Galanin and somatostatin inhibition of neurokinin A and B induced airway mucus secretion in the rat

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Effect of montelukast on platelet activating factor- and tachykinin induced mucus secretion in the rat

Background

Platelet activating factor and tachykinins (substance P, neurokinin A, neurokinin B) are important mediators contributing to increased airway secretion in the context of different types of respiratory diseases including acute and chronic asthma. Leukotriene receptor antagonists are recommended as add-on therapy for this disease. The cys-leukotriene-1 receptor antagonist montelukast has been used in clinical asthma therapy during the last years. Besides its inhibitory action on bronchoconstriction, only little is known about its effects on airway secretions. Therefore, the aim of this study was to evaluate the effects of montelukast on platelet activating factor- and tachykinin induced tracheal secretory activity.

Methods

The effects of montelukast on platelet activating factor- and tachykinin induced tracheal secretory activity in the rat were assessed by quantification of secreted ³⁵SO₄ labelled mucus macromolecules using the modified Ussing chamber technique.

Results

Platelet activating factor potently stimulated airway secretion, which was completely inhibited by the platelet activating factor receptor antagonist WEB 2086 and montelukast. In contrast, montelukast had no effect on tachykinin induced tracheal secretory activity.

Conclusion

Cys-leukotriene-1 receptor antagonism by montelukast reverses the secretagogue properties of platelet activating factor to the same degree as the specific platelet activating factor antagonist WEB 2086 but has no influence on treacheal secretion elicited by tachykinins. These results suggest a role of montelukast in the signal transduction pathway of platelet activating factor induced secretory activity of the airways and may further explain the beneficial properties of cys-leukotriene-1 receptor antagonists.

Analysis of airway secretions in a model of sulfur dioxide induced chronic obstructive pulmonary disease (COPD)

Hypersecretion and chronic phlegm are major symptoms of chronic obstructive pulmonary disease (COPD) but animal models of COPD with a defined functional hypersecretion have not been established so far. To identify an animal model of combined morphological signs of airway inflammation and functional hypersecretion, rats were continuously exposed to different levels of sulfur dioxide (SO2, 5 ppm, 10 ppm, 20 ppm, 40 ppm, 80 ppm) for 3 (short-term) or 20–25 (long-term) days. Histology revealed a dose-dependent increase in edema formation and inflammatory cell infiltration in short-term-exposed animals. The submucosal edema was replaced by fibrosis after long-term-exposure. The basal secretory activity was only significantly increased in the 20 ppm group. Also, stimulated secretion was significantly increased only after exposure to 20 ppm. BrdU-assays and AgNOR-analysis demonstrated cellular metaplasia and glandular hypertrophy rather than hyperplasia as the underlying morphological correlate of the hypersecretion.

In summary, SO2-exposure can lead to characteristic airway remodeling and changes in mucus secretion in rats. As only long-term exposure to 20 ppm leads to a combination of hypersecretion and airway inflammation, only this mode of exposure should be used to mimic human COPD. Concentrations less or higher than 20 ppm or short term exposure do not induce the respiratory symptom of hypersecretion. The present model may be used to characterize the effects of new compounds on mucus secretion in the background of experimental COPD.

Early development of chick embryo respiratory nervous system: an immunohistochemical study

The extrinsic and intrinsic respiratory nervous systems receive specific contributions from the vagal and sympathetic components. Using specific markers for vagal and sympathetic structures, we studied the distribution patterns of immunoreactivity to galanin (GAL), pituitary adenylate cyclase-activating polypeptide-27 (PACAP) and the tachykinin substance P in extrinsic and intrinsic nerve of chick embryo respiratory system, during development from the very early age to hatching. All peptides studied appeared in the intrinsic and extrinsic nervous systems early. We found substance P in both the vagal and sympathetic systems, PACAP in vagal components alone and GAL mainly in the sympathetic system. The intrinsic nervous system showed high immunoreactivity for all peptides studied. These data accord with the well known early trophic functions that peptides have on the development of nervous networks and modulatory activity on the intrinsic nervous system. The GAL again proves to be the main peptide in chick embryo sympathetic respiratory system.

