| BackgroundAtrial fibrillation(AF) is one of the most common encountered cardiac arrhythmia in clinical practice. Serious atrial electrical activity disorder caused by atrial fibrillation make the atrium lose atrial effective shrinkage and expansion, coupled with disorders of the ventricular rate in patients caused by atrioventricular node diminishing rapid atrial exciting conduction, which often make people have bosom frowsty, heart palpitations and other symptoms. The main pathophysiological characteristics of patients with atrial fibrillation are ventricular rate disorder, impaired cardiac function and atrial mural thrombus formation. And the complications such as thrombosis and heart failure result in significantly increased morbidity and mortality in patients with atrial fibrillation, as well as increased medical costs.So far, the treatment of atrial fibrillation is still unsatisfactory. Pharmacotherapy is the basic therapy for atrial fibrillation (AF). Additionally, radiofrequency catheter ablation has greatly improved and its success rate is increasing. However, AF treatments show limited long-term effectiveness probably because of the potential adverse effects of traditional antiarrhythmic drugs and high postoperative AF recurrence rate of catheter ablation therapy, particularly in long-term AF recurrence. All of them are closely related to the underlying mechanism of AF which is not fully elucidated and increase the difficulty of AF treatment. The mechanism of atrial fibrillation is very complex and AF can lead to the changes of atrial electrical and histological characteristics, which is the remodeling of the atrium including the early electrical remodeling and the late structural remodeling of the atrium. Both of them exist in the atrial fibrillation process and progress gradually, influence each other and promote each other.In recent years, many studies found that cardiac autonomic nervous system (autonomic nervous system, ANS) played an important role in the occurrence and maintenance of atrial fibrillation, which including the extrinsic cardiac nervous system (ECNS) and the intrinsic cardiac nervous system (ICNS). Increasing evidence indicates that the excessive regeneration of neurons and nerve fibers and the heterogeneity of the nerve distribution in the atrium make the remodeling of the ICNS, which is named as cardiac autonomic neural remodeling (ANR) and involved in the occurrence and maintenance of AF. It is one of the important pathogenesis of AF. And the ANR often lies with the electrical remodeling and structure remodeling, promote each other, and form a vicious circle, which lead to the occurrence and maintenance of AF.After the occurrence of atrial fibrillation, atrial autonomic nerve remodeling occurs. Studies showed that the ANR including the sympathetic and parasympathetic nerve remodeling played an important role in the maintenance of AF and the formation of the vulnerability substrate in the process. The current studies on atrial ANR after AF mostly focus on the left atrium (LA) and right atrium (RA), but rarely involve the atrial septum (AS). The atrial septum is a distinct structure with high heterogeneity because of its unique configuration. The two sides of the atrial septum are in the endocardium, whereas its middle part contains connective tissues and muscle bundles. The nest egg circle, which is located in the lower atrial septum near the right atrium, is mostly composed of connective tissues. Whether atrial septum undergoes ANR during AF or whether the difference of neural remodeling occurs from various parts of the atrium remains unclear.Statins as an common lipid-lowering drugs is widely used, which has a variety of different roles of anti-inflammatory function, improving endothelial function, reducing myocardial ischemia-reperfusion injury, and so on. The preventive and therapeutic functions of statins in arrhythmia gained considerable attention particular in atrial arrhythmia, such as AF. The recent research showed that statins could prevent the perioperative atrial fibrillation in cardiac surgery. However, the mechanism of the inhibitory effect of statins on AF remains unknown, which was speculated to be related to their anti-inflammatory and antioxidant effects. Furthermore, no study has investigated whether statins can reverse ANR after AF, whether changes in reversal extent occur in different parts of the atrium, and whether statins can inhibit AF. Accordingly, this study aims to observe atrial ANR, the changes in the inducibility of AF, and the effects of rosuvastatin intervention on rapid atrial pacing in a rabbit model, to explore its role and the curative effect in treatment of the AF. This may provide new targets for the treatment of AF and also provides a new way for statins in the clinical prevention and treatment of AF.ObjectiveThe study aimed to observe the effects of pacing on the atrial effective refractory period (AERP), inducibility of atrial fibrillation (AF), and changes of atrial autonomic neural remodeling (ANR) by rosuvastatin intervention on the acute model of rapid-pacing induced AF.Methods1. Grouping and preparation of the rabbit model1.1 GroupingThirty healthy New Zealand rabbits were randomly divided into a control group (C, n=10), rapid-pacing group (P, n=10), and rosuvastatin-intervention group (R, n=10).(1) The rabbits in group C served as the sham operation group and were implanted with pacemakers without pacing, feeded for two weeks.(2)The rabbits in group P were implanted with pacemakers to achieve rapid right atrial pacing at a rate of 600 times/min for two weeks.