The Mineralocorticoid Receptor Promotes Fibrotic Remodeling in Atrial Fibrillation

We studied the role of the mineralocorticoid receptor (MR) in the signaling that promotes atrial fibrosis. Left atrial myocardium of patients with atrial fibrillation (AF) exhibited 4-fold increased hydroxyproline content compared with patients in sinus rhythm. Expression of MR was similar, as was 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), which also increased. 11β-HSD2 converts cortisol to receptor-inactive metabolites allowing aldosterone occupancy of MR. 11β-HSD2 was up-regulated by arrhythmic pacing in cultured cardiomyocytes and in a mouse model of spontaneous AF (RacET). In cardiomyocytes, aldosterone induced connective tissue growth factor (CTGF) in the absence but not in the presence of cortisol. Hydroxyproline expression was increased in cardiac fibroblasts exposed to conditioned medium from aldosterone-treated cardiomyocytes but not from cardiomyocytes treated with both cortisol and aldosterone. Aldosterone increased connective tissue growth factor and hydroxyproline expression in cardiac fibroblasts, which were prevented by BR-4628, a dihydropyridine-derived selective MR antagonist, and by spironolactone. Aldosterone activated RhoA GTPase. Rho kinase inhibition by Y-27632 prevented CTGF and hydroxyproline, whereas the RhoA activator CN03 increased CTGF expression. Aldosterone and CTGF increased lysyl oxidase, and aldosterone enhanced miR-21 expression. MR antagonists reduced the aldosterone but not the CTGF effect. In conclusion, MR signaling promoted fibrotic remodeling. Increased expression of 11β-HSD2 during AF leads to up-regulation of collagen and pro-fibrotic mediators by aldosterone, specifically RhoA activity as well as CTGF, lysyl oxidase, and microRNA-21 expression. The MR antagonists BR-4628 and spironolactone prevent these alterations. MR inhibition may, therefore, represent a potential pharmacologic target for the prevention of fibrotic remodeling of the atrial myocardium.     

Role of ion channels in heart failure and channelopathies

Heart failure (HF) is a complication of multiple cardiac diseases and is characterized by impaired contractile and electric function. Patients with HF are not only limited by reduced contractile function but are also prone to life-threatening ventricular arrhythmias. HF itself leads to remodeling of ion channels, gap junctions, and intracellular calcium handling abnormalities that in combination with structural remodeling, e.g., fibrosis, produce a substrate for an arrhythmogenic disorders. Not only ventricular life-threatening arrhythmias contribute to increased morbidity and mortality but also atrial arrhythmias, especially atrial fibrillation (AF), are common in HF patients and contribute to morbidity and mortality. The distinct ion channel remodeling processes in HF and in channelopathies associated with HF will be discussed. Further basic research and clinical studies are needed to identify underlying molecular pathways of HF pathophysiology to provide the basis for improved patient care and individualized therapy based on individualized ion channel composition and remodeling.     

In Silico Screening of the Key Cellular Remodeling Targets in Chronic Atrial Fibrillation

Chronic atrial fibrillation (AF) is a complex disease with underlying changes in electrophysiology, calcium signaling and the structure of atrial myocytes. How these individual remodeling targets and their emergent interactions contribute to cell physiology in chronic AF is not well understood. To approach this problem, we performed in silico experiments in a computational model of the human atrial myocyte. The remodeled function of cellular components was based on a broad literature review of in vitro findings in chronic AF, and these were integrated into the model to define a cohort of virtual cells. Simulation results indicate that while the altered function of calcium and potassium ion channels alone causes a pronounced decrease in action potential duration, remodeling of intracellular calcium handling also has a substantial impact on the chronic AF phenotype. We additionally found that the reduction in amplitude of the calcium transient in chronic AF as compared to normal sinus rhythm is primarily due to the remodeling of calcium channel function, calcium handling and cellular geometry. Finally, we found that decreased electrical resistance of the membrane together with remodeled calcium handling synergistically decreased cellular excitability and the subsequent inducibility of repolarization abnormalities in the human atrial myocyte in chronic AF. We conclude that the presented results highlight the complexity of both intrinsic cellular interactions and emergent properties of human atrial myocytes in chronic AF. Therefore, reversing remodeling for a single remodeled component does little to restore the normal sinus rhythm phenotype. These findings may have important implications for developing novel therapeutic approaches for chronic AF.     

