Abstract: Objective To explore the relationship between the time-dependent level changes of microRNA and 18S rRNA and the different postmortem interval（PMI） in rat cardiac muscle. Methods SD rats were sacrificed by cervical dislocation and placed at ambient temperature 25 ℃ with a humidity of 50%. Total RNA was extracted from the rat cardiac muscle at different time points after death. The levels of miR-1-2 and 18S rRNA were examined using real-time PCR in rat cardiac muscle. The results were expressed by cycle threshold（Ct） value to explore relationship between PMI and Ct value, and the regression functions were established to estimate PMI. Results The miR-1-2 level in rat myocardial tissue showed no significant changes within 120 h after death, and then began to decline. The 18S rRNA level increased gradually within 96 h after death, and then declined slowly. The nonlinear relationships were established between Ct value （18S rRNA）, ΔCt value （difference between 18S rRNA and miR-1-2 ） and PMI. The R2 of conics fitting were 0.948 7 and 0.807 2, respectively. Conclusion Ct value of 18S rRNA and ΔCt value present a good correlation with PMI, and can be markers for estimating early PMI.

Key words: forensic pathology, microRNAs, RNA, ribosomal, 18S, postmortem interval, polymerase chain reaction, rats