The P2Y receptor family are G protein-coupled receptors which mediate the effects of extracellular nucleotides, primarily through the activation of phospholipase C. To some extent, the P2Y receptors can also activate potassium channels or, alternatively, inhibit adenylate cyclase and N-type calcium channels in response to extracellular nucleotides. Human platelets express two G protein-coupled nucleotide receptors, P2Y1 and P2Y12. P2Y12 is a receptor for ADP and ATP coupled to G-proteins that inhibit the adenylyl cyclase second messenger system. P2Y12 is an integral membrane protein involved in platelet aggregation. It is highly expressed in platelets, with lower levels in the brain, lung, appendix, pituitary and adrenal gland.