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

Department of Animal Sciences, University of Florida, Gainesville, FL 32611

sealy{at}ufl.edu

Abstract

Ractopamine-HCl (RAC) is a beta-adrenergic agonist with variable effects on cattle performance and carcass parameters. Cull cows fed RAC (200 mg·hd^-1·d^-1) demonstrate an increased size of type I and II muscle fibers that does not translate into a larger REA. 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 two breed types (n = 44 each) were fed 0, 100, 200, and 300 mg·hd^-1·d^-1 of ractopamine-HCl (RAC) for the last 28 days of a 54 day feeding period. On day 54 cows were harvested and samples of the Longissimus dorsi (LD) and Semimembranosus (SM) from 16 randomly selected carcasses (n = 4 per treatment) were taken for measurement of β₂-adrenergic receptors, type I, IIA, and IIX myosin heavy chain (MyHC) gene expression. Twenty-four hours postmortem, LD, SM, Infraspinatus (INF), and Vastus lateralis (VL) samples from 40 randomly selected carcasses (n = 10 per treatment) were obtained and frozen for immunohistochemical analysis. Muscle fiber cross-sectional area (CSA) and diameter, myosin heavy chain isoform expression, and fiber associated nuclei (FAN) numbers were measured. Ractopamine dosage exhibited differential effects on muscle morphometrics and MyHC gene expression. Muscle fiber CSA 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 IIX mRNA and tended to increase (P < 0.10) β₂-AR in the SM; a change in mRNA abundance was not detected for either gene in the LD. Treatment with RAC decreased (P < 0.05) FAN numbers in the INF, VL and LD, 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: ractopamine • skeletal muscle • hypertrophy • type I fiber • myosin heavy chain

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