Differences in binding of glucocorticoid receptor to DNA in steroid-resistant asthma

Although glucocorticosteroids are a very effective treatment for asthma and other chronic inflammatory diseases, a small proportion of patients are resistant to their therapeutic effects. The molecular mechanism for this steroid resistance is unclear. Steroid resistance cannot be explained by pharmacokinetic mechanisms, by a defect in the binding of steroids to glucocorticoid receptors, nor by defective nuclear translocation of this receptor, thereby suggesting that the molecular abnormality lies distal to nuclear translocation. We examined the ability of nuclear translocated glucocorticoid receptors to bind to their DNA binding sites (GRE) using electrophoretic mobility shift assays in PBMC from patients with steroid-sensitive and steroid-resistant asthma. The binding of the glucocorticoid receptor to DNA in these patients was also studied using Scatchard analysis. Dexamethasone induced a significant rapid and sustained twofold increase in GRE binding in PBMCs from steroid-sensitive asthmatic patients and nonasthmatic individuals, but this was markedly reduced in steroid-resistant asthmatic patients. Scatchard analysis of glucocorticoid receptor-GRE binding showed no change in binding affinity but did show a reduced number of receptors available for DNA binding in the steroid-resistant patients. These results suggest that the ability of the glucocorticoid receptor to bind to GRE is impaired in steroid-resistant patients because of a reduced number of receptors available for binding to DNA.

Transcriptional down-regulation of m2 muscarinic receptor gene expression in human embryonic lung (HEL 299) cells by protein kinase C

m2 muscarinic receptor gene expression was investigated following stimulation of protein kinase C (PKC) with the phorbol ester 4 beta-phorbol dibutyrate (PDBu) in HEL 299 cells. PDBu (100 nM) caused a time-dependent decrease in the steady-state levels of m2 receptor mRNA and in specific [3H]N-methyl-scopolamine binding. Preincubation with the PKC inhibitor GF-109203X inhibited the reduction in M2 receptor and mRNA levels induced by PDBu, confirming the involvement of PKC. Chronic PDBu treatment also caused desensitization of the receptor as forskolin-stimulated cAMP accumulation, inhibited by carbachol in control cells, was lost upon treatment with PDBu for 24 h. Co-incubation with PDBu and the protein synthesis inhibitor cycloheximide, inhibited PDBu-mediated reduction of m2 receptor mRNA, indicating new protein synthesis is required for down-regulation. Half-life studies using the transcriptional inhibitor actinomycin D suggested that the stability of the m2 receptor mRNA was not altered by PDBu treatment (t1/2 = 2 h). Nuclear run-on assays showed a 50% reduction in the rate of m2 receptor gene transcription after treatment with PDBu for 12 h. In conclusion we have provided evidence for heterologous regulation of m2 receptor gene expression through changes in gene transcription resulting in uncoupling of M2 receptors. Furthermore, the synthesis of an unidentified factor is required for the down-regulation process.

Cyclic nucleotides and phosphodiesterases and airway function

The cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) play an important role in the regulation of airway smooth muscle tone and activation of inflammatory cells. Intracellular concentrations of these nucleotides are tightly regulated by phosphodiesterases (PDEs). Several families of PDEs are now recognized and, within each isoenzyme family, molecular cloning has revealed multiple members. PDE III and IV are important in the breakdown of cAMP; PDE III is involved in the regulation of airway smooth muscle tone, whereas PDE IV is more important in inflammatory cells, including mast cells, macrophages, eosinophils, T-lymphocytes and epithelial cells. PDE V is involved in the breakdown of cGMP in airway and vascular smooth muscle. Regulation of PDE expression in health and disease is now under investigation. Several selective PDE inhibitors have recently been developed, and experimental studies indicate that PDE IV inhibitors may be useful as anti-inflammatory treatments in asthma. Clinical trials in asthma are now in progress.

