Prejunctional alpha 2 adrenoceptors inhibit contraction of tracheal smooth muscle by inhibiting cholinergic neurotransmission

Effects of selective α2 -adrenergic receptor agonists on electrical field-stimulated contractions of isolated bronchi in horses

We investigated the effects of different selective α₂ -adrenergic receptor (AR) agonists (detomidine, medetomidine, xylazine, and brimonidine) on the contractions of horse-isolated bronchi induced by electrical field stimulation (EFS) and by carbachol. No effects were observed on the contraction induced by carbachol, while α₂ -AR agonists reduced EFS-evoked contractions in a concentration-related fashion. The rank order of potency (pD₂ ) was brimonidine (7.40 ± 0.20) >medetomidine (7.09 ± 0.24) >detomidine (6.13 ± 0.55) >xylazine (4.59 ± 0.16). The maximal effects (E_max ) were -56.3% ± 6.3%, -40.4% ± 6.9%, -48.6% ± 9.9%, and -72.7% ± 12.7% for brimonidine, medetomidine, detomidine, and xylazine, respectively. Adrenergic block by guanethidine enhanced the potency (8.10 ± 0.05, 7.30 ± 0.15, 6.83 ± 0.41, and 5.40 ± 0.22) and the efficacy (-95.2% ± 0.7%, -45.2% ± 11.7%, -58.5% ± 9.8%, and -97.9% ± 0.6%) of brimonidine, medetomidine, detomidine, and xylazine, respectively. Selective α₂ -AR antagonist, atipamezole, competitively antagonized the inhibition of EFS-evoked contractions induced by all agonists except xylazine. These results suggest the existence of presynaptic α₂ -ARs on cholinergic neurons, negatively regulating the release of acetylcholine in horse bronchial muscle, and that α₂ -AR agonists may be beneficial against vagally mediated bronchoconstriction.

Dexmedetomidine's inhibitory effects on acetylcholine release from cholinergic nerves in guinea pig trachea: a mechanism that accounts for its clinical benefit during airway irritation

BACKGROUND

Airway instrumentation can evoke upper airway reflexes including bronchoconstriction and cough which can cause serious complications including airway trauma, laryngospasm or bronchospasm which may in turn lead to difficulty with ventilation and hypoxemia. These airway events are mediated in part by irritant-induced neuronal modulation of airway tone and cough responses. We investigated whether the commonly used anesthetic agents dexmedetomidine, lidocaine or remifentanil attenuated neuronal and airway smooth muscle responses in the upper airways of guinea pigs.

METHODS

The ability of dexmedetomidine, lidocaine or remifentanil to attenuate direct cholinergic nerve stimulation, C-fiber stimulation or direct smooth muscle contraction were studied using isolated tracheal rings from male guinea pigs under four paradigms; (1) the magnitude of contractile force elicited by cholinergic electrical field stimulation (EFS); (2) the amount of acetylcholine released during cholinergic EFS; (3) the direct airway smooth muscle relaxation of a sustained acetylcholine-induced contraction and (4) the magnitude of C-fiber mediated contraction.

RESULTS

Dexmedetomidine (1-100 μM) and lidocaine (1 mM) attenuated cholinergic 30Hz EFS-induced tracheal ring contraction while remifentanil (10 μM) had no effect. Dexmedetomidine at 10 μM (p = 0.0047) and 100 μM (p = 0.01) reduced cholinergic EFS-induced acetylcholine release while lidocaine (10 μM-1 mM) and remifentanil (0.1-10 μM) did not. Tracheal ring muscle force induced by the exogenous addition of the contractile agonist acetylcholine or by a prototypical C-fiber analogue of capsaicin were also attenuated by 100 μM dexmedetomidine (p = 0.0061 and p = 0.01, respectively). The actual tracheal tissue concentrations of dexmedetomidine achieved (0.54-26 nM) following buffer application of 1-100 μM of dexmedetomidine were within the range of clinically achieved plasma concentrations (12 nM).

CONCLUSIONS

The α2 adrenoceptor agonist dexmedetomidine reduced cholinergic EFS-induced contractions and acetylcholine release consistent with the presence of inhibitory α2 adrenoceptors on the prejunctional side of the postganglionic cholinergic nerve-smooth muscle junction. Dexmedetomidine also attenuated both exogenous acetylcholine-induced contraction and C-fiber mediated contraction, suggesting a direct airway smooth muscle effect and an underlying mechanism for cough suppression, respectively.

Role of α2-adrenoceptors in electrical field stimulation-induced contraction of pig nasal mucosa and pharmacologic characterization of a novel α2C-adrenoceptor agonist

BACKGROUND

Blood vessels of the nasal mucosa are richly innervated by sympathetic nerves and neural mechanism is of great interest in upper respiratory tract disorders. This study was designed to determine the role of α2-adrenoceptors and, more specifically, α2C-adrenoceptors, on neurogenic sympathetic vasoconstrictor responses in pig nasal mucosa, and to define the pharmacologic profile of a novel selective α2C-adrenoreceptor agonist.

METHODS

Electrical field stimulation (EFS) was applied to nasal mucosa strips placed in an organ bath and attached to force displacement transducers for continuous recording of isometric tension. The affinity and functional activity of compound B for α2C-adrenoceptors were determined by binding analysis and the ability of compound B to stimulate [(35)S]GTPγS binding to the receptors. Compound B was also tested in a postjunctional α2C-adrenoreceptor bioassay.

RESULTS

EFS-induced contractions were partly blocked by the α2-adrenoreceptor antagonist yohimbine (41.1%) and the α2C-adrenoreceptor antagonist JP-1302 had no effect. The α2-adrenoreceptor agonist clonidine, but not compound B, exerted a significant blockade (70.6%). Compound B had high affinity (K(i) = 18 nM), produced potent agonist (EC50 = 279 nM) and good efficacy (E(max) = 73%) responses at the α2C-adrenoceptors, and displayed good functional agonist potency in the human saphenous vein α2C-adrenoreceptor bioassay (pD2 = 6.2).

CONCLUSION

(1) Neurogenic vasomotor contractility is largely regulated through an α-adrenergic mechanism; (2) pig nasal mucosa possesses post- and prejunctional α2-adrenoceptors; (3) the α2C-adrenoreceptor subtype does not seem to be involved; and (4) compound B is a novel, highly selective, and potent α2C-adrenoreceptor agonist.

Effect of clonidine in mice injected with Tityus discrepans scorpion venom

A study was conducted to assess the effect of clonidine (α(2)-adrenoceptor selective agonist) on glycemia, serum and urine α-amylase, blood urea nitrogen (BUN), serum creatinine, white blood cell count, kidney histology and zymogen granule content in pancreatic acini, in mice under the effect of Tityus discrepans (Td) scorpion venom. BALB/c male mice (20 ± 2 g, n = 7-11) were intraperitoneally (ip) injected with a sublethal dose (1 μg/g) of Td venom, and were treated (ip) with 0.1 μg/g of clonidine (Catapresan(®)) or 0.9% NaCl 30 min after the venom injection, and then every 2 h. Six hours later, mice were anesthetized with diethylether and urine and blood samples were withdrawn by cystocentesis and cardiocentesis, respectively. Tissue samples were obtained and fixed immediately in buffered formalin (2%, pH 7.4) and then processed for stain H&E. Td venom did not cause hyperglycemia by itself. However, clonidine induced hyperglycemia, which was synergized by Td venom. Although the venom did not produce hyperamylasemia, clonidine significantly diminished serum α-amylase activity in envenomed mice. Td venom did not significantly increase urinary α-amylase activity, which was unaffected by clonidine. Morphometric analysis using microphotographs of pancreata from mice injected with Td venom showed a reduced zymogen granule content as judged by the acidophilic bidimensional area of acini. This effect was significantly reduced by clonidine. Kidney samples showed histological changes which were partially affected by the drug. Clonidine reduced the increase in BUN and serum creatinine concentration in envenomed mice. Td venom produced neutrophilia and lymphopenia, which were clonidine-resistant at the assayed dose. These results suggest that α(2)-adrenoceptor selective agonists would be able to reduce some scorpion venom-induced renal and pancreatic disturbances, possibly through the inhibition of neurotransmitter release from presynaptic cholinergic and noradrenergic terminals, as well as from adrenal medulla.

