Inhibition of kinin breakdown prolongs retention and action of bradykinin in a myocardial B 2 receptor compartment

1. The high efficacy of ACE inhibitors to potentiate the actions of kinins might be explained by a hypothetical compartment in which B ₂-receptors are colocalized with kinin degrading enzymes. To demonstrate the functional consequence of such a compartment we compared the myocardial uptake and the persistence of action of bradykinin under the influence of kininase inhibitors. 2. Bradykinin-induced vasodilation and uptake of tritiated bradykinin were studied in perfused rat hearts during inhibition of ACE and aminopeptidase P. B ₂-receptors were localized by immuno-gold labelling and electron-microscopy. 3. The EC ₅₀ of bradykinin-induced vasodilation (5.1 ± 0.8 nM) was shifted to 14 fold lower concentrations during inhibition of both kininases. The maximum persistence of vasodilation after termination of bradykinin application (half-life 112 ± 20 s) was increased by kininase inhibitors to 398 ± 130 s. This prolongation was reversed when B ₂-receptors were blocked simultaneously with the termination of bradykinin infusion. 4. Tritiated bradykinin (perfused for 1 rain) was partially (1.7 ± 0.24%) retained by the myocardium and consecutively released with a half-life of 70 ± 9 s. Kinin uptake was increased during kininase inhibition (7.7 ± 2.6%), and was normalized by HOE 140 (2.0 ± 0.34%), or when a tritiated B ₂-receptor antagonist (NPC 17731) was used as label. 5. B ₂-receptors were localized in plasmalemmal and cytosolic vesicles of capillary endothelium. 6. Bradykinin is locally incorporated and can associate with B ₂-receptors repeatedly when kinin breakdown is inhibited. This is the kinetic and functional consequence of a colocalization of kininases and B ₂-receptors in a compartment constituted by endothelial membrane vesicles.