Identification of platelet receptors for bovine von Willebrand factor on human platelets by a new platelet receptor test.

A new test for the quantitation of platelet receptor activity for bovine von Willebrand factor in human platelet extracts is described. The test is based on the competition for the von Willebrand factor occurring between platelets, an antibody against von Willebrand factor, and platelet extracts containing the von Willebrand receptor. Bovine von Willebrand factor aggregates human platelets directly. In the presence of EDTA, anti-bovine von Willebrand factor antibody disaggregates platelet aggregates induced by bovine von Willebrand factor. Disaggregation is measured by following the decrease in light transmission accompanying disaggregation. Platelet extracts inhibited the disaggregating activity of the antibody, and the percent inhibition was directly proportional to the amount of the platelet extract added. The decrease in platelet disaggregating activity of the antibody was used as an indicator of platelet receptor activity in a system consisting of platelet-rich plasma, bovine von Willebrand factor, antibody and platelet extract. With this method we found von Willebrand factor receptors in the whole platelet extract, the Triton X-100 platelet membrane extract and in platelet extract material which bound to a bovine Willebrand factor-Sepharose 6MB affinity column.