SERUM SOMATOMEDIN ACTIVITY AFTER HYPOPHYSECTOMY AND DURING PARTURITION IN FETAL LAMBS

Somatomedins, detectable by sulfation factor activity (SFA) and/or somatomedin-like receptor activity (SmLRA), were measured in the serum of 72 normal fetal lambs of varying gestational ages and in 12 fetuses before and after hypophysectomy. Hypophysectomized fetal lambs [3] were infused with GH [growth hormone] and/or insulin. Intact fetal lambs [3] were infused with glucose. Parturition was induced by infusion of synthetic ACTH into 5 fetal lambs. SmLRA and SFA were compared to a pool of adult sheep serum arbitrarily designated 1.00 U[unit]/ml. Serum SmLRA correlated significantly with gestational age (r [correlation coefficient] = 0.65; P < 0.001), the linear regression line passing through 0.5 U/ml at 50 days and 2 U/ml at 150 days of gestation. Serum SFA also correlated with gestational age (r = 0.40; n = 19; 0.1 > P > 0.05). Serum SmLRA and SFA were unchanged by hypophysectomy of the fetus and GH infusion, but were reduced by insulin infusion. Plasma SmLRA correlated negatively with plasma insulin during glucose infusion (r = -0.67; P < 0.05). During induction of parturition with ACTH, serum SmLRA fell by a mean of 38% and concentrations correlated negatively with plasma cortisol (r = -0.62; P < 0.001). Serum somatomedin activity in fetal sheep is not GH dependent. Somatomedin activity in cord blood after spontaneous delivery may not accurately reflect concentrations prevailing in fetal life.