Porcine von Willebrand disease and atherosclerosis. Influence of polymorphism in apolipoprotein B100 genotype

The relationship of apolipoprotein-B genotype (Lpb) to diet-induced hypercholesterolemia and atherosclerosis was studied in von Willebrand disease (vWD) and normal pigs. Von Willebrand and normal pigs developed comparable levels of hypercholesterolemia (respectively, 757.9 +/- 49.4 versus 772.8 +/- 47.9 mg/dl, P = 0.95). Pigs with Lpb1/5 and Lpb5/8 genotypes, however, developed significantly higher serum cholesterol levels than those with other Lpb genotypes (866.1 +/- 64.0 mg/dl, P = 0.0343). Coronary and aortic atherosclerosis, measured by computer-assisted automated image analyzer, were not significantly different between vWD and normal pigs. Pigs with an Lpb5 allele developed significantly more atherosclerosis than those with the Lpb3/8 or Lpb8/8 genotypes or the rare Lpb1 allele (r greater than or equal to 0.434, P less than or equal to 0.05). Polymorphism in apolipoprotein B100 genotype, then, significantly influenced the severity of diet-induced hypercholesterolemia and atherosclerotic plaque formation in vWD and normal swine without regard to the vWD genotype.

von Willebrand factor and animal models: contributions to gene therapy, thrombotic thrombocytopenic purpura, and coronary artery thrombosis

Use of animal models of von Willebrand factor (vWF) deficiency, both inherited and induced, continues to advance the knowledge of vWF-related diseases. Three examples are reviewed in this article--von Willebrand's disease (vWD), thrombotic thrombocytopenic purpura, and coronary artery thrombosis. The success of gene transfer by liver and bone marrow transplantation in porcine vWD and canine hemophilia A, with a change in phenotype that establishes improved hemostasis, portends imminent testing of gene therapy in these models. With use of recombinant technology, the phenotype of hemophilia B fibroblasts has been transformed to normal, as evidenced by secretion of the normal hemostatically active protein. This result is a prelude to implantation in hemophilic animals. Thrombotic thrombocytopenic purpura is characterized by qualitative and quantitative alterations in vWF. A new animal model induced by the venom factor botrocetin, a cofactor of vWF, closely mimics the human syndrome. A proposed pathophysiologic mechanism for thrombotic thrombocytopenic purpura is outlined. The third contribution is recognition that occlusive coronary thrombosis is a vWF-dependent condition. Without vWF, as in porcine vWD or normal pigs treated with a monoclonal anti-vWF antibody, occlusive thrombi do not develop, even with luminal stenosis. The thrombogenicity of coronary atheromas, including those with fissures of the fibrous cap, is also vWF-dependent.

Role of von Willebrand factor in arterial thrombosis. Studies in normal and von Willebrand disease pigs

With normal and von Willebrand disease (vWD) pigs, we studied the role of von Willebrand factor (vWF) in platelet-vessel wall interactions and occlusive arterial thrombosis. Two methods of arterial injury have been used to determine the thrombotic response of flowing blood in vivo. The first involves balloon catheter injury. After superficial denudation of endothelium from coronary intima, platelets adhere to the subendothelium in a monolayer. Similar numbers of adherent platelets are found in both phenotypes, but platelets in vWD pigs have impaired pseudopod formation and are less well spread morphological indexes of limited platelet activation. Deeper injury, which involves the media, produces nonocclusive platelet-fibrin microthrombi. The second injury method involves pinching the artery at a site of superimposed stenosis, a procedure that almost always exposes media. This procedure induces platelet-fibrin microthrombi in normal and vWD pigs, but only normal pigs develop occlusive thrombosis. Both methods of arterial injury have also been performed in normal and vWD pigs with diet-induced hypercholesterolemia and atherosclerosis. Atherosclerosis promotes platelet spread in vWD pigs but does not abolish the protection from stenosis and injury-induced occlusive thrombosis. In addition, neutralization of vWF activity in normal pigs by a monoclonal antibody prevents the induction of occlusive thrombosis by the stenosis and pinch-injury procedure. This monoclonal antibody also causes performed platelet aggregates to break up. These experimental models of inducing arterial thrombosis have been used in normal and vWD pigs to demonstrate interactions between normal and atherosclerotic vessel wall constituents, circulating platelets and vWF that are fundamental in the process of arterial thrombosis.

