Constitutive activation and internalization of the integrin αIIbβ3 complex in Abcg8^−/− HS platelets. (A) Elevated constitutively bound fibrinogen on the surface of Abcg8^−/− HS platelets. Platelets in whole blood were double-stained with a PE-anti-mouse αIIb mAb and fluorescein isothiocyanate–anti-fibrinogen antibody. (B) Summary of fluorescence-activated cell sorter analysis of integrin αIIb expression on the surface (left: nonpermeabilized) or inside (right: permeabilized) of platelets from Abcg8^−/− mice fed a high plant sterol diet. Mean fluorescence intensity/forward scatter were compared with those obtained from similarly sized GPIbα-null platelets, the latter of which was normalized to 100% over 5 different experiments. Surface expression of integrin αIIb and integrin β3 was decreased in Abcg8^−/− HS platelets (left), whereas total cellular integrin αIIb and integrin β3 were similar to that of GPIbα-null platelets (right). FSC, forward scatter.