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Differential response of cull cow muscles to the hypertrophic actions of ractopamine-hydrogen chloride¹

Department of Animal Sciences, University of Florida, Gainesville 32611

² Corresponding author: sealy{at}ufl.edu

Ractopamine-HCl (RAC) is a β-adrenergic agonist with variable effects on cattle performance and carcass variables. Cull cows fed RAC (200 mg · head^–1 · d^–1) demonstrate an increased size of type I and II muscle fibers that does not translate into a larger ribeye area. The objective of this study was to examine the dose-dependent effects of RAC on cull cow muscle morphometrics. Eighty-eight cull beef cows representing 2 breed types (n = 44 each) were fed 0, 100, 200, and 300 mg · head^–1 · d^–1 of RAC for the last 28 d of a 54-d feeding period. On d 54, cows were slaughtered, and samples of the LM and semimembranosus muscle (SM) from 16 randomly selected carcasses (n = 4 per treatment) were taken for measurement of β ₂-adrenergic receptors and type I, IIA, and IIX myosin heavy chain (MyHC) gene expression. Twenty-four hours postmortem, LM, SM, infraspinatus (INF), and vastus lateralis samples from 40 randomly selected carcasses (n = 10 per treatment) were obtained and frozen for immunohistochemical analysis. Muscle fiber cross-sectional area and diameter, MyHC isoform expression, and fiber-associated nuclei numbers were measured. Ractopamine dosage exhibited differential effects on muscle morphometrics and MyHC gene expression. Muscle fiber cross-sectional area and diameter were increased (P < 0.05) by RAC in INF type I and IIA fibers and SM type IIA fibers. Ractopamine increased (P < 0.05) MyHC type IIX mRNA and tended to increase (P < 0.10) β₂-adrenergic receptors in the SM; a change in mRNA abundance was not detected for either gene in the LM. Treatment with RAC decreased (P < 0.05) fiber-associated nuclei numbers in the INF, vastus lateralis, and LM but did not affect (P > 0.05) MyHC or β-adrenergic receptor expression. These results indicate that cull cow feeding programs may consider supplementing RAC as a means of adding value to cuts within the chuck, such as the INF.

Key Words: hypertrophy • myosin heavy chain • ractopamine • skeletal muscle • type I fiber

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