Role of epoxyeicosatrienoic acids as endothelium-derived hyperpolarizing factor in bovine coronary arteries

Background: Acetylcholine and bradykinin cause endothelium-dependent hyperpolarization and relaxation of vascular smooth muscle. These responses are mediated in part by an endothelium-derived hyperpolarizing factor (EDHF). Epoxyeicosatrienoic acids (EETs) are cytochrome P450 metabolites of arachidonic acid and appear to function as EDHFs in the coronary artery. This conclusion is based on the findings that EETs are synthesized by the vascular endothelium, open calcium-activated potassium (K_Ca) channels, hyperpolarize and relax vascular smooth muscle. Methods: Bovine coronary arteries were precontracted with U46619 and isometric tension was measured to agonists. Whole cell K currents were measured in coronary smooth 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 and cyclooxygenase reduced the response to bradykinin (ED₅₀=2 × 10^-9 M) and these responses were then completely blocked by cytochrome P450 inhibitors. 11, 12-EET relaxed the precontracted bovine coronary artery with an ED₅₀ of 3 × 10^-8 M in the presence and absence of the endothelium. 14, 15-EET caused an increase in outward potassium current that was blocked by the K_Ca channel inhibitor iberiotoxin. A series of 14, 15-EET analogs were tested for agonist activity. Conclusions: These data indicate that cytochrome P450 metabolites contribute to bradykinin-induced, endothelium-dependent relaxations but only following inhibition of NO synthase and cyclooxygenase. EETs relax coronary arteries in an endothelium-independent manner. Carbon-1 carboxyl, 8, 9 and 11, 12-double bonds and 14, 15-epoxy groups are required for full agonist activity. These studies provide further support for the hypothesis that EETs represent EDHFs in the coronary artery and mediate the relaxation responses to bradykinin.