Role of atrial endothelial cells in the development of atrial fibrosis and fibrillation in response to pressure overload.

BACKGROUND: Monocyte chemoattractant protein-1 (MCP-1)-mediated inflammatory mechanisms have been shown to play a crucial role in atrial fibrosis induced by pressure overload. In the present study, we investigated whether left atrial endothelial cells would quickly respond structurally and functionally to pressure overload to trigger atrial fibrosis and fibrillation. METHODS AND RESULTS: Six-week-old male Sprague-Dawley rats underwent suprarenal abdominal aortic constriction (AAC) or a sham operation. By day 3 after surgery, macrophages were observed to infiltrate into the endocardium. The expression of MCP-1 and E-selectin in atrial endothelium and the expression of intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and ED1 in left atrial tissue were enhanced. Atrial endothelial cells were irregularly hypertrophied with the disarrangement of lines of cells by scanning electron microscopy. Various-sized gap formations appeared along the border in atrial endothelial cells, and several macrophages were located just in the endothelial gap. Along with the development of heterogeneous interstitial fibrosis, interatrial conduction time was prolonged and the inducibility of atrial fibrillation by programmed extrastimuli was increased in the AAC rats compared to the sham-operated rats. CONCLUSIONS: Atrial endothelium responds rapidly to pressure overload by expressing adhesion molecules and MCP-1, which induce macrophage infiltration into the atrial tissues. These processes could be an initial step in the development of atrial remodeling for atrial fibrillation.

Splenectomy exacerbates atrial inflammatory fibrosis and vulnerability to atrial fibrillation induced by pressure overload in rats: Possible role of spleen-derived interleukin-10.

BACKGROUND: The spleen is important for cardiac remodeling induced by myocardial infarction. However, the role of the spleen in inflammatory atrial fibrosis induced by pressure overload is unknown. OBJECTIVE: The purpose of this study was to investigate whether splenectomy (SPX) attenuates or exacerbates pressure overload-induced atrial inflammatory fibrosis and vulnerability to atrial fibrillation (AF) in rats. METHODS: Male Sprague-Dawley rats (6 weeks old) were divided into Sham+Sham, Sham+SPX, abdominal aortic constriction (AAC)+Sham, and AAC+SPX groups, and were evaluated for inflammation, fibrosis, and AF on days 2, 4, 14, and 28. RESULTS: On day 4, an AAC-induced rise in interleukin-10 (IL-10) level was observed in the spleen, serum, and left atrium (LA), with SPX showing inhibitory effects in the latter 2 instances. In addition, AAC-induced M2 macrophage recruitment into the LA was decreased by SPX, as determined by immunofluorescence labeling (P <.05). On day 28, AAC-induced heterogeneous interstitial fibrosis of the LA was enhanced by SPX (P <.05). Electrophysiologic recordings revealed that the duration of AF and prolongation of interatrial conduction time induced by AAC were increased by SPX (P < .01 and P <.05, respectively). Furthermore, in the AAC+SPX group, the number of macrophages infiltrating into the LA on day 2 was marginal, but increased on day 28 relative to the AAC+Sham group. IL-10 administration attenuated the AAC-induced atrial remodeling that was aggravated by SPX. CONCLUSION: The study results suggest that SPX exacerbates AAC-induced inflammatory atrial fibrosis and increases vulnerability to AF after 4 weeks, likely because of depletion of spleen-derived IL-10.

Role of leptin signaling in the pathogenesis of angiotensin II-mediated atrial fibrosis and fibrillation.

