Localization of von Willebrand factor and thrombin-interactive domains on human platelet glycoprotein Ib

Platelet membrane glycoprotein Ib (GPIb) functions as receptors for thrombin and von Willebrand factor (vWF) in the presence of ristocetin. To precisely locate the domains on GPIb interacting with vWF and thrombin, we prepared several peptides that have amino acid sequences analogous to that of the GPIb alpha-chain and examined their effects on ristocetin-induced (vWF-dependent) and thrombin-induced platelet aggregations. A peptide extending from residues Asp235 to Lys262 showed the strongest inhibitory effect on ristocetin-induced platelet agglutination, and a group of overlapping peptides composed of 24-28 amino acid residues representing sequences extending from Phe216 to Asp274 was found to inhibit platelet aggregation induced by thrombin. Other peptides did not inhibit platelet aggregations. Moreover, the binding to platelets of the monoclonal anti-GPIb antibody (TM60) which had been shown to inhibit both ristocetin- and thrombin-induced platelet aggregations was strongly inhibited by a peptide extending from Asp249 to Asp274. These data demonstrate that the vWF-binding domain exists in a small region between residues Asp235 and Lys262; the thrombin-interacting domain, in contrast, is located between residues Phe216 and Ala274, with a possible center of interaction in the sequence from Phe216 to Thr240 on the GPIb alpha-chain, and thrombin binding requires a relatively strict conformation in this domain.     

Characterization of an antiglycoprotein Ib monoclonal antibody that specifically inhibits platelet-thrombin interaction

Platelet glycoprotein Ib (GPIb) acts as a high-affinity thrombin binding site and as a receptor for von Willebrand Factor (vWF). A new anti-GPIb monoclonal antibody (mAB) VM16d was produced that specifically inhibited platelet-thrombin but not platelet-vWF interaction. The epitope for VM16d was located within the 45 kDa N-terminal region of the a-chain of GPIb. VM16d inhibited platelet aggregation induced by low dose thrombin (0.05 U/ml) but did not affect platelet aggregation induced by ristocetin, bovine vWF, ADP or collagen. The same inhibitory effects on thrombin-induced platelet aggregation were observed with the whole IgG molecule of VM16d and its F(ab′)₂ and F(ab′) fragments. VM16d also inhibited ¹⁴C-serotonin secretion induced by low dose thrombin and binding of ¹²⁵I-thrombin but not ristocetin-dependent binding of ¹²⁵I-vWF to platelets. These data indicate that the high-affinity thrombin binding site is located on the N-terminal 45 kDa domain of GPIb and that it is topographically separated from the vWF binding site.     

gammaA/gamma' fibrinogen inhibits thrombin-induced platelet aggregation

The minor gammaA/gamma' fibrinogen isoform contains a high affinity binding site for thrombin exosite II that is lacking in the major gammaA/gammaA fibrinogen isoform. We therefore investigated the biological consequences of the gamma' chain binding to thrombin. Thrombin-induced platelet aggregation was inhibited by gammaA/gamma' fibrinogen. Carboxyl terminal peptide fragment gamma'410-427 from the gamma' chain was also inhibitory, with an IC(50) of approximately 200 microM in whole plasma. Deletion of the peptide from either the amino or carboxyl end significantly decreased inhibition. In contrast to thrombin-induced platelet aggregation, aggregation induced by epinephrine, ADP, arachidonic acid, or SFLLRN peptide showed little inhibition by the gamma' peptide. The inhibition of thrombin-induced platelet aggregation was not due to direct inhibition of the thrombin active site, since cleavage of a small peptidyl substrate was 91% of normal even in the presence of 1 mM gamma'410-427. The gamma'410-427 peptide blocked platelet adhesion to immobilized thrombin under both static and flow conditions, blocked soluble thrombin binding to platelet GPIbalpha, and inhibited PAR1 cleavage by thrombin. These results suggest that the gamma' chain of fibrinogen inhibits thrombin-induced platelet aggregation by binding to thrombin exosite II. Thrombin that is bound to the gamma' chain is thereby prevented from activating platelets, while retaining its amidolytic activity.     

