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PubMed1985

Platelet cytoskeleton alpha-actinin in normal and thrombasthenic platelets: distribution and immunologic characterization.

The subcellular localization of alpha-actinin (Mr 100,000) in human skeletal muscle is restricted to the Z line, in which it is believed to anchor actin filaments. Recently, this protein was identified in normal and thrombasthenic human platelets by its antigenic cross-reaction with antibodies to chicken gizzard alpha-actinin. In our study, the biochemical interaction between purified platelet alpha-actinin and striated muscle F-actin was examined by electron microscopy of negatively stained preparations. Like its muscle counterpart, platelet alpha-actinin promotes the cross-linking and bundling of actin filaments. Antibodies prepared to human platelet alpha-actinin cross-reacted with chicken gizzard alpha-actinin as shown by immunoelectrophoresis and the western blotting technique. Immunoblots prepared with normal and thrombasthenic platelets with antibodies to human platelet alpha-actinin revealed that this protein is susceptible to proteolysis. Extracts of freshly drawn platelets showed a protein band of 100 K. When the platelet extracts were incubated at 37 degrees C for various times, the immunoblots showed protein bands of 100 and 80 K. The proportion of the 80 K protein band increased with incubation time. This proteolysis can be prevented by chelating agents such as EDTA or the protease inhibitor leupeptin. Indirect immunofluorescent studies of human skin fibroblasts with antibodies to chicken gizzard actin and human skeletal muscle, chicken gizzard, and platelet alpha-actinin revealed the staining pattern characteristic of each protein. The distribution of alpha-actinin in normal and thrombasthenic platelets was assessed by ferritin-labeled immunoelectron microscopy. Ferritin particles were found in the cytoplasm immediately below the membrane and in some granules. There was no labeling associated with the mitochondria.

Actinin↗
PubMed1979

Alterations in platelet surface sialytransferase activity and platelet aggregation in a group of cancer patients with a high incidence of thrombosis.

Platelet aggregation, platelet surface sialic acid, and platelet surface sialytransferase activity were studied in a group of 12 cancer patients with a high incidence of thrombosis. These patients demonstrated accelerated coagulation, increased Factor VIII antigen and restocetin cofactor, and enhanced adenosine 5'-diphosphate-induced platelet aggregation. Platelet exogenous sialytransferase activity was increased in cancer patients (117.6 +/- 14 pmol/10(9) platelets) as compared to controls (59.0 +/- 4.3 pmol/10(9) platelets, p less than 0.01). Platelet exogenous sialytransferase activity and platelet aggregation were inhibited by aspirin. Thrombosis and bleeding have complicated the clinical course of half of these patients. This platelet membrane analysis provides additional data which may be related to current observations of increased levels of plasma sialytransferase activity and serum sialic acid in cancer patients.

Adult↗
PubMed1980

Identification and separation of secreted platelet proteins by isoelectric focusing. Evidence that low-affinity platelet factor 4 is converted to beta-thromboglobulin by limited proteolysis.

Low-affinity platelet factor 4 and beta-thromboglobulin are low molecular weight platelet secretory proteins that have common antigenic determinants. Four amino acids (Asn-Leu-Ala-Lys) at the amino terminus of beta-thromboglobulin are deleted, but the remaining sequences of the two peptides appear to be identical. Low-affinity platelet factor 4 and beta-thromboglobulin have respective isoelectric points at pH 8.0 and at pH 7.0. Identification, quantitation, and separation of both proteins was achieved by a method combining preparative isoelectric focusing and specific radioimmunoassay with anti-low-affinity platelet factor 4 antibody. It has been determined that the supernate processes immediately after platelet aggregation induced by ionophore A23187 or thrombin contains approximately 80% low-affinity platelet factor 4, 8% beta-thromboglobulin, and 12% highly cationic immunoreactive material (platelet basic protein). Experimental evidence suggests that low-affinity platelet factor 4 is originally secreted by platelets and then converted to beta-thromboglobulin by a platelet-derived, heat-labile protease that is inhibited by phenylmethylsulfonyl fluoride.

Amino Acid Sequence↗
PubMed1984

Characterization of 25 monoclonal antibodies to factor VIII-von Willebrand factor: relationship between ristocetin-induced platelet aggregation and platelet adherence to subendothelium.

