Prostaglandin I2 or prostacyclin (or PGI2) is a member of the family of lipid molecules known as eicosanoids. It is produced in endothelial cells from prostaglandin H2 (PGH2) by the action of the enzyme prostacyclin synthase. It is a powerful vasodilator and inhibits platelet aggregation. Prostaglandin I2 is the main prostaglandin synthesized by the blood vessel wall. This suggests that it may play an important role in limiting platelet-mediated thrombosis. In particular, prostacyclin (PGI2) chiefly prevents formation of the platelet plug involved in primary hemostasis (a part of blood clot formation). The sodium salt (known as epoprostenol) has been used to treat primary pulmonary hypertension. Prostacyclin (PGI2) is released by healthy endothelial cells and performs its function through a paracrine signaling cascade that involves G protein-coupled receptors on nearby platelets and endothelial cells. The platelet Gs protein-coupled receptor (prostacyclin receptor) is activated when it binds to PGI2. This activation, in turn, signals adenylyl cyclase to produce cAMP. cAMP goes on to inhibit any undue platelet activation (in order to promote circulation) and also counteracts any increase in cytosolic calcium levels which would result from thromboxane A2 (TXA2) binding (leading to platelet activation and subsequent coagulation). PGI2 also binds to endothelial prostacyclin receptors and in the same manner raise cAMP levels in the cytosol. This cAMP then goes on to activate protein kinase A (PKA). PKA then continues the cascade by inhibiting myosin light-chain kinase which leads to smooth muscle relaxation and vasodilation. Notably, PGI2 and TXA2 work as antagonists. PGI2 is stable in basic buffers (pH=8), but it is rapidly hydrolyzed to 6-keto PGF1alpha in neutral or acidic solutions. The half-life is short both in vivo and in vitro, ranging from 30 seconds to a few minutes. PGI2 is administered by continuous infusion in humans for the treatment of idiopathic pulmonary hypertension.Prostaglandins are eicosanoids. The eicosanoids consist of the prostaglandins (PGs), thromboxanes (TXs), leukotrienes (LTs), and lipoxins (LXs). The PGs and TXs are collectively identified as prostanoids. Prostaglandins were originally shown to be synthesized in the prostate gland, thromboxanes from platelets (thrombocytes), and leukotrienes from leukocytes, hence the derivation of their names. All mammalian cells except erythrocytes synthesize eicosanoids. These molecules are extremely potent, able to cause profound physiological effects at very dilute concentrations. All eicosanoids function locally at the site of synthesis, through receptor-mediated G-protein linked signalling pathways.