The relative potencies of some agonists at M2 muscarinic receptors in guinea-pig ileum, atria and bronchi

Transcutaneous vagus nerve stimulation attenuates autoantibody-mediated cardiovagal dysfunction and inflammation in a rabbit model of postural tachycardia syndrome

PURPOSE

Previous studies demonstrated M2 muscarinic acetylcholine receptor-activating autoantibodies (M2R-AAb) were present in some patients with postural tachycardia syndrome (POTS). This study examines how these autoantibodies might contribute to the pathophysiology of POTS, and whether low-level tragus stimulation (LLTS) can ameliorate autoantibody-mediated autonomic dysregulation in the rabbit.

METHODS

Five New Zealand white rabbits were immunized with a M2R second extracellular loop peptide to produce cholinomimetic M2R-AAb. Tilt test and infusion studies were performed on conscious rabbits before immunization, 6 weeks after immunization, and 8 weeks after immunization with 2-week daily LLTS treatment. Each rabbit served as its own control.

RESULTS

Compared to preimmune state, an enhanced heart rate increase and decreased parasympathetic activity upon tilting were observed in immunized rabbits. Furthermore, these rabbits demonstrated an attenuated heart rate-slowing response to infusion of the M2R orthosteric agonist arecaidine propargyl ester (APE), suggesting an inhibitory allosteric effect of M2R-AAb. There was also a significant increase in serum inflammatory cytokines in immunized rabbits. LLTS treatment suppressed the postural tachycardia, improved the sympathovagal balance with increased acetylcholine secretion, reduced the levels of inflammatory cytokines, and reversed the attenuated heart rate response to APE in immunized rabbits. No suppression of M2R-AAb expression by LLTS was found during this short-term study period. Receptor-modulating activity of M2R-AAb produced in immunized rabbits was confirmed with in vitro bioassay.

CONCLUSIONS

Autoantibody inhibition of cholinergic ligand activity may be involved in the development of cardiovagal dysfunction and inflammation associated with POTS, both of which can be improved by vagal stimulation.

M2 muscarinic receptor activation inhibits cell proliferation and migration of rat adipose-mesenchymal stem cells

Mesenchymal stem cells (MSCs), also known as stromal mesenchymal stem cells, are multipotent cells, which can be found in many tissues and organs as bone marrow, adipose tissue and other tissues. In particular MSCs derived from Adipose tissue (ADSCs) are the most frequently used in regenerative medicine because they are easy to source, rapidly expandable in culture and excellent differentiation potential into adipocytes, chondrocytes, and other cell types. Acetylcholine (ACh), the most important neurotransmitter in Central nervous system (CNS) and peripheral nervous system (PNS), plays important roles also in non-neural tissue, but its functions in MSCs are still not investigated. Although MSCs express muscarinic receptor subtypes, their role is completely unknown. In the present work muscarinic cholinergic effects were characterized in rat ADSCs. Analysis by RT-PCR demonstrates that ADSCs express M1-M4 muscarinic receptor subtypes, whereas M2 is one of the most expressed subtype. For this reason, our attention was focused on M2 subtype. By using the selective M2 against Arecaidine Propargyl Ester (APE) we performed cell proliferation and migration assays demonstrating that APE causes cell growth and migration inhibition without affecting cell survival. Our results indicate that ACh via M2 receptors, may contribute to the maintaining of the ADSCs quiescent status. These data are the first evidence that ACh, via muscarinic receptors, might contribute to control ADSCs physiology.

M2muscarinic receptors inhibit cell proliferation and migration in urothelial bladder cancer cells

The role of muscarinic receptors in several diseases including cancer has recently emerged. To evaluate the hypothesis that muscarinic acetylcholine receptors may play a role in bladder cancer as well as in other tumor types, we investigated their expression in bladder tumor specimens. All examined samples expressed the M1, M2 and M3 receptor subtypes. We also found that the level of M2 transcripts, but not those of M1 or M3, significantly increased with the tumor histologic grade. In view of these results, we proceeded to investigate whether the M2 agonist Arecaidine had any effect on in vitro cell growth and migration of T24 cells, a bladder tumor cell line expressing the muscarinic receptors, including the M2 subtype. We observed that Arecaidine significantly reduced T24 and 5637 cell proliferation and migration in a concentration dependent manner. The silencing of M2 receptor by siRNA in T24 and 5637 cell lines showed the inability of Arecaidine (100 μM) to inhibit cell proliferation after 48 hours, whereas the use of M1 and M3 antagonists in T24 appeared not to counteract the Arecaidine effect, suggesting that the inhibition of cell proliferation was directly dependent on M2 receptor activation. These data suggest that M2 muscarinic receptors may play a relevant role in bladder cancer and represent a new attractive therapeutic target.

