Comparative studies of the postjunctional activities of some very potent muscarinic agonists

Dioxane and oxathiane nuclei: suitable substructures for muscarinic agonists

Muscarinic agonists, bearing 1,4-dioxane and 1,4-oxathiane nuclei, were synthesized and tested to evaluate their potency at M(1)-M(4) muscarinic receptor subtypes. The stereochemical relationship between the 2-side chain and the 6-methyl group plays an important role in drug-receptor interaction, since the cis isomers are more potent than the corresponding trans isomers. However, the latter are able to discriminate between the muscarinic receptor subtypes. Among them compound 5b proves particularly interesting, since it selectively activates the ileal M(3) receptor subtype and is devoid of agonist activity at the others.

Characterization of the interaction between muscarinic M2 receptors and beta-adrenoceptor subtypes in guinea-pig isolated ileum

1. Contraction of guinea-pig ileum to muscarinic agonists is mediated by M3 receptors, even though they account for only 30% of the total muscarinic receptor population. The aim of this study was to characterize the biochemical and functional effects of stimulation of the predominant M2 muscarinic receptor (70%) and to investigate the hypothesis that M2 receptors specifically oppose beta-adrenoceptor-mediated effects in the ileum. 2. In guinea-pig ileal longitudinal smooth muscle slices, isoprenaline, a non-selective beta-adrenoceptor agonist, and BRL 37344 (sodium-4-[2-[2-hydroxy-2-(3- chlorophenyl)ethylamino]propyl]-phenoxyacetate sesquihydrate), a beta 3-adrenoceptor selective agonist, increased cyclic AMP accumulation with -log EC50 values of 6.6 +/- 0.1 and 5.8 +/- 0.1 respectively. Maximal stimulation by BRL 37344 (10 microM) was 26.4 +/- 5.2% of that observed with isoprenaline (10 microM). Isoprenaline (10 microM)-stimulated cyclic AMP accumulation was significantly, but not completely, inhibited by propranolol (5 microM), with a propranolol-resistant component of 28.2 +/- 6.8% of the maximal stimulation to isoprenaline. In contrast, basal and BRL 37344 responses were resistant to this antagonist. These data provide evidence that both beta 1- and beta 3-adrenoceptors activate adenylyl cyclase in guinea-pig ileum. 3. Isoprenaline (10 microM)-stimulated cyclic AMP accumulation was inhibited (67.4 +/- 0.9%) by the muscarinic agonist (+)-cis-dioxolane (-log EC50 = 7.3 +/- 0.1). The rank order of antagonist affinities against the (+)-cis-dioxolane response was (-log KB values in parentheses): atropine (9.0 +/- 0.2)>methoctramine (7.1 +/- 0.1) >p-fluoro-hexa-hydrosilaphenidol (p-F-HHSiD; 6.5 +/- 0.2) ) pirenzepine(6.3 +/- 0.2). (+)-cis-dioxolane also significantly inhibited BRL 37344 (10 IM; 56.5 +/-2.4%) stimulated cyclic AMP accumulation. These data suggest that M2 receptors mediate inhibition of cyclic AMP accumulation in response to both beta l- and beta 3-adrenoceptor stimulation in guinea-pig ileum.4. 5-Hydroxytryptamine (5-HT), vasoactive intestinal peptide, prostaglandins E2 and E1, all at 10 micro M,significantly increased cyclic AMP accumulation. (+)-cis-Dioxolane (10 micro M) inhibited both basal and agonist-induced cyclic AMP accumulation. Thus the inhibitory effect of M2 receptor agonism does not appear to be restricted to beta-adrenoceptor-stimulated cyclic AMP accumulation.5. The potential for involvement of activation of M2 receptors on responses to beta-adrenoceptor agonists was also studied functionally. Selective M3 receptor alkylation was achieved by pretreatment of tissues with 4-DAMP mustard (40 nM), in the presence of methoctramine (1 micro M; to protect M2 receptors). After washing, tissues were pre-contracted with histamine (0.3 micro M) and relaxed with isoprenaline (0.6 micro M).Under these conditions, oxotremorine M caused concentration-dependent contractions (-log EC50 of 7.8 +/- 0.1), that were surmountably antagonized by methoctramine (1 microM) with a - log KB estimate of 7.4 +/- 0.1. Similar observations were seen versus relaxation produced by BRL 37344 (1 micro M), where the-log KB value for methoctramine was 7.8 +/- 0.2. These data suggest that M2 receptors mediate a functional inhibition of relaxant responses to isoprenaline and BRL 37344.6. These findings are consistent with beta l- and beta 3-adrenoceptors coupling to stimulation of a denylylcyclase in guinea-pig ileum; a response that is inhibited by M2 receptor stimulation. Concordantly, M2 receptor stimulation also inhibits relaxation to both beta l- and beta 3-adrenoceptor stimulation. These results implicate M2 receptors in the modulation of sympathetic control of ileal motility.

