Nutrition, development and efficacy of growth modifiers in livestock species.

Somatotropin (ST) and synthetic beta-adrenergic agonists (beta-AA) are growth-modifying agents that increase the rate and sometimes, the efficiency of protein deposition in lean tissues of livestock species. The ST-induced increase in muscle protein deposition is effected by a relatively modest increase in protein synthetic rate. This is possibly mediated by the endocrine influence of marked increases in circulating IGF (insulin-like growth factor)-I, and other ST-dependent components of the IGF system; mediation by locally expressed IGF-I may also occur. Increased muscle protein accretion in animals treated with beta-AA seems to be directly mediated by binding of the synthetic agonist to muscle beta-1 or beta-2 receptors, leading to increased muscle protein synthesis, possibly accompanied or followed by decreased protein degradation. This response is transient, due to down-regulation of beta-adrenergic receptors. Maximal responses of muscle protein accretion to both ST and beta-AA are attenuated by feeding inadequate levels of total protein or specific, limiting amino acids. For ST, but not beta-AA, this effect in growing pigs is partially offset by increased efficiency of utilization of absorbed amino acids for protein deposition, with predictable consequences for dietary protein and amino acid requirements. Both ST and beta-AA are less efficacious in promoting muscle protein deposition in very young animals. For ST, this is related to postnatal development of the somatotropic axis; a mechanistic explanation for the similar lack of effect of beta-AA is lacking. In both cases, this phenomenon must be considered against the very high inherent capacity and efficiency of lean tissue protein accretion in the neonate.