was produced by subcutaneous injection of monocrotaline into 5-wk-old rats. At 3 d before monocrotaline injection, rats were administered either saline, EPA-Et, or DHA-Et as mentioned above and then killed at 21 d. The study groups were as follows: normal + saline (control), CHF + saline (H group), CHF + EPA-Et (HE group), and CHF + DHA-Et (HD group). QT intervals were significantly shorter (P ≤ 0.05) in the control and HD groups compared with the H and HE groups. Relative to the H group, concentrations of EPA were higher in the HE group and those of DHA were higher in the control and HD groups (P ≤ 0.05). There was less mononuclear cell infiltration in the myocytes of the HD group than in the H group (P = 0.06). The right ventricles in the H, HE, and HD groups showed significantly increased weights (P ≤ 0.05) compared with controls. The administration of EPA-Et or DHA-Et may affect cardiac function by modification of heart fatty acid composition, and the administration of DHA-Et may ameliorate CHF.

Mechanism of spiral formation in heterogeneous discretized excitable media

Spiral waves on excitable media strongly influence the functions of living systems in both a positive and negative way. The spiral formation mechanism has thus been one of the major themes in the field of reaction-diffusion systems. Although the widely believed origin of spiral waves is the interaction of traveling waves, the heterogeneity of an excitable medium has recently been suggested as a probable cause. We suggest one possible origin of spiral waves using a Belousov-Zhabotinsky reaction and a discretized FitzHugh-Nagumo model. The heterogeneity of the reaction field is shown to stochastically generate unidirectional sites, which can induce spiral waves. Furthermore, we found that the spiral wave vanished with only a small reduction in the excitability of the reaction field. These results reveal a gentle approach for controlling the appearance of a spiral wave on an excitable medium.

Regulation of cardiovascular connexins by mechanical forces and junctions

Connexins form a family of transmembrane proteins that consists of 20 members in humans and 21 members in mice. Six connexins assemble into a connexon that can function as a hemichannel or connexon that can dock to a connexon expressed by a neighbouring cell, thereby forming a gap junction channel. Such intercellular channels synchronize responses in multicellular organisms through direct exchange of ions, small metabolites, and other second messenger molecules between the cytoplasms of adjacent cells. Multiple connexins are expressed in the cardiovascular system. These connexins not only experience the different biomechanical forces within this system, but may also act as effector proteins in co-ordinating responses within groups of cells towards these forces. This review discusses recent insights regarding regulation of cardiovascular connexins by mechanical forces and junctions. It specifically addresses effects of (i) shear stress on endothelial connexins, (ii) hypertension on vascular connexins, and (iii) changes in afterload and the composition of myocardial mechanical junctions on cardiac connexins.

Sildenafil improves long-term effect of endothelial progenitor cell-based treatment for monocrotaline-induced rat pulmonary arterial hypertension

BACKGROUND AIMS

We hypothesized that the long-term therapeutic effect of combined sildenafil and bone marrow-derived endothelial progenitor cells (BMDEPCs) on monocrotaline (MCT)-induced rat pulmonary arterial hypertension (PAH) is superior to either treatment alone.

METHODS

Male Sprague-Dawley rats (n = 40) were equally divided into normal controls, MCT (65 mg/kg, subcutaneously) only, MCT + sildenafil (25 mg/kg/day, orally), MCT + BMDEPCs (2.0 × 10(6) autologous cells, intravenously) and MCT + sildenafil+ BMDEPCs. BMDEPCs and sildenafil were given on day 21 after MCT administration. Animals were sacrificed by day 90 after MCT administration.

