Plasma membrane GPIIb/IIIa. Evidence for a cycling receptor pool.

The author used immunofluorescence and digital image processing to investigate the dynamic distribution of GPIIb/IIIa in living platelets. Resting cells were incubated with AP-2, a complex-specific, monoclonal, anti-GPIIb/IIIa antibody. Examination of intact cells demonstrated a rim pattern for GPIIb/IIIa consistent with a surface localization. Permeabilization revealed a time-dependent increase in the labeling of apparent intracellular vacuoles. This pattern is distinct from the "patch-cap" pattern observed when unfixed platelets were incubated with fluoresceinated concanavalin A. Additionally, labeling of this vacuolar pool of GPIIb/IIIa was inhibited by treatment with 2% sodium azide or by incubation at 4 degrees C. Identical staining patterns were obtained with Fab fragments of AP-2. Ultrastructural examination confirmed the presence of labeled intracellular vacuolar structures. Parallel studies performed with AP-1, a monoclonal anti-GPIb antibody, failed to demonstrate internalization of GPIb. Finally, thrombin stimulation of resting platelets, which had been preincubated with AP-2, resulted in the clearing of this newly internalized pool of GPIIb/IIIa; presumably via translocation to the surface. These data suggest the presence of an actively cycling pool of GPIIb/IIIa that has not been described previously. The dynamic distribution of this pool may be important in the regulation of platelet adhesiveness.