Activation of M2 muscarinic receptors in guinea-pig ileum opens cationic channels modulated by M3 muscarinic receptors

Direct modulation of TRPC ion channels by Gα proteins

GPCR-Gi protein pathways are involved in the regulation of vagus muscarinic pathway under physiological conditions and are closely associated with the regulation of internal visceral organs. The muscarinic receptor-operated cationic channel is important in GPCR-Gi protein signal transduction as it decreases heart rate and increases GI rhythm frequency. In the SA node of the heart, acetylcholine binds to the M2 receptor and the released Gβγ activates GIRK (I(K,ACh)) channel, inducing a negative chronotropic action. In gastric smooth muscle, there are two muscarinic acetylcholine receptor (mAChR) subtypes, M2 and M3. M2 receptor activates the muscarinic receptor-operated nonselective cationic current (mIcat, NSCC(ACh)) and induces positive chronotropic effect. Meanwhile, M3 receptor induces hydrolysis of PIP2 and releases DAG and IP3. This IP3 increases intracellular Ca2+ and then leads to contraction of GI smooth muscles. The activation of mIcat is inhibited by anti-Gi/o protein antibodies in GI smooth muscle, indicating the involvement of Gαi/o protein in the activation of mIcat. TRPC4 channel is a molecular candidate for mIcat and can be directly activated by constitutively active Gαi QL proteins. TRPC4 and TRPC5 belong to the same subfamily and both are activated by Gi/o proteins. Initial studies suggested that the binding sites for G protein exist at the rib helix or the CIRB domain of TRPC4/5 channels. However, recent cryo-EM structure showed that IYY58-60 amino acids at ARD of TRPC5 binds with Gi3 protein. Considering the expression of TRPC4/5 in the brain, the direct G protein activation on TRPC4/5 is important in terms of neurophysiology. TRPC4/5 channels are also suggested as a coincidence detector for Gi and Gq pathway as Gq pathway increases intracellular Ca2+ and the increased Ca2+ facilitates the activation of TRPC4/5 channels. More complicated situation would occur when GIRK, KCNQ2/3 (IM) and TRPC4/5 channels are co-activated by stimulation of muscarinic receptors at the acetylcholine-releasing nerve terminals. This review highlights the effects of GPCR-Gi protein pathway, including dopamine, μ-opioid, serotonin, glutamate, GABA, on various oragns, and it emphasizes the importance of considering TRPC4/5 channels as crucial players in the field of neuroscience.

Molecular mechanisms of cholinergic neurotransmission in visceral smooth muscles with a focus on receptor-operated TRPC4 channel and impairment of gastrointestinal motility by general anaesthetics and anxiolytics

Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, wherein it binds to and activates two muscarinic receptors subtypes, M2 and M3, thus causing smooth muscle excitation and contraction. The first part of this review focuses on the types of cells involved in cholinergic neurotransmission and on the molecular mechanisms underlying acetylcholine-induced membrane depolarisation, which is the central event of excitation-contraction coupling causing Ca2+ entry via L-type Ca2+ channels and smooth muscle contraction. Studies of the muscarinic cation current in intestinal myocytes (mICAT) revealed its main molecular counterpart, receptor-operated TRPC4 channel, which is activated in synergy by both M2 and M3 receptors. M3 receptors activation is of permissive nature, while activation of M2 receptors via Gi/o proteins that are coupled to them plays a direct role in TRPC4 opening. Our understanding of signalling pathways underlying mICAT generation has vastly expanded in recent years through studies of TRPC4 gating in native cells and its regulation in heterologous cells. Recent studies using muscarinic receptor knockout have established that at low agonist concentration activation of both M2 receptor and the M2/M3 receptor complex elicits smooth muscle contraction, while at high agonist concentration M3 receptor function becomes dominant. Based on this knowledge, in the second part of this review we discuss the cellular and molecular mechanisms underlying the numerous anticholinergic effects on neuroactive drugs, in particular general anaesthetics and anxiolytics, which can significantly impair gastrointestinal motility. This article is part of the Special Issue on "Ukrainian Neuroscience".

Mechanisms of regulation of motility of the gastrointestinal tract and the hepatobiliary system under the chronic action of nanocolloids

Modern cutting edge technologies of chemical synthesis enable the production of unique nanostructures with excess energy and high reactivity. Uncontrolled use of such materials in the food industry and pharmacology entail a risk for the development of a nanotoxicity crisis. Using the methods of tensometry, mechanokinetic analysis, biochemical methods, and bioinformatics, the current study showed that chronic (for six months) intragastrical burdening of rats with aqueous nanocolloids (AN) ZnO and TiO₂ caused violations of the pacemaker-dependent mechanisms of regulation of spontaneous and neurotransmitter-induced contractions of the gastrointestinal tract (GIT) smooth muscles (SMs), and transformed the contraction efficiency indices (AU, in Alexandria units). Under the same conditions, the fundamental principle of distribution of physiologically relevant differences in the numeric values of the mechanokinetic parameters of spontaneous SM contractions between different parts of GIT is violated, which can potentially cause its pathological changes. Using molecular docking, typical bonds in the interfaces of the interaction of these nanomaterials with myosin II, a component of the contractile apparatus of smooth muscle cells (SMC) were investigated. In this connection, the study addressed the question of possible competitive relations between ZnO and TiO₂ nanoparticles and actin molecules for binding sites on the myosin II actin-interaction interface. In addition, using biochemical methods, it was shown that chronic long-term exposure to nanocolloids causes changes in the primary active ion transport systems of cell plasma membranes, the activity of marker liver enzymes and disrupts the blood plasma lipid profile, which indicates the hepatotoxic effect of these nanocolloids.

The cells and conductance mediating cholinergic neurotransmission in the murine proximal stomach

KEY POINTS

Enteric neurotransmission is essential for gastrointestinal (GI) motility, although the cells and conductances responsible for post-junctional responses are controversial. The calcium-activated chloride conductance (CaCC), anoctamin-1 (Ano1), was expressed by intramuscular interstitial cells of Cajal (ICC-IM) in proximal stomach and not resolved in smooth muscle cells (SMCs). Cholinergic nerve fibres were closely apposed to ICC-IM. Conductances activated by cholinergic stimulation in isolated ICC-IM and SMCs were determined. A CaCC was activated by carbachol in ICC-IM and a non-selective cation conductance in SMCs. Responses to cholinergic nerve stimulation were studied. Excitatory junction potentials (EJPs) and mechanical responses were evoked in wild-type mice but absent or greatly reduced with knockout/down of Ano1. Drugs that block Ano1 inhibited the conductance activated by carbachol in ICC-IM and EJPs and mechanical responses in tissues. The data of the present study suggest that electrical and mechanical responses to cholinergic nerve stimulation are mediated by Ano1 expressed in ICC-IM and not SMCs.

ABSTRACT

Enteric motor neurotransmission is essential for normal gastrointestinal (GI) motility. Controversy exists regarding the cells and ionic conductance(s) that mediate post-junctional neuroeffector responses to motor neurotransmitters. Isolated intramuscular ICC (ICC-IM) and smooth muscle cells (SMCs) from murine fundus muscles were used to determine the conductances activated by carbachol (CCh) in each cell type. The calcium-activated chloride conductance (CaCC), anoctamin-1 (Ano1) is expressed by ICC-IM but not resolved in SMCs, and CCh activated a Cl- conductance in ICC-IM and a non-selective cation conductance in SMCs. We also studied responses to nerve stimulation using electrical-field stimulation (EFS) of intact fundus muscles from wild-type and Ano1 knockout mice. EFS activated excitatory junction potentials (EJPs) in wild-type mice, although EJPs were absent in mice with congenital deactivation of Ano1 and greatly reduced in animals in which the CaCC-Ano1 was knocked down using Cre/loxP technology. Contractions to cholinergic nerve stimulation were also greatly reduced in Ano1 knockouts. SMCs cells also have receptors and ion channels activated by muscarinic agonists. Blocking acetylcholine esterase with neostigmine revealed a slow depolarization that developed after EJPs in wild-type mice. This depolarization was still apparent in mice with genetic deactivation of Ano1. Pharmacological blockers of Ano1 also inhibited EJPs and contractile responses to muscarinic stimulation in fundus muscles. The data of the present study are consistent with the hypothesis that ACh released from motor nerves binds muscarinic receptors on ICC-IM with preference and activates Ano1. If metabolism of acetylcholine is inhibited, ACh overflows and binds to extrajunctional receptors on SMCs, eliciting a slower depolarization response.

Titanium Dioxide Modulation of the Contractibility of Visceral Smooth Muscles In Vivo

Electronic scanning microscopy was used in the work to obtain the image and to identify the sizes of titanium dioxide (TiO2) nanoparticles 21 ± 5 nm. The qualitative and quantitative elemental analysis of the preparations of the caecum, antrum, myometrium, kidneys, and lungs of the rats, burdened with titanium dioxide, was also performed. It was established using the tenzometric method in the isometric mode that the accumulation of titanium dioxide in smooth muscles of the caecum resulted in the considerable, compared to the control, increase in the frequency of their spontaneous contractions, the decrease in the duration of the contraction-relaxation cycle, and the decrease in the indices of muscle functioning efficiency (the index of contractions in Montevideo units (MU) and the index of contractions in Alexandria units (AU)). In the same experimental conditions, there was not the increase, but the decrease in the frequency of spontaneous contractions, the duration of the contraction-relaxation cycle, and the increase in MU and AU indices in the smooth muscles of myometrium (in the group of rats, burdened with TiO2 for 30 days). It was also determined that TiO2 modulates both the mechanisms of the input of extracellular Ca2+ ions and the mechanisms of decreasing the concentration of these cations in smooth muscle cells of the caecum during the generation of the high potassium contraction. In these conditions, there is a considerable increase in the normalized maximal velocity of the contraction phase and the relaxation phase. It was demonstrated in the work that titanium dioxide also changes the cholinergic excitation in these muscles. The impact of titanium dioxide in the group of rats, burdened with TiO2, was accompanied with a considerable impairment of the kinetics of forming the tonic component of the oxytocin-induced contraction of the smooth muscles of myometrium.

