Palmitoylation Augments the Association of Tetraspanin CD63 with the αIIbβ3-CD9 Complex and the Actin Cytoskeleton in Thrombin-Activated Platelets

Blood ◽  
2008 ◽  
Vol 112 (11) ◽  
pp. 5370-5370
Author(s):  
Sara J. Israels ◽  
Eileen M. McMillan-Ward
Keyword(s):  
Actin Cytoskeleton ◽  
Platelet Activation ◽  
Gradient Centrifugation ◽  
Cysteine Residue ◽  
Membrane Microdomains ◽  
Dependent Manner ◽  
Platelet Surface ◽  
Activated Platelets ◽  
Platelet Spreading ◽  
Partner Proteins

Abstract CD63 and CD9 are members of the tetraspanin superfamily of integral membrane proteins that function as organizers of multi-molecular signaling complexes involved in cell morphology, motility and proliferation. In resting platelets, CD63 is located in the membranes of lysosomes and dense granules, and CD9 complexes with the αIIbβ3 integrin on the platelet surface. Following platelet activation and granule exocytosis, CD63 translocates to the plasma membrane, where it co-localizes with the αIIbβ3-CD9 complex and is incorporated in the Triton-insoluble actin cytoskeleton. Tetraspanin complexes cluster dynamically in unique cholesterol-rich membrane microdomains (tetraspanin-enriched microdomains, TEMs) that differ from prototypic lipid rafts. The assembly and maintenance of TEMs depends on the palmitoylation of both tetraspanins and some of their partner proteins. Protein palmitoylation most commonly involves the thioester linkage of palmitate to a cysteine residue and, because the process is reversible, can regulate cellular functions. In cell lines, palmitoylation of tetraspanin juxta-membrane cysteine residues affects subcellular distribution, complex stability, cellular signaling and motility. Recently we demonstrated that tetraspanins and their partner proteins form TEMs in platelets, however the role of palmitoylation in platelet TEMs assembly and maintenance has not been studied. 3[H]-palmitate-labeled, washed human platelets were studied at rest, or following activation with thrombin (0.1U/ml). CD63 and CD9 were isolated by immunoprecipitation, separated by density gradient centrifugation, and 3[H]-palmitate quantitated. Palmitate levels increased in all fractions however the relative inter-fraction distribution did not change, consistent with previous results showing that the distribution of CD63 and CD9 does not change following platelet activation. 2-bromopalmitate (2- BP), which blocked palmitoylation as demonstrated by decreased 3[H]-palmitate-labeling of platelets, inhibited both thrombin-induced platelet aggregation and platelet spreading on immobilized fibrinogen, in a dose and time-dependent manner. 2-BP also inhibited the activation-dependent association of CD63 with CD9 and the incorporation of CD63 into the Triton-insoluble actin cytoskeleton in thrombin-activated platelets. In contrast 2-BP had minimal effect on either the integrity of the α IIb β3-CD9 complex or its agonist-induced association with the cytoskeleton. In summary, inhibition of palmitoylation blocked the activation-dependent association of CD63 with the αIIbβ3-CD9 complex and with the actin cytoskeleton but did not alter the tetraspanin-integrin association present in resting platelets. The effect of 2-BP on platelet spreading on fibrinogen mirrored that previously seen with anti-CD63 MoAbs, and supports the hypothesis that the association of CD63 with αIIbβ3-CD9 modulates outside-in signaling in adherent platelets.

scholarly journals The fate of the open canalicular system in surface and suspension- activated platelets

Blood ◽  
1989 ◽  
Vol 74 (6) ◽  
pp. 1983-1988 ◽  
Author(s):  
G Escolar ◽  
E Leistikow ◽  
JG White
Keyword(s):  
Cell Surface ◽  
Platelet Activation ◽  
Surface Activation ◽  
Resting Cells ◽  
Platelet Surface ◽  
Early Stages ◽  
Activated Platelets ◽  
Surface Contact ◽  
Platelet Spreading ◽  
Membrane Reservoir

