Effect of indomethacin on airway contraction and the release of LTC4-like material

Ovalbumin (OA) and arachidonic acid (AA) were used to induce contractions of sensitized guinea-pig tracheal and lung preparations in the presence and absence of indomethacin. Leukotriene (LT)C4-like material released from these tissues was extracted from the bathing fluid and measured by radioimmunoassay. Challenge with either OA or AA induced release of LTC4-like material from both parenchyma and trachea, AA inducing a greater release than OA although OA induced greater contractions. This suggested that OA-induced the synthesis of other bronchoconstrictor compounds than LTC4. Although indomethacin enhanced OA- and AA-induced contractions of trachea, there was no enhancement of the release of LTC4-like material, suggesting enhancement by indomethacin was a result of the inhibition of the synthesis of prostaglandin E2 and not diversion of AA into the lipoxygenase pathway. Indomethacin had no effect on OA-induced contractions of parenchyma, but attenuated those induced by AA. Indomethacin had no modulatory effect on the release of LTC4-like material in the parenchyma. The results demonstrate that indomethacin does not result in increased synthesis of LTs in the airways.     

Contraction of isolated airway smooth muscle by synthetic leukotrienes C4 and D4

The pharmacology of leukotrienes (LT) C₄ and D₄ in isolated airway smooth muscle was investigated. In rat trachea, neither LTC₄ or D₄ elicited a response. In contrast, LTC₄ was a potent contractile agonist in guinea-pig trachea, bronchus and parenchymal lung strip. Similar effects were obtained with LTD₄ in trachea and parenchyma. In trachea and bronchus, the concentration-response curve to LTC₄ was biphasic: indomethacin converted the biphasic response curve to a simple sigmoidal shape and enhanced the maximum contractile response. The SRS-A antagonist FPL 55712 antagonized the effect of LTD₄ in both trachea and parenchyma. As regards LTC₄-induced contraction of trachea and bronchus, FPL 55712, depending on concentration, either antagonized, or antagonized and enhanced the maximum contractile response. The enhancement of the maximum contractile response by FPL 55712 was not apparent when indomethacin was present. FPL 55712 failed to antagonize the effect of LTC₄ in parenchyma.     

Effect of anti-inflammatory agent etodolac on antigen-induced contractions of the trachea and lung parenchyma of guinea pigs

Etodolac, which inhibits the activity of cyclooxygenase, did not affect antigen-induced contractions of the trachea and lung parenchyma of guinea pigs. Indomethacin tended to enhance antigen-induced contractions of the trachea and significantly enhanced contractions of the lung parenchyma. The inhibitory activity of AA-861, a 5-lipoxygenase inhibitor, in antigen-induced contractions of the trachea and lung parenchyma was more potent than that of ozagrel, a thromboxane A₂ (TXA₂) inhibitor. Thus, lipoxygenase products played a more important role than TXA₂ in antigen-induced contractions of the trachea and lung parenchyma. These results suggest that the enhancement of antigen-induced contractions by indomethacin might be due to an increase in anaphylactic release of lipoxygenase products through the inhibition of cyclooxygenase. Since etodolac did not enhance antigen-induced contractions, we attempted to determine whether or not etodolac inhibits 5-lipoxygenase. Etodolac was found to have no effect on 5-lipoxygenase activity. Therefore, the low adverse effect of etodolac on antigen-induced contractions of the airway may be due to its weak inhibition of cyclooxygenase in the airway. These results suggest that etodolac would have only a very slight, if any, adverse effect on the airway in patients with asthma.     

Leukotriene D4 and platelet-activating factor-acether antagonists on allergic and arachidonic acid-induced reactions in guinea pig airways

