The P2Y1 receptor mediates ADP‐induced p38 kinase‐activating factor generation in human platelets

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Abstract
U46619, a thromboxane A2 mimetic, but not ADP, caused activation of p38 mitogen activated protein (MAP) kinase in aspirin‐treated platelets. In nonaspirinated human platelets ADP activated p38 MAP kinase in both a time‐and concentration‐dependent manner, suggesting that ADP‐induced p38 MAP kinase activation requires generation of thromboxane A2. However, neither a thromboxane A2/prostaglandin H2 receptor antagonist SQ29548 and a thromboxane synthase inhibitor, furegrelate, either alone or together, nor indomethacin blocked ADP‐induced p38 kinase activation in nonaspirinated platelets. Other cycloxygenase products, PGE2, PGD2, and PGF2α, failed to activate p38 kinase in aspirin‐treated platelets. Hence, ADP must be generating an agonist, other than thromboxane A2, via an aspirin‐sensitive pathway, which is capable of activating p38 kinase. AR‐C66096, a P2TAC (platelet ADP receptor coupled to inhibition of adenylate cyclase) antagonist, did not inhibit ADP‐induced p38 MAP kinase activation. The P2X receptor selective agonist, α,β‐methylene ATP, failed to activate p38 MAP kinase. On the other hand, the P2Y1 receptor selective antagonist, adenosine‐2′‐phosphate‐5′‐phosphate inhibited ADP‐induced p38 kinase activation in a concentration‐dependent manner, indicating that the P2Y1 receptor alone mediates ADP‐induced generation of the p38 kinase‐activating factor. These results demonstrate that ADP causes the generation of a factor in human platelets, which can activate p38 kinase, and that this response is mediated by the P2Y1 receptor. Neither the P2TAC receptor nor the P2X1 receptor has any significant role in this response.