ADP induced blood platelet activation: a review

Blood platelet aggregation by ADP plays a major role in the development and extension of arterial thrombosis. ADP is an original physiological agent in that, taking part as a substrate, product or allosteric effector in a large number of intracellular metabolic pathways, it also behaves as an agonist of blood platelet aggregation as do other agents including thrombin, platelet activating factor and collagen, but with probably different, yet unknown signal transduction pathways and also with unknown receptors. We discuss, in this review, how ADP induces the characteristic functional responses of platelets, namely shape change, induction and exposure of the fibrinogen binding sites on the GP IIb-IIIa complex, fibrinogen binding to its receptor and platelet aggregation, and the intracellular biochemical events underlying these functions. The present concepts of ADP receptor(s) are presented with their discrepancies. Platelet diseases related to ADP and pharmacological inhibition of ADP induced platelet aggregation are also discussed. Finally, the idea is put forward that the ADP receptor might belong to the family of G protein coupled receptors.