Effect of ATP and ADP on vWF release. Confluent HUVECs grown in 24-well plates were incubated for 30 minutes at 37°C at the indicated concentrations of ATP (A), ADP (B), and thrombin (C) in either the absence (□) or the presence (○) of IBMX (100 μmol/L). vWF release was measured in the supernatant by ELISA. Results are the mean ± SEM of 8 (A) and 5 (B and C) experiments. *P < .05v unstimulated control cells.