RESULTS

The apoptotic (caspase 3, Bax) and inflammatory (tumor necrosis factor-α, matrix metalloproteinase-9) biomarkers in right ventricle and lung and pulmonary expressions of fibrotic biomarkers (transforming growth factor-β, p-Smad3) and connexin 43 protein were lower in monotherapy groups (i.e., MCT + sildenafil and MCT + BMDEPCs) and further decreased in normal controls and combined treatment groups (i.e., MCT + sildenafil + BMDEPCs) compared with untreated animals (i.e., MCT only) (all P < 0.01). Expressions of anti-fibrotic biomarkers (bone morphogenetic protein-2, p-Smad1/5) and numbers of alveolar sacs and arterioles in lung were higher in monotherapy groups and further increased in normal controls and combined treatment groups compared with untreated animals (all P < 0.005). In right ventricle, connexin 43 and α-myosin heavy chain (MHC) expressions were higher in the monotherapy groups and further elevated in normal controls and combined treatment groups compared with untreated animals, whereas β-MHC exhibited the opposite pattern (all P < 0.01). Right ventricular systolic pressure and weight were lower in the monotherapy animals and further reduced in normal controls and combined treatment groups compared with untreated animals (all P < 0.0001).

CONCLUSIONS

Combined therapy with BMDEPCs and sildenafil was superior to either treatment alone in attenuating rodent MCT-induced PAH.

Stretch of atrial myocytes stimulates recruitment of macrophages via ATP released through gap-junction channels

Atrial inflammation is critical to atrial fibrillation initiation and progression. Although left atrial dilatation is a risk factor for atrial fibrillation, the mechanism linking atrial dilatation and inflammation is unclear. We evaluated the mechanisms underlying infiltration of macrophages induced by stretch of atrial myocytes. Murine macrophages were co-cultured with HL-1 murine atrial myocyte-derived cells. Mechanical stretch applied to atrial myocytes induced transwell macrophage migration. The gap junction-channel blocker carbenoxolone and the non-specific ATP-signaling modifiers apyrase and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate inhibited the enhanced migration. Mechanical stretch of atrial myocytes induced transient increase in extracellular ATP concentration, which was inhibited by carbenoxolone. siRNA knockdown of pannexin-2 inhibited ATP release and macrophage migration. Mice underwent transverse aortic constriction or sham procedure. Transverse aortic constriction procedure induced macrophage infiltration. Daily carbenoxolone administration significantly inhibited macrophage infiltration in the atrium. Thus, mechanical stretch of atrial myocytes induces macrophage migration by ATP released through gap-junction channels, at least in part, in vitro. Administering a gap junction family-channel blocker inhibited this inflammatory change in vivo.

Cardiac-specific over-expression of epidermal growth factor receptor 2 (ErbB2) induces pro-survival pathways and hypertrophic cardiomyopathy in mice

BACKGROUND

Emerging evidence shows that ErbB2 signaling has a critical role in cardiomyocyte physiology, based mainly on findings that blocking ErbB2 for cancer therapy is toxic to cardiac cells. However, consequences of high levels of ErbB2 activity in the heart have not been previously explored.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated consequences of cardiac-restricted over-expression of ErbB2 in two novel lines of transgenic mice. Both lines develop striking concentric cardiac hypertrophy, without heart failure or decreased life span. ErbB2 transgenic mice display electrocardiographic characteristics similar to those found in patients with Hypertrophic Cardiomyopathy, with susceptibility to adrenergic-induced arrhythmias. The hypertrophic hearts, which are 2-3 times larger than those of control littermates, express increased atrial natriuretic peptide and β-myosin heavy chain mRNA, consistent with a hypertrophic phenotype. Cardiomyocytes in these hearts are significantly larger than wild type cardiomyocytes, with enlarged nuclei and distinctive myocardial disarray. Interestingly, the over-expression of ErbB2 induces a concurrent up-regulation of multiple proteins associated with this signaling pathway, including EGFR, ErbB3, ErbB4, PI3K subunits p110 and p85, bcl-2 and multiple protective heat shock proteins. Additionally, ErbB2 up-regulation leads to an anti-apoptotic shift in the ratio of bcl-xS/xL in the heart. Finally, ErbB2 over-expression results in increased activation of the translation machinery involving S6, 4E-BP1 and eIF4E. The dependence of this hypertrophic phenotype on ErbB family signaling is confirmed by reduction in heart mass and cardiomyocyte size, and inactivation of pro-hypertrophic signaling in transgenic animals treated with the ErbB1/2 inhibitor, lapatinib.

