Apolipoprotein B100/Low-Density Lipoprotein Regulates Proteolysis and Functions of von Willebrand Factor under Arterial Shear

BACKGROUND

 Proteolytic cleavage of von Willebrand factor (VWF) by a plasma a disintegrin and metalloproteinase with a thrombospondin type 1 motifs, member 13 (ADAMTS13) is regulated by shear stress and binding of coagulation factor VIII, platelets or platelet glycoprotein 1b, and ristocetin to VWF.

OBJECTIVE

 Current study aims to identify novel VWF binding partners that may modulate VWF functions under physiological conditions.

METHODS

 A deoxyribonucleic acid aptamer-based affinity purification of VWF, followed by tandem mass spectrometry, functional, and binding assays was employed.

RESULTS

 Apolipoprotein B100/low-density lipoprotein (apoB100/LDL) was identified as a novel VWF-binding partner. Purified apoB100/LDL was able to accelerate the proteolytic cleavage of VWF by ADAMTS13 under shear in a concentration-dependent manner. This rate-enhancing activity was dramatically reduced when apoB100/LDL was oxidized. More interestingly, the oxidized apoB100/LDL appeared to compete with native apoB100/LDL for its enhancing activity on VWF proteolysis under shear. As a control, a purified apoA1/high-density lipoprotein (apoA1/HDL) or apoB48 exhibited a minimal or no activity enhancing VWF proteolysis by ADAMTS13 under the same conditions. Both VWF and ADAMTS13 were able to bind native or oxidized apoB100/LDL with high affinities. No binding interaction was detected between VWF (or ADAMTS13) and apoA1/HDL (or apoB48). Moreover, apoB100/LDL but not its oxidized products inhibited the adhesion of platelets to ultra large VWF released from endothelial cells under flow. Finally, significantly reduced ratios of high to low molecular weight of VWF multimers with increased levels of plasma VWF antigen were detected in LDLR-/- mice fed with high cholesterol diet.

CONCLUSION

 These results indicate that apoB100/LDL may be a novel physiological regulator for ADAMTS13-VWF functions.

Associations of plasma von Willebrand factor ristocetin cofactor activity and 5-hydroxyindole acetic acid concentrations with blood flow in lower-leg arteries in Japanese type 2 diabetic patients with normal ankle-brachial index

AIMS

To evaluate the associations of circulating levels of proinflammatory molecules and endothelial factors with blood flow in lower-leg arteries in diabetic patients with normal ankle-brachial index (ABI>0.9).

METHODS

We enrolled 123 type 2 diabetic patients with normal ABI and 30 age-matched nondiabetic subjects consecutively admitted to our hospital. Flow volume and resistive index, an index of peripheral vascular resistance, at the popliteal artery were evaluated using gated two-dimensional cine-mode phase-contrast magnetic resonance imaging. An automatic device was used to measure ABI and brachial-ankle pulse-wave velocity (baPWV) for evaluation of arterial stiffness. Plasma soluble intercellular adhesion molecule-1 (sICAM-1) and monocyte chemoattractant protein-1 (MCP-1) concentrations, serum high-sensitivity C-reactive protein (hsCRP) levels, plasma von Willebrand factor ristocetin cofactor activity (VWF), and plasma vasoconstrictor serotonin metabolite 5-hydroxyindole acetic acid (5-HIAA) concentrations were measured.

RESULTS

Diabetic patients had higher baPWV (P<.0001), resistive index (P<.0001), sICAM-1 (P<.0001), MCP-1 (P=.0224), log hsCRP (P<.0001), VWF (P<.0001), 5-HIAA (P=.0015), and lower blood flow (P<.0001) than nondiabetic subjects. VWF (P=.0019) or 5-HIAA (P=.0011), but not sICAM-1, MCP-1, and log hsCRP, was negatively correlated with blood flow in diabetic patients. A multivariate analysis revealed that the significant independent determinants of blood flow were hypertension, use of renin-angiotensin system inhibitors, VWF and 5-HIAA (r(2)=0.198, P<.0001) in diabetic patients.

CONCLUSIONS

Plasma VWF and 5-HIAA concentrations are associated with blood flow and are involved in the pathogenesis of impaired peripheral circulation due to higher arterial stiffness and greater vascular resistance in lower-leg arteries in diabetic patients with normal ABI.

Improved performance characteristics of the von Willebrand factor ristocetin cofactor activity assay using a novel automated assay protocol

BACKGROUND, OBJECTIVES AND METHODS: An accurate, sensitive and precise assay for reliable determination of the ristocetin cofactor activity of von Willebrand factor (VWF:RCo) in plasma and von Willebrand Factor (VWF)-containing concentrates has been evaluated. The assay is based on a commercially available automated protocol with modifications including a combination of adding additional ristocetin and the use of two calibration curves for the high and low measuring ranges.

