Spasmogenic activity of C5adesArg anaphylatoxin on guinea pig lung parenchymal strips: Sensitivity of the leukotriene-mediated component to cyclooxygenase inhibitors

The leukotriene-dependent component of C5a_desArg-induced contractile activity on guinea pig lung parenchymal strips is inhibited by cyclooxygenase inhibitors. Indomethacin simultaneously increased leukotriene release while inhibiting both cyclooxygenase-dependent mediator release and the contractile force generated. Tissue responses to LTC₄ and LTD₄ are also inhibited by cyclooxygenase blockade, while contractions induced by the thromboxane A₂ analog, U-46619, histamine or acetylcholine are not affected. These data indicate a functional role for cyclooxygenase metabolites in leukotriene-induced contractile responses in lung.     

Bronchoconstrictor effects of leukotriene B4 in the guinea pig in vivo

Administration of leukotriene B4 (LTB4) to anesthetized spontaneously breathing guinea pigs either by the intravenous or aerosol route produced pronounced changes in pulmonary resistance and dynamic compliance. The effects were short lived and were completely abolished by pretreatment of animals with the cyclooxygenase inhibitor indomethacin. Histological examination of lungs following aerosol administration of LTB4 showed a pronounced neutrophil infiltration. These results confirm previous in vitro studies in which LTB4 was shown to produce contractions on guinea pig parenchymal strips indirectly by releasing thromboxane A2.     

Biologic activity of a C2-derived peptide. Demonstration of a specific interaction with guinea pig lung tissues

A synthetic peptide derived from the carboxy terminus of C2b has been investigated for its ability to induce the contraction of guinea pig lung parenchymal strips. This peptide is known to enhance vascular permeability in guinea pig and human skin, and to induce contraction of estrous rat uterus. This C2 peptide (C2 207-223) is active from 5 x 10(-5) M to 5 x 10(-4) M and is not tachyphylactic to itself. No cross-activity between C2 207-223 and C5a or C3a could be demonstrated. C2 207-223 is not inhibited by antihistamines or cyclooxygenase inhibitors. These data indicate that the peptide exerts its action via a mechanism distinct from those of the C3a and C5a anaphylatoxins.     

Evaluation of inhibitors of eicosanoid synthesis in leukocytes: possible pitfall of using the calcium ionophore A23187 to stimulate 5' lipoxygenase

The effect on arachidonate metabolism of two compounds (BW755C and benoxaprofen) which have been reported to inhibit 5' lipoxygenase in leukocytes has been evaluated in human polymorphonuclear leukocytes (PMN) stimulated with the calcium ionophore A23187 and serum-treated zymosan (STZ). The syntheses of leukotriene B4 (LTB4) and thromboxane B2 (TXB2) from endogenous substrate were determined by specific radioimmunoassays as indicators of 5' lipoxygenase and cyclo-oxygenase activity in the PMN respectively. Benoxaprofen inhibited the synthesis of leukotriene B4 by human PMN stimulated with the calcium ionophore A23187, but it was approximately 5 times less potent than BW755C. However, benoxaprofen (IC50 1.6 X 10(-4)M) was approximately 100 times less potent than BW755C (IC50 1.7 X 10(-6)M) at inhibiting leukotriene B4 synthesis induced by serum-treated zymosan. Both drugs inhibited thromboxane synthesis by leukocytes stimulated with A23187 or serum-treated zymosan at similar concentrations (approximately 5 X 10(-6)M). The data obtained using STZ as stimulus are consistent with previous in vivo studies and indicate that benoxaprofen is a relatively selective inhibitor of cyclo-oxygenase. However, this selectivity was far less apparent when A23187 was used as a stimulus to release the eicosanoids which suggests that this inhibition could be via an indirect mechanism and therefore A23187 should be used with caution as a stimulus of 5' lipoxygenase for evaluating inhibitors of eicosanoid synthesis.     

