Muscarinic inhibition of acetylcholine release from a novel in vitro preparation of the guinea-pig trachea

Isolated porcine bronchi provide a reliable model for development of bronchodilator anti-muscarinic agents for human use

BACKGROUND AND PURPOSE

In human airways, muscarinic acetylcholine receptors (mAChRs) exert a predominant role in the control of airways resistance and anti-muscarinic agents are currently included in the pharmacological treatment of chronic obstructive pulmonary disease (COPD). However, the development of more effective mAChR antagonists is hampered by considerable species variability in the ultrastrucural and functional control of airway smooth muscle, making extrapolation of any particular animal model questionable. This study was designed to characterize the mAChRs in a bronchial preparation from pigs, animals considered to provide close models of human biology.

EXPERIMENTAL APPROACH

Smooth muscle bronchial strips were examined by electron microscopy in order to compare their neuromuscular structure with that of human bronchi and used to study the affinity of a series of selective mAChR antagonists, estimated as pKis in competition binding assays with NMS and pA2, by Schild analysis, in contractile experiments.

KEY RESULTS

Pharmacodynamic binding parameters and affinity profiles of a series of antagonists were consistent with the presence of a majority of M2 mAChRs along with a minor population of M3 mAChRs. Functionally, the highly significant correlation between postjunctional pA2 affinities and corresponding affinity constants at human recombinant M1-M5 subtypes indicated that smooth muscle contraction in porcine bronchi, as in human bronchi, was dependent on the M3 subtype.

CONCLUSION AND IMPLICATIONS

Based on the characterization of mAChRs, isolated porcine bronchi provide an additional experimental model for development of mAChR antagonists for the treatment of human airway dysfunctions.

Role of carbon monoxide in electrically induced non-adrenergic, non-cholinergic relaxations in the guinea-pig isolated whole trachea

BACKGROUND AND PURPOSE

Nitric oxide (NO) and vasoactive intestinal peptide (VIP) are considered transmitters of non-adrenergic, non-cholinergic (NANC) relaxations in guinea-pig trachea, whereas the role of carbon monoxide (CO) is unknown. This study was designed to assess the participation of CO, and to investigate the localization of haem oxygenase-2 (HO-2), the CO-producing enzyme, in tracheal neurons.

EXPERIMENTAL APPROACH

NANC responses to electrical field stimulation (EFS) at 3 and 10 Hz were evaluated in epithelium-free whole tracheal segments as intraluminal pressure changes. Drugs used were: L-nitroarginine methyl ester (L-NAME, 100 microM) to inhibit NO synthase (NOS), alpha-chymotrypsin (2 U ml(-1)) to inactivate VIP, zinc protoporphyrin-IX (ZnPP-IX, 10 microM) to inhibit HO-2, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, 10 microM), a soluble guanylyl cyclase inhibitor. For immunohistochemistry, tissues were exposed to antibodies to PGP 9.5, a general neuronal marker, HO-2 and NOS, and processed with an indirect immunofluorescence method.

KEY RESULTS

alpha-Chymotrypsin did not affect NANC relaxations. ODQ inhibited NANC responses by about 60%, a value similar to that obtained by combining L-NAME and ZnPP-IX. The combination of ODQ, L-NAME and ZnPP-IX reduced the responses by 90%. Subpopulations of HO-2 positive neurons containing NOS were detected in tracheal sections.

CONCLUSIONS AND IMPLICATIONS

In the guinea-pig trachea, NANC inhibitory responses at 3 and 10 Hz use NO and CO as main transmitters. Their participation is revealed following inhibition of NOS, HO-2 and soluble guanylyl cyclase. The involvement of CO as a relaxing transmitter paves the way for novel therapeutic approaches in the treatment of airway obstruction.

Treatment with Cyclohexenonic Long-Chain Fatty Alcohol Reverses Diabetes-Induced Tracheal Dysfunction in the Rat

In this study, we tried to elucidate the effect of cyclohexenonic long-chain fatty alcohol (N-hexacosanol) on tracheal dysfunction in diabetic rats. Diabetes was induced in 8-week-old male Sprague-Dawley rats by administering an intraperitoneal injection of 50 mg/kg streptozotocin. Non-diabetic control rats received an injection of citrate-phosphate buffer alone. Four weeks after the induction of diabetes, rats were randomly divided into 5 groups: age-matched non-diabetic control rats (group A); 4-week diabetic rats without N-hexacosanol treatment (group B); diabetic rats treated with vehicle (group C), and diabetic rats treated with N-hexacosanol at a dose of 2 or 8 mg/kg i.p. every day for the following 4 weeks (group D and group E, respectively; n = 6-8 animals in each group). Serum glucose and insulin levels were determined, as were the contractile responses induced by carbachol and 100 mmol/l KCl. The participation of M(2) and M(3) receptors was investigated in the trachea by real-time polymerase chain reaction (PCR), hematoxylin and eosin (HE) and immunohistochemical staining. Hypertrophy of airway smooth muscle was observed in diabetic rats, and was ameliorated by treatment with N-hexacosanol. Treatment with either 2 or 8 mg/kg N-hexacosanol did not alter diabetic rat status, i.e., body weight, serum glucose or serum insulin levels, but it significantly reversed the decrease in tracheal wall thickness and diabetes-induced hypercontractility in the rat trachea. In the immunohistochemical studies, muscarinic M(2) and M(3) receptors were expressed in the airway smooth muscle, the elastic fibers, the fibroblast and the surface of epithelium, and these expressions were not altered by either induction of diabetes or N-hexacosanol treatment. The expression of M(3) muscarinic receptor mRNAs in the trachea tended to be increased by the induction of diabetes and normalized when treated with N-hexacosanol. Our data indicate that N-hexacosanol could reverse diabetes-induced hypercontractility in the rat trachea.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

