A Study On The Mechanism And Change Of The Exercise Tolerance Of No-load Mice Of Amvotrophy

Objective:Prolonged unloading will lead to the loss of muscle mass and fiber type transition, which could cause some changes of muscle contraction characteristics and physiological functions. Whether this will impact exercise endurance in skeletal muscle have not been reported. This study focused on whether unloading-induced muscle atrophy after different time will impact the exercise endurance and its molecular mechanisms.Methods:Male C57BL/6mice were randomly divided into control and different time of unloading(7,14and28days) groups (n=7). After different time of unloading, the mice were placed to run at the speed of15m/min, time and distance were mea.sured until exhaustion to compare the exercise endurance. By using histology, Western blot and immunohistochemistry methods, we explored the molecular mechanisms underlying in the impact of unloading-induced muscle atrophy on exercise endurance by comparing cross-sectional area, TNNll and TNNI2protein expression and the content of slow and fast fiber types.Results:①The mass of hindlimb muscles in mice, gastrocnemius, soleus and plantaris muscles, continued to lose during different time of unloading until28days.②Compared to the control group, the exercise endurance in mice decreased to69.8%(P<0.001),34.5%(P<0.001) and28.3%(P<0.001) of the control group after7,14and28days of hindlimb unloading respectively.③HE stainings showed that after7days of unloading, cross-sectional area of gastrocnemius; soleus and plantaris fiber significantly reduced and reached to the maximum extent until14days of unloading, then slightly increased but still showed significantly reduction compared with control group after28days of unloading.④Western blot and immunohistochemistry results showed that during the process of unload-induced muscle atrophy, the slow-twitch fibers and its relative protein decreased, while the fast-twitch fibers and its relative protein increased. Conclusion:①uring the process of unloading-induced muscle atrophy, with the extension of time after hindlimb unloading, the mass of hindlimb muscles in mice continued to lose, the cross-sectional areas of hindlimb muscles decreased first and then showed an upward trend.②During the process of unloading-induced muscle atrophy, exercise endurance continued to decline with the extension of time after hindlimb unloading.③The changes of contraction characteristics of skeletal muscles resulted from atrophied fibers may be the direct cause of the decline of exercise endurance in mice.④During the process of unloading-induced muscle atrophy, the proportion reduction of slow-twitch fibers and increase of fast-twitch fibers in gastrocnemius, soleus and plantar is muscles may be molecular mechanism underlying in the decline of exercise endurance in mice.