Effects of spironolactone on atrial structural remodeling in rabbit model of chronic atrial fibrillation

AIM: To evaluate the effects and potential mechanism of spironolactone (SP) on atrial structural remodeling in rabbit model of chronic atrial fibrillation (AF). METHODS: The sternotomy was performed and the pacing electrodes were fixed to the left atria of New Zealand white rabbits. The animals were randomly divided into 3 groups. The rabbits were subjected to rapid atrial pacing (RAP) for 3 weeks in RAP group (intragastric administration with placebo) and RAP+SP group (intragastric administration with spironolactone at 20 mg·kg^-1·d^-1), respectively. The rabbits in sham group did not receive RAP and drugs. Before and after RAP, the structure and function of the atria were evaluated and AF inducibility was tested. After RAP, the atrial fibrosis was evaluated, and the expression levels of collagen I, collagen Ⅲ, matrix metalloproteinase (MMP)-2 and MMP-9 were determined. RESULTS: After 3 weeks of RAP, compared with sham group, obvious left atrial enlargement and dysfunction were observed in RAP group and RAP+SP group, but those had no significant differences in these 2 groups. Sustained AF was induced in 7, 5, and 0 rabbits in RAP group, RAP+SP group, and sham group, respectively. Compared with sham group, atrial interstitial fibrosis and the protein expression levels of collagen Ⅰ, collagen Ⅲ, MMP-2 and MMP-9 were all significantly increased in RAP group and RAP+SP group(P<0.05). Compared with RAP group, the the above indexes were all decreased in RAP+SP group(P<0.05). CONCLUSION: Spironolactone suppresses the atrial interstitial fibrosis and collagen expression, thus preventing atrial structural remodeling in rabbit model of chronic AF. The effect of spironolactone on reducing atrial MMP-2 and MMP-9 levels may be the potential mechanism.     

Effects of atorvastatin on atrial electrical remodeling in a rabbit model of chronic atrial fibrillation

AIM: To evaluate the effects of atorvastatin (ATO) on atrial electrical remodeling in a rabbit mo-del of chronic atrial fibrillation (AF) produced by 3 weeks of rapid atrial pacing (RAP). METHODS: The sternotomy was performed and the pacing and testing electrodes were fixed to the left atria of 24 New Zealand white rabbits. The animals were randomly divided into 3 groups. The rabbits in model group and ATO group were subjected to RAP for 3 weeks, and then were treated with placebo and ATO (2.5 mg·kg^-1·d^-1), respectively. The rabbits in sham group did not receive RAP and drugs. Electrophysiological examination was performed to test heart rate, P-wave duration, atrial effective refractory period (AERP) and AF inducibility. The protein expression levels of Cav1.2, Kv4.3 and myeloperoxidase (MPO) were detected by Western blot. RESULTS: Sustained AF was induced in 5 and 4 rabbilts in model group and atorvastatin group and no rabbits in sham group was found. After 3 weeks of RAP, compared with sham group, heart rate and P-wave duration were increased and AERP was shortened in model group and ATO group (P<0.05). Compared with model group, AERP was increased in ATO group (P<0.05), while heart rate and P-wave duration had no difference between these 2 groups. Compared with sham group, the protein levels of Cav1.2 and Kv4.3 were decreased, and protein level of MPO was increased in model group and ATO group (P<0.05). Compared with model group, Cav1.2 was increased and MPO was decreased in ATO group (P<0.05), while Kv4.3 had no difference between these 2 groups. CONCLUSION: Atorvastatin suppresses the down-regulation of atrial Cav1.2 protein level and the shortening of AERP, thus preventing atrial electrical remodeling in a rabbit model of chronic AF. The effect of atrovastatin on reducing atrial MPO level may be the potential mechanism.     

