Vagal Stimulation of the Heartbeat: Muscarinic Receptor Hypothesis

Excitatory effect of M1 muscarinic acetylcholine receptor on automaticity of mouse heart

We have investigated the effects of relatively high concentration of carbachol (CCh), an agonist of muscarinic acetylcholine receptor (mAChR), on cardiac automaticity in mouse heart. Action potentials from automatically beating right atria of mice were measured with conventional microelectrodes. When atria were treated with 100 microM CCh, atrial beating was immediately arrested and diastolic membrane potential (DMP) was depolarized. After exposure of the atria to CCh for approximately 4 min, action potentials were regenerated. The regenerated action potentials had lower frequency and shorter duration when compared with the control. When atria were pre-exposed to pirenzepine (1 microM), an M1 mAChR antagonist, there was complete inhibition of CCh-induced depolarization of DMP and regeneration of action potentials. Pre-exposure to AFDX-116 (11 ({2-[(diethylamino)-methyl]-1 -piperidyl}acetyl)-5,11 -dihydro-6H-pyridol[2,3-b][1,4] benzodiazepine-6-one base, 1 microM), an M2 mAChR antagonist, failed to block CCh-induced arrest of the beating. However, prolonged exposure to CCh elicited gradual depolarization of DMP and slight acceleration in beating rate. Our data indicate that high concentration of CCh depolarizes membrane potential and recovers right atrial automaticity via M1 mAChR, providing functional evidence for the role of M1 mAChR in the atrial myocytes.

Edrophonium-Induced Right Ventricular Outflow Tract Tachycardia

Idiopathic right ventricular outflow tract-ventricular tachycardia (RVOT-VT) generally occurs when sympathetic nervous system activity is increased, though, in a few patients, it develops when parasympathetic nervous activity (PNA) is increased. Among 101 consecutive patients with RVOT-VT confirmed by endocardial catheter mapping, 5 (4.9%) presented with nocturnal RVOT-VT. Autonomic nervous balance was studied by heart rate variability (HRV) analysis from 24-hour ambulatory electrocardiogram (ECG). Standard programmed ventricular stimulation (PVS), ventricular burst pacing, and drug provocation were performed to induce RVOT-VT. In the studied five patients, the average number of mostly nocturnal ventricular premature contractions (VPCs) was 6649 +/- 4472/day. Two patients had nocturnal nonsustained RVOT-VT on 24-hour ambulatory ECG recordings. The HRV analysis revealed that a progressive increase in high-frequency power coincided with an increase in VPCs or development of RVOT-VT at night, whereas low/high frequency ratio did not change significantly during the 24-hour period. RVOT-VT could not be induced by PVS, ventricular burst pacing, or isoproterenol or adenosine triphosphate i.v. However, RVOT-VT could only be induced by edrophonium, 5 mg i.v., in all patients. An increase in PNA was observed in a few patients before the development of RVOT-VT. Edrophonium facilitated induction of RVOT-VT in such patients.

Altered cardiovascular responses in mice lacking the M(1) muscarinic acetylcholine receptor

Although the M(2) muscarinic acetylcholine receptor (mAChR) is the predominant functional mAChR subtype in the heart, some responses of the cardiovascular system to acetylcholine (ACh) may be mediated by other mAChR subtypes. The potential effect of M(1) mAChR on heart function was investigated using M(1) knockout (M(1)-KO) mice. In vivo cardiodynamic analysis showed that basal values of heart rate (HR), developed left ventricular pressure (DLVP), left ventricular dP/dt(max) (LV dP/dt(max)), and mean blood pressure (MBP) were similar between wild-type (WT) and M(1)-KO mice. Injection of the putative M(1)-selective agonist 4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium (McN-A-343) produced an increase in LV dP/dt(max), DLVP, HR, and MBP in WT mice but did not affect hemodynamic function in the M(1)-KO mice. The stimulatory effect of McN-A-343 in WT mice was blocked by pretreatment with propranolol, indicating that stimulation of the M(1) mAChRs on sympathetic postganglionic neurons evoked release of catecholamines. Intravenous injection of ACh in both WT and M(1)-KO mice caused atrioventricular conduction block, without a significant change in the frequency of atrial depolarization, or atrial fibrillation. Immunoprecipitation and reverse transcriptase-polymerase chain reaction failed to detect the expression of M(1) mAChR in cardiac tissue from WT mice. The carbachol-induced increase of phospholipase C activity in cardiac tissues was not different between WT and M(1)-KO mice. These results demonstrate that 1) activation of M(1) mAChR subtype on sympathetic postganglionic cells results in catecholamine-mediated cardiac stimulation, 2) M(1) mAChR is not expressed in mouse heart, and 3) administration of ACh to mice induces arrhythmia.

