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

Impact of ectonucleotidases in autonomic nervous functions

Adenine and uracil nucleotides play key functions in the autonomic nervous system (ANS). For instance, ATP acts as a neurotransmitter, co-transmitter and neuromodulator in the ANS. The purinergic system encompasses (1) receptors that respond to extracellular purines, which are designated as P1 and P2 purinoceptors, (2) purine release and uptake, and (3) a cascade of enzymes that regulate the concentration of purines near the cell surface. Ectonucleotidases and adenosine deaminase (ADA) are enzymes responsible for the hydrolysis of ATP (and other nucleotides such as ADP, UTP, UDP, AMP) and adenosine, respectively. Accordingly, these enzymes are expected to play an important role in the control of neuro-effector transmission in tissues innervated by both the sympathetic and parasympathetic divisions of the ANS. Indeed, ectonucleotidases have the ability to either terminate P2 receptor responses initiated by nucleoside triphosphates (ATP and UTP), and/or to favor the activation of ADP (e.g. P2Y1,12,13) and UDP (e.g. P2Y6) and/or adenosine (P1) specific receptors. In addition, ectonucleotidases can also importantly protect some P2 receptors from desensitization (e.g. P2X1, P2Y1). In this review, we present the (putative) roles of ectonucleotidases and ADA in the ANS with a focus on their regulatory activity at neuro-effector junctions in the following tissues: heart, vas deferens, urinary bladder, salivary glands, blood vessels and the intestine. We also present their implication in nociceptive transmission.

Low-dose salbutamol suppresses airway responsiveness to histamine but not methacholine in subjects with asthma

BACKGROUND

Airway hyperresponsiveness is a cardinal feature of asthma. Although the modulation of cholinergic neuroeffector transmission may play a role in airway responsiveness, in vivo evidence remains scarce. It is well known that histamine causes bronchoconstriction partly via vagal reflex, whereas methacholine does not. To investigate the significance of modulating neuroeffector transmission, we compared the effect of low-dose salbutamol-a β2-adrenoceptor agonist-on airway responsiveness to histamine with that to methacholine.

METHODS

We enrolled 12 subjects with stable asthma. After screening confirmed that inhalation of low-dose salbutamol (1μg) did not change their basic pulmonary function, subjects underwent measurement of airway responsiveness to inhaled histamine and methacholine with or without pretreatment with low-dose salbutamol, in a randomized, crossover fashion. Airway responsiveness was measured by an astograph by which respiratory conductance (Grs) was assessed by the forced oscillation method during continuous inhalation of histamine or methacholine in stepwise incremental concentrations. Airway responsiveness was calculated as the cumulative dose of bronchoconstrictors that induced a decrease of 35% in Grs.

RESULTS

Inhalation of 1μg of salbutamol significantly attenuated airway responsiveness to histamine but not methacholine. This selective attenuation was observed irrespective of disease severity or phenotype, namely atopy or non-atopy.

CONCLUSION

Low-dose salbutamol suppresses airway responsiveness to histamine but not methacholine in subjects with asthma. The present study may provide a novel insight into the bronchoprotective mechanism of β2-adorenoceptor agonist in clinical settings.

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.

Beta-Adrenergic Agonists

Inhaled β₂-adrenoceptor (β₂-AR) agonists are considered essential bronchodilator drugs in the treatment of bronchial asthma, both as symptoms-relievers and, in combination with inhaled corticosteroids, as disease-controllers. In this article, we first review the basic mechanisms by which the β₂-adrenergic system contributes to the control of airway smooth muscle tone. Then, we go on describing the structural characteristics of β₂-AR and the molecular basis of G-protein-coupled receptor signaling and mechanisms of its desensitization/ dysfunction. In particular, phosphorylation mediated by protein kinase A and β-adrenergic receptor kinase are examined in detail. Finally, we discuss the pivotal role of inhaled β₂-AR agonists in the treatment of asthma and the concerns about their safety that have been recently raised.

