Maternal treatment with somatotropin during early gestation affects basic events of myogenesis in pigs

The effects of maternal treatment with somatotropin during early gestation on fetal muscle development were determined. Crossbred gilts received daily injections of either 3 ml of a placebo (n=31) or of 6 mg porcine somatotro pin (n=31) from day (d) 10 to 27 of gestation and samples were collected fr om d 28 embryos, d 37 and 62 fetuses, and from neonates. Administration of somatotropin increased the total number of fibres (primary and secondary fi bres) in neonatal semitendinosus muscle of middle- and low-weight littermat es, whilst no increase was observed in psoas major muscle. Somatotropin ind uced increases in muscular protein concentration, creatine kinase activity, muscle fibre girth, as well as type II to type I fibre conversion which re vealed an advanced degree of differentiation at birth. Treatment effects on prenatal development preceded these changes. Increased DNA concentrations at d 28 of gestation indicate stimulation of cellular proliferation during the embryonic stages. Thereafter, the withdrawal of somatotropin caused a t ransient delay of differentiation as indicated by lower protein concentrati ons and creatine kinase activity compared with controls at d 37 of gestatio n. This was compensated again at d 62, and the number of semitendinosus pri mary fibres was increased in middle- weight fetuses, whereas secondary or t otal fibre number did not yet differ. However, enhanced expression of Myf5 and MyoD indicates higher numbers of initially determined, proliferating my oblasts that may have contributed to increased formation of secondary fibre s. In conclusion, maternal somatotropin is an influential factor in early p regnancy capable of affecting the basic events of myogenesis.