Flow-related platelet deposition on subendothelium

The deposition of platelets on subendothelium is controlled both by flow factors that determine their transport to the surface through convective diffusion, and by the kinetics of their interactions with the surface. Red cells influence platelet transport by several mechanisms, and may also influence platelet reactivity. Platelets initially contact the surface through the binding of membrane GPIb to von Willebrand factor (vWf) that is present in the subendothelium or deposited there from plasma. Platelet spreading on the surface is promoted by the binding of the arg1744 gly1745 asp1746 (RGD) region of vWf (at high shear rates) to platelet GPIIb-IIIa. At high (arterial) shear rates, vWf binding to GPIIb-IIIa also promotes thrombus formation. At these shear rates, thrombin that is generated at the subendothelial surface is not involved in platelet adhesion or thrombus formation that occurs early in thrombogenesis, but is a major mediator of subsequent thrombus growth and/or stabilization.

Mathematical Analysis of Bleeding Time Data in Patients with Platelet Disorders and von Willebrand's Disease

In a previous study the volume of blood obtained from bleeding time incisions was measured every 30 s from normal subjects (n = 15), patients with thrombasthenia (TSA, n=4), idiopathic thrombocytopenic purpura (ITP, n=4), von Willebrand's disease ((v)WD, n=3), Bernard-Soulier syndrome (BSS, n = 2), and δ and αδ-storage pool deficiencies (SPD, n=4 and 5, respectively) and the experimental results analyzed by empirical curve-fitting of the data. In the present investigation, a mathematical model based on blood flow physiology was developed to describe the rate of blood loss over time from these same patients as a function of two parameters, α, which describes the magnitude of vessel contraction following transection, and β, the rate of vessel dilation to its nominal diameter. For the normal controls a third parameter, δ, was used to describe the rate of vessel closure due to the formation of a hemostatic plug. Optimal values for these parameters for the normal subjects and each patient group were determined by least-squared fitting of the experimental bleeding time data. For all subjects, values for the magnitude of vessel contraction were similar (α=0.65±0.02). However, values for β were reduced in both TSA (β=0.22±0.04) and (v)WD (β = 0.30±0.03) and were increased relative to normal controls (β=0.39±0.03) in BSS (β=0.50±0.01) and both δSPD (β=0.50±0.07) and αδSPD (β=0.50±0.05). The initial rate of blood loss was also significantly greater in patients with BSS, ITP, δ-SPD, and αδSPD than in the normal subjects, as determined by a one-way analysis of variance. These results suggest that: (1) the initial contraction of severed blood vessels does not appear to be mediated by any plasma or platelet compounds absent in the various bleeding disorders considered in this study; and (2) the increased initial bleeding observed in SPD may reflect the absence of vasoactive agents, such as ADP or serotonin, released from platelet dense granules following platelet activation. These conclusions are consistent with those reported previously on the same patients and indicate that mathematical modeling of bleeding time measurements, based on assumptions of vascular and platelet reactivity, can provide insights into the complex series of events occurring at sites of vessel injury.

Production and characterization of transformed B-lymphocytes expressing the membrane defect of Scott syndrome

Scott syndrome is a bleeding disorder associated with an isolated defect in expression of membrane coagulant activity by stimulated platelets. This defect represents a decrease in platelet membrane binding sites for coagulation factors Va and VIIIa, reflecting diminished surface exposure of phosphatidylserine (PS). To gain insight into the cellular and genetic basis for this disorder, B-lymphocytes from a patient with Scott syndrome and from normal donors were immortalized by EBV-transformation, and tested for their capacity to expose plasma membrane PS in response to the Ca2+ ionophore, A23187. Upon incubation with A23187, EBV-lymphoblasts derived from normal donors consistently induced surface expression of PS in > 70% of all cells, as detected by membrane association of the PS-binding proteins, factor Va or annexin V. PS exposure in these cells was maximal after 5 min, and saturated at < 100 microM external free [Ca2+]. By contrast, < 30% of Scott syndrome lymphoblasts exposed PS, and saturation was not observed at > 1 mM external free [Ca2+]. Single-cell clones derived from the Scott lymphoblasts all exhibited a diminished response to A23187 comparable with that of the parental cells, suggesting that all lymphocytes from this patient share this membrane abnormality. Hybridomas prepared by fusion of Scott lymphoblasts with the myeloma cell line UC-LUC showed responses to Ca2+ ionophore comparable to those observed for normal lymphoblasts and for hybridomas prepared by fusion of normal lymphoblasts with UC-LUC. This correction of the Scott abnormality suggests possible complementation of an aberrant gene(s) responsible for this disorder.

