Inhibitory effects of sympathomimetic drugs on cholinergically mediated contractions of guinea-pig isolated tracheal muscle

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.

Paradoxical facilitation of acetylcholine release from parasympathetic nerves innervating guinea-pig trachea by isoprenaline

1. Previous studies have provided evidence that activation of beta-adrenoceptors on cholinergic nerve terminals can inhibit neurotransmission in the airways. However, in most cases, this conclusion has been based on indirect evidence obtained from mechanical experiments where changes in airways smooth muscle tone were measured. 2. We have assessed whether modulation of cholinergic neurotransmission by beta-adrenoceptor agonists is due to a pre- or post-junctional action by investigating the effect of isoprenaline on contractile responses evoked by exogenous acetylcholine (ACh) and electrical field stimulation (EFS; 4 Hz, 40 V, 0.5 ms pulse width every 15 s), and on EFS-induced ACh release from cholinergic nerves innervating guinea-pig and human trachea. Furthermore, the subtype of beta-adrenoceptor which modulates neurotransmission and the potential role of cyclic AMP in this response were evaluated. 3. In guinea-pig trachea, isoprenaline (1 nM-1 microM) inhibited the contractile response evoked by exogenous ACh (1 microM) to a similar extent to that evoked by EFS (EC50 = 19.9 and 23 nM, respectively). 4. In epithelium-denuded guinea-pig strips treated with indomethacin (10 microM), isoprenaline significantly enhanced EFS-induced ACh release from cholinergic nerve terminals (by 36% at 0.3 microM). This effect was blocked by propranolol and ICI 118, 551 (each 0.1 microM). In contrast, isoprenaline failed to affect EFS-induced ACh release from parasympathetic nerves innervating human trachea. 5. To evaluate the role of cyclic AMP in the beta-adrenoceptor-induced facilitation of cholinergic neurotransmission, the effects of various cyclic AMP elevating drugs on ACh release were studied. Forskolin (10 microM) significantly augmented (by 17%) EFS-induced ACh release, an effect which was not reproduced by 1,9-dideoxyforskolin (10 microM) which does not activate adenylyl cyclase. Similarly, the cyclic AMP analogue, 8-bromo-cyclic AMP (1 mM) and cholera toxin (1 microgram ml-1) facilitated ACh output by 22 and 47% respectively, whereas prostaglandin E2 (PGE2, 0.1 nM-1 microM) inhibited this response (by 67% at 1 microM). 6. Zardaverine (10 microM), a dual inhibitor of the phosphodiesterase (PDE)3 and PDE4 isoenzyme families, did not affect EFS-induced ACh release and failed to facilitate the actions of either isoprenaline or PGE2. Similarly, neither SK&F 94120 (10 microM) nor rolipram (10 microM), selective inhibitors of PDE3 and PDE4 respectively, significantly affected the release of ACh in response to EFS. 7. The result of this study suggests that isoprenaline facilitates cholinergic neurotransmission in guinea-pig, but not human, trachea by activation of pre-junctional beta 2-adrenoceptors, an effect that may be mediated via activation of the cyclic AMP/cyclic AMP-dependent protein kinase cascade. Furthermore, the data presented herein illustrate the need to undertake direct measurements of neurotransmitter release when examining the effect of agents purported to act pre-junctionally.

Beta 2- but not beta 3-adrenoceptors mediate prejunctional inhibition of non-adrenergic non-cholinergic contraction of guinea pig main bronchi

