Abstract

The possibility that anandamide is an endothelium-derived hyperpolarizing factor was explored in the rat mesenteric vasculature by use of conventional microelectrode techniques. In the main mesenteric artery, anandamide and its more stable analog methanandamide hardly caused a measurable change in membrane potential of the smooth muscle cells, which promptly hyperpolarized to EDHF liberated by acetylcholine. Inhibition of endogenous anandamide breakdown by phenylmethylsulfonyl fluoride did not increase membrane responses to acetylcholine. The CB₁ receptor antagonist SR141716 did not significantly influence EDHF-mediated hyperpolarization except at extremely high concentrations. Smooth muscle cells of third to fourth order branches of the mesenteric artery, which have a more negative resting membrane potential and show smaller responses to acetylcholine, hyperpolarized by about 6 mV to both anandamide and methanandamide, whereas another CB₁receptor agonist, WIN 55,212-2, had no effect. Mechanical endothelium removal or pre-exposure to SR141716A did not affect anandamide- and methanandamide-induced hyperpolarizations. However, in the presence of capsazepine, a selective vanilloid receptor antagonist, these membrane potential changes were reversed to a small depolarization, whereas EDHF-induced hyperpolarizations were not affected. Pretreating small vessels with capsaicin, causing desensitization of vanilloid receptors and/or depletion of sensory neurotransmitter, completely blocked methanandamide-induced hyperpolarizations. These findings show that anandamide cannot be EDHF. In smooth muscle cells of small arteries, anandamide-induced changes in membrane potential are mediated by vanilloid receptors on capsaicin-sensitive sensory nerves. The different membrane response to the cannabinoids between the main mesenteric artery and its daughter branches might be explained by the different density of perivascular innervation.