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

Recent ultrastructural studies have suggested that Glycoprotein Ib (GPIb) has a different distribution on external (surface) versus internal (open canalicular system) membranes in resting discoid platelets. The differential distribution proposed for GPIb differs from that reported for the fibrinogen receptor, GPIIb-IIIa, and could have profound physiological significance when platelets are activated by surfaces. The present study explored the distribution 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 antibodies (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 applied to patients with giant platelets syndromes. The OCS of normal resting platelets was also dilated by exposure of platelets to hypertonic conditions or to cytochalasin-B, an agent that prevents assembly of actin, and, reportedly, movement of GPIb. Morphometric analysis revealed that rates of labeling on internal versus external membranes of giant platelets does not differ significantly (0.93 +/- 0.20), provided the OCS is sufficiently dilated. Platelets exposed to cytochalasin B (1.01 +/- 0.31) or to hypertonic conditions (0.96 +/- 0.20) revealed similar ratios for immunogold particles on external and internal membranes. Results of our study indicate that membranes 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.