| Objective Atrial fibrillation(AF)is the most common sustained atrial arrhythmias in clinical practice.Despite extensive studies,the pathophysiological mechanisms of AF remain unclear.Recently large bodies of evidence have shown that renin-angiotensin-aldosterone system(RAAS)play a role in the onset and persistence of AF.These associations may provide a potentia target for pharm acological interruption of AF and may favorably affect atrial remodeling leading to important clinical benefits.Reports suggest that AF is associated with activation of the RAAS in the atria in humans and in a dogs model of AF.The relationship between the RAAS and AF has been followed with interests in many studies.Some studies with non-antiarrhythmic drugs interfering with the RAAS have demonstrated a positive effect to prevent episodes of AF both in animals and in humans,suggesting a possible role of the RAAS as a mediator of atrial remodeling in AF.Atrial pacing can induce the remodeling of atrial similar as AF.RAAS has been involved in both atrial structural and electrical remodeling.Certain research used long-term atrial tachycardia dogs model and noted that ACEI/ARB could prevent the atrial structural remodeling.However,whether the ACEI and ARB is used alone or in combination therapy,they all increase the plasma renin activity(PRA).In addition,angiotensin II(Ang II)and aldosterone are not always completely suppressed during chronic ACEI/ARB treatment.An incomplete inhibition of the RAAS may be responsible for the residual organ damage and adverse events.Aliskiren,a new nonpeptide direct renin inhibitors(DRI),can inhibites plasma rennin levels.It acts as binding to the active sites of renin,preventing angiotensinogen from binding and being cleaved to form angiotensin ?,eliminate the Ang ? pile-up effect induced by Ang ? receptor antagonist,decrease aldosterone levels in blood and urine,thereby inhibiting the activation of RAAS at the rate limiting step more effectively.So in order to probe into the mechanism and therapeutic methods for AF,in our study,we established a chronic atrial tachycardia canine model to evaluate the effects of different doses of aliskiren on atrial remodeling.Methods In this study,28 mongrel dogs were randomly assigned to the sham-operated group(S,n=7),the pacing-control group(C,n=7),the pacing and aliskiren 1 group(Al,n=7),or the pacing and aliskiren 2 group(A2,n=7).A programmable pacemaker was inserted in a subcutaneous pocket,and a atrial pacing lead was positioned in the right atrial through right jugular vein of each dog.In the group C,A1,and A2,pacing at 500 beats per minute was maintained for 2 weeks.The dogs in the A1 and A2 treated groups received aliskiren(10mg·Kg-1·d-1)?or aliskiren(20 mg · Kg-1·d-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.ACE and Ang?levels in serum and atrial tissue were determined by ELISA,western-blotting was applied to assess the possible changes in cardiac gene expression of ICaLalC(Cavl.2),MEK/ERK,PI3K/Akt signal transduction pathway,and TNF-?1 in right atrial tissue.RT-PCR was applied to assess the possible changes in cardiac gene expression of Cav1.2,INav1.5a subunite(Nav1.5a),MEK/ERK or PI3K/Akt signal transduction pathway and ACE1,ACE2,AT1R in right atrial tissue.Histopathologic studies were performed to identify the fibrosis of atria and above signal transduction pathway by immunohistochemisty.Results After 2-week rapid atrial pacing;ACE and Ang ? levels in serum and atrial tissue were significantly increased.The density of L-Type Ca2+(ICaL)and sodium(INa)currents were significantly reduced in cells from paced dogs(IcaL:group C-4.09±1.46 pA/pF vs group S-6.12 ±0.58 pA/pF,P<0.05;INa:group C 30.48 ±6.0 pA/pF vs group S 46.31 ±4.73 pA/pF,P<0.05).The density of ICaL and