Measurement of Somatomedin-Related Peptides in Fetal, Neonatal, and Maternal Rat Serum by Insulin-Like Growth Factor (IGF) I Radioimmunoassay, IGF-II Radioreceptor Assay (RRA), and Multiplication- Stimulating Activity RRA after Acid-Ethanol Extraction*

Abstract

Previous measurements of somatomedins (Sms) and insulin-like growth factors (IGFs) in maternal and fetal serum have yielded contradictory results. We have, therefore, measured maternal, fetal, and neonatal rat serum with two highly specific assays: 1) IGF-I/Sm-C RIA and 2) a highly specific IGF-II/rat placental membrane radioreceptor assay (RRA). In addition, we have made measurements with a less specific multiplication-stimulating activity (MSA)-rat placental membrane RRA. To avoid possible serious artifacts created by Sm-binding proteins, preliminary acid-ethanol extraction of serum was performed. Results are expressed in terms of a reference human serum with an assigned potency of 1 U/ml. Maternal RIA IGF-I fluctuated between 1.1–1.4 U/ml from the 17th day of pregnancy to the 25th day after delivery (nonpregnant rat serum pool, 1.25 ± 0.22 U/ml). On day 21 of gestation, fetal serum radioimmunoassayable IGF-I was 1.03 ± 0.03 U/ml. After birth, radioimmunoassayable IGF-I fell and reached 0.19 ± 0.03 U/ml at 18 days of age, but rose to 0.71 ± 0.04 U/ml at 25 days of age. At term, maternal radioreceptor assayable IGF-II was 2.18 ± 0.27 U/ml (nonpregnant female pool, 1.4 ± 0.12). By the 25th postpartum day, radioreceptor assayable IGF-II was 1.39 ± 0.12 U/ml. Radioreceptor assayable IGF-II in fetal serum on day 19 was 3.26 ± 0.48 U/ml and rose to 5.37 ± 0.66 U/ml on the day of delivery. A further rise to 8.92 ± 1.03 occurred on day 5. A subsequent fall to 2.41 ± 0.05 U/ml was observed on day 25. The patterns of results of the MSA RRA in fetal and neonatal rat serum were similar to that obtained with the IGF-II RRA. We now conclude that radioimmunoassayable IGF-I is present in higher concentrations than previously reported in term fetal rat serum and that radioreceptor assayable IGF-II is selectively elevated in rat fetal and neonatal life and may have unique metabolic and growth-promoting significance.