The P2 family of membrane-bound receptors is activated by naturally occurring nucleotides. When activated, these widely distributed receptors elicit cellular signaling which mediate multiple tissue-specific effects throughout the body. P2X receptors are ligand-gated ionotropic channels, while P2Y receptors belong to the superfamily of metabotropic G protein-coupled receptors. To date, there are six human cloned and functionally characterized subtypes of the P2Y family: P2Y1, P2Y2, P2Y4, P2Y6, P2Y11 and P2Y12. There exists enough agonist specificity between receptor subtypes to allow functional identification of newly found putative receptors. There are few specific antagonists of the P2Y receptors.