Inhibitors of prostaglandin synthesis block the induction of staphylococcal enterotoxin B-activated T-suppressor cells

A variety of arachidonic acid metabolites possess the ability to modulate immune cell function. Various inhibitors of arachidonic acid metabolism were compared with regard to their effects on T-suppressor (Ts) cell function. Using staphylococcal enterotoxin B (SEB) to activate Lyt-2+ Ts cells, it was shown that indomethacin and 5,8,11,14-eicosatetraynoic acid (ETYA) inhibit the induction phase, but not the expression phase, of suppressor cell activity. Agents which inhibit thromboxane synthetase or lipoxygenase activities (imidazole, nordihydroguaiaretic acid, and pyrogallol) were not found to affect Ts cell induction. Since inhibitors of prostaglandin synthesis are thought to induce lower levels of cyclic adenosine monophosphate, an attempt to overcome the indomethacin inhibition of Ts cell induction by modulating cyclic adenosine monophosphate levels was made. It was found that theophylline and isoproterenol are not able to overcome the inhibition by indomethacin of Ts cell activity. These results strongly suggest that induction of Ts cells by SEB is dependent on the synthesis of products of the prostaglandin synthetase pathway.     

Thromboxane synthetase inhibitors as pharmacological tools: differential biochemical and biological effects on platelet suspensions

The comparative effects of three so called "thromboxane-synthetase-inhibitors" (imidazole, N-0164, and U-51605) on arachidonate metabolism and on platelet aggregation were studied. All three compounds blocked platelet microsomal thromboxane synthesis from prostaglandin endoperoxides without affecting platelet adenyl cyclase. Imidazole, blocked thromboxane synthesis in intact platelets either from arachidonic acid or PGH2, without affecting aggregation. U-51605 simultaneously inhibited thromboxane synthesis and platelet suspension aggregation. N-0164 inhibited aggregation probably at extracellular sites, at concentrations that did not alter arachidonate or PGH2 metabolism. High concentrations of N-0164 simultaneously inhibited PG cyclo-oxygenase and thromboxane synthetase. The lack of specificity of these compounds requires that other actions of these compound must be considered when they are used as pharmacological tools to inhibit thromboxane synthetase.     

Prostacyclin (PGI2) and U-46619 stimulate coronary arteries from diabetic dogs and their action is influenced by inhibitors of prostaglandin biosynthesis

Isolated coronary arteries from diabetic dogs presented different contractile response to U-46619 to prostacyclin (PGI₂) and to arachidonic acid (AA) than those of normal dogs. The stimulatory effect of the synthetic endoperoxide analogue U-46619, was significantly higher in the diabetic condition than in preparations from normal animals. On the other hand, while PGI₂ evoked a dose-dependent relaxation of normal coronary arteries, diabetic vessels were not relaxed by low concentration of PGI₂ whereas higher ones produced a distinct constrictor effect. Additionally, inhibitors of prostaglandins and thromboxane (TX) biosynthesis such as corticosterone, indomethacin, acetylsalicylic acid, imidazole and L-8027, abolished the stimulatory effect of PGI₂ in coronary arteries from diabetic dogs. AA relaxed coronaries from normal dogs and constricted those from diabetic animals, this action being inhibited by imidazol and L-8027.The present results suggests that: a) coronary vessels from diabetic dogs are more reactive to an endoperoxide analogue than normal preparations and b) PGI₂ and AA probably contract diabetic coronary arteries via the participation of a TX like material. It is then plausible that this effect could be tentatively ascribed to the production of a prostaglandin constricting substance including als the probable generation of a TXA₂-like agonist.     

Changes in coronary vascular resistance associated with prolonged hypoxia in isolated rat hearts: a possible role of prostaglandins

Hypoxia caused an initial dilatation in the coronary circulation of perfused male rat hearts but within 15 minutes the coronary vessels became strongly constricted. In hearts from very young (30 days) animals only dilatation was seen. Physiological levels of progesterone in the perfusate prevented the constriction whereas estradiol and testosterone had little effect. Blockade of adrenergic alpha receptors or angiotensin receptors did not prevent the constriction. Two structurally different inhibitors of prostaglandin synthesis, indomethacin and aspirin, and three drugs which can interfere with prostaglandin action, chloroquine, procaine, and propranolol blocked the constriction. Thromboxane A2, a product of PG synthesis, had been reported to be a coronary vasoconstrictor but four drugs which inhibit thromboxane A2 synthesis, dipyridamole, benzydamine, N-0164 and imidazole were not able to prevent the hypoxia-induced constriction. This form of hypoxic coronary constriction seems not to be related to α-adrenergic, angiotensin or thromboxane A2 effects. It may depend on some other product of the prostaglandin pathway.     

