Characterization of muscarinic receptors mediating release of epithelial derived relaxant factor (EpDRF) in guinea-pig isolated trachea

Cromakalim inhibits transmitter acetylcholine release in rat trachea by an action on epithelial cells and a diffusible factor

The present study was undertaken to investigate further the effects of the potassium channel opening drug cromakalim on the release of transmitter acetylcholine from cholinergic nerves of rat isolated trachea by using two tracheal preparations superfused in series. In all experiments, the lower chamber contained an epithelium-denuded preparation which had been incubated with [3H]-choline to incorporate [3H]-acetylcholine into cholinergic transmitter stores, whereas the upper chamber contained an unlabelled, epithelium-intact or epithelium-denuded preparation. When the upper chamber contained an epithelium-intact tracheal preparation, cromakalim (1-100 micro M) significantly reduced the stimulation-induced (S-I) efflux of [3H]-acetylcholine from the radiolabelled, epithelium-denuded tracheal preparation in the lower flow chamber. In contrast, when the upper flow chamber contained an epithelium-denuded preparation, cromakalim (10 micro M) was without effect on the S-I efflux. Glibenclamide (1 micro M), an ATP-sensitive potassium channel blocker, was without effect on the S-I efflux when the upper chamber contained an unlabelled, epithelium-intact tracheal preparation. However, glibenclamide (1 micro M) prevented the inhibition of the S-I efflux by cromakalim (10 micro M). When the upper chamber contained an epithelium-intact tracheal preparation and the lower chamber contained an epithelium-denuded tracheal preparation, cromakalim (10 micro M), when infused through the side-arm of the T-piece, such that only the lower radiolabelled epithelium-denuded tracheal preparation was exposed to the drug, was without effect on the S-I efflux. The findings of the present study have provided evidence of an inhibitory action of the potassium channel opener cromakalim on transmitter acetylcholine release in rat trachea which is dependent on the functional integrity of the tracheal epithelium. The findings suggest that cromakalim may inhibit transmitter acetylcholine release by opening ATP-sensitive potassium channels, probably, on cells in the epithelial layer to release a putative epithelial factor, which in turn acts prejunctionally to mediate the inhibitory effect of cromakalim.

Influence of the epithelium on acetylcholine release from parasympathetic nerves of the rat trachea

