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PubMed1989

Factors influencing the transfusion response to HLA-selected apheresis donor platelets in patients refractory to random platelet concentrates.

Immune and nonimmune causes of platelet refractoriness were evaluated in a group of patients receiving HLA-selected single-donor platelet transfusions. During a 1-year observation period, 1 h and 24 h platelet recoveries wre determined after 522 single-donor platelet transfusions given to 43 patients persistently refractory to pooled random-donor platelet transfusions. 72% of patients tested ultimately developed lymphocytotoxic antibodies suggesting they were alloimmunized. When significant lymphocytotoxic antibodies were demonstrable in these patients, HLA well-matched platelet transfusions consistently produced good transfusion responses. In contrast, patients without lymphocytotoxic antibodies had clinical factors that adversely affected transfusion outcome (P less than 0.0001). Fever and splenomegaly markedly reduced 1 h post-transfusion platelet recoveries, while sepsis compromised the 24 h platelet recovery. Overall, the presence of any clinical factor was most likely to reduce 1 h platelet recovery, while donor-recipient HLA incompatibilities correlated best with poor 24 h post-transfusion platelet recovery. A platelet crossmatch test predicted the transfusion response when non-immune clinical factors were absent.

Antilymphocyte Serum↗
PubMed1990

Platelet count, mean platelet volume and their relation to prognosis in cerebral infarction.

The study was performed on patients with ischaemic cerebral infarction in order to obtain information on serial changes of some platelet parameters and to test their prognostic significance. Platelet count, obtained within 48 h after cerebral infarction, was significantly lower than in the control group (213,611 +/- 65,652 mm-3 vs. 299,525 +/- 60,611 mm-3, P less than 0.001), reaching the normal level on the ninth day and thereafter. The mean platelet volume was significantly greater than in the controls (11.26 +/- 1.29 fl vs. 8.93 +/- 0.93 fl, P less than 0.001), and normalization generally occurred on the forty-fifth day. The mean platelet count was significantly lower in the patients who died than in those who survived (P less than 0.025 and P less than 0.05 respectively on the first to second and fourth day after infarction). The reduction of platelet count and the increase of mean volume appear to be related to an increased platelet consumption in the infarction area, associated with an in vivo platelet activation, as larger platelets are more responsive to platelet activity and aggregability tests. The lower mean platelet count observed in the patients who died suggests that the platelet value might be considered as a prognostic index of cerebral infarction.

Aged↗
PubMed1996

Microenvironmental changes in platelet membranes induced by the interaction of fibrinogen-derived peptide ligands with platelet integrins.

A few studies have confirmed the influence of peptides containing either the RGD or dodecapeptide H-12-V (HHLGGAKQAGDV) sequence on cell membrane structure and function. In order to consider previous findings and to explore microenvironmental changes associated with the interaction of these two fibrinogen-derived peptides with platelet membranes, we employed fluorescence quenching and electron paramagnetic resonance techniques to monitor the possible alterations in platelet membrane dynamics induced by RGDS and H-12-V. The interaction of RGDS with platelet membranes resulted in reduced values of the h+1/ho parameter in both 5-doxylstearic acid and 16-doxylstearic acid spectra indicating a significant rigidification of the membrane lipid bilayer. Otherwise, the fibrinogen-derived peptide that contained the gamma chain C-terminal sequence H-12-V had a fluidizing effect on the platelet membrane lipid bilayer. The labelling of platelet membranes with 1-anilino-8-naphthalenesulphonate (ANS) enabled us to estimate the energy transfer efficiency and the apparent interchromophore distance between membrane protein tryptophan and ANS embedded into the membrane lipid bilayer. As RGDS interacts with platelet membrane this distance decreases, resulting in the relevant increase of energy transfer efficiency. The opposite alterations were recorded upon interaction of platelet membranes with H-12-V. Furthermore, a small shift towards longer wavelengths, which accompanies the spectra of ANS in control platelet membranes, vanishes during the interaction with the peptide H-12-V. This observation can be accounted for by a decrease in the polarity of the ANS environment, and may suggest an enhanced contact of the membrane tryptophan with phospholipid fatty acids. Thus, the data indicate that after the action of H-12-V on platelet membrane receptors, the membrane tryptophan residues become exposed to the external environment and the quenchable fraction of membrane tryptophan becomes smaller. The increase (a) in the relative rotational correlation time (tau c) of 4-(ethoxyfluorophosphinyloxy)-2,2,6,6-tetramethylpiperidine- 1-oxyl (ethoxyfluorophosphinyloxy-TEMPO) and (b) in the hw/hs ratio in the spectra of 4-maleimido-2,2,6,6-tetramethylpiperidine-1-oxyl (maleimido-TEMPO) indicate that under these conditions there is an effective immobilization of some domains located on the hydrated surface of membrane proteins and mobilization of those domains buried inside the membrane protein molecules. The interaction of RGDS with platelet membrane integrins resulted in contrary effects, as compared to H-12-V. In conclusion, our spectroscopic data indicate that these two fibrinogen-derived peptides induce opposite effects in the dynamics of platelet membrane components.

