Growth promotant supplementation alters live performance and carcass characteristics while impacting cellular metabolism, gene expression, and protein synthesis



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Growth promotants have been shown to positively impact production parameters such as live performance and efficiency, carcass characteristics, and ultimately profitability. Therefore, the objectives of this study were to elucidate the molecular mechanisms by which these growth promotants affect production. To asses’ performance, feeding efficiency parameters were monitored followed by carcass characteristic evaluation at harvest. Real-time quantitative reverse transcription polymerase chain reaction, immunohistochemistry, and western blots were preformed on all tissue samples. There was no difference in growth performance or carcass characteristics of steers supplemented with a zinc-methionine complex (ZnMet) or heifers administered recombinant bovine somatotropin (rBST; P > 0.05). Bovine satellite cells, steers and heifers administered the beta-adrenergic agonist, zilpaterol HCl (ZH) increased the abundance of myosin heavy chain (MHC)-IIX mRNA (P < 0.05). Treatment with ZH or ZnMet did not affect the protein abundance of beta-adrenergic receptors (β-AR) in tissue or satellite cells. Zinc-methionine supplemented animals had a greater abundance of MHC-II protein (P < 0.05). The administration of rBST and the supplementation of ZH and ZnMet increased MHC-I, IIA, and IIX fiber cross-sectional area (P < 0.05). Zilpaterol HCl and rBST decreased the proportion of MHC-I fibers (P < 0.05) and increased the proportion of MHC-IIX fibers (P < 0.05). The ZnMet treatment increased the proportion of MHC-I fibers (P < 0.05) and decreased the proportion of MHC-IIX fibers (P < 0.05) in comparison to steers just supplemented with ZH. Supplementation of ZH and ZnMet tended to decrease the density of β2-AR (P < 0.10) and increase the density of internalized β2-AR in skeletal muscle (P < 0.10). The rBST had no affect on β-AR or satellite cell density (P > 0.05). The density of Myf5 satellite cells was decreased (P < 0.05) in ZH heifers. Steers supplemented with ZnMet had a greater density of Pax7 satellite cells (P < 0.05). Overall, these data revealed that different growth promotants impact cellular muscle metabolism via multiple cellular mechanisms, and these cellular differences may ultimately impact performance and profitability.



Myosin heavy chain, Recombinant bovine somatotropin, Zilpaterol hydrochloride, Zinc-methionine complex