Glycoprotein Ib bioassays. Activity levels in Bernard-Soulier syndrome and in stored blood bank platelets

Glycoprotein Ib (GP-Ib) is a major platelet receptor protein concerned with von Willebrand-factor binding, platelet agglutination, and platelet adhesion, and it is required for normal hemostasis. By the use of botrocetin (venom coagglutinin), both quantitative and semiquantitative assays for GP-Ib activity were developed. The latter assay uses limiting dilutions of botrocetin as a measure of GP-Ib activity. Platelets, stored up to 23 days under blood bank conditions, were assayed by the limiting dilution test. Values of GP-Ib were progressively diminished after 9 to 10 days of storage, reaching levels of less than 10% at 23 days. Platelets from a subject with Bernard-Soulier syndrome showed less than 10% GP-Ib activity. These assays appear to be a specific measure of functional GP-Ib activity, and, when combined with GP-Ib antigen measurement by other methods, they provide a means for further characterizing GP-Ib abnormalities.

Coronary atherosclerosis in the pig. Induced plaque injury and platelet response

The thrombogenic potential of atherosclerotic diet-induced coronary atherosclerotic plaques was investigated in normal swine and in bleeder swine with homozygous von Willebrand disease. Fourteen paired normal and bleeder swine were placed on a 1% cholesterol diet for 1 to 16 weeks. Serum cholesterol was elevated in all animals at sacrifice. Foam cell deposits developed in all major epicardial coronary arteries, and lesions progressed over time from small subendothelial foam cell deposits to fibrous cap lesions that contained foam cells, elastic fibers, collagen, degenerative material, and smooth muscle cells. Balloon catheter injury resulted in platelet deposition, largely in a monolayer or as small platelet clumps devoid of fibrin. Lipid debris was present in injured areas of the subendothelium but did not induce thrombus formation. When the injury involved the media, platelet-fibrin thrombi were formed. Lipid was not present in these thrombi. Morphometric analysis of platelet deposition on ballooned atherosclerotic vessels showed similar numbers of platelets in both phenotypes. However, the attached platelets in bleeder pigs showed significantly less spreading than did those in the normal animals. The results show that injury to intimal foam cell and mixed cellular lesions in coronary arteries of cholesterol-fed swine does not promote the development of platelet-fibrin thrombus formation. In contrast, when the injury extended to the media, mixed thrombi were formed.

Phenotypic correction of factor IX deficiency in skin fibroblasts of hemophilic dogs

Primary skin fibroblasts from hemophilic dogs were transduced by recombinant retrovirus (LNCdF9L) containing a canine factor IX cDNA. High levels of biologically active canine factor IX (1.0 micrograms per 10(6) cells per 24 hr) were secreted in the medium. The level of factor IX produced increased substantially if the cells were stimulated by basic fibroblast growth factor during infection. Additionally, we also report that endothelial cells transduced by this virus can produce high levels of biologically active factor IX. We propose that skin fibroblasts and endothelial cells from hemophilia B dogs may serve as potential venues for the development and testing of models for treatment of hemophilia B by retrovirally mediated gene replacement therapy.

von Willebrand factor and occlusive arterial thrombosis. A study in normal and von Willebrand's disease pigs with diet-induced hypercholesterolemia and atherosclerosis

The thrombotic response of atherosclerotic arteries to stenosis and injury was studied in 14 pigs, eight normal and six with von Willebrand's disease (vWD). Atherosclerosis was produced by feeding a 1% to 2% cholesterol diet for 24 weeks. Both groups of pigs developed severe hypercholesterolemia, greater than five times baseline values. Coronary atherosclerosis was detected in all vWD pigs and in all but one normal pig and was not significantly different between groups. At sacrifice under general anesthesia, a Goldblatt clamp (GC) was positioned around the left anterior descending coronary (LAD) and carotid arteries to produce a stenotic segment, which was pinch-injured with needle holders. A 20 MHz Doppler velocity crystal was placed distal to the GC to detect cyclic flow reductions or permanent cessation of flow velocity indicative of occlusive thrombosis. In the phenotypically normal pigs with diet-induced atherosclerosis, occlusive thrombosis was detected in seven of seven LAD and seven of seven carotid arteries. In atherosclerotic vWD pigs, occlusive thrombosis failed to form in six LAD and 10 carotid arteries (p less than 0.003, Wilcoxon rank sum test). Scanning electron micrographs demonstrated platelet-fibrin microthrombi in both groups of pigs; only phenotypically normal pigs had occlusive thrombi. Von Willebrand factor is essential for the development of occlusive thrombosis and appears to support the progression of a mixed microthrombus to an occlusive thrombus.