Amino Acid Sequence↗
PubMed1979

Content and subcellular localization of catecholamines and 5-hydroxytryptamine in human and animal blood platelets: monoamine distribution between platelets and plasma.

1 The content of adrenaline (Ad), noradrenaline (NA) and dopamine was measured in human, guinea-pig, cat, rabbit and rat blood platelets by a highly sensitive and specific radioenzymatic method.2 In all platelet specimens analyzed, the content of the three catecholamines (CA) was several thousand times lower than that of 5-hydroxytryptamine (5-HT).3 In basal conditions, the NA concentration in platelets and plasma always exceeded that of Ad and dopamine.4 In rat and rabbit platelets, Ad, NA and dopamine were present only in the free (unconjugated) form.5 Platelets of rats with storage pool deficiency (Fawn-hooded) contained much less 5-HT and CA than normal rat platelets.6 Following restraint stress, platelets of Fawn-hooded rats, in contrast to normal rat platelets, did not accumulate CA in spite of a dramatic rise in plasma CA.7 Reserpine, a monoamine depletor, released CA as well as 5-HT from rabbit platelets in vivo.8 Subcellular fractionation experiments with rabbit platelets indicate that both CA and 5-HT are most concentrated in the fraction consisting of pure 5-HT organelles.9 Both in humans and rabbits the concentration gradient between platelets and plasma was much lower for CA than for 5-HT, indicating that a high affinity transport mechanism operates in vivo for 5-HT but not for CA.10 In conclusion, the present data show that both human and animal platelets contain Ad, NA and dopamine. The bulk of the CA seems to be stored as unconjugated amines together with 5-HT, histamine and p-octopamine in a multitransmitter storage site, namely the 5-HT organelle.

Adult↗
PubMed1992

Platelets and a platelet-released factor enhance endothelial barrier.

The role of platelets in the maintenance of endothelial barrier is examined in an in vitro model of the microvasculature. Human platelets (6,000/microliters) perfused through a cell column of endothelial-covered microcarriers decrease paracellular permeability of sodium fluorescein (mol wt 342) to 63% of baseline values. This effect is reversible and a second application and removal of platelets produces a similar response. This effect occurs within 5 min and reverses within 10 min after platelet removal. The reduction in permeability is not due to mechanical obstruction of endothelial junctions, since the number of recirculating platelets is not reduced and releasate from unstimulated 2-h platelet incubations also decreases permeability. Releasate from platelets stimulated with 0.1 U/ml of thrombin for 15 min have the same permeability reducing effect. In this system, the platelet factors serotonin (10(-3) M) and ADP (10(-4) M) have no effect on permeability. However, the platelet factors adenosine (10(-4) M), ATP (10(-5) M), and beta-agonists decrease permeability. None of these appear to account for platelet permeability activity, since activity is not blocked by agents directed against these mediators (adenosine deaminase, apyrase, 8-phenyltheophylline, or propranolol). The active factor(s) is stable at -20 degrees C, heat stable, sensitive to trypsin, and has an apparent molecular weight > 100. We conclude that unstimulated platelets release a factor(s) that enhances endothelial barrier in vitro and may be important in maintenance of the normal in vivo barrier.

