The in vivo release of human platelet factor 4 by heparin

Intravenous and subcutaneous injection of heparin or the heparin analogue SSHA into normal volunteers induced release of platelet factor 4 (PF4) but not beta-thromboglobulin (beta-TG). At low heparin doses the amount of PF4 released was related to the plasma heparin concentration achieved. The rise in plasma PF4 was coincident with, and appeared to be a response to, the increase in plasma heparin concentration rather than to the absolute heparin level. After the primary response, the system became refractory to further challenge by the same heparin dose; the full initial magnitude of the response was not regained until 144 h. after heparin was first injected. The maximum amount of PF4 released corresponded to only about 5% of that potentially available from platelets. Moreover, heparin did not stimulate PF4 release from whole blood in vitro. We have demonstrated the presence of PF4 on the vascular endothelium, and suggest that this is the immediate source of the PF4 released by heparin, though it is probably initially derived from platelets. The effect of such binding on the antithrombotic potential of the endothelial surface is discussed.     

The role of platelet factor V in prothrombin conversion

The contribution of platelet factor V to prothrombin conversion was studied in a purified two-stage system designed to measure the ability of factor V to accelerate prothrombin conversion. When unstimulated gel-filtered platelets (GFP) were substituted for both factor V and phospholipid, thrombin evolution was linear following a long lag time. Gel filtration resulted in considerable phospholipid availability with minimal factor V release. Incubating platelets with collagen in increasing concentrations resulted in marked shortening of the lag time, an increase in the initial rate of thrombin formation, and release of platelet factor V. The inhibition of thrombin formation by preincubation of the platelets with metabolic inhibitors is consistent with previous observations that factor V is released from alpha-granules by collagen in a process requiring metabolic energy. Released platelet factor V added to metabolically inhibited platelets reproduces the acceleration of prothrombin conversion demonstrated in GFP incubated with collagen. Furthermore no acceleration of the clotting time at collagen concentrations used in this study was demonstrated in an assay designed to measure available platelet phospholipid in the presence of excess factor V. The rate of increased thrombin generation produced by collagen stimulation is primarily due to released platelet factor V in the system employed.     

Studies on the stimulation of human blood platelets by semi-synthetic platelet-activating factor

"Saturated" and "unsaturated" platelet-activating factor (PAF) obtained from ratfish liver oil were proved to exert potent stimulation on human blood platelets. Using 0.025 to 1.0 mumol/1 PAF a dose-dependent platelet aggregation in platelet-rich plasma was observed. During PAF-induced irreversible aggregation a 9 to 40% release of platelet bound serotonin occurred. The specific effect of PAF, however, seems to be limited to induce reversible aggregation since second wave of aggregation and serotonin release were suppressed by a combination of acetylsalicylic acid and an ADP scavenging system. Incubation of PAF for 30 min in plasma resulted in a 90% loss of its platelet aggregating power. Subthreshold concentrations of PAF enhanced the platelet aggregation triggered by suboptimal concentrations of ADP, epinephrine, or collagen. Vice versa non-aggregating concentrations of ADP, epinephrine, collagen, Ca-ionophore A 23,187, or arachidonic acid amplified PAF-induced platelet aggregation. The synergistic effect of PAF and other stimuli of blood platelet activation can be partly interpreted as a stimulating effect of PAF on the metabolization of arachidonic acid.     

Acetylcholine-induced production of platelet-activating factor by human fetal brain cells in culture

Platelet-activating-factor (PAF) is a potent, biologically active lipid mediator produced by several tissues, including brain. Its role in the central nervous system (CNS) is still unknown, even if its involvement in brain damage and neurotoxicity has been postulated. Its production by neural cells has been demonstrated in different species, but not in man. This paper provides evidence that PAF can be produced by human fetal neurons and/or glial cells in culture. Its synthesis dramatically increased upon stimulation with acetylcholine (ACh), and it was significantly lowered by the cholinergic receptor antagonist atropine. Almost no PAF was detected in the incubation medium, which indicated no release of PAF from cultured cells. Characterization of the cells in culture with specific monoclonal antibodies excluded the presence of endothelial cells or macrophages, which also produce PAF.     