Quantitative investigations of acetylcholinesterase activities in colorectal malformations

In cases of colorectal innervation disturbance the histological and histochemical findings are well placed to classify the morphological state and to indicate to the pediatric surgeon the therapeutic direction. Any limitation of their value results from technical errors, age dependent localization of morphological structures and the subjectivity of the assessment. For this reason a quantitative biochemical investigation of cholinesterase activity in homogenized investigation of cholinesterase activity in homogenized rectal mucosa specimens and resected bowel segments was performed. The biochemical standard value of the AChE-activity amounts up to 3-6 x 10(-7) mol ACh/min/g in healthy test patients and the ratio of specific/unspecific esterases 1.1 to 1.3. The percentage of AChE to the total enzyme activity was 47.5%. The standard distribution of the different globular and asymmetric cholinesterase structures is described by the proportion of the molecular types G1/G2/G4 and A12. These main types determine the biochemical nature of the human rectum. Hypertrophy of cholinergic nerve fibres in aganglionic bowel segments correlates pathobiochemically with the increase of the cholinesterases caused by the high molecular tetramer G4. However, the Hirschsprung's disease could not be differentiated by the percentage of the AChE-activity to the total ChE-activity because of the continuous increase in the BChE in the aganglionic tissue. Neuronal intestinal dysplasias with their main cholinergic nerve fibre hyperplasias are characterized by disturbances of the dualism and interrelation of different cholinesterases. By means of pathogiochemical investigations the subjective factor in the assessment of histochemical states is avoided and the diagnostic accuracy increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a role of glycoprotein IIb-IIIa, distinct from its ability to support aggregation, in platelet activation by ionophores in the presence of extracellular divalent cations

Ionophore A23187-induced 14C-serotonin secretion and thromboxane B2(TxB2) formation were found to be absent in citrated platelet-rich plasma (PRP) from thrombasthenic subjects and in normal PRP treated with glycoprotein (GP) IIb-IIIa complex-specific monoclonal antibodies. Both responses were restored to normal levels when 5 mmol/L EDTA was present, indicating that their absence was not caused by the absence of aggregation per se. In gel-filtered platelets (GFP) incubated with various additions, the blockade or absence of GPIIb-IIIa resulted in reduced A23187-induced secretion and TxB2 formation in media containing 1 mmol/L Ca2+ with or without fibrinogen and 1 mmol/L Mg2+ plus fibrinogen, but not when Ca, Mg-free buffer alone, 1 mmol/L EDTA, or fibrinogen alone were present. In contrast, no such dependence or GPIIb-IIIa was seen in GFP stimulated with thrombin or phorbol myristate acetate in the presence of 1 mmol/L Ca2+, 1 mmol/L EDTA, or buffer alone. The inhibition of ionophore-induced responses seen in both normal GFP treated with antibodies and thrombasthenic GFP was not associated with any significant alteration of the ionophore-mediated [Ca2+]i increase, as measured in both aequorin-loaded GFP stimulated with A23187 and fura-2-loaded GFP stimulated with ionomycin. Incubation of normal GFP with either the monoclonal antibodies or the ligand binding site peptide RGDS in the presence of 1 mmol/L Ca2+ caused virtually complete inhibition of A23187-induced aggregation, measured as the loss of single platelets, but RGDS, in contrast to the antibodies, did not inhibit secretion or TxB2 formation. We conclude that platelet activation induced by ionophores in the presence, but not in the absence, of extracellular divalent cations involves a GPIIb-IIIa-dependent process that most likely involves a property of the ligand-occupied form of the complex distinct from its ability to support aggregation. This could represent another example of an aggregation-independent activity of the receptor-occupied state of the GPIIb-IIIa complex in signal transduction.

