Cloning, characterization, and functional studies of a nonintegrin platelet receptor for type I collagen.

A cDNA (1.6 kb) encoding a platelet protein receptor that binds type I collagen has been isolated from a human bone marrow cDNA library by using a degenerate oligonucleotide probe derived from the amino acid sequence of a CNBr fragment of the purified receptor. Computer search revealed that this cDNA represents the coding sequence of a unique protein. Using the prokaryotic expression system pKK 223-3-65 cDNA, a 54-kD recombinant protein was obtained and purified to apparent homogeneity. In an eukaryotic expression vector (pcDNA3-65 cDNA), a 65-kD protein was identified that was recognized by monoclonal anti-65 kD antibody (anti-65m). The recombinant protein binds to type I, but not to type III collagen by affinity column chromatography. The binding of the recombinant protein to type I collagen-coated Petri dishes is inhibited by anti-65m in a dose-dependent manner. The pcDNA3-65 cDNA-transfected nonadherent T cells express the protein, allowing them to attach to a type I collagen matrix, and are inhibited by anti-65m in a dose-dependent manner. Like the receptor protein purified from platelet membranes, the recombinant protein inhibits type I collagen-induced platelet aggregation and the adhesion of [14C]serotonin-labeled platelets to type I collagen in a dose-dependent manner. The recombinant protein neither binds to type III collagen-coated Petri dishes nor inhibits type III collagen and ADP-induced platelet aggregation, indicating specificity for type I collagen.     

A patient with platelets deficient in glycoprotein VI that lack both collagen-induced aggregation and adhesion.

Molecular level studies on platelets deficient in collagen-induced aggregation provide evidence for identifying possible platelet collagen receptors. We investigated platelets from a patient with mild bleeding time prolongation, but otherwise normal coagulation data. Her platelets lacked collagen-induced aggregation and adhesion, but retained normal aggregation and release by other agonists. Labeling her platelets with 125I or 3H and analysis by SDS-PAGE/autoradiography showed normal levels of glycoproteins Ia, Ib, IIa, IIb, IIIa, and IV. However, there were significantly decreased incorporations of both radioactivities into a 61-kD membrane glycoprotein (GP), which was identified as GPVI from its mobility on unreduced-reduced, two-dimensional SDS-PAGE. Sugiyama et al. (1987. Blood. 69: 1712) reported that the serum from an idiopathic thrombocytopenic purpura (ITP) patient contained an antibody against a 62-kD platelet protein. Our patient's platelets lacked the antigen for the ITP patient's antibody, demonstrating that the ITP serum contains a specific antibody against GPVI. The patient's parents' platelets contained approximately 50% the normal amount of GPVI, but still had normal collagen-induced aggregation and adhesion. The patient's platelets did not bind to types I and III collagen fibrils. Our results suggest that GPVI functions as a collagen receptor.     

Two novel monoclonal antibodies to VWFA3 inhibit VWF-collagen and VWF-platelet interactions

BACKGROUND: The interaction of collagen-von Willebrand factor (VWF)-GPIb is essential for platelet adhesion, especially under high shear conditions. VWF, which acts as a bridge between platelets and exposed subendothelium, interacts with collagen through its A3 domain, which is a new target for the antithrombotic agent. OBJECTIVE: To develop functional blockers that specifically inhibit VWF-dependent adhesion of platelets to collagen under high shear stress. METHODS: To develop murine antihuman VWF A3 monoclonal antibodies (mAbs) by standard hybridoma technology, and characterize their abilities to block interactions between VWF A3 and collagen as well as platelet function. RESULTS: Thirty anti-VWF-A3 mAbs were obtained. Among them, two mAbs, designated as SZ-123 and SZ-125, were found to inhibit VWF-collagen type III interaction. SZ-123 and SZ-125 inhibited the binding of purified human VWF (1.5 or 3 mug mL(-1)) to human placenta collagen type III (IC(50) = 0.07 +/- 0.02 and 0.15 +/- 0.03 mug mL(-1), respectively) or to calf skin collagen type III (IC(50) = 0.48 +/- 0.06 and 0.51 +/- 0.07 mug mL(-1), respectively) coated on plates. Under flow shear condition (1000 s(-1)), SZ-123 and SZ-125 inhibited platelet adhesion on human placenta collagen- or calf skin collagen-coated surfaces. Both mAbs also inhibited platelet aggregation induced by ristocetin, botrocetin or bovine plasma. CONCLUSIONS: SZ-123 and SZ-125 inhibited VWF-collagen and VWF-platelet interactions.     

