The M3 receptor-mediated K(+) current (IKM3), a G(q) protein-coupled K(+) channel

Dimethylethanolamine Decreases Epileptiform Activity in Acute Human Hippocampal Slices in vitro

Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy with about 30% of patients developing pharmacoresistance. These patients continue to suffer from seizures despite polytherapy with antiepileptic drugs (AEDs) and have an increased risk for premature death, thus requiring further efforts for the development of new antiepileptic therapies. The molecule dimethylethanolamine (DMEA) has been tested as a potential treatment in various neurological diseases, albeit the functional mechanism of action was never fully understood. In this study, we investigated the effects of DMEA on neuronal activity in single-cell recordings of primary neuronal cultures. DMEA decreased the frequency of spontaneous synaptic events in a concentration-dependent manner with no apparent effect on resting membrane potential (RMP) or action potential (AP) threshold. We further tested whether DMEA can exert antiepileptic effects in human brain tissue ex vivo. We analyzed the effect of DMEA on epileptiform activity in the CA1 region of the resected hippocampus of TLE patients in vitro by recording extracellular field potentials in the pyramidal cell layer. Epileptiform burst activity in resected hippocampal tissue from TLE patients remained stable over several hours and was pharmacologically suppressed by lacosamide, demonstrating the applicability of our platform to test antiepileptic efficacy. Similar to lacosamide, DMEA also suppressed epileptiform activity in the majority of samples, albeit with variable interindividual effects. In conclusion, DMEA might present a new approach for treatment in pharmacoresistant TLE and further studies will be required to identify its exact mechanism of action and the involved molecular targets.

Role of Muscarinic M1, M2, and M3 Receptors in the Regulation of Electrical Activity of Myocardial Tissue of Caval Veins during the Early Postnatal Ontogeny

We studied the influence of blockers of muscarinic M1, M2, and M3 receptors on the effect of acetylcholine in the myocardial tissue of caval veins in rats at the early stage of ontogeny. The experiments were performed on isolated preparations of the right superior vena cava working under their own rhythm. Action potentials were recorded using the standard microelectrode technique. Acetylcholine (1 μM) suppressed automatic activity in the superior vena cava myocardium. Preliminary perfusion of the preparation with non-selective blocker atropine (1 μM) completely abolished the effect of acetylcholine, treatment with M2 receptor blocker AQ-RA 741 (1 μM) led to partial suppression of the effect of acetylcholine. Blockers of M1 and M3 receptors pirenzepine (1 μM) and 4DAMP (0.1 μM) did not suppress the effect of acetylcholine. Thus, the effect of acetylcholine is predominantly realized via M2 receptors, but M3 receptors can also partially mediate its effect in the superior vena cava myocardium in rats at the early stages of ontogeny.

Exome-Wide Rare Variant Analyses in Sudden Infant Death Syndrome

OBJECTIVE

To determine whether a monogenic basis explains sudden infant death syndrome (SIDS) using an exome-wide focus.

STUDY DESIGN

A cohort of 427 unrelated cases of SIDS (257 male; average age = 2.7 ± 1.9 months) underwent whole-exome sequencing. Exome-wide rare variant analyses were carried out with 278 SIDS cases of European ancestry (173 male; average age = 2.7 ± 1.98 months) and 973 ethnic-matched controls based on 6 genetic models. Ingenuity Pathway Analysis also was performed. The cohort was collected in collaboration with coroners, medical examiners, and pathologists by St George's University of London, United Kingdom, and Mayo Clinic, Rochester, Minnesota. Whole-exome sequencing was performed at the Genomic Laboratory, Kings College London, United Kingdom, or Mayo Clinic's Medical Genome Facility, Rochester, Minnesota.

RESULTS

Although no exome-wide significant (P < 2.5 × 10-6) difference in burden of ultra-rare variants was detected for any gene, 405 genes had a greater prevalence (P < .05) of ultra-rare nonsynonymous variants among cases with 17 genes at P < .005. Some of these potentially overrepresented genes may represent biologically plausible novel candidate genes for a monogenic basis for a portion of patients with SIDS. The top canonical pathway identified was glucocorticoid biosynthesis (P = .01).