Further studies on the presence of functional tissue factor activity on the subendothelium of normal human and rabbit arteries

Although tissue factor (TF) activity has been observed on the subendothelial surface of rabbit aorta and human umbilical cord, immunofluorescent and in situ hybridization methods have failed repeatedly to demonstrate TF in the intima of human blood vessels. In the present study, TF activity on everted, de-endothelialized arteries was studied by two methods. One utilized a flow system and measured fibrin deposition and fibrinopeptide A formation. The other utilized a newly developed rotating probe system and measured the conversion of factor X to factor Xa in the presence of factor VIIa and Ca+2. The study attempted to control, or assess, the possibility that functional TF could have been exposed on the vessel surface by the procedures used to prepare the arterial segments. By both methods, TF activity was detected on the subendothelium of rabbit aortae and human umbilical arteries, and was unaffected by the length of storage or by inclusion of actinomycin D in the storage buffer. TF activity was also observed in the subendothelium of adult human ileo-colic, internal mammary, and renal arteries, studied by the rotating probe method. The latter may underestimate TF activity, as some of the factor Xa formed appears to bind to the subendothelial surface. TF activity (Xa formation) was detected on the luminal surface (subendothelium) of non-everted arteries, but increased activity was observed after eversion of the vessel. The source of the subendothelial TF, and its presence in normal subendothelium in vivo, requires further study. In addition, if any of the TF activity observed in this study was derived from injured endothelial or myointimal cells during preparation of the everted vessel segments, the techniques described could serve as a useful model for studying TF-induced thrombosis and factor Xa formation on injured blood vessels, and for evaluating the anti-thrombotic properties of TF-inhibitors.

Evidence that the arg1744 gly1745 asp1746 sequence in the GPIIb-IIIa-binding domain of von Willebrand factor is involved in platelet adhesion and thrombus formation on subendothelium

Previous studies have strongly suggested that the initial attachment (adhesion) of platelets to subendothelium or collagen at high shear rates is mediated by the binding of von Willebrand factor (vWf) to platelet glycoprotein Ib (GPIb). The finding in the present study that incubating human umbilical artery subendothelium with a monoclonal antibody to the GPIb binding site of vWf markedly inhibited platelet adhesion is fully consistent with this hypothesis. Platelet adhesion (and thrombus formation) also requires GPIIb-IIIa, and previous studies have suggested that the binding of vWf to GPIIb-IIIa may be involved in this process. To explore this further, we utilized two monoclonal antibodies (152B-20 and 152B-6) that recognize the arg1744 gly1745 asp1746 (RGD) sequence of vWf. These antibodies selectively inhibit vWf binding to GPIIb-IIIa and have no cross-reactivity with other RGD-containing proteins that can bind to GPIIb-IIIa. When added to citrated human blood, these antibodies (152B-20 in particular) inhibited platelet adhesion and thrombus formation on rabbit subendothelium at a shear rate of 2600 sec-1 but not at 400 sec-1. The results of the study thus provide direct evidence that the binding of vWf to GPIIb-IIIa is important for platelet adhesion and thrombus formation on subendothelium at high shear rates and that the arg1744 gly1745 asp1746 sequence in the mature vWf is involved in this process.

Studies of thromboxane B2, platelet factor 4, and fibrinopeptide A in bleeding-time blood of patients deficient in von Willebrand factor, platelet glycoproteins Ib and IIb-IIIa, and storage granules