Effect of high-rate left atrial pacing on atrial remodeling and functions of sinoatrial node and atrioventricular node in dogs

AIM: To determine whether sinoatrial node (SAN) and atrioventricular node (AVN) undergo functional remodeling during atrial fibrillation (AF). METHODS: The Beagle dogs were randomized into pacing group (n=9) and control group (n=6). In open-chest dogs, the electrode catheters were sutured at left atria for pacing and data recording. SAN and AVN conductive properties were studied. The dogs in pacing group underwent 4 weeks of high-rate left atrial pacing (400 min-1). The dogs in control group were not subject to pacing. RESULTS: The animal model of chronic AF was successfully established by pacing the left atrium at 400 min-1 in the dogs. Two of those animals were recorded with spontaneous AF at the end of 2 weeks, and the induction rate of AF reached 100% following 4 weeks of pacing. The incidences of paroxysmal AF and permanent AF were significantly increased by pacing compared to control group. After 4 weeks of pacing, atrial effective refractory periods (AERP) at various at pacing cycle lengths (PCL; 250 ms, 300 ms and 350 ms) were all statistically shorter than those in control group. Compared with control group, a longer AVN Wenckebach point ［(294.44±26.06)min-1 vs (328.33±24.01)min-1,P<0.05］ and longer atrioventricular node effective refractory period (AVERP) (P<0.01) in pacing group were observed. The sinus node recovery time (SNRT) and corrected SNRT both showed significant increases. No significant change of P-wave and PA interval between the two groups was found. The left atrial dimensions (anteroposterior, superoinferior and left-right diameters) and the right atrial superoinferior diameter were measured to be significantly increased after 2 weeks of pacing. CONCLUSION: The animal model of left atrium pacing can induce AF occurrence with a higher incidence. The characteristic electrophysiological indexes about atria, AVN and SAN were observed during AF in the canine model, indicating that electrical and structural remodeling accompanies with AVN and SAN remodeling during AF.     

Expression of MPO,MMP-2 and MMP-9 in atrial myocardium of rabbit atrial fibrillation model

ATM: To investigate the expression of myeloperoxidase (MPO), matrix metalloproteinase (MMP)-2 and MMP-9 in the atrial myocardium, and their potential effects on atrial structural remodeling in a rabbit atrial fibrillation (AF) model. METHODS: The sternotomy was performed and the pacing electrode was fixed to the left atria of 20 New Zealand white rabbits. The animals were randomly divided into 2 groups:rapid atrial pacing (RAP) group and sham group. The rabbits in RAP group were subjected to RAP for 3 weeks. The structure and function of the atria and ventricle were analyzed by echocardiography. Atrial burst stimulation was performed to test AF inducibility. The atrial fibrosis was evaluated by Masson trichrome-staining. The mRNA and protein levels of MPO, MMP-2 and MMP-9 were detected by RT-qPCR and Western blot. RESULTS: After 3 weeks of RAP, obvious left atrial enlargement and dysfunction were observed, but almost no change of left ventricular diameter and function was found in RAP group compared with sham group. AF inducibility, atrial interstitial fibrosis and the mRNA and protein levels of MPO, MMP-2 and MMP-9 were all significantly increased in RAP group compared with sham group. CONCLUSION: Obvious atrial structural remodeling is found in the rabbit AF model induced by sustained RAP, and the up-regulation of MPO, MMP-2 and MMP-9 may be the potential molecular mechanism of atrial structural remodeling.     

Effects of the calpain system expression on atrial structural remodeling in canine with atrial fibrillation