Characterization of the thrombin-induced desensitization of platelet activation by thrombin

Brief exposure of platelets to thrombin makes them less sensitive to subsequent activation by thrombin, a phenomenon demonstrated by Shuman, Botney, and Fenton [J. Clin. Invest., 63, 1211-1218, 1979] by incubating prostacyclin-inhibited platelets with thrombin; after removal of thrombin and prostacyclin, the platelets were selectively desensitized to subsequent activation by thrombin. The conditions for this desensitization have been further defined. Inhibition of thrombin-induced platelet activation by prostacyclin was not absolute, it was only temporary, it could be overcome with higher thrombin concentrations, and it varied with platelet concentration and temperature. With low enough thrombin concentrations, high enough prostacyclin concentrations and short enough times of exposure, platelets could be pretreated with thrombin with no evidence of activation. After addition of hirudin to inhibit thrombin, the platelets were washed and tested for thrombin-induced secretion of ATP. Desensitization to thrombin depended on the concentration of thrombin during pretreatment and on the length of pretreatment, consistent with a catalytic modification of a receptor. A less extensive desensitization was observed when platelets without inhibitor were incubated with a sub-threshold level of thrombin before addition of an activating concentration of thrombin. This desensitization also varied with the time of pretreatment and the concentration of sub-threshold thrombin.     

Effect of an impermeant arginine-modifying reagent on the responses of rabbit platelets to agonists

The importance of surface arginyl residues in platelet aggregation was investigated by studying the effects of an impermeant arginine-modifying reagent, p-sulfonylphenylglyoxal (PSPG), on platelet responses to various agonists. Pretreatment of resuspended rabbit platelets with 2-15 mM PSPG resulted in complete inhibition of aggregation responses to ADP and 5-HT, and a concentration-dependent inhibition of the preceding shape change. Aggregation responses to thrombin also were inhibited in a concentration-dependent manner. The protective effects of antagonists of these three agonists (beta, gamma-methylene ATP for ADP, hirudin for thrombin and phentolamine for 5-HT) during pretreatment of platelets with PSPG indicated that intact arginine residues form part of the receptor sites for ADP and for thrombin. Arginine residues are not part of the 5-HT receptor site itself, but seem to be important for the maintenance of the functional integrity of this site.     

Contribution of PAR-1, PAR-4 and GPIbalpha in intracellular signaling leading to the cleavage of the beta3 cytoplasmic domain during thrombin-induced platelet aggregation

Integrin alphaIIbbeta3 plays a pivotal role in platelet aggregation by binding to fibrinogen. The beta3 cytoplasmic domain of alphaIIbbeta3 interacts with cytoskeletal and signaling proteins and is cleaved by micro -calpain, a calcium regulated cysteine protease. In the present study, we have investigated in more detail the cleavage of the beta3 cytoplasmic domain during platelet aggregation induced by thrombin, TRAP-1 and TRAP-4. Our data show that beta3 is cleaved in all three cases. The time course of beta3 cleavage and the amount of cleaved beta3 depends on the way platelets are activated and on the complete activation of micro -calpain, with a maximum of 90% of cleaved beta3 obtained when thrombin is used. Furthermore, our results also show that the cleaved alphaIIbbeta3 is mainly distributed in the Triton soluble fraction, indicating its inability to bind to the cytoskeleton. Interestingly, in the absence of GPIbalpha or following inhibition of thrombin binding to GPIbalpha, there is a reduction in the thrombin-induced calcium flux, beta3 cleavage and micro -calpain activation. These results suggest that cleavage of the beta3 cytoplasmic domain by micro -calpain might be an important step regulating the link between the cytoskeleton and alphaIIbbeta3 during platelet aggregation, and that GPIbalpha could function as a cofactor for the complete activation of platelets by thrombin.     

