Release of a slow-reacting substance from rabbit platelets

Extracellular adenosine 5'-triphosphate in pulmonary disorders

Adenosine 5'-triphosphate (ATP) is found in every cell of the human body where it plays a critical role in cellular energetics and metabolism. ATP is released from cells under physiologic and pathophysiologic condition; extracellular ATP is rapidly degraded to adenosine 5'-diphosphate (ADP) and adenosine by ecto-enzymes (mainly, CD39 and CD73). Before its degradation, ATP acts as an autocrine and paracrine agent exerting its effects on targeted cells by activating cell surface receptors named P2 Purinergic receptors. The latter are expressed by different cell types in the lungs, the activation of which is involved in multiple pulmonary disorders. This succinct review summarizes the role of ATP in inflammation processes associated with these disorders including bronchoconstriction, cough, mechanical ventilation-induced lung injury and idiopathic pulmonary fibrosis. All of these disorders still constitute unmet clinical needs. Therefore, the various ATP-signaling pathways in pulmonary inflammation constitute attractive targets for novel drug-candidates that would improve the management of patients with multiple pulmonary diseases.

Adenosine 5'-triphosphate's role in bradycardia and syncope associated with pulmonary embolism

Adenosine 5′-triphiosphate (ATP) is released from cells under physiologic and pathophysiologic conditions. Extracellular ATP acts as an autocrine and paracrine agent affecting various cell types by activating cell surface P2 receptors (P2R), which include trans-cell membrane cationic channels, P2XR, and G protein coupled receptors, P2YR. We have previously shown that ATP stimulates vagal afferent nerve terminals in the lungs by activating P2X2/3R. This action could lead to bronchoconstriction, cough and the local release of pro-inflammatory neuropeptides. In addition, ATP markedly enhances the IgE-dependent histamine release from human lung mast cells. Thus, we have proposed for the first time that extracellular ATP plays a mechanistic role in pulmonary pathophysiology in general and chronic obstructive pulmonary disease (COPD), and acute bronchoconstriction in asthma in particular. The present review examines whether ATP could also play a role in bradycardia and syncope in a subset of patients with pulmonary embolism.

Adenosine 5'-triphosphate axis in obstructive airway diseases

In recent years, significant progress has been made in our understanding of the pathophysiology behind obstructive airway diseases in general and asthma in particular; this knowledge, however, has not translated to major breakthroughs in the treatment of these disorders. Current therapeutic options are less than optimal and frequently are associated with systemic adverse effects. Recent studies indicate that endogenous purine nucleotides, adenosine 5'-triphosphate (ATP) in particular, could play a mechanistic role in obstructive airway diseases through their actions on multiple cell types relevant to these disorders, including mast cells, eosinophils, dendritic cells, and neurons. The pharmacologic modulation of ATP signal transduction in these cells represents an attractive new therapeutic target.

Novel transcellular interaction: conversion of granulocyte-derived leukotriene A4 to cysteinyl-containing leukotrienes by human platelets

Human platelets dose-dependently converted exogenous leukotriene A4 to leukotriene C4 and efficiently metabolized this compound to leukotrienes D4 and E4. Neither of these compounds were produced after stimulation of human platelet suspensions with ionophore A23187. After LTA4 incubation of subcellular fractions, formation of leukotriene C4 was exclusively observed in the particulate fraction and was separable from the classical glutathione S-transferase activity. This suggested the presence of a specific leukotriene C4 synthase in human platelets. Addition of physiological amounts of autologous platelets to human granulocyte suspensions significantly increased ionophore A23187-induced formation of leukotriene C4. In contrast, the production of leukotriene B4 was decreased. After preincubation of platelets with [35S]cysteine, 35S-labeled leukotriene C4 was produced by A23187-stimulated platelet-granulocyte suspensions, strongly indicating a transcellular biosynthesis of this compound.

