Endogenous TSP1 limits NO/cGMP signaling in murine platelets. Equal numbers of murine C57BL/6 WT and TSP1-null platelets were incubated in Tyrode buffer in the presence of 10 μM DEA/NO for 5 minutes, and cGMP was determined by immunoassay (A). In other experiments WT and TSP1-null platelets were preincubated with exogenous TSP1 (2.2 nM) for 15 minutes and then treated with NO (10 μM DEA/NO) (B) or treated with l-arginine at the indicated doses for 20 minutes and cGMP levels determined via immunoassay (C). Results are the mean (± SD) of at least 3 experiments.