Cardiac expression and atrial fibrillation-associated remodeling of K2P2.1 (TREK-1) K channels in a porcine model

Aims

Effective management of atrial fibrillation (AF) often remains an unmet need. Cardiac two-pore-domain K⁺ (K_2P) channels are implicated in action potential regulation, and their inhibition has been proposed as a novel antiarrhythmic strategy. K_2P2.1 (TREK-1) channels are expressed in the human heart. This study was designed to identify and functionally express porcine K_2P2.1 channels. In addition, we sought to analyze cardiac expression and AF-associated K_2P2.1 remodeling in a clinically relevant porcine AF model.

Main methods

Three pK_2P2.1 isoforms were identified and amplified. Currents were recorded using voltage clamp electrophysiology in the Xenopus oocyte expression system. K_2P2.1 remodeling was studied by quantitative real time PCR and Western blot in domestic pigs during AF induced by atrial burst pacing.

Key findings

Human and porcine K_2P2.1 proteins share 99% identity. Residues involved in phosphorylation or glycosylation are conserved. Porcine K_2P2.1 channels carried outwardly rectifying K⁺ currents similar to their human counterparts. In pigs, K_2P2.1 was expressed ubiquitously in the heart with predominance in the atrial tissue. AF was associated with time-dependent reduction of K_2P2.1 protein in the RA by 70% (7 days of AF) and 80% (21 days of AF) compared to control animals in sinus rhythm. K_2P2.1 expression in the left atrium, AV node, and ventricles was not affected by AF.

Significance

Similarities between porcine and human K_2P2.1 channels indicate that the pig may represent a valid model for mechanistic and preclinical studies. AF-related atrial K_2P2.1 remodeling has potential implications for arrhythmia maintenance and antiarrhythmic therapy.     

The role of atrial dilatation in the domestication of atrial fibrillation

Numerous clinical investigations as well as recent experimental studies have demonstrated that atrial fibrillation (AF) is a progressive arrhythmia. With time paroxysmal AF becomes persistent and the success rate of cardioversion of persistent AF declines. Electrical remodeling (shortening of atrial refractoriness) develops within the first days of AF and contributes to the increase in stability of the arrhythmia. However, ‘domestication of AF’ must also depend on other mechanisms since the persistence of AF continues to increase after electrical remodeling has been completed. During the first days of AF in the goat, electrical and contractile remodeling (loss of atrial contractility) followed exactly the same time course suggesting that they are due to the same underlying mechanism. Contractile remodeling not only enhances the risk of atrial thrombus formation, it also enhances atrial dilatation by increasing the compliance of the fibrillating atrium. In goats with chronic AV-block atrial dilatation increased the duration of artificially induced AF-episodes but did not change atrial refractoriness or the AF cycle length. When AF was maintained a couple of days in these animals, a shortening of the atrial refractory period did occur. However, the AF cycle length did not decrease. Long lasting episodes of AF with a long AF cycle length and a wide excitable gap suggest that in this model AF is mainly promoted by conduction disturbances. Chronic atrial stretch induces activation of numerous signaling pathways leading to cellular hypertrophy, fibroblast proliferation and tissue fibrosis. The resulting electroanatomical substrate in dilated atria is characterized by increased non-uniform anisotropy and macroscopic slowing of conduction, promoting reentrant circuits in the atria. Prevention of electroanatomical remodeling by blockade of pathways activated by chronic atrial stretch therefore provides a promising strategy for future treatment of AF.     

