Action of agonists and antagonists at muscarinic receptors present on ileum and atria in vitro

A study of the muscarinic receptor subtype mediating mucus secretion in the cat trachea in vitro

Mucus secretion from the cat trachea simulated by muscarinic receptor agonists has been studied by monitoring both the weight and acid glycoconjugate content of samples taken from an in vitro preparation. The nature of the receptor has been probed using a number of competitive muscarinic receptor antagonists by estimating their affinities from the degree to which the response could be blocked. Antagonist affinities have also been compared with those obtained in tracheal smooth muscle and atria from the guinea-pig. Atropine had similar affinities for all receptors investigated. 4DAMP and AF-DX116 had relatively high (pA2 = 9) and low (pA2 = 6) affinities respectively for the secretory receptor. The pA2 value of 7.5 calculated for pirenzepine suggested that the receptor was not of the M1 subtype. However, the value was higher than that for pirenzepine in both guinea-pig tissues indicating that the receptor may be an 'intermediate' between M1 and M2 subtypes. The lack of antagonists with absolute selectivity for a particular subtype of the muscarinic receptor prohibits a definitive classification.

Molecular pharmacology of muscarinic receptor heterogeneity

Muscarinic receptors can be pharmacologically classified into 3 types at the present time, however, five genes for the receptor have been identified. The muscarinic receptor types have unique antagonist selectivity, distribution and are linked to specific second messenger systems. The interaction between the muscarinic receptor types and G proteins may depend on the systems in which the receptors are integrated. Expression of the cloned gene in mammalian cells will be useful in delineating the relationships between the pharmacological types of muscarinic receptors and their genes and studying the interactions between the receptor, G proteins, and second messenger coupling.

Assessment of the muscarinic receptor subtype involved in the mediation of pilocarpine-induced purposeless chewing behaviour

Purposeless chewing behaviour in rats was enhanced by intraperitoneal administration of the muscarinic agonists pilocarpine (1.0-8.0 mg/kg), RS 86 (0.5-0.8 mg/kg), oxotremorine (1-2 mg/kg) and arecoline (2-32 mg/kg), but not by nicotine (0.1-3.2 mg/kg). Chewing behaviour was also induced by the ICV administration of the muscarinic agonists carbachol (12.5-100 micrograms) and pilocarpine (50-200 micrograms), but not by the putative M-1 receptor agonist McN-A-343 (50-200 micrograms) or AH 6405 (100-200 micrograms). The muscarinic receptor antagonists scopolamine (0.01-0.1 mg/kg SC), benzhexol (0.075-2.5 mg/kg SC), secoverine (1-10 mg/kg SC), and dicyclomine (1.25-10 mg/kg SC) antagonised purposeless chewing behaviour induced by pilocarpine (4 mg/kg IP). AF-DX 116 (2.5-100 mg/kg SC), an M-2 antagonist, partially inhibited the actions of pilocarpine (4 mg/kg IP). Based on ED40 values the rank order of potency following IP administration was scopolamine greater than benzhexol greater than secoverine greater than dicyclomine greater than AF-DX 116. The ICV administration of the muscarinic antagonists N-methylscopolamine (2.5-10 micrograms) and oxyphenonium (10-40 micrograms) antagonised chewing behaviour induced by pilocarpine (4 mg/kg IP) in a dose-related manner. The M-2 antagonist 4-DAMP (40-160 micrograms ICV), as well as AF-DX 116 (40-160 micrograms ICV), also inhibited the effects of pilocarpine (40-160 micrograms ICV). The putative M-1 receptor antagonist pirenzepine (80-320 micrograms ICV) did not antagonise chewing behaviour induced by pilocarpine (4 mg/kg IP).(ABSTRACT TRUNCATED AT 250 WORDS)

An endogenous factor induces heterogeneity of binding sites of selective muscarinic receptor antagonists in rat heart

According to molecular biological and pharmacological criteria, rat heart membranes normally express only one muscarinic receptor subtype. The selective antagonists pirenzepine and AF-DX 116 bind to this receptor with a single affinity: low and high, respectively. We report here that an endogenous, intracellular factor alters the affinity of selective antagonists for muscarinic receptors in the heart. Thus, when the intracellular fluid is added back to rat heart membranes, both pirenzepine and AF-DX 116 bind to two receptor sites. Approximately 30% of the receptors bind pirenzepine with high affinity and AF-DX 116 with low affinity. Thus, while cardiac muscarinic receptors are coded for by a single mRNA and are therefore genetically homogeneous, the resulting receptor protein might behave like a mixture of receptor subtypes in intact tissues due to the influence of intracellular factors on receptor conformation.