RESULTS

 Addition of extra ristocetin resulted in improved measurement of VWF recoveries from various VWF-containing concentrates that were underestimated using the standard automated protocol. The modifications resulted in improved assay performance over an extended measuring range (2.00-0.03 IUmL(-1) ). Accuracy was tested using VWF deficiency plasma spiked with the 1st international standard (IS) for VWF concentrate. Seven dilutions, ranging from 1.80 to 0.05IUmL(-1) , were analyzed and resulted in measured concentrations between 80% and 100% of the assigned potency of the standard. Linearity was determined from the regression plot of the same concentrate dilutions and resulted in a correlation coefficient of 0.998. The repeatability, expressed as coefficient of variation, was 2% in the normal range (0.90IUmL(-1) ) and 8% at the level of 0.05IUmL(-1) . The corresponding reproducibility results were 2% and 15% at the normal and low measuring ranges, respectively.

CONCLUSIONS

 Analysis of patients with von Willebrand disease (VWD) indicates that the modified automated BCS(®) protocol has a superior discrimination power compared with the standard protocol. This is especially true in samples with low VWF, as in patients with type 3 VWD.

Platelet aggregation and physiological anticoagulants in sickle-cell disease

During the period January 2002-December 2004, we assessed 30 sickle-cell anaemia patients admitted to hospital in Al Khobar with vaso-occlusive crisis for levels of antithrombin (AT) III, protein C (PC) and protein S (PS). We also did platelet aggregation studies. Steady state levels were assessed during follow-up and compared with 36 adult controls. Levels of PC, PS and AT III in the control group were significantly higher than in those in vaso-occlusive crisis and those in steady state (P < 0.001). There was a statistically significant difference between controls and patients for all platelet aggregation factors except adrenaline. There was no significant difference between the levels of PC, PS, AT III and platelet aggregation variables in patients in the steady state and in vaso-occlusive crisis.

Maladie de Willebrand de type 2B et pseudomaladie de Willebrand de type 2B ; à propos de trois observations

Increased affinity of Von Willebrand factor (VWF) for its platelet receptor GPIb-GPIX complex is responsible of an hemorrhagic disease, which is the Von Willebrand disease (VWD) type 2B when the molecular abnormality is located on the VWF, and the platelet-type 2B VWD when the mutation concern the platelet receptor. Haemostatic abnormalities in these bleeding disorders are similar; prolonged bleeding time, fluctuating thrombocytopenia, decreased factor VIII-VWF complex, and an increased response to low dose of ristocetin in platelets rich plasma. High molecular weight VWF multimers are decreased. We report here 2 cases of type 2B VWD and 1 case of platelet type 2B VWD. The distinction between these 2 diseases was established by studying platelet aggregation with weak doses of ristocetin in mixtures of washed platelets (of normal control or patient)+poor platelets plasma (normal or patient). In one case, VWD 2B was discovered late in a 49 years old man, and the factor VIIIC-VWF complex was not diminished. The distinction between these two congenital diseases is important for the treatment of bleeding manifestations which need VWF concentrates infusions in type 2B VWD and administration of platelets concentrates in pseudo type 2B VWD.

Role of the retention test Homburg in evaluating platelet hyperactivity and in monitoring therapy with antiplatelet drugs

Decreased retention in the retention test Homburg (RTH) indicates a loss of platelet function; increase is associated with an increased activation of platelets, for example, in patients with vascular diseases. Compared with other materials (e.g., collagen, glass pearls, etc.) the filter surface in the retention tubes is nonthrombogenic. Therefore, the RTH seems to be well suited for measuring an in vivo over-reactivity of platelets. In a pilot study using the RTH we evaluated the postoperative over-reactivity of platelets in 14 patients and observed a significant heterogeneity of the platelet population concerning size and stickiness. Reliable platelet function tests are also necessary for "drug monitoring," since they deliver important clinical laboratory parameters for efficient control of a therapy with antiplatelet drugs. Therefore, we evaluated in vitro how, after administration of platelet aggregation-inhibiting medications (such as acetylsalicylic acid, Prostaglandin and ReoPro in various concentrations, the adenosine diphosphate (ADP), collagen, ristocetin, or suprarenin increased retention can be reduced. The reaction of the platelets in platelet-rich plasma of different patients or donors to the addition of ADP is variable. The platelet function inhibitor effect is dose dependent. In a clinical pilot study, a significant platelet-inhibiting effect of clopidogrel using the RTH has been shown.