Mechanism of action of arachidonic acid in the isolated perfused rat heart

The purpose of this study was to elucidate the mechanism of action of arachidonic acid in the isolated rat heart perfused with Krebs solution at a constant flow. Administration of arachidonic acid, 3.3-33 nmol, into the heart caused a small transient increase followed by a pronounced decrease in coronary perfusion pressure and increased myocardial tension, heart rate, and the output of prostaglandins (6-keto-PGF1 alpha, PGE2, and PGF2 alpha). Administration of structurally similar fatty acids, dihomo-gamma-linolenic acid, and 8,14,17-eicosatrienoic acid, produced vasoconstriction and decreased myocardial tension without affecting heart rate or the output of prostaglandins. Infusion of PGI2, PGF2 alpha, or PGE2 produced coronary vasodilation and increased myocardial tension, whereas PGF2 alpha increased heart rate, an effect which was not prevented by propranolol. Indomethacin blocked the effect of arachidonic acid on myocardial tension and heart rate, but only reduced the duration of coronary vasodilation. The initial component of arachidonic acid induced coronary vasodilation which was unaffected by indomethacin and also remained unaltered during the infusion of three structurally dissimilar lipoxygenase inhibitors, eicosatetraynoic acid, nordihydroguaiaretic acid, and 1-phenyl-3-pyrazolidone. Indomethacin did not alter the effects of the exogenously administered prostaglandins on perfusion pressure or myocardial tension; however, it blocked the effect of PGF2 alpha on heart rate. The effect of arachidonic acid or PGF2 alpha to increase heart rate was not blocked by thromboxane synthetase inhibitors, imidazole, or OKY-1581. We conclude that the cardiac effects of arachidonic acid are mediated primarily through its conversion to cyclooxygenase products.(ABSTRACT TRUNCATED AT 250 WORDS)     

Prostaglandins and enzyme secretion from dispersed rat pancreatic acinar cells

The role of prostaglandins in exocrine pancreatic enzyme secretion was studied. The effects of three inhibitors of prostaglandin and thromboxane syntheses, were evaluated on release of amylase from dispersed rat pancreatic acinar cells. Mepacrine inhibited, while indomethacin and imidazole had no effect on basal or carbachol or cholecystokinin stimulated enzyme release. Exogenous arachidonic acid or various prostaglandins (E₁, E₂, F_2α, I₂), also did not affect the secretory process. Acinar cells actively incorporated radioactive arachidonic acid, principally into phospholipids (especially phosphatidylcholine), however release of the free fatty acid and subsequent synthesis of radioactive endogenous prostaglandins was not stimulated by the presence of different pancreatic stimulants. Pancreatic microsomes were found to be lacking in cyclo-oxygenase, an enzyme involved in endegenous synthesis of prostaglandins. The data suggest that prostaglandins are not involved directly in excitation-secretion coupling in the exocrine pancreas.     

A role for the lipoxygenase pathway of arachidonic acid metabolism in glucose- and glucagon-induced insulin secretion

Although the cylo-oxygenase pathway of arachidonic acid (AA) metabolism inhibits glucose-stimulatedinsulin release throught synthesis of prostaglandins, very little attention has been given to the effects of lipoxygenase pathway products on beta cell function. We have examined the effects of two structurally-dissimilar lipoxygenase inhibitors on insulin release from mono-layer-cultured rat islet cell. Both nordihydroguaiaretic acid (NDGA, 20–50 μM) and BW755c (100–250μM) caused a dose-responsive inhibition of glucose-induced insulin release. This inhibitory effect occurred despite concomitant inhibition of prostaglandin E synthesis. Lipoxygenase inhibitors also impeded cyclic AMP accumulation. Insulin and cyclic AMP release induced by glucagon were also blunted. These studies suggest the hypothesis that AA released in or near the beta cell is metabolized to lipoxygenase product(s) which have feed-forward properties important to glucose- and glucagon-stimulated cyclic nucleotide accumulation and insulin release.     