Arachidonic acid (AA) and ovalbumin (OA) were used to induce contractions of sensitized guinea pig tracheal spiral (indomethacin-pretreated) and lung parenchymal strip preparations. This model was used to examine the properties of three leukotriene (LT) D4 antagonists and a platelet-activating factor (PAF)-acether receptor antagonist. The three LTD4 antagonists, L-649,923, FPL 57231, and LY163443, inhibited AA-induced contractions of indomethacin-pretreated tracheal spirals selectively. The PAF-acether antagonist, L-652,731, did not inhibit AA-induced contractions of either trachea or parenchyma. This confirmed that AA-induced contractions of trachea involved release and activity of LTD4. The LTD4 antagonists and L-652,731 partially inhibited OA-induced contractions of both trachea and parenchyma. When L-649,923 and L-652,731 or FPL 57231 and L-652,731 were combined, an additive inhibitory effect on OA-induced contractions was observed. When LY163443 and L-652,731 were combined, the inhibitory effect was synergistic. This may be due to the additional effect of LY163443 to inhibit phosphodiesterase. Total inhibition of OA-induced contractions was obtainable with relatively low concentrations when a LTD4 and PAF-acether antagonist were combined. These results suggested that LTD4 and PAF-acether may be the two major mediators in our model of allergic bronchospasm. The LTD4 and PAF-acether antagonists had the capacity to decrease baseline tone, even on tissues that were already relaxed with indomethacin, suggesting that LTD4 and PAF-acether may contribute to intrinsic tone in airway smooth muscle.     

Formation of cysteinyl-containing leukotrienes by human arterial tissues

Human arterial rings incubated in modified Tyrode solution released small amounts of leukotriene (LT) C₄-like material spontaneously and larger amounts upon stimulation with the ionophore A23187 as determined by radioimmunoassay. By reversed phase high pressure liquid chromatography (HPLC) LTC₄-like material was found to comigrate with authentic LTC₄, LTD₄ and LTE₄. Nordihydroguaiaretic acid (NDGA) significantly inhibited the ionophore A23187-induced release of LTC₄-like material, while indomethacin was without effect. Simultaneously the arterial rings released much larger amounts of 6-keto-prostaglandin (PG) F_1α, which were significantly decreased by indomethacin. The results demonstrate that human arterial tissue has the capacity to synthesize cysteinyl-containing LT from endogenous arachidonic acid.     

Inhibitory effects of a lipoxygenase inhibitor nordihydroguaiaretic acid on antagonism of leukotriene C4-induced contractions of isolated guinea-pig trachea

We have studied the effects of a lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) on antagonim of leukotriene (LT) C₄-induced contractions of isolated guinea-pig trachea and the results were compared to that of a cycloocygenase inhibitor indomethacin, NDGA (30 μM) as well as idomethacin (5 μM) inhibited LTC₄-iduced contraction. But in the presence of indomethacin NDGA was ineffective to inhibit the LTC₄ response, whereas two other lipoxygenase inhibitors, phenidone (3–30 μM) and 5,8,11,14-eicostatetraynoic acid (ETYA, 10 μM), markedly inhibited it. The antagonist action of an LTD₄ receptor antagonist FPL55712 against LTC₄-induced contractions was significantly reduced by NDGA (10–30 μM), but indomethacin had no effect on it. NDGA possessed the same inhibitory effect n the LTC₄ antagonism in the presence of indomethacin, but 0.3 μM phenidone and 1 μM ETYA which did not inhibit the LTC₄ response had no effect on it. NDGA also inhibited the relaxant response of isoproterenol on the contraction elicited by 30 nM LTC₄, but did not affect those of forskolin and aminophylline. The relaxant response of isoproterenol on the LCT₄ response was not inhibited by indomethacin, 0.3 μM phenidone and 1 μM ETYA. In the presence of a γ-glutamyltranspeptidase inhibitor, L-serine borate (SB, 45 mM), NDGA had no effect on the LTC₄ antagonism and the relaxant response of isoproterenol. In contrast, NDGA significantly inhibited the relaxant response of isoproterenol on 30 μM histamine- and 30 μM acetylcholine-induced contractions, but it did not affect the histamine antagonism by a histamine H₁-blocker pyrilamine. These results suggest that some putative nonprostanoids are involved in LTC₄-induced contractions of guinea-pig trachea and which regulate the effects of LTD₄ antagonism and β-adrenoceptor activation.     

