Effect of fasting on serum insulin-like growth factor-I (IGF-I) levels and IGF-I-binding activity in cockerels

White Leghorn male chicks of 40 days of age were fasted for 5 days and then refed. Blood samples were collected from these chicks before, during and after fasting and serum levels of GH and insulin-like growth factor-I (IGF-I) and serum IGF-I-binding activity were determined. The fasting-induced reduction in body weight was accompanied by a significant rise in circulating GH and fall in IGF-I, coupled with increased serum IGF-I-binding activity. When pooled serum was chromatographed under neutral conditions, IGF-I binding activity and IGF-I immunoreactivity were mainly associated with a large (M(r) = 150,000) and a small protein (M(r) = 30,000). Fasting induced a marked increase in the IGF-I-binding activity of the 30 kDa IGF-I-binding protein (IGFBP) and refeeding restored activity to the normal levels seen before fasting. Ligand blotting of serum-binding proteins with 125I-labelled IGF-I, after first subjecting the samples to polyacrylamide gel electrophoresis and transfer to nitrocellulose, revealed that four IGFBPs (M(r) = 20,000, 30,000, 35,900 and 41,000) were present in chicken serum, and that the 125I-labelled IGF-I binding of the 30 kDa monomer was increased by fasting and restored to normal by refeeding in agreement with gel filtration profiles of IGF-I-binding activity. Western blot analysis suggested that the 30 kDa IGFBP is homologous to IGFBP-2 found in mammalian blood plasma. The results show that IGFBPs in chicken serum and their responses to fasting are similar to those in mammals.