Abstract We have examined the movement of fibrinogen-gold (fgn-Au) complexes in platelets activated in suspension and by surface contact. Fgn-Au probes did not react with resting cells but were bound to the external membrane of platelets in suspension 5 seconds after addition of 1 U/mL of thrombin. At intervals over a period of 5 to 20 minutes, fgn-Au probes moved from the cell surface to peripheral and then deep channels of the open canalicular system (OCS). When platelets were surface activated by exposure to carbon-stabilized, formvar-coated grids for 5 to 20 minutes and then exposed to fgn-Au complexes for 5 minutes, probes were also observed in the OCS. At 5 minutes, over 40% of the platelets had concentrated fgn-Au in their OCS. Results after 10 minutes revealed 25% with gold-filled channels, 16% after 15 minutes, and 5% after 20 minutes. The decrease in frequency of OCS staining correlated with the increasing frequency of spread platelets, suggesting that tension produced by spreading may cause collapse of the OCS or that the OCS may evaginate onto the platelet during spreading. To evaluate the latter hypothesis, platelets were initially exposed to grids for 5 minutes and then incubated with fgn-Au for intervals of 5 to 20 minutes. The frequency of platelets with fgn-Au concentrated in the OCS was greatest at 5 minutes (44%) and decreased at the same rate as the frequency of spread platelets increased. Only 14.7% of the cells contained fgn-Au in the OCS after 20 minutes. These were primarily dendritic in form, while fully spread platelets rarely contained an OCS filled with the probe. The study indicates that fgn-Au particles are cleared to channels of the OCS independent of the mechanism of platelet activation. Fgn-Au that has been concentrated in the OCS at early stages of surface activation can be externalized during platelet spreading but remain internalized in suspension-activated cells. The OCS represents a membrane reservoir that can be evaginated onto the platelet surface during interaction with surfaces.

scholarly journals The fate of the open canalicular system in surface and suspension- activated platelets

Blood ◽  
1989 ◽  
Vol 74 (6) ◽  
pp. 1983-1988 ◽  
Author(s):  
G Escolar ◽  
E Leistikow ◽  
JG White
Keyword(s):  
Cell Surface ◽  
Platelet Activation ◽  
Surface Activation ◽  
Resting Cells ◽  
Platelet Surface ◽  
Early Stages ◽  
Activated Platelets ◽  
Surface Contact ◽  
Platelet Spreading ◽  
Membrane Reservoir

We have examined the movement of fibrinogen-gold (fgn-Au) complexes in platelets activated in suspension and by surface contact. Fgn-Au probes did not react with resting cells but were bound to the external membrane of platelets in suspension 5 seconds after addition of 1 U/mL of thrombin. At intervals over a period of 5 to 20 minutes, fgn-Au probes moved from the cell surface to peripheral and then deep channels of the open canalicular system (OCS). When platelets were surface activated by exposure to carbon-stabilized, formvar-coated grids for 5 to 20 minutes and then exposed to fgn-Au complexes for 5 minutes, probes were also observed in the OCS. At 5 minutes, over 40% of the platelets had concentrated fgn-Au in their OCS. Results after 10 minutes revealed 25% with gold-filled channels, 16% after 15 minutes, and 5% after 20 minutes. The decrease in frequency of OCS staining correlated with the increasing frequency of spread platelets, suggesting that tension produced by spreading may cause collapse of the OCS or that the OCS may evaginate onto the platelet during spreading. To evaluate the latter hypothesis, platelets were initially exposed to grids for 5 minutes and then incubated with fgn-Au for intervals of 5 to 20 minutes. The frequency of platelets with fgn-Au concentrated in the OCS was greatest at 5 minutes (44%) and decreased at the same rate as the frequency of spread platelets increased. Only 14.7% of the cells contained fgn-Au in the OCS after 20 minutes. These were primarily dendritic in form, while fully spread platelets rarely contained an OCS filled with the probe. The study indicates that fgn-Au particles are cleared to channels of the OCS independent of the mechanism of platelet activation. Fgn-Au that has been concentrated in the OCS at early stages of surface activation can be externalized during platelet spreading but remain internalized in suspension-activated cells. The OCS represents a membrane reservoir that can be evaginated onto the platelet surface during interaction with surfaces.

