Polyphosphoinositide hydrolysis and protein kinase C activation in guinea pig tracheal smooth muscle cells in culture by leukotriene D4 involve a pertussis toxin sensitive G-protein

Transient receptor potentials (TRPs) and anaphylaxis

The transient receptor potential (TRP) superfamily consists of 28 members in mammals (27 in human) that act as polymodal sensors and ion channels. They regulate cellular calcium influx, generate depolarization thereby triggering voltage dependent cellular processes, and in turn they are critical in inducing the metabolic activities of cells. It is increasingly apparent that many of the inflammatory mediators released in allergic reactions involve at least two of these ion channels, the 'Vanilloid' TRPV1 and the 'Ankyrin" TRPA1. This review mainly focuses on TRPV1 and TRPA1 and the role they have in the allergic response and how these receptors may be influenced in exercise-induced anaphylaxis. The threshold to react to an allergen for mast cells and lymphocytes can be reduced by activating the melastatin channel TRPM4. This channel is briefly discussed in the context of allergy.

Olopatadine 0.6% Nasal Spray Protects from Vasomotor Challenge in Patients with Severe Vasomotor Rhinitis

Background

Vasomotor rhinitis (VMR) is a hypersensitivity syndrome with heightened reactivity to environmental triggers.

Methods

Twenty-two patients with severe VMR were treated nasally with either normal saline or 0.6% olopatadine and challenged nasally with a hyperosmolar mannitol solution.

Results

Treatment with 0.6% olopatadine resulted in an improvement in instantaneous nasal symptom scores at 5 and 30 minutes (p < 0.01) compared with baseline and at 30 minutes after hyperosmolar challenge compared with saline-pretreated individuals (p < 0.01). There was also an improvement in nasal peak inspiratory flow rate at 30 minutes after hyperosmolar challenge compared with saline-pretreated individuals (p < 0.01.

Conclusion

In this patient population 0.6% olopatadine appears to be efficacious in symptom reduction in VMR and protects from hyperosmolar challenge.

Hydrogen peroxide reduces lower esophageal sphincter tone in human esophagitis

BACKGROUND & AIMS

We have previously used the normal lower esophageal sphincter (N-LES) of human organ donors to examine the physiologic signal transduction of lower esophageal sphincter (LES) circular muscle. Now, for the first time, we have obtained a human LES specimen with esophagitis (E-LES) and characterized its pathophysiologic mechanical and inflammatory profiles.

METHODS

E-LES was examined histologically, and its in vitro circular muscle contraction and production of inflammatory mediators were compared with those of N-LES.

RESULTS

E-LES exhibited scattered erosions and displayed inflammatory cells in the epithelial layer, basal zone hyperplasia, and elongation of lamina propria papillae, characteristic of chronic reflux esophagitis. E-LES muscle strips developed lower in vitro tone (0.78 g) than N-LES (3.3 +/- 0.2 g). E-LES tone was essentially restored to normal by the H2O2 scavenger catalase, suggesting that H2O2 was responsible for reduction of tone. NOX5 cDNA was higher and H2O2 levels were 4 times higher in E-LES circular muscle (0.85 nmol/mg protein) than in N-LES (0.19 +/- 0.05 nmol/mg protein). When N-LES smooth muscle was incubated in H2O2 (70 micromol/L, 2 hours), platelet activating factor (PAF), prostaglandin E2 (PGE2), and F2-isoprostane increased 2.5, 5.2, and 36 times, respectively. In E-LES, levels of PAF, PGE2, and F2-isoprostane were 4, 6, and 40 times, respectively, higher than in N-LES. PAF, PGE2, and F2 isoprostane produced dose-dependent reductions in tone of N-LES muscle strips.

CONCLUSIONS

We conclude that an excessive production of H2O2 triggers an increased production of PAF, PGE2, and F2-isoprostane, which are responsible for reducing LES tone in human esophagitis.