Pharmacology of peptide leukotrienes on ferret isolated airway smooth muscle

The contractile activities of peptide leukotrienes (LT) on isolated spiral strips of ferret trachea were chracterized pharmacologically. LTC₄ and LTD₄ contracted ferret tracheal strips in a concentration-related manner and were 3- to 8-fold more potent than carbachol. In contrast, high concentrations of LTE₄ evoked either weak contraction or none at all, whereas LTC₄ and D₄ were partial agonists compared to carbachol. In tissues which were unresponsive to LTE₄, this compound antagonized contractile responses to LTC₄ and D₄ in an apparently competitive manner: Carbachol-induced contractions were not altered by LTE₄. The cyclooxygenase inhibitor, indomethacin (5 μM), LT antagonists, FPL55712 (10 μM), atropine (1 μM), phenoxybenzamine (10 μM), and LTB₄ (10 μM) failed to alter LTC₄ and D₄ concentration-response curves. The results in dicate that ferret trachea is sensitive to the contractile activity of LTC₄ and LTD₄ but not LTE₄. The LT-induced contractions appear to be mediated by a direct action of the LT rather than indirectly through release of secondary mediators such as thromboxane, prostaglandin, or acetylcholine. LT receptors in ferret trachea are insensitive to FPL55712 but are antagonized by LTE₄.     

Evidence for lipoxygenase pathway involvement in allergic tracheal contraction

Challenge of actively sensitized guinea-pig trachea in vitro led to a contraction which was enhanced by the cyclo-oxygenase inhibitors, indomethacin and sodium meclofenamate. Cyclo-oxygenase inhibitors eliminated the release of PGE-like material induced by arachidonic acid (AA), histamine, and antigen challenge. AA (10 μg./ml.) and PGE₂ (100 ng./ml.) usually relaxed the trachea, whereas in the presence of cyclo-oxygenase inhibitors a contraction occurred. Phenidone and ETYA, which also blocked the lipoxygenase pathway of AA metabolism inhibited the enhancement of allergic tracheal contraction induced by cyclo-oxygenase inhibitors, decreased the time that the trachea remained contracted, and also eliminated the contraction induced by AA and PGE₂. Thus, cyclo-oxygenase inhibitors may enhance allergic tracheal contraction by diverting AA metabolism into the lipoxygenase pathway and a product of the latter pathway, possibly SRS-A, may be responsible for the enhancement and for the prolonged phase of allergic tracheal contraction. An analogous mechanism may account for aspirin-induced asthma in man.     

Inhibition of leukotriene-induced contraction of guinea pig trachea by 5-lipoxygenase inhibitors

The effects of the 5-lipoxygenase inhibitors nordihydroguiaretic acid (NDGA), 5,8,11,14-eicosatetraynoic acid (ETYA), 1-phenyl-3-pyrazolidone (phenidone) and BW-755c, on the contractile response to LTC₄ or LTD₄ were examined on the isolated guinea pig trachea. Responses to either LTC₄ or LTD₄ or LTD₄ were obtained on indomethacin treated tissues, in the presence of either L-serine-borate complex or L-cysteine, respectively, to exhibit metabolic conversion of the leukotrienes. NDGA (30 μM) and ETYA (100 μM) produced a selective competitive antagonism of LTD₄ - induced contractions, while phenidone antagonized both LTC₄- and LTD₄ - induced responses in a non-competitive manner. In contrast, BW-755c (30 μM) did not significantly antagonize LTC₄ or LTD₄ concentration-response curves. The results suggest that leukotriene antagonism may be produced by large concentrations of some 5-lipoxygenase inhibitors.     

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.     

Leukotriene F4: Comparison of its pharmacological profile with that of the other cysteinyl-containing leukotrienes in guinea-pig ileum and lung parenchyma in vitro

The biological effects of leukotriene (LT)F₄ were compared, on a molar basis, with those of LTC₄, LTD₄ and LTE₄ on isolated superfused strips of guinea-pig ileum smooth muscle (GPISM) and lung parenchyma (GPP). LTF₄ was 1–2 orders of magnitude less active than the other leukotrienes on GPISM (LTD₄ > LTC₄ > LTE₄ > LTF₄) whereas, in the GPP, the activity of LTF₄ was comparable with that of LTE₄, both leukotrienes being about one order of magnitude less active than LTC₄ or LTD₄ (LTC₄=LTD₄ > LTE₄=LTF₄). Further, LTF₄ caused protracted contractions of the GPP which were indistinguishable from those due to LTE₄ and of a much longer duration than responses elicited by either LTC₄ or LTD₄.FPL 55712 (1.9μM) antagonised actions of LTF₄ in both tissue preparations. Indomethacin (2.8μM) inhibited contractions induced by LTF₄ in GPP indicating that part of the bronchoconstriction due to LTF₄, like that elicited by the other leukotrienes, is mediated via release of cyclo-oxygenase products.     

