| Background:Cardiovascular disease is the leading cause of death in China.Ventricular arrhythmias after MI are the main cause for sudden cardiac death(SCD).Objective:To investigate the effect of early metoprolol intervention on myocardial gap junctions remodeling and cardiomyocyte apoptosis in rabbits with MI.Methods:A total of 24 adult male New Zealand white rabbits were randomly divided into sham operation group(SH group,n = 6),myocardial infarction group(MI group,n = 6),early intervention group(EBMI group,n = 6)and routine intervention group(RBMI group,n = 6).MI model was established by ligating the left anterior descending(LAD)coronary artery.The sham group only underwent surgery without LAD coronary artery ligation.The rabbits in EBMI group and RBMI group were treated with intragastric administration of metoprolol(12.5mg/kg,dissolved in 2ml distilled water)after recovery from anesthesia immediately and beyond 24 hours after MI respectively.SH group and MI group were treated with intragastric administration of 2 ml distilled water.Plasma lactate dehydrogenase(LDH)and creatine kinase(CK)level were detected by automatic biochemistry analyzer after 6 hours of the procedure.Ventricular fibrillation threshold(VFT)was measured in vivo by bipolar pacing electrodes after 40 days of the procedure.Cx43,phosphorylated Cx43(p-Cx43)and Cx45 distribution in ventricular tissue were detected by immunofluorescence analyses.The protein levels of Cx43,p-Cx43,Cx45 and Bcl-2,Bax,Caspase-3 in ventricular tissue were detected by Western blot.Cx43 and Cx45 mRNA levels in ventricular tissue were measured by qPCR.Cardiac myocyte apoptosis rate(CMAR)was determined by TUNEL method.Results:1.Plasma LDH and CK increased significantly in the MI group compared with the SH group(both P<0.01).2.Rabbits in the MI group exhibited a marked decrease in VFT compared with the SH group,but the decrease was abolished by metoprolol,in both EBMI group and RBMI group,compared with the MI group(both P<0.05),but no significant difference was observed between the two groups(P>0.05).3.Cx43 was mainly localized in the intercalated disk,which arranged linearly perpendicular to the long axis of cells,and less lateral distribution in the SH group,EBMI group and RBMI group.The opposite change was observed in the MI group according to immunofluorescence analyses.The distribution of p-Cx43 and Cx45 is similar to that of Cx43.4.The level of p-Cx43 was significantly reduced in the MI group compared with the SH group(P<0.05).However,the decrease was reversed by treatment with metoprolol,mo matter in EBMI or RBMI group(both P<0.05),and the former is more significant(P<0.05).This trend is similar to the expression of Cx45 and the P-Cx43/Cx43 ratio,but no difference was observed in the optical density analysis of Cx43 among groups.A significant reduction of both Cx43 and Cx45 mRNA expression was observed in the MI group compared with the SH group(P<0.05).However,the reduction was attenuated by metoprolol in the EBMI and RBMI group(both P<0.05),and early treatment was more effective(P<0.05).5.The MI group untreated with metoprolol showed a higher count of CMAR than that in SH group(P<0.05).This result was reversed in the EBMI and RBMI group,both of which had a lower CMAR value compared with the MI group(both P<0.05),and the former decreased more significantly(P<0.05).The amount of the Bcl-2 was significant decreased in the MI group compared with the SH group(P<0.05).After the application of metoprolol,we observed an increase in Bcl-2 levels by densitometric analysis compared with the MI group,whether early or routine intervention(both P<0.05).More improvements can be achieved with early intervention(P<0.05).Similar change of Bcl-2/Bax ratio was observed,but the levels of Bax and Caspase-3 have the opposite change.Densitometric analysis of Western blot showed a significant increase in the expression of the Bax and Caspase-3 in the MI group compared with the SH group(P<0.05).Both EBMI and RBMI group showed lower levels of Bax and Caspase-3 than the MI group(both P<0.05),and the decrease was more pronounced in the early metoprolol group(P<0.05).Conclusion:Metoprolol can improve the disorganized distribution of connexins in ventricular myocardium,reduce the degradation and dephosphorylation of connexins,decrease the susceptibility of ischemia-induced ventricular arrhythmias and inhibit the cardiomyocyte apoptosis in rabbits with MI.In conclusion,metoprolol attenuates myocardial gap junction remodeling and inhibits apoptosis protecting rabbits with MI and early treatment was more effective. |
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