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Figure 2 Concentration-response curves for the inhibition of plate let cyclooxygenase (COX)-1 activity by aspirin (A) and naproxen (B). One-milliliter aliquots of washed platelets (1.5 x 108 cells) were preincubated with increasing concentrations of aspirin (0.01 to 100 µmol/l) or naproxen (0.01 to 100 µmol/l) for 25 min, and then 0.5 or 10 µmol/l of arachidonic acid (AA) was added for an additional 30 min at 37°C. In panel C, the antagonism of aspirin inhibition of platelet COX-1 by naproxen is shown. Increasing concentrations of naproxen (0.01 to 10 µmol/l) were incubated with washed platelets (1.5 x 108 cells/ml) for 5 min before the addition of aspirin (10 or 100 µmol/l) and the incubation continued for additional 20 min at 37°C. After washing twice, platelets were resuspended in 500 µl of Hanks’ balanced salt solution supplemented with 25 mmol/l HEPES, challenged with 10 µmol/l of AA for 30 min at 37°C, and thromboxane (TX)B2 levels were determined by radioimmunoassay. The data represent the average of inhibition of platelet TXB2 production from five different donors. IC50 = concentrations required to inhibit 50% of enzyme activity.
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