Release of leukotriene C4 from guinea pig trachea

Immunological (ovalbumin) and non-immunological (calcium ionophore A23187) stimulation of guinea pig trachea induces a prolonged contraction that is enhanced by indomethacin (8.5 μM) and inhibited by nordihydroguaiaretic acid (50 μM) pretreatment of the tissue. The mediator released by the above stimuli was identified as leukotriene C₄ by reverse-phase high performance liquid chromatography, and quantitated by bioassay. Indomethacin, and/or arachidonic acid (32.8 μM) did not enhance the release, whereas nordihydrolguaiaretic acid reduced the contraction and release of LTC₄. The results demonstrate the hitherto unproved capability of the large airways to synthesize leukotrienes and emphasize the importance of examining their role in asthma.     

Comparative effects of platelet activating factor, leukotriene D4 and histamine on guinea pig trachea, bronchus and lung parenchyma

The myotropic effect of platelet activating factor (PAF), leukotriene D₄ (LTD₄) and histamine were compared on guinea pig pulmonary tissues. The initial administration of PAF induced a contraction of strips of trachea, bronchus and lung parenchyma. However subsequent injections were characterized by relaxation of trachea and bronchus and a highly reduced (if any) contraction of the parenchyma. The three tissues of the guinea pig respiratory system contracted strongly to leukotriene D₄ and histamine. Indomethacin blocked PAF-induced relaxation of the trachea and bronchus and reduced the contraction of the lung parenchyma. The injection of PAF in the pulmonary circulation stimulated the release of substance(s) causing the contraction of the trachea, bronchus and parenchyma. This study suggests that PAF is not a direct agonist of bronchoconstriction.     

Quantitation of luekotrienes C4, D4 amd E4 released by pathophysiological stimuli

In order to examine the modulation of leukotriene (LT) release, the PAF-acether-mediated stimulation of these compounds in rat lung was studied. Release of LTC₄, LTD₄ and LTE₄ in both perfused and chopped lung preparations was measured using HPLC and radioimmunoassay. Pre-incubation or pre-infusion of the tissue with indomethacin and PGE₂ was conducted to investigate the effect of cyclooxygenase inhibitors and products on the lipoxygenase pathway. In addition, the effects of LT levels of pre-incubation with vasoactive intenstinal polypeptide (VIP) in chopped lung were observed.In perfused rat lung, indomethacin reduced the levels of LTC₄ relative to LTD₄ as measured in the first 2 min after stimulation of the lung by PAF-acether. Chopped lung preparations, incubated for 15 min. exhibited higher levels of LTC₄ and LTD₄ in indomethacin-treated samples, this increases being effectively reversed by PGE₂.In the VIP pre-incubation experiments clear inhibition of peptido -leukotriene synthesis was observed, with no LTC₄ and only low levels of LTD₄ and LTE₄ observed in VIP-incubated samples. In preliminary experiments using rabbit C5a des arg and PAF-acether on rabbit lung parenchyma strips to stimulaet LT release, disodium cromoglycate pre-incubation was observed to inhibit this release.Inhibition of the 5-lipoxygenase pathway of PGE₂ is supported by these experiments. VIP appears to act as an inhibitor of LTC₄ and LTD₄ biosynthesis or release in this model. Too little is known that peptidergic actions to postulate a mechanism by which a neuroendocrine peptide exerts control of release of arachidonate metabolites; however, VIP is associated with muscarinic stimulation (1) and has been found in mast cells (2).     