Enhancement of leukotriene D4-induced contraction of guinea-pig isolated trachea by platelet activating factor

The effect of platelet activating factor (PAF), a potent lipid mediator of inflammation, was examined in the induction of airway hyperreactivity to known mediators of anaphylaxis. Concentration-dependent contractions of the isolated guinea-pig trachea to PAF (10(-7)-10(-5) M) were produced and an EC50 value was found to be 7.5 X 10(-7) M. Pretreatment for 30 min with a known PAF inhibitor, CV-3988 (10(-5) or 10(-4) M), produced significant inhibition of PAF contractions; however, at 10(-6) M, CV-3988 had no effect. In the presence of meclofenamic acid (10(-6) M), the concentration-response curve to PAF was shifted significantly upward and to the left. This potentiation could be reversed by pretreating the tissues with the peptidoleukotriene antagonists, FPL 55712 or SK&F 102922 (10(-5) M). Pretreatment with PAF concentrations having essentially no intrinsic activity (10(-8), 10(-7)) significantly enhanced the contraction of guinea-pig trachea to various concentrations of LTD4 and to certain concentrations of a thromboxane mimic (U-46619). Pretreatment with lyso-PAF failed to potentiate the LTD4 response, while pretreatment with CV-3988 reverse the potentiation by PAF of the lower concentrations of LTD4. However, PAF failed to enhance contractions (with or without the presence of meclofenamic acid) to acetylcholine, histamine, PGD2 or LTC4 (in the presence of serine borate). These results indicate a possible role for PAF as a mediator of airway hyperreactivity.

Pharmacological modulation of the respiratory and haematological changes accompanying active anaphylaxis in the guinea-pig

The intrathoracic accumulation of radiolabelled platelets was recorded in anaesthesized, sensitized guinea-pigs concomitantly with changes in airways resistance during active anaphylaxis. Antigen challenge induced an increased airway resistance associated with intrathoracic platelet accumulation, thrombocytopenia and leukopenia. Bronchoconstriction, but not the haematological changes, was modified by a combination of mepyramine, methysergide and aspirin. The addition of FPL 55712 and PGI2 to this cocktail of drugs additionally inhibited the thrombocytopenia and leukopenia but not the intrathoracic accumulation of platelets induced by antigen challenge. PGI2 was alone able to significantly inhibit the intrathoracic platelet accumulation, but not the bronchoconstriction induced by antigen challenge. These results suggest that active anaphylaxis is associated with activation of blood elements and subsequent pulmonary entrapment of platelets but that the platelet release reaction rather than the embolisation of platelet aggregates is the critical step in the induction of allergic bronchoconstriction in the guinea-pig.

Metabolism of leukotriene A4 into C4 by human platelets

Tritium-labelled leukotriene A4 is converted by a suspension of human platelets into leukotriene C4. The conversion is stimulated by reduced glutathione and is dependent on the platelet concentration. Formation of leukotriene C4 is temperature and time dependent and is destroyed by heating the platelets at 100 degrees C for 5 min. Verification of leukotriene C4 formation was obtained by conversion into leukotriene D4 during reaction of the HPLC-purified platelet-derived leukotriene C4 with commercial gamma-glutamyl transpeptidase. In separate experiments we incubated authentic tritiated leukotriene C4 with human platelets and we showed the formation of tritiated leukotriene D4, demonstrating the presence of gamma-glutamyl transpeptidase activity in these cells. This activity could be blocked by the presence of reduced glutathione in the incubation mixture. In contrast, erythrocytes converted tritiated leukotriene A4 almost exclusively into leukotriene B4. Although platelets have been reported to lack 5-lipoxygenase activity, our study demonstrates that platelets possess the necessary machinery to transform leukotriene A4 into leukotrienes C4 and D4. Our results suggest that an intracellular interaction between platelets and leukotriene A4-forming cells, e.g., polymorphonuclear leukocytes, could lead to the formation of these potent peptidolipids in the circulation.

Kinetics of acetyl glyceryl ether phosphorylcholine (AGEPC)-induced acute lung alterations in the rabbit

Acetyl glyceryl ether phosphorylcholine (AGEPC; 1-O-hexadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine) was infused intravenously into rabbits (0.5 micrograms/kg); subsequently, temporal pulmonary alterations were assessed histologically. Within 30 seconds after AGEPC infusion, widespread platelet and neutrophil aggregates were distributed throughout the pulmonary microvasculature. Concomitantly, small muscular arteries and bronchioles throughout the lungs were contracted. Five minutes after AGEPC infusion, intravascular pulmonary platelet aggregates were less frequent and smaller than those observed at 30 seconds after infusion; however, AGEPC-induced pulmonary neutrophil sequestration persisted. Moreover, at this time, large mononucleated cells and damaged endothelial cells were prevalent throughout the pulmonary microvasculature. Sixty minutes after infusion, neither platelet aggregates nor arterial or bronchiolar constriction was observed. However, in most animals, neutrophils and large mononucleated cells were still abundant, and focal endothelial cell alterations persisted. In addition, discrete areas of interstitial hemorrhage around small and medium-sized arteries were present. These studies suggest that the intravascular release of AGEPC could initiate significant pulmonary injury and therefore could be an important etiologic factor in the development of inflammatory lung diseases.

