Substance P-induced airway hyperreactivity is mediated by neuronal M(2) receptor dysfunction

Neuronal muscarinic (M(2)) receptors inhibit release of acetylcholine from the vagus nerves. Hyperreactivity in antigen-challenged guinea pigs is due to blockade of these M(2) autoreceptors by eosinophil major basic protein (MBP) increasing the release of acetylcholine. In vivo, substance P-induced hyperactivity is vagally mediated. Because substance P induces eosinophil degranulation, we tested whether substance P-induced hyperreactivity is mediated by release of MBP and neuronal M(2) receptor dysfunction. Pathogen-free guinea pigs were anesthetized and ventilated. Thirty minutes after intravenous administration of [Sar(9),Met(O(2))(11)]- substance P, guinea pigs were hyperreactive to vagal stimulation and M(2) receptors were dysfunctional. The depletion of inflammatory cells with cyclophosphamide or the administration of an MBP antibody or a neurokinin-1 (NK(1)) receptor antagonist (SR-140333) all prevented substance P-induced M(2) dysfunction and hyperreactivity. Intravenous heparin acutely reversed M(2) receptor dysfunction and hyperreactivity. Thus substance P releases MBP from eosinophils resident in the lungs by stimulating NK(1) receptors. Substance P-induced hyperreactivity is mediated by blockade of inhibitory neuronal M(2) receptors by MBP, resulting in increased release of acetylcholine.

Eosinophils and airway nerves in asthma

In the lungs, neuronal M2 muscarinic receptors limit the release of acetylcholine from postganglionic cholinergic nerves. However, these receptors are not functional under certain circumstances in animal models of hyperreactivity such as occurs after exposure of sensitised animals to an allergen or during a respiratory tract virus infection. This loss of M2 receptor function leads to an increase in acetylcholine release from cholinergic nerves and thus is a mechanism for the vagally mediated hyperreactivity seen in these animals. Studies in animal models of hyperreactivity have shown that eosinophils localise to the airway nerves of sensitised animals after antigen challenge. Inhibiting this localisation of eosinophils either with an antibody to the eosinophil survival cytokine IL-5 or the eosinophil adhesion molecule VLA-4 prevents loss of M2 muscarinic receptor function. It is likely that eosinophil MBP is responsible for the loss of M2 receptor function, since inhibiting eosinophil MBP with an antibody or neutralising MBP with heparin prevents this loss of function. These data are also supported by ligand binding studies where it has been shown that eosinophil MBP is an allosteric antagonist at neuronal M2 muscarinic receptors. Loss of function of lung neuronal M2 muscarinic receptors may also occur under certain circumstances in patients with asthma, although the mechanisms are not yet established.

Oral corticosteroid trials in the management of stable chronic obstructive pulmonary disease

Although recent guidelines for managing chronic obstructive pulmonary disease (COPD) recommend a trial of oral corticosteroids in the initial assessment, its prognostic value remains unclear. We prospectively studied 127 adults (64% men) with stable COPD (FEV1/FVC < 60%) over 1 year. At entry, we measured lung volumes, gas transfer factor, respiratory symptoms (by questionnaire), and peripheral blood eosinophil count. Skin-prick testing was done, and spirometry after nebulized 5 mg salbutamol and, after 2 weeks, oral prednisolone. Physician A gave all patients inhaled beclomethasone dipropionate (800 mcg/day), whereas physician B prescribed this only to those with a positive oral corticosteroid trial. At 1 year, spirometry and respiratory questionnaire were repeated, with an estimate of overall symptom severity on a visual analogue scale. Follow-up data were available in 104 (82%) patients. Of these, 32 (31%) were unresponsive to salbutamol and prednisolone; 48 (46%) were responsive to beta agonists but not to corticosteroids, and 24 (23%) responded to corticosteroids and salbutamol. Patients in all groups were comparable, except that the prednisolone responders had a higher mean eosinophil count (p < 0.001) and more were ex-smokers (p < 0.001). Only the response to oral prednisolone correlated with the change in prebronchodilator FEV1 over 1 year. Oral prednisolone responders had higher FEV1 at 1 year (p < 0.02) and significantly lower symptom scores (p < 0.02). In COPD, corticosteroid trials contribute information additional to that gained from nebulized bronchodilator reversibility testing. Patients with a positive response to a corticosteroid trial are more likely to have improved symptomatically and spirometrically at 1 year.