Sympathetic nerve-dependent regulation of mucosal vascular tone modifies airway smooth muscle reactivity

The airways contain a dense subepithelial microvascular plexus that is involved in the supply and clearance of substances to and from the airway wall. We set out to test the hypothesis that airway smooth muscle reactivity to bronchoconstricting agents may be dependent on airway mucosal blood flow. Immunohistochemical staining identified vasoconstrictor and vasodilator nerve fibers associated with subepithelial blood vessels in the guinea pig airways. Intravital microscopy of the tracheal mucosal microvasculature in anesthetized guinea pigs revealed that blockade of α-adrenergic receptors increased baseline arteriole diameter by ~40%, whereas the α-adrenergic receptor agonist phenylephrine produced a modest (5%) vasoconstriction in excess of the baseline tone. In subsequent in vivo experiments, tracheal contractions evoked by topically applied histamine were significantly reduced (P < 0.05) and enhanced by α-adrenergic receptor blockade and activation, respectively. α-Adrenergic ligands produced similar significant (P < 0.05) effects on airway smooth muscle contractions evoked by topically administered capsaicin, intravenously administered neurokinin A, inhaled histamine, and topically administered antigen in sensitized animals. These responses were independent of any direct effect of α-adrenergic ligands on the airway smooth muscle tone. The data suggest that changes in blood flow in the vessels supplying the airways regulate the reactivity of the underlying airway smooth muscle to locally released and exogenously administered agents by regulating their clearance. We speculate that changes in mucosal vascular function or changes in neuronal regulation of the airway vasculature may contribute to airways responsiveness in disease.

Acute severe asthma: new approaches to assessment and treatment

The precise definition of a severe asthmatic exacerbation is an issue that presents difficulties. The term 'status asthmaticus' relates severity to outcome and has been used to define a severe asthmatic exacerbation that does not respond to and/or perilously delays the repetitive or continuous administration of short-acting inhaled beta(2)-adrenergic receptor agonists (SABA) in the emergency setting. However, a number of limitations exist concerning the quantification of unresponsiveness. Therefore, the term 'acute severe asthma' is widely used, relating severity mostly to a combination of the presenting signs and symptoms and the severity of the cardiorespiratory abnormalities observed, although it is well known that presentation does not foretell outcome. In an acute severe asthma episode, close observation plus aggressive administration of bronchodilators (SABAs plus ipratropium bromide via a nebulizer driven by oxygen) and oral or intravenous corticosteroids are necessary to arrest the progression to severe hypercapnic respiratory failure leading to a decrease in consciousness that requires intensive care unit (ICU) admission and, eventually, ventilatory support. Adjunctive therapies (intravenous magnesium sulfate and/or others) should be considered in order to avoid intubation. Management after admission to the hospital ward because of an incomplete response is similar. The decision to intubate is essentially based on clinical judgement. Although cardiac or respiratory arrest represents an absolute indication for intubation, the usual picture is that of a conscious patient struggling to breathe. Factors associated with the increased likelihood of intubation include exhaustion and fatigue despite maximal therapy, deteriorating mental status, refractory hypoxaemia, increasing hypercapnia, haemodynamic instability and impending coma or apnoea. To intubate, sedation is indicated in order to improve comfort, safety and patient-ventilator synchrony, while at the same time decrease oxygen consumption and carbon dioxide production. Benzodiazepines can be safely used for sedation of the asthmatic patient, but time to awakening after discontinuation is prolonged and difficult to predict. The most common alternative is propofol, which is attractive in patients with sudden-onset (near-fatal) asthma who may be eligible for extubation within a few hours, because it can be titrated rapidly to a deep sedation level and has rapid reversal after discontinuation; in addition, it possesses bronchodilatory properties. The addition of an opioid (fentanyl or remifentanil) administered by continuous infusion to benzodiazepines or propofol is often desirable in order to provide amnesia, sedation, analgesia and respiratory drive suppression. Acute severe asthma is characterized by severe pulmonary hyperinflation due to marked limitation of the expiratory flow. Therefore, the main objective of the initial ventilator management is 2-fold: to ensure adequate gas exchange and to prevent further hyperinflation and ventilator-associated lung injury. This may require hypoventilation of the patient and higher arterial carbon dioxide (PaCO(2)) levels and a more acidic pH. This does not apply to asthmatic patients intubated for cardiac or respiratory arrest. In this setting the post-anoxic brain oedema might demand more careful management of PaCO(2) levels to prevent further elevation of intracranial pressure and subsequent complications. Monitoring lung mechanics is of paramount importance for the safe ventilation of patients with status asthmaticus. The first line of specific pharmacological therapy in ventilated asthmatic patients remains bronchodilation with a SABA, typically salbutamol (albuterol). Administration techniques include nebulizers or metered-dose inhalers with spacers. Systemic corticosteroids are critical components of therapy and should be administered to all ventilated patients, although the dose of systemic corticosteroids in mechanically ventilated asthmatic patients remains controversial. Anticholinergics, inhaled corticosteroids, leukotriene receptor antagonists and methylxanthines offer little benefit, and clinical data favouring their use are lacking. In conclusion, expertise, perseverance, judicious decisions and practice of evidence-based medicine are of paramount importance for successful outcomes for patients with acute severe asthma.

Sensory nerves and airway irritability

The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.

Adrenergic receptors: classification, ligand binding and molecular properties

The interaction of catecholamines and drugs with adrenergic receptors leads to a set of biochemical reactions which ultimately results in a physiological response. A brief review is given of the classification of adrenergic receptors into subtypes and the use of ligand binding techniques for the identification and characterization of these receptors. Recent advances in the biochemistry of adrenergic receptors are reviewed with special reference to the interaction of the beta and alpha 2-receptors with guanine nucleotide regulatory proteins and adenylate cyclase. The role of calcium and phosphoinositides in the function of the alpha 1-receptor is also discussed.

Immunohistochemical co-localization of transient receptor potential vanilloid (TRPV)1 and sensory neuropeptides in the guinea-pig respiratory system

Electrophysiological studies within the lung have documented the presence of heterogenous groups of afferent fibers composed of Adelta and C-fibers and studies of somatosensory nerves within the skin reveal a complex pattern of distribution of sensory neuropeptides and transient receptor potential vanilloid (TRPV)1 positive nerves. However, the anatomical location of these different subpopulations of nerves within the lung has not been extensively studied. In the present study we have demonstrated that TRPV1 axons represented only a small proportion of the total number of PGP9.5 staining nerves within guinea-pig tracheal epithelium and only half the number of TRPV1 axons was immunopositive for substance P. In contrast, most TRPV1 positive neurones found within guinea-pig intrapulmonary airways were found to co-localize with sensory neuropeptides substance P and calcitonin gene-related peptide within and beneath the epithelium, around blood vessels, within airway smooth muscle and alveoli, indicative of heterogeneity of TRPV1 positive axons throughout the airways. However, in the smooth muscle layer of the trachea there was evidence of substance P and calcitonin gene-related peptide containing nerves that did not stain for TRPV1. We also demonstrated a complete loss of TRVP1 positive axons in the trachea and intrapulmonary airways and associated loss of bronchoconstriction induced by capsaicin, in animals chronically treated with capsaicin. However, some neuropeptide immunoreactive axons remained in the smooth muscle layer of capsaicin-treated animals which could represent the small subset of neuropeptide containing fibers which do not co-localize with TRPV1. We have provided evidence of heterogeneity of TRPV1 positive nerve fibers, including fibers characterized by lack of co-localization with neuropeptides in various regions of the airways and the existence of neuropeptide containing fibers that were not TRPV1 positive in guinea-pigs.