CONCLUSIONS

The lack of exome-wide significant genetic associations indicates an extreme heterogeneity of etiologies underlying SIDS. Our approach to understanding the genetic mechanisms of SIDS has far reaching implications for the SIDS research community as a whole and may catalyze new evidence-based SIDS research across multiple disciplines. Perturbations in glucocorticoid biosynthesis may represent a novel SIDS-associated biological pathway for future SIDS investigative research.

Distribution and function of the muscarinic receptor subtypes in the cardiovascular system

Muscarinic acetylcholine receptors belong to the G protein-coupled receptor superfamily and are widely known to mediate numerous functions within the central and peripheral nervous system. Thus, they have become attractive therapeutic targets for various disorders. It has long been known that the parasympathetic system, governed by acetylcholine, plays an essential role in regulating cardiovascular function. Unfortunately, due to the lack of pharmacologic selectivity for any one muscarinic receptor, there was a minimal understanding of their distribution and function within this region. However, in recent years, advancements in research have led to the generation of knockout animal models, better antibodies, and more selective ligands enabling a more thorough understanding of the unique role muscarinic receptors play in the cardiovascular system. These advances have shown muscarinic receptor 2 is no longer the only functional subtype found within the heart and muscarinic receptors 1 and 3 mediate both dilation and constriction in the vasculature. Although muscarinic receptors 4 and 5 are still not well characterized in the cardiovascular system, the recent generation of knockout animal models will hopefully generate a better understanding of their function. This mini review aims to summarize recent findings and advances of muscarinic involvement in the cardiovascular system.

Chemical activation of a food deprivation signal extends lifespan

Model organisms subject to dietary restriction (DR) generally live longer. Accompanying this lifespan extension are improvements in overall health, based on multiple metrics. This indicates that pharmacological treatments that mimic the effects of DR could improve health in humans. To find new chemical structures that extend lifespan, we screened 30 000 synthetic, diverse drug‐like chemicals in Caenorhabditis elegans and identified several structurally related compounds that acted through DR mechanisms. The most potent of these NP1 impinges upon a food perception pathway by promoting glutamate signaling in the pharynx. This results in the overriding of a GPCR pathway involved in the perception of food and which normally acts to decrease glutamate signals. Our results describe the activation of a dietary restriction response through the pharmacological masking of a novel sensory pathway that signals the presence of food. This suggests that primary sensory pathways may represent novel targets for human pharmacology.

Role of Cholinergic Innervation and RGS2 in Atrial Arrhythmia

The heart receives sympathetic and parasympathetic efferent innervation as well as the ability to process information internally via an intrinsic cardiac autonomic nervous system (ICANS). For over a century, the role of the parasympathetics via vagal acetylcholine release was related to controlling primarily heart rate. Although in the late 1800s shown to play a role in atrial arrhythmia, the myocardium took precedence from the mid-1950s until in the last decade a resurgence of interest in the autonomics along with signaling cascades, regulators, and ion channels. Originally ignored as being benign and thus untreated, recent emphasis has focused on atrial arrhythmia as atrial fibrillation (AF) is the most common arrhythmia seen by the general practitioner. It is now recognized to have significant mortality and morbidity due to resultant stroke and heart failure. With the aging population, there will be an unprecedented increased burden on health care resources. Although it has been known for more than half a century that cholinergic stimulation can initiate AF, the classical concept focused on the M2 receptor and its signaling cascade including RGS4, as these had been shown to have predominant effects on nodal function (heart rate and conduction block) as well as contractility. However, recent evidence suggests that the M3 receptor may also playa role in initiation and perpetuation of AF and thus RGS2, a putative regulator of the M3 receptor, may be a target for therapeutic intervention. Mice lacking RGS2 (RGS2^−/−), were found to have significantly altered electrophysiological atrial responses and were more susceptible to electrically induced AF. Vagally induced or programmed stimulation-induced AF could be blocked by the selective M3R antagonist, darifenacin. These results suggest a potential surgical target (ICANS) and pharmacological targets (M3R, RGS2) for the management of AF.