CONCLUSIONS/SIGNIFICANCE

These studies are the first to demonstrate that increased ErbB2 over-expression in the heart can activate protective signaling pathways and induce a phenotype consistent with Hypertrophic Cardiomyopathy. Furthermore, our work suggests that in the situation where ErbB2 signaling contributes to cardiac hypertrophy, inhibition of this pathway may reverse this process.

Cardiac arrhythmia mechanisms in rats with heart failure induced by pulmonary hypertension

Pulmonary hypertension provokes right heart failure and arrhythmias. Better understanding of the mechanisms underlying these arrhythmias is needed to facilitate new therapeutic approaches for the hypertensive, failing right ventricle (RV). The aim of our study was to identify the mechanisms generating arrhythmias in a model of RV failure induced by pulmonary hypertension. Rats were injected with monocrotaline to induce either RV hypertrophy or failure or with saline (control). ECGs were measured in conscious, unrestrained animals by telemetry. In isolated hearts, electrical activity was measured by optical mapping and myofiber orientation by diffusion tensor-MRI. Sarcoplasmic reticular Ca(2+) handling was studied in single myocytes. Compared with control animals, the T-wave of the ECG was prolonged and in three of seven heart failure animals, prominent T-wave alternans occurred. Discordant action potential (AP) alternans occurred in isolated failing hearts and Ca(2+) transient alternans in failing myocytes. In failing hearts, AP duration and dispersion were increased; conduction velocity and AP restitution were steeper. The latter was intrinsic to failing single myocytes. Failing hearts had greater fiber angle disarray; this correlated with AP duration. Failing myocytes had reduced sarco(endo)plasmic reticular Ca(2+)-ATPase activity, increased sarcoplasmic reticular Ca(2+)-release fraction, and increased Ca(2+) spark leak. In hypertrophied hearts and myocytes, dysfunctional adaptation had begun, but alternans did not develop. We conclude that increased electrical and structural heterogeneity and dysfunctional sarcoplasmic reticular Ca(2+) handling increased the probability of alternans, a proarrhythmic predictor of sudden cardiac death. These mechanisms are potential therapeutic targets for the correction of arrhythmias in hypertensive, failing RVs.

Remodeling of mechanical junctions and of microtubule-associated proteins accompany cardiac connexin43 lateralization

BACKGROUND

Desmosomes and adherens junctions provide mechanical continuity between cardiac cells, whereas gap junctions allow for cell-cell electrical/metabolic coupling. These structures reside at the cardiac intercalated disc (ID). Also at the ID is the voltage-gated sodium channel (VGSC) complex. Functional interactions between desmosomes, gap junctions, and VGSC have been demonstrated. Separate studies show, under various conditions, reduced presence of gap junctions at the ID and redistribution of connexin43 (Cx43) to plaques oriented parallel to fiber direction (gap junction "lateralization").

OBJECTIVE

To determine the mechanisms of Cx43 lateralization, and the fate of desmosomal and sodium channel molecules in the setting of Cx43 remodeling.

METHODS

Adult sheep were subjected to right ventricular pressure overload (pulmonary hypertension). Tissue was analyzed by quantitative confocal microscopy and by transmission electron microscopy. Ionic currents were measured using conventional patch clamp.

RESULT

Quantitative confocal microscopy demonstrated lateralization of immunoreactive junctional molecules. Desmosomes and gap junctions in lateral membranes were demonstrable by electron microscopy. Cx43/desmosomal remodeling was accompanied by lateralization of 2 microtubule-associated proteins relevant for Cx43 trafficking: EB1 and kinesin protein Kif5b. In contrast, molecules of the VGSC failed to reorganize in plaques discernable by confocal microscopy. Patch-clamp studies demonstrated change in amplitude and kinetics of sodium current and a small reduction in electrical coupling between cells.

CONCLUSIONS

Cx43 lateralization is part of a complex remodeling that includes mechanical and gap junctions but may exclude components of the VGSC. We speculate that lateralization results from redirectionality of microtubule-mediated forward trafficking. Remodeling of junctional complexes may preserve electrical synchrony under conditions that disrupt ID integrity.

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.