Immunohistochemical localization of vanilloid receptor subtype 1 (TRPV1) in the guinea pig respiratory system

Transient receptor potential vanilloid-1 (TRPV1) containing nerves are implicated in cough and bronchoconstriction although the significance of their documentation on non-neuronal cells is unclear. We have investigated the anatomical distribution and location of TRPV1 in an animal species often utilized in models of cough and airway inflammation. The distribution and localization of TRPV1 immunoreactivity in the lung was studied using confocal microscopy. Double labelling were carried out using the panaxonal marker, protein gene product 9.5 (PGP) and the neuropeptide substance P. TRPV1 was localized to fine axons within the epithelium of the trachea, however this represented only a fraction of the total axonal innervation of the epithelium. TRPV1 immunoreactive axons were also found in and around subepithelial regions of the airways, including smooth muscle and blood vessels and within the lower airways, found in the vicinity of bronchi and bronchioles, and in and around alveolar tissue. TRPV1 in the epithelium of the trachea was co-localized with substance P containing axons, although TRPV1 immunoreactive neuropeptide negative axons were also discernible. We found evidence for TRPV1 localization to axons throughout the respiratory tract. The distribution was heterogeneous and represented a fraction of the total neuronal innervation of the airways. No TRPV1 was found localized to airway epithelial cells. TRPV1 was often co-localized with the sensory neuropeptide substance P but there was evidence of TRPV1 positive neurones that did not express substance P. This suggests a role for TRPV1 in the airway that is independent of sensory neuropeptides.

Noradrenaline-induced cation currents in isolated rat paratracheal ganglion neurons

The actions of noradrenaline (NA) on the neurons acutely isolated from paratracheal ganglia of rats and the ionic mechanisms involved were studied with nystatin-perforated patch recording configuration. Under current-clamp conditions, application of 10 microM NA produced membrane depolarization followed by repetitive action potentials. NA evoked an inward cationic current under voltage-clamp conditions at a holding potential of -60 mV. Transient tail inward ('hump') current was also induced by washout of NA. The NA-induced current was reduced by extracellular Ca(2+) and Mg(2+), with half-maximal concentrations of 0.7 and 2.6 mM for Ca(2+) and Mg(2+), respectively. Phenylephrine, an alpha(1)-adrenoceptor agonist, mimicked the NA-induced current, but the 'hump' current did not occur upon washout of phenylephrine. The NA-induced current was inhibited by prazosin and WB-4101, alpha(1)-adrenoceptor antagonists. In contrast, in the presence of yohimbine, an alpha(2)-adrenoceptor antagonist, the NA-induced current was potentiated and the washout of NA failed to evoke the 'hump' current. The pretreatment of paratracheal neurons with pertussis toxin also potentiated the NA-induced current. The NA-induced inward current was inhibited by pretreatment with U73122, a phospholipase C inhibitor, and xestospongin-C, a membrane-permeable IP(3) receptor antagonist. On the other hand, thapsigargin, BAPTA-AM and calmidazolium had no effect on the NA-induced current, suggesting that release of Ca(2+) from intracellular Ca(2+) stores via IP(3) receptors is not involved in the NA action. The cationic channels activated by NA play an important physiological role in neuronal membrane depolarization in rat paratracheal ganglia.

Comparison of nebulized epinephrine and terbutaline in patients with acute severe asthma: a controlled trial

PURPOSE

To compare the efficacy and tolerability of nebulized adrenaline and terbutaline in acute severe asthma.

METHODS

DESIGN

Prospective pilot randomized double-blind cross-over trial.

SETTING

Emergency department of a university hospital.

PATIENTS

Thirty-eight patients admitted with severe acute asthma. Each patient received adrenaline (3 mg) and terbutaline (5 mg) nebulizations over 20 min in randomized order. Additional treatment comprised methylprednisolone, intravenous hydration, and oxygen. The efficacy and tolerability of the two drugs were evaluated at the end of each nebulization as well as potential synergistic effects.

RESULTS

Eighteen patients received adrenaline first, and 20 received terbutaline first. Peak expiratory flow (PEF) improved significantly in both groups after the first nebulization (from 157 L/min +/- 111 L/min to 199 L/min +/- 134 L/min with adrenaline, P <.01; and from 142 L/min +/- 65 L/min to 193 L/min +/- 181 L/min with terbutaline, P <.01). Both drugs induced a significant decrease in respiratory frequency. The improvement in PaO2 was only significant with terbutaline. Respiratory frequency, PEF and PaO2 were not improved by the second nebulization. No adverse effects were observed.

CONCLUSIONS

Adrenaline nebulization was as effective and as well tolerated as terbutaline in acute severe asthma. No synergistic effect between terbutaline and adrenaline was observed.

A comparison of the pharmacological modulation of hyperalgesia and bronchial hyperresponsiveness

It has been proposed that there may be similarities in the mechanisms contributing toward hyperalgesia/allodynia observed in various clinical pain syndromes and bronchial hyperresponsiveness (BHR) in asthma. Both features are characterized by an increase in responsivity to various stimuli. In the case of pain, these include increase in responsiveness to mechanical and thermal stimuli whilst asthmatics bronchoconstrict in response to substances that are otherwise innocuous. A variety of mediators can induce hyperalgesia and pain when injected into the skin and these same mediators are also released during an inflammatory insult in the airways and in many cases can induce hyperresponsiveness experimentally. In this review, a comparison of the pharmacological modulation of afferent activity in hyperalgesia and in BHR will be made to ascertain whether any commonality exists between the treatment of these two disease states.

Central pathways for cough in man--unanswered questions

Central processing of afferent cough impulses occurs in a putative 'cough centre' in the dorsal medulla where the reflex is subject to considerable cortical control. Little is known about the central neurotransmitters and mediators which mediate cough in humans. Previous animal and human studies suggest that the antitussive effect of opiates may be mediated at central 5-HT receptors. In three studies in healthy human volunteers, we have investigated the potential role of central cholinergic and dopaminergic receptors in the mediation of cough, and the potential role of 5-HT receptors in the antitussive action of opiates. Intravenous administration of atropine or physostigmine had no effect on capsaicin-induced cough. Similarly, oral administration of L-dopa, bromocriptine or haloperidol had no effect on capsaicin-induced cough. Compared with saline, intravenous morphine significantly suppressed capsaicin-induced cough and increased drowsiness. Compared with placebo, pretreatment with oral pizotifen significantly attenuated the antitussive effect of morphine, but not the sedative effect. This suggests that in humans, an agonist action at 5-HT2 and/or 5-HT1 receptors may be involved in the antitussive effect of morphine, but not its sedative effect. Further knowledge of central cough pathways in humans must await the availability of more selective receptor agonists and antagonists for human studies. This offers the promise of effective antitussive therapy. The challenge is to find an antitussive agent which can return the abnormal sensitivity of the cough reflex to normal, without adverse effects.

