SEXUAL DIMORPHISM IN VASOPRESSIN AND CARDIOVASCULAR-RESPONSE TO HEMORRHAGE IN THE RAT

There is evidence for sex-related differences in the cardiovascular actions of vasopressin. Furthermore, receptors for the gonadal steroid hormones are located in centers in the brain involved in the control of vasopressin release and in cardiovascular regulation. We have, therefore, examined the effects of hemorrhage on mean arterial blood pressure, the plasma vasopressin concentration, and plasma renin concentration in conscious male and female rats. In preliminary experiments, no differences were found in blood and plasma volumes with respect to either sex or phase of the estrous cycle. In separate experiments, rats were subjected to two hemorrhages of 10% of blood volume, separated by an interval of 15 minutes. There were no substantial gender- or cycle-related differences in the ability of hemorrhaged rats to maintain mean arterial blood pressure or increase plasma renin concentration. The increase in plasma vasopressin concentration was greater in proestrous females than in males after the first hemorrhage and in diestrous, proestrous, and metestrous females than in males after the second hemorrhage. Pretreatment with a V1-receptor antagonist was without statistically significant effect on the mean arterial blood pressure responses in males, but it impaired blood pressure compensation in females. There are, then, gender- and cycle-related differences in vasopressin responses to hemorrhage, and vasopressin appears particularly important for blood pressure compensation to hemorrhage in female rats.