POSTNATAL-DEVELOPMENT AND DIFFERENTIATION OF MYOFIBERS IN FUNCTIONALLY DIVERSE PORCINE SKELETAL-MUSCLES

Marked changes in muscle function occur after birth, with the response being dependent on developmental stage. Therefore, postnatal cellular ontogeny of functionally distinct skeletal muscles was investigated i n the pig, a large mammal born at a relatively advanced stage of devel opment. Assessment of myofibre contractile (type I slow/type II fast) and metabolic (oxidative/glycolytic) properties at Days 0, 2, 5 and 14 revealed type-specific differences in hypertrophy and differentiation . Type I fibre proportions increased significantly in soleus and diaph ragm, especially between Days 0 and 5, and rhomboideus showed a simila r trend, but in longissimus there was a slight decrease during Days 0- 2. Cytochrome oxidase activity was relatively high and similar among m yofibres in all muscles at birth, and fibres with low activity were no t detected until Day 5. In contrast with previous reports, glycolytic fibres were present in all muscles at birth; postnatal changes in alph a-glycerophosphate dehydrogenase activity were both muscle-and myofibr e-specific. Hence, although myosin ATPase activity and metabolic prope rties of porcine myofibres are well developed at birth, they continue to mature postnatally. This suggests that postnatal muscle development can be modulated by extrinsic factors, even in mammals born at a rela tively advanced stage of development.