Stxs inhibit ADAMTS13 activity in PRP. (A) HUVECs were stimulated with histamine (20 mM), followed by perfusion of normal platelet-rich-plasma (Nl PRP; containing approximately 100% ADAMTS13 activity). The PRP was mixed with an equal volume of either Tyrode buffer, BSA (1 mg/mL), or Stx-1 or Stx-2 (10 nM), or EDTA (1 mM). The number of ULVWF-platelet strings that remained uncleaved by plasma ADAMTS13 after 2 minutes of perfusion was quantified in 20 fields (original magnification × 200; n = 4-18). (B) Histamine-stimulated HUVECs were perfused with either PRP alone or PRP mixed with an equal volume of buffer, Stx-1, or Stx-2. The number of ULVWF-platelet strings that remained uncleaved by the ADAMTS13 in normal PRP was greater in the presence of Stx-1 or Stx-2 (10 nM) than in the presence of buffer after 2, 3, and 4 minutes of perfusion (n = 4-7).