Atrial Neural Remodeling In Canine Model Of Chronic Atrial Pacing And The Protect Effect Of Angiotensin- (1-7)

Objective: Atrial fibrillation (AF) is the most common sustained atrial arrhythmias in clinical practice, but the exact mechanism of AF hasn't been clarified yet till now.Recently researchers found that autonomic nerve system (ANS) play a very important role in the occurrence and development of AF. ANS releasing various neurotransmitters, activate the corresponding receptors, regulate the activity of the ion channels, and effect the myocardial cell electrophysiological properties, then induced the arrhythmia by the turn-back, trigger, and increased self-discipline mechanism.The ANS effects on the atrial electrophysiology maybe influenced by the heart condition. The latest animal and clinical studies have shown that the activation of rennin-angiotensinsystem (RAS) is confirmed to relate with the initiate and recurrence of AF. Angiotensin-(1-7) [Ang-(1-7)] is an important member of RAS.Ang-(1-7) not only has counterbalance effects of Ang ?, but also to be an inhibitor of the angiotensin converting enzyme (ACE). In our previous research, Ang-(1-7) was affirmed to prevent the atrial electrical remolding and structure remolding in chronic atrial tachycardia canine. In our present study we use chronic atrial tachycardia canine model to investigate the atrial nerve remolding and the protect effect of Ang-(1-7).Methods: In this study, we enrolled 18 mongrel dogs, and randomly assigned to the sham group (S, n=6), the pacing group (P, n=6), and the pacing + Ang-(1-7) group (A,n=6). Pacemakers (AOO) were inserted into the dogs of all three groups, and the pacing lead was positioned into the right atrial. Canines in sham group were instrumented without pacing. In the group P and A, rapid right atrial pacing at 500 beats per minute was maintained for 2 weeks. Dogs in group A additional given Ang-(1-7) (6?g·Kg-1·h-1) through left jugular. After 2 weeks pacing, 10 bipolar recording electrodes were sutured to 10 sites of both atrial and pulmonary during experiments. The atrial effective refractory period (AERP), inducibility and duration of induced AF were measured. Then we isolated both side stellate ganglia, stimulate the both sides stellate ganglia to the heart rate increased 30%, measured the AERP,inducibility and duration of AF under the sympathetic nerve stimulation. Then we isolated the both side cervical vagal nerve trunks, stimulate the vagal nerve to decrease the atrial rate 50%, and measured the AERP, inducibility and duration of AF under the vagal nerve stimulation. At the end of the experiments, the both atrial tissues were dissected for molecule studies. The densities of tyrosine hydroxylase(TH) positive nerve in atrial and choline acetyltransferase (ChAT) positive nerve in atrial were detected by immuno- histochemistry staining. Western blotting was used to detect the expression levels of TH, ChAT and heat shock protein 27 (HSP27). And Real-TimePCR was applied to assess the gene expression of TH, ChAT, NGF and HSP27.Results 2 weeks of chronic atrial pacing markedly shortened the AERP and increased the inducibility of AF in group P. Ang-(1-7) attenuated the pacing induced AERPshortening and AF inducibilityincreased in most of the sitescompared to group P. Sympathetic nerve stimulation (SNS) significantly increased the HR in both groups,but not changed the AERP and AF inducibility in the sham group. SNS significantly decreased the AERP and increased the AF inducible rate in pacing group. Ang-(1-7)attenuated the pacing induced AERP shorten and the increasing of inducibility of AF under SNS. Vagal nerve stimulation (VNS) significantly decreased the AERP and increased the AF inducible rate in all the three groups, there were no statistical difference among three groups.In protein level, compared with the S group, the atrial expression levels of TH and HSP27 were significantly increased in group P,while the expression levels of ChATdid not have significantly changed. Compared with group P, chronic atrial pacinginducedTH and HSP27 expression increase were attenuated in Ang-(1-7)treating dogs. In gene level, the TH, NGF and HSP27 mRNA levels were increased after 2 weeks pacing. Compared with group P, Ang-(1-7) attenuated the increasing of TH, NGF and HSP27 mRNA levels. The expression of ChAT gene level had no significant changes.Immunohistochemical staining showed that the densities of TH positive nerves in the RA and LA were all significantly increased in pacing group than in the sham group. In the sham canines, LA compared with RA had high densities of TH positive nerves; after two weeks of chronic atrial pacing this trend was reversed. While the atrial ChAT positive nerve densities had no significant changes. HE stains show that heterogeneity in the size and arrangement of atrial myocytes were found in the pacing atrial tissues. Masson trichrome stain showed that pacing dog atrial tissues had a large amount of fibrosis distributed throughout the atrial tissue. And these pathological abnormal were attenuated in the Ang-(1-7) treated dogs.Conclusion: The influence of autonomic nerve stimulation on the atrial electrophysiological properties was different in sham and chronic atrial tachycardia canines. SNS increased AF inducible rate only in the chronic pacing canines, while VNS significantly increased AF inducible rate both in the sham and chronic pacing canines. Chronic atrial pacing induced the atrial sympathetic remolding, but the vagal nerve not changed obviously. HSP27 have protecting effect on atrial sympathetic remolding. Ang-(1-7) not only prevent the atrial electrical and structure remolding,but also prevent pacing induced sympathetic nerve remolding.