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.

Characterization of muscarinic receptors in rat kidney

Muscarinic receptors in mammalian kidney seem to be involved in diuresis. In this study we give a detailed characterization of receptors in rat kidney. Specific binding of [3H](-)-quinuclidinylbenzilate ([3H]QNB) to membranes of rat kidney cortex was saturable and of high affinity. A dissociation constant of 0.063 +/- 0.003 nM and a receptor density of 1.46 +/- 0.07 pmol/g wet weight were obtained. The dissociation kinetics could be best described by assuming a mono-exponential function (k-1 = (0.52 +/- 0.1) x 10(-4) s-1). The binding of [3H]QNB reached a maximum in 60 min at 0.6 nM at 37 degrees C. Competition experiments with the enantiomers of benzetimide confirmed the muscarinic nature of the [3H]QNB binding sites. The inhibition constants of pirenzepine (0.23 +/- 0.02 microM), (+-)-hexahydrosiladifenidol (0.040 +/- 0.002 microM), AF-DX 116 (1.45 +/- 0.07 microM), methoctramine (1.67 +/- 0.02 microM) and gallamine (78 +/- 3 microM) classified this receptor as an M3 receptor. Inhibition of [3H]QNB binding by the agonists methylfurtrethonium, arecoline, isoarecoline methiodide, arecaidine propargyl ester and McN-A-343 displayed monophasic inhibition curves. With (+/-)-cis-2-methyl-4-dimethylaminomethyl-1,3- dioxolane methiodide in two out of four experiments a small (11%) population of high affinity agonist sites could be detected. The potassium sparing diuretic amiloride inhibited [3H]QNB binding (36 +/- 3 microM). Although in a way related to the amiloride binding site, the muscarinic receptors in rat kidney are unlikely to be the primary target of diuretic action of this drug.

Quest for agonist and antagonist selectivity at muscarinic receptors in guinea-pig smooth muscles and cardiac atria

Potencies of 11 muscarinic agonists in eliciting contraction of smooth muscle in guinea-pig ileum, trachea, urinary bladder and uterus and in inhibiting the rate of contractions of cardiac atria were compared. While acetylcholine (ACh) was the most potent agonist on the ileum, uterus and cardiac atria, cis-L(+)-dioxolane was equally as potent as ACh on the ileum and more potent on the urinary bladder and trachea. Compared to ACh, methylfurmethide, oxotremorine, acetoxybut-2-inyl-trimethylammonium and cis-L(+)-dioxolane acted weakly on the atria. Aceclidine, arecoline and acetyl-beta-methylcholine displayed selectivity for the urinary bladder and pilocarpine for the tracheal and urinary bladder smooth muscles. Oxotremorine had very low activity on the uterus. The stereoselectivity of muscarinic ACh receptors (mAChRs) for cis-L(+)-and cis-D(-)-dioxolane was low in the urinary bladder and uterus and high in the ileum and trachea. Most antagonists showed little selectivity between different organs, but S(-)-phenylcyclohexylglycoloyl choline was 6 times more active on the urinary bladder than on the ileum and AF-DX 116 was 12-30 times more active on the atria than on the smooth muscles. Among the N-alkyl derivatives of benzilylcholine, the octyl derivative as 400 times more active on the ileum than on the atria, while among the N-alkyl derivatives of QNB, the N-decyl derivative was 41 times more active on the ileum. The observed differences in the potency of various agonists and their stereoisomers on different smooth muscles cannot be explained by differences in the accessibility of receptors or in receptor reserve.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of lithium pretreatment and of cooling on the responsiveness of the rat isolated jejunum and urinary bladder to muscarinic agonists