The cysteinyl-leukotriene-1 receptor antagonist zafirlukast is a potent secretagogue in rat and human airways

Cysteinyl-leukotriene-1 receptor antagonists are important tools in the therapy of asthma. Although many studies have been performed concerning their effects on airway smooth muscle tone, there are no basic data on their effects on airway secretions. Therefore, we assessed the effects of zafirlukast and montelukast on rat tracheal secretion by quantification of secreted 35S04 labelled mucus macromolecules, and determined the influence of the arachidonic acid pathway using the modified Ussing chamber technique. Zafirlukast (432+/-89.99%) and montelukast (167+/-16.74%) stimulated rat tracheal secretion. This was abolished by application of eicosatetraenoic acid, an inhibitor of the arachidonic acid metabolism. Whereas inhibition of cyclooxygenase did not show any significant effect on zafirlukast induced secretion, blockade of the 5-lipoxygenase pathway markedly reduced the secretagogue effects. Furthermore, inhibition of phosphatidylinositol-3-kinase completely inhibited the effects elicited by zafirlukast. Additional experiments revealed secretagogue effects of zafirlukast also in human bronchial tissue. In conclusion, zafirlukast is a potent inducer of tracheal secretion. Obviously, these effects are induced by involvement of a phosphatidylinositol-3-kinase dependent pathway mediated by products of the arachidonic acid metabolism.

Tachykinin NK3 and NK1 receptor activation elicits secretion from porcine airway submucosal glands

1 We presently characterized the tachykinin receptor subtypes, using tachykinin receptor agonists and selective antagonists, that induce submucosal gland fluid flux (J(G)) from porcine tracheal explants with the hillocks technique. We also investigated the effects of the tachykinin receptor agonists on the electrophysiologic parameters of the tracheal epithelium in Ussing chambers. 2 The NK(1) tachykinin receptor agonist substance P (SP, 1 microM) and the NK(3) tachykinin receptor agonist [MePhe(7)]neurokinin B ([MePhe(7)]NKB, 1 microM) induced gland fluid fluxes of 0.29+/-0.03 microl min(-1) cm(-2) (n=26) and 0.36+/-0.05 microl min(-1) cm(-2) (n=24), respectively; while the NK(2) tachykinin receptor agonist [betaAla(8)]neurokinin A (4-10) ([betaAla(8)]NKA (4-10), 1 microM) had no effect on J(G) (n=10). 3 The NK(1) receptor antagonist CP99994 (1 microM, n=9) blocked 93% of the SP-induced J(G), whereas the NK(3) receptor antagonist SB223412 (1 microM, n=12) had no effect on the SP-induced J(G). However, SB223412 (1 microM, n=9) blocked 89% of the [MePhe(7)]NKB-induced J(G) while CP99994 (1 microM, n=10) did not affect the [MePhe(7)]NKB-induced J(G). The NK(2) receptor antagonist SR48968 (1 microM) did not block the J(G) induced by either the NK(1) (n=4) or NK(3) (n=13) receptor agonists. 4 The nicotinic ganglionic acetylcholine receptor antagonist hexamethonium (1 microM) and the muscarinic acetylcholine receptor antagonist atropine (1 microM) also decreased the NK(3) receptor agonist-induced J(G) by 67% (n=10) and 71% (n=12), respectively. 5 The potential difference (PD), short-circuit current (I(SC)), and membrane resistance (R(M)) of the porcine tracheal epithelial membranes were not significantly affected by any of the neurokinin agonists or antagonists (1 microM, basolateral) used in this study, although SP and [betaAla(8)]NKA (4-10) induced a slight transient epithelial hyperpolarization. 6 These data suggest that NK(1) and NK(3) receptors induce porcine airway gland secretion by different mechanisms and that the NK(3) receptor agonists induced secretion is likely due to activation of prejunctional NK(3) receptors on parasympathetic nerves, resulting in acetylcholine-release. We conclude that tachykinin receptor antagonists may have therapeutic potential in diseases with pathophysiological mucus hypersecretion such as asthma and chronic bronchitis.

