Individually, both THC and anandamide cause vasorelaxation in rat mesenteric arteries. However, the interactions between exogenous and endogenous cannabinoids are still largely unknown, and evidence suggests that THC may act as an antagonist to other cannabinoid molecules (Bayewitch et al., 1996; Kelley & Thayer, 2004). We have therefore examined the effects of THC on vasorelaxation to anandamide.

Male Wistar rats (200-300 g) were killed by cervical dislocation. Small mesenteric resistance arteries (G3) or the superior mesenteric artery (G0) were isolated, cut into 2 mm lengths and mounted on a Mulvany-Halpern myograph. Vessels were bathed in oxygenated Krebs’ solution at 37^oC. Vessels were set to a baseline tone of 5 mN, and U46619 was added to increase tension by at least 5 mN.

THC caused significant rightward shifts in the concentration-response curve to anandamide in G3 vessels at 1 µM (control pEC₅₀ = 6.43 ± 0.22, mean ± SEM, n=8 cf pEC₅₀ = 5.09 ± 0.57, n=8, P<0.05, ANOVA) and 10 µM (pEC₅₀ = 4.49 ± 0.20, n=6, P<0.01). THC (10 µM) did not affect the concentration-response curve to the non-cannabinoid vasorelaxant, verapamil (control pEC₅₀ = 7.01 ± 0.12, n=5; & THC 10 µM pEC₅₀ = 7.22 ± 0.09, n=6), the vanilloid receptor agonist capsaicin (capsaicin pEC₅₀ = 5.37 ± 0.11, n=5; & THC 10µM pEC₅₀ = 5.45 ± 0.08, n=4), or the CB₁ receptor agonist CP 55,940 (CP 55,940 pEC₅₀ = 5.86 ± 0.09, n=5; & THC 10 µM pEC₅₀ = 5.77 ± 0.18, n=5). THC (10 µM) also did not affect the actions of anandamide in the superior mesenteric artery (G0; control pEC₅₀ = 5.39 ± 0.29, n=7; & THC 10 µM pEC₅₀ = 5.44 ± 0.34, n=7). When THC (10 µM) was applied in combination with endothelial-denudation in G3 vessels, THC did not significantly inhibit the vasorelaxant effects of anandamide further than was seen with denudation alone (denuded vessels pEC₅₀ = 5.28 ± 0.29, n=7; denuded vessels & THC 10 µM pEC₅₀ = 4.25 ± 0.59, n=6). The inhibitory effect of 1 µM THC on vasorelaxation to anandamide was significantly enhanced in the presence of L-NAME (THC 1 µM pEC₅₀ = 5.59 ± 0.13, n=8; THC 1 µM & L-NAME pEC₅₀ = 3.22 ± 0.93, n=5, P<0.05). The magnitude and duration of vasorelaxation to carbachol (10 µM) in the presence of indomethacin (10 µM) and L-NAME (300 µM) (mediated by endothelium-derived hyperpolarising factor (EDHF)) was also significantly reduced in the presence of THC (10 µM or 100 µM). The effects of THC were similar in profile to the effects of the gap junction inhibitor, 18 -glycyrrhetinic acid (18-GA, 100 µM).

In summary, THC inhibits the vasorelaxant activities of the endogenous cannabinoid anandamide in small isolated mesenteric arteries. This is not through non-specific actions, or actions at the vanilloid receptor or CB₁ receptor, but possibly through inhibition of EDHF activity via inhibition of intercellular communication.

Bayewitch et al. (1996). J. Biol. Chem.,271, 9902-99055.
Kelley & Thayer (2004). Neuropharmacology.,46, 709-715.

This study was funded by the British Heart Foundation (PG2001/150).