Effect of M2 muscarinic receptor antagonist 4-DAMP, on prostaglandin synthesis and mechanical function in the isolated rabbit heart

1. Muscarinic agonist acetylcholine (ACh) (non-selective) and arecaidine propargyl ester (APE) (M2 selective agonist) produced a increase in the output of 6-keto PGF1 alpha and a decrease in the heart rate and myocardial developed tension in a dose-dependent manner. 2. Lower doses of ACh (1.0-5.0 nmol) caused coronary vasodilation, whereas higher doses of ACh (10.0 nmol) and lower as well as higher doses of APE produced a biphasic effect--an initial vasodilation followed by vasoconstriction. 3. The increase in 6-keto PGF1 alpha output elicited by 3 nmol of ACh or APE was inhibited by 10 nM of classical muscarinic receptor antagonist atropine or by the selective M2 beta muscarinic receptor antagonist 4-(diphenylacetoxy-N-methyl piperidine) methiodide (4-DAMP). 4. The decrease in heart rate and myocardial developed tension produced by ACh and APE was attenuated by atropine and 4-DAMP. The coronary vasodilator effect of ACh and APE and the vasoconstrictor effect of APE were also attenuated by both of these muscarinic antagonists.

Affinity and selectivity of biperiden enantiomers for muscarinic receptor subtypes

The affinity of both the (+)- and the (-)-stereoisomer of biperiden for different muscarinic receptor subtypes was investigated in vitro in functional studies with field-stimulated rabbit vas deferens (M1-receptor), guinea-pig ileum (smooth muscle M2 beta-receptor) and rat left atrium (cardiac M2 alpha-receptor). (+)-Biperiden had its highest affinity to M1-receptors (pA2 = 9.07), had low affinity to cardiac M2 alpha-receptors (pA2 = 7.25) and intermediate affinity to ileal M2 beta-receptors (pA2 = 8.27). The ability of (+)-biperiden to discriminate between ileal M2 beta- and cardiac M2 alpha-receptors (factor = 10) was similar to that of 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, factor = 9). In contrast, (-)-biperiden displayed low but nearly undistinguishable affinity for all muscarinic receptor subtypes studied (pA2 = 5.59 +/- 6.38). (+)-Biperiden discriminated strongly between M1- and cardiac M2 alpha-receptors (factor 66), thus being even more selective than pirenzepine (factor 28) which makes it one of the most M1-/cardiac M2 alpha-selective antimuscarinic drugs now available. These results indicate that (+)-biperiden could represent a further valuable tool for the characterization of muscarinic receptor subtypes.

The interaction of methoctramine and himbacine at atrial, smooth muscle and endothelial muscarinic receptors in vitro

