DISPLAY OF THE CHARACTERISTICS OF ENDOTHELIUM-DERIVED HYPERPOLARIZINGFACTOR BY A CYTOCHROME P450-DERIVED ARACHIDONIC-ACID METABOLITE IN THE CORONARY MICROCIRCULATION

1 In addition to nitric oxide (NO) and prostacyclin (PGI(2)) an endoth elium-derived factor, which hyperpolarizes vascular smooth muscle cell s via activation of K+ channels, contributes to the dilator effect of bradykinin in different vascular beds. Since this so-called endotheliu m-derived hyperpolarizing factor (EDHF) also seems to play an importan t role in the coronary circulation, we investigated its nature and mec hanism of action in the rat isolated perfused heart (Langendorff prepa ration). 2 Bolus injections of bradykinin (1, 10, and 100 pmol) elicit ed a transient dose-dependent dilator response (e.g., 12 +/- 2% decrea se in coronary perfusion pressure (CPP) at 10 pmol bradykinin, n = 41) . Administration of the cyclo-oxygenase inhibitor, diclofenac (1 mu M) , augmented the bradykinin-induced dilation approximately twofold (n = 9 P<0.01). Combined treatment with the NO synthase inhibitor, N-G-nit ro-L-arginine (30 mu M) and diclofenac (1 mu M) significantly reduced the duration, but increased the amplitude of the dilator response to b radykinin (27 +/- 2% decrease in CPP, n = 24, P<0.01). 3 The abolition of this N-G-nitro-L-arginine/diclofenac-insensitive dilator response to bradykinin by tetrabutylammonium (0.3 mM), an inhibitor of Ca2+-dep endent K+ channels (4 +/- 1% decrease in CPP, n = 6, P<0.01), supports the view that the dilator compound released in the coronary microcirc ulation is EDHF. 4 This EDHF-type dilation was reversibly inhibited by the phospholipase A(2) inhibitor, quinacrine (3 mu M, 9 +/- 3% decrea se in CPP, n = 6, P<0.01) and by the cytochrome P450 inhibitor SKF525a (3 mu M, 6 +/- 1% decrease in CPP, n = 6, P<0.01). 5 Tetrabutylammoni um, quinacrine or SKF 525a did not affect the endothelium-independent dilator response to sodium nitroprusside (1 nmol), indicating that the se compounds did not affect smooth muscle relaxation in a non-specific manner. 6 These findings suggest that in the coronary microcirculatio n bradykinin stimulates the release of a cytochrome P450-derived arach idonic acid metabolite, which exhibits the characteristic features of EDHF.