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.     

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 antagonism of leukotriene (LT) C4-induced contractions of isolated guinea-pig trachea and the results were compared to that of a cyclooxygenase inhibitor indomethacin. NDGA (30 microM) as well as indomethacin (5 microM) inhibited LTC4-induced contractions. But, in the presence of indomethacin NDGA was ineffective to inhibit the LTC4 response, whereas two other lipoxygenase inhibitors, phenidone (3-30 microM) and 5,8,11,14-eicosatetraynoic acid (ETYA, 10 microM), markedly inhibited it. The antagonist action of an LTD4 receptor antagonist FPL55712 against LTC4-induced contractions was significantly reduced by NDGA (10-30 microM), but indomethacin had no effect on it. NDGA possessed the same inhibitory effect on the LTC4 antagonism in the presence of indomethacin, but 0.3 microM phenidone and 1 microM 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 LTC4 response was not inhibited by indomethacin, 0.3 microM phenidone and 1 microM ETYA. In the presence of a gamma-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 microM histamine- and 30 microM acetylcholine-induced contractions, but it did not affect the histamine antagonism by a histamine H1-blocker pyrilamine. These results suggest that some putative non-prostanoids are involved in LTC4-induced contractions of guinea-pig trachea and which regulate the effects of LTD4 antagonism and beta-adrenoceptor activation.     

Antagonism by ETYA of the effects of leukotrienes on ileum and lung parenchymal strips independent of effects on arachidonic acid metabolism

5,8,11,14-Eicosatetraynoic acid (ETYA), a compound which inhibits both the cyclooxygenase and lipoxygenase pathways of arachidonic acid metabolism, antagonized the contraction of segments of guinea-pig ileal longitudinal muscle produced by SRS-A (IC₅₀ = 2.73 μM). This activity was unaffected by pretreatment of the tissues with 10 μM indomethacin. Phenidone, another mixed cyclooxgenese-lipoxygenese inhibitor, was inactive. FPL-55712, an SRS-A antagonist, was a very potent inhibitor (IC₅₀ = 0.011 μM).BW755C and NDGA nonselectively inhibited the contractions of the guinea-pig ileal longitudinal muscle induced by SRS-A or histamine.ETYA antagonized the contraction of the guinea-pig ileal strip produced by 6 nM synthetic LTC₄ (IC₅₀ = 9.3 μM). FPL-55712 demonstrated an IC₅₀ of 0.3 μM in a similar series of experiments. ETYA, 1, 3 or 10 μM did not inhibit the contractions elicited by 0.5 μM of histamine.This was not a tissue-selective effect since 100 μM ETYA antagonized the LTC₄-induced contraction of the guinea-pig lung parenchymal strip preparation.These data demonstrate that ETYA antagonized the contractile effect of the leukotrienes on tissues from the gastrointestinal tract and lung. Furthermore, the inability of indomethacin or phenidone to inhibit the contractile response suggests that antagonism by ETYA may occur by a mechanism independent of cyclooxygenase and lipoxygenase enzymes.     

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.     

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 were obtained on indomethacin treated tissues, in the presence of either L-serine-borate complex or L-cysteine, respectively, to inhibit metabolic conversion of the leukotrienes. NDGA (30 microM) and ETYA (100 microM) 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 microM) 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.     

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.     

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.     

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.     

The mechanism of action of leukotrienes C4 and D4 in guinea-pig isolated perfused lung and parenchymal strips of guinea pig, rabbit and rat

