Prostacyclin biosynthesis in cultured vascular endothelium is limited by deactivation of cyclooxygenase.

Primary monolayer cultures of human umbilical vein endothelium produce prostacyclin (PGI2) in response to stimulation by thrombin, ionophore A23187, arachidonic acid, and the prostaglandin endoperoxide, PGH2. None of these treatments had a significant effect on the capacity of the endothelium to produce PGI2 in response to subsequent stimulation by PGH2. By contrast, endothelium initially exposed to thrombin, A23187, or arachidonic acid produced approximately 37, 68, and 84% less PGI2, respectively, upon subsequent stimulation by arachidonic acid. These findings suggest that PGI2 biosynthesis in cultured endothelium results in deactivation of cyclooxygenase-hydroperoxidase but not PGI2 synthetase. To test the hypothesis that PGI2 biosynthesis alone causes deactivation of cyclooxygenase, thrombin, A23187, and arachidonic acid were added to monolayers that had been preincubated with ibuprofen (250 microM), 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 cyclooxygenase. After an initial exposure to arachidonic acid, PGI2 production in response to a second stimulation by arachidonic acid was restored to approximately 34, 69, and 74% of maximal, after recovery periods of 1, 24, and 48 h, respectively. We conclude that the regulation of PGI2 biosynthesis in normal vascular endothelium may be in part a function of the activity and biosynthesis of cyclooxygenase-hydroperoxidase and the deactivation of this enzyme may be a primary factor limiting the capacity of the endothelium to produce PGI2.