Effect of ractopamine-HCl and trenbolone acetate on Longissimus muscle fiber area, diameter, and satellite cell numbers in cull beef cows

¹ Department of Animal Sciences, University of Florida, Gainesville, FL 32611
² North Florida Research and Education Center, Marianna, FL 32446

^* To whom correspondence should be addressed. E-mail: sjohnson{at}animal.ufl.edu.

	Abstract

The objective of this study was to evaluate the effects of co-administration of ractopamine-HCl (RAC) and trenbolone acetate plus estradiol (TBA) on LM fiber cross-sectional area (CSA), diameter, fiber-associated myonuclei (FAN), and satellite cell number. Culled crossbred beef cows (n = 98; 11 yr ± 1.8; BCS 4.3 ± 0.03) from a single ranch in south Florida were fed a concentrate diet for 92 d in a 2 x 2 randomized block design. Cows were blocked by BW upon arrival into a light (initial BW = 369.75 ± 2.68 kg; end BW= 501.96 ± 6.90 kg) and heavy (initial BW = 418.31 ± 2.75 kg; end BW= 522.15 ± 7.09 kg) groups before assignment to treatment. Factors included dietary treatment (0, 15 ppm) and implant status (0, 80 mg trenbolone acetate + 16 mg estradiol). Ractopamine was provided in the diet to two pens or half the treatments during the final 35 d of feeding. Cows were slaughtered on d 92. Forty-eight hours postmortem the 6^th rib portions of the LM were harvested from 10 randomly selected carcasses from each treatment group (n = 40). Cyrosections (12 µm) were immunostained for dystrophin and myosin heavy chain I or II for the measurement of fiber CSA and type, respectively. Fiber associated nuclei (FAN) and satellite cell numbers were measured in serial cryosections. There was a (P < 0.05) RAC x TBA interaction. Type I fiber CSA and diameter were increased (P < 0.05) by TBA and RAC. Type I CSA and diameter were larger (P < 0.05) in TBA+RAC than RAC only. Type II fiber CSA and diameter were not affected by TBA (P = 0.48), RAC (P = 0.15) or TBA+RAC (P = 0.60). Satellite cell numbers and FAN were not affected (P > 0.05) by implant status or ractopamine supplementation. These results indicate that TBA and RAC preferentially increase the size of type I fibers in cull cows.

Key Words: Beta-agonist, cull cow, muscle fiber, Ractopamine, satellite cell

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