Expression of multiple subtypes of muscarinic receptors and cellular distribution in the human heart

Five isoforms of the muscarinic acetylcholine receptor (mAChR) have been identified by molecular cloning and designated m(1)-m(5), of which four correspond to the functional subtypes M(1), M(2), M(3), and M(4) in primary tissues. The presence of M(5) receptors in tissues remains uncertain. The present study was designed to explore the diversity and cellular distribution of various mAChR subtypes in human hearts. Competition binding of [N-methyl-(3)H]-scopolamine methyl chloride with various mAChR antagonists yielded data consistent with the presence of multiple subtypes (M(1)/M(2)/M(3)/M(5)) of mAChRs in both human atrial (HA) and ventricular (HV) tissues. Expression of mRNAs encoding all five subtypes was readily detected by reverse transcription-polymerase chain reaction in both HA and HV samples. Immunoblotting with subtype-specific antibodies confirmed the presence of M(1), M(2), M(3), and M(5), but not M(4), proteins in membrane preparations from both HA and HV. The protein levels of M(1) and M(2) were comparable between HA and HV. Although the density of M(3) appeared approximately 10-fold higher in HV than HA, that of M(5) was approximately 5 times lower in HV than in HA. Positive immunostaining of single ventricular myocytes by M(1), M(2), M(3), and M(5) antibodies, respectively, was consistently detected. Under confocal microscopy, M(5) showed characteristic localization to the intercalated discs, whereas other subtypes were more evenly distributed throughout the surface membrane. Our results provide the first molecular evidence for the presence of multiple subtypes of mAChR, including endogenous M(5) receptors, in human hearts and suggest that different subtypes have different tissue distributions and cellular localization.

Electrically induced intracellular Ca2+ transient in single ventricular myocytes: a useful parameter for the study of cardiac drugs

1. Fluorescent Ca2+ indicators, such as fura-2/AM and calcium green-1, have become one of the most popular tools for measuring intracellular calcium ([Ca2+]i). 2. Electrical stimulation triggers a cascade of events in the cardiac muscle, which results in a [Ca2+]i transient and, eventually, contraction. The events that occur in electrically induced cardiac myocytes mimic the normal physiological events in vivo. 3. The electrically induced [Ca2+]i transient represents influx of Ca2+ from outside and mobilization of Ca2+ from the intracellular store and is directly related to contraction. Thus, it is more important to determine the electrically induced [Ca2+]i transient than [Ca2+]i. The [Ca2+]i transient can be easily measured with the spectrofluorescence method using fura 2/AM as the Ca2+ indicator in a single ventricular myocyte preparation. 4. We made use of the results of studies on carbachol, tetrandrine and cardiotoxin to illustrate the usefulness of the electrically induced [Ca2+]i transient in the study of actions of cardiac drugs.