The blood volumes and concentrations of thromboxane B2 (TxB2), platelet factor 4 (PF4), and fibrinopeptide A (FPA) were measured every 30 seconds in bleeding-time blood in normal subjects and in patients with idiopathic thrombocytopenic purpura (ITP), delta and alpha delta storage pool deficiency (SPD), Bernard-Soulier Syndrome (BSS), thrombasthenia (TSA), and von Willebrand's disease (vWD). Data were fitted to second-order (TxB2, PF4, and FPA) or third-order (volumes) polynomials. Average values for various parameters over fixed-time intervals were determined by numerical methods. The bleeding time was greater than 15 minutes in all patient groups and the initial bleeding, as reflected by the initial slope of the fitted blood volume curves, was increased in ITP, BSS, and SPD (delta-SPD in particular), but not in vWD and TSA. The increased values for both the initial slope and the volume of blood collected after 2 minutes in SPD suggest that vascular tone may be modulated, in part, by dense granule substances such as adenosine triphosphate (ATP) or serotonin. In TSA, uniquely, both platelet (TxB2 and PF4) and coagulation (FPA) values were increased in early bleeding samples (initial slope). In vitro studies of TxB2 production, together with previous flow studies of fibrin formation, also suggest enhanced activation and coagulant properties of thrombasthenic platelets. In other patients, reduced values of all substances at later times may reflect impaired platelet-fibrin plug formation in the high-shear regions at the ends of transected blood vessels. However, the initial slopes of the fitted curves for both TxB2 and PF4 were normal in vWD, suggesting that the early appearance of these substances may typically be from platelets that are adherent to collagen within the lower shear environment of the wound surface. The finding that FPA values were not decreased initially in any patient group, including ITP, but were decreased at later times (except for TSA), suggests that early fibrin formation occurs independently of platelets in the low-shear environment of the wound surface, whereas at later times fibrin is formed in a platelet-dependent manner in the high-shear regions at the ends of transected vessels.

Heterogeneous abnormalities of platelet dense granule ultrastructure in 20 patients with congenital storage pool deficiency

Studies on platelet dense granule structure were carried out in 20 patients with various types of congenital storage pool deficiency (SPD), including 15 with specific deficiencies of dense granules and dense granule substances (delta-SPD), and five with combined deficiencies of dense and alpha-granules (alpha delta-SPD). Dense granules were identified by their high affinity for uranyl ions (uranaffin reaction), by their ability to accumulate the fluorescent dye mepacrine, and by their inherent electron opacity on unfixed, unstained whole mount preparations. By all these methods, dense granules were markedly decreased in seven albino patients with the Hermansky-Pudlak syndrome (HPS) variant of delta-SPD. These findings suggest that the basic defect in these patients is a specific abnormality in organelle development which prevents the formation of an intact granule structure, a quantitative abnormality which may differ from that in animals with related pigment disorders. In contrast, eight non-albino patients with delta-SPD had, on average, only a slightly reduced number of uranaffin-positive and mepacrine-positive granules, but a shift in uranaffin-granule distribution towards those lacking a dense core ('empty granules'), suggesting a more qualitative type of dense granule defect. These results are consistent with previous evidence suggesting a decreased uptake of ATP across the granule membrane in delta-SPD. In addition, on whole mounts, these patients' platelets contained substantial numbers of electron dense chains and clusters which contained P and Ca, but with a P/Ca ratio less than that of typical dense granules, and which were retained, along with a larger amount of ATP, after thrombin treatment of the platelets. The various findings in these patients raise the possibility that these structures may represent microvesicles, derived from the Golgi apparatus, which provide a transport mechanism for concentrating adenine nucleotides and calcium in dense granules and which is impaired in some patients with SPD. Additional defects may account for the more extensive granule abnormalities observed in alpha delta-SPD.

Defective Ca(2+)-induced microvesiculation and deficient expression of procoagulant activity in erythrocytes from a patient with a bleeding disorder: a study of the red blood cells of Scott syndrome

The erythrocytes from a patient with Scott syndrome, a bleeding disorder characterized by an isolated defect in expression of platelet procoagulant activity, have been studied. When incubated with the calcium ionophore A23187, Scott syndrome red blood cells (RBCs) expressed less than 10% of the prothrombinase (enzyme complex of coagulation factors Va and Xa) activity of A23187-treated RBCs obtained from normal controls. Consistent with the results from enzyme assay, the ionophore-treated Scott syndrome erythrocytes exhibited diminished membrane vesiculation and decreased exposure of membrane binding sites for factor Va compared with identically treated controls. When examined by scanning electron microscopy, untreated Scott syndrome RBCs were indistinguishable from normal cells. After incubation with A23187, however, the morphology of Scott syndrome RBCs contrasted markedly from normal erythrocytes. Whereas the Ca2+ ionophore induced marked echinocytosis and spiculation of normal RBCs, Scott syndrome RBCs remained mostly discoid under these conditions, with only an occasional echinocyte-like cell observed. These aberrant responses to intracellular Ca2+ were also observed for resealed ghosts prepared from Scott syndrome erythrocytes, indicating that they are related to a defect in the membrane or membrane-associated cytoskeleton. The finding that the erythrocytes of this patient share many of the membrane abnormalities reported previously for Scott syndrome platelets suggests that this defect is common to both cell lines and involves a membrane component required for vesicle formation and for expression of prothrombinase sites.

