BACKGROUND:A high-load exercise can trigger the degradation of titin,leading to skeletal muscle damage.MyoD participates in skeletal muscle generation and plays an important role in the repair of skeletal muscle damage. OBJECTIVE:To observe the expression changes of MyoD,BMAL1 and titin in skeletal muscles at different times during a high-load exercise,as to clarify the role of MyoD and BMAL1 in exercise-induced skeletal muscle damage. METHODS:Twenty-four 8-week-old Sprague-Dawley rats were randomly divided into a control group(n=4)and an exercise group(n=20).Rats in the exercise group were subjected to downhill running(90 minutes).Soleus muscle samples were collected at 0,12,24,48,and 72 hours after exercise.The mRNA expressions of BMAL1 and MyoD were measured by real-time fluorescence quantitative PCR.The ultrastructure of skeletal muscle fibers was observed by transmission electron microscope.Immunofluorescence was used to observe the co-localization of MyoD and BMAL1 as well as BMAL1 and titin. RESULTS AND CONCLUSION:After the single high-load centrifugal exercise,the sarcomere of the soleus muscle was widened and the Z-line was blurred and water wave-like,both of which were most serious at 12 hours after exercise and basically recovered at 72 hours.The results of real-time fluorescent quantitative PCR showed that BMAL1 mRNA expression in the exercise group increased first and then tended to normal,while the mRNA expression of MyoD decreased first and then increased.Immunofluorescence co-localization observation indicated that the co-localization of BMAL1 and MyoD was obviously observed at 12 and 24 hours after exercise,and the co-localization of BMAL1 and titin was observed at 0,12,and 24 hours.All the findings indicate that MyoD and BMAL1 are jointly involved in the repair of exercise-induced skeletal muscle damage probably via titin.