BACKGROUND: We examined the hypothesis that leptin signaling contributes to the atrial fibrosis and atrial fibrillation (AF) evoked by angiotensin II (AngII). METHODS AND RESULTS: Eight-week-old male CL57/B6 (CNT) and leptin-deficient ob/ob mice (Ob) were subcutaneously infused with AngII (2.0 mg/kg per day). Two weeks later, transesophageal burst pacing and an electrophysiological study using isolated perfused hearts were performed. Left-atrial tissues were collected to determine interstitial fibrosis by Masson trichrome staining, and the expressions of mRNAs related to inflammatory profibrotic signals were assessed. Left-atrial fibroblasts were isolated from adult Sprague-Dawley and Zucker rats. The effects of leptin (100 ng/mL) or AngII (100 nmol/L) treatment were evaluated. In CNT-AngII mice, leptin expression in the left atrium was upregulated (P<0.01). Transesophageal burst pacing induced atrial fibrillation in 88% (7/8) of CNT-AngII mice, but not in Ob-AngII mice (0/8; P<0.01). In isolated perfused hearts, atrial fibrillation was induced only in CNT-AngII mice (4/6; 67%). Interatrial conduction time was prolonged in CNT-AngII mice (P<0.01), but not in Ob-AngII mice. The upregulation of collagen 1, collagen 3, transforming growth factor-ß1, α-smooth muscle actin, MCP-1, F4/80, and RANTES mRNA, which was seen in CNT-AngII mice, was attenuated in Ob-AngII mice. In cultured Sprague-Dawley rat atrial fibroblasts, AngII treatment increased leptin expression (P<0.01). Addition of leptin increased transforming growth factor-ß1, α-smooth muscle actin, MCP-1, and RANTES expressions in Sprague-Dawley rat atrial fibroblasts, but not in Zucker rat atrial fibroblasts. CONCLUSIONS: Our results demonstrate for the first time that leptin signaling is essential for the development of atrial fibrosis and atrial fibrillation evoked by AngII.

Novel strategy to prevent atrial fibrosis and fibrillation.

To explore a novel strategy of preventing atrial fibrosis and atrial fibrillation (AF), we have established 3 appropriate experimental models of AF. Firstly, atrial fibrosis was induced by pressure overload by abdominal aortic constriction (AAC). AAC enhanced left atrial (LA) expression of monocyte chemoattractant protein-1. Scanning electron microscopy revealed that LA endothelial cells were irregularly hypertrophied, with disarrangement of lines of cells. Possible "arrested" leukocyte-derived cells were observed on the surface of LA endothelial cells. Treatment with pioglitazone, a peroxisome proliferator-activated receptor-γ agonist, resulted in attenuation of pressure overload-induced LA fibrosis. Secondly, LA fibrosis was induced by continuous infusion of angiotensin II (AII). Repeated whole-body hyperthermia led to the induction of heat shock protein (HSP) 72, which resulted in attenuation of AII-induced LA fibrosis. Thirdly, atrial fibrosis was induced by 5/6 nephrectomy as a model of AF associated with chronic kidney disease. Because the amount of nicotinamide adenine dinucleotide phosphate oxidase was increased and the potent antioxidant agent was effective, oxidative stress may be involved in the pathogenesis of LA fibrosis and enhanced AF vulnerability in this model. These observations suggest that inflammatory profibrotic processes are essential for the development of atrial fibrosis in these 3 models. Pioglitazone, induction of HSPs and antioxidant agent could be novel therapeutic approaches to preventing atrial fibrosis and AF.

Establishment of a model of atrial fibrillation associated with chronic kidney disease in rats and the role of oxidative stress.

BACKGROUND: An animal model of atrial fibrillation (AF) associated with chronic kidney disease (CKD) has not been available. OBJECTIVE: The purpose of this study was to test the validity of 5/6 nephrectomy (5.6Nx) as an appropriate model of AF associated with CKD and to investigate the role of oxidative stress. METHODS: Male Sprague-Dawley rats were subjected to 5.6Nx. A novel derivative of lipoic acid, sodium zinc dihydrolipoylhistidinate (DHLHZn), was subcutaneously infused. Four weeks later, hearts were isolated. RESULTS: We observed 5 main findings. (1) 5.6Nx induced renal dysfunction with elevation of systolic blood pressure and impaired glucose tolerance. (2) In the left atrium (LA), expressions of α-smooth muscle action and collagen type I, the compositional proteins of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, and malondialdehyde were increased by 5.6Nx, which was reversed by DHLHZn treatment. (3) In the LA, the tissue content of angiotensin II was elevated by 5.6Nx, which was also reversed by DHLHZn. (4) Masson trichrome staining revealed that heterogeneous LA interstitial fibrosis was induced by 5.6Nx, which was attenuated by DHLHZn. (5) In isolated perfused heart experiments, 5.6Nx caused slowing of interatrial conduction. In the hearts of rats of the 5.6Nxgroup, right atrial extrastimuli invariably induced AF (8/8 [100%]), which were suppressed by DHLHZn (3/8 [38%], P <.05). CONCLUSION: We successfully established an appropriate model of AF associated with CKD in rats. Because the amount of NADPH oxidase was increased and the potent antioxidant agent DHLHZn was effective, oxidative stress may be involved in the pathogenesis of LA fibrosis and enhanced AF vulnerability in our model.

