Biogenesis of prostanoids is under the control of some polypeptide growth factors. Cytosolic phospholipase A2, a form specific for arachidonic acid containing phospholipids, is activated by a translocation mechanism regulated by growth factors, while prostaglandin H synthase isoforms are induced de novo in several cell types. No information is available as far as PGI2 synthase is concerned. Human umbilical vein endothelial cells were cultured under conditions favoring proliferation or differentiation or capillary-like network formation in the presence of collagen gels. Basic fibroblast growth factor (bFGF 0.5-4 ng/ml) was used as a mitogen, interleukin-1 alpha (IL-1 alpha 10-60 UI/ml) as a differentiating agent, and prostacyclin (PGI2) biosynthesis was evaluated. Under the first condition, basal PGI2 production was unaffected while, in the presence of IL-1 alpha, a marked stimulation of PGI2 synthesis was observed. It is known that IL-1 alpha is a potent inducer of PGH synthase, while it is not known whether PGI2 synthase is also induced. Two lines of evidence indicate that PGI2 synthase is a constitutively expressed not inducible enzyme: (a) proliferating nonproducing cells when added with PGH2 produce an amount of PGI2 not different from the amount produced by cells stimulated with IL-1 alpha; (b) under this condition PGI2 synthase was immunodetectable either by immunofluorescence detected by confocal microscopy or by ELISA and, on microsomes isolated from endothelial cells, by Western blotting. It is concluded that the limiting step in the conversion arachidonate-PGI2 is represented solely by the level of PGH synthase. These results strongly suggest, but do not prove, the constitutive nature of the enzyme. The final demonstration requires the availability of a probe to detect mRNA level, a trial we are carrying out at the moment.