E-ring 8-isoprostanes inhibit ACh release from parasympathetic nerves innervating guinea-pig trachea through agonism of prostanoid receptors of the EP3-subtype

1. In the present study, we examined the effect of E-ring 8-isoprostanes on cholinergic neurotransmission in guinea-pig trachea and identified the receptor(s) involved. As isoprostanes are isomeric with prostaglandins, PGE(2) and sulprostone (a selective EP(3)-receptor agonist) were examined in parallel. 2. 8-Iso-PGE(1), 8-iso-PGE(2) (0.1 nm-1 microM), sulprostone (1 nm-1 microM) and PGE(2) (1 microM) suppressed EFS-evoked [(3)H]ACh release from guinea-pig trachea in a concentration-dependent manner, producing 39.5, 53.9, 61.2 and 59.9% inhibition, respectively, at 1 microM. It should be noted that an established maximum effective concentration was not determined. 3. Neither SQ 29,548 (1 microm; a TP-receptor antagonist) nor AH 6809 (10 microM; an EP(1)-/EP(2)-/DP-receptor antagonist) reversed the inhibitory effect of these compounds. 4. L-798,106, a novel and highly selective EP(3)-receptor antagonist, produced a parallel shift to the right of the concentration-response curves that described the inhibitory action of sulprostone on EFS-evoked contractile responses in guinea-pig vas deferens (an established EP(3)-receptor-expressing tissue), from which a mean pA(2) of 7.48 was derived. On guinea-pig trachea, L-798,106 also antagonised sulprostone-induced inhibition of EFS-induced twitch responses, with similar potency (mean pA(2)=7.82). 5. The inhibitory effects of 8-iso-PGE(1), 8-iso-PGE(2), sulprostone and PGE(2) on EFS-induced [(3)H]ACh release was blocked by L-798,106 at a concentration (10 microM) that binds only weakly to human recombinant EP(1)-, EP(2)- and EP(4)-receptor subtypes expressed in HEK 293 cells. 6. These data suggest that E-ring 8-isoprostanes, PGE(2) and sulprostone inhibit EFS-evoked [(3)H]ACh release from cholinergic nerves innervating guinea-pig trachea, by interacting with prejunctional prostanoid receptors of the EP(3)-subtype.

Modulation of cholinergic responsiveness through the [beta]-adrenoceptor signal transmission pathway in bovine trachealis

The effects of pharmacological stimulation at different levels of the beta-adrenoceptor (AR) pathway, including the receptor, the receptor-coupled Gs protein, and adenylyl cyclase, were studied by simultaneous measurements of acetylcholine (ACh) release and isometric force evoked by electric stimulation in isolated bovine trachealis. The beta-AR agonists isoproterenol (10-6 and 10-5 M) and salbutamol (10-7 to 10-5 M) significantly attenuated both ACh release and contractile force. Forskolin, at 10-6 M, significantly increased ACh release without effect on contractile force, whereas at 10-5 M it increased ACh release but significantly decreased force. Activation of Gs protein by cholera toxin (10 microg/ml) significantly attenuated both ACh release and contractile force, but its effect on ACh release was abolished by calcium-activated potassium (KCa)-channel blocker iberiotoxin (10-7 M). The KCa-channel opener NS-1619 (10-4 M) attenuated significantly both ACh release and contractile force. It is concluded that beta-AR agonists attenuate cholinergic neurotransmission in isolated bovine trachealis model by a mechanism not involving cAMP but KCa channels.

