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Elevation of a Specific mRNA in Longissimus Muscle of Steers Fed Ractopamine¹

Texas A&M University, College Station 77843

Abstract

To address the hypothesis that some classes of growth promoters stimulate muscle protein synthesis in growing cattle, 23 crossbred steers were fed diets containing the phenethanolamine growth promoter ractoparnine in a 140-d feeding trial. Steers received either no ractopamine, .18 or .36 mg·kg BW^–1·d^–1 ractopamine for 140 d or .36 or .72 mg·kg BW^–1·d^–1 ractopamine for 56 d. Longissimus muscle was obtained at slaughter and frozen in liquid N₂. RNA was extracted by homogenization of pulverized frozen muscle in guanidinium isothiocyanate and centrifugation through cesium chloride. Polyadenylated mRNA was extracted by capture on oligo-dT columns. Ractopamine had no effect on total RNA or mRNA concentrations (P > .25). Hybridization of the RNA to a putative myosin light chain-1/3 (MLC-1/3) cDNA clone in a Northern blot indicated one heavy band (approximately 1 kb) with no evidence of extensive destruction of the RNA. A second, minor band (approximately 3 kb) also was observed in some samples. The MLC-1/3 cDNA clone was hybridized to 1- or 5-µg samples of total RNA, and the intensity of me resultant autoradiographs was quantified by laser densitometry. There was a statistical correlation between MLC-1/3 mRNA·µg RNA^–1 and longissimus cross-sectional area (P < .05) and average daily gain (P < .025). The results suggest that ractopamine either increased the transcription of the putative MLC-1/3 gene and(or) increased the stability of MLC-1/3 mRNA in bovine longissimus muscle, either of which could result in an increase in specific myofibrillar protein synthesis.

¹ Tech. article no. 23414, Texas Agric. Exp. Sta. Funded in part by Lilly Research Laboratories and the Texas Adv. Technol. Res. Program.

² Dept. of Anim. Sci.

³ Lilly Research Laboratories, Greenfield, IN.

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