In order to clarify the potential role of IGF-I in fetal growth, the dynamics of IGF receptor were investigated in comparison with insulin receptor in human and rat fetuses.
In humans, the serum levels of IGF-I measured by IGF-I radioimmunoassay after acid-ethanol extraction were significantly lower in fetuses than in adult controls. Conversely, serum insulin levels in fetuses were not indistinguishable from those in adults. On the other hand, specific binding of
125 I-IGF-I to human fetal erythrocytes was 2.3%, which was significantly higher than that of adult women (1.6%). Insulin binding to the erythrocytes was also higher in human fetuses than in adult controls (cord : 3.8%, adult : 3.0%). By Scatchard analysis, the changes in the specific binding of IGF-I and insulin were mainly due to the changes in the binding capacity rather than those in the binding affinity. Additionally, IGF receptor on human fetal erythrocytes was recognized as type I IGF receptor, because the binding of
125I-labelled IGF-I and
125 I-labelled IGF-II could be displaced by the addition of cold IGF-I, IGF-II and insulin.
In rats, serum levels of IGF-I were also much lower in fetuses than in adult controls. Specific binding of
125 I-IGF-I to liver microsomal membranes was 34.3% (per 400/tg protein) in fetal rats (D
20), which was significantly higher than that of adult rats (3.2%). Scatchard analysis indicated that these changes in
125 I-IGF-I binding was chiefly due to the changes in binding capacity. On the other hand, serum insulin levels increased with progress of pregnancy, and specific binding of insulin to microsomal membranes of fetal liver increased on D
21 of gestation, mainly due to the increase in binding affinity rather than binding capacity.The bindings of
125 I-labelled IGF-I and
125 I-labelled IGF-II to fetal liver microsomal membranes were clearly displaced by cold IGF-I and IGF-II but not by excess of cold insulin, indicating that the receptor was type 2 IGF receptor.
These results suggest that IGF-I possesses much more receptor in fetal tissues than in adult tissues despite its lower concentration in fetal sera. It is speculated that IGF-I may play an important role in fetal growth and development.
抄録全体を表示