Under normal conditions, small disk-shape platelets circulate in the blood freely and without interaction with one another. ADP is stored in dense bodies inside blood platelets and is released upon platelet activation. ADP interacts with a family of ADP receptors found on platelets (P2Y1, P2Y12, and P2X1), which leads to platelet activation.
* P2Y1 receptors initiate platelet aggregation and shape change as a result of interactions with ADP.
* P2Y12 receptors further amplify the response to ADP and draw forth the completion of aggregation.
ADP in the blood is converted to adenosine by the action of ecto-ADPases, inhibiting further platelet activation via adenosine receptors.