Signaling mechanisms underlying muscarinic receptor-mediated increase in contraction rate in cultured heart cells

We have investigated the mechanisms by which stimulation of cardiac muscarinic receptors result in paradoxical stimulatory effects on cardiac function, using cultured neonatal rat ventricular myocytes as a model system. Application of low concentrations of carbachol (CCh) (EC50 = 35 nM) produced an atropine-sensitive decrease in spontaneous contraction rate, while, in cells pretreated with pertussis toxin, higher concentrations of CCh (EC50 = 26 microM) elicited an atropine-sensitive increase in contraction rate. Oxotremorine, an m2 muscarinic acetylcholine receptor (mAChR) agonist, mimicked the negative but not the positive chronotropic response to CCh. Reverse transcription followed by polymerase chain reaction carried out on mRNA obtained from single cells indicated that ventricular myocytes express mRNA for the m1, m2, and, possibly, m4 mAChRs. The presence of m1 and m2 mAChR protein on the surface membranes of the cultured ventricular myocytes was confirmed by immunofluorescence. The CCh-induced positive chronotropic response was significantly inhibited by fluorescein-tagged antisense oligonucleotides directed against the m1, but not the m2 and m4, mAChR subtypes. The response was also inhibited by antisense oligonucleotides against Gqalpha protein. Finally, inhibition of CCh-induced phosphoinositide hydrolysis with 500 microM neomycin or 5 microM U73122 completely abolished the CCh-induced positive chronotropic response. These results are consistent with the stimulatory effects of mAChR activation on the rate of contractions in cultured ventricular myocytes being mediated through the m1 mAChR coupled through Gq to phospholipase C-induced phosphoinositide hydrolysis.

Deferoxamine blocks interactions of fluoride and carbachol in isolated mammalian cardiac preparations

In papillary muscles, carbachol reduced the positive inotropic effects of isoprenaline (10 nmol/l). The negative inotropic effects of carbachol in isoprenaline-stimulated guinea pig papillary muscles were attenuated by additionally applied sodium fluoride (3 mmol/l). These effects of sodium fluoride were blocked by deferoxamine (200 micromol/l). In guinea pig left atria, sodium fluoride alone greatly reduced force of contraction. These effects in atria were blocked by 200 micromol/l deferoxamine, and positive inotropic effects of sodium fluoride were observed. It is suggested that the cardiac effects of muscarinic M2 receptor agonists in the ventricle involve, at least in part, the activation of phosphatases which are blocked by fluoride and reactivated by deferoxamine.

Muscarinic M1 receptor activation reduces maximum upstroke velocity of action potential in mouse right atria

We investigated whether acetylcholine affects cardiac action potentials through the muscarinic M1 in addition to M2 receptors in spontaneously beating mouse isolated right atria. A conventional glass microelectrode technique was used for the purpose. Acetylcholine (3-10 microM) reduced the maximum upstroke velocity of the action potentials (Vmax), followed by an increase. It shortened action potential duration at 90% repolarization, hyperpolarized the resting membrane and decreased the rate of beating. Atropine (3-100 nM) concentration dependently antagonized these effects of acetylcholine. Pirenzepine (10 and 30 nM), a selective muscarinic M1 receptor antagonist, antagonized acetylcholine (5 microM)-induced reduction of Vmax without affecting other effects of acetylcholine. In addition, pirenzepine (30 nM) induced an immediate and linear acceleration of the VmaX reduced by acetylcholine. In contrast, AF-DX 116 (11(¿2-[(diethylamino)-methyl]-1-piperidyl¿acetyl)-5,11-dihydro-6 H-pyridol[2,3-b][1,4]benzodiazepine-6-one base, 30-300 nM), a selective muscarinic M2 receptor antagonist, failed to antagonize acetylcholine-induced reduction of Vmax, but abolished its increase. It antagonized the shortening of action potential duration, membrane hyperpolarization and decreased the beating rate. McN-A-343 (4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium chloride, 100 and 300 microM), a muscarinic M1 receptor agonist, reduced Vmax and prolonged action potential duration, while oxotremorine (100-300 nM), a muscarinic M2 receptor agonist, evoked reverse effects. These results suggest that acetylcholine exerts a mixed effect on Vmax, consisting of a reduction and a facilitation, possibly mediated by concurrent activation of muscarinic M1 and M2 receptors, respectively, in isolated right atria of mice.

