A comparison of an A1 adenosine receptor agonist (CVT-510) with diltiazem for slowing of AV nodal conduction in guinea-pig.
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1. The purpose of this study was to compare the pharmacological properties (i.e. the AV nodal depressant, vasodilator, and inotropic effects) of two AV nodal blocking agents belonging to different drug classes; a novel A1 adenosine receptor (A1 receptor) agonist, N-(3(R)-tetrahydrofuranyl)-6-aminopurine riboside (CVT-510), and the prototypical calcium channel blocker diltiazem. 2. In the atrial-paced isolated heart, CVT-510 was approximately 5 fold more potent to prolong the stimulus-to-His bundle (S-H interval), a measure of slowing AV nodal conduction (EC50 = 41 nM) than to increase coronary conductance (EC50 = 200 nM). At concentrations of CVT-510 (40 nM) and diltiazem (1 microM) that caused equal prolongation of S-H interval (approximately 10 ms), diltiazem, but not CVT-510, significantly reduced left ventricular developed pressure (LVP) and markedly increased coronary conductance. CVT-510 shortened atrial (EC50 = 73 nM) but not the ventricular monophasic action potentials (MAP). 3. In atrial-paced anaesthetized guinea-pigs, intravenous infusions of CVT-510 and diltiazem caused nearly equal prolongations of P-R interval. However, diltiazem, but not CVT-510, significantly reduced mean arterial blood pressure. 4. Both CVT-510 and diltiazem prolonged S-H interval, i.e., slowed AV nodal conduction. However, the A1 receptor-selective agonist CVT-510 did so without causing the negative inotropic, vasodilator, and hypotensive effects associated with diltiazem. Because CVT-510 did not affect the ventricular action potential, it is unlikely that this agonist will have a proarrythmic action in ventricular myocardium. (+info)
Identification and characterization of multiple subtypes of muscarinic acetylcholine receptors and their physiological functions in canine hearts.
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M2 receptors have long been believed to be the only functional subtype of muscarinic acetylcholine receptor (mAChR) in the heart, although recent studies have provided evidence for the presence of other subtypes. We performed a detailed study to clarify this issue. In the presence of tetramethylammonium (1 microM to 10 mM), a novel K+ current with both delayed rectifying and inward rectifying properties (IKTMA) was activated in single canine atrial myocytes. 4-Aminopyridine (0.05-2 mM) also induced a K+ current (IK4AP) with characteristics similar to but distinct from those of IKTMA. Both IKTMA and IK4AP were abolished by 1 microM atropine. IK4AP, but not IKTMA, was minimized by treatment with pertussis toxin. IKTMA was markedly decreased by 4-diphenylacetoxy-N-methylpiperidine methiodide (a selective antagonist for M3 subtype) but was not altered by pirenzepine (for M1), methoctramine (for M2), and tropicamide (for M4). Tropicamide substantially reduced IK4AP, but the antagonists for other mAChR subtypes had no effects on IK4AP. By comparison, IKACh (ACh-induced K+ current) was significantly depressed by methoctramine but was unaltered by other antagonists. Results from displacement binding of [methyl-3H]N-scopolamine methyl chloride with pirenzepine, methoctramine, 4-diphenylacetoxy-N-methylpiperidine methiodide, or tropicamide revealed the coexistence of multiple mAChR subtypes in canine atrium. Cloning of cDNA fragments and detection of mRNAs coding for M2, M3, and M4 provided further supporting evidence. Our results suggest that 1) multiple subtypes of mAChRs (M2/M3/M4) coexist in the dog heart and 2) different subtypes of mAChRs are coupled to different K+ channels. Our findings represent the first functional evidence for the physiological role of cardiac M3 and M4 receptors. (+info)
Regional differences in the recovery course of tachycardia-induced changes of atrial electrophysiological properties.