AIM: To evaluate the influence of the calpain system mRNA and protein expression on the progress of atrial structural remodeling in fibrillating canine.METHODS: 17 dogs were randomly divided into 2 groups: normal control group (SR,n=6) and atrial fibrillation (AF,n=11) group.AF was induced by rapid pacing for 8 weeks and all dogs underwent transthoratic echocardiography before and after rapid pacing.The mRNA and protein expression of calpainⅠ,calpainⅡand calpastatin were assessed by real-time quantitative PCR and Western blotting,respectively.RESULTS: Compared with SR group,the left atrial diameters and the content of calcium in atrial myocardium increased significantly in AF group (P<0.05).There was no significant difference in mRNA expression of calpainⅠand calpainⅡ (P>0.05) between two groups.The expression of calpastatin mRNA was upregulated significantly in AF group (P<0.05).The levels of calpainⅠand calpainⅡprotein were significantly increased in AF group compared with SR group (P<0.05).The expression of calpastatin protein was significantly decreased in AF group (P<0.05).The calpainⅠand calpainⅡprotein levels were positively correlated with the left atrial diameter.The calpastatin protein level was negatively correlated with the left atrial diameter (P<0.05).CONCLUSION: The changes of the calpain system protein expression in AF result in the disturbance of calpain/calpastatin system and degradation of many proteins,which may play an important role in mechanism of atrial remodeling.     

Relationship between fibrosis and remodeling of gap junction in atrium of patients with atrial fibrillation

AIM: To examine fibrosis and remodeling of gap junction in atrial myocardium of patients with or without atrial fibrillation and to investigate the relationship between them. METHODS: Right atrial appendage (RAA) samples were collected from 44 patients with rheumatic heart disease during heart operation, 28 of which were clinically diagnosed as atrial fibrillation (AF), the left remained sinus rhythm (SR). Fibrosis and remodeling of connexin 43 were examined by polarization microscope and microscopy respectively, and analyzed with an image analyzer. Meanwhile, intercalated disc was counted under transmission electron microscope. The collagen volume fraction of type I (CVF-I) and the volume fraction of Cx43 (Cx43VF) were studied between groups of atrial fibrillation and sinus rhythm. The relationship between CVF-I and fraction of remodeled intercalated disc was studied as well. RESULTS: (1) Polarization microscope demonstrated that CVF-I collagen increased (P<0.01) in atrial fibrillation group. (2) The ratio of remodeled of intercalated disc in patients with AF was higher (P<0.01) than that in SR group whereas the number of intercalated disc was not different (P>0.05) between the two groups. (3) Cx43VF decreased (P<0.01) in the AF patients compared to those with SR. (4) A positive correlation between fibrosis and the remodeling of intercalated disc (r=0.96, P<0.01) was observed. The CVF-I was negatively correlated with the Cx43VF (r=-0.98, P<0.01). CONCLUSION: These results suggest that both fibrosis of atrial muscle and remodeling of intercalated disc are involved in the pathogenesis of human atrial fibrillation. Fibrosis of atrial muscle may play an important role in the process of atrial fibrillation by interfering with remodeling of intercalated disc and thereby involves in the remodeling of connexins.     

Over-expressed calreticulin interacts with integrin-α5 and calcineurin system to induce atrial remodeling in patients with atrial fibrillation and valvular disease

AIM: To determine whether calreticulin over-expression contributes to atrial fibrosis in the patients with atrial fibrillation (AF) and valvular heart disease (VHD).METHODS: Right and left atrial specimens were obtained from 78 patients undergoing valve replacement surgery. The patients were divided into sinus rhythm (SR) group, paroxysmal AF (PaAF) group and persistent AF (PeAF, AF lasting >6 months) group. The protein expression of calreticulin, integrin-α5, and transforming growth factor-β1 (TGF-β1) was measured. Immunoprecipitation was also performed to determine whether calreticulin interacted with either calcineurin B or integrin-α5. RESULTS: The protein expression of calreticulin, integrin-α5 and TGF-β1 was increased in AF groups, especially in the left atrium of the patients with mitral valve disease as compared with SR group. Calreticulin interacted with both calcineurin B and integrin-α5. The expression level of integrin-α5 was significantly correlated with the expression level of TGF-β1, while the expression level of calreticulin was correlated with that of integrin-α5 and TGF-β1. Under similar classification of the cardiac function, the expression level of calreticulin in PeAF group was higher than that in SR group. CONCLUSION: The expression of calreticulin, integrin-α5, and TGF-β1 is increased in the atrial tissues of the AF patients and is related to the AF type, suggesting that calreticulin is involved in atrial remodeling in AF and VHD patients.     