Human and rabbit platelets form platelet-activating factor in response to calcium ionophore

Homogenous preparations of human and rabbit platelets synthesized platelet-activating factor (PAF) when stimulated by calcium ionophore A23187. PAF was isolated by high performance liquid chromatography (HPLC) on a Lichrosorb Si60 column. PAF was well separated from other phospholipids especially sphingomyelin and lysolecithin. The retention times of PAF derived from human or rabbit platelets were identical to the commercially available semisynthetic PAF and radiolabeled PAF. The amount of PAF was determined by the extent to which it induced the aggregation of washed rabbit platelets.     

Properties of PAF-acether-induced platelet aggregation and secretion. Studies in gel-filtered human platelets

Human platelets rapidly lose their responsiveness to PAF-acether after blood collection. We collected blood from fasting donors and prepared gel-filtered platelets that remained responsive to PAF-acether for about 6 hours. Log-dose response studies showed bi-phasic aggregation between 20 and 100 nM PAF-acether with secretion of dense-, - and lysosomal granule contents during the second wave of aggregation. Between 0.2 and 10 nM PAF-acether aggregation was weak and no secretion occurred whereas 300 nM PAF-acether or more induced maximal aggregation and secretion. Secretion, however, was never more than 70, 55, and 30% of maximal secretable amount of 5HT, βTG and βN, respectively. Aggregation and secretion were enhanced by fibrinogen (optimal concentration 0.3–0.7 g.l⁻¹), required Ca²⁺ or Mg²⁺ but were inhibited when Mg²⁺ or Ca²⁺ were present at a concentration of 2 mM or more. These date show that human platelets are almost equally sensitive to PAF-acether as rabbit platelets, and respond with incomplete secretion of dense-, - and lysosomal granule contents.     

血小板活化因子介导神经元损伤与天门冬氨酸信号通路的研究

目的　探讨血小板活化因子 (PAF)诱导的神经元凋亡是否涉及N 甲基 D 天门冬氨酸 (NMDA)信号传导通路。方法　用原代小鼠大脑皮质神经元培养系统 ,不同剂量PAF处理神经元 2 4h或PAF受体拮抗剂 (BN 5 2 0 2 1)、NMDA受体拮抗剂 (MK 80 1)和一氧化氮合酶抑制剂 (L NAME)预处理 30min ;碘化物 (PI ) /calcein(钙黄绿素 )染色 ,荧光显微镜摄像 ,计算细胞死亡率。同时免疫印迹蛋白测定神经型一氧化氮合成酶(nNOS)表达水平及用3 H标记放免法检测nNOS的活性。结果　 (1)不同剂量 (0 0 1、0 1、0 3、0 6 μmol/L)的PAF处理神经元 2 4h ,均可致神经元死亡 ,0 3和 0 6 μmol/L组与对照组相比 ,差异均有显著性 (均 P <0 0 1)。(2 )PAF神经毒性作用不仅被BN 5 2 0 2 1所拮抗 ,MK 80 1、L NAME也可减轻其作用。 (3)PAF增加神经元的nNOS蛋白的表达 ,同时也增加其活性。结论　PAF对神经元的损伤作用与NMDA信号通路激活有关。     

Factors influencing shear-induced platelet alterations: platelet lysis is independent of platelet aggregation and release

