EFFECTS OF HYPOBARIC HYPOXIA ON HISTOCHEMICAL FIBER-TYPE COMPOSITION AND MYOSIN HEAVY-CHAIN ISOFORM COMPONENT IN THE RAT SOLEUS MUSCLE

Histochemical fibre-type composition and myosin heavy chain isoform co mponent in the soleus muscle were studied in normoxic rats at postnata l ages of 5, 10, 15, and 20 weeks and in rats exposed to hypobaric hyp oxia (460 torr) for 5 weeks from postnatal ages of 5, 10, and 15 weeks . The increase in the percentage of type I fibres and the concomitant decrease in that of type IIa fibres in the soleus muscle of normoxic r ats were observed until 15 weeks of age. On the other hand, no change in the fibre-type composition of the muscle during postnatal developme nt was observed in hypoxic rats, irrespective of the age at which they were exposed to hypoxia. The changes in the myosin heavy chain isofor m component (MHC I and MHC IIa) of the muscle during postnatal develop ment and by hypoxia corresponded well with those in the muscle fibre-t ype composition. It is concluded that hypobaric hypoxia inhibits the g rowth-related shift of muscle fibre-types from type IIa to type I and of myosin heavy chain isoforms from MHC IIa to MHC I in the rat soleus muscle, and that there are no changes in the muscle fibre-type compos ition or the myosin heavy chain isoform component caused by hypoxia af ter the shifts in these parameters which occur during postnatal develo pment are completed.