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PubMed1989

Synthesis of platelet-activating factor by human blood platelets and leucocytes. Evidence against selective utilization of cellular ether-linked phospholipids.

Synthesis of platelet activating factor (PAF) in blood platelet suspensions may be due to leucocyte contamination. We therefore investigated PAF synthesis in human blood platelet suspensions and granulocyte- (PMN)-enriched leucocyte suspensions upon stimulation by thrombin and Ca2+-ionophore A23187, both in the presence and absence of the presumed PAF catabolism inhibitor phenylmethylsulfonyl fluoride (PMSF). PAF synthesis was measured by aggregation of washed rabbit platelets and by [3H]acetate incorporation. In contrast to A23187, thrombin was unable to stimulate PAF synthesis by leucocytes. As thrombin did induce PAF synthesis by platelet suspensions, this was evidently not due to leucocyte contamination. A23187 also induced PAF synthesis by platelets, but this was dependent upon the platelet isolation method and possibly associated activation. The ratio of [3H]acetate incorporation into 1-alkyl- versus 1-acyl-2-acetylglycerophosphocholine upon stimulation of non-PMSF-treated leucocytes and platelets amounted to 12.8 and 1.2, respectively. These values are at least 10-fold higher than the ratio of 1-alkyl versus 1-acyl species in the cellular phosphatidylcholine precursor for PAF. By PMSF pretreatment, the distribution of incorporated [3H]acetate between 1-ether- and 1-ester-linked species became similar to that in the precursor phosphatidylcholines of the respective cell type, due to increased recovery of [3H]acetate in the acyl compounds. Both leucocyte and platelet homogenates rapidly degraded acylacetylglycerophosphocholine to (acetyl)glycerophosphocholine, and this deacylation was inhibited by PMSF pretreatment of the cells. We conclude that upon cell stimulation a phospholipase A2 converts both alkylacylglycerophosphocholine and diacylglycerophosphocholine to the 2-lysoanalogs in a ratio similar to the occurrence of the parent compounds. The acetyltransferase subsequently acetylates both compounds to acylacetylglycerophosphocholine and alkylacetylglycerophosphocholine (PAF), respectively. Deacylation of the 1-ester-linked species, either before or after acetylation, gives the impression of selective utilization of 1-ether-linked species for PAF production. It is only after inhibition of the deacylation by pretreatment of the cells with PMSF that a mainly nondiscriminative use of 1-ether- and 1-ester-linked species by both phospholipase A2 and acetyltransferase becomes evident.

Acetates↗
PubMed1993

[Relation between traditional Chinese medicinal syndrome differentiation and blood platelet function in 310 cases of blood stasis].

310 patients (153 male, 157 female) of Blood Stasis were divided into various groups by Syndrome Differentiation of TCM and 35 healthy subjects were taken as control. The relationship of TCM Syndrome and function of blood platelet were studied with blood platelet counting, its volume, adherence and aggregation, plasma thromboxane B2 (TXB2) and 6-keto-prostaglandin F1 alpha(6k-PGF1 alpha) as indices of observation. The results were: (1) Blood platelet aggregation of Blood Stasis patients was higher than that in control (P > 0.001). (2) There were some differences of platelet aggregation among different TCM Syndromes. Platelet aggregation of Blood Stasis patients with Qi Deficiency was higher than that with other Syndromes significantly, and the aggregating rate in those patients was higher than that in control (P < 0.05). (3) The platelet aggregation and TXB2 of patient with Heart Qi Deficiency were significantly higher than those of the control (P < 0.05, P < 0.001), and they were also higher than of other groups of blood stasis patient with Spleen, Lung or Kidney Qi Deficiency.

6-Ketoprostaglandin F1 alpha↗
PubMed1982

Inhibitors of blood platelet aggregation. Activity of some 1H-benz[de]isoquinolinecarboximidamides on the in vivo blood platelet aggregation induced by collagen.

A series of 33 1H-benz[de]isoquinolinecarboximidamides has been prepared and tested in the rat after intraperitoneal (ip) and/or oral (po) administration for their ability to inhibit the in vivo blood platelet aggregation induced by collagen. In this aggregation test, a considerable number of active compounds were found. Fourteen compounds were active when administered in [0.2 (mmol/kg)/day], five of which also exhibited significant po activity. One compound was toxic after ip administration but was found to be active after po administration without apparent toxicity. It is thought that the solubility of the drug in water is an important factor for the resorption after oral administration and, hence, for its oral activity.

Animals↗
PubMed1994

Blood platelets in experimental lung emphysema. Comparative analysis of the number and aggregation abilities of platelets in left and right ventricular blood of the heart.

Quantitative and morphological analyses (in SEM) of blood platelets collected from the left and right ventricles of the rat heart in the course of experimental lung emphysema were done. Platelet aggregation index was estimated, too. Emphysema was induced by a single intratracheal instillation of papain solution in a dose of 20 mg/kg b.w/1 ml PBS. The animals were sacrificed after 2 and 24 hours and 7, 14, 28 days later. Within 24 hours of the experiment a slight decrease was observed in the number of platelets in the blood collected from the left ventricle compared to the right one as well as to control animals. Also a reduction in platelet aggregation coefficient value was noted. However, in the later period of emphysema progression (after 7th day), a statistically significant increase was found in the number of blood platelets in the left ventricle. A relation was noted between quantitative changes of blood platelets and emphysema progression evaluated morphometrically. The ultrastructural examinations in SEM suggest the occurrence of platelet satellitosis in animals intratracheally injected with papain solution. The present results indicate the possibility of a significant contribution of blood platelets to the pathogenesis of experimental lung emphysema.

