Effects of High-intensity IntervalTraining on Skeletal Muscle Autophagy and Its Regulatory Mechanisms in Rats

Objective: To investigate the effects of 10-wkhigh-intensity interval training(HIIT) on autophagy activities of skeletal muscle and its regulatory mechanisms in rats. Methods: 34 male SD rats were dividedinto three groups: sedentary control(SC,n =10), moderate-intensity continuous training(MICT, n =12), and HIIT(n =12). The exercise protocol of HIIT group was 4 intervals of 9-minute running on a treadmill(i.e.,4 minutes at 42 m/min and 5 minutes at 18 m/min), and the MICT group was 34 min of continuous exercise at a moderate intensity(28 m/min). Both HIIT and MICT groups were trained for 10 weeks, 5 days/wk. After that, the runningtime toexhaustion were measured throughan exhaustive exercise test, and the blood lactate levels were detected before and after the test. The protein expressions of autophagy markers(LC3-II、Beclin-1、ATG-3/-5/-7/-12/-16 L), mitochondrial markers(COXIV and SDH), and SIRT3 and ALDH2 in the soleus and rectus femoris muscles were detected by Western Blotting.The form of autophagic vacuole and the ultrastructure of mitochondria in soleus muscle were observed using a transmission electron microscopy(TEM).Results: Both HIIT and MICT groups were significantly longer in runningtime to exhaustion and lower in blood lactate level after the exhaustive exercise testby comparing with SC group(P<0.01). In addition, the runningtime to exhaustion was longer in HIIT group than that of MICT group(P<0.05), and HIIT groupresulted in alower blood lactate level at 10 min after exhaustive exercise test than SC group(P<0.05).The protein expressions ofthe SDH, COXIV and SIRT3 were significantly higher in the soleus and rectus femoris muscle compared with SC group after HIIT(P<0.01)and MICT training(P<0.05).The higher expression of COXIV protein was observed in HIIT group than MICT group in the soleus muscles(P<0.05);additionally, the HIIT group was greater in the expression of LC3-II/LC3-I ratio, LC3-II, Beclin-1, and ATG-3/-5/-7/-12 proteinsin the soleus muscles compared with SC group(P<0.05). Meanwhile, HIIT group was higher in the expression of ALDH2(P<0.01), Beclin-1(P<0.05),and ATG-5/-7(P<0.05)proteinin the rectus femoris muscle compared with SC group as well. The greater protein expressions of ALDH2, SIRT3, and COXIV in the rectus femoris muscle was observed in HIIT group than that of MICT group(P<0.05).Furthermore, TEM found the appearance of autophagic vacuole and the larger mitochondrial size in soleus muscle in HIIT group.The relative protein contents of COXIV and SDH in the soleus and rectus femoris muscles were positively correlated with runningtime to exhaustion, and the LC3-II/LC3-I ratio was positively correlated with SIRT3 and SDH protein contentsin the soleus muscles.Additionally, ALDH2 protein content was positively correlated with SIRT3 and COXIV protein contents in the rectus femoris muscle. Conclusion:HIIT can increase autophagy activities and autophagy related protein expressions in the soleus muscle butnotthe rectus femoris muscle, and the possible mechanism may be related with mitochondrial regulatory factor SIRT3.