Characteristics of hepatic receptors for somatomedin-C/insulin-like growth factor I and insulin in the developing human.

Insulin and the somatomedins (Sms) are putative regulators of cell proliferation and metabolism in the fetus. Since the liver is a potential target tissue of these hormones during fetal life, we characterized the hepatic receptors for Sm-C/insulin like growth factor I (Sm-C/IGF-I) and insulin during the second trimester of human fetal development. Membrane-enriched fetal liver homogenates specifically bound 8.9 +/- 1.5% (+/- SD) of added [125I]insulin and 5.1-7.2% of [125I]Sm-C/IGF-I. Binding of both hormones was constant from 12-20 weeks gestation and was much greater than that in adult liver membranes. Analysis of dose-response data indicated high affinity between each receptor and its respective ligand (Kd for the Sm-C/IGF-I receptor, 2.2 X 10(-10) M; Kd for insulin receptor, 5.2 X 10(-10) and 7.7 X 10(-9) M). Limited cross-reactivity (approximately 1:1,000) of insulin with the Sm-C/IGF-I receptor and Sm-C/IGF-I with the insulin receptor was found. Affinity labeling studies showed that each receptor possessed an approximately 135,000-dalton subunit which was a part of a larger disulfide-linked complex. Thus, the human fetal liver has specific receptors for Sm-C/IGF-I and insulin that are similar to those described for other tissues in terms of both hormone-binding characteristics and subunit structure, suggesting that these receptors mediate important cellular functions at this stage of fetal development.