Mechanism of the potentiation of neurally-induced bronchoconstriction by gallamine in the guinea-pig

Characterization of the muscarinic receptor subtype(s) mediating contraction of the guinea-pig lung strip and inhibition of acetylcholine release in the guinea-pig trachea with the selective muscarinic receptor antagonist tripitramine

1. The muscarinic receptor subtypes mediating contraction of the guinea-pig lung strip and inhibition of the release of acetylcholine from cholinergic vagus nerve endings in the guinea-pig trachea in vitro have previously been characterized as M2-like, i.e. having antagonist affinity profiles that are qualitatively similar but quantitatively dissimilar compared to cardiac M2 receptors. The present study sought to establish definitely the identity of these receptor subtypes by using the selective muscarinic receptor antagonist, tripitramine. Guinea-pig atria and guinea-pig trachea (postjunctional contractile response) were included for reference. 2. It was found that tripitramine antagonized methacholine-induced contractions of the guinea-pig lung strip with pKB value of 8.76 +/- 0.05. Both the parallel shifts of the concentration-response curves and the slope of the Schild plot begin not significantly different from unity (when antagonist preincubation was for 2 h) indicated the involvement of a single population of receptors in the contractile response. From the pKB values obtained with tripitramine and a range of other selective muscarinic receptor antagonists (cf. Roffel et al., 1993), this single population of receptors can only be classified as M2-like. 3. Tripitramine antagonized methacholine-induced chronotropic and inotropic responses in guinea-pig right and left atria with apparent pKB values of 9.4-9.6. However, such values were only obtained when antagonist preincubation was relatively long and/or antagonist concentration relatively high (e.g with 1 h at 100 or 300 nM but 3 h at 30 nM). It thus appears that low concentrations of tripitramine do not readily equilibrate with M2 receptors in guinea-pig atria nor with M2-like receptors in the guinea-pig lung strip. 4. Tripitramine increased electrical field stimulation-induced cholinergic twitch contractions in guinea-pig trachea in concentrations of 0.3-100 nM, by blocking prejunctional muscarinic inhibitory autoreceptors; with higher concentrations, twitch contractions were progressively diminished, as a result of blocking postjunctional M3 receptors (apparent pKB value 6.07 +/- 0.15). The pEC20 value (-log concentration that increases twitch by 20% maximum) was 8.29 +/- 0.08, which would suggest that M4 receptors are involved in this response. 5. Oxotremorine-induced inhibition of the release of prelabelled [3H]-acetylcholine from guinea-pig trachea, under conditions where there is no auto-feedback, was blocked by tripitramine (2 h preincubation) with a pKB value of 8.56 +/- 0.06. The slope of the corresponding Schild plot was not significantly different from unity, which together with the parallel shifts of the concentration-response curves indicated the involvement of a single muscarinic receptor subtype. 6. Since the pKB value for tripitramine at prejunctional receptors in guinea-pig trachea is in between the affinities towards M2 and M4 receptors, correlation plots were constructed to compare the pKB values obtained with tripitramine and a range of other selective muscarinic receptor antagonists (cf. Kilbinger et al., 1995) to reported affinities at M1-M4 receptors. This showed rather similar distribution patterns of the data points around the line of equality in the case of M2 and M4 receptor subtypes. However, the correlation coefficient was markedly better for M2 (0.9667) than for M4 (0.5976). Since recent evidence suggests that M4 receptors are not expressed in cholinergic nerves from guinea-pig trachea, it is concluded that prejunctional muscarinic autoinhibitory receptors in this tissue exhibit an atypical M2 type character, with a pharmacological profile distinct from cardiac M2 receptors.