Animals↗
PubMed1994

Lack of effect of ouabain on calcium homeostasis in rat platelets: comparative study with human platelets.

The precise mechanisms of Ca2+ handling in rat platelets are not fully understood. We sought to determine whether rat platelets possess a Na(+)-Ca2+ exchanger. First, we investigated the time course of the effect of ouabain (10(-4) M) on cytosolic sodium concentration ([Na+]i) and Ca2+ homeostasis in platelets from Wistar rats in comparison with those from humans. Ouabain increased platelet [Na+]i in both rat and human platelets. Whereas ouabain induced a time-dependent increase in basal and thrombin-stimulated (0.3 units/ml) cytosolic calcium concentration ([Ca2+]i) in human platelets, no change was found in rat platelets. Furthermore, 90-min pretreatment of rat platelets with ouabain did not affect the [Ca2+]i response to thrombin (0.1-1.0 units/ml) or a maximal dose of ionomycin (5 microM). Also the decline in [Ca2+]i after the peak response evoked by these agonists in the absence of extracellular Ca2+ was not changed by ouabain pretreatment. Similarly, replacement of extracellular Na+ had no influence on any of these determinations. Thus decreasing the plasma membrane Na+ gradient did not affect basal [Ca2+]i, thrombin-induced mobilization of Ca2+ from intracellular stores, internal Ca2+ discharge capacity, or Ca2+ extrusion from cytosol of rat platelets. In contrast to human platelets, a Na(+)-Ca2+ exchange mechanism does not appear to play a significant role in Ca2+ homeostasis of rat platelets.

Animals↗
PubMed1970

Glycogenesis and glyconeogenesis in human platelets. Incorporation of glucose, pyruvate, and citrate into platelet glycogen; glycogen synthetase and fructose-1,6-diphosphatase activity.

Washed human platelets are capable of depositing 1-4 as well as probable 1-6 glucosyl linkages onto preexistent glycogen primer. They are also capable of degrading (glycogenolysis) newly synthesized 1-4 as well as probable 1-6 glucosyl linkages. A higher rate of glycogen synthesis was found in platelet suspensions containing lower concentrations of platelets. This was shown to result from decreased glycogen degradation and consequent increased residual glycogen primer in low platelet suspensions. The increased glycogen content of low platelet suspensions was not a result of platelet washing, removal of platelets from plasma, or release of platelet metabolites into the media. The enzyme glycogen synthetase was found to be present at a rate of 5.2 mumoles of uridine diphosphate (UDP) glucose incorporated into glycogen per gram platelets per hour at 37 degrees C. The K(m) for UDP glucose was 6.6 mmoles/liter. At optimum concentration of glucose 6-phosphate, the K(m) was reduced 4.6 fold and V(max) was increased 4.3-fold. Human platelets contain the glyconeogenic pathway. They incorporate pyruvate-(14)C and citrate-(14)C into platelet glycogen and contain an apparent fructose-1,6-diphosphatase. The apparent fructose-1,6-diphosphatase was activated by adenosine monophosphate (AMP) and adenosine diphosphate (ADP), inhibited by adenosine triphosphate (ATP), and shown to be rate limiting for glyconeogenesis at physiologic concentration of adenine nucleotide.

Adenosine Triphosphate↗
PubMed1976

Similarities between platelet contraction and cellular motility during mitosis: role of platelet microtubules in clot retraction.

