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This Article Full Text Full Text (PDF) All Versions of this Article: jas.2008-1049v1 86/12/3568    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Gonzalez, J. M. Articles by Johnson, S. E. Search for Related Content PubMed PubMed Citation Articles by Gonzalez, J. M. Articles by Johnson, S. E.

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

This article has been cited by other articles:

				J. M. Gonzalez, S. E. Johnson, T. A. Thrift, J. D. Savell, S. E. Ouellette, and D. D. Johnson Effect of ractopamine-hydrochloride on the fiber type distribution and shelf-life of six muscles of steers J Anim Sci, May 1, 2009; 87(5): 1764 - 1771. [Abstract] [Full Text] [PDF]