LTD4 induces hyperresponsiveness to histamine in bovine airway smooth muscle: role of SR-ATPase Ca2+ pump and tyrosine kinase

Airway hyperresponsiveness is a key feature of asthma, but its mechanisms remain poorly understood. Leukotriene D(4) (LTD(4)) is one of the few molecules capable of producing airway hyperresponsiveness. In this study, LTD(4), but not leukotriene C(4) (LTC(4)), produced a leftward displacement of the concentration-response curve to histamine in bovine airway smooth muscle strips. Neither LTC(4) nor LTD(4) modified the concentration-response curve to carbachol. In simultaneous measurements of intracellular Ca(2+) ([Ca(2+)](i)) and contraction, histamine or carbachol produced a transient Ca(2+) peak followed by a plateau, along with a contraction. LTD(4) increased the histamine-induced transient Ca(2+) peak and contraction but did not modify responses to carbachol. Enhanced responses to histamine induced by LTD(4) were not modified by staurosporine or chelerythrine but were abolished by genistein. Western blot showed that carbachol, but not histamine, caused intense phosphorylation of extracellular signal-regulated kinase 1/2 and that LTD(4) significantly enhanced the phosphorylation induced by histamine, but not by carbachol. L-type Ca(2+) channel participation in the hyperresponsiveness to histamine was discarded because LTD(4) did not modify the [Ca(2+)](i) changes induced by KCl. In tracheal myocytes, LTD(4) enhanced the transient Ca(2+) peak induced by histamine (but not by carbachol) and the sarcoplasmic reticulum (SR) Ca(2+) refilling. Genistein abolished this last LTD(4) effect. Partial blockade of the SR-ATPase Ca(2+) pump with cyclopiazonic acid reduced the Ca(2+) transient peak induced by histamine but not by carbachol. These results suggested that LTD(4) induces hyperresponsiveness to histamine through activation of the tyrosine kinase pathway and an increasing SR-ATPase Ca(2+) pump activity. L-type Ca(2+) channels seemed not to be involved in this phenomenon.

Mechanisms of leukotriene D4-induced constriction in human small bronchioles

We examined the mechanisms underlying leukotriene D4- (LTD4) induced constriction of human small (300 - 500 micron i.d.) bronchioles, and the effect of LTD4 on ion currents and Ca2+ transients in smooth muscle cells (SMC) isolated from these bronchioles. LTD4 caused a concentration-dependent bronchoconstriction with an EC50=0.58+/-0.05 nM (n=7) which was not easily reversible upon washout. This bronchoconstriction was entirely dependent on extracellular Ca2+. Blockade of L-type Ca2+ channels with nifedipine (10 microM) reduced LTD4 response by 39+/-2% (n=8), whilst La3+, Gd3+ and SK&F 96,365 abolished LTD4-induced bronchoconstriction completely and reversibly, suggesting the majority of Ca2+ entry was via non-selective cation channels. Antagonists of PI-PLC (U73,122 and ET-18-OCH3), PLD (propranolol) and PKC (cheleretrine and Ro31-8220) were without any effect on LTD4-induced bronchoconstriction, whilst the PC-PLC inhibitor D609 caused complete relaxation. Inhibition of protein tyrosine kinase with tyrphostin A23 (100 microM) caused about 50% relaxation, although the inactive analogue tyrphostin A1 was without effect. In freshly isolated SMC from human small bronchioles LTD4 caused a slow increase of intracellular Ca2+ concentration, with a consequent rise of the activity of large conductance Ca2+-dependent K+ channels and the amplitude of depolarization-induced outward whole-cell current. Again, no effect of LTD4 could be observed in the absence of extracellular Ca2+. We conclude that LTD4 causes constriction of these small bronchioles primarily by activating Ca2+ entry via non-voltage gated channels, possibly by a PC-PLC mediated pathway.