Binding of collagen alpha1 chains to human platelets.

We previously reported that purified alpha1 chains of type 1 chick skin collagen induce platelet aggregation. We now describe immunological and biochemical evidence that the peptide binds to intact platelets as an early event in the induction of platelet aggregation and the release reaction. Antibody against alpha1 (I) was obtained by immunizing rabbits with complete Freund's adjuvant mixed with purified alpha1. Immunofluorescence studies showed that alpha1(I)-treated platelets exhibited strong immunofluorescence. The intensity of fluorescence was markedly decreased by the pretreatment of platelets with alpha1-CB5 and glucosylgalactosylhydroxylysine. Dose-response curves of platelet aggregation induced by alpha1 and the binding of alpha1 by washed intact platelets are correlated. The biochemical studies showed that the binding of the alpha1 chain to washed intact platelets was platelet concentration and temperature dependent, and that it reached a maximum in 10 min. The process was reversible and specific, with an association constant of 1.7 muM. The inhibitor of alpha1-induced platelet aggregation, glucosylgalactosyl hydroxylysine, inhibited the alpha1 binding. These results suggest that alpha1(I) chains bind to specific receptor site(s) on platelet membranes to trigger aggregation and the release reaction.     

Type-specific antibody prevents platelet aggregation induced by group B streptococci type III

Group B streptococci (GBS) type III organisms readily induced platelet aggregation and serotonin release in human platelet-rich plasma (PRP). In a system using a GBS/platelet ratio of 1.5, aggregation occurred after 2 to 9 minutes (maximum aggregation, 73% +/- 11%). Serotonin release began within the first minute, reaching 40% before aggregation was detected. Maximum release was 65% +/- 9%. The addition of type-specific rabbit antisera inhibited aggregation and release in a dose-dependent fashion, whereas rabbit antisera against GBS type II and a pneumococcus type 14 and 19 had no effect. To test the activity of different isotypes, monoclonal antibodies against the sialic acid determinant of the GBS type III antigens were used. IgG, IgM, and IgA antibodies were all effective in blocking aggregation and serotonin release. Although the significance of this phenomenon is not clear, it may represent a protective function of antibody that is not directly related to opsonization and phagocytosis.     

Type IV and type "A-B" collagens do not elicit platelet aggregation or the serotonin release reaction.

Human collagens were isolated from kidney, lung, skin, aorta, cartilage, and placenta. Five different types were obtained, including two new molecular species, one characteristic of basement membranes, or type IV collagen, and the other the recently described "A-B" collagen derived from fetal membranes. All the collagens were purified and separated by combination of heat-gelation fractionation and salt fractionation. In neutral solution at 37 degrees neither type IV nor type "A-B" collagen elicited platelet aggregation or 14C-serotonin release. Preincubation of platelets with both types IV and "A-B" collagen did not inhibit aggregation upon subsequent addition of collagen types I, II, or III.     