1. The aim of the present study was to see whether contractile responses induced by muscarinic agonists in the rat jejunum and urinary bladder were differently affected by procedures that mainly influence the steps following agonist-receptor interaction. Thus, the effects of ex vivo lithium pretreatment (6.8 mmol kg-1 i.p. for 3 days) and in vitro cooling from 37 degrees C to 20 degrees C) on the contractile responses to full and partial agonists, carbachol, oxotremorine, muscarine and pilocarpine were studied. 2. Lithium pretreatment did not affect muscarinic responses on the urinary bladder. It significantly reduced responses to carbachol and oxotremorine but not to muscarine and pilocarpine on the rat jejunum. 3. A decrease of the bath temperature from 37 degrees C to 20 degrees C potentiated responses to carbachol, muscarine and oxotremorine and abolished those to pilocarpine in the urinary bladder. The same lowering of the bath temperature potentiated responses to carbachol, did not affect those to muscarine and to oxotremorine and reduced those to pilocarpine in the jejunum. 4. Together the findings indicate that differences exist in the stimulus-response coupling induced by muscarinic agonists between the two tissues and that, in a given tissue, differences exist among agonists in their ability to activate the agonist-receptor-transducer complex.

Analysis of muscarinic cholinoceptors mediating phosphoinositide hydrolysis in guinea pig cardiac muscle

The muscarinic receptor mediating stimulation of PI hydrolysis in guinea pig atria and ventricles has been studied. The non-selective muscarinic agonist (+)-cis-dioxolane elicited this response, concentration-dependently, with a potency indicative of a low receptor reserve. The potency of a novel, M2-selective agonist, L-660,863 (-log EC50 = 6.3, atria; 6.0, ventricles) was observed to be lower than its apparent affinity (-log KA = 7.6) for M2 receptors, indicating an action probably mediated by a population distinct from that producing negative inotropy in the same tissue. The inhibition of the response to (+)-cis-dioxolane by several muscarinic antagonists (atropine, pirenzepine, AF-DX 116, methoctramine, HHSiD and pFHHSiD) generated an affinity profile for this receptor also dissimilar to that described for the receptor mediating the classical cardiac 'M2' response. Although no other muscarinic receptor mRNA has been detected in this tissue, these data suggest the presence of a second population of muscarinic sites, which may signify an M2 receptor diversity.

A pharmacological study of the responses induced by muscarinic agonists on the isolated superior cervical ganglion of the guinea-pig

We have studied the muscarinic agonist induced responses on the guinea-pig superior cervical ganglion in vitro, as recorded from the internal carotid nerve using a grease-gap. The principal response was a depolarization, but a small hyperpolarizing response could be revealed under certain conditions. We determined the pA2 of a number of muscarinic antagonists against the muscarine induced depolarization. Four selective antagonists and atropine appeared to act competitively. The rank order of their pA2s was 4-DAMP (8.5), atropine (8.4), pirenzepine (8.0), methoctramine (7.2) and AF-DX 116 (6.3). In addition to muscarine, we assessed the potency and relative maximum response of nine other muscarinic compounds to depolarize this preparation: carbachol, 5-methylfurmethide, oxotremorine, oxotremorine-M, pilocarpine, RS 86, AF102B and two novel compounds L-670548 and L-679512. L-670548 was the most potent and AF102B was the least potent agonist tested. Only AF102B evoked a maximum depolarization that was significantly smaller than muscarine. A hyperpolarizing response to carbachol (1 microM) could be recorded when the superfusing medium contained 0.3 microM pirenzepine and only 0.1 mM CaCl2 (cf. usual 2.5 mM). This response was relatively small compared to that evoked on the superior cervical ganglion of the rat. It was blocked by the cardioselective antagonists methoctramine (0.1-0.3 microM) and AF-DX 116 (0.3-1.0 microM). Of the 10 agonists tested, only carbachol, oxotremorine and oxotremorine-M reproducibly evoked a hyperpolarizing response. It was concluded that muscarinic agonists can induce a depolarization of the guinea-pig superior cervical ganglion mediated by M1 receptors. The activation of cardiac-like M2 receptors resulted in a hyperpolarizing response that was relatively small.