Evidence for presynaptic cannabinoid CB(1) receptor-mediated inhibition of noradrenaline release in the guinea pig lung

Using neurochemical method, evidence was obtained that cannabinoid CB(1) receptors are localized on noradrenergic terminals and their stimulation by WIN-55,212-2 reduces the release of [3H]noradrenaline evoked by axonal activity in a frequency-dependent manner. At stimulation rates of 1 and 3 Hz, there was significant inhibition of noradrenaline release, with IC(50) of WIN-55,212-2 41.5+/-2.6 and 320.5+/-28.2 nM, for 1 and 3 Hz, respectively. Cannabinoid CB(1) receptor antagonist SR 141716A completely prevented WIN-55,212-2 from reducing the release. The release of noradrenaline is negatively modulated by presynaptic alpha(2)-adrenoceptors. Because BRL-44408, an alpha(2B)-adrenoceptor, and prazosin, an alpha(1)- and alpha(2B)-adrenoceptor antagonist, both increased the release of [(3)H]noradrenaline, it seems likely that the alpha(2B) subtype is responsible for the negative feedback modulation of noradrenaline release. In the presence of alpha(2)-adrenoceptor antagonism, cannabinoid CB(1) receptor activation by WIN-55,212-2 was much more effective in inhibiting the release of [(3)H]noradrenaline. Using a specific antibody against the C-terminus of the rat cannabinoid CB(1) receptor and also against neuropeptide Y, ultrastructural evidence was obtained that cannabinoid CB(1) receptors are exclusively localized on neuropeptide Y-positive noradrenergic varicosities. Since the sympathetic innervation of the human airway smooth muscle is sparse, and mainly the circulating adrenaline relaxes the airways via activation of beta(2)-adrenoceptor localized on the smooth muscle, it is suggested that inhibition of noradrenaline release by cannabinoids, and the subsequent bronchospasm, may be limited to those cases when noradrenaline released from sympathetic varicosities is involved in airway relaxation.

Moxonidine acting centrally inhibits airway reflex responses

We examined the role of I1-imidazoline (I1-IR) receptors in control of airway function, by testing the effects of systemic administration of the I1-IR agonist moxonidine on reflex responses of tracheal smooth muscle (TSM) tone to either lung deflation or mechanical stimulation of intrapulmonary rapidly adapting receptors. Experiments were performed in either alpha-chloralose anesthetized or decorticate, paralyzed, and mechanically ventilated beagle dogs. Moxonidine (10-100 micrograms/kg) administered via three different routes (femoral vein, muscular branch of superior thyroid artery, and vertebral artery) attenuated TSM responses to stimulation of airway sensory nerve fibers by two different ways and caused a decrease in arterial pressure and heart rate. These effects were dose dependent and were significantly reversed by efaroxan (an I1-IR and alpha 2-adrenergic blocker) administered via the vertebral artery. Intravertebral efaroxan abolished the hemodynamic effects of moxonidine. Intravenous moxonidine (10-100 micrograms/kg) did not alter airway smooth muscle responses to electrical stimulation of the peripheral vagus nerve. In addition, in vitro moxonidine (1-100 micrograms/ml) had no effect on contractile responses to increasing doses of acetylcholine. These findings indicate that moxonidine may act at a central site to suppress reflex airway constriction, even when given into the systemic circulation. Given the presence of I1-IR sites and alpha 2-adrenergic receptors in brain regions participating in airway reflexes, these receptor classes may be involved in brainstem control of the cholinergic outflow to the airways.

Role of adrenergic nervous system in cigarette smoke-induced bronchoconstriction in guinea pigs

The goal of this study was to clarify the role of the adrenergic nervous system in bronchoconstriction induced by exposure to cigarette smoke in guinea pigs. Artificially ventilated animals were exposed to 160 puffs of smoke for 8 min. Bronchoconstriction was assessed as a percentage of the baseline total pulmonary resistance (RL). The effects of pretreatment with phentolamine (0.1 mg/kg, i.v.), propranolol (1 mg/kg, i.v.), and/or atropine (1 mg/kg, i.v.) were evaluated. Exposure to cigarette smoke caused significant bronchoconstriction. Phentolamine, an alpha-adrenoceptor antagonist, significantly inhibited cigarette smoke-induced bronchoconstriction, while propranolol, a beta-adrenoceptor antagonist, significantly enhanced it. Combined use of these compounds further enhanced the bronchoconstriction. All of modulations of the bronchoconstriction by adrenoceptor antagonists were completely abolished by pretreatment with atropine. Phentolamine and/or propranolol had no effect on the bronchoconstriction induced by inhaled acetylcholine. Pretreatment with yohimbine (0.5 mg/kg, i.v.), a selective alpha2-adrenoceptor antagonist, showed modulatory effects similar to those of phentolamine on cigarette smoke-induced bronchoconstriction. These results suggest that cigarette smoke-induced bronchoconstriction is regulated by the prejunctional modulation of the cholinergic system via alpha- and beta-adrenoceptors. This mechanism may be modulated by the autoregulation of adrenergic nerves via the alpha2-autoreceptor.

I1-imidazoline receptors and cholinergic outflow to the airways

We examined the role of I1-imidazoline receptors in the control of airway function, by testing the effects of systemic administration of the I1-imidazoline agonist moxonidine on reflex responses of tracheal smooth muscle (TSM) tone to either lung deflation or mechanical stimulation of intrapulmonary rapidly adapting receptors. Experiments were performed in either alpha-chloralose anaesthetized or decorticate, paralyzed and mechanically ventilated beagle dogs. Moxonidine (10-100 microg/kg) administered via three different routes (the femoral vein, muscular branch of superior thyroid artery, and vertebral artery) attenuated TSM responses to stimulation of airway sensory nerve fibers by two different ways, and caused a decrease in arterial pressure and heart rate. These effects were dose-dependent, and were significantly reversed by efaroxan (an I1-imidazoline and alpha2-adrenergic blocker) administered via the vertebral artery. Intravertebral efaroxan abolished the hemodynamic effects of moxonidine. Intravenous moxonidine (10-100 microg/kg) did not alter airway smooth muscle responses to electrical stimulation of the peripheral vagus nerve. In addition, in vitro moxonidine (1-100 microg/ml) had no effect on contractile responses to increasing doses of acetylcholine. These findings indicate that moxonidine may act at a central site to suppress reflex airway constriction, even when given into the systemic circulation. Given the presence of I1-imidazoline sites and alpha2-adrenergic receptors in brain regions participating in airway reflexes, these receptor classes may be involved in brainstem control of the cholinergic outflow to the airways.

Heterologous expression of the cloned guinea pig alpha 2A, alpha 2B, and alpha 2C adrenoceptor subtypes. Radioligand binding and functional coupling to a CAMP-responsive reporter gene

Functional studies have shown that 6-chloro-9-[(3-methyl-2-butenyl)oxy]-3-methyl-1H-2,3,4,5-tetrahydro-3- benzazepine (SKF 104078) has very low affinity for prejunctional alpha 2-adrenoceptors (alpha 2-AR) in the guinea pig atrium. In this study, we have cloned guinea pig homologues of the human alpha 2-C10, alpha 2-C4 AR subtypes and have studied them in isolation by heterologous expression in cultured mammalian cells. Oligonucleotide primers, designed from conserved areas of the human alpha 2-ARs were used in a polymerase chain reaction (PCR) with template cDNA synthesized from guinea pig atrial mRNA. Three PCR products were obtained that shared identity with the three human alpha 2-AR subtypes. A guinea pig (gp) genomic library was screened with a cDNA clone encoding a portion of the gp-alpha 2A, and genes containing the complete coding sequences of the guinea pig alpha 2A, alpha 2B, and alpha 2C AR subtypes were obtained. These guinea pig genes were subcloned into a eukaryotic expression plasmid and were expressed transiently in COS-7 cells. The binding of the alpha 2-selective antagonist [3H]MK-912 to membranes prepared from these cells was specific and of high affinity with Kd values of 810 pM for gp-alpha 2A, 2700 pM for gp-alpha 2B and 110 pM for gp-alpha 2C. Competition for the binding of [3H]MK-912 by SKF 104078 indicated that it was of moderately high affinity (approximately 100 nM) but that it was not selective for any of the guinea pig alpha 2-AR subtypes. Co-expression of guinea pig alpha 2-AR subtypes with a cyclicAMP-responsive chloramphenicol acetyltransferase (CAT) reporter gene resulted in agonist-dependent modulation of CAT activity. For the gp-alpha 2 A, a biphasic response was obtained with low concentrations of noradrenaline (NE) decreasing forskolin-stimulated CAT activity and high concentrations causing a reversal. For the gp-alpha 2B, NE produced mostly potentiation of forskolin-stimulated activity, and for the gp-alpha 2C, NE caused mainly inhibition. Overall, the pharmacology of the cloned guinea pig alpha 2-AR subtypes was in agreement with data obtained for the native guinea pig receptors and was functionally similar to that of the cloned human alpha 2-AR subtypes.