Lipoxygenase- and cyclooxygenase-reaction products and incorporation into glycerolipids or radiolabeled arachidonic acid in the bovine retina

The metabolism of radiolabeled arachidonic acid (AA) by the intact bovine retina in vitro has been studied. Synthesis of prostaglandins (PGs) and hydroxyeicosatetraenoic acids (HETEs), and incorporation of AA into glycerolipids has been measured by reverse-phase and straight-phase high performance liquid chromatography with flow scintillation detection, and by thin-layer chromatography. AA was actively acylated into glycerolipids, particularly triglycerides, phosphatidylcholine and phosphatidylinositol. AA was also converted to the major PGs, PGF2 alpha, PGE2, PGD2, 6-keto-PGF1 alpha and TXB2, and to the lipoxygenase reaction products, 12-HETE, 5-HETE, and other monohydroxy isomers. Approximately 6% of the radiolabeled AA was converted to eicosanoids. The synthesis of HETEs was inhibited in a concentration-dependent manner (IC50 = 8.3 nM) by nordihydroguaiaretic acid (NDGA). PG synthesis was inhibited by aspirin (10 microM), indomethacin (1 microM) and NDGA (IC50 = 380 nM). Metabolism of AA via lipoxygenase, cyclooxygenase and activation-acylation was inhibited by boiling retinal tissue prior to incubation. These studies demonstrate an active system for the uptake and utilization of AA in the bovine retina, and provide the first evidence of lipoxygenase-mediated metabolism of AA, resulting in the synthesis of mono-hydroxyeicosatetraenoic acids, in the retina.     

Formation of thromboxane B2 and hydroxyarachidonic acids in purified human lymphocytes in the presence and absence of PHA.

The metabolism of exogenous and endogenous [14C] arachidonc acid was studied in purified human peripheral blood lymphocytes carefully freed of contaminating platelets. Formation of products co-migrating in a number of different solvent systems with 5-hydroxyarachidonic acid (5-HETE), thromboxane B2 (TB2), prostaglandins and probably 12-hydroxyarachidonic acid (12-HETE) was demonstrated. In cells prelabeled with [14C] arachidonic acid, phytohemagglutinin (PHA) produced substantial (3.5- to 12-fold) increases in 5-HETE, 12-HETE, and TB2 radiolabeling. The metabolism of exogenous [14C] arachidonic acid was much less affected by PHA. Since PHA releases cell-bound arachidonic acid, it appears that the response involving endogenous label is due to increased availability of free arachidonic acid rather than induction of arachidonic acid-metabolizing enzymes. Various inhibitors of arachidonic acid metabolism exerted similar effects in lymphocytes to those described previously in other tissues providing a possible basis for interpreting their inhibitory effects on mitogenesis, described in the preceding paper.     

Inhibitors of arachidonic acid metabolism eliminate the increase in cytosolic free calcium induced by the mitogen concanavalin A in rat thymocytes

Using inhibitors of arachidonic acid (AA) metabolism, the possible involvement of AA products in the generation of [Ca2+]i and the pHi rise induced by the mitogen concanavalin A (Con A) in rat thymocytes has been studied. The lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA, 10 microM) and the phospholipase A2 inhibitor bromophenacyl bromide (10 microM) eliminated the [Ca2+]i signal induced by Con A; the cyclooxygenase blocker indomethacin also inhibited it. However, neither NDGA nor indomethacin suppressed the pHi rise stimulated by Con A. Exogenous AA induced an increase in [Ca2+]i but not in the pHi. These results indicate that AA metabolites, probably of the lipoxygenase pathway, take part in the generation of the [Ca2+]i response to the mitogen. In contrast, they appear not to be involved in the pHi rise evoked by Con A.     

Possible involvement of prostaglandins in vasoconstriction induced by zymosan and arachidonic acid in the perfused rat liver

Exposure of perfused livers to zymosan, arachidonic acid or phenylephrine but not to latex particles, stimulates hepatic constriction. The effects of arachidonic acid are rapid, reach a maximum after 2-3 min and then decline. They are blocked by the cyclooxygenase inhibitor indomethacin but not by the lipoxygenase inhibitor nordihydroguaiaretic acid. This suggests a role for prostaglandins in this action. Zymosan progressively increases hepatic pressure after a lag time of about 1 min. Perfusion of bromophenacyl bromide, indomethacin and nordihydroguaiaretic acid only partially inhibits the zymosan-induced vasoconstriction. None of these inhibitors effect the phenylephrine-induced response. Repeated infusion of arachidonic acid leads to homologous desensitization of the response whereas the response of the liver to phenylephrine is unaffected. The present data indicate that prostaglandins, produced and released within the liver, affect vasoconstriction in this organ.     

