Evidence for reset of regulated cortisol in pregnancy: studies in adrenalectomized ewes.

These studies test the hypothesis that the increased adrenocorticotropic hormone (ACTH) and cortisol in pregnancy reflect a reset of regulated plasma cortisol concentrations. Ewes were sham operated (Sham) or adrenalectomized (ADX) at approximately 108 days gestation. Adrenalectomized ewes were replaced with aldosterone (3 micrograms.kg-1.day-1) and with cortisol at either of two doses (ADX + 0.6 and ADX + 1.0 mg.kg-1.day-1); the ewes were also studied postpartum. Plasma cortisol concentrations in ADX + 0.6 ewes (5.3 +/- 1.3 ng/ml) were similar to the Sham ewes postpartum (5.5 +/- 0.6 ng/ml), whereas ADX + 1.0 concentrations (8.9 +/- 1.0 ng/ml) were similar to pregnant Sham ewes (9.5 +/- 1.9 ng/ml). Plasma ACTH concentrations were significantly increased in the pregnant ADX + 0.6 ewes (273 +/- 44 pg/ml) relative to pregnant Sham ewes (84 +/- 9 pg/ml) or the same ewes postpartum (42 +/- 9 pg/ml). Plasma ACTH concentrations were not different among the groups postpartum. Acute increases in plasma cortisol to 15-25 ng/ml produced similar inhibition in all groups. These results suggest that pregnancy resets the basal cortisol concentration required for normalization of basal ACTH concentration.