Somatomedin in the rhesus monkey fetus

To assess the role of somatomedin in the maturing fetus, a RIA was developed for Multiplication Stimulating Activity (MSA), a major somatomedin. Purified MSA was conjugated to hemocyanin and specific antiserum induced in rabbits. In the RIA there was parallel displacement with unlabelled MSA and acid-extracted human or rhesus monkey serum. There was no cross-reactivity with physiological concentrations of insulin or with the other somatomedins. Immunoreactive MSA(I-MSA) in acromegalic serum extracts was approximately twice that of normal controls, while levels were reduced in growth hormone (GH) deficiency. I-MSA in pooled extracts from sera of 5 adult male monkeys was 84.0 ng/ml. Fetal samples were obtained from rhesus monkeys through catheterized interplacental vessels. At 90 days gestation (term 165 days), I-MSA in 5 fetuses was 70.2 ± 21.0 ng/ml (mean ± SEM), and at 160 days, 172.0 ± 34.3 ng/ml (P <0.05).Corresponding GH levels were 50.7 ± 2.0 ng/ml and 24.3 ± 2.3 ng/ml (P< 0.001)(reported 61st.End. Soc. 1979, Abst. 601). Because of marked variability,the mean level of I-MSA in day-old neonates was not significantly different from fetal values. The rising levels of I-MSA during the latter half of gestation in these studies suggest that:(1)somatomedins may play an important role in fetal growth; (2) somatomedin may exert feedback inhibition on GH secretion during this period.Supported by MRC Canada Grant No 9360.