Inhibitory effect of sanguinarine on PKC-CPI-17 pathway mediating by muscarinic receptors in dispersed intestinal smooth muscle cells

This study investigated the inhibitory effects of sanguinarine (SA) on PKC-CPI-17 pathway in rat intestinal smooth muscle cells (ISMC). Previous studies indicate that the inhibitory effects of SA on ISMC contraction are possibly mediated by the Ca(2+) influx. ISMC was treated with 1 μM SA for 24h remarkably inhibited the mRNA expression of m2 and m3 receptors. ISMC treated with 1 or 3 μM SA for 30 min significantly decreased the mRNA expression of PKC-δ, PKC-ε, PKC-η, and CPI-17. 1 μM SA could markedly inhibit carbachol (CCh)-mediated increase PKC-δ, PKC-η, and CPI-17 mRNA but had no effect in PKC-ε.Treatment of ISMC with SA (1 μM, 30 min) caused a decrease in protein expression of PKC-δ. However, the expression of CPI-17 was significantly inhibited in a time-dependent manner. These results demonstrate that the inhibitory effect of SA is coupled with alteration of PKC-mediated signal transduction and intracellular Ca(2+) concentration.

Muscarinic agonists and antagonists: effects on gastrointestinal function

Muscarinic agonists and antagonists are used to treat a handful of gastrointestinal (GI) conditions associated with impaired salivary secretion or altered motility of GI smooth muscle. With regard to exocrine secretion, the major muscarinic receptor expressed in salivary, gastric, and pancreatic glands is the M₃ with a small contribution of the M₁ receptor. In GI smooth muscle, the major muscarinic receptors expressed are the M₂ and M₃ with the M₂ outnumbering the M₃ by a ratio of at least four to one. The antagonism of both smooth muscle contraction and exocrine secretion is usually consistent with an M₃ receptor mechanism despite the major presence of the M₂ receptor in smooth muscle. These results are consistent with the conditional role of the M₂ receptor in smooth muscle. That is, the contractile role of the M₂ receptor depends on that of the M₃ so that antagonism of the M₃ receptor eliminates the response of the M₂. The physiological roles of muscarinic receptors in the GI tract are consistent with their known signaling mechanisms. Some so-called tissue-selective M₃ antagonists may owe their selectivity to a highly potent interaction with a nonmuscarinic receptor target.

Signaling mechanisms mediating muscarinic enhancement of GABAergic synaptic transmission in the spinal cord

Activation of muscarinic acetylcholine receptors (mAChRs) inhibits spinal nociceptive transmission by potentiation of GABAergic tone through M(2), M(3), and M(4) subtypes. To study the signaling mechanisms involved in this unique mAChR action, GABAergic spontaneous inhibitory postsynaptic currents (sIPSCs) of lamina II neurons were recorded using whole-cell patch clamp techniques in rat spinal cord slices. The mAChR agonist oxotremorine-M caused a profound increase in the frequency of GABAergic sIPSCs, which was abolished in the Ca(2+)-free solution. Inhibition of voltage-gated Ca(2+) channels with Cd(2+) and Ni(2+) largely reduced the effect of oxotremorine-M on sIPSCs. Blocking nonselective cation channels (NSCCs) with SKF96365 or 2-APB also largely attenuated the effect of oxotremorine-M. However, the KCNQ channel blocker XE991 and the adenylyl cyclase inhibitor MDL12330A had no significant effect on oxotremorine-M-induced increases in sIPSCs. Furthermore, the phosphoinositide-3-kinase (PI3K) inhibitor wortmannin or LY294002 significantly reduced the potentiating effect of oxotremorine-M on sIPSCs. In the spinal cord in which the M(3) subtype was specifically knocked down by intrathecal small interfering RNA (siRNA) treatment, SKF96365 and wortmannin still significantly attenuated the effect of oxotremorine-M. In contrast, SKF96365 and wortmannin both failed to alter the effect of oxotremorine-M on sIPSCs when the M(2)/M(4) mAChRs were blocked. Therefore, our study provides new evidence that activation of mAChRs increases synaptic GABA release through Ca(2+) influx and voltage-gated Ca(2+) channels. The PI3K-NSCC signaling cascade is primarily involved in the excitation of GABAergic interneurons by the M(2)/M(4) mAChRs in the spinal dorsal horn.

Neuronally released acetylcholine acts on the M2 muscarinic receptor to oppose the relaxant effect of isoproterenol on cholinergic contractions in mouse urinary bladder

We investigated whether M(2) muscarinic receptor activation opposes isoproterenol-induced relaxation in mouse urinary bladder and whether endogenous acetylcholine acts through a similar M(2) mechanism. When measured in urinary bladder from M(3) receptor knockout mice, the muscarinic agonist oxotremorine-M elicited only very weak contractions. In the presence of alpha,beta-methylene ATP (30 microM) and isoproterenol (1 microM), however, oxotremorine-M elicited a robust contractile response. This response was completely absent in bladder from M(2)/M(3) double knockout mice, indicating that activation of the M(2) receptor inhibits the relaxant effect of isoproterenol on the contraction to alpha,beta-methylene ATP. Similar results were obtained when prostaglandin F(2alpha) (5 microM) was used as the contractile agent but not when serotonin was used. Electrical field stimulation of the urinary bladder from wild-type mouse elicited contractions that were inhibited 20% by atropine and 40% by desensitization with alpha,beta-methylene ATP. When measured in the presence of alpha,beta-methylene ATP to desensitize the purinergic component of contraction, isoproterenol exhibited moderately greater relaxant activity in field-stimulated bladder from the M(2) knockout mouse compared with that observed in wild-type bladder. This differential relaxant effect of isoproterenol was greatly increased in the presence of physostigmine. In contrast, no differential effects were noted for isoproterenol in similar experiments on bladders from M(3) knockout and M(2)/M(3) double knockout mice in the presence of physostigmine. Our results suggest that neuronally released acetylcholine acts on the M(2) muscarinic receptor to inhibit the relaxant effect of isoproterenol on the minor, cholinergic component of contraction in the field-stimulated mouse urinary bladder.

Muscarinic receptor-activated cationic channels in murine ileal myocytes

BACKGROUND AND PURPOSE

There is little information about the excitatory cholinergic mechanisms of mouse small intestine although this model is important for gene knock-out studies.

EXPERIMENTAL APPROACH

Using patch-clamp techniques, voltage-dependent and pharmacological properties of carbachol- or intracellular GTPgammaS-activated cationic channels in mouse ileal myocytes were investigated.

KEY RESULTS

Three types of cation channels were identified in outside-out patches (17, 70 and 140 pS). The voltage-dependent behaviour of the 70 pS channel, which was also the most abundantly expressed channel (approximately 0.35 micro(-2)) was most consistent with the properties of the whole-cell muscarinic current (half-maximal activation at -72.3+/-9.3 mV, slope of -9.1+/-7.4 mV and mean open probability of 0.16+/-0.01 at -40 mV; at near maximal activation by 50 microM carbachol). Both channel conductance and open probability depended on the permeant cation in the order: Cs+ (70 pS) >Rb+ (66pS) >Na+ (47 pS) >Li+ (30 pS). External application of divalent cations, quinine, SK&F 96365 or La3+ strongly inhibited the whole-cell current. At the single channel level the nature of the inhibitory effects appeared to be very different. Either reduction of the open probability (quinine and to some extent SK&F 96365 and La3+) or of unitary current amplitude (Ca2+, Mg2+, SK&F 96365, La3+) was observed implying significant differences in the dissociation rates of the blockers.

CONCLUSIONS AND IMPLICATIONS

The muscarinic cation current of murine small intestine is very similar to that in guinea-pig myocytes and murine genetic manipulation should yield important information about muscarinic receptor transduction mechanisms.

Regulation of TRP-like muscarinic cation current in gastrointestinal smooth muscle with special reference to PLC/InsP3/Ca2+ system

Acetylcholine, the main enteric excitatory neuromuscular transmitter, evokes membrane depolarization and contraction of gastrointestinal smooth muscle cells by activating G protein-coupled muscarinic receptors. Although the cholinergic excitation is generally underlined by the multiplicity of ion channel effects, the primary event appears to be the opening of cation-selective channels; among them the 60 pS channel has been recently identified as the main target for the acetylcholine action in gastrointestinal myocytes. The evoked cation current, termed mI(CAT), causes either an oscillatory or a more sustained membrane depolarization response, which in turn leads to increases of the open probability of voltage-gated Ca2+ channels, thus providing Ca2+ entry in parallel with Ca2+ release for intracellular Ca2+ concentration rise and contraction. In recent years there have been several significant developments in our understanding of the signaling processes underlying mICAT generation. They have revealed important synergistic interactions between M2 and M3 receptor subtypes, single channel mechanisms, and the involvement of TRPC-encoded proteins as essential components of native muscarinic cation channels. This review summarizes these recent findings and in particular discusses the roles of the phospholipase C/InsP3/intracellular Ca2+ release system in the mI(CAT) physiological regulation.