We have studied the role of factor VIII-von Willebrand factor (FVIII-vWF) in both platelet adherence to subendothelium and ristocetin-induced platelet aggregation using monoclonal antibodies to human FVIII-vWF. Twenty-five monoclonal antibodies were obtained, two of which were directed to the factor VIII moiety of FVIII-vWF; one of these two completely inhibited the procoagulant activity (FVIII:C). The remaining 23 monoclonal antibodies were directed to the von Willebrand factor moiety of FVIII-vWF. The ability of the latter monoclonal antibodies to inhibit platelet adherence to arterial subendothelium was investigated with a perfusion model. According to the number of platelets adhering to the subendothelium, three groups of monoclonal antibodies could be discerned: (A) antibodies not affecting platelet adherence; (B) antibodies that inhibited platelet adherence to the level as observed when von Willebrand's disease plasma was tested; and (C) antibodies that completely inhibited both platelet adherence to subendothelium and ristocetin-induced platelet aggregation. The two antibodies present in group C competed for the same or closely related epitope(s) present on FVIII-vWF. These results demonstrate that a domain is present on the FVIII-vWF molecule that is associated both with ristocetin-induced aggregation and with the ability of FVIII-vWF to support platelet adherence to the subendothelium. Based on these observations, it is concluded that ristocetin-induced binding of FVIII-vWF to platelets reflects, at least in part, a physiologic mechanism regulating the function of FVIII-vWF in primary hemostasis.

Antibodies, Monoclonal↗
PubMed1984

A comparison of spontaneous platelet aggregation in whole blood with platelet rich plasma: additional evidence for the role of ADP.

ADP, generated from red blood cells is believed to be responsible for the spontaneous aggregation of platelets in whole blood. This notion is based mainly on the use of enzymes which remove ADP. We have studied spontaneous platelet aggregation in whole blood and autologous platelet rich plasma obtained from 12 healthy male and female volunteers. Platelet aggregation was quantitated by measuring the fall in the number of single platelets counted using a whole blood platelet counter (Ultra Flo 100). In a rotating tube model, the mean fall in the number of platelets due to spontaneous aggregation was 56% in whole blood but, only 3% in platelet rich plasma prepared from the same blood samples. Spontaneous platelet aggregation in whole blood was unaffected by apyrase grade I, but was reduced to 15% by apyrase grade II, to 38% by creatine phosphokinase/creatine phosphate and to 9% by pyruvate kinase/phosphoenolpyruvate. The results of this study provide additional evidence that ADP generated in whole blood triggers the spontaneous aggregation of platelets.

Adenosine Diphosphate↗
PubMed1981

Binding of factor VIII-von Willebrand factor to human arterial subendothelium precedes increased platelet adhesion and enhances platelet spreading.

Time-dependent studies of platelet adhesion and binding of F VIII-VWF to human arterial subendothelium were carried out in vitro, using 51Cr-labeled platelets and 125I-labeled F VIII-VWF. Rapid binding of F VIII-VWF was found, resulting in a constant surface concentration after 1 to 2 min of perfusion. The number of platelets adhering in the first minute is similar in the absence of F VIII-VWF, but the platelet coverage is about twofold higher perfusions of 2 to 5 min in the presence of F VIII-VWF. Thus binding of F VIII-VWF precedes the increase in platelet adhesion, in line with our previous studies on the role of subendothelium-bound F VIII-VWF. By morphological evaluation it was found that the number of contact platelets (nonspread platelets) is lower in the presence of F VIII-VWF. The ratio of contact platelets to spread decreases in time, however, at a higher rate when F VIII-VWF is present. This indicates that F VIII-VWF enhances platelet spreading arterial subendothelium and consequently that the spreading is impaired in VWD. The enhancement may be caused directly by F VIII-VWF or indirectly as the result of a better attachment of contact platelets.

Arteries↗
PubMed1983

The effect of aspirin and linoleic acid on platelet aggregation, platelet fatty acid composition and haemostasis in man.

