Ristocetin-mediated ¹²⁵I-VWF binding. (A) Transfected CHO cells are incubated for 30 minutes with increasing concentrations of ¹²⁵I-VWF in the presence of ristocetin 1 mg/mL. Cell-surface GPIbα levels were measured by flow cytometry on a separate aliquot simultaneously with each binding experiment. The Mut cells bound significantly higher amounts of VWF than WT (n = 3, P = .01 and 0.008 at VWF levels of 0.4 and 0.8, respectively). (B-D) CHO cells incubated with increasing concentrations of ¹²⁵I-VWF (0.2-1.6 μg/mL) under ristocetin levels of 0, 0.2, and 0.4 mg/mL. The Mut cells bound significantly higher amounts of VWF in the absence of ristocetin (n = 3, P = .02-.002 at various levels of VWF) and bound significantly more VWF than WT under different low levels of ristocetin. At ristocetin 0.2 and 0.4 mg/mL, P was .04 to .000 004 at various VWF levels. DPM indicates disintegrations per minute.