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

Neural control of the lower airways: Role in cough and airway inflammatory disease

Airway function is under constant neurophysiological control, in order to maximize airflow and gas exchange and to protect the airways from aspiration, damage, and infection. There are multiple sensory nerve subtypes, whose disparate functions provide a wide array of sensory information into the CNS. Activation of these subtypes triggers specific reflexes, including cough and alterations in autonomic efferent control of airway smooth muscle, secretory cells, and vasculature. Importantly, every aspect of these reflex arcs can be impacted and altered by local inflammation caused by chronic lung disease such as asthma, bronchitis, and infections. Excessive and inappropriate activity in sensory and autonomic nerves within the airways is thought to contribute to the morbidity and symptoms associated with lung disease.

Anticholinergic Treatment of Watery Rhinorrhea

When the anticholinergic drug ipratropium bromide is given as a nasal spray, it can reach the glandular cholinoceptors and block the secretory response to methacholine. Nine placebo-controlled clinical trials have shown that the treatment significantly reduces watery rhinorrhea in patients with perennial rhinitis not responsive to other types of treatment. To avoid a sensation of nasal dryness as a side effect from spraying, it is important in the individual patient to match the dosage to the severity of hypersecretion. Ipratropium has no effect on sneezing or on nasal blockage.

Mechanisms of airway responses to esophageal acidification in cats

Acid in the esophagus causes airway constriction, tracheobronchial mucous secretion, and a decrease in tracheal mucociliary transport rate. This study was designed to investigate the neuropharmacological mechanisms controlling these responses. In chloralose-anesthetized cats (n = 72), we investigated the effects of vagotomy or atropine (100 μg·kg(-1)·30 min(-1) iv) on airway responses to esophageal infusion of 0.1 M PBS or 0.1 N HCl at 1 ml/min. We quantified 1) diameter of the bronchi, 2) tracheobronchial mucociliary transport rate, 3) tracheobronchial mucous secretion, and 4) mucous content of the tracheal epithelium and submucosa. We found that vagotomy or atropine blocked the airway constriction response but only atropine blocked the increase in mucous output and decrease in mucociliary transport rate caused by esophageal acidification. The mucous cells of the mucosa produced more Alcian blue- than periodic acid-Schiff (PAS)-stained mucosubstances, and the mucous cells of the submucosa produced more PAS- than Alcian blue-stained mucosubstances. Selective perfusion of the different segments of esophagus with HCl or PBS resulted in significantly greater production of PAS-stained mucus in the submucosa of the trachea adjacent to the HCl-perfused esophagus than in that adjacent to the PBS-perfused esophagus. In conclusion, airway constriction caused by esophageal acidification is mediated by a vagal cholinergic pathway, and the tracheobronchial transport response is mediated by cholinergic receptors. Acid perfusion of the esophagus selectively increases production of neutral mucosubstances of the apocrine glands by a local mechanism. We hypothesize that the airway responses to esophageal acid exposure are part of the innate, rather than acute emergency, airway defense system.

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.

Autonomic neural control of intrathoracic airways

Autonomic neural control of the intrathoracic airways aids in optimizing air flow and gas exchange. In addition, and perhaps more importantly, the autonomic nervous system contributes to host defense of the respiratory tract. These functions are accomplished by tightly regulating airway caliber, blood flow, and secretions. Although both the sympathetic and parasympathetic branches of the autonomic nervous system innervate the airways, it is the later that dominates, especially with respect to control of airway smooth muscle and secretions. Parasympathetic tone in the airways is regulated by reflex activity often initiated by activation of airway stretch receptors and polymodal nociceptors. This review discusses the preganglionic, ganglionic, and postganglionic mechanisms of airway autonomic innervation. Additionally, it provides a brief overview of how dysregulation of the airway autonomic nervous system may contribute to respiratory diseases.