Arrhythmogenic substrate in hearts of rats with monocrotaline-induced pulmonary hypertension and right ventricular hypertrophy

Mechanisms associated with right ventricular (RV) hypertension and arrhythmias are less understood than those in the left ventricle (LV). The aim of our study was to investigate whether and by what mechanisms a proarrhythmic substrate exists in a rat model of RV hypertension and hypertrophy. Rats were injected with monocrotaline (MCT; 60 mg/kg) to induce pulmonary artery hypertension or with saline (CON). Myocardial levels of mRNA for genes expressing ion channels were measured by real-time RT-PCR. Monophasic action potential duration (MAPD) was recorded in isolated Langendorff-perfused hearts. MAPD restitution was measured, and arrhythmias were induced by burst stimulation. Twenty-two to twenty-six days after treatment, MCT animals had RV hypertension, hypertrophy, and decreased ejection fractions compared with CON. A greater proportion of MCT hearts developed sustained ventricular tachycardias/fibrillation (0.83 MCT vs. 0.14 CON). MAPD was prolonged in RV and less so in the LV of MCT hearts. There were decreased levels of mRNA for K(+) channels. Restitution curves of MCT RV were steeper than CON RV or either LV. Dispersion of MAPD was greater in MCT hearts and was dependent on stimulation frequency. Computer simulations based on ion channel gene expression closely predicted experimental changes in MAPD and restitution. We have identified a proarrhythmic substrate in the hearts of MCT-treated rats. We conclude that steeper RV electrical restitution and rate-dependant RV-LV action potential duration dispersion may be contributing mechanisms and be implicated in the generation of arrhythmias associated with in RV hypertension and hypertrophy.

Enhanced protection against pulmonary hypertension with sildenafil and endothelial progenitor cell in rats

BACKGROUND

Sildenafil and bone marrow-derived endothelial progenitor cells (BMDEPCs) have been shown to ameliorate monocrotaline (MCT)-induced pulmonary arterial hypertension (PAH) in the rat. We test whether combined sildenafil and BMDEPC treatment exerts additional protection against MCT-induced PAH in rats.

METHODS

Male Sprague-Dawley rats were randomized to receive saline injection only (group 1), MCT (70 mg/kg) only (group 2), MCT plus autologous BMDEPC (2.0×10(6) cells) transplantation (group 3), MCT with sildenafil (30 mg/kg/day) (group 4), and MCT with combined BMDEPCs-sildenafil (30 mg/kg/day) (group 5). Intravenous BMDEPC and oral sildenafil were given on day 3 after MCT administration. Hemodynamics were analyzed using Labchart software, whereas cellular and molecular parameters were measured using flow cytometry, real-time PCR, TUNEL assay, Western blot, and immunohistochemical staining.

RESULTS

By day 35 following MCT treatment, lower expression of connexin43, protein kinase C-ε, Bcl-2, and endothelial nitric oxide synthase and higher expression of tumor necrosis factor-α and caspase 3 were found in right ventricle (RV) and lung in group 2 compared with other groups (all p<0.05). The number of alveolar sacs and lung arterioles were also lower in group 2 than in other groups (all p<0.05). Furthermore, RV systolic pressure (RVSP), RV weight, and RV-to-final body weight ratio were substantially increased in group 2 than in other groups, and notably higher in groups 3 and 4 than in groups 1 and 5 (all p<0.0001).

CONCLUSIONS

Combined therapy with autologous BMDEPC and sildenafil is superior to either BMDPEC or sildenafil alone for preventing MCT-induced PAH.

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.

Sildenafil limits monocrotaline-induced pulmonary hypertension in rats through suppression of pulmonary vascular remodeling

