The effect of zinc on the β - adrenergic receptor in bovine satellite cells and the use of β - agonists and steroidal implants on muscle protein and mRNA levels in feedlot cattle
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There are multiple methods cattle growers utilize in order to increase the amount of lean tissue deposition in feedlot cattle. β – adrenergic agonists are a commonly provided growth promotant that aids in increasing average daily gain (ADG), increasing animal efficiency, and increasing muscle mass. Ractopamine hydrochloride (RH) is a currently marketed β – agonist that is available for both steer and beef heifer use. Experiments were conducted implementing RH in an in vitro and in vivo environment. The in vitro study focused on the interactive effects of RH and the micromineral, zinc, on bovine satellite cells (BSC). While the in vivo study observed how RH and steroid hormones affect feedlot heifers. The in vitro experiment was designed to identify if zinc alters the binding affinity of RH to the β – agonist receptor. In order to determine if this interaction occurred, the concentration of cAMP produced by BSC was recorded in cells treated in a 2x2 factorial of either 0 or 1 μM zinc and 0 or 10 μM RH. Treatments were provided for short (6 h), mid (24), and long (96 h) time periods. Muscle related protein and mRNA levels were also measured in the study at 24 and 48 h. At 6 h, no differences (P >0.05) were detected in cAMP production between any of the treatments. However, at 24 h the 10 μM RH, 1 μM zinc treatment had a greater concentration of cAMP (P <0.05) compared to all other treatments. At 96 h the 10 μM RH, 0 μM zinc treatment had a lower concentration of cAMP (P =0.05) compared to the control. A tendency for an interaction of Zn and RH (P <0.10) was determined at 96 h. There were no significant results recorded from gene quantification methods. Genes of interest included the β1 adrenergic receptor, the β2 adrenergic receptor, AMPKα, myosin heavy chain I, myosin heavy chain IIA, and myosin heavy chain IIX. Protein quantification was performed via western blotting procedures two assess the abundance of the β1 adrenergic receptor and the β2 adrenergic receptor; however, no differences were detected in protein abundance between treatments. The in vivo experiment’s design was to determine the effects of a β-agonist and terminal implant date on feedlot heifer performance. It was our goal to determine the effect of implant strategy and β-agonist administration in beef feedlot heifers (n = 264). Terminal implants (TI; estradiol/trenbolone acetate implant, Component TE-200) were provided to heifers on day 0 of the trial (TI d 0), day 40 (TI d 40), and day 80 (TI d 80). The trial lasted for 140 days and animals that received the later two implants were first implanted on day 0 with Component TE-IH. The second treatment the cattle received was ractopamine-hydrochlorie (RH) in the form of Optaflexx® (OPT) over the final 28 days of the trial. Heifers either received no OPT (0 mg/head/d; No) or 200 mg/head/d (Yes) of OPT. Thirty animals were subjected to longissimus muscle (LM) biopsies on d 0, 40, 80, 112, and at slaughter on d 140 to view mRNA levels of myogenic related genes and protein quantities of the β1-adrenergic receptor (β1 AR) and β2-adrenergic receptor (β2 AR). On the same days, blood samples were taken from 108 animals to assess changes in plasma blood urea nitrogen (BUN), non-esterified fatty acids (NEFA) and progesterone due to treatments. Relative mRNA levels of myosin heavy chain IIX (MHC IIX), AMPKα, and IGF-I were increased (P < 0.05) in animals receiving a TI d 40 over the other two implant dates after RH was fed to animals. After RH administration myosin heavy chain IIA (MHC IIA) mRNA levels tended to decrease (P = 0.09) due to RH. An interaction between TI d and RH administration caused an increase (P < 0.05) in MHC IIA mRNA level in the TI d 80/Yes treatment group over all other treatments except the TI d 40/No treatment group. Protein intensity of the β2 AR was decreased (P < 0.05) by the latest TI d (TI d 80) during RH feeding, while β1 AR protein intensity tended to be lower (P < 0.10) in animals fed RH. BUN plasma levels were reduced (P < 0.05) after terminal implants and RH feeding; while progesterone was decreased (P < 0.05) by RH alone. Plasma NEFA levels were not affected by any treatment.