New insights into the mechanism and actions of growth hormone (GH) in poultry

Despite well documented anabolic effects of GH in mammals, a clear demonstr ation of such responses in domestic poultry is lacking. Recently, comprehen sive dose-response studies of GH have been conducted in broilers during lat e post-hatch development (8 to 9 weeks of age). GH reduced feed intake (FI) and body weight gain in a dose-dependent manner, whereas birds pair-fed to the level of voluntary Fl of GH-infused birds did not differ from controls . The reduction in voluntary FI may involve centrally mediated mechanisms, as hypothalamic neuropeptide Y protein and mRNA were reduced with GH, coinc ident with the maximal depression in Fl. Growth of breast muscle was also r educed in a dose-dependent manner. Circulating IGF-I was not enhanced by GH , despite evidence that early events in the GH signaling pathway were intac t. A GH dose-dependent increase in circulatings 3,3',5-triiodothyronine(T3) paralleled decreases in hepatic 5D-III monodeiodinase activity, whereas 5' D-I activity was not altered. This confirms that a marked hyperthyroid resp onse to GH occurs in late posthatch chickens, resulting from a decrease in the degradative pathway of T3 metabolism. This secondary hyperthyroidism wo uld account for the decreased skeletal muscle mass (52) and lack of enhance d IGF-I (53) in GH-treated birds. Based upon these studies, it is now evide nt that GH does in fact have significant effects in poultry, but metabolic responses may confound the anabolic potential of the hormone. (C) 1999 Else vier Science Inc. All rights reserved.