1. The present study was undertaken to investigate the influence of the airway epithelium on the release of acetylcholine (ACh) from parasympathetic nerves of the rat trachea. Epithelium-intact and epithelium-denuded preparations of rat trachea were incubated with [3H]-choline to incorporate [3H]-ACh into the cholinergic transmitter stores. Release of radiolabelled transmitter ACh was evoked by electrical field stimulation (60 s trains of 1 ms pulses, 5 Hz, 15 V). 2. Field stimulation both of epithelium-intact and epithelium-denuded radiolabelled tracheal preparations evoked an increase in the efflux of radioactivity; however, the mean stimulation-induced (S-I) efflux from epithelium-denuded preparations (2932 +/- 190 d.p.m., n = 9) was approximately 60% of that from epithelium-intact preparations (4802 +/- 820 d.p.m., n = 11). We have shown previously that, in epithelium-intact (but not epithelium-denuded) tracheal preparations, a substantial proportion of the S-I efflux is resistant to tetrodotoxin (1 microM) and to the removal of extracellular Ca2+, indicating that much of the S-I efflux is not caused by exocytotic release of neuronal [3H]-ACh. In epithelium-denuded tracheal preparations, superfused individually, phosphorylcholine (1 and 100 microM) did not alter S-I efflux. In epithelium-intact tracheal preparations, both in the absence and in the presence of atropine (1 microM), neither N(G)-nitro-L-arginine (100 microM), superoxide dismutase (100 units ml(-1)), indomethacin (10 microM), capsaicin (30 microM) nor alpha-chymotrypsin (1 unit ml(-1)) altered S-I efflux. 3. Experiments were also performed using two tracheal preparations superfused in series. When unlabelled epithelium-intact preparations were present in the upper chamber (superfused first), the S-I efflux from radiolabelled epithelium-denuded preparations in the lower chamber (superfused second) did not differ significantly from radiolabelled epithelium-denuded preparations superfused individually. Moreover, there was no significant difference in the S-I efflux from radiolabelled epithelium-denuded preparations in the lower chamber between experiments in which the upper chamber contained epithelium-intact or epithelium-denuded preparations. 4. Field stimulation of epithelium-intact tracheal preparations in the upper chamber with 90, 120 and 300-s periods (trains of 1 ms pulses, 5 Hz, 15 V) did not significantly alter the S-I efflux from radiolabelled epithelium-denuded tracheal preparations in the lower chamber. 5. When introduced into the upper (unlabelled epithelium-intact) and subsequently allowed to superfuse the lower (radiolabelled epithelium-denuded) tracheal preparations, the stable cholinomimetic carbachol (3 microM) markedly reduced the S-I efflux whereas ACh (0.1 and 1 microM) had no significant effect. However, in the presence of the anti-cholinesterase neostigmine (1 microM), ACh (1 microM) significantly reduced S-I efflux, indicating that ACh is subject to rapid hydrolysis by cholinesterase enzymes. When atropine (10 microM) was only exposed to radiolabelled epithelium-denuded preparations in the lower chamber, the inhibitory effects of ACh (1 microM) and carbachol (3 microM) on S-I efflux were prevented. 6. In conclusion, the findings of the present study do not support the notion that the airway epithelium exerts an inhibitory influence on ACh release from parasympathetic nerves of the rat trachea. Alternatively, if epithelium-dependent modulation of cholinergic transmission does occur in the rat trachea, then the mechanism does not appear to involve phosphorylcholine, nitric oxide, superoxide radicals, cyclo-oxygenase products of arachadonic acid, capsaicin-sensitive neuropeptides or vasoactive intestinal peptide. Moreover, the inhibitory effect of carbachol and ACh on transmitter ACh release in the rat trachea appears to be due solely to activation of prejunctional inhibitory muscarinic cholinoceptors on parasympathetic nerves and does not involve the liberation of a putative epithelium-derived inhibitory factor.

Muscarinic cholinoceptor subtypes mediating tracheal smooth muscle contraction and inositol phosphate generation in guinea pig and rat

The effects of the muscarinic cholinoceptor antagonists atropine (non-selective), pirenzepine (M1-selective), methoctramine (M2-selective) and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; M3-selective) were examined on the responsiveness of guinea pig and rat tracheal tissue to acetylcholine and carbachol. Results indicate that smooth muscle contraction in isolated tracheal tissue from both species was mediated primarily by muscarinic M3 cholinoceptors. The effects of atropine, pirenzepine and 4-DAMP were similar against the contractile actions of acetylcholine and carbachol in both species and in epithelium-intact and epithelium-denuded tissue. In contrast, differences in the effects of methoctramine in antagonising contractile responses to acetylcholine and carbachol were observed between the two species and following epithelium removal in the guinea pig. Thus, whilst this study has found that tracheal smooth muscle contraction in the guinea pig and rat is mediated primarily by muscarinic M3 cholinoceptors, anomalies in the functional inositol phosphate generation results obtained with the muscarinic cholinoceptor antagonists highlight species differences in the actions of acetylcholine and carbachol in eliciting smooth muscle contraction suggesting the possible existence of functional non-M3 muscarinic cholinoceptors.

