Parainfluenza virus type 1 reduces the affinity of agonists for muscarinic receptors in guinea-pig lung and heart

Treatment with LPS plus INF-γ induces the expression and function of muscarinic acetylcholine receptors, modulating NIH3T3 cell proliferation: participation of NOS and COX

BACKGROUND AND PURPOSE

LPS and IFN-γ are potent stimuli of inflammation, a process in which fibroblasts are frequently involved. We analysed the effect of treatment with LPS plus IFN-γ on the expression and function of muscarinic acetylcholine receptors in NIH3T3 fibroblasts with regards to proliferation of these cells. We also investigated the participation of NOS and COX, and the role of NF-κB in this process.

EXPERIMENTAL APPROACH

NIH3T3 cells were treated with LPS (10 ng·mL(-1)) plus IFN-γ (0.5 ng·mL(-1)) for 72 h (iNIH3T3 cells). Cell proliferation was evaluated with MTT and protein expression by Western blot analysis. NOS and COX activities were measured by the Griess method and radioimmunoassay respectively.

KEY RESULTS

The cholinoceptor agonist carbachol was more effective at stimulating proliferation in iNIH3T3 than in NIH3T3 cells, probably due to the de novo induction of M3 and M5 muscarinic receptors independently of NF-κB activation. iNIH3T3 cells produced higher amounts of NO and PGE2 than NIH3T3 cells, concomitantly with an up-regulation of NOS1 and COX-2, and with the de novo induction of NOS2/3 in inflamed cells. We also found a positive feedback between NOS and COX that could potentiate inflammation.

CONCLUSIONS AND IMPLICATIONS

Inflammation induced the expression of muscarinic receptors and, therefore,stimulated carbachol-induced proliferation of fibroblasts. Inflammation also up-regulated the expression of NOS and COX-2, thus potentiating the effect of carbachol on NO and PGE2 production. A positive crosstalk between NOS and COX triggered by carbachol in inflamed cells points to muscarinic receptors as potential therapeutic targets in inflammation.

The Use of Antiallergic and Antiasthmatic Drugs in Viral Infections of the Upper Respiratory Tract

Despite their frequency, upper respiratory tract infections (URTIs) constitute an area with few, if any, effective treatment remedies. Asthma and airway allergies share similar pathogenetic mechanisms to URTIs and it is not surprising, therefore, that agents used to treat allergic disorders have also been studied in URTIs. Their possible effects, limitations and hypothetical modes of action in URTIs are reviewed. In controlled clinical trials of satisfactory scientific standard, symptom reductions in both experimental rhinovirus infections and natural colds have occurred with topical anticholinergics, oral antihistamines and topical chromones. Future treatment alternatives for URTIs may include the intranasal anticholinergic ipratropium bromide, new nonsedating antihistamines and sodium cromoglycate (cromolyn sodium). The latter has a record of safety and an absence of adverse effects that would make it an attractive alternative for this common but not particularly serious condition in otherwise healthy individuals.

Role of parasympathetic nerves and muscarinic receptors in allergy and asthma

Parasympathetic nerves control the symptoms and inflammation of allergic diseases primarily by signaling through peripheral muscarinic receptors. Parasympathetic signaling targets classic effector tissues such as airway smooth muscle and secretory glands and mediates acute symptoms of allergic disease such as airway narrowing and increased mucus secretion. In addition, parasympathetic signaling modulates inflammatory cells and non-neuronal resident cell types such as fibroblasts and smooth muscle contributing to chronic allergic inflammation and tissue remodeling. Importantly, muscarinic antagonists are experiencing a rebirth for the treatment of asthma and may be useful for treating other allergic diseases.

Muscarinic receptor antagonists: effects on pulmonary function

In healthy lungs, muscarinic receptors control smooth muscle tone, mucus secretion, vasodilation, and inflammation. In chronic obstructive pulmonary disease (COPD) and asthma, cholinergic mechanisms contribute to increased bronchoconstriction and mucus secretion that limit airflow. This chapter reviews neuronal and nonneuronal sources of acetylcholine in the lung and the expression and role of M₁, M₂, and M₃ muscarinic receptor subtypes in lung physiology. It also discusses the evidence for and against the role of parasympathetic nerves in asthma, and the current use and therapeutic potential of muscarinic receptor antagonists in COPD and asthma.

Eosinophil-Mediated Cholinergic Nerve Remodeling

Eosinophils are observed to localize to cholinergic nerves in a variety of inflammatory conditions such as asthma, rhinitis, eosinophilic gastroenteritis, and inflammatory bowel disease, where they are also responsible for the induction of cell signaling. We hypothesized that a consequence of eosinophil localization to cholinergic nerves would involve a neural remodeling process. Eosinophil co-culture with cholinergic IMR32 cells led to increased expression of the M2 muscarinic receptor, with this induction being mediated via an adhesion-dependent release of eosinophil proteins, including major basic protein and nerve growth factor. Studies on the promoter sequence of the M2 receptor indicated that this induction was initiated at a transcription start site 145 kb upstream of the gene-coding region. This promoter site contains binding sites for a variety of transcription factors including SP1, AP1, and AP2. Eosinophils also induced the expression of several cholinergic genes involved in the synthesis, storage, and metabolism of acetylcholine, including the enzymes choline acetyltransferase, vesicular acetylcholine transferase, and acetylcholinesterase. The observed eosinophil-induced changes in enzyme content were associated with a reduction in intracellular neural acetylcholine but an increase in choline content, suggesting increased acetylcholine turnover and a reduction in acetylcholinesterase activity, in turn suggesting reduced catabolism of acetylcholine. Together these data suggest that eosinophil localization to cholinergic nerves induces neural remodeling, promoting a cholinergic phenotype.

Virus-induced airway dysfunction: pathogenesis and biomechanisms

BACKGROUND

Viral respiratory tract infections cause significant morbidity and mortality. Respiratory viruses are suspected to play a role in the inception of asthma early in life. Respiratory syncytial virus (RSV) is the most common cause of infant bronchiolitis, which is associated with the development of childhood wheezing and asthma. However, it is not clear whether this association is "causal" or "circumstantial."

METHODS

Animal models have been pivotal in studying the pathophysiology of viral respiratory infections. Various approaches to assessing airway inflammation and function have been used to define the mechanisms of virus-induced airway dysfunction and to address clinically relevant questions regarding the role of RSV in wheezing and asthma after bronchiolitis.

