Effect of RO318220 on thrombin-stimulated platelet secretion and aggregation. Aspirin-treated, washed, and gel-filtered platelets treated with vehicle or inhibitor were incubated in a lumiaggregometer with calcium, firefly luciferase, with or without fibrinogen (100 μg/mL) before activation with thrombin (0.2 U/mL). Light transmission and ATP production were recorded for 5 to 10 minutes and analyzed for (A) ATP secretion, (B) extent of aggregation (% total), (C) initial rate of aggregation (mm/min), and (D) delay to aggregate formation (minute). ▪ indicates no fibrinogen added; and ▦, fibrinogen present at 100 μg/mL. Means and standard errors were derived from 3 to 7 experiments.