Interplay of platelet polymorphisms, risk factors, and von [corrected] Willebrand factor [corrected] in determining collagen-adenosine diphosphate PFA-100 results in patients with coronary artery disease

Platelets play a pivotal role in thrombus formation in patients with coronary artery disease (CAD), since the high shear generated in the presence of severe coronary stenoses can increase platelet reactivity (PR) and trigger thrombogenesis. Several reports have suggested a functional effect of human platelet antigen (HPA)-1 and HPA-2 gene polymorphisms on PR. However, the true determinants of high-shear PR in CAD patients taking their usual medications are still incompletely understood. In 104 patients with stable CAD we analyzed the possible clinical, biochemical and genetic factors affecting high-shear PR, measured by the ex vivo platelet function analyzer (PFA-100) collagen-adenosine diphosphate method. In univariate analysis, a lower PR was associated with decreased plasma von Willebrand factor-ristocetin cofactor activity, increased blood levels of triglycerides, female sex, use of thienopyridines, lower platelet count, and HPA-1b carriership. All variables, except HPA-1b, remained associated with lower PR in multivariate analysis. However, the introduction in the model of the HPA-1 and HPA-2 genotypes as interaction terms led to a significant improvement in the prediction of PR, although the quantitative effect was small (about 3% improvement, P=0.046).Thus, in CAD patients, there seems to be only a mild effect of the platelet glycoprotein HPA-1 and HPA-2 polymorphisms on collagen-adenosine diphosphate-stimulated PR after the effect of well-established clinical and biochemical determinants are considered.

A new L1446P mutation is responsible for impaired von Willebrand factor synthesis, structure, and function

We report on a new mutation (4337T-->C) in exon 28 of the von Willebrand factor (VWF) gene, resulting in a substitution of L with P at residue 1446 (L1446P) of pre-pro-VWF. The defect is transmitted as a dominant trait and induces a reduced VWF synthesis, an abnormal VWF multimer pattern and a deficient VWF-platelet glycoprotein Ib interaction. The proband had low plasma and platelet VWF antigen levels, a reduced VWF collagen-binding capacity, and a disproportionately low VWF ristocetin cofactor activity, associated with the absence of ristocetin-induced platelet aggregation. Multimer analysis showed that the smaller multimers were slightly low, whereas the larger ones were significantly reduced or absent, with a clear cutoff between the two patterns. Similar hemostatic findings were observed in the proband's sister and nephew. Desmopressin administration restored VWF levels to near normal, but this was not so for VWF ristocetin cofactor activity or ristocetin-induced platelet aggregation. VWF multimers improved after desmopressin, moreover, with the larger forms restored and the smaller ones still relatively more represented. Recombinant P1446 VWF synthesis was reduced at heterozygous level, and its multimer pattern was similar to that observed in plasma VWF. These findings confirm the role of L1446P mutation in determining the von Willebrand disease (VWD) phenotype observed in our patients. Given the lack of large and intermediate VWF multimers, and the fact that the VWF-platelet interaction defect appears to be partially independent of multimer pattern, the VWD associated with L1446P mutation may belong to the type 2A/2M VWD variant.

Platelet function tests in childhood. Measuring aggregation and release reaction in whole blood

Blood samples from 42 newborns, 78 infants and schoolchildren, and 81 healthy adults were tested for the parameters of primary hemostasis. Only whole blood techniques were used. Agonist-induced aggregation and release-reaction studies were performed in a whole blood lumi-aggregometer simultaneously. The release of adenosine triphosphate (ATP) was detected by the luciferin-luciferase method. The in vitro bleeding time was measured by the PFA 100 system. The results of these studies were ostensibly influenced by blood cells. Many aggregation phenomena were correlated with the platelet count. Aggregation and release reaction by collagen were inversely correlated with the hematocrit. In the PFA 100, hematocrit and leukocyte count were also inversely correlated with the closure time and the maximal blood flow velocity. Both parameters were diminished in newborns. The aggregation response to adenosine diphosphate (ADP) was similar in the three groups. The same was true for the aggregation and release reaction by arachidonic acid and for the agglutination by ristocetin. The aggregation and release reaction by collagen were diminished in the specimens from newborns. For the explanation of this transient hypofunction, only theoretical considerations exist. Beyond the postnatal period and during childhood, no remarkable differences from the adult norm were found.

Inverse relationships between haemoglobin and ristocetin-induced platelet aggregation in haemodialysis patients under erythropoietin therapy

BACKGROUND

Amelioration of the anaemia of chronic renal failure and subsequent improved haemorheology result in correction of bleeding diathesis as evidenced by shortening of the skin bleeding time (BT). However, the relationship between the haematocrit and platelet-vessel wall interactions in haemodialysis (HD) patients under recombinant human erythropoietin (rHuEpo) therapy, assessed by platelet aggregation in response to ristocetin is more complex and somewhat inconsistent.

METHODS

We investigated the relationship between haemoglobin (Hb) levels and whole blood ristocetin-induced platelet aggregation (electric impedance method) in 28 HD patients treated with rHuEpo, and with normal BT. The measurements were repeated in 16 subjects after having reduced platelet aggregability with orally administered ketanserin.