Immunological Evidence for Prothrombin in Human Platelets

1986 ◽  
Vol 55 (02) ◽  
pp. 268-270
Author(s):  
R J Alexander
Keyword(s):  
Electrophoretic Mobility ◽  
Platelet Activation ◽  
Double Diffusion ◽  
Human Platelets ◽  
Secreted Proteins ◽  
Platelet Surface ◽  
Activated Platelets ◽  
Similar Protein ◽  
Diffusion Assay ◽  
Supernatant Solution

SummaryAn attempt was made to isolate from plasma the platelet surface substrate for thrombin, glycoprotein V (GPV), because a GPV antigen was reported to be present in plasma (3). Plasma fractionation based on procedures for purification of GPV from platelets revealed a thrombin-sensitive protein with appropriate electrophoretic mobility. The protein was purified; an antiserum against it i) reacted with detergent-solubilized platelet proteins or secreted proteins in a double diffusion assay, ii) adsorbed a protein from the supernatant solution of activated platelets, and iii) inhibited thrombin-induced platelet activation, but the antiserum did not adsorb labeled GPV. The purified protein was immunochemically related to prothrombin rather than to GPV. Other antibodies against prothrombin were also able to adsorb a protein from platelets. It is concluded that 1) plasma does not contain appreciable amounts of GPV, and 2) platelets contain prothrombin or an immunochemically similar protein.

Extracellular fibrinogen binding protein, Efb, from Staphylococcus aureus binds to platelets and inhibits platelet aggregation

2004 ◽  
Vol 91 (04) ◽  
pp. 779-789 ◽  
Author(s):  
Oonagh Shannon ◽  
Jan-Ingmar Flock
Keyword(s):  
Platelet Aggregation ◽  
Potent Inhibitor ◽  
Specific Binding ◽  
Binding Protein ◽  
Dependent Manner ◽  
Platelet Surface ◽  
Putative Receptor ◽  
Activated Platelets ◽  
Fibrinogen Binding ◽  
Dose Dependent

Summary S. aureus produces and secretes a protein, extracellular fibrinogen binding protein (Efb), which contributes to virulence in wound infection. We have shown here that Efb is a potent inhibitor of platelet aggregation. Efb can bind specifically to platelets by two mechanisms; 1) to fibrinogen naturally bound to the surface of activated platelets and 2) also directly to a surface localized component on the platelets. This latter binding of Efb is independent of fibrinogen. The specific binding of Efb to the putative receptor on the platelet surface results in a stimulated, non-functional binding of fibrinogen in a dose dependent manner, distinct from natural binding of fibrinogen to platelets. The natural binding of fibrinogen to GPIIb/IIIa on activated platelets could be blocked by a monoclonal antibody against this integrin, whereas the Efb-mediated fibrinogen binding could not be blocked. The enhanced Efb-dependent fibrinogen binding to platelets is of a nature that does not promote aggregation of the platelets; instead it inhibits aggregation. The anti-thrombotic action of Efb may explain the effect of Efb on wound healing, which is delayed in the presence of Efb.

scholarly journals Factor XIIIa binding to activated platelets is mediated through activation of glycoprotein IIb-IIIa

Blood ◽  
1994 ◽  
Vol 83 (4) ◽  
pp. 1006-1016 ◽  
Author(s):  
AD Cox ◽  
DV Devine
Keyword(s):  
Monoclonal Antibody ◽  
Flow Cytometry ◽  
Platelet Activation ◽  
Factor Xiiia ◽  
Cross Linking ◽  
Platelet Surface ◽  
Activated Platelets ◽  
Fibrinogen Binding ◽  
A Chain ◽  
Functional Receptor

Abstract Stabilization of a clot is dependent on fibrin cross-linking mediated by the transglutaminase, factor XIIIa (FXIIIa). In addition to fibrin stabilization, FXIIIa acts on a number of platelet-reactive proteins, including fibronectin and vitronectin, as well as the platelet proteins, glycoprotein (GP) IIb-IIIa, myosin, and actin. However, conditions inducing the platelet-activation dependent binding of FXIIIa have not been characterized nor have the sites mediating FXIIIa binding been identified. The generation of FXIIIa and consequent detection of FXIIIa on the platelet surface were compared with other thrombin- induced activation events; the rate at which FXIIIa bound to activated platelets was much slower than platelet degranulation or fibrin(ogen) binding. Whereas platelets could be rapidly induced to express a functional receptor for FXIIIa, the rate of FXIIIa binding to platelets is limited by the rate of conversion of FXIII to FXIIIa. Immunoprecipitation of radiolabeled platelets using polyclonal anti- FXIII A-chain antibody identified two proteins corresponding to GPIIb and GPIIIa. Preincubation of intact platelets with 7E3, a monoclonal antibody that blocks the fibrinogen binding site, or GRGDSP peptide inhibited FXIIIa binding by about 95% when measured by flow cytometry; FXIIIa binding to purified GPIIb-IIIa was also inhibited by 7E3. The binding of FXIIIa to purified GPIIb-IIIa was enhanced by the addition of fibrinogen, but not by that of fibronectin or thrombospondin, suggesting that FXIIIa also binds to fibrinogen associated with the complex. These observations suggest that activated platelets bearing FXIIIa may enhance stabilization of platelet-rich thrombi through surface-localized cross-linking events.

