Blood pressure control by the renin-angiotensin system in normotensive subjects: Assessment by angiotensin converting enzyme and renin inhibition

Abstract

Background. The participation of the renin-angiotensin system in the control of blood pressure in normal, sodium-replete subjects is not clear. The use of a specific inhibitor of human renin should allow a better delineation of the importance of this system. Methods and Results. Blood pressure responses were measured 1 hour after randomized, double-blind administration of the renin inhibitor Ro 42-5892 (600 mg p.o.) or the angiotensin converting enzyme inhibitor captopril (50 mg p.o.) in 20 healthy men on an ad libitum sodium diet. Effective inhibition of the renin-angiotensin system by either compound was indicated by increases of immunoreactive renin associated with an increase of angiotensin I production rate of 67.8±33.6% after captopril and a decrease of 79.5±16.4% after Ro 42-5892. Furthermore, Ro 42-5892 decreased plasma renin activity by 64%. Whereas intra-arterial diastolic (60±5.1 to 51.4±7.2 mm Hg, p<0.01) and mean arterial (77.7±6.0 to 71.4±8.5 mm Hg, p<0.001) pressures decreased after captopril, they remained unchanged after Ro 42-5892. Captopril, but not Ro 42-5892, increased forearm blood flow (2.4±0.8 versus 1.9±0.8 ml/min/100 ml, p<0.01) and significantly enhanced the increase of forearm blood flow to brachial artery infusions of bradykinin (0.15, 1.5, 5, 15, and 50 ng/min/100 ml; 5 minutes each) from 744±632% to 1,383±514% (p<0.01). Furthermore, repeat bradykinin infusions resulted in further decreases of blood pressure (from mean pressure of 71.4±8.5 to 63.2±7.6 mm Hg, p<0.01) only after captopril. Changes of blood pressure after captopril were unrelated to baseline plasma renin activity but correlated with captopril-induced enhancement of vasodilation to bradykinin (r=0.68, p<0.05). Conclusions. The lack of blood pressure effects of renin inhibition in contrast to angiotensin converting enzyme inhibition suggests that the renin-angiotensin system does not contribute significantly to blood pressure control in normotensive, sodium-replete subjects. The hypotensive activity of angiotensin converting enzyme inhibitors may result from additional hormonal effects, for example, inhibition of bradykinin degradation and/or subsequent increases of vasodilating prostaglandins or endothelium-derived relaxing factor(s).