Muscarinic receptors--characterization, coupling and function

Differential effects of carbachol on calcium entry and release in CHO cells expressing the m3 muscarinic receptor

Calcium signalling was examined in CHO-k1 cells that stably express the m3 subtype of the muscarinic receptor. The calcium indicator Fura-2 was retained in these cells only in the presence of probenecid (1 mM), suggesting that Fura-2 efflux was mediated by an organic anion transporter. The addition of carbachol (CCh) to Fura-2 loaded cells in suspension caused a rapid transient increase in intracellular calcium [Ca]i followed by a smaller sustained plateau phase. The transient rise in [Ca]i was dose-dependent with a threshold response of 89 +/- 18 nM above baseline with 10 nM CCh and a maximum stimulation of 734 +/- 46 nM with 10 microM CCh. This phase was accompanied by a similar dose-dependent stimulation of total inositol phosphate production and was assumed to be generated by release from intracellular stores of the endoplasmic reticulum (ER). The sustained increase in [Ca]i was generated by entry from the extracellular bath since it was blocked by pretreatment with La3+ (1 microM) and was absent when bath calcium was chelated with EGTA. This phase was not dependent on CCh dose, and a stimulation of [Ca]i of approximately 90 nM above baseline was observed with CCh concentrations between 50 nM and 10 microM. With this dose range, the rate of Mn2+ quenching of Fura-2 at the Ca-insensitive excitation wavelength of 360 nm was likewise maximally stimulated. At lower CCh concentrations (10-50 nM), it was clear that the activation of Ca entry could not be dissociated from a threshold release of Ca from intracellular stores. The phorbol ester PMA, which uncouples the muscarinic receptor from phospholipase C, reduced the transient rise in [Ca]i by approximately 50% with little or no effect on Ca entry at higher CCh levels (> or = 1 microM). At lower CCh concentrations (< or = 100 nM) however, pretreatment with PMA completely blocked all Ca mobilization and supports the contention that Ca entry is coupled to Ca release from stores or to store depletion. The emptying of inositol trisphosphate-sensitive stores with thapsigargin (10 nM) stimulated Ca entry and also the rate of Mn2+ quenching. Store depletion by incubation in Ca-free media likewise stimulated Mn2+ uptake without a rise in [Ca]i. Our data are therefore consistent with a 'capacitative' coupling model, whereby the activation of the plasma membrane receptor leads to an InsP3-induced change in the degree of filling of the ER Ca pool.(ABSTRACT TRUNCATED AT 400 WORDS)

Binding profile of the selective muscarinic receptor antagonist tripitramine

The binding selectivity of the muscarinic antagonist tripitramine has been tested on the five cloned human muscarinic receptor subtypes (Hm1 to Hm5) expressed in chinese hamster ovary (CHO-K1) cells. The results indicate that tripitramine binds to the muscarinic Hm2 receptor with a Ki value of 0.27 +/- 0.02 nM. Tripitramine distinguishes Hm2 vs. Hm4 by a factor of 24 and vs. Hm3 and Hm5 by a factor of 142 and 125, respectively. A lower affinity ratio, about 6-fold, was found between muscarinic Hm2 and Hm1 receptors. A comparative study with the well-known selective muscarinic M2 receptor antagonist methoctramine indicates that tripitramine has gained both potency and selectivity for the muscarinic Hm2 receptor subtype.

A comparison between muscarinic receptor occupancy, inositol 1,4,5-trisphosphate accumulation and Ca2+ mobilization in permeabilized SH-SY5Y neuroblastoma cells

Electrically permeabilized SH-SY5Y neuroblastoma cells have been used to examine the relationship between receptor occupation by muscarinic agonists, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) accumulation and Ca2+ mobilization from intracellular stores. The kinetics, concentration-dependence and guanine nucleotide-sensitivity of these responses have been characterized for the agonists, carbachol, arecoline and oxotremorine. Carbachol stimulated Ins(1,4,5)P3 accumulation and Ca2+ mobilization with an EC50 value approximately 50 microM, only slightly lower than the apparent affinity of this agonist for the "free" receptor (100 microM). Arecoline and oxotremorine were partial agonists, mobilizing 45 and 21% of the Ca2+ mobilized by carbachol, and yielded EC50 values for both Ins(1,4,5)P3 and Ca2+ responses, similar to their binding affinity. Guanosine 5'-O-3 thio-triphosphate (GTP gamma S) markedly enhanced the responses elicited by all three agonists. Carbachol became significantly more potent for both Ins(1,4,5)P3 accumulation (EC50 = 4.1 microM) and Ca2+ mobilization (EC50 = 0.25 microM), revealing a separation of the dose-response relationships. GTP gamma S caused a smaller separation of the responses elicited by arecoline (Ca2+ mobilization EC50 = 0.9 microM; Ins(1,4,5)P3 accumulation EC50 = 3.6 microM), and only enhanced maximal responses for oxotremorine. These data reveal that the functional coupling of muscarinic receptors to activation of phosphoinositidase C and subsequent Ca2+ mobilization from intracellular stores is maintained after electrical permeabilization. Furthermore, this model has been used to reveal differences in the relative activities of muscarinic agonists and how they are influenced by a hydrolysis-resistant guanine nucleotide.

