| BackgroundAtrial fibrillation (AF) is the most common sustained tachyarrhythmia in clinical practice. The presense of AF is an independent predictor of morbidity and mortality, increasing the risk of thromboembolism, cardiovascular disfunction and even death. Up today antiarrhythmic grugs are still the most common used trestment for AF, but conversion to and maintenance of sinus rhythm by pharmacological method becomes increasingly difficult the longer the arrhythmia exists. One reason for this is that the underling mechanism of AF remains unclear. Therefor, a potential new approach to AF therapy is to target the underling mechnism.Resently lots of studies have shown that atrial electrical remodeling contributed to the recurrence and maintenance of AF, and the intracellular Ca2+ overload is a main characteristic in atrial electrical remodeling due to the alterations in gene expression and dysfunction of L-type Ca2+ channel and sarcoplasmic reticulum calcium handling proteins. To determine what factor induces the alterations of calcium handling proteins and intracellular Ca2+ overload leading to atrial electrical remodeling is of great importance as to seeking for new AF therapy. It has been previously well known that the activation of renin-angiotensin system(RAS) was harmful to cardiovascular system. Pedersen et al demonstrated that ACEI treatment with trandolapril reduced the risk of developing atrial fibrillation by 55% during a 2-to-4 year follow-up period in patients with left ventricular dysfunction after acute myocardial infarction.The detailed mechanism of the role of ACEI in preventing atrial fibriiation has not yet been clarified. ATI receptor antagonists and ACEI are not anti-arrhythmic drugs , but they do have the effect of antiarrhymia , the underling mechanism is unclear, but one thing is certain that the RAS is activated whenever AF occurrence. Recently Nakashima et al investigated the effects of candesartan and captopril on atrial electrical remodeling in dogs with short-term rapid atrial pacing , and found for the first time that Ang II contributed to atrial electrical remodeling. However the electrophysiological effect of Angll on chronic AF animal model has not yet been studied. It is important to determine whether ACEI can prevent atrial electrical remodling in chronic AF dog model through inhibiting the activation of RAS, and can be an effective agent for AF in clinical practice.In this study, we examined the changes in AERP, atrial tissue Ang II concentration, intracellular Ca2+ concentration and the gene expression of calcium handling proteins in longterm(8 weeks) rapid atrial pacing dog model, and investigated the prohibiting effects of ACEI in atrial electrical remodeling, in order to demonstrate the better therapeutic effect of ACEI in human AF patients. Objective(J) To investigate the electrophsiological character of atrial electrical remodling by methodology of Monophasic action potential(MAP) technique and P wave changes on standard electrocardiogram, and the influence of captopril on it .(2) To investigate the changes of atrial tissue Ca2+ and Ang II concentration, and the gene express! on of ACE after 8 weeks of atrial electrical remodling, and the influence of captopril on it.(3) To investigate the effect of captopril on the gene expression of Ca2+ handling protein in chronic AF dog model.(4) To investigate the changes of the activation of RAS in atrial tissue ofchronic AF dog model and the protecting effect of captoprial on it . Methods(1) Twenty-six mongrel dogs(weighing 11-19 kg) were randomly divided into three groups, pacing group(n=ll): all dogs were subject to rapid right atrial pacing(350-430bpm) for 8 weeks; treating group(n=9): the pacing procedure was the same as that in pacing group except that all dogs were given captopril 50 mg bid from three days before pacing to eight weeks later. Another 6 sham-operated dogs were severed as control.