Increased function of inhibitory neuronal M2 muscarinic receptors in trachea and ileum of diabetic rats

1. Release of acetylcholine from parasympathetic nerves is inhibited by neuronal M(2) muscarinic receptors. The effects of streptozotocin-induced diabetes on prejunctional M(2) and postjunctional M(3) muscarinic receptor function in rat trachea and ileum were investigated in vitro. 2. Neuronal M(2) muscarinic receptor function was tested by measuring the ability of an agonist, pilocarpine, to inhibit and an antagonist, methoctramine, to potentiate electrical field stimulation (EFS)-induced contraction of trachea and ileum. Concentration-response curves to pilocarpine and methoctramine were shifted to the left in both to a greater degree in diabetics than controls. 3. In trachea, post-junctional M(3) muscarinic receptor function was increased since maximum contractile responses to the muscarinic agonists acetylcholine and carbachol were greater in diabetics than controls. This increase offset the increased function of the inhibitory neuronal M(2) muscarinic receptors since EFS-induced, frequency-dependent contraction was equal in control and diabetic rats. 4. In contrast, post-junctional M(3) muscarinic receptor function was unchanged by diabetes since concentration-response curves to acetylcholine and carbachol were not different between groups. Thus, EFS-induced contractions of the ileum were decreased in diabetics versus controls. 5. In conclusion, inhibitory M(2) muscarinic receptors on parasympathetic nerves in the trachea and ileum are hyperfunctional in diabetic rats. The function of post-junctional M(3) muscarinic receptors in the trachea, but not the ileum, is also increased in diabetes. 6. The dysfunction of inhibitory, neuronal M(2) muscarinic receptors in the airways may protect against hyperreactivity and in the ileum may contribute to gastrointestinal dysmotility associated with diabetes.

Effect of M2 and M3 muscarinic receptors on airway responsiveness to carbachol in bronchial-hypersensitive (BHS) and bronchial-hyposensitive (BHR) guinea pigs

The expression balance of M2 and M3 muscarinic receptor subtypes on the pathogenesis of airway hyperresponsiveness was investigated by using two congenitally related strains of guinea pigs, bronchial-hypersensitive (BHS) and bronchial-hyposensitive (BHR). CCh-induced airway responses in vivo and in vitro were investigated by comparing the effects of muscarinic receptor subtype antagonists, and the relative amounts of M2 and M3 muscarinic receptor mRNA in tracheal smooth muscle and lung tissue were investigated. After treatment with muscarinic receptor subtype antagonists, the ventilatory mechanics (VT, Raw, and Cdyn) of response to CCh aerosol inhalation were measured by the bodyplethysmograph method. The effects of these antagonists on CCh-induced tracheal smooth muscle contraction were also investigated. The effects of M2 muscarinic receptor blockade were less but the effects of M3 muscarinic receptors blockade on the airway contractile responses were greater in BHS than in BHR. In M3 muscarinic receptor blockades, CCh-induced tracheal contractions in BHS were significantly greater than those in BHR. In tracheal smooth muscle from BHS, the relative amount of M2 muscarinic receptors mRNA was less but that of M3 muscarinic receptor mRNA was more than those in BHR. These results suggest that the high ACh level as a consequence of dysfunction of M2 muscarinic autoreceptors and the excessive effect of M3 muscarinic receptors on the airway smooth muscle may play an important role in the pathogenesis of airway hyperresponsiveness.

Modulation of acetylcholine release in the guinea-pig trachea by the nitric oxide donor, S-nitroso-N-acetyl-DL-penicillamine (SNAP)

The effects of the nitric oxide (NO) donor S-nitroso-N-acetyl-DL-penicillamine (SNAP) and the NO synthase inhibitor L-N(G)-nitroarginine (L-NOARG) on the electrically evoked [(3)H]-acetylcholine release were studied in an epithelium-free preparation of guinea-pig trachea that had been preincubated with [(3)H]-choline. SNAP (100 and 300 microM) caused small but significant increases of the electrically evoked [(3)H]-acetylcholine release (121+/-4% and 124+/-10% of control). Resting outflow of [(3)H]-ACh was not affected by SNAP. The increase by SNAP was abolished by the specific inhibitor of soluble guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-alpha]quinoxalin-1-one (ODQ, 1 microM). The facilitatory effect of SNAP (100 and 300 microM) was reversed into inhibition of release (to 74+/-4% and to 78+/-2%) after pretreatment of the trachea with capsaicin (3 microM). ODQ prevented the inhibition. Capsaicin pretreatment alone did not significantly alter the release of [(3)H]-acetylcholine. A significant inhibition by SNAP (100 microM) of [(3)H]-acetylcholine release (78+/-3%) was also seen in the presence of the NK(2) receptor antagonist SR 48968 (30 nM). L-NOARG (10 and 100 microM) significantly enhanced the electrically-evoked smooth muscle contractions, but caused no significant increases of the evoked release from capsaicin pretreated trachea strips. This might indicate that the inhibitory effect of endogenous NO on acetylcholine release is too small to be detected by overflow studies. It is concluded that NO has dual effects on the evoked acetylcholine release. NO enhances release in the absence of modifying drugs, but NO inhibits acetylcholine release after blockade of the NK(2) receptor or after sensory nerve depletion with capsaicin. This suggests that NO and endogenous tachykinins act in series to produce an increase in acetylcholine release.

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.