Muscarinic cationic current in gastrointestinal smooth muscles: signal transduction and role in contraction

1 The muscarinic receptor plays a key role in the parasympathetic nervous control of various peripheral tissues including gastrointestinal tract. The neurotransmitter acetylcholine, via activating muscarinic receptors that exist in smooth muscle, produces its contraction. 2 There is the opening of cationic channels as an underlying mechanism. The opening of cationic channels results in influxes of Ca2+ via the channels into the cell and also via voltage-dependent Ca2+ channels which secondarily opened in response to the depolarization, providing an amount of Ca2+ for activation of the contractile proteins. 3 Electrophysiological and pharmacological studies have shown that the cationic channels as well as muscarinic receptors exist in many visceral smooth muscle cells. However, the activation mechanisms of the cationic channels are still unclear. 4 In this article, we summarize the current knowledge of the muscarinic receptor-operated cationic channels, focusing on the receptor subtype, G protein and other signalling molecules that are involved in activation of these channels and on the molecular characteristics of the channel. This will improve strategies aimed at developing new selective pharmacological agents and understanding the activation mechanism and functions of these channels in physiological systems.

M(2) and M(3) muscarinic receptor-mediated contractions in longitudinal smooth muscle of the ileum studied with receptor knockout mice

Isometric contractile responses to carbachol were studied in ileal longitudinal smooth muscle strips from wild-type mice and mice genetically lacking M(2) or M(3) muscarinic receptors, in order to characterize the mechanisms involved in M(2) and M(3) receptor-mediated contractile responses. Single applications of carbachol (0.1-100 microM) produced concentration-dependent contractions in preparations from M(2)-knockout (KO) and M(3)-KO mice, mediated via M(3) and M(2) receptors, respectively, as judged by the sensitivity of contractile responses to blockade by the M(2)-preferring antagonist methoctramine (300 nM) or the M(3)-preferring antagonist 4-DAMP (30 nM). The M(2)-mediated contractions were mimicked in shape by submaximal stimulation with high K(+) concentrations (up to 35 mM), almost abolished by voltage-dependent Ca(2+) channel (VDCC) antagonists or depolarization with 140 mM K(+) medium, and greatly reduced by pertussis toxin (PTX) treatment. The M(3)-mediated contractions were only partially inhibited by VDCC antagonists or 140 mM K(+)-depolarization medium, and remained unaffected by PTX treatment. The contractions observed during high K(+) depolarization consisted of different components, either sensitive or insensitive to extracellular Ca(2+). The carbachol contractions observed with wild-type preparations consisted of PTX-sensitive and -insensitive components. The PTX-sensitive component was functionally significant only at low carbachol concentrations. The results suggest that the M(2) receptor, through PTX-sensitive mechanisms, induces ileal contractions that depend on voltage-dependent Ca(2+) entry, especially associated with action potential discharge, and that the M(3) receptor, through PTX-insensitive mechanisms, induces contractions that depend on voltage-dependent and -independent Ca(2+) entry and intracellular Ca(2+) release. In intact tissues coexpressing M(2) and M(3) receptors, M(2) receptor activity appears functionally relevant only when fractional receptor occupation is relatively small.

In vitro effects of bethanechol on equine gastrointestinal contractility and functional characterization of involved muscarinic receptor subtypes

The goal of this study was to investigate the effect of bethanechol (BeCh) on contractility patterns of smooth muscle preparations of equine duodenum descendens, jejunum, caecum and pelvic flexure in vitro. Concentration-response relationships were developed for BeCh using in vitro assays with and without preincubation of muscarinic (M) receptor antagonists for M2 and M3 receptors. BeCh induced a significant, concentration-dependent increase in contractile response in equine intestine in specimens with circular orientation. The maximal effect was largest for jejunal specimens with no difference in EC50 within the different locations investigated. The M2 antagonist, AF-DX 116, caused a rightward shift of the concentration-response curve and the M3 antagonist, 4-DAMP (1,1-dimethyl-4-diphenylacetoxypiperidinium iodide), almost completely inhibited the effect of BeCh over the entire concentration-response curve. These data provide evidence that, although the effect of BeCh is predominantly mediated by M3 receptors, M2 muscarinic receptors also play a role in BeCh-induced contraction in specimens of equine intestine. The involvement of other muscarinic receptor subtypes cannot be excluded. Further studies are necessary to understand the effect of BeCh in vivo including diseased animals.

Muscarinic receptor subtypes of the bladder and gastrointestinal tract

The parasympathetic nervous system is responsible for maintaining normal intestinal and bladder function, contracting the smooth muscle by releasing the neurotransmitters acetylcholine (ACh) and ATP and relaxing sphincters by releasing nitric oxide. ACh is the main transmitter released and smooth muscle contraction is mediated via a mixed M2/M3 receptor population; M3 receptors acting via phospholipase C and M2 receptors acting via inhibition of adenylate cyclase. In ileal, colonic, gastric and bladder (detrusor) smooth muscle the density of M2 receptors is far greater than the density of M3 receptors, the M2:M3 ratio being 3:1 in most species including man. Despite the predominance of M2-receptors, direct contraction of intestinal and detrusor smooth muscle is mediated via the M3-receptor subtype and only this subtype is involved in contraction in vitro. Furthermore, knocking out the M3-receptor gene can have severe consequences on intestinal and bladder responses. In some tissues however M2-receptors may mediate an indirect "re-contraction" whereby a reduction in adenylate cyclase activity reverses the relaxation induced by beta-adrenoceptor stimulation. Thus, intestinal and bladder responses to muscarinic agonists are slightly depressed in M2 receptor knockout mice. The role of receptor subtypes in disease is unclear, but an enhancement of M2 receptor mediated responses has been reported to occur in diabetes. Animal models suggest that M2 receptors may play a greater role in some situations such as in the denervated bladder and intestine. In human disease the mechanisms operating are not so clear. Detrusor sensitivity to muscarinic agonists is enhanced in the neurogenic overactive bladder, but there is controversy surrounding the role of M2 receptors and conflicting results have been reported. Thus, the main muscarinic receptor mediating contraction in normal smooth muscle is the M3 receptor, but M2 receptors are also present and possibly may have an enhanced role in disease.

Regulation of muscarinic cationic current in myocytes from guinea-pig ileum by intracellular Ca2+ release: a central role of inositol 1,4,5-trisphosphate receptors

The dynamics of carbachol (CCh)-induced [Ca(2+)](i) changes was related to the kinetics of muscarinic cationic current (mI(cat)) and the effect of Ca(2+) release through ryanodine receptors (RyRs) and inositol 1,4,5-trisphosphate receptors (IP(3)Rs) on mI(cat) was evaluated by fast x-y or line-scan confocal imaging of [Ca(2+)](i) combined with simultaneous recording of mI(cat) under whole-cell voltage clamp. When myocytes freshly isolated from the longitudinal layer of the guinea-pig ileum were loaded with the Ca(2+)-sensitive indicator fluo-3, x-y confocal imaging revealed CCh (10 microM)-induced Ca(2+) waves, which propagated from the cell ends towards the myocyte centre at 45.9 +/- 8.8 microms(-1) (n = 13). Initiation of the Ca(2+) wave preceded the appearance of any measurable mI(cat) by 229 +/- 55 ms (n = 7). Furthermore, CCh-induced [Ca(2+)](i) transients peaked 1.22 +/- 0.11s (n = 17) before mI(cat) reached peak amplitude. At -50 mV, spontaneous release of Ca(2+) through RyRs, resulting in Ca(2+) sparks, had no effect on CCh-induced mI(cat) but activated BK channels leading to spontaneous transient outward currents (STOCs). In addition, Ca(2+) release through RyRs induced by brief application of 5 mM caffeine was initiated at the cell centre but did not augment mI(cat) (n = 14). This was not due to an inhibitory effect of caffeine on muscarinic cationic channels (since application of 5 mM caffeine did not inhibit mI(cat) when [Ca(2+)](i) was strongly buffered with Ca(2+)/BAPTA buffer) nor was it due to an effect of caffeine on other mechanisms possibly involved in the regulation of Ca(2+) sensitivity of muscarinic cationic channels (since in the presence of 5 mM caffeine, photorelease of Ca(2+) upon cell dialysis with 5 mM NP-EGTA/3.8 mM Ca(2+) potentiated mI(cat) in the same way as in control). In contrast, IP(3)R-mediated Ca(2+) release upon flash photolysis of "caged" IP(3) (30 microM in the pipette solution) augmented mI(cat) (n = 15), even though [Ca(2+)](i) did not reach the level required for potentiation of mI(cat) during photorelease of Ca(2+) (n = 10). Intracellular calcium stores were visualised by loading of the myocytes with the low-affinity Ca(2+) indicator fluo-3FF AM and consisted of a superficial sarcoplasmic reticulum (SR) network and some perinuclear formation, which appeared to be continuous with the superficial SR. Immunostaining of the myocytes with antibodies to IP(3)R type 1 and to RyRs revealed that IP(3)Rs are predominant in the superficial SR while RyRs are confined to the central region of the cell. These results suggest that IP(3)R-mediated Ca(2+) release plays a central role in the modulation of mI(cat) in the guinea-pig ileum and that IP(3) may sensitise the regulatory mechanisms of the muscarinic cationic channels gating to Ca(2+).