Canine hemophilia B resulting from a point mutation with unusual consequences

We have used the polymerase chain reaction to amplify the entire coding region of canine factor IX from a hemophilia B animal. When the sequence was compared to that which codes for normal canine factor IX, a single missense mutation was identified. This mutation (G----A at nucleotide 1477) results in the substitution of glutamic acid for glycine-379 in the catalytic domain of the molecule. The mutation creates a new restriction site that allowed confirmation of the abnormal sequence in both hemophilic and carrier animals. Amino acid 379 in canine factor IX corresponds to position 381 in human factor IX, a location at which no human mutations have been described. Moreover, it occurs at one of the few amino acids that have been rigorously conserved among the trypsin-like serine proteases throughout evolution. The mutation responsible for canine hemophilia B results in a complete lack of circulating factor IX in the affected animals. As it is unusual for a missense mutation to result in a complete absence of protein product, structural modeling of the mutant and normal proteins was pursued. These studies suggest that the observed mutation would have major adverse effects on the tertiary structure of the aberrant factor IX molecule. The elucidation of this mutation sheds light on structure-function relationships in factor IX and should facilitate future experiments directed toward gene therapy of this disease.

Role of botrocetin in platelet agglutination: formation of an activated complex of botrocetin and von Willebrand factor

Botrocetin (venom coagglutinin) induces binding of von Willebrand factor (vWF) to platelet glycoprotein Ib (GPIb), resulting in platelet agglutination. A mechanism whereby botrocetin causes vWF to change to an active platelet-agglutinating form is proposed. Incubation of native vWF with botrocetin yielded an increasingly active vWF with slower migration in two-dimensional immunoelectrophoresis but with no apparent change in vWF multimer pattern. The "activated" vWF eluted mainly in the void volume (Vo) (Bio-Gel A-15m column chromatography). Botrocetin eluted in the included volume (Vi). Vo peaks appeared to contain a vWF-botrocetin complex, based on bioassays and immunoassays. 125I-Botrocetin mixed with vWF eluted in two peaks: in the Vo, coincident with active vWF, and in the Vi. With von Willebrand disease (vWD) plasma lacking vWF, 125I-Botrocetin eluted in the Vi only. It did not bind to platelets without vWF. In aggregometric studies, antibodies (Ab) against botrocetin, vWF, and GPIb prevented botrocetin-induced platelet agglutination and caused dissolution of preformed platelet agglutinates. Immunostaining of aggregates with antibotrocetin Ab revealed a positive reaction. Botrocetin appears to act in a two-step manner, first binding with vWF to form a complex, which then binds to GPIb to cause agglutination. All three components, vWF, botrocetin, and GPIb, appear to be required for maintenance of stable platelet agglutinates.

Effect of recombinant factor VIIa on the hemostatic defect in dogs with hemophilia A, hemophilia B, and von Willebrand disease

Recombinant factor VIIa (rF.VIIa) is a two-chain procoagulant enzyme (Mr, approximately 50,000) active only when complexed with tissue factor in the extrinsic clotting system. We administered human rF.VIIa to hemophilic and von Willebrand disease (vWD) dogs to determine its hemostatic effectiveness and survival in the circulation. Hemophilia A dogs lacking factor VIII demonstrated an immediate increase in plasma rF. VIIa and prompt stoppage of hemorrhage at bleeding time (BT) sites. In seven studies in two dogs, the range of dose of rF. VIIa was 50-220 micrograms/kg, with an apparent 7- to 11-fold increase in plasma factor VII and a mean recovery in plasma of 34%. The t1/2 was 2.8 +/- 0.5 hr. The BT was normalized except in an animal given the minimum dose. In four studies in two hemophilia B dogs lacking factor IX, BT was normalized. The elevation in plasma factor VII was by a factor of 8-30, with a mean recovery of rF.VIIa in plasma of 44%. In two studies in a homozygous vWD dog lacking von Willebrand factor, which is needed for platelet function, BT was not corrected even though large doses of rF. VIIa were given. The human rF. VIIa protein was immunogenic for dogs. These studies indicate that factor VIIa corrects the hemostatic defect in dogs with hemophilia A and B, diseases primarily of the intrinsic clotting system, but does not correct the hemostatic defect in vWD.

