1. The effects of angiotensin II infusion without and with simultaneous infusion of prostaglandin E2 were studied in 25 women in second trimester pregnancy. Twenty received one infusion of angiotensin II alone, followed by its infusion simultaneously with prostaglandin E2; five received two identical infusions of angiotensin II alone as controls.
2. Angiotensin II infusion alone was associated with suppression of plasma renin concentration to levels inversely proportional to the evoked change in diastolic blood pressure. Plasma renin substrate concentration was unchanged, but plasma aldosterone concentration rose markedly. This rise was inversely proportional to the threshold for pressor effect of angiotensin II.
3. Prostaglandin E2 administration alone was associated with increased plasma renin concentrations.
4. The pressor effect of angiotensin II was blunted when given together with prostaglandin E2 and plasma concentrations of angiotensin II reached were lower.
5. Plasma renin concentration was again suppressed during the joint infusion regimen; the degree of suppression was inversely proportional to the change in diastolic pressure. Plasma aldosterone concentration rose, but did not differ in the control and experimental groups.
6. Thus although the renin-angiotensin system is stimulated in normal pregnancy, the normal control mechanisms are still functional, and the capacity for further increases in activity exists.