Electrophysiological characters in a noble model of atrial fibrillation induced by left atrium pacing after right atrial infarction

AIM: To explore a noble chronic atrial fibrillation model induced by left atrium pacing after right atrial infarction and to investigate the atrial electrophysiological characters of the model. METHODS: 24 rabbits were randomly divided into 3 groups: control group (C group, n=8), pacing group (P group, n=8), artial infarction+pacing group (I group, n=8). C group: a pacing pole was fixed under adventitia of the left atrium without pacing; P group: a pacing pole was fixed under adventitia of the left atrium with 1 000 beats/min of pacing; I group: the animals were placed under 1 000 beats/min of left atrial pacing after ligating the atrial branch of right coronary artery. The technique of programmed stimulating was used to measure electrophysiological indexes of atrial in the groups. RESULTS: (1) After 3 weeks pacing AF was induced with a higher rates, and reached to 100% in I group. (2) At driving cycle length of 200 ms, ERPA was (115.0±7.6) ms in C group, (81.3±12.5) ms in P group and (87.5±12.8) ms in I group, which were statistically shorter in the later two groups compared to control group (both P<0.01). (3) I group and P group showed a significantly poor performance of frequency adaptability compared to control group after 3 week stimulation (P<0.01, P<0.05, respectively). (4) 3 weeks after pacing, the interval of P-wave in I group was significantly prolonged compared to P group and C group (P<0.05, P<0.01, respectively). (5) ERPA was obviously shortened and RRPA was prolonged significantly in I group compared to control group (P<0.01, respectively). Inter-atrial conduction defect (IACD) was significantly prolonged in I group compared to C group and P group after 1 h to 3 week stimulation (P<0.01, respectively). CONCLUSION: Compared to the traditional AF model induced by pacing only, a noble model of left atrium pacing after right atrial infarction has a higher AF incidence. The apparent electrophysiological changes of the AF model include: shortening of ERPA, the frequency inadaptability, extension of P-wave interval and prolonged RRPA as well as IACD.     

Protein kinase A regulates small-conductance calcium-activated potassium type 2 channel in chronic atrial fibrillation patients

AIM：To investigate the alteration of small-conductance calcium-activated potassium type 2 (SK2) channel currents in atrial myocytes from atrial fibrillation (AF) patients, and the relationship between protein kinase A (PKA) and SK2 channel. METHODS：Right auricular tissues were obtained from the patients undergoing open-heart surgery with extracorporeal circulation. Single atrial myocytes were isolated by modified enzymatic dissociation method. The SK2 channel currents in the isolated human atrial myocytes were recorded using whole-cell patch-clamp technique. The alteration of SK2 channel currents and the regulation of SK2 channel by PKA were compared between sinus rhythm (SR) group and AF group. The total protein and PKA levels in human atrial tissues were detected by BCA assay and ELISA, respectively. RESULTS：The SK2 channel current densities and the proportion of SK2 channel currents in the integrated inward currents were significantly increased in AF group (all P<0.05 vs SR group). PKA-selective inhibitor H-89 reduced SK2 channel current densities and the proportion of SK2 channel currents in the integrated inward currents in both SR and AF groups, with larger reduction in AF group (all P<0.05 vs SR group). The PKA level was significantly decreased in AF atrial tissues (P<0.05 vs SR group). CONCLUSION: The increase in SK2 channel currents underlies the occurrence and maintenance of AF. PKA-dependent regulation may be involved in the remodeling of SK2 channel in both SR and AF human atrial myocytes, with a more powerful effect in AF.     