Activation of CaMKII as a key regulator of reactive oxygen species production in diabetic rat heart.

Diabetes mellitus is a risk factor for heart failure. Increased reactive oxygen species (ROS) have been proposed as a possible mechanism of cardiac dysfunction in diabetic patients. However, the mechanisms of ROS increase are still elusive. We hypothesized that activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) induced by impaired intracellular Ca(2+) ([Ca(2+)](i)) metabolism may stimulate ROS production in the diabetic heart. Cultured cardiomyocytes from neonatal rats were exposed to high glucose concentrations (25 mmol/L) and ROS levels were analyzed in 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, acetyl ester (CM-H(2)DCFDA)-loaded cells by flow cytometry analysis. Exposure to high glucose concentrations for 24h significantly increased CM-H(2)DCFDA fluorescence, which was significantly inhibited by 1,2-bis (o-aminophenoxy) ethane- N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester (BAPTA-AM), a [Ca(2+)](i) chelator, and KB-R7943, an inhibitor of the Na(+)-Ca(2+) exchanger (NCX) in the reverse mode. These results indicate that [Ca(2+)](i) increase by NCX activation may induce ROS increase following exposure to high glucose concentrations. We confirmed that exposure to high glucose concentrations significantly increased [Ca(2+)](i), which was inhibited by KB-R7943. Na(+)-H(+) exchanger (NHE) is a key factor in [Ca(2+)](i) metabolism, and is known to activate NCX by increasing the intracellular Na(+) ([Na(+)](i)) level. We showed that the expression of NHE isoform 1 and NHE activity increased following exposure to high glucose concentrations by evaluating protein expressions and intracellular pH recovery from acid loading. Exposure to high glucose concentrations up-regulated phosphorylated CaMKII expression in cardiomyocytes that was inhibited by KB-R7943. Further, autocamtide 2-related inhibitory peptide (AIP), a CaMKII inhibitor, significantly attenuated the ROS increase following exposure to high glucose concentrations. We confirmed these results obtained in in vitro experiments in an animal model of diabetes. ROS level and components of NADPH oxidase, p47phox and p67phox were up-regulated in streptozotocin-induced diabetic rat heart, which were attenuated by KN-93, a CaMKII inhibitor. Consistently, expression of phosphorylated CaMKII was increased in the diabetic heart. Activation of CaMKII by impaired [Ca(2+)](i) metabolism may be a mechanism of ROS increase in the heart with diabetes mellitus.

Inhibition of Na⁺-H⁺ exchange as a mechanism of rapid cardioprotection by resveratrol.

BACKGROUND AND PURPOSE Resveratrol is a polyphenol abundantly found in grape skin and red wine. In the present study, we investigated whether resveratrol exerts protective effects against cardiac ischaemia/reperfusion and also explored its mechanisms. EXPERIMENTAL APPROACH Infarct size and functional recovery in rat isolated perfused hearts subjected to no-flow global ischaemia followed by reperfusion were measured. Cultured neonatal rat cardiomyocytes were exposed to H(2)O(2) (100 µmol·L(-1)) to induce cell injury. Intracellular ion concentrations were measured using specific dyes. Western blotting was used to quantify protein expression levels. KEY RESULTS In rat isolated perfused hearts, treatment with resveratrol (20 and 100 µmol·L(-1)) 15 min before ischaemia considerably improved left ventricular functional recovery and infarct size. In cultured neonatal rat cardiomyocytes, resveratrol significantly attenuated the increase in reactive oxygen species (ROS) and loss of mitochondrial inner membrane potential. Resveratrol also suppressed the increase in intracellular concentrations of Na(+) ([Na(+)](i)) and Ca(2+) ([Ca(2+)](i)) after H(2)O(2) application; however, it did not suppress the ouabain-induced [Ca(2+) ](i) increase. By measuring changes in intracellular pH recovery after acidification, we also confirmed that acid-induced activation of the Na(+)-H(+) exchanger (NHE) was prevented by pretreatment with resveratrol. Furthermore, resveratrol inhibited the H(2)O(2)-induced translocation of PKC-α from the cytosol to the cell membrane; this translocation is believed to activate NHE. CONCLUSION AND IMPLICATIONS Resveratrol exerts cardioprotection by reducing ROS and preserving mitochondrial function. The PKC-α-dependent inhibition of NHE and subsequent attenuation of [Ca(2+)](i) overload may be a cardioprotective mechanism.

