In Vivo Evidence for Increased Adrenal Sensitivity to Adrenocorticotropin-(l–24) in the Lamb Fetus Late in Gestation*

Plasma concentrations of immunoreactive ACTH and cortisol were monitored daily in chronically cannulated sheep fetuses during the last 3 weeks of gestation. A prepartum increase in fetal plasma cortisol occurred without a concomitant rise in fetal plasma ACTH. When fetal lambs were injected with various doses of ACTH-(1-24) and the plasma cortisol responses were integrated over time, the resulting changes in the log dose-response curves indicate that fetal adrenal sensitivity increases late in gestation. Thus, the marked rise in fetal plasma cortisol before birth can be explained, at least in part, by an increase in fetal adrenal sensitivity to ACTH.