Effect of calcium on the availability of platelet von Willebrand factor.
The binding of a polyclonal immune-purified rabbit anti-human von Willebrand factor (VWF) antibody to activated and nonactivated normal human platelets was studied in the presence and absence of extracellular calcium. Extracellular Ca++ in the incubation buffer results in no increase in the amount of antibody bound to the surface of unstimulated platelets when compared with that bound in the absence of Ca++. Alpha thrombin stimulation of the platelets increases antibody binding to the platelets in both the presence and absence of extracellular Ca++; however, the increase is approximately four times as great in the presence of extracellular Ca++. The amount of VWF detected in the incubation supernatants of unstimulated platelets is similar in either the presence or absence of Ca++, and this amount is increased without demonstrating a Ca++ dependency after thrombin stimulation. Measurement of thrombin-induced beta thromboglobulin (beta TG) release reveals some parallel between anti-VWF antibody binding and beta TG release. However, the amount of beta TG release after thrombin stimulation is independent of extracellular Ca++, whereas the amount of bound anti-VWF is markedly increased in the presence of Ca++. These studies demonstrate that thrombin induces an increase in the surface exposure of platelet VWF even in the absence of extracellular Ca++. The data also suggest that there may be a quantity of platelet VWF that can become exposed on the platelet surface by an action of thrombin that is independent of the Ca++-dependent granule release reaction induced by thrombin.
Parker
R I
RI
Rick
M E
ME
Gralnick
H R
HR
D000818:Animals; D000906:Antibodies; D001779:Blood Coagulation Factors / Q000032:analysis*; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology*; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism; D007163:Immunosorbent Techniques; D066298:In Vitro Techniques; D011817:Rabbits; D013917:Thrombin / Q000494:pharmacology; D013997:Time Factors; D001620:beta-Thromboglobulin / Q000378:metabolism; D014841:von Willebrand Factor / Q000032:analysis* / Q000276:immunology
D000906:Antibodies; D001779:Blood Coagulation Factors; D007074:Immunoglobulin G; D001620:beta-Thromboglobulin; D014841:von Willebrand Factor; D013917:Thrombin; D002118:Calcium