The roles of mediators, irritants and allergens in causing mucin secretion from the trachea

The Rabbit Maxillary Sinus: A Review of the Effects of Innervation on Mucociliary Activity

The stable bone walls of the maxillary sinus of the rabbit permit undisturbed recordings of the mucociliary activity by a photoelectric technique in vivo. The distribution of peptidergic neurotransmittors in the rabbit maxillary sinus was compared with their mucociliary effects on the same preparation. Nerve fibers containing neuropeptide Y were abundant around blood vessels and glands in the lamina propria. A few nerve fibers were seen just beneath the epithelium. Injections with neuropeptide Y decreased mucociliary wave frequency, the maximum decrease was about 15% for the doses 1.0 and 5.0 μg/kg. Vasoactive intestinal polypeptide (VIP) was found in the subepithelial layer and also distributed close to glands and blood vessels in the lamina propria. Administration of VIP alone did not change the mucociliary activity. However, if the rabbits were challenged with threshold doses of the mucociliary stimulant methacholine, the response to the cholinergic agonist was potentiated by VIP. Substance P (SP)-containing nerve fibers were found in the subepithelial layer, with single varicose endings sometimes penetrating into the epithelium. Challenges with SP produced a prompt increase of the mucociliary activity (the maximum increase was about 50% above baseline level), presumably due to increased chloride flux toward the lumen. Similar effects were produced by neurokinin A, which emanates from the same precursor molecule as SP. Another sensory peptide, calcitonin gene-related peptide was without effect on the mucociliary system. Our data indicate that there is a correlation between the occurrence of nerve fibers just beneath the epithelium and mucociliary effects. A localization close to the epithelium seems a logical prerequisite for epithelial effects.

Chemical aspects of tracheal glycoproteins

The chemical characteristics of tracheal mucus obtained directly from the epithelial surface of the trachea indicate that the mucus from each animal source consists of a group of sulphated sialic acid-containing glycoproteins. Fractionation of the native glycoprotein from the cat by gel chromatography in the presence of urea and dithiothreitol suggests a value of about 3 X 10(6) for the molecular weights. The chief monosaccharide constituents are N-acetylneuraminic acid, N-acetylglucosamine, N-acetylgalactosamine, fucose and galactose. In the goose tracheal mucin, mannose is present (serum proteins being absent). Doubly labelled cat mucus, obtained by giving Na235SO4 and [3H]glucose simultaneously into the lumen of the trachea, is massively released by parasympathetic agents, e.g. pilocarpine. The resulting mucus has a high content of 35S and is derived largely from submucosal gland cells. Subsequent exposure to an irritant, ammonia, releases a low sulphation fraction, highly labelled with 3H, arising from goblet cells. Evidence supports the view that the overall mucus is composed of mixed secretions, chemically distinct, from different cellular synthesizing sites. Differential nervous stimulation of the various sites may cause far-reaching changes in the chemical and physical properties of the mucus by selective action on the secretion of one or more of the contributing glycoproteins.

Structure and function of mucus-secreting cells of cat and goose airway epithelium

In the cat and goose, studies have been undertaken to determine the ultrastructure of airway epithelia, the concentration and distribution of the secretory cells which produce respiratory tract mucus, and the histochemistry of mucins located within cells and on their luminal surfaces. By electron microscopy all the 11 cell types so far described can be found in the airways of the cat but not the goose. Both goblet cells and submucosal glands are abundant in the cat whereas the trachea of goose lacks the latter, having instead abundant goblet cells many of which form 'intraepithelial glands'. Histochemically, the goblet cells of the cat and goose are similar in that they contain mucins with a predominance of sulphate esters. A surface mucosubstance can be demonstrated which, histochemically, is similar to that described in dog and man. Interestingly, this surface layer may be sloughed in response to an inhaled irritant such as ammonia and thereby contribute to the respiratory tract mucus recovered experimentally. Incorporation into macromolecules of radioactively labelled mucin precursors is assessed by autoradiography of tissue sections, and preliminary results of experiments designed to test the response of mucus-secreting cells to airway irritation and the parasympathomimetic drug, pilocarpine, are also presented.