Nitric oxide and asthmatic inflammation

Asthmatic patients show an increased expression of inducible nitric oxide synthase (iNOS) in airway epithelial cells and an increased level of nitric oxide (NO) in exhaled air. The NO derived from airway epithelial cells may be a mechanism for amplifying and perpetuating asthmatic inflammation, through inhibition of T helper 1 (Th1) cells and their production of interferon gamma (IFN-gamma). This would result in an increase in the number of Th2 cells and the cytokines interleukin 4 (IL-4) (which is important for IgE expression) and IL-5 (which plays a critical role in the recruitment of eosinophils into the airways). Although this mechanism may be part of our nonspecific airway defence against parasite invasion, it appears to have been activated inappropriately in asthma. Here, Peter Barnes and Eddy Liew argue that the development of specific iNOS inhibitors may represent a novel therapeutic approach for asthma and other allergic diseases.

Evidence for the involvement of cGMP in neural bronchodilator responses in humal trachea

1. We have investigated the correlation between relaxation and changes in cyclic nucleotide content of human tracheal smooth muscle (HTSM) in vitro following inhibitory non-adrenergic non-cholinergic (i-NANC) neural bronchodilator responses evoked by electrical field stimulation (EFS), and compared these with changes seen with sodium nitroprusside (SNP), 3-morpholinosydnonimine (SIN-1) and vasoactive intestinal peptide (VIP). The effects of N omega-nitro-L-arginine methyl ester (L-NAME), Methylene Blue and alpha-chymotrypsin (alpha-CT) were studied. 2. EFS (10 Hz, 1 ms, 40 V for 30 s) evoked a time-dependent relaxation accompanied by a concurrent rise in cGMP, both of which were maximal at 30 s and unaffected by epithelium removal. Levels of cAMP were more variable than those of cGMP and were not significantly changed at any time point. 3. SIN-1 (1 mM) and SNP (100 microM) also produced time-dependent relaxations which were maximal between 2 and 8 min, accompanied by concomitant rises in cGMP; however, these changes were larger than those associated with i-NANC relaxations. cAMP levels were unchanged at all time points. 4. EFS-evoked i-NANC relaxations and cGMP increases (time, t = 30 s) were inhibited by L-NAME. The effects were partially reversed by L-arginine (1 mM), but not by D-arginine. D-NAME and alpha-CT (2 u ml-1) had no effect on either relaxation or cGMP accumulation. Tetrodotoxin (TTX, 3 microM) inhibited both relaxation and cGMP accumulation. 5. VIP (1 microM) also produced a time-dependent relaxation associated with a concurrent rise in cAMP levels with no change in cGMP levels. 6. Methylene Blue (10 microM) partially inhibited EFS (10 Hz)-evoked i-NANC relaxation and cGMP accumulation, and almost completely inhibited both relaxation and cGMP accumulation evoked by SIN-1 (1 mM). Methylene Blue had no significant effect on relaxation or cGMP accumulation evoked by SNP (100 microM). 7. Neural i-NANC relaxations in HTSM are associated with a concurrent selective accumulation of cGMP which is unaffected by epithelium removal. This is inhibited in a stereoselective manner by L-NAME and mimicked by SNP and SIN-1; however, cGMP accumulation was greatly increased with SNP and SIN-1 suggesting compartmentalized changes in cGMP content. VIP also caused relaxation associated with an increase of cAMP; however, no evidence was found for VIP being involved in i-NANC relaxation. Hence nitric oxide (NO), or a NO-containing complex, appears to mediate i-NANC responses in human trachea in vitro.

Ozone-induced airway hyperresponsiveness: role of superoxide anions, NEP, and BK receptors