Functional M3 cholinoreceptors are present in pacemaker and working myocardium of murine heart

The presence of M3 cholinoreceptors and their role in mediation of action potential waveform modulation were determined by immunolabeling of receptor proteins and standard microelectrode technique, respectively. The sinoatrial node (SAN), which was determined as a connexin 43 negative area within the intercaval region, the surrounding atrial tissue, and the working ventricular myocardium exhibited labeling of both M3 and M2 receptors. However, the density of M3 and M2 labeling was about twofold higher in the SAN compared to working myocardium. The stimulation of M3 receptors was obtained by application of nonselective M1 and M3 muscarinic agonist pilocarpine (10(-5) M) in the presence of selective M2 blocker methoctramine (10(-7) M). Stimulation of M3 receptors provoked marked shortening of action potential duration in atrial and ventricular working myocardium. In the SAN, M3 stimulation leads to a significant reduction of sinus rhythm rate accompanied with slowing of diastolic depolarization and increase of action potential upstroke velocity. All electrophysiological effects of selective M3 stimulation were suppressed by specific blocker of M3 receptors 4-DAMP (10(-8) M). We conclude that M3 cholinoreceptors are present in pacemaker and working myocardium of murine heart, where they mediate negative cholinergic effects: slowing of sinus rhythm and shortening of action potentials.

Overexpression of M₃ muscarinic receptor is a novel strategy for preventing sudden cardiac death in transgenic mice

The present study was designed to investigate the cardiac benefits of M₃ muscarinic receptor (M₃-mAChR) overexpression and whether these effects are related to the regulation of the inward rectifying K⁺ channel by microRNA-1 (miR-1) in a conditional overexpression mouse model. A cardiac-specific M₃-mAChR transgenic mouse model was successfully established for the first time in this study using microinjection, and the overexpression was confirmed by both reverse transcriptase-polymerase chain reaction and Western blot techniques. We demonstrated that M₃-mAChR overexpression dramatically reduced the incidence of arrhythmias and decreased the mortality in a mouse model of myocardial ischemia-reperfusion (I/R). By using whole-cell patch techniques, M₃-mAChR overexpression significantly shortened the action potential duration and restored the membrane repolarization by increasing the inward rectifying K⁺ current. By using Western blot techniques, M₃-mAChR overexpression also rescued the expression of the inward rectifying K⁺ channel subunit Kir2.1 after myocardial I/R injury. This result was accompanied by suppression of upregulation miR-1. We conclude that M₃-mAChR overexpression reduced the incidence of arrhythmias and mortality after myocardial I/R by protecting the myocardium from ischemia in mice. This effect may be mediated by increasing the inward rectifying K⁺ current by downregulation of arrhythmogenic miR-1 expression, which might partially be a novel strategy for antiarrhythmias, leading to sudden cardiac death.

Constitutive overexpression of muscarinic receptors leads to vagal hyperreactivity

Background

Alterations in muscarinic receptor expression and acetylcholinesterase (AchE) activity have been observed in tissues from Sudden Infant Death Syndrome (SIDS). Vagal overactivity has been proposed as a possible cause of SIDS as well as of vasovagal syncopes. The aim of the present study was to seek whether muscarinic receptor overexpression may be the underlying mechanism of vagal hyperreactivity. Rabbits with marked vagal pauses following injection of phenylephrine were selected and crossed to obtain a vagal hyperreactive strain. The density of cardiac muscarinic receptors and acetylcholinesterase (AchE) gene expression were assessed. Blood markers of the observed cardiac abnormalities were also sought.