Effects of Bronchial Transection and Reanastomosis on Mucociliary System

STUDY OBJECTIVES

The mechanisms involved in the impairment of mucociliary function after lung transplantation are not completely understood. The purpose of the present study was to isolate the effects of unilateral bronchial transection and reanastomosis in a rat model.

DESIGN

In situ bronchial mucociliary transport (MCT) was determined proximal and distal to the bronchial anastomosis, as well as in the right bronchus, in 48 rats classified into six groups: intact rats, and rats at 1 day, 2 days, 7 days, 15 days, and 30 days after bronchial transection and reanastomosis of the left main stem bronchus. In vitro mucus transportability and mucus contact angle were studied in another group of eight rats after 1 week of surgery.

RESULTS

Distal to the anastomosis site, left bronchus in situ MCT (mean +/- SD) was 0.26 +/- 0.19 mm/min for the intact group, and 0.11 +/- 0.13 mm/min, 0.07 +/- 0.04 mm/min, 0.03 +/- 0.04 mm/min, 0.07 +/- 0.12 mm/min, and 0.05 +/- 0.06 mm/min for 1 day, 2 days, 7 days, 15 days, and 30 days after surgery, respectively (all significantly reduced, p < 0.05). No intergroup differences were found proximal to the anastomosis (p = 0.30). When comparing the left and right bronchi, differences were detected in both distal (p < 0.0001) and proximal sides (p = 0.0001). No significant differences in mucus transportability in vitro were found (p = 0.15). Mucus contact angle of the left bronchus (52.8 +/- 20.5 degrees ) was significantly greater than that of the mucus from the right bronchus (34.4 +/- 12.9 degrees; p < 0.05).

CONCLUSIONS

We conclude that bronchial transection and reanastomosis lead to a marked impairment of MCT in distal airways, which can in part be explained by alterations in the surface properties of mucus.

Distribution of NADPH-diaphorase activity in the feline laryngeal mucosa

We studied nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase activity in the feline laryngeal mucosa using a histochemical technique in an effort to clarify the role of nitric oxide (NO) in the larynx. Many NADPH-diaphorase-positive nerve fibres were distributed around the blood vessels and the laryngeal glands. The majority of neuronal cells in the intralaryngeal ganglia were NADPH-diaphorase-positive. It is likely that NADPH-diaphorase-positive nerve fibres around the blood vessels and glands in the laryngeal mucosa originate from the intralaryngeal ganglia, and that NO regulates circulation and secretion in the larynx.

On muscarinic control of neurogenic mucus secretion in ferret trachea

1. Muscarinic receptor subtypes mediating neurogenic mucus secretion in ferret trachea were characterized in vitro and in vivo using 35SO4 as a label for secreted mucus, and the muscarinic receptor antagonists telenzepine for the M1 receptor subtype, methoctramine for the M2 subtype and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) for the M3 receptor. We also performed receptor binding and mapping studies. 2. Each muscarinic antagonist displaced [N-methyl-3H]scopolamine binding with high-affinity binding constant (KH) values of 1.9, 2.7 and 5.0 nM for telenzepine, methoctramine and 4-DAMP, respectively. Muscarinic M1 and M3 receptors localized to submucosal glands, whereas M2 receptors did not. 3. In vitro, electrical stimulation (50 V, 10 Hz, 0.5 ms for 5 min) increased 35SO4 output by 160%. Telenzepine did not inhibit the neurogenic secretory response at concentrations two-or twentyfold its KH value, nor did it inhibit secretion induced by acetylcholine (ACh). 4-DAMP inhibited neurogenic secretion by 80 and 95%, respectively, at concentrations two-and twentyfold its KH value, and also inhibited ACh-induced secretion. Methoctramine potentiated neurogenic secretion induced at 2.5 Hz (50 V, 0.5 ms for 5 min) in a dose-related (5.4-100 nM) manner with increases of 33-451% above electrically stimulated values. Methoctramine did not potentiate secretion induced at 10 Hz and did not have any effect on ACh-induced secretion. 4. In vivo, vagal stimulation (10 V, 10 Hz, 2 ms for 8 min) increased output of 35SO4 by approximately 120%. Telenzepine had no significant effect on neurogenic secretion. Methoctramine approximately doubled the stimulated response, whereas 4-DAMP abolished the stimulated secretory response. 5. We conclude that in ferret trachea, cholinergic nerve stimulation increases mucus secretion via muscarinic M3 receptors on the submucosal glands. The magnitude of the secretory response is regulated by neuronal M2 muscarinic receptors. The muscarinic M1 receptors localized to the submucosal glands do not appear to be involved with mucus secretion.