scholarly journals Molecular requirements for the interaction of thrombospondin with thrombin-activated human platelets: modulation of platelet aggregation

Blood ◽  
1992 ◽  
Vol 79 (8) ◽  
pp. 1995-2003 ◽  
Author(s):  
C Legrand ◽  
V Thibert ◽  
V Dubernard ◽  
B Begault ◽  
J Lawler
Keyword(s):  
Platelet Aggregation ◽  
Binding Domain ◽  
Platelet Glycoprotein ◽  
Dependent Manner ◽  
Heparin Binding ◽  
Platelet Surface ◽  
Amino Terminal ◽  
Activated Platelets ◽  
Heparin Binding Domain ◽  
Induced Aggregation

Abstract We have investigated the molecular requirements for thrombospondin (TSP) to bind to the platelet surface and to support the subsequent secretion-dependent platelet aggregation. For this, we used two distinct murine monoclonal antibodies (MoAbs), designated MAI and MAII, raised against human platelet TSP, and three polyclonal antibodies, designated R3, R6, and R5, directed against fusion proteins containing the type 1 (Gly 385-Ile 522), type 2 (Pro 559-Ile 669), and type 3 (Asp 784-Val 932) repeating sequences, respectively. Among them, R5 and R6, but not R3, inhibited thrombin-induced aggregation of washed platelets and the concomitant secretion of serotonin. These antibodies, however, did not inhibit the expression of TSP on thrombin-activated platelets, as measured by the binding of a radiolabeled MoAb to TSP, suggesting that they may inhibit platelet aggregation by interfering with a physiologic event subsequent to TSP binding. In contrast, MoAb MAII, which reacts with an epitope located within the heparin-binding domain of TSP, inhibited both TSP surface expression and platelet aggregation/secretion induced by thrombin. In addition, this MoAb inhibited in a dose-dependent manner (IC50 approximately 0.5 mumol/L) the interaction of 125I-TSP with immobilized fibrinogen and platelet glycoprotein IV, both potential physiologic receptors for TSP on thrombin-activated platelets. These results indicate that the interaction of TSP with the surface of activated platelets can be modulated at the level of a specific epitope located within the amino terminal heparin-binding domain of the molecule. Thus, selective inhibition of the platelet/TSP interaction may represent an alternative approach to the inhibition of platelet aggregation.

DETECTION OF ACTIVATED PLATELETS IN WHOLE BLOOD BY FLOW CYTOMETRY

1987 ◽  
Author(s):  
S J Shattil ◽  
J A Hoxie ◽  
M Cunningham ◽  
C S Abrahms ◽  
J O’Brien ◽  
...  
Keyword(s):  
Flow Cytometry ◽  
Monoclonal Antibodies ◽  
Platelet Activation ◽  
Whole Blood ◽  
Thrombus Formation ◽  
Membrane Surface ◽  
White Blood Cells ◽  
Color Flow ◽  
Platelet Surface ◽  
Activated Platelets

Platelets may become activated in a number of clinical disorders and participate in thrombus formation. We have developed a direct test for activated platelets in whole blood that utilizes dual-color flow cytometry and requires no washing steps. Platelets were distinguished from erythrocytes and white blood cells in the flow cytometer by labeling the platelets with biotin-AP1, an antibody specific for membrane glycoprotein lb, and analyzing the cells for phycoerythrin-streptavidin fluorescence. Membrane surface changes resulting from platelet activation were detected with three different FITC-labeled monoclonal antibodies: 1) PAC1, an antibody specific for the fibrinogen receptor on activated platelets; 2) 9F9, which binds to the D-domain of fibrinogen and detects platelet-bound fibrinogen; and 3) S12, which binds to an alpha-granule membrane protein that associates with the platelet surface during secretion. Unstimulated platelets demonstrated no PAC1, 9F9, or S12-specific fluorescence, indicating that they did not bind these antibodies. Upon stimulation with agonists, however, the platelets demonstrated a dose-dependent increase in FITC-fluorescence. The binding of 9F9 to activated platelets required fibrinogen. Low concentrations of ADP and epinephrine, which induce fibrinogen receptors but little secretion, stimulated near-maximal PAC1 or 9F9 binding but little S12 binding. On the other hand, a concentration of phorbol myristate acetate that evokes full platelet aggregation and secretion induced maximal binding of all three antibodies. When blood samples containing activated and non-activated platelets were mixed, as few as 0.8% activated platelets could be detected by this technique. There was a direct correlation between ADP-induced FITC-PAC1 binding and binding determined in a conventional 125I-PAC1 binding assay (r = 0.99; p < 0.001). These studies demonstrate that activated platelets can be reliably detected in whole blood using activation-dependent monoclonal antibodies and flow cytometry. This method may be useful to assess the degree of platelet activation and the efficacy platelet inhibitor therapy in thrombotic disorders.

