Abstract

We tested the hypothesis that an endogenous cannabinoid (CB) receptor agonist, such as N-arachidonylethanolamine (anandamide), is the transmitter that mediates perivascular sensory nerve-dependent Ca²⁺-induced relaxation. Rat mesenteric branch arteries were studied using wire myography; relaxation was determined after inducing contraction with norepinephrine. Cumulative addition of Ca²⁺ caused dose-dependent relaxation (ED₅₀ = 2.2 ± 0.09 mM). The relaxation was inhibited by 10 mM TEA and 100 nM iberiotoxin, a blocker of large conductance Ca²⁺-activated K⁺ channels, but not by 5 μM glibenclamide, 1 mM 4-aminopyridine, or 30 nM apamin. Ca²⁺-induced relaxation was also blocked by the selective CB receptor antagonist SR141716A and was enhanced by pretreatment with 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (pefabloc; 30 μM), an inhibitor of anandamide metabolism. Anandamide also caused dose-dependent relaxation (ED₅₀ = .72 ± 0.3 μM). The relaxation was not inhibited by endothelial denudation, 10 μM indomethacin, or 1 μM miconazole, but was blocked by 3 μM SR141716A, 10 mM TEA, precontraction with 100 mM K⁺, and 100 nM iberiotoxin, and was enhanced by treatment with 30 μM pefabloc. Mesenteric branch arteries were 200-fold more sensitive to the relaxing action of anandamide than arachidonic acid (ED₅₀ = 160 ± 7 μM). These data show that: 1) Ca²⁺ and anandamide cause hyperpolarization-mediated relaxation of mesenteric branch arteries, which is dependent on an iberiotoxin-sensitive Ca²⁺-activated K⁺channel, 2) relaxation induced by both Ca²⁺ and anandamide is inhibited by CB receptor blockade, and 3) relaxation induced by anandamide is not dependent on its breakdown to arachidonic acid and subsequent metabolism. These findings support the hypothesis that anandamide, or a similar cannabinoid receptor agonist, mediates nerve-dependent Ca²⁺-induced relaxation in the rat.