Binding and functional properties of hexocyclium and sila-hexocyclium derivatives to muscarinic receptor subtypes

1. We have compared the binding properties of several hexocyclium and sila-hexocyclium derivatives to muscarinic M1 receptors (in rat brain, human neuroblastoma (NB-OK 1) cells and calf superior cervical ganglia), rat heart M2 receptors, rat pancreas M3 receptors and M4 receptors in rat striatum, with their functional antimuscarinic properties in rabbit vas deferens (M1/M4-like), guinea-pig atria (M2), and guinea-pig ileum (M3) muscarinic receptors. 2. Sila-substitution (C/Si exchange) of hexocyclium (-->sila-hexocyclium) and demethyl-hexocyclium (-->demethyl-sila-hexocyclium) did not significantly affect their affinities for muscarinic receptors. By contrast, sila-substitution of o-methoxy-hexocyclium increased its affinity 2 to 3 fold for all the muscarinic receptor subtypes studied. 3. The p-fluoro- and p-chloro-derivatives of sila-hexocyclium had lower affinities than the parent compound at the four receptor subtypes, in binding and pharmacological studies. 4. In binding studies, o-methoxy-sila-hexocyclium (M1 = M4 > or = M3 > or = M2) had a much lower affinity than sila-hexocyclium for the four receptor subtypes, and discriminated the receptor subtypes more poorly than sila-hexocyclium (M1 = M3 > M4 > M2). This is in marked contrast with the very clear selectivity of o-methoxy-sila-hexocyclium for the prejunctional M1/M4-like heteroreceptors in rabbit vas deferens. 5. The tertiary amines demethyl-hexocyclium, demethyl-sila-hexocyclium and demethyl-o-methoxy-sila-hexocyclium had 10 to 30 fold lower affinities than the corresponding quaternary ammonium derivatives.

Effects of muscarinic M2 and M3 receptor stimulation and antagonism on responses to isoprenaline of guinea-pig trachea in vitro

1. In guinea-pig and canine airway smooth muscle, there is reduced beta-adrenoceptor agonist sensitivity in tissues pre-contracted with muscarinic agonists when compared to tissues pre-contracted with other spasmogens, such as histamine or leukotriene D4. This reduced sensitivity may be the result of an interaction between muscarinic receptors and beta-adrenoceptors. In this study the effects of M2 receptor antagonism and stimulation have been investigated on the relaxant potency of isoprenaline in guinea-pig isolated tracheal smooth muscle. 2. (+)-cis-Dioxolane contracted isolated tracheal strips in a concentration-dependent manner (EC50 = 11.5 +/- 0.9 nM). The rank order of antagonist apparent affinities (with pA2 values in parentheses) was atropine (9.4 +/- 0.1) > zamifenacin (8.2 +/- 0.1) > para-fluoro-hexahydro-siladiphenidol (p-F-HHSiD, 7.2 +/- 0.1) > pirenzepine (6.5 +/- 0.1) > methoctramine (5.5 +/- 0.1). Schild slopes were not significantly different from unity. This was consistent with a role of muscarinic M3 receptors in mediating contraction. 3. In tissues pre-contracted to 3 g isometric tension using (+)-cis-dioxolane (0.2 microM, approximately EC80), the relaxant potency of isoprenaline was significantly (P < 0.05) increased by 0.3 microM methoctramine (control EC50 = 32.2 +/- 4.3 nM, plus methoctramine EC50 = 19.1 +/- 4.5 nM). This concentration of methoctramine had no effect on contractile responses to (+)-cis-dioxolane (control, EC50 = 17.6 +/- 3.2 nM, plus methoctramine, EC50 = 21.0 +/- 4.4 nM). 4 When acetylcholine (non-selective), (+)-cis-dioxolane (non-selective), L-660,863 ((+/- )-3-(3-amino-1,2,4-oxadiazole-5-yl)-quinuclidine, M2-selective) or SDZ ENS 163 (thiopilocarpine, mixed M2 antagonist,partial M3 agonist) were used to achieve isometric tensions of 3 g, the relaxant potency of isoprenaline ranged from 3.7 +/- 0.3 nM (SDZ ENS 163) to 49.4 +/- 3.2 nM ((+)-cis-dioxolane). Reducing the concentration of these agonists (and therefore the level of developed tension to 2 g), significantly(P<0.05) increased the relaxant potency of isoprenaline. In contrast, when histamine was used to pre-contract tissues to either 2 or 3 g (EC50 = 4.2 +/- 0.6 and 3.8 +/- 1.1 nM, respectively), there was no significant effect on the relaxant potency of isoprenaline.5. There was a slight but significant (P<0.05) reduction in the relaxant potency of isoprenaline, in tissues pre-contracted to 3 g using histamine in combination with (+ )-cis-dioxolane (30 nM). This effect was reversed by M2 receptor antagonism, using methoctramine (1 MicroM).6. These data suggest that in guinea-pig isolated trachea, the relaxant potency of isoprenaline may depend not only on the level of developed tension but also, on the level of muscarinic M2 receptor stimulation/blockade of the spasmogen inducing the tension. However, the lack of selective M2 agonist and the low M2/M3 selectivity of antagonists in this tissue do not permit definitive conclusions to be made about the role of these receptors in modulating isoprenaline potency.

