BACKGROUND The storage of platelet concentrates (PCs) induces a reduction in the platelet surface expression of glycoprotein (GP) Ib alpha. The location of the platelets' high-affinity binding site for thrombin has been postulated as being located on GPIb alpha. This study attempts to determine whether loss or alteration of GPIb alpha during storage of PCs is related to impairment in the reactivity of platelets to thrombin. METHODS In this study, platelet surface expression of GPIb alpha was monitored by means of flow cytometry, throughout standard storage of PCs for up to 10 days. Two thrombin-induced platelet responses, the binding of radiolabeled fibrinogen and the platelet surface expression of P-selectin, were evaluated. Thrombin-binding assays were also performed to assess the number of thrombin receptors in platelets. RESULTS The surface expression of the GPIb/IX complex declines during storage of PCs. The thrombin-induced maximal binding of fibrinogen in platelets stored for 3, 7, and 10 days was 77 +/- 7 percent, 60 +/- 20 percent, and 34 +/- 25 percent, respectively, of that found in fresh platelets. Moreover, the concentration of thrombin needed for 50 percent of platelets to express the CD62 antigen P-selectin at the surface increased from 0.05 U per mL in fresh platelets to 0.11, 0.56, and 1.2 U per mL in platelets stored for 3, 7, and 10 days, respectively. Thrombin-binding experiments demonstrated a significant reduction in the number of high-affinity binding sites throughout storage of PCs (55 +/- 21 sites/platelet in 10-day-stored platelets vs. 73 +/- 25 in fresh platelets). A significant correlation was also observed between the number of high-affinity thrombin-binding sites and surface expression of GPIb alpha. Selective blockage of the thrombin-binding site on GPIb alpha with monoclonal antibody LJ-Ib10 also inhibited the response of fresh platelets to thrombin, up to a level equivalent to that found in 3-day-stored platelets. CONCLUSIONS The loss of the GPIb alpha-located high-affinity thrombin-binding site may impair the ability of platelets to become activated by thrombin as storage time increases.