Mechanisms of perpetuation of atrial fibrillation in chronically dilated atria

The progressive nature of atrial fibrillation (AF) has been demonstrated in numerous experimental as well as clinical investigations. Electrical remodeling (shortening of atrial refractoriness) develops within the first days of AF and contributes to the increase in stability of the arrhythmia. However, "domestication of AF" must also depend on other mechanisms since the stability of AF continues to increase after electrical remodeling has been completed. Chronic atrial stretch induces activation of numerous signaling pathways leading to cellular hypertrophy, fibroblast proliferation and tissue fibrosis. The resulting electro-anatomical substrate is characterized by increased non-uniform anisotropy and local conduction heterogeneities facilitating reentry in the dilated atria. Atrial fibrosis may lead to disruption of the electrical side-to-side junctions between muscle bundles. This can result in electrical dissociation between neighboring muscle bundles, i.e. they become activated out-of-phase. Recent mapping studies in goats with persistent AF showed that electrical dissociation can not only occur between neighboring muscle bundles but also in the third dimension, i.e. between the epicardial layer and the endocardial bundle network. Such endo-epicardial dissociation will significantly increase the number of wavefronts which can simultaneously be present in the atrial wall. This article reviews data suggesting a role of endo-epicardial dissociation in dilated and fibrillating atria, for the self-perpetuating nature of AF as well as its possible implications for therapeutic interventions.     

Structural atrial remodeling alters the substrate and spatiotemporal organization of atrial fibrillation: a comparison in canine models of structural and electrical atrial remodeling

Several animal models of atrial fibrillation (AF) have been developed that demonstrate either atrial structural remodeling or atrial electrical remodeling, but the characteristics and spatiotemporal organization of the AF between the models have not been compared. Thirty-nine dogs were divided into five groups: rapid atrial pacing (RAP), chronic mitral regurgitation (MR), congestive heart failure (CHF), methylcholine (Meth), and control. Right and left atria (RA and LA, respectively) were simultaneously mapped during episodes of AF in each animal using high-density (240 electrodes) epicardial arrays. Multiple 30-s AF epochs were recorded in each dog. Fast Fourier transform was calculated every 1 s over a sliding 2-s window, and dominant frequency (DF) was determined. Stable, discrete, high-frequency areas were seen in none of the RAP or control dogs, four of nine MR dogs, four of six CHF dogs, and seven of nine Meth dogs in either the RA or LA or both. Average DFs in the Meth model were significantly greater than in all other models in both LA and RA except LA DFs in the RAP model. The RAP model was the only one with a consistent LA-to-RA DF gradient (9.5 +/- 0.2 vs. 8.3 +/- 0.3 Hz, P < 0.00005). The Meth model had a higher spatial and temporal variance of DFs and lower measured organization levels compared with the other AF models, and it was the only model to show a linear relationship between the highest DF and dispersion (R(2) = 0.86). These data indicate that structural remodeling of atria (models known to have predominantly altered conduction) leads to an AF characterized by a stable high-frequency area, whereas electrical remodeling of atria (models known to have predominantly shortened refractoriness without significant conduction abnormalities) leads to an AF characterized by multiple high-frequency areas and multiple wavelets.     

Icariin attenuates excessive alcohol consumption-induced susceptibility to atrial fibrillation through SIRT3 signaling

Excessive alcohol consumption has long been identified as a risk factor for adverse atrial remodeling and atrial fibrillation (AF). Icariin is a principal active component from traditional Chinese medicine Herba Epimedii and has been demonstrated to exert potential antiarrhythmic effect. The present study was designed to evaluate the effect of icariin against alcohol-induced atrial remodeling and disruption of mitochondrial dynamics and furthermore, to elucidate the underlying mechanisms. Excessive alcohol-treated C57BL/6 J mice were infected with serotype 9 adeno-associated virus (AAV9) carrying mouse SIRT3 gene or negative control virus. Meanwhile, icariin (50 mg/kg/d) was administered to the animals in the presence or absence of AAV9 carrying SIRT3 shRNA. We noted that 8 weeks of icariin treatment effectively attenuated alcohol consumption-induced atrial structural and electrical remodeling as evidenced by reduced AF inducibility and reversed atrial electrical conduction pattern as well as atrial enlargement. Furthermore, icariin-treated group exhibited significantly enhanced atrial SIRT3-AMPK signaling, decreased atrial mitoSOX fluorescence and mitochondrial fission markers, elevated mitochondrial fusion markers (MFN1, MFN2) as well as NRF-1-Tfam-mediated mitochondrial biogenesis. Importantly, these beneficial effects were mimicked by SIRT3 overexpression while abolished by SIRT3 knockdown. These data revealed that targeting atrial SIRT3-AMPK signaling and preserving mitochondrial dynamics might serve as the novel therapeutic strategy against alcohol-induced AF genesis. Additionally, icariin ameliorated atrial remodeling and mitochondrial dysfunction by activating SIRT3-AMPK signaling, highlighting the use of icariin as a promising antiarrhythmic agent in this circumstance.     

