| 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,A mount of researches confirmed that AF induced atrial electrical remodeling(AER).including the shortening of action potential duration(APD)and effective refractory period(ERP),even the inducibility and duration of AF.As it is known to us,the overload of intracellular calcium is the main factor to induce AF.Moreover,rapid atrial pacing(RAP)may imitate AF to induce AER and the changes of some ion channels and related genes’ expression.In latest studies,the activation of rennin-angiotensin-system(RAS)is confirmed to related with AF,which has caused more attention.At present,the discovery of angiotensin-(1-7)[Ang-(1-7)]and the cloning of ACE2 have led to a new perception of the intrinsic mechanisms through which the RAS regulates homeostasis.The heptapeptide Ang-(1-7)not only appears to counterbalance most of the Ang II effects,but has been reported to be an inhibitor of the carboxy-terminal catalytic domain of ACE.In our previous research,Ang-(1-7)was affirmed to prevent the shortening of APD and the reduction of APD rate adaption induced by atrial pacing.Angiotensin-(1-7)also can preserve the density of ICaL and ITO currents,and the mRNA levels of Kv4.3 in paced atrial cells.Through the early phase of our study,it was speculated that Ang-(1-7)promoted atrial natriuretic peptide(ANP)secreting to prevent cardiac remodeling through the Mas/PI3K/Akt/NO axis.Our present study is to investigate the role of ANP in the cardiac protection of Ang-(1-7)in chronic RAP cannin model.Methods For this study,20 mongrel dogs were randomly assigned to the sham-operated group(S,n=5),the pacing-control group(C,n=5),the pacing +Ang-(1-7)group(A,n=5)and the pacing + Ang-(1-7)+ A-71915(the antagonist of ANP receptor)(N,n=5).A programmable pacemaker was inserted in a subcutaneous pocket,and an atrial pacing lead was positioned in the right atrial through right jugular vein of each dog.In the group C,A and N,pacing at 500 beats per minute was maintained for 2 weeks.Group A and N was given Ang-(1-7)(6μg· Kg-1· h-1)through left jugular.Besides that,group N was administrated A-71915(0.30μg·Kg-1· h-1)simultaneously.After that,six bipolar recording electrodes were sutured to 6 sites of both atrial during experiments.Some electrophysiological index were measured.including atrial ERPs,inducibility and duration of induced AF)under different basic pacing cycle length(BCL)(300.250 and 200 ms).At the end of the experiments,the whole-cell patch-clamp technique was used to record left atrial ionic currents concluding ICaL、INa.And Real-Time PCR applied to assess possible changes in cardiac gene expression of Cav1.2、INav1.5 a subunite(Nav1.5 α),TGF-β or ANP.Meanwhile,western-blotting was applied to determine the expression of TGF-β or ANP in protein level.Results APR for 2 weeks markedly shortened the atrial ERPs in BCLs and increased the inducibility of induced AF in group C.Ang-(1-7)attenuated the shortening of atrial ERPs and the increasing of inducibility of AF in most of the sites,compared to group C.Those indexes in group N showed no difference to group C.The density of IcaL and INa was decreased in group C.But in Ang-(1-7)treating dogs,the decreasing of the density of ICaL and INa was attenuated.Those indexes in group N showed no difference to group C.In gene level,in group C,the expression of Cav1.2 and Nav1.5 a mRNA was decreased,while the expression of TGF-β、ANP mRNA was increased.Compared to group C,in Ang-(1-7)treating dogs,the decreasing of Cav1.2 and Nav1.5 α mRNA and the increasing of TGF-β expression induced ARP was attenuated,while the increasing of ANP mRNA was augmented.The aforementioned changes in group A seemed abolished by A-71915.In protein level,the expression of TGF-β and ANP was increased after ARP for 2 weeks.Compared to group C,Ang-(1-7)attenuated the increasing of TGF-β expression and augmented the increasing of ANP expression.Those indexes in group N showed no difference to group C.Atrial tissue from the paced dogs showed a large amount of interstitial fibrosis distributed throughout the tissue,evidenced by Masson trichrome stain.In addition,heterogeneity in the size and arrangement of atrial myocytes was found in these tissues.But these pathological abnormal findings were attenuated in the angiotensin-(1-7)-treated dogs.But the atrial fibrosis wasn’t improved in group N.Conclusion Ang-(1-7)can prevent the shortening of atrial ERPs and the increasing of AF inducibility induced by ARP.And Ang-(1-7)attenuated the decreasing of the density of ICaL and INa and the related gene expressing including Cavl.2 and Nav1.5 α.Ang-(1-7)can attenuated TGF-β expression to improve the atrial fibrosis induced by ARP.Furthermore,Ang-(1-7)promoted ANP secretion and ANP played crucial role in the cardiac protection of the former. |
