Mediators of bradykinin-induced vasorelaxation in human coronary microarteries

To investigate the mediators of bradykinin-induced vasorelaxation in human coronary microarteries (HCMAs), HCMAs ( diameter approximate to 300 mum) obtained from 42 heart valve donors ( 20 men and 22 women; age range, 3 to 65 years; mean age, 46 years) were mounted in Mulvany myographs. In the presence of the cyclooxygenase inhibitor indomethacin, bradykinin relaxed preconstricted HCMAs in a concentration-dependent manner. N-G-nitro-L-arginine methyl ester and ODQ ( inhibitors of nitric oxide [ NO] synthase and guanylyl cyclase, respectively) and the NO scavenger hydroxocobalamin, either alone or in combination, shifted the bradykinin concentration-response curve to the right. Removal of H2O2 ( with catalase), inhibition of cytochrome P450 epoxygenase ( with sulfaphenazole or clotrimazole) or gap junctions ( with 18alpha-glycyrrhetinic acid or carbenoxolone), and blockade of large- (BKCa) and small- (SKCa) conductance Ca2+-dependent K+ channels (with iberiotoxin and apamin), either alone or in addition to hydroxocobalamin, did not affect bradykinin. In contrast, complete blockade of bradykinin-induced relaxation was obtained when we combined the nonselective BKCa and intermediate-conductance (IKCa) Ca2+-dependent K+ channel blocker charybdotoxin and apamin with hydroxocobalamin. Charybdotoxin plus apamin alone were without effect. Inhibition of inwardly rectifying K+ channels (K-IR) and Na+/K+-ATPase ( with BaCl2 and ouabain, respectively) shifted the bradykinin concentration-response curve 10-fold to the right but did not exert an additional effect in the presence of hydroxocobalamin. In conclusion, bradykinin-induced relaxation in HCMAs depends on ( 1) the activation of guanylyl cyclase, K-IR, and Na+/ K+-ATPase by NO and (2) IKCa and SKCa channels. The latter are activated by a factor other than NO. This factor is not a cytochrome P450 epoxygenase product or H2O2, nor does it depend on gap junctions or BKCa channels.