Pharmacological evidence for cardiac muscarinic receptor subtypes

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.

Functional M3 muscarinic acetylcholine receptors in mammalian hearts

In contrast to most peripheral tissues where multiple subtypes of muscarinic acetylcholine receptor (mAChR) coexist, with each of them playing its part in the orchestra of parasympathetic innervation, the myocardium has been traditionally considered to possess a single mAChR subtype. Although there is much evidence to support the notion that one receptor subtype (M2) orchestrates myocardial muscarinic transduction, there is emerging evidence that M1 and M3 receptors are also expressed and are of potential physiological, pathophysiological and pharmacological relevance. Clarifying this issue has a profound impact on our thinking about the cholinergic control of the heart function and disease and approaches to new drug development for the treatment of heart disease associated with parasympathetic dysfunction. This review article presents evidence for the presence of the M3 receptor subtype in the heart, and analyzes the controversial data from published pharmacological, functional and molecular studies. The potential roles of the M3 receptors, in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias, are discussed. On the basis of these considerations, we have made some proposals concerning the future of myocardial M3 receptor research.

Effects of arecoline on calcium channel currents and caffeine-induced calcium release in isolated single ventricular myocyte of guinea pig

The effects of Arecoline (Are) on calcium mobilization were investigated. In isolated single ventricular myocyte of guinea pig, patch clamp whole cell recording techniques were used to record the current of L-type calcium channel and cytosolic Ca2+ level ([Ca2+]i) labeled with fluorescence probe Fluo-3/AM was measured under a laser scanning confocal microscope. Results revealed that Are (3-100 mumol/L) could inhibit L-type calcium current in a concentration-dependent manner and the value of IC50 was 33.73 mumol/L (n = 5). In the absence of extracellular calcium, the resting levels of [Ca2+]i was not affected by Are (n = 6, P > 0.05), but pretreatment with Are (30 mumol/L) could significantly inhibit the [Ca2+]i elevation induced by caffeine (10 mmol/L, n = 6, P < 0.01). It was concluded that Are could inhibit not only calcium influx through L-type calcium channel but also calcium release from sarcoplasmic reticulum.

Muscarinic receptor subtypes mediating negative chrono- and inotropic responses in isolated, blood-perfused dog right atria

1. Negative chrono- and inotropic responses to both carbachol (CCh) into the sinus node artery and electrical stimulation of vagal nerve fibres (ES) were studied in the isolated, blood-perfused canine right atrium, using four muscarinic receptor antagonists, atropine, 4-DAMP, AF-DX 116 and pirenzepine. 2. ES and CCh evoked negative chrono- and inotropic responses in a frequency-dependent manner and in a dose-related manner, respectively. 3. Each antagonist inhibited these negative chrono- and inotropic responses in a dose-dependent manner. The ranking order of blocking potency (ID50) was atropine greater than or equal to 4-DAMP greater than AF-DX 116 greater than pirenzepine. 4. The ID50 of atropine, 4-DAMP or AF-DX 116 against sinus rate decreases induced by CCh or ES was not significantly different from that against the atrial tension decreases. In contrast, the ID50 of pirenzepine against sinus rate decreases evoked by CCh (17 nmol) or ES (20 nmol) was significantly smaller than that for the atrial tension decrease (CCh, 200 and ES, 53 nmol, respectively). 5. These results suggest that, in the isolated dog atrium, M2-receptor-mediated-responses are predominant. However, M1-receptor activation may also be involved in sinus rate response.

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.

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.

Autoradiographic localisation of muscarinic receptors on guinea pig intracardiac neurones and atrial myocytes in culture

Muscarinic receptors were localised on cells cultured from the atria and interatrial septum of newborn guinea pig heart by autoradiography using [3H]propylbenzilylcholine mustard. All of the intracardiac neurones observed in these cultures were specifically labelled: both the neuronal cell body and processes were evenly labelled over their entire surface. Autoradiographic grains were also uniformly distributed over atrial myocytes in culture. This even pattern of labelling of both neurones and atrial myocytes was not changed by the substitution of serum-supplemented growth medium with serum-free, hormone-supplemented, defined medium. The demonstration of muscarinic receptors on intracardiac neurones has important implications for studies on the roles of these neurones in the mammalian heart.