Tachykinin NK(2) receptors facilitate acetylcholine release from guinea-pig isolated trachea

The release of newly synthesised [3H]acetylcholine was evoked by electrical field stimulation (5 Hz, 600 pulses) of epithelium-deprived guinea-pig trachea strips after sensory neuropeptides depletion with 3 microM capsaicin. The selective tachykinin NK(2) receptor agonist [betaAla(8)]neurokinin A-(4-10) increased in a concentration-dependent manner the electrically-induced release of [3H]acetylcholine. The facilitatory effect was antagonised by the selective non-peptide tachykinin NK(2) receptor antagonist, SR 48968 (apparent pK(B) 8.9). The tachykinin NK(1) and NK(3) receptor agonists substance P methyl ester and senktide (both 10 and 100 nM), respectively, did not affect the evoked release of [3H]acetylcholine. It is concluded that the cholinergic nerves of guinea-pig trachea are endowed with prejunctional facilitatory tachykinin receptors of the NK(2) subtype.

Antigen-induced bronchial hyperresponsiveness in the rabbit is not dependent on M(2)-receptor dysfunction

We have assessed the effect of sensitization to allergen on airway smooth muscle responsiveness and acetylcholine (ACh) release from cholinergic nerves in tracheal preparations from rabbits immunized at birth to Alternaria tenuis and littermate control rabbits injected with saline. ACh release induced by EFS was significantly greater in tracheal preparations obtained from immunized rabbits compared with littermate controls. The ability of the muscarinic-receptor agonist, oxotremorine, to inhibit ACh release to EFS (4 Hz) was not altered by immunization. The contractile response evoked by electrical field stimulation (EFS), ACh and 5-hydroxytryptamine (5-HT) was not significantly altered in tracheal preparations from antigen immunized rabbits compared with littermate controls. Antigen challenge of immunized rabbits did not affect the release of ACh from isolated trachea following EFS, or the ability of oxotremorine to inhibit ACh release. Furthermore, antigen challenge of immunized rabbits failed to alter the contractile response to EFS or ACh, but reduced the contractile potency of 5-HT. These results demonstrate increased ACh release in tracheal preparations following immunization which had no functional consequence on airway smooth muscle responsiveness. Moreover, the increased release in ACh was not associated with an alteration in M(2)-receptor function. Thus, antigen-induced bronchial hyperresponsiveness in the rabbit does not appear to depend upon M(2)-receptor dysfunction.

Prejunctional muscarinic receptors regulating neurotransmitter release in airways

Prejunctional pA2 values of five muscarinic antagonists were determined in the guinea-pig trachea under stimulation conditions in which the antagonists alone did not enhance acetylcholine release. The antagonists were partly selective at M1 (pirenzepine), M2 (AQ-RA 741, himbacine) and M3 receptors (hexahydrosiladifenidol, dicyclomine). The profile of the antagonist affinities was different from that obtained at cardiac M2 receptors but resembled the profile reported in the literature for the cloned m4 receptor. This suggests that autoinhibition of acetylcholine release in the trachea is mediated via M4 receptors.

Function, signal transduction mechanisms and plasticity of presynaptic muscarinic receptors in the urinary bladder

Presynaptic M1 muscarinic receptors on parasympathetic nerve terminals in rat urinary bladder strips are involved in an autofacilitatory mechanism that markedly enhances acetylcholine release during continuous electrical field stimulation. The facilitatory muscarinic mechanism is dependent upon a PKC mediated second messenger pathway and influx of extracellular Ca2+ into the parasympathetic nerve terminals via L and N-type Ca2+ channels. Prejunctional muscarinic facilitation has also been detected in human bladders. The muscarinic facilitatory mechanism is upregulated in hyperactive bladders from chronic spinal cord transected rats; and the facilitation in these preparations is primarily mediated by M3 muscarinic receptors. Presynaptic muscarinic receptors represent a new target for pharmacological treatment of bladder hyperactivity. If presynaptic facilitation is restricted to the bladder and not present in other tissues then drugs acting at this site might be expected to exhibit uroselectivity.

Role of cAMP and neuronal K+ channels on alpha 2-AR-induced inhibition of ACh release in equine trachea

To investigate the effects of changes in intracellular cAMP on alpha 2-adrenoceptor (AR)-induced inhibition of airway acetylcholine (ACh) release, we examined the effects of the alpha 2-AR agonist clonidine on electrical field stimulation-evoked ACh release from equine tracheal parasympathetic nerves before and after treatment with 8-bromo-cAMP or forskolin. We also tested whether charybdotoxin (ChTX)- or iberiotoxin (IBTX)-sensitive Ca(2+)-activated K+ channels mediate alpha 2-AR-induced inhibition by examining the effect of clonidine in the absence and presence of ChTX or IBTX on ACh release. The amount of released ACh was measured by HPLC coupled with electrochemical detection. Clonidine (10(-7) to 10(-5) M) dose dependently inhibited ACh release before and after treatment with 8-bromo-cAMP (10(-3) M) or forskolin (3 x 10(-5) M). ChTX and IBTX, both at the concentration of 5 x 10(-7) M, significantly increased ACh release; however, they did not alter the magnitude of clonidine-induced inhibition. These results indicated that in equine tracheal parasympathetic nerves, alpha 2-AR-induced inhibition of ACh release is via an intracellular cAMP-independent pathway. Activation of both ChTX- and IBTX-sensitive Ca(2+)-activated K+ channels inhibits the electrical field stimulation-evoked ACh release, but these channels are not involved in the alpha 2-AR-induced inhibition of ACh release.

