Anandamide (arachidonoylethanolamide, AnNH) is shown to activate human platelets, a process which was not inhibited by acetylsalicylic acid (aspirin). Unlike AnNH, hydroperoxides generated thereof by lipoxygenase activity, and the congener (13-hydroxy)linoleoylethanolamide, were unable to activate platelets, though they counteracted AnNH-mediated stimulation. On the other hand, palmitoylethanolamide neither activated human platelets nor blocked the AnNH effects. AnNH inactivation by human platelets was afforded by a high-affinity transporter, which was activated by nitric oxide-donors up to 225% of the control. The internalized AnNH could thus be hydrolyzed by a fatty acid amide hydrolase (FAAH), characterized here for the first time.z 1999 Federation of European Biochemical Societies.

The endocannabinoid 2-arachidonoylglycerol (2-D 4 AchGro) activates human platelets in platelet-rich plasma at physiological concentrations. The activation was inhibited by selective antagonists of CB 1 and CB 2 cannabinoid receptors, but not by acetylsalicylic acid. Human platelets can metabolize 2-D 4 Ach-Gro by internalization through a high affinity transporter (K m 300^30 nm, V max 10^1 pmol´min 21´m g protein 21 ), followed by hydrolysis by a fatty acid amide hydrolase (K m 8^1 mm, V max 400^50 pmol´min 21´m g protein 21 ). The anandamide transport inhibitor AM404, and anandamide itself, were ineffective on 2-D 4 Ach-Gro uptake by platelets, whereas anandamide competitively inhibited 2-D 4 Ach-Gro hydrolysis (inhibition constant 10^1 mm). Platelet activation by 2-D 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-D 4 Ach-Gro induced an approximately threefold increase in [ 3 H]2-D 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-D 4 Ach-Gro, whereas 5-hydroxytryptamine (serotonin) enhanced and extended its effects. Remarkably, ADP and collagen also reduced [ 3 H]2-D 4 AchGro release from 2-D 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.

The aggregation of platelets induced by soluble and particulate stimuli is enhanced by the addition of minute amounts of H2O2. Externally added catalase strongly inhibits the aggregation induced by particulate stimuli and by phorbol myristate acetate (PMA). The addition of aminotriazole to stimulated platelets causes a significant inhibition of intracellular catalase. This indicates the formation of H2O2 inside the platelets during activation. No effects were observed when the platelets were stimulated by the ionophore A23187.

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