Regulation of maternal ACTH in ovine pregnancy: does progesterone play a role?
Pregnancy is characterized by increased plasma adrenocorticotropic hormone (ACTH) and cortisol. Studies suggest that progesterone acts as an antagonist at mineralocorticoid receptors. Therefore, we tested the hypothesis that chronic progesterone, produced by treatment of nonpregnant ewes or during pregnancy, will result in increased plasma ACTH relative to the plasma cortisol concentrations. We studied three groups of ewes: ovariectomized nonpregnant, nonpregnant treated with progesterone, and pregnant ewes. In two series of studies, ewes were adrenalectomized and replaced with 0.35 mg·kg–1·day–1 or 0.5 mg·kg–1·day–1 cortisol. In both studies, aldosterone was infused at 3 µg·kg–1·day–1. In the first study, additional infusions of cortisol over 24 h were used to increase daily replacement doses to 0.5, 1, or 1.5 mg·kg–1·day–1, and intact pregnant and nonpregnant ewes were studied with infusions of cortisol at 0, 0.5, and 1 mg·kg–1·day–1. In adrenalectomized ewes chronically replaced to