1. The action of methoctramine and himbacine at muscarinic receptors has been studied using guinea-pig isolated trachea, oesophageal muscularis mucosae, paced left atria, and rat aortic preparations. 2. Methoctramine (1 x 10(-6)-3.2 x 10(-4) M), but not himbacine, elicited positive inotropic responses. These responses were enhanced by pretreating the animals with reserpine. The responses in reserpine-treated animals were not antagonized by phentolamine (1 x 10(-6) M) but were antagonized by propranolol (1 x 10(-6) M). 3. Methoctramine, but not himbacine, exhibited allosteric inhibitory effects at cardiac muscarinic receptors, resulting in a curvilinear Schild plot. Deviations from competitive antagonism were also observed in combination dose-ratio experiments using atropine and methoctramine. At 1 x 10(-6) M, the pKB value for methoctramine was 7.88 +/- 0.15 (mean +/- s.e.mean, n = 5). The pA2 value for himbacine at cardiac muscarinic receptors was 8.52 +/- 0.06 (n = 3). 4. At tracheal and oesophageal muscularis mucosal smooth muscle receptors, the Schild plots for both antagonists were linear. The pA2 values for methoctramine at receptors in these two preparations were similar (6.08 +/- 0.05 and 6.03 +/- 0.09 respectively, n = 4) and were approximately 60 fold less than those values observed at atrial receptors. Himbacine, also exhibited similar values at muscarinic receptors in the trachea and oesophageal muscularis mucosae (7.61 +/- 0.05 and 7.57 +/- 0.04 respectively, n = 4). 5. Muscarinic receptors mediating relaxation of the rat aortic endothelium exhibited pA2 values for methoctramine (5.87 +/- 0.12, n = 6) which were similar to those observed in the smooth muscle, but not the atria. The pA2 values for himbacine at endothelial muscarinic receptors were approximately 0.5 pA2 units lower than those observed at muscarinic receptors in smooth muscle (6.92 + 0.80, n = 6). In addition, the Schild slopes for methoctramine and himbacine at these receptors were significantly (P < 0.05) less than unity. 6. Methoctramine, and to a lesser extent himbacine, are potent and selective antagonists for cardiac muscarinic receptors. However, caution should be used in interpretation of the data with methoctramine in view of the inhibitory allosteric properties and direct inotropic actions of this compound.

Comparison of the muscarinic receptors of the guinea-pig oesophageal muscularis mucosae and trachea in vitro

1. The muscarinic receptor profile of the guinea-pig oesophageal muscularis mucosae has been compared to that of the trachea in vitro. There was no significant difference in the potency of the following muscarinic agonists at muscarinic receptors in the two tissues: carbachol, RS-86, ethoxyethyltrimethyl-ammonium, bethanechol and pilocarpine. RS-86 was 6-fold more potent at receptors in the muscularis mucosae in comparison to the trachea. There was no difference in the affinity of either carbachol or RS 86 at receptors in the two tissues. 2. The affinities (pA2) of the majority of antagonists (atropine, 4 diphenylacetoxy-N-methylpiperidine methiodide, AF-DX 116, silabenzhexol, methoctramine and gallamine) were similar at receptors in the muscularis mucosae and trachea. In contrast, the affinity of pirenzepine was approximately 5-fold greater at receptors in the muscularis mucosae (pA2 = 7.4) in comparison to receptors in the trachea (pA2 = 6.8). 3. It is concluded that the muscarinic receptor profile of the oesophageal muscularis and trachea are similar and only differ slightly with respect to the affinity of pirenzepine. Consequently, it is difficult to justify receptor heterogeneity in these two tissues.

Differences in antagonist affinities at muscarinic receptors in chick and guinea-pig

1. The antimuscarinic effects of a number of muscarinic antagonists in left atrium and ileum of the chick and guinea-pig were evaluated. 2. In agreement with findings in the guinea-pig, pirenzepine did not differentiate between muscarinic receptors in the left atrium and ileum of the chick. However, its affinity in this species was 5- to 10-fold higher than that observed in similar mammalian tissues. 3. Gallamine, methoctramine and himbacine are cardioselective antagonists in the guinea-pig with a selectivity index of 20, 25 and 10 respectively. However, in the chick, while himbacine maintained its cardioselective effect, gallamine was non-selective and methoctramine was only 4-fold more selective. 4. Secoverine, a non-selective antagonist in guinea-pig exhibited a small degree of cardioselectivity in the chick. 5. The ileoselective effect of 4-DAMP in the chick was much greater than that observed in the guinea-pig. 6. It is concluded that muscarinic receptors in cardiac and smooth muscles of the chick differ from those in the guinea-pig.