scholarly journals Prostaglandin E2 potentiates platelet aggregation by priming protein kinase C

Blood ◽  
1993 ◽  
Vol 82 (9) ◽  
pp. 2704-2713 ◽  
Author(s):  
R Vezza ◽  
R Roberti ◽  
GG Nenci ◽  
P Gresele
Keyword(s):  
Protein Kinase C ◽  
Platelet Aggregation ◽  
Protein Kinase ◽  
Prostaglandin E2 ◽  
Platelet Activation ◽  
Human Platelets ◽  
Platelet Surface ◽  
Kinase C ◽  
Activated Platelets ◽  
Induced Aggregation

Abstract Prostaglandin E2 (PGE2) is produced by activated platelets and by several other cells, including capillary endothelial cells. PGE2 exerts a dual effect on platelet aggregation: inhibitory, at high, supraphysiologic concentrations, and potentiating, at low concentrations. No information exists on the biochemical mechanisms through which PGE2 exerts its proaggregatory effect on human platelets. We have evaluated the activity of PGE2 on human platelets and have analyzed the second messenger pathways involved. PGE2 (5 to 500 nmol/L) significantly enhanced aggregation induced by subthreshold concentrations of U46619, thrombin, adenosine diphosphate (ADP), and phorbol 12-myristate 13-acetate (PMA) without simultaneously increasing calcium transients. At a high concentration (50 mumol/L), PGE2 inhibited both aggregation and calcium movements. PGE2 (5 to 500 nmol/L) significantly enhanced secretion of beta-thromboglobulin (beta TG) and adenosine triphosphate from U46619- and ADP-stimulated platelets, but it did not affect platelet shape change. PGE2 also increased the binding of radiolabeled fibrinogen to the platelet surface and increased the phosphorylation of the 47-kD protein in 32P- labeled platelets stimulated with subthreshold doses of U46619. Finally, the amplification of U46619-induced aggregation by PGE2 (500 nmol/L) was abolished by four different protein kinase C (PKC) inhibitors (calphostin C, staurosporine, H7, and TMB8). Our results suggest that PGE2 exerts its facilitating activity on agonist-induced platelet activation by priming PKC to activation by other agonists. PGE2 potentiates platelet activation at concentrations produced by activated platelets and may thus be of pathophysiologic relevance.

A FVIII C1 Domain Loop Containing Phe2093 Contributes Affinity to the Binding of Recombinant FVIII C1C2 Proteins to Activated Platelets

Blood ◽  
2008 ◽  
Vol 112 (11) ◽  
pp. 3069-3069
Author(s):  
Ting-Chang Hsu ◽  
Kathleen P. Pratt ◽  
Arthur R. Thompson
Keyword(s):  
Membrane Binding ◽  
Cysteine Residue ◽  
Wild Type ◽  
Platelet Surface ◽  
Proteolytic Activation ◽  
Mutant Proteins ◽  
Activated Platelets ◽  
C1 Domain ◽  
Beta Hairpin ◽  
The Difference