虚拟生理心脏模型及房颤机制研究进展

房颤是临床上最常见的持续性心律失常.揭示房颤的发病机制和病理生理过程是其诊断、预防、治疗、药物研发及临床设备设计的关键,而实验和临床只能呈现细胞或亚细胞的局部特性及房颤病症的宏观结果.随着生物信息技术、统计分析技术等的发展,运用多物理尺度的虚拟生理心脏模型,来实现宏观结果与微观机制相统一的研究方法备受关注.本文综述了离子通道、心肌细胞、心脏组织及器官等多尺度的虚拟生理心脏模型研究进展,探讨了近年来基于虚拟生理心脏模型的房颤机制研究以及房颤的治疗手段,提示了房颤研究的挑战和未来的发展方向.     

Intact female stroke-prone hypertensive rats lack responsiveness to mineralocorticoid receptor antagonists

Data from the Framingham Heart Study suggest that women may be more sensitive to the deleterious cardiovascular remodeling effects of aldosterone. Previous studies from our laboratory have shown that chronic treatment with spironolactone, a mineralocorticoid receptor (MR) antagonist, decreases ischemic cerebral infarct size and prevents remodeling of the middle cerebral artery (MCA) in male spontaneously hypertensive stroke-prone rats (SHRSP). Therefore, we hypothesized that MR antagonism would reduce ischemic infarct size and prevent MCA remodeling in female SHRSP. Six-week-old female SHRSP were treated for 6 wk with spironolactone (25 or 50 mg.kg(-1).day(-1)) or eplerenone (100 mg.kg(-1).day(-1)) and compared with untreated controls. At 12 wk, cerebral ischemia was induced for 18 h using the intraluminal suture occlusion technique, or the MCA was isolated for analysis of passive structure using a pressurized arteriograph. MR antagonism had no effect on infarct size or passive MCA structure in female SHRSP. To study the potential effects of estrogen, the above experiments were repeated in bilaterally ovariectomized (OVX) female SHRSP treated with spironolactone (25 mg.kg(-1).day(-1)). Infarct size and vessel structure in OVX SHRSP were not different from control SHRSP. Spironolactone had no effect on infarct size in OVX SHRSP. However, MCA lumen and outer diameters were increased in spironolactone-treated OVX SHRSP, suggesting an effect of estrogen. Cerebral artery MR expression, assessed by Western blotting, was increased in female, compared with male, SHRSP. These studies highlight an apparent sexual dimorphism of MR expression and activity in the cerebral vasculature from hypertensive rats.     

心房颤动与Kv1.5钾通道阻滞剂及其研究进展

心房颤动是临床常见的心律失常,药物是心房颤动的主要治疗方法。胺碘酮和心律平等药物虽然可以治疗和转复心房颤动,但长期应用会引起恶性心律失常和心脏外的副作用。抑制Kv1.5钾通道电流,可选择性延长心房肌动作电位时程及有效不应期,改善心房肌的电重构和组织重构。近年来关于Kv1.5钾通道及其阻滞剂的研究迅速发展并引起广泛关注。为进一步探讨Kv1.5钾通道是否可能成为心房颤动的治疗靶点,我们对目前相关研究进展作一综述。     

Routine versus aggressive upstream rhythm control for prevention of early atrial fibrillation in heart failure: background, aims and design of the RACE 3 study

Background

Rhythm control for atrial fibrillation (AF) is cumbersome because of its progressive nature caused by structural remodelling. Upstream therapy refers to therapeutic interventions aiming to modify the atrial substrate, leading to prevention of AF.