Arachidonate lipoxygenase inhibitors in guinea-pig isolated trachea. Effects on contractions to antigen and various agonists

We compared the effects of the leukotriene (LT) D4 receptor antagonist FPL55712 and some lipoxygenase inhibitors on contractions of isolated guinea-pig trachea induced by antigen (ovalbumin, OA) and calcium ionophore A23187 in the presence of the cyclooxygenase inhibitor indomethacin (5 microM), and by arachidonic acid (AA), melittin and LTD4. FPL55712 (0.1 and 1 microM) inhibited contractions induced by AA (100 microM) and the phospholipase A2 activator melittin (3 micrograms/ml), while the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA, 10 microM) was a more effective inhibitor of the melittin response than the AA response. FPL55712 inhibited contractions induced by OA (100 micrograms/ml) more than by A23187 (1 microgram/ml), and these inhibitory effects of FPL55712 were much less in the presence of l-serine-borate complex (45 mM), an inhibitor of LTC4 conversion to LTD4. NDGA (10 microM) had no significant effect on the OA response, whereas the lipoxygenase inhibitors 1-phenyl-3-pyrazolidone (phenidone, 10 microM) and 5,8,11,14-eicosatetraynoic acid (ETYA, 10 microM) clearly inhibited it. In contrast, NDGA and phenidone inhibited the A23187 response, but ETYA had no effect on it. FPL55712, phenidone and ETYA, but not NDGA, had a large inhibitory effect on LTD4-induced contractions, but these inhibitors had no effect on histamine-induced contractions. These results suggest that in the guinea-pig trachea inhibitors of LTD4-induced contractions decrease antigen-induced contractions, whereas lipoxygenase inhibitors reduce the contraction to A23187.     

Differentiation of the mechanisms by which leukotrienes C4 and D4 elicit contraction of the guinea pig trachea

The contraction elicited by leukotriene (LT) C₄ and D₄ on isolated guinea pig trachea were characterized under conditions in which LTC₄ to LTD ₄ metabolism was blocked by presence of 45 mM ?-serine-borate complex (SB). The presence of Sb caused a shift of the LTC₄-concentration-response curve to the left by 7.5-fold, and blocked the bioconversion of LTC₄ to LTD₄ by the trachea as estimated by HPLC analysis of the LTs present in the tissue bath fluid. The potency of FPL 55712 as an antagonist of the LTC₄-induced contractions in the presence of SB was 15-30-fold less than its potency as an antagonist of the LTD₄-induced contractions. In contrast, another LT antagonist, Sk&F 101132, equally antagonized the contractions elicited by LTC₄ and LTD₄ in either the presence or absence of SB. The differential antagonism of LTC₄ and LTD₄ implies the existence of multiple pharmacologic receptors for the LTs. The calcium channel entry blockers, nifedipine and verapamil, at concentrations as high as 10 μM, suppressed the maximal LTC₄-induced contraction by no more than 20%. whereas the purported intracellular calcium antagonist, TMB-8, completely suppressed the LTC₄ concentration-response curve in the presence of SB, a profile identical to that previously reported for LTD₄. Thus, if multiple LT receptors exist, they appear to mobilize calcium in a qualitatively similar fashion following LT stimulation.     

Pharmacological comparison of L-serine borate and glutathione as inhibitors of metabolism of LTC4 to LTD4 by the isolated guinea pig trachea

Cumulative dose-response curyes to leukotriene C₄ (LTC₄) and leukotriene D₄ (LTD)₄ were obtained on indomethacin (5 μM) treated isolated guinea pig tracheal spiral strips. LTC₄ curves, in the presence of either glutathione (GSH; 10 mM) or L-serine borate (SB; 45 mM), were not antagonized by FPL-55712 (3 μM), a selective LTD₄ receptor antagonist. LTC₄ curves on trachea treated with a lower concentration of GSH (1 mM), and LTD₄ curves were competitively antagonized by FPL-55712. LTC, curves on GSH (10 mM) treated trachea were 2 fold to the left of those on SB treated tissues. This effect of GSH was blocked by pretreatment with nordihydro-guiaretic acid (30 μM), an inhibitor of 5-lipoxygenase.GSH (10 μM) and SB (45 mM) are effective inhibitors of conversion of LTC₄ into functionally important levels of LTD₄ by the guinea pig trachea. In addition, GSH appeares to enhance LTC₄ responsiveness by increasing synthesis of a contractile 5-lipoxygenase product(s), possibly LTC₄. From the data it is suggested that for inhibition of LTC₄ metabolism, SB may be more usefull when examining responses to exogenously applied LTC₄, while GSH (10 mM) may be useful when examining responses to endogenously generated LTC₄.     