CTP:phosphocholine cytidylyltransferase inhibition by ceramide via PKC-alpha, p38 MAPK, cPLA2, and 5-lipoxygenase

In a companion paper (Vivekananda J, Smith D, and King RJ. Am J Physiol Lung Cell Mol Physiol 281: L98-L107, 2001), we demonstrated that tumor necrosis factor (TNF)-alpha inhibited the activity of CTP:phosphocholine cytidylyltransferase (CT), the rate-limiting enzyme in the de novo synthesis of phosphatidylcholine (PC), and that its actions were likely exerted through a metabolite of sphingomyelin. In this paper, we explore the signaling pathway employed by TNF-alpha using C2 ceramide as a cell-penetrating sphingolipid representative of the metabolites induced by TNF-alpha. We found that in H441 cells, as reported in other cell types, cytosolic phospholipase A2 (cPLA2) is activated by TNF-alpha. We also observed that the inhibiting action of C2 ceramide on CT requires protein kinase C-alpha, p38 mitogen-activated protein kinase, and cPLA2. The actions of C2 ceramide on CT activity can be duplicated by adding 2 microM lysoPC to these cells. Furthermore, we found that the effects of C2 ceramide are dependent on 5-lipoxygenase but that cyclooxygenase II is unimportant. We hypothesize that CT activity is inhibited by the lysoPC generated as a consequence of the activation of cPLA2 by protein kinase C-alpha and p38 mitogen-activated protein kinase. The other product of the activation of cPLA2, arachidonic acid, is a substrate for the synthesis of leukotrienes, which raise intracellular Ca2+ levels and complete the activation of cPLA2.

Leukotriene D4-induced activation of smooth-muscle cells from human bronchi is partly Ca2+-independent

Cysteine-containing leukotrienes (cysteinyl-LTs) are potent bronchoconstrictors and play a key role in asthma. We found that histamine and LTD4 markedly constrict strips of human bronchi (HB) with similar efficacy. However, in human airway smooth-muscle (HASM) cells, LTD4, at variance with histamine, elicited only a small, transient change in intracellular calcium ion concentration. HASM cells express both Ca2+-dependent and -independent isoforms of protein kinase C (PKC) (i.e., PKC-alpha and PKC-alpha ). Western blot analysis showed that PKC-alpha is activated by histamine and, to a lesser extent, by LTD4, whereas only LTD4 translocates PKC-alpha. This translocation was specifically inhibited by the LTD4 antagonist pobilukast. Phorbol-dibutyrate ester (PDBu) (a PKC activator) contracted HB strips to the same extent in the presence as in the absence of extra- and intracellular Ca2+. In the absence of Ca2+, LTD4 contracted HB strips to the same extent as did PDBu, suggesting the involvement of a Ca2+-independent PKC in LTD4-mediated signal transduction. PDBu-induced desensitization and the PKC inhibitor H7 abolished the slow and sustained LTD4-triggered contraction of HB strips in the absence of Ca2+, although H7 did not greatly affect the response in the presence of the ion. Thus, in human airways, we identified a novel LTD4 transduction mechanism linked to bronchial smooth-muscle contraction, which is partly independent of Ca2+ and involves the activation of PKC-alpha.

G(s) protein dysfunction in allergen-challenged human isolated passively sensitized bronchi

We studied the intracellular mechanisms of allergen-induced beta(2)-adrenoceptor dysfunction in human isolated passively sensitized bronchi. Sensitization was obtained by overnight incubation of bronchial rings with serum containing a high specific IgE level to Dermatophagoides but a low total IgE level. Allergen challenge was done by incubation with a Dermatophagoides mix. The G(s) protein stimulant cholera toxin (2 microg/ml) displaced the carbachol (CCh) concentration-response curves of control and sensitized but not of challenged rings to the right. Cholera toxin (10 microg/ml) displaced the concentration-response curves to CCh of control, sensitized, and challenged rings to the right, but this effect was less in challenged rings. The effects of the G(i) protein inhibitor pertussis toxin (250 ng/ml or 1 microg/ml) on salbutamol concentration-relaxation curves did not differ significantly between challenged and sensitized rings. The adenylyl cyclase activator forskolin and the Ca(2+)-activated K(+)-channel opener NS-1619 relaxed CCh-contracted bronchial rings without significant differences between control, sensitized, and challenged rings. Neither G(i) nor G(s) alpha-subunit expression differed between control, sensitized, and challenged tissues. We conclude that G(s) protein dysfunction may be a mechanism of allergen-induced beta(2)-adrenoceptor dysfunction in human isolated passively sensitized bronchi.

