Objective:To discuss the effect of bone marrow mesenchymal stem cells(BMSCs)-derived exosomes(Exo)on isoproterenol(ISO)-induced myocardial fibrosis in the rats,and to clarify its mechanism.Methods:The Exo was isolated from the BMSCs and characterized by transmission electron microscope,nanoparticle tracking analysis,and Western blotting methods.Forty SD rats were divided into control group,model group,BMSCs-Exo group,and BMSCs-Exo+ferroptosis activator(Erastin)group,and there were 10 rats in each group.The myocardial fibrosis models were established by subcutaneous injection of ISO in all the rats except control group.The rats in BMSCs-Exo and BMSCs-Exo+Erastin groups were given BMSCs-Exo,and the rats in BMSCs-Exo+Erastin group were additionally injected with Erastin intraperitoneally.After 4 weeks,echocardiography was used to detect the left ventricular ejection fraction(LVEF),left ventricular fractional shortening(LVFS),left ventricular end diastolic diameter(LVEDD),and left ventricular end systolic diameter(LVESD)of the rats in various groups;HE staining was used to observe the pathomorphology of myocardium tissue of the rats in various groups;Masson staining was used to observe the fibrosis degrees of myocardium tissue of the rats in various groups;immunohistochemistry was used to detect the positive expression rates of α-smooth muscle actin(α-SMA),type Ⅰ collagen(COL-Ⅰ),and type Ⅲ collagen(COL-Ⅲ)in myocardium tissue of the rats in various groups;colorimetry was used to detect the Fe2+level in myocardium tissue of the rats in various groups;Western blotting method was used to detect the expression levels of acyl-CoA synthetase long chain family member 4(ACSL4),ferritin heavy chain 1(FTH1),glutathione peroxidase 4(GPX4),and solute carrier family 7 member 11(SLC7A11)proteins in myocardium tissue of the rats in various groups.Results:The particles isolated from BMSCs had a typical lipid bilayer structure,and most particle sizes distributed around 100 nm.High expression levels of CD63,CD9,tumor susceptibility gene 101(TSG101),and heat shock protein 70(HSP70)proteins confirmed the particles as Exo.Compared with control group,the LVEF and LVFS of the rats in model group were significantly decreased(P＜0.05),LVEDD and LVESD were increased(P＜0.05),the myocardial cell arrangement was disordered,some nuclear shrinkage and necrosis were seen,the collagen volume fraction(CVF)was significantly increased(P＜0.05),the positive expression rates of α-SMA,COL-Ⅰ,and COL-Ⅲ were significantly increased(P＜0.05),the Fe2+level in myocardium tissue was significantly increased(P＜0.05),the expression level of ACSL4 protein was significantly increased(P＜0.05),and the expression levels of FTH1,GPX4,and SLC7A11 proteins were significantly decreased(P＜0.05).Compared with model group,the LVEF and LVFS of the rats in BMSCs-Exo group were significantly increased(P＜0.05),the LVEDD and LVESD were significantly decreased(P＜0.05),the myocardial tissue damage was significantly alleviated,the CVF was significantly decreased(P＜0.05),the positive expression rates of α-SMA,COL-Ⅰ,and COL-Ⅲ were significantly decreased(P＜0.05),the Fe2+level in myocardium tissue was significantly decreased(P＜0.05),the expression level of ACSL4 protein was significantly decreased(P＜0.05),and the expression levels of FTH1,GPX4,and SLC7A11 proteins were significantly increased(P＜0.05).Compared with BMSCs-Exo group,the LVEF and LVFS of the rats in BMSCs-Exo+Erastin group were significantly decreased(P＜0.05),the LVEDD and LVESD were significantly increased(P＜0.05),the myocardial cell edema and necrosis were seen,the CVF was significantly increased(P＜0.05),the positive expression rates of α-SMA,COL-Ⅰ,and COL-Ⅲ were significantly increased(P＜0.05),the Fe2+level in myocardium tissue was significantly increased(P＜0.05),the expression level of ACSL4 protein was significantly increased(P＜0.05),and the expression levels of FTH1,GPX4,and SLC7A11 proteins were significantly decreased(P＜0.05).Conclusion:BMSC-derived Exo can improve the myocardial fibrosis in the rats induced by ISO,and its mechanism may be related to the inhibition of ferroptosis.