Skeletal muscle collagen expression was studied in normobaric sedentary (NS) and training (NT) and hypobaric sedentary (HS) and training (HT) rats after experimental periods of 10, 21, and 56 days. The weights of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus muscles were increased between the experimental period of 21 and 56 days so that EDL weight was 57 (P < 0.01) and 36% (P < 0.05) higher in 56 days HS (56HS) and 56 days HT (56HT), respectively, than in 56 days NS (56NS). Soleus muscle weight was higher in 56HS (61%; P < 0.01) and in 56HT (27%; P < 0.05) than in 56NT. In EDL muscle, collagen type I mRNA level was lower in 56HT than in 56NS (36%; P < 0.05) and 56NT (44%; P < 0.01). In 56HT, collagen type III mRNA level was 39 (P < 0.01) and 42% (P < 0.05) lower than in 56NS and 56HS, respectively. In soleus muscle, prolyl 4-hydroxylase activity was greater (P < 0.05) in 56NT, 56HS, and 56HT than in 56NS. Total hydroxyproline content in EDL muscle was increased in 56HS and 56HT and in soleus muscle of 56HS. In conclusion, although collagen types I and III mRNA levels in EDL muscle decreased in 56HT, the prolyl 4-hydroxylase data suggest unchanged synthesis of total collagen. Exposure to hypobaric conditions as such, its combination to endurance training, as well as training in normobaric conditions increased prolyl 4-hydroxylation capacity in soleus muscle, which may indicate respective change in collagen synthesis rate.
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