Evidence for the involvement of platelet glycoproteins other than GP Ib in heparin-associated thrombocytopenia and thrombosis

Heparin-associated thrombocytopenia and thrombosis (HATT) is a potentially fatal complication of heparin therapy which is characterized by a progressive fall in the platelet count associated with arterial or venous thrombosis. The etiology is consistent with the development an antibody which, in the presence of heparin, induces intravascular platelet aggregation. Biochemical analysis has demonstrated that the platelet membrane glycoprotein (GP) Ib binds heparin. Recent studies have shown that polyclonal antisera or monoclonal antibodies to GP Ib can inhibit platelet aggregation induced by HATT plasma in the presence of heparin implicating GP Ib as the site of heparin-antibody interaction. The Bernard-Soulier syndrome (BSS) is an inherited bleeding disorder in which the platelets fail to adhere to exposed subendothelium due to a deficiency of GP Ib. We have used the platelets from a patient with documented BSS to further investigate the role of GP Ib in the heparin- dependent platelet aggregation induced by the plasma of three patients with HATT. We have shown that platelet aggregation by HATT plasma in the presence of heparin occurred as readily with BSS platelets as with normal donor platelets. These results indicate that glycoprotein Ib cannot be the only site of platelet-heparin-antibody interactions.     

Glycoprotein V hydrolysis by thrombin. Lack of correlation with secretion

Platelet membrane glycoprotein V (GPV) was hydrolyzed during thrombin-induced platelet aggregation releasing a fragment GPV_f1 into the supernatant. Hydrolysis of GPV required catalytically active thrombin and was diminished by chemical modification of the fibrinogen binding site of thrombin. Half maximal liberation of GPV_f1 occurred at a 10-fold higher concentration of thrombin than was required for half-maximal release. Time course studies at several thrombin concentrations showed disparate release of GPV_f1 and thrombospondin. These results emphasize the complexity of the initial events in thrombin-induced platelet activation.     

The amino acid sequence in fibrin responsible for high affinity thrombin binding

Human fibrin has a low affinity thrombin binding site in its E domain and a high affinity binding site in the carboxy-terminal region of its variant gamma' chain (gamma'408-427). Comparison of the gamma' amino acid sequence (VRPEHPAETEYDSLYPEDDL) with other protein sequences known to bind to thrombin exosites such as those in GPIbalpha, the platelet thrombin receptor, thrombomodulin, and hirudin suggests no homology or consensus sequences, but Glu and Asp enrichment are common to all. Tyrosine sulfation in these sequences enhances thrombin exosite binding, but this has not been uniformly investigated. The fibrinogen gamma' chain mass determined by electrospray ionization mass spectrometry, was 50,549 Da, a value 151 Da greater than predicted from its amino acid/carbohydrate sequence. Since each sulfate group increases mass by 80 Da, this indicates that both tyrosines at 418 and 422 are sulfated. A series of overlapping gamma' peptides was prepared for evaluation of their inhibition of 125I-labeled PPACK-thrombin binding to fibrin. gamma'414-427 was as effective an inhibitor as gamma'408-427 and its binding affinity was dependent on all carboxy-terminal residues. Mono Tyr-sulfated peptides were prepared by substituting non-sulfatable Phe for Tyr at gamma'418 or 422. Sulfation at either Tyr residue increased binding competition compared with non-sulfated peptides, but was less effective than doubly sulfated peptides, which had 4 to 8-fold greater affinity. The reverse gamma' peptide or the forward sequence with repositioned Tyr residues did not compete well for thrombin binding, indicating that the positions of charged residues are important for thrombin binding affinity.     