Effects of serum from patients with atrial fibrillation on chemotaxis of cardiac fibroblasts

AIM: To investigate the effects of the serum from the patients with atrial fibrillation (AF) on the chemotaxis of rat cardiac fibroblasts.METHODS: Cardiac fibroblasts were isolated from the ventricles of neonatal Sprague-Dawley rats and primarily cultured with digestion and differential adhesion. The cells in 3 to 4 passages were used for Transwell chamber assay to determine the chemotatic effects of the sera.RESULTS: Compared with control group, the cells that migrated through the polycarbonate membrane were obviously increased in AF group. The strongest chemotaxis was induced by the serum from the patients with persistent atrial fibrillation(pers-AF).The number of migrated cells in non-AF atrial arrhythmia(AA) group was higher than that in paroxysmal atrial fibrillation(paro-AF) group, and that in control group was the lowest. The results of multiple Logistic regression analysis showed that the migrated cells were related to AF and left atrial diameter.CONCLUSION: The chemotactic effect of AF serum is obviously higher than that of control serum. The differences of AF sera among groups show that myocardial fibrosis is a chronic, insidious and delayed process. The migration and infiltration of cardiac fibroblasts indirectly reflect the presence, severity and extent of the myocardial damage. The changes of migrated cells precede the changes of left atrial diameter, indicating that the cause of fibrosis is more important, and the positive correlation between AF and left atrial diameter may not be the direct causality.     

Resveratrol reduces electrical remodeling in atrial fibrillation by down-regulating microRNA-21 in neonatal rat atrial myocytes

AIM: To detect the effects of resveratrol (RSV) on the expression of microRNA-21 (miR-21) in primarily cultured neonatal rat atrial myocytes with electric remodeling induced by rapid electrical stimulation (RES). Furthermore, to find out the possible mechanism of miR-21 regulating electrical remodeling. METHODS: The neonatal rat atrial myocytes were isolated by double-enzyme (trypsin and collagenase I) digestion and differential adhesion method. The atrial fibrillation (AF) model was induced by RES. Atrial myocytes were randomly divided into 4 groups:control group, RSV group, RES group, and RSV+RES group. To further detect whether RSV regulated electric remodeling by miR-21, except the 4 groups, we add miR-21 over-expression group and miR-21 inhibitor group:RES+negative control (NC) group, RES+miR-21 mimics group, RES+miR-21 mimics+RSV group, RES+miR-21 inhibitor group, and RES+miR-21 inhibitor+RSV group. The optimal concentration and pretreatment time of resveratrol were determined by CCK-8 assay. The expression of miR-21 and the mRNA expression of L-type calcium channels CACNA1C and CACNB2 in atrial myocytes were detected by qPCR. The protein expression of L-type calcium channels Cav1.2 and Cavβ₂ in the atrial myocytes was analyzed by Western blot. RESULTS: The expression of miR-21 in RES group was significantly increased compared with control group, while preconditioning with RSV decreased the expression of miR-21. Compared with RES+miR-21 mimics group, the expression of miR-21 in RES+miR-21 mimics+RSV group was significantly decreased. Meanwhile, the mRNA expression of CACNA1C and CACNB2, and the protein levels of Cav1.2 and Cavβ₂ were increased (P<0.05). Compared with RES group, the expression of miR-21 in RES+miR-21 inhibitor group and RES+miR-21 inhibitor+RSV group was decreased, while the mRNA expression of CACNA1C and CACNB2, and the protein levels of Cav1.2 and Cavβ₂ were increased. However, no difference of the expression of miR-21, the mRNA expression of CACNA1C and CACNB2, and the protein levels of Cav1.2 and Cavβ₂ among RSV+RES, RES+miR-21 inhibitor and RES+miR-21 inhibitor+RSV groups was observed (P<0.05).CONCLUSION: In AF model induced by RES, RSV may reduce electric remodeling by inhibiting the expression of miR-21 and regulating the downstream target genes.     