Anti-inflammatory agents and allergen-induced beta2-receptor dysfunction in isolated human bronchi

Antigen challenge causes beta2-adrenoceptor dysfunction in sensitized human bronchi (Am. J. Respir. Crit. Care Med. 1997;155:1230-1234). This study investigated whether the dysfunction can be prevented by anti-inflammatory agents. Human bronchial rings (2 to 4 mm) from surgery were passively sensitized to house dust mite and challenged (1) with allergen only, (2) with allergen plus indomethacin (10(-)5 M), (3) with allergen plus nedocromil sodium (10(-)7 M to 10(-)5 M), (4) with allergen plus the H1-receptor antagonist cetirizine (10(-)7 M to 10(-)5 M), and (5) with allergen plus the peptido-leukotriene receptor antagonist iralukast (10(-)7 M to 10(-)5 M). Rings were first contracted with 10(-)6 M carbachol and then relaxed with salbutamol (10(-)9 M to 10(-)4 M). The concentration-relaxation curve to salbutamol was shifted significantly to the right in the rings challenged with allergen only compared with control rings. In the rings challenged with allergen plus nedocromil sodium (10(-)6 M and 10(-)5 M) or iralukast (10(-)6 M and 10(-)5 M) the concentration-relaxation curves to salbutamol were significantly shifted to the left compared with rings challenged in saline alone, suggesting a protective effect against beta2-adrenoceptor dysfunction. Neither allergen plus cetirizine nor allergen plus indomethacin shifted significantly the concentration-relaxation curves to salbutamol compared with rings challenged in saline alone. We conclude that the release of peptido-leukotrienes may play a significant role in causing the allergen-induced beta2-receptor dysfunction in passively sensitized human bronchi.

Leukotriene D4-induced contraction of cat esophageal and lower esophageal sphincter circular smooth muscle

BACKGROUND & AIMS

In esophageal circular muscle, acetylcholine activates phosphatidylcholine-specific phospholipases C and D and phospholipase A2, producing diacylglycerol and arachidonic acid, which cause contraction by interacting synergistically to activate protein kinase C. In a model of acute esophagitis, leukotriene D4 (LTD4) contributes to acetylcholine-induced contraction. We examined intracellular signaling in LTD4-induced contraction.

METHODS

Esophageal and lower esophageal sphincter (LES) cells, isolated by enzymatic digestion, were contracted by LTD4 in the absence or presence of inhibitors. Permeabilization by saponin allowed use of G-protein antibodies and heparin.

RESULTS

Esophageal contraction was inhibited by pertussis toxin, Gi3 antibodies, D609 (phosphatidylcholine-specific phospholipase C inhibitor), propranolol (phospholipase D pathway inhibitor), and chelerythrine (protein kinase C antagonist) but not W7 (calmodulin antagonist). LES contraction was unaffected by pertussis toxin. It was inhibited by Gq antibodies, U-73122 (phosphatidylinositol-specific phospholipase C inhibitor), heparin (inositol 1,4,5-trisphosphate inhibitor), and W7 and reduced by D609.

CONCLUSIONS

In the esophagus, LTD4 activates a protein kinase C-dependent pathway through pertussis toxin-sensitive Gi3 proteins and phosphatidylcholine-specific phospholipase. In the LES, LTD4 activates a calmodulin-dependent pathway through pertussis toxin-insensitive Gq proteins and phosphatidylinositol-specific phospholipase C. The intracellular pathways activated by LTD4 in the esophagus and the LES are similar to those activated by acetylcholine and other agonists.

