Effects Of Enalapril, Irbesartan And Angiotensin-(1-7) On Atrial Remodeling In A Canine Model Of Chronic Atrial Fibrillation

Objective Atrial tachycardia-induced remodeling plays a key role in the pathophysiology of atrial fibrillation(AF).A variety of ionic abnormalities have been reported in atrial myocytes from patients with AF.Patch-clamp studies of the atrial myocyte from experimental models or human chronic AF revealed that AF reduces the density of some ion currents,including the L-type Ca²⁺current(I_CaL)and transient outward K⁺ current(I_TO).Clinical studies also showed decrease inα1C Ca²⁺channel subunit and I_TOKv4.3 subunit mRNA concentrations in patients with AF.The renin-angiotensin system(RAS)has been shown to be involved in many cardiovascular diseases.Reports suggest that AF is associated with activation of the RAS in the atria in humans and in a dog model of AF.Some studies with non-antiarrhythmic drugs interfering with the renin-angiotensin system(RAS)have demonstrated a positive effect to prevent episodes of AF both in animals and in humans,suggesting a possible role of the RAS as a mediator of atrial remodeling in AF.Although angiotensinâ…¡plays a key role in the biology of the RAS,it is certainly not the only biologically active peptide produced by this system,particularly within tissues.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 Angâ…¡effects,but has been reported to be an inhibitor of the carboxy-terminal catalytic domain of ACE.So in order to probe into the mechanism and therapeutic methods for AF remodeling,in our study,we established a chronic atrial tachycardia canine model to evaluate the effects of enalapril,irbesartan and Ang-(1-7)on atrial remodeling.Methods For this study,30 mongrel dogs were randomly assigned to the sham-operated group(S,n=6),the pacing-control group(C,n=6),the pacing and enalapril group(EN,n=6),the pacing and irbesartan group(IB,n=6)or the pacing and Ang-(1-7)group(A,n=6).A programmable pacemaker was inserted in a subcutaneous pocket,and a atrial pacing lead was positioned in the right atrium through right jugular vein of each dog.In the group C,EN,IB and A,pacing at 500 beats per minute was maintained for 2 weeks.The dogs in the enalapril,irbesartan and Ang-(1-7)treated groups received enalapril(2mg·Kg^-1·d^-1),irbesartan(60 mg·Kg^-1·d^-1)or Ang-(1-7)(6μg·Kg^-1·h^-1)during the pacing respectively.At the end of the experiments,the whole-cell patch-clamp technique was used to record left atrial ionic currents and action potential duration(APD).And RT-PCR was applied to assess possible changes in cardiac gene expression of I_CaLα1C,Kv4.3, ERK1/ERK2,and mas in right atrial tissue.Histopathologic studies were performed to identify the fibrosis of atria.Results Rapid pacing for 2 weeks markedly shortened APD90(P＜0.05 vs group S),and APD90 adaption to rate were decreased(P＜0.05 vs group S)by the pacing. The density of L-type Ca²⁺(I_CaL)and transient outward K⁺(I_TO)currents was significantly reduced in cells from paced dogs(I_CaL:group C-2.96±1.55 pA/pF vs group S-5.84±2.29 pA/pF,P＜0.01;I_TO:group C 8.72±4.35 pA/pF vs group S 17.60±6.49 pA/pF,P＜0.01).After 2-week rapid atrial pacing,although enanapril couldn't prevent the APD changes induced by atrial pacing,APD90 and APD90 rate adaptation in irbesartan or Ang-(1-7)-treated group were preserved.The density of I_CaLand I_TOcurrents in the Ang-(1-7)treated group was significantly higher than that in the pacing-control group(I_CaL:group A-5.74±3.43pA/pF vs group C -2.96±1.55pA/pF,P＜0.05;I_TO:group A 14.11±5.04pA/pF vs group C 8.72±4.35 pA/pF, P＜0.05),and there was no significant difference between the Ang-(1-7)treated group and the sham-operated group.Enalapril or irbesartan didn't change the density of I_CaL compared with the pacing-control group,but enalapril markedly increased the density of I_TO(group EN 29.99±14.78 pA/pF vs group C 8.72±4.35 pA/pF,P＜0.01;vs group S 17.60±6.49 pA/pF,P＜0.01).In keeping with the changes in I_CaLand I_TO currents,the mRNA levels of I_CaLα1C and Kv4.3 decreased in the pacing-control group compared with the sham-operated group(I_CaLα1C:group C 0.28±0.07 vs group S 0.50±0.11,P＜0.01;Kv4.3:group C 0.32±0.09 vs group S 0.53±0.06,P＜0.01). Enalapril,irbesartan or angiotensin-(1-7)couldn't prevent the gene expression decrease of I_CaLα1C induced by chronic rapid atrial pacing,but enalapril and angiotensin-(1-7)increased the mRNA levels of Kv4.3 compared with the pacing-control group(group EN 0.77±0.09,group A 0.63±0.13 vs group C 0.32±0.09,P＜0.01;group EN vs group S 0.53±0.06 P＜0.01).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 enalapril,irbesartan and angiotensin-(1-7)-treated dogs.Consisted with the pathological findings,extracellular signal-regulated kinases(ERK1 and ERK2)mRNA expression increased in the pacing-control group after atrial pacing(ERK1:group C 1.31±0.12 vs group S 1.00±0.11,P＜0.05;ERK2:group C 1.66±0.63 vs group S 0.90±0.45,P＜0.05),but in the enalapril,irbesartan or angiotensin-(1-7)-treated groups the ERK1/ERK2 mRNA expression was decreased compared with the pacing-control group(ERK1:group EN 0.67±0.11,group IB 0.55±0.38,group A 0.64±0.37 vs group C P＜0.01;ERK2:group EN 0.73±0.25,group IB 0.96±0.19,group A 0.97±0.32 vs group C P＜0.05). Moreover,the mas receptor mRNA expression was unaltered after atrial pacing,but enalapril and angiotensin-(1-7)decreased the mas mRNA level during pacing(group EN 0.91±0.26,group A 0.92±0.19 vs group S 1.23±0.18,P＜0.05).Conclusion Enalapril,irbesartan and angiotensin-(1-7)can attenuate the fibrosis of atria by suppressing the expression of ERK1/ERK2.Enalapril up-regulates Kv4.3 mRNA expression and I_TOdensity in paced atrial.Irbesartan and angiotensin-(1-7) can prevent the shortening of APD and the reduction of APD rate adaption induced by atrial pacing.Angiotensin-(1-7)can preserve the density of I_CaLand I_TOcurrents, and the mRNA levels of Kv4.3 in paced atrial cells.