RECOVERY OF ENDOTHELIAL CELL PROSTACYCLIN PRODUCTION AFTER INHIBITION BY LOW-DOSES OF ASPIRIN

Endothelial cells synthesize prostacyclin (PGI2), an unstable prostaglandin that inhibits platelet aggregation and serotonin release. Because cyclooxygenase, which is necessary for synthesis of PGI2, is inactivated by aspirin, we examined the effect of aspirin on PGI2 production by cultured human endothelial cells. Endothelial cells synthesize PGI2 (20.1 ± 7.2 ng/106 cells, mean ± SD) when stimulated with 20 μM sodium arachidonate for 2 min. PGI2 production is inhibited by low-dose aspirin (5 μM); the t( 1/2 ) of inactivation is 6.0 ± 1.3 min (mean ± SEM, n = 3). Thus, endothelial cell cyclooxygenase is as sensitive to aspirin as the enzyme in platelets. After 1 h incubation with aspirin, endothelial cell PGI2 production was inhibited 50% by 2.1 ± 4.0 μM aspirin and was inhibited 90% by 6.2 ± 0.9 μM aspirin (mean ± SEM, n = 4). When endothelial cells were incubated with 100 μM aspirin, washed, and recultured, their ability to synthesize PGI2 returned to control levels in 35.6 ± 1.0 h (mean ± SEM, n = 4). Recovery of endothelial PGI2 production after aspirin depended on de novo protein synthesis because treatment with cycloheximide (3 μg/ml) inhibited recovery by 92%. These results indicate that although endothelial cell cyclooxygenase in vitro is inhibited by low concentrations of aspirin, endothelial cells rapidly resynthesize their cyclooxygenase after the aspirin is removed. This rapid resynthesis of cyclooxygenase lessens the likelihood that aspirin used in clinical doses promotes thrombosis.