EVIDENCE AGAINST ANANDAMIDE AS THE HYPERPOLARIZING FACTOR MEDIATING THE NITRIC OXIDE-INDEPENDENT CORONARY VASODILATOR EFFECT OF BRADYKININ IN THE RAT

The mediator of nitric oxide-(NO) independent vasodilation attributed to endothelium-derived hyperpolarizing factor remains unidentified alt hough there is evidence for a cytochrome P450-derived eicosanoid. Anan damide, the ethanolamide of arachidonic acid and an endogenous ligand for cannabinoid receptors, was proposed as an endothelium-derived hype rpolarizing factor-mediating mesenteric vasodilation to acetylcholine and the hypotensive effect of bradykinin. Using pharmacological interv entions that attenuate responses to bradykinin, we examined the possib ility of anandamide as a mediator of the NO-independent vasodilator ef fect of bradykinin in the rat perfused heart by determining responses to anandamide and arachidonic acid. Hearts were treated with indometha cin to exclude prostaglandins and nitroarginine to inhibit NO synthesi s and elevate perfusion pressure. The cannabinoid receptor antagonist, SR 141716A (2 mu M), reduced dose-dependent vasodilator responses to anandamide (1-10 mu g) but was without effect on responses to AA (1-10 mu g), bradykinin (10-1000 ng) or cromakalim (1-10 mu g). Inhibition of voltage-dependent Ca++ channels with nifedipine (5 nM) attenuated v asodilation to anandamide and arachidonic acid whereas inhibition of C a++-activated K+ channels with charybdotoxin (10 nM) reduced responses to arachidonic acid but had no effect on vasodilation induced by anan damide. Inhibition of cytochrome P450 with clotrimazole (1 mu M) great ly reduced vasodilator responses to bradykinin with less effect on tho se to anandamide. Finally, the time course of the coronary vasodilator responses to anandamide and bradykinin were dissimilar. These results argue against a role of anandamide in the vasodilator effect of brady kinin in the rat heart.