'Sensory-efferent' neural control of mucus secretion: characterization using tachykinin receptor antagonists in ferret trachea in vitro

1. We characterized the tachykinin receptor(s) mediating 'sensory-efferent' neural control of release of 35SO4-labelled macromolecules (mucus) from ferret trachea in vitro in Ussing chambers using selective tachykinin antagonists. Secretion was induced by substance P (SP), neurokinin A (NKA), capsaicin, the NK1 tachykinin receptor agonist [Sar9, Met(O2)11]substance P ([Sar9]SP), or acetylcholine (ACh), or by electrical stimulation of nerves. Antagonists used were FK888 and L-668,169, selective for the NK1 receptor, SR 48968, selective for the NK2 receptor, and FK224, a dual antagonist at NK1 and NK2 receptors. The selectivity of FK888 and SR 48968 was examined on NKA-induced contraction of ferret tracheal smooth muscle in vitro. 2. SP (1 microM) increased mucus secretion by 695% above vehicle controls. FK888 (0.1 microM-30 microM) inhibited SP-induced secretion in a dose-dependent manner, with complete inhibition at 10 microM and an IC50 of 1 microM. L-668,169 (1 microM) also completely inhibited SP-induced secretion. 3. NKA (1 microM) significantly increased mucus secretion by 271% above baseline, a response which was completely inhibited by FK888 (10 microM) or L-668,169 (microM). Secretion induced by ACh (10 microM: 317% above baseline) was not inhibited by FK888 but was inhibited by atropine. Capsaicin (10 microM)-induced secretion (456% above vehicle controls) was significantly inhibited by FK888 and by L-668,169 (111% and 103% inhibition respectively). 4. Electrical stimulation (50 V, 10 Hz, 0.5 ms, 5 min) increased mucus output above baseline (increased by 12 to 26 fold), a response blocked by tetrodotoxin (0.1 microM). FK888 (10 microM) inhibited the increase in secretion due to electrical stimulation by 47%. Atropine, propranolol and phentolamine in combination(APP) inhibited the response to electrical stimulation by 48%. The remaining NANC response, i.e. in the presence of APP, was further reduced by 66% with FK888. FK224 (10 microM) inhibited neurally evoked secretion by 73%. SR 48968 (0.1 fLM) had no effect on electrically-stimulated or [Sar9]SP-induced secretion.5. NKA (10nM- 1O microM: in the presence of DMSO control vehicle) induced tracheal smooth muscle contraction in a concentration-dependent manner with a maximal contraction of 30% of the maximal response to ACh (10 mM) and an ECm of 0.3 JAM. SR 48968 (0.1 microM in DMSO) inhibited the NKA induced contraction whereas FK888 did not. Neither antagonist had any inhibitory effect on ACh induced contraction.6. We conclude that 'sensory-efferent' neurogenic mucus secretion in ferret trachea in vitro is mediated via tachykinin NK, receptors with no involvement of NK2 receptors. Potent and selective tachykinin antagonists may have therapeutic potential in bronchial diseases such as asthma and chronic bronchitis in which neurogenic mucus hypersecretion may be aetiologically important.

