Changes in TM cofactor activity for protein C activation and TF cofactor activity on U937 cells, monocytes, or HUVECs surfaces after exposure to 1,25(OH)₂D₃. Cells were exposed to 1,25(OH)₂D₃ (0.1 μmol/L) for 24 hours. Cell-surface TM activity was determined for suspended cells as described in Materials and Methods. Basal ▵OD_405nm/min levels were 0.063 ± 0.002/min/5 × 10⁶ U937 cells (61 ± 2 ng activated protein C/10⁶ cells), 0.062 ± 0.005/min/10⁶ monocytes (300 ± 24 ng activated protein C/10⁶ cells), and 0.251 ± 0.027/min/10⁶HUVECs (1,305 ± 235 ng activated protein C/10⁶ cells). Cell-surface TF activity was determined by normal plasma-based one-stage recalcification clotting time and was quantitated by reference to standard curves constructed using human placenta TF as described in Materials and Methods. These assays were repeated independently three times and the results are expressed as the mean ± SD. The difference between TM cofactor activity on the surface of 1,25(OH)₂D₃-treated U937 cells or monocytes and that of untreated cells is statistically significant (P < .05). The difference between TF cofactor activity on the surface of U937 cells which had been 1,25(OH)₂D₃-treated for 7 days and that of untreated cells is also statistically significant (P < .05).