M2 muscarinic receptor activation regulates Schwann cell differentiation and myelin organization

Glial cells express acetylcholine receptors. In particular, rat Schwann cells express different muscarinic receptor subtypes, the most abundant of which is the M2 subtype. M2 receptor activation causes a reversible arrest of the cell cycle. This negative effect on Schwann cell proliferation suggests that these cells may possibly progress into a differentiating program. In this study we analyzed the in vitro modulation, by the M2 agonist arecaidine, of transcription factors and specific signaling pathways involved in Schwann cell differentiation. The arecaidine-induced M2 receptor activation significantly upregulates transcription factors involved in the promyelinating phase (e.g., Sox10 and Krox20) and downregulates proteins involved in the maintenance of the undifferentiated state (e.g., c-jun, Notch-1, and Jagged-1). Furthermore, arecaidine stimulation significantly increases the expression of myelin proteins, which is accompanied by evident changes in cell morphology, as indicated by electron microscopy analysis, and by substantial cellular re-distribution of actin and cell adhesion molecules. Moreover, ultrastructural and morphometric analyses on sciatic nerves of M2/M4 knockout mice show numerous degenerating axons and clear alterations in myelin organization compared with wild-type mice. Therefore, our data demonstrate that acetylcholine mediates axon-glia cross talk, favoring Schwann cell progression into a differentiated myelinating phenotype and contributing to compact myelin organization.

Effects of muscarinic agents on chick choroids in intact eyes and eyecups: evidence for a muscarinic mechanism in choroidal thinning

PURPOSE

In chicks, ocular growth inhibition is associated with choroidal thickening and growth stimulation with choroidal thinning, suggesting a mechanistic link between the two responses. Because muscarinic antagonists inhibit the development of myopia in animal models by a non-accommodative mechanism, we tested the hypothesis that agonists would stimulate eye growth and thin the choroid. We also hypothesized that the effective growth-inhibiting antagonists would thicken the choroid.

METHODS

Chicks, age 12-16 days, were used. In vivo: Agonists: Single intravitreal injections (20 μL) of oxotremorine (oxo), pilocarpine (pilo), carbachol (carb), or arecaidine (arec) were given to otherwise untreated eyes. A-scan ultrasonography was done prior to injections, and at 3, 24, 48 and 72 h. Antagonists: -10D lenses were worn on one eye for 4 days. Atropine (atro), pirenzepine (pirz), oxyphenonium (oxy) or dicyclomine (dicy) were injected (20 μL) daily into lens-wearing eyes; saline injections were done as controls. Ultrasonography was done on d1 and on d4; on d4 measurements were done before and 3 h after injections. In vitro: Paired eyecups of retinal pigment epithelium (RPE), choroid and sclera were made from 1-week old chicks. All drugs except atropine were tested on one eyecup, its pair in plain medium. Choroidal thickness was measured at various times over 48 h.

RESULTS

Agonists: In vivo, oxotremorine caused an increase in the rate of axial elongation (drug vs saline: 24-72 h: 338 μm vs 250 μm; p < 0.001). All except pilocarpine caused choroidal thinning by 24 h (oxo, carb and arec vs saline: -25, -35 and -46 μm vs 3 μm). In vitro, all agonists thinned choroids by 24 h (oxo: -6 vs 111 μm; pilo: 45 vs 212 μm; carb: -58 vs 65 μm; arec: 47 vs 139 μm; p < 0.05). Antagonists: Atropine, pirenzepine and oxyphenonium inhibited the development of myopia in negative lens-wearing eyes, and also caused choroidal thickening (drug vs saline: 42, 80, 88 vs 10 μm per 3 h). In vitro, pirenzepine thickened choroids by 3 h (77 vs 2 μm, p < 0.01).

CONCLUSIONS

Muscarinic agonists caused choroidal thinning in intact eyes and eyecups, supporting a role for acetylcholine in the choroidal response to hyperopic defocus or form deprivation. Only oxotremorine stimulated eye growth, which is inconsistent with a muscarinic receptor mechanism for antagonist-induced eye growth inhibition. The dissociation between choroidal thinning and ocular growth stimulation for the other agonists in vivo suggest separate pathways for the two.

M2 receptor activation inhibits cell cycle progression and survival in human glioblastoma cells

Muscarinic receptors, expressed in several primary and metastatic tumours, appear to be implicated in their growth and propagation. In this work we have demonstrated that M2 muscarinic receptors are expressed in glioblastoma human specimens and in glioblastoma cell lines. Moreover, we have characterized the effects of the M2 agonist arecaidine on cell growth and survival both in two different glioblastoma cell lines (U251MG and U87MG) and in primary cultures obtained from different human biopsies. Cell growth analysis has demonstrated that the M2 agonist arecaidine strongly decreased cell proliferation in both glioma cell lines and primary cultures. This effect was dose and time dependent. FACS analysis has confirmed cell cycle arrest at G1/S and at G2/M phase in U87 cells and U251 respectively. Cell viability analysis has also shown that arecaidine induced severe apoptosis, especially in U251 cells. Chemosensitivity assays have, moreover, shown arecaidine and temozolomide similar effects on glioma cell lines, although IC50 value for arecaidine was significantly lower than temozolomide. In conclusion, we report for the first time that M2 receptor activation has a relevant role in the inhibition of glioma cell growth and survival, suggesting that M2 may be a new interesting therapeutic target to investigate for glioblastoma therapy.

M2 muscarinic receptors inhibit cell proliferation in human glioblastoma cell lines

AIMS

In the present work we investigated the expression of M2 muscarinic receptor subtype in two glioblastoma cell lines and its role in the control of cell proliferation.

MAIN METHODS

The M2 receptor transcript and protein expression was studied using RT-PCR and western blot analysis. (3)[H]-thymidine incorporation was used to evaluate cell proliferation in the presence or in the absence of M(2) agonist arecaidine.

