The central role of the P(2T) receptor in amplification of human platelet activation, aggregation, secretion and procoagulant activity

Adenosine diphosphate (ADP) is an important platelet agonist and ADP released from platelet dense granules amplifies responses to other agonists. There are three known subtypes of ADP receptor on platelets: P2X(1), P2Y(1) and P(2T) receptors. Sustained ADP-induced aggregation requires co-activation of P2Y(1) and P(2T) receptors. AR-C69931MX, a selective P(2T) receptor antagonist and novel antithrombotic agent, was studied to characterize further the function of the P(2T) receptor. The roles of the P2Y(1) receptor and thromboxane A(2) were assessed using the selective P2Y(1) antagonist A2P5P and aspirin respectively. Aggregation was measured by whole blood single-platelet counting and platelet-rich plasma turbidimetry, using hirudin anticoagulation. Dense granule release was estimated using (14)C]-5-hydroxytryptamine (HT)-labelled platelets. Ca(2+) mobilization, P-selectin expression, Annexin V binding and microparticle formation were determined by flow cytometry. P(2T) receptor activation amplified ADP-induced aggregation initiated by the P2Y(1) receptor, as well as amplifying aggregation, secretion and procoagulant responses induced by other agonists, including U46619, thrombin receptor-activating peptide (TRAP) and collagen, independent of thromboxane A(2) synthesis, which played a more peripheral role. P(2T) receptor activation sustained elevated cytosolic Ca(2+) induced by other pathways. These studies indicate that the P(2T) receptor plays a central role in amplifying platelet responses and demonstrate the clinical potential of P(2T) receptor antagonists.