Interactive effects of zinc and zilpaterol hydrochloride on the bovine β-adrenergic receptors

Abstract Text: Beta-adrenergic receptors (β-AR) are a member of the G-coupled protein receptor family and are responsible for binding endogenous catecholamines and synthetically produced β-adrenergic agonists (β-AA).  These β-AAs are established growth promoters that increase muscle accretion via hypertrophy of the muscle fiber, decrease lipogenesis, and improve feed efficiency when fed to finishing cattle.  Zinc (Zn) has been shown to have allosteric binding sites on β₂-ARs, enhancing agonist affinity, and inhibiting antagonist binding.  Zilpaterol hydrochloride (ZH) is a β-AA, shown to have a high affinity for β₂-ARs, which are the predominant receptor on skeletal muscle and adipocytes in cattle.  The objective of this study was to determine if utilizing Zn in combination with ZH affected the interaction of ZH with the β₂-AR by altering cAMP production, mRNA quantity, and protein abundance.  Cultures of bovine satellite cells were established and treated at 120 h with: 1) 0 µM Zn/ZH; 2) 0 µM Zn/10 µM ZH; 3) 1 µM Zn/0 µM ZH; 4) 1 µM Zn/10 µM ZH in differentiation media for 0, 6, 24, 48 and 96 h.  Protein and mRNA were measured at 24 and 96 h, and cAMP was measured at 0, 6, 24, 48 and 96 h.  At 0, 24, 48 and 96 h, no differences (P > 0.05) were detected in cAMP production.  At 6 h, 1µM Zn had the greatest concentration of cAMP (P < 0.05) compared to ZH treatments.  Using RT-QPCR analysis, no differences (P > 0.05) were detected in mRNA abundance between any of the treatments for the genes evaluated at 24 h.  Genes of interest included β₁-AR, β₂-AR, AMPKα, IGF-I, myosin heavy chain (MHC)-I, MHC-IIA, MHC-IIX, GPR43, SCD, CEBPβ, and PPARγ. At 96 h, 10 µM ZH cells had an increased quantity of MHC-I mRNA (P < 0.05) compared to 0 µM Zn/ZH, and a greater abundance of MHC-IIX mRNA (P < 0.05) and a tendency for higher levels of IGF-I mRNA (P < 0.15) compared to 0 µM Zn/ZH and 1µM Zn/10 µM ZH.  Furthermore, at 96 h, 1µM Zn/10 µM ZH cells tended to express more GPR43 and PPARγ mRNA (P < 0.15) compared to 0 µM Zn/ZH. Protein quantification was performed via western blotting procedures to assess the abundance of the β_1­-AR and the β₂-AR; however, no differences (P > 0.05) were detected.  These results indicated that Zn and ZH in combination decreased myogenic gene expression.

Keywords: β-adrenergic receptor, zilpaterol hydrochloride, zinc