The relationship between reproductive performance and blood pressure in the spontaneously hypertensive rat

Blood pressure was measured serially throughout pregnancy in spontaneously hypertensive rats and in normotensive control rats of the Wistar-Kyoto strain. Changes in blood pressure were also correlated with the outcome of pregnancy. In control rats there was a small but significant decrease in blood pressure between days 20 and 22 of pregnancy (day of birth = day 22.5). In the spontaneously hypertensive rat this decrease occurred earlier, between days 18 and 19 of pregnancy, and was of greater magnitude. There was a positive correlation in the spontaneously hypertensive rat between the number of live-born pups and the magnitude of the decrease in blood pressure. Perinatal mortality, but not litter size, was greater in the spontaneously hypertensive rat than in control rats. Thus the physiologic mechanisms responsible for the decrease in blood pressure in the normal rat are preserved in the spontaneously hypertensive rat, and the successful completion and outcome of pregnancy depend on the capacity of the hypertensive rat to amplify these processes. Consequently, the spontaneously hypertensive rat is not an appropriate experimental model for hypertension of human pregnancy, in particular for preeclampsia, in which the presence of the conceptus characteristically causes blood pressure to rise, especially during the last trimester. However, an investigation of the factors responsible for the profound anti hypertensive effect of pregnancy in rats could provide new insights into the mechanisms by which blood pressure is regulated during pregnancy and suggest new therapeutic approaches.