The uptake of radiolabelled precursors of mucus glycoconjugates by secretory tissues in the feline trachea

1. We gave one of three radiolabelled precursors of mucus glycoconjugates ([3H]proline, [3H]glucose and [35S]sulphate) into the tracheas of anaesthetized cats for 3 h. In other cats [35S]sulphate was given by intravenous injection. 2. After a further 2 h, tracheas were removed and fixed. Serial actions were cut and alternate sections were stained with Haematoxylin and Eosin or prepared as unstained autoradiographs. Points on submucosal gland and surface epithelium were chosen with a grid on photomicrographs of the stained sections. Absorbance, which is proportional to autoradiographic grain density, was estimated on corresponding points on unstained autoradiographs by flying-spot microdensitometry. 3. With [3H]proline as precursor, the grain densities were greater over surface epithelium than over submucosal gland. With [3H]glucose, grain densities were greater over the surface epithelium in three cases, equal in one and greater over submucosal gland in the last. [35S]Sulphate, given either into the tracheal segment or intravenously, yielded grain densities that were greater over the submucosal glands than over surface epithelium. 4. The areas of submucosal gland the surface epithelium were estimated by point counting and the total content of radioactivity in the two structures estimated by multiplying mean absorbance by area. Ratios of the total radiolabel in surface epithelium to that in submucosal gland were consistently high when [3H]proline was the precursor and low with [35S]sulphate, given by either route. [3H]Glucose gave intermediate ratios. 5. Secretions washed from the trachea were subjected to gel-exclusion chromatography. Washings from tracheas labelled with [3H]proline contained some molecules eluting in the void volume of a Sepharose CL-4B column (suggesting a relative molecular mass of greater than 10(6) Da), but more of the radiolabel eluted in three peaks in the partially included volume. Density gradient ultracentrifugation of the void volume material gave radiolabelled peaks at densities of approximately 1.60 and 1.50 g ml-1, consistent with glycosylated proteins, as well as less dense material (less than 1.30 g ml-1), probably proteins with little or no glycosylation. 6. We discuss the justification of using these radiolabelled precursors to give relatively selective labelling of secretory products from submucosal gland and surface epithelium.

The effects of peptides and mediators on mucus secretion rate and smooth muscle tone in the ferret trachea

The effects of a number of peptides and mediators were measured on the secretion rate of tracheal mucus and tracheal smooth muscle tone in the ferret in vitro whole trachea. The comparison of secretion rate and smooth muscle tone, measured simultaneously in the same preparation, shows that there are wide differences in sensitivity between the two systems; there appears to be no relationship between mucus volume output and smooth muscle contraction. The comparison of mucus secretion rate and glycoprotein output in other models and species to these drugs in the same concentrations indicates that there may be mucus glycoprotein output without an increase in volume output.

Synthesis of mucous glycoproteins by rabbit tracheal cells in vitro. Modulation by substratum, retinoids and cyclic AMP

One function of airway epithelium is the secretion of mucins, which comprise an important component of the mucous lining layer. We demonstrate that rabbit tracheal epithelial cells grown in primary culture incorporate [3H]glucosamine into material released into the medium which is characterized as mucin by the following criteria: high Mr, monosaccharide composition, ion-exchange behaviour different from that of glycosaminoglycans and oligosaccharides attached via N-acetylgalactosamine. The production of mucin by the cells requires growth on a substratum of collagen gel and is enhanced by retinoids in the extracellular medium. In the presence of retinoids, 8-bromo cyclic AMP and factors present in medium from 3T3 fibroblasts each further stimulate mucin production. These results indicate that an isolated epithelial-cell culture system, in the absence of nervous, mesenchymal or other tissue types, can be used to answer questions about the regulation of mucin production at the cellular level.

