SEARCH · Search PubMed

Results for “Platelets”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 289 records · Page 16Linked to original sources

PubMed1982

The quantitation of platelet-associated IgG on cohorts of platelets separated from healthy individuals by buoyant density centrifugation.

Evidence suggests that as platelets age in the circulation they become progressively smaller and less dense through the loss of protein. The smallest, least dense platelets have a significantly shortened survival, but the mechanism of clearance of these platelets is not known. To evaluate whether the binding of IgG could play a role in the clearance of senescent platelets, we measured platelet size, total protein, and platelet-associated IgG on subpopulations of platelets isolated from 6 healthy individuals using a discontinuous iso-osmotic arabinogalactan (stractan) gradient. There was a close correlation between density, size, and total protein content (r greater than 0.9) for all platelet fractions. There was also a relationship between the amount of platelet-associated IgG (PAIgG), total protein, and platelet size (r greater than 0.9) for the first 3 progressively less dense platelet factions. However, the fourth platelet fraction containing the smallest, least dense, and on current evidence, oldest platelets had very elevated amounts of IgG. This amount was approximately 10 times higher than the mean platelet IgG for the same individual and was similar to the amount of PAIgG found on platelets from patients with immune thrombocytopenia. A progressive increase in the ratio of PAIgG measured after platelet solubilization to PAIgG measured on intact platelets was also noted for the first three populations, indirectly suggesting that platelets clear IgG from their surface during aging. Increased binding of IgG to senescent platelets may mediate their destruction.

Blood Platelets↗

PubMed2005

The effect of time of day and exercise on platelet functions and platelet-neutrophil aggregates in healthy male subjects.

Platelet activation state changes by exercise. The effect of exercise time on platelet activation state and formation of platelet-neutrophil aggregates are not known yet. In this study the effect of exercise and time of day were examined on platelet activity with platelet-neutrophil aggregates. Ten moderately active males aged 27+/- 1.63 (mean+/-S.D.) years completed sub-maximal (70% VO(2max)) exercise trials for 30 min. Blood pressure (BP) was recorded. Venous blood samples were obtained at rest, immediately post-exercise and after 30 min of recovery. Whole blood was analysed for haematocrit (Hct), haemoglobin (Hb), platelet count (PC), mean platelet count (MPV) and platelet aggregation (PA). Platelet-neutrophil aggregates and beta-thromboglobulin (beta-TG) levels were assayed. Platelet count showed significant increase after morning exercise ((236+/- 32)x10(9) l(-1) versus (202+/- 34)x10(9) l(-1) baseline, p < 0.05). Exercise resulted in significantly lower MPV after the evening exercise (9.16+/- 0.5 fl versus 9.65+/- 0.36 fl, p < 0.05). Platelet aggregation by adenosine diphosphate (ADP) decreased after morning exercise and the recovery aggregation levels were significantly different at two different times of the day (68+/- 20% a.m. versus 80+/- 12% p.m., p < 0.05). It was also showed that platelet-neutrophil aggregates increased significantly from baseline after both exercises. Exercise-induced platelet-neutrophil aggregates were higher in the evening (10.7+/- 1.3% p.m. versus 6.4+/- 1.8% a.m., p < 0.0001). It is therefore concluded that besides platelet-platelet aggregation, exercise can cause platelet- neutrophil aggregates. In addition, time of day has an effect on platelet activation related events. Circadian variations of physiological parameters may have an effect on thrombus formation by platelet activation.

Adenosine Diphosphate↗

PubMed1996

In vivo tracking of platelets: circulating degranulated platelets rapidly lose surface P-selectin but continue to circulate and function.

