Human platelets bind and degrade 2-arachidonoylglycerol, which activates these cells through a cannabinoid receptor

The endocannabinoid 2-arachidonoylglycerol (2-Delta (4)AchGro) activates human platelets in platelet-rich plasma at physiological concentrations. The activation was inhibited by selective antagonists of CB1 and CB2 cannabinoid receptors, but not by acetylsalicylic acid. Human platelets can metabolize 2-Delta (4)Ach-Gro by internalization through a high affinity transporter (K-m = 300 +/- 30 nM, V-max = 10 +/- 1 pmol.min(-1).mg protein(-1)), followed by hydrolysis by a fatty acid amide hydrolase (K-m = 8 +/- 1 muM, V-max = 400 +/- 50 pmol.min(-1).mg protein(-1)). The anandamide transport inhibitor AM404, and anandamide itself, were ineffective on 2-Delta (4)Ach-Gro uptake by platelets, whereas anandamide competitively inhibited 2-Delta (4)Ach-Gro hydrolysis (inhibition constant = 10 +/- 1 muM). Platelet activation by 2-Delta (4)Ach-Gro was paralleled by an increase of intracellular calcium and inositol-1,4,5-trisphosphate, and by a decrease of cyclic AMP. Moreover, treatment of preloaded platelet-rich plasma with 2-Delta (4)Ach-Gro induced an approximately threefold increase in [H-3]2-Delta (4)Ach-Gro release, according to a CB receptor-dependent mechanism. On the other hand, ADP and collagen counteracted the activation of platelets by 2-Delta (4)Ach-Gro, whereas 5-hydroxy tryptamine (serotonin) enhanced and extended its effects. Remarkably, ADP and collagen also reduced [H-3]2-Delta (4)AchGro release from 2-Delta (4)Ach-Gro-activated platelets, whereas 5-hydroxytryptamine further increased it. These findings suggest a so far unnoticed interplay between the peripheral endocannabinoid system and physiological platelet agonists.