Methodology/Principal Findings

Cardiac muscarinic M₂ and M₃ receptors were overexpressed in hyperreactive rabbits compared to control animals (2.3-fold and 2.5-fold, respectively) and the severity of the phenylephrine-induced bradycardia was correlated with their densities. A similar overexpression of M₂ receptors was observed in peripheral mononuclear white blood cells, suggesting that cardiac M₂ receptor expression can be inferred with high confidence from measurements in blood cells. Sequencing of the coding fragment of the M₂ receptor gene revealed a single nucleotide mutation in 83% of hyperreactive animals, possibly contributing for the transcript overexpression. Significant increases in AchE expression and activity were also assessed (AchE mRNA amplification ratio of 3.6 versus normal rabbits). This phenomenon might represent a compensatory consequence of muscarinic receptors overexpression. Alterations in M₂ receptor and AchE expression occurred between the 5th and the 7th week of age, a critical period also characterized by a higher mortality rate of hyperreactive rabbits (52% in H rabbits versus 13% in normal rabbits) and preceeded the appearance of functional disorders.

Conclusions/Significance

The results suggest that cardiac muscarinic receptor overexpression plays a critical role in the development of vagal hyperreactivity, whereas AchE hyperactivity appears as a compensatory consequence of it. Since similar vagal disorders were observed recently by us in SIDS, muscarinic receptor overexpression could become a marker of risk of vasovagal syncopes and SIDS.

Evidence for enhanced M3 muscarinic receptor function and sensitivity to atrial arrhythmia in the RGS2-deficient mouse

Atrial fibrillation (AF) is the most common arrhythmia seen in general practice. Muscarinic ACh receptors (M2R, M3R) are involved in vagally induced AF. M2R and M3R activate the heterotrimeric G proteins, G(i) and G(q), respectively, by promoting GTP binding, and these in turn activate distinct K(+) channels. Signaling is terminated by GTP hydrolysis, a process accelerated by regulator of G protein signaling (RGS) proteins. RGS2 is selective for G(q) and thus may regulate atrial M3R signaling. We hypothesized that knockout of RGS2 (RGS2(-/-)) would render the atria more susceptible to electrically induced AF. One-month-old male RGS2(-/-) and C57BL/6 wild-type (WT) mice were instrumented for intracardiac electrophysiology. Atrial effective refractory periods (AERPs) were also determined in the absence and presence of carbachol, atropine, and/or the selective M3R antagonist darifenacin. Susceptibility to electrically induced AF used burst pacing and programmed electrical stimulation with one extrastimulus. Real-time RT-PCR measured atrial and ventricular content of RGS2, RGS4, M2R, M3R, and M4R mRNA. AERP was lower in RGS2(-/-) compared with WT mice in both the high right atrium (HRA) (30 +/- 1 vs. 34 +/- 1 ms, P < 0.05) and mid right atrium (MRA) (21 +/- 1 vs. 24 +/- 1 ms, P < 0.05). Darifenacin eliminated this difference (HRA: 37 +/- 2 vs. 39 +/- 2 ms, and MRA: 30 +/- 2 vs. 30 +/- 1, P > 0.4). RGS2(-/-) were more susceptible than WT mice to atrial tachycardia/fibrillation (AT/F) induction (11/22 vs. 1/25, respectively, P < 0.05). Muscarinic receptor expression did not differ between strains, whereas M2R expression was 70-fold higher than M3R (P < 0.01). These results suggest that RGS2 is an important cholinergic regulator in the atrium and that RGS2(-/-) mice have enhanced susceptibility to AT/F via enhanced M3 muscarinic receptor activity.

Reciprocal regulation between M3 muscarinic acetylcholine receptor and protein kinase C-epsilon in ventricular myocytes during myocardial ischemia in rats

We have studied the association between M(3) muscarinic acetylcholine receptors (M(3)-mAChR) and protein kinase C-epsilon (PKC-epsilon) during ischemic myocardial injury using Western blot analysis and immunoprecipitation technique. Myocardial ischemia (MI) induced PKC-epsilon translocation from cytosolic to membrane fractions. This translocation participated in the phosphorylation of M(3)-mAChR in membrane fractions, which could be abolished by the inhibitor of PKC, chelerythrine chloride. On the other hand, M(3)-mAChR could also regulate the expression of PKC-epsilon in ischemic myocardium. Choline (choline chloride, an M(3) receptor agonist, administered at 15 min before occlusion) strengthened the association between PKC-epsilon and M(3)-mAChR. However, blockade of M(3)-mAChR by 4-diphenylacetoxy-N-methylpiperidine methiodide (an M(3) receptor antagonist, administered at 20 min before occlusion) completely inhibited the effect of choline on the expression of PKC-epsilon. We conclude that the translocation of PKC-epsilon is required for the phosphorylation of M(3)-mAChR; moreover, increased PKC-epsilon activity is associated with M(3)-mAChR during MI. This reciprocal regulation is likely to play a role in heart signal transduction during ischemia between ventricular myocytes.