To gain further insight into the mechanisms involved in fluid shear-induced platelet alterations, we examined conditions and factors that might affect shear-induced platelet reactions in human citrated platelet-rich plasma (C-PRP) under well-defined laminar flow conditions at shear stresses between 0 and 160 dyn/cm² using a Couette rotational viscometer. Prevention of excessive alkalinization of C-PRP during shear due to CO₂ loss did not appreciably affect shear-induced platelet aggregation (PAG), adenine nucleotide (AN) release or platelet lysis. Shear-induced PAG and AN release were significantly greater in C-PRP stored and sheared at 24°C as compared to C-PRP stored at 24°C or 37°C and sheared at 37°C whereas platelet lysis was not affected by temperature. When C-PRP was sheared in the presence of EDTA or EGTA, shear-induced PAG up to shear stresses of 80 dyn/cm² was almost completely suppressed whereas AN release and lysis were unaffected. Exposure of C-PRP to PGE₁ and theophylline before shear virtually abolished shear-induced PAG and AN release at shear stresses up to 80 dyn/cm² but had no demonstrable effect on shear-induced platelet lysis. These findings seem to indicate that ADP released from platelets by shear and extracellular Ca⁺⁺ or in the presence of PGE₁ and theophylline. These findings seem to indicate that the structural and biochemical changes associated with shear-induced PAG and release in our system do not predispose platelets to shear-induced lysis.     

Expression of coagulant activity in human platelets: release of membranous vesicles providing platelet factor 1 and platelet factor 3

The relationship between the appearance of membrane-associated factor V-like activity (platelet factor 1, PF1) and phospholipid-like catalytic activity (platelet factor 3, PF3) has been examined, in vitro, in collagen-stimulated, human platelets. Both activities increased 7 fold upon collagen treatment relative to stirred controls. After sedimentation of stimulated platelets, 31% of total PF1 and 41% of PF3 remained in the supernatant fraction. PF1 eluted from a Sepharose CL-4B column in the same void volume fractions as PF3, phospholipid, and vesicular particles. These fractions had roughly 100 fold (lipid basis) or 1000 fold (protein basis) enhanced specific activity when compared to the stimulated platelet suspension. Freeze-fracture electron microscopy demonstrated that these void volume fractions contain two populations of membranous vesicles (80-200 nm and 400-600 nm in diameter). Upon centrifugation of the void volume fractions, PF1 and PF3 activities, phosphate-containing material, and ultraviolet-absorbing material all sedimented at the same rate, indicating that PF1 and PF3 are activities associated with one or both of the platelet-derived vesicle populations. Finally, we examined the effects of inhibitors on the appearance of PF1, PF3, platelet factor 4, total intrinsic factor V activity, and serotonin as well as on platelet aggregation. These studies suggest that the collagen-stimulated release of PF1 and PF3 is not coupled to either platelet aggregation or PF4 release but is probably a separate phase of the release reaction.     

重组人血小板活化因子乙酰水解酶保护小鼠脑缺血再灌注的研究

目的观察重组人血小板活化因子乙酰水解酶(rPAF-AH)对小鼠脑缺血再灌注的保护作用。方法采用线栓法建立小鼠大脑中动脉缺血再灌注模型,尾静脉注射rPAF-AH预处理后观测模型小鼠神经行为学和脑梗死灶大小的改变,采用Westernblot研究MMP-2的表达及活性,同时与奥扎格雷钠注射液、金纳多注射液及假手术对照组比较。结果 rPAF-AH预处理明显降低脑梗死小鼠行为障碍的神经行为学评分和脑梗死灶的大小(P0.01)。Westernblot分析结果证明,rPAF-AH预处理可降低脑缺血再灌注损伤所诱导的MMP-2蛋白的表达及其活性水平(P0.01)。结论 rPAF-AH预处理对小鼠脑缺血再灌注损伤有一定的保护作用,这可能得益于其抑制了MMP-2蛋白的高表达及活性增高。     

