Receptor for extracellular adenine nucleotides such as ADP (PubMed:7779087). 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). Expressed in muscle, heart, liver, kidney, lung, brain, spleen, but not in testis. Belongs to the G-protein coupled receptor 1 family.