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

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Abstract
The endocannabinoid 2‐arachidonoylglycerol (2‐Δ4Ach‐Gro) 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‐Δ4Ach‐Gro by internalization through a high affinity transporter (Km = 300 ± 30 nm, Vmax = 10 ± 1 pmol·min−1·mg protein−1), followed by hydrolysis by a fatty acid amide hydrolase (Km = 8 ± 1 µm, Vmax = 400 ± 50 pmol·min−1·mg protein−1). The anandamide transport inhibitor AM404, and anandamide itself, were ineffective on 2‐Δ4Ach‐Gro uptake by platelets, whereas anandamide competitively inhibited 2‐Δ4Ach‐Gro hydrolysis (inhibition constant = 10 ± 1 µm). Platelet activation by 2‐Δ4Ach‐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‐Δ4Ach‐Gro induced an approximately threefold increase in [3H]2‐Δ4Ach‐Gro release, according to a CB receptor‐dependent mechanism. On the other hand, ADP and collagen counteracted the activation of platelets by 2‐Δ4Ach‐Gro, whereas 5‐hydroxytryptamine (serotonin) enhanced and extended its effects. Remarkably, ADP and collagen also reduced [3H]2‐Δ4Ach‐Gro release from 2‐Δ4Ach‐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.