大鼠脑缺血再灌注后血小板活化因子及其受体的病理作用

目的探讨血小板活化因子(PAF)及其受体(PAFR)在大鼠缺血再灌注后的病理性神经毒性作用及其可能的干预方法。方法选用SD大鼠建立一侧大脑中动脉缺血再灌注模型,于缺血再灌注后3h、6h、12h用高效薄层层析法测定大脑皮质缺血侧和缺血对侧PAF含量、RT-PCR检测PAFR基因表达。使用原代野生型小鼠皮质神经元细胞培养系统,将体外培养的神经元分为空白对照组、PAF组和PAF 拮抗剂组,应用碘化物/钙黄绿素染色分别检测各组细胞凋亡情况。结果缺血侧的大鼠皮质在缺血再灌注后3h、6h、12h PAF含量(单位为ng/g)分别为13·71±1·23、17·90±1·14和17·89±1·54;PAFR基因表达(单位为密度)分别为0·5892±0·1222、2·0512±0·1519、2·0168±0·1653,缺血再灌注6h、12h组缺血侧皮质PAF含量及PAFR基因表达与缺血对侧、再灌注3h比较差异有极显著性(均P<0·01);在进一步的体外小鼠皮质神经元细胞培养中,PAF组、PAF 受体拮抗剂组细胞凋亡率分别为(41·92±1·39)%和(18·94±1·18)%,与空白对照组[(10·23±0·59)%]比较差异有极显著性(均P<0·01),PAF 受体拮抗剂组细胞凋亡率与PAF组比较明显降低(P<0·01)。结论缺血性神经损伤作用是由PAF及其受体介导,用PAFR拮抗剂进行干预,对缺血缺氧性神经元损伤具有保护作用。     

Unsaturated platelet-activating factor: influence on aggregation, serotonin release and thromboxane synthesis of human thrombocytes

Unsaturated platelet-activating factor (paf-acether) aggregated thrombocytes of healthy male volunteers like saturated paf-acether. Unsaturated paf-acether released serotonin in the presence of imipramine. Within one minute the release increased depending on the concentrations of unsaturated paf-acether up to 45% of the serotonin. Human thrombocytes synthesized only a small amount of thromboxane B2 (TXB2) after aggregation induced by unsaturated paf-acether. Unsaturated paf-acether prevented the binding of radiolabelled saturated paf-acether to intact washed thrombocytes in the same extent as saturated paf-acether.     

A new bleeding disorder: lack of platelet aggregatory response to adrenaline and lack of secondary aggregation to ADP and platelet activating factor (PAF)

A bleeding disorder, probably familial, with absent adrenaline-induced platelet aggregation and lack of secondary aggregation response to ADP and platelet activating factor (PAF), is described. The laboratory findings do not fit any hitherto recognized hemorrhagic disease. The disorder was not caused by alpha-adrenergic receptor deficiency, but the ultimate defect has not yet been unraveled. This patient illustrates that a normal response to more than one aggregating stimulus is necessary for normal hemostasis, and indicates a physiopathological role for adrenaline not hitherto recognized. Whether this also applies to PAF remains to be proven.     

The effects of the second generation antipsychotics and a typical neuroleptic on collagen-induced platelet aggregation in vitro

Objective. Antipsychotics are widely used in psychiatry, and consequently a lot of their side effects have been reported. One of them is cardiovascular disease leading to increased risk of stroke, thrombosis, pulmonary, embolism, in which hyperactivation of blood platelets is involved. The purpose of the present study was to examine the effects of the second generation antipsychotics (SGAs) such as clozapine, risperidone, and olanzapine, and a typical neuroleptic – haloperidol – on the one step of platelet activation–platelet aggregation induced by collagen in vitro. Blood was collected into buffered sodium citrate (3.8%) and centrifuged to get platelet-rich plasma (PRP). In PRP (2×10⁸ platelets/ml) obtained from healthy volunteers that was incubated with antipsychotics (clozapine, risperidone, olanzapine, haloperidol; 30 min) aggregation of blood platelets was measured using a Chrono-Log Lumi-aggregometer. Aggregation of platelets was measured after stimulation of platelets with 1 µl of collagen (2 µg/ml). Results. Clozapine, like haloperidol reduced platelet aggregation induced by collagen (inhibition of platelet aggregation reached about 20%) (P=1×10^?5 and P=0.003, respectively). Risperidone had also a weak antiaggregatory effect (P=0.05). Among tested antipsychotics only olanzapine had no effect on collagen-stimulated platelet aggregation (P>0.05). Conclusion. The obtained results indicate that the difference in action of tested drugs on platelet aggregation may dependent on the various chemical structures of these drugs. Clozapine, risperidone and haloperidol are structurally diverse, and they all significantly reduce platelet aggregability induced by collagen. On the other hand, a close structural analog of clozapine – olanzapine – did not inhibit platelet aggregation. However, mechanism of antipsychotics action on blood platelets is not clear. Moreover, it seems that clozapine, risperidone and haloperidol treatment due to antiaggregatory action may have even some antithrombotic effects.     

