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

Glucose fluctuations increase the incidence of atrial fibrillation in diabetic rats

AIMS

We investigated whether glucose fluctuations aggravate cardiac fibrosis and increase the occurrence of atrial fibrillation (AF) in rats with diabetes mellitus (DM).

METHODS AND RESULTS

Streptozotocin-induced diabetic rats were randomly divided into three groups: uncontrolled DM (U-STZ) group, controlled DM (C-STZ) group, and DM with glucose fluctuations (STZ-GF) group. Glucose fluctuations were induced by fasting for 24 h and additional regular insulin injections (0.5 IU/kg) administered three times per week for three consecutive weeks. C-STZ rats were administered long acting insulin (20 IU/kg) twice a day to control blood glucose levels. Cardiac fibrosis evaluated by Masson trichrome staining and the expressions of collagen type 1, collagen type 3, and α-smooth muscle actin were increased in U-STZ rats compared with C-STZ rats, which were more pronounced in STZ-GF rats. The inducibility of AF was significantly larger in U-STZ rats than C-STZ rats and was greatest in STZ-GF rats. To explore the mechanism of cardiac fibrosis, we investigated the levels of reactive oxygen species (ROS) and apoptosis. The expression of malondialdehyde, an indicator of ROS levels, was significantly upregulated in STZ-GF rats compared with U-STZ rats, along with increased thioredoxin-interacting protein (Txnip) expression in STZ-GF rats. Furthermore, caspase-3 expression and the number of TUNEL-positive cells were significantly increased in STZ-GF rats compared with U-STZ and C-STZ rats.

CONCLUSION

Glucose fluctuations increase the incidence of AF by promoting cardiac fibrosis. Increased ROS levels caused by upregulation of Txnip expression may be a mechanism whereby in glucose fluctuations induce fibrosis.

Bone morphogenetic protein-4 induces upregulation of Cav3.1 Ca²⁺ channels in HL-1 atrial myocytes

Cardiac T-type Ca(2+) channels are reexpressed in atrial and ventricular myocytes under various pathological conditions such as post-myocardial infarction, hypertrophy, and heart failure, but relatively little is known about the mechanisms. Our previous study found that bone morphogenetic protein-4 (BMP4) was reexpressed in pathological cardiac hypertrophy models and BMP4-mediated cardiomyocyte hypertrophy. We hypothesized that BMP4 could upregulate cardiac T-type Ca(2+) channels in HL-1 atrial myocytes. The T-type Ca(2+) currents were recorded by using the patch-clamp technique, and the expressions of Cav3.1 and Cav3.2 were measured by real-time PCR method in HL-1 cells. BMP4 and Cav3.1 mRNA expressions increased in the left atrium from the pressure overload-induced hypertrophy of mice hearts. BMP4 treatment for 48 h induced increase of Cav3.1 but not Cav3.2 mRNA expression in HL-1 cells, and the increase was inhibited by BMP4 inhibitor noggin. Acute treatment with BMP4 did not affect T-type Ca(2+) currents, but chronic treatment (48 h) significantly increased the amplitude of T-type Ca(2+) currents in HL-1 cells. Chronic treatment with BMP4 induced upregulation of NADPH oxidase-4 (NOX4), increase of reactive oxygen species (ROS) level, and activation of mitogen-activated protein kinase (MAPK)-activated protein kinases c-jun N-terminal kinases (JNK) and p38. BMP4-induced upregulation of Cav3.1 mRNA was inhibited by NADPH oxidase inhibitor apocynin, the radical scavenger tempol, JNK inhibitor SP600125, and p38 inhibitor SB203580. In conclusion, BMP4 induces upregulation of Cav3.1 Ca(2+) channels and T-type Ca(2+) currents in HL-1 atrial myocytes through ROS/MAPK pathways.

Beneficial effects of pioglitazone on atrial structural and electrical remodeling in vitro cellular models