We investigated the role of reactive oxygen species in ozone-induced airway hyperresponsiveness (AHR) in Brown Norway rats. Airway responsiveness to inhaled acetylcholine (ACh) and bradykinin (BK) and inflammatory cell recruitment in bronchoalveolar lavage fluid (BALF) were measured in vivo. Neutral endopeptidase (NEP) activity assay and measurement of BK-receptor binding sites in Brown Norway rat lungs were carried out in vitro. Apocynin (5 mg/kg), an inhibitor of superoxide anion-generating NADPH oxidase, was administered perorally 30 min before a 3- or 6-h exposure to 3 ppm of ozone, and the animals were studied 18-24 h postexposure. Ozone induced increases in airway responsiveness to ACh and BK and in neutrophil counts in BALF. Apocynin inhibited the increase in airway responsiveness to BK but not to ACh without affecting the neutrophil counts in BALF. The antioxidants allopurinol and deferoxamine prevented ozone-induced AHR to both ACh and BK but did not reduce neutrophil counts. To further examine the mechanisms of ozone-induced AHR to BK, we measured NEP activity and the density of BK receptors in vitro after ozone exposure. Ozone exposure had no significant effect either on NEP activity or on the affinity and the number of BK receptors in lungs from rats treated with or without apocynin. We conclude that superoxide anions released from inflammatory cells in the airway may be involved in ozone-induced AHR. Inactivation of NEP or upregulation of BK receptors do not appear to be involved, but the possibility of localized changes cannot be excluded.

Role of neutral endopeptidase in bronchial hyperresponsiveness to bradykinin induced by IL-1 beta

Interleukin-1 beta (IL-1 beta) induces bronchial hyperresponsiveness (BHR) to bradykinin but not to acetylcholine. We examined whether this was mediated through the inhibition of neutral endopeptidase (NEP) activity and/or through the enhancement of airway microvascular leakage (AML) by IL-1 beta. We administered human recombinant IL-1 beta (500 U) or saline intratracheally and 24 h later measured the airway responses to bradykinin (1 mM; 45 breaths). IL-1 beta-treated rats showed a decrease of 18.5 and 21.1% of NEP activity in the lungs and tracheobronchial tree, respectively (P < 0.05), associated with an augmented response in total lung resistance to bradykinin but with no increase in Evans blue dye extravasation used as a marker of AML. Phosphoramidon (0.1 and 1 mM; 90 breaths), an NEP inhibitor, induced a dose-dependent increase in lung resistance to bradykinin without further enhancing BHR induced by IL-1 beta. Bradykinin-induced AML was not enhanced by phosphoramidon in either saline- or IL-1 beta-treated rats. Similarly, after captopril (1 mM; 90 breaths), an inhibitor of angiotensin-converting enzyme, there was no further enhancement of BHR to bradykinin induced by IL-1 beta. BHR to bradykinin induced by IL-1 beta may result from an inhibition of peptidase activity, such as NEP and angiotensin-converting enzyme, and is not associated with an enhancement of AML.

L-arginine increases exhaled nitric oxide in normal human subjects

1. Endogenous nitric oxide plays an important physiological role and is synthesized by several isoforms of nitric oxide synthase from the semiessential amino acid L-arginine. Nitric oxide is detectable in the exhaled air of normal individuals and may be used to monitor the formation of nitric oxide in the respiratory tract. 2. We have investigated the effect of orally administered L-arginine (0.05, 0.1, 0.2 g/kg) compared with matched placebo on the concentration of nitric oxide in the exhaled air in 23 normal individuals. 3. L-Arginine caused significant increases in the concentration of nitric oxide in exhaled air at doses of 0.1 and 0.2 mg/kg, which was maximal 2 h after administration. This was associated with an increase in the concentration of L-arginine and nitrate in plasma. There were no significant changes in heart rate, blood pressure or forced expiratory volume in 1 s. 4. These results suggest that an increase in the amount of substrate for nitric oxide synthase can increase the formation of endogenous nitric oxide. This may have therapeutic relevance in diseases in which there is defective production of nitric oxide.

Increased nitric oxide in exhaled air of normal human subjects with upper respiratory tract infections

Viral infection may induce the expression of nitric oxide (NO) synthase, resulting in increased NO formation that has an antiviral effect. NO may be produced by various cells of the upper and lower respiratory tract, and may be detected in the exhaled air. We have studied the levels of exhaled NO in 18 normal subjects during symptomatic upper respiratory tract infections and during recovery 3 weeks later. Exhaled NO was measured using a modified chemiluminescence analyser. At the time of symptoms of upper respiratory tract infection, the peak exhaled NO values were 315 +/- 57 ppb (mean +/- SEM) and decreased to 87 +/- 9 ppb during recovery. Recovery values of exhaled NO were similar to those reported in age-matched normal control subjects (88 +/- 3 ppb, n = 72). These findings suggest that symptomatic upper respiratory tract infections markedly increase the concentration of NO in exhaled air. This may reflect the induction of nitric oxide synthase (NOS) in upper and lower respiratory tract, and may be relevant to viral exacerbations of asthma.