Release of prostaglandins and thromboxanes by guinea pig isolated type II pneumocytes

Lung cells have been isolated by enzymatic digestion of guinea pig lungs and mechanical dispersion to obtain a suspension of viable cells (approximately 500 X 10(6) cells). Type II pneumocytes have been purified to approximately 92% by centrifugal elutriation (2000 rpm, 15 ml/min) followed by a plating in plastic dishes coated with guinea pig IgG (500 micrograms/ml). We have investigated the arachidonic acid metabolism through the cyclooxygenase pathway in this freshly isolated type II cells (2 x 10(6) cells/ml). Purified type II pneumocytes produced thromboxane B2 (TxB2) predominantly and to a smaller extent the 6-keto prostaglandin PGF1 alpha (6-keto-PGF1 alpha) and prostaglandin E2 (PGE2) after incubation with 10 microM arachidonic acid. The stimulation of pneumocytes with 2 microM calcium ionophore A23187 released less eicosanoids than were produced when cells were incubated with 10 microM arachidonic acid. There was no additive effect when the cells were treated with both arachidonic acid and the ionophore A23187. Guinea pig type II pneumocytes failed to release significant amounts of TxB2, 6-keto-PGF1 alpha and PGE2 after stimulation with 10 nM leukotriene B4, 10 nM leukotriene D4, 10 nM platelet-activating factor, 5 microM formyl-methionyl-leucyl-phenylalanine, 0.2 microM bradykinin and 10 nM phorbol myristate acetate. Our findings indicate that guinea pig type II pneunomocytes possess the enzymatic machinery necessary to convert arachidonic acid to specific cyclooxygenase products, which may suggest a role for these cells in lung inflammatory processes.     

Bronchoconstrictor effects of leukotriene B4 in the guinea pig

Administration of leukotriene B₄ (LTB₄) to anesthetized spontaneously breathing guinea pigs either by the intravenous or aerosol route produced pronounced changes in pulmonary resistance and dynamic compliance. The effects were short lived and were completely abolished by pretreatment of animals with the cyclooxygenase inhibitor indomethacin. Histological examination of lungs following aerosol administration of LTB₄ showed a pronounced neutrophil infiltration. These results confirm previous studies in which LTB₄ was shown to produce contractions on guinea pig parenchymal strips indirectly by releasing thromboxane A₂.     

Pharmacology of L-660,711 (MK-571): a novel potent and selective leukotriene D4 receptor antagonist

L-660,711 (3-(3-(2-(7-chloro-2-quinolinyl)ethenyl)phenyl) ((3-dimethyl amino-3-oxo propyl)thio)methyl)thio)propanoic acid is a potent and selective competitive inhibitor of [3H]leukotriene D4 binding in guinea pig (Ki value, 0.22 nM) and human (Ki value, 2.1 nM) lung membranes but is essentially inactive versus [3H]leukotriene C4 binding (IC50 value in guinea pig lung, 23 microM). Functionally it competitively antagonized contractions of guinea pig trachea and ileum induced by leukotriene (LT) D4 (respective pA2 values, 9.4 and 10.5) and LTE4 (respective pA2 values, 9.1 and 10.4) and contractions of human trachea induced by LTD4 (pA2 value, 8.5). L-660,711 (5.8 x 10(-8)M) antagonized contractions of guinea pig trachea induced by LTC4 in the absence (dose ratio = 28) but not in the presence of 45 mM L-serine borate (dose ratio less than 2). L-660,711 (1.9 x 10(-5)M) did not block contractions of guinea pig trachea induced by histamine, acetylcholine, 5-hydroxytryptamine, PGF2 alpha, U-44069, or PGD2. In the presence of atropine, mepyramine, and indomethacin, L-660,711 (1.9 x 10(-5)M) inhibited a small component of the response to antigen on guinea pig trachea but completely blocked anti-IgE-induced contractions of human trachea. L-660,711 (i.v.) antagonized bronchoconstriction induced in anesthetized guinea pigs by i.v. LTC4, LTD4, and LTE4 but did not block bronchoconstriction to arachidonic acid, U-44069, 5-hydroxytryptamine, histamine, or acetylcholine. Intraduodenal L-660,711 antagonized LTD4 (0.2-12.8 micrograms/kg)-induced bronchoconstriction in guinea pigs, and p.o. L-660,711 blocked LTD4- and Ascaris-induced bronchoconstriction in conscious squirrel monkeys and ovalbumin-induced bronchoconstriction in conscious sensitized rats treated with methysergide (3 micrograms/kg). The pharmacological profile of L-660,711 indicates that it is a potent, selective, orally active leukotriene receptor antagonist which is well suited to determine the role played by LTD4 and LTE4 in asthma and other pathophysiologic conditions.     