The biological actions of pure slow-reacting substance of anaphylaxis (SRS-A) from guinea-pig lung, pure slow-reacting substances (SRS) from rat basophilic leukaemia cells (RBL-1) and synthetic leukotrienes C₄ (LTC₄) and D₄ (LTD₄) have been investigated on lung tissue from guinea pig, rabbit and rat. In the guinea pig, the leukotrienes released cyclo-oxygenase products from the perfused lung and contracted strips of parenchyma. The effects of SRS-A, SRS and LTD₄ were indistinguishable. LTC₄ and LTD₄ had similar actions although LTD₄ was more potent than LTC₄. Indo-methacin (1 μg/ml) inhibited the release of cyclo-oxygenase products from perfused guinea-pig lung and caused a marked reduction in contractions of guinea-pig parenchymal strips (GPP) due to LTC₄ and LTD₄. The residual contraction on the GPP was abolished by FPL 55712 (0.5 – 1.0 μg/ml). It appears, therefore, that a major part of the constrictor actions of LTC₄ and LTD₄ in guinea-pig lung are mediated by myotropic cyclo-oxygenase products, i.e. thromboxane A₂ (TxA₂) and prostaglandins (PGs).In rabbit and rat lung, however, SRS-A, SRS and the leukotrienes were much less potent in contracting parenchymal strips and there was little evidence of the release of cyclo-oxygenase products. FPL 55712 at a concentration of 1 μg/ml failed to antagonise leukotriene-induced contractions.     

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.     

Specific enhancement of LTD4-induced contractions of isolated guinea pig trachea by 5-hydroxyeicosatetraenoic acid (5-HETE)

In view of the likely production of monohydroxyeicosatetraenoic acid (HETE's) in bronchial asthma, the role of these lipoxygenase products in the development of a classical clinical element of airway disease, namely airway hyperreactivity, has been investigated. Tracheas removed from guinea-pigs actively sensitized to ovalbumin produced, upon antigenic challenge (0.01 μg/ml), a 17-fold increase (0.97 ± 0.34 ng/ml to 16.73 ± 1.58 ng/ml) in the amount of 5-hydroxyeicosatetraenoic acid (5-HETE) as measured by radioimmunoassay of the tissue-bath fluid, indicating that this tissue is capable of producing 5-HETE. While 5-HETE alone, at concentrations equal to or greater than those found during the above antigenic response (0.001 to 1.0 μM), failed to produce intrinsic contractions of normal, nonsensitized guinea-pig trachea, a 30 min pretreatment with 5-HETE (1.0 μM) enhanced subsequent LTD₄-induced contractions. Pretreatment with either 12- or 15-HETE, at similar concentrations and conditions, failed to potentiate LTD₄ concentration-response curves. The effect of 5-HETE was time-dependent, since pretreatment for either 15 or 60 min had little or no effect on subsequent LTD₄ responses. Also, the 5-HETE-induced enhancement seemed specific fot LTD₄, since contractions to LTC₄ (in the presence of l-serine borate), acetylcholine, histamine, PGD₂ or U-46619 were unaffected by 5-HETE. Therefore, 5-HETE may have a role in the development of airway hyperreactivity by interacting with released LTD₄ to exacerbate airway smooth muscle contraction in asthma.     

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.     

Further evidence for role of leukotrienes as mediators of decidualization in the rat

Inhibitors of leukotrieens were utilized to investigate the role of leukoteines (LTs) in the induction of decidualization in the rat. Alzet osmotic minipumps, filled with either FPL 55712 (FPL, a specific antagonist of peptidoleukotrienes), nordihydroguaiaretic acid (NDGA, an inhibitor of LT synthesis) or in combination with leukotriene C₄ (LTC₄) and/or prostaglandin E2 (PGE₂), were instilled at the ovarian end of uterine horns of day 5 pseudopregnant rats. Intraluminal infusion of FPL or DNGA, for 4 days, induced a dose dependent decrease in the uterine wet weights when compared to that induced by the infusion of their corresponding vehicles (1 μl/h). Furthermore, simultaneous infusion of LTC₄ (10 ng/h) with different doses of FPL (1, 0.5, or 0.25 μg/h) produced an increase in uterine weights as compared to that produced by FPL alone. Maximum response, however, was noted when LTC₄ (n0 ng/h) was infused with FPL at a rate of 0.5 μg/h. The infusion of LTC₄ (10 ng/h) or PGE₂ (1 μg/h) with NDGA, at 1 and 5 μg/h, could not overcome its inhibitory effect on decidualization. On the contrary, a combination of LTC₄ (10 ng/h) and PGE₂ (1 μg/h) was comparable to that induced by the infusion of the vehicle. To determine if the synthesis of PGs and LTs was inhibited by NDGA, one uterine horn was infused with NDGA (5 μg/h) and the other horn with the vehicle. The intrauterine infusion of NDGA for 24 h inhibited the release of PGE₂, PGF_2α, LTC₄ and LTB₄ as compared to those released by the vehicle-infused horns. These data suggest that both PGs and LTs are required for the induction and progression of decidualization.     

