Effect of thrombin stimulation on the functional binding of tPA to VSMCs. Serum-deprived cells were stimulated with thrombin for 18 hours at the concentrations shown under serum-free conditions. The plasminogen activator activity of bound tPA was determined as in Fig 5 under conditions preferential for the detection of the high-affinity site (preincubation with 10 nmol/L tPA). Cells were also assayed in suspension (as in Fig 4) with similar results. Using conditions preferential for detection of the low-affinity site (see legend to Table 3), plasmin generation was essentially unchanged (<5% increase). Data shown represent the mean ± SD (n = 4).