Bronchoconstrictor effects of leukotriene B4 in the guinea pig in vivo

Administration of leukotriene B₄ (LTB₄) to anesthetized spontaneously breathing guinea pigs either by the intravenous or aerosol route produced pronounced changes in pulmonary resistance and dynamic compliance. The effects were short lived and were completely abolished by pretreatment of animals with the cyclooxygenase inhibitor indomethacin. Histological examination of lungs following aerosol administration of LTB₄ showed a pronounced neutrophil infiltration. These results confirm previous $\text{in vitro}$ studies in which LTB₄ was shown to produce contractions on guinea pig parenchymal strips indirectly by releasing thromboxane A₂.     

Detection of a novel cyclooxygenase metabolite produced by human promyelocytic leukemia (HL-60) cells

Arachidonic acid metabolism via the lipoxygenase pathway was examined in HL-60 cells before and after N,N-dimethylformamide induced differentiation along granulocytic lines. Untreated HL-60 cells produced small amounts of the 5-lipoxygenase products, 5-hydroxy-eicosatetraenoic acid and leukotriene B4 upon stimulation with calcium ionophore A23187. N,N-dimethylformamide treatment, caused a 10 to 20 fold increase in the amount of ionophore A23187-induced 5-lipoxygenase metabolites. An additional, and as yet unidentified arachidonic acid metabolite was routinely observed during reverse-phase high pressure liquid chromatography analyses of lipoxygenase products. Sensitivity to inhibition by less than 10(-7)M indomethacin coupled with other characteristics of its production, strongly suggest the compound is a cyclooxygenase product. The unusual UV absorbance and chromatographic elution pattern, however, suggest that it is not a typical prostaglandin, thromboxane or prostacyclin product.     

Mechanisms of pulmonary vasoconstriction induced by chemotactic peptide FMLP in isolated rabbit lungs.

The chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) has been shown to constrict both bronchial and coronary vascular smooth muscle through the action of cyclooxygenase or lipoxygenase products. We observed that intravenous FMLP increased pulmonary vascular resistance (PVR) in isolated buffer-perfused rabbit lungs. FMLP increased the PVR (primarily in the middle segment of the pulmonary vascular bed) at concentrations greater than or equal to 10(-7) M. Maximum vasoconstriction occurred at 5 min and then slowly declined to a level that remained above baseline at 30 min. Tachyphylaxis was observed in response to FMLP. When polymorphonuclear leukocytes (PMNs) were added to the perfusate, FMLP caused a greater increase in PVR. PMN depletion with dimethylmyleran significantly reduced the PVR response to FMLP. Pretreatment with two dissimilar cyclooxygenase inhibitors, meclofenamate and ibuprofen, and the leukotriene synthesis blocker MK 886 had no effect on the FMLP-induced vasoconstriction. However, the reactive oxygen species scavenger catalase significantly reduced the vasoconstriction. These results suggest that FMLP induces vasoconstriction that is dependent on PMNs and mediated by reactive oxygen species with no involvement of cyclooxygenase or lipoxygenase products.     

