Receptor for extracellular adenine nucleotides such as ADP (PubMed:9442040, PubMed:9038354, PubMed:25822790). 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 (PubMed:9442040). ATP functions as antagonist and inhibits ADP-induced mobilization of Ca(2+) (PubMed:9038354). The P2Y1 receptor-specific antagonists A3P5PS, A3P5P and A2P5P inhibit downstream signaling mediated by mobilization of Ca(2+) from intracellular stores, and platelet shape changes in response to extracellular ADP (PubMed:9442040). Belongs to the G-protein coupled receptor 1 family.