The effect of p38, MEK inhibitors, and aspirin on thrombin and collagen-induced p-selectin expression. Washed human platelets were resuspended in Tyrode buffer and preincubated with either 20 μM SB203580 (A,C), 10 μM MEK inhibitor, PD98059 (B,D), 1 mM aspirin (E,F), or the appropriate vehicle control for 5 minutes. Platelets were then treated with either 0.5 μg/mL collagen (A,B,F) or 0.025 U/mL thrombin (C-E) at 37°C for 5 minutes and fixed by adding paraformaldehyde. Fixed platelets were incubated with the monoclonal anti–human p-selectin antibody SZ51 at 22°C for 30 minutes and, after washing, were further incubated with a FITC-conjugated goat anti–mouse Ig antibody. Surface expression of the α-granule membrane protein p-selectin was analyzed using flow cytometry. (G) Quantitative results from 3 experiments are expressed as the p-selectin expression index (fluorescence intensity of platelets stimulated with an agonist/fluorescence intensity of unstimulated platelets). Statistical differences between inhibitor-treated samples and controls were examined by Student t test.