The Prostacyclin receptor, also termed the prostaglandin I2 receptor or just IP, is a receptor belonging to the prostaglandin (PG) group of receptors. IP binds to and mediates the biological actions of prostacyclin (also termed Prostaglandin I2, PGI2, or when used as a drug, epoprostenol). IP is encoded in humans by the PTGIR gene. While possessing many functions as defined in animal model studies, the major clinical relevancy of IP is as a powerful vasodilator: stimulators of IP are used to treat severe and even life-threatening diseases involving pathological vasoconstriction. Gene The gene is located on human chromosome 19 at position q13.32 (i.e. 19q13.32), contains 6 exons, and codes for a G protein coupled receptor (GPCR) of the rhodopsin-like receptor family, Subfamily A14 (see rhodopsin-like receptors#Subfamily A14). Expression IP is most highly expressed in brain and thymus and is readily detected in most other tissues. It is found throughout the vascular network on endothelium and smooth muscle cells. Ligands Activating ligands Standard prostanoids have the following relative efficacies as receptor ligands in binding to and activating IP: PGI2>>PGD2=PGE2=PGF2α>TXA2. In typical binding studies, PGI2 has one-half of its maximal binding capacity and cell-stimulating actions at ~1 nanomolar whereas the other prostaglandins are >50-fold to 100-fold weaker than this. However, PGI2 is very unstable, spontaneously converting to a far less active derivative 6-keto-PGF1 alpha within 1 minute of its formation. This instability makes defining the exact affinity of PGI2 for IP difficult. It also makes it important to have stable synthetic analogs of PGI2 for clinical usage. The most potent of these receptor agonists for binding to and activating IP are iloprost, taprostene, and esuberaprost which have Kd values (i.e. concentrations which bind to half of available IP receptors) in the low nanomole/liter range (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=345/). Inhibiting ligands Several synthetic compounds bind to, but do not activate, IP and thereby inhibit its activation by the activating ligands just described. These receptor antagonists include RO1138452, RO3244794, TG6-129, and BAY-73-1449, all of which have Kd values for IP at or beneath low nanomol/liter levels (http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=345/). Mechanism of cell activation IP is classified as a relaxant type of prostenoid receptor based on its ability, upon activation, to relax certain pre-contracted smooth muscle preparations and smooth muscle-containing tissues such as those of pulmonary arteries and veins. When bound to PGI2 or other of its agonists, IP stimulates one or more of three types of G protein complexes,