KEY FINDINGS

We demonstrated that M(2) receptor is expressed in both cell lines, although U251 cells show a higher expression level, compared to U87 cells. The activation of M(2) receptors by the agonist arecaidine decreases cell growth in a dose and time dependent manner. The anti-proliferative effect of arecaidine is also confirmed by the significant decrease of (3)[H]-thymidine incorporation in both cell lines. Moreover the M2 antagonist gallamine counteracts the arecaidine effects confirming M2 receptor involvement in glioma cell growth inhibition.

SIGNIFICANCE

These data suggest a role for M2 receptors in the inhibition of glioma cell proliferation and the possibility of exploiting these receptors as new promising tools for glioblastoma therapy.

Muscarinic agonists and antagonists: effects on the urinary bladder

Voiding of the bladder is the result of a parasympathetic muscarinic receptor activation of the detrusor smooth muscle. However, the maintenance of continence and a normal bladder micturition cycle involves a complex interaction of cholinergic, adrenergic, nitrergic and peptidergic systems that is currently little understood. The cholinergic component of bladder control involves two systems, acetylcholine (ACh) released from parasympathetic nerves and ACh from non-neuronal cells within the urothelium. The actions of ACh on the bladder depend on the presence of muscarinic receptors that are located on the detrusor smooth muscle, where they cause direct (M₃) and indirect (M₂) contraction; pre-junctional nerve terminals where they increase (M₁) or decrease (M₄) the release of ACh and noradrenaline (NA); sensory nerves where they influence afferent nerve activity; umbrella cells in the urothelium where they stimulate the release of ATP and NO; suburothelial interstitial cells with unknown function; and finally, other unidentified sites in the urothelium from where prostaglandins and inhibitory/relaxatory factors are released. Thus, the actions of muscarinic receptor agonists and antagonists on the bladder may be very complex even when considering only local muscarinic actions. Clinically, muscarinic antagonists remain the mainstay of treatment for the overactive bladder (OAB), while muscarinic agonists have been used to treat hypoactive bladder. The antagonists are effective in treating OAB, but their precise mechanisms and sites of action (detrusor, urothelium, and nerves) have yet to be established. Potentially more selective agents may be developed when the cholinergic systems within the bladder are more fully understood.

Cholinergic activation of phasic activity in the isolated bladder: possible evidence for M3- and M2-dependent components of a motor/sensory system

OBJECTIVES

To analyse pressure changes induced by muscarinic agonists on the isolated bladder in order to examine whether there are different responses representing different components of a motor/sensory system within the bladder wall.

MATERIALS AND METHODS

Whole isolated bladders from 19 female guinea-pigs (280-400 g) were used. A cannula was inserted into the urethra to monitor intravesical pressure and the bladder was suspended in a heated chamber containing carboxygenated physiological solution at 33-36 degrees C. Initially, the responses to the cholinergic agonists, arecaidine but-2-ynyl ester tosylate and carbachol were assessed. Then, in an attempt to identify the muscarinic receptor subtypes involved, the effects of selective muscarinic antagonists on the arecaidine-induced bladder responses were assessed. The antagonists used were the relatively M(3)-selective 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) and darifenicin, and relatively M(2)-selective AFDX-116. All drugs were added to the solution bathing the ablumenal surface of the bladder.

RESULTS

The whole bladders exposed to cholinergic agonists respond with complex changes in intravesical pressure. Immediately after application of the agonist there was a burst of high frequency transient contractions. During continued application of agonist the frequency of the transients decreased and their amplitude increased. Thus, there appear to be two components to the response: an initial fast phase and a later slow component. The maximum frequency of the initial burst increased with increasing concentrations of agonist. By contrast, the frequency of the transients in the steady state showed little dependence on agonist concentration. There were quantitative differences between the responses to arecaidine and carbachol. Arecaidine was less effective in generating the initial burst of high-frequency activity and the transients were significantly larger. At low dose, arecaidine was more effective in producing the large transients in the steady state. Pre-exposure of the bladder to 4-DAMP (0.1-10 nM) or darifenicin (0.1-10 nM) significantly reduced the frequency of the initial burst of activity; 0.3 nM 4-DAMP reduced the frequency by half. In this concentration range, 4-DAMP reduced the amplitude of the initial transients but did not affect the frequency of the transients in the steady state. There were similar results with darifenicin. However, darifenicin was less effective in reducing the amplitude of the initial transients. By contrast, ADFX-116 had little effect on the frequency of the initial transients but did reduce amplitude; 300 nM AFDX-116 was needed to reduce the frequency of the initial burst by half.

CONCLUSIONS

This analysis suggests that there are different but interrelated mechanisms in the isolated bladder contributing to complex contractile activity. Three components can be identified: a mechanism operating during voiding to produce a global contraction of the whole bladder and two mechanisms, pacemaker and conductive, involved in generating and propagating local contractions in the bladder wall. The pacemaker component is more sensitive to darifenicin and 4-DAMP than to AFDX-116 suggesting that the underlying processes rely predominantly on M(3) receptors and less so on M(2) (M(3) > M(2)). The phasic activity in the later stages is less affected by M(3) antagonists and might therefore involve predominantly M(2) receptors (M(2) > M(3)). The potential importance of these results in terms of the general physiology and pharmacology of the bladder is discussed.

