Evidence against Anandamide as the Hyperpolarizing Factor Mediating the Nitric Oxide-Independent Coronary Vasodilator Effect of Bradykinin in the Rat1

The mediator of nitric oxide-(NO) independent vasodilation attributed to endothelium-derived hyperpolarizing factor remains unidentified although there is evidence for a cytochrome P450derived eicosanoid. Anandamide, the ethanolamide of arachidonic acid and an endogenous ligand for cannabinoid receptors, was proposed as an endothelium-derived hyperpolarizing factor-mediating mesenteric vasodilation to acetylcholine and the hypotensive effect of bradykinin. Using pharmacological interventions that attenuate responses to bradykinin, we examined the possibility of anandamide as a mediator of the NOindependent vasodilator effect of bradykinin in the rat perfused heart by determining responses to anandamide and arachidonic acid. Hearts were treated with indomethacin to exclude prostaglandins and nitroarginine to inhibit NO synthesis and elevate perfusion pressure. The cannabinoid receptor antagonist, SR 141716A (2 /j.M), reduced dose-dependent vasodilator responses to anandamide (1-10 -4g) but was without effect on responses to AA (1-10 jug), bradykinin (10-1000 ng) or cromakalim (1-10 /xg). Inhibition of voltage-dependent Ca+ + channels with nifedipine (5 nM) attenuated vasodilation to anandamide and arachidonic acid whereas inhibition of Ca++activated K+ channels with charybdotoxin (10 nM) reduced responses to arachidonic acid but had no effect on vasodilation induced by anandamide. Inhibition of cytochrome P450 with clotrimazole (1 ju,M) greatly reduced vasodilator responses to bradykinin with less effect on those 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 bradykinin in the rat heart.