To examine the hypothesis that surface P-selectin-positive (degranulated) platelets are rapidly cleared from the circulation, we developed novel methods for tracking of platelets and measurement of platelet function in vivo. Washed platelets prepared from nonhuman primates (baboons) were labeled with PKH2 (a lipophilic fluorescent dye), thrombin-activated, washed, and reinfused into the same baboons. Three-color whole blood flow cytometry was used to simultaneously (i) identify platelets with a mAb directed against glycoprotein (GP)IIb-IIIa (integrin alpha 11b beta 3), (ii) distinguish infused platelets by their PKH2 fluorescence, and (iii) analyze platelet function with mAbs. Two hours after infusion of autologous thrombin-activated platelets (P-selectin-positive, PKH2-labeled), 95 +/- 1% (mean +/- SEM, n = 5) of the circulating PKH2-labeled platelets had become P-selectin-negative. Compared with platelets not activated with thrombin preinfusion, the recovery of these circulating PKH2-labeled, P-selectin-negative platelets was similar 24 h after infusion and only slightly less 48 h after infusion. The loss of platelet surface P-selectin was fully accounted for by a 67.1 +/- 16.7 ng/ml increase in the plasma concentration of soluble P-selectin. The circulating PKH2-labeled, P-selectin-negative platelets were still able to function in vivo, as determined by their (i) participation in platelet aggregates emerging from a bleeding time wound, (ii) binding to Dacron in an arteriovenous shunt, (iii) binding of mAb PAC1 (directed against the fibrinogen binding site on GPIIb-IIIa), and (iv) generation of procoagulant platelet-derived microparticles. In summary, (i) circulating degranulated platelets rapidly lose surface P-selectin to the plasma pool, but continue to circulate and function; and (ii) we have developed novel three-color whole blood flow cytometric methods for tracking of platelets and measurement of platelet function in vivo.

Animals↗

PubMed1986

Activation of plasminogen by tissue plasminogen activator on normal and thrombasthenic platelets: effects on surface proteins and platelet aggregation.

Tissue plasminogen activator (TPA) converts plasminogen to plasmin within the fibrin clot, thus localizing activation of fibrinolysis. To determine the extent to which platelets promote activation of plasminogen by TPA, we studied the interaction of TPA and plasminogen with unstimulated platelets. Normal washed platelets incubated in the presence of physiologic concentrations of plasminogen (180 micrograms/mL) and TPA (20 ng/mL) failed to generate plasmin activity. In contrast, incubation of platelets with TPA concentrations achieved during thrombolytic therapy (40 to 800 ng/mL) produced a tenfold to 50-fold increase in plasmin activity. After exposure to plasminogen and 200 ng/mL of TPA for one hour, platelets failed to agglutinate in the presence of ristocetin. Incubation of platelets suspended in autologous plasma with 400 ng/mL of TPA for one hour also inhibited ristocetin-induced agglutination. Exposure of platelets to plasminogen and increasing concentrations of TPA correlated with a decrease in glycoprotein Ib (GPIb) and an increase in glycocalicin, as shown by immunoblotting. The glycoprotein IIb/IIIa (GPIIb/IIIa) complex and a 250,000-dalton protein also disappeared from washed platelets after incubation with plasminogen and 200 ng/mL of TPA for one hour. These platelets failed to aggregate in the presence of adenosine diphosphate (ADP) or gamma thrombin, although aggregation in response to calcium ionophore A23187 and arachidonic acid remained intact. However, aggregation in response to all four agonists was normal when platelets were incubated with TPA in the presence of autologous plasma. Platelets from a patient with Glanzmann's thrombasthenia also generated plasmin in the presence of TPA. Hydrolysis of GPIb and inhibition of ristocetin-induced agglutination occurred to a lesser extent with these platelets than with control platelets. We conclude that platelets provide a surface for activation of plasminogen by pharmacologic amounts of TPA. Plasmin generation leads to degradation of GPIb and decreased ristocetin-induced agglutination in normal and thrombasthenic platelets, as well as degradation of GPIIb/IIIa in normal washed platelets and inhibition of ADP and gamma thrombin-induced aggregation. These findings suggest that pharmacologic concentrations of TPA may cause platelet dysfunction due to plasmin generation on the platelet surface.

Adenosine Diphosphate↗

PubMed1998

Interaction of wheat germ agglutinin and concanavalin A with platelets. Stimulation of platelet functional reactions and binding with membrane glycoproteins.

