Thrombin plus S1P induces a rapid, synergistic, and enduring TF activity expression on ECs. (A) Confluent monolayers of ECs were incubated with 4.8 nM thrombin (▴) and 2 μM S1P (○) used alone or in combination (•). TF activity on the EC surface was determined at 0, 1, 2, 3, 5, 8, 12, and 24 hours after incubation as described in “Materials and methods.” (B) The monolayers were treated briefly (30 minutes) with 4.8 nM thrombin (▴) and 2 μM S1P (○) used alone or in combination (•). TF activity on the EC surface was determined after 0, 1, 2, 3, 5, 8, 12, and 24 hours of stimulation as described in “Materials and methods.” These data represent the means ± SDs of 4 points per condition. Similar results were observed in 2 independent experiments using ECs of different lots.