Expression of muscarinic receptor subtypes in rat gastric smooth muscle: effect of M3 selective antagonist on gastric motility and emptying

Expression of muscarinic receptor subtypes in rat gastric smooth muscle was examined with reverse transcriptase-polymerase chain reaction (RT-PCR). Under the condition for detecting the messages of m1-m4 subtypes in brain, atrium, and gastric mucosa, only the fragments of m2 and m3 subtypes were amplified with RNA prepared from rat gastric smooth muscles. Furthermore, the amplified fragments were digested by restriction enzymes, reconfirming that the predicted size products of m2 and m3 contain the partial DNA sequences of m2 and m3 subtypes, respectively. We measured gastric motility in rats with a pressure transducer system under the continuous venous infusion of the muscarinic antagonists atropine and butylscopolamine (nonselective), AF-DX 116 (M2), zamifenacine (M3), and glucagon. Heart rate was monitored simultaneously in the tail. Gastric motility was inhibited in the presence of glucagon and zamifenacine without alteration of heart rate, whereas there was no inhibition in the presence of AF-DX 116 even after the augmentation of heart rate was observed. Gastric emptying was also suppressed in the presence of zamifenacine, which had an effect comparable with that of atropine, butylscopolamine, and glucagon. These results indicate that the activation of the M3 subtype in gastric smooth muscle causes its contraction, and the M3 selective antagonist could be a potentially useful drug without an adverse effect on the heart for radiological and endoscopic examination in the upper gastrointestinal tract.

Functional role of M2 and M3 muscarinic receptors in the urinary bladder of rats in vitro and in vivo

1. Urinary bladder smooth muscle is enriched with muscarinic receptors, the majority of which are of the M2 subtype whereas the remaining minority belong to the M3 subtype. The objective of the present study was to assess the functional role of M2 and M3 receptors in the urinary bladder of rat in vitro and in vivo by use of key discriminatory antagonists. 2. In the isolated bladder of rat, (+)-cis-dioxolane produced concentration-dependent contractions (pEC50 = 6.3) which were unaffected by tetrodotoxin (0.1 microM). These contractions were antagonized by muscarinic antagonists with the following rank order of affinity (pA2) estimates: atropine (9.1) > 4-diphenyl acetoxy-methyl piperidine methiodide (4-DAMP) (8.9) > darifenacin (8.5) > para fluoro hexahydrosiladifenidol (p-F-HHSiD) (7.4) > pirenzepine (6.8) > methoctramine (5.9). These pA2 estimates correlated most favourably (r = 0.99, P < 0.001) with the binding affinity (pKi) estimates of these compounds at human recombinant muscarinic m3 receptors expressed in Chinese hamster ovary cells, suggesting that the receptor mediating the direct contractile responses to (+)-cis-dioxolane equates with the pharmacologically defined M3 receptor. 3. As M2 receptors in smooth muscle are negatively coupled to adenylyl cyclase, we sought to determine whether a functional role of M2 receptors could be unmasked under conditions of elevated adenylyl cyclase activity (i.e., isoprenaline-induced relaxation of KCl pre-contracted tissues). Muscarinic M3 receptors were preferentially alkylated by exposing tissues to 4-DAMP mustard (40 nM, 1 h) in the presence of methoctramine (0.3 microM) to protect M2 receptors. Under these conditions, (+)-cis-dioxolane produced concentration-dependent reversal (re-contraction) of isoprenaline-induced relaxation (pEC50 = 5.8) but had marginal effects on pinacidil-induced, adenosine 3':5'-cyclic monophosphate (cyclic AMP)-independent, relaxation. The re-contractions were antagonized by methoctramine and darifenacin, yielding pA2 estimates of 6.8 and 7.6, respectively. These values are intermediate between those expected for these compounds at M2 and M3 receptors and were consistent with the involvement of both of these subtypes. 4. In urethane-anaesthetized rats, the cholinergic component (approximately 55%) of volume-induced bladder contractions was inhibited by muscarinic antagonists with the following rank order of potency (ID35%inh, nmol kg-1, i.v.): 4-DAMP (8.1) > atropine (20.7) > methoctramine (119.9) > darifenacin (283.3) > pirenzepine (369.1) > p-F-HHSiD (1053.8). These potency estimates correlated most favourably (r = 0.89, P = 0.04) with the pKi estimates of these compounds at human recombinant muscarinic m2 receptors. This is consistent with a major contribution of M2 receptors in the generation of volume-induced bladder contractions, although the modest potency of darifenacin does not exclude a role of M3 receptors. Pretreatment with propranolol (1 mg kg-1, i.v.) increased the ID35%inh of methoctramine significantly from 95.9 to 404.5 nmol kg-1 but had no significant effects on the inhibitory responses to darifenacin. These data suggest an obligatory role of beta-adrenoceptors in M2 receptor-mediated bladder contractions in vivo. 5. The findings of the present study suggest that both M2 and M3 receptors can cause contraction of the rat bladder in vitro and may also mediate reflex bladder contractions in vivo. It is proposed that muscarinic M3 receptor activation primarily causes direct contraction of the detrusor whereas M2 receptor activation can contract the bladder indirectly by reversing sympathetically (i.e. beta-adrenoceptor)-mediated relaxation. This dual mechanism may allow the parasympathetic nervous system, which is activated during voiding, to cause more efficient and complete emptying of the bladder.