Objective

The Routine versus Aggressive upstream rhythm Control for prevention of Early AF in heart failure (RACE 3) study hypothesises that aggressive upstream rhythm control increases persistence of sinus rhythm compared with conventional rhythm control in patients with early AF and mild-to-moderate early systolic or diastolic heart failure undergoing electrical cardioversion.

Design

RACE 3 is a prospective, randomised, open, multinational, multicenter trial. Upstream rhythm control consists of angiotensin converting enzyme inhibitors and/or angiotensin receptor blockers, mineralocorticoid receptor antagonists, statins, cardiac rehabilitation therapy, and intensive counselling on dietary restrictions, exercise maintenance, and drug adherence. Conventional rhythm control consists of routine rhythm control therapy without cardiac rehabilitation therapy and intensive counselling. In both arms, every effort is made to keep patients in the rhythm control strategy, and ion channel antiarrhythmic drugs or pulmonary vein ablation may be instituted if AF relapses. Total inclusion will be 250 patients. If upstream therapy proves to be effective in improving maintenance of sinus rhythm, it could become a new approach to rhythm control supporting conventional pharmacological and non-pharmacological rhythm control.     

Decreased expression of small-conductance Ca2+-activated K+ channels SK1 and SK2 in human chronic atrial fibrillation

AimsSmall-conductance Ca^2 +-activated K⁺ (SK) channels are recognized as new ion channel candidates in atrial fibrillation (AF), with pivotal implications as novel drug targets due to their atrial-selective distribution in humans. The purpose of this study was to investigate whether SK channels and the Ca^2 +-activated K⁺ current (I_K,Ca) are involved in electrical remodeling of human chronic AF (cAF) and whether they display the differential distribution between the right (RA) and left atria (LA).Main methodsThe right (RAA) and left atrial appendage (LAA) myocytes were obtained from 29 sinus rhythm (SR) and 22 cAF patients. The I_K,Ca and action potential (AP) were recorded using the patch-clamp technique. Three SK channel subtypes (SK1–3) expressions were assayed by western blot and real-time quantitative PCR analysis.Key findingsThe I_K,Ca was decreased and its role in AP repolarization was attenuated in cAF, concomitant with a significant decrease in protein and mRNA levels of SK1 and SK2. In either SR or cAF, there was no difference in the I_K,Ca density and protein and mRNA expression levels of SK1–3 between RAA and LAA myocytes.SignificanceOur results demonstrated that SK1 and SK2 are involved in electrical remodeling of cAF. SK1–3 and I_K,Ca do not display the inter-atrial differential distribution in SR or cAF. These findings provide a new insight into mechanisms of electrical remodeling of human cAF.     

Role of HDACs in cardiac electropathology: Therapeutic implications for atrial fibrillation

Perpetuation of atrial fibrillation (AF) is caused by electropathology, which is defined as impairment of electrical activation caused by structural and metabolic remodeling of cardiomyocytes. We previously dissected the molecular mechanisms underlying electropathology and identified an important role for histone deacetylases (HDACs). HDACs catalyze the removal of acetyl-groups from lysine residues within nucleosomal histone tails and many non-histone proteins. Various HDAC inhibitors are efficacious in attenuating electropathology, and improve contractile function in experimental AF. Emerging evidence reveals novel mechanisms by which HDAC inhibitors prevent cardiac electropathology and thereby benefit the heart during AF. These mechanisms include post-translational modification of contractile and structural proteins and changes in gene expression. In this review paper, we summarize recent findings on novel functions of zinc-dependent HDACs in electropathology and discuss the potential for pharmacological HDAC inhibition as a strategy to treat AF.     

Direction-dependent conduction abnormalities in a canine model of atrial fibrillation due to chronic atrial dilatation