Role of prostaglandins in controlling plasma fibronectin levels

Fibronectin is a normal glycoprotein component of plasma, interstitial fluid, and extracellular matrix which has binding sites for collagen, gelatin, actin, glycosaminoglycans, fibrin, Staphylococcus aureus, and some cells. Since it is a dimer, it can crosslink these substances to each other or to extracellular components of basement membrane, thereby affecting many physiological processes. The level of circulating fibronectin is markedly reduced following even moderate blunt or operative trauma, thermal injury, starvation, advanced cancers, hemorrhage, etc. Replacement therapy has been tried with some success in patients who become septic following multiple injuries. The reduction in plasma fibronectin has been attributed to several causes including consumption by binding to cell debris at the site of injury, binding to circulating cell debris and its subsequent removal by elements of the phagocytic system, and degradation by proteolytic cleavage. However, the amount of fibronectin removed from circulation raises some question about this. In this paper, we used indomethacin, ibuprofen, imidazole, and essential fatty acid deprivation to inhibit the synthesis of prostaglandins in young adult rats. Thirty minutes after ip administration of one of the inhibitors, the rats were subjected to a midline laparotomy and mild intestinal manipulation. Blood samples were taken at intervals following closure of the incision and analyzed for fibronectin. In all cases, the normal decline in plasma fibronectin seen in untreated rats was abrogated by inhibiting prostaglandin synthesis. Since imidazole specifically inhibits thromboxane A synthesis, this strongly suggests that thromboxanes directly or indirectly control the trauma-induced reduction in circulating fibronectin. This was confirmed by ip injection of thromboxane into the rats which resulted in a decline in plasma fibronectin levels.     

Arachidonic acid metabolism and casein secretion in lactating rabbit mammary epithelial cells: effects of inhibitors of prostaglandins and leukotrienes synthesis

The metabolism of arachidonic acid (AA) in fragments of lactating rabbit mammary glands in vitro was studied by considering the distribution of 13-[14C]AA in the cells, and the effects of inhibitors of cyclooxygenase and lipoxygenase pathway on the basal and prolactin (PRL)-stimulated casein secretion. 13-[14C]AA was incorporated in all classes of lipids and PRL increased transiently the percentage of free fatty acid after 1 and 5 min. Ten microM ETYA (5,8,11,14-Eicosatetraynoic acid), a tetrayne analogue of AA inhibited prostaglandins F2 alpha (PGF2 alpha) production but not leukotrienes B4 and C4 (LTB4 and LTC4) production and increased basal casein secretion. 10(-4) M DCHA (Docosahexaenoic acid) a competitive inhibitor of prostaglandin-synthetase inhibited PGF2 alpha production but did not affect basal nor PRL-stimulated casein secretion. Fourteen microM indomethacin inhibited PGF2 alpha and LTC4 production and PRL-stimulated casein secretion. Ten microM NdgA (nordihydroguaiaretic acid) an inhibitor of lipoxygenase pathway, inhibited LTB4 and LTC4 production, increased basal level of casein secretion and inhibited PRL-stimulated casein secretion. Hundred microM caffeic acid, an inhibitor of glutathione-S-transferase (GST), a class of enzymes implied in the transformation of LTA4 into LTC4, had the same effect that NDGA on basal and PRL-stimulated casein secretion. These findings show that inhibitors of AA metabolites alter casein secretion.     

Lipoxygenase- and cyclooxygenase-reaction products and incorporation into glycerolipids of radiolabeled arachidonic acid in the bovine retina