Methoctramine induces nonspecific airway hyperresponsiveness in vivo

We investigated the effects of subtype-selective muscarinic receptor antagonists upon aerosol antigen-induced bronchoconstriction in anesthetized guinea pigs. Neither pirenzepine (muscarinic M1 receptor-selective), 4-methylpiperidine methiodide (4-DAMP, muscarinic M3 receptor-selective), [N-iminomethyl-N'-[(2-hydroxy-2-phenyl-2-cyclohexyl)-ethyl] piperazine HCl (DAC-5945, muscarinic M3 receptor-selective), ipratropium or atropine inhibited bronchoconstriction, but methoctramine (muscarinic M2 receptor-selective) produced a dose-dependent increase in bronchoconstriction (up to 46%). Methoctramine also produced increases in bronchoconstriction induced by aerosols of histamine (up to 45%) and platelet activating factor (up to 118%), demonstrating nonspecific airway hyperresponsiveness. This effect of methoctramine was not inhibited by atropine, DAC-5945 or vagotomy and could not be attributed to altered arachidonic acid metabolism or beta-adrenergic antagonism. However, propranolol prevented methoctramine-induced airway hyperresponsiveness, suggesting that this effect resulted from the reported ganglionic blocking activity of methoctramine. In conclusion, muscarinic receptors do not appear to play an important role in antigen-induced bronchoconstriction in anesthetized guinea pigs. Furthermore, caution should be exercised in using methoctramine to characterize the roles of muscarinic receptors in airway inflammatory responses in vivo.

The interaction of selective and non-selective antagonists with pre- and postjunctional muscarinic receptor subtypes in the guinea pig trachea

Muscarinic receptor antagonists were used to study prejunctional M2 and postjunctional M3 receptors in the isolated guinea pig trachea. The effects of four M2-selective muscarinic receptor antagonists (gallamine, methoctramine, AQ-RA 741 and AF-DX 116) were studied on twitch contractions, elicited by electrical field stimulation, of tracheal ring preparations. M1-selective (pirenzepine, (+)- and (-)-telenzepine), M3-selective (4-DAMP-methobromide and UH-AH 371) and non-selective (atropine and ipratropium) muscarinic receptor antagonists were also used. The clear potentiation of the twitch contractions and the subsequent strong inhibition observed with M2-selective antagonists demonstrate antagonism at prejunctional M2 and postjunctional M3 muscarinic receptors, respectively. The maximal potentiation correlated well with the M2/M3-selectivity known from binding experiments: gallamine > methoctramine > AQ-RA 741 > AF-DX 116. Strong correlations were also found between the pEC20 values for potentiation of the twitch response and the pKi values for bovine cardiac M2 muscarinic receptors and between the pIC50 values for inhibition of the twitch response and the pA2 values for M3 muscarinic receptors as determined on non-stimulated methacholine-contracted tracheal smooth muscle preparations. Thus, study of the effects of a wide concentration range of putative M2-selective muscarinic receptor antagonists on the twitch contractions of single tracheal rings induced by low-intensity electrical field stimulation yields information about M2/M3 receptor selectivity and about prejunctional M2 and postjunctional M3 receptor affinity within the same experiment.

(R)-alpha-methylhistamine augments neural, cholinergic bronchospasm in guinea pigs by histamine H1 receptor activation

In the airways, activation of histamine H3-receptors with (R)-alpha-methylhistamine inhibits neurally induced cholinergic contractions in vitro and peptidergic responses in vivo. The role of histamine H3-receptors on the cholinergic bronchoconstriction induced by electrical stimulation of the dorsal medulla in guinea pigs was assessed in this study. There was no evidence for an H3-receptor mediated inhibition of cholinergic bronchospasm in vivo. However, there was potentiation of central cholinergic bronchoconstriction by (R)-alpha-methylhistamine or histamine by a mechanism involving H1-receptors. I.v. (R)-alpha-methylhistamine (0.3-3 mg/kg) or histamine (0.001-0.01 mg/kg) produced a transient bronchospasm and potentiated the bronchoconstriction due to medullary stimulation. These effects of (R)-alpha-methylhistamine and histamine were blocked by the histamine H1-antagonist, chlorpheniramine (30 micrograms/kg i.v.) but not by H2- or H3-receptor antagonists. (R)-alpha-Methyl-histamine did not potentiate the bronchoconstriction due to i.v. methacholine. Other bronchoconstrictor agents such as methacholine and serotonin did not potentiate the CNS-induced bronchospasm.