The pituitary adenylate cyclase-activating polypeptide is a physiological inhibitor of platelet activation

The pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide of the vasoactive intestinal peptide/secretin/glucagon superfamily. Studies in two related patients with a partial trisomy 18p revealed three copies of the PACAP gene and elevated PACAP concentrations in plasma. The patients suffer from severe mental retardation and have a bleeding tendency with mild thrombocytopenia, and their fibroblasts show increased PACAP mRNA levels. The PACAP receptor (vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor 1 [VPAC1]) in platelets and fibroblasts is coupled to adenylyl cyclase activation. Accordingly, we found increased basal cAMP levels in patients' platelets and fibroblasts, providing a basis for the reduced platelet aggregation in these patients. Megakaryocyte-specific transgenic overexpression of PACAP in mice correspondingly increased PACAP release from platelets, reduced platelet activation, and prolonged the tail bleeding time. In contrast, the PACAP antagonist PACAP(6-38) or a monoclonal PACAP antibody enhanced the collagen-induced aggregation of normal human platelets, and in PACAP knockout mice, an increased platelet sensitivity toward collagen was found. Thus, we found that PACAP modulates platelet function and demonstrated what we believe to be the first hemostatic defect associated with PACAP overexpression; our study suggests the therapeutic potential to manage arterial thrombosis or bleeding by administration of PACAP mimetics or inhibitors, respectively.     

Comparison of type I,type III and type VI collagen binding assays in diagnosis of von Willebrand disease

Summary. Background: von Willebrand factor (VWF) plays a key role in coagulation by tethering platelets to injured subendothelium through binding sites for collagen and platelet GPIb. Collagen binding assays (VWF:CB), however, are not part of the routine work‐up for von Willebrand disease (VWD). Objectives: This study presents data on collagen binding for healthy controls and VWD subjects to compare three different collagens. Patients/Methods: VWF antigen (VWF:Ag), VWF ristocetin cofactor activity and VWF:CB with types I, III and VI collagen were examined for samples obtained from the Zimmerman Program. Results: Mean VWF:CB in healthy controls was similar and highly correlated for types I, III and VI collagen. The mean VWF:CB/VWF:Ag ratios for types I, III and VI collagen were 1.31, 1.19 and 1.21, respectively. In type 1 VWD subjects, VWF:CB was similar to VWF:Ag with mean VWF:CB/VWF:Ag ratios for types I, III and VI collagen of 1.32, 1.08 and 1.1, respectively. For type 2A and 2B subjects, VWF:CB was uniformly low, with mean ratios of 0.62 and 0.7 for type I collagen, 0.38 and 0.4 for type III collagen, and 0.5 and 0.47 for type VI collagen. Conclusions: Normal ranges for type I, III and VI collagen are correlated, but higher values were obtained with type I collagen as compared with types III and VI. The low VWF:CB in type 2A and 2B subjects suggests that VWF:CB may also supplement analysis of multimer distribution. However, these results reflect only one set of assay conditions per collagen type and therefore may not be generalizable to all collagen assays.     

Nucleotide receptor signaling in platelets

Upon injury to a vessel wall the exposure of subendothelial collagen results in the activation of platelets. Platelet activation culminates in shape change, aggregation, release of granule contents and generation of lipid mediators. These secreted and generated mediators trigger a positive feedback mechanism potentiating the platelet activation induced by physiological agonists such as collagen and thrombin. Adenine nucleotides, adenosine diphosphate (ADP) and adenosine triphosphate (ATP), released from damaged cells and that are secreted from platelet-dense granules, contribute to the positive feedback mechanism by acting through nucleotide receptors on the platelet surface. ADP acts through two G protein-coupled receptors, the Gq-coupled P2Y1 receptor, and the Gi-coupled P2Y12 receptor. ATP, on the other hand, acts through the ligand-gated channel P2X1. Stimulation of platelets by ADP leads to shape change, aggregation and thromboxane A2 generation. ADP-induced dense granule release depends on generated thromboxane A2. Furthermore, costimulation of both P2Y1 and P2Y12 receptors is required for ADP-induced platelet aggregation. ATP stimulation of P2X1 is involved in platelet shape change and helps to amplify platelet responses mediated by agonists such as collagen. Activation of each of these nucleotide receptors results in unique signal transduction pathways that are important in the regulation of thrombosis and hemostasis.     