Effect of chronic atrial fibrillation on Ca2+/calmodulin dependent protein kinase Ⅱ expression in human atrial myocytes

AIM: To investigate the effect of chronic atrial fibrillation (AF) on free calcium concentration and expression of Ca2+/calmodulin dependent protein kinase Ⅱ (CaMKⅡ) in human atrial myocytes. METHODS: The intracellular free calcium concentration in acute isolated atrial myocytes and the expression of CaMKⅡ in atrial tissue of rheumatic heart disease patients with atrial fibrillation (AF) and with normal sinus rhythm were measured by laser scanning cofocal microscopy technique and Western blotting, respectively. RESULTS: The intracellular Ca2+ concentration in patients with atrial fibrillation was significantly higher than that in patients with normal sinus rhythm ［(276.38±38.12） nmol/L vs （122.28±45.63) nmol/L, P<0.05］. Western blotting analysis of atrial samples showed that CaMKⅡ expression was enhanced during chronic atrial fibrillation (10.14±0.31 vs 6.86±0.89，P<0.05). CONCLUSION: Chronic AF leads to intracellular calcium overload in human atrial myocytes. Ca2+/calmodulin dependent protein kinase signaling cascades may play an important role in maintenance of chronic AF.     

Up-regulation of miR-133a expression attenuates myocardial fibrosis in spontaneously hypertensive rats

AIM: To investigate the effect of up-regulated expression of microRNA-133a (miR-133a) on myocardial fibrosis in spontaneously hypertensive rats (SHR). METHODS: Wistar-Kyoto (WKY) rats with homologous normal blood pressure served as the normal control group. SHR were divided into SHR group, SHR+ adeno-associated virus (AAV) group and SHR+miR-133a-AAV group randomly. miR-133a carried by miR-133a-AAV was transfected into SHR heart by coronary perfusion. The rat tail artery pressure was monitored. The myocardial collagen deposition was observed by Masson staining. The expression of miR-133a in myocardial tissue was detected by real-time PCR. The protein levels of transforming growth factor-β1 (TGF-β1) and connective tissue growth factor (CTGF) were determined by immunohistochemistry and Western blot. RESULTS: Compared with the WKY rats, the tail artery pressure of the SHR increased significantly. The expression of miR-133a in heart decreased, and the expression levels of TGF-β1 and CTGF increased (P<0.05), and myocardial fibrosis occurred. After up-regulating the expression level of miR-133a in the heart of SHR, the myocardial fibrosis was significantly reduced, and the expression levels of TGF-β1 and CTGF decreased (P<0.05). CONCLUSION: Up-regulation of the miR-133a expression improves myocardial fibrosis induced by hypertension, which may be related to inhibiting the protein expression of TGF-β1 and CTGF in myocardium.     

Non-contact mapping and frequency analysis of vagal-mediated atrial fibrillation

AIM: To explore the mechanism of the initiation and maintenance of vagal-mediated atrial fibrillation (AF) by non-contact mapping and frequency analysis of vagal-mediated atrial fibrillation in canine.METHODS: Atrial effective refractory period (AERP) and dispersion of AERP were measured in 8 canine during baseline and bilateral cervical vagal nerve stimulation (CVNS). Left atrium (LA) and right atrium (RA) electrical activity of AF was assessed by non-contact mapping and frequency analysis.RESULTS: Compared with baseline, CVNS attenuated left and right AERP, but only increased the dispersion of left AERP. During CVNS, AF was easily induced and maintained, repetitive organized activations rotated around a preferential route were only found in the LA, and dominant frequencies (DFs) from LA were higher than those of the RA ［(12.5±1.5)Hz vs (9.3±1.2)Hz, P<0.01］. After the cessation of CVNS, DFs of AF decreased in the LA and RA ［(9.2±0.5)Hz vs (8.5±0.6)Hz, P>0.05］, and AF was spontaneously terminated.CONCLUSION: The change of electrophysiological character, difference of activation pattern, and frequency gradient between the LA and RA suggest that the initiation and maintenance of vagal-mediated AF dependent on the LA.     