On the mechanism of the spermine-exerted inhibition on alpha-thrombin-induced platelet activation

Previous reports have shown that various amines inhibited platelet activation, but no definitive conclusions on their action mechanism were drawn. We have further investigated the action of spermine on platelet responses evoked by alpha-thrombin and other agonists. Spermine inhibited in a concentration-dependent manner (1-10 mM), and more efficiently than spermidine and putrescine, the alpha-thrombin-induced (1.5 nM) platelet activation. Spermine added at a concentration that inhibited completely aggregation only partially affected the thrombin-induced increase in cytosolic Ca(2+) concentration, protein phosphorylation, and ATP secretion. The polyamine had little effect on the morphology of resting platelets, as measured by electron microscopy, thrombin hydrolytic activity, and fibrinogen clotting capacity but decreased the thrombin binding to platelets and isolated glycocalicin. Spermine partially inhibited the aggregation elicited by ADP, vasopressin, platelet-activating factor, thrombin receptor-activating peptide, fluoroaluminate, ionomycin, and dioctanoylglycerol but did not affect the cytosolic Ca(2+) increase induced by these agonists. The polyamine bound to both glycocalicin and platelets, and it inhibited the fibrinogen binding to stimulated platelets. The amount of 14C-spermine bound to resting cells decreased in the presence of the glycoprotein GPIb-antibody LJIB1, whereas the polyamine bound to activated platelets, which was higher than that tied to resting cells, was markedly reduced by LJCP8 or decorsin, a GPIIb/IIIa antibody and antagonist-peptide, respectively. These results indicate that spermine specifically inhibits the thrombin binding to GPIb of resting platelets and the fibrinogen binding to GPIIb/IIIa (integrin alpha(IIb)beta(3)) of activated platelets.     

A novel tetrameric venom protein, agglucetin from Agkistrodon acutus, acts as a glycoprotein Ib agonist.

A novel platelet agglutination inducer, agglucetin, was purified from the Formosan Agkistrodon acutus snake venom. It migrated as a single band with an apparent molecular mass of 58.8 kDa and two distinct bands of 16.2/14.5 kDa under non-reducing and reducing conditions by SDS-PAGE, respectively. Further confirmed by FPLC, electrospray ionization mass spectrometry and 2D-PAGE, native agglucetin exists as a tetramer composed of disulfide-linked alpha1, alpha2, beta1 and beta2 subunits. Partial N-terminal sequence of agglucetin subunit showed a high degree of homology to those of C-type lectin-like glycoprotein (GP) Ib binding proteins. Functional studies showed that agglucetin. in the absence of von Willebrand factor (vWF), dose-dependently induced platelet agglutination and caused a negligible elevation of intracellular Ca+2 mobilization and thromboxane B, formation in human platelet suspensions. Anti-GP Ib monoclonal antibodies (mAbs), AP1 or LJ-Ib1, specifically inhibited agglucetin-induced platelet agglutination in a dose-dependent manner. However, EDTA, arietin (a long chain RGD-containing disintegrin), 7E3 (an anti-GP IIb/IIIa mAb), heparin, hirudin, PGE1, or indomethacin exhibited no inhibitory effect on agglucetin-induced platelet agglutination. Furthermore, flow cytometric analysis revealed that FITC-agglucetin dose-dependently bound to human formalin-fixed platelets in a saturable manner, and its binding was specifically blocked by anti-GP Ib mAb. It is concluded that agglucetin, acts specifically on an epitope of platelet membrane GP Ib overlapping with that of API, causing platelet agglutination in a Ca+2- and GP IIb/IIIa-independent manner.     

Unexpected effects of aurin tricarboxylic acid on human platelets.

