Parturition-related changes in insulin-like growth factors-I and -II in the perinatal lamb.
Specific radioligand assays for the two somatomedins, insulin-like growth factor (IGF)-I and IGF-II, have been used to study the ontogeny of somatomedin secretion in the perinatal lamb. Plasma samples were obtained from sheep fetuses from 54 days of gestation to term (147 days) and from neonatal lambs. All samples were first extracted in acid-ethanol to remove the somatomedin-binding proteins. Concentrations of IGF-I measured by radioimmunoassay were lower (P less than 0.01) in the fetus than in the adult sheep (0.96 +/- 0.17 (S.D.) units/ml, n = 11). Fetal IGF-I values rose (P less than 0.01) from 0.29 +/- 0.15 units/ml (n = 6) at 50-80 days to 0.79 +/- 0.18 units/ml (n = 13) at 140-150 days. While values were similar 0-2 days after birth, they rose (P less than 0.01) to 2.4 +/- 1.3 units/ml (n = 15) 3-7 days after birth. By 60 days they had fallen to adult values. In contrast, IGF-II levels measured by rat placental membrane radioreceptor assay were higher (P less than 0.001) in the fetus (2.71 +/- 1.06 units/ml, n = 18) than in the adult (1.0 +/- 0.17 units/ml) and showed no gestational trend between 50 and 140 days of gestation. Longitudinal studies showed a fall (P less than 0.01) in IGF-II values starting several days before birth. By 12 h after birth, IGF-II concentrations were similar to the adult and showed no subsequent postnatal change. These results demonstrate that IGF-I and IGF-II are not secreted in parallel in the perinatal lamb. There are major changes in the regulation of both IGF-I and IGF-II in relationship to birth. It is suggested that the high fetal IGF-II concentrations appear to be maintained by a stimulus withdrawn before birth. The postnatal rise in IGF-I may be related to the increase in hepatic somatogenic receptors at this age.