Myosin heavy chain composition of skeletal muscles in young rats growing under hypobaric hypoxia conditions

This study investigated the effects of voluntary wheel running on the myosi n heavy chain (MHC) composition of the soleus (Sol) and plantaris muscles ( Pla) in rats developing under hypobaric choronic hypoxia (CH) conditions du ring 4 wk in comparison with those of control rats maintained under local b arometric pressure conditions (C) or rats pair-fed an equivalent quantity o f food to that consumed by CH animals (PF). Compared with C animals, sedent ary rats subjected to CH conditions showed a significant decrease in type I MHC in Sol (-12%, P < 0.01). Although strongly decreased under hypoxia, sp ontaneous running activity increased the expression of type I MHC (P < 0.01 ) so that no difference in the MHC profile of Sol was shown between CH acti ve and C active rats. The MHC distribution in Sol of PF rats was not signif icantly different from that found in C animals. CH resulted in a significan t decrease in type I (P < 0.01) and type IIA (P < 0.005) MHC, concomitant w ith an increase in type IIB MHC in Pla (P < 0.001), compared with C and PF animals. In contrast to results in Sol muscle, this slow-to-fast shift in t he MHC profile was unaffected by spontaneous running activity. These result s suggest that running exercise suppresses the hypoxia-induced slow-to-fast transition in the MHC expression in Sol muscles only. The hypoxia-induced decrease in food intake has no major influence on MHC expression in develop ing rats.