RESULTS

Ristocetin-induced platelet aggregation in the whole group was comparable to those found in 21 age-matched healthy subjects (normals) and in 25 HD patients not treated with rHuEpo (uraemics). Interestingly, a significant inverse correlation between this aggregation and Hb concentration was found (r = -0.392, P < 0.05). In the group of 16 patients, the pre-ketanserin aggregation was more intensive than in the normals and uraemics (P < 0.05). Ketanserin produced a fall in ristocetin-induced platelet aggregation (P < 0.02), prolongation of the BT (P < 0.02) and, unexpectedly, a decrease in serum Epo concentration (P < 0.0002) and the Hb level (P < 0.001). Again, an inverse correlation between depressed ristocetin-induced platelet aggregation and lowered Hb concentration was found (r = -0.590, P < 0.02). Moreover, a strong positive correlation between the extent of preketanserin platelet aggregation and the decrease in the intensity of this process that followed the trial was observed (r = 0.919, P < 0.000005). There were no changes in other haematological parameters or arterial blood pressure.

CONCLUSIONS

Considering the role of von Willebrand factor and fibrinogen in mediating ristocetin-induced platelet aggregation, and enhanced synthesis and/or release of these macromolecules in response to uraemia or inflammation, we suggest that exaggerated whole-blood platelet aggregability to ristocetin points to blunted erythropoiesis in HD patients on rHuEpo therapy.

Familial infantile thrombotic thrombocytopenic purpura

PURPOSE

To further define familial infantile thrombotic thrombocytopenic purpura and clarify its pathophysiology, we describe a family with two infants presenting with this rare syndrome.

RESULTS

Complete, but temporary remission followed the transfusion of whole blood in the first sibling and fresh frozen plasma (FFP) in the second. Periodic FFP transfusions have kept the surviving proband in a prolonged clinical remission. The presence of unusually large von Willebrand factor multimers was demonstrated in the proband and the processing activity of these large multimers was found to be normal.

CONCLUSION

The occurrence of this rare disorder, in siblings who are products of a consanguinous union, suggests an as yet uncharacterized genetic defect.

Elevated von Willebrand factor antigen predicts deterioration in diabetic peripheral nerve function

We have studied the temporal relationship of plasma von Willebrand Factor (vWF), a marker of endothelial damage, with the development of complications in 63 young diabetic patients (56 of whom were insulin-dependent) who took part in a prospective study. Results are presented from baseline to follow-up. In the group as a whole, no significant changes were found in any autonomic function tests, temperature discrimination threshold or nerve conduction velocities. Median motor and peroneal latency were significantly increased, while median motor, peroneal motor and sural sensory potentials were significantly decreased at follow-up compared with baseline (p < 0.001). There was a significant fall in mean plasma vWF antigen and ristocetin co-factor activity in the entire group. Within the group, we identified eight patients whose peroneal motor and sural sensory conduction velocities had decreased by over 2 ms-1. In these patients (Group A), baseline vWF antigen and activity were significantly higher than in the rest of the patients (Group B), (p = 0.04) and vWF antigen was still significantly higher after 3 years (p = 0.02). There were no differences between the groups in incidence of retinopathy, urinary albumin excretion rate or macrovascular disease. To assess the influence of glycaemic control on vWF, we first compared a matched group (C) of diabetic patients with similar HbA1 to that of group A, but with normal nerve conduction velocities: vWF was still significantly higher in group A compared with group C (p = 0.02). Furthermore, hierarchical regression showed that vWF predicted deteriorating nerve function independently of glycaemic control or the type of diabetes. Endothelial dysfunction may be associated with development of peripheral neuropathy in young diabetic patients.

Haemostatic variables in patients with unstable angina

To assess the contribution of thrombus formation in the pathogenesis of unstable angina, we employed the recently developed assays of small fragments which reflect the degree of activation of various components of the haemostatic system. Such haemostatic measurements were undertaken in patients with unstable angina (n = 47) from the time of their admission to the coronary care unit (CCU) at 8-h intervals in the first 24 h and then daily for a total of 5 days. The results obtained were compared with healthy control values. Patients exhibited lower ATIII, prolongation of the APTT and TT, but not PT or the reptilase time, which is a consequence of heparinization. There was significant elevation of fibrinogen, factor VIII:C, von Willebrand factor:antigen and von Willebrand factor:ristocetin cofactor throughout the study period. There was also evidence of thrombin generation as indicated by the elevated levels of fibrinopeptide A (FPA) and thrombin-antithrombin complexes. The platelet release proteins, beta-thromboglobulin (BTG) and platelet factor 4 (PF4), were markedly elevated in the first 2 days and dropped gradually thereafter. The fibrinolytic inhibitor, plasminogen activator inhibitor (PAI), levels were elevated throughout. Proteins C and S, plasminogen and alpha 2-antiplasmin remained unchanged. It was concluded that in patients with unstable angina, there is significant activation of the clotting system and inhibition of fibrinolysis which confirms the existence of a tendency towards thrombus formation in patients with unstable angina.