M3 muscarinic acetylcholine receptor is associated with beta-catenin in ventricular myocytes during myocardial infarction in the rat

1. The present study was designed to investigate whether the M(3) muscarinic acetylcholine receptors (mAChR) is associated with beta-catenin in the ventricular myocardium during ischaemic myocardial injury and to determine the possible mechanism/s involved. 2. Rat hearts were subjected to coronary artery ligation for 1 and 6 h or 1 month to establish a myocardial ischaemia (MI) model. In the acute MI model, 16 rats were randomized into four groups: (i) control; (ii) ischaemia (rats were subjected to 20 min coronary occlusion); (iii) choline (10 mg/kg, i.v., choline chloride, an M(3) receptor agonist, was administered 15 min before occlusion); and (iv) 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; 0.12 mg/kg 4-DAMP, an M(3) receptor antagonist, was administered 20 min before occlusion, followed 5 min later by 10 mg/kg, i.v., choline chloride). Immunochemistry, western blot analysis and immunoprecipitation were used to determine the expression and localization of beta-catenin and the M(3) mAChR. 3. Myocardial ischaemia caused a time-dependent increase in the expression of beta-catenin. Moreover, a physical association was found between beta-catenin and the M(3) mAChR in intercalated discs. This association was enhanced by prolonged ischaemia. Administration of choline before ischaemia not only increased beta-catenin expression, but also strengthened the association between beta-catenin and the M(3) mAChR. However, blockade of M(3) mAChR by 4-DAMP completely inhibited the effect of choline on the expression of beta-catenin. In addition, MI increased phosphorylation of the M(3) mAChR. 4. The results indicate that increased beta-catenin activity is associated with M(3) mAChR during MI. This association is likely to play a role in heart signal transduction during ischaemia between neighbouring ventricular myocardiocum.

Drugs and their molecular targets: an updated overview

About 330 targets bind approved drugs, 270 encoded by the human genome and 60 belonging to pathogenic organisms. A large number of druggable targets have been recently proposed from preclinical and first clinical data, but a huge reservoir of putative drug targets, possibly several thousands, remains to be explored. This overview considers the different types of ligands and their selectivity in the main superfamilies of drug targets, enzymes, membrane transporters and ion channels, and the various classes of membrane and nuclear receptors with their signalling pathway. Recently approved drugs such as monoclonal antibodies, tyrosine kinase and proteasome inhibitors, and major drugs under clinical studies are reviewed with their molecular target and therapeutic interest. The druggability of emerging targets is discussed, such as multidrug resistance transporters and cystic fibrosis transmembrane conductance regulator (CFTR), hyperpolarization-activated cyclic nucleotides-gated (HCN), cyclic nucleotide-gated (CNG) and transient receptor potential (TRP) ion channels, tumour necrosis factor (TNF) and receptor activator of NFkappaB (RANK) receptors, integrins, and orphan or recently deorphanized G-protein-coupled and nuclear receptors. Large advances have been made in the therapeutical use of recombinant cytokines and growth factors (i.e. tasonermin, TNFalpha-1a; becaplermin, platelet-derived growth factor (PDGF); dibotermin-alpha, bone morphogenetic proteins (BMP)2; anakinra, interleukin-1 receptor antagonist protein (IRAP), and in enzyme replacement therapy, i.e. algasidase (alpha-galactosidase) and laronidase (alpha-l-iduronidase). New receptor classes are emerging, e.g. membrane aminopeptidases, and novel concepts are stimulating drug research, e.g. epigenetic therapy, but the molecular target of some approved drugs, such as paracetamol and imidazolines, still need to be identified.

