Persistence of mobile receptors on surface- and suspension-activated platelets

We examined the distribution of glycoprotein Ib (GPIb) and glycoprotein IIb-IIIa (GPIIb-IIIa) receptors on suspension- and surface-activated platelets before and after exposure to thrombin (1 U/ml, 10 minutes). Frozen thin sections prepared from fixed suspension-activated platelets or grids containing fixed surface-activated platelets were stained with specific antibodies to GPIb (antiglycocalicin) and GPIIb-IIIa (AP2 or 7E3), incubated with the corresponding gold-labeled secondary antibody, and examined in the electron microscope. GPIb and GPIIb-IIIa were evenly distributed on membranes of resting and suspension-activated platelets. GPIb and GPIIb-IIIa were present in the open canalicular system (OCS) of resting and, more prominently, in dilated OCS channels of thrombin suspension-activated platelets. On surface-activated platelets more intense labeling for GPIb was observed along pseudopods of dendritic cells whereas GPIIb-IIIa receptors were slightly increased over the peripheral zone. Morphometric study of labeling on fully spread, surface-activated platelets revealed that the density of GPIb increased significantly after thrombin treatment (60.7 +/- 13.1 vs 40.9 +/- 8.3 gold particles/microns 2, p < 0.05). A flow cytometry assay employing the same antiglycocalicin antibody revealed no down-regulation or clearance of GPIb after exposure of platelets to thrombin. GPIIb-IIIa distribution on spread platelets after exposure to thrombin remained basically unchanged (28.4 +/- 10.5 vs 32.6 +/- 10.9 particles/microns2 in nonactivated platelets). These findings indicate that clearance of GPIIb-IIIa and GPIb on suspension-activated platelets does not take place to the extent suggested in previous studies and does not occur spontaneously or after thrombin activation on surface-activated platelets. Although the presence of mobile receptors on platelets is essential for spreading on immobile surfaces and each other, their clearance to the OCS is not a fundamental mechanism regulating adhesion.