von Willebrand factor antigen compared with other factors in vasculitic syndromes

In order to analyse their role as a specific marker of vascular damage and their value in monitoring disease activity the plasma concentrations of von Willebrand factor antigen (vWFAg) and the ristocetin cofactor (RiCoF) activities were determined in 43 children with vasculitis and 20 controls. These patients were sub-divided into three groups according to diagnosis: Henoch-Schönlein purpura (n = 18), polyarteritis nodosa (n = 16), and systemic lupus erythematosus (n = 9). High concentrations of vWFAg and activities of RiCoF were found in all the patient groups. vWFAg and RiCoF returned to normal as the patients became symptom free and remained above normal in those with continuing symptoms. The amount of vWFAg did not correlate with the acute phase reactants. vWFAg acted as a specific marker of vascular damage and was useful for the monitoring of disease activity both in small vessel vasculitis and systemic necrotising arteritis.

Acquired von Willebrand's disease in association with essential thrombocythemia: regression following treatment

Acquired von Willebrand's disease in a 40-year-old woman affected with essential thrombocythemia (ET) is reported. The profile of plasma von Willebrand factor (vWF) revealed decreased ristocetin cofactor activity and diminished large multimers of vWF in spite of a normal vWF antigen level. There was no evidence of circulating inhibitor against the factor VIII complex. The vWF abnormality improved by controlling the platelet count following treatment for ET with interferon-alpha 2b and ranimustine. The possible mechanism of the development of AvWD in ET is briefly discussed.

Hemostatic effect of normal platelet transfusion in severe von Willebrand disease patients

Platelet von Willebrand factor (vWF) has been suggested to play an important role in the hemostatic process. Clinical and experimental data indicate that bleeding time (BT) and platelet-vessel wall interaction cannot be normalized unless the defect of platelet vWF is also corrected. We have examined the effect of normal platelet concentrate transfusion 1 hour after cryoprecipitate infusion in five type III von Willebrand disease (vWD) patients. The cryoprecipitate infusion attained normal circulating levels of ristocetin cofactor, vWF antigen, and factor VIII activity. In two patients, cryoprecipitate infusion did not modify the BT (greater than 30 minutes), whereas in the remaining three patients BT was only partially corrected (from greater than 30 to 12, 18, and 21 minutes). However, the immediate platelet transfusion completely corrected the BT in four cases, and in one case it shortened the BT to 8.30 minutes (n = 3 to 8 minutes). In the perfusion study, cryoprecipitate infusion only resulted in a slight increase in platelet deposition (surface coverage range: 2.4% to 11.3%), whereas the platelet concentrate transfusion elicited a more marked improvement (range: 8.2% to 26.4%; P less than .02 v post-cryoprecipitate). These results suggest an important in vivo role of the platelet vWF in supporting platelet-vessel wall interaction. They also give support to the occasional addition of normal platelet transfusion to the cryoprecipitate infusion for the control of serious bleeding episodes resistant to cryoprecipitate in severe vWD patients.

Studies on an unusual von Willebrand's variant--type IID

We describe a man and his daughter from a large New Zealand family with the extremely rare variant von Willebrand's (VW) disease type IID. These two patients had a severe bleeding history following minor surgery and displayed easy bruising. However, routine laboratory screening tests and factor VIII studies were essentially normal except for slightly reduced ristocetin cofactor activity and prolonged skin bleeding times. Although lacking higher molecular weight forms of VW antigen in common with VW type II's the patients' multimer patterns were clearly different from types IIA and IIB. Instead of showing the characteristic 'triplet' pattern in each multimer band the patients gave a single prominent band with faint satellite bands different in mobility to those in normals. Von Willebrand factor from the patients' platelets gave a similar abnormal pattern. DDAVP failed to correct the bleeding time in either patient and multimer analysis confirmed that there was no increase in the higher molecular weight VWF antigen.

Abnormality of von Willebrand factor in patients with hemoglobin E-beta (0) thalassemia

The authors have identified six Southeast Asian patients ranging in age from 14 to 21 years with hemoglobin E-beta(0) thalassemia and a coagulopathy involving von Willebrand factor (vWF). These patients had normal or only slightly decreased plasma clotting factor levels. The activated partial thromboplastin time was prolonged in four of the patients. The abnormal feature common to all patients was a qualitative loss of high molecular weight multimers of vWF by crossed immunoelectrophoresis (vWF:CIE). Plasma vWF antigen concentration (vWF:Ag) and ristocetin cofactor activity (vWF:RCo) also were decreased and bleeding time prolonged in three patients. Epistaxis was present in two. No family history of increased bleeding tendency was present in any patient. Coagulation parameters and vWF:CIE were normal in two first-degree relatives without this hemoglobinopathy. vWF abnormalities and clinical manifestations were greatest in those patients with the most severe anemia and hepatosplenomegaly. These six patients appear to have an acquired abnormality of vWF, although they lack the clinical characteristics of acquired von Willebrand disease. While the etiology of this abnormality is unclear, the authors speculate that proteolysis of vWF secondary to extramedullary hematopoiesis or loss through high cardiac output shear stress in these anemic patients may be involved.

