Intravascular and interstitial degradation of bradykinin in isolated perfused rat heart

1 Bradykinin (BK) has been shown to exert cardioprotective effects which are potentiated by inhibitors of angiotensin I-converting enzyme (ACE). In order to clarify the significance of ACE within the whole spectrum of myocardial kininases we investigated BK degradation in the isolated rat heart. 2 Tritiated BK (H-3-BK) or unlabelled BK was either repeatedly perfused through the heart, or applied as an intracoronary bolus allowing determination of its elution kinetics. BK metabolites were analysed by HPLC. Kininases were identified by ramiprilat, phosphoramidon, diprotin A and 2-mercaptoethanol or apstatin as specific inhibitors of ACE, neutral endopeptidase 24.11 (NEP), dipeptidylaminopeptidase IV and aminopeptidase P (APP), respectively. 3 In sequential perfusion passages, H-3-BK concentrations in the perfusate decreased by 39% during each passage. Ramiprilat reduced the rate of H-3-BK breakdown by 54% and nearly abolished [1-5]-BK generation. The ramiprilat-resistant kininase activity was for the most part inhibited by the selective APP inhibitor apstatin (IC50 0.9 mu M). BK cleavage by APP yielded the intermediate product [2-9]-BK, which was rapidly metabolized to [4-9]-BK by dipeptidylaminopeptidase IV. 4 After bolus injection of H-3-BK, 10% of the applied radioactivity were protractedly eluted, indicating the distribution of this fraction into the myocardial interstitium. In samples of such interstitial perfusate fractions, H-3-BK was extensively (by 92%) degraded, essentially by ACE and APP. The ramiprilat- and mercaptoethanol-resistant fraction of interstitial kininase activity amounted to 14%, about half of which could be attributed to NEP. Only the product of NEP, [1-7]-BK, was continuously generated during the presence of H-3-BK in the interstitium. 5 ACE and APP are located at the endothelium and represent the predominant kininases of rat myocardium. Both enzymes form a metabolic barrier for the extravasated fraction of BK. Thus, only interstitial, but not intravascular concentrations of BK are increased by kininase inhibitors to the extent that a significant potentiation of BK effects could be explained. NEP contributes less than 5% to the total kininase activity, but is the only enzyme which is exclusively present in the interstitial space.