The effects of inhibitors of mitosis, energy metabolism and protein synthesis on clot retraction were investigated. The results show that (1) Incubation of colchicine (0-01-0-1 mM) with platelet-rich plasma (PRP) inhibits the subsequent retraction of clots derived from diluted PRP. (2) Inhibition of clot retraction by high concentrations of colchicine (up to 40 mM) can be overcome by increasing the platelet concentration in the system. (3) Incubation of clots in colchicine or 80% D2O solutions inhibits their retraction. Exposure of partially retracted clots to these agents is without effect. (4) Hydrostatic pressure retards clot retraction. (5) Incubation of PRP with either 2-deoxy-D-glucose or antimycin alone does not affect clot retraction, but a combination of these agents is inhibitory. (6) Clot retraction is not inhibited by puromycin or cycloheximide. (7) Platelets in retracting clots have constricted regions containing microfilaments and pseudopods containing microtubules. Fibrin strands are progressively condensed around the constricted regions as retraction advances. (8) The development of platelet constriction, platelet pseudopods and the intracellular microfilaments are delayed in colchicinized clots, corresponding to the retardation of retraction. Following the initial delay of retraction colchicinized clots, like controls, show condensation of fibrin strands adjacent to these constricted areas of platelets containing microfilaments. The formation of pseudopods is impaired and no microtubules are found in platelets in the presence of colchicine. The above results suggest that the thrombin-induced platelet contraction during clot retraction is a coordinated movement, which, under optimal conditions involves both microtubules and microfilaments. The contraction of microfilaments produces the constriction of platelets and brings about clot retraction by reducing the angle between fibrin strands. Platelet microtubules are related to the development of pseudopods and play a supplementary role in facilitating microfilament-mediated cellular constriction. The similarities between platelet contraction and cellular motility in mitosis is discussed.

Antimycin A↗
PubMed2000

[Use of platelet function analyzer PFA-100 and whole blood aggregometry to monitor blood platelet sensitivity to acetylsalicylic acid (aspirin). Is it possible to reliably monitor antiplatelet treatment using routine laboratory diagnostic methods?].

Introduction of the antiplatelet agents of new generations and the occurrence of the phenomenon of "aspirin-resistance" triggered the search for better, simpler and more reliable routine diagnostic methods to monitor platelet reactivity. Our objective was to evaluate the usefulness and reliability of two simple methods: platelet function analyzer (PFA-100) and whole blood platelet aggregometry for monitoring of platelet function in 18 healthy blood donors and 35 patients with ischaemic heart disease (IHD) subjected to small doses/75 mg and 150 mg a day) of acetylsalicylic acid (aspirin). In 50% of healthy blood donors the intake of 75 mg ASA a day resulted in the prolongation of PFA-100 collagen/epinephrine closure time (CEPI = (relevant to reduced platelet reactivity) of over 150 s, whereas 75% donors responded to 150 mg ASA-. Otherwise, the daily dose of 150 mg ASA resulted in a prolonged CEPI merely in 23% of in IHD patients. At both doses ASA completely inhibited the arachidonic acid-induced whole blood platelet aggregation in all healthy donors and in all but 3 IHD patients. Collagen-induced platelet aggregation was only negligibly affected by either dose of ASA. Our results point that the simultaneous monitoring of the PFA-100 collagen/epinephrine closure time and whole blood platelet aggregometry (Chrono-Log) enables to reliably evaluate the inhibition of platelet function by ASA and discriminate the partial or complete platelet insensitivity to aspirin. The phenomenon of more frequent platelet aspirin-resistance in IHD patients requires to be verified in randomised clinical prospective studies.

Adult↗
PubMed1989

Mechanisms of platelet destruction in immune-mediated thrombocytopenia: in vitro studies with canine platelets exposed to heterologous and isologous antiplatelet antibodies.

Normal canine platelets coated with heterologous or isologous antiplatelet antibody were interacted with viable canine neutrophils in vitro. Platelet phagocytosis was assessed by detecting nitroblue tetrazolium (NBT) reduction, measuring uptake of opsonised 51Cr-labelled platelets, and electron microscopy. The NBT reduction and uptake of 51Cr-labelled opsonised platelets were markedly increased. Electron microscopy revealed phagocytosis of antibody-coated platelets and their degradation intracellularly. Exposure of canine platelets to rabbit anti-canine platelet antibody in vitro produced morphological changes in platelets and caused serotonin release. Serotonin was not released in the absence of antiplatelet antibody or in the presence of normal rabbit gamma-globulin. Morphological changes in the platelets included disappearance of alpha and dense granules and exaggeration of the open canalicular system. These observations indicate that circulating platelets may be vulnerable to an antiplatelet antibody and that antibody-mediated phagocytosis of platelets is an important mechanism in the pathogenesis of immune-mediated thrombocytopenia.

Animals↗
PubMed1988

Inhibition of thrombin-induced platelet activation by leupeptin. Implications for the participation of calpain in the initiation of platelet activation.