Muscarinic M1 acetylcholine receptors regulate the non-quantal release of acetylcholine in the rat neuromuscular junctionviaNO-dependent mechanism

Nitric oxide (NO), previously demonstrated to participate in the regulation of the resting membrane potential in skeletal muscles via muscarinic receptors, also regulates non-quantal acetylcholine (ACh) secretion from rat motor nerve endings. Non-quantal ACh release was estimated by the amplitude of endplate hyperpolarization (H-effect) following a blockade of skeletal muscle post-synaptic nicotinic receptors by (+)-tubocurarine. The muscarinic agonists oxotremorine and muscarine lowered the H-effect and the M1 antagonist pirenzepine prevented this effect occurring at all. Another muscarinic agonist arecaidine but-2-ynyl ester tosylate (ABET), which is more selective for M2 receptors than for M1 receptors and 1,1-dimethyl-4-diphenylacetoxypiperidinium (DAMP), a specific antagonist of M3 cholinergic receptors had no significant effect on the H-effect. The oxotremorine-induced decrease in the H-effect was calcium and calmodulin-dependent. The decrease was negated when either NO synthase was inhibited by N(G)-nitro-L-arginine methyl ester or soluble guanylyl cyclase was inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. The target of muscle-derived NO is apparently nerve terminal guanylyl cyclase, because exogenous hemoglobin, acting as an NO scavenger, prevented the oxotremorine-induced drop in the H-effect. These results suggest that oxotremorine (and probably also non-quantal ACh) selectively inhibit the non-quantal secretion of ACh from motor nerve terminals acting on post-synaptic M1 receptors coupled to Ca(2+) channels in the sarcolemma to induce sarcoplasmic Ca(2+)-dependent synthesis and the release of NO. It seems that a substantial part of the H-effect can be physiologically regulated by this negative feedback loop, i.e., by NO from muscle fiber; there is apparently also Ca(2+)- and calmodulin-dependent regulation of ACh non-quantal release in the nerve terminal itself, as calmidazolium inhibition of the calmodulin led to a doubling of the resting H-effect.

Activation of muscarinic K+ channels by arecaidine propargyl ester in isolated guinea-pig atrial myocytes

Arecaidine propargyl ester (APE) was developed as a potential candidate compound for the treatment of Alzheimer's disease. APE has been shown to have cardiovascular effects. APE produces negative chronotropic and inotropic effects in isolated atria. However, the ionic mechanisms underlying the cardiovascular effects of APE in guinea-pig atria are unclear. The aims of this study were: (1) to examine the shortening effect of APE on action potential duration (APD) and to compare the difference in potency between APE and muscarine in isolated single guinea-pig atrial myocytes by using the current clamp method, (2) to examine by using patch clamp techniques the ionic mechanisms underlying the cardiac effects of APE, and (3) to determine whether the cardiac effects caused by APE affect the usefulness of APE as a potential candidate for the treatment of Alzheimer's disease. The APE significantly reduced the APD in guinea-pig atria and produced no direct effect on ventricular myocytes. APE is approximately 20 times as potent as muscarine in shortening the APD. Attenuation of the APD was consistently accompanied by a hyperpolarization of the resting membrane potential in a concentration-dependent manner. The APE activated muscarinic K+ channels and increased potassium conductance in guinea-pig atrial myocytes. In the cell-attached configuration, the APE contained in the pipette increased the channel-opening probability and decreased the closed-state time interval. The proposal that APE can be used as a potential remedy for the treatment of Alzheimer's disease should be taken into consideration the undesirable cardiovascular side effects that APE causes at lower concentrations.

Cholinergic regulation of the evoked quantal release at frog neuromuscular junction

The effects of cholinergic drugs on the quantal contents of the nerve-evoked endplate currents (EPCs) and the parameters of the time course of quantal release (minimal synaptic latency, main modal value of latency histogram and variability of synaptic latencies) were studied at proximal, central and distal regions of the frog neuromuscular synapse. Acetylcholine (ACh, 5 x 10(-4) M), carbachol (CCh, 1 x 10(-5) M) or nicotine (5 x 10(-6) M) increased the numbers of EPCs with long release latencies mainly in the distal region of the endplate (90-120 microm from the last node of Ranvier), where the synchronization of transmitter release was the most pronounced. The parameters of focally recorded motor nerve action potentials were not changed by either ACh or CCh. The effects of CCh and nicotine on quantal dispersion were reduced substantially by 5 x 10(-7) M (+)tubocurarine (TC). The muscarinic agonists, oxotremorine and the propargyl ester of arecaidine, as well as antagonists such as pirenzepine, AF-DX 116 and methoctramine, alone or in combination, did not affect the dispersion of the release. Muscarinic antagonists did not block the dispersion action of CCh. Cholinergic drugs either decreased the quantal content m(o) (muscarinic agonist, oxotremorine M, and nicotinic antagonist, TC), or decreased m(o) and dispersed the release (ACh, CCh and nicotine). The effects on m(o) were not related either to the endplate region or to the initial level of release dispersion. It follows that the mechanisms regulating the amount and the time course of transmitter release are different and that, among other factors, they are altered by presynaptic nicotinic receptors.