Lateral nasal gland secretion in the anaesthetized dog

The effects of pharmacological and nervous stimuli on the flow of secretion from the dog lateral nasal gland following catheterization are described. Drugs were injected close-arterially into the arterial supply to the nose, or intravenously. Cholinergic agonists (pilocarpine, methacholine), given intravenously (I.V.) or intra-arterially (I.A.), and stimulation of the vidian nerve produced a copious flow of secretion which was blocked by atropine. The adrenoceptor agonists phenylephrine (alpha) and salbutamol (beta 2), given I.V. or I.A., and stimulation of the vagosympathetic nerve produced a small but consistent flow of secretion. Histamine (50 micrograms), substance P (0.1 micrograms) and prostaglandin E1 (1-5 micrograms), injected I.A., produced small flows of secretion. Bradykinin (25 ng-50 micrograms), 5-hydroxytryptamine (100 ng-50 micrograms) and vasoactive intestinal peptide (VIP) (10 ng-50 micrograms) did not cause secretion. The total protein content, the composition of secretions as revealed by sodium dodecyl sulphate-polyacrylamide agarose gel electrophoresis, and changes in [Na] and [K] in relation to flow of secretion are described. Differences in ion and protein concentrations, and in protein composition, are described for vidian nerve-induced and vagosympathetically induced secretions. Electron microscopy revealed that the gland contains serous cells in the secretory region, and ducts morphologically similar to the intercalated, striated and excretory ducts of salivary glands.

Control of mucoglycoprotein output from the rabbit nose

A method for the collection of rabbit nasal washings to analyse outputs of mucous glycoproteins is described. The radiolabels sodium [35S]sulphate and [3H]glucose are bound to the glycoproteins. The release of bound 35S and 3H was enhanced by cervical sympathetic nerve stimulation. Adrenoceptor agonists (phenylephrine, dobutamine and salbutamol) given I.V. increased the output of 35S, and the last two drugs increased the output of 3H. The blocking effects of thymoxamine and propranolol on these responses are described. Pilocarpine (given I.V. or intranasally) produced large increases in 35S release; histamine had little effect. Irritants (ammonia and cigarette smoke) and diluted serum or plasma, given intranasally, produced large increases in 3H output, and sometimes enhanced 35S release. Radiolabelled nasal washings fractionated on Sepharose CL-4B gel chromatography columns formed two peaks, with most of the radioactivity in the high molecular weight (mucous glycoprotein) peak.

Effects of prostaglandins E1, E2, and F2 alpha on mucin secretion from human bronchi in vitro

The effects of three prostaglandins on the output of 35S labelled mucus glycoproteins (mucins), from explants of human bronchial tissue suspended in Ussing chambers, have been investigated. Prostaglandin F2 alpha, added to the Krebs-Henseleit solution bathing both luminal and submucosal sides of the tissue, significantly increased mucin output at concentrations of 0.1 and 1.0 micrograms/ml (0.28 and 2.8 microM), 11 preparations being used for each dose. Since prostaglandin F2 alpha has been shown to be released from human airways challenged by antigen, it may be important in the regulation of mucus secretion in these circumstances. Prostaglandins E1 and E2 had no significant effects on mucin output even at the highest concentration (1.0 micrograms/ml), though in half the tissues tested PGE2 stimulated secretion consistently. These results contrast with the findings in a previous study, which showed an inhibition of mucin output by PGE2.

Evidence for non-cholinergic, non-adrenergic nervous control of mucus secretion into the cat trachea

We have investigated the effects of electrical stimulation of the vagus nerves on the output of mucus glycoproteins (mucins), radiolabelled with 3H and 35S, into the trachea of anaesthetized cats. In five control experiments, stimulation of the vagus nerves on four successive occasions, separated by 1 h, caused significant rises in the output of radiolabelled mucins. In these experiments repetition of stimulation did not appear to lessen the response. In a parallel series of five experiments the vagus nerves were again stimulated on four occasions, but atropine was administered in increasing doses between the stimuli. Large responses, not significantly less than those seen in the corresponding control stimulations, were seen even in the presence of the highest dose of atropine. In this series of experiments, however, the effect of the last vagal stimulation (with the highest dose of atropine) was significantly less then the first (no atropine). Administration of phentolamine and l-propranolol in addition to atropine failed to reduce the response to vagal stimulation significantly. We conclude that, while cholinergic nerves can probably explain part of the increase in mucin output which occurs with vagus nerve stimulation, there is a large response mediated by a non-cholinergic, non-adrenergic neurotransmitter. Possible neurotransmitters and the relationship of these findings to those of earlier studies are discussed.