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

An isolated preparation of the guinea-pig trachea is described which allows the simultaneous measurement of acetylcholine release and smooth muscle contraction. Incubation of the epithelium-free preparation with [3H]choline resulted in the formation of [3H]acetylcholine. Electrical stimulation caused the release of [3H]acetylcholine and a contractile response. Tetrodotoxin and omission of calcium from the medium abolished both the evoked release and contractions. The muscarinic agonists oxotremorine, carbachol and pilocarpine concentration-dependently inhibited the electrically evoked acetylcholine release and contracted the tracheal smooth muscle. Pre- and postsynaptic EC50 values for a given agonist were not different. Atropine (100 nmol/l) significantly facilitated the evoked acetylcholine release. A concentration of 10 nmol/l atropine did not change the evoked release but antagonized the inhibitory effect of oxotremorine. It is concluded that presynaptic muscarine autoreceptors inhibit the release of acetylcholine from parasympathetic nerves of the guinea-pig trachea.

Distribution of dissociation constant values of muscarinic agonists

The frequency distributions of dissociation constant values of some muscarinic agonists (carbachol, muscarone and cis-2-methyl-5-trimethylammoniummethyl-1,3-oxathiolane) obtained on guinea-pig ileum and atria and rat urinary bladder have been examined to see if the means of the dissociation constant values and the statistical tests for their significance, should be based on geometric rather than on arithmetic means. For the three compounds the distributions on a logarithmic scale did not significantly deviate from normality while the distributions on an arithmetic scale tended to deviate from normality.

Comparison of the effects of some muscarinic agonists on smooth muscle function and phosphatidylinositol turnover in the guinea-pig taenia caeci

1. The effects of the muscarinic agonists acetylcholine (ACh), carbachol (CCh), AHR-602, and McN-A-343 on contractility and on inositol phosphate accumulation in the presence of lithium were compared in the taenia of the guinea-pig caecum. 2. Compared to CCh, ACh was a full agonist for contraction but AHR-602 and McN-A-343 were partial agonists producing 80-85% of the maximal response to CCh. Similar to previous findings with CCh, tonic contractions produced by AHR-602 and McN-A-343 were less sensitive to inhibition by nifedipine or verapamil than tonic contractions to ACh. 3. CCh and ACh produced similar increases in inositol phosphate accumulation and the effect of CCh (0.1 mM) was inhibited by atropine (IC50 8.5 nM) and pirenzepine (IC50 450 nM). The accumulation of inositol phosphates in the presence of AHR-602 or McN-A-343 was not significantly different (P greater than 0.05) from basal levels. 4. A concentration of 0.2 mM AHR-602 produced a parallel shift of the concentration-response curve to CCh on inositol phosphate accumulation. The IC50 value for inhibition of CCh (0.1 mM) was greater than 50 fold higher than the EC50 value for contraction produced by the partial agonist. McN-A-343 (20 microM) produced a flattening of the concentration-response curve to CCh for inositol phosphate accumulation. 5. The results suggest that the increase in phosphatidylinositol turnover produced by muscarinic agonists, like the contractile response, involves an M2-muscarinic receptor. AHR-602 and McN-A-343 are partial agonists for the contractile response and while producing no significant increase in phosphatidylinositol turnover inhibit the response to CCh.

