Thrombin enhanced adhesion of platelets to von Willebrand factor substrates.

When exposed to thrombin, the adhesion of platelets to a von Willebrand factor (vWf) substrate, relative to a control substrate, is selectively increased. Adhesion to a vWf substrate is dependent upon the concentration of vWf, the duration of the adhesion assay, the concentration of thrombin, and the presence of divalent cations. The enhanced adhesion results from an action of thrombin on the platelets; no effect on the vWf substrate is involved. Once adherent to the substrate, the platelets undergo a profound change in morphology from the spiny sphere phenotype characteristic of activated platelets to a flattened and highly spread state. The adhesion of activated platelets to solid phase vWf is not inhibited by physiological concentrations of fibrinogen.

Inhibition of platelet adhesion to fibronectin, fibrinogen, and von Willebrand factor substrates by a synthetic tetrapeptide derived from the cell-binding domain of fibronectin.

The role in platelet function of the cell-binding region of fibronectin was explored by the use of synthetic peptides. The prototypical peptide gly-arg-gly-asp-ser was capable of inhibiting thrombin-induced platelet aggregation without altering the degree of platelet activation as judged by the secretion of 14C-serotonin. The peptide also effectively inhibited, in a concentration-dependent manner, the binding of radiolabeled fibronectin to platelets and the adhesion of platelets to fibronectin substrates. The smallest peptide from the cell-binding region of fibronectin which retained full activity was arg-gly-asp-ser. Transposition of amino acids or conservative substitutions of amino acids within this short sequence resulted in inactive peptides. Peptides containing the arg-gly-asp-ser sequence were also capable of inhibiting the adhesion of platelets to fibrinogen and von Willebrand factor substrates. Examination of the entire panel of synthetic peptides for ability to inhibit adhesion to fibrinogen or von Willebrand factor substrates revealed the same structure-function relationships that had been determined in the studies with fibronectin.

Mechanism of inhibition by trinitrophenylation of the ristocetin cofactor activity of von Willebrand factor.

In the presence of ristocetin, von Willebrand factor is capable of agglutinating washed platelets. Modification of only a small percentage of amino groups of von Willebrand factor with trinitrobenzenesulfonic acid markedly inhibits this platelet agglutinating activity. 90% of the platelet agglutinating activity is lost after modification of only 10% of the von Willebrand factor amino groups. Since only the higher molecular weight forms of the heterogeneous von Willebrand factor polymers possess this platelet agglutinating activity, it was important to demonstrate that trinitrophenylation did not alter the degree of von Willebrand factor polymerization. This was accomplished by agarose gel electrophoresis. Subsequent direct binding and competitive binding studies demonstrated that trinitrophenylation markedly impairs the ability of von Willebrand factor to bind to the platelet surface. Thus the loss of platelet agglutinating activity upon modification of only a small fraction of the amino groups of von Willebrand factor is attributable to impaired binding of the modified von Willebrand factor to the platelet surface.

Adsorption of von Willebrand factor by fibrillar collagen--implications concerning the adhesion of platelets to collagen.

Von Willebrand factor is adsorbed from plasma by fibrillar collagen in a manner which is dependent upon the time of incubation and collagen concentration. The adsorption does not require divalent cations and is temperature independent. As purified von Willebrand factor is also adsorbed by fibrillar collagen it is unlikely that the adsorption is mediated by other plasma proteins. Denatured collagen has no effect on von Willebrand factor activity and does not inherit the adsorption of the factor by native fibrillar collagen. The adsorbed von Willebrand factor can be eluted from fibrillar collagen with 1M NaCl. The similarities between the adhesion of platelets to collagen and the adsorption of von Willebrand factor by collagen suggest that von Willebrand factor may have a role in collagen-platelet adhesion. The observed inhibition of platelet adhesion to collagen by antiserum to von Willebrand factor is consistent with this hypothesis.

An improved method for evaluation of blood coagulation in heparinized blood.

A simple method designed to allow coagulation studies in heparinized blood was evaluated. Citrated plasma containing heparin was incubated with an anion-exchange resin commercially available in tablet form (Heparsorb). After centrifugation of the mixture, coagulation studies were carried out on the supernatant plasma. One heparin neutralizer (HN) tablet was capable of removing as much as 43 units of heparin from 1 ml plasma within 10 min of incubation. When normal plasma was exposed to the heparin neutralizer, little or no change in the activities of clotting factors XII, XI, VIII, VII, X, V, and II was observed. However, the resin caused a substantial (44%) loss of factor IX-activity from normal plasma. This loss of factor IX-activity was not observed with plasma from patients receiving coumadin therapy. The results of coagulation tests (aPTT, PT, TT) and determination of fibrinogen/fibrin degradation products performed with HN-treated plasmas from nine patients receiving heparin therapy for thromboembolic disease and seven patients undergoing cardiopulmonary bypass operations were virtually identical to those obtained for the same persons before heparin was administered. The method, which is suitable for the routine clinical laboratory, may be useful in the hemostatic evaluation of critically ill patients who experience bleeding complications while receiving heparin therapy, in the laboratory control of coumadin therapy during the heparin-coumadin overlap period, and in the rapid identification of heparin contamination of blood specimens as a cause of an unexplained aPTT prolongation.

Acquired von Willebrand's disease. Evidence for a quantitative and qualitative factor VIII disorder.

To define further the factor VIII abnormality in acquired von Willebrand's disease, we performed immunoelectrophoresis of factor VIII antigen, as well as quantitative measurements of the antigen, factor VIII procoagulant activity and von Willebrand factor activity on plasma from an affected 57-year-old man who also had a poorly differentiated lymphocytic lymphoma. No evidence for an inhibitor against factor VIII procoagulant activity or von Willebrand factor activity was detected, but immunoelectrophoresis showed none of the less anodic forms of factor VIII antigen. There were concomitant decreases in total antigen (0.19 U per milliliter) and von Willebrand factor levels (0.12 U per milliliter). Factor VIII-procoagulant activity was borderline low (0.45 U per milliliter). Correction of both the abnormal immunoelectrophoresis pattern and the quantitative abnormalities followed radiotherapy of the lymphoma. The factor VIII abnormalities might have resulted from binding or destruction of theless anodic forms of factor VIII antigen by the malignant lymphocytes.

Rapid loss of factor XII and XI activity in ellagic acid-activated normal plasma: role of plasma inhibitors and implications for automated activated partial thromboplastin time recording.

Rapid prolongation of the aPTT of normal plasma upon incubation with ellagic acid containing aPTT reagents was observed. The aPTT prolongation was not due to time-dependent changes in pH in the incubation mixture or loss of activity of the labile coagulation factors VIII and V but occurred as a result of rapid progressive inactivation of ellagic acid-activated factors XII and XI. Prolongation of the aPTT and loss of contact factor activities was not observed in plasma incubated with particulate activator reagents. This finding seemed to indicate that adsorption of factors XII and XI to larger particles during the activation process might protect these factors from inactivation by naturally occurring plasma inhibitors. Evidence is presented which supports previous findings that C1-INH, alpha1-AT, and antithrombin (in the presence of heparin) contribute to factor XIIa and XI a inactivation in ellagic acid-activated plasma and that plasma albumin may compete with factor XII for ellagic acid binding. The data indicate that ellagic acid-containing aPTT reagents have unfavorable properties which seriously limit their usefulness in the clinical laboratory, particularly in respect to recording of the aPTT with certain fully automated clot timers.