Does a decrease in cortisol negative feedback efficacy precede ovine parturition?

In sheep and other ruminants parturition is stimulated by increased secretion of fetal cortisol. The mechanism of this increased fetal adrenal activity is not known, but may be dependent on a decreased fetal hypothalamopituitary sensitivity to the negative feedback inhibition by cortisol. Seven fetal sheep (129-142 days gestation), chronically prepared with vascular catheters, were infused with cortisol (10 micrograms/min; n = 5) or vehicle (n = 4) for 5 h. Cortisol infusion increased fetal plasma cortisol to 50.8 +/- 4.3 ng/ml, approximately 33 ng/ml above the corresponding plasma cortisol concentration in the vehicle-infused fetuses. One hour after the end of the cortisol or vehicle infusion, infusion of sodium nitroprusside (50 micrograms/min, iv) increased fetal plasma adrenocorticotropin hormone (ACTH) concentration in both groups of fetuses. Results of another study (15) demonstrated that increases in fetal plasma cortisol of only 1.7 ng/ml for 5 h in younger (117-131 days gestation) fetuses completely blocked the fetal ACTH response to the same dose of nitroprusside. The results indicate that the preparturient rise in fetal ACTH is accompanied by a decrease in cortisol negative feedback.