HUVECs produce similar quantities of ULVWF strings following stimulation by histamine in millimolar or Stxs in nanomolar concentrations. (A) HUVECs were incubated with histamine, buffer, or 10 nM Stx-1 or Stx-2 prior to perfusion with washed normal human platelets in Ca⁺²/Mg⁺²-free Tyrode buffer. The number of VWF-platelet strings formed in 20 microscope fields at × 400 magnification was quantified after 2 minutes of flow (n = 10-20). (B) In separate experiments, the multimeric patterns of VWF secreted into culture media were compared following HUVEC stimulation for 5 minutes with histamine (His, 20 mM) or Stx-2 or Stx-1 (10 nM). Separation and detection of VWF multimers were as described in Figure 3. The VWF multimeric pattern in normal plasma (NP) is shown for comparison. The vertical line indicates the location of ULVWF forms (n = 10-15).