Control and modulation of airway epithelial cells and their secretions

Information on the control and modulation of airway epithelial cells and their secretion is obtained by three techniques: 1) in vivo studies of animal models of disease, 2) in vitro studies by organ culture of human and animal model airways, and 3) chemical analysis of human and animal bronchial secretion. The contribution of each of these techniques is described in this paper, including recent or new information. In vivo models of mucous hypersecretion can be produced by irritants, infection, and drugs more quickly than previously expected. In the rat, beta 1 and beta 2 receptors are present with evidence of different activity in various airway regions. Organ culture studies combine autoradiographic analysis of cell activity with chemical analysis of secretory product, and describe inhibitory effect of new agents such as VIP. The application of density-gradient ultracentrifugation gives total recovery of undegraded macromolecules from bronchial mucus; it is now possible to recover mucous glycoprotein of molecular weight larger than that previously isolated. The organ culture studies and density-gradient ultracentrifugation studies indicate that a proteoglycan is a significant constituent of total bronchial secretion. Differences between diseases are emerging in the macromolecular partitioning between sol and gel obtained at 160,000 X g, a higher speed than that previously applied systematically in such studies.

Directional Ca2+ effect on stimulation of mucin secretion from chicken trachea in vitro

Chicken tracheal mucosa in vitro transported and incorporated radioactive precursors into mucins, which were secreted at a steady rate into the tracheal lumen. Secretion of mucins labelled with (35)S and (3)H after pulse-labelling of the mucosal layer with Na(2) (35)SO(4) and d-[1-(3)H]glucosamine as precursors was an energy-dependent process, as it was strongly inhibited by the action of respiratory-chain inhibitors, an uncoupler of oxidative phosphorylation, a metabolic blocker and a temperature shift from 41 degrees C to 5 degrees C. On the other hand, both cholinergic and parasympathomimetic agents considerably increased the secretion of dual-radiolabelled mucins when applied on the submucosal side of the trachea. The effect of Ca(2+) was directional, since only high submucosal (3.6 or 18mm) or low luminal (zero or 0.18mm) Ca(2+) massively enhanced the secretion of radiolabelled mucin compared with the mucin output measured under physiological Ca(2+) conditions (1.8mm). Whereas application of ionophore A23187 on either side of the trachea significantly increased mucin output, its presence in the appropriate tracheal compartment and under appropriate Ca(2+) conditions further accentuated the output of radiolabelled mucins. Addition of acetylcholine under appropriate conditions also had an additive effect on the Ca(2+)-stimulated secretion of mucins. Ca(2+) stimulation of mucin secretion appears to be dependent on the metabolic integrity of the mucosal cells. Mucins secreted in response to high submucosal and low luminal [Ca(2+)] appear to consist of a number of different types of glycoproteins, as judged from their ion-exchange-chromatographic behaviour.

The effect of leukotriene C4 on mucin release into the cat trachea in vivo and in vitro

We have tested the effect of leukotriene C4 (LTC4, 6 X 10(-8) to 6 X 10(-5) M) on the output of radiolabelled mucins from the trachea of the anaesthetized cat. Doses between 6 X 10(-7) M and 6 X 10(-5) M stimulated mucin release. FPL 55712 (9.5 X 10(-6) M) partially antagonized the effect of the highest dose of LTC4. Tests of LTC4 (6 X 10(-8) to 6 X 10(-6) M) on cat trachea in vitro failed to show any effect on mucin secretion. We conclude that leukotrienes may be one of the mediators of mucus secretion into the inflamed airway.