It has been demonstrated that atrial remodeling contributes toward atrial fibrillation (AF) maintenance and angiotensin II (AngII) is involved in the pathogenesis of atrial remodeling. Peroxisome proliferator-activated receptor-γ (PPAR-γ) agonists have been shown to inhibit atrial remodeling. However, the underlying mechanisms are poorly understood. In the present study we investigated the regulating effects of PPAR-γ agonist on AngII-induced atrial structural and electrical remodeling in vitro cellular models. The effects of pioglitazone on AngII-induced connective tissue growth factor (CTGF) expression and cell proliferation were assessed in primary-cultured mouse atrial fibroblasts. The influences of pioglitazone on AngII-induced L-type calcium channel (ICa-L) α1c expression and current density were evaluated in atrial myocytes (HL-1). Pioglitazone attenuated AngII-induced CTGF expression and proliferation in atrial fibroblasts, and pioglitazone also inhibited the expression or phosphorylation of AngII-induced transforming growth factor-β1 (TGF-β1), tumor necrosis factor receptor associated factor 6 (TRAF6), TGF-β-associated kinase 1 (TAK1) and Smad2/3. In HL-1 cells, pioglitazone suppressed AngII-induced ICa-L α1c expression and current density as well as CAMP responsive element binding protein (CREB) phosphorylation. Besides, pioglitazone inhibited AngII-induced production of AngII type I receptor (AT1R) and downregulation of PPAR-γ in both atrial fibroblasts and HL-1 cells. In conclusion, Pioglitazone suppresses AngII-induced CTGF expression and proliferation in atrial fibroblasts, which might be at least in part related with its inhibitory effects on TGF-β1/Smad2/3 and TGF-β1/TRAF6/TAK1 signaling pathways. Moreover, pioglitazone also attenuates AngII-induced ICa-L remodeling in HL-1 cells, which might be at least in part associated with its inhibitory effect on CREB phosphorylation. It is suggested that PPAR-γ agonist may have potential applications in preventing atrial remodeling.

Atrial remodeling is directly related to end-diastolic left ventricular pressure in a mouse model of ventricular pressure overload

Background

Atrial fibrillation (AF) is often preceded by underlying cardiac diseases causing ventricular pressure overload.

Objective

It was our aim to investigate the progression of atrial remodeling in a small animal model of ventricular pressure overload and its association with induction of AF.

Methods

Male mice were subjected to transverse aortic constriction (TAC) or sham operation. After four or eight weeks, echocardiographic measurements and hemodynamic measurements were made and AF induction was tested. The hearts were either fixed in formalin or ventricles and atria were separated, weighed and snap-frozen for RNA analysis.

Results

Four weeks of pressure overload induced ventricular hypertrophy and minor changes in the atria. After eight weeks a significant reduction in left ventricular function occurred, associated with significant atrial remodeling including increased atrial weight, a trend towards an increased left atrial cell diameter, atrial dilatation and increased expression of markers of hypertrophy and inflammation. Histologically, no fibrosis was found in the left atrium. But atrial gene expression related to fibrosis was increased. Minor changes related to electrical remodeling were observed. AF inducibility was not different between the groups. Left ventricular end diastolic pressures were increased and correlated with the severity of atrial remodeling but not with AF induction.

Conclusion

Permanent ventricular pressure overload by TAC induced atrial remodeling, including hypertrophy, dilatation and inflammation. The extent of atrial remodeling was directly related to LVEDP and not duration of TAC per se.

Atrial arrhythmia in ageing spontaneously hypertensive rats: unraveling the substrate in hypertension and ageing

Background

Both ageing and hypertension are known risk factors for atrial fibrillation (AF) although the pathophysiological contribution or interaction of the individual factors remains poorly understood. Here we aim to delineate the arrhythmogenic atrial substrate in mature spontaneously hypertensive rats (SHR).

Methods

SHR were studied at 12 and 15 months of age (n = 8 per group) together with equal numbers of age-matched normotensive Wistar-Kyoto control rats (WKY). Electrophysiologic study was performed on superfused isolated right and left atrial preparations using a custom built high-density multiple-electrode array to determine effective refractory periods (ERP), atrial conduction and atrial arrhythmia inducibility. Tissue specimens were harvested for structural analysis.

Results

Compared to WKY controls, the SHR demonstrated: Higher systolic blood pressure (p<0.0001), bi-atrial enlargement (p<0.05), bi-ventricular hypertrophy (p<0.05), lower atrial ERP (p = 0.008), increased atrial conduction heterogeneity (p = 0.001) and increased atrial interstitial fibrosis (p = 0.006) & CD68-positive macrophages infiltration (p<0.0001). These changes resulted in higher atrial arrhythmia inducibility (p = 0.01) and longer induced AF episodes (p = 0.02) in 15-month old SHR. Ageing contributed to incremental bi-atrial hypertrophy (p<0.01) and atrial conduction heterogeneity (p<0.01) without affecting atrial ERP, fibrosis and arrhythmia inducibility. The limited effect of ageing on the atrial substrate may be secondary to the reduction in CD68-positive macrophages.

Conclusions

Significant atrial electrical and structural remodeling is evident in the ageing spontaneously hypertensive rat atria. Concomitant hypertension appears to play a greater pathophysiological role than ageing despite their compounding effect on the atrial substrate. Inflammation is pathophysiologically linked to the pro-fibrotic changes in the hypertensive atria.