Human platelets deficient in dense granules contain normal amounts of pp60c-src

We have determined that pp60c-src, a protein tyrosine kinase abundant in normal platelets, is present at comparable levels in platelets that are deficient in dense granules (Hermansky-Pudlak syndrome). Relative quantitation of pp60c-src was performed by immunoblot analysis after protein separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our data suggest that human platelet dense granules, unlike chromaffin cell secretory granules, are not the major intracellular site of localization of pp60c-src.

Upstream thrombus growth impairs downstream thrombogenesis in non-anticoagulated blood: effect of procoagulant artery subendothelium and non-procoagulant collagen

In the present experiments we have investigated the influence of wall shear rate and axial position on platelet and fibrin deposition which results when flowing human non-anticoagulated blood is exposed to either non-procoagulant fibrillar collagen (human type III) or procoagulant subendothelium (rabbit aorta). Platelet adhesion, thrombus volume and fibrin deposition were morphometrically evaluated at axial positions of 1 and 13 mm following perfusions for 5 min at shear rates of 100, 650 and 2,600 s-1. An axially-dependent decrease of platelet adhesion (34-57%, p less than 0.01-0.05) and thrombus volume (57-80%, p less than 0.05) was observed on collagen at all shear rates. On subendothelium, an axially-dependent decrease was observed for platelet adhesion only at 100 s-1 (29%; p less than 0.01) and for thrombus volume at shear rates of 650 s-1 and above (49-58%, p less than 0.01). Deposition of fibrin on subendothelium was axially decreased (16-42%, p less than 0.05) at all shear rates, while no significant axial differences were seen on collagen. However, substantially more fibrin was deposited on the subendothelium (p less than 0.05), and the upstream platelet adhesion and thrombus volume were lower than on collagen (p less than 0.05) at 100 s-1 and 650 s-1. The axially-dependent phenomena on the two surfaces are consistent with the concept of rapid-growing upstream thrombi which deplete the blood layer streaming adjacent ot the surface of platelets, leading to decreased platelet deposition further downstream.(ABSTRACT TRUNCATED AT 250 WORDS)

Decreased content and surface expression of alpha-granule membrane protein GMP-140 in one of two types of platelet alpha delta storage pool deficiency

To determine whether alpha-granule membranes are present in platelets of patients with storage pool deficiencies of both alpha and dense granules (alpha delta-SPD), we examined the content and surface expression of the alpha-granule membrane protein GMP-140 in one patient (J.C.) with a severe alpha-granule deficiency and in three members of a family (family C) with milder alpha-granule deficiencies. Surface expression of GMP-140 in stimulated platelets, assessed by flow cytometric measurements of the binding of two anti-GMP-140 monoclonal antibodies, was 24-38% of normal values in platelets from patient J.C., vs. 60-95% of normal values in family C. Total platelet content of GMP-140, determined in platelet lysates by antigen-capture ELISA, was 49% of normal in patient J.C., but normal in the members of family C. Platelets of patient J.C. were found to be heterogeneous with respect to GMP-140 content and surface expression by both flow cytometry and immunogold electron microscopy. Approximately 80% of her platelets expressed little or no GMP-140 after stimulation, whereas the remaining 20% expressed normal amounts of GMP-140 and showed extensive immunogold labeling of typical alpha-granules and clear vacuoles. No such heterogeneity was found in platelets from family C. These findings in the severe alpha delta-SPD patient are in clear contrast to the observations of normal GMP-140 content in the three other alpha delta-SPD patients, and in patients with the gray platelet syndrome, reported previously by others. These results illustrate the phenotypic heterogeneity of alpha-granule deficiencies in human platelets, and suggest that a defect in granule formation in the megakaryocytes may account for the alpha-granule defect in at least one form of alpha delta-SPD.

