Submitted to: Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 15, 1999
Publication Date: N/A
Interpretive Summary: Dietary protein influences many aspects of the growth process. Specifically, the quality and quantity of dietary protein control the synthesis and secretion of growth regulating hormones such as growth hormone and the insulin-like growth factors (IGFs). In addition, the function of the IGFs are directly influenced by a family of circulating IGF-binding proteins. The purpose of this study was to evaluate the overall effects of dietary protein levels on the circulating levels of IGF-binding proteins in broiler chickens. Chickens were fed an inadequate level of dietary protein(12%), an optimal level (21%) or excessive amount of dietary protein (30%). Compared to chickens consuming optimal or excessive protein, those consuming low protein had higher levels of growth hormone and lower levels of IGF-I. Two IGF- binding proteins were markedly lower while two remained unchanged in chickens consuming low dietary protein, compared with chickens consuming optimal or excessive protein. These results demonstrate that specific IGF-binding proteins can be used as an indicator of IGF metabolism and growth in chickens.
An experiment was conducted to determine the effect of dietary protein on the distribution of insulin-like growth factor (IGF) binding proteins in chicken plasma. Three groups of male broilers (n=6 per group) were fed (ad libitum) isocaloric diets containing 12, 21 or 30% dietary protein. Birds were fed respective diets beginning at 7 d of age and killed at 28 d. No differences were observed between adequate (21%) and high (30%) protein intakes for any of the parameters investigated. Feeding protein deficient diets (12%) resulted in 34% decrease in body weight, 17% decrease in feed intake and a 39% increase in feed:gain ratio. IGF binding proteins in plasma samples were separated by SDS-PAGE and transferred to nitrocellulose sheets. Nitrocellulose blots were probed with 125I-chicken IGF-II. Four regions of binding activity corresponding to 70, 43, 32 and 24 kDa were observed in all samples. Birds consuming 12% dietary protein had less than 50% of the 43 kDa-binding activity of birds consuming 21 or 30% dietary protein. The 32 kDa-binding activity was 42% lower in the 12% dietary protein compared to birds consuming adequate protein. In contrast, 70 kDa and 24 kDa binding activities were not influenced by dietary protein. Chickens consuming 12% dietary protein had higher levels of growth hormone and lower levels of IGF-I than those consuming 21 or 30% dietary protein. These data indicate that in chickens, the circulating levels of at least two independent IGF-binding proteins are influenced by dietary protein.