2,3-dinor-6-keto-prostaglandin F1 alpha is a major urinary prostacyclin metabolite, and is significantly higher in 9 patients with severe atherosclerosis and evidence of platelet activation. Prostacyclin is a potent vasodilator and platelet inhibitor produced by vascular endothelium. Endogenous production of prostacyclin under physiologic conditions is extremely low, far below the capacity of vascular tissue to generate this substance in response to stimulation in vitro. This may reflect a low frequency or intensity of stimulation of prostacyclin production. PGI2 synthase (PGIS), a catalyst of PGI2 formation from prostaglandin H2, is widely distributed and predominantly found in vascular endothelial and smooth muscle cells. PGI2 plays an important cardioprotective role increasingly appreciated in recent years in light of adverse effects of COX-2 inhibitors in clinical trials. This cardioprotection is thought to be mediated, in part, by prostacyclin inhibition of platelet aggregation. Multiple lines of evidence suggest that prostacyclin additionally protects from cardiovascular disease by pleiotropic effects on vascular smooth muscle. PGI2 inhibits proliferation of cultured vascular SMCs by inhibiting cell cycle progression from G1 to S phase. (PMID: 6231483, 7000774, 6231483, 16303599, 16533160, 17073611, 17164138)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.