Receptor for extracellular UTP > ADP = 2-methylthio-ATP > ADP-beta-S > ATP = ATP-gamma-S. The activity of this receptor is mediated by G proteins which activate a phosphatidylinositol-calcium second messenger system. Functionally coupled to phospholipase C. Abundantly expressed in various tissues including lung, stomach, intestine, spleen, mesentery, heart, and, most prominently, aorta. Belongs to the G-protein coupled receptor 1 family.