Aurin tricarboxylic acid (ATA) is a potent inhibitor of ristocetin-mediated platelet agglutination and of shear-induced, von Willebrand factor (vWf)-mediated platelet aggregation, probably via inhibition of vWf interaction with glycoprotein Ib (GPIb). We examined the effects of ATA (both the sodium salt and a solution of ATA in ethanol) on platelet functions in citrated plasma (PRP) and in suspensions of washed platelets in Tyrode-albumin solution (contains 2 mM Ca2+). ATA (42-211 micrograms/ml) blocked aggregation and release of granule contents induced by thrombin (0.15 U/ml in PRP; 0.03 U/ml in platelet suspension). Responses to higher concentrations of thrombin were not inhibited. ATA also prolonged thrombin-induced clotting of fibrinogen. Since ATA had no effect on fibrinogen-induced responses of chymotrypsin-treated platelets, ATA probably acts on thrombin rather than on fibrinogen. In PRP and platelet suspensions, ATA (acid form 106 micrograms/ml; sodium salt 122 micrograms/ml) had little effect on ADP-induced platelet aggregation. The sodium salt of ATA (61-122 micrograms/ml) enhanced collagen-induced aggregation and release by platelets in citrated plasma and by washed platelets; the enhancement was extensively inhibited by aspirin. With platelet suspensions, ATA significantly enhanced aggregation and release caused by low concentrations of sodium arachidonate (15-50 microM); aggregation and release caused by higher concentrations of arachidonate were somewhat inhibited by ATA. Arachidonate-induced aggregation and release were also enhanced by ATA in PRP. ATA enhanced aggregation and release induced by the calcium ionophore A23187; aspirin had little effect on the enhancement.(ABSTRACT TRUNCATED AT 250 WORDS)     

Platelet membrane fluidity and receptor exposition in patients with Alzheimer's disease

The majority of reports indicate that blood platelet membrane fluidity is increased in Alzheimer's disease (AD) patients. The increased membrane fluidity implies that platelet membrane receptors are less exposed to the external environment. To verify this hypothesis we tried to estimate platelet membrane fluidity at its two different depths and receptors' exposure after platelet activation in 12 AD sufferers. Platelet membrane fluidity was measured by the EPR with the use of 2 spin-labelled markers. In AD patients both "near surface" (p < 0.04) and "deeper depth" (p < 0.005) fluidity was significantly increased. The exposure of platelet granule membrane protein--P-selectin and membrane glycoprotein receptors: alpha subunit of glycoprotein Ib and beta3 subunit of GP complex IIb/IIIa were measured by flow cytometry with the use of human platelet monoclonal antibodies labelled with fluorescein and ficoerithrin. The exposure of GPIb alpha subunit was significantly decreased both in resting state (p < 0.0001) and after thrombin activation (p < 0.005). In thrombin-activated platelets the expression of P-selectin and beta3 subunit of fibrinogen receptor were also significantly decreased (p < 0.00001 and p < 0.04, respectively). The authors conclude that both platelet membrane fluidity and receptor exposure might serve as an adjunct marker of in vivo AD diagnosis.     

Actinoidin: a new inhibitor of ristocetin- and ristomycin-induced platelet agglutination

Actinoidin, like vancomycin, inhibits ristocetin-and ristomycin-induced VIIIR:WF-dependent agglutination of untreated or formaldehyde-fixed human platelets but does not interfere with bovine factor VIII-induced agglutination. The socalled “direct effect” of ristocetin and ristomycin on fixed platelets, which is represented by an immediate increase in light absorbancy in the absence of cofactor, was not blocked by actinoidin at concentrations which totally inhibited agglutination in the presence of VIIIR:WF. Actinoidin does not inhibit thrombin-induced platelet aggregation. This represents a further argument against a common platelet membrane receptor site for thrombin and ristocetin or a ristocetin-VIIIR:WF complex on the platelet membrane.     

The effect of platelet inhibitors on glycoprotein V hydrolysis by thrombin

Platelet membrane glycoprotein V ( GPV ) is hydrolyzed during thrombin-induced platelet aggregation. The present studies were undertaken to determine whether cleavage of GPV was a direct result of thrombin action or was secondary to platelet activation. Acetylsalicylic acid or vincristine had little or no effect on platelet activation or cleavage of GPV by thrombin. Prostacyclin or antimycin A and 2-deoxyglucose inhibited platelet activation but had no effect on hydrolysis of GPV . These findings suggest that the hydrolysis of GPV occurs independently of the biochemical and morphological changes associated with platelet activation and is therefore a direct effect of thrombin.     

