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William B Campbell, John R Falck, Kathryn Gauthier
Med Sci Monit 2001; 7(4): BR578-584
BACKGROUND: Acetylcholine and bradykinin cause endothelium-dependent hyperpolarizationand relaxation of vascular smooth muscle. These responses are mediated in part by an endothelium-derivedhyperpolarizing factor (EDHF). Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonicacid and appear to function as EDHFs in the coronary artery. This conclusion is based on the findingsthat EETs are synthesized by the vascular endothelium, open calcium-activated potassium (KCa) channels,hyperpolarize and relax vascular smooth muscle. METHODS: Bovine coronary arteries were precontractedwith U46619 and isometric tension was measured to agonists. Whole cell K currents were measured in coronarysmooth muscle cells by the patch clamp method. RESULTS: Bradykinin caused a concentration-related relaxation(ED50=10(-10) M) which was not affected by inhibitors of cytochrome P450. Inhibition of NO synthase andcyclooxygenase reduced the response to bradykinin (ED(50)=2 x 10(-9) M) and these responses were thencompletely blocked by cytochrome P450 inhibitors. 11, 12-EET relaxed the precontracted bovine coronaryartery with an ED50 of 3 x 10(-8) M in the presence and absence of the endothelium. 14, 15-EET causedan increase in outward potassium current that was blocked by the KCa channel inhibitor iberiotoxin. Aseries of 14, 15-EET analogs were tested for agonist activity. CONCLUSIONS: These data indicate thatcytochrome P450 metabolites contribute to bradykinin-induced, endothelium-dependent relaxations but onlyfollowing inhibition of NO synthase and cyclooxygenase. EETs relax coronary arteries in an endothelium-independentmanner. Carbon-1 carboxyl, 8, 9 and 11, 12-double bonds and 14, 15-epoxy groups are required for fullagonist activity. These studies provide further support for the hypothesis that EETs represent EDHFsin the coronary artery and mediate the relaxation responses to bradykinin.