Inhibitors of calcium-dependent proteases (calpains) such as leupeptin and antipain have been shown to selectively inhibit platelet activation by thrombin. Based upon this observation, it has been proposed that calpains play a role in the initiation of platelet activation. In the present studies, we have examined the effect of leupeptin on the earliest known event in thrombin-induced platelet activation: the interaction between the agonist, its receptors, and the guanine nucleotide-binding proteins which stimulate phospholipase C (Gp) and inhibit adenylyl cyclase (Gi). We found that leupeptin inhibited thrombin's ability to stimulate phosphoinositide hydrolysis, suppress cAMP formation, and dissociate Gp and Gi into subunits. Leupeptin had no effect, however, on the same responses to other agonists or on thrombin binding to platelets. Although these observations might suggest, as others have concluded, that calpain is involved in the initiation of platelet activation by thrombin, we also found that: 1) substituting platelet membranes for intact platelets and decreasing the free Ca2+ concentration below the threshold required for calpain activation did not diminish the effects of leupeptin on phosphoinositide hydrolysis and cAMP formation, 2) washing the platelets after incubation with leupeptin reversed the effects of the inhibitor, 3) permeabilizing the platelets with saponin did not enhance the inhibitory effects of leupeptin, and 4) leupeptin inhibited the proteolysis of fibrinogen and the hydrolysis of S2238 by thrombin. Similar results in these assays were obtained with antipain. Therefore, our observations suggest that the inhibition of platelet activation by leupeptin is due to a direct interaction with thrombin and need not reflect a role for calpain in the initiation of platelet activation.

Adenosine Diphosphate Ribose↗
PubMed1988

Platelet IgG, IgA, IgM, and albumin: correlation of platelet and plasma concentrations in normal subjects and in patients with ITP or dysproteinemia.

IgG, IgA, IgM, and albumin are primarily known as plasma proteins. Their presence in platelets is poorly understood. The total platelet content of IgG, IgA, and albumin, measured in solubilized platelets by an enzyme-linked immunosorbent-assay (ELISA) technique, was greater than 90% secreted after stimulation by thrombin, consistent with an alpha-granule location. The platelet concentrations of these proteins correlated with their plasma concentrations in normal subjects and over a wide range of abnormalities in patients with IgG or IgA myeloma or liver cirrhosis. IgM was not detectable in normal platelets but was measurable and related to the plasma IgM concentration in patients with macroglobulinemia. In patients with idiopathic thrombocytopenic purpura (ITP), the platelet concentrations of IgG, IgA, and albumin were all twofold to threefold higher than normal despite normal plasma concentrations. Platelet surface IgG, measured by 125I-monoclonal antibody binding, constituted less than 1% of the total platelet IgG, and it appeared to be a pool distinct from the alpha-granule IgG since its concentration in normal subjects and patients did not correlate with either plasma or total platelet IgG concentrations. These observations are consistent with hypotheses that megakaryocytes incorporate plasma proteins into developing alpha-granules by pinocytosis and that the increased ratio of platelet to plasma of IgG, IgA, and albumin in ITP may reflect a younger average age of these platelets.

Blood Platelets↗
PubMed1986

Functional characterization of platelet-bound factor XIa: retention of factor XIa activity on the platelet surface.

Previously we have shown that both factor XI and factor XIa are bound specifically to distinct, high-affinity sites on the surface of activated platelets in the presence of high Mr kininogen. To determine the functional significance of factor XIa binding to platelets, bound factor XIa has now been compared with the unbound enzyme. Platelets incubated with thrombin, high Mr kininogen, and 125I-labeled factor XIa bound 130 to 500 molecules of factor XIa per platelet. Scatchard analysis of binding data give a dissociation constant (Kd) of 822 pmol/L +/- 140 (SEM). Rates of factor IX activation, assayed by release of trichloroacetic acid-soluble 3H-labeled activation peptide from purified [3H]-factor IX, were similar when factor XIa was bound to platelets and when it was free in solution. The platelet-bound factor XIa was isolated by centrifugation through 20% sucrose and was functionally characterized both in a factor XIa coagulation assay and in the factor IX activation peptide release assay in comparison with unbound factor XIa in the presence of treated platelets. The functional activity of platelet-bound factor XIa as a factor IX activator as well as its structural integrity were shown to be fully retained on the platelet surface. Since platelets bind factor XI and promote its proteolytic activation to factor XIa, factor XIa binding to platelets may serve to localize factor IX activation to the hemostatic plug, where factor XIa is protected from inactivation by plasma protease inhibitors and where acceleration of subsequent coagulation reactions can occur.