Characterization of guinea-pig cardiac muscarinic receptors by radioligand dissociation kinetics

The question of cardiac muscarinic receptor heterogeneity was studied in guinea-pig auricles and ventricle. Radioligand dissociation kinetics were analyzed in the absence and presence of alinidine and AQ-A 39, two muscarinic agents and allosteric modulators of radiotracer dissociation. The dissociation kinetics were monophasic with all 3 radiotracers used and in both auricles and ventricle. [125I]3-Quinuclidinyl 4-iodobenzilate ([125I]QNB) dissociated with an identical half-life (t1/2 off) in ventricle and in left and right auricle, respectively. Alinidine (1 mM) decreased t1/2 off by 50% in ventricle but had no significant influence in auricles. AQ-A 39 (= falipamil) uniformly increased t1/2 off 4-fold in all 3 tissues. The binding of tritium-labelled quinuclidinyl benzilate ([3H]QNB) dissociated equally rapidly from both ventricular and auricular receptors under control conditions but was not affected by alinidine. AQ-A 39 (1 mM) slowed [3H]QNB dissociation 6.5-fold in ventricle but only 3-fold in both auricles. [3H]N-Methyl scopolamine ([3H]NMS) dissociation was uniform in both auricles but was somewhat slower in the ventricle, both in the absence and presence of alinidine. AQ-A 39 was without effect. These results demonstrate differences in ventricular versus auricular receptors detected by [125I]QNB in the presence of alinidine and by [3H]QNB in the presence of AQ-A 39. The quaternary ligand [3H]NMS was unable to detect receptor heterogeneity. No differences were found between right and left auricular receptors. The results presented provide no evidence for the existence of different subtypes of muscarinic receptors for the negative chronotropic and inotropic actions of muscarinic agonists.

Receptor binding profiles of some selective muscarinic antagonists

The binding of hexahydrosiladifenidol, procyclidine, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine) and AF-DX 116 to muscarinic receptors in the heart, ileum, urinary bladder, parotid gland and cerebral cortex from guinea pig was studied in competition experiments with (-)-[3H]QNB. The affinity of AF-DX 116 was higher in the heart than in the cortex and it was extremely low in the parotid gland. The affinities of hexahydrosiladefinidol, procyclidine and 4-DAMP were higher in the cortex and parotid gland than in the heart, bladder and ileum. Hexahydrosiladifenidol and 4-DAMP recognized two classes of muscarinic binding sites in the cortex. However, in contrast to functional data, binding results showed that 4-DAMP hexahydrosiladifenidol and procyclidine did not distinguish between the sites in the smooth muscles and those in the heart. Nevertheless, the present data support the view that the putative M2-receptors are heterogeneous, since the four drugs examined were found to distinguish between the muscarinic binding sites in the parotid gland and those in smooth muscles and heart.

Quantitative effects of some muscarinic agonists on evoked surface-negative field potentials recorded from the guinea-pig olfactory cortex slice

1. The effects of muscarinic receptor agonists on the electrically-evoked surface-negative field potential (N-wave) were measured in the guinea-pig olfactory cortex slice maintained in vitro. 2. Bath-superfusion of (+/-)-muscarine, acetylcholine (ACh), carbachol (CCh), or methacholine (MCh) (10-200 microM) produced reversible, dose-dependent depressions of the N-wave (ACh and MCh effects were observed in the presence of 10 microM neostigmine). The order of potencies (based on agonist dose causing 50% field depression: IC50) was: ACh greater than or equal to muscarine greater than CCh greater than MCh. All four agonists depressed the field potential by 100% at doses greater than 500 microM. 3. Pilocarpine and bethanechol were weak agonists and only produced measurable effects at high doses (1-2 mM). Neither agonist evoked a maximum response at doses up to 10 mM. 4. The muscarinic ganglion stimulant, McN-A-343 yielded inconsistent results, depressing the field potential in some slices, but having no effect in others. Pre-application of a conditioning dose (100 microM) of McN-A-343 reduced subsequent responses to CCh, suggesting possible partial agonist properties. 5. Oxotremorine (up to 100 microM) did not depress the field potential, but it reversibly antagonized the effects of CCh. 6. It is concluded that reproducible, quantifiable responses to muscarinic agonists can be evoked in the olfactory cortex slice. We suggest this preparation may be useful for conducting pharmacological studies of 'intact' central muscarinic receptors.

