Functional analysis of the activity of newly synthesized PAI-1. Platelets were incubated in the presence of 1, 10, and 100 ng tPA for 6 hours as described in Figure 3. (A) Addition of 100 ng tPA resulted in a different migration position corresponding to the expected position of the tPA/PAI-1 complex, indicating that newly synthesized PAI-1 is active. (B) Addition of increasing concentrations of tPA reduced the free PAI-1 protein and increased the tPA/PAI-1 complex.