Cholinergic stimulation enhances colonic motor activity, transit, and sensation in humans

The cholinesterase inhibitor neostigmine indirectly stimulates muscarinic M(1)/M(2)/M(3) receptors, thereby reducing colonic distension in acute colonic pseudo-obstruction. We investigated the dose-response profile for the colonic sensorimotor effects of neostigmine and bethanechol, a direct muscarinic M(2)/M(3) agonist in humans. A barostat-manometric assembly recorded phasic pressures, tone, and pressure-volume relationships (compliance) in the descending colon and rectum of 30 healthy subjects who received intravenous neostigmine (0.25, 0.75, or 1.5 mg; n = 15) or subcutaneous bethanechol (2.5, 5, or 10 mg; n = 15). Sensation to luminal distension was also assessed. Thereafter, the effects of neostigmine and bethanechol on colonic transit (geometric center) were compared with those of saline by scintigraphy in 21 subjects. Both drugs increased colonic phasic pressure activity, reduced rectal compliance, and enhanced urgency during rectal distension. Neostigmine also reduced colonic and rectal balloon volumes, reflecting increased tone by an average of 12% and 25% for the highest dose, respectively. Only neostigmine reduced colonic compliance, accelerated colonic transit [mean geometric center at 90 min 2.5 vs. 1.0 (placebo)], and increased pain perception during colonic distension. We conclude that neostigmine has more prominent colonic motor and sensory effects than bethanechol. Moreover, neostigmine induces coordinated colonic propulsion, perhaps by stimulating muscarinic M(1) receptors in the myenteric plexus.

Carbachol and acetylcholine delay the early postdenervation depolarization of muscle fibres through M1-cholinergic receptors

The resting membrane potential (RMP) of denervated muscle fibres of rat diaphragm muscle is depolarized by approximately 8-10 mV during the first 3 h after nerve section and this early postdenervation depolarization is reduced substantially by the presence of 5x10(-8) M acetylcholine (ACh) or carbachol (CB). The muscarinic antagonist atropine (Atr; 5x10(-9) to 5x10(-6) M) reduced the effect of CB in a dose-dependent manner (K(i)=7x10(-8) M) and increased the rate of the early postdenervation depolarization. In lower doses (5x10(-7) M), Atr acted only in the presence of an allosteric stabilizator hexamethylene-bis-[dimethyl-(3-phtalimidopropyl)ammonium] (W-84). Also pirenzepine, a specific inhibitor of the M1 subtype of muscarinic receptor, blocked the action of CB in a dose-dependent manner with an apparent inhibition constant K(i)=1x10(-7) microM. DAMP, a specific M3 antagonist, was without effect on the muscle hyperpolarization induced by CB. CB also hyperpolarized the membrane potentials of muscles which were denervated for 1-3 days. It is concluded that ACh and CB protect the muscle fibres from early depolarization through M1-cholinergic receptors on the muscle membrane. These particular receptors can apparently mediate the 'trophic', non-impulse regulation of RMP in skeletal muscles when they are activated by acetylcholine released non-quantally.

Stereoselective recognition of the enantiomers of phenglutarimide and of six related compounds by four muscarinic receptor subtypes

1. We have compared the binding properties of the enantiomers of phenglutarimide (1) and of six related compounds to M1 receptors in NB-OK-1 cells, M2 receptors in rat heart, M3 receptors in rat pancreas and the M4 receptors of rat striatum, with their functional (antimuscarinic) properties in rabbit vas deferens (M1/M4-like), guinea-pig atria (M2) and guinea-pig ileum (M3) receptors. The binding properties of the enantiomers of three of the compounds were also measured on cloned human m1-m4 receptors expressed by CHO cells, using [3H]-N-methylscopolamine ([3H]-NMS) as radioligand. 2. The high affinity enantiomers behaved as competitive antagonists in binding and pharmacological studies. (S)-phenglutarimide (pKi-M1 = 9.0/9.3) and (R)-thienglutarimide (pKi-M1 = 8.6/9.2) recognized selectively the native M1 > M4 > M3 > M2 receptors in tissues as well as the respective cloned receptors. 3. The pA2 values at the inhibitory heteroreceptors in the rabbit vas deferens, and at the guinea-pig atria and ileum for the seven more potent enantiomers were compatible with the previous classification of these receptors as M1/M4-like, M2 and M3, respectively. 4. Replacement of the phenyl by a thienyl ring or of the diethylamino by a piperidino group in the phenglutarimide molecule did not affect markedly the potencies of the high affinity enantiomer. In contrast, replacement of the phenyl by a cyclohexyl ring decreased 20 fold the active enantiomers potency. Methylation of the piperidine-2,6-dione nitrogen also reduced markedly the eutomers' affinities, more on the M1 than on the other subtypes. 5. The selectivity profiles (recognition of four receptor subtypes) of six of the seven less active enantiomers were different from the corresponding more active enantiomers selectivity profiles, suggesting that the preparations used in this study were pure. However, we cannot not exclude the hypothesis that the batch of (S)-thienglutarimide used in this study was contaminated by less than 0.02% of the eutomer. 6. In contrast with the eutomer binding site, replacement of the phenyl ring by a thienyl or cyclohexyl ring did not affect binding of the low affinity enantiomers to the muscarinic receptor or the [3H]-NMS-receptor complex. The replacement of the diethylamino group by a piperidine ring, and N-methylation of the piperidine-2,6 dione moiety increased slightly these enantiomers' potencies. 7. The muscarinic receptors were extremely stereoselective, and had up to 20000 fold lower affinity for the less active enantiomers. However, the stereochemical requirements of the muscarinic receptor subtypes were different for the enantiomers of compounds 1-7, being most stringent at M1 receptors. 8. The weaker enantiomers behaved as competitive antagonists in pharmacological studies, at least in the concentration-range investigated.