Peptidoleukotrienes induce an endothelium-dependent relaxation of guiena pig main pulmonary artery and thoratic aorta

The purpose of our investigation was to assess the role of the endothelium in the vasomotor effects of leukotrienes. Norepinephrine-preconstricted rings isolated from guinea pig main pulmonary artery and thoraic aorta responded to LTC₄ and LTD₄ with a concentration-dependent relaxation. In endothelium-denuded rings, both LTC₄ and LTD₄ caused a concentration-dependent contraction. The LTD₄ receptor antagonist ICI 198, 615 inhibited both LTC₄- and LTD₄-induced relaxation and contraction. Inhibition of γ-glutamyl transpeptidase with AT-125 prevented the effects of LTC₄, but not those of LTD₄. The relaxant effect of LTD₄ was not modified by indomethacin, but was abolished by methylene blue. We conclude that: 1)LTD₄ induces a receptor-mediated endothelium-dependent relaxation of cavian pulmonary artery and aorta; 2) the vasorelaxant effect of LTC₄ requires its conversion to LTD₄; 3) the vasorelaxant effect of LTD₄ is unrelated to PGI₂ release, and is probably due to the release of an “EDRF”; 4) the removal of the endothelium reveals a direct receptor-mediated vasoconstricting effect of leukotrienes.     

Pharmacologic characterization of the contractile activity of peptide leukotrienes in guinea-pig pulmonary artery

The actions of the peptide leukotriene (LT) LTC₄, LTD₄ and LTE₄ and phenylephrine (PE) were studied in isolated left branches of the guiena-pig pulmonary artery (GPPA). Indomethacin 5 × 10⁻⁶ M enhanced both the potency and maximal response of all agonists, but the effect on LTD₄ and LTE₄ was larger. The influence of indomethacin suggests the release of an endogenous vasodilating cyclooxygenase product in GPPA. In the pressence of indomethacin the rank-order of potency was LTC₄ > LTD₄ > LTE₄ ≥ PE with respective pD₂ vaues of 7.65, 7.39, 6.35 and 6.26. All further studies were carried out in the presence of 5 × 10⁻⁶ indomethacin. Removal of the endothelium further increased both potency (> 3-fold) and the maximal response of all agonists tested, indicating that a non-clycooxygenase endothelium-dependent relaxing factor may be present in GPPA. In separate studies, GPPA was demonstrated capable of metabolizing ³H-LTC₄ by an L-serine borate inhibitable γ-glutamyl transpeptidase. In contrast, relatively little formation of ³H-LTE₄ was apparent either from ³H-LTC₄ or ³H-LTD₄. The LTD₄-selective antagonists, LY 171,883 and ICI 198,615 had -log molar K_B values of 6.07 ± 0.14 and 9.38 ± 0.32, respectively, against LTD₄ in the absence of endothelium. The ability of LY 171,883 to antagonize LTC₄ was eliminated in the presence of 45 mM serine borate in endothelium denuded tissues. LT receptors in GPPA appear to be heterogeneous and similar to guinea pig ariway receptors.     