G-protein-gated TRP-like cationic channel activated by muscarinic receptors: effect of potential on single-channel gating

There is little information about the mechanisms by which G-protein–coupled receptors gate ion channels although many ionotropic receptors are well studied. We have investigated gating of the muscarinic cationic channel, which mediates the excitatory effect of acetylcholine in smooth muscles, and proposed a scheme consisting of four pairs of closed and open states. Channel kinetics appeared to be the same in cell-attached or outside-out patches whether the channel was activated by carbachol application or by intracellular dialysis with GTPγS. Since in the latter case G-proteins are permanently active, it is concluded that the cationic channel is the major determinant of its own gating, similarly to the K_ACh channel (Ivanova-Nikolova, T.T., and G.E. Breitwieser. 1997. J. Gen. Physiol. 109:245–253). Analysis of adjacent-state dwell times revealed connections between the states that showed features conserved among many other ligand-gated ion channels (e.g., nAChR, BK_Ca channel). Open probability (P_O) of the cationic channel was increased by membrane depolarization consistent with the prominent U-shaped I-V relationship of the muscarinic whole-cell current at negative potentials. Membrane potential affected transitions within each closed-open state pair but had little effect on transitions between pairs; thus, the latter are likely to be caused by interactions of the channel with its ligands, e.g., Ca²⁺ and Gαo-GTP. Channel activity was highly heterogeneous, as was evident from the prominent cycling behavior when P_O was measured over 5-s intervals. This was related to the variable frequency of openings (as in the K_ACh channel) and, especially, to the number of long openings between consecutive long shuttings. Analysis of the underlying Markov chain in terms of probabilities allowed us to evaluate the contribution of each open state to the integral current (from shortest to longest open state: 0.1, 3, 24, and 73%) as P_O increased 525-fold in three stages.

Effects of polyamines on the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle myocytes

The effects of extracellular and intracellular polyamines (PAs), spermine and putrescine, on the cation current (mI(CAT)) evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgammaS (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal myocytes. Extracellular spermine and putrescine rapidly and reversibly inhibited mI(CAT) in a concentration- and voltage-dependent manner with the IC(50) values at -40 mV of about 1 and 5 mM, respectively. Membrane depolarization relieved the blocking action of PAs although cation conductance activation curve remained N-shaped. The inhibition was similar for both carbachol- and GTPgammaS-evoked currents, suggesting that the cation channel rather than the muscarinic receptor was the primary site of the PA action. In outside-out membrane patches, both cation channel unitary conductance and open probability were reduced. In perforated-patch experiments used to retain cytoplasmic PAs sustained 100 microM carbachol-induced mI(CAT) was significantly smaller (478 +/- 76 pA, n = 7) compared to that recorded using conventional whole-cell configuration with nominally PA-free pipette solution (1314 +/- 76 pA, n = 12), but comparable in size to mI(CAT) with 0.3 mM spermine in the pipette solution (509 +/- 41 pA, n = 19). Intracellular putrescine inhibited mI(CAT) less potently compared to spermine. In conclusion, these results show a novel role of intestinal PAs in mI(CAT) inhibition, which can contribute to their well-known suppressing effect on the gastrointestinal smooth muscle excitability and contractility.

Mechanisms mediating cholinergic antral circular smooth muscle contraction in rats

AIM: To investigate the pathway (s) mediating rat antral circular smooth muscle contractile responses to the cholinomimetic agent, bethanechol and the subtypes of muscarinic receptors mediating the cholinergic contraction.

METHODS: Circular smooth muscle strips from the antrum of Sprague-Dawley rats were mounted in muscle baths in Krebs buffer. Isometric tension was recorded. Cumulative concentration-response curves were obtained for (+)-cis-dioxolane (cD), a nonspecific muscarinic agonist, at 10^-8-10^-4 mol/L, in the presence of tetrodotoxin (TTX, 10^-7 mol/L). Results were normalized to cross sectional area. A repeat concentration-response curve was obtained after incubation of the muscle for 90 min with antagonists for M1 (pirenzepine), M2 (methoctramine) and M3 (darifenacin) muscarinic receptor subtypes. The sensitivity to PTX was tested by the ip injection of 100 mg/kg of PTX 5 d before the experiment. The antral circular smooth muscles were removed from PTX-treated and non-treated rats as strips and dispersed smooth muscle cells to identify whether PTX-linked pathway mediated the contractility to bethanechol.

RESULTS: A dose-dependent contractile response observed with bethanechol, was not affected by TTX. The pretreatment of rats with pertussis toxin decreased the contraction induced by bethanechol. Lack of calcium as well as the presence of the L-type calcium channel blocker, nifedipine, also inhibited the cholinergic contraction, with a reduction in response from 2.5 ± 0.4 g/mm² to 1.2 ± 0.4 g/mm² (P < 0.05). The dose-response curves were shifted to the right by muscarinic antagonists in the following order of affinity: darifenacin (M₃) > methocramine (M₂) > pirenzepine (M₁).

CONCLUSION: The muscarinic receptors-dependent contraction of rat antral circular smooth muscles was linked to the signal transduction pathway(s) involving pertussis-toxin sensitive GTP-binding proteins and to extracellular calcium via L-type voltage gated calcium channels. The presence of the residual contractile response after the treatment with nifedipine, suggests that an additional pathway could mediate the cholinergic contraction. The involvement of more than one muscarinic receptor (functionally predominant type 3 over type 2) also suggests more than one pathway mediating the cholinergic contraction in rat antrum.

Receptor-activated calcium entry channels--who does what, and when?

The recent Science STKE E-Conference on Defining Calcium Entry Signals highlighted many of the outstanding problems and questions regarding the nature and regulation of the receptor-activated entry of Ca2+, particularly as it relates to Ca2+ signaling in nonexcitable cells. Frequently, these stem from the current lack of any clear candidates for the molecular identity of many of the major conductances involved. Moreover, there is considerable confusion in the field, largely as a result of the use of sometimes inappropriate or imprecise methodologies and inconsistent terminology. Nevertheless, much useful information is beginning to be revealed about the biophysical characterization of the fundamental properties of the channels involved and, at least in some cases, the specific conditions under which they are active. As a result, it is becoming clear that cells often contain various Ca2+ entry channels in addition to the ubiquitous store-operated, or capacitative, channels. These different channels are activated in distinct ways and operate under different conditions of stimulation.

The M2 muscarinic receptor mediates in vitro bladder contractions from patients with neurogenic bladder dysfunction

Bladder muscle specimens from seven patients with neurogenic bladder dysfunction were analyzed to determine whether the muscarinic receptor subtype mediating contraction shifts from M(3) to the M(2) subtype as found in the denervated, hypertrophied rat bladder. Seven bladder specimens were analyzed from six female and one male patients. Six of the patients had traumatic cervical spinal cord injuries (C(4)-C(7)), and the other patient had an L(1) congenital myelomeningocele. This was compared with results from bladder specimens obtained from eight organ transplant donors. The affinities of three subtype-selective muscarinic receptor antagonists for inhibition of carbachol-induced contractions were determined. The affinity of the M(3) selective antagonists darifenacin or p-fluoro-hexahydrosiladifenadol (p-F-HHSiD) was determined in six of the seven spinal injury patient specimens. The affinity was consistent with M(2)-mediated contractions in four of these six specimens, intermediate between M(2) and M(3) in one specimen, and within the M(3) range in one specimen. The other specimen, tested only with the M(2) selective antagonist methoctramine, showed an M(3) affinity. In the organ donors, the affinity of p-F-HHSiD was within the M(2) range for six of seven specimens, whereas the affinity of darifenacin was within the M(3) range for five of six and intermediate between M(2) and M(3) for the other specimen tested. The affinity of methoctramine in both organ donor specimens tested was within the M(3) range. Whereas normal detrusor contractions are mediated by the M(3) receptor subtype, in patients with neurogenic bladder dysfunction as well as certain organ transplant donors, contractions can be mediated by the M(2) muscarinic receptor subtype.

Contractile role of M2 and M3 muscarinic receptors in gastrointestinal, airway and urinary bladder smooth muscle

Both M(2) and M(3) muscarinic receptors are expressed in smooth muscle and influence contraction through distinct signaling pathways. M(3) receptors interact with G(q) to trigger phosphoinositide hydrolysis, Ca(2+) mobilization and a direct contractile response. In contrast, M(2) receptors interact with G(i) and G(o) to inhibit adenylyl cyclase and Ca(2+)-activated K(+) channels and to potentiate a Ca(2+)-dependent, nonselective cation conductance. Ultimately, these mechanisms lead to the prediction that the influence of the M(2) receptor on contraction should be conditional upon mobilization of Ca(2+) by another receptor such as the M(3). Mathematical modeling studies of these mechanisms show that the competitive antagonism of a muscarinic response mediated through activation of both M(2) and M(3) receptors should resemble the profile of the directly acting receptor (i.e., the M(3)) and not that of the conditionally acting receptor (i.e., the M(2)). Using a combination of pharmacological and genetic approaches, we have identified two mechanisms for the M(2) receptor in contraction: 1) a high potency inhibition of the relaxation elicited by agents that increase cytosolic cAMP and 2) a low potency potentiation of contractions elicited by the M(3) receptor. The latter mechanism may be involved in muscarinic agonist-mediated heterologous desensitization of smooth muscle, which requires activation of both M(2) and M(3) receptors.