(2) Examine protocol: (T) Establish a canine chronic atrial fibrillation model by rapid atrial pacing. (2) ECG were recorded in anaesthetized dogs before and after 1, 4, 8 weeks pacing. The amplitude, maxiurn duration, minnium duration, average duration of P wave on 12 lead ECG, dispersion of P wave and ventricular heart rate were calculated. ?Before and after 8 weeks pacing, left ventricular end diastolic pressure and right atrial pressure were recorded by cardiac catheterization while animals were in sinus rhythm. ?Electrophysiological examination were performed to calculate intra right atrial conduct time and right atrial effective refractory period. AF was induced with two times threshold intesity by programmed stimuli or burst pacing. (5)The amplitude, duration, 90% repolarization duration, 50% repolarization of monophasic action potential and MAPD90-50 were measured according to MAP recording. (6) Angiotensin II concentration in atrial tissue was determined by radio-immunity. ?the messenger ribonucleic acid (mRNA) level of angiotensin converting enzyme(ACE) was measured by reverse transcription polymerase chain reaction(RT-PCR), and normalized to the mRNA level of {3 actin. (8)The content of myocardial calcium was assayed by spectrocomparator in left and right myocardium from all dogs. (9) The messenger ribonucleic acid (mRNA) levels of sarcoplasmic reticular Ca2+-ATPase(SR Ca2+-ATPase) and L- type calcium channel were measured by reverse transcription - polymerase chain reaction(RT-PCR) and normalized to the mRNA level of J3 actin. (Kj) study the changes of above all indexproduced by captopril.(3) Statistical analysis. All continuous data were expressed as means + standard deviation. SPSS 10.0 for windows was used to analyze all the data. Comparisons between groups were performed with unpaired Student' s t-test. Comparisons of continuous variables among multiple groups were performed by single- factor ANOVA. Pearson' s correlation coefficient (r) was used to determind the relation between groups. A value of /K0.05 was considered to be statistically significant. ResultsAF model: Twenty anesthetized mongrel dogs underwent insertion of transvenous lead at the right atrial appendage that was continuously paced at 350-430 beat per minute for 8 weeks. Among those dogs, Two had a sudden death during pacing period due to ventricular fibrillation, one showed heart failure symptom after 4 weeks pacing and echocardiography showed thrombosis in right atrium, and pacemaker stop generating pulse in another dog at the end of study. Totally 8 dogs in treating and pacing group respectively, complete the procedure.(1) Electrocardiogram, compared with control, ventricular heart rate was significantly decreased and the amplitude of P wave was elevated markedly in pacing group after 8 weeks pacing (K0. 05). Average P wave duration was significantly enlarged in pacing group after 4 weeks pacing compared with that in control group and become more signifiant after 8 weeks pacing. P dispersion and P maximum were also significantly longer in pacing group than that in control after 4,8 weeks pacing. P minimum was not significantly changed during pacing. In treating group, ventricular heat rate also showed a tendance of decline after 8 weeks pacing,but there was no siginificant difference compared with pacing and control groups. The amplitude of P ware was no longer elevated, compared with pacing group(K0. 05) .No significant changes were found in P dispersion after 4,8 week pacing in treating group comparded with that in control group. Pmaxshowed a slight prolonging after 8 weeks pacing, but it was significant shorter than that in pacing group (KO. 05).(2) Hemodynamics. Left ventricular end disatolic pressure and right atrium have no difference among three groups before rapid atrial pacing, but elevated significantly in pacing group campared with control group (KO. 05), and treating group(y°(0. 05) after 8 weeks pacing.(3) Electrophysiological examination, in pacing group, right atrial ERP, measured at circle length of 200ms and 300ms, were markedly shortened whereas right atrial intra-atrial conduction time increased significantly compared with control. In treating group, there were no differences in AERP before and after 8 weeks rapid pacing. No AERP shortening and atrial intra-atrial conduction time elevating were found in treating group compared with control. Before rapid atrial pacing, AF was not induced in any dog by programmed stimuli and burst pacing. In pacing group, spontaneous AF was occurred in two dogs after 1,4 weeks pacing respectively, when stop pacing for electrocardiogram exam. After 8 weeks pacing, AF was occurred spontaneously in 2 dogs, sustained AF was induced in 4 dogs by programmed stimuli, and in 2 dogs by burst pacing. In treating group, spontaneous AF was occurred in one dog after 4 weeks pacing, when stop pacing for electrocardiogram exam. After 8 weeks pacing, sustained AF was induced in 2 dogs by programmed stimuli, but no sustained AF was found in another 6 dogs by programmed stimuli and burst pacing.