Antigen-induced hyperreactivity to histamine: role of the vagus nerves and eosinophils

M2 muscarinic receptors limit acetylcholine release from the pulmonary parasympathetic nerves. M2 receptors are dysfunctional in antigen-challenged guinea pigs, causing increased vagally mediated bronchoconstriction. Dysfunction of these M2 receptors is due to eosinophil major basic protein, which is an antagonist for M2 receptors. Histamine-induced bronchoconstriction is composed of a vagal reflex in addition to its direct effect on airway smooth muscle. Because hyperreactivity to histamine is seen in antigen-challenged animals, we hypothesized that hyperreactivity to histamine may be due to increased vagally mediated bronchoconstriction caused by dysfunction of M2 receptors. In anesthetized, antigen-challenged guinea pigs, histamine-induced bronchoconstriction was greater than that in control guinea pigs. After vagotomy or atropine treatment, the response to histamine in antigen-challenged animals was the same as that in control animals. In antigen-challenged animals, blockade of eosinophil influx into the airways or neutralization of eosinophil major basic protein prevented the development of hyperreactivity to histamine. Thus hyperreactivity to histamine in antigen-challenged guinea pigs is vagally mediated and dependent on eosinophil major basic protein.

Role of insulin in antigen-induced airway eosinophilia and neuronal M2 muscarinic receptor dysfunction

In the lungs, neuronal M2 muscarinic receptors limit ACh release from parasympathetic nerves. In antigen-challenged animals, eosinophil proteins block these receptors, resulting in increased ACh release and vagally mediated hyperresponsiveness. In contrast, diabetic rats are hyporesponsive and have increased M2 receptor function. Because there is a low incidence of asthma among diabetic patients, we investigated whether diabetes protects neuronal M2 receptor function in antigen-challenged rats. Antigen challenge of sensitized rats decreased M2 receptor function, increased vagally mediated hyperreactivity by 75%, and caused a 10-fold increase in eosinophil accumulation around airway nerves. In antigen-challenged diabetic rats, neuronal M2 receptor function was preserved and there was no eosinophil accumulation around airway nerves. Insulin treatment of diabetic rats completely restored loss of M2 receptor function, vagally mediated hyperresponsiveness, and eosinophilia after antigen challenge. These data demonstrate that insulin is required for development of airway inflammation, loss of neuronal M2 muscarinic receptor function, and subsequent hyperresponsiveness in antigen-challenged rats and may explain decreased incidence of asthma among diabetic humans.

Pulmonary neuronal M2 muscarinic receptor function in asthma and animal models of hyperreactivity

In the lungs neuronal M2 muscarinic receptors limit acetylcholine release from postganglionic cholinergic nerves. These inhibitory M2 receptors are dysfunctional in antigen challenged guinea pigs and in humans with asthma which leads to an increase in vagally mediated hyperreactivity. In vitro, eosinophil products act as allosteric antagonists at neuronal M2 muscarinic receptors. In vivo, displacing or neutralising MBP preserves neuronal M2 muscarinic receptor function and prevents hyperreactivity. Thus, there is good evidence from animal studies that after antigen challenge pulmonary M2 muscarinic receptors become dysfunctional because MBP inhibits their function. Loss of function of pulmonary neuronal M2 muscarinic receptors has also been reported in patients with asthma, although the clinical significance of this dysfunction and the mechanisms underlying it are not yet established.