Binding and covalent cross-linking of purified von Willebrand factor to native monomeric collagen.

We have analyzed the interaction of the adhesive glycoprotein, von Willebrand factor (vWF), with native monomeric collagen monolayers by adsorbing acid soluble Types I and III collagen derived from calf skin to polystyrene microtiter wells and incubating the wells with purified human 125I-vWF. The binding of 125I-vWF was saturable, reversible, specific, and was abolished by heat denaturation of the collagen monomers. Binding was half-maximal at 5 micrograms/ml, and, at saturation, 7.5 ng 125I-vWF were bound to each microgram of immobilized collagen. 125I-vWF did not bind to wells coated with other extracellular matrix or plasma proteins such as fibronectin, fibrinogen, gelatin, or the q subunit of the first component of complement (C1q). In addition, bound 125I-vWF could not be displaced from collagen by the addition of either fibronectin or fibrinogen. After incubation with Factor XIIIa, plasma transglutaminase, 125I-vWF bound to collagen could no longer be displaced by vWF, which suggests covalent cross-linking of vWF to collagen monomers. Factor XIIIa-dependent covalent cross-linking of vWF to collagen, but not to fibronectin or laminin, was also demonstrated by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate.     

Decreased type II/type I TGF-beta receptor ratio in cells derived from human atherosclerotic lesions. Conversion from an antiproliferative to profibrotic response to TGF-beta1.

Atherosclerosis and postangioplasty restenosis may result from abnormal wound healing. The present studies report that normal human smooth muscle cells are growth inhibited by TGF-beta1, a potent wound healing agent, and show little induction of collagen synthesis to TGF-beta1, yet cells grown from human vascular lesions are growth stimulated by TGF-beta1 and markedly increase collagen synthesis. Both cell types increase plasminogen activator inhibitor-1 production, switch actin phenotypes in response to TGF-beta1, and produce similar levels of TGF-beta activity. Membrane cross-linking of 125I-TGF-beta1 indicates that normal human smooth muscle cells express type I, II, and III receptors. The type II receptor is strikingly decreased in lesion cells, with little change in the type I or III receptors. RT-PCR confirmed that the type II TGF-beta1 receptor mRNA is reduced in lesion cells. Transfection of the type II receptor into lesion cells restores the growth inhibitory response to TGF-beta1, implying that signaling remains responsive. Because TGF-beta1 is overexpressed in fibroproliferative vascular lesions, receptor-variant cells would be allowed to grow in a slow, but uncontrolled fashion, while overproducing extracellular matrix components. This TGF-beta1 receptor dysfunction may be relevant for atherosclerosis, restenosis and related fibroproliferative diseases.     

Quercetin inhibits collagen-stimulated platelet activation through inhibition of multiple components of the glycoprotein VI signaling pathway

Summary.  Background : The regulation of platelet function by pharmacological agents that modulate platelet signaling has proven a successful approach to the prevention of thrombosis. A variety of molecules present in the diet have been shown to inhibit platelet activation, including the antioxidant quercetin. Objectives : In this report we investigate the molecular mechanisms through which quercetin inhibits collagen-stimulated platelet aggregation. Methods : The effect of quercetin on platelet aggregation, intracellular calcium release, whole cell tyrosine phosphorylation and intracellular signaling events including tyrosine phosphorylation and kinase activity of proteins involved in the collagen-stimulated glycoprotein (GP) signaling pathway were investigated. Results : We report that quercetin inhibits collagen-stimulated whole cell protein tyrosine phosphorylation and intracellular mobilization of calcium, in a concentration-dependent manner. Quercetin was also found to inhibit various events in signaling generated by the collagen receptor GPVI. This includes collagen-stimulated tyrosine phosphorylation of the Fc receptor γ-chain, Syk, LAT and phospholipase Cγ2. Inhibition of phosphorylation of the Fc receptor γ-chain suggests that quercetin inhibits early signaling events following stimulation of platelets with collagen. The activity of the kinases that phosphorylate the Fc receptor γ-chain, Fyn and Lyn, as well as the tyrosine kinase Syk and phosphoinositide 3-kinase was also inhibited by quercetin in a concentration-dependent manner, both in whole cells and in isolation. Conclusions : The present results provide a molecular basis for the inhibition by quercetin of collagen-stimulated platelet activation, through inhibition of multiple components of the GPVI signaling pathway, and may begin to explain the proposed health benefits of high quercetin intake.     