More types than one: multiple muscarinic receptor coupled K+ currents undergo remodelling in an experimental model of atrial fibrillation

The common cardiac arrhythmia atrial fibrillation (AF) tends to show progression in its severity, which is associated with 'remodelling': structural and electrophysiological changes that facilitate arrhythmia induction and maintenance. In this issue of the BJP, Yeh and colleagues demonstrate for the first time, down-regulation of three types of muscarinic cholinergic receptor (mAChR) coupled K+ currents (IKM2, IKM3 and IKM4) and of M2, M3 and M4 mAChR subtype proteins, in a canine model of atrial tachycardia (AT) induced remodelling. The IKMs and their extent of AT-induced remodelling were similar in left-atrial and pulmonary vein (PV) myocytes, so remodelling of M2-M4 receptor-linked currents appears not to underlie the unique contribution of PVs to AF. Parasympathetic stimulation can increase susceptibility to AF; thus remodelling of M2-M4 receptors and K+ currents could be adaptive in AT. Further work is warranted to determine whether or not remodelling of multiple mAChRs and currents also contributes to human AF.

Atrial tachycardia induces remodelling of muscarinic receptors and their coupled potassium currents in canine left atrial and pulmonary vein cardiomyocytes

BACKGROUND AND PURPOSE

Both parasympathetic tone and atrial tachycardia (AT) remodelling of ion channels play important roles in atrial fibrillation (AF) pathophysiology. Different muscarinic cholinergic receptor (mAChR) subtypes (M2, M3, M4) in atrial cardiomyocytes are coupled to distinct K+-currents (called IKM2, IKM3, IKM4, respectively). Pulmonary veins (PVs) are important in AF and differential cholinergic current responses are a potential underlying mechanism. This study investigated AT-induced remodelling of mAChR subtypes and K+-currents in left-atrial (LA) and PV cardiomyocytes.

EXPERIMENTAL APPROACH

Receptor expression was assayed by western blot. IKM2, IKM3 and IKM4 were recorded with whole-cell patch-clamp in LA and PV cardiomyocytes of nonpaced control dogs and dogs after 7 days of AT-pacing (400 bpm).

KEY RESULTS

Current densities of IKM2, IKM3 and IKM4 were significantly reduced by AT-pacing in LA and PV cardiomyocytes. PV cardiomyocyte current-voltage relations were similar to LA for all three cholinergic currents, both in control and AT remodelling. Membrane-protein expression levels corresponding to M2, M3 and M4 subtypes were decreased significantly (by about 50%) after AT pacing. Agonist concentration-response relations for all three currents were unaffected by AT pacing.

CONCLUSIONS AND IMPLICATIONS

AT downregulated all three mAChR-coupled K+-current subtypes, along with corresponding mAChR protein expression. These changes in cholinergic receptor-coupled function may play a role in AF pathophysiology. Cholinergic receptor-coupled K+-currents in PV cardiomyocytes were similar to those in LA under control and AT-pacing conditions, suggesting that differential cholinergic current properties do not explain the role of PVs in AF.

Estrogen modulates in vitro atrial bradycardia induced by Indian red scorpion venom via G-protein coupled mechanisms

Role of estrogen on cardiac dysrhythmia produced by Indian red scorpion (Mesobuthus tamulus) venom was examined using rat right atrial preparations in vitro. In females, the M. tamulus venom produced an increase, a decrease and an increase in rate at 0.03, 0.3 and 3 microg/ml of venom, respectively, producing N-shaped response curve, whereas no such response pattern was observed in males. Force of contraction in females was increased at all the concentrations of the venom, while in males the increase was seen only at 3 microg/ml. Castration of male rats did not alter the venom response to female type, while 'estrogenisation of castrated male rats' (pseudofemales) produced a response similar to females. Tamoxifen reversed the venom-induced responses both in females and pseudofemales. Further in females, the venom action at 0.3 microg/ml was blocked by atropine. Response at this concentration was also blocked by pertussis toxin and methylene blue. Results suggest that the cholinergic component of venom response is modulated by estrogen receptors via G(i)-protein-guanylyl cyclase mechanism.

