Receptor for extracellular adenine nucleotides such as ADP (PubMed:8508924). 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). Brain, spinal cord, gastrointestinal tract, spleen and leg muscle. Is not detected in the heart, liver, stomach, lung and kidney. Belongs to the G-protein coupled receptor 1 family.