Effects of two lectins, wheat germ agglutinin (WGA) and concanavalin A (Con A), on platelet functional reactions and interaction of lectins with the platelet membrane glycoproteins (GPs) have been studied. Both lectins stimulated platelet aggregation and secretion of serotonin from platelet dense granules. The effects of WGA and Con A were blocked by specific sugars, N-acetyl-D-glucosamine and alpha-methyl-D-mannopyranoside, respectively, by adenylate cyclase activator prostaglandin E1, and by anti-GP IIb-IIIa monoclonal antibody (monAB), CRC64, that inhibits platelet interaction with fibrinogen. The data indicate that both lectins interacting with the carbohydrate moiety on the platelet surface stimulated not passive agglutination but fibrinogen--GP IIb-IIIa-dependent platelet aggregation which is coupled with the secretion from granules and activation of the intracellular systems of signal transduction. However, there were significant differences between the stimulatory effects of WGA and Con A. WGA induced more pronounced and quick platelet aggregation and stimulated several times higher serotonin secretion than Con A. In addition, adhesion studies showed that plastic-adsorbed WGA appeared to be a nonadhesive substrate, whereas Con A effectively stimulated platelet adhesion. Unlike Con A-induced platelet aggregation, adhesion to Con A substrate was not inhibited by monAB CRC64, i.e., was not dependent on GP IIb-IIIa--fibrinogen interaction. Binding of lectins with major platelet GPs was studied using immobilized WGA and Con A and platelet lysate as a source of GPs. Platelet lysate was incubated with immobilized lectins and then binding of individual GPs was evaluated using specific mono- and polyclonal antibodies. WGA binds with GP Ib and P-selectin but not with other GPs tested. Interaction of Con A with platelet GPs was less specific. This lectin binds with GP IIb-IIIa, GP Ib, GP IV, and P-selectin. Although GP Ib appeared to be the main protein which bound WGA on platelet surface, anti-GP Ib antibodies failed to affect WGA-induced platelet aggregation, but inhibited WGA-induced agglutination of fixed platelets. Thus, interaction of the WGA with GP Ib could not be considered as a major stimulus initiating WGA-dependent platelet activation and aggregation.

Blood Platelets↗

PubMed2003

Platelet hyperactivity after statin treatment discontinuation.

Hydroxymethyl-glutaryl-CoA-reductase inhibitors (statins) reduce cardiovascular events by cholesterol lowering as well as by non-lipid related actions. Among them, the modulation of platelet activity could play a relevant role in vascular protection. Furthermore withdrawal of statins has been associated with increased cardiovascular event rate. The aim of our study was to evaluate platelet activity after cerivastatin discontinuation in eighteen subjects that did not accept other drugs and in sixteen subjects continuing treatment with simvastatin. Fourteen subjects at the end of the discontinuation period decided to receive other drugs (simvastatin) and they were evaluted six weeks later. We measured complete lipid profile by the chromogenic method (LDL-C was calculated); oxidized-LDL (ox-LDL; ELISA), platelet P-selectin (P-sel) expression (flow cytometry detection), platelet aggregation (% change of transmitted light), intracellular citrullin production (iCit; HPLC) as an indicator of intracellular NO synthase activity at baseline and 7, 14, 28, 60 days after statin discontinuation. P-sel expression and platelet aggregation were increased at 14 days (p < 0.001 and p < 0.05) in association with raised ox-LDL (r = 0.30, p < 0.05) and decreased iCit (r = 0.53, p < 0.01). Increased LDL-C was related to P-sel and platelet aggregation at 28 days (r = 0.30, p < 0.05). Subjects continuing statin treatment had no significant changes of P-sel at 28 (p = 0.221) and 60 days (p = 0.238). Subjects treated with simvastatin after 60 days of diet showed a significant reduction of P-sel and platelet aggregation after six weeks of treatment (p < 0.01). Our data suggest a platelet hyperactivation state in the second week after statin discontinuation which is partially related to raised LDL-C. Such a finding could participate in the increased cardiovascular event rate after statin discontinuation.

Adult↗

PubMed1981

Evidence that platelet buoyant density, but not size, correlates with platelet age in man.

Following infusion of 51Cr-labeled autologous platelets into normal subjects, high-density (HD) and low-density (LD) platelet cohorts were isolated by prolonged centrifugation in isosmotic arabino-galactan (Stractan). Specific radio-activity of LD platelets declined rapidly post-infusion (T1/2 = 1.5 days), but specific radioactivity of HD platelets remained constant or increased over a 3--4-day period and gradually declined for 6--7 days thereafter. These differences were exaggerated when platelet cohorts enriched in LD or HD cells by slow centrifugation in high-density albumin were labeled and transfused. Mean survival of a platelet cohort enriched with HD cells was significantly (P less than 0.02) shorter (7.73 days) than that of a cohort enriched with LD cells (9.33) days). In normal subjects treated with aspirin, capacity for thromboxane synthesis was regained more rapidly (P less than 0.05) in LD than in HD platelets. HD and LD platelets differed only slightly in mean volume (HD platelets = 7.57 mu3, LD platelets = 6.87 mu3, 0.05 less than P less than 0.01). We believe the most logical interpretation of these findings is that under normal conditions in man, newly formed platelets are less dense on the average than total platelets and become more dense as they age in the circulation. Thus, specific radioactivity of LD platelets declines rapidly as these platelets move into a more dense compartment and are replaced by newly formed, unlabelled cells; specific radioactivity of HD platelets remains constant or increases as labelled platelets enter this compartment in numbers equal to or greater than the number leaving it at the end of their life span. The similarity in mean volumes of LD and HD platelets suggests that platelet size is unrelated to platelet age under normal conditions.