Distribution of substance P immunoreactive nerve fibers in the tracheal submucosal gland of cats

Immunohistochemistry combined with electron microscopy was employed to investigate the distribution of substance P-immunoreactive (SP-IR) nerve fibers in the tracheal submucosal gland of cats. The SP-IR nerve fibers were found to form a network around the glands. Numerous varicosities were also detected within the basement membrane of the acini and secretory tubules. All the intraglandular varicosities showed close spatial contact with serous cells, mucous cells, and myoepithelial cells. Our findings suggest that substance P-induced mucus secretion from tracheal submucosal glands in cats may be caused not only by a glandular contractile response of myoepithelial cells, but also by direct stimulation to both serous and mucous cells.

Vagal control of mucus glycoconjugate secretion into the feline trachea

1. We examined the effects of frequencies and patterns of electrical stimulation of the peripheral cut ends of the vagus nerves on the release of mucus glycoconjugates into feline trachea in vivo. Mucus glycoconjugates, radiolabelled biosynthetically with [35S]sulphate and [3H]glucose, were washed from a tracheal segment in situ, and dialysed before being counted and assayed chemically by the periodic acid-Schiff (PAS) method. 2. Vagal stimulation with regular pulses (10 V, 2 ms duration) at 1, 2.25, 4.5, 9 and 18 Hz produced frequency-dependent increases in the output of mucus glycoconjugates. 3. The muscarinic agonist pilocarpine (0.1-10 microM), given intrasegmentally, produced dose-dependent increases in the output of mucus glycoconjugates. 4. Pretreatment with atropine, phentolamine and propranolol reduced but did not abolish the effects of vagal stimulation. Vagus nerve stimulation still caused frequency-dependent increases in the output of mucus glycoconjugates. 5. High frequency stimulations at 22.5 and 47.5 Hz given intermittently (1 s burst then 4 s rest), whether in the absence or presence of cholinergic and adrenergic blockade, produced similar secretory responses as the same number of pulses delivered in regular trains at 4.5 and 9.5 Hz. This suggests that neither cholinergic nor non-adrenergic, non-cholinergic (NANC) nerve mechanisms in this system are potentiated by high frequency, intermittent burst stimulation. 6. In the absence of atropine, regular vagal stimulation had a greater effect on heart rate than did the same number of pulses delivered in bursts. 7. High molecular weight glycoconjugates from secretions were taken from the void volume of a Sepharose CL-2B gel filtration column and separated further by density-gradient centrifugation. Macromolecular components were observed at two densities, a typical mucin at 1.52 g ml-1, and a high density atypical component at 1.63 g ml-1. In secretions collected during vagal stimulation, either in the absence or presence of cholinergic and adrenergic blockade, the ratio of low density to high density macromolecules was higher than in unstimulated secretions. This can be explained if both cholinergic and NANC nervous vagal mechanisms stimulate the output of typical (density = 1.52 g ml-1) mucins into the feline trachea.

Innervation of lower airways and neuropeptide effects on bronchial and vascular tone in the pig

