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More pieces of the platelet activation puzzle slide into place.

Platelets are normally activated at sites of vascular injury by a combination of collagen exposure, local thrombin generation, and the release of ADP from damaged cells and other activated platelets. Drugs targeting ADP receptors have proved to be clinically useful in settings where platelet activation contributes to arterial thrombosis. However, until the past few years, relatively little was known about the mechanisms by which platelets respond to ADP. Indirect evidence suggested that 1 or more G proteins and G protein–coupled receptors were involved, but none of the receptors known to respond to adenine nucleotides appeared to have the necessary pharmacological profile. In vitro, ADP causes platelets to change shape (undergo cytoskeletal reorganization), secrete their storage granules, expose binding sites for fibrinogen on the αIIbβ3 integrin, and aggregate. These events all require an increase in cytosolic Ca2+ concentration and are facilitated by the ability of ADP to inhibit cAMP formation by adenylyl cyclase (1, 2).