Insulin-like growth factors I and II in maternal and fetal guinea pig serum

The role of insulin-like growth factors (IGFs) in fetal development has been the subject of much speculation. We undertook studies of maternal and fetal IGF I and II in the guinea pig because the long gestation period and greater size of the fetuses permitted blood sampling over a longer period of gestation and maturation than is possible in the rat. Acid gel filtrates of fetal and maternal serum were prepared, and the IGF I was measured by RIA; IGF II was measured by rat placental membrane radioreceptor assay. Fetal IGF I levels were lower than maternal levels from the 33rd day of estimated gestation to term. Fetal IGF II levels from the 33rd day to the 49th day of gestation were not significantly different from those of maternal serum [1597 +/- 377 (SE) ng/ml vs. 1295 +/- 224] ng/ml. Very high levels of IGF II, in excess of 5000 ng/ml, were observed in fetuses at 50, 55, and 60 days of gestation. Thereafter, fetal IGF II levels fell markedly before term. Fetal and maternal IGFs after 49, 50, 60, and 65 days of pregnancy were compared by isoelectric focusing. The guinea pig normally has two major basic peaks of IGF I, which were present both in maternal and fetal serum. Most maternal and fetal guinea pig sera contained only a single, slightly acidic peak of IGF II. No evidence of a unique fetal IGF was detected by our methods. The very high levels of IGF II reached in fetal guinea pig sera suggest that it may have a role in fetal development.