Inhibition of allergen-induced lung eosinophilia by type-III and combined type III- and IV-selective phosphodiesterase inhibitors in brown-Norway rats

We examined the effect of a type IV (rolipram) and a combined type III and IV (Org 20421) isoenzyme-selective phosphodiesterase inhibitor upon allergen-induced pulmonary eosinophil recruitment in sensitised Brown Norway rats. Rats were sensitised with ovalbumin intraperitoneally and later challenged with ovalbumin aerosol which induced a significant increase in the total eosinophil and neutrophil count in bronchovalveolar lavage fluid at 24 hours (from 0.38 +/- 0.12 to 1.36 +/- 0.18 x 10(6), p < 0.01 and from 0.06 +/- 0.01 to 0.33 +/- 0.07 x 10(6), p < 0.01) respectively. Pretreatment with rolipram (30 mumol/kg) and Org 20421 (30 mumol/kg) abolished the eosinophilia and neutrophilia evoked by ovalbumin. We conclude that type IV and possibly type III isozyme phosphodiesterase inhibitors may regulate, directly or indirectly, eosinophil and neutrophil activity and/or those cells responsible for attracting them into the lung.

Inhibition of fetal calf serum-stimulated proliferation of rabbit cultured tracheal smooth muscle cells by selective inhibitors of protein kinase C and protein tyrosine kinase

Severe chronic asthma is associated with structural changes in the airway wall including airway smooth muscle (ASM) hyperplasia. We have used cultured ASM cells isolated from rabbit trachealis as a model with which to investigate possible mechanisms of accelerated ASM growth to mitogenic stimuli. To elucidate the role that protein kinase C (PKC)- and protein tyrosine kinase (PTK)-dependent pathways play in the control of ASM mitogenesis, we have investigated the effect of reportedly selective inhibitors of PKC (3-[1-[3-(amidinothio)propyl]-3-indolyl]-4-(1-methyl-3-indolyl)-1H - pyrrole-2,5-dionemethanesulfonate [Ro31-8220] and 3-[1-(aminopropyl)indolyl]-4-(1-methyl-3-indolyl)-1H-pyrrole-2,5-dione acetate [Ro31-7549]) and PTK (alpha-cyano-3-ethoxy-4-hydroxy-5-phenylthiomethylcinnamamide [ST638]) on partially purified PKC, fetal calf serum (FCS)-stimulated protein phosphotyrosine content and on FCS-induced proliferation. Anion-exchange chromatography of lysed ASM cells resolved two peaks of Ca(2+)-activated, phospholipid-dependent PKC activity and one peak of Ca(2+)- and phospholipid-independent PKC activity. The selective PKC inhibitors, Ro31-8220 and Ro31-7549, abolished the main peak of PKC activity and the Ca(2+)- and phospholipid-independent peak that co-eluted with the main peak. The inhibition was dependent on the concentration of ATP in the reaction cocktail (IC50: 10 microM ATP: Ro31-8220 0.026 microM, Ro31-7549 0.073 microM; 100 microM ATP: Ro31-8220 0.065 microM, Ro31-7549 0.271 microM), consistent with these compounds inhibiting PKC at the ATP-binding site. Ro31-8220 was more potent (2- to 3-fold) than Ro31-7549. Concentrations of each inhibitor that produced maximal inhibition of the pooled kinase activity also abolished the second peak of Ca(2+)-dependent activity. The PTK inhibitor, ST638, had no effect on the kinase activity associated with any of the Ca(2+)-dependent or -independent peaks that eluted from the column. ST638, however, maximally inhibited FCS-stimulated PTK activity (IC50 25 microM). FCS-stimulated PTK was also inhibited by Ro31-8220 (IC50 0.15 microM), but only by 60%, revealing an Ro31-8220-insensitive component to the response. The ability of each protein kinase inhibitor to inhibit proliferation was also studied using four independent indices of ASM cell growth and division: 3-(4,5-dimethylthiazol-2-yl)2,5-diphenyl tetrazolium bromide (MTT) dye conversion, Coomassie blue protein determination, hemacytometer cell counts, and DNA synthesis. Ro31-8220 and Ro31-7549 produced concentration-dependent inhibition of FCS-stimulated proliferation of growth-arrested ASM cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of glucocorticoids on transcription factor activation in human peripheral blood mononuclear cells

