Receptor for ADP. Coupled to G(i)-proteins. May play a role in hematopoiesis and the immune system. Strong expression in spleen and adult brain. Lower expression in placenta, lung, liver, spinal cord, thymus, small intestine, uterus, stomach, testis, fetal brain, and adrenal gland. Not detected in pancreas, heart, kidney, skeletal muscle, ovary or fetal aorta. Clearly detected in lymph node and bone marrow, weakly detected in peripheral blood mononuclear cells (PBMC) and in peripheral blood leukocytes (PBL), but not detected in polymorphonuclear cells (PMN). In the brain, detected in all brain regions examined. Stimulation by ADP in stably transfected CHO cells resulted in inhibition of adenylyl cyclase and the phosphorylation of the MAP kinases MAPK3 and MAPK1 in a pertussis toxin-sensitive way. Inhibition of adenylyl cyclase and phosphorylation of the MAP kinases are transduction mechanisms that involve G(i) proteins.