Influence of dietary protein on insulin-like growth factor (IGF) binding proteins in the chicken

In plasma from avian and mammalian species, IGFs are bound to specific binding proteins (BPs). When chicken plasma proteins are separated by electrophoresis (SDS-PAGE), electroblotted onto nitrocellulose and probed with I251-IGF-I, a distribution pattern oflGF binding analogous to a mammalian pattern is observed; however, relative levels of IGF-BPs are several-fold lower in chickens. If chicken plasma proteins are probed with 135I-IGF-II, a unique binding specie of approximately 70 kD is observed. An experiment was conducted to determine the effect of dietary protein (DP) on the distribution of IGF binding activity in chicken plasma. Three groups of male chickens (n=6 per group) were fed (ad libitum) isocaloric diets containing 12, 21 or 30% DP. Birds were fed respective diets beginning at 7 d of age and killed at 28 d. Plasma was collected at sacrifice and frozen. Binding proteins in plasma samples (3pl) were separated by SDS-PAGE on 15 lane minigels. Nitrocellulose blots were probed with ml-chicken IGF-II; Xray film was exposed to blots for 14 d. Samples were analyzed in duplicate and intensity of each binding specie was normalized to its respective signal from pooled control samples which were loaded on each gel as internal standards. Birds consuming 12% DP had less than 50% of the 43 kD-binding activity of birds consuming 21 or 30% DP (P < 0.01); the 32 kD-binding activity was also lower (P < 0.01) in the 12% DP group. In contrast, 70 kD and 24 kD binding activities were not influenced by DP. Chickens consuming 12% DP had higher levels of somatotropin and lower levels of IGF-I than those consuming higher levels of DP. These data indicate that in chickens, as in mammals, circulating IGF, somatotropin and specific IGF-BPs are influenced by protein intake.