Time course of changes in fatigability of highª²frequency tetanic contraction in rat unloaded soleus

    GAO Fang, YU Zhiª²Bin

    Department of Aerospace Physiology, School of Aerospace Medicine, Fourth Military Medical University, Xi¬ðan 710033, China

    ¡¾Abstract¡¿ AIM: To explore the effects of unloaded hindlimb on tetanic contraction fatigability of soleus. METHODS: Isometric twitch and highª²frequency tetanic contraction were examined by perfusion technique of isolated skeletal muscle strip. The contractile functions of soleus were observed in 1ª²week, 2ª²week, or 4ª²week tailª²suspended rats. RESULTS: The threshold voltage induced isometric twitch contraction significantly increased in 1 week of unloaded soleus (P<0.01) as compared with that of control. There was no difference in the threshold voltage between control and 2ª²week unloaded soleus (P>0.05). But the threshold voltage increased again in 4ª²week unloaded soleus (P<0.05). In highª²frequency tetanic contraction, the maximal tension (Po) of soleus was reduced respectively by 32.8%, 60.1% and 77.6 % after one (P<0.05), two and four weeks of (P<0.01) tailª²suspension (SUS). The tension at the thirtyª²third second of tetanic contraction (P33) in control decreased 18.0%ª²26.0%, while the P33/Po in the SUS group decreased 40.9% after one week of unloading (P<0.05), 51.1% after two weeks and 73.1% after four weeks of SUS, respectively. CONCLUSION: These results suggest that the excitability of unloaded soleus is decreased and the fatigability of unloaded soleus is increased. The changes in sarcolemma ion channels in unloaded soleus may be one of the underlying mechanisms of increasing the fatigability. ......