Arachidonate lipoxygenase inhibitors in guinea-pig isolated trachea. Effects on contractions to antigen and various agonists

We compared the effects of the leukotriene (LT) D₄ receptor antagonist FPL55712 and some lipoxygenase inhibitors on contractions of isolated guinea-pig trachea induced by antigen (ovalbumin, OA) and calcium ionophore A23187 in the presence of the cyclooxygenase inhibitor indomethacin (5 μM), and by arachidonic acid (AA), melittin and LTD₄. FPL55712 (0.1 and 1 μM) inhibited contractions induced by AA (100 μM) and the phospholipase A₂ activator melittin (3 μg/ml), while the lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA, 10 μM) was a more effective inhibitor of the melittin response than the response. FPL55712 inhibited contractions induced by OA (100 μg/ml) more than by A23187 (1 μg/ml), and these inhibitory effects of FPL55712 were much less in the presence of l-serine-borate complex (45 mM), an inhibitor of LTC₄ conversion to LTD₄. NDGA (10 μM) had no significant effect on the OA response, whereas the lipoxygenase inhibitors 1-phenyl-3-pyrazolidone (phenidone, 10 μM) and 5,8,11,14-eicosatetraynoic acid (ETYA, 10 μM) clearly inhibited it. In contrast, NDGA and phenidone inhibited the A23187 response, but ETYA had no effect on it. FPL55712, phenidone and ETYA, but not NDGA, had a large inhibitory effect on LTD₄-induced contractions, but these inhibitors had no effect on histamine-induced contractions. These results suggest that in the guinea-pig trachea inhibitors of LTD₄-induced contractions decrease antigen-induced contractions, whereas lipoxygenase inhibitors reduce the contraction to A23187.     

The effects of indomethacin, NDGA, allopurinol and superoxide dismutase on prostaglandin E2 and leukotriene C4 levels after mesenteric ischemia-reperfusion injury

In this study, the changes of arachidonic acid metabolites after an ischemia-reperfusion (I/R) period are investigated. The cyclooxygenase and lipoxygenase metabolites were found to be significantly increased after a 45 min period of ischemia followed by 5 min of reperfusion. Prostaglandin E₂ (PGE₂)- and leukotriene C₄ (LTC₄)-like activities did not change in the ischemic period, but they both increased after reperfusion. A cyclooxygenase inhibitor indomethacin and lipoxygenase inhibitor nordehydroguaretic acid (NDGA) decreased PGE₂- and LTC₄-like activities, respectively, while allopurinol and superoxide dismutase (SOD) decreased both activities.According to our results, it can be assumed that free oxygen radicals are responsible for the elevation of PGE₂- and LTC₄-like activities and both of these arachidonic acid metabolites and free oxygen radicals are the main necrotizing agents in ischemia-reperfusion induced damage.     

Release of slow-reacting substance of anaphylaxis and leukotriene C4-like immunoreactivity from guinea pig colonic tissue

Colonic mucosa, muscularis propria and subserosa from ovalbumin-sensitized guinea pigs were incubated and challenged with antigen in vitro. Slow-reacting substance of anaphylaxix (SRS-A) was determined biologically as well as radioimmunologically in terms of leukotriene (LT) C₄-like immunoreactivity. Before antigenic challenge release of immunoreactive LTC₄ by all tissues was below or close to the detection limit of the radioimmunoassay. After addition of antigen colonic mucosa released considerable amounts of LTC₄-like immunoreactivity, while muscularis propria and subserosa were less active. The biological activity of the SRS-A formed after challenge was antagonized by FPL 55712. Contrary to LTC₄-like immunoreactivity release of 6-keto-prostaglandin (PG) F1_α was predominant in the subsero and smaller amounts were released from the smooth muscular and mucosal layers. Synthesis of SRS-A and LTC₄-like immunoreactivity, respectively, as well as synthesis of 6-keto-PGF_1α was inhibited by the dual inhibitor of lipoxygenase and cyclooygenase BW755C. The results suggest a role for LTs as local mediators of inflammatory reactions in colonic disease states, particularly those with possible involvement of immunological processes.