Further evidence that glycoprotein IIb-IIIa mediates platelet spreading on subendothelium

In order to explore further the mechanism by which glycoprotein GPIIb-IIIa promotes platelet vessel wall interaction, platelet adhesion to subendothelium was studied in an annular chamber in which subendothelium from rabbit aorta was exposed at a shear rate of 2,600 s-1 to blood from patients with thrombasthenia. Perfusions were conducted for each of 5 exposure times (1, 2, 3, 5 and 10 min), and the percent surface coverage of the vessel segment with platelets in the contact (C) and spread (S) stage was determined. Increased values of platelet contact (C) were obtained in thrombasthenia at all exposure times; this finding is consistent with a defect in platelet spreading, based on a previously described kinetic model of platelet attachment to subendothelium. According to this model of attachment, increased values of platelet contact (C) at a single exposure time may be indicative of either a defect in spreading (S) or initial contact (C), but multiple exposures will result in increased contact only for defects which are related to defective platelet spreading (S). The results obtained over a broad range of exposure times provide more conclusive evidence that GPIIb-IIIa mediates platelet spreading than those previously obtained at single exposure times.

Studies on platelet protein phosphorylation in patients with impaired responses to platelet agonists

The phosphorylation responses of platelet proteins after platelet stimulation with agonists were studied in patients with clinical bleeding disorders and various types of impaired platelet functional responses. Impaired collagen-induced phosphorylation, particularly of the 47 kd substrate (P47) for protein kinase C, was observed in one patient whose platelet defect appears to be an impaired initial response to weak platelet agonists but whose platelet secretory mechanism is normal. Reduced phosphorylation of a 31 kd polypeptide was also observed. The phosphorylation defect in this patient differs from that seen in another patient in whom impaired P47 and myosin light chain phosphorylation was observed but whose functional defect may be more closely related to secretion. The results provide further evidence that phosphorylation of P47 may play a role in platelet activation mechanisms preceding secretion and that abnormalities of phosphorylation of both P47 and myosin light chain may be associated with platelet functional defects in some patients with bleeding disorders.

Time dependence of aequorin-indicated calcium levels in stimulated and unstimulated platelets: evidence for multiple aequorin environments in platelets

Aequorin-induced calcium signals were examined in human unstimulated platelets and platelets stimulated with various agonists as a function of time. The total aequorin response in unstimulated platelets, obtained by Triton x-100 lysis in the presence of 1 mM Ca, decreased in a distinctly non-linear manner over 20-60 min. This decrease was slightly, but significantly, greater in platelets maintained in the continuous presence of 1 mM Ca than in platelets maintained without external Ca, and could not be accounted for completely by leakage of aequorin from the cells. Basal Ca levels in unstimulated platelets also decreased in a non-linear manner, with a similar sensitivity to the continuous presence or absence of external Ca. These observed changes in aequorin response thus appear to be at least partially due to an intracellular discharge of aequorin, and are therefore consistent with the view that aequorin in platelets is heterogeneously distributed among localized environments differing in Ca concentration. The aequorin signals observed initially in platelets stimulated by ADP or epinephrine were lost completely over a period of 30-60 min in almost all cases studied, while initial rates of aggregation were either unchanged (epinephrine) or only partially decreased (ADP) over this same time period. In contrast, thrombin- and A23187-induced aequorin signals were virtually unchanged over periods up to 90 min. Minimal changes with time also occurred in the aequorin signals induced by phorbol ester or by collagen in the presence of indomethacin. These differences in time dependence suggest that the signals generated by ADP and epinephrine may derive from different sources of aequorin than those associated with the signals induced by other agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Assembly of the platelet prothrombinase complex is linked to vesiculation of the platelet plasma membrane. Studies in Scott syndrome: an isolated defect in platelet procoagulant activity

