Objective To explore the effect and mechanism of miR-208b-3p on energy metabolism in mice with heart failure(HF)induced by transverse aortic constriction(TAC).Methods Twenty-four mice were randomly divided into sham operation group(Sham group,n=6)and surgery group(n=18).TAC was used to establish an HF model in the surgical group,the sham group received the same surgical procedures as TAC,but no ligation of the transverse aortic arc.At the second week after TAC,the surgery group was randomly divided into Antagomir group(n=6),Antagomir-NC group(n=6)and TAC group(n=6).The mice of the Antagomir group and the Antagomir-NC group were injected with miR-208b-3p antagomir reagent(800 μg)and miR-208b-3p antagomir negative control reagent(800 μg),respectively by tail vein,twice a week,for 4 consecutive weeks.Echocardiography was performed at the 6th week after surgery to evaluate the cardiac function.HE staining and Sirius red staining were used to observe myocardial histopathology in mice.ATP assay was employed to detect the ATP level in myocardial tissues.RT-qPCR was applied to detect the expression of miR-208b-3p,mitochondrial genes(ND1,ND2,ND3,ND4,ND4L,ND5,ND6,CO1,CO2,CO3,CYTB,ATP6 and ATP8),POLRMT and 12S rRNA in myocardial tissues.Double luciferase reporter assay was conducted to detect the interaction between miR-208b-3p and the potential target gene POLRMT.Western blotting was utilized to detect the changes in the protein levels of POLRMT and ND1,CO2,CYTB and ATP8 in myocardial tissues.Results The expression of miR-208b-3p was significantly higher in myocardial tissues of the TAC group than the Sham group(P＜0.05).Echocardiography revealed that the ejection fraction,systolic and diastolic functions were significantly improved in the Antagomir group than the TAC group(P＜0.05).Pathological observation showed significantly improved cardiomyocyte hypertrophy,arrangement disorder and myocardial interstititial cell infiltration in the Antagomir group(P＜0.05).Compared with the TAC group,the ATP level was significantly increased(P＜0.05),the expression levels of POLRMT and mitochondrial gene transcripts(12SrRNA and 13 mitochondrial gene-coding polypeptides)were significantly increased(P＜0.05),but there were no changes in SDHA and SDHB levels in the Antagomir group.Double luciferase reporter assay indicated that miR-208b-3p bound to the CDS region of POLRMT.The protein levels of POLRMT,ND1,CO2,CYTB and ATP8 were significantly increased in the myocardial tissues in the Antagomir group than the TAC group(P＜0.05).Conclusion MiR-208b-3p inhibits the expression of mitochondrial genes by targeting POLRMT,aggravates mitochondrial energy metabolism disorder,and deteriorates cardiac insufficiency and ventricular remodeling in HF mice.