Adult↗

PubMedDate not supplied

Kinetics and mobilization from the spleen of indium-111-labeled platelets during platelet apheresis.

The rate and extent of platelet mobilization from the spleen were measured, and their relationship to the removal of platelets from the peripheral blood during discontinuous flow platelet apheresis was determined in four normal volunteers. Autologous platelets were labeled with Indium-111-oxine and in vivo whole body and organ In-111 radioactivity quantitated with a scintillation camera and a computer-assisted imaging system. Dynamic changes in splenic radioactivity were monitored during 12 cycles of platelet apheresis. The number of platelets harvested and changes in whole body and blood In-111 activity were determined during the procedure. The platelet life-span was estimated, and the sites of sequestration of labeled platelets was measured. Platelet apheresis removed a mean of 64 percent of platelets in the circulation; i.e., 48 percent of all platelets in the body. During the procedures, 28.0 +/- 9.4 percent In-111-labeled platelets in the body were removed, splenic radioactivity decreased by 36.5 +/- 13.2 percent, and whole body activity decreased by 34.5 +/- 9.7 percent. In-111 activity in the spleen and whole body decreased in parallel, indicating a dynamic equilibrium between these pools. The life-span of the labeled platelets was 226 +/- 25 hours, similar to that of normal subjects. The major sites of sequestration of senescent platelets were the spleen (37.9 +/- 20%) and liver (30.3 +/- 5.6%); this is similar to that found in normal subjects. We conclude that as platelets are removed from the peripheral blood, the blood pool is rapidly and effectively replenished from the splenic platelet pool. These two pools are in dynamic equilibrium and permit removal of large numbers of platelets without resultant thrombocytopenia. Platelet apheresis does not adversely effect platelet life-span, and the sequestration pattern in the reticuloendothelial system is normal.

Adult↗

PubMed1999

Platelet surface p-selectin, platelet-granulocyte heterotypic aggregates, and plasma-soluble p-selectin during plateletpheresis.

BACKGROUND: Plateletpheresis components have been shown to contain p-selectin-positive platelets after collection and storage. P-selectin mediates binding of activated platelets to granulocytes and monocytes. This study was undertaken to assess platelet activation, granulocyte activation, platelet-granulocyte heterotypic aggregate formation, and the plasma-soluble p-selectin level during plateletpheresis performed on a particular instrument (MCS+, Haemonetics). STUDY DESIGN AND METHODS: Flow cytometry was used to assay platelet surface p-selectin, granulocyte iC3b receptor, and platelet-granulocyte aggregates in the platelet component, residual blood in the disposable polycarbonate bowl of the MCS+, and in the donor blood with and without the addition of in vitro agonists before, during, and after plateletpheresis. The plasma-soluble p-selectin levels in the platelet component, disposable bowl, and donor venous blood were measured by an enzyme-linked immunosorbent assay. RESULTS: Levels of p-selectin-positive platelets, activated granulocytes, and platelet-granulocyte aggregates were greater in the disposable bowl than in the preapheresis donor blood. Levels of p-selectin-positive platelets, activated granulocytes, and platelet-granulocyte aggregates in the postapheresis donor blood were similar to those in the preapheresis donor blood. The platelet components contained no activated granulocytes or detectable platelet-granulocyte heterotypic aggregates, and only about 10-percent activated platelets. The plasma-soluble p-selectin level in the platelet component was significantly greater than that in the preapheresis donor blood, the residual blood in the disposable bowl, or the postapheresis donor blood. CONCLUSIONS: Measurements of platelet surface p-selectin, platelet-granulocyte heterotypic aggregates, and plasma-soluble p-selectin can be used to detect platelet activation during plateletpheresis.

Antibodies, Heterophile↗

PubMed2002

A randomized controlled trial comparing the frequency of acute reactions to plasma-removed platelets and prestorage WBC-reduced platelets.