Synthesis and pharmacological characterization of enantiomerically pure muscarinic agonists: difluoromuscarines

The four homochiral 4-deoxy-4,4-difluoromuscarine stereoisomers (difluoromuscarines) were prepared in very high enantiomeric excess. A convenient sequence based on the use of natural as well as "unnatural" ethyl lactate allowed the synthesis of target compounds, whose absolute configuration is dictated by that of the starting synthon. Quaternary ammonium salts (+)-5, (-)-5, (-)-6, and (+)-6 were tested in vitro on guinea pig tissues, and their muscarinic potency was evaluated at M2 (heart) and M3 (ileum and bladder) muscarinic receptor subtypes. The eutomer (+)-5 and distomer (-)-5 were also tested in vivo on pithed rat, and their muscarinic activity at the M1 receptor subtype was compared with those of racemic muscarine [(+/-)-1] and (2S,4R,5S)-4-deoxy-4-fluoromuscarine [(+)-4]. Further pharmacological parameters such as affinity, relative efficacy, and enantioselectivity have been determined for compounds (+)-5 and (-)-5 at M2 (heart force and rate) and M3 (ileum and bladder) receptors in order to investigate muscarinic receptor heterogeneity. The four homochiral difluoromuscarines behave as muscarinic agonists in all the tests with a potency trend which is different from that previously observed with the 4-deoxy-4-fluoromuscarines and (+/-)-1, thus indicating the intervention of the second fluorine atom on the receptor-ligand interaction. Moreover, the second fluorine atom produces significant differences in the affinity and relative efficacy values of compounds (+)-5 and (-)-5 at M2 and M3 subtypes, which could be attributed to a heterogeneity between the muscarinic receptors mediating heart rate and heart force and those involved in the contraction of ileum and bladder.

[Ca2+]i oscillations induced by muscarinic stimulation in airway smooth muscle cells: receptor subtypes and correlation with the mechanical activity

1. Cytosolic calcium concentration ([Ca2+]i) by indo 1 microspectrofluorimetry in freshly isolated cells and isometric contraction of isolated rings were measured in response to muscarinic cholinoceptor stimulation in rat tracheal smooth muscle. 2. In isolated myocytes, acetylcholine (ACh, 0.03-1 microM) caused a rapid and graded increase in [Ca2+]i up to a net amplitude of 492 +/- 26 nM (n = 19) which gradually declined. The EC50 for ACh was 0.13 microM. This first [Ca2+]i peak was followed, when the ACh concentration increased, in approximately 50-60% of the cells, by successive peaks of decreased amplitude ([Ca2+]i oscillations) superimposed on the plateau phase. Whereas the percentage of cells exhibiting [Ca2+]i oscillations remained consistent, the frequency of these oscillations increased to up to 10 min-1 with an ACh concentration of 100 microM. 3. Removal of extracellular calcium (in the presence of EGTA, 0.4 mM) or addition of the voltage-dependent Ca(2+)-channel blocker verapamil (10 microM) did not alter the first [Ca2+]i peak, the plateau or the oscillations induced by ACh or carbachol. In contrast, the specific inhibitor of the sarcoplasmic Ca(2+)-ATPase, thapsigargin (1 microM), completely abolished the [Ca2+]i response. Thapsigargin (1 microM) also blocked the caffeine (5 mM)-induced transient rise in [Ca2+]i. 4. Atropine (a non-selective muscarinic cholinoceptor antagonist) and 4-diphenyl acetoxy N-methyl piperidine (4-DAMP, a selective M3 antagonist) inhibited the [Ca2+]i response to muscarinic cholinoceptor activation with an IC50 of 13 and 20 nM, respectively. Pirenzepine (a selective M1 antagonist) also totally inhibited the [Ca2+]i response to ACh but with a higher IC50 of 2 microM. Methoctramine (a selective M2 antagonist) up to a concentration of 10 microM caused only a 40% inhibition. The effect of muscarinic antagonists on cumulative concentration-response curves (CCRC) for carbachol was assessed at the following concentrations: atropine and 4-DAMP at 3, 10 and 30 nM; pirenzepine 0.3, 1 and 3 microM, and methoctramine at 1, 3 and 10 microM. For these concentrations, all of the antagonists produced a rightward shift of the CCRC for carbachol and pA2 values were 9.2, 8.8, 6.7 and 6.3, respectively. 5. In conclusion, the present study indicates that muscarinic stimulation of rat isolated tracheal smooth muscle cells induces [Ca2+]i oscillations. The occurrence of these oscillations depends on the graded amplitude of the first [Ca2+]i rise and their frequency may play a role in the amplitude of the mechanical activity in response to muscarinic cholinoceptor activation. Both the [Ca2+]i and the contractile responses are primarily dependent on activation of the M3 receptor subtype.

