Involvement of CB1 cannabinoid receptors in the EDHF-dependent vasorelaxation in rabbits.

1. It was recently suggested that an endogenous cannabinoid could represent an endothelium-derived hyperpolarizing factor (EDHF). The aim of the present study was to clarify whether CB1 cannabinoid receptors are involved in the nitric oxide (NO)- and prostanoid-independent vasodilation produced by acetylcholine in rabbits. 2. Pithed rabbits received indomethacin. Noradrenaline was infused to raise blood pressure, and vasodilation was elicited by bolus injections of acetylcholine. The NO-synthase inhibitor Nomega-nitro-L-arginine methylester inhibited the acetylcholine-evoked vasodilation by about 40%. The remaining vasodilation was unaffected by the CB1 cannabinoid receptor antagonist SR141716A, but was inhibited by the potassium channel blocker tetraethylammonium. In addition, the mixed CB1/CB2 cannabinoid receptor agonist WIN55212-2 did not elicit vasodilation. 3. No CB1 cannabinoid receptors were involved in the prostanoid- and NO-independent vasodilation produced by acetylcholine. An exogenous cannabinoid also did not cause vasodilation. Therefore, it is unlikely that an endogenous cannabinoid serves as an EDHF acting at smooth muscle CB1 cannabinoid receptors in the rabbit.