In order to address the issue of mechanistic differences between arterial vessel types, we compared the vasorelaxant actions of anandamide in resistance and conduit mesenteric vessels.

Male Wistar rats (250-350 g) were stunned and killed by cervical dislocation. Small mesenteric resistance arteries (G3) and the superior mesenteric artery (G0) were isolated and mounted in a Mulvany-Halpern myograph (Mulvany & Halpern, 1977). 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. The vasorelaxant effects of anandamide were assessed as cumulative concentration-response curves.

Anandamide produced concentration-dependent relaxations of pre-constricted G3 arteries (pEC₅₀=5.74 ± 0.20 mean ± SEM, maximal relaxation at 100 µM (R_max)=89.8 ± 7.8 %, n=9) and of G0 (pEC₅₀=5.02 ± 0.21, R_max=33.4 ± 3.3 %, n=7). The CB₁ receptor antagonist, SR141716A (100 nM), caused a reduction in the vasorelaxant response to anandamide in both arteries (G3 pEC₅₀=4.87 ± 0.26, n=6, P<0.01, ANOVA; G0 pEC50=4.10 ± 0.76, n=6, P<0.01). Both vessels also had reduced maximal vasorelaxant responses to anandamide after treatment with capsaicin (10 µM for 1 hr) to deplete sensory neurotransmitters (G3 R_max=51.2 ± 12.4 %, n=6, P<0.01; G0 R_max=11.1 ± 3.9 %, n=6, P<0.01). Only G3 arteries were sensitive to antagonism of the novel endothelial cannabinoid receptor by the compound O-1918 (1 µM, Offertaler et al., 2003) (G3 pEC₅₀=4.85 ± 0.22, n=6, P<0.01; G0 pEC₅₀=5.31 ± 0.38, n=6). Similarly, only G3 arteries were sensitive to removal of the endothelium (G3 pEC₅₀=5.08 ± 0.29, P<0.05, R_max=65.2 ± 13.4 %, n=6, P<0.05; G0 pEC₅₀=4.82 ± 0.31, R_max=37.6 ± 6.6, n=7). Vasorelaxation to anandamide was not affected by the nitric oxide synthase inhibitor N^G-nitro-L-arginine methyl ester (300 µM) in either artery. Only G3 arteries were sensitive to inhibition of EDHF activity with a combination of charybdotoxin (100 nM) and apamin (500 nM) in the presence of L-NAME (300 µM) and indomethacin (10 µM) (Randall & Kendall, 1998) (G3 pEC₅₀=3.44 ± 0.20, P<0.01, R_max=47.9 ± 10.2 %, P<0.01, n=6; G0 pEC₅₀=4.44 ± 0.27, R_max=44.33 ± 9.2, n=7).

The results of the present study demonstrate that there are fundamental differences in the mechanisms of action of anandamide between arteries. In small resistance vessels, vasorelaxation occurs through stimulation of the vanilloid receptor, the CB₁ receptor, and an endothelial cannabinoid receptor that is coupled to EDHF release. By contrast, in the larger mesenteric artery, vasorelaxation is almost entirely due to stimulation of the vanilloid receptor and CB₁ receptor, and is not endothelium-dependent.

Mulvany MJ & Halpern W (1977) Circ Res 41, 19-26.
Offertaler et al. (2003) Mol Pharmacol 63, 699-705.
Randall MD & Kendall DA (1998) Eur J Pharmacol 346, 51-53.

This study was funded by the BHF (PG2001/150). We thank Dr G. Kunos for the generous gift of the O-1918 compound.