Pharmacological characterization of the muscarinic receptor subtypes responsible for the contractile response in the rat urinary bladder

1. Contractile responses of smooth muscle from the Wistar rat urinary bladder were studied with the use of muscarinic agonists and antagonists. 2. McN-A-343 induced only weak contractile responses of the bladder muscle. In contrast, oxotremorine showed higher potency than either acetylcholine or bethanechol in inducing a contractile response (the respective pD2 values were 6.38 +/- 0.25, 4.82 +/- 0.24 and 4.42 +/- 0.14). 3. The M2 antagonists, methoctramine (10(-9) M to 10(-5) M) and gallamine (10(-9) M to 10(-5) M), did not reduce acetylcholine-induced (10(-5) M) contractions of the bladder muscle strip. On the other hand, 4-diphenyl-acetoxy-N-methyl piperidine methiodide (4-DAMP, 10(-10) M to 10(-7) M), an M3 receptor blocker, effectively antagonized the acetylcholine-induced contractions in a concentration-dependent manner. 4-DAMP had a similar pA2 value to those of the non-selective antagonists, atropine and scopolamine (pA2 values were 8.26 +/- 0.05, 8.36 +/- 0.05 and 8.41 +/- 0.11, respectively). Pirenzepine, and M1 blocker, antagonized the contractions at higher concentrations (10(-8) M to 10(-5) M, pA2 = 6.23 +/- 0.04). 4. It is concluded that (1) the dominant muscarinic receptor subtype responsible for smooth muscle contraction in the rat urinary bladder is M3; and (2) the muscarinic agonist oxotremorine was more potent than acetylcholine and bethanechol in inducing a contractile response.

Non-selectivity of the monoclonal antibody M35 for subtypes of muscarinic acetylcholine receptors

The monoclonal antibody M35, one of the first monoclonal antibodies successfully raised against muscarinic acetylcholine receptors, has been widely used to study the distribution of this protein in a variety of tissues and cell types of different species. It is not fully known, however, to which muscarinic acetylcholine receptor subtypes M35 binds. Knowledge of subtype-selectivity of M35 is a necessary step towards a functional interpretation of the obtained immunocytochemical data. The aim of the present study was to determine the subtype-selectivity of M35 employing transfected CHO-K1 cells stably expressing human m1-m5 muscarinic acetylcholine receptors separately, and to study M35 immunoreactivity in areas of rat central and peripheral tissues known to be specifically enriched in a single muscarinic acetylcholine receptor subtype. The results show that (a) all five transfected cell lines were immunopositive for M35, (b) nontransfected control cells were immunonegative, (c) the number of mAChRs expressed per cell correlated positively with the intensity of M35 immunoreactivity, and (d) cell types in aldehyde-fixed rat tissue enriched in a single m1-m4 subtypes revealed clear M35 immunoreactivity. Taken together, the present results show that M35 does not discriminate between muscarinic acetylcholine receptor subtypes. Evidently, the epitope of M35 on the receptor-protein is preserved on all muscarinic acetylcholine receptor subtypes. The epitope for M35 must, therefore, be localized on a homologous part of each subtype.

The M5 (m5) receptor subtype: fact or fiction?

By comparison to the other subtypes of muscarinic receptors, very little is known about the binding properties, locations, mechanisms and physiological functions of the M5 (m5)* receptor subtype. Studies of the m5 receptor have been hampered by the lack of m5-selective ligands or antibodies and a source that endogenously expresses predominantly the m5 receptor subtype. We have developed a pharmacological labeling strategy using the non-selective muscarinic antagonist [3H]NMS, in the presence of muscarinic antagonists and toxins in green mamba venom to occlude the m1-m4 receptor subtypes, to selectively label the m5 receptor subtype. This m5-selective labeling approach, along with those developed for the other four receptor subtypes, has permitted for the first time a comparison of the relative expression levels and anatomical localizations of the five muscarinic receptor subtypes in the brain. The distribution profile of the m5 receptor is distinct from the other four subtypes and is enriched in the outer layers of the cortex, specific subfields of the hippocampus, caudate putamen, olfactory tubercle and nucleus accumbens. These studies have also demonstrated that the levels of m5 receptor protein expression are apparently higher and more widespread than anticipated from previous in situ hybridization and immunoprecipitation studies. Taken together, the results suggest a unique and potentially physiologically important role for the m5 receptor subtype in modulating the actions of acetylcholine in the brain.