Characterization of the muscarinic receptor subtypes in the rat urinary bladder

We investigated the nature of the muscarinic receptors present in the rat urinary bladder by performing binding studies with various selective (pirenzepine, AF-DX 116, hexahydrosiladifenidol, benzhexol, 4-diphenyl-acetoxy-N-methyl piperidine methiodide, dicyclomine, secoverine) and classical (N-methylscopolamine, atropine) antagonists. Competition experiments were carried out against [3H]N-methyl scopolamine at 30 degrees C in Na+/Mg2+ HEPES buffer; non-specific binding was determined in the presence of 1 microM 3-quinuclidinyl benzilate. Of all the antagonists examined, only AF-DX 116 exhibited a heterogeneous binding profile (nH less than 1). Computer-assisted analysis showed that the data fitted best to a two-binding site model, revealing the existence of high and low affinity receptors. The affinity values of AF-DX 116, determined in binding experiments carried out in heart and gland homogenates, allowed us to classify the rat urinary bladder receptors into cardiac and glandular subtypes. We suggest that the glandular receptor subtype is involved in smooth muscle contraction, since AF-DX 116 was equally potent in inhibiting smooth muscle contraction and the secretion of saliva.

Chiral muscarinic agonists possessing a 1,3-oxathiolane nucleus: enantio- and tissue-selectivity on isolated preparations of guinea-pig ileum and atria and of rat urinary bladder

Racemate and corresponding enantiomers of muscarinic agonists carrying a 1,3-oxathiolane nucleus were studied on isolated preparations of guinea-pig ileum and atria and of rat urinary bladder. The efficacy of these agonists were determined according to the method of Furchgott and Bursztyn (1967) and enantio-selectivity and tissue-selectivity were investigated. The enantio-selectivities of the most potent compounds studied (expressed as the ratio of potencies or affinities of the enantiomers) vary significantly from tissue to tissue, supporting the view that M2 receptors are not homogeneous. In particular, the data all indicate that the ileal receptors are different to the atrial and bladder ones.

Methoctramine reveals heterogeneity of M2 muscarinic receptors in longitudinal ileal smooth muscle membranes

Direct binding studies on longitudinal ileal and atrial muscarinic receptors revealed that most of the ileal or atrial selective antagonists identified in functional studies did not differentiate between these muscarinic receptors in direct binding studies. Methoctramine, an atrial selective muscarinic receptor antagonist in functional studies was, however, able to partially discriminate between these two receptors in our binding studies. Furthermore the binding data obtained using this compound indicated that longitudinal ileal muscarinic receptors were heterogeneous. The predominant population of ileal muscarinic receptors displayed a similar pharmacology to the cardiac type M2 muscarinic receptor. The minor population of muscarinic receptors identified in binding studies displayed a similar pharmacology to the ileal muscarinic receptor identified in functional studies and were pharmacologically similar to the exocrine gland type M2 muscarinic receptor.

Methoctramine, a polymethylene tetraamine, differentiates three subtypes of muscarinic receptor in direct binding studies

A newly described cardiac selective antagonist methoctramine was examined in direct binding studies for affinity at three putative subtypes of the muscarinic receptor. The compound proved to be highly selective (158 fold) for cardiac as opposed to exocrine gland type M2 muscarinic receptors. In addition the compound was also 16 fold selective for M2 cardiac over M1 muscarinic receptors. These results indicate that methoctramine is a useful compound for the classification of muscarinic receptor subtypes.

Direct binding studies on ileal and cardiac muscarinic receptors

1 Functional studies have indicated that muscarinic receptors in cardiac tissue differ from those in the ileum. In the present study ileal and cardiac muscarinic receptors identified by [3H]-N-methyl scopolamine ([3H]-NMS) were characterized and the selectivity of currently available ileal and atrial selective antagonists determined. 2 In terms of the current functional classification of muscarinic receptors both ileal and cardiac muscarinic receptors were of the M2 subtype based upon their low affinity for pirenzepine. 3 Cyclohexylphenyl(2-piperidinoethyl)silanol (CPPS), a highly ileal selective antagonist in functional studies, was unable to distinguish between ileal and atrial muscarinic receptors identified in binding studies. Furthermore, although AF-DX 116 and dicyclomine were able to differentiate atrial and ileal muscarinic receptors, neither compound was more than 2 fold selective. These data indicate that it is not possible to subclassify ileal and atrial muscarinic receptors using direct ligand binding studies with these antagonists. 4 In circular ileal smooth muscle, apparent heterogeneity of the M2 muscarinic receptor population was observed. Thus AF-DX 116 identified two populations of sites with affinities differing by 30 fold. These two populations of M2 muscarinic receptors may represent the typical M2 muscarinic receptors identified in cardiac tissue and the more recently discovered 'gland type' M2 muscarinic receptors. 5 The circular ileal smooth muscle tissue homogenate was able to decrease dramatically the apparent affinity of adiphenine. This activity, which appeared to result from a phenylmethylsulphonylfluoride (PMSF) sensitive protease effect, should be considered when conducting studies using this tissue preparation and compounds of similar structure to adiphenine.

