Fibrinogen transfusion recovered platelet surface P-selectin in Fg^−/− mice. (A) Plasma Fg was determined by Western blot 30 minutes (30′), 6 hours (6h), 1 day (1d), 2 days (2d), and 4 days (4d) after transfusion. Thirty minutes after transfusion, significant plasma Fg levels were observed compared with the pretransfused plasma (time 0). Subsequently, the plasma Fg decreased gradually throughout the 4-day observation period (left panel). Platelet Fg was assayed before transfusion (0), as well as 2 and 4 days after transfusion. Significant levels of Fg were detected in Fg^−/− platelets by day 2. Vinculin was used as a loading control (right panel). The immunoblots in each panel were developed from the same PVDF membrane and the experiments were repeated at least 3 times. (B) P-selectin expression on the surface of Fg^−/− platelets before transfusion (shaded), as well as 1, 2, and 4 days after transfusion (arrow indicated), was analyzed via flow cytometry after incubation with either thrombin (left panel) or TRAP (right panel). Plasma Fg transfusion completely reversed the defect in P-selectin expression on Fg^−/− platelets. (C) P-selectin expression on thrombin- or TRAP-activated platelets was plotted to show the time course. A significant increase in cell surface P-selectin expression on Fg^−/− platelets was observed 1, 2, and 4 days after transfusion (n = 6, **P < .001), and demonstrated a tendency to increase with time following transfusion. Dashed line indicates cell surface P-selectin expression on wild-type platelets.