Sympathetic regulation of fructose secretion in the seminal vesicle of the guinea-pig

Fructose secretion of everted guinea-pig seminal vesicles was studied in vitro. Carbachol produced dose dependent increase in fructose secretion. The effect was blocked by scopolamine but not by hexamethonium, mecamylamine, tetrodotoxin or previous denervation. High concentrations of acetylcholine also increased fructose secretion. This response was not augmented by physostigmine. Methoxamine reduced secretion. Methoxamine, terbutaline, clonidine and vasoactive intestinal peptide counteracted carbachol. Field stimulation produced increased secretion that was not blocked by autonomic drugs, tetrodotoxin or previous denervation. Stimulation of the hypogastric nerve produced frequency dependent increase in fructose secretion. The effect was blocked by tetrodotoxin and scopolamine but not enhanced by physostigmine. If the hypogastric nerve was stimulated close to the seminal vesicle the response was unaffected by hexamethonium but proximal stimulation was blocked. After chronic proximal denervation of the hypogastric nerve, stimulation close to the seminal vesicle produced enhanced response. Destruction of the peripheral ganglia at the base of the seminal vesicle abolished the response. Sections showed that most secretory nerves enter the organ at its base. Phentolamine or yohimbine but not prazosine or propranolol or guanethidine enhanced the secretory response to distal hypogastric nerve stimulation. Tyramine counteracted the response but after reserpinization it was enhanced by tyramine. It is concluded that the secretory cells of the guinea-pig seminal vesicle have a sympathetic secretomotor innervation by short cholinergic neurones with a preganglionic supply via the hypogastric nerve. Inhibitory alpha 1 and beta 2-adrenoreceptors are present on the cells but neurogenic adrenergic inhibition of the secretion is essentially prejunctional and due to activation of inhibitory alpha 2-receptors on the secretomotor nerves.

A controlled trial of nebulized salbutamol and adrenaline in acute severe asthma

OBJECTIVE

To compare efficacy and safety of nebulisation of adrenaline (2 mg over 10 min) and salbutamol (5 mg over 10 min) in acute severe asthma.

DESIGN

Prospective randomized and double blind study.

SETTING

Intensive care unit of a University teaching hospital.

PATIENTS AND PARTICIPANTS

22 asthmatic patients presenting to the emergency room with acute severe asthma.

INTERVENTIONS

Patients were randomly assigned to receive either adrenaline (n = 11) or salbutamol (n = 11) via a nebulizer. Additional treatment comprised hydrocortisone hemisuccinate (100 mg) and supplemental oxygen (71/min). The efficacy and safety of both drugs were evaluated at 20 and 40 min.

RESULTS

A statistically significant increase in the Peak Expiratory Flow (PEF) was achieved at the 20th min in both groups (from 85 +/- 38 l/min to 120 +/- 45 l/min; p < 0.001; and from 107 +/- 28 l/min to 145 +/- 19 l/min; p < 0.001; in adrenaline group and salbutamol group respectively). With both drugs, PEF further increased at 40 min to a level that was statistically significant when compared to the 20 min evaluation. The magnitude of the absolute variation in PEF was similar with both drugs. Both drugs induced a significant decrease in heart rate, respiratory frequency and PaCO2 while the increase of PaO2/FIO2 ratio was not significant. The decrease of respiratory frequency at 40 min was more important with salbutamol (p = 0.03). No side effects were recorded in both groups.

CONCLUSION

After a single dose, nebulized adrenaline (2 mg) proved as effective and safe as salbutamol (5 mg) in acute severe asthma.

Comparison of the effects of salbutamol and adrenaline on airway smooth muscle contractility in vitro and on bronchial reactivity in vivo

BACKGROUND

The effect of adrenergic agonists in asthma depends on their net effect on microvascular leakage, mucosal oedema, vascular clearance of spasmogens, inhibition of cholinergic neurotransmission, and airway smooth muscle contractility. It has been postulated that adrenaline, by virtue of its alpha effects on the vasculature and cholinergic neurotransmission, may have additional useful properties in asthma compared with selective beta agonists such as salbutamol.

METHODS

The airway effects of adrenaline (a non-selective adrenoreceptor agonist) were compared with the selective beta 2 agonist salbutamol. Their airway smooth muscle relaxant potencies and effect on histamine contraction in human bronchi in vitro were compared with their effects on airway calibre and histamine reactivity in asthmatic subjects in vivo. For the in vitro studies changes in tension were measured in response to these agents in thoracotomy specimens of human airways. In vivo the effects of adrenaline and salbutamol on airway calibre and histamine reactivity were measured in eight subjects with mild to moderate asthma in a randomised crossover study.

RESULTS

Salbutamol and adrenaline had approximately equivalent airway smooth muscle relaxant potencies in vitro and bronchodilator potency in vivo. However, their effects on histamine induced contraction in vitro were significantly different from their effects on histamine reactivity in vivo. Salbutamol was less potent in vitro producing a mean (SE) 2.4 (0.15) doubling dose increase in the histamine EC20 and adrenaline a 5.2 (0.18) doubling dose increase (mean difference between salbutamol and adrenaline 2.8 doubling doses; 95% CI 1.1 to 4.5). Salbutamol had no effect on the maximal response to histamine whereas adrenaline reduced it by 54%. In contrast, salbutamol was more potent in vivo producing a mean (SE) increase in PD20 histamine of 1.84 (0.5) doubling doses whereas adrenaline was without effect increasing PD20 by only 0.06 (0.47) doubling doses (mean difference between adrenaline and salbutamol 1.78, 95% CI 0.26 to 3.29 doubling doses).

CONCLUSIONS

These findings suggest that the alpha adrenergic airway effects of non-selective adrenoreceptor agonists such as adrenaline offer no additional protection against histamine-induced broncho-constriction in vivo than beta 2 selective drugs such as salbutamol, despite adrenaline providing greater protection against histamine-induced contraction in vitro. The differences between the effects of these agents in vitro and in vivo may be related to their opposing vascular effects in vivo.

Adrenergic and peptidergic innervation of the trachealis muscle in the normal horse: a preliminary report

The tone of respiratory smooth muscle is largely determined by the input from autonomic nerves. The distribution of adrenergic and selected nonadrenergic, non-cholinergic (NANC) nerves in the normal equine trachealis muscle was investigated using immunohistochemistry. The smooth muscle of the trachealis was found to contain numerous nerves immunoreactive for an enzymatic marker of adrenergic nerves, as well as many nerves immunoreactive for a putative NANC neurotransmitter, peptide histidine isoleucine, a potent bronchodilator. The tissue surrounding the respiratory smooth muscle contained numerous nerves immunoreactive for the neuropeptides substance P and calcitonin gene-related peptide, which can cause marked vasodilation and bronchoconstriction. The complex innervation of the equine trachea should be kept in mind when interpreting the results of physiological experiments.

