Vasorelaxant properties of the novel endocannabinoid N-arachidonoyl-dopamine (NADA) S.E. O'Sullivan, D.A. Kendall & M.D. Randall, School of Biomedical Sciences, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK.

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Studies on the vascular effects of endocannabinoids have focused on the prototypic member, anandamide. One mechanism mediating the effects of anandamide is stimulation of vanilloid receptors (Zygmunt et al., 1999). We have now examined the vascular effects of N-arachidonoyl-dopamine (NADA), a novel compound that stimulates vanilloid and cannabinoid receptors (Bisogno et al., 2000).

Male Wistar rats (250-350 g) were stunned and killed by cervical dislocation. Small mesenteric resistance arteries (G3, 100-400 µm internal diameter) were isolated, cut into 2 mm lengths and mounted on a Mulvany-Halpern myograph (Mulvany & Halpern, 1977). The superior mesenteric artery (G0) was also examined. Vessels were bathed in oxygenated Krebs-Henseleit solution at 37^oC, set to a baseline tone of 5 mN, and allowed to equilibrate. U46619 (10-300 nM) was added to increase tension by at least 5 mN and the effects of NADA were assessed.

NADA caused relaxation in G3 (pEC₅₀=6.07 ± 0.17 mean ± SEM, maximal relaxation at 100 µM (R_max)=83.8 ± 5.3 %, n=10), in G0 (pEC₅₀=4.97 ± 0.19, R_max=42.9 ± 4.3 %, n=7) and in the aorta (pEC₅₀=5.62 ± 0.46, R_max=17.6 ± 3.3 %, n=7). In G3 vessels, responses to NADA were not affected by the dopamine receptor antagonist SCH23390 (1 µM, n=7) or addition of the nitric oxide synthase inhibitor N^G-nitro-L-arginine methyl ester (300 µM, n=6). Responses to NADA were inhibited in the presence of 60 mM KCl (R_max=28.3 ± 2.2 %, P<0.01, ANOVA, n=4), and in endothelium-denuded vessels (R_max=47.9 ± 8.5 %, P<0.01, n=6). Pre-treatment with capsaicin (10 µM) for 1 hr reduced the vasorelaxant effects of NADA (R_max=61.8 ± 8.5 %, P<0.05, n=10) (Zygmunt et al., 1999). The VR receptor antagonist capsazepine (1 µM, n=5) and the CB₁ receptor antagonist SR141716A (1 µM, n=6) caused rightward shifts in the concentration-response curves to NADA (capsazepine, pEC₅₀=5.39 ± 0.13, P<0.01, n=5; SR141716A, pEC₅₀=5.42 ± 0.22, P<0.01, n=6). However, SR141716A at 100 nM (n=5), and a second CB₁ receptor antagonist (AM251; 100 nM or 1 µM, n=6) had no effect. SR141716A at 1 µM after endothelial denudation did not reduce the relaxant responses further (n=6). In G0, vasorelaxation was not dependent on an intact endothelium (n=7), but was sensitive to capsaicin (n=6) and SR141716A at 1 µM (n=6) and 100 nM (n=7).

The results of the present study demonstrate for the first time that NADA is a potent vasorelaxant. Its effects are mediated by stimulation of vanilloid receptors, and an endothelial receptor that is sensitive to SR141716A, possibly the novel endothelial cannabinoid receptor (Jarai et al., 1999).

This study was funded by the BHF (PG2001/150).

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