Abstract-Endothelium-dependent hyperpolarization and relaxation of vascular smooth muscle are mediated by endothelium-derived hyperpolarizing factors (EDHFs). EDHF candidates include cytochrome P-450 metabolites of arachidonic acid, K ϩ , hydrogen peroxide, or electrical coupling through gap junctions. In bovine coronary arteries, epoxyeicosatrienoic acids (EETs) appear to function as EDHFs. A 14,15-EET analogue, 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EEZE) was synthesized and identified as an EET-specific antagonist. In bovine coronary arterial rings preconstricted with U46619, 14,15-EET, 11,12-EET, 8,9-EET, and 5,6-EET induced concentration-related relaxations. Preincubation of the arterial rings with 14,15-EEZE (10 mol/L) inhibited the relaxations to 14,15-EET, 11,12-EET, 8,9-EET, and 5,6-EET but was most effective in inhibiting 14,15-EET-induced relaxations. 14,15-EEZE also inhibited indomethacin-resistant relaxations to methacholine and arachidonic acid and indomethacin-resistant and L-nitroarginineresistant relaxations to bradykinin. It did not alter relaxation responses to sodium nitroprusside, iloprost, or the K ϩ channel activators (NS1619 and bimakalim). Additionally, in small bovine coronary arteries pretreated with indomethacin and L-nitroarginine and preconstricted with U46619, 14,15-EEZE (3 mol/L) inhibited bradykinin (10 nmol/L)-induced smooth muscle hyperpolarizations and relaxations. In rat renal microsomes, 14,15-EEZE (10 mol/L) did not decrease EET synthesis and did not alter 20-hydroxyeicosatetraenoic acid synthesis. This analogue acts as an EET antagonist by inhibiting the following: (1) EET-induced relaxations, (2) the EDHF component of methacholineinduced, bradykinin-induced, and arachidonic acid-induced relaxations, and (3) the smooth muscle hyperpolarization response to bradykinin. Thus, a distinct molecular structure is required for EET activity, and alteration of this structure modifies agonist and antagonist activity. These findings support a role of EETs as EDHFs. Key Words: epoxyeicosatrienoic acids Ⅲ arachidonic acid Ⅲ endothelium-derived hyperpolarizing factors Ⅲ endothelium Ⅲ 20-hydroxyeicosatetraenoic acid E poxyeicosatrienoic acids (EETs) are cytochrome P-450 metabolites of arachidonic acid. They are synthesized by the vascular endothelium and released in response to vasoactive agonists, such as bradykinin and acetylcholine, and are stimulated by cyclic stretch. [1][2][3][4][5][6] Additionally, in the coronary circulation, they cause vascular smooth muscle hyperpolarization and relaxation and, therefore, function as endothelium-derived hyperpolarizing factors (EDHFs). 2 To investigate the role of endogenous EETs in the relaxations induced by endothelium-dependent vasoactive substances, inhibitors of cytochrome P-450 are used, but these investigations have given variable results. In some studies, inhibitors of cytochrome P-450 block the relaxations to bradykinin and acetylcholine, whereas in other studies, these inhibitors are without effect or also inhibit relaxations to K ϩ ...