Levalbuterol hydrochloride

Racemic salbutamol (racemic albuterol) ameliorates symptoms of asthma by activating beta-adrenoceptors on nerve, smooth muscle and inflammatory cells within the airways. Racemic salbutamol comprises equal proportions of 2 isomers: (S)-salbutamol and (R)-salbutamol, with the latter being exclusively responsible for activation of beta-adrenoceptors. Accordingly, within racemic salbutamol it is (R)-salbutamol that efficiently relieves obstruction of asthmatic airways and affords highly effective protection from bronchoconstrictor stimuli, including allergens. During regular use of racemic salbutamol, there is a progressive decline of protective efficacy and a corresponding intensification of airway responsiveness. This decline is largely absent during regular use of (R)-salbutamol. Consequently, bronchodilator responses to sub-maximal doses of (R)-salbutamol exceed responses to the equivalent dose of (R)-salbutamol given as the racemate. For example, in asthmatics with baseline FEVs <or= 60%, 1.25 mg of nebulised (R)-salbutamol achieved a maximal 52% change in FEV while 2.5 mg of racemic salbutamol only achieved a 38% change in FEV. Since extrapulmonary effects (e.g., tremor, heart rate) of beta agonists are related to dose and limit the use of beta agonist therapy, (R)-salbutamol at 0.63 mg provides uncompromised efficacy with marked reduction of side-effects. In addition to quantitative differences, the constituent isomers of salbutamol also exhibit qualitative differences. Thus, (R)-salbutamol inhibits activation of human eosinophils in vitro whereas, under the same conditions and concentrations, (S)-salbutamol augments activation of these cells. This property of (S)-salbutamol may explain why eosinophilia in induced sputum from subjects with allergic asthma is increased by regular use of racemic salbutamol. Similarly, the capacity of (R)-salbutamol to suppress hyperresponsiveness of the airways can be contrasted with the capacity of (S)-salbutamol to intensify hyperresponsiveness. This action of (S)-salbutamol would explain why regular use of racemic salbutamol intensifies the bronchoconstrictor response to antigen in subjects with allergic asthma. Taken together, these findings imply that replacement of racemic salbutamol by (R)-salbutamol will diminish, or even eliminate, the anomalous actions that have curtailed the efficacy of racemic salbutamol. Pharmacokinetically, (R)-salbutamol exhibits near absolute conformational stability (i.e., no conversion to (S)-salbutamol). If in vitro anti-inflammatory actions of (R)-salbutamol are also manifest in asthmatic airways, (R)-salbutamol could provide a novel approach to asthma therapy which combines bronchodilation and bronchoprotection with anti-inflammatory efficacy.

Adenosine Deaminase Deficiency: Metabolic Basis of Immune Deficiency and Pulmonary Inflammation

Genetic deficiencies in the purine catabolic enzyme adenosine deaminase (ADA) in humans results primarily in a severe lymphopenia and immunodeficiency that can lead to the death of affected individuals early in life. The metabolic basis of the immunodeficiency is likely related to the sensitivity of lymphocytes to the accumulation of the ADA substrates adenosine and 2'-deoxyadenosine. Investigations using ADA-deficient mice have provided compelling evidence to support the hypothesis that T and B cells are sensitive to increased concentrations of 2'-deoxyadenosine that kill cells through mechanisms that involve the accumulation of dATP and the induction of apoptosis. In addition to effects on the developing immune system, ADA-deficient humans exhibit phenotypes in other physiological systems including the renal, neural, skeletal, and pulmonary systems. ADA-deficient mice develop similar abnormalities that are dependent on the accumulation of adenosine and 2'-deoxyadenosine. Detailed analysis of the pulmonary insufficiency seen in ADA-deficient mice suggests that the accumulation of adenosine in the lung can directly access cellular signaling pathways that lead to the development and exacerbation of chronic lung disease. The ability of adenosine to regulate aspects of chronic lung disease is likely mediated by specific interactions with adenosine receptor subtypes on key regulatory cells. Thus, the examination of ADA deficiency has identified the importance of purinergic signaling during lymphoid development and in the regulation of aspects of chronic lung disease.