Potentiation with epinephrine of macaque platelet aggregation by other agonists: implications for studies on human atherosclerosis

The effects of low concentrations of epinephrine on the aggregation of macaque and human platelets by arachidonic acid (AA), collagen, and thrombin were studied. When epinephrine (0.05 to 1 microM) was added to macaque or human citrated or macaque heparinized platelets, either before or after the addition of near-threshold concentrations of AA, significant increases in aggregability were always seen. Epinephrine alone did not aggregate macaque platelets from citrated blood. When near-threshold concentrations of collagen or thrombin were present in the medium, low concentrations of epinephrine (0.05 to 0.50 microM) potentiated the aggregation of macaque and human citrated platelets and macaque heparinized platelets. The P values for the addition of epinephrine were less than 0.01 in all series. The ability of low epinephrine concentrations to potentiate aggregation of macaque platelets by other agonists is of particular significance because in humans the most important effect of epinephrine on platelets in vivo is probably the potentiation, by low concentrations, of aggregation induced by other aggregatory agents normally present in the blood in low concentrations.     

The influence of glutathione and other thiols on human platelet aggregation

The platelet membrane contains sulfhydryl groups which are essential for normal platelet function. Reduced glutathione (GSH) and other thiols such as cysteine and 6-mercaptopurine were found to inhibit human platelet aggregation induced by adenosine diphosphate (ADP), collagen and arachidonic acid. The inhibition of ADP-induced aggregation by GSH (IC₅₀=0.61 ± 0.05 mM) was greater than that by cysteine (IC₅₀=13 ± 1 mM) or 6-mercaptopurine (IC₅₀=5.4 ± 0.2 mM). Two other thiols, dithiothreitol and beta-mercaptoethanol were found to cause platelet aggregation instead of inhibition. The interaction of GSH with the ADP receptor was noncompetitive in nature.     

Disappearance of human platelet factor 4 (PF4) in rabbits: Does an immediate component exist?

Twelve male New Zealand rabbits were injected with 21 micrograms/kg of human platelet factor 4 antigen (PF4). The decay of the protein followed a monoexponential curve for the first 5 mins, with a half-life (t 1/2) of 1.94 mins and a calculated concentration at 0 time (CO) of 79.4 ng/ml. Five rabbits were pre-treated with heparin (2.500 I.U. i.v.) and 3 mins later were injected with the same amount of PF4. PF4 decay followed a monoexponential curve with a t 1/2 of 25.3 mins, and with CO of 380.8 ng/ml. This value is not greatly different from the one calculated assuming an immediate and uniform distribution in plasma (482.7 ng/ml for a plasma volume of 43.5 ml/kg). The 12 rabbits injected with PF4 were divided in 3 groups, in which heparin was given at 10', 30' or 60' after PF4, respectively. After heparin the peak levels of PF4 were 139.9 ng/ml, 65.3 ng/ml and 52.7 ng/ml, respectively. The following monoexponential PF4 decay had t 1/2 of 20.7, 25.6 and 26.0 mins, respectively. In a separate group of 4 animals, we studied heparin decay after an intravenous bolus of 2.500 I.U. Heparin decay could not be described by a monoexponential equation and was different from the decay of PF4 injected after heparin. On the basis of the present data we suggest the presence of an immediate component of PF4 decay, most likely due to uptake by the tissues. Heparin pretreatment may avoid this uptake process.