Activation of human platelets by complement proteins C5b-9 is accompanied by the release of small plasma membrane vesicles (microparticles) that are highly enriched in binding sites for coagulation factor Va and exhibit prothrombinase activity. We have now examined whether assembly of the prothrombinase enzyme complex (factors VaXa) is directly linked to the process of microparticle formation. Gel-filtered platelets were incubated without stirring with various agonists at 37 degrees C, and the functional expression of cell surface receptors on platelets and on shed microparticles was analyzed using specific monoclonal antibodies and fluorescence-gated flow cytometry. In addition to the C5b-9 proteins, thrombin, collagen, and the calcium ionophore A23187 were each found to induce formation of platelet microparticles that incorporated plasma membrane glycoproteins GP Ib, IIb, and IIIa. These microparticles were enriched in binding sites for factor Va, and their formation paralleled the expression of catalytic surface for the prothrombinase enzyme complex. Little or no microparticle release or prothrombinase activity were observed when platelets were stimulated with epinephrine and ADP, despite exposure of platelet fibrinogen receptors by these agonists. When platelets were exposed to thrombin plus collagen, the shed microparticles contained activated GP IIb-IIIa complexes that bound fibrinogen. By contrast, GP IIb-IIIa incorporated into C5b-9 induced microparticles did not express fibrinogen receptor function. Platelets from a patient with an isolated defect in inducible procoagulant activity (Scott syndrome) were found to be markedly impaired in their capacity to generate microparticles in response to all platelet activators, and this was accompanied by a comparable decrease in the number and function of inducible factor Va receptors. Taken together, these data indicate that the exposure of the platelet factor Va receptor is directly coupled to plasma membrane vesiculation and that this event can be dissociated from other activation-dependent platelet responses. Since a catalytic membrane surface is required for optimal thrombin generation, platelet microparticle formation may play a role in the normal hemostatic response to vascular injury.

Inhibition of human platelet function in vitro and ex vivo by acetaminophen

The effects of acetaminophen (APAP) in vitro, or ex vivo following APAP ingestion, on human platelet aggregation, 14C-5HT secretion, and thromboxane B2 (TxB2) formation were assessed. APAP added in vitro to citrated platelet-rich plasma (PRP) inhibited aggregation, secretion, and TxB2 formation induced by collagen, epinephrine, arachidonate, and the ionophore A23187, but had no effect on the responses induced by the endoperoxide analog U44069. Arachidonate-induced responses were inhibited by lower concentrations of APAP than were the responses to the other agonists. In PRP obtained 1 hour after ingestion of 650 mg or 1000 mg APAP, arachidonate-induced TxB2 formation was inhibited by 40-99% in five subjects tested, whereas inhibition of collagen- or epinephrine-induced TxB2 formation was less consistent. Aggregation and secretion responses were not altered by APAP ingestion in 4 of the 5 subjects, but were inhibited in the remaining subject, who had the highest plasma APAP levels. In contrast to aspirin and indomethacin, APAP-induced inhibition of collagen-stimulated TxB2 formation could be partially overcome with increasing collagen concentrations. No such partial correction occurred with epinephrine, however. In washed platelet suspensions labeled with 3H-arachidonate, both APAP and aspirin inhibited the formation of labeled PGD2 and PGE2, as well as TxB2. These results suggest that APAP acts in human platelets as a reversible inhibitor of cyclo-oxygenase, as found previously in other tissues, and that recent APAP ingestion can, on occasion, produce inhibition of platelet functional responses measured in vitro.

