Endogenous TSP1 is necessary for platelet aggregation in the presence of NO. Equal numbers of murine C57BL/6 WT and TSP1-null platelets were incubated under standard high shear (A,B) or static aggregation (C) conditions. Aggregation profiles were determined in the presence of a titrated dose of thrombin (0.1 U/mL added at time points indicated by arrows, A) or a fixed thrombin dose (0.2 U/mL, B,C) plus or minus DEA/NO (10 μM). Alternatively, equal numbers of WT and TSP1-null platelets were treated with a fixed dose of thrombin (0.2 U/mL) and 8-Br-cGMP (10 μM) under high shear conditions and aggregation was determined (D). Data presented are representative of at least 3 experiments.