title: Insulin-like growth factor (IGF)-I but not IGF-II promotes lean growth and feed efficiency in broiler chickens.
abstract: The efficacy of exogenous IGFs to stimulate growth and modulate protein and fat deposition was examined in a number of broiler chicken lines. From around 600 g body weight the chickens received a continuous infusion of vehicle (0.1 M acetic acid), human recombinant IGF-I or [Gly1]IGF-II at 300 microg/kg body weight per day, or a combined infusion of 150 microg/kg/day of each IGF for 2 weeks. Experiment 1 used commercial broiler female chickens and included measurements of nitrogen balance, Ntau-methylhistidine excretion and muscle protein synthesis rates. In Experiment 2 the same treatments were applied to three experimental lines of chickens selected for high food consumption (relatively fat), high food utilisation efficiency (relatively lean), or at random (control). IGF-I, but not IGF-II, significantly increased growth rate and food utilisation efficiency by around 10-15% in each experiment, an effect which was consistent across all genotypes. Nitrogen balance was significantly increased by IGF-I in Experiment 1 as was carcass nitrogen content in Experiment 2, indicating that the increased growth was in lean tissue. Carcass fat was consistently reduced in chickens receiving IGF-I and was related to the levels of circulating IGF-I (r2 = 0.30, P < 0.01) but not triiodothyronine. Protein synthesis rates were unaffected by treatment and could not account for increased growth rate. However, there was a significant reduction in Ntau-methylhistidine excretion indicating a reduced rate of muscle protein breakdown in IGF-I-treated chickens (1. 56%/day vs 2.05%/day for IGF-I-treated vs controls, P < 0.05). The efficiency of feed utilisation was inversely related to the rate of protein breakdown (r2 = 0.25, P < 0.01). In conclusion, these experiments are the first to report an enhancement of growth and food utilisation efficiency by broiler chickens receiving exogenous IGF-I. Our results show that IGF-I may be important in controlling the growth and efficiency of food utilisation of young chickens at least in part by modulating the rates of protein breakdown.
authors.affiliation: Cooperative Research Centre for Tissue Growth and Repair, CSIRO Division of Human Nutrition, Gouger Street, Adelaide, South Australia, 5000, Australia.
authors.forename: F M
authors.initials: FM
authors.lastname: Tomas
authors.forename: R A
authors.initials: RA
authors.lastname: Pym
authors.forename: J P
authors.initials: JP
authors.lastname: McMurtry
authors.forename: G L
authors.initials: GL
authors.lastname: Francis
mesh_terms: D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D001786:Blood Glucose / Q000187:drug effects; D001823:Body Composition / Q000187:drug effects; D002645:Chickens / Q000097:blood / Q000254:growth & development*; D005260:Female; D006801:Humans; D015918:Infusion Pumps, Implantable / Q000662:veterinary; D007328:Insulin / Q000097:blood; D007334:Insulin-Like Growth Factor I / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D007335:Insulin-Like Growth Factor II / Q000008:administration & dosage / Q000378:metabolism / Q000494:pharmacology*; D009929:Organ Size / Q000187:drug effects; D011506:Proteins / Q000187:drug effects / Q000378:metabolism; D014284:Triiodothyronine / Q000097:blood / Q000187:drug effects
chemical_list: D001786:Blood Glucose; D007328:Insulin; D011506:Proteins; D014284:Triiodothyronine; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II