Factors modulating recovery rate after intermittent tetanic fatigue in atrophic soleus / 生理学报

ABSTRACT

Fatigue occurs when the interval of intermittent tetanic contraction of skeletal muscle is shortened to a certain degree and the contractile tension declines. After fatigue, prolongation of the contraction interval can make the contractile tension recover. In atrophic soleus, the recovery rate is slower. It has been shown that a decrease in the contractile tension is caused by the inhibition of the myofibrils and sarcoplasmic reticulum Ca(2+) release channels during fatigue. So the mechanism of the recovery of contractile tension is the recovery of the inhibited myofibrils and sarcoplasmic reticulum Ca(2+) release channels. But how the inhibition affects the recovery course is still unclear. To specify the factors modulating the recovery rate after intermittent tetanic fatigue in soleus, and to seek the reasons for the decrease in recovery rate in atrophic soleus, we observed the recovery time course of different types of fatigue in isolated soleus muscle strips. The 10% or 50% decrease in the maximal tetanic contractile tention (P(0)) was defined respectively as slight or moderate fatigue. After short-term (S10P, 10 s) and long-term (L10P, 300 s) slight fatigue, the tetanic contractile tension recovered to nearly 100% P(0) at the 20th minute. In both slight fatigue groups, perfusion with 10 mumol/L of ruthenium red (an inhibitor of Ca(2+) release channels in sarcoplasmic reticulum) slowed down the recovery rate. It was suggested that slight fatigue only induced inhibition of myofibrils. After short-term (S50P, 60 s) or long-term (L50P, 300 s) moderate fatigue, the tetanic contractile tension at the 20th minute recovered to about 95% P(0) in S50P group and 90% P(0) in L50P group, respectively. The recovery rate in L50P group was significantly lower than that in S50P group. So the recovery rate after moderate fatigue was related to the tetanic contraction duration. In both moderate fatigue groups, perfusion with 5 mmol/L of caffeine (an opener of Ca(2+) release channels in sarcoplasmic reticulum) resulted in nearly 100% recovery at the 5th minute. It was suggested that moderate fatigue induced inhibition of myofibrils and sarcoplasmic reticulum Ca(2+) release channels. In 1-week tail-suspended rats, soleus muscles showed a 40% of atrophy. After slight fatigue, the tetanic contractile tension in unloaded soleus recovered to 94% P(0) in S10P group and 95% P(0) in L10P. After moderate fatigue, the tetanic contractile tension in unloaded soleus recovered to 92% P(0) in S50P and 84% P(0) in L50P at the 20th minute. There were significant decreases in all of the fatigue groups as compared with the control groups. These results suggest that both slight and moderate fatigue inhibit the myofibrils and sarcoplasmic reticulum Ca(2+) release channels in 1-week unloaded soleus, so the recovery rate after tetanic fatigue is slower than that in the control group.