Induction of receptor clustering, patching, and capping on surface-activated platelets.
Interaction of selective ligands or specific antibodies with mobile glycoprotein IIb-IIIa receptors on suspended human platelets is associated with reorganization of receptor-ligand complexes. Words used to describe the reorganization have been adapted from similar, antigen-antibody redistribution on lymphocytes, and include descriptive terms such as clustering, patching, capping, and endocytosis. Studies on the interaction of fibrinogen coupled to colloidal gold (Fgn/Au) with glycoprotein IIb-IIIa on surface-activated cells failed to reveal clustering, patching, capping, and endocytosis observed by others on agonist-stimulated or immunoreacted platelets in suspension. The present study sought to induce these phenomena in order to distinguish them from the ligand-receptor interaction and translocation that do develop on surface-activated platelets. Treatment of platelets with cytochalasin B after spreading, but before exposure to Fgn/Au, resulted in clustering of electron-dense probes. Addition of cytochalasin B to fully spread platelets after they had been incubated with Fgn/Au produced patching. Allowing grids of fully spread platelets reacted with Fgn/Au for 5 minutes to rest on drops of Hanks' balanced salt solution for 10 minutes resulted in capping. Platelets exposed to cytochalasin B in suspension before interaction with Formvar grids failed to spread fully or develop pseudopods, but channels of the open canalicular system became filled with Fgn/Au-receptor complexes. The Fgn/Au-filled channels were concentrated in cell centers, and could be confused with endocytosis identified previously by immunofluorescence. Thus, phenomena resembling clustering, patching, capping, and endocytosis can be produced on plasma membranes of surface-activated platelets, but do not appear required for response to agonists or surface activation.
Pediatrics, Laboratory Medicine and Pathology, University of Minnesota, Minneapolis.
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JG
White
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Escolar
D001792:Blood Platelets / Q000166:cytology / Q000187:drug effects; D003102:Colloids; D003571:Cytochalasin B / Q000494:pharmacology; D004705:Endocytosis; D005340:Fibrinogen / Q000378:metabolism / Q000494:pharmacology; D006046:Gold / Q000378:metabolism / Q000494:pharmacology; D006801:Humans; D015539:Platelet Activation; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism*; D011940:Receptor Aggregation / Q000187:drug effects
D003102:Colloids; D010980:Platelet Membrane Glycoproteins; D003571:Cytochalasin B; D006046:Gold; D005340:Fibrinogen