I、II型胶原蛋白对人软骨细胞生物学特性的影响

背景：实验证明胶原蛋白底物具有刺激成软骨的作用,但关于不同类型胶原蛋白刺激成软骨作用的能力仍存在争议。 目的：观察I、Ⅱ型胶原蛋白对体外培养人软骨细胞生物学特性的影响。 方法：将P3代人软骨细胞分别加入普通培养板、I型胶原蛋白包被培养板、Ⅱ型胶原蛋白包被培养板继续培养。培养10d内,MTT法绘制细胞生长曲线;培养28d后,采用ELISA法、聚合酶链反应、二甲基亚甲基蓝比色等方法检测3种培养板中软骨细胞分泌I胶原蛋白、Ⅱ型胶原蛋白及糖胺多糖的量。 结果与结论：Ⅱ型胶原蛋白包被培养板中软骨细胞数量最多,增殖速度为I型胶原蛋白包被培养板的2倍、普通培养板的5倍。Ⅱ型胶原蛋白包被培养板中软骨细胞分泌I型胶原蛋白最少,与普通培养板板检测结果差异有显著性意义（P〈0．01）,与I型胶原蛋白包被培养板检测结果差异无显著性意义;Ⅱ型胶原蛋白包被培养板中软骨细胞分泌Ⅱ型胶原蛋白、糖胺多糖最多,与其他两种培养板检测结果差异有显著性意义（P〈0．01）。表明胶原蛋白包被培养板培养软骨细胞优于普通培养板,其中Ⅱ型胶原蛋白包被培养板在培养软骨细胞时更能维持细胞形态,延长去分化现象出现的时间,更利于细胞再分化。     

Investigations of Platelet Function in Whole Blood with BAPA as Anticoagulant

BACKGROUND: Citrate anticoagulation of blood results in non-physiologically low calcium concentration and rapid deterioration of platelet viability. Benzylsulfonyl-D-Arg-Pro-4-amidinobenzylamide (BAPA) as anticoagulant maintains the physiological calcium level and seems to retain platelet function (PF) over a time period of at least 24 h. We evaluated PF in BAPA-anticoagulated peripheral whole blood (WB) between 0.5 and 30 h after blood collection. METHODS: In WB from 21 healthy volunteers (15 women, 6 men, age range 19-57 years) platelet aggregation (PA) was determined by impedance method and ATP release by luminometry. Platelet response was tested by ADP (10 and 20 μmol/l) and collagen (1 and 2 μg/ml) between 0.5 and 30 h after blood collection. RESULTS: Parameters of ADP-induced PA showed stable values until 6.5 h after blood collection followed by a significant decline. PA in response to collagen was stable up to 25 h of storage. ATP release induced by collagen displayed a continuous, significant decrease over time. CONCLUSION: Preservation of platelet response in BAPA-anticoagulated blood depends on the applied agonist showing that collagen-induced PA is more stable compared to ADP known as a weak agonist in WB.     