Prostaglandin E2 suppression of acetylcholine release from parasympathetic nerves innervating guinea-pig trachea by interacting with prostanoid receptors of the EP3-subtype

1. We have demonstrated recently that exogenous prostaglandin E2 (PGE2) inhibits electrical field stimulation (EFS)-induced acetylcholine (ACh) release from parasympathetic nerve terminals innervating guinea-pig trachea. In the present study, we have attempted to characterize the pre-junctional prostanoid receptor(s) responsible for the inhibitory action of PGE2 and to assess whether other prostanoids modulate, at a prejunctional level, cholinergic neurotransmission in guinea-pig trachea. To this end, we have investigated the effect of a range of both natural and synthetic prostanoid agonists and antagonists on EFS-evoked [3H]-ACh release. 2. In epithelium-denuded tracheal strips pretreated with indomethacin (10 microM), PGE2 (0.1 nM-1 microM) inhibited EFS-evoked [3H]-ACh release in a concentration-dependent manner with an EC50 and maximal effect of 7.62 nM and 74% inhibition, respectively. Cicaprost, an IP-receptor agonist, PGF2alpha and the stable thromboxane mimetic, U46619 (each at 1 microM), also inhibited [3H]-ACh release by 48%, 41% and 35%, respectively. PGD2 (1 microM) had no significant effect on [3H]-ACh release. 3. The selective TP-receptor antagonist, ICI 192,605 (0.1 microM), completely reversed the inhibition of cholinergic neurotransmission induced by U-46619, but had no significant effect on similar responses effected by PGE2 and PGF2alpha. 4. A number of EP-receptor agonists mimicked the ability of PGE2 to inhibit [3H]-ACh release with a rank order of potency: GR63799X (EP3-selective) > PGE2 > M&B 28,767 (EP3 selective) > 17-phenyl-omega-trinor PGE2 (EP1-selective). The EP2-selective agonist, AH 13205 (1 microM), did not affect EFS-induced [3H]-ACh release. 5. AH6809 (10 microM), at a concentration 10 to 100 times greater than its pA2 at DP-, EP1- and EP2-receptors, failed to reverse the inhibitory effect of PGE2 or 17-phenyl-omega-trinor PGE2 on [3H]-ACh release. 6. These results suggest that PGE2 inhibits [3H]-ACh release from parasympathetic nerves supplying guinea-pig trachea via an interaction with prejunctional prostanoid receptors of the EP3-receptor subtype. Evidence for inhibitory prejunctional TP- and, possibly, IP-receptors was also obtained although these receptors may play only a minor role in suppressing [3H]-ACh release when compared to receptors of the EP3-subtype. However, the relative importance of the different receptors will depend not only on the sensitivity of guinea-pig trachea to prostanoids but on the nature of the endogenous ligands released locally that have activity on parasympathetic nerves.

Neuromuscular transmission and its pharmacological blockade. Part 1: Neuromuscular transmission and general aspects of its blockade

Blockade of neuromuscular transmission is an important feature during anaesthesia and intensive care treatment of patients. The neuromuscular junction exists in a prejunctional part where acetylcholine is synthesized, stored and released in quanta via a complicated vesicular system. In this system a number of proteins is involved. Acetylcholine diffuses across the junctional cleft and binds to acetylcholinereceptors at the postjunctional part, and is thereafter metabolized by acetylcholinesterase in the junctional cleft. Binding of acetylcholine to its postjunctional receptor evokes muscle contraction. Normally a large margin of safety exists in the neuromuscular transmission. In various situations, apart from up-and-down regulation of acetylcholine receptors, adjustment of acetylcholine release can occur. Pharmacological interference can interrupt the neuromuscular transmission and causes muscle relaxation. For this reason both depolarizing and non-depolarizing muscle relaxants are clinically used. The characteristics of an ideal clinical muscle relaxant are defined. In the description of the pharmacology of the relaxants the importance of pharmacodynamic and pharmacokinetic parameters are defined. Stereoisomerism plays a role with the relaxants. Toxins and venoms also interfere with neuromuscular transmission, through both pre- and postjunctional mechanisms.

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.