The effect of linoleic acid and of aspirin on platelet aggregation has been measured in six healthy volunteers with a new platelet aggregometer (Wellcome) designed to be used with whole blood. The subjects were given a controlled diet for 6 weeks during which their platelet aggregation, platelet fatty acid composition, dilute blood clot lysis time, bleeding time and serum cholesterol and triglycerides were measured. A basal diet typical of that normally eaten in the UK was fed for 3 weeks, then for a further 2 weeks 60 ml/d of safflower seed oil was added to the diet. Finally there was a further week on the basal diet and on the last day the subjects each took 900 mg of aspirin. The effects of the safflower seed oil was to increase platelet linoleic acid (C18:2 omega 6) content from 5.53 +/- 0.52 micrograms to 10.1 +/- 0.92 micrograms/100 micrograms total fatty acids (P less than 0.001), to decrease platelet aggregation to ADP, and to decreased serum cholesterol. Fibrinolysis and bleeding times were unaltered. Aspirin decreased platelet aggregation, prolonged bleeding time and increased platelet arachidonic acid (C20:4 omega 6) from 24.7 +/- 0.38 micrograms to 25.8 +/- 0.61 micrograms/100 micrograms total fatty acids (P less than 0.01). The Wellcome whole blood aggregometer is a sensitive test of platelet function and using it linoleic acid has been shown to reduce aggregation in conjunction with an increase in polyunsaturated fatty acid content of the platelet membrane.

Adult↗
PubMed1980

Platelet arachidonic acid metabolism and platelet function in ten patients with chronic myelogenous leukemia.

We have compared the pathways of arachidonic acid (C 20:4) metabolism in platelets from ten patients with Philadelphia chromosome-positive CML with those of seven normal subjects. Platelets were incubated with 3H-arachidonic acid, gel-filtered, and treated with thrombin (5 U/ml). The cyclooxygenase and lipoxygenase-derived products and free arachidonic acid released from the platelets were separated by high pressure liquid chromatography and their radioactivity determined. The total uptake of 3H-C 20:4 by platelets from CML patients did not differ from controls, but the release of radioactivity in response to thrombin was significantly lower (p < 0.01) in CML patients (32.3% +/- 4.9% of total radioactivity was released from control platelets; 19.0% +/- 7.4% from CML platelets). Both cyclooxygenase and lipoxygenase-derived products were reduced, but there was no specific pattern of abnormality. Although there was no direct correlation between either the WBC or platelet count and impairment of platelet C 20:4 metabolism, the platelets from three patients with accelerated disease released the lowest total amount of 3H-C 20:4 metabolites. In a single patient, studied before and after successful chemotherapy (hydroxyurea), severe abnormalities in platelet arachidonic acid metabolism returned to normal after treatment.

Adult↗
PubMed1977

Effects of C-reactive protein on platelet function. III. The role of cAMP, contractile elements, and prostaglandin metabolism in CRP-induced inhibition of platelet aggregation and secretion.

It was previously demonstrated that C-reactive protein (CRP) inhibits platelet aggregation and release reactions, activation of platelet factor 3, and platelet-dependent clot retraction. Multiple considerations including selective inhibition of secondary wave aggregation suggested that CRP exerted its inhibitory effects by interfering with the release of endogenous ADP. In the present investigation, CRP was found by direct assay to inhibit the release of endogenous ADP and/or serotonin concomitant with inhibition of platelet aggregation stimulated by ADP, epinephrine, thrombin, and AHGG. CRP did not induce an increase in the basal level of platelet cAMP, suggesting independence of a direct effect upon this mediator system. Furthermore, CRP did not inhibit the aggregation and secretion induced by the antibiotic ionophore A23187, suggesting the absence of a direct effect upon the activation of platelet contractile elements. By contrast, CRP did inhibit both thrombin-induced release of malondialdehyde, a prostaglandin endoperoxide nonprostanoate endproduct, and platelet aggregation induced by the prostaglandin endoperoxide precursor arachidonic acid. These data, therefore, raise the possibility that CRP inhibits platelet reactivities by interfering with an aspect of porstaglandin metabolism, and that this occurs subsequent to the hydrolytic accumulation of arachidonic acid and prior to the movement of calcium from the platelet dense tubules. These studies support the concept that CRP serves to modulate platelet reactivities during acute inflammatory reactions.

Adenosine Diphosphate↗
PubMed1982

Comparison of the enzymatic sensitivities of the platelet receptor for human C-reactive protein and its functional relationship to the platelet IgG Fc receptor.

Thermally modified human C-reactive protein (H-CRP) and IgG (AHGG) each activate isolated human platelets to reactions of aggregation and secretion. As these molecules exhibit many functional similarities, we questioned whether they might also share a receptor on the platelet membrane. Neither plasmin nor phospholipase C altered the platelet response to H-CRP or AHGG, although these reagents enhanced the platelet expression to acid soluble collagen (ASC). Conversely, chymotrypsin treatment of platelets resulted in an elevated response to each H-CRP and AHGG, but not to ASC. These data suggest that the H-CRP and AHGG platelet receptors share characteristics which contrast with those of the receptor for collagen. However, monomeric IgG, which can bind with the platelet and inhibit the response to AHGG, exerted no effect on the platelet response to H-CRP. Further, a functional receptor for thermally modified human or rabbit CRP was detected on rabbit platelets in the absence of a demonstrable Fc receptor for aggregated IgG. These data indicate that the platelet receptors for the modified forms of CRP and IgG are distinct.