Leukotriene D4 induces a rapid increase in cAMP in the human epithelial cell line, Int 407: a potential role for this signal in the regulation of calcium influx through the plasma membrane

Although the LTD4-induced Ca2+ influx in human epithelial cells has been shown to be regulated by a pertussis toxin-sensitive heterotrimeric G-protein, most likely a G alpha i3 protein [Adolfsson J.L.P., Ohd J.F., Sjölander A. Leukotriene D4-induced activation and translocation of the G-protein alpha i3-subunit in human epithelial cells. Biochem Biophys Res Commun 1996; 226: 413-419], the signalling pathway further downstream is still unclear. In the present study, we investigated the possible involvement of cAMP and protein kinase A activity in the LTD4-induced Ca2+ influx in the epithelial cell line Int 407. Stimulation with LTD4, but not with the calcium ionophore ionomycin, triggered a rapid increase (peak at 7 s) in the cellular cAMP level, an effect that was totally abolished by pertussis toxin. Furthermore, the LTD4-induced Ca2+ signal was reduced by 60% when cells that had been pre-incubated with the protein kinase A inhibitor Rp-cAMPS (50 microM for 30 min) were stimulated in a calcium containing medium. In contrast, Rp-cAMPS had no apparent effect on the LTD4-induced Ca2+ signal when the cells were stimulated in a calcium-depleted medium. The immediate LTD4-induced protein tyrosine phosphorylation (15 s), previously shown to be necessary for the subsequent Ca2+ influx, was abolished not only by pretreatment with pertussis toxin but also by exposure to Rp-cAMPS. Furthermore, direct activation of the cellular adenylyl cyclase activity by treatment with forskolin alone induced a prompt Ca2+ signal in the presence, but not in the absence, of extracellular Ca2+, identical results were obtained with the cell permeable cAMP analogue 8-bromo-cAMP. In addition, forskolin induced protein tyrosine phosphorylation similar to that seen with LTD4. These results suggest that protein kinase A activity participates in the regulation of the LTD4-induced Ca2+ influx at a site that is downstream of the activation of the pertussis toxin-sensitive G-protein but upstream of a LTD4-stimulated tyrosine kinase(s).

Inhibitory effect of a novel phosphodiesterase IV inhibitor, T-440, on antigen- and chemical mediator-induced bronchoconstrictions in guinea pigs in vivo

We demonstrated the effect of a novel selective type IV phosphodiesterase (PDE) IV inhibitor, T-440 (1-[1-(2-methoxyethyl)pyrid-2-one-4-yl]-2,3-bis (hydroxymethyl)-6,7-diethoxynaphthalene), on antigen- and chemical mediator-induced bronchoconstrictions in anesthetized guinea pigs in vivo. Intravenously (i.v.) administered T-440 inhibited antigen-induced bronchoconstriction dose-dependently in passively sensitized guinea pigs (ED50 = 2.3 mg/kg). Histamine-, leukotriene (LT) D4-, U-46619-, acetylcholine (ACh)-, neurokinin A- and endothelin-1-induced bronchoconstrictions were also inhibited by i.v. injected T-440. Most potent suppression was produced against the bronchoconstriction induced by LTD4 (ED50 = 0.89 microgram/kg), whereas the effect against ACh was very weak (ED50 = 1.8 mg/kg). Additionally, T-440 inhibited histamine-induced bronchoconstriction by intraduodenal and intratracheal administration (ED50 and EC50 = 1.6 mg/kg and 0.50 mg/ml, respectively). Bronchoconstrictions induced by antigen and chemical mediators were also suppressed by theophylline. However, all of these anti-spasmolytic effects of theophylline were less potent than those of T-440 (1.8-110 times). Our results indicate the importance of PDE IV in bronchodilation, and PDE IV inhibitors may have potential as anti-asthma drugs.

Effect of leukotriene D4 on tracheal mucociliary transport velocity in quails

We investigated the effect of leukotriene D4 (LTD4) on tracheal mucociliary transport in quails. Topical application of LTD4 (0.2-2 ng) to tracheal mucosa dose-dependently increased mucociliary transport velocity (MCTV) in 5 or 10 min after application. Forty minutes after application of 2 ng of LTD4, MCTV was decreased to about 84% of that in the control group. Both the transient increase and the subsequent decrease induced by 2 ng of LTD4 were blocked by ONO-1078 (Pranlukast: 4-oxo-8-[4-(4-phenylbutoxy)-benzoylamino]-2-(tetrazol-5-yl)- 4H- 1-benzopyran) (0.03-3 mg/kg, i.m.), a specific leukotriene antagonist. These results suggest that LTD4 possesses a biphasic effect on tracheal mucociliary transport through leukotriene receptors.

