METABOLISM OF BRADYKININ BY THE RAT CORONARY VASCULAR BED

Objective: To study the metabolism of bradykinin (BK) after a single p assage through the coronary bed in isolated Langendorff rat hearts. Me thods: BK was infused into the aortic flow line to obtain a final conc entration of 10 nM, and the coronary effluent was collected to quantif y BK and des-Arg(9)-BK by competitive enzyme immunoassay. The nature o f immunoreactive material was confirmed by immunograms after HPLC sepa ration. 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 V-max/K-m 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 enz yme (ACE) inhibitor, enalaprilat (130 nM), reduced BK degradation at a ll flow rates. Inhibition of neutral endopeptidase with retrothiorphan (25 nM) had no effect on BK degradation. However, the combined treatm ent with enalaprilat and retrothiorphan reduced BK degradation to lowe r values than enalaprilat alone. The effect of enzyme inhibitors on BK recovery was inversely related to coronary flow: inhibiting BK degrad ation markedly increased BK recovery at 1 ml/min, but had no effect at 10 ml/min. The kininase I metabolite of BK, des-Arg(9)-BK, could not be detected under these experimental conditions, Conclusions: ACE is t he major enzyme responsible for BK degradation during a single passage through the coronary bed. Neutral endopeptidase contributes to BK deg radation only when ACE activity is impaired. The effect of enzyme inhi bitors on the coronary concentration of BK is highly dependent on coro nary flow rate. (C) 1998 Elsevier Science B.V.