The role of the somatotrophic axis in the metabolism of the chicken

As it is for mammalian species, growth hormone (GH) is indispensable for no rmal growth and development of avian species. In contrast to mammals, exoge nous GH administration has little, if any, potential for improving the grow th rate and feed efficiency of rapidly growing broilers; it is more likely to do so in older birds. This is at least partly because of age-related cha nges in tissue GH-binding activity and GH-receptor mRNA expression. The eff ects of GH on lipid deposition depends on the age of the bird and pattern o f GH administration. Pulsatile, but not continuous, GH administration to ol der broilers seems to reduce fat deposition. As in rats, the bioactivity of GH might also depend on the pulse-induced cyclicity in GH receptors and GH -binding proteins. In chickens, GH is also a very potent lipolytic hormone, but seems to have no diabetogenic effect, which is reported in mammalian s pecies. Both insulin-like growth factors have apparently no growth-promotin g effects in normal growing broilers, but seem to have opposite effects on fat deposition. In contrast to GH, both insulin-like growth factors have a marked hypoglycemic effect. Whether all these effects are direct effects, o r are mediated by secondary mechanisms, awaits further investigations. (C) 1999 Elsevier Science Inc. All rights reserved.