Effects of birth weight and postnatal nutrition on neonatal sheep: II. Skeletal muscle growth and development

This study investigated effects of birth weight and postnatal nutrition on growth and development of skeletal muscles in neonatal lambs. Low (L; mean +/- SD 2.289 +/- .341 kg, n = 28) and high (H; 4.840 +/- .446 kg, n = 20) b irth weight male Suffolk x (Finnsheep x Dorset) lambs were individually rea red on a liquid diet to grow rapidly (ad libitum fed, ADG 337 g, n = 20) or slowly (ADG 150 g, n = 20) from birth to live weights (LW) up to approxima tely 20 kg. At birth, weight of semitendinosus (ST) muscle in L lambs was 4 3% that in H lambs; aggregate weights of ST and seven other dissected muscl es were similarly reduced. In ST muscle of L lambs, mass of DNA, RNA, and p rotein were also significantly reduced to levels 67, 60, and 34%, respectiv ely, of those in H lambs. However, myofiber numbers of ST, tibialis caudali s, or soleus muscles did not differ between the L and H birth weight lambs and did not change during postnatal growth. During postnatal rearing, daily accretion rate of dissected muscle was lower in L than in H lambs. Accreti on of muscle per kilogram of gain in empty body weight (EBW) was reduced in the slowly grown L lambs compared with their H counterparts, although the difference was less pronounced between the rapidly grown L and H lambs. Thr oughout the postnatal growth period, ST muscle of L lambs contained less DN A with a higher protein:DNA ratio at any given muscle weight than that of H lambs. Slowly grown lambs had heavier muscles at any given EBW than rapidl y grown lambs. Content of DNA and protein:DNA ratio in ST muscle were unaff ected by postnatal nutrition, but RNA content and RNA:DNA were greater and protein:RNA was lower at any given muscle weight in rapidly grown. lambs. R esults suggest that myofiber number in fetal sheep muscles is established b efore the presumed, negative effects of inadequate fetal nutrient supply on skeletal muscle growth and development become apparent. However, prolifera tion of myonuclei may be influenced by fetal nutrition in late pregnancy. R educed myonuclei number in severely growth-retarded newborn lambs may limit the capacity for postnatal growth of skeletal muscles.