Effect of phospholipase A2 activation on the receptor function in the rat left atrium: unmasking of a positive inotropic effect of methacholine

1. To elucidate the functional consequence of endogenous phospholipase A2 activation, the effect of pretreatment with melittin on (-)-isoprenaline, forskolin and methacholine inotropic responses in isolated rat left atria was studied. 2. Melittin pretreatment resulted in a significant decrease of the positive inotropic response of (-)-isoprenaline, whereas no significant change in the forskolin response was seen. 3. A more prominent negative inotropic effect of methacholine could be perceived after melittin pretreatment. These effects of melittin were not simply reversed by mepacrine, a phospholipase A2 inhibitor. 4. After combined mepacrine/melittin pretreatment, a positive inotropic response was elicited by methacholine, which could be blocked by atropine but not by (+/-)-propranolol. 5. It is suggested that the combination of mepacrine/melittin abolishes the activity of an inhibitory G protein.

High carbachol increases the electrically induced [Ca2+]i transient in the single isolated ventricular myocyte of rats

In order to investigate the mechanisms responsible for the inotropic effects of muscarinic acetylcholine receptor stimulation by high concentrations of muscarinic receptor agonists, we studied the effects of carbachol at 30-300 microM on the electrically induced [Ca2+]i transient of rat isolated ventricular myocytes. Carbachol at this dose range increased the amplitude and duration of the electrically induced [Ca2+]i transient time and dose dependently. It also increased the resting fluorescence ratio and time to 80% decline of amplitude from the peak. At 100-300 microM the increase in [Ca2+]i transient was followed by a cluster of Ca2+ oscillations in 50-83% of the cells studied. The effects were blocked by atropine, but not pertussis toxin. Depletion of Ca2+ from sarcoplasmic reticulum by ryanodine, which itself reduced the amplitude of the [Ca2+]i transient and increase resting fluorescence, abolished the effect of carbachol on the [Ca2+]i transient without affecting its effect on resting fluorescence ratio. The caffeine-induced [Ca2+]i transient was unaffected by prior addition of carbachol in a Ca2+ free and low Na+ solution. Inhibition of Ca2+ by the L-type Ca2+ channel blocker, verapamil, which itself reduced the amplitude of the [Ca2+]i transient without affecting the resting fluorescence ratio, attenuated the augmentation of the amplitude of the [Ca2+]i transient elicited by carbachol. Ni2+, a non-specific Ca2+ channel blocker and an inhibitor of Na(+)-Ca2+ exchange, abolished the effects of carbachol on both [Ca2+]i transient and resting fluorescence ratio. Low external Na+, which increased the resting fluorescence ratio due to its inhibitory effect on Na(+)-Ca2+ exchange, also abolished the effects of carbachol. The results indicate that the inotropic effect of muscarinic acetylcholine receptor stimulation by high concentrations of a muscarinic receptor agonist may be due to an increase in the electrically induced [Ca2+]i transient in ventricular myocytes via a process which is not pertussis toxin sensitive. The increase in the electrically induced [Ca2+]i transient may result from increases in Na2(+)-Ca2+ exchange and influx of Ca2+ via voltage-gated Ca2+ channels, and mobilization of Ca2+ from the intracellular store. The mobilization of Ca2+ from the intracellular store is a secondary event. The study has provided for the first time that muscarinic acetylcholine receptor stimulation by high concentrations of carbachol increases Ca2+ influx via the Ca2+ channel and mobilization of Ca2+ from its intracellular store. The study has also demonstrated for the first time the occurrence of Ca2+ oscillations induced by high concentrations of carbachol.

Does mammalian heart contain only the M2 muscarinic receptor subtype?