INa currents in the A2 group was significantly higher than that in the pacing-control group(ICaL:group A2-6.23 ± 1.35 pA/pF vs group C-4.09±1.46 pA/pF,P<0.05;INa:group A2 58.62± 16.17 pA/pF vs group C 30.48 ±6.08 pA/pF,P<0.01),Al group didn't change the density of ICal and INa compared with the pacing-control group.The protein levels of Cav1.2 decreased in the pacing-control group compared with the sham-operated group(Cav1.2:group C 0.31 ±0.03 vs group S 1.0,P<0.01).Al group didn't change the decrease of Cav1.2 compared with the pacing-control group,but the protein levels of Cav1.2 was upregulated in aliskiren A2 group(Cav1.2:group A2 0.75±0.04 vs group C 0.31 ±0.03,P<0.01),The protein levels of Cav1.2 in A2 group was higher than Al group(Cav1.2:group A2 0.75±0.04 vs group A10.49±0.05,P<0.01).The protein expression levels of MEK/ERK signal transduction pathway increased in the pacing-control group compared with the sham-operated group,but the PI3K-Akt signal transduction pathway downregulated in the pacing-control group compared with the sham-operated group.Aliskiren may inverse the change of the above protein expression.After 2-week rapid atrial pacing,The mRNA levels of Cavl.2 and Nav1.5? decreased in the pacing-control group compared with the sham-operated group(Cav1.2:group C 0.46±0.08 vs group S 1.0,P<0.01;Nav1.5?:group C 0.5210.08 vs group S 1.0,P<0.01).A2 group increased the mRNA levels of Cavl.2 and Nav1.5a compared with the pacing-control group and higher than A1 group(Cavl.2:group A2 0.83±0.10 vs group C 0.46±0.08,P<0.01;Nav1.5?:1.07±0.08 vs group C 0.52±0.08;P<0.01;group A2:1.07±0.08 vs group Al:0.76±0.07;P<0.01).A1 group didn't change the decrease of Cavl.2 mRNA expression compared with group C.Extracellular signal-regulated kinases(ERK1/2)mRNA expression increased in group C after atrial pacing(ERK1:group C 1.37±0.12 vs group S 1.0,P<0.01;ERK2:group C 1.30±0.11 vs group S 1.0,P<0.01),After group A1 and A2 therapy,ERK1/2 mRNA expression levels were decreased(ERKl:group Al:0.85±0.08,group A2:0.70±0.08 vs group C:1.37±0.12;P<0.01;ERK2:group Al;0.82±0.05,group A2:0.83±0.07 vs group C:1.30±0.11;P<0.01).Moreover,mitogen-activated protein kinase(MEK)mRNA expression increased in the group C after atrial pacing(group C 1.24±0.06 vs S ? 1.0,P<0.01)After group A1 and A2 therapy,MEK mRNA expression levels were decreased(group Al:0.91±0.26,group A2:0.92±0.19 vs group C:1.24±0.06,P<0.01).PI3K-Akt mRNA expression decreased in group C after atrial pacing.Aliskiren may inverse the change of the above gene expression.However,in the mRNA expression of PI3K,group A1 and group A2 with no significance.in the mRNA expression of Akt,there is significantly different between group A1 and A2.Some gene expression related with RAAS,ACE1 and AT1R mRNA expression increased in the pacing-control group after atrial pacing(group C:ACE1:2.25±0.11;AT1R 1.26±0.10 vs group S 1.0,P<0.01).ACE2 and AT1R mRNA expression downregulated in the pacing-control group after atrial pacing(group C:0.66±0.06 vs group S 1.0,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.Moreover,immunohistochemisty shown that MEK/ERK and PI3K/Akt siganal transduction pathway immunization positive increased in the pacing-control group after atrial pacing,aliskiren seems can attenuate the above change.Conclusion Direct renin inhibitor aliskiren can attenuate the fibrosis or apoptosis of cardiac myocyte and reverse the ion channel remodeling caused by rapid atrial pacing.Aliskiren may attenuate the atrial remodeling by affect RAAS,especially the inhibition of Ang?,and modulate MEK/ERK and PI3K/Akt siganal transduction pathway gene expression.Aliskiren may be proved useful agents in the prevention and therapy of AF. |
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