Chronic rapid atrial pacing (RAP) leads to changes that perpetuate atrial fibrillation (AF). Chronic atrial dilatation due to mitral regurgitation (MR) also increases AF inducibility, but it is not clear whether the underlying mechanism is similar. Therefore, we have investigated atrial electrophysiology in a canine MR model (mitral valve avulsion, 1 mo) using high-resolution optical mapping and compared it with control dogs and with the canine RAP model (6-8 wk of atrial pacing at 600 beats/min, atrioventricular block, and ventricular pacing at 100 beats/min). At followup, optical action potentials were recorded using a 16 x 16 photodiode array from 2 x 2-cm left atrial (LA) and right atrial (RA) areas in perfused preparations, with pacing electrodes around the field of view to study direction dependency of conduction. Action potential duration at 80% repolarization (APD(80)) was not different between control and MR but was reduced in RAP atria. Conduction velocities during normal pacing were not different between groups. However, the MR LA showed increased conduction heterogeneity during pacing at short cycle lengths and during premature extrastimuli, which frequently caused pronounced regional conduction slowing. Conduction in the MR LA during extrastimulation also displayed a marked dependence on propagation direction. These phenomena were not observed in the MR RA and in control and RAP atria. Thus both models form distinctly different AF substrates; in RAP dogs, the decrease in APD(80) may stabilize reentry. In MR dogs, regional LA conduction slowing and increased directional dependency, allowing unidirectional conduction block and preferential paths of conduction, may account for increased AF inducibility.     

Aldosterone regulation of T-type calcium channels

Voltage-operated calcium channels play a crucial role in signal transduction in many excitable and non-excitable cell types. While a rapid modulation of their activity by hormone-activated kinases and/or G proteins has been recognized for a long time, a sustained control of their expression level has been only recently demonstrated. In adrenal H295R cells, for example, aldosterone treatment selectively increased low threshold T-type calcium current density without affecting L-type currents. Antagonizing the mineralocorticoid receptor (MR) with spironolactone prevented aldosterone action on T-type currents. By RT-PCR, we detected in these cells the presence of two different isoforms of L-type channels, alpha(1)C and alpha(1)D, and one isoform of T channel, alpha(1)H. A second T channel isoform (alpha(1)G) was also observed under particular culture conditions. Quantification of the specific messenger RNA by real time RT-PCR allowed us to show a 40% increase of the alpha1H messenger levels upon aldosterone treatment (alpha(1)G was insensitive), a response that was also completely prevented by spironolactone. Because T-type, but not L-type channel activity is linked to steroidogenesis, this modulation represents a positive, intracrine feed back mechanism exerted by aldosterone on its own production.Aldosterone has been also implicated in the pathogenesis and progression of ventricular hypertrophy and heart failure independently of its action on arterial blood pressure. We have observed that, in rat neonatal cardiomyocytes, aldosterone increases (by two-fold) L-type calcium current amplitude in ventricular but not in atrial cells. No significant effect of aldosterone could be detected on T-type currents, that were much smaller than L-type currents in these cells. However, aldosterone exerted opposite effects on T channel isoform expression, increasing alpha(1)H and decreasing alpha(1)G. Although the functional role of T channels is still poorly defined in ventricular cardiomyocytes, an overexpression of alpha(1)H could be partially responsible for the arrhythmias linked to hyperaldosteronism.Finally, T channels also appear to be involved in the neuroendocrine differentiation of prostate epithelial cells, a poor prognosis in prostate cancer. We have shown that the only calcium channel expressed in the prostatic LNCaP cells is the alpha(1)H isoform and that induction of cell differentiation with cAMP leads to a concomitant increase in both T-type current and alpha(1)H mRNA. In spite of the presence of MR in these cells, aldosterone only modestly increased alpha(1)H mRNA levels. A functional role for these channels was suggested by the observation that low nickel concentrations prevent neuritic process outgrowth.In conclusion, it appears that T-type calcium channel expression vary in different patho-physiological conditions and that aldosterone, in several cell types, is able to modulate this expression.     

Signal-transduction pathways that regulate smooth muscle function. II. Receptor-ion channel coupling mechanisms in gastrointestinal smooth muscle

Regulation of membrane ion channels by second messengers is an important mechanism by which gastrointestinal smooth muscle excitability is controlled. Receptor-mediated phosphorylation of Ca(2+) channels has been known for some time; however, recent findings indicate that these channels may also modulate intracellular signaling. The plasmalemma ion channels may also function as a point of convergence between different receptor types. In this review, the molecular mechanisms that link channel function and signal transduction are discussed. Emerging evidence also indicates altered second-messenger modulation of the Ca(2+) channel in the pathophysiology of smooth muscle dysmotility.