The occurrence and distribution of peptide-containing nerve fibres [substance P (SP), calcitonin gene-related peptide (CGRP), vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine (PHI), neuropeptide Y (NPY)] and noradrenergic nerve fibres [tyrosine hydroxylase (TH)- and dopamine beta hydroxylase (DBH)-positive] in the airways of the pig were studied by means of immunohistochemistry. SP- and CGRP-immunoreactive (-IR) nerve fibres were present close to and within the lining respiratory epithelium, around blood vessels, within the tracheobronchial smooth muscle layer and around local tracheobronchial ganglion cells. The content of CGRP- and neurokinin A (NKA)-like immunoreactivity (-LI) measured by radioimmunoassay (RIA) was twice as high in the trachea compared to that in the peripheral bronchi. SP was a more potent constrictor agent than NKA on pig bronchi in vitro. CGRP had a relaxant effect on precontracted pig bronchi. On blood vessels CGRP exerted a relaxant effect that was more pronounced on pulmonary arteries than on bronchial arteries. VIP/PHI-IR fibres were seen in association with exocrine glands and in the tracheobronchial smooth muscle layer. VIP-positive nerve fibres were abundant around blood vessels in the trachea but sparse or absent around blood vessels in the peripheral bronchi. This histological finding was supported by RIA; it was shown that the content of peptides displaying VIP-like immunoreactivity (-LI) was 18 times higher in the trachea compared to peripheral bronchi. VIP was equally potent as CGRP in relaxing precontracted pig bronchi in vitro. Both bronchial and pulmonary arteries were relaxed by VIP. NPY was colocalized with VIP in tracheal periglandular nerve fibres and in nerve fibres within the tracheobronchial smooth muscle layer. NPY was also present in noradrenergic (DBH-positive) vascular nerve fibres. The content of NPY was much higher (15-fold) in the trachea compared to small bronchi. NPY caused a contraction of both pulmonary and bronchial arteries. The bronchial smooth muscle contraction to field stimulation in vitro was purely cholinergic. A noncholinergic relaxatory effect following field stimulation was observed after bronchial precontraction. Capsaicin had no effect on pig bronchi in vitro.

Neuropeptides as modulators of airway function

Many neuropeptides have recently been identified in human and animal airways. These peptides, which may coexist with classical transmitters, have potent effects on airway calibre, blood vessels and secretions, raising the possibility that they may be involved in airway diseases such as asthma. Vasoactive intestinal peptide and peptide histidine methionine have potent relaxant effects on both vascular and bronchial smooth muscle, and may be neurotransmitters of non-cholinergic vasodilatation and non-adrenergic bronchodilation. Several neuropeptides which are found in sensory nerves, such as substance P, neurokinin A and calcitonin gene-related peptide, have both direct inflammatory effects and influence inflammatory cells, and might also contribute to the pathology of asthma if released from sensory nerve endings by an axon reflex.

Neural control of airway mucus secretion

Neural mechanisms contribute to the control of secretion of mucus in the airways of a number of animal species including humans. The nerves involved are adrenergic, cholinergic and non-adrenergic, non cholinergic (NANC) and contribute to greater or lesser degrees, depending upon the species, to secretion from submucosal glands and epithelial goblet cells. Experimental studies implicate abnormalities in neural control in the pathophysiology of certain bronchial diseases in humans which are associated with mucus hypersecretion. New observations indicate a number of novel interventions with therapeutic potential for control of mucus in chronic bronchitis and asthma.

The effects of local anaesthetic agents upon mucus secretion in the feline trachea in vivo

The actions of lignocaine and tetrodotoxin (TTX) in a tracheal segment of the cat were tested on secretion of mucus macromolecules radiolabelled with 35S and 3H. Lignocaine, 4.3-43 mM, given into the segment, caused a concentration dependent increase of secretion of 3H-and 35S-labelled macromolecules. At 43 mM, lignocaine increased secretion: delta 3H = +433 +/- 191%, delta 35S = +327 +/- 34.5% (n = 8). This effect lessened over 15-45 min. Atropine (1 mg/kg) had little effect on these responses. All concentrations of lignocaine tested (4.3-43 mM) abolished the effect of vagus nerve stimulation on secretion and diminished the effect of a submaximal concentration of pilocarpine (5 microM) in the segment in a dose-dependent manner. TTX in the segment did not alter the resting secretion. At 50 microM it abolished, and at 10 microM diminished, vagal control of secretion without affecting the secretory response to pilocarpine. The study shows that lignocaine, in concentrations which block vagal control of secretion (greater than or equal to 4.3 mM), stimulates the release of mucus macromolecules. Resting secretion is unaltered by TTX, and so does not appear to be under neurogenic inhibition. Larger concentrations of lignocaine (greater than or equal to 13 mM) also diminish pilocarpine-induced secretion, whereas TTX may inhibit nervous control of mucus secretion selectively. The results suggest that clinical anaesthesia of the airways with lignocaine may stimulate mucus secretion.

