Effects of Pregnancy and Estrogen on the Angiotensin II Pressor Response of the Rabbit Using Serial Systolic Blood Pressure Measurement in the Ear

Abstract

Serial systolic blood pressure measurements were made in order to assess the effects of pregnancy and estrogen on the angiotensin II pressor response of the rabbit. The systolic blood pressure of the ear in 5 pregnant rabbits did not change throughout the gestational period, being compatible with that in 14 nonpregnant rabbits. However, vascular resistance to infused angiotensin II (over 13.1 ng/kg/min required to elicit a pressor response of 20 mmHg in systolic pressure) was demonstrated as early as the 10th day of pregnancy, and maximum mean resistance occurred on the 20th day: the mean pressor dose required was 34.5 ± 3.6 ng/kg/min (mean ± S.D.). In a separate series, vascular resistance in castrated rabbits was demonstrated as early as the 10th day of estrogen administration, reached a maximum at the 4th week, and then gradually decreased to the control level (pretreatment, 10.5 ± 2.5 ng/kg/min; 10 days, 18.9 ± 2.3 ng/kg/min; 4 weeks, 32.4 ± 5.4 ng/kg/min; 10 weeks, 18.3 ± 3.6 ng/kg/min; and 24 weeks, 11.0 ± 2.0 ng/kg/min). These results indicate that (1) pregnant rabbits have a decreased sensitivity to angiotensin II, (2) estradiol may modulate the angiotensin II pressor response in castrated rabbits, and (3) serial systolic blood pressure measurement may provide an appropriate test for the angiotensin II pressor response in rabbits.