Cardiac activity and blood pressure in rats during selective blockade of various subtypes of muscarinic cholinoceptors

Cardiac activity and blood pressure in adult rats were recorded during selective blockade of cholinoceptors. Blockade of muscarinic M1 and M2-cholinoceptors had little effect on cardiac activity. Blockade of muscarinic M3-cholinoceptors was followed by heart acceleration. The data suggest that the tonic inhibitory influence of the vagus nerve is mediated via cardiac muscarinic M3-cholinoceptors. Electrostimulation of the right vagus nerve during selective blockade of various subtypes of muscarinic cholinoceptors was followed by the decrease in heart rate. Our results indicate that muscarinic cholinoceptors play a role in the immediate inhibition of cardiac activity upon vagus nerve stimulation.

Ischemia impairs the association between connexin 43 and M3 subtype of acetylcholine muscarinic receptor (M3-mAChR) in ventricular myocytes

We used Western blot analysis to examine the expression of connexin 43 and M2/M3 acetylcholine muscarinic receptors (mAChR) and their interaction in ventricular myocytes from control and the ischemic heart. We confirmed that the connexin 43 and M2/ M3-mAChR were expressed in ventricular myocytes. Moreover, we showed that M3-mAChR was expressed in non-glycosylated (72 kDa) and glycosylated forms (115 kDa). Immunostaining showed that connexin 43 is closely associated with M3-mAChR in parts of cell membranes of myocytes. Immunoprecipitation of lysate of cardiac myocytes with M2/M3-mAChR antibody pulled down a 44 kDa protein recognized by connexin 43 antibody. Ischemia increased the expression of M3-mAChR in myocytes. The ischemiainduced increase in the M3-mAChR expression was specific because ischemia did not affect the expression of M1, M2, M4 and M5- mAChR in the heart. On the other hand, ischemia decreased the expression of connexin 43 in myocardium. We also examined the effect of ischemia on the interaction between M2/M3-mAChR and connexin 43. Ischemia suppressed the association of M3-mAChR with connexin 43 but did not affect the association of connexin 43 with M2-mAChR. Administration of choline before ischemia not only partially restored the expression of connexin 43 but also attenuated the ischemia-induced suppression of the association between connexin 43 and M3-mAChR. We conclude that connexin 43 interacts with M2/M3-mAChR and that ischemia specifically impairs the association between M3-mAChR and connexin 43.

Choline produces cytoprotective effects against ischemic myocardial injuries: evidence for the role of cardiac m3 subtype muscarinic acetylcholine receptors

BACKGROUND/AIMS

Accumulating evidence indicates the presence of functional M3 subtype of acetylcholine muscarinic receptors (M(3)-mAChR), in addition to the well-recognized M(2)-mAChR, in the heart of various species including man. However, the pathophysiological role of the cardiac M(3)-mAChR remain undefined. This study was designed to explore the possible role of M(3)-mAChR in cytoprotection of myocardial infarction and several related signaling pathways as potential mechanisms.

METHODS

Studies were performed in a rat model of myocardial infarction and in isolated myocytes.

RESULTS

We found that choline relieved myocardial injuries during ischemia or under oxidative stress, which was achieved by correcting hemodynamic impairment, diminishing ventricular arrhythmias and protecting cardiomyocytes from apoptotic death. The beneficial effects of choline were reversed by the M(3)-selective antagonists but not by the M(2)-selective antagonist. Choline/M(3)-mAChR activated several survival signaling molecules (antiapoptotic proteins Bcl-2 and ERKs), increased endogenous antioxidant reserve (SOD), and reduced apoptotic mediators (proapoptotic proteins Fas and p38 MAPK) and intracellular Ca2+ overload.

CONCLUSION

Choline improves cardiac function and reduces ischemic myocardial injuries via stimulating the cardiac M(3)-mAChRs which in turn result in alterations of multiple signaling pathways leading to cytoprotection. The findings suggest M(3)-mAChR as a new target for drug development for improving cardiac function and preventing cardiac injuries during ischemia/reperfusion.