Evidence for the presence of tissue factor activity on subendothelium

By a variety of methods, tissue factor activity was demonstrated in the subendothelium of rabbit aorta and human umbilical artery. In one method, everted segments of de-endothelialized vessels were mounted in an annular perfusion chamber and the subendothelial surface was exposed to nonanticoagulated human blood under controlled flow. Procoagulant activity was assessed by measuring fibrin deposition on subendothelium and fibrinopeptide A (FPA) levels in post chamber blood. Both fibrin deposition and FPA were decreased with rabbit vessel segments exposed (at a shear rate of 650 seconds-1) to blood from patients with factor VII deficiency and with umbilical artery segments (at shear rates of 90 to 180 seconds-1) that had been pretreated with a monoclonal antibody to human tissue factor. In a second method, everted umbilical artery segments were mounted on a stir bar and the subendothelial surface was exposed, with stirring, to plasma or purified coagulation factors. The capacity of the surface to clot plasma on addition of calcium was inhibited by the antibody to tissue factor. The surface also activated purified 3H-factor X in the presence of factor VIIa, but not in its absence, and this surface property was almost entirely eliminated by pretreating the vessel segments with antitissue factor. Tissue factor activity in subendothelium could play a role in both the arrest of bleeding and in promoting the formation of thrombi at sites of vascular injury.

Fibrinogen-independent platelet adhesion and thrombus formation on subendothelium mediated by glycoprotein IIb-IIIa complex at high shear rate

Platelet adhesion and thrombus formation on subendothelium, studied at a shear rate of 2,600 s-1, were inhibited by two synthetic peptides known to interact with GPIIb-IIIa. One peptide (HHLGGAKQAGDV) corresponds to the carboxyl terminal segment of the fibrinogen gamma-chain (gamma 400-411) and the other (RGDS) contains the amino acid sequence Arg-Gly-Asp (RGD) common to fibronectin, von Willebrand factor, vitronectin and the alpha-chain of fibrinogen. Neither platelet adhesion nor thrombus formation were decreased in a patient with severe congenital fibrinogen deficiency and this was equally true when his blood was further depleted of the small amounts of fibrinogen present utilizing an anti-fibrinogen antibody. In normal subjects, adhesion and thrombus formation were inhibited by the Fab' fragments of a monoclonal anti-GPIIb-IIIa antibody (LJ-CP8), which interferes with the interaction of platelets with all four adhesive proteins in both the fluid and solid phase. However, another anti-GPIIb-IIIa antibody (LJ-P5) that had minimal effects on the interaction of platelets with fibrinogen, but inhibited to varying degrees platelet interaction with other adhesive proteins, was equally effective. The findings demonstrate that, at a shear rate of 2,600 s-1, adhesive proteins other than fibrinogen are involved in GPIIb-IIIa-mediated platelet adhesion and thrombus formation on subendothelium. In addition, since LJ-P5 inhibited the binding of soluble von Willebrand factor and vitronectin, these adhesive proteins may be involved in platelet thrombus formation. In contrast to the results obtained at a shear rate of 2,600 s-1, fibrinogen could play a role in mediating platelet-platelet interactions with weak agonists or lower shear rates.

The response of platelets to epinephrine in storage pool deficiency--evidence pertaining to the role of adenosine diphosphate in mediating primary and secondary aggregation

Aggregation responses and thromboxane (Tx) formation in ten patients with storage pool deficiency (SPD) specific to the dense granules (delta-SPD) were studied to assess further the role of dense granule adenosine diphosphate (ADP) in mediating platelet aggregation by epinephrine. The ability of epinephrine to elicit secondary aggregation (SA) responses was highly variable in delta-SPD when tested at 5 mumol/L epinephrine, but was consistently abnormal when tested over a range of concentrations. The occurrence of SA in both delta-SPD patients and normal subjects was correlated with the magnitude of the rate of primary aggregation (PA). This PA rate was normal, on average, for the entire patient group but was greater in patients with more consistent SA responses. The PA findings were related to the Kd value obtained in binding studies with 3H-yohimbine, but not with the number of alpha 2-receptor sites. Studies on Tx production (assessed by radioimmunoassay of TxB2) showed that the ability to synthesize Tx from arachidonate was not impaired in delta-SPD, and that there was an absolute positive correlation between epinephrine-induced SA and Tx production. Aggregation in delta-SPD platelets in response to the Tx receptor agonist U44069 was consistently decreased, but could be corrected by addition of ADP. The results of the study suggest that dense granule-derived ADP is not required for PA by epinephrine, but mediates SA as a synergistic agonist with TxA2. This role of ADP in SA may be elucidated more precisely by further studies on platelet activation processes in delta-SPD.