Selective blockade of M2 and M3 muscarinic receptors by hexahydrobenzyl-fourdapine and a comparison with zamifenacin

1. 4-Diphenylacetoxy-N-cyclohexylmethyl-piperidine HCl (hexahydro-benz-4DAP) is more active as an antagonist of carbachol at receptors in guinea-pig isolated ileum, log K (pA2) = 6.64 +/- 0.14 (s.e. 7 results), than at receptors in guinea-pig isolated atria, log K = 5.43 +/- 0.14 (7). In the presence of neostigmine bromide (0.2 microM) the value for atria was 5.62 +/- 0.19 (4), so the lower activity on atria cannot be attributed to hydrolysis of the compound by cholinesterases present in this tissue. 2. The limit of solubility of the free base in Krebs solution (pH 7.6) is about 50 microM for both hexahydrobenz-4DAP and benzyl-fourdapine (benz-4DAP). 3. In experiments on guinea-pig isolated ileum with hexahydro-benz-4DAP given together with 4-DAMP methobromide, the combined dose-ratio was consistent with competition: similar results were obtained with benz-4DAP. 4. In rats anaesthetized with pentobarbitone, hexahydro-benz-4DAP antagonized the effects of bethanechol on blood-pressure in doses that had little effect on heart rate or airflow. There was a limit to the effect which could be obtained, however, and the slopes of the Schild plots were less than one. The effects on rat blood-pressure had a half-life of at least 30 min. 5. In similar experiments with zamifenacin the slopes of the Schild plots were close to 1 and the compound was 10 to 20 times as active on blood-pressure at it was on heart-rate. 6. The limited solubility of the base probably accounts for the flat Schild plots obtained with hexahydro-benz-4DAP, which had about 10 fold selectivity for effects on blood-pressure and was more active than expected from tests on isolated ileum.

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

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

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)

Role of muscarinic receptor subtypes in central antinociception

1. The ability to modify the pain threshold by the two M1-muscarinic agonists: McN-A-343 and AF-102B and by the specific M2-agonist arecaidine was examined in mice and rats by using three different noxious stimuli: chemical (writhing test), thermic (hot-plate test) and mechanical (paw pressure test). 2. In the mouse hot-plate test McN-A-343 (20-50 micrograms per mouse i.c.v.) and AF-102B (1-10 mg kg-1 i.p.) produced significant antinociception which was prevented by atropine (1 microgram per mouse i.c.v.) and by the two selective M1 antagonists: pirenzepine (0.01 micrograms per mouse i.c.v.) and dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by the specific M2-antagonist AFDX-116 (0.1 micrograms per mouse i.c.v.), naloxone (1 mg kg-1 i.p.) or by the acetylcholine (ACh) depletor hemicholinium-3 (HC-3) (1 micrograms per mouse i.c.v.). McN-A-343 and AF-102B were able to increase the pain threshold also in the mouse acetic acid writhing test and in rat paw pressure test. These antinociceptive effects were completely prevented by dicyclomine (0.08 micrograms per mouse i.c.v. or 10 mg kg-1 i.p.) but not by AFDX-116 (0.1 microgram per mouse or rat i.c.v.). 3. In contrast with the M1-agonists, the M2-agonist arecaidine (0.1-2 micrograms per mouse or rat i.c.v.) did not induce antinociception in all three analgesic tests. However, arecaidine, at the same i.c.v. doses, was able to reduce the pain threshold in the hot-plate and paw pressure tests.4. The site of muscarinic control of the pain threshold is localized in the CNS since drugs which do not cross the blood-brain barrier such as McN-A-343, pirenzepine and arecaidine exerted their effects only if injected i.c.v.5. On the basis of the above findings and existing literature we suggest that the postsynaptic muscarinic receptors involved in antinociception belong to the M1 subtype. Nevertheless, presynaptic autoreceptors (M2 subtype) may play a role in pain regulation since they are involved in modulation of endogenous ACh release.

Some differential effects of 4-diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride on guinea-pig atria and ileum

4-Diphenylacetoxy-N-(2-chloroethyl)-piperidine hydrochloride (I) cyclizes at neutral pH to form an aziridinium salt. The formation and breakdown of the salt depend on the temperature (in the range 25 to 37 degrees C). In solution at 30 degrees C, peak levels, corresponding to 60-80% conversion, are reached after around 60 min and the half-life exceeds 100 min. In the presence of 0.9% NaCl conversion was reduced to 45-60%. I blocks muscarinic receptors in guinea-pig ileum and atria irreversibly and it is possible to produce dose-ratios on ileum with 10 nM I which are about 100 times those on atria. After about 30 min exposure to solutions of I (prepared 15-20 min previously so that formation of aziridinium ions is well-established) the graph of log (dose-ratio) against time is linear and similar plots were obtained with two different agonists, carbachol and ethoxyethyltrimethylammonium. With results for the ileum, extrapolation of the line suggests that it does not start from zero (dose-ratio = 1): this is because of an initial relatively rapid reversible block. This early phase is similar to that seen on ileum with 10 nM 4DAMP methobromide, which is a competitive antagonist, so is probably caused by competitive block by the aziridinium ion, which closely resembles 4DAMP metho-salts. The subsequent irreversible phase should be caused by alkylation of the receptors. I is easy to make and should be a valuable tool for the study of muscarinic receptors.