Phospholipase C, but not InsP3 or DAG, -dependent activation of the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle cells

1. In visceral smooth muscles, both M(2) and M(3) muscarinic receptor subtypes are found, and produce two major metabolic effects: adenylyl cyclase inhibition and PLCbeta activation. Thus, we studied their relevance for muscarinic cationic current (mI(CAT)) generation, which underlies cholinergic excitation. Experiments were performed on single guinea-pig ileal cells using patch-clamp recording techniques under conditions of weakly buffered [Ca(2+)](i) (either using 50 microm EGTA or 50-100 microm fluo-3 for confocal fluorescence imaging) or with [Ca(2+)](i) 'clamped' at 100 nm using 10 mm BAPTA/CaCl(2) mixture. 2. Using a cAMP-elevating agent (1 microm isoproterenol) or a membrane-permeable cAMP analog (10 microm 8-Br-cAMP), we found no evidence for mI(CAT) modulation through a cAMP/PKA pathway. 3. With low [Ca(2+)](i) buffering, the PLC blocker U-73122 at 2.5 microm almost abolished mI(CAT), in some cases without any significant effect on [Ca(2+)](i). When [Ca(2+)](i) was buffered at 100 nm, U-73122 reduced both carbachol- and GTPgammaS-induced mI(CAT) maximal conductances (IC(50)=0.5-0.6 microm) and shifted their activation curves positively. 4. U-73343, a weak PLC blocker, had no effect on GTPgammaS-induced mI(CAT), but weakly inhibited carbachol-induced current, possibly by competitively inhibiting muscarinic receptors, since the inhibition could be prevented by increasing the carbachol concentration to 1 mm. Aristolochic acid and D-609, which inhibit PLA(2) and phosphatidylcholine-specific PLC, respectively, had no or very small effects on mI(CAT), suggesting that these enzymes were not involved. 5. InsP(3) (1 microm) in the pipette or OAG (20 microm) applied externally had no effect on mI(CAT) or its inhibition by U-73122. Ca(2+) store depletion (evoked by InsP(3), or by combined cyclopiazonic acid, ryanodine and caffeine treatment) did not induce any significant current, and had no effect on mI(CAT) in response to carbachol when [Ca(2+)](i) was strongly buffered to 100 nm. 6. It is concluded that phosphatidylinositol-specific PLC modulates mI(CAT) via Ca(2+) release, but also does so independently of InsP(3), DAG, Ca(2+) store depletion or a rise of [Ca(2+)](i). Our present results explain the previously established 'permissive' role of the M(3) receptor subtype in mI(CAT) generation, and provide a new insight into the molecular mechanisms underlying the shifts of the cationic conductance activation curve.

Muscarinic Agonist-Mediated Heterologous Desensitization in Isolated Ileum Requires Activation of Both Muscarinic M2 and M3 Receptors

We investigated the subtypes of the muscarinic receptor mediating short-term heterologous desensitization in the isolated ileum. Treatment of the ileum from C57BL/6 mice with acetylcholine (30 microM) for 20 min caused a subsequent decrease in contractile sensitivity to both prostaglandin F2alpha (PGF2alpha) and the muscarinic agonist, oxotremorine-M. This subsensitivity was characterized by 7- and 3-fold increases in the EC50 values of the agonists, respectively, with no significant effect on the maximal response. The subsensitivity to PGF2alpha was prevented in both M2 and M3 muscarinic receptor knockout mice. Similarly, the subsensitivity to oxotremorine-M was prevented in M2 knockout mice. Acetylcholine-mediated desensitization of histamine-induced contractions in the guinea pig ileum was inhibited by both M2- and M3-selective muscarinic antagonists with high potency, although careful analysis of the data suggested behavior more consistent with an M2 antagonistic profile. Modeling studies showed that the competitive antagonism of response contingent upon activation of two receptor subtypes should exhibit a pharmacological profile similar to that of the least sensitive signaling pathway. Our results demonstrate that muscarinic agonist-mediated short-term heterologous desensitization of intestinal smooth muscle is contingent upon activation of both M2 and M3 muscarinic receptors and that activation of either receptor by itself is insufficient to cause desensitization.

Effects of G-protein-specific antibodies and G beta gamma subunits on the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle cells

(1) The effects on the whole-cell carbachol-induced muscarinic cationic current (mIcat) of antibodies against the alpha-subunits of various G proteins, as well as the effect of a Gbetagamma subunit, were studied in single guinea-pig ileal smooth muscle cells voltage-clamped at -50 mV. Ionized intracellular calcium concentration, [Ca(2+)](i), was clamped at 100 nM using a 1,2-bis(2-aminophenoxyl-ethane-N,N,N',N'-tetraacetic acid)/Ca(2+) mixture. (2) Application of ascending concentrations of carbachol (1-300 micro M) activated mIcat (mean amplitude 0.83 nA at 300 micro M carbachol; EC(50) 8 micro M; Hill slope 1.0). A 20 min or longer intracellular application via the pipette solution of G(i3)/G(o) or G(o) antibodies resulted in about a 70% depression of the maximum response without change in the EC(50) value. In contrast, antibodies against alpha-subunits of G(i1), G(i1)/G(i2), G(i3), G(q)/G(11) or G(s) protein over a similar or longer period did not significantly reduce mIcat. Antibodies to common Gbeta or infusion of the Gbetagamma subunit itself had no effect on mIcat. (3) If cells were exposed briefly to carbachol (50 or 100 micro M) at early times (<3 min) after infusion of antibodies to Galpha(i3)/Galpha(o) or to Galpha(o) had begun, carbachol responses remained unchanged even after 20-60 min; that is, the depression of mIcat by these antibodies was prevented. (4) These data show that Galpha(o) protein couples the muscarinic receptor to the cationic channel in guinea-pig ileal longitudinal smooth muscle and that Gbetagamma is not involved. They also show that prior activation of the muscarinic receptor presumably causes a long-lasting postactivation change of the G protein, which is not reflected in mIcat, but acts to hinder antibody binding.

Receptor signaling mechanisms underlying muscarinic agonist-evoked contraction in guinea-pig ileal longitudinal smooth muscle

1 In guinea-pig ileal longitudinal muscle, muscarinic partial agonists, 4-(N-[3-chlorophenyl]-carbomoyloxy)-2-butynyl-trimethylammonium (McN-A343) and pilocarpine, each produced parallel increases in tension and cytosolic Ca(2+) concentration ([Ca(2+)]c) with a higher EC(50) than that of the full agonist carbachol. The maximum response of [Ca(2+)]c or tension was not much different among the three agonists. The Ca(2+) channel blocker nicardipine markedly inhibited the effects of all three agonists 2 The contractile response to any agonist was antagonized in a competitive manner by M(2) receptor selective antagonists (N,N'-bis[6-[[(2-methoyphenyl)methyl]amino]hexyl]-1,8-octanediamine tetrahydrochloride and 11-[[2-[(diethlamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4] benzodiazepine-6-one), and the apparent order of M(2) antagonist sensitivity was McN-A343>pilocarpine>carbachol. M(3) receptor selective antagonists, 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide and darifenacin, both severely depressed the maximum response for McN-A343, while darifenacin had a similar action in the case of pilocarpine. Both M(3) antagonists behaved in a competitive manner in the case of the carbachol response. 3 McN-A343 failed to release Ca(2+) from the intracellular stores, and the Ca(2+)-releasing action of pilocarpine was very weak compared with that of carbachol. All three agonists were capable of increasing Ca(2+) sensitivity of the contractile proteins. 4 McN-A343 rarely produced membrane depolarization, but always accelerated electrical spike discharge. Pilocarpine effect was more often accompanied by membrane depolarization, as was usually seen using carbachol. 5 The results suggest that muscarinic agonist-evoked contractions result primarily from the integration of Ca(2+) entry associated with the increased spike discharge and myofilaments Ca(2+) sensitization, and that Ca(2+) store release may contribute to the contraction indirectly via potentiation of the electrical membrane responses. They may also support the idea that an interaction of M(2) and M(3) receptors plays a crucial role in mediating the contraction response.

Increased relaxant action of forskolin and isoproterenol against muscarinic agonist-induced contractions in smooth muscle from M2 receptor knockout mice

The ability of forskolin and isoproterenol to inhibit the contractile action of the muscarinic agonist, oxotremorine-M, was investigated in smooth muscle from wild-type and M(2) muscarinic receptor knockout mice. Forskolin (5.0 micro M) caused a significant reduction in the contractile activity of oxotremorine-M in ileum, trachea, and urinary bladder from both wild-type and M(2) muscarinic receptor knockout mice. This reduction in contractile activity was characterized by decreases in potency or maximal response, but not always both. Similar results were obtained with isoproterenol (1.0 micro M). The relaxant effects of forskolin in ileum, trachea, and urinary bladder from M(2) receptor knockout mice were approximately 3- to 9-fold greater than those observed in the same tissues from wild-type mice. Similar results were obtained with isoproterenol in ileum and urinary bladder, although the differences between wild-type and M(2) receptor knockout tissues were less than those observed with forskolin. In contrast, there was no significant difference between the relaxant effect of isoproterenol in trachea from wild-type and M(2) receptor knockout mice. In contrast to the results observed with oxotremorine-M as the contractile agent, forskolin and isoproterenol did not exhibit greater relaxant activity against KCl-induced contractions in M(2) receptor knockout mice compared with wild-type mice. These results suggest that a component of the contractile response to muscarinic agonists in smooth muscle involves an M(2) muscarinic receptor-mediated inhibition of the relaxant effects of agents that increase cAMP levels.