Abstract Factor VIII (FVIII) circulates bound to von Willebrand factor, and upon proteolytic activation it dissociates and attaches to activated membranes, e.g. at the platelet surface, that expose negatively-charged phosphatidylserine. This membrane association is mediated entirely by the FVIIIa light chain (A3-C1-C2), and the C2 domain is known to be a primary contributor to the membrane affinity. We previously demonstrated that the C1 domain, which is homologous to C2, also contributes to the affinity for activated platelets, (Hsu et al., Blood111, 200–208, 2008). Our earlier work showed that the platelet affinity of recombinant C1C2, as well as the total number of binding sites per platelet, were significantly higher than those measured for recombinant C2. Thus C1 either interacts with the platelet surface directly, or it stabilizes a conformation of C2 that promotes membrane binding. In this study, the affinities for activated platelets of a series of mutant C1C2 proteins were evaluated to determine residues involved in FVIII binding to platelets. C1C2 and C2 proteins were generated with a free cysteine residue substituted for the wild-type serine at position 2296 in C2 (S2296C does not disrupt the protein structure or affect membrane binding), to which a sulfyhydryl-linked fluorescein probe was attached covalently (C1C2* and C2*). Single or double alanine substitutions were introduced at the two beta-hairpin turns in C2 that mediate membrane binding (M2199A, F2200A, L2251A, L2252A), and at C1 residue F2093, which aligns with C2 residue L2252. Washed platelets were activated with SFLLRN-amide, incubated for 1 hr with the wild-type or mutant proteins, and analyzed by flow cytometry on a FACSCaliber. The relative binding affinities were estimated by using multiple protein concentrations and comparing the fluorescent signals to the values for saturation binding of C1C2* and C2*. Alanine substitutions at all of these positions resulted in decreased binding of C1C2*, comparable to the difference in affinity of C2* versus C1C2* (figure 1A). C1C2-F2093C was then generated, and the introduced sulfhydryl was blocked by adding free cysteine, thus introducing the bulky, charged cystine residue at this putative membrane-binding site (C1C2-F2093C-C). Binding assays utilizing detection by monoclonal antibody ESH8, which does not interfere with membrane attachment, followed by a PE-labeled secondary antibody showed that C1C2-F2093C-C had markedly reduced binding to activated platelets compared to C1C2 (figure 1B). These results are consistent with the hypothesis that the C1 domain contacts the membrane directly when FVIIIa becomes attached to the activated platelet surface and indicate that F2093 contributes significantly to this interaction. Figure Figure

scholarly journals Direct detection of activated platelets and platelet-derived microparticles in humans

Blood ◽  
1990 ◽  
Vol 75 (1) ◽  
pp. 128-138 ◽  
Author(s):  
CS Abrams ◽  
N Ellison ◽  
AZ Budzynski ◽  
SJ Shattil
Keyword(s):  
Flow Cytometry ◽  
Cardiopulmonary Bypass ◽  
Platelet Activation ◽  
Bleeding Time ◽  
Relative Proportion ◽  
Open Heart Surgery ◽  
Normal Range ◽  
In Vivo Models ◽  
Platelet Surface ◽  
Activated Platelets

Flow cytometry was used to determine whether activated platelets and platelet-derived microparticles can be detected directly in whole blood after a hemostatic insult. Two different in vivo models of platelet activation were examined: (1) a standardized bleeding time, and (2) cardiopulmonary bypass. Platelets and microplatelets were identified with a biotinylated anti-glycoprotein (GP)lb antibody and a fluorophore, phycoerythrin-streptavidin. Microparticles were distinguished from platelets by light scatter. Activated platelets were detected with three fluorescein-labeled monoclonal antibodies (MoAbs): (1) PAC1, which binds to the activated form of GPIIb-IIIa; (2) 9F9, a newly developed antibody that is specific for fibrinogen bound to the surface of activated platelets; and (3) S12, which binds to an alpha- granule membrane protein expressed on the platelet surface after granule secretion. In nine normal subjects, bleeding times ranged from 4.5 to 7.5 minutes. Over this time, there was a progressive increase in the amount of PAC1, 9F9, and S12 bound to platelets in blood emerging from the bleeding time wound. With all three antibodies, platelet activation was apparent as early as 30 seconds after the incision (P less than .03). Activation was accompanied by a progressive decrease in the concentration of platelets in blood from the wound, while the concentration of microparticles increased slightly. In nine patients undergoing open heart surgery, 1 hour of cardiopulmonary bypass caused a 2.2-fold increase in the relative proportion of microparticles in circulating blood (P less than .001). Moreover, bypass caused platelet activation as evidenced by a mean two- to threefold increase in PAC1 binding to platelets. Although this increase was significant (P less than .02), PAC1 binding exceeded the normal range for unstimulated control platelets in only 5 of 9 patients, and 9F9 and S12 binding exceeded the normal range in only two patients. Taken together, these studies demonstrate that it is now feasible using flow cytometry to evaluate the extent of platelet activation and the presence of platelet- derived microparticles in the circulation of humans.

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