Prostacyclin Biosynthesis in Cultured Vascular Endothelium is Limited by

umbilical vein endothelium produce prostacyclin (PGI2) in response to stimulation by thrombin, ion-ophore A23187, arachidonic acid, and the prostaglan-din endoperoxide, PGH2. None of these treatments had a significant effect on the capacity of the endo-thelium to produce PGI2 in response to subsequent stimulation by PGH2. By contrast, endothelium ini-tially exposed to thrombin, A23187, or arachidonic acid produced-37, 68, and 84 % less PGI2, respec-tively, upon subsequent stimulation by arachidonic acid. These findings suggest that PGI2 biosynthesis in cultured endothelium results in deactivation of cy-clooxygenase-hydroperoxidase but not PGI2 synthe-tase. To test the hypothesis that PGI2 biosynthesis alone causes deactivation of cyclooxygenase, thrombin, A23187, and arachidonic acid were added to mono-layers that had been preincubated with ibuprofen (250 AM), a rapidly reversible, competitive inhibitor of this enzyme. After removal of the ibuprofen and the initial stimulus, PGI2 production in response to subsequent stimulation by arachidonic acid was maximal. These findings suggest that the metabolism of arachidonic acid itself causes a direct deactivation of cyclooxy-genase. After an initial exposure to arachidonic acid, PGI2 production in response to a second stimulation by arachidonic acid was restored to-34, 69, and 74% of maximal, after recovery periods of 1, 24, and 48 h