Effects of two new pirenzepine analogs on the contractile response of the guinea-pig oesophageal muscularis mucosae to acetylcholine, bethanechol, histamine and high potassium

The guinea-pig oesophageal muscularis mucosae was used to determine the affinity for muscarinic receptors of two new tricyclic compounds, DF 545 and DF 594, which are structurally related to pirenzepine. Both acetylcholine and bethanechol induced a concentration-dependent contraction of the muscularis mucosae. This contraction was competitively antagonized by DF 545 and DF 594 over the dose range 10(-7)-10(-5) M, while at higher concentrations both antagonists caused a depression of the maximal response to the cholinomimetics. The potency of DF 545 and DF 594 appeared to be comparable to that of pirenzepine and approximately 50 times lower than that of atropine. By comparing the affinities of DF 545 and DF 594 with those of selective antagonists (methoctramine and 4-DAMP) which discriminate between M2/M3 muscarinic receptor subtypes, it emerged that pirenzepine as well as DF 545 and DF 594 might act on M3 receptors, which seem to be predominant in the guinea-pig oesophageal muscularis mucosae. McN-A-343 exhibited no agonist activity while it acted as a competitive antagonist against acetylcholine and bethanechol. None of the compounds exhibited calcium antagonist properties. DF 545 inhibited the contractile responses to histamine, but DF 594 and pirenzepine did not.

Binding and functional properties of antimuscarinics of the hexocyclium/sila-hexocyclium and hexahydro-diphenidol/hexahydro-sila-diphenidol type to muscarinic receptor subtypes

1. In an attempt to assess the structural requirements for the muscarinic receptor selectivity of hexahydro-diphenidol (hexahydro-difenidol) and hexahydro-sila-diphenidol (hexahydro-sila-difenidol), a series of structurally related C/Si pairs were investigated, along with atropine, pirenzepine and methoctramine, for their binding affinities in NB-OK 1 cells as well as in rat heart and pancreas. 2. The action of these antagonists at muscarinic receptors mediating negative inotropic responses in guinea-pig atria and ileal contractions has also been assessed. 3. Antagonist binding data indicated that NB-OK 1 cells (M1 type) as well as rat heart (cardiac type) and pancreas (glandular/smooth muscle type) possess different muscarinic receptor subtypes. 4. A highly significant correlation was found between the binding affinities of the antagonists to muscarinic receptors in rat heart and pancreas, respectively, and the affinities to muscarinic receptors in guinea-pig atria and ileum. This implies that the muscarinic binding sites in rat heart and the receptors in guinea-pig atria are essentially similar, but different from those in pancreas and ileum. 5. The antimuscarinic potency of hexahydro-diphenidol and hexahydro-sila-diphenidol at the three subtypes was influenced differently by structural modifications (e.g. quaternization). Different selectivity profiles for the antagonists were obtained, which makes these compounds useful tools to investigate further muscarinic receptor heterogeneity. Indeed, the tertiary analogues hexahydro-diphenidol (HHD) and hexahydro-sila-diphenidol (HHSiD) had an M1 = glandular/smooth muscle greater than cardiac selectivity profile, whereas the quaternary analogues HHD methiodide and HHSiD methiodide were M1 preferring (M1 greater than glandular/smooth muscle, cardiac).

Subclassification of atrial and intestinal muscarinic receptors of the rat--direct binding studies with agonists and antagonists

1. Although extensively investigated, the extent of differences between receptors mediating negative inotropic and chronotropic responses is still unclear. In the present study atrial and intestinal muscarinic receptors were identified by [3H]-N-methyl-scopolamine ([3H]-NMS) binding and the affinities of some presumably inotropy- or chronotropy-selective agonists and several antagonists determined. 2. All the agonists tested showed similar affinity for right and left atrial receptors. Accepting an affinity difference of 0.4 log units as experimental error, none of the agonists tested was selective for either atrium. 3. Affinity differences of the cardioselective antagonists himbacine, AF-DX 116 and methoctramine and the M1-selective antagonist dicyclomine for right and left atrial muscarinic receptors were also minimal (less than 2 fold selective). When compared to intestinal receptors, AF-DX 116 was 3 to 4 fold, methoctramine 10 to 13 fold selective and himbacine and dicyclomine non-selective. 4. These data provide evidence for differences between atrial and intestinal but not between right and left atrial muscarinic receptors.

Structure-activity relationships of new analogues of arecaidine propargyl ester at muscarinic M1 and M2 receptor subtypes

1. The potency of arecaidine propargyl ester (APE) and of several analogues containing a modified ester side chain has been assessed at M1 and M2 muscarinic receptor subtypes. APE was shown to act as a potent agonist at ganglionic M1 receptors in the pithed rat, at M2 receptors in guinea-pig isolated atria (-log EC50 = 8.22) and ileum (-log EC50 = 7.77). 2. The arecaidine 2-butynyl and 2-pentynyl esters were approximately equipotent with APE at M1 and M2 receptors, whereas the 2-hexynyl derivative was found to be less potent than APE in atria (-log EC50 = 6.80) and ileum (-log EC50 = 6.70) by about one order of magnitude. The 2-heptynyl and 3-phenyl propargyl esters exhibited no agonist actions in atria and ileum. 3. Shifting the triple bond from the 2 to the 3 position and introducing a bulky group at position 1 of the ester side chain of APE and analogues resulted in competitive antagonists (pA2 ranging from 4.9 to 7.3). 4. APE and its 2-butynyl analogue showed some agonistic selectivity for cardiac M2 receptors (potency ratio, ileum/atria = 2.8 and 4.6 respectively). All antagonists in this series of compounds were not selective in terms of affinity since their pA2 values at cardiac and ileal M2 receptors were similar (potency ratios, ileum/atria = 0.4 to 1.2).