M1 muscarinic receptor-mediated facilitation of acetylcholine release in the rat urinary bladder

1. Release of [3H]ACh in response to electrical field stimulation (10 Hz) was measured in strips of rat urinary bladder and cardiac atrial tissues previously incubated with [3H]choline. 2. The volley output of [3H]ACh release was positively correlated with frequency of stimulation in the urinary bladder but negatively correlated in the atrium. 3. The quantity of [3H]ACh release was influenced by the pattern and duration of stimulation. Continuous stimulation (CS) with trains of 100 shocks released 10 times larger amounts of ACh than the same number of shocks presented as short trains of intermittent stimulation (IS): ten shocks per train with 5 s inter-train intervals. 4. The facilitation of transmitter release was antagonized completely by the administration of atropine (1 microM) or pirenzepine (0.05 microM), a selective M1 antagonist. Eserine, an anticholinesterase agent, markedly facilitated ACh release induced by CS and IS. This effect was blocked by atropine. 5. Release of ACh from atrial strips did not exhibit CS-induced facilitation. Eserine decreased IS- and CS-evoked ACh release in the atrium. 6. It is concluded that continuous stimulation of postganglionic cholinergic nerves in the rat urinary bladder leads to the activation of M1 muscarinic, facilitatory presynaptic receptors which enhance the release of ACh. Presynaptic facilitation may be an important mechanism for modulating neural input to the bladder during micturition.

Poly-L-arginine-mediated release of acetylcholine from parasympathetic nerves in rat and guinea-pig airways

1. The synthetic cationic polypeptide, poly-L-arginine (0.03-1 mg ml-1) induced concentration-dependent contraction of guinea-pig and rat isolated trachea. In guinea-pig isolated trachea, this response was attenuated in the presence of the muscarinic cholinoceptor antagonist, atropine (0.1 microM) and augmented by the acetylcholinesterase inhibitor, ecothiophate (0.1 microM). The neuronal sodium channel blocker, tetrodotoxin (3 microM) failed to alter the contractile response to poly-L-arginine and acetylcholine. 2. The contractile response to poly-L-arginine in rat isolated trachea was inhibited in the presence of atropine (0.1 microM) and the 5-hydroxytryptamine (5-HT) receptor antagonist, methysergide (1 microM). Treatment of rat tracheal preparations with capsaicin (100 microM) or tetrodotoxin (3 microM) failed to alter the contractile response to poly-L-arginine. In contrast, ecothiophate (0.1 microM) augmented the contractile response to poly-L-arginine in rat isolated trachea. 3. Electrical field stimulation (5 Hz, 2 min) of epithelium-denuded guinea-pig tracheal preparations preloaded with [3H]-choline resulted in a contractile response and the simultaneous efflux of radioactivity into the superfusate. Both these responses were abolished in the presence of tetrodotoxin (1.5 microM). Poly-L-arginine (1 mg ml-1) also increased the efflux of total radioactivity from epithelium-denuded guinea-pig isolated tracheal preparations preloaded with [3H]-choline, but this response was tetrodotoxin-insensitive. The negatively charged polyanion, heparin (1 mg ml-1) failed to increase significantly the efflux of radioactivity from epithelium-denuded preparations. 4.In conclusion, the synthetic cationic polypeptide, poly-L-arginine, caused contraction of guinea-pig isolated tracheal preparations via the release of acetylcholine from parasympathetic nerves. Similarly,poly-L-arginine-induced contraction of rat isolated trachea is secondary to the release of acetylcholine from parasympathetic nerves and/or the release of mast cell-derived 5-HT.

Selectivity of LG50643 for postjunctional muscarinic-receptor subtype in the guinea-pig trachea

The effects of (+/-)-LG50643, a new N-quaternary tropinic ester of phenylcyclohexene carboxylic acid, endowed with a potent antimuscarinic activity, have been investigated on muscarinic receptor-mediated responses of the guinea-pig trachea to electrical field stimulation. An isolated preparation which allows the simultaneous measurement of tritiated acetylcholine release (prejunctional effect) and smooth muscle contraction (postjunctional effect) was used. The guinea-pig epithelium-deprived trachea was stimulated with 500 pulses (20 Hz, 1 ms, 9 V for 5 s, 30 s apart) in the presence of indomethacin (1 microM). Three successive pre- and postjunctional responses were observed. The potencies (-logEC50) of (+/-)-LG50643 for pre- and postjunctional muscarinic receptors were determined and compared with those of selective muscarinic antagonists. In addition, the affinity values of (+/-)-LG50643 for muscarinic-receptor subtypes were determined in radioligand binding experiments in cerebral cortex, heart and salivary glands of rat as target tissues for M1, M2 and M3 receptors, respectively. The results obtained in both functional and binding assays indicate (+/-)-LG50643 is a potent and selective antagonist for the M3-receptor subtype.

Muscarinic acetylcholine receptor subtypes in smooth muscle

Muscarinic acetylcholine M2 and M3 receptor subtypes are coexpressed in many types of smooth muscle including gastrointestinal smooth muscle, urinary bladder and vascular and airway tissue. Activation of M3 receptors, via the G protein Gq, results in increased polyphosphoinositide hydrolysis, release of Ca2+ ions from the sarcoplasmic reticulum and consequently causes contraction. Quantitation of the relative expression of M2 and M3 receptors has shown that the proportion of M2 receptors often predominates over the M3 receptor population by 4:1 or more. Although it is established that M2 receptors preferentially link, via a pertussis-toxin-sensitive G protein Gi, to inhibition of adenylate cyclase activity, relatively little is known concerning the physiological role of the M2 receptor population. In this review, Richard Eglen and colleagues discuss recent data concerning the possible role(s) of muscarinic receptor subtypes in smooth muscle and appraise the pharmacological methods for dissecting the function of muscarinic receptor subtypes in tissues co-expressing multiple receptors.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

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.

