Pressor responsiveness to and secretion of vasopressin during the estrous cycle

Previously, we observed a sexual dimorphism of the pressor responsiveness to vasopressin in the rat. We have now determined that the pressor action of vasopressin is influenced by the phase of the estrous cycle. Conscious unrestrained male and female rats were given graded intravenous 30-min infusions of vasopressin. The pressor responses of male and estrous female rats were similar, whereas the pressor responses of diestrous, proestrous, and metestrous females were substantially (P less than 0.01) attenuated. This effect could not be attributed to differences in baroreflex sensitivity, which was unaffected by the sex of the rat or phase of the estrous cycle. Baroreflex sensitivity was assessed by the slope of the relationship between changes in heart rate and mean arterial blood pressure in response to graded injections of phenylephrine and nitroprusside. The basal plasma vasopressin concentration was higher (P less than 0.05) in male than in female rats in each phase of the estrous cycle. This was due to a difference in vasopressin release, since there were no sex-related differences in the metabolic clearance rate of vasopressin. These findings suggest that the gonadal hormones influence the cardiovascular actions of vasopressin and its secretion.