Receptor for extracellular adenine nucleotides such as ADP (PubMed:8058061). In platelets, binding to ADP leads to mobilization of intracellular calcium ions via activation of phospholipase C, a change in platelet shape, and ultimately platelet aggregation (By similarity). Mainly found in blood, brain, and lung. To a lesser extent in stomach, gut and skeletal muscle. Belongs to the G-protein coupled receptor 1 family.