Adenosine mediates IL-13-induced inflammation and remodeling in the lung and interacts in an IL-13-adenosine amplification pathway

IL-13 is an important mediator of inflammation and remodeling. We hypothesized that adenosine accumulation, alterations in adenosine receptors, and adenosine-IL-13 autoinduction are critical events in IL-13-induced pathologies. To test this, we characterized the effects of IL-13 overexpression on the levels of adenosine, adenosine deaminase (ADA) activity, and adenosine receptors in the murine lung. We also determined whether adenosine induced IL-13 in lungs from ADA-null mice. IL-13 induced an inflammatory and remodeling response that caused respiratory failure and death. During this response, IL-13 caused a progressive increase in adenosine accumulation, inhibited ADA activity and mRNA accumulation, and augmented the expression of the A1, A2B, and A3 but not the A2A adenosine receptors. ADA enzyme therapy diminished the IL-13-induced increase in adenosine, inhibited IL-13-induced inflammation, chemokine elaboration, fibrosis, and alveolar destruction, and prolonged the survival of IL-13-transgenic animals. In addition, IL-13 was strongly induced by adenosine in ADA-null mice. These findings demonstrate that adenosine and adenosine signaling contribute to and influence the severity of IL-13-induced tissue responses. They also demonstrate that IL-13 and adenosine stimulate one another in an amplification pathway that may contribute to the nature, severity, progression, and/or chronicity of IL-13 and/or Th2-mediated disorders.

Too much of a good thing: adenosine overload in adenosine-deaminase-deficient mice

Chronic lung diseases are associated with persistent lung inflammation and damage. The mechanisms that govern the chronic nature of these disorders are not known. Adenosine is a signaling nucleoside that is generated in hypoxic environments such as that found in the inflamed lung, which suggests that it might serve a regulatory role in chronic lung diseases. Support for this hypothesis comes from studies in adenosine-deaminase-deficient mice where lung adenosine levels accumulate in association with increased lung inflammation and damage. Furthermore, lowering adenosine levels or antagonizing adenosine receptors can reverse pulmonary phenotypes in this model, suggesting that chronic adenosine elevations can affect signaling pathways that mediate aspects of chronic lung disease.

Characterization of adenosine receptors involved in adenosine-induced bronchoconstriction in allergic rabbits

1. Recent work has suggested that adenosine may be involved in asthma via the activation of A1 receptors. However, the role of the recently cloned A3 receptor in airways is largely unknown. In the present study, we have investigated the role of the A3 receptor in adenosine-induced bronchoconstriction in allergic rabbits. 2. Aerosol challenge of antigen (Ag) immunized rabbits with the adenosine precursor, adenosine 5'-monophosphate (AMP), resulted in a dose-dependent fall in dynamic compliance (Cdyn). The maximum fall in Cdyn in these rabbits was significantly greater than that in litter matched, sham immunized animals (P < 0.05). However, there was no significant difference in the maximum increase in airways resistance (Rt) between Ag and sham immunized rabbits (P > 0.05). 3. Aerosol challenge of Ag immunized rabbits with cyclopentyl-adenosine (CPA) (A1-receptor agonist) elicited a dose-dependent fall in Cdyn in Ag immunized rabbits and the maximum fall in Cdyn in these rabbits was significantly greater than that observed in sham immunized rabbits (P < 0.05). Similarly, CPA induced dose-dependent increases in R1 in Ag immunized rabbits whereas sham immunized rabbits failed to respond to CPA within the same dose range. The maximum increase in RL in Ag immunized rabbits was significantly greater than that of sham immunized rabbits (P < 0.05). 4. Aerosol challenge of either Ag or sham immunized rabbits with the A3 agonist aminophenylethyladenosine (APNEA) did not elicit dose-dependent changes in either RL or Cdyn. Moreover, there was no significant difference in the maximum response, measured by either parameter, between the two animal groups (P > 0.05). 5. These data provide further evidence for a role of the A1 receptor in the airways, but do not support a role for the A3 receptor in adenosine-induced bronchoconstriction in the allergic rabbit.