RESULTS

Viral lower respiratory tract infections alter airway function in humans and animals. The extent and duration of the alterations may depend on the virus itself, host factors and environmental factors. Animal studies demonstrated that viral infection induces airway hyperresponsiveness and enhances this alteration in the allergen-sensitized and exposed host. This altered airway function is mediated by immune and neurogenic inflammatory mechanisms. Recent studies in mice show that neonatal RSV infection sensitizes the newborn to develop an asthma-like phenotype on reinfection, providing further opportunities to investigate the role of RSV in postbronchiolitis wheezing and asthma in this animal model.

CONCLUSIONS

Further studies are needed to fully establish the mechanisms underlying the pathophysiology of viral respiratory tract infections and to clarify their role in the inception and/or progression of chronic airway diseases such as asthma. The results of ongoing therapeutic studies promise to minimize the impact of such viral infections.

Anesthesia for the Child with an Upper Respiratory Tract Infection: Still a Dilemma?

One of the most controversial issues in pediatric anesthesia has revolved around the decision to proceed with anesthesia and surgery for the child who presents with an upper respiratory tract infection (URI). In the past, doctrine dictated that children with URIs have their surgery postponed until the child was symptom free. This practice was based on the empirically supported premise that anesthesia increased the risk of serious complications and complicated the child's postoperative course. Although recent clinical data confirm that some children with URIs are at increased risk of perioperative complications, these complications can, for the most part, be anticipated, recognized, and treated. Although the child with a URI still presents a challenge, anesthesiologists are now in a better position to make informed decisions regarding the assessment and management of these children, such that blanket cancellation has now become a thing of the past.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Muscarinic acetylcholine receptors and airway diseases

Parasympathetic nerves provide the dominant autonomic innervation of the airways. Release of acetylcholine from parasympathetic nerves activates postjunctional muscarinic receptors present on airway smooth muscle, submucosal glands, and blood vessels to cause bronchoconstriction, mucus secretion, and vasodilatation, respectively. Acetylcholine also feeds back onto prejunctional muscarinic receptors to enhance or inhibit further acetylcholine release. In asthma and chronic obstructive pulmonary disease, bronchoconstriction and mucus secretion is increased and the airways are hyperresponsive to contractile agents. These changes are due to increased parasympathetic nerve activity. The number and function of postjunctional muscarinic receptors in the airways are unchanged in animal models of asthma. Rather, it is the supply of acetylcholine to the postjunctional cells (smooth muscle and submucosal gland) that is increased. The increase in acetylcholine release occurs because prejunctional, inhibitory M(2) muscarinic receptors on the parasympathetic nerves are dysfunctional. M(2) muscarinic receptor dysfunction and subsequent airway hyperreactivity have been demonstrated to occur in animals in response to a variety of triggers, including antigen challenge, virus infection, ozone exposure, and vitamin A deficiency. In humans, there is evidence that loss of M(2) muscarinic receptor function is related to asthma. The mechanisms by which neuronal M(2) muscarinic receptor function is lost and its relevance to human airway disease are discussed in this review.

Parainfluenza viruses

Human parainfluenza viruses (HPIV) were first discovered in the late 1950s. Over the last decade, considerable knowledge about their molecular structure and function has been accumulated. This has led to significant changes in both the nomenclature and taxonomic relationships of these viruses. HPIV is genetically and antigenically divided into types 1 to 4. Further major subtypes of HPIV-4 (A and B) and subgroups/genotypes of HPIV-1 and HPIV-3 have been described. HPIV-1 to HPIV-3 are major causes of lower respiratory infections in infants, young children, the immunocompromised, the chronically ill, and the elderly. Each subtype can cause somewhat unique clinical diseases in different hosts. HPIV are enveloped and of medium size (150 to 250 nm), and their RNA genome is in the negative sense. These viruses belong to the Paramyxoviridae family, one of the largest and most rapidly growing groups of viruses causing significant human and veterinary disease. HPIV are closely related to recently discovered megamyxoviruses (Hendra and Nipah viruses) and metapneumovirus.

Mutation screening of the muscarinic m2 and m3 receptor genes in asthmatics, outgrow subjects, and normal controls

Muscarinic receptors are important in the development of airway hyperresponsiveness. In some patients with asthma and in animal models of hyperreactivity, functional abnormalities in these receptors are suggested to contribute to disease. Here, we have screened for single nucleotide polymorphisms in the coding region of human muscarinic m2 and m3 receptor genes using direct fluorescence sequencing. DNA samples from 102 current asthmatics and 58 who had outgrown asthma ("outgrow" patients) were compared with 70 random non-asthmatic controls. A mutation characterized by a single base substitution (A1050G, Ser350Ser) was identified in the muscarinic m2 receptor gene. This polymorphism was common and was represented in all three groups studied. In contrast, in the m3 receptor coding region examined, we found a very rare nucleotide variant (C261T, Ile87Ile), identified in only one of the 230 samples genotyped. Therefore, neither A1050G in the m2 receptor nor C261T in the m3 receptor is likely to be functionally significant for airway hyperresponsiveness in asthma. Our data suggest that both the m2 and m3 receptor genes are highly conserved, and no significant genetic mutations are related to their possible functional changes in human asthma.

Upper airway infection and pediatric anesthesia: how is the evidence based?

Anesthesia for the child with an upper respiratory infection remains one of the most common, yet contentious, issues facing the pediatric anesthesiologist. A general lack of evidence-based research has led to disparities in the manner in which children with upper respiratory infections have been traditionally managed. More recent research, however, suggests that children with uncomplicated infections can be managed safely, given that most complications can be anticipated, recognized, and treated. This review summarizes the evolving literature regarding cancellation of surgery for the child with an upper respiratory infection, perioperative outcomes, and anesthetic management.

