Maintenance of serum somatomedin activity in hypophysectomized pregnant rats

Pregnant female rats were hypophysectomized on the 14th day of pregnancy without fetal loss or premature delivery. Completeness of hypophysectomy was confirmed by measurements of serum growth hormone and prolactin. Serum somatomedin activities were measured by the hypophysectomized rat costal cartilage in vitro assay using a human serum standard. In this assay somatomedin activity of normal rat serum is about four times that of normal human serum. Forty-eight hours after hypophysectomy serum somatomedin was maintained at 3.9 +/- 1.0 units/ml. At this time after operation somatomedin activity of nonpregnant hypophysectomized rat serum would be virtually undetectable. Somatomedin concentrations were still maintained on the 18th and 20th day of pregnancy. However, within 24 hours after delivery somatomedin had fallen to less than 0.6 units/ml and by three days post delivery were less than 0.2 units/ml. These observations indicate that the fetal-placental unit, probably by secreting a chorionic somatomammotropin, can maintain the concentration of growth hormone-dependent somatomedin in tha absence of the pituitary.