Analysis of release rate of PAI-1 from activated platelets. To study the release rate of PAI-1, platelets were incubated for 0, 3, 8 and 24 hours and then pelleted and lysed. The PAI-1 concentration in medium and lysate was determined by ELISA. During basal conditions 67% of total PAI-1 was released in 24 hours. As expected, thrombin stimulated the release of PAI-1, and after 3 hours 85% of the total PAI-1 pool was found in the medium (mean ± SEM).