Mutation screening of the muscarinic M(2) and M(3) receptor genes in normal and asthmatic subjects

1. Muscarinic receptors are important in the development of airway hyperresponsiveness, and dysfunction of these receptors has been suggested to be present in asthma. 2. The human muscarinic M(2) and M(3) receptor genes were screened for polymorphic variation using single-stranded conformation polymorphism (SSCP) analysis, complemented by direct fluorescent sequencing. Forty-six random DNA samples and 46 respiratory physician diagnosed asthmatic samples were used as a template for analysis. 3. Within the muscarinic M(2) receptor gene, we identified two degenerate single base substitutions (1197T-->C, Thr-->Thr and 976A-->C, Arg-->Arg) in one random and one asthmatic sample respectively. Analysis of the 3' UTR region revealed an additional 'A' at bp 1793 (c.f. ATG). This was present in all of 49 samples analysed by sequencing or BsmI digest, suggesting that the published sequence (GenBank Accession NO: M16404) is incorrect. A common 3' UTR polymorphism (T-->A) was found at bp 1696 (c.f. ATG) (allelic frequency=65%, n=60), but this does not alter transcription factor recognition sites. 4. We were unable to identify any polymorphic variation within the muscarinic M(3) coding region or the flanking regions investigated, using the methods described. 5. The coding regions for the human muscarinic M(2) and M(3) receptor genes are both highly conserved. These data suggest that polymorphic variation within these coding sequences is unlikely to account for inter-individual variability in response to methacholine or anticholinergic therapy. The potential functional significance of the muscarinic M(2) receptor 3' UTR polymorphism (bp 1696) remains to be determined.

Mice that overexpress Cu/Zn superoxide dismutase are resistant to allergen-induced changes in airway control

Within the respiratory epithelium of asthmatic patients, copper/zinc-containing superoxide dismutase (Cu/Zn SOD) is decreased. To address the hypothesis that lung Cu/Zn SOD protects against allergen-induced injury, wild-type and transgenic mice that overexpress human Cu/Zn SOD were either passively sensitized to ovalbumin (OVA) or actively sensitized by repeated airway exposure to OVA. Controls included nonsensitized wild-type and transgenic mice given intravenous saline or airway exposure to saline. After aerosol challenge to saline or OVA, segments of tracheal smooth muscle were obtained for in vitro analysis of neural control. In response to electrical field stimulation, wild-type sensitized mice challenged with OVA had significant increases in cholinergic reactivity. Conversely, sensitized transgenic mice challenged with OVA were resistant to changes in neural control. Stimulation of tracheal smooth muscle to elicit acetylcholine release showed that passively sensitized wild-type but not transgenic mice released more acetylcholine after OVA challenge. Function of the M(2) muscarinic autoreceptor was preserved in transgenic mice. These results demonstrate that murine airways with elevated Cu/Zn SOD were resistant to allergen-induced changes in neural control.

Effects of neurokinin receptor antagonists in virus-infected airways

We investigated the effects of a neurokinin-1 (NK(1)) receptor antagonist (SR-140333) and a NK(2) receptor antagonist (SR-48968) on airway responsiveness and on the function of neuronal M(2) muscarinic receptors, which normally inhibit vagal acetylcholine release, in guinea pigs infected with parainfluenza virus. Antagonists were given 1 h before infection and daily thereafter. Four days later, bronchoconstriction induced by either intravenous histamine (which is partly vagally mediated) or electrical stimulation of the vagus nerves was increased by viral infection compared with control. In addition, the ability of the muscarinic agonist pilocarpine to inhibit vagally induced bronchoconstriction was lost in virus-infected animals, demonstrating loss of neuronal M(2) receptor function. Macrophage influx into the lungs was inhibited by pretreatment with both antagonists. However, only the NK(1) receptor antagonist prevented M(2) receptor dysfunction and inhibited hyperresponsiveness (measured as an increase in either vagally induced or histamine-induced bronchoconstriction). Thus virus-induced M(2) receptor dysfunction and hyperresponsiveness are prevented by a NK(1) receptor antagonist, but not by a NK(2) receptor antagonist, whereas both antagonists had similar anti-inflammatory effects.

The role of respiratory viruses in acute and chronic asthma

Respiratory infections can have dual effects related to asthma. First, there is increasing evidence that severe infections with RSV and PIV in infancy can alter lung development and physiology to increase the risks of subsequent wheezing and asthma. Second, infections with common cold viruses and influenza commonly precipitate wheezing symptoms in children and adults who already have established asthma, and RV appears to be the most important virus in producing exacerbations of the disease. The principal mechanisms by which this occurs appears to be viral replication in epithelial cells, triggering a cascade of inflammation involving granulocytes, macrophages, T cells, and secreted cytokines and mediators. The inflammatory process, although essential to clear the infection, augments pre-existing airway inflammation in asthma, leading to increased airway obstruction and lower respiratory tract symptoms. Greater understanding of virus-induced changes in inflammation and corresponding changes in airway physiology may lead to new therapeutic approaches to the treatment and prevention of virus-induced airway dysfunction.

The role of viruses in development or exacerbation of atopic asthma

Respiratory viral infections in early childhood have been linked to the development of persistent wheezing and asthma. Epidemiologic data indicate that, for the majority of children, virus-induced wheezing is a self-limited condition, with no long-term consequences. For a substantial minority, however, virus-induced wheezing is associated with persistent asthma and the potential for enhanced allergic sensitization. For the most part, this subset of patients is genetically predisposed; they are atopic children in whom respiratory viral infections trigger the early development of asthma by mechanisms that have not been fully elucidated. Both inflammatory and noninflammatory mechanisms may be involved. It does not appear that viral infection per se in early life is responsible for the induction of atopic asthma. Data from animal models provide support for the concept that enhanced allergic sensitization caused by increased uptake of allergen during infection may play a critical role, as well as T-cell-mediated immune responses to viral infection, which may favor eosinophilic inflammatory responses and the development of altered airway function to inhaled methacholine. Recent advances in our understanding of the interactions between respiratory viruses and the development of reactive airway disease offer new possibilities for preventive treatment in children at risk for developing persistent wheezing and asthma exacerbation as a result of viral infection.

Interaction of viral infections with muscarinic receptors

Both in humans and in experimental animals, much of the airway hyperresponsiveness that accompanies viral infections is the result of increased reflex bronchoconstriction. The M3 muscarinic receptors on the airway smooth muscle function normally during viral infections so that the direct effects of acetylcholine on the smooth muscle are not altered. In contrast, the M2 muscarinic receptors on the vagal nerve endings, which normally inhibit acetycholine release, are markedly dysfunctional during viral infections. This leads to substantial increases in acetylcholine release and potentiated reflex bronchoconstriction. Multiple mechanisms account for virus-induced M2 receptor dysfunction. Viral neuraminidase may deglycosylate the M2 receptor, decreasing acetylcholine affinity. Furthermore, both viruses and interferon-gamma decrease M2 receptor gene expression. Finally, in atopic hosts, viral infection causes M2 receptor dysfunction by activating eosinophils, causing them to release major basic protein which binds to the M2 receptor, functioning as an endogenous antagonist.

