[No influence of increased frequency on fatigability of tetanic contraction in rat atrophic soleus]
The present study was performed to observe the time course and features of intermittent tetanic contractile function changes in soleus and extensor digitorum longus (EDL) muscles of tail-suspended rats. The optimal stimulating frequency, fatigability and time-dependent recovery after fatigue were measured in isolated muscle strips. The optimal stimulating frequency of soleus and EDL was 60 Hz and 120 Hz in control rats, respectively. It was not changed in 1-week unloaded soleus, but shifted to 80 Hz and 100 Hz in 2- and 4-week unloaded soleus, respectively. The maximal isometric tension (P(o)) of tetanic contraction at optimal stimulating frequency did not alter in 1- and 2-week unloaded soleus, but significantly decreased in 4-week unloaded soleus. After 5 min of fatigue, tetanic contractile tension of control soleus was decreased to 22.8% P(o), but significantly decreased to 10.4%, 10.0% and 11.6% P(o) in 1-, 2- and 4-week unloaded soleus, respectively. The tetanic contractile tension recovered to 98% P(o) in control soleus at the twentieth minute after fatigue, but only recovered to 79.0%, 83.6% and 78.5% P(o) in 1-, 2- and 4-week unloaded soleus. The optimal stimulating frequency, P(o), fatigability and time-dependent recovery of intermittent tetanic contraction were not altered in unloaded EDL compared with control. These results indicate that higher stimulating frequency can compensate the P(o) reduction in 1- and 2-week unloaded soleus, but not in 4-week unloaded soleus. The unloaded soleus, but not EDL, is more susceptible to fatigue than the synchronous controls. The unloaded soleus not only fatigues to a greater extent but also recovers significantly less than the control.