Leukotriene receptors

The current challenge in research on leukotriene receptors is to clone these molecules. Traditional protein purification approaches have not been successful in providing sequence information. Solubilization of cys-LT1 has been achieved but results in the dissociation of G-proteins and the loss of high affinity binding (Mong et al., 1986b; Mong and Sarau, 1990), while cys-LT2 activity cannot be monitored by other than functional assays and there have not been any purification attempts. Partial purification of B-LT has been reported but has not been continued to homogeneity (Sherman et al., 1992; Votta et al., 1990; Miki et al., 1990). Nor have attempts to clone these receptors through either homology screening or expression cloning been successful. The cloning of the prostanoid receptors, described in detail elsewhere in this volume, has shown that these receptors belong to a distinct family within the G-protein-coupled receptor superfamily. It is probable, therefore, that the leukotriene receptors will also belong to a separate group within this superfamily since phylogenic comparisons have shown that receptors displaying high affinity for structurally related ligands exist as discrete families. Recently, a human cDNA encoding an orphan FMLP-related receptor cloned from HL60 cells of myeloid lineage was identified as the receptor for another eicosanoid, lipoxin A (Fiore et al., 1994). FMLP has a similar profile of biological actions to LTB4. Moreover, LTD4 showed a high degree of cross-reactivity with this receptor with an affinity only 20-fold less that of lipoxin A, although LTB4 was inactive. It remains to be determined whether the leukotriene receptors will fall into this class of receptors. The cloning of the leukotriene receptors will allow identification of the different receptor types and subtypes and potentially splice variants. Evaluation of currently developed antagonists at these receptor types could also open the way for novel therapies for inflammatory conditions.

Leukotriene D4-induced signal transduction

In a human epithelial cell line LTD4 induces a calcium signal that is dependent on both intracellular mobilization and influx of calcium. This calcium signal is generated via the activation of dual G protein pathways. Whereas the intracellular mobilization of calcium is regulated by a pertussis toxin-insensitive G protein, the subsequent influx of calcium is regulated by a pertussis toxin-sensitive G protein. Furthermore, a LTD4-induced cellular elevation of cAMP also participates in the regulation of this calcium signal. The increase in cAMP is directly related to the LTD4-induced influx of calcium, perhaps by an activation of protein kinase A and a subsequent phosphorylation of a plasma membrane channel. This model of the LTD4-induced signaling pathway in epithelial cells is outlined in Figure 2.

Effect of phospholipase C inhibitor U-73122 on antigen-induced airway smooth muscle contraction in guinea pigs

The importance of phospholipase C (PLC) in airway smooth muscle contraction was studied, using an inhibitor of PLC, 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl] amino]hexyl]-1H-pyrrole-2,5-dione (U-73122). Tracheas from ovalbumin (OA)-sensitized guinea pigs contracted rapidly after exposure to low concentrations of antigen (OA). However, tracheas treated with U-73122 for 10 min prior to the addition of antigen, demonstrated a 3 log rightward shift in the OA dose-response curve with an IC50 of 7 microM. The analogue of U-73122, 1-[6[[17 beta-3-methoxyestra-1,3,5 trien-17-yl]amino]hexyl]-2,5-pyrrolidine-dione (U-73433), was approximately 5-fold less active in inhibiting smooth muscle contraction. In addition to the inhibition of antigen-induced smooth muscle contraction, U-73122 inhibited carbachol- and leukotriene D4-induced smooth muscle contraction. Furthermore, U-73122 inhibited in a dose-dependent manner antigen-induced histamine release from guinea pig tracheal tissue. The inhibition of smooth muscle contraction by U-73122 correlated well with the inhibition of polyphosphoinositide mediates smooth muscle contractile responses to muscarinic agonists and leukotrienes as well as antigenic-induced contraction.

