Platelet adhesion to fluid and solid phospholipid membranes.

We have studied platelet adhesion to phospholipid model membranes in vitro. Our results showed that films made of egg lecithin, dioleoyllecithin or phosphatidylethanolamine from two different sources (egg yolk and E. coli) are unadhesive for platelets. Platelets adhered to films made of distearoyllecithin, dipalmitoyllecithin and N--stearoylsphingomyelin. According to electron spin resonance measurements, the former lipids were present during incubation with platelet-rich plasma above the phase transition temperature, whereas the latter were present below this temperature. Cross-linking of phosphatidylethanolamine films with glutaraldehyde or egg lecithin, as well as dioleoyllecithin with OsO4, abolishes the phase transition of the lipids in these films, transforming them to the solid state. After such treatment the films become adhesive for platelets. Thus fluid liquid crystalline phospholipid membranes are unadhesive for platelets; solid crystalline (gel) films are adhesive for these cells. We suggest that the fluidity of the plasma membrane has an essential role in making the endothelium unadhesive for platelets in vivo.