Characterization of vasorelaxant responses to anandamide in the rat mesenteric arterial bed

Abstract

The endogenous cannabinoid anandamide has recently been identified as a vasorelaxant but the underlying mechanisms are controversial. The vasorelaxant responses to anandamide have now been examined in the rat mesenteric arterial bed. Anandamide caused potent vasorelaxations (pD2= 6.24 ± 0.06; Rmax= 89.4 ± 2.2 %) which were unaffected by inhibition of nitric oxide synthase with NG‐nitro‐l‐arginine methyl ester (l‐NAME; 300 μm). The responses were also predominantly endothelium independent and were unaffected by the cannabinoid CB1 receptor antagonist SR141716A (1 μm), although at higher concentrations (3 and 10 μm) SR141716A was inhibitory. Both 1 mm ouabain (pD2= 5.90 ± 0.07; Rmax= 50.4 ± 6.5 %) and 100 μm 18α‐glycyrrhetinic acid (pD2= 6.04 ± 0.14; Rmax= 40.9 ± 5.8 %) opposed anandamide‐induced vasorelaxation. However, the gap junction inhibitors carbenoxolone (100 μm) and palmitoleic acid (50 μm) did not affect vasorelaxation to anandamide. Relaxation to anandamide was significantly attenuated by both capsaicin pretreatment to deplete the sensory nerves of neurotransmitters (pD2= 5.86 ± 0.18; Rmax= 56.3 ± 5.2 %) and the vanilloid antagonist ruthenium red (10 μm; pD2= 5.64 ± 0.09; Rmax= 33.7 ± 3.9 %). However, these inhibitory effects were prevented by the additional presence of l‐NAME, when the relaxation to anandamide was unaffected (pD2= 6.19 ± 0.07; Rmax= 81.9 ± 2.8 %). The inhibitor of neuronal nitric oxide synthase, 7‐nitroindazole, also prevented capsaicin from inhibiting the responses to anandamide. The results of this study point to anandamide acting via several mechanisms, which include the involvement of sensory nerves, but only in the presence of nitric oxide.

Characterisation of the vasorelaxant properties of the novel endocannabinoid N‐arachidonoyl‐dopamine (NADA)

“…Also, some vessels were incubated for 1 h with the vanilloid agonist capsaicin (10 mM) to deplete the sensory nerves of neurotransmitters. This was followed by a 20-min washout period (Zygmunt et al, 1999;Harris et al, 2002).…”

“…They found that, in isolated mesenteric arteries, vasorelaxation to anandamide was sensitive to the VR antagonist capsazepine, capsaicin pretreatment and CGRP receptor antagonism. A role for sensory nerves in vasorelaxation to CBs has been confirmed since then (Ralevic et al, 2000;Harris et al, 2002), although it is not thought that stimulation of the VR accounts for the entire relaxant response to anandamide (White et al, 2001;Harris et al, 2002;Ho & Hiley, 2003). Since NADA has also been found to stimulate VRs (Huang et al, 2002), their potential involvement in the vasorelaxant response to NADA was investigated.…”

“…However, this is complicated by the wide-ranging actions of SR141716A at various concentrations. For example, SR141716A can inhibit myoendothelial gap junctions (Chaytor et al, 1999), which themselves have been implicated in the actions of anandamide via EDHF (Chaytor et al, 1999;Harris et al, 2002). SR141716A may also antagonise the novel non-CB 1 endothelial CB receptor proposed by Kunos and colleagues (Ja´rai et al, 1999;Offerta´ler et al, 2003).…”

“…Although nitric oxide does not appear to mediate responses to anandamide, the vasorelaxation is partly endothelium-dependent, in some, but not all preparations (White & Hiley, 1997;Chaytor et al, 1999;Ja´rai et al, 1999). It is therefore likely that anandamide stimulates the release of the endothelium-derived hyperpolarising factor (EDHF), which is communicated to smooth muscle cells via myoendothelial gap junctions (Chaytor et al, 1999;Harris et al, 2002).…”

“…Whether this action can account for the entire endocannabinoid vasorelaxant response has yet to be clarified, as further studies have not found that the entire response to anandamide is mediated via sensory nerves (Ralevic et al, 2000;Harris et al, 2002).…”