Mechanism of action of leukotrienes on the guinea pig bronchus

The pharmacological activity of leukotrienes (LT) A4, C4, D4, E4, and histamine was investigated on guinea pig upper and lower bronchi. The contractions of the upper bronchi to histamine, LTA4, C4 and D4 were enhanced by cyclooxygenase inhibitors aspirin (1.67 X 10(-5) and 1.67 X 10(-6) M) and indomethacin (2.8 X 10(-6) and 2.8 X 10(-5) M) whereas the responses to LTE4 were not affected. The myotropic activity of the lower bronchi to all agonists were either very slightly or not at all modified by the presence of cyclooxygenase inhibitors. The thromboxane synthetase inhibitor OKY-046 (1.77 X 10(-5) and 1.77 X 10(-6) M) did not change the responses of higher bronchi to the agonists which suggested that the response of the upper bronchi may be mediated by prostaglandins but not by thromboxanes. The responses of the lower bronchi to leukotrienes A4, C4, D4 and E4 were inhibited by compound OKY-046. Blockade of thromboxane receptors together with inhibition of lipoxygenases by compound L-655,240 (2.53 X 10(-8) to 2.53 X 10(-5) M) had a slight effect on the stimulation of upper and lower bronchi by leukotrienes and histamine. The compound FPL-55712 (1.92 X 10(-6) and 1.92 X 10(-5) M) strongly reduced the contractions of the upper and lower bronchi to leukotrienes but did not affect the responses to histamine. These results suggest that the contractile effects of leukotrienes on upper bronchi is modulated by bronchorelaxant prostaglandins whereas the responses of the lower bronchi are mediated by thromboxanes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Endotoxin-induced lung injury in rats: role of eicosanoids

We studied lung vascular injury and quantitated lung eicosanoids in rats after intraperitoneal injection of Salmonella enteritidis endotoxin. Within 40 min after endotoxin injection (20 mg/kg), lung tissue thromboxane B2 doubled, although 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) increased by 8- to 10-fold. Lung 5-hydroxyeicosatetraenoic acid and leukotriene C4 were variably increased by endotoxin. The levels of all eicosanoids returned to base line 6 h after endotoxin challenge. Lung vascular injury, as assessed by the extravascular accumulation of 125I-albumin and water in isolated perfused lungs, was observed 90 min after endotoxin injection (0.02-20 mg/kg) in vivo. Inhibition of the cyclooxygenase pathway with indomethacin and the lipoxygenase pathway with diethylcarbamazine and 2-(12-hydroxydodeca-5,10-dinyl)-3,5,6-trimethyl-1,4-benzoqui none failed to attenuate endotoxin-induced lung injury. In addition, essential fatty acid deficiency, which markedly reduced lung tissue levels of 6-keto-PGF1 alpha, thromboxane B2, and leukotriene C4, did not protect against endotoxin injury. We conclude that although lung eicosanoids are activated during endotoxemia, they do not play a crucial role in the development of acute lung vascular injury in rats.     

Pulmonary microvascular responses to arachidonic acid in isolated perfused guinea pig lung

We examined the effects of arachidonic acid (AA) on pulmonary hemodynamics and fluid balance in Ringer- and blood-perfused guinea pig lungs during constant-flow conditions. Mean pulmonary arterial (Ppa), venous (Pv), and capillary pressures (Pcap, estimated by the double-occlusion method) were measured, and arterial (Ra) and venous resistances (Rv) were calculated. Bolus AA injection (500 micrograms) caused transient increases (peak response 1 min post-AA) in Ppa, Pcap, and Rv without affecting Ra in both Ringer- and blood-perfused lungs. The response was sustained in blood-perfused lungs. AA had no effect on the capillary filtration coefficient in either Ringer- or blood-perfused lungs. AA stimulated the release of thromboxane B2 and 6-ketoprostaglandin F1 alpha in both Ringer- and blood-perfused lungs, but the responses were sustained only in the blood-perfused lungs. Meclofenamate (1.5 X 10(-4) M), a cyclooxygenase inhibitor, abolished the AA-induced pulmonary hemodynamic responses in both Ringer- and blood-perfused lungs, whereas U-60257 (10 microM), a lipoxygenase inhibitor, attenuated the response only in the blood-perfused lungs. In conclusion, AA does not alter pulmonary vascular permeability to water in either Ringer- or blood-perfused lungs. AA mediates pulmonary venoconstriction and thus contributes to the rise in Pcap. The venoconstriction results from the generation of cyclooxygenase-derived metabolites from lung parenchymal cells and blood-formed elements. Lipoxygenase metabolites may also contribute to the vasoconstriction in the blood-perfused lungs.     

