Two distinct receptors for the insulin-like growth factors (IGF-I and IGF-II) have been identified in mammalian tissues, but so far only a receptor structurally related to the type I receptor has been identified in chicken embryonic tissues. This study was designed to characterize binding sites for IGF peptides in chicken liver microsomal membranes prepared from hatch to 10 weeks of age which is the period of most rapid growth.
Binding of both human (h) IGF-I and hIGF-II was displaceable by either peptide and exhibited similar pH, time and temperature dependency. Human IGF-II was more potent than hIGF-I in competing for the binding of the iodinated ligands with half-maximum effective concentrations of 3–5 μg/l and 7–13 μg/l respectively. Porcine insulin was also a potent competitor. Affinity cross-linking studies, followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis under reducing conditions demonstrated that both IGF peptides were linked to a protein with a molecular weight of about 130 000 Da characteristic of the α-subunit of the type I receptor. There was no evidence for the presence of a type II receptor similar to that found in mammals.
Specific binding of both peptides was low on the day of hatch, increased about threefold by day 3 of age and remained high for the first 3 weeks of life before returning to a lower steady state level up to 10 weeks of age. This was the result of variation in receptor number, with no change in the affinity of the binding site for either ligand. The affinity constant for IGF-II (4·5 ± 0·5 (s.e.m.) litres/nmol) was higher than for IGF-I (1·4 ± 0·3 litres/nmol).
Insulin-like growth factors have previously been reported to stimulate the metabolism and growth of hepatocytes in vitro and are produced by these cells. The occurrence of a higher number of receptors at the period of most rapid growth of the liver suggests that they may have a role in regulating normal liver growth in an autocrine or paracrine manner. Furthermore, present evidence suggests that this is through a single receptor related to the type I receptor.