Glycoprotein Ib is homogeneously distributed on external and internal membranes of resting platelets

Recent ultrastructural studies have suggested that Glycoprotein Pb (GPIb) h as a different distribution on external (surface) vel sus internal (open ca nalicular system) membranes in resting discoid platelets. The differential distribution proposed for GPIb differs from that reported for the fibrinoge n receptor, GPIIb-IIIa, and could have profound physiological significance when platelets are activated by surfaces. The present study explored the di stribution of GPIb, on external and internal membranes of resting platelets . Immunogold cytochemical techniques were applied to ultrathin cryosections of washed platelets. Polyclonal antibodies or mixtures of monoclonal antib odies (AP1 and 6D1) were used for labeling. To avoid the technical problem posed by limited accessibility of antigens located in very narrow portions of the open canalicular system (OCS) to antibodies, the same methods were a pplied to patients with giant platelets syndromes. The OCS of normal restin g platelets was also dilated by exposure of platelets to hypertonic conditi ons of to cytochalasin-B, an agent that prevents assembly of actin, and, re portedly, movement of GPIb. Morphometric analysis revealed that rates of la beling on internal versus external membranes of giant platelets does not di ffer significantly (0.93 +/- 0.20), provided the OCS is sufficiently dilate d. Platelets exposed to cytochalasin B (1.01 +/- 0.31) or to hypertonic con ditions (0.96 +/- 0.20) revealed similar ratios for immunogold particles on external and internal membranes. Results of our study indicate that membra nes of the exposed surface and lining OCS channels of resting platelets are continuous, identical structures and GPIb is homogeneously distributed on external and internal membranes.