Interaction of ebeinone, an alkaloid from Fritillaria imperialis, at two muscarinic acetylcholine receptor subtypes

The ability of the alkaloid, ebeinone, isolated from Fritillaria imperialis, to act at muscarinic M2 and M3 acetylcholine receptors was investigated. In functional studies with guinea-pig left atrium, ebeinone was found to be ca. 10-fold more active as an antagonist of responses to carbachol (CCh) than in either guinea-pig ileum or trachea. Estimates of dissociation constants (KB values) in the three tissues were 77.3, 931.1 and 547.0 nM, respectively. Inhibition binding studies in rat atria with the non-selective antagonist [3H]N-methylscopolamine ([3H]NMS) showed ebeinone to have a KI value of 80.9 nM. Comparison of ebeinone with pancuronium, another steroid-like compound with a similar KB value at the muscarinic M2 receptor, found both compounds able to retard the dissociation rate of [3H]NMS in atria, indicating an allosteric mode of interaction at the M2 receptor. It is concluded that ebeinone exhibited a higher affinity for muscarinic M2 receptors than for M3 receptors in the guinea-pig and that it interacted allosterically at rat atrial M2 receptors.

Cholinergic transmission in pancreatic ganglia of the cat

Although there is considerable evidence indicating the existence of important cholinergic neural regulation of pancreatic function, very little is known about the action of acetylcholine on pancreatic ganglion neurons. The present study was undertaken to determine the effect of pressure microejection of acetylcholine and muscarine on ganglion cell excitability of the cat pancreas. Recordings were made in vitro from ganglion neurons located in the head region of the pancreas. Acetylcholine evoked a fast- and a slow-developing membrane depolarization in the majority of neurons tested. A decrease in membrane input resistance accompanied the fast depolarizing response, whereas an increase in input resistance accompanied the slow depolarizing response. The fast response was mimicked by 1,1-dimethyl-4-phenylpiperazinium iodide and nicotine and antagonized by hexamethonium. The slow depolarizing response was mimicked by muscarine and antagonized by atropine and pirenzepine. The dependence of the slow depolarization on extracellular K+ and the distinct voltage dependence of the slow depolarization suggest that activation of muscarinic receptors was due to inactivation of IM. The slow excitatory postsynaptic potential and associated increase in excitability evoked by repetitive nerve stimulation was partially cholinergic dependent in the majority of neurons tested. It was concluded that cholinergic transmission in cat pancreatic ganglia involves nicotinic and M1 receptors that mediate fast and slow synaptic transmission, respectively, and that activation of M1 receptors modifies the output firing frequency.

Muscarinic acetylcholine receptor expression in memory circuits: implications for treatment of Alzheimer disease

Cholinergic transmission at muscarinic acetylcholine receptors (mAChR) has been implicated in higher brain functions such as learning and memory, and loss of synapses may contribute to the symptoms of Alzheimer disease. A heterogeneous family of five genetically distinct mAChR subtypes differentially modulate a variety of intracellular signaling systems as well as the processing of key molecules involved in the pathology of the disease. Although many muscarinic effects have been identified in memory circuits, including a diversity of pre- and post-synaptic actions in hippocampus, the identities of the molecular subtypes responsible for any given function remain elusive. All five mAChR genes are expressed in hippocampus, and subtype-specific antibodies have enabled identification, quantification, and localization of the encoded proteins. The m1, m2, and m4 mAChR proteins are most abundant in forebrain regions and they have distinct cellular and subcellular localizations suggestive of various pre- and postsynaptic functions in cholinergic circuits. The subtypes are also differentially altered in postmortem brain samples from Alzheimer disease cases. Further understanding of the molecular pharmacology of failing synapses in Alzheimer disease, together with the development of new subtype-selective drugs, may provide more specific and effective treatments for the disease.