Possible functional modulation by acetylcholine of nitric oxide on guinea pig isolated trachea

The aim of this study was to evaluate whether acetylcholine induces NO release. We determined the responses on the cholinergic component of the response to electrical field stimulation (EFS) the effects of L-nitro-arginine-methyl-ester (L-NAME; 1 mM), an inhibitor of NO synthase, of L-Arginine (L-ARG; 1 mM), a precursor of NO synthesis, and methoctramine (0.01-0.1-1 microM), an antagonist of M2 receptors, alone or associated with L-NAME. The experiments were performed on guinea pig isolated intact- or denuded-epithelium tracheal rings contracted in a frequency-dependent manner to EFS. At the maximum frequency tested (30 Hz), the contractile response elicited was 60.36 +/- 0.61% of acetylcholine (100 microM) contraction, while the maximal relaxant effect induced by EFS was -28.40 +/- 0.61% in epithelium intact preparations. A pretreatment with L-NAME significantly (P<0.05) increased the contraction (76.08 +/- 1.39%) and reduced the relaxation elicited by EFS. L-NAME effect on both EFS induced responses were statistically (P<0.05) reversed by the association L-NAME + L-ARG. Methoctramine (1 microM) enhanced contractile (P<0.05) (79.20 +/- 2.21%), as well as relaxant responses (-38.73 +/- 0.99%) elicited by EFS in guinea pig epithelium-intact tracheal rings; in a separate series of experiments, performed on guinea pig epithelium-intact rings, L-NAME increased the contractile responses to methoctramine (82.6 +/- 2.31), but reduced the relaxant ones (26.38 +/- 1.29). In contrast, at the maximum frequency tested, it increased only the contractile response, but not modify the relaxant one, in epithelium denuded rings. In conclusion, the present data showed that the release of acetylcholine from postganglionic cholinergic nerves plays an important role on NO formation and this effect may be modulate by epithelium.

Epithelial factors: modulation of the airway smooth muscle tone

The airway epithelium is composed of a heterogeneous population of cells. This epithelial layer is not only a physical barrier but also a target responding to a variety of inflammatory mediators. These cells can respond by releasing contracting and relaxing factors to modulate airway responsiveness. They can also metabolize some of the inflammatory mediators. Epithelial damage is a consistent feature of some respiratory conditions, but whether or not such damage contributes to airway disease is for the moment unknown. This review summarizes the literature on the known and proposed roles of the epithelium in the modulation of the airway smooth muscle tone.

Characterization of muscarinic receptors that mediate contraction of guinea-pig isolated trachea to choline esters: effect of removing epithelium

1. The muscarinic receptor subtype that mediates contraction of guinea-pig trachea, in the presence and absence of epithelium, to acetic and carbamic acid choline esters was determined by use of preferential muscarinic receptor antagonists: pirenzepine (M1 receptor), methoctramine (M2 receptor) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) (M3 receptor). 2. Acetylcholine (ACh), methacholine (MeCh), carbachol (CCh), bethanechol (BeCh) and oxotremorine induced concentration-dependent contraction of guinea-pig isolated tracheal strips in the presence and absence of epithelium. Contraction to acetic choline esters (ACh and MeCh) was augmented by removal of the epithelium, whereas contraction to carbamic acid choline esters (CCh and BeCh) and oxotremorine was not influenced by removal of the epithelium. 3. Pirenzepine, methoctramine and 4-DAMP caused parallel rightward displacements of the concentration-contraction curves to the muscarinic agonists. The pA2 values (determined from Arunlakshana-Schild graphs) for pirenzepine and 4-DAMP in guinea-pig trachea in the presence of epithelium were: ACh as the agonist, 7.6 and 9.0, respectively; CCh as the agonist, 7.6 and 9.1, respectively. The apparent pKB values for methoctramine with the same system were: ACh as the agonist, 5.6; CCh as the agonist, 5.6. Similar values were obtained with MeCh, BeCh and oxotremorine as the agonists. These values were agonist- and epithelium-independent. 4. It is concluded from the pA2 and apparent pKB values obtained for the muscarinic receptor antagonists used in this study that contraction of guinea-pig isolated trachea, with and without epithelium, to both acetic and carbamic acid choline esters is mediated via the muscarinic M3 receptor subtype.Differential contractile responses of guinea-pig trachea to acetic and carbamic acid choline esters upon the mechanical removal of the epithelium may not be explained by activation of different muscarinic receptor subtypes by these agonists.