Actions of methoctramine, a muscarinic M2 receptor antagonist, on muscarinic and nicotinic cholinoceptors in guinea-pig airways in vivo and in vitro

1. The effects of the muscarinic M2 receptor antagonist methoctramine, on contractions of airway smooth muscle induced by cholinergic nerve stimulation and by exogenously applied acetylcholine (ACh), have been investigated in vivo and in vitro in guinea-pigs. 2. Stimulation of the preganglionic cervical vagus nerve in anaesthetized guinea-pigs, caused bronchoconstriction and bradycardia which were mimicked by an intravenous dose of ACh. The muscarinic M2 antagonist, methoctramine (7-240 nmol kg-1), inhibited the bradycardia induced by both vagal stimulation and ACh (ED50: 38 +/- 5 and 38 +/- 9 nmol kg-1, respectively). In this dose-range, methoctramine facilitated vagally-induced bronchoconstriction (ED50: 58 +/- 5 nmol kg-1), despite some inhibition of ACh-induced bronchoconstriction (ED50: 81 +/- 11 nmol kg-1). The inhibition of ACh-induced bronchoconstriction and hypotension was dose-dependent, but was not statistically significant until doses of 120 nmol kg-1 and 240 nmol kg-1 respectively. 3. In the guinea-pig isolated, innervated tracheal tube preparation, methoctramine (0.01-1 microM) caused facilitation of contractions induced by both pre- and postganglionic nerve stimulation, whereas contractions induced by exogenously applied ACh were unaffected. Higher concentrations of methoctramine (greater than or equal to 10 microM), reduced responses to both nerve stimulation and exogenous ACh, indicating blockade of post-junctional muscarinic M3 receptors. 4 ACh caused a slow maintained increase in tone of the tracheal tube and at the same time reduced the contractions induced by nerve stimulation. This inhibitory effect of ACh on neuronally mediated responses was antagonized by methoctramine (0.01-1 microM) in a concentration-dependent manner. However, the ACh-induced tone change was unaffected by methoctramine in this concentration-range, indicating a lack of muscarinic M3 receptor antagonist activity in this concentration-range.5. The effect of methoctramine on responses induced by pre- and postganglionic nerve stimulation was not identical. At concentrations of methoctramine of 1 ,microM and greater, preganglionic stimulation-induced contractions were reduced when compared to those induced by postganglionic stimulation, suggesting an inhibitory effect of methoctramine on ganglionic transmission. This ganglion blocking action of methoctramine was not due to its reported M1 receptor antagonist activity (blocking facilitatory Ml receptors in the ganglia) since pirenzepine was without effect in this preparation. We believe that the ganglionic blocking action of metoctramine is due to its nicotinic receptor antagonist properties, since the concentration of methoctramine inhibiting ganglionic transmission in the tube preparation (1 microM) was shown to inhibit contractions induced by the nicotinic agonist, 1,1-dimethyl-4-phenyl-piperazine in tracheal strips.6. These results show that methoctramine is able to demonstrate adequately the presence of autoinhibitory receptors functionally both in vivo and in vitro and confirms their pre-junctional location on pulmonary cholinergic nerve terminals and their classification as muscarinic M2 subtypes. These results also indicate that while methoctramine is a potent muscarinic M2 receptor antagonist, it does not possess the required selectivity to discriminate between cholinoceptor subtypes in preparations, such as the airways, where mixed populations of muscarinic and nicotinic cholinoceptors exist.

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.

Ganglion-blocking agents enhance neurally mediated bronchoconstriction in the guinea-pig: possible role of sensory neuropeptides

The effect of ganglion blockade by hexamethonium bromide (0.1-100 mumol.kg-1) and pentolinium tartrate (0.01-3 mumol.kg-1) on the bronchoconstriction induced by vagal nerve stimulation (15 Hz, 0.2 ms, 3 s, 7-20 V) was evaluated in the anaesthetized guinea-pig. Both ganglion-blocking agents potentiated this response dose dependently. When the neural bronchoconstriction was suppressed by atropine, hexamethonium restored this response dose dependently. Hexamethonium produced inhibitory effects on vagally induced bronchoconstriction in capsaicin-desensitized and in propranolol- or reserpine-pretreated guinea-pigs. Propranolol (0.03-3 mumol.kg-1) produced a marked dose-dependent increase of neural bronchoconstriction (which was markedly reduced, about 10 times) in capsaicin-desensitized animals. Our results show that ganglion-blocking agents potentiate neural bronchoconstriction in the guinea-pig and that sensory neuropeptides may have a role in this effect. Moreover, beta-adrenergic modulation of the release of neuropeptides from vagal sensory fibers is suggested.