The relative potencies of cholinomimetics and muscarinic antagonists on the rat iris in vivo: effects of pH on potency of pirenzepine and telenzepine

The effects of cholinomimetics and muscarinic antagonists were compared following topical administration to the eyes of anaesthetized rats. For tests with cholinomimetics, clonidine (0.3 mg/kg) was used to induce mydriasis via central inhibition of parasympathetic tone. Full, dose-dependent miosis was induced by acetylcholinesterase inhibitors [physostigmine greater than neostigmine greater than tetrahydroaminoacridine (THA)] and by membrane channel blockers (4-aminopyridine greater than 3,4-diaminopyridine). Oxotremorine was the most potent direct agonist tested [oxotremorine greater than arecaidine propargylester (APE) greater than arecoline greater than carbachol greater than ethoxyethyltrimethyl-ammonium iodide (EOE) greater than RS 86]. Some putative M1 selective agonists were weakly active or behaved as partial agonists (pilocarpine greater than AH6405 greater than Mc-A-343 greater than isoarecoline). Of the antagonists, compared in non-clonidine treated rats, scopolamine hydrochloride was the most potent. Of the receptor selective antagonists the M2 (ileal) selective compounds hexahydrosiladifenidol and 4-DAMP were more potent than either M1 selective (pirenzepine, telenzepine) or M2 (atrial) selective (AF DX 116) drugs. These data tentatively suggest the involvement of an M2 (ileal) type muscarinic receptor. Potency was lower for quaternary structures, probably due to impaired corneal penetration. The potency of pirenzepine and telenzepine was increased 60-fold at low pH following topical administration. Acid induced corneal damage does not appear to account for this potency shift as the effects of scopolamine and several agonists (oxotremorine, pilocarpine and McNA-343) were not substantially altered by acid media. For pirenzepine the potency shift appears to be related to protonation of the second amino group (N1) in the piperazine tail (pKa = 2.05).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Desethylaprophen: a metabolite of aprophen with antimuscarinic activities

The metabolic fate of aprophen hydrochloride (2-diethylaminoethyl 2,2-diphenylpropionate) was studied in rats after intravenous administration. Both 14C-labeled and unlabeled aprophen were used in these studies. Blood samples were collected and analyzed to determine the identities of the metabolites formed. Utilizing high-performance liquid chromatography, desethylaprophen was identified as a major metabolite in ether-extracted samples from rats, and could be detected in blood samples 1 min after intravenous administration. It was most likely formed by N-de-ethylation of aprophen by a cytochrome P-450-dependent monooxygenase. Synthetic desethylaprophen was found to possess cholinolytic activity (i.e., it functioned as a muscarinic antagonist by blocking the contraction of acetylcholine-stimulated guinea pig ileum, the release of alpha-amylase from pancreatic acinar cells stimulated by carbachol, and also by inhibiting the binding of [3H]N-methyl scopolamine to the muscarinic receptors of guinea pig ileum). It was interesting that although the biological effects of desethylaprophen were 100-fold lower than those of aprophen, it was equally able to compete for the binding sites of muscarinic receptors of the guinea pig ileum.

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.

The selectivity of the (-)-and (+)-forms of hyoscine methiodide and of hyoscyamine camphorsulphonate for muscarinic (M2) receptors

The affinities of (-)-S-hyoscyamine (+)-camphorsulphonate, (+)-R-hyoscyamine (-)-camphorsulphonate, (-)-S-hyoscine methiodide and (+)-R-hyoscine methiodide for muscarinic acetylcholine receptors in guinea-pig atria and ileum at 30 degrees C and in ileum at 37 degrees C have been measured in dose-ratio experiments. The agonists were carbachol, arecaidine propargyl ester (APE) and ethoxyethyl trimethylammonium iodide (EOE). The effects produced by the agonists confirmed that, relative to carbachol, arecaidine propargyl ester is more active on atria than on ileum whereas ethoxyethyl trimethylammonium iodide is more active on ileum than on atria. There was no striking difference between estimates of affinity based on the effects of agonists on atrial size and the effects on atrial rate nor was there any striking difference between the affinities measured with the different agonists. With the isomers of hyoscyamine there was no striking difference between the affinity for receptors in atria and those in ileum, which is consistent with the low selectivity reported for atropine (racemic hyoscyamine). (-)-S-Hyoscine methiodide had greater affinity for muscarinic receptors in ileum than for those in atria, though the difference is smaller than has been previously observed. (+)-R-Hyoscine methiodide had no detectable selectivity. The phenomenon of selectivity cannot be wholly ascribed to differences in physiochemical properties of the antagonists: the three-dimensional structures with which the antagonists interact cannot be identical.