Animals↗
PubMed1981

Platelet lipid composition and platelet aggregation in human liver disease.

Abnormal plasma lipoproteins in patients with liver disease are associated with an increase in erythrocyte cholesterol concentration and a raised erythrocyte cholesterol/phospholipid molar ratio. We hypothesized that their platelets would also have an increased cholesterol/phospholipid ratio and that this might affect aggregation in vitro. Platelet aggregates by adrenaline and ADP was measured in 34 patients with a variety of liver diseases and in 20 normal subjects and the values were related to platelet lipid composition. The platelet cholesterol/phospholipid ratio was 13% higher in the patients and correlated closely with erythrocyte cholesterol/phospholipid ratio. Platelet aggregation was reduced in most of the patients and inversely correlated with the cholesterol/phospholipid ratio. Cross-incubation and hemostasis studies indicated that there were no inhibitory factors present in the plasma; the defect was in the platelets. In contrast, other workers have shown that cholesterol-rich platelets, either from patients with Type IIa hyperlipoproteinemia or prepared in vitro, aggregate more readily than normal platelets. However, the phospholipid and fatty acid compositions of our patient platelets were also abnormal: the lecithin/sphingomyelin ratio was increased and was inversely correlated with aggregation; the proportion of arachidonic acid was decreased and positively correlated with the aggregation. In our patients with liver diseases the effects of the altered phospholipid and fatty acid composition presumably overrode those of the increased cholesterol content so that instead of enhanced aggregation, only reduced or normal aggregation was seen. We conclude that the reduced platelet aggregation seen in liver disease may reflect a decrease in arachidonic acid availability for prostaglandin and/or thromboxane production.

Blood Platelets↗
PubMed1981

Modulation of platelet aggregation by native DNA - initial description of platelet receptor type, number and discrimination for native DNA.

Native DNA (dsDNA) induces the aggregation of isolated human platelets. Using isotopically labeled dsDNA (125I-dsDNA) and Scatchard analysis, a single class of platelet receptor was detected with a KD = 190 pM and numbering approximately 275/platelet. This receptor was discriminatory in that heat denatured dsDNA, poly A, poly C, poly C x I and poly C x poly I failed to substantially inhibit either the platelet binding of, or platelet aggregation induced by, dsDNA; by themselves, these polynucleotides were ineffective as platelet agonists. However, poly G, poly I and poly G x I effectively and competitively inhibited platelet binding of the radioligand, independently activated the platelet and when used at a sub-activating concentration decreased the extent of dsDNA stimulated platelet aggregation. These data depict a receptor on human platelets for dsDNA and perhaps certain additional polynucleotides and relate receptor-ligand interactions to a physiologic platelet function.

Animals↗
PubMed1994

Fc-independent cross-linking of a novel platelet membrane protein by a monoclonal antibody causes platelet activation.

A monoclonal antiplatelet antibody (MA-13G8E1) is described that dose-dependently induces platelet aggregation and serotonin release in an Fc-independent fashion. Whereas platelets were equally aggregated by F(ab')2 fragments of this monoclonal antibody (MoAb), its Fab fragments, on the other hand, were inactive, indicating that divalent interaction is an essential requirement to induce platelet activation by MA-13G8E1. In addition, we could show that platelet epitope cross-linking by MA-13G8E1 occurred on the same platelet. MA-13G8E1 stimulated platelet phospholipase C (PLC) and induced activation of protein kinase C (PKC), both of which were almost unaffected by aspirin pretreatment. Furthermore, PLC activation appeared to be a direct antibody-mediated effect, since intracellular Ca2+ rises were not inhibited by EGTA, cytochalasin B, or aggregation-blocking MA-16N7C2 (antiglycoprotein [anti-GP]IIb/IIa). The MA-13G8E1 antigen is constitutively expressed on resting platelets of different species (7,100 +/- 800 molecules per human platelet), but not on other cell types tested. Both immunoprecipitation and affinity isolation by MA-13G8E1 showed two low-molecular weight proteins (45 and 36 kD), having slightly acidic isoelectric pH levels (4.5 to 5.5) and forming multimolecular complexes. In conclusion, we found an MoAb that is able to induce platelet activation in an Fc-independent fashion. The mechanism involves cross-linking of a hitherto undescribed platelet membrane protein, leading to PLC and PKC stimulation.