Binding Sites↗
PubMed1987

Platelet density-dependent partition of platelet-von Willebrand factor between alpha granule and non-alpha granule pools.

In this study, we demonstrate that platelets contain a small but significant amount of platelet-von Willebrand factor (vWf) not associated with alpha-granules. When platelets free of plasma proteins are exposed to micromolar concentrations of digitonin, plasma membrane permeabilization occurs without disruption of platelet granules. Employing this technique, we have found that upon exposure of a total platelet population to 8 microM digitonin, 5% of total platelet-vWf is released into the supernatant; this occurs without release of beta-TG from alpha-granules. When platelets of discrete buoyant density profiles are tested, this extragranular platelet-vWf increased with decreasing platelet density. These findings suggest that a redistribution of platelet-vWf from alpha-granule to non-granule sites occurs coincident with a decrease in platelet buoyant density.

Blood Platelets↗
PubMed1987

Studies with a monoclonal antibody against activated platelets: evidence that a secreted 53,000-molecular weight lysosome-like granule protein is exposed on the surface of activated platelets in the circulation.

To define the role of activated platelets we have attempted to prepare monoclonal antibodies specific for activated platelets. The IgG2b antibody of one of the clones, designated 2.28, was studied in more detail. Native platelets from normal individuals bound 650 125I-2.28 molecules/platelet, whereas thrombin-activated platelets bound 12,600 molecules/platelet with high affinity (4.6 nmol/L). Immunoelectrophoretic analysis revealed that 2.28 reacted with a 53,000-mol wt protein. Immunocytochemistry showed that the antigen is located in a special subclass of platelet granules in unstimulated platelets and is exposed on the surface of thrombin-activated platelets. Double-labeling studies with immunogold labels disclosed simultaneous localization of 2.28 binding sites and cathepsin D in the same granules both in megakaryocytes and endothelial cells, thereby indicating that the antigen may be localized in lysosomes. By using flow cytofluorometry, in vivo platelet activation was studied in patients undergoing cardiac surgery with cardiopulmonary bypass. Increased numbers of platelets that expressed the 2.28 antigen on their surface were observed after extracorporeal perfusion. The percentage of 2.28-positive platelets in the circulation was 3.9% +/- 2.7% (SD) in controls (n = 20), 5.5% +/- 3.0% in patients (n = 10) before cardiopulmonary bypass surgery, 24.6% +/- 13.5% after the bypass, and 8.5% in two patients with acute deep venous thrombosis. These data indicate that 2.28 may serve as a useful probe of in vitro and in vivo platelet activation.

Antibodies, Monoclonal↗
PubMed1976

Studies on platelet plasma membranes. I. Characterization of surface proteins of human platelets labeled with diazotized (125i)-diiodosulfanilic acid.

A polar, nonpenetrating compound of high specific activity, diazotized (125I)-diiodosulfanilic acid (DD125ISA), has been developed as a label for exposed proteins of the human platelet plasma membrane, and platelet proteins and the pattern of labeling have been studied with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). That DD125ISA binds specifically to membrane proteins was demonstrated by: (1) the specific activity of isolated membrane protein was five to seven times that of whole platelet protein and (2) no proteins of intact platelets were labeled which were not represented in the isolated plasma membrane. That the DD125ISA-labeled membrane proteins were exposed on the cell surface was demonstrated by: (1) DD125ISA-labeled proteins were altered by trypsin treatment of intact, labeled platelets and (2) the pattern of labeling produced by reaction of isolated membranes with DD125ISA was quite different from that produced by the labeling of intact platelets. Analysis of platelet membrane proteins by SDS-PAGE demonstrated the glycoproteins previously described at 150,000 daltons (termed glycoprotein I) and 92,000 daltons (glycoprotein III) but we could discriminate two apparently distinct glycoproteins in the intermediate region (IIa: 125,000 daltons, and II: 118,000 daltons). Glycoproteins I and III were constant whereas IIa was clearly visible only in unreduced samples and II was predominant in reduced samples. Reaction of DD125ISA with intact platelets resulted in equal labeling of three of these four membrane glycoproteins (IIa, II, and III). The pattern of exposed proteins on the platelet surface labeled by DD125ISA was different from lactoperoxidase-131I, which labeled predominantly the 92,000 dalton glycoprotein, as demonstrated by simultaneous SDS-PAGE analysis. Therefore three glycoproteins of the human platelet plasma membrane are exposed to a radioisotope probe on the platelet surface and are accessible for contact interactions.