Neuronal control of airways smooth muscle

Sensory afferent nerves relay impulses from the airways to the central nervous system so that appropriate changes in bronchomotor tone and breathing patterns may occur. The dominant efferent control of airways smooth muscle is exerted via bronchoconstrictor parasympathetic cholinergic nerves. In some species this is opposed by bronchodilator sympathetic noradrenergic nerves. In addition, there exist both excitatory bronchoconstrictor and inhibitory bronchodilator non-adrenergic, non-cholinergic pathways. This review examines the role of the different branches of the autonomic nervous system in the control of airways smooth muscle tone with particular reference to modulation of these branches and the interactions which may exist between them.

Atypical presynaptic alpha-adrenoceptor modulation of neurally-mediated cholinergic responses in guinea-pig tracheal smooth muscle

Cholinergic excitatory nerves in guinea-pig trachea are subject to inhibitory control by presynaptic alpha 2-adrenoceptors. Recently, the nature of these receptors has come into question insofar as the presynaptic inhibitory effects of the alpha 2-adrenoceptor agonist, clonidine, in the guinea-pig trachea have been shown to be antagonized by the alpha 2-adrenoceptor antagonist, yohimbine, as well as the alpha 1-adrenoceptor antagonist, prazosin. This inhibitory action of prazosin had not been described previously in the airways and may relate to the use of norepinephrine rather than clonidine as the alpha-adrenoceptor agonist in earlier studies. The present study evaluates the susceptibility of norepinephrine-induced inhibition of neurally-mediated cholinergic excitatory responses to antagonism by prazosin and yohimbine under conditions identical to those which showed clonidine to be sensitive to these antagonists. In tissues pretreated with guanethidine, propranolol and indomethacin, norepinephrine (1 microM) induced a 37-fold rightward shift of the frequency-response curve for neurally-mediated cholinergic contractions which was reversed partially by pretreatment of tissues with yohimbine. Norepinephrine also caused a concentration-dependent inhibition of cholinergic 'twitch' responses induced by intermittent (1 Hz) nerve stimulation. This action of norepinephrine was antagonized in a concentration-dependent manner by yohimbine but was unaffected by prazosin. These results indicate that in guinea-pig trachea the presynaptic inhibitory actions of norepinephrine on cholinergic nerves are mediated via classical alpha 2-adrenoceptors, i.e. receptors that can be blocked by yohimbine but not by prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of inhaled alpha 2-adrenoceptor and GABAB receptor agonists on citric acid-induced cough and tidal volume changes in guinea pigs

The effects of alpha 2-adrenoceptor and GABA receptor agonists on citric acid-induced cough and increased tidal volume were investigated in conscious guinea pigs. Inhalation of low doses of B-HT 920 (5-allyl-2-amino 5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine dihydrochloride), and xylazine significantly inhibited citric acid-induced cough and tidal volume increases. Intraperitoneal administration of higher doses of B-HT 920 than those given by aerosol were ineffective. The inhibitory effects of B-HT 920 were antagonised by prior intraperitoneal administration of yohimbine, but not atropine. Inhalation of GABA or baclofen inhibited tidal volume increases, but had no effect on cough. Inhaled alpha 2-adrenoceptor or GABA agonists had no effect on the reduced respiratory rate after citric acid inhalation. It is concluded that alpha 2-adrenoceptor agonists inhibit cough via a mechanism which may not be related to their ability to reduce citric acid-induced tidal volume increases, since GABA and baclofen inhibited tidal volume increases but not cough. We suggest that alpha 2-adrenoceptor agonists may have therapeutic potential in the treatment of cough.

Role of K+ channels in the modulation of cholinergic neural responses in guinea-pig and human airways

1. Several agonists modulate cholinergic neurotransmission in airways raising the possibility that there may be a common inhibitory mechanism, such as the activation of a common K+ channel in the nerve ending. To test this hypothesis, we examined whether blockers of K+ channels are able to depress the prejunctional inhibitory modulation of cholinergic contractile responses by various agonists in guinea-pig and human airways in vitro. 2. Electrical field stimulation (40 V, 0.5 ms) was applied to guinea-pig (0.5 Hz) or human (1 Hz) tracheal strips every 4 min to elicit cholinergic neural responses. The effects of the K+ channel blockers, charybdotoxin (ChTX, 10 nM), apamin (100 nM) and glibenclamide (1 microM), on the prejunctional inhibition of cholinergic contraction evoked by neuropeptide Y (NPY, 100 nM), an alpha 2-agonist, clonidine (10 nM), a mu-opioid agonist, [D-Ala2, NMePhe4, Gly-ol5]-enkephalin (DAMGO, 100 nM), and a KATP channel opener, lemakalim (300 nM) were tested in guinea-pigs. In human tissues, the effect of ChTX (10 nM) on the mu-opioid (DAMGO, 300 nM)-induced inhibition of cholinergic nerves was examined. 3. In guinea-pigs, ChTX (10 nM) significantly reversed the prejunctional inhibition of cholinergic contraction by NPY (84.2 +/- 16.2%), clonidine (71.9 +/- 22.4%), DAMGO (67.3 +/- 13.1%) and lemakalim (20.9 +/- 9.4%) (n = 5, P < 0.05, respectively), while apamin (100 nM) had no effect. Glibenclamide (10 microM) reduced only the lemakalim-induced inhibitory modulation. ChTX (10 nM) itself potentiated cholinergic contraction (24.6 +/- 9.4%, n = 5, P < 0.05) without affecting exogenously applied acetylcholine dose-response curves. Pretreatment with ChTX (10 nM) significantly reduced the inhibitory modulation of cholinergic nerves by NPY, clonidine and DAMGO, but not by lemakalim. 4. In human tissues, ChTX significantly reduced DAMGO-induced prejunctional inhibition of cholinergic contraction (13.6 +/- 8.5% with and 46.5 +/- 5.5% without ChTX, respectively; n = 5, P < 0.05). 5. These results may support a hypothesis that the activation of ChTX-sensitive K+ channels is involved in the inhibitory modulation of cholinergic neuro-transmission by agonists acting on presynaptic receptors in guinea-pig and human airways.

Inhibitory action of neuropeptide Y on agonist-induced responses in isolated guinea pig trachea

The effect of neuropeptide Y (NPY) was tested on isolated guinea pig trachea. At 30 nM, NPY induced a weak but significant contractile response which was on average less than 6% of responses elicited by standard spasmogens. This myotropic action of NPY was blocked by indomethacin. In addition to its contractile activity, NPY greatly reduced the maximal response to vasoactive intestinal peptide (VIP), noradrenaline (NA), substance P (SP) and 5-hydroxytryptamine (5-HT), without affecting their pD2 values. However, NPY did not influence the response induced by histamine and carbamylcholine. Pretreatment of tracheal spirals with indomethacin (10(-6) M) abolished the NPY-evoked inhibition of VIP, SP and 5-HT responses but failed to reduce the action of NPY on NA-elicited relaxation. This latter effect was however blocked in the presence of tetrodotoxin. In conclusion, NPY inhibits responses elicited by various agonists in the guinea pig trachea. This effect seems to be mediated at both pre- and postjunctional levels. The postjunctionally mediated inhibitory action of NPY appears to be expressed especially with agents that generate prostaglandins concomitantly with inducing their response. In contrast, the NPY-evoked inhibition of NA-evoked relaxation seems to be mediated via a prejunctional mechanism.