Opioid inhibition of neurally mediated mucus secretion in human bronchi

Capsaicin, which induces release of neuropeptides such as substance P from sensory nerves, stimulated mucus secretion in surgically resected human bronchi in vitro. Pretreatment of the tissue with the opioid antagonist naloxone significantly enhanced secretion, possibly by blocking the inhibitory effect of opiate premedication before surgery. Capsaicin-induced mucus secretion was completely blocked by morphine, and this effect was reversed by naloxone. Thus, sensory nerve stimulation increases mucus secretion in human airways, which might contribute to the mucus hypersecretion seen after inhalation of irritants such as cigarette smoke. Secretion can be completely inhibited by opioid drugs, so they may represent a new therapeutic approach to airway hypersecretion in chronic bronchitis and asthma, in which axon reflex mechanisms have been implicated.

Effect of peptide histidine valine on cardiovascular and respiratory function in normal subjects

Non-adrenergic inhibitory nerves may have an important role in regulating airway calibre. A recently discovered peptide, peptide histidine valine, is a potent relaxer of airway smooth muscle in vitro and has been proposed as a possible neurotransmitter in this tissue. The cardiovascular and respiratory effects of graded infusions of this peptide (2.5-10 pmol kg-1 min-1) have been examined in six normal subjects in a placebo controlled, randomised double blind study. The mean (SEM) peak plasma concentration of peptide histidine valine during the highest infusion rate was 2392 (170) pmol/l, representing a 29 fold increase above the basal concentration. This was accompanied by flushing, a significant increase in heart rate of 28 (3.7) beats/min and skin temperature of 1.8 degrees (0.16 degrees) C, but no effect on systolic or diastolic blood pressure. Despite these high plasma concentrations of the peptide and the substantial tachycardia and increase in skin blood flow, there was no change in partial expiratory flow at 40% of vital capacity (Vp40) or in the airway response to inhaled histamine (geometric PD40 9.37 and 9.73 mumol during saline and peptide histidine valine infusion respectively). Although these findings provide no support for a physiological role of peptide histidine valine in controlling airway function in healthy subjects, important effects of locally released peptides in the vasoactive intestinal peptide family cannot be excluded.

Comparison of mucus flow rate, radiolabelled glycoprotein output and smooth muscle contraction in the ferret trachea in vitro

1. The concentration-response curves for rate of mucus output, labelled-glycoprotein output and smooth muscle contraction in response to methacholine, phenylephrine and salbutamol were determined in the ferret trachea in vitro. 2. The potencies of methacholine and phenylephrine are both in order: smooth muscle contraction, glycoprotein output, rate of mucus output. 3. At lower concentrations methacholine is more potent than is phenylephrine on smooth muscle contraction, glycoprotein output and rate of mucus output. 4. Concentration-response curves for salbutamol show very little change in rate of mucus output but a large increase in glycoprotein output. 5. It is concluded that the glycoprotein output induced by salbutamol may come from a source different from those induced by methacholine and phenylephrine.

Regulatory peptides in the respiratory system

Many regulatory peptides have been described in the respiratory tract of animals and humans. Some peptides (bombesin, calcitonin, calcitonin gene-related peptide) are localised to neuroendocrine cells and may have a trophic or transmitter role. Others are localised to motor nerves. Vasoactive intestinal peptide and peptide histidine isoleucine are candidates for neurotransmitters of non-adrenergic inhibitory fibres and may be cotransmitters in cholinergic nerves. These peptides may regulate airway smooth muscle tone, bronchial blood flow and airway secretions. Sensory neuropeptides (substance P, neurokinin A and B, calcitonin gene-related peptide) may contract airway smooth muscle, stimulate mucus secretion and regulate bronchial blood flow and microvascular permeability. If released by an axon reflex mechanism these peptides may be involved in the pathogenesis of asthma. Other peptides, such as galanin and neuropeptide Y, are also present but their function is not yet known.