Effects of inflammatory cells on neuronal M2 muscarinic receptor function in the lung

In the lungs, acetylcholine released from the parasympathetic nerves stimulates M3 muscarinic receptors on airway smooth muscle inducing contraction and bronchoconstriction. The amount of acetylcholine released from these nerves is limited locally by neuronal M2 muscarinic receptors. These neuronal receptors are dysfunctional in asthma and in animal models of asthma. Decreased M2 muscarinic receptor function results in increased release of acetylcholine and in airway hyperreactivity. Inflammation has long been associated with hyperreactivity and the role of inflammatory cells in loss of neuronal M2 receptor function has been examined. There are several different mechanisms for loss of neuronal M2 receptor function. These include blockade by endogenous antagonists such as eosinophil major basic protein, decreased expression of M2 receptors following infection with viruses or exposure to pro inflammatory cytokines such as gamma interferon. Finally, the affinity of acetylcholine for these receptors can be decreased by exposure to neuraminidase.

Muscarinic receptors and control of airway smooth muscle

Contraction of airway smooth muscle is mediated by M3 muscarinic receptors on the airway smooth muscle. However, there is no evidence suggesting that hyperresponsiveness results from any alterations in function of these M3 muscarinic receptors. In contrast, there is clearly increased release of the neurotransmitter acetylcholine in animal models of hyperactivity and in asthma. Release of acetylcholine is controlled by inhibitory M2 muscarinic receptors, and it appears that it is these M2 receptors that are dysfunctional in animal models of hyperresponsiveness. Allergen-induced M2 receptor dysfunction is absolutely dependent upon an influx of eosinophils into the airways. Activated eosinophils release major basic protein, which binds to M2 receptors and prevents binding of acetylcholine. Thus, the normal negative feedback control of acetylcholine release is lost, and acetylcholine release is increased. In conclusion, loss of function of inhibitory M2 muscarinic receptors on the airway parasympathetic nerves causes vagally mediated bronchoconstriction and hyperresponsiveness following antigen challenge.

Virus- and interferon-induced loss of inhibitory M2 muscarinic receptor function and gene expression in cultured airway parasympathetic neurons

Viral infections increase vagally mediated reflex bronchoconstriction. Decreased function of inhibitory M2 muscarinic receptors on the parasympathetic nerve endings is likely to contribute to increased acetylcholine release. In this study, we used cultured airway parasympathetic neurons to determine the effects of parainfluenza virus and of interferon (IFN)-gamma on acetylcholine release, inhibitory M2 receptor function, and M2 receptor gene expression. In control cultures, electrically stimulated acetylcholine release increased when the inhibitory M2 receptors were blocked using atropine (10(-)5 M) and decreased when these receptors were stimulated using methacholine (10(-)5 M). Acetylcholine release was increased by viral infection and by treatment with IFN-gamma (300 U/ml). In these cells, atropine did not further potentiate, nor did methacholine inhibit, acetylcholine release, suggesting decreased inhibitory M2 receptor function and/or expression. Using a competitive reverse transcription-polymerase chain reaction method, we demonstrated that M2 receptor gene expression was decreased by more that an order of magnitude both by virus infection and by treatment with IFN. Thus, viral infections may increase vagally mediated bronchoconstriction both by directly inhibiting M2 receptor gene expression and by causing release of IFN-gamma which inhibits M2 receptor gene expression.

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.

Influence of respiratory tract viral infection on endothelin-1-induced potentiation of cholinergic nerve-mediated contraction in mouse trachea

1. This study examined the influence of respiratory tract infection with influenza A/PR-8/34 virus on endothelin receptor-mediated modulation of contraction induced by stimulation of cholinergic nerves in mouse isolated trachea. 2. The ETB receptor-selective agonist, sarafotoxin S6c (30 nM) induced large transient contractions (118 +/- 5% Cmax, n = 13; where Cmax is the contraction induced by 10 microM carbachol) of isolated tracheal segments from control mice. The peak contractile response to 30 nM sarafotoxin S6c was significantly lower in preparations from virus-inoculated mice at day 2 (57 +/- 8% Cmax, n = 3, P < 0.05) and 4 post-inoculation (90 +/- 8% Cmax, n = 9, P < 0.05), consistent with virus-induced attentuation of the ETB receptor-effector system linked to airway smooth muscle contraction. The mean peak contraction to 30 nM sarafotoxin S6c of preparations from virus-inoculated mice at day 8 post-inoculation (94 +/- 17% Cmax, n = 4) was not significantly different from that of control. 3. Electrical field stimulation (EFS; 90 V, 0.5 ms duration, 10 s train, 0.1-30 Hz) of preparations from control and virus-inoculated mice, caused contractions that were abolished by 0.1 microM atropine or 3 microM tetrodotoxin, indicating that these responses were mediated by neuronally released acetylcholine. Sarafotoxin S6c markedly potentiated contractions induced by a standard stimulus (0.3 Hz, every 3 min) in tracheal segments from control and virus-inoculated mice. In tracheal tissue from control mice, 30 nM sarafotoxin S6c significantly increased a standard EFS-induced contraction of 24 +/- 4% Cmax by a further 24 +/- 3% Cmax (i.e. 2 fold increase, n = 11). Sarafotoxin S6c (30 nM) also markedly potentiated standard EFS-induced contractions in preparations from virus-inoculated mice at day 2 (17 +/- 2% Cmax, n = 3), day 4 (17 +/- 5% Cmax, n = 9) and day 8 (26 +/- 5% Cmax, n = 4) post-inoculation. The level of potentiation of EFS-induced contractions in preparations from virus-inoculated mice was similar to that in tissue from control mice at days, 2, 4 and 8 post-inoculation. In contrast, sarafotoxin S6c (30 nM) did not enhance contractile responses of tracheal segments from control and virus-inoculated mice to exogenously applied acetylcholine (n = 3). 4. Endothelin-1 (1 nM) caused similar potentiations of standard EFS-induced contractions in tracheal segments from control (13 +/- 2% Cmax, n = 23) and virus-inoculated mice at day 2 (13 +/- 1% Cmax, n = 5), day 4 (16 +/- 5% Cmax, n = 6), and day 8 (13 +/- 3% Cmax, n = 8) post-inoculation. In contrast, 1 nM endothelin-1 did not enhance contractile responses of tracheal segments from control and virus-inoculated mice to exogenously applied acetylcholine (n = 4). Neither the ETA receptor-selective antagonist, BQ-123 (3 microM) nor the ETB receptor-selective antagonist, BQ-788 (1 microM) alone had any significant inhibitory effect on endothelin-1-induced potentiations of tracheal segments from control or virus-inoculated mice at days 2, 4 and 8 post-inoculation. However, simultaneous pre-incubation with BQ-123 (3 microM) and BQ-788 (1 microM) prevented endothelin-1-evoked potentiations, indicative of a role for both ETA and ETB receptors in this system. 5. These data clearly demonstrate that respiratory tract viral infection attenuated the function of the postjunctional ETB receptor-effector system linked directly to airway smooth muscle contraction. However, the function of prejunctional ETA and ETB receptor-effector systems linked to augmentation of cholinergic nerve-mediated airway smooth muscle contraction remained unaffected during respiratory tract viral infection in mice.