We studied the effects of selective beta-adrenoceptor agonists on the cholinergic and non-adrenergic non-cholinergic (excitatory NANC) contractions elicited by electrical field stimulation of guinea pig main bronchi in vitro. Addition of the selective beta 2-adrenoceptor agonists, fenoterol and salbutamol, and the selective beta 3-adrenoceptor agonist, BRL 37344 (4-[2-[(2-hydroxy-2-(3-chlor-phenyl)ethyl)amino]-propyl]-phenoxyac etic acid), induced a dose-dependent inhibition of the cholinergic contraction (pD2 7.89, 6.71 and 4.56, respectively) and the excitatory NANC response (pD2 9.11, 8.16 and 7.42, respectively). Fenoterol- and BRL 37344-induced inhibition of the excitatory NANC response was blocked with high potency (pKB 8.77 and 9.07, respectively) by the selective beta 2-adrenoceptor antagonist, ICI 118,511 (erythro-1-(7-methylindan-4-yloxy)-3-(isopropylamino)-but an-2-ol). A comparable contraction induced by neurokinin A (2 or 5 nM) was also inhibited by fenoterol, salbutamol and BRL 37344, but at significantly higher concentrations than for the inhibition of the excitatory NANC response (pD2 8.72, 7.56 and 6.66, respectively). Such a preferential inhibition of electrical field stimulation- versus agonist-induced effects was not observed for cholinergic contractions (pD2 versus methacholine-induced tone 7.86, 6.93 and 5.10, respectively). The results clearly exclude the involvement of beta 3-adrenoceptors in these responses. Furthermore they show that beta 2-adrenoceptors are involved in the prejunctional inhibition of excitatory NANC contractions, presumably via modulation of tachykinin release from sensory nerves, and solely in the postjunctional inhibition of cholinergic contractions.

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 anti-asthma drugs and potassium channel openers on neurally-mediated contraction of isolated guinea pig trachea

The effects of anti-asthma drugs, isoproterenol, aminophylline and hydrocortisone, and potassium channel openers on the contraction induced by electrical stimulation or exogenously applied acetylcholine were investigated in isolated guinea pig trachea. Isoproterenol and aminophylline non-selectively inhibited both the contraction evoked by vagus nerve- and that by transmural field-stimulation, but had no effect on the response induced by exogenously applied acetylcholine. Hydrocortisone and potassium channel openers, NIP-121 and cromakalim, preferentially inhibited vagus nerve-mediated response. These results suggest that anti-asthma drugs may have an ability to inhibit neurally-mediated contraction in the guinea pig trachea.

Inhibitory effects of non-narcotic antitussive drugs on cholinergically and non-cholinergically mediated neurogenic contractions of guinea-pig isolated bronchial muscle

We have examined the actions of non-narcotic antitussive drugs on the response evoked by electrical field stimulation or by acetylcholine (ACh) and neurokinin A (NKA) on guinea-pig bronchial strip chain. Electrical field stimulation (1-32 Hz, 0.5 ms, 30 V for 5 s) evoked a biphasic contraction in a frequency-dependent manner, consisting of a cholinergically mediated fast contraction followed by a non-cholinergically mediated slow contraction. Dextromethorphan (1-300 microM) and tipepidine (0.1-100 microM) caused a concentration-dependent inhibition in the height of the biphasic contraction, but noscapine (1-300 microM) was less effective. Submaximal contractions of bronchial muscle evoked by exogenous ACh (1-30 microM) were inhibited by tipepidine (10-100 microM), but not by dextromethorphan (10-100 microM) or noscapine (10-100 microM), while those evoked by exogenous NKA (10-300 nM) were augmented by these drugs. The results indicate that in guinea-pig isolated bronchial muscle, dextromethorphan inhibited both neurally-mediated responses but not those to the exogenously applied agent. Tipepidine caused an inhibition similar to the non-cholinergically mediated response of dextromethorphan, it also caused a more profound inhibition of the cholinergically mediated response and selectively antagonized ACh. Noscapine had no effect.

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)

Morphine and opioid peptides selectively inhibit the non-cholinergically mediated neurogenic contraction of guinea-pig isolated bronchial muscle

The experiments examine the actions of morphine and opioid peptides on the responses evoked by electrical field stimulation or by acetylcholine (ACh) and substance P (SP) in guinea-pig bronchial strip chain. Electrical field stimulation evoked a biphasic contraction, consisting of a cholinergically mediated fast contraction followed by a non-cholinergically mediated slow contraction. Morphine and opioid peptides caused a concentration-dependent inhibition in the height of the non-cholinergic contraction. The order of inhibitory activity was BW443C greater than dynorphin greater than morphine greater than beta-endorphin greater than leucine-enkephalin greater than methionine-enkephalin. Cholinergically mediated contractions were less potently inhibited by these opioids. Submaximal contractions of bronchial muscle evoked by exogenous ACh (2 microM) or SP (0.2 microM) were not inhibited by morphine (100 microM) or opioid peptides (3-10 microM), rather, they were augmented. The results indicate that in guinea-pig isolated bronchial muscle, morphine and opioid peptides can selectively inhibit excitatory non-cholinergic neurotransmission via prejunctional opioid receptors.

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.

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.