Coupling of M2 muscarinic receptor to L-type Ca channel via c-src kinase in rabbit colonic circular smooth muscle

BACKGROUND & AIMS

The L-type Ca(2+) channel is a major pathway for Ca(2+) influx in colonic smooth muscle and is modulated by endogenous levels of nonreceptor tyrosine kinase, c-src. Tyrosine kinases are also activated by G-protein-coupled receptors (GPCR). This study determined whether muscarinic receptor couples to Ca(2+) channels via c-src kinase.

METHODS

Currents were measured in rabbit colonic smooth muscle cells and in transfected HEK293 cells by patch-clamp technique. Tyrosyl phosphorylated proteins were detected by Western blots and the interaction of c-src with the c-terminus of alpha subunit of Ca(2+) channel was determined by a GST pull-down assay.

RESULTS

Methacholine (10 micromol/L) enhanced Ca(2+) channel currents by 30% under conditions whereby the M(3) receptor pathway was blocked by either 4-DAMP or by intracellular dialysis with anti-Galphaq antibody. Similar effects were observed by blocking intracellular Ca(2+) release with heparin. Enhancement was abolished by intracellular anti-Galphai antibody and by the c-src inhibitor, PP2 but unaffected by the inactive analog PP3. Immunoblot with anti-src antibody revealed increased src phosphorylation by muscarinic receptor stimulation. Purified c-src directly associated with the c-terminus of alpha1c subunit of the Ca(2+) channel. In M(2) receptor transfected HEK293 cells, currents were enhanced 2-fold by carbachol.

CONCLUSIONS

These studies demonstrate stimulation of Ca(2+) current in colonic smooth muscle cells by M2 receptor coupled to Galphai-G protein and c-src activation. They also suggest a central role of c-src kinase in the cross-talk between tyrosine kinase receptor and GPCR.

Possible involvement of M5 muscarinic receptor in the enhancing actions of the novel gastroprokinetic agent Z-338 on nifedipine-sensitive voltage-dependent Ca2+ currents in guinea pig stomach

We investigated the effects of the novel gastroprokinetic agent Z-338 (N-(N-N'-diisopropylaminoethyl)-[2-(2-hydroxy-4,5-dimethoxybenzoylamino)-1,3-thiazole-4-yl] carboxyamide monohydrochloride trihydrate) on L-type voltage-dependent Ca2+ currents (ICa) in guinea pig gastric myocytes by using the whole-cell patch clamp technique. Bath-applied acetylcholine (ACh) produced biphasic effects on ICa, i.e., enhancement (1-100 nM) and inhibition (1-100 microM), both of which were abolished by pretreatment with atropine (10 microM) or intracellular perfusion of GDPbetaS (500 microM). Z-338 (> or = 1 nM, ED50: 120 nM) mimicked the enhancing effects of ACh, but did not inhibit ICa. The effects of Z-338 and ACh were non-additive and blocked by atropine and GDPbetaS, but not by pertussis toxin (PTX) pretreatment (500 ng/ml). ACh (> or = 1 microM) induced slow inward currents via activation of the muscarinic receptor/PTX-sensitive G-protein pathway, but Z-338 was devoid of these effects. Neither pirenzepine (1 microM), AF-DX116 (1 microM), nor oxybutynin (100 nM) could prevent Z-338 (1 microM) and ACh (10 nM) from enhancing ICa, whilst 4-DAMP (100 nM) blocked the effects of Z-338 and ACh. Bath-application of protein kinase C (PKC) activator PDBu (phorbol-12,13-dibutyrate) (250 nM) enhanced ICa, and conversely, pipette inclusion of PKC inhibitor peptide (150 microM) abolished the effects of ACh and Z-338 on ICa. These results collectively suggest that although contribution of the M3 receptor is not excluded, the major actions of Z-338 on gastric myocytes are potentiation of ICa through activation of M5-like receptor.

Muscarinic agonist potencies at three different effector systems linked to the M(2) or M(3) receptor in longitudinal smooth muscle of guinea-pig small intestine

1. The abilities of muscarinic agonists (arecoline, bethanechol, carbachol, McN-A343, methacholine, pilocarpine) to inhibit isoprenaline-induced cyclic AMP production in chopped fragments (via M(2) receptors), and to evoke cationic current (I(cat)) (via M(2) receptors) or calcium store release (via M3 receptors) in enzyme-dispersed, single voltage-clamped cells from longitudinal smooth muscle of the guinea-pig small intestine were examined. 2. All muscarinic agonists (1 - 300 microM) examined inhibited isoprenaline (1 microM)-induced accumulation of cyclic AMP, the IC(50) varying from 52 to 248 microM. However, their relative potencies to evoke this M(2) effect were not significantly correlated with their ability to evoke I(cat), also a M(2) effect, whether or not calcium stores were depleted; pilocarpine and McN-A343 inhibited the I(cat) response to carbachol. 3. Muscarinic agonists (concentration 300 or 1000 microM), except pilocarpine and McN-A343 which were ineffective, evoked Ca(2+)-activated K(+) current (I(K-Ca)) resulting from Ca(2+) store release (M(3) effect). Their effectiveness was tested by estimating residual stored calcium by subsequent application of caffeine (10 mM). The relative potencies to evoke Ca(2+) store release (M(3)) and for I(cat) activation (M(2)) were closely correlated (P<0.001). 4. These data might be explained if M(2)-mediated adenylyl cyclase inhibition and I(cat) activation involve different G proteins, or involve different populations of M(2) receptors. The observed correlation of agonist potency between I(cat) activation and Ca(2+) store release supports the proposal (Zholos & Bolton, 1997) that M(3) activation can potentiate M(2)-cationic channel coupling through Ca(2+)-independent mechanisms.

Inhibition by clonidine of the carbachol-induced tension development and nonselective cationic current in guinea pig ileal myocytes

Effects of clonidine, an imidazoline derivative as well as alpha2-adrenoceptor agonist, on carbachol (CCh)-evoked contraction in guinea pig ileal smooth muscle were studied using isometric tension recording. To investigate the cellular mechanisms of the inhibitory effect of clonidine, its effects on CCh-evoked nonselective cationic current (I(CCh)), voltage-dependent Ca2+ current (I(Ca)) and voltage-dependent K+ current (I(K)) was also studied using patch-clamp recording techniques in single ileal cells. Clonidine inhibited the contraction evoked by CCh (1 microM) in a concentration-dependent manner with an IC50 valve of 61.7 +/- 2.5 microM. High K+ (40 mM)-evoked contraction was only slightly inhibited even when clonidine was used at 300 microM. Externally applied clonidine inhibited I(CCh) dose-dependently with an IC50 of 42.0 +/- 2.6 microM. When applied internally via patch pipettes, clonidine was without effect. An I(CCh)-like current induced by GTPgammaS was also inhibited by bath application of clonidine. None of KU14R and BU224, both imidazoline receptor blockers, and yohimbine, an alpha2-adrenergic blocker, significantly affects the inhibitory effect of clonidine on I(CCh). Clonidine (300 microM) only slightly decreased membrane currents flowing through voltage-gated Ca2+ channels or K+ channels. These data indicate that clonidine relaxes smooth muscle contraction produced by muscarinic receptor activation and suggest that the effect of clonidine seems due mainly to inhibition of I(CCh) via acting directly on the involved cationic channel.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

The mechanism for the contraction induced by leukotriene C4 in guinea-pig taenia coli

The mechanism underlying the LTC(4)-induced contraction of guinea-pig taenia coli was determined using the simultaneous measurements of [Ca(2+)](i) and force in whole muscle preparations. Additional experiments were performed in receptor coupled permeabilized preparation. For comparison purposes, the contraction which was induced by a typical G-protein mediated agonist, carbachol was also characterized. LTC(4) induced a contraction in the guinea-pig taenia coli in a concentration-dependent manner. The maximal response was obtained at 100 nM and the EC(50) value was 5.4+/-1.9 nM. Both LTC(4) and carbachol induced increases in [Ca(2+)](i) and force. The maximum force induced by 100 nM LTC(4) was significantly smaller than that induced by 10 microM carbachol, although an increase in [Ca(2+)](i) produced by both agonists was similar. In the permeabilized preparations, carbachol, but not LTC(4), induced an additional force development at a fixed Ca(2+) concentration. LTC(4) induced no increase in [Ca(2+)](i) and force in the Ca(2+)-free solution, while carbachol induced transient increases in both [Ca(2+)](i) and force in a Ca(2+)-free solution. Both diltiazem and SK&F 96365 significantly inhibited the LTC(4)- and carbachol-induced increases in [Ca(2+)](i) and force in normal PSS. The inhibitory pattern of [Ca(2+)](i) by these drugs was also similar. We thus conclude that LTC(4) induces the contraction of the guinea-pig taenia coli mainly through Ca(2+) influx via both the diltiazem-sensitive and SK&F 96365-sensitive Ca(2+) channels, without affecting either the Ca(2+)-sensitivity or the intracellular Ca(2+) release. These results indicated that the mechanism underlying the LTC(4)-induced contraction differs greatly from that for conventional G-protein mediated agonists, such as carbachol.

