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 (IG-FBP) and refeeding restored activity to the normal levels seen before fasting. Ligand blotting of serum-binding proteins with I-125-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 I-125- 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.