Thrombotic thrombocytopenia with von Willebrand factor deficiency induced by botrocetin. An animal model

Thrombotic thrombocytopenia with severe von Willebrand factor (vWF) depletion was induced in rats by intraperitoneal injection of botrocetin, a Bothrops factor requiring vWF for platelet aggregation. The vWF deficiency state with thrombocytopenia was produced in 12 animals by single or multiple injections of botrocetin. Single botrocetin injection (450 units/kg) reduced plasma vWF activity to less than 0.01 mu/ml for 24 hours and less than 0.2 units/ml up to 72 hours. During this period, multimeric analysis of plasma vWF:Ag revealed loss of intermediate and high molecular weight forms. A moderate reduction in vWF:Ag (Laurell) and FVIII:C was observed. Bleeding time remained prolonged during the deficiency state. Recovery of plasma vWF and platelet count occurred in 4 to 5 days. Multiple botrocetin injections (450 units/kg/injection) at 48-hour intervals produced a sustained severe vWF depletion (less than 0.2 units/ml) for 1 week. On the 8th day, vWF and platelet count had recovered to preinjection levels. Recovery was associated with the appearance of increased levels of very high molecular weight multimers of vWF:Ag in the plasma. After each botrocetin administration (single or repeat), a transient thrombocytopenia developed which was most severe at 1 to 5 hours postinjection. Another group of 10 animals was given a single botrocetin injection and specimens of lung, liver, kidney, and spleen were obtained at varying time intervals postinjection. Transmission electron micrographs of the lung and spleen 2 hours after botrocetin administration showed platelet aggregates and platelet microthrombi in the lung and spleen. The platelet thrombi in the lung had disappeared within 24 hours, and in the spleen by 48 hours. Liver sinusoids and kidney glomeruli were free of thrombi. A comparison is made of the thrombocytopenia and the ultrastructure of the platelet microthrombi induced by botrocetin with that of other agents.

Prevention of occlusive coronary artery thrombosis by a murine monoclonal antibody to porcine von Willebrand factor

A murine monoclonal antibody (mAb) against porcine von Willebrand factor (vWF) induced an antithrombotic state in normal pigs. Thrombosis was induced by a standard procedure of stenosis and mechanical injury of the artery. The mAb was an IgG1 kappa that inhibited vWF-induced platelet aggregation at a titer of 1:6250 and bound to immobilized vWF at a maximal dilution of 1:512,000. The antibody did not affect two other vWF functions, platelet adhesion and binding of coagulant factor VIII (factor VIII:C). The antithrombotic state was characterized by a prolonged bleeding time and lack of plasma vWF activity, but with near-normal levels of factor VIII:C and von Willebrand antigen. The circulating Ag.mAb complex demonstrated a multimeric distribution comparable to that of native plasma vWF. Three groups of pigs were studied: group A consisted of nine untreated animals, eight of which developed occlusive coronary thrombosis; group B, four treated animals with a long bleeding time, none of which developed occlusive thrombosis; and group C, two animals with preexisting thrombosis treated with mAb, in which stable blood flow was reestablished. Morphologically, the group B animals showed adherent platelets covering the injured intima but no thrombosis. This mAb is an antithrombotic agent that prevents platelet thrombosis without affecting intrinsic platelet function.

Purified human factor VIII procoagulant protein: comparative hemostatic response after infusions into hemophilic and von Willebrand disease dogs