Potentiation by viral respiratory infection of ovalbumin-induced guinea-pig tracheal hyperresponsiveness: role for tachykinins

1. We investigated whether virus-induced airway hyperresponsiveness in guinea-pigs could be modulated by pretreatment with capsaicin and whether viral respiratory infections could potentiate ovalbumin-aerosol-induced tracheal hyperresponsiveness. 2. Animals were inoculated intratracheally with bovine parainfluenza-3 virus or control medium 7 days after treatment with capsaicin (50 mg kg-1, s.c.). Four days after inoculation, tracheal contractions were measured to increasing concentrations of substance P, histamine and the cholinoceptor agonist, arecoline. 3. In tracheae from virus-infected guinea-pigs, contractions in response to substance P, histamine and arecoline were significantly enhanced (P < 0.01) by 144%, 46% and 77%, respectively. Capsaicin pretreatment inhibited the hyperresponsiveness to substance P partly (62%) and to histamine and arecoline completely. 4. In another series of experiments animals were first sensitized with ovalbumin (20 mg kg-1, i.p.). After 14 days animals were exposed to either saline or ovalbumin aerosols for 8 days. After 4 aerosol exposures (4 days) animals were inoculated with either parainfluenza-3 virus or control medium. One day after the last ovalbumin aerosol, tracheal contraction in response to increasing concentrations of substance P, histamine and arecoline was measured. 5. Tracheae from ovalbumin-aerosol-exposed control inoculated animals showed a similar degree of airway hyperresponsiveness to saline-aerosol-exposed virus-treated guinea-pigs. Virus inoculation of ovalbumin-treated animals significantly potentiated the tracheal contractions to substance P compared to either of the treatments alone. The contractions in response to histamine and arecoline were only slightly enhanced. 6. In conclusion, sensory nerves and/or tachykinins are involved in virus-induced airway hyperresponsivenessin guinea-pigs and viral respiratory infections can potentiate the increase in tracheal responsiveness to bronchoconstrictor agonists after ovalbumin exposure.

Prejunctional muscarinic autoreceptors on horse airway cholinergic nerves

Muscarinic autoreceptors on horse airway cholinergic nerves were studied by examining the effects of muscarinic receptor antagonists on electrical field stimulation (EFS)-induced acetylcholine (ACh) release in trachealis preparations. All the antagonists including atropine (non-selective), pirenzepine (M1-selective), AF-DX 116 (M2-selective), and hexahydrosiladifenidol (M3-selective) augmented ACh release concentration-dependently. The augmentation was not due to displacement of ACh molecules from tissue receptors into the bath liquid because incubation with atropine after EFS had no influence on the measured amount of ACh. Hexahydrosiladifenidol was more potent in inhibiting ACh-induced muscle contraction, which is known to be mediated by M3 receptors, than in augmenting ACh release. The maximal ACh release rate in response to the selective antagonists was much less than that following atropine. Furthermore, the concentrations of the selective antagonists required to augment ACh release far exceeded their KdS for M1, M2, or M3 receptors. These observations suggest that the muscarinic autoreceptors on horse airway cholinergic nerves may belong to a novel subtype.

ETB but not ETA receptor-mediated contractions to endothelin-1 attenuated by respiratory tract viral infection in mouse airways

1. The current study investigated the effects of respiratory tract viral infection on the density of ETA and ETB receptors in murine tracheal smooth muscle and on the contractile response to endothelin-1 mediated by these receptors. 2. Quantitative autoradiographic studies using [125I]-endothelin-1 revealed that tracheal smooth muscle from control mice contained ETA and ETB receptors in the ratio of 42%:58% (+/- 4%, n = 10 mice), respectively. In contrast, tracheal smooth muscle obtained from mice 2 days post-inoculation with Influenza A/PR-8/34 virus contained 23 +/- 2% fewer receptors for [125I]-endothelin-1 (n = 10, P < 0.01). This reflected a selective reduction in ETB receptor density and a change in the ratio of ETA and ETB receptors to 77%:23% (+/- 5%, n = 10 mice), respectively. 3. The ETB receptor-selective agonist, sarafotoxin S6c, was a potent spasmogen of murine isolated tracheal smooth muscle and the EC50 for contraction was similar in preparations from control (3.6 nM [95% confidence limits, 2.7-4.8 nM], n = 16 preparations from 8 mice) and virus-inoculated mice (3.0 nM [2.4-3.7 nM], n = 16 preparations from 8 mice). However, the maximum contractions induced by sarafotoxin S6c (100 nM) in the preparations from virus-inoculated mice (37 +/- 5% Cmax, where 100% Cmax was the response to 10 microM carbachol) were significantly smaller than those from control mice (85 +/- 4% Cmax, P < 0.01). 4. Contractions induced by endothelin-1 in tracheal smooth muscle preparations obtained from virus inoculated mice (EC50 for contraction, 6.5 nM [95% confidence limits, 2.7-16 nM]; maximum contraction,112 +/- 5% Cm.; n = 4) were similar to those induced by endothelin-1 in control preparations (ECm9.3 nM (4.2-21); maximum contraction, 110 +/- 3% Cmax; n = 4). Endothelin-1-induced contractions in control preparations were only marginally inhibited by the ETA receptor-selective antagonist BQ-123 (in the presence of 3 micro M BQ-123; EC50 for contraction, 5.9 nM [4.1-8.5]; maximum contraction, 82 +/- 4%Cmax; n = 4). In contrast, 3 microM BQ-123 caused a 50 fold rightward shift (17-160, n =4) of the concentration-effect curve to endothlin-1 in preparations obtained from virus-inoculated mice (measured at the 30% Cmax level of contraction).5. Tracheal smooth muscle preparations exposed to 100 nM sarafotoxin S6c for 30 min (followed by a 30 min washout period) did not contract to subsequently administered sarafotoxin S6c (1-100 nM;n = 8), but contracted normally in response to endothelin-1 (EC50 6.5 nM (2.3-18); maximum contraction,109 +/- 2% Cmax; n = 4). Endothelin-l-induced contractions in these ETB receptor desensitized preparations were markedly inhibited by 3 microM BQ-123, irrespective of whether the preparations were obtained from control (63 fold shift (10-400) at the 30% Cma. level of contraction, n = 4) or virus inoculated mice (46 fold shift (18-120), n = 4).6. In summary, tracheal smooth muscle obtained from mice infected with a respiratory tract virus,Influenza A/PR-8/34 had a reduced density of ETB receptors and an attenuated ETB receptor-mediated contractile response to sarafotoxin S6c and endothelin-1. Virus-inoculation was also associated with a modest increase in tracheal smooth muscle ETA receptor density, although no significant change in ETA receptor-mediated contractile activity was seen. Thus, virus infection in murine airways produced profound alterations in endothelin receptor density, some of which were associated with changes in receptor-mediated contractile activity.

