Stx-1 and Stx-2 inhibit ULVWF-platelet string cleavage by rhADAMTS13. Washed normal human platelets in Ca⁺²/Mg⁺²-free Tyrode buffer were perfused continuously over HUVECs (A-B) or GMVECs (C-D) that had been stimulated for 10 minutes with histamine (20 mM; A,C), Stx-1 (10 nM; B) or Stx-2 (10 nM; D). ULVWF-platelet strings formed during the initial 2 minutes were photographed in a single field in the next 2 minutes. Washed platelets in buffer containing rhADAMTS13 (1:10 dilution; no additional histamine or Stx) were then perfused over the endothelial cells, and the ULVWF-platelet strings were photographed repeatedly in the same microscopic field (original magnification × 400) at the time intervals shown. A vertical line indicates the addition of rhADAMTS13. Arrows indicate ULVWF-platelet strings that were cleaved by rhADAMTS13 in about 2 minutes during the perfusion of endothelial cells that had been stimulated previously by 20 mM histamine (A,C), but persisted for more than 3 to 5 minutes (> 10 minutes in many experiments) during the perfusion of endothelial cells that had been stimulated previously by 10 nM Stx-1 (B) or Stx-2 (D). Each of these photos is representative of 3 to 20 experiments.