Effects of tachykinin NK1 receptor antagonists on vagal hyperreactivity and neuronal M2 muscarinic receptor function in antigen challenged guinea-pigs

1. The role of tachykinin NK1 receptors in the recruitment of eosinophils to airway nerves, loss of inhibitory neuronal M2 muscarinic receptor function and the development of vagal hyperreactivity was tested in antigen-challenged guinea-pigs. 2. In anaesthetized guinea-pigs, the muscarinic agonist, pilocarpine (1-100 microg kg(-1), i.v.), inhibited vagally induced bronchoconstriction, in control, but not in antigen-challenged guinea-pigs 24 h after antigen challenge. This indicates normal function of neuronal M2 muscarinic receptors in controls and loss of neuronal M2 receptor function in challenged guinea-pigs. Pretreatment of sensitized guinea-pigs with the NK1 receptor antagonists CP99994 (4 mg kg(-1), i.p.), SR140333 (1 mg kg(-1), s.c.) or CP96345 (15 mg kg(-1), i.p.) before antigen challenge, prevented M2 receptor dysfunction. 3. Neither administration of the NK1 antagonists after antigen challenge, nor pretreatment with an NK2 receptor antagonist, MEN10376 (5 micromol kg(-1), i.p.), before antigen challenge, prevented M2 receptor dysfunction. 4. Electrical stimulation of the vagus nerves caused a frequency-dependent (2-15 Hz, 10 V, 0.2 ms for 5 s) bronchoconstriction that was significantly increased following antigen challenge. Pretreatment with the NK1 receptor antagonists CP99994 or SR140333 before challenge prevented this increase. 5. Histamine (1-20 nmol kg(-1), i.v.) caused a dose-dependent bronchoconstriction, which was vagally mediated, and was significantly increased in antigen challenged guinea-pigs compared to controls. Pretreatment of sensitized animals with CP99994 before challenge prevented the increase in histamine-induced reactivity. 6. Bronchoalveolar lavage and histological studies showed that after antigen challenge significant numbers of eosinophils accumulated in the airways and around airway nerves. This eosinophilia was not altered by pretreatment with the NK1 receptor antagonist CP99994. 7. These data indicate that pretreatment of antigen-sensitized guinea-pigs with NK1, but not with NK2 receptor antagonists before antigen challenge prevented the development of hyperreactivity by protecting neuronal M2 receptor function. NK1 receptor antagonists do not inhibit eosinophil accumulation around airway nerves.

Pretreatment with antibody to eosinophil major basic protein prevents hyperresponsiveness by protecting neuronal M2 muscarinic receptors in antigen-challenged guinea pigs

In antigen-challenged guinea pigs there is recruitment of eosinophils into the lungs and to airway nerves, decreased function of inhibitory M2 muscarinic autoreceptors on parasympathetic nerves in the lungs, and airway hyperresponsiveness. A rabbit antibody to guinea pig eosinophil major basic protein was used to determine whether M2 muscarinic receptor dysfunction, and the subsequent hyperresponsiveness, are due to antagonism of the M2 receptor by eosinophil major basic protein. Guinea pigs were sensitized, challenged with ovalbumin and hyperresponsiveness, and M2 receptor function tested 24 h later with the muscarinic agonist pilocarpine. Antigen-challenged guinea pigs were hyperresponsive to electrical stimulation of the vagus nerves compared with controls. Likewise, loss of M2 receptor function was demonstrated since the agonist pilocarpine inhibited vagally-induced bronchoconstriction in control but not challenged animals. Pretreatment with rabbit antibody to guinea pig eosinophil major basic protein prevented hyperresponsiveness, and protected M2 receptor function in the antigen-challenged animals without inhibiting eosinophil accumulation in the lungs or around the nerves. Thus, hyperresponsiveness is a result of inhibition of neuronal M2 muscarinic receptor function by eosinophil major basic protein in antigen-challenged guinea pigs.