The relevance of the association between inflammation and atrial fibrillation

BACKGROUND

The relevance of the association between inflammation and atrial fibrillation (AF) is not firmly established. The clinical importance is considerable because inflammation is usually not targeted as a treatment option, minimizing a probable benefit.

MATERIALS AND METHODS

We have used a case-control study with a Mendelian randomization rationale to assess whether proposed risk factors that have a genetic component and are readily detected in circulating blood are causally related to AF. The studied variables were C-reactive protein (CRP) and a representative of the chemokine system, the monocyte chemoattractant protein-1 (CCL2).

RESULTS

Plasma CRP and CCL2 concentrations were significantly higher in AF patients than in the unaffected population. However, when segregated between paroxysmal and permanent, the difference for CRP was only observed in patients with a permanent condition. Plasma CCL2 was raised in both subgroups. Confounding factors were carefully considered, and multivariable analyses revealed that circulating CCL2 was significant and CRP was negligible to explain the presence of AF. The duration of the episode also bore a significant predictive value. Odd ratios for AF as a function of genotype did not differ from 1·0 for any of the individual CRP and CCL2 polymorphisms, or any combinations.

CONCLUSIONS

Elevated plasma CRP concentration per se does not increase atrial fibrillation risk. Values obtained for CCL2 suggest that inflammation is probably a consequence of AF. Our data also suggest that the effect of the duration of the episode should be further studied in the assessment of the actual role of inflammation.

Mechanism of and therapeutic strategy for atrial fibrillation associated with diabetes mellitus

Diabetes mellitus (DM) is one of the most important risk factors for atrial fibrillation (AF) and is a predictor of stroke and thromboembolism. DM may increase the incidence of AF, and when it is combined with other risk factors, the incidence of stroke and thromboembolism may also be higher; furthermore, hospitalization due to heart failure appears to increase. Maintenance of well-controlled blood glucose and low levels of HbA1c in accordance with guidelines may decrease the incidence of AF. The mechanisms of AF associated with DM are autonomic remodeling, electrical remodeling, structural remodeling, and insulin resistance. Inhibition of the renin-angiotensin system is suggested to be an upstream therapy for this type of AF. Studies have indicated that catheter ablation may be effective for AF associated with DM, restoring sinus rhythm and improving prognosis. Catheter ablation combined with hypoglycemic agents may further increase the rate of maintenance of sinus rhythm and reduce the need for reablation.

Antioxidant therapies for the management of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia in clinical practice, representing a major public health problem. Recent evidence suggests oxidative stress may play an important role in the pathogenesis and perpetuation of AF. In the past few years, experimental data and clinical evidence have tested the concept of antioxidant therapies to prevent AF. Besides statins, ACE-inhibitors (ACEIs) and/or angiotensin-receptor blockers (ARBs), and omega-3 polyunsaturated fatty acids, several other interventions with antioxidant properties, such as Vitamin C and E, thiazolidinediones, N-acetylcysteine, probucol, nitric oxide donors or precursors, NADPH oxidase inhibitors, Xanthine oxidase inhibitors have emerged as novel strategies for the management of AF. We aim to review recent evidence regarding antioxidant therapies in the prevention and treatment of atrial fibrillation.

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.

A pilot study of circulating PPAR-γ receptor protein in elderly patients with atrial fibrillation

INTRODUCTION

The present study aimed to investigate the relationship between serum peroxisome proliferator-activated receptor-γ (PPAR-γ) protein concentration and inflammatory markers in elderly patients with atrial fibrillation (AF).

MATERIAL AND METHODS

We enrolled a total of 73 elderly patients: 45 with AF as the test group and 28 in sinus rhythm as a control group. We assayed serum PPAR-γ receptor protein, high-sensitivity C-reactive protein (hs-CRP), interleukin-6 (IL-6), and tumour necrosis factor-α (TNF-α). The study protocol and written informed consent were approved by the Ethics Committee of Clinical Research, Peking University First Hospital.

RESULTS

The concentration of PPAR-γ receptor protein was lower in AF patients than that in the control group (p < 0.01), and the concentrations of hs-CRP, IL-6, and TNF-α were higher than those in the control group (all p < 0.01). The PPAR-γ level was negatively correlated with hs-CRP, IL-6, and left atrium diameter (LAD) level (all p < 0.05). On logistic regression analysis, PPAR-γ, hs-CRP, TNF-α and LAD level were associated with AF.

CONCLUSIONS

Elderly patients with AF show an inflammatory state and atrial remodeling. The PPAR-γ receptor protein concentration is inversely linked with inflammation in AF. As an important transcription factor regulating inflammatory gene expression, PPAR-γ may take part in the pathogenesis of AF.