TSP-1 blocks the inhibitory actions of PGE₁ on platelet aggregation. Platelets (3 × 10⁸/mL) were preincubated with TSP-1 (10 μg/mL) or fibrinogen (1 mg/mL) for 15 minutes, followed by addition of PGE₁ (0.1μM) for 1 minute and then stimulated with collagen (5 μg/mL) for 4 minutes under stirring conditions. (A) Shown are representative aggregation traces. (B) As in panel A, except platelets were incubated with TSP-1 (0.1-10 μg/mL) for 1-15 minutes before addition of PGE₁ (0.1μM). Data are expressed as mean ± SEM from 4 independent experiments performed in duplicate. §P < .05 compared with absence of PGE₁; *P < .05 compared with PGE₁ in the absence of TSP-1. (C) Platelets were incubated with TSP-1 (10 μg/mL) or vehicle for 15 minutes and then stimulated with collagen (0.25-2 μg/mL) and aggregation measured. (i) Representative traces of experiments performed with collagen at either 0.75 or 2 μg/mL. (ii) Collated data of 3 independent experiments expressed as mean ± SEM of aggregation. Black bars indicate the presence of TSP-1; gray bars, absence of TSP-1.