Pulmonary platelet kinetics in asthma

Platelets produce a range of bronchoconstrictor mediators. Measurements of plasma factors have implicated platelet activation in allergic asthma, and sensitised guinea pigs challenged with ovalbumin show pulmonary platelet aggregation accompanying bronchoconstriction. To investigate this further we injected autologous platelets labelled with indium 111 and red cells labelled with technetium 99m into three young volunteers with atopic asthma and three non-asthmatic volunteers and, after equilibration of platelets between blood and splenic pool, monitored lung 99mTc and 111In activities continuously. Comparison with the corresponding activities in blood samples allowed calculation of pulmonary platelet to red cell transit time ratio (tp/tr). This ratio was 0.9, 1.02, and 0.98 in the non-asthmatic subjects compared with 1.04, 0.97, and 1.17 in the asthmatic subjects. This argues against the existence of an intrapulmonary platelet pool in normal subjects; transpulmonary transit time was slightly prolonged in one asthmatic subject. Bronchial challenge with Dermatophagoides pteronyssinus was performed in the asthmatic subjects and monitoring continued for a further 30 minutes. Antigen induced falls in FEV1 of 20-50% were accompanied by small decreases in the 111In but not in the 99mTc lung signal. In line with this tp/tr fell to 0.89, 0.89, and 1.05. Antigen induced bronchoconstriction was therefore not accompanied by intrapulmonary platelet accumulation. Platelet survival was normal at 10.2 days in both groups of subjects.

Formation of prostaglandins, leukotrienes and paf-acether by macrophages

Prostaglandins (PG) and leukotrienes (LT)--arachidonic acid-dependent metabolites--and paf-acether (platelet-activating factor)--an ether phospholipid--are potent mediators of allergic and inflammatory reactions. Their structures, chemical synthesis and biosynthetic pathways have been recently described. These mediators are produced by various cells with proinflammatory activities including the macrophages upon interaction with a specific secretagogue stimulus (phagocytosis of zymosan particles, immune-complexes); in IgE-dependent hypersensitivity reactions; upon interaction with one of these mediators. Formation of these mediators by macrophages depends upon their local environment. Qualitative and/or quantitative variations in their synthesis are observed depending on the tissue they are derived from (alveole or peritoneum) and on the type of inflammation (immunologic specific or not). Their potent biological activities (increase of vascular permeability, smooth muscle contraction, cardiac and vascular effects and/or chemotactism) suggest a role for these mediators in various pathologies.

Platelet-activating factor stimulation of peptidoleukotriene release: inhibition by vasoactive polypeptide

Leukotriene (LT) release stimulated by platelet activating factor was inhibited by vasoactive intestinal polypeptide (VIP) in an in vitro rat lung preparation. This was detected by HPLC and radioimmunoassay. LTC4, although the major species in the stimulatory model used, was not detected in peptide-treated preparations and LTD4 and LTE4 levels were considerably reduced.

Leukotrienes: clinical significance

The leukotrienes, so named because of their initial identification in leukocyte preparations and the presence of three conjugated double bonds (a conjugated triene), are metabolites of the same polyunsaturated fatty acids (e.g., arachidonic acid) that give rise to the prostaglandins, thromboxanes, and several other families of biologically active lipids. Their potential clinical importance derives from their effects on vascular and other smooth muscle reactivity and on leukocyte function. Several leukotrienes may markedly influence the cellular and vascular responses that constitute an integral part of hypersensitivity and inflammatory reactions of the skin. Preliminary data from several laboratories have been presented that implicate a specific leukotriene in the evolution of the lesions of psoriasis.

Mediators of arachidonic acid-induced contractions of indomethacin-treated guinea-pig airways: leukotrienes C4 and D4

