Inhibitory effects of brefeldin A, a membrane transport blocker, on the bradykinin‐induced hyperpolarization‐mediated relaxation in the porcine coronary artery

To elucidate the mechanism of the relaxation mediated by endothelium‐derived hyperpolarizing factors (EDHFs), the effect of brefeldin A, a membrane transport blocker, on cytosolic Ca 2+ concentration ([Ca 2+ ] i ) and tension was determined in the porcine coronary arterial strips. We also examined the effect of brefeldin A on [Ca 2+ ] i in the endothelial cells of the porcine aortic valve. In the presence of 10 μ M indomethacin and 30 μ M N G ‐nitro‐ L ‐arginine ( L ‐NOARG), both bradykinin and substance P induced a transient decrease in [Ca 2+ ] i and tension in arterial strips contracted with 100 n M U46619 (thromboxane A 2 analogue). A 6 h pre‐treatment with 20 μg ml −1 brefeldin A abolished the bradykinin‐induced relaxation, while it had no effect on the substance P‐induced relaxation. In the absence of indomethacin and L ‐NOARG, brefeldin A had no effect on the bradykinin‐induced relaxation during the contraction induced by U46619 or 118 m M K + . The indomethacin/ L ‐NOARG‐resistant relaxation induced by bradykinin was completely inhibited by 3 m M tetrabutylammonium (non‐specific Ca 2+ ‐activated K + channel blocker), while that induced by substance P was not inhibited by 3 m M tetrabutylammonium or 1 m M 4‐aminopyridine (voltage‐dependent K + channels blocker) alone, but completely inhibited by their combination. Brefeldin A had no effect on the [Ca 2+ ] i elevation in endothelial cells induced by bradykinin or substance P. In conclusion, bradykinin produce EDHF in a brefeldin A‐sensitive mechanism in the porcine coronary artery. However, this mechanism is not active in a substance P‐induced production of EDHF, which thus suggests EDHF to be more than a single entity.British Journal of Pharmacology (2001) 134 , 168–178; doi: 10.1038/sj.bjp.0704246