Potentiation of the Actions of Bradykinin by Angiotensin I–Converting Enzyme Inhibitors

Abstract

Abstract Part of the beneficial effects of angiotensin I–converting enzyme (ACE) inhibitors are due to augmenting the actions of bradykinin (BK). We studied this effect of enalaprilat on the binding of [ 3 H]BK to Chinese hamster ovary (CHO) cells stably transfected to express the human BK B 2 receptor alone (CHO-3B) or in combination with ACE (CHO-15AB). In CHO-15AB cells, enalaprilat (1 μmol/L) increased the total number of low-affinity [ 3 H]BK binding sites on the cells at 37°C, but not at 4°C, from 18.4±4.3 to 40.3±11.9 fmol/10 6 cells ( P <.05; K d , 2.3±0.8 and 5.9±1.3 nmol/L; n=4). Enalaprilat preserved a portion of the receptors in high-affinity conformation ( K d , 0.17±0.08 nmol/L; 8.1±0.9 fmol/10 6 cells). Enalaprilat decreased the IC 50 of [Hyp 3 -Tyr(Me) 8 ]BK, the BK analogue more resistant to ACE, from 3.2±0.8 to 0.41±0.16 nmol/L ( P <.05, n=3). The biphasic displacement curve of the binding of [ 3 H]BK also suggested the presence of high-affinity BK binding sites. Enalaprilat (5 nmol to 1 μmol/L) potentiated the release of [ 3 H]arachidonic acid and the liberation of inositol 1,4,5-trisphosphate (IP 3 ) induced by BK and [Hyp 3 -Tyr(Me) 8 ]BK. Moreover, enalaprilat (1 μmol/L) completely and immediately restored the response of the B 2 receptor, desensitized by the agonist (1 μmol/L [Hyp 3 -Tyr(Me) 8 ]BK); this effect was blocked by the antagonist, HOE 140. Finally, enalaprilat, but not the prodrug enalapril, decreased internalization of the receptor from 70±9% to 45±9% ( P <.05, n=7). In CHO-3B cells, enalaprilat was ineffective. ACE inhibitors in the presence of both the B 2 receptor and ACE enhance BK binding, protect high-affinity receptors, block receptor desensitization, and decrease internalization, thereby potentiating BK beyond blocking its hydrolysis.