Bradykinin potentiation by ACE inhibitors:A matter of metabolism

1. Studies in isolated cells overexpressing ACE and bradykinin type 2 (B₂) receptors suggest that ACE inhibitors potentiate bradykinin by inhibiting B₂ receptor desensitization, via a mechanism involving protein kinase C (PKC) and phosphatases. Here we investigated, in intact porcine coronary arteries, endothelial ACE/B₂ 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 ≈ 10 fold leftward shift of the bradykinin CRC observed with ACE inhibition alone. 3. In arteries that, following repeated exposure to 0.1 μM 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 ([Phe⁸Ψ(CH₂-NH)Arg⁹]-bradykinin and [ΔPhe⁵]-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 ≈ 25-30%, thereby confirming that B₂ 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-B₂ receptor co-localization on the endothelial cell membrane. NEP does not appear to affect the bradykinin levels in close proximity to B₂ receptors, and the ACE inhibitor-induced bradykinin potentiation precedes B₂ receptor coupling to eNOS in caveolae.