Potentiation of bradykinin actions by angiotensin I converting enzyme inhibitors

The goal of our research was to understand better the mechanism behind the clinical benefits of angiotensin I converting enzyme (ACE) inhibitors, used by now in 5-10 million people world wide for the treatment of cardiovascular diseases. Many of the benefits of ACE inhibitor therapy in heart diseases are attributed to their effects in augmenting bradykinin (BK) activity. We tested the hypothesis that ACE inhibitors potentiate BK effects by a mechanism which is in addition to, and independent of the inhibition of BK degradation. We employed 3 preparations: the guinea pig ileum, guinea pig left atrium, and cultured Chinese hamster ovary (CHO) cells transfected with human cDNAs encoding the BK {dollar}rm Bsb2{dollar} receptor with and without somatic ACE.; In the guinea pig ileum, BK effects were potentiated immediately although the peptide was inactivated slowly. ACE inhibitors potentiated BK effects even after ACE was inactivated by sequestering the {dollar}rm Znsp{lcub}2+{rcub}{dollar} cofactor of the enzyme, and BK analogues which are hydrolyzed much slower than BK, such as dextran-BK and methionyl-lysyl-BK, were also potentiated by ACE inhibitors, suggesting that an alternative BK potentiating mechanism exists. This mechanism was further examined on guinea pig atria another ACE-resistant analogue, {dollar}rmlbrack Hypsp3{dollar}-{dollar}rm Tyr(Me)sp8rbrack {dollar}-BK. Enalaprilat not only increased the maximum and steady-state inotropic effects of the kinin analogue but actually restored the positive inotropic response after it very much decreased because the continued presence of the agonist desensitized the receptor. Thus, enalaprilat inhibited desensitization and reactivated the receptor. To determine whether ACE inhibitors interact directly with the BK {dollar}rm Bsb2{dollar}-receptor when potentiating BK, we measured receptor binding and agonist-stimulated responses (inositol 1,4,5-trisphosphate generation and {dollar}rmlbrack sp3Hrbrack {dollar}arachidonic acid release) in CHO cells expressing only the {dollar}rm Bsb2{dollar}-receptor but saw enalaprilat had no effect in the system. However, when in cells ACE and the B{dollar}sb2{dollar}-receptor were co-expressed, enalaprilat inhibited B{dollar}sb2{dollar}-receptor desensitization and internalization, and potentiated functional responses. Thus, the data indicate a novel hypothesis whereby ACE inhibitors, indirectly via ACE, potentiate and sustain the actions of BK by reactivating or maintaining the active conformation of the BK {dollar}rm Bsb2{dollar}-receptor.