Commercial Preparations Hormone Delivery Responses and Mode of Action

The anabolic preparations available in the United States[1] contain naturally occurring testosterone (T) and its propionate ester, estradiol-17p (E) and its benzoate ester, progesterone (Pr), and the xenobiotic compounds trenbo-lone acetate (TA), zeranol (Z), and melengestrol acetate (MGA). Active ingredients may be further characterized as estrogens (E is a steroid hormone synthesized mainly in gonadal tissues; Z is a resorcylic acid lactone derivative of the nonsteroidal fungal estrogen zearalenone), steroidal androgens (T is a hormone synthesized in gonads and adrenal cortex with potential for conversion to estrogens; TA with effects produced mainly by its active metabolite trenbolone-17pOH, exhibits both androgenic and anti-corticosteroid properties), and progestagens (naturally occurring steroid Pr; synthetic steroidal compound MGA). Formulations containing single, or certain combinations of, ingredients are applied as impregnated silastic rubber implants or compressed pellets under the skin of the upper surface of the ear, or for MGA, inclusion in the diet.

The relative quantities of active ingredients (up to 43.9 mg for E and 200 mg for T, TA, and Pr) in implants[1] reflect amounts required to produce effects in vivo. These result in variably circulating concentrations of total E in the range of 5 to 80 pg/ml with those for T, TA, and Pr in excess of 250 pg/ml.[2] Typical improvements in growth, feed conversion efficiency, and carcass leanness have been summarized1-2-1 in the range of 10 to 30%, 5 to 15%, and 5 to 8%, respectively, with greatest effects occurring in steers in the relative absence of endogenous sex hormones. Smaller responses occur in postpubertal heifers and bulls.[3] Estrogens are considered to have the greatest anabolic activity with potentiation by androgens and in particular when combined with trenbolone acetate. For implants, the growth responses are affected by rates and quantities of systemic uptake of hormonal compounds from the implant, their transport by carrier proteins such as sex hormone binding globulin or serum albumin, and the diffusion of free forms into target cells. These events precede interaction with specific members of the steroid nuclear hormone family of receptor transcription factors for estrogens, androgens, and progestagens and associated chaperone proteins.[4-6] These ligand-bound steroid receptors form activated, usually dimer, complexes that along with co-activators bind to specific nuclear hormone response elements. Depending on recognition sites, these may activate or repress DNA expression to affect gene transcription and translation directly in skeletal muscle or adipose cells or indirectly by stimulating expression of other hormonal compounds, such as IGF-I with suppression of thyroid or corticosteroid hormone function.[2] Although poorly understood, changes in these messaging systems are considerable to produce alterations in the balance of anabolism and catabolism of protein and fat. Synthesis of protein may be influenced directly at a gene level, with catabolism mediated by proteolysis, such as produced by lysosome, ubiquitin, and/or proteasome-dependent pathways.[7]

The maintenance of activity of hormonal preparations is determined by continued availability from the implant and by retention in tissues in the active form. These, along with metabolized and variably inactivated forms, contribute to the presence of residues in meat postmortem. Metabolic inactivation is effected predominantly by liver CYP450 systems with elimination, for example, following hydroxylation or sulphation in urine or if more lipophilic via bile, with the additional possibility of reabsorption.[4] An issue of increasing contemporary importance is persistence in the environment of excreted compounds, including the nonabsorbed fraction for MGA, and subsequent re-entry to the water or food chain.[8]

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