Somatomedin-C receptor ontogeny and levels in porcine fetal and human cord serum.

The ontogeny of somatomedin receptors in tissues of fetal pigs and levels of somatomedin-C in fetal pig serum at various gestational ages and in human cord serum was investigated. Specific binding of 125I somatomedin-C by particulate membranes prepared from fetal organs from a variety of gestational ages almost always exceeds specific 125I insulin binding. In liver, kidney, heart and the maternal portion of the placenta, apparent binding affinity for somatomedin is relatively constant throughout gestation and is the same for membranes from fetal and adult animals. In contrast, in the fetal portion of the placenta, specific somatomedin-C binding and apparent binding affinity increases as gestation progresses. The changes in this tissue correlate temporally with the acceleration of growth of the pig fetus. Membranes prepared from fetal lungs exhibit higher specific binding of somatomedin and higher affinity constants than adult lung membranes. Somatomedin levels in fetal pig serum are about 25% of those observed in the sow and are constant throughout fetal life. Somatomedin in human cord serum is likewise low compared to adult levels. Small-for-gestational age infants and large, postmature infants have lower mean somatomedin levels than normal weight, full-term infants. The identification of specific somatomedin receptors in fetal tissues opens the possibility that somatomedin-C stimulates growth of the fetus. Although not resolved, the relatively low levels of somatomedin in fetal serum may reflect low levels of the somatomedin binding protein rather than an absolute deficiency of biologically active somatomedin.