Enkephalinase inhibitors potentiate substance P-induced secretion of 35SO4-macromolecules from ferret trachea

To study the roles of substance P (SP) and endogenous peptidases in regulating mucus secretion from ferret trachea, we measured the SP-induced release of 35SO4-labeled macromolecules after incubating segments of trachea in Ussing chambers in the presence and absence of selected inhibitors of proteolytic enzymes. Our strategy was based on the idea that if endogenous peptidases degrade SP, then inhibitors of these enzymes should potentiate SP-induced secretion. We found that tracheas of ferrets contained SP-like immunoreactivity, and that SP stimulated the release of bound 35SO4 with rapid onset and offset. Eighty-five percent of the total macromolecular radioactivity released was contained in fractions of molecular weights greater than 10(6). The response to SP was concentration-dependent and reproducible. Thiorphan potentiated the secretory response to SP in a concentration-dependent fashion and phosphoramidon potentiated SP-induced secretion, whereas other inhibitors of proteinases and peptidases were without effects. These results suggest that substance P may regulate mucus secretion in ferrets, and that enkephalinase (dipeptidyl carboxypeptidase II, EC 3.4.24.11) in the airway degrades SP in a physiologically significant fashion, and thereby regulates peptide-induced secretion.

The effect of airflow on mucus secretion into the trachea of the cat

We have investigated the effects of the passage of air and of instillation of hyperosmolal solutions in a segment of trachea of the pentobarbitone-anaesthetized cat on the release of radiolabelled mucus glycoproteins (mucins) into that segment. Ambient air passed through the segment at 11 min-1 increased the output of both 35S- and 3H-labelled mucins. It also stimulated the output of mucins measured chemically. Passage of ambient air warmed to body temperature caused a similar effect on the output of radiolabelled mucins unless the air had also been saturated with water vapour at that temperature. Instillation of cold Krebs-Henseleit solution into the tracheal segment had no effect on the release of radiolabelled mucins. The action of warmed dry air persisted after extrinsic nerves (sympathetic and parasympathetic) to the trachea had been cut. Hyperosmolal solutions placed in the tracheal segment also increased the output of both 35S- and 3H-labelled mucins. We conclude that passage of ambient air through the trachea of the cat increases mucin output. This is probably by its drying action rather than by the mechanical disturbance from the flow or by cooling. The response might occur during eupnoea but is more likely to be important during hyperpnoea such as occurs during exercise. We discuss the relevance of the response to exercise-induced asthma.

Neuropeptides degranulate serous cells of ferret tracheal glands

To determine whether serous or mucous cells in tracheal submucosal glands respond to the neuropeptides substance P (SP) and vasoactive intestinal peptide (VIP), we studied the peptide-induced changes in gland cell morphology accompanying release of 35SO4-labeled macromolecules from tracheal explants of ferrets. Explants were labeled for 1 h in medium containing 35SO4 and washed for 3.5 additional hours. Base-line secretion in the absence of drugs declined between 1.5 and 3.5 h after the pulse. Between 2.5 and 3.5 h, the average percent change in counts per minute recovered per sample period was not significantly different from zero (P greater than 0.3; n = 6). Substance P (10(-5) M) and VIP (2 X 10(-6) M) added 4 h after labeling each increased greatly the release of 35SO4-labeled macromolecules (SP, 219%; VIP, 180%) above base line. Bethanechol, a muscarinic-cholinergic agonist (10(-5) M), increased secretion by an average of 142% above base line (each effect, P less than 0.05; n = 6 each). Light and electron microscopy of the control tissues showed glands with narrow lumens and numerous secretory granules. Glands treated with SP or VIP had enlarged lumens and the serous cells were markedly degranulated. These phenomena were documented by morphometry and suggest that SP and VIP cause secretion from glands at least partially by stimulating exocytosis from serous cells.