Role of in vitro cholesterol depletion in mediating human platelet aggregation

Summary.  We investigated the direct role of cholesterol lowering on human platelet aggregation by in vitro cholesterol depletion using methyl-β-cyclodextrin. Collagen and thrombin receptor agonist peptide induced maximal aggregation was significantly decreased in cholesterol depleted platelets. In contrast, anti-CD9 antibody, mAb7, or anti-β₃ antibody, D3, induced percent maximal aggregation was unaffected by cholesterol depletion. Surface and total α_IIbβ₃ levels were equivalent in both groups. Morphological and ultrastructural analysis of collagen induced aggregates revealed that normal and cholesterol depleted platelets changed shape and aggregated; however, cholesterol depletion impaired microtubule ring formation and aggregate size. Cholesterol depletion also diminished the extent of the open canalicular system and collagen induced platelet ATP release. These data suggest cholesterol depletion impairs platelet aggregation by altering platelet ultrastructure critical in mediating secretion. Temporal differences and differences in tyrosine phosphoprotein levels following collagen stimulation were observed, thereby indicating that platelet signaling was concurrently affected by cholesterol depletion.     

Dose-dependent platelet stimulation and inhibition induced by anti-PIA1 IgG

The PIA1 antibody produces several clinically distinct and severe thrombocytopenias. Investigations have demonstrated divergent effects on platelet function; prior reports demonstrated inhibition, while a conflicting publication showed platelet activation. We have resolved this conflict using anti-PIA1 IgG produced by a patient with posttransfusion purpura. Relatively low concentrations stimulated platelet aggregation and release of adenosine triphosphate (ATP) whereas high concentrations inhibited platelet function, producing a thrombasthenia-like state. The number of molecules of platelet-associated IgG necessary to initiate aggregation and ATP release (2,086 +/- 556) or produce maximum aggregation (23,420 +/- 3,706) or complete inhibition (63,582 +/- 2654) were measured with a quantitative radiometric assay for bound anti-PIA1. Preincubation of platelets with high concentrations of PIA1 antibody inhibited platelet aggregation with 10 mumol/L adenosine diphosphate and blocked 125I-labeled fibrinogen platelet binding. Platelet activation with nonfibrinogen dependent agonist, 1 U/ml thrombin, was not inhibited by this high concentration of PIA1 IgG. In conclusion, anti-PIAI IgG produces (1) stimulation of platelet aggregation and ATP release that is initiated with 2000 molecules IgG per platelet and is associated with an increase of 125I-fibrinogen binding; (2) conversely, inhibition of platelet aggregation is observed with maximum antibody binding, 63,000 molecules IgG per platelet, and is mediated via a blockade of fibrinogen binding.     

Aprotinin: is it prothrombotic?

Controversy continues as to whether aprotinin (Trasylol) is prothrombotic. The recent discovery of the thrombin receptor family, known as the protease-activated receptor family (PAR) has been essential in aiding our understanding of the mechanism of action of aprotinin. Our results show that aprotinin has no effect on platelet aggregation induced by adrenaline, adenosine diphosphate, phorbol-12-myristate-13-acetate, collagen or PAR 1 agonist peptide. However, aprotinin inhibits thrombin-induced platelet activation as assessed by macroaggregation, microaggregation and platelet membrane calcium flux. Aprotinin inhibits proteolytic activation of platelets, but platelets can still be activated by non-proteolytic mechanisms.     

Aspirin resistance detected with aggregometry cannot be explained by cyclooxygenase activity: involvement of other signaling pathway(s) in cardiovascular events of aspirin-treated patients

