Abstract: BACKGROUND: Proper exercise can improve antioxidant capacity, inhibit and delay oxidative stress, and activate autophagy in the body. However, the mechanism of oxidative stress and autophagy in the liver tissue of rats after exhaustive exercise due to a long-term endurance training is still very complicated. Dendrobium officinale flavonoids have antioxidant capacity, immune regulation, anti-inflammatory and analgesic effects. It is still unclear whether Dendrobium officinale flavonoids can protect liver tissue before exercise. 
OBJECTIVE: To investigate the effect of Dendrobium officinale flavonoids on oxidative stress and autophagy in the liver tissue of exhaustive exercise rats.
METHODS: Sixty male Sprague-Dawley rats aged 10 weeks old were randomly divided into a quiet control group, an exercise control group, and low-, medium-, and high-dose Dendrobium officinale flavonoid groups. Except for the quiet control group, the rest rats underwent 5-day adaptive treadmill training, followed by 6-week formal exercise training. Exhaustion was achieved in the last training at the end of 6 weeks. Rats in the low-, medium- and high-dose Dendrobium officinale flavonoids groups were intragastrically administrated with 1.5, 3.0, and 4.5 g/(kg•d) Dendrobium officinale flavonoids half an hour before exercise, respectively. The intragastric volume was 5 mL/kg, once a day, until the end of the experiment. Hematoxylin-eosin staining was used to observe the pathological changes of rat liver tissue, and to detect rat serum alanine aminotransferase and aspartate aminotransferase, malondialdehyde, catalase, superoxide dismutase, and glutathione peroxidase levels, and total antioxidant capacity. Changes in autophagy-related indexes in rat liver tissue were detected by RT-qPCR and western blot assay. 
RESULTS AND CONCLUSION: Hematoxylin-eosin staining results revealed that in the exercise control group, hepatocytes were arranged in a disorderly manner, the hepatic sinusoids were swollen, the portal area was infiltrated with inflammatory cells, and there were a large number of lipid droplets in the cytoplasm. The lipid droplets and inflammatory cell infiltration in the liver tissue of rats were significantly reduced in the low-dose Dendrobium officinale flavonoid group, while there were no significant changes in the liver tissue of the rats in the medium- and high-dose Dendrobium officinale flavonoid groups compared with the quiet group. Compared with the quiet control group, the expression levels of serum alanine aminotransferase, aspartate aminotransferase, malondialdehyde and total antioxidant capacity were significantly increased (P < 0.01), while the serum activities of catalase, superoxide dismutase and glutathione peroxidase were significantly decreased in the exercise control group (P < 0.01). Compared with the exercise control group, the activities of alanine aminotransferase and aspartate aminotransferase were significantly decreased (P < 0.01), and the activities of serum catalase, superoxide dismutase and glutathione peroxidase were significantly increased in the low-, medium- and high-dose Dendrobium officinale flavonoid groups (P < 0.01). Moreover, the serum level of malonaldehyde and total antioxidant capacity were decreased in the medium- and high-dose Dendrobium officinale flavonoid groups (P < 0.05). Compared with the quiet control group, the mRNA and protein expression levels of Beclin1, Atg7, LC3-II, p62 and Rab7 in the liver tissue of rats were significantly increased in the exercise control group (P < 0.01). Compared with the exercise control group, the mRNA and protein expression levels of Beclin1, Atg7, LC3 - II, p62 and Rab7 in the liver tissues of rats showed a downward trend in the medium- and high-dose Dendrobium officinale flavonoid groups (P < 0.05 or P < 0.01). To conclude, long-term endurance exercise until exhaustive exercise can induce oxidative stress and damage to liver tissue in rats. Dendrobium officinale flavonoids can maintain the stability of liver function by regulating liver oxidative metabolism and autophagy in rat liver tissue. Thereby, liver protection is achieved.

Key words: Dendrobium officinale flavonoid, exhaustive exercise, liver, oxidative stress, autophagy, rat