Prostacyclin modulates the responses to leukotrienes C4 and D4 of guinea-pig airway smooth muscle

Lipid metabolites as regulators of airway smooth muscle function

Compelling evidence identifies airway smooth muscle (ASM) not only as a target but also a cellular source for a diverse range of mediators underlying the processes of airway narrowing and airway hyperresponsiveness in diseases such as asthma. These include the growing family of plasma membrane phospholipid-derived polyunsaturated fatty acids broadly characterised by the prostaglandins, leukotrienes, lipoxins, isoprostanes and lysophospholipids. In this review, we describe the enzymatic and non-enzymatic biosynthetic pathways of these lipid mediators and how these are influenced by drug treatment, oxidative stress and airways disease. Additionally, we outline their cognate receptors, many of which are expressed by ASM. We describe potential deleterious and protective roles for these lipid mediators in airway inflammatory and remodelling processes by describing their effects on diverse functions of ASM in asthma that have the potential to contribute to asthma pathogenesis and symptoms. These functions include contractile tone development, cytokine and extracellular matrix production, and cellular proliferation and migration.

Pharmacological evidence for a novel cysteinyl-leukotriene receptor subtype in human pulmonary artery smooth muscle

1. To characterize the cysteinyl-leukotriene receptors (CysLT receptors) in isolated human pulmonary arteries, ring preparations were contracted with leukotriene C(4) (LTC(4)) and leukotriene D(4) (LTD(4)) in either the absence or presence of the selective CysLT(1) receptor antagonists, ICI 198615, MK 571 or the dual CysLT(1)/CysLT(2) receptor antagonist, BAY u9773. 2. Since the contractions induced by the cysteinyl-leukotrienes (cysLTs) in intact preparations failed to attain a plateau response over the concentration range studied, the endothelium was removed and the tissue treated continuously with indomethacin (Rubbed+INDO). In these latter preparations, the pEC(50) for LTC(4) and LTD(4) were not significantly different (7.61+/-0.07, n=20 and 7.96+/-0.09, n=22, respectively). However, the LTC(4) and LTD(4) contractions were markedly potentiated when compared with data from intact tissues. 3. Leukotriene E(4) (LTE(4)) did not contract human isolated pulmonary arterial preparations. In addition, treatment of preparations with LTE(4) (1 microM; 30 min) did not modify either the LTC(4) or LTD(4) contractions. 4. Treatment of preparations with the S-conjugated glutathione (S-hexyl-GSH; 100 microM, 30 min), an inhibitor of the metabolism of LTC(4) to LTD(4), did not modify LTC(4) contractions. 5. The pEC(50) values for LTC(4) were significantly reduced by treatment of the preparations with either ICI 198615, MK 571 or BAY u9773 and the pK(B) values were: 7.20, 7.02 and 6.26, respectively. In contrast, these antagonists did not modify the LTD(4) pEC(50) values. 6. These findings suggest the presence of two CysLT receptors on human pulmonary arterial vascular smooth muscle. A CysLT(1) receptor with a low affinity for CysLT(1) antagonists and a novel CysLT receptor subtype, both responsible for vasoconstriction. Activation of this latter receptor by LTC(4) and LTD(4) induced a contractile response which was resistant to the selective CysLT(1) antagonists (ICI 198615 and MK 571) as well as the non-selective (CysLT(1)/CysLT(2)) antagonist, BAY u9773.

OKY-046 inhibits anaphylactic bronchoconstriction and reduces histamine level in bronchoalveolar lavage fluid in sensitized guinea pigs

We investigated the effects of OKY-046, a potent and selective thromboxane A2 (TxA2) synthetase inhibitor, on anaphylactic bronchoconstriction and release of chemical mediators into airway lumen in sensitized guinea pigs in vivo. OKY-046 dose-dependently inhibited antigen-induced anaphylactic bronchoconstriction with or without mepyramine, a histamine H1 antagonist. In the presence of mepyramine, OKY-046 (300 mg/kg, p.o.) elicited significant reductions in histamine (1 min) and TxB2 increases (1-15 min) in bronchoalveolar lavage (BAL) fluid but significantly increased the plasma level of 6-keto-PGF1 alpha, a stable PGI2 metabolite, after antigen challenge. On the contrary, indomethacin only significantly reduced increases in TxB2 levels. These results suggest that the antiasthmatic effect of OKY-046 is probably due to inhibition of TxA2 synthesis and suppression of histamine release via a PGI2 shunting mechanism.