Neuronal control of airways smooth muscle

Sensory afferent nerves relay impulses from the airways to the central nervous system so that appropriate changes in bronchomotor tone and breathing patterns may occur. The dominant efferent control of airways smooth muscle is exerted via bronchoconstrictor parasympathetic cholinergic nerves. In some species this is opposed by bronchodilator sympathetic noradrenergic nerves. In addition, there exist both excitatory bronchoconstrictor and inhibitory bronchodilator non-adrenergic, non-cholinergic pathways. This review examines the role of the different branches of the autonomic nervous system in the control of airways smooth muscle tone with particular reference to modulation of these branches and the interactions which may exist between them.

Muscarinic receptor subtypes in airways

Muscarinic receptor subtypes in the airways appear to subserve different physiological functions. M1-receptors facilitate neurotransmission through parasympathetic ganglia and enhance cholinergic reflexes, but are also localized to alveolar walls. M2-receptors act as autoreceptors on post-ganglionic cholinergic nerves and inhibit acetylcholine release. There is some evidence that they may be defective in asthma (as a consequence of airway inflammation?) and this may enhance cholinergic reflexes and account for beta-blocker-induced asthma. M2-receptors in airway smooth muscle may also counteract the bronchodilator action of beta-agonists. M3-receptors mediate contractile responses in airway smooth muscle via phosphoinositide hydrolysis, and are the predominant receptors on submucosal glands and airway vascular endothelium. M4- and M5-receptors have not been identified in human airways, but in rabbit lung M4-receptors are expressed on alveolar walls and smooth muscle. Anticholinergic drugs which selectively block M3 and M1-receptors may have an advantage over currently used non-selective antagonists in the treatment of airway obstruction.

Functional relevance of presynaptic muscarinic autoreceptors

Pre- and postsynaptic muscarinic receptors have been characterized in the isolated trachea and ileal circular muscle of the guinea pig. The muscarinic autoreceptors mediating inhibition of acetylcholine release in the circular muscle belong to the M1 subtype, whereas those inhibiting acetylcholine release in the trachea are M2 or M4 receptors. In both tissues the postsynaptic muscarinic receptors are M3 receptors. Blockade of the autoreceptors by selective M1 and M2/M4 receptor antagonists leads to facilitation of cholinergic neurotransmission.

Role of acetylcholinesterase in airway epithelium-mediated inhibition of acetylcholine-induced contraction of guinea-pig isolated trachea

To seek evidence for the involvement of acetylcholinesterase activity in the modulatory influence of the airway epithelium, we examined responses to acetylcholine (ACh), bethanechol, histamine or KCl in isolated epithelium-intact and epithelium-denuded guinea-pig trachealis preparations. The concentration-response curves to ACh were shifted 26-fold to the left by epithelial denudation but the contractile response to KCl was not altered. The response to histamine in epithelium-denuded preparations increased 4-fold with no attenuation in the presence of physostigmine (30 nM). Physostigmine (30 nM) potentiated the response to ACh in epithelium-intact tissues more (about 26-fold) than in epithelium-denuded tissues (about 3.5-fold). Thus, in the presence of physostigmine removing the epithelium had only a slight effect (not statistically significant) on the potency of ACh to contract the trachea. Removing the epithelium had no effect on the potency of bethanechol, a muscarinic receptor agonist that is not a substrate for cholinesterases. Physostigmine itself contracted the trachealis muscle but the pD2 values and maximum responses in epithelium-intact and denuded preparations were not significantly different. The frequency-response curves to electrical field-stimulated cholinergic contractions were unaffected by removing the epithelium. In conclusion, the principal mechanism by which the epithelium inhibits contraction of guinea-pig trachea to exogenously applied ACh is via epithelium-derived acetylcholinesterase activity.

Muscarinic receptor subtypes in equine tracheal smooth muscle

Selective muscarinic receptor antagonists were used to identify muscarinic receptor subtypes in equine trachealis strips. The M1 receptor antagonist pirenzepine (10(-7) mol/L to 3 x 10(-5) mol/L) and the M3 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP, 10(-9) mol/L to 3 x 10(-7) mol/L3) dose dependently inhibited the contractile responses to electrical field stimulation (EFS) and exogenous acetylcholine (ACh). Schild plots yielded a pA2 value for pirenzepine vs ACh of 6.75 +/- 0.09, which is consistent with the affinity for M2 or M3 receptors, and a pA2 value for 4-DAMP vs ACh of 8.47 +/- 0.09, which is in agreement with the affinity for M3 receptors. The M2 receptor antagonist gallamine (10(-5) mol/L and 10(-4) mol/L) did not affect the response of trachealis to exogenous ACh and low-frequency EFS (0.1-2 Hz) but decreased the responses to high-frequency EFS (4-16 Hz). These results suggest that the muscarinic receptors mediating contractions induced by ACh in equine tracheal smooth muscle are of the M3 subtype. The lack of an increase in the response to EFS following gallamine suggests that functional prejunctional inhibitory M2 receptors are not present on the cholinergic nerves innervating equine tracheal smooth muscle.

