Serum levels of immunoreactive somatomedin A in the rat: some developmental aspects.

Immunoreactive somatomedin A (SMA) was determined in rat serum collected at 20 days of gestation and at 5, 15, 25, and 50 days of postnatal age. Adult levels were shown to be GH dependent. A rapid rise in immunoreactive SMA was observed after 15 days of postnatal age, and this was attributed to the triggering of the SMA-GH regulatory system by nutritional factors at weaning. Circulating levels of immunoreactive SMA were reduced during early development and in pregnancy, raising the possibility that early growth is regulated by an embryonic form of SM which is not detected by the SMA RIA.