Arachidonic acid (AA) (66 microM) induced contractions of indomethacin-treated (8.4 microM) guinea-pig tracheal and lung parenchymal preparations. Mepacrine (210 microM) treatment did not affect the magnitude of contraction induced by AA. Normal and ovalbumin-sensitized tissues responded identically to AA, and released equivalent amounts of the contractile mediators. Nordihydroguaiaretic acid (100 microM) markedly reduced release of the contractile mediators and reduced AA-induced contractions of the airways. The mediators of AA-induced, calcium ionophore A23187-induced, and antigen-induced contraction of the trachea and lung parenchyma were purified and identified by reverse-phase high performance liquid chromatography to be leukotrienes C4 and D4, being present in an approximate ratio of 20:1. Mepacrine-treated trachea exhibited a smaller contractile response to stimuli (A23187 for normal tissues and ovalbumin for sensitized tissues). Addition of exogenous AA (66 microM) increased the magnitude of contraction, although not to the level observed on tissues not treated with mepacrine. There was no observable effect of AA on the response of mepacrine-treated parenchyma to the ionophore or antigen. It was concluded that (a) immunological sensitization does not alter AA metabolism via the lipoxygenase pathway in guinea-pig airways and (b) the mediators of AA-induced contraction are leukotrienes C4 and D4.

Release of platelet-activating factor, slow-reacting substance, and vasoactive amines from isolated rat kidneys

The present work brings the first evidence for the simultaneous release of Paf-acether (platelet-activating factor), slow-reacting substance (SRS), histamine, and serotonin from isolated rat kidneys stimulated with ionophore A 23187. However, with compound 48/80 we detected only SRS, histamine, and serotonin. Upon addition of antigen, kidneys from sensitized rats released Paf-acether and SRS of anaphylaxis. Paf-acether released in the perfusate was identified by its ability to aggregate aspirin-treated washed rabbit platelets in the presence of an ADP scavenger complex. It was also characterized by its inactivation by phospholipase A2, and it was eluted from high pressure liquid chromatography (HPLC) after 16 to 18 min, a retention identical to that of synthetic Paf-acether, that is, between sphingomyelin and lysophosphatidylcholine. The biological activity of SRS was detected after several purification steps including Amberlite XAD-2 and reverse phase HPLC (RP-HPLC). Kidney SRS exhibited a typical spasmogenic activity in isolated guinea-pig ileum preparation that was reversed by FPL 55712. When kidneys were incubated with [3H]arachidonic acid, radioactivity and biological activity comigrated in RP-HPLC with leukotrienes C and D. These results indicate that the kidney is capable of actively released inflammatory mediators.

Spasmogenic properties of platelet-activating factor: evidence for a direct mechanism in the contractile response of pulmonary tissues

Platelet-activating factor (1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine; PAF) induces a specific, dose-dependent contraction of guinea pig lung parenchymal strips with an ED50 value of 10(-9) M. The smooth muscle contractile activity of PAF in this system was not effected by the H1-blocking antihistamine, pyrilamine (10(-6) M), the prostaglandin synthesis inhibitors, indomethacin (10(-5) M), aspirin (10(-4) M), or sulfinpyrazone (5 X 10(-4) M), the leukotriene synthesis inhibitor, nordihydroguaiaretic acid (NDGA; 10(-5) M), the leukotriene antagonist FPL 55712 (10(-6) M) or the inhibitor of arachidonic acid metabolism, eicosatetraynoic acid (ETYA; 2 X 10(-5) M). The role of platelets in this system was also investigated. PAF-mediated contractions were not attenuated following platelet depletion using nitrogen mustard, nor were they augmented by the addition of exogenous platelets. Furthermore, isolated platelets incubated with PAF did not release stable substances spasmogenic for lung parenchymal strips. Finally, contractile activity of PAF was demonstrated in lung parenchymal strips from rats, a species whose platelets are insensitive to PAF at elevated concentrations. Taken together, these data show that PAF contracts smooth muscle of guinea pig lung parenchyma independently of endogenous histamine, arachidonic acid metabolites, or platelets trapped within the pulmonary vasculature. It is concluded, therefore, that PAF may act directly on contractile cells of the lung.

Dependency of the Paf-acether induced bronchospasm on the lipoxygenase pathway in the guinea-pig

The Paf-acether (platelet-activating factor) induced bronchospasm (Paf-BCS) was studied in the anesthetized guinea-pig. The SRS antagonist, FPL-55712, as well as inhibitors of both lipoxygenase and cyclooxygenase, phenidone, nordihydroguaiaretic acid (NDGA), and benoxaprofen, caused a dose-related antagonism of Paf-BCS. By contrast, selective inhibitors of cyclooxygenase, indomethacin and aspirin, exerted moderate antagonism at intermediate doses, but had no effect at high doses. Furthermore, diethylmaleate (DEM), which impairs leukotriene synthesis by interfering with glutathione (GSH), suppressed Paf-BCS. Taken together, these results demonstrate that the lipoxygenase pathway plays a major part in the bronchospasmogenic effect of Paf-acether in the guinea-pig.