| The purpose of this article is to investigate the effects of hypoxia or exercise on protein catabolism, the property of cross-bridge and the gene expression of myosin heavy chain (MHC) isoforms in rat skeletal muscle. 1 Material and methods1.1 Acute experiment: 152 female SD rats were randomly divided to four groups : (1) Control at normoxic condition(C), (2) Exhausting at normoxic condition(NE), (3) Control at hypoxic condition(HC) ,(4)Exhausting after hypoxic condition(HE).The rats of HC and HE were laid in hypoxic cabin(corresponding to an altitude of 4000m, oxygen level is 12.7%, 23h hypoxia /d ) for 3 days' hypoxic exposure and hypoxic recovery after exhaustion. After 3 days the ones of HE and NE were given a acute exhaustive exercise (16 meter/min, -160 downhill running on treadmill) at normoxic condition. Finally the rats of four groups were killed respectively at 0, 12h, 1d, 2d, 4d, 7d after exhaustion, and the samples of quadriceps were collected immediately and stored in liquid nitrogen. 1.2 Chronic experiment: 32 male SD rats were randomly divided to four groups : (1) Control at normoxic condition(Cm), (2) Training at normoxic condition(NT), (3) Control at hypoxic condition(HC) ,(4)Training at hypoxic condition(HT).The rats of HC and HT were laid in hypoxic cabin(corresponding to an altitude of 4000m,oxygen level 12.7%) for 22h/days, and the ones of HE and NE were trained for 1h/day on the treadmill (20meter/min,+100 uphill running) . After 28 day, the rats of four groups were all killed and the samples of quadriceps were collected immediately and stored in liquid nitrogen.1.3 Mesured indexes: The skeletal muscle cathepsin D activity was measured by means of ultraviolet spectroscopy, and myofibril ATPase activity by common spectroscopy. The gene expression of MHC—I,IIa,IIx,IIb isoforms mRNA was determined by using the semi-quantitative reverse transcription PCR technology. The composition of MHC isoforms was detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis(SDS-PAGE) and immunoblotting.2 Results and Analysis2.1 Acute experiment:The effects showed that the cathepsin D activities of HE group and HC group were higher significantly than that of C group(P<0.05)(P<0.01), but NE group didn't change significantly. Moreover, the cathepsin D activity of HE group was remarkably higher than those of NE group and HC groups(P<0.01). It indicated that hypoxia companied exercise can give a greater effect on the cathepsin D activity in skeletal muscle lysosome. Along the time course, the cathepsin D activities of HC and NE groups increased at 0, 12h after exhaustion and recovered after 1d to 7d, but HE group keep the high level all the time. It was found that the myofibril ATPase activity of HC group were higher significantly than that of C group(P<0.01), and NE group also increased after 24h. But HE group didn't change significantly. It indicated that hypoxia or exercise at normoxic condition can respectively cause the upregulation of myofibril ATPase activity, but hypoxia companied exercise can not cause a greater enhance of myofibril ATPase activity .There isn't benefit to myofibril ATPase activity to carry out recovery at hypoxic condition after exhaustion .The MHC band of SDS-PAGE marked by MHC marker was proved by its monoclonal antibody by immunoblotting. It was showed that the percentage of MHC Ⅱa+Ⅱx of HE group was lower significantly than that of C group(P<0.01),but that of MHCⅡb increased with a change trend of "slow to fast".There was also a change trend of "slow to fast" in HC group , but exhausting at normoxic condition(NE) can cause a trend of "fast to slow" without a significant statistical deference (P>0.05). The effects of MHC gene expression showed that the mRNA contents of the MHC isoforms in HC group at all sampling time spots were higher significantly than those in C group(P<0.05), and those in other groups were also upregulated by hypoxia or exhausting exercise as a whole. The percentage analysis of the MHC isoforms mRNA expr...
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