Five muscarinic acetylcholine receptor (mAChR) subtypes, m1-m5, have been cloned and sequenced to date. The question as to which mAChR subtypes exist in mammalian heart has been studied extensively and is still under considerable debate. We used the reverse transcriptase-polymerase chain reaction to amplify mRNA from adult rat ventricular myocytes, and found that these cells express mRNA for m1 and m2 mAChRs. Immunocytochemical analysis confirmed that m1 and m2, but not m3, mAChR proteins are present on the surface of these cells. Finally, the functional significance of these receptors was examined. Administration of the m1 mAChR antagonist pirenzepine inhibited the stimulatory effect of the muscarinic agonist carbachol on Ca transients. These findings are consistent with the presence of at least two mAChR subtypes in mammalian heart, m1 and m2, and suggest that activation of m1 mAChRs is involved in the stimulatory effects of muscarinic agonists in mammalian heart.

Molecular and functional identification of m1 muscarinic acetylcholine receptors in rat ventricular myocytes

The expression of muscarinic acetylcholine receptor (mAChR) subtypes in freshly isolated adult rat ventricular myocytes was investigated by reverse transcription of cellular mRNA followed by amplification of cDNA using the polymerase chain reaction (PCR). After reverse-transcriptase PCR, bands were obtained corresponding to the expected sizes for the m1 and m2 but not for the m3 to m5 mAChRs. The identity of the m1 and m2 bands was confirmed by single-cell PCR, restriction digest mapping, and Southern blot analysis. The presence of m1 and m2, but not m3, mAChR protein in these cells was shown by indirect immunofluorescence studies using subtype-specific antibodies. It was further investigated whether the identified m1 mAChR was responsible for the stimulatory effects on Ca2+ transients by high concentrations of carbachol ( > 10 mumol/L) known to occur in these cells. In pertussis toxin-treated ventricular myocytes electrically stimulated at 1 Hz, carbachol (300 mumol/L) increased the basal Ca2+ level from 96 +/- 7 to 118 +/- 8 nmol/L and the peak Ca2+ transient level from 519 +/- 32 to 640 +/- 36 nmol/L (mean +/- SEM P < .05 for both, n = 8). These effects of carbachol on Ca2+ transients were antagonized by 10 nmol/L pirenzepine, an m1 mAChR-selective antagonist. In contrast, the m2 mAChR-selective antagonist methoctramine (up to 100 nmol/L) did not inhibit the response. These results are the first to use single-cell PCR to probe cardiomyocyte-specific gene expression and indicate that m1 mAChRs are expressed on adult rat ventricular myocytes in addition to m2 mAChRs. The results further suggest that m1 mAChRs mediate the stimulatory responses on Ca2+ transients to high concentrations of cholinergic agonists seen in these cells.

Muscarinic receptor heterogeneity in neonatal rat ventricular myocytes in culture

Carbachol increased ventricular automaticity in a concentration-dependent fashion from a control rate of 72 +/- 5 (mean +/- SEM) to 86 +/- 4 beats per minute at 10(-4) M carbachol. Pirenzepine, an M1-selective antagonist, and AFDX 116, an M2-selective antagonist, both at 10(-7) M, did not block the carbachol-induced positive chronotropic response. In contrast, 10(-7) M HHSiD, an M3-selective antagonist, completely blocked the positive chronotropic effect of carbachol. Carbachol stimulated the accumulation of IP1 in a concentration-dependent manner at concentrations > or = 3 x 10(-6) M. AFDX 116 had no effect on carbachol-induced IP1 accumulation. HHSiD significantly inhibited IP1 accumulation at concentrations > or = 3 x 10(-8) M, while pirenzepine inhibited IP1 accumulation only at concentrations > or = 10(-5) M. McN A343 and methacholine, two muscarinic receptor agonists with minimal M2 activities, and carbachol did not alter basal cAMP concentration, but all three agonists significantly attenuated the increase in cAMP accumulation in response to isoproterenol. Carbachol inhibited isoproterenol-mediated cAMP accumulation at concentrations > or = 10(-7) M. AFDX 116, HHSiD, and pirenzepine blocked the carbachol-induced inhibition of isoproterenol-stimulated cAMP accumulation. At equimolar concentrations, the inhibitory effects of HHSiD and AFDX-116 were similar, while that of pirenzepine was much less. Pretreatment with pertussis toxin for 24 h did not prevent the carbachol-mediated positive chronotropic response or accumulation of IP1 but completely abolished the inhibition of isoproterenol-stimulated cAMP accumulation. These results indicate that (a) neonatal ventricular myocytes in culture have a heterogeneous population of muscarinic (M2 and M3) receptors, (b) the M3 receptor is coupled to pertussis toxin-sensitive and pertussis toxin-insensitive G proteins, (c) M3 receptor stimulation activates phosphoinositide hydrolysis and increases automaticity via a pertussis toxin-insensitive G protein-dependent pathway, and (d) both M2 and M3 receptors couple to pertussis toxin-sensitive G protein(s) to mediate the inhibition of intracellular cAMP accumulation in response to isoproterenol stimulation.