Gly-pro-arg-pro (GPRP) enhances free thrombin

Fibrinogen-related peptides, which inhibit the interaction of fibrinogen with the platelet membrane glycoprotein IIb/IIIa complex (GPIIb/IIIa) are under preclinical investigation now (1). Whereas peptides containing the Arg-Gly-Asp (RGD) sequence inhibit fibrinogen-dependent platelet aggregation by direct binding to GPIIb/IIIa (2), GPRP inhibits fibrinogen polymerisation by direct binding to the fibrinogen polymerisation sites and modifying the glutamine residues in the - and γ-chains of fibrinogen (3, 4). Since GPRP has been shown to inhibit ADP-induced platelet aggregation it has been suggested as antithrombotic agent (5).

It has been demonstrated that in defibrinated plasma the amount of free thrombin generated after clotting activation is significantly higher than in normal plasma (6). The explanation for this observation is that in normal plasma free thrombin is partially adsorbed on generated fibrin (7). Since GPRP inhibits fibrinogen polymerisation, we investigated the generation of thrombin in platelet-rich plasma (PRP) containing GPRP.     

GPV is a marker of in vivo platelet activation--study in a rat thrombosis model

Thrombin plays a central role in the genesis of thrombotic events and is the most potent known platelet agonist. This enzyme activates platelets by cleaving G-protein coupled protease activated receptors (PARs) and by binding to glycoprotein (GP) Ib. Thrombin also cleaves platelet GPV to liberate a soluble 69 kDa fragment (GPVf1), leaving a 20 kDa fragment (GPVf2) attached to the membrane. The aim of this study was to assess the value of GPV as an in vivo marker of the activation of platelets by thrombin. Newly developed monoclonal and polyclonal antibodies recognizing rat GPVf1 and GPVf2 respectively were used to detect soluble GPV by ELISA and the new NH2-terminus exposed by thrombin using flow cytometry. These assays were employed in a rat thrombosis model designed to trigger thrombin formation in vivo. When thromboplastin (4.8 ml/kg/h) was infused for 30 min, thrombin generation was reflected by a rapid increase in thrombin-antithrombin (TAT) complexes in plasma and by the appearance of GPVf2 at the surface of circulating platelets. Simultaneously, GPVf1 disappeared from the surface of platelets and accumulated as a soluble fragment in plasma, where it was detected by GPV ELISA. These effects were inhibited by pretreatment of the rats with hirudin. Levels of plasma PF4 also increased in this model, but unlike GPV levels which returned slowly (> 2 hours) to baseline, PF4 had a very short half-life. In conclusion, GPV is cleaved by thrombin in vivo, circulates and is a reliable in vivo marker of the activation of platelets by thrombin. Monitoring of GPV levels in rats should be useful to evaluate the effects of antithrombotic and antiplatelet drugs, while further studies will be required to confirm the potential interest of GPV as a marker of thrombotic states in humans.     

Thrombin receptor occupancy modulates aggregation efficiency and platelet surface expression of vWF and thrombospondin at low thrombin concentrations.

Previous studies evaluating requirements for occupancy of thrombin receptors in normal platelet secretion and aggregation, using the thrombin antagonists hirudin and PPACK (D-Phe-Pro-Arg-chloromethylketone), have suggested that at low thrombin activating concentrations (0.025-0.13 U/ml), occupancy was required only in the first 45-60 s following activation. In our study, we differentiate between thrombin receptor occupancy requirements for surface expression of secreted adhesive proteins, for activation of GPIIb-IIIa receptors, and for aggregation of washed platelets (WP) in laminar shear flow. Platelets activated with 0.05 U/ml thrombin for 10 min to allow maximal secretion (hereafter referred to as "pre-activated platelets"), then sheared, showed a 50-70% decrease in platelet counts after 60 s of shear. Treatment of pre-activated platelets with hirudin or PPACK produced a 65% reduction of capture efficiencies, alphaG (reflecting experimental/theoretical initial rates of aggregation), as well as a 30-40% decrease in the surface expression of von Willebrand factor (vWF) and thrombospondin (TSP). However, alpha-granule membrane P-selectin expression and numbers of activated GPIIb-IIIa receptors were comparable for treated and non-treated platelets. No significant difference in any of the parameters tested was observed when platelets were similarly pre-activated with 0.2 U/ml thrombin, due to treatment with thrombin antagonists. Binding of soluble FITC-vWF (GRGDSP-sensitive) to pre-activated, thrombin antagonist treated platelets, was greatly reduced (> or =80%). Soluble Fg was shown to bind to antagonist-treated pre-activated platelets, but could not significantly enhance platelet aggregation. Although occupancy of thrombin receptors by catalytically active thrombin is required transiently for secretion and activation of platelets, there is a further requirement for thrombin occupancy at low thrombin concentrations, for optimizing initial rates of platelet aggregation, surface expression of vWF and TSP, and activated GPIIb-IIIa ligand recognition.     

