Investigations on possible presynaptic effect of adenosine and noradrenaline on cholinergic neurotransmission in guinea pig trachea

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.

Prejunctional stimulation of cholinergic nerves in rat airway smooth muscle by an adenosine analogue

The effect of 5'-N-ethylcarboxamidoadenosine (NECA) on rat bronchial smooth muscle was examined in vitro. Both the nerve mediated muscle contraction induced by electrical stimulation and the potassium evoked release of [3H]ACh were enhanced by NECA. The apparent affinity (EC50) of NECA in the contraction experiments was 0.30 +/- 0.06 microM. The adenosine (ADO) receptor antagonist, 8-phenyltheophylline (8-PT), inhibited the NECA induced potentiation of both the electrical induced contraction and the potassium evoked release of [3H]ACh. The EC50 and intrinsic activity of exogenous ACh were not altered in the presence of NECA (1 microM) in experiments where smooth muscle contraction were measured, indicating that NECA has a prejunctional effect and not a postjunctional effect on muscarinic receptors. The new A2 specific ADO receptor agonist 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) and ADO also enhanced the nerve-mediated contraction (EC50 = 35 +/- 8 microM and 69 +/- 20 microM, respectively). 8-PT (10 microM) and enprofylline (ENPF) (10 microM) inhibited the electrically induced contraction by 55 +/- 16% and 45 +/- 5% respectively. The potassium evoked release, however, was stimulated 56 +/- 6% and 39 +/- 7% by 50 microM 8-PT and ENPF respectively. The results provide evidence for a NECA specific ADO receptor in rat bronchi that is most likely prejunctional. Stimulation of this receptor, which may be of an A2 receptor subtype, enhances the nerve mediated release of ACh and thereby induce contraction of the bronchial smooth muscle.

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.

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.

Evidence for A1 and A2 adenosine receptors in guinea pig trachea

The adenosine analogs [5'-N-ethylcarboxamideadenosine (NECA), 2-Chloro-adenosine (2-ClA), R-phenylisopropyladenosine (R-PIA), N6-cyclohexyl adenosine (CHA), and N6-cyclopentyladenosine (CPA)] produced both relaxation and contraction responses in isolated guinea-pig trachea. A concentration-related relaxation response was observed in trachea which were precontracted with either histamine or KC1. This response followed an order of analog potency that was indicative of the A2 receptor subtype (NECA greater than 2-ClA greater than R-PIA greater than CPA greater than CHA). Theophylline, an adenosine-receptor antagonist, blocked this relaxation response. In addition, a concentration-related contractile response was produced with adenosine analogs in those trachea that were not previously contracted. In contrast, the contractile response followed an analog potency indicative of the A1 receptor subtype (R-PIA greater than 2-ClA = CPA = CHA). This contractile response was not mediated by cholinergic, adrenergic or histaminergic receptors. 2-ClA induced a biphasic response, while NECA only relaxed these tissue under basal tone. Unlike the relaxation response, these contractile responses were not attenuated by theophylline, but were blocked by 1,3 dipropyl-8-(2 amino-4-chlorophenyl)xanthine (PACPX). These findings confirm the existence of two subpopulations of adenosine receptors in guinea pig trachealis muscle.

Innervation of airway smooth muscle. Efferent mechanisms

The classical view, with one excitatory (cholinergic) and one inhibitory (noradrenergic) component, of the innervation of airway smooth muscle is incomplete and at least two other, possibly peptidergic, types of innervation must be included when the innervation of airways is considered. A summary of these neuronal components is given in Fig. 1 and their possible origin is outlined. Besides the inhibitory noradrenergic innervation of the airways observed in some species, an inhibitory NANC (i-NANC) innervation has been demonstrated. The polypeptide, VIP, seems to be the most likely candidate for the neurotransmitter in the i-NANC innervation of the airways. The excitatory cholinergic innervation is present in the airways from the trachea down to the peripheral bronchi. In the guinea-pig bronchi an excitatory NANC (e-NANC) innervation has been demonstrated as well. The e-NANC nerves may correspond to chemosensitive primary afferent nerves with substance P or a related tachykinin as transmitter. When the innervation of airway smooth muscle of different mammalian species is compared it is evident that all nerve components except the cholinergic, show a considerable variability among species. The cholinergic innervation seems to be present in all mammalian species whereas the other components may be completely absent from some species. Distinct regional variations in the innervation of the airways may occur, which is exemplified by the distribution of the autonomic innervation in the guinea-pig tracheo-bronchial tree. Cholinergic neurotransmission in for example the guinea-pig and human airways can be modulated by NA via prejunctional inhibitory alpha 2-adrenoceptors. Furthermore, the e-NANC neurotransmission in the guinea-pig airways may be modulated by NA or by selective alpha 2-adrenoceptor agonists, acting via prejunctional inhibitory alpha 2-adrenoceptors. The clinical importance of the NANC innervation in relation to asthma is discussed. The i-NANC nerves may exert a modulating effect on bronchoconstriction, and a functional defect would presumably lead to an exaggerated response to constrictor stimuli. The e-NANC nerves in the airways may also be clinically relevant since the transmitter (tachykinins) from these nerves can produce bronchoconstriction and promote inflammation of the airway epithelium, either by direct mechanisms or indirectly by activation of mast cells, and thus contribute to the features of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic inhibition of acetylcholine release