Paraoxon-induced desensitization and its reversal by tetrodotoxin

1. Paraoxon (10 microM for 20 min) induced a desensitization of the taenia caecum of the guinea-pig for contractions produced by a number of cholinomimetics. 2. Tetrodotoxin (0.3 microM) reversed the desensitization suggesting involvement of Na+ channels.

Formation and release of [3H]acetylcholine in the rat urinary bladder strip

The relationship between different frequencies of loading stimulation and [3H]acetylcholine (ACh) formation and release from nerve terminals has been investigated in extratrigonal strips of the urinary bladder of the rat. An increase in frequency (0.2, 0.4 and 0.8 Hz) for the 30 min incubations with [3H]choline produced an enhancement of storage of [3H]ACh from 19.5 to 34% of total tritium content in the tissue. Higher frequencies (1.6 and 3.2 Hz) failed to increase storage further on. The [3H]choline content did not vary significantly. Electrical field stimulation at 2 Hz (360 shocks) produced a release of tritium. The evoked outflow was higher when the strip was loaded at 0.8 Hz than at the other frequencies tested. Both [3H]ACh and [3H]choline were measured in the perfusate of strips preloaded at 0.8 Hz. Most of the induced outflow was found to be [3H]ACh, as in previous experiments carried out using 0.2 Hz as a loading frequency. The findings suggest that in the rat urinary bladder strip loading at 0.8 Hz is suitable for increasing the formation and the resulting release of [3H]ACh during electrical stimulation.

Pharmacological differences between two muscarinic responses of the rat superior cervical ganglion in vitro

1 Pharmacological differences have been observed between the muscarinic agonist-induced depolarizing and hyperpolarizing responses of the rat isolated superior cervical ganglion. 2 Pirenzepine (0.3 microM) selectively reduced the depolarizing response and unmasked the hyperpolarizing response. No such selectivity was observed with a concentration of N-methylatropine which was equipotent with pirenzepine in antagonizing the depolarizing response. 3 The neuromuscular blocking agents gallamine (10 microM) and pancuronium (3 microM) exhibited the oppositive selectivity to pirenzepine, both dramatically reduced the hyperpolarization but only slightly antagonized the depolarization. 4 The potencies of a range of agonists in evoking the depolarizing and hyperpolarizing responses, the latter in the presence of 0.3 microM pirenzepine, have been determined. Methylfurmethide failed to hyperpolarize the ganglion at concentrations which evoked maximal depolarizations. 5 The muscarinic hyperpolarization did not appear to be mediated by the secondary release of catecholamines. 6 It was concluded that the two muscarinic responses on the rat superior cervical ganglion, the slow depolarization and faster hyperpolarization, are mediated by different muscarinic receptor subtypes.

Determination of dissociation constants and relative efficacies of some potent muscarinic agonists at postjunctional muscarinic receptors

This study was undertaken to determine dissociation constants (KA and relative efficacies (er) of seven muscarinic agonists (methylfurtrethonium; dioxolane, oxathiolane, carbachol, muscarine, muscarone and oxotremorine) in three isolated tissues (guinea-pig ileum and atria and rat urinary bladder). The rank order of affinities (-log KA) of the various compounds varied depending on the tissue used. er values for the different agonists did not differ significantly from each other in any of the three tissues, except that the er of muscarine in the guinea-pig ileum was higher than those of the other compounds and that of oxotremorine in the rat urinary bladder was lower than those of the other agonists. Comparisons among tissues show that KA and er values were the same in different tissues for some compounds (muscarone, muscarine and methylfurtrethonium), while significant differences were found for the other compounds. This suggests the existence of a discrete receptor population recognized by some but not all agonists. For oxotremorine er as well as -log KA, is greater in atria than in smooth muscle: these factors combine to determine the cardioselectivity of this compound which can now ascribed to receptor selectivity.