Sensitivity of cortisol-induced inhibition of ACTH and renin in fetal sheep.

Previous experiments demonstrated that increases in ovine fetal plasma cortisol concentration to maximal stress levels suppressed fetal plasma renin activity and completely inhibited fetal adrenocorticotropin (ACTH) responses to subsequent stress. This study was designed to quantitate the suppressive action of cortisol on both ACTH and renin. Fetal sheep between 117 and 131 days gestation were surgically prepared with chronically implanted catheters. At least 4 days after surgery, vehicle or cortisol (0.25, 0.5, 1, 2, or 3 micrograms/min) were infused for 5 h. One hour after the end of the vehicle or cortisol infusion, fetal ACTH and renin secretion were stimulated by intravenous infusion of sodium nitroprusside. Cortisol infusions suppressed basal plasma renin activity (caused by suppression of plasma renin concentration) to degrees that were related to the increases in fetal plasma cortisol concentration. After cortisol infusions, renin responses to hypotension were apparently suppressed to degrees not obviously related to the rate of cortisol infusion. Fetal plasma ACTH responses to hypotension were completely suppressed by increases in total and unbound fetal plasma cortisol concentration 1.6 and 1.7 ng/ml, respectively. These results demonstrate a high sensitivity of the fetal hypothalamopituitary unit and renin-angiotensin system to cortisol.