BACKGROUND: Removal of the plasma supernatant from platelets before transfusion is effective in preventing acute reactions to platelets caused by cytokines. Prestorage WBC reduction of platelets may be even more effective at preventing reactions as the WBCs are removed and WBC-derived cytokines do not accumulate in this component. This study evaluates the effectiveness of plasma removal and two methods of prestorage WBC reduction for preventing acute reactions to platelets. STUDY DESIGN AND METHODS: Platelets given to adults with hematologic malignancies were randomly allocated to one of three types: plasma supernatant removed and a platelet storage solution added, whole blood-derived platelets that are prestorage WBC reduced by filtration before storage, and prestorage WBC-reduced apheresis platelets. Patients were monitored before, during, and after transfusion, and the severity of reactions was graded on a Likert scale. RESULTS: A total of 129 patients from four centers were given 1190 platelet transfusions. The overall frequency of reactions was 13.6 percent (162 of 1190), 21.3 percent (36 of 169) for the plasma-removed platelets, 11.4 percent (59 of 517) for random donor WBC-reduced platelets, and 13.3 percent (67 of 504) for apheresis WBC-reduced platelets (p=0.384). The overall frequency of severe reactions was 4.1 percent with plasma-removed platelets, 1.7 percent for whole blood-derived, prestorage WBC-reduced platelets, and 1.4 percent for prestorage WBC-reduced apheresis platelets. CONCLUSION: The frequency of reactions to plasma-removed platelets and prestorage WBC-reduced platelets was not significantly different; however, the power of the study for this comparison was low. There was no difference in the frequency of reactions to the two types of prestorage WBC-reduced platelets. The frequency of severe reactions to prestorage WBC-reduced platelets is low, occurring in only 1 to 2 percent of transfusions.

Adolescent↗

PubMed1986

Platelet-activating factor and the release of a platelet-derived coronary artery vasodilator substance in the canine.

Platelet-activating factor (acetyl-glyceryl-ether-phosphorylcholine; 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphorylcholine), which is released by stimulated neutrophils and platelets, possesses the ability to alter vascular tone and permeability and to activate various formed blood elements. We have characterized the hemodynamic effects of intracoronary injections of platelet-activating factor and the influences of pharmacological blockade and platelet depletion on its activity. Intracoronary injections of platelet-activating factor produced maximum increases in left circumflex coronary artery blood flow of 55 +/- 8, 52 +/- 8, and 52 +/- 7 ml/min at 0.5, 1.0, and 2.0 nM, respectively. Only modest changes in systemic arterial blood pressure and regional developed isometric contractile force were associated with the intracoronary artery administration of platelet-activating factor over the range of doses studied. The increase in left circumflex coronary artery blood flow in response to platelet-activating factor was attenuated (44%), but not prevented, by pretreatment with diphenhydramine, (4 mg/kg, iv), and was not affected by pretreatment with aspirin (20 mg/kg, iv) or the systemic administration of the serotonin receptor antagonist, methysergide. The coronary vasodilator response to platelet-activating factor was reduced significantly by the induction of thrombocytopenia (95 +/- 3% platelet depletion) through the administration of sheep-derived canine platelet antiserum. The intracoronary artery injection of platelet-rich plasma activated with platelet-activating factor into thrombocytopenic dogs produced a significantly greater increase in coronary artery blood flow than the injection of either non-activated platelet-rich plasma or platelet-depleted plasma to which platelet-activating factor was added. Similar changes in coronary artery blood flow could be obtained with the intracoronary artery injection of cell-free supernates from washed platelets activated with platelet-activating factor. The observed results suggest that circulating platelets, when exposed to platelet-activating factor, can release a coronary dilator substance, and that the coronary artery dilation is not prevented by pharmacological receptor antagonists for histamine, serotonin, or inhibitors of cyclooxygenase.

Animals↗

PubMed1969

Heterogeneity of human platelets. II. Functional evidence suggestive of young and old platelets.

In the previous communication, suggestive evidence was presented for large-heavy platelets being "young" platelets and light-small platelets being "old" platelets. Large-heavy, light-small, and total human platelet populations were compared with respect to their platelet function. After addition of adenosine diphosphate (ADP), thrombin, or epinephrine, platelet aggregation time was 3.0-, 4.5-, and 3.3-fold shorter with large-heavy platelets compared with light-small platelets, and large-heavy platelets released 3.7-, 7.6-, and 8.1-fold greater adenosine triphosphate (ATP) into the medium, respectively, than did light-small platelets. After platelet aggregation by thrombin or epinephrine, large-heavy platelets released 6.0- and 3.8-fold more ADP into the medium than did light-small platelets. After platelet aggregation by ADP, light-small platelets consumed 5.9-fold greater added extracellular ADP than did large-heavy platelets.Large-heavy platelets aggregated by ADP, thrombin, or epinephrine released 9.1-, 8.5-, and 12.7-fold greater platelet factor 4 than light-small platelets similarly treated.