Functional properties of muscarinic receptor subtypes Hm1, Hm3 and Hm5 expressed in Sf9 cells using the baculovirus expression system

The human muscarinic ACh receptor subtypes m1, m3 and m5 have been expressed in Sf9 cells using the baculovirus expression system. Stimulation of all three subtypes with CCh caused an increase in inositol-1,4,5-trisphosphate and intracellular Ca++. The increase in cytosolic free Ca++ was to a large extent due to influx. The levels of receptors (< 0.1-1 pmol/mg protein) increased with infection time in a narrow time span (24-36 h). The changes in the receptor densities did not significantly affect the EC50 values of CCh-mediated Ca++ mobilization with the m3 or the m5 subtype. The EC50 value was higher with the m1 receptor at low expression levels (approximately 100 fmol/mg protein), and it decreased with an increase in receptor density. The receptor subtypes displayed no gross differences in their response to oxotremorine-M, which behaved as a full agonist, or to oxotremorine and pilocarpine, which were less active. With the m3 subtype, there was an increase in the maximal response to oxotremorine with longer infection times. The results demonstrate that the recombinant muscarinic receptors, expressed in Sf9 cells, show many of the characteristics of endogenously expressed receptors when studied at low expression levels and that the receptor density may significantly affect the receptor pharmacology.

Differential coupling of m1, m3, and m5 muscarinic receptors to activation of neuronal nitric oxide synthase

The selectivity of coupling of m1, m3, and m5 muscarinic receptors to activation of the neuronal type of nitric oxide synthase was investigated. Stimulation with the agonist carbachol of all three receptor subtypes expressed in Chinese hamster ovary cells resulted in a rapid and transient activation of the enzyme, as measured by stimulation of guanylate cyclase in reporter neuroblastoma cells. Carbachol was more potent and efficacious at m5 receptors than at the other two receptor subtypes. Stimulation of all three muscarinic receptors resulted in an increased concentration of intracellular calcium, with a time course that preceded activation of nitric oxide synthase. At each receptor subtype, there was a close relationship between the magnitude of the maximal calcium response and that of enzyme activation.

Possible involvement of muscarinic M1 and M3 receptor subtypes mediating vasodilation in isolated, perfused canine lingual arteries

1. Using the cannula insertion method, muscarinic receptor subtypes were analysed in isolated, perfused canine lingual arteries preconstricted with phenylephrine. 2. Both acetylcholine and McN-A-343 induced a profound vasodilation in a dose-related manner. Acetylcholine-induced dilations were approximately 1000-times more potent than McN-A-343-induced dilation. 3. Acetylcholine-induced dilations were abolished after removal of the endothelium by intraluminal treatment with 1 mg saponin. 4. Acetylcholine-induced dilations were markedly inhibited by an M1/M3 receptor antagonist, 4-DAMP. Moreover, they were slightly, but significantly, inhibited by an M1 antagonist, pirenzepine, but never influenced by an M2 antagonist, AF-DX 116. Mc-N-A-343-induced vasodilations were inhibited by both 4-DAMP or pirenzepine. 5. These results suggest that there are abundant functional M3 and a few M1 receptors in the canine lingual artery that mediate vasodilation and that this vasodilation is dependent on the presence of an intact endothelium.

Molecular and functional identification of m1 muscarinic acetylcholine receptors in rat ventricular myocytes

The expression of muscarinic acetylcholine receptor (mAChR) subtypes in freshly isolated adult rat ventricular myocytes was investigated by reverse transcription of cellular mRNA followed by amplification of cDNA using the polymerase chain reaction (PCR). After reverse-transcriptase PCR, bands were obtained corresponding to the expected sizes for the m1 and m2 but not for the m3 to m5 mAChRs. The identity of the m1 and m2 bands was confirmed by single-cell PCR, restriction digest mapping, and Southern blot analysis. The presence of m1 and m2, but not m3, mAChR protein in these cells was shown by indirect immunofluorescence studies using subtype-specific antibodies. It was further investigated whether the identified m1 mAChR was responsible for the stimulatory effects on Ca2+ transients by high concentrations of carbachol ( > 10 mumol/L) known to occur in these cells. In pertussis toxin-treated ventricular myocytes electrically stimulated at 1 Hz, carbachol (300 mumol/L) increased the basal Ca2+ level from 96 +/- 7 to 118 +/- 8 nmol/L and the peak Ca2+ transient level from 519 +/- 32 to 640 +/- 36 nmol/L (mean +/- SEM P < .05 for both, n = 8). These effects of carbachol on Ca2+ transients were antagonized by 10 nmol/L pirenzepine, an m1 mAChR-selective antagonist. In contrast, the m2 mAChR-selective antagonist methoctramine (up to 100 nmol/L) did not inhibit the response. These results are the first to use single-cell PCR to probe cardiomyocyte-specific gene expression and indicate that m1 mAChRs are expressed on adult rat ventricular myocytes in addition to m2 mAChRs. The results further suggest that m1 mAChRs mediate the stimulatory responses on Ca2+ transients to high concentrations of cholinergic agonists seen in these cells.

