Surface membrane clearing of receptor ligand complexes in human blood platelets

Human blood platelets were challenged sequen-tially in vitro with polyclonal anti-platelet anti-bodies and cationized ferritin. Both ligands bound to the surface membrane and were seques-tered by internalization into a surface-connected membrane system (SCS) with a cleared surface membrane as the eventual result. Patching and capping of bound antibody preceded internaliz-ation, and platelets cooperated in the clearing process by adhering to each other at capped areas and by mutual covering up, a process dubbed platelet hugging. The internalization process was repeated upon challenge 'with the second ligand, the two ligands being sequestered as separate deposits in the SCS, mirroring the two cycles of internalization. Platelets were activated during the internalization process and tended to aggre-gate. In aggregates, surfaces exposed to the me-dium were cleared of ligand, which accumulated in the intercellular spaces and within the SCS of the aggregated platelets. Aggregation but not in-ternalization and hugging was prevented by ade-nosine and adenosine monophosphate. Key words: blood platelets, membrane dynamics, antibodies, sequestration, spreading