BACKGROUND: MicroRNAs (miRNAs) are endogenous small RNAs of approximately 23 nucleotides that can pair with sites in 3Ј untranslated regions in mRNAs of protein-coding genes to down-regulate their expression. Recently, circulating miRNAs have been reported as promising biomarkers for various pathologic conditions. We assessed the hypothesis that miRNAs may leak into the circulating blood from injured cells and thereby serve as biomarkers for identifying the injured cell type. METHODS: We used

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... oproterenol-induced myocardial injury in rats as a model and miRNA array analyses to identify candidate miRNAs specifically produced in the ventricles of the heart. Individual miRNA concentrations were measured by real-time reverse-transcription PCR. Plasma cardiac troponin I (cTnI) concentrations were measured with an ELISA. RESULTS: Array analyses revealed miR-208 to be produced exclusively in the heart, and we selected this miRNA as a possible biomarker of myocardial injury. Plasma concentrations of miR-208 increased significantly after isoproterenol-induced myocardial injury and showed significant correlation with the concentration of cTnI, a classic biomarker of myocardial injury. CONCLUSIONS: The plasma concentration of miR-208 may be a useful indicator of myocardial injury. Our results suggest that profiling of circulating miRNAs may help identify promising biomarkers of various pathologic conditions.