Role of lipoxygenase and cytochrome P-450 in production of endothelium-derived relaxing factors in canine femoral veins

We wished to determine whether the metabolism of arachidonic acid, through lipoxygenase and cytochrome P-450 pathways, is involved in production of endothelium-derived relaxing factor(s) (EDRFs) in canine femoral veins. Veins were removed from anesthetized dogs and cut into rings. Endothelium was deliberately removed from some rings. In separate sets of experiments, rings were incubated with either AA861 (10^-5 M) or TMK777 (10^-6 M), inhibitors of 5-lipoxygenase, nor-dihydroguaiaretic acid (NDGA 3 × 10^-6 M), an inhibitor of lipoxygenase or proadifen (SKF 525A, 10^-6 M), an inhibitor of cytochrome P-450. In addition, some rings were incubated with a combination of indomethacin (10^-5 M) and N^G-monomethyl-L-arginine (L-NMMA 10^-4 M) or, where appropriate, a solvent control. Concentration-response curves were obtained for acetylcho-line, adenosine diphosphate, thrombin, A23187, and nitric oxide in rings contracted with a submaximal concentration of prostaglandin F_2α. AA861 and TMK777 did not alter endothelium-dependent relaxations to the agonists, whether with or without indomethacin and L-NMMA. However, indomethacin plus L-NMMA reduced endothelium-dependent relaxations to thrombin. These results suggest that metabolism of arachidonic acid, through lipoxygenase and cytochrome P-450 pathways, does not produce an EDRF in veins. However, thrombin receptor-activated relaxations are mediated in part by products of the cyclooxygenase pathway and nitric oxide.