Regional differences in anandamide- and methanandamide-induced membrane potential changes in rat mesenteric arteries

The possibility that anandamide is an endothelium-derived hyperpolarizing f actor was explored in the rat mesenteric vasculature by use of conventional microelectrode techniques. In the main mesenteric artery, anandamide and i ts more stable analog methanandamide hardly caused a measurable change in m embrane potential of the smooth muscle cells, which promptly hyperpolarized to EDHF liberated by acetylcholine. Inhibition of endogenous anandamide br eakdown by phenylmethylsulfonyl fluoride did not increase membrane response s to acetylcholine. The CB1 receptor antagonist SR141716 did not significan tly influence EDHF-mediated hyperpolarization except at extremely high conc entrations. Smooth muscle cells of third to fourth order branches of the me senteric artery, which have a more negative resting membrane potential and show smaller responses to acetylcholine, hyperpolarized by about 6 mV to bo th anandamide and methanandamide, whereas another CB1 receptor agonist, WIN 55,212-2, had no effect. Mechanical endothelium removal or pre-exposure to SR141716A did not affect anandamide- and methanandamide-induced hyperpolar izations. However, in the presence of capsazepine, a selective vanilloid re ceptor antagonist, these membrane potential changes were reversed to a smal l depolarization, whereas EDHF-induced hyperpolarizations were not affected . Pretreating small vessels with capsaicin, causing desensitization of vani lloid receptors and/or depletion of sensory neurotransmitter, completely bl ocked methanandamide-induced hyperpolarizations. These findings show that a nandamide cannot be EDHF. In smooth muscle cells of small arteries, anandam ide-induced changes in membrane potential are mediated by vanilloid recepto rs on capsaicin-sensitive sensory nerves. The different membrane response t o the cannabinoids between the main mesenteric artery and its daughter bran ches might be explained by the different density of perivascular innervatio n.