VWF-mediated Erk phosphorylation in platelets. Platelets were stimulated with VWF/ristocetin (8 and 300 μg/mL, respectively) for various amounts of time (A) or for 1 minute in the absence (vehicle) or presence of the Src kinase inhibitor PP2 or its inactive analog, PP3 (B). Both figures were analyzed to detect either phospho-Erk (top panels) or total Erk (bottom panels), as described in “Materials and methods.” Arrows denote the 2 molecular forms of Erk in platelets.