Glucocorticoids have an inhibitory effect on inflammatory and immune responses, and this may be through the modulation of transcription factor binding to DNA. The interaction of the transcription factors, activator protein-1 (AP-1), nuclear factor kappa B (NF kappa B), and cAMP-responsive element binding protein (CREB) with DNA and glucocorticoid receptors (GR) was analyzed in human peripheral blood mononuclear cells by gel mobility shift assays. TNF-alpha, IL-1 beta and phorbol myristate acetate (PMA) treatment increased AP-1 and NF kappa B DNA binding by up to 200% but decreased CREB binding (38%) over a 60-min time course. Dexamethasone produced a rapid and sustained increase in glucocorticoid response element binding and a concomitant 40-50% decrease in AP-1, NF kappa B, and CREB DNA binding that was blocked by combined dexamethasone and cytokine or PMA treatment. These latter effects were due to increases in the nuclear localization of GR, not to reduced amounts of the other transcription factors. This suggests that in these cells GR within the nucleus interacts with cytokine-stimulated transcription factors by the process of cross coupling. This may be an important molecular site of steroid action.

Effects of neuropeptides on human lung fibroblast proliferation and chemotaxis

An increase in subepithelial mesenchymal cells and associated connective tissue is a feature of bronchial asthma. We determined whether neuropeptides could modulate fibroblast activity, particularly with respect to proliferation and chemotaxis. Human lung fibroblasts were cultured with neurokinin A (NKA), substance P (SP), vasoactive intestinal peptide (VIP), and calcitonin-gene-related peptide (CGRP). After 48 h, fibroblast proliferation was measured by a colorimetric assay based on the uptake and subsequent release of methylene blue. The chemotactic response to neuropeptides was determined with the use of a modified Boyden chamber. Both NKA and SP (10(-7)-10(-4) M) stimulated human lung fibroblast proliferation in HFL1 and IMR-90 fibroblasts. VIP and CGRP had no effect on fibroblast proliferation. NKA alone stimulated fibroblast chemotaxis maximally at 10(-10) M. Neutral endopeptidase (NEP) activity of 0.52 and 5.2 pmol/10(6) cells was assayed in IMR-90 and Hs68 fibroblasts, respectively. Phosphoramidon (5 x 10(-6)-10(-5) M), an NEP inhibitor, enhanced fibroblast proliferation in a dose-dependent manner. Thus neuropeptides have the potential to cause activation of mesenchymal cells, and neuropeptide release may contribute to the structural abnormalities observed in asthmatic airways.

Neurogenic goblet cell secretion and bronchoconstriction in guinea pigs sensitised to trimellitic anhydride

Trimellitic anhydride is a cause of occupational asthma in humans. We have previously found that tracheal instillation of trimellitic anhydride conjugated to guinea pig serum albumin induces acute bronchoconstriction and airway plasma exudation in sensitised animals, responses mediated primarily via histamine release. In the present study, neural mechanisms mediating bronchoconstriction and goblet cell secretion were determined in trimellitic anhydride-sensitised guinea pigs using the ganglionic blocker hexamethonium to eliminate efferent reflex mechanisms, pretreatment with capsaicin to eliminate afferent mechanisms, or cimetidine and mepyramine to eliminate histamine-mediated mechanisms. The magnitude of secretion of intracellular mucus from tracheal goblet cells was quantified morphometrically as a mucus score which is inversely related to the degree of discharge. Guinea pigs were injected intradermally either with 0.1 ml 0.3% trimellitic anhydride in corn oil or with corn oil alone as control. Fourteen to eighteen days later all sensitised animals had developed specific immunoglobulin (Ig) G1 antibodies whereas the controls had not. Tracheal instillation of conjugated trimellitic anhydride in anaesthetised animals significantly increased airway lung resistance (RL) 24-fold in sensitised guinea pigs (34.3 +/- 7.9 cm H2O.ml-1.s) compared with controls (1.4 +/- 0.1 cm H2O.ml-1.s). Mucus score was significantly reduced by 51% (indicating goblet cell secretion) in sensitised guinea pigs (183 +/- 22 mucus score units) compared with controls (372 +/- 41 mucus score units). The antihistamines significantly inhibited conjugated trimellitic anhydride-induced bronchoconstriction by 89%, but did not significantly affect goblet cell discharge. Hexamethonium alone did not significantly affect conjugated trimellitic anhydride-induced bronchoconstriction or goblet cell secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticosteroids increase beta 2-adrenergic receptor transcription in human lung