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)

Effects of various pharmacological agents on isolated human bronchial and pulmonary arterial and venous muscle preparations contracted by leukotriene D4

The response of isolated human bronchial muscle preparations to leukotriene D4 (LTD4, 0.1 microM; 0.22 +/- 0.03 g/mm2) was similar to that of histamine (50 microM; 0.21 +/- 0.02 g/mm2). Isolated pulmonary venous preparations also contracted to these same concentrations of both agonists (LTD4, 0.32 +/- 0.19 g/mm2; and histamine, 0.36 +/- 0.07 g/mm2). However, pulmonary arterial preparations responded to histamine (50 microM, 0.59 +/- 0.10 g/mm2) but exhibited a reduced response to LTD4 (0.3 microM, 0.06 +/- 0.01 g/mm2). Bronchial and pulmonary venous muscle preparations from the human lung had the same sensitivities to LTD4 (pD2 values: bronchus, 7.95 +/- 0.08 and vein, 7.76 +/- 0.07). When bronchial or pulmonary venous muscle preparations were incubated for 30 min with either diltiazem (10 microM), indomethacin (1.7 microM) or L-cysteine (3 microM), the LTD4 cumulative concentration-effect curves following these drug treatments were similar to controls. However, FPL-55712 (10 microM) significantly shifted the LTD4 concentration-effect curves produced in bronchial preparations to the right. In isolated pulmonary arterial preparations none of these drug treatments enhanced the LTD4 response. These results show that isolated human pulmonary arterial preparations are less responsive to LTD4 than bronchial or venous preparations. In addition, the data obtained subsequent to the various drug treatments indirectly suggest that the LTD4 contraction is not modified by a calcium channel blocker or by inhibition of the endogenous products of the cyclooxygenase pathway.

Prostaglandin F2 stimulates the generation of lipoxygenase products in guinea pig isolated trachea

The generation of lipoxygenase products on the contraction elicited by prostaglandin (PG) F2 alpha was investigated in the guinea-pig isolated trachea. Indomethacin (5 x 10(-6) M) inhibited the response at low concentrations of PGF2 alpha while enhanced the response at higher concentrations of PGF2 alpha. Phenidone (10(-4) M) and nordihydroguaiaretic acid (NDGA, 3 x 10(-5) M) appeared to inhibit the PGF2 alpha response. The PGF2 alpha response augmented by indomethacin was dose-dependently inhibited by NDGA and a leukotriene (LT) antagonist, FPL55712. NDGA had no effect on the contraction elicited by histamine but markedly inhibited the contraction elicited by LTD4. The inhibition by NDGA of the LTD4-induced contraction was abolished in the presence of indomethacin (5 x 10(-6) M). FPL55712 inhibited the LTD4-induced contraction but the extent of the antagonism was not changed by indomethacin. In the presence of indomethacin PGF2 alpha (10(-8) M) did not affect the LTD4 (3 x 10(-9) M) response but significantly enhanced the arachidonic acid (AA, 6.6 x 10(-5) M)-induced contraction. FPL55712 (3 x 10(-6) M) completely inhibited the AA response augmented by PGF2 alpha. These results suggest that lipoxygenase-mediated LT-like substances are released in the response at higher concentrations of PGF2 alpha on the guinea-pig isolated trachea, and the mode of action of PGF2 alpha is different from those of histamine and LTD4.

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 microM). This activity was unaffected by pretreatment of the tissues with 10 microM indomethacin. Phenidone, another mixed cyclooxygenase-lipoxygenase inhibitor, was inactive. FPL-55712, an SRS-A antagonist, was a very potent inhibitor (IC50 = 0.011 microM). 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 microM). FPL-55712 demonstrated an IC50 of 0.3 microM in a similar series of experiments. ETYA, 1, 3 or 10 microM did not inhibit the contractions elicited by 0.5 microM of histamine. This was not a tissue-selective effect since 100 microM 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.