Characterization of muscarinic receptors in salivary and lacrimal glands of the rat

The binding characteristics of muscarinic receptors in rat salivary and lacrimal glands were studied by means of radioligand binding techniques. In competition experiments against [3H]N-methylscopolamine, classical muscarinic antagonists ipratropium bromide, N-methylscopolamine and N-methylatropine exhibited very similar KD values in all the glands and their binding behavior was well described by a one binding site model (nH congruent to 1). The novel cardioselective antimuscarinic compound, AF-DX 116, displayed an equally low affinity in all the tissues examined. Pirenzepine and dicyclomine, two other selective muscarinic antagonists, showed a similar behaviour in all but the sublingual gland, where their binding profile indicated the presence of a heterogeneous receptor population (nH = 0.74 and 0.84, respectively). Histological studies of the sublingual-submandibular glandular complex demonstrated the presence of ganglionic structures mainly located in the hilum of the sublingual-submandibular glandular complex connected with the sublingual gland. Binding studies carried out with pirenzepine on the hilum and on a synaptosomal preparation from this region again revealed the presence of two populations of muscarinic receptors with KD values of 22-25 and 270-463 nM. These results are best explained by the presence of M1 and M2 receptors located on neuronal and glandular structures.

Muscarinic receptor heterogeneity in guinea pig intestinal smooth muscle: binding studies with AF-DX 116

Muscarinic receptor subtypes in longitudinal and circular smooth muscles of the guinea pig ileum were characterized with the use of the cardioselective antagonist AF-DX 116 in binding competition experiments against 0.3 nM [3H] N-methylscopolamine [( 3H]NMS). This compound recognized a heterogeneous receptor population in both smooth muscles, revealing the existence of different percentages of the cardiac (KD = 92-110 nM) and the glandular (KD = 1150-2541 nM) muscarinic receptor subtypes. These results, together with the low potency displayed by AF-DX 116 to inhibit the agonist-stimulated smooth muscle contraction and salivary secretion allow the suggestion that the glandular muscarinic receptor subtype, showing a low affinity for AF-DX 116, is involved in smooth muscle contraction.

Characterization of muscarinic receptors in canine bronchial smooth muscle

Rings of canine bronchi were suspended for isometric tension recording. Contractions produced by exogenously added acetylcholine were inhibited by pirenzepine and pancuronium. The pKB values were 6.76 for pirenzepine (calculated at 10(-6) M) and 5.30 and 5.13 for pancuronium (calculated at 10(-5) and 3 X 10(-5) M, respectively). Contractile responses evoked by cholinergic nerve stimulation (0.2-16 Hz, 9 V) were depressed by pancuronium in a concentration-dependent manner, while concentrations of pirenzepine selective for M1-muscarinic receptors were without effect. The results indicate that exogenous and nerve-released acetylcholine activate a homogenous population of M2-muscarinic receptors in isolated preparations of canine bronchial smooth muscle.