Leukotriene D4 receptors are not negatively coupled to adenylyl cyclase in guinea-pig lung parenchyma

1. The possibility that receptors for the peptide-containing leukotrienes may be negatively coupled to adenylyl cyclase in guinea-pig lung parenchyma was investigated by comparing the effect of leukotriene D4 (LTD4) on the intracellular cyclic nucleotide (cyclic AMP and cyclic GMP) content and on the activity of cyclic AMP-dependent protein kinase (PKA). In addition, the potential association between changes in the cyclic nucleotide content and the ability of LTD4 to increase lung parenchymal tone was also evaluated. 2. Non-cumulative challenge of parenchymal lung strips with LTD4 elicited concentration-dependent contractions (pD2 = 8.23) that were paralleled by concentration-related increases in the intracellular level of cyclic AMP and cyclic GMP, and in the activation state of PKA (Kact = 33 nM). Temporally, these biochemical effects of LTD4 were transient, peaking after approximately 5 min drug contact thereafter decaying, despite the continued generation of tone. Both the biochemical and mechanical effects of LTD4 were antagonized by the LTD4-receptor blocking drug, ICI 198,615 (1 microM for 60 min), indicating that they were receptor-mediated events. 3. Challenge of guinea-pig lung with LTD4 (200 nM; EC100 for tension generation) stimulated a 150 and 70 fold increase in the elaboration of thromboxane B2 (TXB2) and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) respectively, relative to that generated spontaneously. 4. Pretreatment of lung strips for 60 min with an irreversible inhibitor of cyclo-oxygenase, flurbiprofen,at a concentration (8 microM) that abolished both basal and LTD4 (200 nM)-induced TXB2 and 6-keto-PGF1alpha release, relaxed rapidly the spontaneous tone of the tissues, reduced the cyclic AMP content by ~50%and lowered the PKA activity ratio from 29% to 17%. In addition, flurbiprofen abolished the ability of LTD4 (200 nM) to increase the cyclic AMP content and to activate PKA. Functionally, the magnitude of LTD4 (200 nM)-induced tone and the increase in cyclic GMP content were attenuated by approximately 20% and 50% respectively in flurbiprofen-treated tissues.5. In flurbiprofen-treated tissues, isoprenaline (10 microM for 10 min) increased the cyclic AMP content(from 4 to 27 pmol mg-1 protein) and activated PKA (from 15% to 26%). Preincubation (30 s or 5 min)of lung with LTD4 (200 nM) did not inhibit (or enhance) these isoprenaline-induced effects.6. Pretreatment of lung strips for 60 min with the thromboxane synthetase inhibitor, dazmegrel (10 microM),relaxed the spontaneous tone of the tissues, abolished the LTD4 (200 nM)-stimulated release of TXB2 and significantly enhanced (~two fold) the elaboration of 6-keto-PGF1alpha. In addition, dazmegrel attenuated (by ~50%) LTD4 (200 nM)-induced cyclic GMP accumulation but approximately doubled both the cyclic AMP content and PKA activity ratio. LTD4-induced contractions, in contrast, were not affected by dazmegrel.7. EP 092 (1 microM for 60 min), a selective TP-receptor blocking drug, had no effect on spontaneous tone,eicosanoid formation or on the cyclic GMP content of guinea-pig lung parenchymal strips. Likewise,EP 092 exerted no significant mechancial effect in lung challenged with LTD4 (200 nM) although it did potentiate, to a small extent, the ability of LTD4 (200 nM) to increase the cyclic AMP content.8. It is concluded that LTD4 can increase the intracellular level of cyclic AMP in guinea-pig parenchyma and activate PKA by a leukotriene-receptor-mediated mechanism sensitive to ICI 198,615. However,these biochemical actions of LTD4 are induced indirectly by an arachidonic acid-derived cyclo-oxygenase product(s) other than TXA2. Thus, contrary to reports of other investigators, no evidence was found to corroborate the finding that stimulation of leukotriene receptors on guinea-pig lung parenchyma results in a rapid lowering of the cyclic AMP content even in cyclo-oxygenase-blocked tissues. These data,therefore, do not support the hypothesis that leukotriene-induced tension generation is dependent upon a prior reduction in the cyclic AMP content.