Measurement of platelet von Willebrand factor is dependent on method of platelet isolation

The authors have compared platelet von Willebrand factor antigen and ristocetin cofactor activity measurements in a normal population, using two different previously published platelet isolation techniques. Preparation of a platelet lysate by platelet washes using Tyrode's albumin solution and inhibitors yielded a threefold higher vWF antigen and a twofold higher ristocetin cofactor activity measurement as compared to platelets isolated by tris buffered saline washes containing 0.1% (weight/volume) Na2EDTA. Contamination by plasma von Willebrand factor to account for the higher values found by the first method was excluded by monitoring the presence of added purified I125 von Willebrand factor to platelet rich plasma. vWF multimer analysis showed the same distribution of multimers in both preparations but suggested a relative decrease in lower molecular weight multimers with the second method. Platelet isolation can result in significant loss of platelet von Willebrand factor, with a preferential loss of lower molecular weight multimers, likely resulting from unintentional platelet release.

In vitro and in vivo characterization of a high-purity, solvent/detergent-treated factor VIII concentrate: evidence for its therapeutic efficacy in von Willebrand's disease

A factor VIII (FVIII) concentrate, virus-inactivated by the solvent/detergent procedure, was studied in vitro. In contrast with most high-purity, virus-inactivated FVIII concentrates, it contains not only high levels of von Willebrand factor (vWF) antigen and ristocetin cofactor activity but also high molecular weight forms of von Willebrand factor. Furthermore, it is able to promote platelet adhesion on collagen in a perfusion system. In vivo studies performed in patients with different types of von Willebrand's disease provided evidence that this concentrate corrects Duke's bleeding time and prevents or stops haemorrhages. Thus, the particular advantages of this FVIII/vWF preparation are safety, low content of contamination proteins, and efficacy in von Willebrand's disease.

Supranormal von Willebrand factor multimers in scleroderma

Platelet adhesion-aggregation reactions play an early and pivotal role in the pathogenesis of systemic sclerosis in scleroderma, but the mechanisms are incompletely understood. We determined whether or not plasma from 11 consecutive patients with scleroderma contained a subset of larger than normal ("supranormal") multimers of von Willebrand factor (vWF) that are potent inducers of platelet aggregation and adhesion. Supranormal multimers were found in all patients on at least one of two different occasions 9 to 12 months apart, whatever the duration and severity of the disease, but in none of the normal controls. Administration of low-dose aspirin (40 mg) to five of the 11 patients for ten days to inhibit the platelet release reaction slightly reduced the amounts of supranormal multimers suggesting that they might originate in part from platelets. Supranormal multimers may contribute to the pathogenesis of systemic sclerosis by inducing platelet aggregation and enhancing adhesion to subendothelium under the conditions of elevated shear stress occurring in the partially occluded vessels of the arterial microcirculation of scleroderma.

Effects of physiological concentrations of vasopressin on haemostatic function in man

Plasma concentrations of vasopressin (aVP) attained under conditions of stress were simulated by infusing four volunteers with 0.25, 0.5, 1.0 and 2.0 pressor units of aVP over 1 h (units/h). Three subjects had all four infusions and one received only 1.0 unit/h. Blood samples were taken for assay of factor VIII coagulant activity (FVIIIC), factor VIII related antigen (FVIIIRAg), the ristocetin cofactor (FVIIIRiCof), euglobulin lysis time (ELT) and aVP concentrations before infusion (time 0) and every 20 min for 80 min. Fibrinopeptide A (FPA) generation time was measured at time 0, 60 and 80 min. At infusion rates of 0.25 unit/h median aVP levels peaked at 6.5 pg/ml and there was no change in FVIII or FPA generation time, and plasminogen activator activity (10(6)/ELT2) rose from 100 to 400 units. At 1.0 unit/h, aVP levels rose to 25.4 pg/ml, FVIIIC rose by 160% and activator activity from 87 to 360 units. At 2.0 units/h, aVP concentrations reached 83 pg/ml, there was an increase in all modalities of FVIII and activator activity rose from 251 to 452 units. FPA generation time shortened and circulating plasma levels of FPA were increased. There was a highly significant correlation between the percentage increases in all three components of FVIII and plasma aVP levels (FVIIIC: r = 0.87, P less than 0.0001; FVIIIRAg: r = 0.61, P less than 0.0001; FVIIIRiCof: r = 0.80, P less than 0.0001) and between the increase in plasminogen activator activity and aVP levels (r = 0.56, P less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