Adenine Nucleotides↗

PubMed1989

Platelet destruction in autoimmune thrombocytopenic purpura: kinetics and clearance of indium-111-labeled autologous platelets.

Using autologous 111In-labeled platelets, platelet kinetics and the sites of platelet destruction were assessed in 16 normal subjects (13 with and three without spleens), in 17 studies of patients with primary autoimmune thrombocytopenic purpura (AITP), in six studies of patients with secondary AITP, in ten studies of patients with AITP following splenectomy, and in five thrombocytopenic patients with myelodysplastic syndromes. In normal subjects, the spleen accounted for 24 +/- 4% of platelet destruction and the liver for 15 +/- 2%. Untreated patients with primary AITP had increased splenic destruction (40 +/- 14%, p less than 0.001) but not hepatic destruction (13 +/- 5%). Compared with untreated patients, prednisone treated patients did not have significantly different spleen and liver platelet sequestration. Patients with secondary AITP had similar platelet counts, platelet survivals, and increases in splenic destruction of platelets as did patients with primary AITP. In contrast, patients with myelodysplastic syndromes had a normal pattern of platelet destruction. In AITP patients following splenectomy, the five nonresponders all had a marked increase (greater than 45%) in liver destruction compared to five responders (all less than 40%). Among all patients with primary or secondary AITP, there was an inverse relationship between the percent of platelets destroyed in the liver plus spleen and both the platelet count (r = 0.75, p less than 0.001) and the platelet survival (r = 0.86, p less than 0.001). In a stepwise multiple linear regression analysis, total liver plus spleen platelet destruction, the platelet survival and the platelet turnover were all significant independent predictors of the platelet count. Thus platelet destruction is shifted to the spleen in primary and secondary AITP. Failure of splenectomy is associated with a marked elevation in liver destruction. The magnitude of spleen and liver destruction appears to be of considerable importance in the severity of the disease, as reflected in the platelet survival and platelet count.

Autoimmune Diseases↗

PubMed1976

Studies on platelet plasma membranes. II. Characterization of surface proteins of rabbit platelets in vitro and during circulation in vivo using diazotized (125i)-diiodosulfanilic acid as a label.

Diazotized (125I)-diiodosulfanilic acid (DD125ISA) binds specifically to the exposed proteins on the surface of the rabbit platelet plasma membrane. This was demonstrated by the following observations with the use of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of whole platelets and the isolated plasma membrane fraction: (1) the specific activity of isolated membrane protein was sevenfold that of whole platelet protein, (2) no proteins of intact platelets were labeled which were not represented in the isolated plasma membrane, (3) DD125ISA-labeled proteins were altered by trypsin treatment of intact, labeled platelets, and (4) the pattern of labeling produced by reaction of isolated membranes with DD125ISA differed from that produced by the labeling of intact platelets. Reaction of DD125ISA with intact platelets produced labeling of only the three membrane glycoproteins (molecular weights: 180,000, 125,000, and 92,000 daltons) with greatest labeling of the largest glycoprotein and least labeling of the smallest glycoprotein. When rabbit platelets were labeled simultaneously with DD125ISA and 51Cr, the two isotopes were similarly distributed in various density populations of platelets. Some DD125ISA was solubilized from labeled and washed platelets by sonication, but all platelet DD125ISA was recovered in the plasma membrane fraction after 30 minutes' circulation in vivo. In vivo 51Cr recovery and survival were not altered by simultaneous labeling of platelets with DD125ISA. The disappearance of DD125ISA from circulating platelets (T 1/2 = 17 hours) was more rapid than 51Cr (T 1/2 = 30 hours) and appeared exponential in contrast to the linear 51Cr disappearance. On the other hand, DD125ISA did not disappear from platelets faster than 51Cr when doubly labeled platelets were harvested after 3 hours' circulation and incubated in autologous plasma (T 1/2 of DD125ISA elution = 43 hours, 51Cr = 33 hours). SDS-PAGE analysis of DD125ISA-labeled platelets after 14 to 20 hours' circulation in vivo demonstrated the same pattern of DD125ISA distribution on membrane glycoproteins as on the platelets prior to infusion. We interpret this symmetrical loss of the membrane label to indicate symmetrical loss of membrane proteins, suggesting that the platelet may lose pieces of membrane and not specific surface proteins during circulation. This could occur during reversible adhesion encounters during the process of hemostasis and cause the smaller size and decreased effectiveness of older platelets.

