Platelet transport and surface reactions in mural thrombosis
McIntire, Larry V.
Doctor of Philosophy
Due to the complexity of mural thrombosis, involving blood flow patterns, blood cellular and protein components, and multiple surface reactions, some fundamental mechanisms of the blood clot formation process are still difficult to resolve. This thesis focuses on three key areas that contribute to an understanding of flow effects and platelet-surface interactions in mural thrombosis. First, a mathematical model based on a constitutive equation that accounts for lateral motions of red blood cells (RBCs) by collision and viscosity effects is presented. It predicts the steady-state RBC distributions and blood velocity profiles under different shear rates and hematocrits in a parallel-plate flow chamber. The platelet drift velocity toward the wall due to the plasma counter flow is calculated directly from these RBC distributions, and higher wall concentrations of platelets are predicted at higher shear rates and larger hematocrits, as have been observed experimentally. Second, utilizing a novel flow cytometric analysis, measurements of the platelet activation status after platelets adhere onto the microspheres coated with vWf, insoluble collagen fibrils, or soluble collagen monomers are presented. The platelets adhering on immobilized fibrillar collagen express a higher percentage of activated GpIIb-IIIa receptors and more P-selectin than do those on vWf- and soluble collagen-coated surfaces. This finding explains why platelets adhere separately on vWf-coated surfaces but form large aggregates on fibrillar collagen-coated surfaces in flow chamber experiments. In addition, it indicates that immobilized collagen structures affect the thrombogenicity of surfaces. Further studies using this protocol reveal that Ca2+ released from intracellular pools is involved in signal transduction pathways that lead to the conformation change of GpIIb-IIIa and the surface expression of P-selectin after platelets interact with collagen. Finally, using a combination of viscometric and flow cytometric methods, originally designed to study receptor-ligand binding forces, an unexpected interaction phenomenon was discovered between the platelet receptor GpIb-IX-V complex and immobilized vWf. Platelet adhesion via GpIb-IX-V onto the microspheres with high vWf coating density is enhanced as the shear rate increases. This enhancement is not due to the shear-induced platelet activation, cytoskeletal rearrangement by calpains, or secondary flows in the cone-and-plate viscometer at high shear rates.
Cell biology; Biomedical engineering