High potassium (51 mM) has been shown to evoke release of acetylcholine ([3H]ACh and endogenous ACh) from cholinergic nerves in rat bronchial smooth muscle. The release of [3H]ACh was reduced by 85% when the Ca2+ concentration was changed from 2 to 0.1 mM. The veratridine-induced release was completely inhibited by tetrodotoxin, but tetrodotoxin did not reduce the potassium-evoked release. The muscarinic agonist, oxotremorine, reduced the potassium stimulated release of [3H]ACh, without affecting the basal release. In contrast, scopolamine substantially potentiated the potassium-evoked release. Adenosine had a dual effect in the rat bronchi. Adenosine inhibited the potassium-evoked release of [3H]ACh and this presynaptic effect of adenosine was antagonized by 8-phenyltheophylline. Adenosine also induced contraction of the bronchial smooth muscle and there was potentiation by adenosine of the ACh-induced contraction. The results indicate that cholinergic nerve terminals in the rat bronchi possess muscarinic receptors which inhibit the release of ACh. Adenosine may have analogous effects, e.g. presynaptic inhibition of transmitter release in addition to postsynaptic enhancement of bronchial smooth muscle contraction.

Apparent enhancement of cholinergic transmission in rabbit bronchi via adenosine A2 receptors

Adenosine and its derivatives enhanced the contractile responses to transmural nerve stimulation in rabbit isolated bronchial smooth muscle. 5'-N-Ethylcarboxamideadenosine (NECA) was the most potent adenosine analogue studied. Enhancement of contractile responses by NECA was competitively antagonized by 8-p-sulfophenyltheophylline. Guanethidine, mepyramine, capsaicin or eicosatetraynoic acid did not antagonize the enhancement elicited by adenosine or NECA. NECA did not enhance the contractile responses to exogenously applied acetylcholine or contractile responses elicited after administration of tetrodotoxin. We suggest that adenosine, via an action at A2 receptors, enhances contractile responses to nerve stimulation in rabbit bronchial muscle. Methylxanthines are competitive antagonists at these extracellular receptors. The enhancement probably involves a sodium-dependent mechanism but not adrenergic mechanisms or release of histamine, substance P or arachidonate metabolites. The enhancement indicates increased cholinergic transmitter release or action, but release of a secondary spasmogenic or decreased release of an inhibitor mediator cannot be excluded. The results may indicate a role for adenosine in asthma.

Inhibition of the excitatory non-adrenergic, non-cholinergic neurotransmission in the guinea pig tracheo-bronchial tree mediated by alpha 2-adrenoceptors

Guinea pig main bronchi were studied in vitro for the presence of motor innervation. Field stimulation of the main bronchi revealed that besides the excitatory cholinergic component a slow and atropine resistant contraction similar to that previously found in the hilus bronchi (Grundström et al. 1981a) was present also at this level of the airways. This slow contraction was blocked by tetrodotoxin whereas it was left unaffected by a number of conventional antagonists (i.e. atropine, guanethidine, propranolol, yohimbine and prazosin) suggesting that it was due to activation of non-adrenergic, non-cholinergic neurones. In the presence of a beta-adrenoceptor blocking drug, noradrenaline inhibited the slow contractions induced by field stimulation in both the main and hilus bronchi. By contrast, contractions elicited by histamine were left unaffected by noradrenaline. In order to characterize the receptor for noradrenaline the effects of alpha1-and alpha2-adrenoceptor blockers were evaluated. The results indicate that in the guinea pig bronchi noradrenaline inhibits non-adrenergic, non-cholinergic neurotransmission by acting on prejunctional alpha 2-adrenoceptors.

Relaxant beta-receptors in the trachea, but not prejunctional alpha 2-receptors in the tracheal cholinergic neurones, are subjected to homologous desensitization

Guinea-pigs were pretreated with either isoprenaline, terbutaline or the alpha 2 agonist B-HT 920 in order to asses the hypothesis that beta-and alpha 2 receptors in trachea are subjected to homologous desensitization. In these experiments the beta receptor activity was investigated on tracheal ring preparations contracted with carbacholine. Both in the isoprenaline and the terbutaline pretreated group the relaxant responses to beta agonists were diminished. Pretreatment with B-HT 920 did not affect the sensitivity of the trachea to beta stimulation. In order to asses the responsiveness of alpha 2 receptors the trachea was contracted by electrical field stimulation in the presence of propranolol. During these conditions contractions were mediated by activation of cholinergic neurones and inhibitory effects of alpha stimulation were due to inhibition of the cholinergic neurotransmission by stimulation of prejunctional alpha 2 receptors. In these tests neither isoprenaline, terbutaline nor B-HT 920 pretreatment affected the responsiveness of alpha stimulation to inhibit the electrically induced contractions.