Characterization of the human somatostatin receptor type 4 promoter

Somatostatin (SRIF) exerts inhibitory effects on virtually all endocrine and exocrine secretions. Five distinct SRIF receptor subtypes (sst 1-5) have been identified. In contrast to the other subtypes, very little is known about specific functions of sst4. We investigated structure and regulation of the human sst4 gene. A genomic clone containing the 5' region of the sst4 gene was isolated. 1.5 kb of the promoter was sequenced and putative transcription factor binding sites were identified. The transcription start site was located 88 nucleotides upstream of the translation start site. A -984 sst4 promoter directed significant levels of luciferase expression in GH4 rat pituitary cells, Skut-1B endometrium cells, and BEAS-2B human bronchial epithelial cells, whereas only low activity was detected in JEG3 chorion carcinoma cells or COS-7 monkey kidney cells. A minimal -209 promoter allowed cell specific expression, its activity in COS-7 cells is not enhanced by co-transfection of the pituitary-specific transcription factor Pit-1. An enhancer element was localized between nt -459 and -984. We did not find any regulation of the sst4 promoter region analyzed by SRIF, forskolin, TPA, IGF-1, EGF, T3, glucocorticoids or 17beta-estradiol. These studies identify the 5' region of the sst4 gene. Furthermore, specific activity of the promoter in various cell lines is demonstrated.

Effects of the nitric oxide/cGMP system compared with the cAMP system on airway mucus secretion in the rat

Mucus secretion of the airways is under the control of a variety of intracellular second messenger systems. Cyclic nucleotides such as cGMP, coupled to the recently discovered nitric oxide system, and cAMP are of outstanding interest in this respect. The present study used the modified Ussing chamber technique and mucins labelled with (35)SO(4) to investigate mucus secretion in the rat trachea to clarify the contribution of these different second messenger systems to the control of mucin secretion.A variety of drugs affecting either the generation or the breakdown of the respective cyclic nucleotides were used. Neither drugs interfering with nitric oxide synthase nor the phosphodiesterase isoenzyme responsible for cGMP breakdown nor cGMP analogues were able to affect mucus secretion. In contrast, stimulation of adenylate cyclase or inhibition of the respective phosphodiesterase resulted in a potent increase of mucus secretion. In conclusion, we failed to show the involvement of the nitric oxide/cGMP system, whereas the cAMP system seems to be a very efficient regulator of mucus secretion in the rat trachea.

Effects of VIP and related peptides on airway mucus secretion from isolated rat trachea

Vasoactive intestinal polypeptide (VIP) is known as an important regulator of airway function. It has been suggested that VIP is involved in the pathogenesis of asthma due to its relaxant effects on smooth muscles. The present study was designed to characterize the effects of the peptides of the VIP family on airway mucus secretion. The peptides VIP, PHI, PACAP-27, PACAP-38, GLP-I, exendin-4, helodermin, helospectin I and helospectin II were investigated using isolated rat trachea. Data show that PACAP-27 is the most potent stimulator of airway mucus secretion (225% stimulation). The rank order of potency was PACAP-27 > VIP > helospectin II > PHI > exendin-4 = helodermin = helospectin I = PACAP-38. The addition of the protease inhibitor thiorphan enhanced the effects of PHI and helodermin, but not of the other peptides. These data show that the peptides of the VIP family stimulate airway mucus secretion differently.

Identification and pharmacological characterization of somatostatin receptors in rat lung