Generation of leukotrienes by human monocytes pretreated with cytochalasin B and stimulated with formyl-methionyl-leucyl-phenylalanine

The N-formylated tripeptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) initiated the generation of immunoreactive C-6 sulfidopeptide leukotrienes and of leukotriene B4 (LTB4) in a dose-dependent manner from monolayers of human monocytes pretreated for 10 min with 5 micrograms/ml of cytochalasin B. The EC50 for the immunoreactive C-6 sulfidopeptide leukotrienes was 10(-8) M FMLP and for immunoreactive LTB4 was 5 X 10(-8) M FMLP. The maximal response to FMLP occurred within 10 min, and the sum of the two classes of leukotrienes generated was about 1/6 that obtained from monocytes stimulated with calcium ionophore A23187. The requirement for cytochalasin B in order for FMLP, but not the calcium ionophore, to stimulate leukotriene generation is compatible with the ability of cytochalasin B to augment in other cells certain stimulus-specific transmembrane responses that are not dependent on the integrity of the cytoskeleton. Resolution by reverse phase high performance liquid chromatography of the products released from monocytes pretreated with cytochalasin B and stimulated with FMLP or calcium ionophore yielded a single peak of immunoreactive LTB4 eluting at the same retention time as the synthetic standard; immunoreactive C-6 sulfidopeptide leukotrienes eluted at the retention times of leukotriene C4 (LTC4) and leukotriene D4 (LTD4). [3H]LTB4 was not metabolically altered by monocytes pretreated with cytochalasin B and activated with FMLP in comparison with cells treated with buffer alone, whereas [3H]LTC4 was partially converted to [3H]LTD4. The leukotriene-generating response of monolayers of human monocytes pretreated with cytochalasin B to FMLP is receptor-mediated, as indicated by the inactivity of the structural analog N-acetyl-methionyl-leucyl-phenylalanine and by the capacity of the FMLP receptor antagonist carbobenzoxyphenylalanyl-methionine to inhibit the agonist action of FMLP in a dose-response fashion.     

Differential inhibition of thromboxane B2 and leukotriene B4 biosynthesis by two naturally occurring acetylenic fatty acids

The seed oil of the plant Ixiolaena brevicompta is a rich source of crepenynic acid (octadec-cis-9-en-12-ynoic acid), which has been linked with extensive sheep mortalities in Western New South Wales and Queensland, Australia. A number of acetylenic fatty acids have been found to interfere with lipid and fatty acid metabolism and inhibit cyclooxygenase and lipoxygenase enzymes in a variety of tissues. We have investigated the effects of crepenynic acid and ximenynic acid (octadec-trans-11-en-9-ynoic acid) on leukotriene B4 and thromboxane B2 production in rat peritoneal leukocytes and compare them with non-acetylenic compounds linoleic and ricinoleic acids. In concentrations ranging from 10 to 100 microM linoleic acid and ricinoleic acid had only minimal effects on leukotriene B4 and thromboxane B2 production in ionophore-stimulated cells. Ximenynic acid gave dose-dependent inhibition of leukotriene B4, thromboxane B2 and 6-ketoprostaglandin F1 alpha production. Ximenynic acid appears to be a more effective inhibitor of leukotriene B4 than crepenynic acid with an IC50 of 60 microM compared to 85 microM. On the other hand, crepenynic acid is a much more effective inhibitor of the cyclooxygenase products, having an IC50 for thromboxane B2 of less than 10 microM. Both acetylenic fatty acids inhibited phospholipase activity in these cells by 40-50% at a concentration of 100 microM but had no inhibitory effect at 10 microM. These results indicate that crepenynic acid and ximenynic acid differentially inhibit the cyclooxygenase and lipoxygenase products of stimulated leukocytes, and that at high doses of these fatty acids the effect on these products may be partially due to inhibition of phospholipase A2.     