Sodium fluoride attenuates the negative inotropic effects of muscarinic M2 and adenosine receptor agonists

Sodium fluoride increased the force of contraction in isolated guinea-pig papillary muscles concentration dependently, starting at 3 mmol/1. Sodium fluoride inhibited phosphorylase phosphatase activity in homogenates from guinea pig hearts, starting at 1 mmol/1. The positive inotropic effect of 3 mmol/1 sodium fluoride was not accompanied by an increase in cAMP content in guinea-pig papillary muscles. In papillary muscles, carbachol or (-)-N(6)-phenylisopropyladenosine reduced the positive inotropic effect of isoprenaline (10 nmol/1) or the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (60 mu mol/1). These negative inotropic effects of carbachol and (-)-N(6)-phenylisopropyladenosine were attenuated by additional sodium fluoride (3 mmol/l). It is concluded that sodium fluoride can impair the signal transduction of muscarinic M2 (carbachol) and adenosine receptor (-)-N(6)-phenylisopropyladenosine) agonists. This effect of sodium fluoride could support the hypothesis that the cardiac effects of muscarinic M2 and adenosine receptor agonists involve, at least in part, the activation of phosphatases.

Muscarinic agonist-induced positive inotropic response in chick atria

Carbachol (10(-6)-3X10(-4)M) induces a positive inotropic response in paced, pertussis toxin-treated fibers which is atropine-sensitive and independent of endogenous catecholamines. At the same concentrations in atria from saline-treated chicks, carbachol's negative inotropic effect on the steady state contractions (SSC) is attenuated and the rested state contraction (RSC) is increased. The RSC and SSC in pertussis toxin-treated fibers are increased by carbachol (EC50 = 30 microM) indicating that repetitive electrical depolarization is not essential for the inotropic response. The inotropic response of the SSC is frequency-independent from 0.10-1.0 Hz; however it is decreased (approximately 50%) at a high frequency (3.0 Hz). In control untreated atrial muscle, carbachol (10(-4)M) selectively increases the early component of the RSC. The late component of the RSC, representing activation of transmembrane Ca2+ inward current, is not changed. Carbachol's positive inotropic effect is perhaps exerted by enhancing Ca2+ release and/or Ca2+ content of the sarcoplasmic reticulum. The ability of various muscarinic agonists to induce a positive inotropic response was: carbachol > acetylcholine > oxotremorine. This order correlates with the ability of these agents to induce a tetrodotoxin-resistant Na+ inward current that increases intracellular Na+ and to promote phosphatidylinositol hydrolysis. These data are consistent with the hypothesis that the carbachol-induced positive inotropic response may result from greater intracellular Ca2+ availability secondary to enhanced Na-Ca exchange. The greater Ca2+ availability, together with increased production of the Ca-mobilizing messenger, inositol 1,4,5-trisphosphate (InsP3), can exert a synergistic effect to regulate force generation.

