Von Willebrand factor (vWF) is a large multimeric protein found in plasma, intracellular stores of platelets and endothelial cells, as well as in the extracellular matrix. VWF has been implicated in venous and arterial thrombosis. Its importance in the primary adhesion of platelets at sites of vascular injury, and for platelet aggregation at very high shear rates was clearly demonstrated by other investigators. We have investigated the role of vWF in the recruitment of platelets activated with low concentrations of agonist, such as may be found with partial ADP secretion or thrombin generation in vivo, at physiologic shear rates (G) ranging from 100--2000 s--1. / In the presence of ristocetin, soluble vWF bound to the glycoprotein Ib receptor (GPIb), and mediated shear-associated aggregation independently of the glycoprotein IIb-IIIa receptor (GPIIb-IIIa), with very few vWF monomer equivalents required to achieve high capture efficiencies (alphaG; reflecting initial rates of aggregation). Activation of washed platelets in the absence of soluble ligands, with low concentrations of the physiologic agonists, ADP or thrombin, resulted in good aggregation. Participation by GPIb was shearrate dependent, with the extent of contribution further varying with activation conditions. Inhibition of vWF-GPIb interactions in ADP and thrombin-induced aggregation, caused 25 and 50% inhibition of alpha G at G = 300 and 1000 s--1 respectively. alpha G's were similar for both agonists, and showed an absolute dependence on activated GPIIb-IIIa in each case. Further investigations using thrombin versus ADP as activator, however, revealed differences in participation by other surface-expressed ligands, in particular Fg. Thus, antibodies against TSP and Fg inhibited aggregation differentially, depending on shear rate and agonist activating conditions. Removal of catalytically-active thrombin from its receptors by antagonists hirudin and PPACK at ≥1 minute following activation, allowed normal secretion to occur from alpha-granules (monitored using P-selectin). However, the thrombin antagonists significantly decreased both platelet aggregation and surface-expression of vWF and TSP for platelets activated at low (0.05 U/ml), but not intermediate (0.2 U/ml) thrombin concentrations. In conclusion, all our studies demonstrated a consistently important cross-bridging role for vWF, but multi-ligand, multi-receptor participation was required for optimal shear-associated aggregation of platelet suspensions activated at very low agonist concentrations.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.36023 |
| Date | January 1999 |
| Creators | Kasirer-Friede, Ana. |
| Contributors | Frojmovic, Mony M. (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Division of Experimental Medicine.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001681347, proquestno: NQ55344, Theses scanned by UMI/ProQuest. |
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