QUANTITATIVE REVIEW ON THE USE OF ANABOLIC HORMONES WITH STEROIDAL ACTIVITY IN RUMINANTS REARED FOR MEAT PRODUCTION .2. MODES OF ACTION

The hypotheses on the modes of action of hormonal anabolic agents in growing animals have been reviewed in more than 120 recent publications. The mechanisms of action are still not fully understood. Androgens such as testosterone and estrogens such as oestradiol-17beta (E-17beta) may act in different ways: firstly, testosterone (and probably also E-17beta) acts directly on different tissues, and particularly at the level of the muscle cell by binding to a specific receptor. The hormone-receptor complex interacts with the nuclear receptor located in the chromatin and enhances protein synthesis (and probably also protein degradation). Trenbolone acetate (TBA) reduces protein synthesis and to a greater extent protein degradation. This action of TBA could take place via a reduction in the activity of catabolic glucocorticoids, either by a diminution in their secretion, or by displacing them from their receptor, or by reducing the number of receptors. Secondly, an indirect action of anabolic hormones is probable via the modifications in activity of other growth-regulating hormones. Growth hormone and insulin-like growth factor-I concentrations are enhanced by E-17beta, diethylstilbestrol, zeranol and testosterone but not by TBA. Insulin appears to be indirectly enhanced by estrogens through an increase in growth hormone, whereas androgens reduce insulin levels. Thyroid hormone (tri- and tetra-iodothyronine) activity is reduced by androgens, whereas the action of oestrogens depends on the physiological maturity of the animal. The modes of action of these anabolic hormones are discussed in relation to growth rate and body composition.