Localization of eosinophils to airway nerves and effect on neuronal M2 muscarinic receptor function

Neuronal M2 muscarinic receptors inhibit acetylcholine release from pulmonary parasympathetic nerves but are dysfunctional in antigen-challenged animals and asthmatics. Deletion of pulmonary eosinophils protects M2 receptor function in antigen-challenged guinea pigs. Therefore, the association of eosinophils with airway nerves was investigated. Nerve-associated eosinophils were significantly increased in challenged animals compared with controls (0.75 +/- 0.05 vs. 0.28 +/- 0.05 eosinophils/nerve). In antigen-challenged animals, eosinophil density was greatest around airway nerves, suggesting recruitment to the nerves. M2 receptor function was inversely correlated with the number of eosinophils per nerve, thus eosinophils are associated with airway nerves in antigen-challenged guinea pigs, where they impair M2 receptor function. In airways from three patients with fatal asthma, 196 of 637 eosinophils (30%) were associated with nerves, and release of eosinophil major basic protein was evident; conversely, in three control patients 1 of 11 (9%) eosinophils were in contact with nerves. Thus eosinophils and their granule proteins are also seen in association with airway nerves in patients with asthma.

Antibody to VLA-4, but not to L-selectin, protects neuronal M2 muscarinic receptors in antigen-challenged guinea pig airways

Antigen challenge of sensitized guinea pigs decreases the function of inhibitory M2 muscarinic autoreceptors on parasympathetic nerves in the lung, potentiating vagally induced bronchoconstriction. Loss of M2 receptor function is associated with the accumulation of eosinophils around airway nerves. To determine whether recruitment of eosinophils via expression of VLA-4 and L-selectin is critical for loss of M2 receptor function, guinea pigs were pretreated with monoclonal antibodies to VLA-4 (HP1/2) or L-selectin (LAM1-116). Guinea pigs were sensitized and challenged with ovalbumin, and M2 receptor function was tested. In controls, blockade of neuronal M2 muscarinic receptors by gallamine potentiated vagally induced bronchoconstriction, while in challenged animals this effect was markedly reduced, confirming M2 receptor dysfunction. Pretreatment with HP1/2, but not with LAM1-116, protected M2 receptor function in the antigen-challenged animals. HP1/2 also inhibited the development of hyperresponsiveness, and selectively inhibited accumulation of eosinophils in the lungs as measured by lavage and histology. Thus, inhibition of eosinophil influx into the lungs protects the function of M2 muscarinic receptors, and in so doing, prevents hyperresponsiveness in antigen-challenged guinea pigs.

Compliance at night with low flow oxygen therapy: a comparison of nasal cannulae and Venturi face masks

BACKGROUND

The factors that influence nocturnal compliance among patients prescribed low flow oxygen therapy were determined and tolerance of nasal cannulae and Venturi face masks compared.

METHODS

Two studies were performed: (1) a prospective study of 99 hospitalised patients prescribed low flow oxygen therapy, 49 on nasal cannulae and 50 on Venturi face mask; and (2) a prospective study of 20 separate patients with an acute respiratory disorder requiring low flow oxygen therapy who were given nasal cannulae and Venturi face mask on successive nights in random order.

RESULTS

In study 1, 49 patients dislodged their device at least once during the night. Those using Venturi face masks and those over 75 years of age had the most dislodgements. In study 2 patients also dislodged Venturi face masks more frequently (mean (SD) 2.0 (2.4)) than nasal cannulae (0.7 (1.4)). Most patients expressed a preference for nasal cannulae.

CONCLUSIONS

Nocturnal tolerance of nasal cannulae is superior to Venturi face masks, and this factor should be considered when choosing the method of oxygen delivery.

Ulcerative jejunitis: are we missing cases?

We present two cases of ulcerative jejunitis unassociated with coeliac disease. The condition is probably being underdiagnosed especially since the reduction in the investigative laparotomy.