OBJECTIVE: The aim of this study was to explore the influence of micro ribonucleic acid (miR)-145-5p on myocardial cell apoptosis in rats with myocardial infarction (MI) through the phosphatidylinositol 3-hydroxy kinase/protein kinase B (PI3K/Akt) pathway. MATERIALS AND METHODS: In this study, Sprague-Dawley rats were used as research objects to establish the acute MI model in vivo. Infarction tissues and non-infarction tissues were both collected from rats. The expression level of miR-145-5p was determined using quantitative Polymerase Chain Reaction (qPCR), and the pathological changes in myocardial tissues of rats were observed through hematoxylin-eosin (HE) staining. In addition, H9c2 rat myocardial cells were cultured under hypoxia or normal oxygen concentration to simulate hypoxia in MI tissues. The changes in the expression of miR-145-5p in H9c2 cells in normal oxygen and hypoxia were determined. Meanwhile, the ratio of apoptotic cells to viable cells, and the changes in the expressions of proteins B-cell lymphoma 2 (Bcl-2), Bcl-2 associated X protein (Bax), Caspase-3 and Caspase-9 were evaluated through flow cytometry assay and Western blotting, respectively. The expression levels of crucial proteins in the PI3K/Akt pathway were measured as well. Additionally, H9c2 cells were transfected with miR-145-5p control and miR-145-5p mimic to evaluate cell apoptosis. RESULTS: QPCR results revealed that the expression level of miR-145-5p was substantially elevated in MI tissues (p<0.05). HE results indicated that the soma exhibited deformation after MI, suggesting that there were more necrotic and apoptotic cells. Compared with those cultured under normal oxygen concentration, H9c2 cells cultured in hypoxia environment exhibited significantly upregulated expression level of miR-145-5p, downregulated expression level of anti-apoptosis protein Bcl-2, upregulated level of pro-apoptosis protein Bax, activated Caspase-3 and Caspase-9, and downregulated expression level of functional proteins in the PI3K/Akt pathway (p<0.05). Furthermore, the expression levels of apoptosis-associated proteins significantly rose in H9c2 cells transfected with miR-145-5p mimic compared with those transfected with miR-145-5p control, showing statistically significant differences (p<0.05). CONCLUSIONS: MiR-145-5p is notably raised in MI tissues of rats. After infarction, there are evidently more apoptotic myocardial cells. The expression of miR-145-5p is markedly elevated in H9c2 rat myocardial cells in hypoxia. Compared with those cultured in normal oxygen, H9c2 cells cultured in hypoxia showed increased apoptosis. The apoptosis of myocardial cells transfected with miR-145-5p mimic is notable higher than that of myocardial cells transfected with miR-145-5p control. Moreover, the expressions of active Akt and PI3K proteins decrease remarkably. The results of this study demonstrate that miR-145-5p inactivates the PI3K/Akt pathway to promote the apoptosis of MI cells.