beta 2-Adrenergic receptors (beta 2R) are widely distributed and mediate a wide range of cellular responses in lung. Because glucocorticosteroids increase expression of beta 2R in cell lines, we have investigated the effects of glucocorticoids on the beta 2R mRNA level and the number of beta 2R in human peripheral lung in vitro. Incubation of lung tissues with dexamethasone (Dex) elevated both beta 2R mRNA level (as measured by Northern blot analysis) and beta 2R number (as measured by [125I]iodocyanopindolol binding). The increased accumulation of beta 2R mRNA could be detected at 15 min (1.27 +/- 0.1-fold) and the maximal accumulation occurred at 2 h (2.73 +/- 0.5-fold). The Dex-induced increase in beta 2R mRNA returned to the control level by 17 h. The increase in beta 2R number (1.58 +/- 0.2-fold) was slower, reaching a maximum between 17 and 24 h. Dex increased beta 2R mRNA in a time- and concentration-dependent manner that was abolished by the steroid receptor antagonist mifepristone (RU-38486 or RU-486). The stability of beta 2R mRNA was unchanged by Dex, and a nuclear run-on assay revealed that Dex approximately doubled the transcriptional rate of the beta 2R gene. These observations suggest that glucocorticoids act on steroid receptors to increase beta 2R expression by increasing the rate of beta 2R gene transcription.

Characterization and autoradiographic mapping of [3H]CP96,345, a nonpeptide selective NK1 receptor antagonist in guinea pig lung

We have studied binding and distribution of NK1 receptors in guinea pig lung using [3H]CP96,345. Kinetic studies showed that specific binding of [3H]CP96,345 was rapid and reversible, giving a kinetic dissociation constant (Kd) of 0.28 +/- 0.05 nM. The specific binding was also saturable and Scatchard analysis indicated a single class of binding site with an equilibrium Kd of 0.12 +/- 0.03 nM and maximum binding capacity (Bmax) of 107.0 +/- 10.3 fmol/mg of protein. Competition studies showed the rank order of affinity for agonists and antagonists as follows: SP > NKA = septide >> NKB = senktide; CP96,345 > FK888 > FK224 > L668169. NK3 agonists, NK2-selective antagonists, and a calcium channel blocker, diltiazem, showed no displacement, indicating high selectivity for NK1 receptors. Autoradiographic mapping showed specific labeling over airway smooth muscle from central to peripheral airways, submucosal glands, and nerve fibers of trachea. The labeling of airway epithelium was increased with diminishing size of airways. Pulmonary blood vessels were also moderately labeled and there was sparse labeling over alveolar walls. [3H]CP96,345 may provide a useful tool to evaluate NK1 receptor expression in peripheral organs.

