We have previously reported some initial findings of mechanistic differences between resistance vessels and the superior mesenteric artery in the vasorelaxant effects of NADA (O'Sullivan et al., 2003). We have now further investigated this phenomenon.

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 NADA were assessed as cumulative concentration-response curves.

NADA caused relaxation of pre-constricted arteries that was more effective in G3 (pEC₅₀=6.39 ± 0.12 mean ± SEM, maximal relaxation at 100 µM (R_max)= 79.1 ± 4.4 %, n=10) than G0 vessels (pEC₅₀=5.45 ± 0.15, R_max=37.2 ± 3.0 %, n=7, P<0.01, ANOVA). Both vessels had reduced maximal vasorelaxant responses to NADA after treatment with capsaicin (10 µM for 1 hr) to deplete sensory neurotransmitters (G3 R_max=60.3 ± 6.4 %, n=10, P<0.01; G0 R_max=17.6 ± 6.4 %, n=6, P<0.01). The CB₁ receptor antagonists SR141716A (100 nM) only caused a reduction in the vasorelaxant response to NADA in G0 (G3 R_max =75.3 ± 6.8, n=5; G0 R_max =16.3 ± 2.3 %, n=7, 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=5.26 ± 0.20, n=6, P<0.01; G0 pEC₅₀=5.11 ± 0.14, n=6). Similarly, only G3 arteries were sensitive to removal of the endothelium (G3 pEC₅₀=5.70 ± 0.29, P<0.01, R_max=39.1 ± 4.7 %, n=6, P<0.01; G0 pEC₅₀=5.29 ± 0.17, R_max=48.7 ± 5.0 %, n=7). Vasorelaxation to NADA was not affected by the nitric oxide synthase inhibitor N^G-nitro-L-arginine methyl ester (300 µM) in either artery. Responses to NADA in G3 arteries only were sensitive to inhibition of EDHF activity with the combination of charybdotoxin (100 nM) and apamin (500 nM) in the presence of L-NAME (300 µM) and indomethacin (10 mM) (Randall & Kendall, 1998) (G3 pEC₅₀=5.73 ± 0.29, P<0.01, R_max=40.0 ± 7.0%, P<0.01, n=5).

The results of the present study demonstrate that there are fundamental differences in the mechanisms of actions of NADA between arteries. In small resistance vessels, vasorelaxation occurs through stimulation of the vanilloid 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 Pharm 63, 699-705.
O'Sullivan et al. (2003) B J Pharm (In press).
Randall MD & Kendall DA (1998) Eur J Pharm 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.