The effect of allosteric antagonists in modulating muscarinic M2-receptor function in guinea-pig isolated trachea

1. We have assessed the influence of a range of synthetic cationic polypeptides with putative inhibitory actions at prejunctional muscarinic M2-receptors on electrical field stimulation-induced contraction of guinea-pig isolated tracheal preparations. Electrical field stimulation of epithelium-denuded guinea-pig trachea resulted in frequency-dependent contractile responses. As expected, tracheal smooth muscle sensitivity to electrical field stimulation was increased in tissues pretreated with the muscarinic M2-receptor antagonist, gallamine. In contrast, gallamine did not significantly alter the contractile potency to acetylcholine. 2. Unlike gallamine, the synthetic cationic polypeptides, poly-L-arginine, poly-L-lysine, poly-D-lysine, the cationic dye ruthenium red and the anionic polysaccharide, heparin, failed to increase significantly tracheal smooth muscle sensitivity to electrical field stimulation. 3. Poly-L-arginine, ruthenium red and heparin had no effect on the contractile response to exogenously applied methacholine. 4. These data are consistent with the concept that in guinea-pig tracheal smooth muscle, gallamine is an allosteric antagonist of guinea-pig tracheal muscarinic M2-receptors, whereas the various cationic polypeptides and the polyanion, heparin, are not.

The effect of leukocyte depletion on pulmonary M2 muscarinic receptor function in parainfluenza virus-infected guinea-pigs

1. Parainfluenza infections of the airways cause dysfunction of inhibitory M2 muscarinic receptors on the pulmonary parasympathetic nerves. To distinguish the direct effects of virus from the effects of virus-induced airway inflammation on M2 muscarinic receptor function, guinea-pigs were depleted of leukocytes by pretreating with cyclophosphamide (30 mg kg-1, i.p. daily for 7 days) after which they were infected with parainfluenza virus type 1 (Sendai virus). 2. Guinea-pigs were anaesthetized, tracheotomized, and ventilated. The vagus nerves were isolated and cut, and the distal ends were electrically stimulated causing bronchoconstriction. In control animals, pilocarpine (1-100 micrograms kg-1, i.v.) inhibited and gallamine (0.1-10 mg kg-1, i.v.) potentiated vagally-induced bronchoconstriction by stimulating or blocking M2 muscarinic receptors on the vagus. These effects of pilocarpine and gallamine were almost completely lost in virus-infected animals, demonstrating loss of M2 receptor function. 3. Cyclophosphamide depleted peripheral blood leukocytes and inhibited the virus-induced influx of inflammatory cells into the lung. Depletion of leukocytes protected M2 receptor function from viral infection in some, but not all, guinea-pigs tested. 4. Among infected animals that had been depleted of leukocytes, the viral content (expressed as the log of the number of tissue culture infectious doses per g lung tissue) of those that retained normal M2 receptor function was 4.29 +/- 0.05 (mean +/- s.e. mean), while the viral content of those that lost M2 receptor function despite leukocyte depletion was 5.45 +/- 0.20 (P = 0.011). Thus the viral content of the lungs in which M2 receptor function was lost was 16 times greater than that of the lungs in which M2 receptor function was preserved. Viral content correlated with the inhibition of vagally-mediated bronchoconstriction after the maximum dose of pilocarpine (100 Microg kg-1; r2 = 0.81, P =0.0004).5. In antigen-challenged animals, inhibitory M2 muscarinic receptor function is restored when positively charged inflammatory cell proteins are bound and neutralized by heparin. However, heparin(2000 micro kg-1, i.v.) did not reverse virus-induced loss of M2 muscarinic receptor function, even in those guinea-pigs with a lower viral titer.6. Because leukocyte depletion protected M2 muscarinic receptor function only in animals with mild viral infections, it appears that viruses have both an indirect, leukocyte-dependent effect on M2 receptors and, in animals with more severe infections, a leukocyte-independent effect on M2 receptors. The failure of heparin to restore M2 receptor function demonstrates that the leukocyte-dependent loss of M2 receptor function is not mediated by positively charged inflammatory cell proteins.

Anesthetic implications of an upper respiratory infection in children

Pediatricians and pediatric anesthesiologists are frequently confronted with the dilemma of a child scheduled for elective surgery with or recently recovered from an upper respiratory tract infection. Modifications of routine anesthetic practice may decrease but not eliminate risks of associated complications. Guidelines for the evaluation and triage of these children are presented.

