Mechanisms involved in the angiotensin II-independent hypotensive action of ACE inhibitors

1. The blood pressure-lowering and cardioprotective actions of angiotensin converting enzyme (ACE) inhibitors are thought to be based primarily on a reduction in vascular angiotensin II (Ang-II) formation. However, since ACE also degrades the potent endothelium-dependent vasodilator bradykinin, it has been proposed that the local accumulation of this peptide represents an additional mechanism by which ACE inhibitors exert their cardiovascular effects. 2. Incubation of endothelial cells with ACE inhibitors indeed causes an enhanced formation of nitric oxide (NO) and prostacyclin (PGI2) which can be completely blocked by the B2-kinin receptor antagonist Hoe 140, suggesting that the vascular endothelium is capable of generating vasoactive kinins from an endogenous source. 3. Moreover, ACE inhibitors not only prevent the breakdown of bradykinin but, by virtue of an as yet unidentified mechanism, also enhance the potency of bradykinin at the receptor level and reverse the desensitization of the B2-kinin receptor following continuous exposure to bradykinin. Both of these effects may enhance or sustain the bradykinin-induced formation of NO and PGI2 by the endothelium. 4. Furthermore, ACE inhibition leads to the accumulation of Ang-I which can be metabolised to Ang-(1-7) by another endothelial enzyme, neutral endopeptidase 24.11. By activating an as yet unidentified angiotensin receptor, Ang-(1-7), but not other known angiotensin peptides, stimulates endothelial NO release in porcine coronary arteries as well as in the isolated perfused rat heart. This effect is, albeit to a different degree, dependent on the release of vasoactive kinins from the endothelium. The shift in Ang-I metabolism towards an enhanced formation of Ang-(1-7) in the presence of an ACE inhibitor may thus contribute to the hypotensive action of this class of compounds as well.