Muscarinic stimulation increases basal Ca(2+) and inhibits spontaneous Ca(2+) transients in murine colonic myocytes

Localized Ca(2+) transients in isolated murine colonic myocytes depend on Ca(2+) release from inositol 1,4,5-trisphosphate (IP(3)) receptors. Localized Ca(2+) transients couple to spontaneous transient outward currents (STOCs) and mediate hyperpolarization responses in these cells. We used confocal microscopy and whole cell patch-clamp recording to investigate how muscarinic stimulation, which causes formation of IP(3), can suppress Ca(2+) transients and STOCs that might override the excitatory nature of cholinergic responses. ACh (10 microM) reduced localized Ca(2+) transients and STOCs, and these effects were associated with a rise in basal cytosolic Ca(2+). These effects of ACh were mimicked by generalized rises in basal Ca(2+) caused by ionomycin (250-500 nM) or elevated external Ca(2+) (6 mM). Atropine (10 microM) abolished the effects of ACh. Pretreatment of cells with nicardipine (1 microM), or Cd(2+) (200 microM) had no effect on responses to ACh. An inhibitor of phospholipase C, U-73122, blocked Ca(2+) transients and STOCs but did not affect the increase in basal Ca(2+) after ACh stimulation. Xestospongin C (Xe-C; 5 microM), a membrane-permeable antagonist of IP(3) receptors, blocked spontaneous Ca(2+) transients but did not prevent the increase of basal Ca(2+) in response to ACh. Gd(3+) (10 microM), a nonselective cation channel inhibitor, prevented the increase in basal Ca(2+) after ACh and increased the frequency and amplitude of Ca(2+) transients and waves. Another inhibitor of receptor-mediated Ca(2+) influx channels, SKF-96365, also prevented the rise in basal Ca(2+) after ACh and increased Ca(2+) transients and development of Ca(2+) waves. FK-506, an inhibitor of FKBP12/IP(3) receptor interactions, had no effect on the rise in basal Ca(2+) but blocked the inhibitory effects of increased basal Ca(2+) and ACh on Ca(2+) transients. These results suggest that the rise in basal Ca(2+) that accompanies muscarinic stimulation of colonic muscles inhibits localized Ca(2+) transients that could couple to activation of Ca(2+)-activated K(+) channels and reduce the excitatory effects of ACh.

Signalling pathway for histamine activation of non-selective cation channels in equine tracheal myocytes

1. The signalling pathway underlying histamine activation of non-selective cation channels was investigated in single equine tracheal myocytes. Application of histamine (100 microM) activated the transient calcium-activated chloride current (ICl(Ca)) and sustained, low amplitude non-selective cation current (ICat). The H1 receptor antagonist pyrilamine (10 microM) blocked activation of ICl(Ca) and ICat. Simultaneous application of histamine (100 microM) and caffeine (8 mM) during H1 receptor blockade activated ICl(Ca), but not ICat. Neither the H2 receptor antagonist cimetidine (20 microM) nor the H3 receptor antagonist thioperamide (20 microM) prevented activation of ICl(Ca) and ICat. 2. Intracellular dialysis of anti-Galphai/Galphao antibodies completely blocked activation of ICat by histamine, whereas ICl(Ca) was not affected. By contrast, anti-Galphaq/Galpha11 antibodies greatly inhibited ICl(Ca), but did not alter activation of ICat. 3. 1-Oleoyl-2-acetyl-sn-glycerol (OAG, 20-100 microM) did not induce any current or affect currents activated by histamine or methacholine (mACH). Simultaneous application of OAG and caffeine activated ICl(Ca), but not ICat, indicating that a rise in [Ca2+]i and stimulation of diacylglycerol-sensitive protein kinase C (PKC) is not sufficient to activate ICat. The phospholipase C inhibitor U73122 (2 microM) blocked histamine activation of ICl(Ca) and ICat, but simultaneous exposure of myocytes to histamine and caffeine restored both ICl(Ca) and ICat in the presence of U73122. 4. Histamine and mACH activated currents with equivalent I-V relationships. The currents activated by these agonists were not additive; following activation of ICat by mACH, histamine failed to induce an additional membrane current. Similarly, mACH did not induce an additional current after full activation of ICat by histamine. 5. We conclude that H1 histamine receptors activate ICat through coupling to Gi/Go proteins. Activation of ICat also requires intracellular calcium release, mediated by H1 receptors coupling to Gq/G11 proteins. This coupling is analogous to the activation of ICat by co-stimulation of M2 and M3 receptors.

Voltage-dependent inhibition of the muscarinic cationic current in guinea-pig ileal cells by SK&F 96365

The effects of SK&F 96365 on cationic current evoked either by activating muscarinic receptors with carbachol or by intracellularly applied GTPgammaS (in the absence of carbachol) were studied using patch-clamp recording techniques in single guinea-pig ileal smooth muscle cells. SK&F 96365 reversibly inhibited the muscarinic receptor cationic current in a concentration-, time- and voltage-dependent manner producing concomitant alteration of the steady-state I-V relationship shape which could be explained by assuming that increasing membrane positivity increased the affinity of the blocker. The inhibition was similar for both carbachol- and GTPgammaS-evoked currents suggesting that the cationic channel rather than the muscarinic receptor was the primary site of the SK&F 96365 action. Increased membrane positivity induced additional rapid inhibition of the cationic current by SK&F 96365 which was more slowly relieved during membrane repolarization. Both the inhibition and disinhibition time course could be well fitted by a single exponential function with the time constants decreasing with increasing positivity for the inhibition (e-fold per about 12 mV) and approximately linearly decreasing with increasing negativity for the disinhibition. At a constant SK&F 96365 concentration, the degree of cationic current inhibition was a sigmoidal function of the membrane potential with a potential of half-maximal increase positive to about +30 mV and a slope factor of about -13 mV. Increasing the duration of voltage steps at -80 or at 80 mV, increased the percentage inhibition; the degree of inhibition was almost identical at both potentials providing evidence that the same cationic channel was responsible for the cationic current both at negative and at positive potentials. It is concluded that the distinctive and unique mode of SK&F 96365 action on the muscarinic receptor cationic channel is a valuable tool in future molecular biology studies of this channel.

Muscarinic receptor subtypes modulating smooth muscle contractility in the urinary bladder

Normal physiological voiding as well as generation of abnormal bladder contractions in diseased states is critically dependent on acetylcholine-induced stimulation of contractile muscarinic receptors on the smooth muscle (detrusor) of the urinary bladder. Muscarinic receptor antagonists are efficacious in treating the symptoms of bladder hyperactivity, such as urge incontinence, although the usefulness of available drugs is limited by undesirable side-effects. Detrusor smooth muscle is endowed principally with M2 and M3 muscarinic receptors with the former predominating in number. M3 muscarinic receptors, coupled to stimulation of phosphoinositide turnover, mediate the direct contractile effects of acetylcholine in the detrusor. Emerging evidence suggests that M2 muscarinic receptors, via inhibition of adenylyl cyclase, cause smooth muscle contraction indirectly by inhibiting sympathetically (beta-adrenoceptor)-mediated relaxation. In certain diseased states, M2 receptors may also contribute to direct smooth muscle contraction. Other contractile mechanisms involving M2 muscarinic receptors, such as activation of a non-specific cationic channel and inactivation of potassium channels, may also be operative in the bladder and requires further investigation. From a therapeutic standpoint, combined blockade of M2 and M3 muscarinic receptors would seem to be ideal since this approach would evoke complete inhibition of cholinergically-evoked smooth muscle contractions. However, if either the M2 or M3 receptor assumes a greater pathophysiological role in disease states, then selective antagonism of only one of the two receptors may be the more rational approach. The ultimate therapeutic strategy is also influenced by the extent to which pre-junctional M1 facilitatory and M2 inhibitory muscarinic receptors regulate acetylcholine release and also which subtypes mediate the undesirable effects of muscarinic receptor blockade such as dry mouth. Finally, the consequence of muscarinic receptor blockade in the central nervous system on the micturition reflex, an issue which is poorly studied and seldom taken into consideration, should not be ignored.