Azo Compounds↗
PubMed1984

The relationship among platelet-associated IgG, platelet lifespan, and reticuloendothelial cell function.

Platelet-associated IgG (PAIgG) has been reported to be elevated in nonthrombocytopenic patients who have a normal platelet lifespan. This has been interpreted as indicating that PAIgG is a nonspecific finding in these patients and not a determinant of platelet survival. It is important to recognize that the reticuloendothelial (RE) system plays an important role in the clearance of antibody-sensitized cells. In this study, we related the level of PAIgG and the platelet lifespan to the RE function in patients with: (A) idiopathic thrombocytopenic purpura (ITP), and (B) five patients with elevated levels of PAIgG yet normal or near-normal platelet counts. RE function was assessed by measuring the clearance of autologous chromium-labeled red cells sensitized with a precise amount of alloantibody (2,000-3,600 molecules of IgG/cell). Eight patients with immune thrombocytopenia had significantly shortened platelet survivals (less than 2-113 hr). In contrast, the five patients with elevated PAIgG, yet normal or near-normal platelet counts, all had normal autologous platelet survivals (186-222 hr). These patients also had significantly impaired clearance of IgG-sensitized red cells, with an average of 85% of the infused red cells remaining in the circulation at 60 min (normal 42% +/- 14%, n = 10). In this study, every patient with elevated PAIgG and normal RE function had a shortened platelet lifespan. Those patients with elevated PAIgG and impaired RE function did not invariably have a shortened platelet lifespan. The observation that the PAIgG is elevated in some patients whose platelet survival is normal does not indicate that PAIgG is not biologically relevant. It indicates that these patients may have RE blockade and do not clear IgG-sensitized cells.

Blood Platelets↗
PubMed1976

Platelet glycocalicin. I. Orientation of glycoproteins of the human platelet surface.

The orientation of proteins and glycoproteins of the platelet surface has been studied using various surface probes and labeling reagents. A fourth major glycoprotein has now been detected in platelet plasma membranes by sodium dodecyl sulfate-gel electrophoresis in addition to the previously recognized glycoproteins I, II, and III. Glycoprotein IV Mr, = approximately 87,000) appears to be present on the inner aspect of the membrane or buried within it since it is not accessible to surface probes such as lactoperoxidase-catalyzed iodination, radiolabeling with transglutaminase and [14C]glycine ethyl ester, or proteolytic enzymes. The ratio of these four major membrane-bound glycoproteins is approximately 10:4:2:3. Contrary to previous reports, only one glycoprotein, glycoprotein III, is accessible to lactoperoxidase-catalyzed iodination in intact platelets. Differences in the rate of destruction of glycoprotein II in intact platelets by trypsin suggests that two components may be migrating in this region. Examination of the soluble fraction obtained following platelet homogenization showed the presence of a single soluble glycoprotein of molecular weight 148,000 comprising about 10% of total platelet sialic acid. Treatment of intact platelets with neuraminidase resulted in the quantitative loss of siliac acid from the soluble glycoprotein, and it was strongly labeled in the intact platelet by [14C]glycine ethyl ester in the presence of transglutaminase. Treatment of intact platelets with chymotrypsin which does not cause the platelet release reaction, caused the rapid conversion of the soluble glycoprotein to a macroglycopeptide. These results indicate a surface origin for the soluble glycoprotein rather than a cytoplasmic or granular origin. The term glycocalicin is suggested for this glycoprotein in view of its origin in the platelet glycocalyx.

Blood Platelets↗