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In a previous study using a microRNA (miRNA/miR) microarray assay, we demonstrated that miR-133b-5p was upregulated in response to hypoxic preconditioning (HPC). The present study was designed to investigate the role of the miR-133b-5p in HPC-induced cardioprotection and the underlying mechanisms involving caspase-8 and caspase-3 apoptotic signaling. Adult rats were subjected to myocardial ischemia/reperfusion (I/R) injury with or without ischemic preconditioning (IPC), and the level of miR-133b-5p in myocardium was measured. Neonatal rat cardiomyocytes were isolated and subjected to hypoxia/reoxygenation (H/R) injury, with or without HPC. miR-133b-5p antagomir was transfected into the cardiomyocytes to observe whether it could block HPC-induced cardioprotection. Cellular injury was evaluated by detecting cell viability, lactate dehydrogenase (LDH) activity and apoptotic rate. Reverse transcription-quantitative polymerase chain reaction was used to measure the level of miR-133b-5p. The activation of caspase-8 and caspase-3 were measured by western blot analysis to detect the cleaved fragments as well as a colorimetric assay. Following myocardial I/R injury, the expression of miR-133b-5p was decreased in myocardium, while this decrease was restored by IPC. HPC protected neonatal rat cardiomyocytes against H/R injury by increasing cell viability, while reducing LDH release and cell apoptosis. These protective effects were coupled with the upregulation of miR-133b-5p. However, the knockdown of miR-133b-5p in the cardiomyocytes blocked HPC-mediated cardioprotection as reflected by the aggravation of cell injury and apoptosis. HPC upregulated miR-133b-5p level was markedly suppressed by the antagomir. In addition, the cleavage and activities of caspase-8 and caspase-3 were inhibited by HPC while reversed by knockdown of miR-133b-5p. Upregulation of miR-133b-5p contributes to HPC-mediated cardioprotection in cardiomyocytes, and the mechanism may be associated with inhibition of caspase-8 and caspase-3 apoptotic signaling.

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