von Willebrand factor. A protein which binds at the cell surface interface between platelets

von Willebrand factor (VWF) functions in platelet aggregation, a form of cellular interaction. In vitro analysis of platelet aggregation, as measured by the platelet aggregometer, requires addition of a promoter such as the glycopeptide antibiotic ristocetin. Native multimeric VWF (Mr = 1-20 X 10(6)) can be reduced with sulfhydryl reagents to a monomeric state (Mr = 2 X 10(5)). In this study, the binding of bovine VWF and ristocetin to bovine platelets was investigated using fluorescence anisotropy of derivatized monomer protein and ristocetin and also by radioisotope methods using 125I-labeled monomer and native protein. Ristocetin bound to bovine platelets but not to VWF. VWF binding to formaldehyde-fixed platelets was dependent on the presence of a promoter such as ristocetin. The monomer and multimer VWF bound equally well in the presence of low ristocetin concentrations. Under these conditions, plots of VWF binding versus platelet concentration were sigmoidal, indicating positive cooperativity with respect to platelets. At higher (100 micrograms/ml) ristocetin concentrations, the binding curve was no longer sigmoidal. Ristocetin promoted the formation of small platelet aggregates, an effect that was amplified by the presence of VWF. In fact, all conditions which resulted in monomer or multimer VWF binding to platelets also caused formation of platelet aggregates observed by light microscopy. These combined results were consistent with VWF binding only to the interface between proximal platelets. High affinity binding could be provided by the presence of two cell surfaces and the resulting multiple binding interactions. Polycations, such as poly(L-lysine) and Polybrene, also promoted the formation of platelet aggregates and facilitated the binding of VWF to platelets. Physiological platelet activators such as thrombin, ADP, and collagen also facilitated VWF binding to native platelets and caused platelet aggregation. It appears possible that any process which causes the surface membranes of platelets to become spatially close will allow expression of VWF activity.

Factor VIII complex in chronic renal failure: influence of protein C, fibrinolysis and diabetes mellitus

Chronic renal failure causes elevations of factor VIII coagulant activity and Factor VIII-related antigen even before the patients enter chronic hemodialysis. The change from control of Factor VIII ristocetin cofactor does not reach significance. The elevations are not effected by entering onto hemodialysis. These parameters are the same for non-diabetic and diabetic patients. Protein C, plasminogen and total fibrinolytic capacity are normal in diabetic and non-diabetic patients, with or without hemodialysis for chronic renal failure. However, before entering onto hemodialysis some of these parameters had negative correlation coefficients with parts of the factor VIII complex among the diabetic and non-diabetic patients. These negative correlates turned positive after hemodialysis. Thus, there are differences in these catabolic mechanisms for factor VIII when hemodialysis is used for diabetic and non-diabetic patients with chronic renal failure.

Biological properties of ristocetin-ps-aglycone

Ristocetin-ps-aglycone obtained by acid hydrolysis of ristocetin A, has a substantially greater antimicrobial activity against Gram-positive bacteria than the parent compound. None of the substances are active against Gram-negative bacteria, yeast or fungi. The ps-aglycone is several times more toxic than ristocetin A when administered intravenously. Both substances are well-tolerated when given subcutaneously, intraperitoneally and perorally. Both ristocetin A and the ps-aglycone have a very low absorption after oral administration. Plasma levels following intravenous administration of ristocetin A and the ps-aglycone are comparable, with both showing a rapid decline during the first 60 minutes followed by a somewhat slower elimination. The aggregating properties of the ps-aglycone could not be determined due to its low solubility at neutral pH.

Ristocetin-Willebrand factor activity in dog plasma

A sensitive and reproducible method for the quantitation of ristocetin-Willebrand factor activity in canine plasma is described. This assay measures the initial velocity of aggregation of suspensions of canine washed platelets in the presence of ristocetin and ristocetin-Willebrand factor. The washed platelets are stable for 4 h and when prepared from the same donor vary little in their day-to-day response to ristocetin. The calculated mean plasma ristocetin-Willebrand factor activity in 37 normal dogs using this method is 98 +/- 26% (mean +/- SD) Ristocetin-Willebrand activity is 106 +/- 25% in dogs with severe hemophilia A and 45 +/- 13% in dogs with moderate forms of von Willebrand's disease.