1. [125I]-[LTT]SRIF-28 and [125I]-SMS 201-995 were used to identify and characterize somatostatin (SRIF) receptors localized in rat lung tissue. In vitro autoradiography of rat lung tissue sections showed the existence of specific, high affinity binding sites for [125I]-[LTT]SRIF-28 without any significant specific binding of the sst2/sst5-receptor selective ligand [125I]-SMS 201-995. 2. In radioligand binding studies, specific binding of [125I]-[LTT]SRIF-28 to membranes of rat lung was linearly related to the concentration of membrane protein used with only a small portion of nonspecific binding. With [125I]-SMS 201-995 no specific binding could be observed up to a membrane concentration of 0.1 mg of protein/assay tube. 3. [125I]-[LTT]SRIF-28 bound rapidly to rat lung membranes with an apparent association rate constant (kapp) of 1.8 +/- 0.1 h-1 (n = 3). The equilibrium of specific binding was reached after an incubation period of approximately 90 min at room temperature and remained constant for the next 3 h. The association rate constant (k1) was calculated to be 3.7 x 10(10) M-1 h-1. The dissociation reaction followed first order kinetics with a dissociation rate constant (k-1) = 0.44 +/- 0.07 h-1 corresponding to a half-time of 95 +/- 15 min (n = 3). From these kinetic experiments an equilibrium dissociation constant (KD) for the binding of [125I]-[LTT]SRIF-28 was calculated to be 11.9 pM. 4. Saturation binding of [125I]-[LTT]SRIF-28 revealed an equilibrium dissociation constant (KD) of 50.1 pM (pKD = 10.3 +/- 0.1; n = 3) and a receptor density (Bmax) of 78 +/- 3 fmol mg-1 protein. A Hill coefficient not significantly different from 1 indicated saturable binding to a single class of high affinity binding sites. 5. Specific binding of [125I]-[LTT]SRIF-28 to rat lung membranes was inhibited by SRIF-14, SRIF-28 and different SRIF analogues. SRIF and different synthetic short chain SRIF analogues exhibited the following rank order of potency: SRIF-28 > SRIF-14 > CGP 23996 >> RC 160 > BIM 23014 > SMS 201- 995 > BIM 23056 > MK 678. 6. The binding affinities for SRIF and the various SRIF analogues determined using rat lung tissue were in close correlation to those obtained with Chinese hamster ovary (CHO) cells stably expressing sst, (r = 0.92) and sst4 (r = 0.95) receptors, respectively. 7. Reverse transcriptase--polymerase chain reaction (RT-PCR) showed the predominant expression of mRNA specific for sst4 receptors as well as some weak sst1 mRNA expression. 8. The findings suggest that sst4 receptor expression is the predominant form of the somatostatin receptors identified in rat lung tissue. In this study we demonstrated for the first time the existence of sst4 receptors in mammalian tissue.

Effects of selective and non-selective phosphodiesterase inhibitors on tracheal mucus secretion in the rat

The present study was designed to characterize the effects of unselective and isoenzyme-selective phosphodiesterase inhibitors on airway mucus secretion. The isolated rat trachea was incubated in a modified Ussing chamber. Mucus macromolecules were metabolically labelled with 35S. The inhibitors were applied at the luminal side. The unselective phosphodiesterase inhibitors theophylline, enprofylline and 3-isobutyl-methylxanthine stimulated mucus secretion in a concentration-dependent manner with half-maximum effects (EC50 values) at 690 microM, 400 microM and 46 microM, respectively. The adenosine antagonist 8-phenyltheophylline did not significantly stimulate mucus output, suggesting a negligible role of adenosine in the cellular mechanisms of mucus secretion. Adenosine itself did not increase radiolabel output. Rolipram, an inhibitor of phosphodiesterase isoenzyme IV, and zardaverine, which inhibits the isoenzymes III and IV, increased potently macromolecule output with EC50 values of 40 nM and 6 microM, respectively. The selective inhibitors of phosphodiesterase isoenzymes III and V, motapizone and zaprinast, did not influence airway mucus release, suggesting a relatively low activity of isoenzymes III and V in glands of rat trachea. The stimulatory effect of theophylline on airway mucus secretion may contribute to its beneficial action in chronic obstructive airway disease. Our data suggest that this effect is mediated predominantly by phosphodiesterase isoenzyme IV.

Amylin immunoreactivity in the rat trachea and characterization of the interaction of amylin and somatostatin on airway mucus secretion

Amylin is a peptide containing 37 amino acids that is mainly expressed in pancreatic B-cells and cosecreted with insulin. It is the major component of the islet amyloid typically found in non-insulin-dependent diabetes mellitus. The amylin mRNA is present in RNA isolated from lung, and amylin receptors have been detected in lung membranes. Recently, amylin was shown to be a potent stimulator of airway mucus secretion. In this study, we characterized the site of amylin expression in rat trachea using a highly specific antiserum and the functional interaction of amylin with somatostatin-14 in mucus secreting cells. Amylin-like immunoreactivity is present in epithelial cells of submucous gland acini. The expression pattern varies, since some acini showed strong staining while others were negative. In addition, some columnar cells of the tracheal lining epithelium are strongly stained. Amylin applied submucosally is a potent stimulator of airway mucus secretion. Somatostatin inhibits this effect. Amylin may influence airway mucus secretion by paracrine and endocrine mechanisms, and our data suggest that amylin and somatostatin belong to the increasing number of peptides that are known to influence airway function.