Animals↗
PubMed1979

Shape-changing agents produce abnormally large platelets in a hereditary "giant platelets syndrome (MPS)".

Geometries of platelets in citrated PRP obtained from normal donors (17) and donors (5) with a hereditary dominant giant platelet syndrome, herein referred to as "Montreal platelet syndrome" (MPS), are compared. The measured geometric axial ratio (rp = thickness/diameter) is used to classify platelet morphologies into three groups: discocytes (rp less than 0.5), disco-echinocytes (rp = 0.5 to 0.9), sphero-echinocytes (rp greater than 0.9). MPS discocytes are normal sized; however, MPS sphero-echinocytes and disco-echinocytes have mean volumes approximately two times larger than normal. It is demonstrated that these larger-than-normal sized MPS platelets can be produced directly from MPS discocytes by treatment with agents known to induce platelet shape change (adenosine diphosphate, thrombin, and incubation at 4 degrees C). Treatment of platelets obtained from normal donors which have been resuspended in MPS PPP and ADP or incubation at 4 degrees C causes the formation of normal-sized disco-echinocytes and sphero-echinocytes. The diameters of MPS disco-echinocytes are identical to the diameters of MPS platelets observed on peripheral blood smear, whereas those of MPS sphero-echinocytes are approximately 20% lower. It is suggested that the appearance of abnormally large platelets in MPS is related to a defect in the mechanism which regulates platelet size and shape during shape change.

Adult↗
PubMedDate not supplied

Cryopreservation of human platelets using dimethyl sulfoxide and glycerol-glucose: effects on "in vitro" platelet function.

BACKGROUND: The technique of freezing blood platelets could be very useful in the transfusion support of thrombocytopenic patients. The best method of platelet cryopreservation still remains an object of debate, though it has been suggested that dimethyl sulfoxide (DMSO) is more effective than glycerol-glucose as a cryopreservative. However, few studies have directly compared platelets cryopreserved with different methods. METHODS: We compared "in vitro" function of platelets cryopreserved with 5% dimethyl sulfoxide (DMSO) or 3% glycerol-glucose at -140 degrees C. Platelet aggregation and release reaction were studied with a Lumi aggregometer, thromboxane B2 (TxB2) production by radioimmunoassay, and Ca++ movement by the Fura 2 method. RESULTS: Cryopreservation with both of the methods dramatically reduced the ability of platelets to release ATP and to aggregate in response to single agonists. In contrast, cryopreserved platelets maintained their ability to aggregate after stimulation with paired agonists and to produce TxB2. Cytoplasmic Ca++ increase induced by thrombin was observed in the glycerol-preserved platelets, while it was nearly absent in the DMSO-preserved ones. CONCLUSIONS: We suggest that cryopreservation with glycerol-glucose or DMSO induces similar defects of platelet function. The damage is severe, but platelets are still able to respond to strong stimulation.

Blood Platelets↗
PubMed1996

Platelet lipoxygenase inhibitors attenuate thrombin- and thromboxane mimetic-induced intracellular calcium mobilization and platelet aggregation.

Platelets metabolize arachidonic acid via cyclooxygenase and lipoxygenase (LO) enzymatic pathways. Although platelets produce large amounts of arachidonic acid metabolites via the LO pathway, little is known regarding the physiological significance of these products. We used three structurally dissimilar LO inhibitors, 5,8,11-eicosatriynoic acid (ETI), baicalein and phenidone, and found that LO inhibition attenuated thrombin- and U46619 (a thromboxane mimetic)-induced increases of platelet intracellular calcium ([Ca++]i) in washed human platelets. LO inhibitors also reduced platelet aggregation induced by thrombin and U46619. The effect of ETI on reducing the thrombin-induced [Ca++]i elevation persisted even when cation channels were blocked, suggesting that LO inhibitors modify release of Ca from intracellular stores. Stimulating endogenous LO product formation potentiated thrombin-induced [Ca++]i responses and aggregation, and these effects were eliminated by ETI. ETI did not alter inositol 1,4,5-trisphosphate production in stimulated platelets, but increased platelet cyclic AMP production in thrombin- or forskolin-stimulated platelets. These results suggest that LO products are regulators of platelet [Ca++]i mobilization and aggregation in response to some agonists, and that LO inhibitors may work in part by modifying platelet cyclic AMP metabolism.