Clinical interactions with alpha-2-adrenergic agonists in anesthetic practice

With the continued use of alpha-2-adrenergic agonists in anesthetic practice, careful attention should be given to the potential for drug interactions. Based on a review of the basic and applied pharmacology of this class of compound, we have made recommendations for the safe and efficacious use of alpha-2-adrenergic agonists in the clinical setting.

Hypoxia modulates mediator responses and neurotransmission in guinea-pig trachea in vitro

To discover whether hypoxia affects mediator responses and neurotransmission in tracheal smooth muscle, we studied in vitro tracheal segments from guinea-pigs under isometric conditions. Hypoxia itself did not alter the basal tone. The maximum response to acetylcholine and histamine under hypoxia was less than that under oxygenated conditions. The logarithm of 50% effective concentration (log EC50) of the response to acetylcholine under hypoxia was not altered, but the log EC50 of the response to histamine decreased significantly. In contrast to the response to exogenous acetylcholine, the maximum contractile response to electrical field stimulation (EFS) under hypoxia was not different from that under oxygenated conditions, but the logarithm of 50% effective frequency of contractions caused by EFS under hypoxia decreased significantly. On the other hand, non-adrenergic-non-cholinergic relaxation caused by EFS was unaffected by hypoxia. These observations suggest that hypoxia can modulate the responses of tracheal smooth muscle to acetylcholine, histamine and nerve stimulation.

Analysis of preganglionic nerve evoked cholinergic contractions of the guinea pig bronchus

We compared cholinergic bronchial muscle contractions induced by vagus nerve (preganglionic) stimulation (VNS) with those induced by electrical field (postganglionic) stimulation (EFS). When normalized to their respective maximum response, the frequency-response curves (10 s trains) between 4 and 16 Hz were similar between VNS and EFS; however, at frequencies of 0.1-2 Hz, and at frequencies greater than 32 Hz, the VNS contractions were significantly less than EFS. When contractions elicited by 100 pulses were examined, it was found that the responses to VNS were maximal at 10-30 Hz then declined significantly to 82-35% of maximal between 40 and 200 Hz, whereas the response to EFS was essentially unchanged at frequencies up to 60 Hz and declined only to 72% of maximal up to 200 Hz. At frequencies as low as 20 Hz, the contractions evoked by VNS faded to 45 +/- 9% of the peak contraction during 60 sec of continuous stimulation, whereas those evoked by 60 sec continuous EFS remained constant. This fade observed during prolonged VNS was not blocked by the antagonists, pirenzepine and AFDX-116, at concentrations selective for M1 and M2 muscarinic receptors, respectively; nor was the fade blocked by pre-treatment with indomethacin, propranolol, phentolamine, or choline. At frequencies greater than 10 Hz, the amplitude of the preganglionic compound action potential also faded during repetitive stimulation. The results support the hypothesis that the airway ganglion neurons innervating guinea pig bronchial smooth muscle effectively filter preganglionic stimuli, especially at low and relatively high frequencies. During continuous vagus nerve stimulation, preganglionic mechanisms may also play a role in limiting the ultimate output of airway ganglia.

Muscarinic receptors and parasympathetic neurotransmission in guinea-pig trachea

Muscarinic receptors involved in cholinergic neurotransmission were studied in isolated innervated guinea-pig tracheas using preganglionic (nerve) and postganglionic (field) stimulation, after blocking sympathetic effects with bethanidine (5 microM). Neostigmine (10 nM) significantly increased responses to nerve and field stimulation. The M1 antagonist pirenzepine (0.1-100 nM) selectively reduced tracheal responses to nerve stimulation in control and in neostigmine-treated tissues. The M2 antagonist gallamine (0.1-100 microM) significantly increased tracheal responses to nerve and field stimulation in control and in neostigmine-treated preparations. At concentrations that increased baseline tone, oxotremorine, arecoline and pilocarpine decreased responses to nerve and field stimulation. Gallamine (30 microM) selectively reduced the inhibitory effects of these agonists on responses to nerve and field stimulation. The findings indicate that cholinergic neurotransmission in guinea-pig trachea is modulated by facilitatory M1 receptors at parasympathetic ganglia and inhibitory M2 receptors at the postganglionic nerve endings.

Prejunctional GABA-B inhibition of cholinergic, neurally-mediated airway contractions in guinea-pigs

GABA is a known inhibitory neurotransmitter in the CNS. Recent studies have also demonstrated the presence of GABA in peripheral tissue, including lung. To delineate a role for GABA in lung, the effect of GABA and selective GABA agonists and antagonists on neuronally-induced airway contractions in guinea pigs were studied. In vitro, tracheal contractions induced by electrical field stimulation (EFS) were inhibited by tetrodotoxin and atropine indicating that the contractions were mediated by neuronal release of acetylcholine. The contractions caused by EFS, but not those by exogenous acetylcholine, were inhibited by GABA (EC50 = 4.5 microM) and the selective GABA-B agonist baclofen (EC50 = 9 microM), but not by the GABA-A agonist, muscimol. The inhibitory effect of baclofen was not affected by the GABA-A antagonist, bicuculline, but was significantly reversed with the GABA-B antagonists, 3-aminopropylphosphonic acid (3-APPA) (pA2 = 4.5) and 2-hydroxysaclofen (pA2 = 4.1). In vivo, vagal nerve stimulation (5 V, 20 Hz, 0.5 ms, 5 s) in anesthetized, mechanically ventilated guinea-pigs caused cholinergic-dependent bronchospasms that were inhibited by intravenous GABA (3 and 10 mg/kg) and baclofen (1-10 mg/kg), but not by muscimol. The inhibitory effects of GABA and baclofen against vagal bronchospasm were blocked by 3-APPA (5 mg/kg, i.v.), but not by bicuculline. Responses to the GABA-B agonists were unaltered after the treatment of animals with phentolamine or propranolol to block alpha-adrenergic and beta-adrenergic receptors, respectively. Bronchospasm due to intravenous methacholine was also unchanged by GABA and baclofen.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of catecholamines on cholinergically and non-cholinergically mediated contractions of guinea-pig isolated bronchial muscle

The actions of catecholamines on the responses evoked by electrical field stimulation or by acetylcholine and substance P in guinea-pig bronchial strip chain have been examined. Electrical field stimulation evoked a biphasic contraction, consisting of a cholinergically-mediated fast contraction followed by a non-cholinergically-mediated slow contraction. All catecholamines tested caused a concentration-dependent reduction in the height of the biphasic contraction, where non-cholinergic contractions were more potently inhibited. The inhibitory effect of isoprenaline was largely prevented by propranolol (2 microM) alone, whereas those of noradrenaline and adrenaline were prevented by treatment with both propranolol (2 microM) and yohimbine (2 microM). The inhibitory effect of dopamine was unaffected either by propranolol (2 microM), yohimbine (2 microM) or haloperidol (10 microM). Submaximal contractions of bronchial muscle evoked by exogenous acetylcholine (2 microM) or substance P (0.2 microM) were also inhibited by catecholamines, except dopamine, but the effects were antagonized by propranolol (2 microM) alone. The results suggest that in guinea-pig isolated bronchial muscle, catecholamines can inhibit both cholinergic and non-cholinergic excitatory neurotransmissions not only by postjunctional beta-adrenoceptors but also by prejunctional alpha 2-adrenoceptors.

