Schematic overview of the contribution of the three P2 purinergic receptor subtypes, P2Y1, P2Y12, and P2X1 in platelet activation. P2Y12 receptor plays a central role in ADP-mediated platelet activation. Platelet exposure to primary platelet agonists including thrombin, collagen and Von Willebrand factor (vWF) results in their adhesion followed by the release of ATP and ADP from dense granules. Extracellular ATP is rapidly metabolized into ADP, AMP and finally to adenosine by the ecto-enzymes CD39 and CD73. Secreted ATP and ADP, as well as ADP deriving from ATP degradation, activate P2Y1, P2Y12, and P2X1 receptors (as depicted). P2Y12 receptor stimulation modulates the growth and stability of thrombus by potentiating platelet dense granule release, platelet aggregation and procoagulant activity (Kunapuli et al., 2003).