Excitatory prejunctional beta 2-adrenoceptor distribution within equine airway cholinergic nerves

We examined the effect of activation of beta 2-adrenoceptor (AR) by isoproterenol (ISO) on acetylcholine (ACh) release evoked by electrical field stimulation (EFS: 20 V, 0.5 Hz, 0.5 msec) from cholinergic nerves in five regions of equine airways. We also tested if the effect of ISO was dependent on epithelium or prostanoids by examining the effect of ISO on ACh release in the presence and absence of epithelium or cyclooxygenase inhibition. Trachealis strips or bronchial rings were preincubated for 60 min with 10(-7) M atropine, 10(-6) M neostigmine, and 10(-5) M guanethidine. The ACh amount was measured by high-performance liquid chromatography with electrochemical detection. Isoproterenol (10(-8)-10(-6) M) augmented ACh release throughout the whole airway in a concentration-dependent manner. Rubbing off the epithelium potentiated EFS-induced ACh release but neither epithelium removal nor cyclooxygenase inhibition affected the magnitude of ISO-induced augmentation of ACh release. These results indicate that in equine airway parasympathetic nerves: (1) excitatory prejunctional beta 2-adrenoceptors are distributed throughout the tracheobronchial tree; (2) the function of this excitatory beta 2-adrenoceptor is independent of endogenous prostanoids and epithelium; and (3) there is an epithelium-derived factor that inhibits ACh release.

Beta 2 adrenergic receptors in asthma: a current perspective

The role of the beta 2-adrenergic receptor in both the pathogenesis and treatment of asthma has been a subject of intense speculation and investigation for 25 years. The physiological effects of endogenous circulating catecholamines and exogenous adrenergic agonists in the lung are mediated by the beta 2-adrenergic receptor, which is present on a variety of cell types. Documented effects of beta 2-adrenergic receptor activation in the human lung include smooth muscle relaxation, inhibition of acetylcholine release from cholinergic nerve terminals, stimulation of serous and mucous cell secretion, increases in ciliary beat frequency, promotion of water movement into the airway lumen by stimulation of ion secretion across the apical membrane of epithelial cells, increase in bronchial blood flow, reduction in venular permeability, and inhibition of mediator release from some, but not all, inflammatory cells. Beta 2-Adrenergic receptors are present in normal or increased numbers on asthmatic airway smooth muscle but are uncoupled in severe asthma, leading to functional hyporesponsiveness, probably due to the effects of inflammatory mediators. There is also evidence for dysfunction of beta 2-adrenergic receptors on circulating inflammatory cells following mediator release. However, dysfunction of the receptors on airway smooth muscle and inflammatory cells is unlikely to be of primary importance in the pathogenesis of asthma. There is increasing concern that regular beta 2-adrenergic receptor agonist use in the therapy of asthma is deleterious. Although a number of theories have been advanced to explain such an effect, none is well established and further research is urgently required.

Adenosine bronchoconstriction in asthma: investigations into its possible mechanism of action

1. Inhaled adenosine and its parent nucleotide, adenosine 5'-monophosphate (AMP) provoke bronchoconstriction in atopic and asthmatic individuals but not in normal subjects. 2. In clinical studies, histamine H1-receptor antagonists, cyclo-oxygenase inhibitors and the mast cell 'stabilising' drugs, sodium cromoglycate and nedocromil, protect against the effects of adenosine bronchoprovocation suggesting the involvement of secondary mast cell mediator release. 3. Adenosine and its analogues potentiate histamine and leukotriene release from mast cells activated by other stimuli in vitro, and may also increase net mediator release from mast cells by counteracting the inhibitory effect of circulating adrenaline. 4. Although adenosine fulfils many of the criteria required for a mediator in asthma, its importance is not fully understood, and the mechanisms by which it provokes bronchoconstriction in asthmatic subjects is far from concluded. 5. Two possibilities are that either adenosine acts directly on luminal mast cells to upregulate histamine secretion, or it acts to initiate neuronal reflexes which stimulate histamine release indirectly and possibly activate peptidergic and/or cholinergic pathways.