Stereoselective interaction of uncharged esters at four muscarinic receptor subtypes

We investigated the binding and pharmacological properties of the esters of 3,3-dimethylbutan-1-ol (the carbon analogue of choline) with either diphenylglycolic acid, (R)-phenylcyclohexylglycolic acid, or (S)-phenylcyclohexylglycolic acid [BS-6181, (R)-BS-7826 and (S)-BS-7826, respectively] at muscarinic M1, M2, M3 (Hm3) and M4 receptors. The three uncharged compounds were muscarinic receptor antagonists, with pA2 or pKi values between 7.9 and 5.6. The achiral ester BS-6181 displayed highest affinity for M1, M3 (Hm3) and M4 receptors (pA2 or pKi = 7.2-7.6) and lower affinity for M2 receptors (pA2 or pKi = 6.7 and 6.8). The four muscarinic receptor subtypes were able to distinguish between the two enantiomers of the cyclohexyl derivative of BS-6181 [(R)- and (S)-BS-7826], with a preference for the (R)-isomer (up to 79-fold). Interestingly, the (S)-enantiomer of BS-7826, being the distomer, was found to be M4 selective (pKi/M4 = 6.9; pA2 or pKi/M1-M3 (Hm3) = 5.6-6.2). These results indicate that uncharged compounds may (stereo)selectively bind to muscarinic receptors via hydrophobic interactions. Thus, an ionic bond between muscarinic ligands and an anionic site of the receptor is not absolutely necessary for recognition of muscarinic receptors.

In vitro pharmacological properties of 4-bromodexetimide for muscarinic receptors

The decrease of m-AChR density observed in neurodegenerative disorders has generated considerable interest in non-invasive mapping of muscarinic acetylcholine receptors (m-AChR) in the central nervous system. The aim of our study was to evaluate the selectivity of 4-bromodexetimide for the M1, M2, M3 and M4 m-AChR subtypes using in vitro binding analysis to determine the potential use of the bromine-76 labelled 4-bromodexetimide in the investigation of m-AChR subtypes in human brain with Positron Emission Tomography. Subtype selectivity of 4-bromodexetimide was determined in competition studies against tritiated subtype selective ligands using various rat or rabbit structure homogenates reflecting a single binding site and in optimal saturation and low non specific binding conditions. These conditions were reached for every subtype studied by analyzing the data from the saturation experiments of the tritiated ligands. 4-bromodexetimide displayed nanomolar affinities for the four m-AChR subtypes and a preferential selectivity for the M1 and M4 subtypes. The saturation analysis of [76Br]4-bromodexetimide, performed with rat cortex membranes showed high affinity for m-AChR receptors (Kd = 1.8 nM). As in vivo studies of [76Br]4-bromodexetimide showed preferential localization in the cortex and the striatum which are M1 and M4 rich structures and since it binds preferentially to the M1 and M4 subtypes, this radiotracer can still allow a combined subtype specific measurement of these muscarinic receptors.

Muscarinic cholinergic receptor subtypes in human peripheral blood lymphocytes

The subtypes of muscarinic cholinergic receptors were studied in human peripheral blood lymphocytes with radioligand binding techniques and the non-selective muscarinic cholinergic receptor antagonist [3H]quinuclidinyl benzylate (QNB) as a ligand. [3H]QNB was bound to human peripheral lymphocytes in a manner consistent with the labelling of muscarinic cholinergic receptors. The dissociation constant (Kd) value was 0.60 +/- 0.08 nM and the maximum density of binding sites (Bmax) was 2.33 +/- 0.03 fmol/2.2 x 10(6) cells. The binding was time-, temperature- and concentration-dependent, belonging to a single class of high affinity sites. Analysis of the pharmacological profile of [3H]QNB binding in the presence of compounds specific for the different muscarinic receptor subtypes suggests that human peripheral blood lymphocytes express mainly muscarinic cholinergic M2 and M3 receptor subtypes and to a lesser extent muscarinic M4 receptors. The characterization of the subtypes of muscarinic cholinergic recognition sites expressed by human peripheral blood lymphocytes may represent a tool for investigating the possible relationships between immune and cholinergic systems in normal and pathologic conditions.

