title: Does a decrease in cortisol negative feedback efficacy precede ovine parturition?
abstract: 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.
mesh_terms: D000818:Animals; D005260:Female; D005312:Fetal Blood / Q000032:analysis; D006706:Homeostasis; D006854:Hydrocortisone / Q000097:blood*; D007022:Hypotension / Q000503:physiopathology; D007030:Hypothalamo-Hypophyseal System / Q000196:embryology; D007743:Labor, Obstetric; D009599:Nitroprusside / Q000494:pharmacology; D011247:Pregnancy; D012756:Sheep
chemical_list: D009599:Nitroprusside; D006854:Hydrocortisone