We hypothesize that sildenafil attenuates pulmonary hypertension through suppressing pulmonary vascular remodeling. Thirty male adult Sprague-Dawley rats were randomized to receive saline injection (Group 1), subcutaneous monocrotaline (MCT) (60 mg/kg) (Group 2), and MCT plus oral sildenafil (30 g/kg per day) (Group 3) 5 days after MCT administration. By Day 35, Western blot showed lower connexin43 and membranous protein kinase C epsilon expressions but higher oxidative stress in right ventricle in Group 2 compared with the other groups. Additionally, pulmonary Smad1/5 was lowest, whereas connexin43 and Smad3 were highest in Group 2. Pulmonary mRNA expressions of tumor necrosis factor-alpha, caspase-3, plasminogen activator inhibitor-1, and transforming growth factor-beta were higher, whereas bone morphogenetic protein Type II receptor, Bcl-2, and endothelial nitric oxide synthase were lower in Group 2 than in the other groups. Similarly, mRNA expressions of tumor necrosis factor-alpha, caspase-3, and beta-myosin heavy chain were increased, whereas Bcl-2, endothelial nitric oxide synthase, and alpha-myosin heavy chain expressions in right ventricle were reduced in Group 2 compared with the other groups. Number of lung arterioles was lowest, whereas number of arterioles with muscularization of the medial layer was highest in Group 2. Right ventricle systolic pressure and weight were elevated in Group 2 compared with the other groups. In conclusion, sildenafil effectively alleviates MCT-induced pulmonary hypertension through suppressing pulmonary vascular remodeling.

Effect of pravastatin on ventricular arrhythmias in infarcted rats: role of connexin43

Epidemiologic studies showed that men treated with statins appear to have a lower incidence of sudden death than men without statins. However, the specific factor for this remained disappointingly elusive. We assessed whether pravastatin enhanced connexin43 expression after myocardial infarction through attenuation of endothelin-1. Twenty-four hours after ligation of the anterior descending artery, male Wistar rats were randomized to vehicle, pravastatin, mevalonate, bosentan, or a combination of pravastatin and mevalonate or pravastatin and bosentan for 4 wk. Myocardial endothelin-1 levels were significantly elevated in vehicle-treated rats at the border zone compared with sham-operated rats. Myocardial connexin43 expression at the border zone was significantly decreased in vehicle-treated infarcted rats compared with sham-operated rats. Attenuated connexin43 expression was blunted after administration of pravastatin, as assessed by immunofluorescence analysis, Western blotting, and real-time quantitative RT-PCR of connexin43. Bosentan enhanced connexin43 amount in infarcted rats and did not have additional beneficial effects on pravastatin-treated rats. Arrhythmic scores during programmed stimulation in vehicle-treated rats were significantly higher than scores in those treated with pravastatin. In contrast, the beneficial effects of pravastatin-induced connexin43 were abolished by the addition of mevalonate and a protein kinase C inducer. In addition, the amount of connexin43 showed significant increase after addition of bisindolylmaleimide, implicating that protein kinase C is a relevant target in endothelin-1-mediated connexin43 expression. Thus chronic use of pravastatin after infarction, resulting in enhanced connexin43 amount by attenuation of mevalonate-dependent endothelin-1 through a protein kinase C-dependent pathway, may attenuate the arrhythmogenic response to programmed electrical stimulation.

Exacerbation of acute viral myocarditis by tobacco smoke is associated with increased viral load and cardiac apoptosis

Exposure to tobacco smoke is known to have deleterious cardiovascular effects. In this study, we tested whether exposure to tobacco smoke exacerbates the severity of viral myocarditis in mice. Viral myocarditis was generated in 4-week-old male BALB/c mice by injection of Encephalomyocarditis virus (EMCV). Four groups were studied: (1) control (C, no smoke and no virus); (2) smoke only (S, exposure to cigarette smoke for 90 min/day for 15 days); (3) virus only (V); and (4) exposure to smoke for 5 days before plus 10 days following virus injection (S+V). We found that viral inoculation preceded by smoke exposure increased mortality more than twofold compared with virus inoculation alone. In addition, the mRNA level of atrial natriuretic factor was significantly higher in S+V than among any of the other 3 groups. Virus injection significantly decreased cardiac function compared with controls, with further deterioration observed in the S+V group. We also observed a significantly increased rate of apoptosis, with an increased activation of apoptosis-inducing factor in hearts exposed to S+V compared with those exposed to V alone. Our results suggest that preexposure to smoke significantly exacerbates the severity of viral myocarditis, likely through increased viral load and increased cardiomyocyte cell death.

Ultrastructure and regulation of lateralized connexin43 in the failing heart

RATIONALE

Gap junctions mediate cell-to-cell electric coupling of cardiomyocytes. The primary gap junction protein in the working myocardium, connexin43 (Cx43), exhibits increased localization at the lateral membranes of cardiomyocytes in a variety of heart diseases, although the precise location and function of this population is unknown.