(4) MAP: ? Morphology. Each phase of monophasic action potential is easily discerned and the baseline is stable in control dogs. After 8 weeks rapid atrial pacing, the top of MAP is flat, the plateau is significantly shortened, and the amplitude, velocity of depolarization were reduced. Atrial fibrillation circle length is markedly decreased and absolutely irregular. Morphology of MAP is various. The repolarization phasic is overlap and diastolic interval is disappear. But in treating group, Morphology of MAP was almost the same as that in control dogs. (2) MAPA of pacing group has a tendency to decrease. Comparedwith control, MAPD, MAPD90, MAPD50 and MAPD90-50 were shortened by 16.20%, 19. 65%, 13.59% and 31. 25%, respectively in pacing group, but in treating group only a slight shortening was found.(5) Compared with control, the level of angiotensin II of both atrial myocardium was significantly increased in pacing group(K0. 05 respectively), while in treating dogs the level of angiotensin II of both atrial myocardium had no difference compared with control after 8 weeks pacing , but significantly longer than that in treating group.(6) Compared with control, the concentration of calaium in left and right atrial myocardium was elevated by 46. 39% and 36. 17% in pacing group , while in treating group , the concentration of calcium in both atrial myocardium was almost the same as that in control, but significantly lower than that in pacing dogs(K0. 001,0. 05 respectively).(7) Compared with control, the mRNA levels of angiotensin converting enzyme from left and right atrial myocardium were elevated by 54. 50% and 45. 00% in pacing group, while in treating group only the mRNA levels of angiotensin converting enzyme from left atrial myocardium were found significantly down regulated compared with that in control and pacing group(/X0.001,0.01 respectively).(8) Changes of gene expression in atrial myocardium Ca2+ handling protein : compared with control,the mRNA levels of L-type calcium channel from left and right atrial myocardium were reduced by 19.51% and 25.92% respectively. After 8 weeks treating with captopril the mRNA levels of L-type calcium channel from left atrial myocardium were upregulated significantly compared with control. In pacing group,the mRNA levels of the sarcoplasmic reticulum Ca2+ -ATPase in left atrial tissue were increased but it had no significant changes in right atrial tissue compared with control,but after 8 weeks treating with captopril,the mRNA levels of sarcoplasmic reticulum Ca2+-ATPase in both atria were significantly upregulated comparded with control and pacing group(KO. 001). Conclusion(1) Atrial fibrillation electrical remodeling is characterized by shortening of atrial effective refractory period, duration of action potential and plateau of action potential. These changes can be prevented by treatment with captopril. Monophasic action potential technique can accurately reflect the effect of treatment on atrial fibrillation electrical remodeling.(2) Average duration P wave, P maximum and P dispersion on surface standard electrocardiogram recorded in AF animal were increased with pacing continuing . These parameters reflect the extent of severity of electrical remodeling in atrial fibrillation, and can be used as uninvasive parameters to predict atrial fibrillation occurence.(3) These studies have demonstrated that atrial electrical remodeling in atrial fibrillation is associated with intracellular calcium overload. The mRNA upregulation of L-type calcium channel and SR Ca2+-ATPase in atrial myocardium maybe the mechanism of intracellular calcium overload. SR Ca2+- ATPase maybe a time-dependent process.(4) The levels of angiotensin converting enzyme gene expression and angiotensin II concentration in atrial myocardium were elevated in chronic atrial fibrillation model, and the angiotensin II concentration was positively correlated to intracellular Ca2t concentration, indicating that RAS activation maybe involved in the underling mechanism of intracellular Ca2+ overload.(5) ACEI captopril might have benefit in preventing atrial myocardium intracellular Ca2+ overload, upregulating the mRNA level of L-type Ca2+channel and SR Ca2+-ATPase in chronic AF dog model by inhibiting the activation of RAS and reducing the concentration of Angll in atrial myocardium.(6) These studies indicated that Angll may be involved in the mechanism of atrial electrical remodeling and the blockade of Angll by ACEI captopril may lead to a better therapeutic management of human atrial fibrillation. |
PDF Full Text Request