Skeletal muscle protein turnover in young male pigs

A 6-h continuous infusion of L-[U-14C]tyrosine was used to estimate fractional protein synthesis rates (FSR) in the longissimus dorsi (LD), semitendinosus (ST) and brachialis (BR) muscles of intact male pigs weighing 22 or 45 kg. The FSR was approximately 20% lower for pigs at 45 kg (4.2 vs. 5.2%/d, 5.2 vs. 6.4%/d and 5.1 vs. 6.4%/d for the LD, ST and BR muscles, respectively) compared with pigs at 22 kg. Fractional protein accretion or growth rates (FGR) were estimated over a 2-wk period. At 22 kg, FGR for the LD, ST and BR were 0.7, 2.4 and 1.7%/d, respectively; at 45 kg, FGR for these muscles were 1.7, 1.9 and 0.7%/d. Fractional protein breakdown rates (FBR) derived by difference (FBR = FSR - FGR) were 44, 16 and 8% lower for the LD, ST and BR muscles, respectively, at 45 kg compared with those at 22 kg. Therefore, it is suggested that muscle growth rate is modulated by alterations in FBR. By using the fractional rates found in these muscles to determine total-muscle protein synthesized or degraded in pigs at 22 and 45 kg, the proportion of protein retained was approximately 28% of that synthesized by the pigs at each weight. Since individual skeletal muscles of the pigs differed in protein turnover rates during postnatal growth and development, selection of a muscle(s) for turnover rates in growth studies is critical.