INFLUENCE OF COMBINED THROMBIN STIMULATION, SURFACE ACTIVATION, AND RECEPTOR OCCUPANCY ON ORGANIZATION OF GPIB IX RECEPTORS ON HUMAN PLATELETS/

Down-regulation and clearance of as many as 60-80% of GPIb/IX receptor s from exposed surfaces on thrombin-activated platelets to channels of the open canalicular system (OCS) is considered to be a fundamental m echanism regulating platelet adhesivity in vitro and in vivo. The pres ent study has combined thrombin stimulation in suspension, surface act ivation on formvar grids, receptor occupancy by von Willebrand factor (vMF) and exposure to anti-vWF antibody in an effort to demonstrate th e removal of GPIb/IX receptors from activated cells. Individually the stimuli failed to cause any change in the frequency of GPIb/ TX recept ors. Combined, the stimuli were no more effective than when each was u sed alone. The only way to cause GPIb/IX to move was to add anti-vWF t o thrombin-activated platelets allowed to spread on formvar grids and covered with multimers of ristocetin-activated human or bovine VWF. Tr anslocation of GPIb/IX-vWF-anti-vWF complexes from peripheral margins into caps over cell centres, however, did not clear the peripheral zon e of vWF binding capacity. Exposure of capped platelets after fixation to a second incubation with vWF demonstrated as many multimers extend ing from the central cap to the peripheral margins as were seen on pla telets exposed a single time to VWF. Antibodies to GPIb, but not to GP IIb/IIIA, prevented the second labelling by vWF. Down-regulation or cl earance of GPIb/IX, in light of this study, does not appear to be a fu ndamental mechanism modulating platelet adhesivity.