Regulation of platelet adhesion by NO is blocked by TSP1. Fresh, washed human platelets were added to 35 × 10-mm plastic dishes precoated with either type I collagen (3 μg/mL) or fibrinogen (5 μg/mL) (A) and incubated in Tyrode buffer and the indicated treatment agents for 1 hour at 37°C. Wells were washed and platelets fixed, stained, and counted. Human platelets were incubated on collagen-coated plates in the presence of DEA/NO (10 μM) plus or minus ODQ (10 μM) (B) or collagen and fibrinogen coated plates plus or minus exogenous TSP1 (2.2 nM) (C), and adhesion was determined. Results are the mean (± SD) of at least 3 experiments.