PROPOSED MECHANISMS FOR THE REGULATION OF GROWTH-HORMONE ACTION IN POULTRY - METABOLIC EFFECTS

Growth hormone (GH) administration to nonruminant red meat animals markedly alters carcass composition so that dramatic reductions in adipose tissue accretion and enhancement of lean tissue growth occur. These repartitioning effects of GH in the pig are reported to reflect antagonism of the lipogenic effect of insulin on adipose tissue, the primary site of fatty acid synthesis in this species, so that glucose disposal and utilization by adipose tissue are markedly reduced, and substrate availability to muscle is increased. In poultry, a significant positive response to GH administration is not consistently achieved, and factors such as posthatch period of development and the pattern of tissue exposure to GH are important determinants of the response to GH enhancement. This may relate to the status of target tissue GH receptors that appear subject to down regulation in the adult chicken. Sensitivity of the bird to the appetite-suppressive effects of GH and interaction between this effect and energy intake have recently been demonstrated and need to be further explored. Growth hormone clearly influences hepatic lipogenesis and net lipid deposition in the broiler chicken. However, future research emphasis on the regulation of GH receptor binding activity and gene expression and their relationship to GH action, as well as on newer components of the GH axis such as GH-binding proteins, will help to clarify controlling mechanisms in poultry.