Cortisol differentially regulates pituitary-adrenal function in the sheep fetus after disconnection of the hypothalamus and pituitary

We have investigated the effects of a 5 day infusion of cortisol into fetal sheep, in which the hypothalamus and pituitary were surgically disconnected (HPD), on fetal pituitary-adrenal function. Fetal HPD and vascular catheterization were carried out at between 104 and 124 days gestation. Cortisol was administered (3.5 mg 24 h-1) for 120 h between 134 and 140 days (HPD + F group; n = 5) and saline was administered during the same gestational age range to HPD (HPD group; n = 12) and intact fetal sheep (Intact group; n = 6). Cortisol infusion into the HPD fetal sheep did not suppress the mRNA levels for Proopiomelanocortin (POMC) in the fetal anterior pituitary at 139/140 days gestation (POMC mRNA: 18S rRNA: Intact 0.40 +/- 0.05; HPD 0.56 +/- 0.07; HPD + F 0.49 +/- 0.07). Similarly, there was no significant effect of either HPD or cortisol infusion on the plasma concentrations of immunoreactive (ir) ACTH or ACTH(1-39). The adrenal: fetal body weight ratio was significantly higher, however, in the HPD + F (88.4 +/- 8.7 mg kg-1) and Intact groups (84.1 +/- 5.6 mg kg-1) when compared with the HPD fetal sheep (63.7 +/- 5.4 mg kg-1). The ratio of total IGF-II mRNA: 18S rRNA was similar in the adrenals of the Intact (0.48 +/- 0.09), HPD (0.78 +/- 0.09) and HPD + F (0.71 +/- 0.11) groups. The ratios of CYPIIA1, 3 beta-HSD and CYP21A1 mRNA: 18S rRNA were significantly lower in adrenals from the HPD group when compared to those in the Intact group and were not restored to normal by cortisol infusion. We have therefore demonstrated that cortisol does not act directly at the fetal pituitary to suppress POMC synthesis or ACTH secretion in late gestation. Cortisol does, however, stimulate fetal adrenal growth after HPD in the absence of any effects on adrenal IGF-II or steroidogenic enzyme mRNA levels. The data provide evidence that an intact hypothalamic-pituitary axis and cortisol each play an important role in the stimulation of adrenal growth and steroidogenesis which occurs during the last 10-15 days of gestation in the sheep.J. T. Ross, I. D. Phillips, J. A. Owens and I. C. McMille