Bronchial effects of alpha 2-adrenoceptor agonists and of other antihypertensive agents in asthma

The respective prevalence of hypertension and asthma is sufficient for their combined existence to be far from rare. The effects of certain antihypertensive drugs, e.g., alpha 2-adrenoceptor agonists, on the bronchi may be either harmful or beneficial. When inhaled, alpha 2-agonists reduce the immediate bronchial response to allergens, whereas when ingested they aggravate the bronchial response to histamine and all the more so when their effect on the central nervous system is greater. Therefore, there has been much interest in agents such as the new oxazoline derivative, rilmenidine, which has less central effects than clonidine, an imidazoline compound of reference. Calcium antagonists inhibit smooth muscle contraction and release of mast cell inflammatory mediators. In asthmatic subjects, their short-term administration leads to a modest improvement in spontaneous bronchial obstruction, has only a partial protective action against various nonspecific or allergenic stimuli, and slightly reinforces the beneficial effect of beta 2-agonists. Beta-adrenoceptor antagonists aggravate bronchial obstruction and nonspecific bronchial hyperreactivity in asthmatic subjects. These harmful effects are dose-dependent, have even been reported after the administration of eyedrops, and are common to all beta-blockers. Angiotensin-converting enzyme inhibitors increase bronchial hyperreactivity in patients who develop cough during treatment and may, in certain cases, worsen or even induce asthma, probably by opposing inactivation by hydrolysis of tachykinins and of bradykinins.

New concepts in the pathogenesis of bronchial hyperresponsiveness and asthma

Recent studies have suggested that inflammation may play an important role in the characteristic bronchial hyperresponsiveness and symptoms of chronic asthma. The mechanisms by which inflammatory cells, mediators, and nerves interact to produce the features of asthma are still uncertain, however. Although mast cells play an important role in the immediate response to allergen (and probably exercise), pharmacologic evidence argues against a critical role in the late response or bronchial hyperresponsiveness in which other cells, such as macrophages and eosinophils, may play a more important role. Many mediators have been implicated in asthma, but only PAF causes a prolonged increase in bronchial responsiveness. PAF attracts eosinophils into tissues and potently activates these cells, which may lead to epithelial damage, a key feature of asthmatic airways. PAF is also a potent inducer of microvascular leakage in airways, which may result in submucosal edema and plasma exudation into the airway lumen in the future. PAF antagonists will reveal whether PAF plays an important role in the eosinophilic inflammation of asthma. Neural mechanisms may also make an important contribution. Inflammatory mediators may influence neurotransmitter release from airway nerves, and neurotransmitters may be proinflammatory. Neural control is complex and cholinergic, adrenergic, and NANC mechanisms may contribute to bronchial hyperresponsiveness. Many neuropeptides, which may be the transmitters of NANC nerves, have been identified in airways. Neuropeptides in airway sensory nerves, such as substance P, have potent proinflammatory effects and, if these are released by an axon reflex, may amplify the inflammatory response in asthma. Since asthma may be chronic eosinophilic bronchitis, it is logical that the primary treatment should involve drugs that suppress this inflammatory response. At present, corticosteroids appear to be the most effective therapy; they have potent effects against eosinophils and macrophages (but not on mast cells) and reduce bronchial hyperresponsiveness and symptoms. By contrast, bronchodilators, such as beta-agonists, although they reduce symptoms, do not reduce the chronic inflammatory response or bronchial hyperresponsiveness and may mask the underlying inflammation. New therapies should be directed toward controlling eosinophil infiltration and activation in airways.

Inhibition of cholinergic and non-adrenergic, non-cholinergic bronchoconstriction in the guinea pig mediated by neuropeptide Y and alpha 2-adrenoceptors and opiate receptors

The mechanisms underlying the regulatory influence of neuropeptide Y (NPY) and of alpha 2-adrenoceptor and opiate receptor activation on cholinergic and excitatory non-adrenergic, non-cholinergic (e-NANC) neurotransmission were studied in guinea pig hilus bronchi in vitro. NPY inhibited both the cholinergic and e-NANC bronchial contractions evoked by field stimulation. The NPY attenuation of the e-NANC contraction could not be antagonized by the alpha 2-antagonist, idazoxan, or naloxone. UK 14,304 a specific alpha 2-agonist, also reduced the two nervous components of bronchial contraction and this action was inhibited by idazoxan. NPY and UK 14,304 exerted a minor influence on the bronchial smooth muscle tone per se or on contractions evoked by acetylcholine or neurokinin A. This suggested that the inhibitory responses were caused by a prejunctional action reducing the release of transmitter substances from sensory and cholinergic nerve endings. Furthermore NPY (10(-7) M) seemed to be more potent to inhibit both contractile components than noradrenaline (10(-6) M) in the presence of propranolol (3 X 10(-6) M). Morphine was able to reduce the e-NANC response via a naloxone-sensitive mechanism. The capsaicin-evoked bronchoconstriction and the bronchodilator NANC effect evoked by field stimulation were, however, not influenced by UK 14,304. It is concluded that NPY, alpha 2-receptor and opiate receptor activation inhibit the release of sensory transmitters evoked by field stimulation but not by capsaicin.

Modulation of cholinergic neurotransmission by vasoactive intestinal peptide and peptide histidine isoleucine in guinea-pig tracheal smooth muscle

There is increasing evidence that vasoactive intestinal peptide (VIP) and peptide histidine isoleucine (PHI) are non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmitters in airway smooth muscle. The possibility that VIP and PHI may also have neuromodulatory effects on excitatory responses, mediated by cholinergic nerves, to electrical field stimulation (EFS) was studied in guinea-pig isolated trachea. VIP (0.5 nM) pre-junctionally, inhibited the release of acetylcholine (ACh), whereas post-junctionally, responses to methacholine (MCh) were enhanced. At a maximum relaxant concentration (100 nM), VIP inhibited cholinergic neurotransmission both pre- and post-junctionally. Similarly, PHI (30 nM) inhibited neuronal ACh release, but enhanced transmitter action post-junctionally. At 3 microM, PHI inhibited ACh release. VIP- and PHI-induced inhibition of EFS was not affected by methysergide, pyrilamine, naloxone, phentolamine and propranolol. These data suggest that, in airway smooth muscle VIP and PHI may modulate cholinergic transmission via specific receptors.

Effects of adrenergic agonists and adenosine on cholinergic neurotransmission in human tracheal smooth muscle

There is only limited information available on the prejunctional regulation of acetylcholine (ACh) release from cholinergic nerves in human airway smooth muscle. Stimulation of cholinergic nerves in fresh postmortem tracheal muscle strips with electrical field stimulation (EFS) causes reproducible contractions. We have studied the effect on contractile responses of: 1) The alpha 2-adrenoceptor agonist effect of noradrenaline (NA, 0.1-30 microM) and clonidine (10 nM-30 microM), in the presence of 1 microM propranolol and prazosin +/- idazoxan (0.1 microM); 2) The beta-adrenoceptor agonist effect of fenoterol (FEN) and isoprenaline (ISO, 1 nM-30 microM) +/- ICI 118,551 (10 nM), comparing EFS responses to comparable responses to exogenous ACh; 3) The A1 and A2 adenosine receptor agonists effects of L-PIA and NECA (1 nM-10 microM). NA caused a concentration-dependent depression of the cholinergic frequency-response curve. However responses at 5 Hz were not modified by the addition of idazoxan. Similarly clonidine did not reduce contractile responses. The concentrations of isoprenaline (56 nM) and fenoterol (165 nM) required to inhibit EFS (5 Hz) by 50% (IC50) were significantly less than those required to inhibit closely matched ACh responses to a comparable degree (ISO = 117 and FEN = 304 nM), and the maximum inhibition of EFS was greater. Following isoprenaline and the beta 2-antagonist ICI 118,551 the IC50's for EFS and ACh were not different. NECA and PIA had no effect on cholinergic EFS. We conclude that a prejunctional beta 2 receptor may be present on cholinergic nerves in post-mortem tracheal smooth muscle but no evidence for alpha 2-adrenoceptor or adenosine-receptor regulation was obtained.