Effects of pregnancy, estradiol, and progesterone on pressor responsiveness to angiotensin II.

The pressor and heart rate responses to infused angiotensin (ANG) II were measured in conscious pregnant Long-Evans rats. Responses were recorded 7 days after chronic indwelling venous and arterial cannulas were implanted (virgin rats) and again at days 7, 14, and 21 of pregnancy. A significant reduction in the pressor response was noted as early as day 7 of pregnancy; this is comparable to the human condition. It was also noted that the reflex bradycardia associated with the pressor response was entirely absent at 21 days. To determine a possible cause of these pregnancy-induced changes, ovariectomized rats were injected subcutaneously for 10 days with estradiol, progesterone, or a combination of both. They were then tested for pressor responses to ANG II. None of the hormone-injected groups showed any significant deviation in their pressor response compared with saline-injected controls. Similar results were obtained in acutely prepared rats under chloroform anesthesia. However, the magnitude of the pressor response in the chloroformed animals was significantly reduced compared with the conscious unrestrained animals. It is concluded that 1) the rat is an appropriate model of human pregnancy in which to study changes in ANG responsiveness in pregnancy and 2) neither estradiol nor progesterone is responsible for the reduced pressor response.