Abstract-ACE inhibitors block B 2 receptor desensitization, thereby potentiating bradykinin beyond blocking its hydrolysis. Angiotensin (Ang)-(1-7) also acts as an ACE inhibitor and, in addition, may stimulate bradykinin release via angiotensin II type 2 receptors. In this study we compared the bradykinin-potentiating effects of Ang-(1-7), quinaprilat, and captopril. Porcine coronary arteries, obtained from 32 pigs, were mounted in organ baths, preconstricted with prostaglandin F 2␣ , and exposed to quinaprilat, captopril, Ang-(1-7), and/or bradykinin. Bradykinin induced complete relaxation (pEC 50 ϭ8.11Ϯ0.07, meanϮSEM), whereas quinaprilat, captopril, and Ang-(1-7) alone were without effect. Quinaprilat shifted the bradykinin curve to the left in a biphasic manner: a 5-fold shift at concentrations that specifically block the C-domain (0.1 to 1 nmol/L) and a 10-fold shift at concentrations that block both domains. Captopril and Ang-(1-7) monophasically shifted the bradykinin curve to the left, by a factor of 10 and 5, respectively. A 5-fold shift was also observed when Ang-(1-7) was combined with 0.1 nmol/L quinaprilat. Repeated exposure of porcine coronary arteries to 0.1 mol/L bradykinin induced B 2 receptor desensitization. The addition of 10 mol/L quinaprilat or Ang-(1-7) to the bath, at a time when bradykinin alone was no longer able to induce relaxation, fully restored the relaxant effects of bradykinin. Angiotensin II type 1 or 2 receptor blockade did not affect any of the observed effects of Ang-(1-7). In conclusion, Ang-(1-7), like quinaprilat and captopril, potentiates bradykinin by acting as an ACE inhibitor. Bradykinin potentiation is maximal when both the ACE C-and N-terminal domains are inhibited. The inhibitory effects of Ang-(1-7) are limited to the ACE C-domain, raising the possibility that Ang-(1-7) synergistically increases the blood pressure-lowering effects of N-domain-specific ACE inhibitors. is a heptapeptide that is formed endogenously from both Ang I and Ang II. 1 In rats and dogs, Ang-(1-7) exerts direct vasodilatory effects via nonangiotensin II type 1 (AT 1 ), non-angiotensin II type 2 (AT 2 ) receptors, possibly by stimulating bradykinin and NO release. 1,2 In contrast, in humans or pigs, no direct vasodilatory effects of Ang-(1-7) were observed, 3-7 although Ang-(1-7) did antagonize the pressor effects of Ang II, suggesting that it may cause vasodilation indirectly, by acting as an AT 1 receptor antagonist. 5,6 In addition, Ang-(1-7) potentiates bradykinin, either via an AT 2 receptor-dependent mechanism or through inhibition of ACE. 3,5,8 The latter effect is not necessarily based on blockade of bradykinin hydrolysis, because recent studies have shown that ACE inhibitors, including Ang-(1-7), potentiate bradykinin by inhibiting desensitization of its receptor. 9 -11 Somatic ACE has 2 homologous domains, each containing an active center. According to their position (N-or C-terminal), these domains are designated as the N-or C-domain, respectively. Interestingly, Ang-(1-7) inhibits t...

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1 NO synthase (NOS) inhibitors partially block bradykinin (BK)-mediated vasorelaxation. Here we investigated whether this is due to incomplete NOS inhibition and/or NO release from storage sites. We also studied the mechanism behind ACE inhibitor-mediated BK potentiation. 2 Porcine coronary arteries (PCAs) were mounted in organ baths, preconstricted, and exposed to BK or the ACE-resistant BK analogue Hyp 3 -Tyr(Me) 8 -BK (HT-BK) with or without the NOS inhibitor L-NAME (100 mM), the NO scavenger hydroxocobalamin (200 mM), the Ca 2+ -dependent K + -channel blockers charybdotoxin+apamin (both 100 nM), or the ACE inhibitor quinaprilat (10 mM). 3 BK and HT-BK dose-dependently relaxed preconstricted vessels (pEC 50 8.0+0.1 and 8.5+0.2, respectively). pEC 50 's were &10 fold higher with quinaprilat, and &10 fold lower with L-NAME or charybdotoxin+apamin. Complete blockade was obtained with hydroxocobalamin or L-NAME+ charybdotoxin+apamin. 4 Repeated exposure to 100 nM BK or HT-BK, to deplete NO storage sites, produced progressively smaller vasorelaxant responses. With L-NAME, the decrease in response occurred much more rapidly. L-Arginine (10 mM) reversed the e ect of L-NAME. 5 Adding quinaprilat to the bath following repeated exposure (with or without L-NAME), at the time BK and HT-BK no longer induced relaxation, fully restored vasorelaxation, while quinaprilat alone had no e ect. Quinaprilat also relaxed vessels that, due to pretreatment with hydroxocobalamin or L-NAME+charybdotoxin+apamin, previously had not responded to BK. 6 In conclusion, L-NAME-resistant BK-induced relaxation in PCAs depends on NO from storage sites, and is mediated via stimulation of guanylyl cyclase and/or Ca 2+ -dependent K + -channels. ACE inhibitors potentiate BK independent of their e ect on BK metabolism.

Abstract-Somatic angiotensin-converting enzyme (ACE) contains 2 domains (C-domain and N-domain) capable of hydrolyzing angiotensin I (Ang I) and bradykinin. Here we investigated the effect of the selective C-domain and N-domain inhibitors RXPA380 and RXP407 on Ang I-induced vasoconstriction of porcine femoral arteries (PFAs) and bradykinin-induced vasodilation of preconstricted porcine coronary microarteries (PCMAs). Ang I concentrationdependently constricted PFAs. RXPA380, at concentrations Ͼ1 mol/L, shifted the Ang I concentration-response curve (CRC) 10-fold to the right. This was comparable to the maximal shift observed with the ACE inhibitors (ACEi) quinaprilat and captopril. RXP407 did not affect Ang I at concentrations Յ0.1 mmol/L. Bradykinin concentrationdependently relaxed PCMAs. RXPA380 (10 mol/L) and RXP407 (0.1 mmol/L) potentiated bradykinin, both inducing a leftward shift of the bradykinin CRC that equaled Ϸ50% of the maximal shift observed with quinaprilat. Ang I added to blood plasma disappeared with a half life (t 1/2 ) of 42Ϯ3 minutes. Quinaprilat increased the t 1/2 Ϸ4-fold, indicating that 71Ϯ6% of Ang I metabolism was attributable to ACE. RXPA380 (10 mol/L) and RXP407 (0.1 mmol/L) increased the t 1/2 Ϸ2-fold, thereby suggesting that both domains contribute to conversion in plasma. In conclusion, tissue Ang I-II conversion depends exclusively on the ACE C-domain, whereas both domains contribute to conversion by soluble ACE and to bradykinin degradation at tissue sites. Because tissue ACE (and not plasma ACE) determines the hypertensive effects of Ang I, these data not only explain why N-domain inhibition does not affect Ang I-induced vasoconstriction in vivo but also why ACEi exert blood pressure-independent effects at low (C-domain-blocking) doses.