Identification of guinea pig eosinophil chemotactic factor of anaphylaxis as leukotriene B4 and 8(S),15(S)-dihydroxy-5,9,11,13(Z,E,Z,E)-eicosatetraenoic acid.

We have purified and characterized the guinea pig eosinophil chemotactic factor of anaphylaxis (ECF-A), an activity previously described in diffusates from sensitized lung challenged with specific Ag that appeared to selectively attract eosinophils from mixed leukocyte populations. Time course studies showed that the release of ECF-A from challenged presensitized guinea pig lung fragments closely paralleled the release of immunoreactive leukotriene B4 (iLTB4) and histamine. However, the majority of ECF-A (greater than 80%) and iLTB4 (greater than 79%) was extractable with the lipid fraction from the methanol wash of Sep-Pak-extracted diffusate, whereas histamine remained in the aqueous phase. A comparable neutrophil chemotactic activity was also found in the methanol extracts of the anaphylactic diffusates. By using a combination of HPLC and specific RIA, greater than 60% of ECF-A was attributable to LTB4. A second eosinophil chemotactic activity was also identified and coeluted (on both reverse phase and straight phase HPLC) with the synthetic standard 8(S),15(S)-dihydroxy-5,9,11,13(Z,E,Z,E)eicosatetraenoic acid (8(S),15(S)-diHETE). This was confirmed as 8(S),15(S)-diHETE by gas chromatography-mass spectrometry. Platelet-activating factor and histamine had negligible activity for guinea pig eosinophils, compared with synthetic LTB4 (p less than 0.05, 10(-9) and 10(-8) M; p less than 0.01, 10(-7) to 5 x 10(-6) M). In addition, synthetic 8(S),15(S)-diHETE had 3 times less activity than LTB4 at optimal chemotactic concentrations (10(-6) and 10(-7) M, respectively). Thus, guinea pig ECF-A appears to be largely attributable to lipoxygenase products of arachidonic acid, namely LTB4 and 8(S),15(S)-diHETE. Because guinea pig ECF-A was equally active on neutrophils (greater than 96% purity), it can no longer be considered a selective eosinophil chemoattractant.     

Caffeic acid is a selective inhibitor for leukotriene biosynthesis

.eukotrienes are significantly involved in immunoregulation and in a variety of diseases, including asthma, inflammation and various allergic conditions. They are initially biosynthesized by 5-lipoxygenase from arachidonic acid, which can also be metabolized to prostaglandin endoperoxide by cyclooxygenase. The specific inhibitors for 5-lipoxygenase would be useful not only as tools for investigating the regulation mechanism of leukotriene biosynthesis, but also as drugs for clinical use. Although recently a few selective inhibitors have been reported, most of them are difficult to obtain, since they are new compounds. We found that caffeic acid, which is one of the most common reagents, is a selective inhibitor for 5-lipoxygenase and therefore for leukotriene biosynthesis. The inhibitory effect of its methyl ester on 5-lipoxygenase (ID50 = 4.8 X 10(-7) M) was stronger than that of caffeic acid itself (ID50 = 3.7 X 10(-6) M). Caffeic acid inhibited 5-lipoxygenase in a non-competitive manner. Caffeic acid and its methyl ester did not inhibit prostaglandin synthase activity at all, at least up to 5 X 10(-4) M, but rather stimulate at higher doses. The biosynthesis of leukotriene C4 and D4 in mouse mast tumor cells was also inhibited completely with 10(-4) caffeic acid. Besides, caffeic acid had little effect on arachidonic acid metabolism in platelet at less than 1 X 10(-5) M, but at higher doses it showed a definite inhibitory effect, i.e., thromboxane B2, HHT (12(S)-hydroxy-5,8,10-heptadecatetraenoic acid) and 12-HETE (12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid) syntheses were inhibited 33, 40 and 80% at 1 X 10(-4) M, respectively. Platelet aggregation induced by arachidonic acid was also inhibited by caffeic acid at high dose, while platelet aggregation induced by ADP is not influenced by caffeic acid at all. The observations on caffeic acid and its derivatives may contribute to leukotriene research.     