Muscarinic autoreceptors of Torpedo electric organ are of the M1 subtype: evidence by radioligand binding using selective antagonists

The presynaptic muscarinic autoreceptor of Torpedo marmorata electric organ has been characterised by radioligand binding studies using the subtype-selective antagonists pirenzepine, (+)-telenzepine, methoctramine, and AF-DX 116. The presynaptic receptor had relatively high affinity for the M1 antagonists pirenzepine and (+)-telenzepine (Ki = 35 and 7 nM, respectively) and lower affinities for the M2 antagonists AF-DX 116 and methoctramine (Ki = 311 and 277 nM, respectively). Comparison of these binding data with those from an M2 receptor (rat heart membranes) assayed under identical conditions and with data in the recent literature suggests that the Torpedo muscarinic autoreceptor has a pharmacology most similar to the M1 pharmacological subtype of muscarinic acetylcholine receptor.

Release of [3H]acetylcholine from the isolated rat or guinea-pig trachea evoked by preganglionic nerve stimulation; a comparison with transmural stimulation

Basal and stimulated outflow of radioactive acetylcholine, phosphorylcholine and choline from rat and guinea-pig isolated tracheae were measured by reverse phase HPLC followed by liquid-scintillation-spectrometry. Tracheae were stimulated either by an electrical field (transmural stimulation) or by a local stimulation of the innervating parasympathetic nerves (preganglionic stimulation). Epithelium was removed in most experiments, as the epithelium inhibits acetylcholine release. The basal tritium efflux (1,600 dpm/3 min) from rat isolated tracheae incubated with [3H]choline consisted of 56% [3H]phosphorylcholine and 38% [3H]choline. Preganglionic stimulation (15 Hz, 1,200 pulses) caused a 2-fold increase in tritium outflow that was abolished by the removal of extracellular calcium or by the addition of tetrodotoxin. The stimulated outflow of tritium induced by preganglionic nerve stimulation was caused by an exclusive release of [3H]acetylcholine, whereas the efflux of [3H]phosphorylcholine and [3H]choline remained unaffected by this stimulation mode. Transmural stimulation of the rat or guinea-pig trachea, however, caused, in addition to the release of [3H]acetylcholine, the outflow of [3H]phosphorylcholine. Hexamethonium (300 mumol/l) or tubocurarine (100 mumol/l) inhibited (80%) the increase in tritium outflow evoked by preganglionic stimulation, but did not affect tritium outflow evoked by transmural stimulation. Oxotremorine reduced [3H]acetylcholine release evoked by both stimulation modes, but oxotremorine was less potent with transmural stimulation. Scopolamine (0.3 mumol/l) enhanced (120%) the release of [3H]acetylcholine evoked by preganglionic nerve stimulation indicating the blockade of an endogenous negative muscarinic feedback mechanism. Epithelium-dependent inhibition of [3H]acetylcholine release was evident with both preganglionic and transmural stimulation. The present experiments demonstrate that release of [3H]acetylcholine evoked from the isolated trachea by stimulation of the preganglionic trunk of the parasympathetic cholinergic nerves.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of prejunctional muscarinic autoreceptors in the guinea-pig trachea

1. The effects of ten muscarinic antagonists on electrically evoked [3H]-acetylcholine release and muscle contraction were compared in an epithelium-free preparation of the guinea-pig trachea that had been preincubated with [3H]-choline. 2. The M3-selective antagonists UH-AH 37, 4-diphenyl-acetoxy-N-piperidine methobromide and para-fluorohexahydrosiladiphenidol were more potent in reducing the contractile response than in facilitating the evoked [3H]-acetylcholine release. Hexahydrosiladiphenidol did not discriminate between pre- and postjunctional effects. The rank order of the postjunctional potencies of the ten antagonists as well as the postjunctional pA2 values obtained for hexahydrosiladiphenidol (7.95) and AQ-RA (7.08) identified the muscular receptor as an M3 subtype. 3. The M2-selective antagonists methoctramine, AF-DX 116 and AQ-RA 741 were more potent in facilitating the evoked [3H]-acetylcholine release than in inhibiting the contractile response. The increase in release by low concentrations of methoctramine, AF-DX 116 and AQ-RA 741 was paralleled by an enhancement of the stimulation-evoked contractions. 4. Comparison of the pre- and postjunctional potencies of the M1-, M2- and M3-selective antagonists suggests that autoinhibition of acetylcholine release is mediated via an 'M2-like' receptor which differs from the cardiac type M2 receptor in its relatively high affinity for hexahydrosiladiphenidol.