Effects of p38 inhibitors and aspirin on VWF-induced integrin-dependent platelet aggregation. (A,B) PRP was preincubated at 37°C for 5 minutes in the presence of p38 inhibitors SB203580 (A), SB202190 (B), or 0.5% DMSO (vehicle for SB203580 and SB202190). The VWF modulator ristocetin (1.25 mg/mL) was then added to induce VWF-GPIb-IX interaction. Ristocetin-induced platelet aggregation/agglutination was recorded using a platelet turbidometric aggregometor. (C) Washed human platelets (3 × 10⁸/mL) were preincubated with various concentrations of aspirin or buffer (control) for 5 minutes. Arachidonic acid (50 μM) was added to induce platelet aggregation. (D) PRP was incubated without (control) or with aspirin (1 mM) at 37°C for 5 minutes in the presence of 0.5% dimethyl sulfoxide (DMSO). PRP also was incubated with aspirin in the presence of SB202190 (20 μM) or RGDS (1 mM). Ristocetin was then added to PRP to induce platelet aggregation/agglutination. (E) Washed human platelets (3 × 10⁸/mL) were preincubated with aspirin (1 mM), SB203580 (20 μM), or SB202190 (20 μM) for 5 minutes. As a control, platelets also were preincubated with DMSO. A low dose of α-thrombin (0.05 U/mL) was added to induce platelet aggregation.