OBJECTIVES: Although the concept of aspirin resistance is extensively reported in medical literature, its precise mechanisms and clinical outcomes are largely unknown. In this study, we examined individual thromboxane biosynthesis and platelet aggregation in aspirin-treated patients, and whether the results of a platelet aggregation test influenced clinical outcomes. RESULTS: Subjects taking 81 mg of aspirin (n = 50) and controls (n = 38) were evaluated for platelet aggregation and platelet cyclooxygenase-1 (COX-1) activity by measuring collagen-induced thromboxane B2 production. For aggregometry, both light transmission (LT) and laser-light scattering methods were employed to quantitatively evaluate aggregate sizes and numbers. Aspirin treatment resulted in the inhibition of collagen-induced platelet aggregation, particularly the transition from small to large platelet aggregates. Although platelet COX-1 activity seemed to be uniformly inhibited in all patients, platelet aggregation studies showed great inter-individual differences; variation in platelet COX-1 activity only accounted for 6-20% of the individual aggregations. Factor analysis revealed the existence of a common factor (other than platelet COX-1) that explained 48.4% of the variations in platelet aggregation induced by collagen, adenosine diphosphate (ADP), and collagen-related peptide. We then prospectively enrolled 136 aspirin-treated patients in our study, and we found that being in the upper quartile level of LT, or with large aggregate formation induced by collagen, was an independent risk factor for developing cardiovascular events within 12 months [hazard ratio (HR) = 7.98, P = 0.008 for LT; HR = 7.76, P = 0.007 for large aggregates]. On the other hand, the existence of diabetes mellitus was an independent risk factor for overall outcomes (HR 1.30-11.9, P = 0.015-0.033). CONCLUSIONS: Aspirin resistance expressed as unsuppressed platelet COX-1 activity is a rare condition in an out-patient population. Other factor(s) affecting collagen-induced platelet aggregation may influence early outcomes in aspirin-treated patients.     

Platelet aggregation response and adenosine triphosphate secretion after abdominal total hysterectomy

This study was designed to clarify the quantitative relationship between platelet aggregation and the secretion of adenosine triphosphate (ATP) after surgery. Peripheral blood was collected from 41 patients who underwent abdominal total hysterectomy. Platelet count, volume, aggregation and the amount of secreted ATP were determined using live platelets before, one day after and two weeks after surgery. Platelet aggregation and ATP secretion were investigated using a lumi-aggregometer. The aggregating reagents used were 5 microM of adenosine diphosphate (ADP) (final concentration) and 5 microg/ml of collagen. Structural alterations of platelets at these time points were also investigated by electron microscopy. Platelet aggregation induced by collagen was significantly lower (p<0.05) one day after surgery. ADP-induced aggregation two weeks after surgery was more intense than before (p<0.05) and one day after (p<0.05) surgery. The amount of secreted ATP induced by each of ADP and collagen was significantly lower (p<0.05-0.01) one day after surgery and correlation coefficients between platelet aggregation and secretion of ATP showed lower values in both ADP- and collagen-induced aggregation. One day after surgery electron microscopy showed that granule concentrations were markedly reduced in platelets. In conclusion, after consumption of circulating platelets at the site of operation, in addition to being lost by bleeding, the remaining platelets in circulation consist of platelet subpopulations different from those present before surgery, exhibiting low values of correlation coefficients between platelet aggregation and secretion of ATP and low concentrations of granules.     

Role of thromboxane A2 as a mediator of platelet-activating-factor-induced aggregation of human platelets

1. At present it is unclear whether platelet-activating-factor (PAF)-induced aggregation is mediated by thromboxane. To obtain further information about this event we have compared the affects of aspirin on platelet aggregation and secretion induced by PAF and collagen. 2. Collagen and PAF induced aggregation and secretion in human platelets in a dose-related manner. 3. Aspirin inhibited the magnitude of both platelet aggregation and secretion induced by PAF and collagen, but the degree of inhibition was much greater for collagen. 4. Aspirin strongly inhibited the aggregation rate of collagen-induced platelet aggregation, but had no measurable effect on the rate of PAF-induced aggregation. 5. Inconsistencies reported in previous studies of the effect of aspirin on PAF-induced platelet aggregation may be explained, in part, by the doses of PAF used and the method of inactivating cyclo-oxygenase (in vitro compared with in vivo). 6. Our results suggest that the initial events of PAF-induced aggregation are independent of thromboxane A2 formation and that thromboxane A2 plays only a minor role in the later phase of PAF-induced aggregation.