Hormone Ontogeny in the Ovine Fetus. III. The Effect of Exogenous Somatostatin*

Abstract

We infused synthetic cyclic somatostatin (SRIF) into the chronically catheterized ovine fetus and neonate to study its effect on the secretion of GH. After a 2-h control period, eight fetuses (92–138 days gestation) and five neonatal lambs received 50 μg SRIF as an iv bolus, followed by 150 μg SRIF over 55 min as an infusion. Two additional fetuses (132 and 133 days) received a 50-μg bolus, followed by 300 μg SRIF over 115 min. In all fetuses, GH concentration fell during the infusion. The maximum suppression was 52.6 ± 4.5% (72.0 ± 15.5 ng/ml) of basal GH concentration (P < 0.01). After the infusion, GH concentration rose to 137.0 ± 9.0% of basal concentration at 90 min. In the only neonatal lamb with elevated basal GH concentration, GH fell during the SRIF infusion. PRL concentration fell slightly but consistently during SRIF infusion, and no rebound was observed after the infusion. The maximum suppression was 8.5 ± 2.2 ng/ml (P < 0.01). In three of four lambs with basal PRL levels >5 ng/ml, PRL concentration decreased during SRIF infusion. In three fetuses with a basal insulin concentration of >5 μU/ml, a falling trend in plasma insulin was noted during SRIF infusion. SRIF infusion into the fetus had no effect on the concentration of chorionic somatomammotropin in fetal plasma or on maternal GH, PRL, or chorionic somatomammotropin concentrations. These results suggest the presence of SRIF receptors in the fetal pituitary gland by midgestation and are compatible with the hypothesis that the fall in plasma GH concentration in midgestation, in both human and ovine fetuses, is the result of the development of inhibitory mechanisms mediated by hypothalamic SRIF. The results do not exclude the potential role of decreased secretion of GH-releasing factor.