The procoagulant protein F.VIII:C is noncovalently bound to von Willebrand factor (vWF) to give the factor VIII macromolecular complex. New highly purified preparations of isolated human F.VIII:C, devoid of vWF and about 500,000-fold purified, were administered to hemophilia A and von Willebrand disease (vWD) dogs to determine their hemostatic effectiveness and survival in the circulation. Two preparations of F.VIII:C were used: peak 1, with active components of Mr 185,000-280,000, and peak 2, with a single component of Mr 170,000. In hemophilic dogs, with no plasma F.VIII:C but normal vWF, both preparations immediately elevated plasma F.VIII:C to expected levels, promptly stopped induced and spontaneous hemorrhages, and gave sustained plasma levels of F.VIII:C. The isolated F.VIII:C immediately complexed with endogenous vWF in hemophilic plasma and was eliminated exponentially, with a half-life (t1/2) of about 9 hr. Survival of peak 2 F.VIII:C was longer than that of peak 1 material. In contrast, F.VIII:C complexed to vWF in a therapeutic concentrate administered to hemophilic dogs was eliminated biexponentially with first-phase t1/2 of 3.2 hr and second-phase t1/2 of 9 hr. In vWD dogs with no vWF and reduced F.VIII:C levels, the isolated F.VIII:C produced supernormal levels of F.VIII:C without effect on induced bleeding. It was rapidly eliminated from plasma with a t1/2 of about 1 hr, as was the complexed F.VIII:C in the concentrate. These data indicate that isolated F.VIII:C promptly complexes with vWF and in this form is highly effective in controlling hemophilic hemorrhages with good survival in plasma. Without endogenous vWF with which to complex, the F.VIII:C is promptly eliminated.

Determinants of von Willebrand factor activity elicited by ristocetin and botrocetin: studies on a human von Willebrand factor-binding antibody

The three main probes for functional vWF activity--ristocetin, botrocetin, and the PAggF test--and similarities and differences in their elicited vWF activities have been reviewed. Emphasis has been placed on the technologies dependent on these probes, with a brief description of a series of relatively simple and sensitive tests developed in this laboratory. These tests include the development of the PAF test for vWF in certain animal plasmas; the development and use of fixed lyophilized platelets that retain receptor activity for vWF; the purification of botrocetin (venom coagglutinin) freed of thrombinlike enzymes and its use in vWF assays; the development of macroscopic platelet aggregation tests for screening and assay of vWF; and the application of the macroscopic test for rapid screening and quantitation of human plasmas for acquired inhibitors of vWF utilizing each of the three probes. Historically, the similarities of the ristocetin and botrocetin probes were first observed. For normal human plasmas and for patients with classic vWD, both homozygous or heterozygous, similar values for vWF were obtained with these two probes. Similar platelet binding of vWF in the presence of the two probes was likewise noted. However, further studies of these two probes revealed striking differences. Especially important for study of animal plasmas generally as well as a canine model of vWD was the observation that the vWF in all animal plasmas tested with botrocetin was highly reactive, whereas with ristocetin nearly all plasmas were resistant. Similarly, all animal platelets tested for vWF-dependent aggregation with the two probes were highly reactive with botrocetin, but inactive with ristocetin.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of inhibitory and binding characteristics of an antibody causing acquired von Willebrand syndrome: an assay for von Willebrand factor binding by antibody

An acquired inhibitor of von Willebrand factor (vWF) activity occurring in a patient with benign gammopathy and von Willebrand syndrome (vWS) has been partially characterized. The inhibitor-induced syndrome resulted in low to undetectable plasma levels of vWF/ristocetin, vWF/botrocetin, FVIIIR:Ag, and FVIII:C with a normal to slightly prolonged bleeding time. Platelet vWF was normal. Intensive and continuous infusion of a heat-treated factor VIII concentrate (Hemofil-T, Hyland, Glendale, Calif) elevated the FVIII:C plasma levels to about 100%, with an increase in FVIIIR:Ag levels to about 340% and vWF/ristocetin levels to about 40%, much lower than expected based on the dose of Hemofil-T and its content of vWF and FVIII:C activities. The inhibitor bound to staphylococcal protein A (SpA) with high affinity, indicating an IgG antibody (Ab). An assay for the vWF-binding capacity was developed on the basis of absorption of the Ab from serially diluted plasma by SpA and removal of vWF and FVIII:C activities from normal plasma by the SpA-Ab complex. The Ab-binding site was on the vWF component of the factor VIII complex. The Ab was unable to bind isolated FVIII:C. The combined use of the new vWF-binding assay and a battery of tests for inhibition of vWF-dependent platelet aggregation with ristocetin (which detects high molecular weight vWF), with botrocetin (which detects high and low molecular weight vWF), and with platelet-aggregating factor (which detects high molecular weight vWF) provided a means of analysis of Ab effect on in vitro vWF function. Using these tests, a comparison was made of the effects of the vWS Ab with those of an Ab inhibitor occurring in homozygous von Willebrand's disease. The Ab of the vWS patient had weak inhibitory action on vWF/ristocetin without having an effect on vWF/botrocetin and platelet-aggregating factor, a high titer vWF-binding capacity, and no anamnestic response following concentrate therapy. These findings contrasted with those of the Ab occurring in inhibitor von Willebrand's disease in which vWF inhibitor and binding values were similar, with a strong anamnestic response. The findings indicate that the vWS Ab binds to an epitope on the molecular vWF in such a way that causes only limited inhibition of vWF/ristocetin function and no inhibition of vWF/botrocetin function, suggesting that these two functional domains are at separate sites.