The metabolism of radiolabeled arachidonic acid (AA) by the intact bovine retina has been studied. Synthesis of prostaglandins (PGs) and hydroxyeicosatetraenoic acids (HETEs), and incorporation of AA into glycerolipids has been measured by reverse-phase and straight-phase high performance liquid chromatography with flow scintillation detection, and by thin-layer chromatography. AA was actively acylated into glycerolipids, particularly triglycerides, phosphatidylcholine and phosphatidylinositol. AA was also converted to the major PGs, PGF_2α, PGE₂, PGD₂, 6-keto-PGF_1α and TXB₂, and to the lipoxygenase reaction products, 12-HETE, 5-HETE, and other monohydroxy isomers. Approximately 6% of the radiolabeled AA was converted to eicosanoids. The synthesis of HETEs was inhibited in a concentration-dependent manner (IC₅₀ = 8.3 NM) by nordihydroguaiaretic acid (NDGA). PG synthesis was inhibited by aspirin (10 μM), indomethacin (1 μM) and NDGA (IC₅₀ = 380 nM). Metabolism of AA via lipoxygenase, cyclooxygenase and activation-acylation was inhibited by boiling retinal tissue prior to incubation. These studies demonstrate an active system for the uptake and utilization of AA in the bovine retina, and provide the first evidence of lipoxygenase-mediated metabolism of AA, resulting in the synthesis of mono-hydroxyeicosatetraenoic acids, in the retina.     

Phospholipase A2, an in vivo immunomodulator

Arachidonic acid (AA) can be released from membrane phospholipids by the action of phospholipase A2 (PLA2). There is evidence that unsaturated fatty acids, particularly AA, released from membrane phospholipids are required to activate the respiratory burst of macrophages. The data reported here indicate that peritoneal macrophages harvested 30 min after i.p. injection of PLA2 can phagocytose Candida albicans more efficiently and emit more chemoluminescence (CL) than normal cells when stimulated by zymosan. PLA2 injection also enhances the CL of peritoneal cells from mice already stimulated by immunomodulators such as trehalose dimycolate (TDM), bestatin, or oncostatic drugs such as aclacinomycin (ACM). CL is not sensitive to potassium cyanide (KCN), but is inhibited by catalase, superoxide dismutase (SOD), nordihydroguaiaretic acid (NDGA) and high doses of indomethacin (10(-3) M). In vivo PLA2 treatment stimulates the synthesis of both cyclooxygenase and lipoxygenase derivatives of AA metabolism (PGE2, 6-keto, PGF1 alpha TXB2 and LTC4). Inhibitors of AA metabolism (NDGA, indomethacin) modulate the production of free oxidizing radicals in this experimental model, partly because of their effect on AA metabolism, as determined by the measuring immunoreactive products. However, this work indicates that the effects of these inhibitors, which have been extensively used in CL studies, should be interpreted with caution, since their specificity for AA metabolism is relative.     

Hypotensive action of arachidonic acid, slow acting analphylactic substance (SRS-A) and thromboxane A2 in rats]

In the rat, diethylcarbamazine, imidazole and L 8027 do not modify the hypotensive activity of PGE2 and arachidonic acid. The formation of SRS-A from arachidonic acid does not compete with the synthesis of PG in the cardiovascular system of the rat. The thromboxane A2 does not participate in the hypotensive activity of arachidonic acid in the rat.     

A formyl peptide contracts guinea pig lung: role of arachidonic acid metabolites

We studied the role of cyclooxygenase and lipoxygenase products of arachidonic acid metabolism in mediating N-formyl-methionyl-leucyl-phenylalanine- (FMLP) induced contractions of guinea pig lung parenchymal strips. The cyclooxygenase inhibitors indomethacin (10(-5) M) and aspirin (3 X 10(-5) to 10(-4) M), the lipoxygenase inhibitor nordihydroguaiaretic acid (10(-5) to 3 X 10(-5) M), and the combined cyclooxygenase/lipoxygenase inhibitors 1-phenyl-3-pyrazolidinone (Phenidone) (3 X 10(-5) to 3 X 10(-4) M) and BW 755C (10(-5) to 10(-4) M) each caused a decrease in the maximum force induced by FMLP (Fmax) and an increase in the concentration of FMLP required to produce 50% of Fmax (EC50). The thromboxane synthesis inhibitor imidazole (3 X 10(-3) M) also decreased Fmax. The leukotriene D4 receptor antagonist FPL 55712 (5.7 X 10(-6) to 1.9 X 10(-5) M) increased the EC50 for FMLP, whereas desensitization of lung parenchymal strips to leukotriene B4 by pretreatment with this leukotriene (10(-7) M) had no effect on FMLP-induced contraction. After exposure to FMLP (10(-6) M), guinea pig lung produced (as determined by high-performance liquid chromatography and radioimmunoassay) leukotrienes C4 and B4, thromboxane A2 (as measured by its stable degradation product thromboxane B2), and prostaglandin F2 alpha. Lung strips not exposed to FMLP showed no evidence of leukotriene production. We conclude that thromboxane A2 and leukotriene C4 generated in response to FMLP mediate a substantial fraction of the force induced by this peptide in guinea pig lung parenchymal strips.     

