Inhibition of Both Aminopeptidase P and Angiotensin-Converting Enzyme Prevents Bradykinin Degradation in the Rat Coronary Circulation

Bradykinin (Bk), which is produced locally in the heart, exhibits potent cardioprotective effects. However, these effects appear to be limited by rapid degradation of the peptide. To determine the mechanism of Bk metabolism in the coronary circulation, [3H]Bk was perfused through the isolated rat heart via the aorta in the presence and absence of specific peptidase inhibitors. The radiolabeled metabolites were collected from the pulmonary artery and then separated, identified, and quantified by reversed-phase high-performance liquid chromatography (HPLC) by using a radioactive flow detector. In the absence of inhibitors, only 45 +/- 2% of the radioactivity eluted from the coronary circulation as intact [3H]Bk. The chromatograms suggested that Bk was being hydrolyzed at the Arg1-Pro2 bond by aminopeptidase P and at the Pro7-Phe8 bond by angiotensin-converting enzyme. When the aminopeptidase P inhibitor, apstatin (200 microM), was coperfused with [3H]Bk, cleavage at the Arg1-Pro2 bond was blocked and the amount of intact [3H]Bk in the perfusate increased to 57 +/- 5% (p < 0.05 vs. control). Coperfusion with the angiotensin-converting enzyme inhibitor, ramiprilat (0.5 microM), alone blocked cleavage at the Pro7-Phe8 bond and increased intact [3H]Bk to 75 +/- 3% (p < 0.001 vs. control). When both apstatin and ramiprilat were present, almost all of the radioactivity (96 +/- 1%) eluted as intact [3H]Bk (p < 0.01 vs. ramiprilat alone). The results indicate that the degradation of Bk in the rat coronary circulation can be fully accounted for by aminopeptidase P (approximately 30%) and angiotensin-converting enzyme (approximately 70%).