Morphological study of early phases of platelet adhesion to foreign surfaces: effect of calcium

The role of calcium in the interfacial reactions occurring during the early phases of blood contact with glass surfaces has been studied using a rheologic technique that permits enumeration of adherent platelets as well as ultrastructural visualization of the blood-glass interface by both transmission and scanning electron microscopy . In the presence of calcium, platelets adhere to the glass surface at a rapid rate. Within 4 minutes, most of the adherent platelets have lost their normal discoid shape and show pseudopod formation and cytoplasmic spreading. These platelets conform with the glass surface and show central apposition of platelet organelles similar to that seen in activated platelets. Fibrin strands and platelet aggregates are also seen. Chelation of calcium using sodium citrate or EDTA results in decreased platelet adhesion or retention to glass. Platelets that are adherent to glass in the presence of citrate and EDTA retain their discoid shape, although an occasional platelet may show pseudopod formation and centralization of organelles. Calcium appears to be an important factor governing the adhesion of platelets to glass and appears to exert this effect on the platelets themselves in their plasma milieu and not on the preceding adsorption of plasma proteins.

Botrocetin (venom coagglutinin): reaction with a broad spectrum of multimeric forms of factor VIII macromolecular complex

Botrocetin, originally called venom coagglutinin, is a Bothrops factor that causes aggregation of blood platelets in the presence of the von Willebrand component of the factor VIII macromolecular complex. The complex consists of a series of multimers with a molecular weight of about 1-20 x 10(6). Ristocetin, another agent that causes platelet aggregation dependent on von Willebrand factor, reacts with only the higher molecular weight multimers. We report on the reactivity of botrocetin in relation to the multimeric structure of the factor VIII complex. Several plasmas or plasma fractions with abnormal distribution of the multimeric sizes were examined, including variant von Willebrand disease type IIA with lack of the higher molecular weight forms, commercial antihemophilic factor concentrates with a preponderance of lower molecular weight forms, cryoprecipitate-free plasma containing mainly the smaller multimers, and a chromatographic fraction of plasma containing only the highest molecular weight polymers. Factor VIII-related antigen content was adjusted to 25-100%. All of the preparations lacking the high molecular weight forms caused prompt platelet aggregation with botrocetin, but none of them caused aggregation in the ristocetin test made isochronal with the botrocetin test. The very high molecular weight polymers were equally effective with botrocetin and ristocetin. These findings indicate that the Bothrops factor is reactive with a broad spectrum of high to low molecular weight forms of the factor VIII complex, suggesting that bioassays of von Willebrand factor with botrocetin should correlate better with immunoassays for factor VIII-related antigen and could reflect better the full platelet-aggregating function of the complex than do ristocetin determinations.

Venom coagglutinin for detection of von Willebrand factor activity in animal plasmas

Platelet-aggregating vWF activity in several animal species is difficult to detect with either the ristocetin co-factor test or the PAF test using human platelets, indicating considerable species restriction in applicability of these procedures. VCA, a recently described second "activator" of vWF, was found not to be restrictive as to species and readily elicited the vWF response in all 13 animal plasmas tested. For any one of the animal plasmas, the vWF-dependent platelet aggregation reaction can be demonstrated by using VCA with either homologous or heterologous species of platelets. VCA was employed with lyophilized platelets in two types of platelet aggregating test systems, a macroscopic procedure and an aggregometric method. Many of the animal plasmas (dog, rabbit, horse, cow, cat, sheep, and goat) had about three to five times the plasma vWF content of human plasma. Pig and rat plasmas had vWF levels approximating those of human plasma. Guinea pig plasma had about half as much vWF as human plasma. VCA also caused a well-defined vWF-dependent platelet response in PRPs. The data suggest considerable differences in the rate of aggregation of platelets from different species. Bioassay of vWF can be performed with the different animal plasmas if the source of platelets and the VCA concentration are controlled. VCA thus appears to provide a broad-spectrum probe for vWF and is effective in the various species combinations of plasma and platelets.