Bradykinin potentiation by ACE inhibitors: a matter of metabolism

1 Studies in isolated cells overexpressing ACE and bradykinin type 2 (B,) receptors suggest that ACE inhibitors potentiate bradykinin by inhibiting B-2 receptor desensitization, via a mechanism involving protein kinase C (PKC) and phosphatases. Here we investigated, in intact porcine coronary arteries, endothelial ACE/B-2 receptor 'crosstalk' as well as bradykinin potentiation through neutral endopeptidase (NEP) inhibition. 2 NEP inhibition with phosphoramidon did not affect the bradykinin concentration-response curve (CRC), nor did combined NEP/ACE inhibition with omapatrilat exert a further leftward shift on top of the approximate to 10 fold leftward shift of the bradykinin CRC observed with ACE inhibition alone. 3 In arteries that, following repeated exposure to 0.1 muM bradykinin, no longer responded to bradykinin ('desensitized' arteries), the ACE inhibitors quinaprilat and angiotensin-(1-7) both induced complete relaxation, without affecting the organ bath fluid levels of bradykinin. This phenomenon was unaffected by inhibition of PKC or phosphatases (with calphostin C and okadaic acid, respectively). 4 When using bradykinin analogues that were either completely or largely ACE-resistant ([Phe(8)psi(CH2-NH)Arg(9)]-bradykinin and [DeltaPhe]-bradykinin, respectively), the ACE inhibitor-induced shift of the bradykinin CRC was absent, and its ability to reverse desensitization was absent or significantly reduced, respectively. Caveolar disruption with filipin did not affect the quinaprilat-induced effects. Filipin did however reduce the bradykinin-induced relaxation by approximate to 25-30%, thereby confirming that B, receptor-endothelial NO synthase (eNOS) interaction occurs in caveolae. 5 In conclusion, in porcine arteries, in contrast to transfected cells, bradykinin potentiation by ACE inhibitors is a metabolic process, that can only be explained on the basis of ACE-B-2 receptor co-localization on the endothelial cell membrane. NEP does not appear to affect the bradykinin levels in close proximity to B-2 receptors, and the ACE inhibitor-induced bradykinin potentiation precedes B, receptor coupling to eNOS in caveolae.