Discrimination by N-ethylmaleimide between the chronotropic and inotropic response to muscarinic receptor stimulation in rat atrium

N-ethylmaleimide (NEM) rapidly blocked the negative chronotropic effect of carbachol on rat right atrium. In contrast, NEM did not reduce the negative inotropic response to muscarinic (M) receptor stimulation. Carbachol inhibited the specific binding of [3H]-N-methylscopolamine [( 3H]-NMS) to membranes of rat atria as reflected by a shallow inhibition curve. Both guanosine triphosphate (GTP) and NEM shifted the [3H]-NMS inhibition curves of carbachol to the right. Pretreatment of the atrial membranes with NEM abolished the GTP-induced rightward shift. However, when instead of the membranes the intact atria were pre-incubated with NEM, no interaction between NEM and GTP in the membranal preparation was observed. The results indicate that NEM sharply discriminated between the inotropic and chronotropic effects to M-receptor stimulation in rat atria. The inhibitory effect of NEM on the M-receptor-mediated negative chronotropic effect in rat atrium cannot be explained by an interaction of the sulfhydryl reagent with GTP-binding proteins, like Ni or No.

Receptors on individual neurones

Membrane potential or ionic conductance of neurones of the mammalian central or peripheral nervous system maintained in vitro can be measured over periods of several hours. Drugs or transmitters which change potential or conductance can be applied repeatedly under equilibrium conditions, and pharmacological null methods used to characterize the receptors with which they interact. The method offers an advantage over ligand binding studies on nervous tissue because both agonist and antagonist affinities can be estimated on individual functioning cells. The results to date suggest the hypothesis that a given receptor subtype is always associated with the same change in ion conductance, and the corollary that distinct ion conductances affected by the same transmitter result from interactions with different receptor subtypes.

Acetylcholine hyperpolarizes central neurones by acting on an M2 muscarinic receptor

Acetylcholine (ACh) is considered to act as a neurotransmitter in the mammalian brain by binding to membrane receptors and bringing about a change in neurone excitability. In the case of muscarinic receptors, cell excitability is usually increased; this effect results from a closure of membrane potassium channels in cortical cells. However, some central neurones are inhibited by ACh, and we hypothesized that these two opposite effects of ACh resulted from interactions with different subtypes of muscarinic receptor. We made intracellular recordings from neurones in the rat nucleus parabrachialis, a group of neurones in the upper pons some of which themselves synthesize ACh. ACh and muscarine caused a membrane hyperpolarization which resulted from an increase in the membrane conductance to potassium ions. The muscarinic receptor subtype was characterized by determining the dissociation equilibrium constant (KD) for pirenzepine during the intracellular recording; the value of approximately 600 nM indicates a receptor in the M2 class. This muscarinic receptor is quite different from that which brings about a decrease in potassium conductance in other neurones, which has a pirenzepine KD of approximately 10 nM (M1 receptors). It is possible that antagonists selective for this kind of M2 receptor would be useful in the management of conditions, such as Alzheimer's disease, which are associated with a reduced effectiveness of cholinergic neurones.

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

The action of 'selective' agonists and antagonists at muscarinic receptors mediating ileal contractions, and the rate and force of atrial contractions has been assessed. The effect of nicotinic receptor stimulation, catecholamine release and acetylcholinesterase (AChE) action on muscarinic activity has also been assessed. The nicotinic actions of carbachol did not affect its agonist potency nor the antagonist affinity data obtained when this agonist was used in atrial and ileal preparations. Antagonist data indicated that muscarinic receptors mediating the rate and force of atrial contractions did not differ. Differences in agonist potencies at these two muscarinic receptors were attributable to either differences in intrinsic efficacy or susceptibility to the action of acetylcholinesterase. The small differences in agonist potency observed between atrial and ileal muscarinic receptors were considered not sufficient to indicate receptor heterogeneity. The pirenzepine affinity data indicated that all three receptors are of the M2 type. Affinity data using secoverine and 4-diphenyl-acetoxy-N-methyl piperidine methiodide indicated that ileal and atrial muscarinic receptors differ. Data obtained using gallamine, pancuronium and stercuronium cannot be regarded as indicative of receptor affinity since the antagonism is not competitive; it did nonetheless corroborate the conclusion that ileal and atrial muscarinic receptors are different.