Measurement of canine von Willebrand factor using ristocetin and Polybrene. Diagnosis of canine von Willebrand's disease

Dogs with VWD provide useful models for the study of the factor VIII complex. However, the study of canine VIIIR:WF has been hampered by the lack of a routine plasma assay for canine RCF, an activity that is usually used as a measure of VIIIR:WF. This study shows that canine plasma can be assayed for RCF with a macroscopic tilt-tube method using formalin-fixed human platelets, ristocetin, and extra canine albumin (5.0 mg/ml) to prevent plasma precipitation. An assay was also developed for canine plasma PBCF, an activity that is closely related to RCF. In 45 normal canine plasmas, VIII:C was not correlated with RCF, PBCF, or VIIIR:AG. However, RCF, PBCF, and VIIIR:AG were well correlated with each other. In 26 canine VWD plasmas, VIII:C was frequently normal, whereas VIIIR:AG, RCF, and PBCF were almost always deficient. The patterns of VIIIR:AG, RCF, and PBCF deficiencies in the canine VWD plasmas suggested that some canine breeds have a "classic" form of VWD whereas other breeds have "variant" forms of disease.

Studies on the mechanism of ristocetin-induced platelet agglutination: binding of ristocetin to platelets

Ristocetin was trace-labeled with [3H] by the reductive methylation method. It was shown to agglutinate human platelets in the presence of VIIIR:WF in a manner indistinguishable from unlabeled ristocetin. The binding of the labeled ristocetin to normal and enzyme-modified human platelets was studied both in the presence and absence of VIIIR:WF and at nonagglutinating and agglutinating concentrations of ristocetin. Virtually no difference in [3H]ristocetin binding was seen whether VIIIR:WF was present or not. Platelets treated with chymotrypsin, which destroys their ability to agglutinate to VIIIR:WF and ristocetin, did not bind less ristocetin than did control platelets. A pronounced, direct relationship was found between [3H]ristocetin bound by normal platelets and total ristocetin concentration. This implies that at the higher (agglutinating) concentrations of ristocetin either more binding sites are exposed or, more probably, aggregation of ristocetin occurs.

Classification of von Willebrand's disease

The classification of von Willebrand's disease should be based primarily on the mode of inheritance: thus, the main division would between autosomal dominant and autosomal recessive types. Phenotypic subdivisions within these major types may then be possible. A subgroup of the dominant type seems to be well defined by increased electrophoretic mobility of the VIII-related antigen.

The effects of ristocetin and von Willebrand factor on platelet electrophoretic mobility

Ristocetin will induce the agglutination of platelets in the presence of von Willebrand factor. In previous studies, an electrostatic mechanism was proposed for this phenomenon wherein first the platelet's surface charge is reduced by the binding of ristocetin and then the von Willebrand factor acts as a bridge between platelets. To test this hypothesis, the effects of ristocetin and von Willebrand factor, singly and together, on the electrophoretic mobility of normal, trypsinized, and Bernard-Soulier platelets was measured. Ristocetin alone, at concentrations of 0.5 mg/ml or more, produced a statistically significant reduction in the electrophoretic mobility of fresh or fixed platelets. Control experiments showed that the reduction was not due to changes in the ionic milieu of the solution. Therefore, the decrease in platelet mobility is evidence for binding of ristocetin to the platelet surface. Bernard-Soulier and trypsinized platelets also had reductions in mobility with ristocetin, suggesting that ristocetin binds to the platelet at sites other than the binding site for von Willebrand factor. The presence of plasma from a patient with von Willebrand's disease did not alter the reduction in mobility of normal platelets by ristocetin. However, the reduction was markedly enhanced in the presence of normal plasma. This enhancement did not occur with Bernard-Soulier platelets and was inhibited by anti-Factor VIII/von Willebrand factor antiserum or trypsinization of the platelets. Thus, the enhanced reduction appears to be associated with the binding of von Willebrand factor to the platelet surface. These studies indicate that platelets undergo two changes with ristocetin and von Willebrand factor, both of which facilitate agglutination: reduction in net surface charge and binding of von Willebrand factor, a large molecule which can serve as a bridge between platelets. In parallel studies, bovine von Willebrand factor, without ristocetin, agglutinated and reduced the electrophoretic mobility of normal but not Bernard-Soulier or trypsinized platelets; this indicates a similar mechanism of agglutination.

Antihemophilic factor concentrate therapy in von Willebrand disease. Dissociation of bleeding-time factor and ristocetin-cofactor activities

Treatment of von Willebrand disease with two plasma antihemophilic factor (AHF) concentrates, cryoprecipitate and glycine-precipitated AHF, was compared. Both concentrates were equally effective in immediately raising the plasma levels of factor VIII, the factor VIII-related antigen, and the ristocetin-related von Willebrand factor (vWF) and in stimulating a secondary rise in plasma factor VIII. Given either concentrate, the vWF activity, the antigen, and factor VIII levels were normalized in a patient with von Willebrand disease. However, correction of the prolonged bleeding time and control of bleeding occurred only with the cryoprecipitate. The bleeding-time corrective factor and the ristocetin-related vWF or platelet-aggregating factor are dissociable, distinct activites.