Contractile role of M2 and M3 muscarinic receptors in gastrointestinal smooth muscle

Muscarinic agonists elicit contraction through M3 receptors in most isolated preparations of gastrointestinal smooth muscle, and not surprisingly, several investigators have identified M3 receptors in smooth muscle using biochemical, immunological and molecular biological methods. However, these studies have also shown that the M2 receptor outnumbers the M3 by a factor of about four in most instances. In smooth muscle, M3 receptors mediate phosphoinositide hydrolysis and Ca2+ mobilization, whereas M2 receptors mediate an inhibition of cAMP accumulation. The inhibitory effect of the M2 receptor on cAMP levels suggests an indirect role for this receptor; namely, an inhibition of the relaxant action of cAMP-stimulating agents. Such a function has been rigorously demonstrated in an experimental paradigm where gastrointestinal smooth muscle is first incubated with 4-DAMP mustard to inactivate M3 receptors during a Treatment Phase, and subsequently, the contractile activity of muscarinic agonists is characterized during a Test Phase in the presence of histamine and a relaxant agent. When present together, histamine and the relaxant agent (e.g., isoproterenol or forskolin) have no net contractile effect because their actions oppose one another. However, under these conditions, muscarinic agonists elicit a highly potent contractile response through the M2 receptor, presumably by inhibiting the relaxant action of isoproterenol or forskolin on histamine-induced contractions. This contractile response is pertussis toxin-sensitive, unlike the standard contractile response to muscarinic agonists, which is pertussis toxin-insensitive. When measured under standard conditions (i.e., in the absence of histamine and without 4-DAMP mustard-treatment), the contractile response to muscarinic agonists is moderately sensitive to pertussis toxin if isoproterenol or forskolin is present. Also, pertussis toxin-treatment enhances the relaxant action of isoproterenol in the field-stimulated guinea pig ileum. These results demonstrate that endogenous acetylcholine can activate M2 receptors to inhibit the relaxant effects of beta-adrenoceptor activation on M3 receptor-mediated contractions. An operational model for the interaction between M2 and M3 receptors shows that competitive antagonism of the interactive response resembles an M3 profile under most conditions, making it difficult to detect the contribution of the M2 receptor.

Muscarinic M2 receptor-mediated contraction in the guinea pig Taenia caeci: possible involvement of protein kinase C

Contraction of the guinea pig taenia caeci is mediated by muscarinic M3 receptors; however, they comprise only 30% of the muscarinic receptors present. This study investigated the role of the predominant M2 receptor population in contractions and possible second messengers involved after M3 receptors were selectively alkylated by 4-DAMP mustard [N-(2-chloroethyl)-4-piperidinyldiphenylacetate] (60 nM) in the presence of otenzepad (AF-DX 116; 1 microM). Concentration-response curves to oxotremorine-M (oxo-M) in the presence of histamine and isoprenaline were performed in the presence of otenzepad (1 and 3 microM), resulting in a mean apparent pK(B) of 6.49, indicative of an M2 response. As the taenia has intrinsic tone, precontraction with histamine was not necessary and, therefore, in some experiments only isoprenaline was included. In these studies, an M3 response to oxo-M was observed, as the mean apparent pK(B) for otenzepad was 5.89. To investigate protein kinase C (PKC) involvement in the M2 response following M3 inactivation, the inhibitor chelerythrine (1 microM) was included with histamine and isoprenaline in the absence and presence of otenzepad. The oxo-M concentration-response curve was shifted by otenzepad with an apparent pK(B) value of 6.05, a value significantly different from that seen in the absence of chelerythrine (P < 0.05). These results suggest that activation of PKC by a spasmogen such as histamine is necessary to see an M2 response following M3 receptor inactivation.

G protein-coupled receptors in gastrointestinal physiology. IV. Neural regulation of gastrointestinal smooth muscle

G protein-coupled receptors receive many of the neural, hormonal, and paracrine inputs to gastrointestinal (GI) smooth muscle cells. This article examines the major G protein-coupled receptors, G proteins, and effectors that mediate responses to enteric neuromuscular transmitters. Excitatory transmitters primarily couple through Gq/11 and Gi/Go proteins and elicit responses via formation of inositol trisphosphate and diacylglycerol and inhibition of adenylyl cyclase. Several inhibitory transmitters couple through Gs and activation of adenylyl cyclase. There are interesting examples, however, of inhibitory transmitters apparently using pathways regulated by Gq/11 to elicit responses by localized Ca2+ release and activation of Ca2+-dependent ion channels. G protein-coupled receptors may also be differentially expressed by smooth muscle cells and interstitial cells of Cajal, which may increase the diversity of responses and allow specialized innervation of GI muscle tissues.

CCK regulates nonselective cation channels in guinea pig gastric smooth muscle cells

CCK has widespread effects in the gastrointestinal tract, stimulating pancreatic secretion and contraction of smooth muscles. The cellular mechanisms by which CCK causes smooth muscle contraction are poorly understood. We investigated the effects of CCK on guinea pig gastric smooth muscle cells using patch-clamp techniques. CCK caused contraction of cells accompanied by inward current. The conductance activated by CCK was nonselective for cations and showed little voltage dependence. Because ACh also activates nonselective cation current, we examined interactions between CCK and ACh. When CCK activated inward current, ACh caused no further effect. When CCK failed to activate current, subsequent ACh-activated current was larger and no longer exhibited its characteristic voltage dependence. Intracellular dialysis with guanosine 5'-O-(3-thiotriphosphate) caused similar changes in the voltage dependence of the ACh-activated current, suggesting a role for G proteins in regulation of the current. Activation of nonselective cation current would depolarize muscle and may contribute to the excitation mediated by CCK in tissues. These findings provide evidence that multiple types of receptors converge to regulate nonselective cation current.

M2 and M3 muscarinic receptors couple, respectively, with activation of nonselective cationic channels and potassium channels in intestinal smooth muscle cells

Smooth muscle cells of guinea pig ileum express both M2 and M3 subtypes of muscarinic receptors. Under voltage clamp, activation of the muscarinic receptors with carbachol (CCh) induces Ca2+-activated K+ current (I[K-Ca]) and nonselective cationic current (Icat). Receptor subtypes mediating the current responses were characterized by using pirenzepine, AF-DX116, 4-DAMP and atropine, which have different profiles of the affinity constants for muscarinic receptor subtypes. The muscarinic antagonists inhibited either CCh-evoked I(K-Ca) or Icat with different potencies. Their relative potencies for I(K-Ca) and Icat inhibition resembled the relative affinity constants for M3 and M2 subtypes, respectively. Thus, the I(K-Ca) is mediated via the M3 subtype and the Icat via the M2 subtype.

Subtypes of the muscarinic receptor in smooth muscle

Muscarinic receptors are expressed in smooth muscle throughout the body. In most instances, the muscarinic receptor population in smooth muscle is composed of mainly the M2 and M3 subtypes in an 80% to 20% mixture. The M3 subtype mediates phosphoinositide hydrolysis and calcium mobilization, whereas the M2 subtype mediates an inhibition of cAMP accumulation. In addition, a variety of ionic conductances are elicited by muscarinic receptors. Muscarinic agonists stimulate a nonselective cation conductance that is pertussis toxin-sensitive and dependent on calcium. The pertussis toxin-sensitivity of this response suggests that it is mediated by M2 receptors. Following agonist induced depolarization of smooth muscle, voltage dependent calcium channels are activated to enable an influx of calcium. In some instances, muscarinic agonists enhance this conductance through a mechanism involving protein kinase C, whereas in other instances, muscarinic agonists suppress this calcium conductance. Smooth muscle often contains calcium activated potassium channels that tend to repolarize the membrane following calcium influx. Activation of muscarinic receptors suppresses this potassium conductance in some smooth muscles. Under standard conditions, muscarinic agonists elicit pertussis toxin-insensitive contractions through activation of the M3 receptor. When most of the M3 receptors are inactivated, it is possible to measure a pertussis toxin-sensitive contractile response to muscarinic agonists that is most likely mediated through M2 receptors. M2 receptors also cause an indirect contraction by inhibiting the relaxant effects of agents that increase cAMP (e.g., forskolin and isoproterenol).

Muscarinic receptor subtypes controlling the cationic current in guinea-pig ileal smooth muscle

1. The effects of muscarinic antagonists on cationic current evoked by activating muscarinic receptors with the stable agonist carbachol were studied by use of patch-clamp recording techniques in guinea-pig single ileal smooth muscle cells. 2. Ascending concentrations of carbachol (3-300 microM) activated the cationic conductance in a concentration-dependent manner with conductance at a maximally effective carbachol concentration (Gmax) of 27.4+/-1.4 nS and a mean -log EC50 of 5.12+/-0.03 (mean+/-s.e.mean) (n=114). 3. Muscarinic antagonists with higher affinity for the M2 receptor, methoctramine, himbacine and tripitramine, produced a parallel shift of the carbachol concentration-effect curve to the right in a concentration-dependent manner with pA2 values of 8.1, 8.0 and 9.1, respectively. 4. All M3 selective muscarinic antagonists tested, 4-DAMP, p-F-HHSiD and zamifenacin, reduced the maximal response in a concentration-dependent and non-competitive manner. This effect could be observed even at concentrations which did not produce any increase in the EC50 for carbachol. At higher concentrations M3 antagonists shifted the agonist curve to the right, increasing the EC50, and depressed the maximum conductance response. Atropine, a non-selective antagonist, produced both reduction in Gmax (M3 effect) and significant increase in the EC50 (M2 effect) in the same concentration range. 5. The depression of the conductance by 4-DAMP, zamifenacin and atropine could not be explained by channel block as cationic current evoked by adding GTPgammaS to the pipette (without application of carbachol) was unaffected. 6. The results support the hypothesis that carbachol activates M2 muscarinic receptors so initiating the opening of cationic channels which cause depolarization; this effect is potentiated by an unknown mechanism when carbachol activates M3 receptors. As an increasing fraction of M3 receptors are blocked by an antagonist, the effects on cationic current of an increasing proportion of activated M2 receptors are disabled.