Functional involvement of thrombospondin in platelet aggregation induced by low versus high concentrations of thrombin

Thrombospondin (TSP) is a major platelet secretory glycoprotein. Earlier studies of various investigators demonstrated that TSP is the endogenous platelet lectin and is responsible for the hemagglutinating activity expressed on formaldehyde-fixed thrombin-treated platelets. The direct effect of highly purified TSP on thrombin-induced platelet aggregation was studied. It was observed that aggregation of gel-filtered platelets induced by low concentrations of thrombin (less than or equal to 0.05 U/ml) was progressively inhibited by increasing concentrations of exogenous TSP (greater than or equal to 60 micrograms/ml). However, inhibition of platelet aggregation by TSP was not observed when higher than 0.1 U/ml thrombin was used to activate platelets. To exclude the possibility that TSP inhibits platelet aggregation by affecting thrombin activation of platelets, three different approaches were utilized. First, by using a chromogenic substrate assay it was shown that TSP does not inhibit the proteolytic activity of thrombin. Second, thromboxane B2 synthesis by thrombin-stimulated platelets was not affected by exogenous TSP. Finally, electron microscopy of thrombin-induced platelet aggregates showed that platelets were activated by thrombin regardless of the presence or absence of exogenous TSP. The results indicate that high concentrations of exogenous TSP (greater than or equal to 60 micrograms/ml) directly interfere with interplatelet recognition among thrombin-activated platelets. This inhibitory effect of TSP can be neutralized by anti-TSP Fab. In addition, anti-TSP Fab directly inhibits platelet aggregation induced by a low (0.02 U/ml) but not by a high (0.1 U/ml) concentration of thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hirudin insensitive thrombin-stimulated platelet release

It has been previously proposed that platelet stimulation may involve two platelet-thrombin complexes: an initial platelet-thrombin complex (P-T) which is converted to an activated platelet-thrombin complex (P*-T). By using the release of radioactive serotonin as a measure of thrombin stimulation, we have demonstrated that under appropriate conditions, a hirudin sensitive and a hirudin insensitive complex can be differentiated. At short platelet-thrombin preincubation times (0-2 minutes) at 4 degrees C, in a buffer containing 18.7 mM phosphate, added hirudin almost completely inhibited the release of radioactive serotonin obtained upon subsequent warming to 37 degrees C (only the hirudin sensitive complex exists). If platelets were preincubated with thrombin for longer periods of time (30 minutes), hirudin became less effective in inhibiting the release obtained upon subsequent warming to 37 degrees C. The same results were obtained whether or not the platelets were washed after incubation at 4 degrees C and before warming to 37 degrees C. We postulate that this change in hirudin sensitivity may reflect a slow conversion of the first platelet-thrombin complex (P-T) to an activated platelet-thrombin complex (P*-T) which can undergo release upon warming. This transition appears to be much faster in acetatetris buffer since at short platelet-thrombin incubation times at 4 degrees C, added hirudin had little or no effect on the release obtained upon warming to 37 degrees C. The difference in the ability of hirudin to inhibit thrombin-induced release in the two buffers was shown to depend upon the presence of phosphate and on variations in ionic strength, and not due to a change in the inhibition constant (Ki) for hirudin.