Hormone ontogeny in the ovine fetus. XVI. Plasma immunoreactive somatomedin C/insulin-like growth factor I in the fetal and neonatal lamb and in the pregnant ewe.

Abstract

The basic somatomedins [SMC/insulin-like growth factor I (IGF-I)] and the neutral somatomedins (multiplication-stimulating activity/IGF-II) exhibit different patterns during pregnancy and ontogeny. We have adapted a specific RIA for SMC/IGF-I to study the pattern of change of this growth factor in fetal, neonatal, and adult sheep plasma. Acid-ethanol extraction of the samples was performed to minimize the interference of the somatomedin-binding proteins in the RIA. Results are expressed in terms of a neonatal lamb reference plasma with an arbitrarily assigned potency of 1 U/ml (equivalent to 500 ng/ml purified human IGF-I). In chronically catheterized fetal and neonatal lambs, plasma SMC/IGF-I rose from 0.14 +/- 0.02 U/ml before 100 days of gestational age (term 147 days) to 0.22 +/- 0.01 U/ml between 101 and 144 days. In neonates (1-33 days), the mean plasma concentration of SMC/IGF-I was 0.93 +/- 0.07 U/ml. In adult sheep, plasma SMC/IGF-I was twice as high in rams as in nonpregnant ewes (0.88 +/- 0.06 vs. 0.44 +/- 0.03 U/ml); in pregnant ewes plasma SMC/IGF-I did not change significantly between the second and the last third of gestation and was always higher than the fetal levels. The rise in fetal plasma SMC/IGF-I around 100 days of gestation parallels the rise in fetal GH and PRL concentrations. The discrepancy between the present results and earlier reports based on bioassays and radioreceptor assays may be due to the presence of high circulating concentrations of IGF-II-like peptides in the ovine fetus. The striking sex difference in the plasma concentrations of SMC/IGF-I in adult sheep suggests that SMC/IGF-I generation or disposition is influenced by sex steroids.