OBJECTIVE

To define the subcellular location of lateralized gap junctions at the light and electron microscopic level, and further characterize the biochemical regulation of gap junction turnover.

METHODS AND RESULTS

By electron microscopy, we characterized gap junctions formed between cardiomyocyte lateral membranes in failing canine ventricular myocardium. These gap junctions were varied in structure and appeared to be extensively internalizing. Internalized gap junctions were incorporated into multilamellar membrane structures, with features characteristic of autophagosomes. Intracellular Cx43 extensively colocalized with the autophagosome marker GFP-LC3 when both proteins were exogenously expressed in HeLa cells, and endogenous Cx43 colocalized with GFP-LC3 in neonatal rat ventricular myocytes. Furthermore, a distinct phosphorylated form of Cx43, as well as the autophagosome-targeted form of LC3 (microtubule-associated protein light chain 3) targeted to lipid rafts in cardiac tissue, and both were increased in heart failure.

CONCLUSIONS

Our data demonstrate a previously unrecognized pathway of gap junction internalization and degradation in the heart and identify a cellular pathway with potential therapeutic implications.

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.

Administration of connexin43 siRNA abolishes secretory pulse synchronization in GnRH clonal cell populations

GnRH is released from hypothalamic neurons in coordinated pulses, but the cellular basis for this process is poorly understood. Previously, we found that secretory pulses from GT1-7 cells became synchronized with time in culture. Using this culture model, we investigated whether the gap junction proteins connexin43 (Cx43) and/or connexin26 (Cx26) are involved in this synchronization. Our results reveal that cytoplasmic densities immunoreactive for Cx43, and mRNA or protein for Cx43 increase with time in culture. Also, microinjection of day-3 cultures with siRNA for Cx43 abolished synchronized activity at day 7. Interestingly, cytoplasmic plaques, mRNA, or protein for Cx26 remained stable with culture time and Cx26 siRNA administration did not alter secretory activity. Our findings demonstrate that Cx43, but not Cx26 is necessary for synchronized secretory activity in these GT1-7 cultures and raise the possibility that Cx43-related gap junctions may be important in GnRH neuronal coordination in the hypothalamus.

Connexin43 knockdown or overexpression modulates cell coupling in control and failing rabbit left ventricular myocytes

AIMS

We have shown that failing human and rabbit left ventricle (LV) exhibits downregulation and dephosphorylation of connexin43 (Cx43) and that Cx43 dephosphorylation in heart failure (HF) contributes to reduced cell coupling. However, the role of Cx43 downregulation per se in impaired coupling in HF is unclear.

METHODS AND RESULTS

First, we used adenovirus (Ad) encoding a Cx43 siRNA sequence to knock down Cx43 protein levels in cultured control rabbit LV myocytes. Cells cultured for up to 48 h with intermittent pacing maintained Cx43 protein levels and phosphorylation status. Cell coupling in Cx43 knockdown myocyte pairs (by Lucifer Yellow dye transfer) was markedly reduced after 24 h infection (associated with approximately 40% Cx43 knockdown) and after 48 h (associated with approximately 70% Cx43 knockdown). The phosphorylation status, distribution of remaining Cx43 proteins, and levels of other cardiac connexins (Cx40 and Cx45) were unchanged. Second, we overexpressed Cx43 to levels comparable to control using an adenovirus encoding wild-type Cx43 (Cx43WT) gene in isolated LV myocytes from our arrhythmogenic HF rabbit model. We found 87% more Cx43WT proteins improved dye coupling [vs. Ad-beta-galactosidase (LacZ) infected HF controls]. Overexpressed Cx43 protein was located throughout the myocyte membrane (same pattern as in controls), and the phosphorylation status of Cx43 remained comparable to that in AdLacZ infected HF controls.

CONCLUSION

In addition to Cx43 dephosphorylation, downregulation of Cx43 plays an essential role in reduced cell coupling in the failing rabbit heart. Modulation of Cx43 expression could be a novel therapeutic approach to improve conduction and decrease sudden death in HF.