Effect of inflammatory cell mediators on M2 muscarinic receptors in the lungs

Acetylcholine released from vagal nerve endings constricts airways by stimulating M3 muscarinic receptors on the airway smooth muscle. At the same time, released acetylcholine feeds back onto inhibitory M2 muscarinic autoreceptors on the nerve endings, limiting further release of acetylcholine. Loss of function of these M2 receptors increases vagally-mediated bronchoconstriction after viral airway infections, exposure to ozone, or antigen inhalation. Viral infections may decrease M2 receptor function by inducing inflammation or via direct damage to the receptors as a result of cleavage of sialic acid residues by viral neuraminidase. Inflammation appears to be critical in the loss of M2 receptor function after ozone exposure. Antigen-induced loss of M2 receptor function can be reversed acutely by administering the poly-anionic substances heparin or poly-l-glutamate, possibly by binding and neutralizing positively charged eosinophil proteins. Such positively charged eosinophil proteins, particularly major basic protein, may be acting as endogenous inhibitors at the M2 receptors, as can be demonstrated in in vitro ligand binding studies.

The impact of respiratory infections on asthma

It is apparent that the effects of viral respiratory infections on the development of airway hyperresponsiveness are multiple and interrelated and involved the production of viral specific IgE, upregulation of leukocyte inflammatory activity, enhancement of the factors involved in the generation of late phase allergic responses, altered beta-adrenergic and cholinergic nervous system activity, and damage to the airway epithelium. The summation of these effects is the development of airway inflammation rather than a direct effect on bronchial smooth muscle, per se. An understanding of this pathogenesis underscores the relative importance of anti-inflammatory rather than antimicrobial therapy in viral-induced exacerbations in asthma symptoms.

Respiratory infections and asthma

Airway responsiveness is increased during respiratory virus infections, both in subjects with asthma and without underlying pulmonary disease. Furthermore, the airway hyperresponsiveness is altered for a prolonged period of time, weeks or months after the viral illness has subsided. This article reviews the possible mechanisms of virus-associated airway hyperresponsiveness, including the complex interplay of IgE-dependent reactions, changes in autonomic nervous system function and inflammation, epithelial damage, effects of viruses on the cellular immune response, and enhanced late-phase response.

Effects of Neuraminidase on Airway Reactivity in the Guinea Pig

We investigated the effects of neuraminidase, a viral enzyme that cleaves alpha ketosidic cell-bound sialic acids, to see if it accounts for parainfluenza and influenza virus-induced airway hyperreactivity. Accordingly, Vibrio cholerae neuraminidase was administered intratracheally in guinea pigs, and airway reactivity was assessed 3 h later. Removal of sialic acid residues was evaluated by histologic studies. Airway responsiveness was determined in anesthetized, tracheotomized, and mechanically ventilated guinea pigs by exposing them to increasing concentrations of aerosolized bronchoconstrictor agents. Respiratory system conductance was measured by the occlusion method. Neuraminidase injected intratracheally did not change airway reactivity to 10(-4) to 10(-2) M acetylcholine or 10(-4) to 2.5 x 10(-3) M histamine; nor did it prevent aerosolized albuterol from inhibiting histamine-induced bronchoconstriction. Substance P (10(-6) to 5 x 10(-5) M) had no significant bronchoconstrictor effect on guinea pigs pretreated with saline or neuraminidase. In guinea pigs pretreated with aerosols of the neutral endopeptidase inhibitor phosphoramidon (10(-4) M) before the concentration curve to aerosolized substance P was recorded, neuraminidase significantly reduced substance P-induced bronchoconstriction. When bronchoconstriction was induced by the 4-11 fragment of substance P (10(-5) to 10(-2) M), which is devoid of positive charges, it did not differ significantly in guinea pigs pretreated with saline and those pretreated with neuraminidase. These results indicate that in the guinea pig, neuraminidase injected intratracheally does not induce non-specific airway hyperreactivity and may alter the binding of substance P to its receptors.

Neuraminidase reduces the number of super-high-affinity [3H]oxotremorine-M binding sites in lung

The effects of neuraminidase on the binding of the radioligand agonist [3H]oxotremorine-M ([3H]oxo-M) were investigated in lung membranes. [3H]Oxo-M labelled super-high-affinity binding sites (KD of 1.36 nM), as indicated by the very high affinity displayed by carbachol when tested in competition with 0.5 mM [3H]oxo-M. Neuraminidase reduced the number of [3H]oxo-M binding sites with no change occurring in the KD. These results suggest that the effects of neuraminidase may explain virus-induced airway hyperresponsiveness.

Evidence for the absence of a functional role for muscarinic M2 inhibitory receptors in cat trachea in vivo: contrast with in vitro results

1. The effect of the selective muscarinic M2 receptor antagonist, gallamine and the selective M2 receptor agonist, pilocarpine, on airway constriction induced by vagal stimulation was studied in anaesthetized cats. In addition, the effect of gallamine on contraction of cat isolated tracheal and bronchi preparations induced by electrical field stimulation was also investigated. 2. In in vivo experiments, extrathoracic airway constriction was measured with an electromechanical caliper that was attached to the outer surface of tracheal ring 4. Intrathoracic airway constriction was determined by measuring the changes in total lung resistance and dynamic compliance during vagal stimulation. 3. Intravenous gallamine (0.1, 1, and 10 mg kg-1) augmented the rise in total lung resistance induced by vagal stimulation in a dose- and frequency-dependent manner. At stimulation frequencies of 8 and 12 Hz the fall in dynamic compliance provoked by vagal stimulation was also significantly increased by gallamine (10 mg kg-1). Gallamine was without effect on airway constriction induced by acetylcholine. 4. Vagal stimulation at 4 Hz produced significant tracheal constriction, but the amount of constriction did not change following injection of increasing doses of gallamine. Similarly, there was no difference in tracheal constriction at any frequency of stimulation (0.5-16 Hz) when frequency-response curves before and after gallamine injection (10 mg kg-1) were compared. 5. Pilocarpine (0.01-10 micrograms kg-1, i.v.) diminished changes in total lung resistance and dynamic compliance induced by vagal stimulation, an effect that was reversed by gallamine (10 mg kg-1, i.v.). Vagally-induced tracheal constriction was not significantly affected by any dose of pilocarpine, nor was it modified by gallamine (10mg kg- ') given subsequently.6. Atropine (0.5 mgkg-') completely blocked tracheal constriction induced by vagal stimulation, indicating that the changes in tracheal ring diameter provoked by stimulation were mediated by muscarinic receptors and that intravenous drugs could reach the cervical trachealis muscle.7. In vitro tissue bath studies demonstrated a significant leftward shift of the frequency-response curve to electrical field stimulation in both tracheal strips and bronchial rings following gallamine (10-4M) administration.8. Although the functional presence of muscarinic M2 autoreceptors was demonstrated in feline isolated tracheal and bronchial preparations, a corresponding functional role was not detected in cat trachea in vivo. This was despite repeated demonstration of muscarinic M2 receptor-mediated limitation of airway constriction of intrathoracic airways in vivo.