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.

Himbacine discriminates between putative muscarinic M1 receptor-mediated responses

This study describes the antagonistic properties of himbacine, in comparison with those of pirenzepine, at muscarinic receptors mediating the depolarization of rat superior cervical ganglion, the inhibition of electrically-induced twitch contractions of rabbit vas deferens and the contraction of dog saphenous vein, currently classified as putative muscarinic M1 sites. The affinity of himbacine for the vas deferens site (pA2 8.08) was nearly ten times higher than those for the M1 receptors of rat ganglion and dog saphenous vein (pA2 7.14 and 7.16, respectively); affinity estimates for pirenzepine were similar throughout the different preparations. The present data are consistent with the allocation of ganglion and saphenous vein receptors into the M1 subclass; the profile of the vas deferens site, conversely, appears to be different, and possibly more closely related to that reported for the M4/m4 receptor.

Receptor subtypes or species homologues: relevance to drug discovery

Cell surface receptors are targets for the pharmacological manipulation of physiological processes and thus represent a key direction for the development of selective therapeutic agents. Traditional pharmacological techniques, together with the development of synthetic ligands, have led to the identification of differences in receptor recognition properties and the proposal of multiple receptor subtypes. Molecular biological studies have confirmed the existence of receptor subtypes within a single species by demonstrating differences in receptor primary sequences. However, equivalent receptors between species also show differences in primary structure, albeit to a much lower degree. This review by Judith Hall and colleagues addresses the question of how differences in receptor primary structure between species relate to changes in pharmacology. The relevance of this to the choice of screens in the testing of potential therapeutic drugs is discussed.

Pharmacological characterization of acetylcholine-stimulated [35S]-GTP gamma S binding mediated by human muscarinic m1-m4 receptors: antagonist studies

1. We have used dose-ratio analysis to estimate functionally the affinity constants (pKb) and Schild slope factors of a range of selective or atypical antagonists at human muscarinic m1-m4 receptors. 2. The functional response was the stimulation by acetylcholine of [35S]-GTP gamma S binding to membranes from Chinese hamster ovary (CHO) cells stably expressing individual receptor subtypes. 3. A novel experimental design and analysis was used which allowed the estimation of affinity and Schild slope factor from a single antagonist inhibition curve, and the results were compared with other methods of analysis, both theoretically valid and invalid. 4. In general, the affinity estimates were very similar to previously reported values obtained in binding studies with animal tissues and cloned human receptors and the Schild slope factors were close to unity. 5. These results demonstrate the validity of the assay and provide no evidence for species differences in antagonist affinity for muscarinic receptor subtypes. 6. The results confirm both the utility of himbacine in distinguishing between m1 and m4 receptors and a previously reported modest m4-selectivity for tropicamide and secoverine. 7. The cholinesterase inhibitor, tacrine (THA), had a potency profile similar to that of gallamine but with less selectivity. Its affinity could not be determined since it had Schild slope factors of about 2 at all subtypes. 8. o-Methoxy-sila-hexocyclium had only a modest selectivity for the m1 subtype.

MCI-826 is a potent and selective antagonist of peptide leukotrienes (p-LTs) and has characteristics distinctive from those of FPL 55712

Antagonistic effects of a newly synthesized compound, (E)-2,2-diethyl-3'-[2-[2-(4-isopropyl)thiazolyl]ethenyl]succinanilic+ ++ acid sodium salt (MCI-826) on the contraction of the isolated guinea pig trachea and human bronchus induced by various agonists including peptide leukotrienes (p-LTs), histamine, acetylcholine (ACh), prostaglandin (PG) D2 and others were investigated and compared with the effects of a p-LT antagonist, FPL 55712, in some experiments. MCI-826 potently antagonized LTD4- and LTE4-induced contractions at extremely low concentrations in the isolated guinea pig trachea with pA2 values of 8.3 and 8.9, respectively, on a molar basis. These values indicated that MCI-826 is over 100 times stronger than FPL 55712. Similarly, MCI-826 at 10(-8) g/ml (2.4 x 10(-8) M) markedly antagonized LTD4-induced contractions of the isolated human bronchus. Although FPL 55712 fairly inhibited the 10(-9) g/ml LTC4-induced contraction of the isolated guinea pig trachea, MCI-826 had little effect on the contraction at high concentrations like 3 x 10(-6) g/ml (7.1 x 10(-6) M). MCI-826 modestly affected the other agonist-induced contractions and the resting tonus of the isolated guinea pig trachea at 10(-6) g/ml (2.4 x 10(-6) M) or higher concentrations, but FPL 55712 caused fair inhibition of some of those contractions and gradually lowered the resting tonus with time. These results indicate that MCI-826 is a highly potent and selective antagonist of LTD4 and LTE4 and can be a useful tool for biological and pharmacological experiments on p-LTs.