The interaction of 4-DAMP mustard with subtypes of the muscarinic receptor

The compound 4-DAMP mustard (N-2-chloroethyl-4-piperidinyl diphenylacetate) is a 2-chloroethylamine derivative of the selective muscarinic antagonist 4-DAMP (N,N-dimethyl-4-piperidinyl diphenylacetate). At neutral pH, 4-DAMP mustard cyclizes spontaneously into an oziridinium ion that binds covalently with muscarinic receptors. Analysis of the kinetics of receptor alkylation showed that the interaction of 4-DAMP mustard with M2 and M3 receptors was consistent with a model in which the aziridinium ion rapidly forms a reversible complex with the receptor which converts to a covalent complex at a relatively slower rate. The rate constant (k2) for alkylation of M2 and M3 receptors was approximately the same (k2 = 0.1 min-1); however, the affinity of the aziridinium ion for the M3 receptor (KD = 7.2 nM) was approximately 6.3-fold greater than that for the M2 receptor (KD = 43 nM). The results of competitive binding experiments on Chinese hamster ovary cells transfected with the M1 - M5 subtypes of the muscarinic receptor showed that the affinity of the aziridinium ion for the M1, M3, M4 and M5 subtypes was approximately the same and about 11-fold greater than that for the M2 receptor. 4-DAMP mustard is a useful tool for selectively inactivating all non-M2 muscarinic receptors, particularly when it is used in the presence of a reversible M2 selective antagonist to protect the M2 receptor from alkylation. The results of studies on isolated smooth muscle preparations that have had their M3 receptors alkylated with 4-DAMP mustard are consistent with the postulate that the M2 receptor can elicit contraction by inhibiting the relaxant effect of isoproterenol and forskolin on histamine induced contractions.

Regulation of intracellular Ca2+ in response to muscarinic and glutamate receptor agonists during the differentiation of NTERA2 human embryonal carcinoma cells into neurons

Single cell microfluorimetry was used to study intracellular calcium ion signals ([Ca(2+)](i)) evoked by acetylcholine (ACh), glutamate receptor agonists and by KCI-induced membrane depolarization, during neuronal differentiation of the human embryonal carcinoma (EC) cell line, NTERA2. In undifferentiated NTERA2 EC cells, [Ca(2+)](i) was elevated in response to ACh, but not to the glutamate receptor agonists NMDA, kainate or AMPA. The ACh-induced rise in [Ca(2+)](i) was dependent upon both Ca(2+) influx and Ca(2+) mobilization from cytoplasmic calcium stores. Three other human EC cell lines responded similarly to ACh but not to glutamate or KCI-induced depolarization. In neurons derived from NTERA2 cells by retinoic acid induction, [Ca(2+)](i) signals were evoked by ACh, NMDA, kainate and by an elevation of the extracellular KCI concentration. As in undifferentiated EC cells, the ACh-mediated increases in [Ca(2+)](i) were governed by both Ca(2+) influx and Ca(2+) mobilization. In contrast, the effects of NMDA, kainate and KCI did not involve intracellular Ca(2+) mobilization. The appearance of glutamate and KCI responsiveness was not detected in non-neuronal differentiated derivatives of NTERA2 cells. Using a number of pharmacologically defined muscarinic receptor antagonists we found that NTERA2 EC cells express M(1), M(3), M(4) and possibly M(5) receptor subtypes linked to changes in [Ca(2+)](i), whilst only M(3) and M(5) are present in NTERA2-derived neurons. The results were supported by PCR analysis of the muscarinic mRNA species expressed in the cells. The data demonstrate that differentiation of NTERA2 EC cells into neurons involves the induction of functional glutamate receptors coupled to rises in [Ca(2+)](i), and changes in the expression of muscarinic ACh receptor subtypes.

Involvement of the M2 muscarinic receptor in contractions of the guinea pig trachea, guinea pig esophagus, and rat fundus

The involvement of the M2 muscarinic receptor in contractile responses of the guinea pig trachea, guinea pig esophagus, and rat fundus was investigated. In the standard assay, oxotremorine-M elicited contractions of the trachea with an EC50 value of approximately 73 nanoM.--2- -(Diethylamino)methyl- -1-piperidinyl-acetyl--5,11- dihydro-6H-pyrido-2,3-b--1,4- benzodiazepine-6-one (AF-DX 116) at 1 and 10 microM antagonized these contractions by 2.1- and 9.0-fold increases in the EC50 value for oxotremorine-M. These effects are consistent with antagonism of an M3-mediated contractile response. In subsequent experiments, the M3 receptors were first inactivated selectively by incubation with N-(2-chloroethyl)-4- piperidinyl diphenylacetate (4-DAMP mustard) (40 nanoM) for 1 hr in the presence of AF-DX 116 (1 microM) followed by extensive washing. In 4-DAMP mustard treated trachea, oxotremorine-M elicited contractions with an EC50 value of 0.31 microM in the presence of histamine (10 microM) and forskolin (4 microM). Under these conditions, AF-DX 116 at 1 and 10 microM antagonized contractions to oxotremorine-M by 8- and 59-fold increases in the EC50, respectively, while para- fluorohexahydrosiladiphenidol(p-F-HHSiD) (0.1 microM) had no effect. These effects are consistent with a contraction being mediated by an M2 receptor. In the guinea pig esophagus and rat fundus, AF-DX 116 and p-F-HHSiD blocked contractions measured under similar conditions with magnitudes intermediate between what would be expected from an M2 and an M3 receptor, suggesting that perhaps both subtypes contribute to the overall contractile response under these conditions. In addition, contractions of the guinea pig trachea measured in the presence of histamine and forskolin were pertussis toxin sensitive. These results that, in the trachea, M2 receptors can dominate the contractile response after a majority of the M3 receptors have been inactivated, whereas in the guinea pig esophagus and rat fundus, M2 receptors may contribute to, but do not play a dominant role in the overall response.