Cardioprotective effects of pravastatin against lethal ventricular arrhythmias induced by reperfusion in the rat heart.

BACKGROUND: Statins are reported to reduce mortality in patients with coronary artery disease and that mortality benefit might be related to the drugs' antiarrhythmic properties. METHODS AND RESULTS: Male rats were fed with or without pravastatin (0.1 mg·kg⁻¹·day⁻¹) for 7 days, and thereafter subjected to 10 min of ischemia by coronary artery ligation followed by 20 min reperfusion. Treatment with pravastatin reduced the frequency and duration of ventricular tachycardia and fibrillation (VT/VF) and improved the arrhythmia score after reperfusion. To investigate the rapid effects of pravastatin, isolated perfused rat hearts were subjected to 20 min of global ischemia followed by 30 or 60 min of reperfusion. Treatment with pravastatin (10 nmol/L) from 10 min before ischemia shortened the total duration of reperfusion-induced VT/VF. Interestingly, pravastatin administered from the beginning of reperfusion also exerted antiarrhythmic effects. These results indicate that pravastatin exerts antiarrhythmic effects not only with daily oral intake but also when administered just before ischemia or even after ischemia. Intracellular calcium ([Ca²âº](i)) overload and collapse of mitochondrial inner membrane potential (Δψ(m)) are associated with the arrhythmogenesis during ischemia-reperfusion. In cultured cardiomyocytes, pretreatment with pravastatin (10 nmol/L) suppressed [Ca²âº](i) overload and prevented Δψ(m) loss induced by H2O2. CONCLUSIONS: Pravastatin attenuated reperfusion-induced lethal ventricular arrhythmias. Inhibition of [Ca²âº](i) overload and preserving Δψ(m) may be the mechanisms of the observed antiarrhythmic effects of pravastatin.

Pioglitazone attenuates inflammatory atrial fibrosis and vulnerability to atrial fibrillation induced by pressure overload in rats.

BACKGROUND: Inflammatory processes are involved in the pathogenesis of atrial fibrillation (AF). OBJECTIVE: The purpose of this study was to test the hypothesis that atrial fibrosis and enhanced vulnerability to AF evoked by pressure overload can be attenuated by pioglitazone, a peroxisome proliferator-activated receptor-γ agonist, via suppression of inflammatory profibrotic signals. METHODS: Male Sprague-Dawley rats were subjected to abdominal aortic constriction (AAC). Pioglitazone 3 mg/kg/day or vehicle was orally administered for 4 weeks. RESULTS: Western blot analysis revealed that AAC enhanced expression of monocyte chemoattractant protein (MCP)-1, transforming growth factor-ß1 and α-smooth muscle actin in the left atrium (LA), which was suppressed by pioglitazone. Messenger RNA expression of collagen type 1 and atrial natriuretic peptide in the LA was increased by AAC, which was suppressed by pioglitazone. Gelatin zymography demonstrated that activity of matrix metalloproteinase-9 was increased by AAC, which was suppressed by pioglitazone. Pioglitazone attenuated AAC-induced LA fibrosis. In isolated-perfused heart experiments, AAC did not alter the refractory period of the LA or the right atrium (RA), but it did prolong the interatrial conduction time. Programmed extrastimuli from the RA induced AF in all of the AAC-treated rats (8/8 [100%]). This was suppressed by pioglitazone (2/8 [25%], P <.05) with normalization to interatrial conduction time. CONCLUSION: The results of this study suggest that inflammatory profibrotic mechanisms are involved in this pressure-overloaded AF model. The results also suggest that pioglitazone is effective at attenuating atrial fibrosis, possibly via suppression of MCP-1-mediated inflammatory profibrotic processes.