OBJECTIVE To study the metabolism of bradykinin (BK) after a single passage through the coronary bed in isolated Langendorff rat hearts. METHODS BK was infused into the aortic flow line to obtain a final concentration of 10 nM, and the coronary, effluent was collected to quantify BK and des-Arg9-BK by competitive enzyme immunoassay. The nature of immunoreactive material was confirmed by immunograms after HPLC separation. The experiments were performed with hearts perfused at either one of the following coronary flow rates: 1, 5 or 10 ml/min. RESULTS BK recovery without inhibitors was 86.3 +/- 2.9, 60.8 +/- 6.3, and 29.6 +/- 6.8% at 10, 5, and 1 ml/min, respectively. The Vmax/Km ratios at these coronary flow rates were 2.19 +/- 0.72, 4.81 +/- 0.64, and 2.59 +/- 0.33 min-1 g-1), respectively. The angiotensin-converting enzyme (ACE) inhibitor, enalaprilat (130 nM), reduced BK degradation at all flow rates. Inhibition of neutral endopeptidase with retrothiorphan (25 nM) had no effect on BK degradation. However, the combined treatment with enalapril and retrothiorphan reduced BK degradation to lower values than enalaprilat alone. The effect of enzyme inhibitors on BK recovery was inversely related to coronary flow: inhibiting BK degradation markedly increased BK recovery at 1 ml/min, but had no effect at 10 ml/min. The kininase I metabolite of BK, des-Arg9-BK, could not be detected under these experimental conditions. CONCLUSIONS ACE is the major enzyme responsible for BK degradation during a single passage through the coronary bed. Neutral endopeptidase contributes to BK degradation only when ACE activity is impaired. The effect of enzyme inhibitors on the coronary concentration of BK is highly dependent on coronary flow rate.