Effects of progesterone on blood pressure, plasma volume, and responses to hypotension

Gonadal steroids have been implicated in the control of blood pressure and fluid homeostasis. These experiments test the effect of progesterone in ovariectomized ewes on blood pressure, volume, and hormone responses to hypotension. Eight ewes were each studied in four conditions: ovariectomized, progesterone and estrogen replaced, progesterone replaced, or sham treated. During each treatment mean arterial pressure (MAP), plasma volume (PV), baroreflex responsiveness, and adrenocorticotropic hormone (ACTH), arginine vasopressin (AVP), and renin responses to hypotension were determined. Progesterone treatment significantly reduced resting MAP and increased PV compared with ovariectomy or sham treatments. Heart period at 85 mmHg was reduced with progesterone treatment. There was no effect of progesterone treatment on ACTH, AVP, or renin or heart rate responses to hypotension. Basal AVP levels were increased, and angiotensin II concentrations were decreased, by estrogen and progesterone and by sham treatments; plasma Na+ also tended to be increased during these treatments. These results suggest that a small increase in progesterone can reset resting MAP and PV without altering reflex heart rate or endocrine responses to hypotension.