Endothelium-dependent metabolism by endocannabinoid hydrolases and cyclooxygenases limits vasorelaxation to anandamide and 2-arachidonoylglycerol

Background and purpose: The endocannabinoids, N-arachidonoylethanolamide (anandamide) and 2-arachidonoylglycerol (2-AG) are rapidly degraded by fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL). Whilst these lipid mediators are known to modulate vascular tone, the extent to which they are inactivated via local metabolism in the vasculature remains unclear.

Experimental approach: In rat isolated small mesenteric arteries, the regulatory role of FAAH, MGL and cyclooxygenase (COX) in relaxant responses to anandamide and 2-AG was evaluated by using inhibitors of these enzymes. Relaxations to nonhydrolysable analogues of endocannabinoids and arachidonic acid were also examined.

Key results: Relaxation to anandamide but not 2-AG was potentiated by the selective FAAH inhibitor, URB597 (1 mM). In contrast, MAFP (10 mM; an inhibitor of FAAH and MGL) enhanced responses to both anandamide and 2-AG. Inhibition of COX-1 by indomethacin (10 mM) potentiated relaxations to 2-AG, whereas inhibition of COX-2 by nimesulide (10 mM) potentiated anandamide-induced relaxation. With the exception of MAFP, effects of FAAH and COX inhibitors were dependent on the endothelium. Relaxation to methanandamide and noladin ether, the non-hydrolysable analogues of anandamide and 2-AG respectively, were insensitive to the enzyme inhibitors.

Conclusion and implications: This study shows that local activity of FAAH, MGL and COX, which is present largely in the endothelium, limits the vasodilator action of endocannabinoids in rat small mesenteric arteries. Despite the differential roles played by these enzymes on relaxation to anandamide versus 2-AG, our results suggest that inhibitors of these enzymes enhance the vascular impact of endocannabinoids.