Skeletal Muscle Protein Turnover in Young Male Pigs1 DONALD R. MULVANEY,2 ROBERT A. MERKEL AND WERNER G. BERGEN Growth Biology Program, Department oj Animal Science, Michigan State University, East Lansing, MI 48824-1225 ABSTRACT 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 %Id, 5.2 vs. 6.4 %Id 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% Id, respectively; at 45 kg, FGR for these muscles were 1.7, 1.9 and 0.7 %Id. Fractional protein break down 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 post natal growth and development, selection of a muscle(s) for turnover rates in growth studies is critical. J. Nutr. 115: 1057-1064, 1985. INDEXING KEY WORDS pigs « fractional protein synthesis rate •frac tional protein breakdown rate •skeletal muscle protein accretion Muscle comprises as much as 45 % of an ing instances of metabolic emergencies such animal's body mass (1) and hence there is as starvation (4, 5). much interest in understanding the regula- Knowledge of the relationship of protein tion of muscle growth. Postnatal muscle synthesis and breakdown to protein accre- growth occurs primarily through hypertrophy tion in specific muscles, as well as total (2). Rates of protein accretion during growth muscle mass of livestock, is important for are dependent on and represent the differ- development of methodology for efficient enees between protein synthesis and break- skeletal muscle protein deposition during down (3). The relative importance of protein animal production. While some data on synthesis and degradation on the regulation fractional rates of protein synthesis in skele- of muscle mass in rats has been studied (4, 5). tal muscle are available from pigs (6-9), Muscle protein accretion in rats is controlled none of these studies assessed muscle protein primarily through changes in synthesis accretion and breakdown rates, while breakdown acts as a secondary regula tor of protein accretion, since fractional protein accretion and fractional protein © 1985American InsUtute ^ Nutrition Received forpublicatlon breakdown are highly correlated. Net pro- 3January i9ss.Accepted torpublication: ioAprii «ss. tein breakdown Of mUScle Or an elevation in ;M¡chiganAgriculturalExperiment Station Journal Article 11517. - Present address for D. R. Mulvaney is: Animal and Dairy Sciences breakdown rates does not OCCUr except dur- Department, Auburn University, Auburn, AL 36849. 1057 by guest on August 18, 2015 jn.nutrition.org Downloaded from