The role of muscarinic M1 and M2 receptors in airway constriction in the cat

The role of prejunctional inhibitory and facilitatory muscarinic receptors was investigated in cats with tracheal hyperresponsiveness to vagal stimulation. Intrathoracic airway caliber (total lung resistance (RL) and dynamic compliance (Cdyn] and the diameter of tracheal ring 4 were measured during vagal stimulation and local acetylcholine (ACh) injection before and after administration of the M1 receptor antagonist pirenzepine or the M2 receptor antagonist gallamine. The responses of tracheal ring 4, RL, and Cdyn to ACh were unaltered by gallamine or pirenzepine. Changes in RL and Cdyn during vagal stimulation were enhanced by gallamine, but the magnitude of tracheal constriction was unchanged. Vagally induced tracheal constriction was decreased by pirenzepine in hyperresponsive but not in control cats. The M2 receptors limit intrathoracic airway constriction, but a functional role for M2 receptors in the cervical trachea could not be demonstrated. However, these data suggest that M1 excitatory receptors may play a role in vagally mediated tracheal hyperreactivity.

Effect of respiratory tract viral infection on murine airway beta-adrenoceptor function, distribution and density

1. The effects of a respiratory tract viral infection on beta-adrenoceptor density, distribution and function were investigated in murine airways. 2. Following intranasal inoculation of CBA/CaH mice with influenza A/PR-8/34 virus, the virus proliferated rapidly in trachea (peak titres 2 days post-inoculation) and lung (peak titres 4-6 days post-inoculation). Respiratory tract viral infection was associated with a significant increase in lung weight (88% higher than control mice at day 6 post-inoculation) that was related temporally to the development of peripheral lung inflammation and consolidation. 3. Analysis of specific binding of [125I]-cyanopindolol to beta-adrenoceptors revealed that on days 2, 4 and 8 post-inoculation with virus, mouse isolated tracheal sections contained, on average, 40% more beta-adrenoceptors than tracheal sections from time matched control mice. Subsequent quantitative autoradiographic studies demonstrated that this increase in total tracheal beta-adrenoceptors was due primarily to a 90% increase in the density of beta-adrenoceptors in the tracheal epithelium in virus-infected mice. 4. In contrast, virus-infection had no significant effect on the density of beta-adrenoceptors in tracheal airway smooth muscle, although within 2 days of inoculation with virus, mouse tracheal smooth muscle segments were approximately 2 fold less sensitive to the beta-adrenoceptor agonist, noradrenaline (mean pD2 = 6.57 +/- 0.04, n = 24) and to the adenylyl cyclase-activator forskolin (mean pD2 = 6.78 +/- 0.04, n = 12) compared to segments from control mice (mean pD2 = 6.84 +/- 0.06 for noradrenaline; mean pD2 = 7.03 +/- 0.07 for forskolin). Similar values were obtained 8 days post-inoculation. At day 2, but not day 8 post-inoculation with virus, relaxation responses to theophylline were also marginally attenuated compared with controls.5. Mouse isolated tracheal segments obtained 2 days after virus inoculation and segments from timematched control mice were equisensitive to the spasmogenic actions of the muscarinic cholinoceptor agonist, carbachol. However, tracheal segments from mice inoculated with virus were less responsive to carbachol on day 4 (mean pD2 = 6.45 + 0.04, n = 8) and day 8 (mean pD2 = 6.45 +/- 0.02, n = 12) compared to control preparations (day 4, mean pD2 = 6.73 +/- 0.06, n = 8; day 8, mean pD2= 6.65 +/- 0.04, n = 12, P < 0.05). In contrast, endothelin-l-induced contractions of tracheal smooth muscle were notaffected by virus-infection.6. These data demonstrate that respiratory tract viral infection can produce significant tissue-selective changes in airway /beta-adrenoceptor density as well as small reductions in airway smooth muscle muscarinic cholinoceptor and /beta-adrenoceptor function.

Parainfluenza virus infection damages inhibitory M2 muscarinic receptors on pulmonary parasympathetic nerves in the guinea-pig

1. The effect of viral infection on the function of neuronal M2 muscarinic autoreceptors in the lungs was studied in anaesthetized guinea-pigs. 2. Guinea-pigs were inoculated intranasally with either parainfluenza type 3 or with a vehicle control. Four days later the animals were anaesthetized, paralysed and artificially ventilated. Pulmonary inflation pressure, tidal volume, blood pressure, and heart rate were recorded. Both vagus nerves were cut and electrical stimulation of the distal portions caused bronchoconstriction (measured as an increase in pulmonary inflation pressure) and bradycardia. 3. In control animals, pilocarpine (1-100 micrograms kg-1, i.v.) attenuated vagally-induced bronchoconstriction by stimulating inhibitory M2 muscarinic receptors on parasympathetic nerves in the lungs. Conversely, blockade of these receptors with the antagonist gallamine (0.1-10 mg kg-1, i.v.) produced a marked potentiation of vagally-induced bronchoconstriction. These results confirm previous findings. 4. In guinea-pigs infected with parainfluenza virus, pilocarpine did not inhibit vagally-induced bronchoconstriction. Furthermore, gallamine did not potentiate vagally-induced bronchoconstriction to the same degree as in uninfected controls. 5. There was no increase in baseline pulmonary inflation pressure in the infected animals over the controls. Receptors on airway smooth muscle were unchanged by viral infection since large doses of pilocarpine caused equivalent bronchoconstriction in both groups of animals. Gallamine inhibited the vagally-induced fall in heart rate equally in both groups of animals indicating that virus-induced changes in M2 receptor function on pulmonary parasympathetic nerves are not part of a generalized decrease in M2 receptor function. 6. These results demonstrate that the M2 muscarinic receptor-mediated inhibition of acetylcholine release from parasympathetic nerves in the lungs is decreased in animals infected with parainfluenza virus. Loss of this inhibition would result in increased release of acetylcholine from the parasympathetic nerves and may explain virus-induced airway hyperresponsiveness.