Effects of a simulated estrous cycle on sodium, volume, ACTH, and AVP in sheep

The studies were designed to test for effects of acute increases in estradiol and progesterone, similar in magnitude and duration to those in the ovine estrous cycle, on adrenocorticotropic hormone (ACTH) and plasma vasopressin (AVP) under resting conditions and in response to hypotension. Ewes (7 per group) were studied as intact, ovariectomized, ovariectomized and treated with progesterone for 7-8 days, or subsequently treated with estradiol. During progesterone treatment plasma sodium and AVP were increased significantly. However, neither plasma volume nor blood pressure was altered. Plasma AVP responses to hypotension were not altered by either progesterone or estradiol treatment. The peak plasma ACTH response to hypotension was not altered by steroid treatment; however, the duration of the response was greater in progesterone-treated ewes than in intact ewes. The results indicate that changes in gonadal steroids similar to those in the ovine estrous cycle cause a small increase in plasma sodium that stimulates AVP, but do not alter regulation of blood pressure or volume or AVP or ACTH responses to hypotension.