text: 
  TxA2, generated from arachidonic acid by cyclooxygenase 1 (COX-1) and Tx synthase, further amplifies platelet activation. COX-1 converts arachidonic acid into prostaglandin endoperoxides PGG2 and PGH2, the latter being, in turn, transformed by Tx synthase into TxA2, a potent amplifier of platelet aggregation with vasoconstrictive properties. At the site of the atheroma rupture, platelet-released TxA2 leads to downstream micro-vessel contraction and thrombus propagation (7).
  In addition to its role in coagulation, thrombin at extremely low concentrations is one of the major platelet activators (8). Human platelets express two cell surface G-protein-coupled protease-activated receptors (PARs) for thrombin: PAR-1 and PAR-4. By binding the hirudin-like extracellular amino terminal domain (the so-called thrombin receptor), thrombin activates platelets and smooth muscle cells, thus promoting platelet pro-coagulant activity, shape change, secretion and release of agonists (ADP and TxA2), expression of P-selectin, activation of the alphaIIbbeta3 integrin receptor, and aggregation. Thrombin also binds to GpIbalpha on the surface of platelets, thought to act as a co-factor that localizes the enzyme on the platelet surface and accelerates the hydrolysis of PAR-1 (9). PAR-1 and P2Y12 cross-react in platelet activation, and Galphaq and Galphai-coupled receptors are involved in this process. Thrombin-dependent platelet aggregation is mediated in part by secreted ADP, acting on the Galphai-linked ADP receptor. By blocking Galphaq via PAR-1 and Gbetai via P2Y12, combined inhibition of thrombin and P2Y12 receptors leads to a synergistic inhibitory effect on thrombin-induced platelet aggregation (10).
  5-HT is a vasoconstrictor agent that binds to 5HT-2A receptors and amplifies the platelet response by stimulating shape change and enhancing platelet recruitment at sites of injury (3). It may also play a pro-coagulant role by promoting the retention of fibrinogen and thrombospondin on the platelet surface. Intraplatelet 5-HT stores are implicated in shear-induced platelet aggregation and thrombus propagation (3).
  
entities: ['platelet']
entities: Cell
entities: PMC-3231843-sec-03_T1
entities: ['platelet']
entities: Cell
entities: PMC-3231843-sec-03_T2
entities: ['atheroma']
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entities: ['platelet']
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entities: ['micro-vessel']
entities: Tissue
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entities: ['thrombus']
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entities: Cellular_component
entities: PMC-3231843-sec-03_T9
entities: ['platelets']
entities: Cell
entities: PMC-3231843-sec-03_T11
entities: ['smooth muscle cells']
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entities: ['platelet']
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entities: ['Intraplatelet']
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