title: Effect of aspirin on platelet-von Willebrand factor surface expression on thrombin and ADP-stimulated platelets.
abstract: Platelets contain a pool of endogenous platelet-von Willebrand factor (vWF) that becomes expressed on the platelet surface when platelets are stimulated by a variety of agonists. Maximal platelet-vWF expression occurs in concert with platelet alpha-granule secretion. Aspirin (ASA) is known to impair platelet activation and alpha-granule secretion by irreversible inhibition of platelet cyclo-oxygenase. We studied native and ASA-treated platelets for their ability to mobilize and to express platelet-vWF in response to adenosine diphosphate (ADP) or thrombin. We found that each agonist was effective in promoting increased platelet-vWF surface expression on native and ASA-treated platelets. ASA-treated platelets responded identically to native platelets to low (0.01 U/mL) and high (1.0 U/mL) concentrations of thrombin, while the ADP-induced increase in ASA-treated platelets was only 50% to 60% of that for control platelets. Measurement of secreted platelet-vWF and beta-thromboglobulin indicated that the increase seen with ADP was largely independent of alpha-granule secretion. Using monoclonal antibodies (MoAbs) against the platelet glycoproteins (GP) IIb/IIIa and Ib (MoAbs 10E5 and 6D1, respectively), we demonstrated that the ADP-induced increase in platelet-vWF expression on control platelets primarily involved the binding of secreted platelet-vWF to the platelet GPIIb/IIIa. In contrast, the increase in platelet-vWF that occurred following ADP stimulation of ASA-treated platelets was largely insensitive to GPIIb/IIIa blockade. No effect of GPIb blockade in platelet-vWf expression was noted for either control or ASA-treated platelets. When platelet shape change was prevented by the addition of cytochalasin D, ADP-induced platelet-vWf surface expression on ASA-treated platelets was reduced by more than 80%. Our data indicate that platelets in which the cyclooxygenase pathway is blocked by the action of aspirin can increase surface expression of platelet-vWf as a consequence of platelet shape change. We speculate that this process exposes platelet-vWf bound to GPIIb/IIIa, or possibly GPIb, within the surface connected canalicular system.
authors.affiliation: Clinical Pathology Department, National Institutes of Health, Bethesda, MD 20892.
authors.forename: R I
authors.initials: RI
authors.lastname: Parker
authors.forename: H R
authors.initials: HR
authors.lastname: Gralnick
mesh_terms: D000244:Adenosine Diphosphate / Q000494:pharmacology*; D001241:Aspirin / Q000494:pharmacology*; D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D015638:Cytochalasin D / Q000494:pharmacology; D003594:Cytoplasmic Granules / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D015539:Platelet Activation / Q000187:drug effects*; D010974:Platelet Aggregation / Q000187:drug effects; D010980:Platelet Membrane Glycoproteins / Q000378:metabolism; D013917:Thrombin / Q000494:pharmacology*; D001620:beta-Thromboglobulin / Q000378:metabolism; D014841:von Willebrand Factor / Q000378:metabolism*
chemical_list: D010980:Platelet Membrane Glycoproteins; D001620:beta-Thromboglobulin; D014841:von Willebrand Factor; D015638:Cytochalasin D; D000244:Adenosine Diphosphate; D013917:Thrombin; D001241:Aspirin