Animals↗

PubMed1999

Platelet-derived growth factor inhibits platelet activation in heparinized whole blood.

We previously have demonstrated that human platelets have functionally active platelet-derived growth factor alpha-receptors. Studies with gel-filtered platelets showed that an autocrine inhibition pathway is transduced through this tyrosine kinase receptor during platelet activation. The physiological significance of this inhibitory effect of platelet-derived growth factor on gel-filtered platelets activation is, however, not known. In the present study, we investigated whether platelet-derived growth factor inhibits platelet activation under more physiological conditions in heparinized whole blood, which represents a more physiological condition than gel-filtered platelets. Using flow cytometric assays, we demonstrate here that platelet-derived growth factor inhibits thrombin-, thrombin receptor agonist peptide SFLLRN-, and collagen-induced platelet aggregation and shedding of platelet-derived microparticles from the platelet plasma membrane during platelet aggregation in stirred heparinized whole blood. The inhibitory effect of platelet-derived growth factor was dose dependent. However, under nonaggregating conditions (no stirring), we could not demonstrate any significant effect of platelet-derived growth factor on thrombin- and thrombin receptor agonist peptide-induced platelet surface expression of P-selectin. Our results demonstrate that platelet-derived growth factor appears to be a true antithrombotic agent only under aggregating conditions in heparinized whole blood.

Antigens, CD↗

PubMed1993

Detection of an epitope specific for the dissociated form of glycoprotein IIIa of platelet membrane glycoprotein IIb-IIIa complex and its expression on the surface of adherent platelets.

Glycoproteins (GPs) IIb and IIIa form a Ca(2+)-dependent complex in platelet membrane and change their conformation upon platelet activation and dissociation of the complex. A new anti-GPIIIa monoclonal antibody (mAb), CRC54, is described which could distinguish different conformational states of GPIIIa. This antibody (i) precipitated GPIIb-IIIa from platelet Triton X-100-lysate, (ii) recognized the GPIIIa band in Western blotting of platelet SDS-lysate, and (iii) did not react with platelets from a Glanzmann's thrombasthenia patient lacking GPIIb-IIIa. Immunoblotting of chymotryptic digestion products of purified GPIIb-IIIa has shown that CRC54 epitope is located within residues 1-100 at the N-terminus of GPIIIa. CRC54 bound weakly to platelets in the presence of Ca2+ and Mg2+, 2.34 +/- 0.28 x 10(3) molecules per platelet at saturation. The same level of binding was observed without any divalent cations in the medium. However, binding of CRC54 was increased by several times after treatment of platelets with EDTA, 10.04 +/- 0.28 x 10(3) molecules per platelet. Increase of CRC54 binding correlated with the dissociation of GPIIb-IIIa complex which was followed by the decrease of the binding of another mAb, CRC64, directed against complex-specific epitope of GPIIb-IIIa. Binding of CRC54 to platelets was changed neither by platelet activation in suspension with thrombin or ADP nor by the occupancy of GPIIb-IIIa ligand binding site with GRGDSR peptide. However, binding was significantly stimulated by platelet adhesion to polystyrene plastic. As measured using 51Cr-labelled platelets, binding of 125I-CRC54 to adherent platelets in the presence of divalent cations was about 4 times higher than to platelets in suspension, 8.68 +/- 0.48 x 10(3) per platelet. This increase was not due to the dissociation of GPIIb-IIIa since complex-specific antibody CRC64 still bound effectively to the surface of adherent platelets. The data obtained indicated that: (1) CRC54 recognized an epitope specific for the dissociated form of GPIIIa; (2) the CRC54-reactive epitope of GPIIIa is also expressed on the surface of adherent platelets.

Antibodies, Monoclonal↗

PubMed1985

Modulation of cytoplasmic calcium in human platelets by the phospholipid platelet-activating factor 1-O-alkyl-2-acetyl-SN-glycero-3-phosphorylcholine.