12-Hydroxy-5,8,10,14-eicosatetraenoic Acid↗
PubMed1996

Inhibition of binding of anti-PLA1 antibodies to platelets with monoclonal antibody LK-4. Evidence for multiple PLA1 receptor sites on platelet GPIIIa.

The PLA1 epitope on platelet GPIIIa has a sulfhydryl-dependent conformation and is dependent on a leucine 33/proline33 polymorphism. Monoclonal antibody LK-4 differentiates PLA1/PLA1 from PLA2/PLA2 platelet lysates on solid phase enzyme-linked immunosorbent assay (ELISA), as well as immunoblot. To determine whether LK-4 reacts at or near the binding site(s) for human anti-PLA1, nine such antibodies (Abs) (six neonatal; three posttransfusion) were examined in the presence and absence of LK-4 for binding to platelets, as well as rGPIIIa 1-66, a recombinant glutathione S-transferase fusion peptide. All nine human Abs bound to rGPIIIa 1-66, as well as platelets, in a saturation-dependent manner, employing both solid phase ELISA, as well as flow cytometry. Binding of all nine Abs to rGPIIIa 1-66 or platelets was inhibited by LK-4. IC50's for inhibition of binding of anti-PLA1 to rGPIIIa 1-66 varied from 8 to 160 micrograms/mL (5 x 10(-8)- 1 x 10(-6) mol/L). However, IC50's for LK-4 inhibition of binding to platelets was strikingly different. Six of the nine Abs had IC50's of 1 to 10 micrograms/mL (8-fold to 16-fold greater inhibition than with rGPIIIa 1-66), whereas three neonatal Abs had IC50's of 380 to 1,013 micrograms/mL (6-fold to 48-fold less inhibition than with rGPIIIa 1-66). Similar results were noted with intact GPIIIa, rGPIIIa 1-66 blocked the binding of anti-PLA1 Abs to platelets and served to segregate the nine patients into two groups: a sensitive group of anti-PLA1 Abs from six patients in which binding to platelets was progressively inhibited by increasing concentrations of rGPIIIa 1-66 with inhibition at 1 micrograms/mL of 18% and inhibition at 256 micrograms/mL of 78%; a second resistant group of three anti-PLA1 Abs from three patients in which inhibition was first noted at 16 micrograms/mL of 4% with 35% inhibition at 256 micrograms/mL. Thus, LK-4 binds to GPIIIa at the 1-66 N-terminal region, inhibits binding of anti-PLA1 Ab to platelets, and segregates, anti-PLA1 Abs into two groups. These data are compatible with two or more receptor sites for anti-PLA1 Ab: one that is present on rGPIIIa 1-66 and sensitive to LK-4 inhibition, another that is present on rGPIIIa 1-66, as well as other site(s) on platelet GPIIIa and insensitive to inhibition.

Antibodies, Monoclonal↗
PubMed1998

A fifty percent reduction of platelet surface glycoprotein Ib does not affect platelet adhesion under flow conditions.

Glycoprotein (GP) Ib is an adhesion receptor on the platelet surface that binds to von Willebrand Factor (vWF). vWF becomes attached to collagens and other adhesive proteins that become exposed when the vessel wall is damaged. Several investigators have shown that during cardiopulmonary bypass (CPB) surgery and also during platelet activation in vitro by thrombin or thrombin receptor activating peptide (TRAP) GPIb disappears from the platelet surface. Such a disappearance is presumed to lead to a decreased adhesive capacity. In the present study, we show that a 65% decrease in platelet surface expression of GPIb, due to stimulation of platelets in Orgaran anticoagulated whole blood with 15 micromol/L TRAP, had no effect on platelet adhesion to both collagen type III and the extracellular matrix (ECM) of human umbilical vein endothelial cells under flow conditions in a single-pass perfusion system. In contrast to adhesion, ristocetin-induced platelet agglutination was highly dependent on the presence of GPIb. Immunoelectron microscopic studies showed that GPIb almost immediately returned to the platelet surface once platelets had attached to collagen. In a subsequent series of experiments, we showed that when less than 50% of GPIb was blocked by an inhibitory monoclonal antibody against GPIb (6D1), platelet adhesion under flow conditions remained unaffected.

Blood Platelets↗