Skeletal muscle growth and protein turnover in neonatal boars and barrows.

The objective of this study was to determine the effect of castration, within 24 h after birth, on skeletal muscle growth and protein metabolism in neonatal pigs at 1, 2, and 4 wk of age. Four additional pigs were slaughtered at birth to obtain initial body composition. All other pigs were infused with [14C]tyrosine for 6 h before slaughter to determine in vivo fractional protein synthesis rates (FSR). At slaughter, muscle bundles were removed from the semitendinosus and incubated with [3H]tyrosine to determine in vitro protein synthesis rates. Nucleic acids and protein were determined on the semitendinosus muscle. Testosterone concentrations, determined at weekly intervals, peaked in boars at 3 wk of age. Castration at birth did not affect combined weights of the semitendinosus, longissimus, triceps brachii, and brachialis muscles. Likewise, neither in vitro protein synthesis rates nor in vivo FSR were affected by castration. However, a developmental decline in in vivo FSR and in vitro protein synthesis rates occurred from 1 to 4 wk. Neither concentrations nor total quantity of protein, RNA, or DNA in the semitendinosus muscle differed between neonatal boars and barrows at any age. Concentrations of DNA and RNA at 4 wk were two- and threefold lower, respectively, than at birth. Protein:DNA and protein:RNA ratios increased three- and sixfold, respectively, from birth to 4 wk. Testosterone concentrations had little effect on skeletal muscle growth and protein turnover rates during this neonatal period.