The calcium-sensitive, fluorescent dye Quin 2 was used to quantitate changes in free intracellular calcium [( Ca2+]i) induced in platelets by the phospholipid platelet-activating factor 1-O-alkyl-2-acetyl-SN-glycero-3-phosphorylcholine (AGEPC). The Ca2+]i of unstimulated platelets was 91 +/- 18 nM (mean +/- SD, n = 8), and treatment with 1 to 16 nM AGEPC increased [Ca2+]i in a dose-related manner, with 16 nM AGEPC increasing [Ca2+]i by 102 +/- 20 nM. [Ca2+]i was not increased by analogs of AGEPC which do not activate platelets including the lysophospholipid precursor of AGEPC, the optical isomer, and a C-2 benzoyl analog. The capacity of AGEPC to increase [Ca2+]i exceeded that required to induce maximal platelet aggregation. In four experiments, 100% platelet aggregation was induced by 4.5 +/- 2.4 nM AGEPC (mean +/- SD) and was associated with a submaximal increase in [Ca2+]i of 56 +/- 22 nM. Pretreatment of platelets with AGEPC rendered the platelets specifically unresponsive to repeat stimulation with AGEPC in terms of both platelet aggregation and increased [Ca2+]i, whereas the platelet response to thrombin was undiminished by pretreatment with AGEPC. In contrast, the platelet response to 0.5 microM calcium ionophore A23187 was undiminished by pretreatment with the same concentration of ionophore, suggesting that AGEPC does not activate platelets by an ionophore-like mechanism. IgG aggregates and AGEPC in combination activate platelets synergistically, as shown by the observation that a 1-min exposure of platelets to 60 micrograms/ml of IgG aggregates increased the platelet aggregation response to 2 nM AGEPC from 44 to 100%. In contrast, sequential exposure of platelets to IgG aggregates and AGEPC increased [Ca2+]i additively, suggesting that increased [Ca2+]i contributes to but does not fully mediate synergistic platelet activation by IgG aggregates and AGEPC. Quantitation of free intracellular calcium with the fluorescent dye Quin 2 is a highly sensitive technique for delineating the role of calcium in mediating platelet activation.

Adult↗

PubMed1986

Platelet-collagen interaction: inhibition by ristocetin and enhancement by von Willebrand factor-platelet binding.

The contribution of von Willebrand factor (vWF)-platelet binding to platelet-collagen interaction was examined in vitro. The binding of vWF to platelets was mediated and regulated by ristocetin. Subthreshold concentrations of ristocetin (less than or equal to 1 mg/mL), insufficient to cause ristocetin-induced platelet aggregation (RIPA), were added to platelet-rich plasma (PRP) prior to the addition of collagen. The collagen-induced platelet aggregation (CIPA) was modified by ristocetin and the degree of alteration was dependent on the ristocetin concentration. Response as a function of ristocetin concentration was designated the Collagen-Platelet Aggregation Response (CoI-PAR). In normal PRP the CoI-PAR was a progressive inhibition followed by decreasing inhibition and then an enhanced response. The enhanced response occurred over a narrow range of ristocetin concentrations (0.8 to 1.0 mg/mL). In the absence of vWF (severe von Willebrand's disease, Type I, vWF less than 1%) the CoI-PAR was a progressive, eventually complete inhibition with no enhanced response (with ristocetin concentrations up to 3.0 mg/mL). With addition of vWF to this PRP an enhanced response was observed at a ristocetin concentration inversely proportional to the vWF level. PRP from a patient with severe Hemophilia A showed a response within the normal range. Subthreshold ristocetin did not cause plasma protein precipitation or platelet release of 3H-serotonin, nor induce micro platelet aggregate formation. Digestion of platelet membrane glycoproteins (GP(s] with chymotrypsin demonstrated that upon removal of GPI, RIPA was absent, CIPA retained and the CoI-PAR was progressive inhibition, with no enhancement. With removal of GPs I, II, and III, RIPA, CIPA, and the CoI-PAR were absent. A dose-response 125I-vWF-platelet binding occurred with increasing ristocetin concentrations which was unchanged by the addition of collagen. These results demonstrated that ristocetin-platelet association inhibited CIPA, and vWF-platelet binding enhanced platelet-collagen adhesion and platelet aggregation. The in vitro-enhanced CIPA represents a vWF-dependent aggregation of sufficient magnitude to overcome the inhibitory effect of ristocetin. These studies demonstrate an influential interaction of ristocetin, vWF, and collagen with the platelet membrane and imply an important hemostatic contribution of vWF-platelet binding in platelet-collagen interaction.

Blood Platelets↗