Sensitivity of ovine myometrial tissue to various eicosanoids in vitro

The sensitivity of sheep myometrial tissue to prostaglandin F_2α (PGF_2α), PGE₂, the thromboxane analog U-44069, and leukotrienes C₄ (LTC₄) and LTD₄ was investigated in a superfusion system. Tissues were obtained from eight oophorectomized ewes, with or without pretreatment with estradiol-17β. After equilibration, spontaneous activity was abolished by adding indomethacin to the superfusion fluid. The dose needed to induce a contraction with a peak level of 50% of the median peak level of spontaneous contractions increased from PGE₂ to PGF_2α, U-44069, LTC₄, and LTD₄. The differences between the doses required were significant for all compounds, except between LTC₄ and LTD₄. Estradiol-17β pretreatment caused an increase in the required dose of PGF_2α. The results of this study do not support the hypothesis that leukotrienes are involved in the regulation of myometrial activity.     

Peptidoleukotriene binding in guinea pig uterine membrane preparations

Peptidoleukotrienes are known to be potent smooth muscle contractile agents in many tissues, including guinea pig uterus. In order to characterize the receptors at which the leukotrienes interact, guinea pig uteri were homogenized in 50nM Tris-HCl, pH 7.4 at 40°C and centrifuged at 1000xg fpr 10 min. The supernatant was centrifuged at 40,000 xg and the washed pellet was used to measure the binding of ³H-LTC₄ and ³H-LTD₄. Specific binding of ³H-LTD₄ was not detected, but specific, saturable binding of ³H-LTC₄ was measured at 40°C, was complete in 10 min. and was rapidly reversible on addition of unlabeled LTC₄. Binding was linear with protein concentration and stimulated by CaCl₂ and L-serine borate. Scatchard and kinetic analysis of binding in the presence of calcium suggested a K_d of 10–12 nM. LTC₄ was a more potent competitor of binding than LTD₄ (IC₅₀ − 40nM and 30 μM, respectively). FPL 55712 inhibited binding from 10–100 μM but stimulated binding at lower concentrations. Thus, the guinea pig uterus has specific receptors for LTC₄, but not LTD₄, that can be demonstrated by radioligand binding.     

Vasoconstrictor effects of leukotrienes C4 and D4 in the feline mesenteric vascular bed

The effects of leukotriene C₄ (LTC₄) and leukotriene D₄ (LTD₄) in the feline mesenteric vascular bed were investigated under conditions of controlled blood flow so that changes in perfusion pressure directly reflect changes in vascular resistance. Intra-arterial injections of LTC₄ and LTD₄ (0.3–3.0 μg) increased perfusion pressure in a dose-related fashion. Vasoconstrictor responses to LTC₄ and LTD₄ were similar to norepinephrine (NE) whereas mesenteric vasoconstrictor response to the thromboxane analog, U46619, was markedly greater than were responses to LTC₄ and LTD₄. Meclofenamate in a dose that greatly attenuated the systemic depressor response to arachidonic acid was without effect on vasoconstrictor responses to LTC₄ and LTD₄, NE and U46619 in the mesenteric vascular bed. The present data show that LTC₄ and LTD₄ possess significant vasoconstrictor activity in the feline mesenteric vascular bed. In addition, the present data suggest that products of the cyclooxygenase pathway do not mediate vasoconstrictor responses to LTC₄ and LTD₄ in the intestinal circulation of the cat.     

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.     

Contraction of guinea pig gail bladder strips by leukotrienes and other agonists

In the presence of indomethacin, Leukotriene C₄ (LTC₄), LTD₄ and LTE₄ were shown to be contractile agents on guinea pig gall bladder strips. The respective pD₂ values for LTC₄, LTD₄ ad LTE₄ were 9.1, 9.1 and 7.7. The contractile effects of LTD₄ were not mediated through the generation of cyclooxygenase products and were antagonized by the SRS-A antagonist FPL-55712. The effects of PGE₁, PGF_2α, the endoperoxide analogue U44069 and histamine on gall bladder strips were also examined. All these agents caused dose-related contractions but were considerably less potent than the leukotrienes. Leukotrienes are therefore potent contractile agents on the guinea pig gall bladder and may contribute to gall bladder contractions or spasms .