Activation of extracellular-signal regulated kinase and protein phosphatases in human atria during atrial fibrillation

AIM: The purpose of this study was to determine whether the signal transduction systems were activated at the molecular atrial tissue level in patients with atrial fibrillation (AF) and whether atrial expression of extracellular-signal regulated kinase (ERK) and protein phosphatases is altered. METHODS: Atrial tissue sample of 30 patients undergoing cardiac surgery were examined. 20 patients had AF, 10 patients had no history of AF. The mRNA expression of calcineurin B and MKP-1 were detected by semi-quantitative RT-PCR. ERK1 and phospho-ERK1 were analyzed at the protein level by Western blot. RESULTS: Western blot analysis showed that atrial fibrillation did not induce significant change in ERK1 expression level in the left atrium. In contrast , phospho-ERK1 content was increased in the patients with AF in comparison with those who had sinus rhythm (SR). The mRNA expression of calcineurin B and MKP-1 in the patients with AF were significantly higher than that in patients with SR. CONCLUSION: The activation of extracellular-signal regulated kinase and protein phosphatases may have correlation with the initiation or maintenance of atrial fibrillation.     

Relationship of microRNA-133a and TGF-β1 protein in myocardial tissues of spontaneously hypertensive rats

AIM：To observe the changes of microRNA-133a and transforming growth factor β1 (TGF-β1) protein in the myocardium of spontaneously hypertensive rats (SHR). METHODS：Male SHR (18 weeks old, n=12) and male Wistar-Kyoto rats (WKY, 18 weeks old, n=12) served as SHR group and control group, respectively. Caudal arterial blood pressure was detected by a noninvasive blood pressure measurement and analysis system. Myocardial collagen volume fraction (CVF) and perivascular collagen area ratio (PVCA) were determined by Masson staining. The level of miR-133a in the heart was detected by real-time quantitative PCR. The protein level of TGF-β1 in the heart was also analyzed by the methods of immunohistochemisty and Western blotting. RESULTS：Compared with control group, systolic and diastolic blood pressure, CVF and PVCA significantly increased, the expression of TGF-β1 protein was significantly up-regulated, and the level of miR-133a was significantly reduced in SHR group. In SHR group, the expression of miR-133a was decreased to (23.9±4.6)% in control group. A negative correlation between the levels of miR-133a and TGF-β1 protein in SHR group was observed (r=-0.791, P<0.01). CONCLUSION：The level of miR-133a is down-regulated along with the up-regulation of TGF-β1 protein expression and collagen synthesis in the myocardial tissues of SHR. miR-133a and TGF-β1 may be involved in myocardial fibrosis in SHR.     

Effect of PPARδ activation on expression of MMP-9 and fibronectin in infarcted myocardium

AIM:To investigate the effect of peroxisome proliferator-activated receptor δ (PPARδ) activation with dietary GW610742X on the expression of matrix metalloproteinase-9 (MMP-9) and fibronectin (FN) in infarcted and remodeling myocardium. METHODS: Wistar rats were divided into 4 groups: control group, sham group, myocardial infarction (MI) group and MI+GW610742X (GW) group. The left coronary artery was ligated to establish the MI model. PPARδ activator GW610742X (100 mg·kg^-1·d^-1) was given to the rats in GW group. At the 3rd month of the procedure, the expression of PPARδ, MMP-9 and FN at mRNA and protein levels in the left ventricular free wall(LVFW) of the heart from each group was identified and the distribution of FN was detected by immunofluorescence. RESULTS: After 3 months following the procedure, obvious necrosis and fibrosis in LVFW were observed in MI group. The expression of PPARδ in MI group was higher than that in control, sham and GW groups (P<0.01), and PPARδ expression in GW group was lower than that in control and sham group (P<0.05). In MI and GW groups, the expression of MMP-9 was higher while the expression of FN was lower than those in control and sham group (P<0.05 or P<0.01). In GW group, the expression of MMP-9 was lower (P<0.05) while the expression of FN was higher (P<0.01) than those in MI group. Meanwhile, the expression of MMP-9 and FN in sham group was similar to those in control group (P>0.05). CONCLUSION: MMP-9 is upregulated and FN is downregulated in infarcted myocardium during the remodeling process. Activation of PPARδ inhibits the upregulation of MMP-9 and degradation of FN, thus ameliorating the myocardial remodeling.     