Tissue-specific regulation of muscarinic acetylcholine receptor expression during embryonic development

We used solution hybridization, immunoprecipitation, and immunoblot analyses to examine the developmental expression of chicken m2 (cm2), cm3, and cm4 muscarinic acetylcholine receptor (mAChR) mRNA and protein in embryonic and post-hatched chick heart and retina in order to correlate developmental expression patterns with known physiological events. cm2 is the predominant mAChR subtype expressed in chick heart. cm3 and cm4 protein and mRNA expression is very low in chick heart, and cm3 expression is highest early in development. The decrease in cm3 expression correlates well with the developmental decrease in mAChR-mediated activation of phospholipase C. cm4 is the predominant mAChR subtype expressed in chick retina. The expression of both cm4 protein and mRNA is highest early in development and decreases as development progresses. cm2 and cm3 mAChR are expressed at approximately equivalent levels and have similar patterns of expression. The cm2 and cm3 protein levels increase throughout development, while cm2 and cm3 mRNA levels peak at embryonic day 15 and then decrease after hatching. Our data indicate that the three mAChR subtypes are differentially regulated in chick heart and retina and that the patterns of expression of mAChR may be important in the development and physiology of these tissues.

Computer simulation of the electrotonic modulation of pacemaker activity in the sinoatrial node by atrial muscle

Electrotonic interaction between the sinoatrial (SA) node and surrounding atrial muscle was investigated in a computer simulation using a modified Oxsoft HEART model (Oxsoft, Oxford, UK). When an SA node cell model was coupled to a passive atrial membrane model (RC circuit) with various coupling conductances (Gc), there was a Gc-dependent prolongation of spontaneous cycle length (SCL). At a sufficiently high value of Gc, the spontaneous activity was finally stopped. A nonlinear relationship between Gc and SCL was obtained, similar to that observed in experiments on rabbit SA node cells. When the muscarinic potassium current (iK,ACh) was activated in the SA node cell model, the coupling-induced inhibition of pacemaker activity was potentiated. Although coupling current and iK,ACh were additive, their effects on SCL were more than additive because of the nonlinear dependence of SCL on net current. A decrease in the input resistance of the atrial membrane model to stimulate the activation of iK,ACh in atrial muscle was also shown to potentiate the coupling-induced inhibition of SA node spontaneous activity.

Differential time course for desensitization to muscarinic effects on K+ and Ca2+ channels

The time course of muscarinic effects on K and Ca currents was investigated at 22-24 degrees C in guinea-pig atrial myocytes, using the whole-cell voltage clamp. At a holding potential of -40 or -50 mV, short exposures to 100 microM acetylcholine (ACh) or carbachol (CCh) reproducibly induced outward K currents (IK,ACh). During long exposures to these agonists, IK,ACh faded with time. In cells not dialysed with guanosine triphosphate (GTP), IK,ACh could dissipate completely following 15-20 min of agonist exposure. After agonist washout, lost sensitivity was not recovered. In cells dialysed with GTP (0.2-1 mM), IK,ACh still faded but normal sensitivity to agonists was restored with washout. Fade of IK,ACh was not prevented by intracellular heparin or dextran, excluding the involvement of either beta-adrenergic or muscarinic receptor kinase. IK,ACh induced by bethanechol or adenosine also faded, and subsequent CCh application after washout revealed a diminished response. Intracellular guanosine-5'-o-(3-thiotriphosphate (GTP gamma S) induced IK,ACh which faded, and subsequent exposure to CCh was without effect. Equally, after full desensitization with CCh, GTP gamma S failed to induce IK,ACh. The Ca current (ICa) was activated by voltage steps to 0 mV and increased with 1-3 microM isoproterenol. This increase could be reversed by CCh, even when IK,ACh had completely faded. Prolonged muscarinic agonist exposure sometimes also caused fade of the effect on ICa, which always occurred after loss of IK,ACh. The results show that desensitization is heterologous and may involve the guanine nucleotide-binding (G) protein. The differential desensitization to the effects on IK,ACh and ICa suggests the involvement of two different signalling pathways in the muscarinic control of K and Ca channels.