CONTRIBUTION OF K+ CHANNELS TO ARACHIDONIC ACID-INDUCED ENDOTHELIUM-DEPENDENT VASODILATION IN RAT ISOLATED PERFUSED MESENTERIC-ARTERIES

The contribution of K+ channels and cytochrome P450 generated arachidonic acid (AA) metabolites to the endothelium-dependent vasodilation produced by this fatty acid in the perfused rat isolated mesenteric arteries was examined using a variety of compounds known to inhibit transmembrane K+ channels and cytochrome P450 enzymes. AA (1-1000 nmol) caused dose-and endothelium-dependent vasodilation in the presence of indomethacin and the effect was neither altered by lipoxygenase (AA 861) nor cytochrome P450 monooxygenase (alpha-naphthoflavone, ketoconazole and metyrapone) inhibitors indicating that AA-induced, endothelium-dependent vasodilation in this vascular bed was not mediated by product(s) of AA metabolism. The vasodilator effect of AA was also not altered by L-N(G)-nitroarginine, methylene blue (50-mu-M), oxyhemoglobin (5-mu-M) or superoxide dismutase (50 U/ml), thus ruling out nitric oxide being its mediator. Conversely, arterial perfusion with K+-free or excess (50 mM) K+ Krebs' solution, but not ouabain infusion, minimized the vasodilator effect of AA, suggesting that this action of the fatty acid is due to changes in membrane K+ conductance that is independent of Na+/K+-adenosine triphosphatase activity. The vasodilator action of BRL 34915 (a K+ channel activator) was also minimized by extracellular K+ depletion or excess K+ (50 mM), but not by ouabain. Apamin (0.5-mu-M) and crude scorpion venom (2.5-mu-g/ml) attenuated AA- but not BRL 34915-induced vasodilation. Glyburide (inhibitor of ATP-activated K+ channel) abolished the vasodilator action of AA and BRL 34915. Procaine, a nonspecific K+ channel blocker did not affect AA-induced vasodilation even though it attenuated that caused by BRL 34915. These data suggest that endothelium-derived relaxing factor generated by AA in the rat mesenteric arteries is not an arachidonate metabolite, but a factor that appears to produce vascular smooth muscle relaxation by activating K+ channels that are sensitive to apamin and the sulphonylurea, glyburide.