Identification and characterization of muscarinic cholinergic receptors in the human urinary bladder and parotid gland

The binding characteristics of [3H]quinuclidinyl benzilate (QNB) to muscarinic sites in isolated plasma membrane fractions of the human urinary bladder and parotid gland were studied. QNB binding to both preparations was of high affinity and low capacity. Mean values for the apparent dissociation constants (Kd) for binding to membrane preparations from the urinary bladder and parotid glands were 22 and 34 pM and the Bmax values 234 and 456 fmol/mg protein, respectively. Significance of difference between Kd and Bmax values from the two tissues was at the level of P less than 0.005 and P less than 0.05, respectively. QNB binding was inhibited by muscarinic receptor antagonists with varying degree of effectiveness. The mean values for the inhibition constant (Ki) were significantly lower for oxybutynin, amitriptyline, and pirenzepine but higher for secoverine in preparations of the urinary bladder than of the parotid gland. The mean Ki values for quinidine and verapamil were lower in the urinary bladder than that in the parotid gland. Carbachol exhibited a marked selectivity for the urinary bladder (about 30-fold) compared with the parotid gland. The present data obtained in two human tissues that are highly cholinergic in their innervation give support to the argument for heterogeneity of the muscarinic cholinergic receptors.

Comparison of muscarinic acetylcholine binding in the urinary bladder and submandibular gland of the rabbit

In order to explore the possibility of heterogeneity in peripheral muscarine receptors, receptors were characterized in membrane fractions isolated from rabbit urinary bladder and submandibular gland. With [3H]QNB as radioligand, specific binding with very high affinity was found in both preparations. Although the BmaxS for binding in the two preparations were very similar, the mean KD value in the submandibular gland was significantly higher (P less than 0.005) than that in the bladder. Among the anticholinergic drugs, oxybutynin had a significantly lower value for the inhibition constant (Ki) in the submandibular gland whereas Ki for both secoverine and pirenzepine was significantly higher in this tissue than in urinary bladder. The Ki for carbacholine was about 7-fold higher in submandibular gland than in the bladder. Although quinidine and verapamil showed relatively weak binding to the muscarine receptor site, their Ki in the submandibular gland was significantly higher than that in the bladder. The results indicate that although there is a considerable similarity between muscarinic receptors in urinary bladder and submandibular gland, the differences in Ki values for different compounds in the two tissues support the argument favouring heterogeneity of muscarinic acetylcholine receptors in peripheral effector organs.

Competitive and non-competitive antagonism exhibited by 'selective' antagonists at atrial and ileal muscarinic receptor subtypes

The affinity of a number of 'selective' agonists and antagonists has been assessed at atrial or ileal muscarinic receptors by use of in vitro functional analysis. The most selective compound for ileal muscarinic receptors was silabenzhexol (approx. 50 fold), and to a lesser extent benzhexol (approx. 5 fold). Conversely, the most selective compound for the atrial muscarinic receptors was AF-DX 116 (approx. 6 fold). The novel M1-receptor antagonist, telenzepine and other antagonists such as propantheline and isopropamide did not distinguish between atrial and ileal receptors. Dicyclomine, adiphenine, hexahydroadiphenine and oxyphenonium exhibited competitive antagonism at atrial receptors but non-competitive antagonism at ileal receptors. No conclusions could, therefore, be drawn with regard to their selectivity. The agonists, arecaidine propargyl ester (APE), ethoxyethyltriethylammonium (EOE) and carbachol, exhibited some selectivity in potency but little difference in affinity. It is concluded that the study supports the existence of ileal and atrial muscarinic receptor subtypes. However, the use of dicyclomine and related compounds in receptor classification is limited.

Pancuronium and gallamine are antagonists for pre- and post-junctional muscarinic receptors in the guinea-pig lung

The effects of atropine, pancuronium and gallamine were tested on pre- and post-junctional muscarinic receptors in the lung. Inhibition of bronchoconstriction induced by intravenous injection of acetylcholine (ACh) was used as a measure of post-junctional receptor blockade. All three antagonists reduced ACh-induced bronchoconstriction. The effects were dose-related for atropine and pancuronium and complete inhibition was obtained with 0.01 mg/kg and 10 mg/kg respectively. Gallamine was much less potent than the other two drugs; the inhibitory effect was not dose-related and never exceeded 50% even at a dose of 10 mg/kg. In contrast, blockade of pre-junctional inhibitory muscarinic receptors in pulmonary parasympathetic nerves by these three antagonists, produced potentiation of bronchoconstriction induced by vagal-nerve stimulation. Consequently, the effect of the three antagonists on vagally-induced bronchoconstriction is dependent on the balance between their pre- and post-junctional blocking activity. Gallamine was the most effective and atropine the least effective antagonist for potentiating nerve-induced bronchoconstriction. At doses which produce 100% neuromuscular blockade, both pancuronium (0.04 mg/kg) and gallamine (4 mg/kg) potentiated vagally-induced bronchoconstriction. At these doses, pancuronium doubled and gallamine caused a four-fold increase in vagally-induced bronchoconstriction, despite partial concurrent blockade of muscarinic receptors in the smooth muscle of the airways.