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BACKGROUND: Regional differences in recovery of tachycardia-induced changes of atrial electrophysiological properties have not been well studied. METHODS AND RESULTS: In the control group (5 dogs), atrial effective refractory period (AERP) and inducibility of atrial fibrillation (AF) were assessed before and every 4 hours for 48 hours after complete atrioventricular junction (AVJ) ablation with 8-week VVI pacing. In experimental group 1 (15 dogs), AERP and inducibility of AF were assessed before and after complete AVJ ablation with 8-week rapid right atrial (RA) pacing (780 bpm) and VVI pacing. In experimental group 2 (7 dogs), AERP and inducibility of AF were assessed before and after 8-week rapid left atrial (LA) pacing and VVI pacing. AERP and inducibility and duration of AF were obtained from 7 epicardial sites. In the control group, atrial electrophysiological properties obtained immediately and during 48-hour measurements after pacing did not show any change. In the 2 experimental groups, recovery of atrial electrophysiological properties included a progressive recovery of AERP shortening, recovery of AERP maladaptation, and decrease of duration and episodes of reinduced AF. However, recovery of shortening and maladaptation of AERP and inducibility of AF was slower at the LA than at the RA and Bachmann's bundle. CONCLUSIONS: The LA had a slower recovery of tachycardia-induced changes of atrial electrophysiological properties, and this might play a critical role in initiation of AF. (+info)
Rate-dependent blockade of a potassium current in human atrium by the antihistamine loratadine.
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The antihistamine loratadine is widely prescribed for the treatment of symptoms associated with allergies. Although generally believed to be free of adverse cardiac effects, there are a number of recent reports suggesting that loratadine use may be associated with arrhythmias, in particular atrial arrhythmias. Nothing is known regarding the potassium channel blocking properties of loratadine in human cardiac cells. Using the whole-cell patch clamp technique, the effects of loratadine on the transient outward K current (Ito), sustained current (Isus), and current measured at -100 mV (IK1 and Ins), the major inward and outward potassium currents present in human atrial myocytes, were examined in order to provide a possible molecular mechanism for the observed atrial arrhythmias reported with loratadine use. Loratadine rate-dependently inhibited Ito at therapeutic concentrations with 10 nM loratadine reducing Ito amplitude at a pacing rate of 2 Hz by 34.9+/-6.0%. In contrast, loratadine had no effect on either Isus or current measured at -100 mV. These results may provide a possible mechanism for the incidences of supraventricular arrhythmias reported with the use of loratadine. (+info)
Inhibitory effects of aprindine on the delayed rectifier K+ current and the muscarinic acetylcholine receptor-operated K+ current in guinea-pig atrial cells.
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In order to clarify the mechanisms by which the class Ib antiarrhythmic drug aprindine shows efficacy against atrial fibrillation (AF), we examined the effects of the drug on the repolarizing K+ currents in guinea-pig atrial cells by use of patch-clamp techniques. We also evaluated the effects of aprindine on experimental AF in isolated guinea-pig hearts. Aprindine (3 microM) inhibited the delayed rectifier K+ current (IK) with little influence on the inward rectifier K+ current (IK1) or the Ca2+ current. Electrophysiological analyses including the envelope of tails test revealed that aprindine preferentially inhibits IKr (rapidly activating component) but not IKs (slowly activating component). The muscarinic acetylcholine receptor-operated K+ current (IK.ACh) was activated by the extracellular application of carbachol (1 microM) or by the intracellular loading of GTPgammaS. Aprindine inhibited the carbachol- and GTPgammaS-induced IK.ACh with the IC50 values of 0.4 and 2.5 microM, respectively. In atrial cells stimulated at 0.2 Hz, aprindine (3 microM) per se prolonged the action potential duration (APD) by 50+/-4%. The drug also reversed the carbachol-induced action potential shortening in a concentration-dependent manner. In isolated hearts, perfusion of carbachol (1 microM) shortened monophasic action potential (MAP) and effective refractory period (ERP), and lowered atrial fibrillation threshold. Addition of aprindine (3 microM) inhibited the induction of AF by prolonging MAP and ERP. We conclude the efficacy of aprindine against AF may be at least in part explained by its inhibitory effects on IKr and IK.ACh. (+info)
Aryl propanolamines: comparison of activity at human beta3 receptors, rat beta3 receptors and rat atrial receptors mediating tachycardia.
