增加刺激频率不影响大鼠萎缩比目鱼肌强直收缩疲劳性

No influence of increased frequency on fatigability of tetanic contraction in rat atrophic soleus

为观察模拟失重对大鼠比目鱼肌(soleus,SOL)与趾长伸肌(extensor digitorum longus,EDL)间断强直收缩功能的影响, 以及对刺激频率的调节作用,采用离体骨骼肌条灌流技术,观测其产生强直收缩最大张力的最适刺激频率、疲劳性与疲 劳后恢复过程。结果表明:对照组大鼠SOL强直收缩的最适刺激频率为60 Hz,尾部悬吊1周大鼠SOL的最适刺激频率 亦为60 Hz,尾部悬吊2周后,其最适刺激频率增高为80 Hz,4周后则为100 Hz;在最适刺激频率作用下,悬吊大鼠 SOL间断强直收缩的最大张力(Po)在悬吊1与2周未见改变,第4周才呈现显著性降低(P<0．01)。间断强直收缩5 min后, 对照组大鼠SOL张力降低到22．8％Po;悬吊1、2与4周组疲劳性均增加,与其同步对照组相比均有显著性差异(P< 0．01)。疲劳性强直收缩后,在20 min内对照大鼠SOL张力基本恢复到疲劳前水平,而悬吊1、2与4周组则不能完全恢 复(P<0．05)。对照组大鼠EDL的最适刺激频率为120 Hz,悬吊1、2与4周组EDL的最适刺激频率、疲劳性以及疲劳后恢 复过程均未发生改变。以上结果提示,增加刺激频率可对悬吊1与2周大鼠SOL强直收缩最大张力的降低有代偿作用,但 不能代偿悬吊4周大鼠SOL最大收缩张力的降低,亦不影响悬吊大鼠SOL间断强直收缩疲劳性的增加与疲劳后恢复的减缓。

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 （Po） 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 rain of fatigue, tetanic contractile tension of control soleus was decreased to 22.8% Po, but significantly decreased to 10.4%, 10.0% and 11.6% Po in 1-, 2- and 4-week unloaded soleus, respectively. The tetanic contractile tension recovered to 98% Po in control soleus at the twentieth minute after fatigue, but only recovered to 79.0%, 83.6% and 78.5% Po in 1-, 2- and 4-week unloaded soleus. The optimal stimulating frequency, Po, 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 Po 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.