Relaxant responses of rabbit aorta: influence of cytochrome P450 inhibitors.

Based on the use of inhibitors, cytochrome P450 has been implicated in endothelium-dependent relaxant responses via metabolism of arachidonic acid (AA). However, the contribution of cytochrome P450 and its AA metabolites to the regulation of vascular tone has not been established due, in part, to questions of specificity of cytochrome P450 inhibitors which have not been extensively characterized in terms of their vascular effects. Consequently, we addressed the effects of several inhibitors on vasorelaxant responses of phenylephrine-contracted, rabbit, aortic rings to agents that utilize different transduction mechanisms to determine any actions unrelated to inhibition of cytochrome P450 and/or AA metabolism. Octadecynoic acid (2.5 and 5 microM), a mechanism-based inhibitor of cytochrome P450 metabolism of fatty acids, and eicosatetrayenoic acid (10 and 20 microM), an inhibitor of AA metabolism, were without effect on vasorelaxant responses to acetylcholine, sodium nitroprusside, isoproterenol and diazoxide. 7-Ethoxyresorufin (2-10 microM), a substrate for cytochrome P450, and clotrimazole (2.5-10 microM) which binds to the heme moiety of the enzyme, concentration-dependently reduced responses to acetylcholine but not the other agonists indicating an effect on nitric oxide synthesis although neither affected the conversion of L-arginine to L-citrulline by endothelial cells. SKF 525A (50-200 microM), the prototypical inhibitor of cytochrome P450, which is metabolized to an inhibitory intermediate, also reduced responses to acetylcholine and, in addition, impaired the vasorelaxant activities of isoproterenol and diazoxide.