Distribution of galanin immunoreactivity in the respiratory tract of pig, guinea pig, rat, and dog

Galanin, a newly discovered peptide isolated from porcine intestine, is known to cause contraction in rat smooth muscle preparations and to induce hyperglycaemia in dogs. By the use of radioimmunoassay and immunohistochemical techniques the concentration and distribution of galanin immunoreactivity were determined in several areas of the respiratory tract of five dogs, five guinea pigs, five rats, and two pigs. Antibodies were raised in rabbits to whole unconjugated natural porcine galanin. The highest galanin concentrations were found in the bronchus and the trachea of the dog, guinea pig, rat (2 pmol/g in each case), and pig (less than 1 pmol/g). The lowest galanin concentrations were found in the lung parenchyma. Gel chromatographic analysis in the pig showed one molecular form of galanin coeluting with the porcine galanin standard. By means of the indirect immunofluorescence technique on sections of tissues fixed in benzoquinone solution, galanin was found to be confined to nerve fibres in different regions of the respiratory tract. In the nasal mucosa of the pig nerve fibres containing galanin were distributed around seromucous glands and blood vessels and beneath the epithelium. In the trachea, bronchus, and major intrapulmonary airways of the pig, dog, and guinea pig galanin immunoreactive fibres were detected predominantly in smooth muscle, as well as around seromucous glands and in the adventitia of blood vessels. Rarely, galanin immunoreactive nerve fibres were found in the lung parenchyma. A few galanin immunoreactive ganglion cells also containing vasoactive intestinal polypeptide were found in the adventitia of the tracheobronchial wall of the pig and dog. The distribution of galanin suggests that it may have some influence on airway, vascular, and secretory functions in the mammalian respiratory tract.

Nervous control of mucin secretion into human bronchi

Pieces of ferret trachea and human bronchi were mounted in Ussing chambers and given [35S]sulphate as a radiolabelled precursor of mucous glycoproteins (mucins). The output of 35S bound to macromolecules was studied as an index of mucin secretion. In the ferret trachea, electrical field stimulation increased the rate of mucin secretion. Tetrodotoxin (10(-7) M or 10(-6) M) abolished this effect. Pilocarpine (25 microM) stimulated the output of mucins from human bronchus. Atropine (10(-5) M) abolished this effect. Electrical field stimulation of human bronchus stimulated mucin secretion. Tetrodotoxin (10(-6) M) abolished this effect. Field stimulation in the presence of either atropine (10(-5) M) or atropine with l-propranolol (10(-5) M) and phentolamine (10(-5) M) caused no stimulation of mucin secretion rate. Some bronchi were treated with noradrenaline (10(-5) M) for 1 h to allow the adrenergic nerves to take up transmitter. Even in these, atropine prevented the effect of field stimulation. We conclude that activity in cholinergic nerves can stimulate mucin secretion in the bronchi in man. Our results provide no evidence that the adrenergic nerves or non-adrenergic, non-cholinergic nerves have a direct action on bronchial secretory cells in man.

The action of dust in the airways on secretion into the trachea of the cat

The effects of inert dusts administered into cat airways on the release of radiolabelled mucus glycoproteins (mucins) from an isolated tracheal segment have been investigated. Charcoal dust or barium sulphate powder, when placed in the segment, stimulated release of both 35S- and 3H-labelled mucins. The effect of dust was not significantly reduced after denervation of the segment. Charcoal dust, when added to the inspired air, caused an increase in release of mucins labelled with both radio-isotopes. Bilateral vagotomy significantly reduced this effect, but atropine and l-propranolol administered together did not. It is concluded that inert dust can stimulate tracheal mucin output both by a local mechanism and by activation of a reflex. The types of receptor that may be involved in the reflex, and the possible role of the mucotropic effects of dust in airway clearance, are discussed.