Eicosanoid-mediated increase in glucose and lactate output as well as decrease and redistribution of flow by complement-activated rat serum in perfused rat liver

Rat serum, in which the complement system had been activated by incubation with zymosan, increased the glucose and lactate output, and reduced and redistributed the flow in isolated perfused rat liver clearly more than the control serum. Heat inactivation of the rat serum prior to zymosan incubation abolished this difference. Metabolic and hemodynamic alterations caused by the activated serum were dose dependent. They were almost completely inhibited by the cyclooxygenase inhibitor indomethacin and by the thromboxane antagonist 4-[2-(4-chlorobenzesulfonamide)-ethyl]-benzene-acetic acid (BM 13505), but clearly less efficiently by the 5'-lipoxygenase inhibitor nordihydroguaiaretic acid and the leukotriene antagonist N-(3-[3-(4-acetyl-3-hydroxy-2-propyl-phenoxy)-propoxy]-4-chlorine-6-meth yl- phenyl)-1H-tetrazole-5-carboxamide sodium salt (CGP 35949 B). Control serum and to a much larger extent complement-activated serum, caused an overflow of thromboxane B2 and prostaglandin F2 alpha into the hepatic vein. It is concluded that the activated complement system of rat serum can influence liver metabolism and hemodynamics via release from nonparenchymal liver cells of thromboxane and prostaglandins, the latter of which can in turn act on the parenchymal cells.     

On the role of arachidonic acid metabolites in mast-cell mediated mitogenesis in the rat

We investigated whether the mitogenic response induced by local mast-cell secretion in the rat mesentery was affected by suppression of phospholipase A2, lipoxygenase, or cyclooxygenase in arachidonic acid metabolism. Enzyme inhibitor was given in a single intravenous dose 5 min before intraperitoneal injection of the mast-cell secretagogue 48/80. Mepacrine, a phospholipase A2 inhibitor, suppressed the generation of both leukotrienes (SRS) and prostaglandins (PG), whereas the lipoxygenase inhibitor BW 755C reduced the generation of SRS, and the cyclooxygenase inhibitor indomethacin significantly suppressed the generation of PG. None of the enzyme inhibitors affected the basal mesenteric histamine content or histamine release in the mesentery after exposure to 48/80, and none of them affected mast-cell-mediated mitogenesis in the mesentery as judged by specific DNA activity and mitosis counting. The stimulation of DNA synthesis and mitosis initiated by secreting mast cells is apparently not mediated or modulated by synthesis of leukotrienes, prostaglandins, or other known arachidonic acid metabolites.     

Mechanism of action of platelet-activating factor on guinea-pig lung parenchyma strips

The contribution of thromboxane A2 to platelet-activating factor (PAF)induced contraction of guinea-pig lung parenchyma strips (GPLPS) was investigated using an experimental design that allowed us to record the contractions of the tissues in parallel with the determination of thromboxane B2 (TXB2) levels in the organ baths by enzyme immunoassay. It was found that the first injection of PAF induced the contraction of GPLPS and the release of TXB2. Following subsequent additions of PAF to the same tissue, the contractile response was abolished but TXB2 levels were not significantly reduced. Pretreatment of the tissue with the thromboxane synthetase inhibitor OKY-046 (3.5, 170, and 350 microM) strongly inhibited the release of TXB2 but had no effect on the contraction of the tissues induced by PAF. The mechanism of PAF-induced contraction of GPLPS was further investigated using several drugs that interfere with arachidonic acid metabolism. It was found that pretreatment of the tissues with the cyclooxygenase and thromboxane synthetase inhibitors indomethacin (2.8, 28, and 56 microM) and OKY-046 (170 microM) or with the thromboxane antagonist SKF-88046 (1.25 and 12.5 microM) had no significant effect on the contractile response to PAF. The compound L-655,240 (2.5, 25, and 50 microM), which acts simultaneously as an antagonist of thromboxane and inhibitor of lipoxygenase, significantly reduced GPLPS contractions induced by PAF. Another lipoxygenase inhibitor, nordihydroguaiaretic acid (33 microM), and the inhibitor of both pathways of arachidonic acid metabolism, BW775c (110 microM), both reduced PAF-induced contractions of GPLPS.(ABSTRACT TRUNCATED AT 250 WORDS)