Animals↗
PubMedDate not supplied

The role of blood platelets in nucleoside metabolism: regulation of platelet thymidine phosphorylase.

Blood platelets are the smallest cellular elements in mammalian blood. Because of their small size, platelets have an unusually large surface area: volume ratio and are exquisitely sensitive to a multitude of physiological and environmental stimuli. Platelets lack nuclei, but most possess functional mitochondria and remain capable of both anaerobic and aerobic energy metabolism, for which they utilise a variety of substrates including many which are cytotoxic and genotoxic for other (nucleated) cells. Nucleic acid precursors are amongst the potentially genotoxic compounds for which platelets have an apparently insatiable appetite. In particular platelets actively scavenge adenine and adenosine, which they convert to nucleotides and use in energy metabolism, but they also rapidly phosphorylase thymidine and liberate thymine into the extracellular medium. In addition, platelets contain non-metabolisable membrane-bound pools of adenine nucleotides which they secrete in response to strong agonists. Taken together, these observations suggest that blood platelets play an important role in nucleic acid precursor metabolism. In the previous paper we have shown that most thymidine phosphorylase activity present in normal human blood resides in the cytoplasm of platelets. Here we demonstrate that this enzyme activity can be modulated in a dose-dependent fashion, not only by substances recognised as platelet agonists and antagonists, but also by some compounds which are considered to be toxic, mutagenic and/or carcinogenic. The data which we present provide additional support for our previous suggestion that platelets regulate thymidine homeostasis and further imply that this is the normal, physiological, platelet function. Preliminary results suggest that assays of blood platelet thymidine metabolism may provide data with a wide variety of applications.

Alcohols↗
PubMed1991

[Signal transduction in blood platelets].

When a blood vessel is disrupted, subendothelial structures such as collagen come into contact with circulating blood platelets. These will adhere and recruit additional platelets to form a platelet aggregate which will close the leak, but which can, under certain circumstances, give rise to the formation of a thrombus. In this work our personal contribution to a better understanding of this process is given. We could demonstrate the presence of an antibody interfering with the platelet-collagen interaction in two patients with a bleeding problem. One of the antibodies is directed against glycoprotein (GP) Ia, a known collagen receptor, the other one recognizes a less well characterized protein of 85-90 kD. It therefore can be concluded that activation of blood platelets requires the simultaneous interaction of collagen with multiple receptors. Activation of platelets following binding of an agonist in many instances involves activation of phospholipase C via a GTP-binding protein or G-protein. We have further studied this by using a direct stimulator of G-proteins, AlF4-, which in platelets indeed activates phospholipase C, together with other systems. Furthermore, we could demonstrate that activation of phospholipase C in a GTP-dependent manner also occurs in platelet cytosol, indicating that the action of G-proteins is not restricted to membrane-linked phenomena. Activation of phospholipase C gives rise to the formation of inositol phosphates, of which mainly inositol 1, 4, 5 trisphosphate increases intracellular Ca(2+)-levels. Following this, the Ca(2+)-dependent phospholipase A2 releases arachidonic acid from the membranes. In platelets arachidonic acid is metabolised to another platelet activator: thromboxane A2. We have studied the effects of the inhibition of this aggregation-amplifying pathway by using specific inhibitors of the synthesis of thromboxane A2 and of thromboxane A2 receptor antagonists both in vitro and in vivo. One of the conclusions that were reached from these studies was that theoretically the combination of these two classes of drugs should yield a significant stronger antiplatelet effect than either class used alone. We could later on confirm this hypothesis, which stimulated some pharmaceutical companies to look for dual action compounds, of which we have studied two so far.(ABSTRACT TRUNCATED AT 400 WORDS)

Blood Platelets↗
PubMed1988

Characterization of an anti-thrombospondin monoclonal antibody (P8) that inhibits human blood platelet functions. Normal binding of P8 to thrombin-activated Glanzmann thrombasthenic platelets.

Stimulated human blood platelets release thrombospondin, an alpha-granule glycoprotein of 450 kDa. The aim of this work was to characterize an anti-thrombospondin monoclonal antibody (P8) in order to study the role of thrombospondin in platelet functions. The presence of thrombospondin receptor sites on resting and thrombin-stimulated platelets of three Glanzmann's thrombasthenia patients and normal donors was investigated using the P8 monoclonal antibody. Monoclonal antibody P8 was extensively characterized using ELISA, immunoprecipitation, immunoadsorbent affinity chromatography combined with tryptic peptide map analysis and crossed immunoelectrophoretic techniques. Labelled P8 bound strongly to thrombin-stimulated normal platelets (n = 14917 +/- 420, mean +/- SD) (Kd = 9.2 +/- 3.0 nM) and poorly to resting platelets (n = 2697 +/- 1278) (Kd = 24.8 +/- 18.6 nM). Moreover, the number of binding sites for P8 on thrombin-stimulated platelets from three Glanzmann's thrombasthenia patients, lacking the IIb-IIIa glycoprotein complex, were found similar to normal samples. F(ab')2 fragments of P8 inhibited aggregation of, and reduced secretion from, washed platelets stimulated by low concentrations of thrombin (0.05-0.06 U/ml) and collagen (0.5-0.6 microgram/ml). F(ab')2 fragments of P8 inhibited thrombin-induced platelet aggregation, but did not reduce fibrinogen binding (n) nor affect its dissociation constant (Kd). Inhibition of platelet aggregation by P8 suggests that thrombospondin plays an active role in promoting platelet aggregation, at low concentrations of thrombin and collagen. Normal binding of P8 to thrombin-stimulated Glanzmann thrombasthenic platelets indicates the presence of a thrombospondin receptor on the platelet surface distinct from the GPIIb-IIIa complex.

Antibodies, Monoclonal↗