(3)The rapid right atrial pacing model was also built in group R with rosuvastatin lavage (5 mg/kg d) administered for two weeks postoperatively. The two other groups were given placebo for two weeks.1.2 Preparation of the rabbit modelUsing the method of rapid atrial pacing, the rabbit model with atrial fibrillation was set up. After intravenous anesthesia with 3% pentobarbital sodium (30 mg/kg), through the right external jugular vein, the electrode of a pacemaker was implanted into the high right atrium under x-ray guidance to record the intracardiac electrogram. Then electrode was fixed, separating the neck skin to make pouch and positioning the pacemaker in a subcutaneous pocket of the rabbit's neck. Gentamicin was used to flush pouch. The entire process followed the aseptic operation. After operation, penicillin was used to prevent postoperative wound infection at 800,000 U/d for 3 days.2. Measurement of the intracardiac electrophysiology indexAtrial effective refractory period (AERP) and inducibility of AF were measured immediately and repeated after two weeks. The AERP was defined as the longest S1-S2 interval failing a propagated response. Briefly, following a 30 s conditioning stimulation period at basic cycle lengths of 200 ms, the S1-S2 intervals started at 150 ms, followed by decrements of 5 ms (S1:S2 = 8:1) until no more atrial response was propagated. AF was induced by PES with burst stimulation twice the diastolic threshold (cycle length of 120 ms, lasting 2 min to 3 min). The inducibility of AF was measured using a multi-channel electrophysiological controller. In the operation, standard electrocardiogram (ECG) lead II and blood pressure were continuously monitored on the Electrophysiology Management System.3. ECG monitoringBefore the operation, the surface ECG of the animals was monitored everyday. After the operation, surface ECGs were recorded every day to examine the occurrence of AF by turning off the pacemakers. Rapid irregular atrial rhythms lasting >30 s were regarded as successful inductions of AF.4. Western blotThe sympathetic and parasympathetic nerves were marked with tyrosine hydroxylase (TH) and choline acetyl transferase (CHAT), respectively. Western blot analysis was used for detecting the expression of sympathetic and parasympathetic nerve in myocardial tissues of LA, AS and RA through examining the expression of tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT).5.1mmunohistochemical stainingThe density and distribution of the two types of nerves were measured using immunohistochemical staining. The sympathetic and parasympathetic nerves labeled with TH and CHAT were detected by immunohistochemistry. The nerve fiber density was measured using a computer-aided morphology analyzer (Image Proplus 6.0).Statistical analysis:Statistical analysis of all data was performed with SPSS 17.0, and the measurement data were expressed as x ± s. The nerve expression in the Western blot analysis among multiple groups was compared using ANOVA. The least significant difference test was employed for pairwise comparison. The inducibility of AF for each group was compared using a chi-square test, with a significance level of ?= 0.05. A two-tailed test was adopted for significance testing, and P< 0.05 was considered statistically significant.Results1. Measurement of the intracardiac electrophysiology indexAfter two weeks, the AERP of group P significantly decreased compared with that of group C (P< 0.05). And the inducibility of AF in group P was significantly higher than that in group C. The AERP of group R showed no significant difference compared with that of group C (P> 0.05), but it was significantly prolonged compared with that of group P and AF could not be induced as easily(P< 0.05).2. ECG monitoringSurface ECGs were recorded every day to examine the occurrence of AF. No spontaneous AF or other arrhythmia was detected in group C. Eight of the rabbits with rapid right atrial pacing in group P exhibited AF. Thus, the inducibility of AF in group P was significantly higher than that in group C. Only one rabbit had AF in group R and the inducibility of AF in group R was significantly lower compared with that in group P (P<0.01).3. Western blotThe expression of TH and CHAT proteins in left atrium, right atrium, and atrial septum in group P significantly increased (P< 0.05) compared with that in group C, as shown by the Western blot assay. The expression of these proteins was two to four times higher after two weeks of rapid pacing (P< 0.05). The expression of the sympathetic and parasympathetic nerve proteins in the left atrium, atrial septum, and right atrium of group R was higher than that of group C, but was significantly lower compared with that of group P (P< 0.05).4. Immunohistochemical stainingThe densities of TH-and CHAT-positive nerve fibers in the left atrium, atrial septum, and right atrium had no significant differences in group C. However, the heterogeneity of the nerve distribution was the most evident in the atrial septum (P< 0.05). In group P, the densities of the TH-and CHAT-positive nerve fibers and the heterogeneity of the nerve distribution in the left atrium, right atrium, and atrial septum were significantly higher than that of group C (P< 0.01). Moreover, the nerve fibers in the myocardial cell space of group P evidently increased with a more disordered distribution, RA> AS> LA (P< 0.05). After two weeks of rosuvastatin intervention in group R, the density and heterogeneity of the TH-and CHAT-positive nerve fibers were still higher than those in group C, but significantly lower compared with that in group P (P< 0.01). The nerve fiber densities in the right atrium and atrial septum decreased more significantly than that in left atrium (P< 0.05).ConclusionPersistent rapid atrial pacing can lead to heterogeneous ANR in different parts of the rabbit atrium and may cause AF, which can be reversed by rosuvastatin. The inhibitory function of rosuvastatin may be associated with its role in reversing atrial ANR. |
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