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Porcine Skeletal Muscle Myofibrillar Protein Synthesis is Stimulated by Ractopamine^1,2,

Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario N1G 2W1, Canada

Thirty-two 64-kg individually penned barrows were fed protein at 130 or 170 g/kg diet and ractopamine at 0 or 20 mg/kg in a randomized complete block design for 28 d. Fractional synthesis rates of myofibrillar, sarcoplasmic and connective tissue proteins in longissimus dorsi, biceps femoris and gastrocnemius muscles were examined by primed-continuous infusion of L-[ring-2,6-³H(N)]phenylalanine over a 4-h period. Fractional synthesis, accretion and breakdown rates of protein in the muscles were not affected by dietary ractopamine, regardless of whether estimated on the basis of plasma or tissue homogenate specific activity. Absolute rates of protein synthesis (P < 0.05) and breakdown (P < 0.1) in biceps femoris muscle were both elevated by ractopamine feeding at 170 g protein/kg diet. Also in the diet containing 170 g protein/kg, ractopamine increased (P < 0.05) protein contents of longissimus dorsi and biceps femoris muscles. Dietary ractopamine increased (P < 0.05) fractional rates of synthesis of myofibrillar but not sarcoplasmic proteins in longissimus dorsi and biceps femoris muscles of pigs fed the 170 g protein/kg diet. This result demonstrates that ractopamine treatment results in a stimulation of myofibrillar protein synthesis.

KEY WORDS: • beta-adrenergic agonists • pigs • myofibrillar protein synthesis

¹ We thankfully acknowledge the financial support provided by Ontario Ministry of Agriculture and Food, Natural Sciences and Engineering Research Council and ELANCO Division, Eli Lilly Canada Inc., London, ON. Financial support for Olayiwola Adeola's graduate study was generously provided by the Canadian Commonwealth Scholarship and Fellowship Committee.

² Presented in part at the 81st Annual Meeting of the American Society of Animal Science, 1989, University of Kentucky, Lexington, KY. [Adeola, O., Ball, R. O. & Young, L. G. (1989) Ractopamine stimulates porcine myofibrillar protein synthesis. J. Anim. Sci. 67 (suppl. 1): 191 (abs. 475)].

³ Current address: Department of Animal Sciences, Purdue University, West Lafayette, IN 47907.

Manuscript received 28 February 1991. Revision accepted 26 August 1991.

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