Inhibition of arachidonic acid-induced contraction of guinea pig lung strips

The effects of several enzyme inhibitors on arachidonic acid-induced contractions of guinea pig lung strips were studied. Varying concentrations of indomethacin, an inhibitor of cyclooxygenase, produced only a limited effect on contraction of tissue strips. By contrast, nordihydroguaiaretic acid (NDGA), 5,8,11,14-eicosatetraynoic acid (ETYA), and phenidone, which inhibit either lipoxygenase, or both lipoxygenase and cyclooxygenase, caused a dose-related antgonism of the arachidonic acid-induced contraction. The effects of these latter agents were similar to that of FPL 55712. Results indicate that the products of cyclooxygenase are predominantly involved in the early phase and the products of lipoxygenase are predominantly related to the late phase of arachidonic acid-induced contraction.     

Influence of platelet-activating factor on leukotriene D4-induced contractions of the guinea pig parenchymal strip

Platelet-activating factor (PAF) and sulphidopeptide leukotrienes, such as leukotriene D4 (LTD4), are potent constrictors that are probably released simultaneously in a variety of inflammatory respiratory events. The purpose of the present study was to determine whether LTD4-induced contractions of guinea pig parenchymal lung strips (GPPS) are modified in the presence of PAF. The contractile responses of isolated GPPS to cumulative doses of LTD4, acetylcholine, histamine, and potassium chloride in the presence of PAF (0.1 nM, 0.1 microM) were compared with parallel controls. There was no significant alteration of the response to acetylcholine and potassium chloride and the PAF-induced inhibition of the response to histamine, although significant, was not concentration dependent. In contrast, PAF in a concentration range from 0.1 nM to 1.0 microM caused a marked, concentration-dependent reduction of LTD4-induced contractions. Pretreatment with the PAF receptor antagonist, BN52021, prevented the attenuation of LTD4-induced contraction by PAF. The attenuation of LTD4-induced contraction by PAF was also prevented by pretreatment with indomethacin or with the thromboxane synthase inhibitor U63,557A, but not by pretreatment with the lipoxygenase inhibitors BW755c or nordihydroguaiaretic acid. Thus inhibition of LTD4-induced GPPS contraction by PAF is receptor dependent and probably secondary to thromboxane generation. The respiratory smooth muscle response to leukotrienes may be modified significantly by concomitant PAF release.     

Mechanism of phosgene-induced lung toxicity: role of arachidonate mediators

We have previously shown that phosgene markedly increases lung weight gain and pulmonary vascular permeability in rabbits. The current experiments were designed to determine whether cyclooxygenase- and lipoxygenase-derived mediators contribute to the phosgene induced lung injury. We exposed rabbits to phosgene (1,500 ppm/min), killed the animals 30 min later, and then perfused the lungs with a saline buffer for 90 min. Phosgene markedly increased lung weight gain, did not appear to increase the synthesis of cyclooxygenase metabolites, but increased 10-fold the synthesis of lipoxygenase products. Pre- or posttreatment with indomethacin decreased thromboxane and prostacyclin levels without affecting leukotriene synthesis and partially reduced the lung weight gain caused by phosgene. Methylprednisolone pretreatment completely blocked the increase in leukotriene synthesis and lung weight gain. Posttreatment with 5,8,11,14-eicosatetraynoic acid (ETYA), a nonmetabolized competitive inhibitor of arachidonic acid metabolism, or the leukotriene receptor blockers, FPL 55712 and LY 171883, also dramatically reduced the lung weight gain caused by phosgene. These results suggest that lipoxygenase products contribute to the phosgene-induced lung damage. Because phosgene exposure did not increase cyclooxygenase synthesis or pulmonary arterial pressure, we tested whether phosgene affects the lung's ability to generate or to react to thromboxane. Infusing arachidonic acid increased thromboxane synthesis to the same extent in phosgene-exposed lungs as in control lungs; however, phosgene exposure significantly reduced pulmonary vascular reactivity to thromboxane but not to angiotension II and KCl.