Effect of spironolactone on hyperthyroxine-induced atrial fibrillation and atrial remodeling in rabbits

AIM: To detect the effect of spironolactone on hyperthyroxine-induced atrial remodeling. METHODS: New Zealand rabbits were divided into control group (C), hyperthyroxine group (H) and spironolactone group (S). Thyroxin was given to the rabbits in group H and group S by intraperitoneal injection for 4 weeks, and then spironolactone was given in group S by gavage for 2 weeks. Atrial fibrillation (AF) was induced by "burst" stimulation after administration. The inducing rate of AF and atrial effective refractory period (AERP) were tested by intra-cardiac electrophysiologic instrument. The expression of AF-related Ca²⁺ channel (Cav1.2), K⁺ channels (Kv1.5 and Kv4.3) and connexins (Cx40 and Cx43) at mRNA and protein levels was detected by real-time PCR, immunohistochemistry and Western blot. RESULTS: Spironolactone reduced the inducing rate of AF. No significant difference of AERP between group H and group S was observed (CONCLUSION: Spironolactone attenuates the hyperthyroxine-induced atrial remodeling in rabbits, and reduces the susceptibility of the myocardium to AF.     

An approach to calcium signal transmission between L-type calcium channel and sarcoplasmic reticulum of atrial myocytes during atrial fibrillation

AIM: To inquire into the Ca2+ signal transmission from L-type calcium channel to the sarcoplasma reticulum in atrial fibrillation. METHODS: Ten adult cross-bred dogs were used in the experiment. Five dogs underwent continuous rapid atrial pacing (500±20 beats/min) for twenty-four weeks to create persistent atrial fibrillation. Another group of size-matched dogs (n=5) without pacemaker implantation was used as a control group. Canine atrial myocytes were isolated by enzymatic dissociation. The Ca2+ cytosolic transient in atrial myocytes was analyzed by confocal imaging after loading myocytes with the acetoxymethyl ester of fluo-3 (Fluo-3/AM). Ca2+ signal transmission from L-type Ca2+ channels in the plasma membrane to the sarcoplasma reticular IP3R and RyR in atrial myocytes during atria fibrillation were measured. RESULTS: (1) Ca2+ signal transmission from L-type calcium channel to IP3R in the sarcoplasma reticulum:intracellular Ca2+concentration was slightly increased in two groups after blocking T-type calcium channel and RyR, but showed no statistic significance (P＞0.05) between them. (2) Ca2+ signal transmission from L-type calcium channel to RyR in the sarcoplasma reticulum: intracellular Ca2+concentration was risen (1.5576±0.1989) in control groups after blocking T-type calcium channel and IP3R, and no significance was observed (P＞0.05) compared with that in atrial fibrillation group (1.5372±0.2952). CONCLUSIONS: Ca2+ signal transmission possibly exists from L-type calcium channel to RyR and IP3R in the sarcoplasma reticulum, but it does not play an important role in intracellular Ca2+-overload and abnormal Ca2+ signal transmission during atrial fibrillation.     

Mitochondrial reactive oxygen species and atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia in clinical practice. Mitochondrial oxidative stress is supposed to contribute to development, progression and self-perpetuation of AF. Reactive oxygen species (ROS) is the major molecule mediating mitochondrial oxidative stress damage. ROS can alter the redox status of various molecular targets, which quite specifically leads to functional alterations of ion channel activity or activation of a variety of redox sensitive signal transduction pathways. Eventually, it leads to atrial electrical remodeling and promotes the development of AF. Therefore, mitochondrial oxidative stress pathways may be a new target for the therapy of atrial fibrillation.