Muscarinic activity of McN-A-343 and its value in muscarinic receptor classification

The affinity and potency of McN-A-343 (4-(m-chlorophenyl-carbamoyloxy) -2-butynyltrimethylammonium chloride) has been assessed at a range of M1 and M2 muscarinic receptors. McN-A-343 was shown to act as a full agonist at M2 receptors present in the guinea-pig isolated taenia caeci (-log EC50 = 5.14). McN-A-343 exhibited no agonist action in the guinea-pig ileum, atria, bladder or trachea. McN-A-343 was not selective in terms of affinity since its dissociation constants at M1 and M2 binding sites in the rat cerebral cortex and myocardium respectively, were very similar (cortical pPKi = 5.05; myocardial pKi = 5.22). The selectivity previously reported for the compound may be due to differences in intrinsic efficacy and/or tissue receptor reserve. Based on differential antagonist affinities, the muscarinic receptor profile of the taenia caeci, trachea and bladder was similar to that observed in the ileum, but dissimiliar to that observed in the atria.

Evidence for a receptor mediated action of norepinephrine distinct from alpha- and beta-adrenoceptors

The mode of action of (-) norepinephrine (NE) and UK-14,304-18 has been investigated using the cholinergically-evoked 'twitch' response of the electrically stimulated guinea-pig ileum. St 587 and benextramine were employed as antagonists. St 587 acted as a competitive antagonist toward UK-14,304-18, yielding an apparent pA2 value of 7.3. In contrast, St 587 failed to act competitively toward NE. Similarly, benextramine (1 X 10(-5) mol/l) blocked the inhibitory responses to UK-14,304-18 but was considerably less active toward NE. Remaining responses to NE after benextramine were not antagonized by St 587, even at a concentration of 3 X 10(-5) mol/l. It is postulated that NE acts to inhibit the 'twitch' response be evoking two different receptor-mediated events: 1. agonism at the alpha 2-adrenoceptor and 2. agonism at a site which is distinct from the alpha- and beta-subtypes. In the concentrations studied, UK-14,304-18, St 587 and benextramine are postulated to lack affinity for the proposed site. The effect of NE and UK-14,304-18 was also investigated on the contractile responses to exogenously applied histamine. These experiments were done in the presence of muscarinic cholinergic and adrenoceptor blockade. NE inhibited responses to histamine but UK-14,304-18 was inactive. Furthermore, the inhibitory action of NE was stereoselective with the (-) form being 25 times more potent than the (+) enantiomer. These findings suggest the presence of a receptor site for NE which is distinct from cholinergic mechanisms and established alpha and beta-adrenoceptors.

Pharmacological analysis of the inhibition by pirenzepine and atropine of vagal-stimulated acid secretion in the isolated stomach of the mouse

The muscarinic receptors involved in the vagal stimulation of gastric acid secretion in the mouse isolated stomach assay have been examined by analysing the effects of pirenzepine and atropine on fully-defined frequency-effect curves. Both atropine and pirenzepine produced concentration-dependent inhibition of vagal-stimulated acid secretion in a manner consistent with a model describing competitive antagonism of endogenous acetylcholine, which was assumed to be released by vagal stimulation. The results obtained are quite compatible with the hypothesis that vagal stimulation involves muscarinic receptors which are homogeneous with those previously found on histamine and oxyntic cells in the mouse stomach assay. These results find no evidence for muscarinic receptor heterogeneity and reinforce the hypothesis that the selectivity of pirenzepine in vivo relative to atropine is due to the loss of atropine into the gastric secretion.