Specificity of coupling of muscarinic receptor isoforms to a novel chick inward-rectifying acetylcholine-sensitive K+ channel

The G-protein-gated inward-rectifying K+ channel GIRK1 has been demonstrated in heart and brain. These tissues also both express the M2, M3, and M4, muscarinic acetylcholine receptors (mAChR) (Gadbut, A.P., and Galper, J.B. (1994),J. Biol. Chem. 269,25823-25829). Only the M2 mAChR has been demonstrated to couple to GIRK1 (Kubo, Y., Reuveny, E., Slesinger, P. A., Jan, Y. N., and Jan, L. Y. (1993) Nature 264, 802-806). In this study we determined the specificity of coupling of the M3 and M4 mAChR to a new GIRK1 cloned from a chick brain cDNA library. This clone codes for a 492-amino acid protein that is 93% identical to rat GIRK1 and is expressed in brain, atrium, and ventricle, but not skeletal muscle. In Xenopus laetis oocytes co-expression of GIRK1 with either the chick M2 or M4 mAChR gave carbamylcholine (10 microm)-stimulated K+ currents of 308 +/-26 nA and 298 +/-29 nA, respectively, which were both Ba2+- and pertussis toxin-sensitive. Activation of the M3 receptor produced 2382 +/-478 nA of current which was insensitive to Ba2+ and pertussis toxin, but was 85% inhabitable by the Cl channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (10-20 microm) consistent with coupling to an endogenous Ca2+-activated Cl- channel via a phosphatidylinositol-dependent mechanism. Co-expression of the cardiac inward rectifier CIR with chick M2 or M4 mAChR and GIRK1 increased currents more than 10-fold, but had no effect on specificity of coupling. These data demonstrate a new function for the M4 mAChR and a high degree of specificity for coupling of each receptor subtype to GIRK1.

Localization and characterization of the subtypes(s) of muscarinic receptor involved in prostacyclin synthesis in rabbit heart

The present study was conducted to localize and characterize the subtype(s) of muscarinic receptor involved in prostacyclin production elicited by the cholinergic transmitter acetylcholine (ACh) in various cell types in the rabbit heart. ACh increased prostacyclin synthesis, measured as 6-keto-prostaglandin(1 alpha) (6-keto-PGF(1 alpha)), in cultured coronary endothelial cells and freshly dissociated ventricular myocytes in a dose-dependent manner, but not in cultured coronary smooth muscle cells of rabbit heart. McN-A-343 {(4-hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride}, a selective M1 muscarinic ACh receptor (mAChR) agonist, did not alter 6-keto-PGF(1 alpha) synthesis in these cell types. ACh induced 6-keto-PGF(1 alpha) synthesis in coronary endothelial cells and ventricular myocytes was not altered by a low concentration (0.01 microM) of pirenzepine, an M1 mAChR antagonist, but was reduced by a higher concentration (1 microM). In coronary endothelial cells, ACh-induced 6-keto-PGF(1 alpha) production was reduced by hexahydrosila-difendial (HHSiD), an M3 mAChR antagonist, and in ventricular myocytes by both AF-DX 116 [11-{2-[(diethylamino)methyl]-1-piperidinyl]acetyl-5,11-dihydro-6H- pyrido[2,3-b]-benzodiazepine-6 one}], an M2 receptor antagonist, and HHSiD. The decrease by ACh of isoproterenol-stimulated cAMP accumulation was minimized by AF-DX 116, but not by HHSiD or pirenzepine. Pertussis toxin treatment minimized ACh-induced decrease in isoproterenol-stimulated rise in cAMP, but not ACh-induced 6-keto-PGF(1 alpha) synthesis. These data suggest that ACh stimulates prostacyclin production in coronary endothelial cells via M3 mAChR and in ventricular myocytes via M2 and M3 mAChR, and may contribute to its cardioprotective effects. Moreover, ACh induced decrease in cAMP, but not the increase in 6-keto-PGF (1 alpha) production, is mediated by pertussis toxin-sensitive G(alpha i) proteins in these cells.