Nitric oxide as a neurotransmitter in human airways

Human airway smooth muscle possesses a prominent nonadrenergic noncholinergic (i-NANC) bronchodilator response. Nitric oxide (NO) appears to account for all the i-NANC response in human central and peripheral airways in vitro. Furthermore, it appears that i-NANC relaxations in human trachea are associated with a concomitant selective elevation of cGMP, but not cAMP levels, which are inhibited by an NO synthase (NOS) inhibitor. This confirms the hypothesis that the L-arginine/NO/cGMP pathway is responsible for mediating the i-NANC response in this tissue. It is not certain from where the NO is formed or the location of the NOS enzyme. However, in human trachea, NOS immunoreactivity (NOS-IR) has been described in nerve fibres originating from intrinsic neurons. In addition, the density of NOS-IR is reduced from proximal to distal airways and this correlates with functional data describing a reduced i-NANC relaxation response from central to peripheral airways. The i-NANC bronchodilator nerves are the only neural relaxant pathway in human airways and, therefore, it is important to determine whether there is any defect in the ability of these nerves to function in diseased airways. In fact, functional and immunohistochemical data suggest that there may be a deficiency in NOS-IR nerves leading to a decreased i-NANC response in tissue from patients with cystic fibrosis. NOS inhibitors appear to enhance the cholinergic bronchoconstriction in human airways in vitro. Therefore, if the nitrergic innervation is dysfunctional in inflammatory conditions, its absence may lead to exaggerated bronchoconstriction.

Stimulation of guinea-pig tracheal afferent fibres by non-isosmotic and low-chloride stimuli and the effect of frusemide

1. Inhalation of low-chloride or non-isosmotic solutions evokes cough or reflex bronchoconstriction in humans that is inhibited by frusemide (furosemide), whilst capsaicin-evoked cough is unaffected. Here we have examined the responses of single vagal afferent fibres innervating the isolated guinea-pig trachea to these stimuli, and tested the effect of frusemide on fibre responses. 2. Both distilled water and hypertonic saline applied for 30 s onto identified receptive fields produced marked excitation of all A delta and C fibres tested. Isotonic glucose, a low-chloride solution, was a less potent stimulant and caused excitation in 37% of A delta fibres and 69% of C fibres. There was no difference in the distribution of low-chloride sensitive and insensitive receptive fields. 3. In the presence of frusemide, responses of A delta fibres to isotonic glucose were significantly inhibited to 34.2 +/- 6.2% of the pre-drug control level. However, frusemide was without effect either on responses of A delta fibres to distilled water or hypertonic saline, or on responses of C fibres to capsaicin. 4. These data support a role for tracheo-bronchial A delta and C fibres in airway reflexes evoked by hypotonic, hypertonic and low-chloride stimuli. The protective effect of frusemide against airway responses to low-chloride but not to non-isosmotic solutions may reflect an action on sensory nerve endings.

Suppression of human eosinophil respiratory burst and cyclic AMP hydrolysis by inhibitors of type IV phosphodiesterase: interaction with the beta adrenoceptor agonist albuterol

The cyclic AMP phosphodiesterase (PDE) III/IV inhibitor, zardaverine, and the PDE IV-selective inhibitor, rolipram, both caused concentration-dependent inhibition of opsonized zymosan-stimulated superoxide anion generation by purified human peripheral blood eosinophils with approximate IC50 values of 30 and 40 microM, respectively. In contrast, the selective PDE III inhibitor, SK&F 94120, was ineffective in suppressing this functional response at concentrations below 100 microM. The inhibitory effects of rolipram and zardaverine on superoxide anion generation were increased in the presence of the beta-2 adrenoceptor agonist, albuterol, which itself was an inhibitor of eosinophil respiratory burst (IC50 = 20 microM). The effects of albuterol and the PDE inhibitors in combination were simply additive. Paradoxically, both rolipram and zardaverine significantly potentiated albuterol-induced cyclic AMP accumulation in a synergic fashion. Cyclic AMP PDE activity of eosinophil homogenates was inhibited by both zardaverine (IC50 = 515 nM) and rolipram (IC50 = 550 nM) as well as two other PDE IV-selective inhibitors, Ro 20-1724 (IC50 = 3.0 microM) and denbufylline (IC50 = 360 nM), whereas SK&F 94120 was ineffective. These data suggest that cyclic AMP levels in human eosinophils are regulated by the action of a type IV PDE isoenzyme and that elevation of the intracellular cyclic AMP concentration by PDE IV inhibition can suppress the functional activity of these cells. However, the suppressor effect of the PDE IV inhibitors appears to be independent of that of a beta-2 adrenoceptor agonist, implying a possible adenylyl cyclase-independent mechanism of action for beta agonists in eosinophils.