Signaling pathways that activate platelets intraarterially. Platelets possess different receptors that are specific for physiological stimuli such as collagen, ADP, TxA2, thrombin, or vWF. Receptors for ADP are the purinergic P2Y1 and P2Y12 receptors, Collagen binds a GPIa/IIa integrin-receptor and activates platelets through the GPVI receptor. Thrombin activates via protease-activated receptors (PAR), of which 4 isotypes have been described. It also binds GPIb/V/IX, which is equally a receptor for vWF. TxA2 activates through the TP-receptor, a prostanoid-receptor (not shown are epinephrine and serotonin, which may also activate platelets in vivo). Following receptor mediated platelet activation the platelet GPIIb/IIIa integrin receptor is activated and secreted from platelet granules, which ultimately mediates platelet-platelet binding through fibrinogen and thus is indispensable for aggregation. Other sequele of platelet activation include activation of platelet cyclooxygenase-1 (COX) which induces formation and release of auto-activating TxA2.
Abbreviations: ADP, adenosine diphosphate; TxA2, thromboxane A2; vWF, von-Willebrandt Factor.