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1. The in vitro activity of four aryl propanolamines was compared to two prototypic beta3 receptor agonists, CGP 12177 and CL316243 at the human beta3 receptor, the rat beta3 receptor in the stomach fundus and receptors mediating atrial tachycardia. 2. L-739,574 was the most potent (EC50 = 9 nM) and selective agonist at the human beta3 receptor with high maximal response (74% of the maximal response to isoproterenol). 3. A phenol-biaryl ether analogue possessed modest affinity for the human beta3 receptor (EC50 = 246 nM), but was highly efficacious with a maximal response 82% of the maximal response to isoproterenol. The other derivatives were intermediate in potency with low maximal responses. 4. These agonists at the human beta3 receptor did not activate the rat beta3 receptor in the rat stomach fundus. In fact, the aryl propanolamines (10(-6) M) inhibited CL316243-induced activation of the rat beta3 receptor. Thus, agonist activity at the human beta3 receptor translated into antagonist activity at the rat beta3 receptor. 5. L739,574 and the phenol biaryl ether increased heart rate via beta1 receptors. 6. Although CGP12177 produced atrial tachycardia, neither the indole sulphonamide nor biphenyl biaryl ether did, although both had high affinity for the human beta3 receptor. Thus, the atrial tachycardic receptor was not identical to the human beta3 receptor. 7. These studies (a) characterized four aryl propanolamines with high affinity at the human beta3 receptor, (b) found that they were antagonists at the rat beta3 receptor, an observation with profound implications for in vivo rat data, and (c) established that the rodent atrial non-beta1, beta2 or beta3 tachycardic receptor was also unrelated to the human beta3 receptor. (+info)
Altered kinetics of contraction of mouse atrial myocytes expressing ventricular myosin regulatory light chain.
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To investigate the role of myosin regulatory light chain isoforms as a determinant of the kinetics of cardiac contraction, unloaded shortening velocity was determined by the slack-test method in skinned wild-type murine atrial cells and transgenic cells expressing ventricular regulatory light chain (MLC2v). Transgenic mice were generated using a 4.5-kb fragment of the murine alpha-myosin heavy chain promoter to drive high levels of MLC2v expression in the atrium. Velocity of unloaded shortening was determined at 15 degrees C in maximally activating Ca2+ solution (pCa 4.5) containing (in mmol/l) 7 EGTA, 1 free Mg2+, 4 MgATP, 14.5 creatine phosphate, and 20 imidazole (ionic strength 180 mmol/l, pH 7.0). Compared with the wild type (n = 10), the unloaded shortening velocity of MLC2v-expressing transgenic murine atrial cells (n = 10) was significantly greater (3.88 +/- 1.19 vs. 2.51 +/- 1.08 muscle lengths/s, P < 0.05). These results provide evidence that myosin light chain 2 regulates cross-bridge cycling rate. The faster rate of cycling in the presence of MLC2v suggests that the MLC2v isoform may contribute to the greater power-generating capabilities of the ventricle compared with the atrium. (+info)
Mapping of atrial activation with a noncontact, multielectrode catheter in dogs.
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BACKGROUND: Endocardial mapping of sustained arrhythmias has traditionally been performed with a roving diagnostic catheter. Although this approach is adequate for many tachyarrhythmias, it has limitations. The purpose of this study was to evaluate a novel noncontact mapping system for assessing atrial tachyarrhythmias. METHODS AND RESULTS: The mapping system consists of a 9F multielectrode-array balloon catheter that has 64 active electrodes and ring electrodes for emitting a locator signal. The locator signal was used to construct a 3-dimensional right atrial map; it was independently validated and was highly accurate. Virtual electrograms were calculated at 3360 endocardial sites in the right atrium. We evaluated right atrial activation by positioning the balloon catheter in the mid right atrium via a femoral venous approach. Experiments were performed on 12 normal mongrel dogs. The mean correlation coefficient between contact and virtual electrograms was 0.80+/-0.12 during sinus rhythm. Fifty episodes of atrial flutter induced in 11 animals were evaluated. In the majority of experiments, complete or almost complete reentrant circuits could be identified within the right atrium. Mean correlation coefficient between virtual and contact electrograms was 0.85+/-0.17 in atrial flutter. One hundred fifty-six episodes of pacing-induced atrial fibrillation were evaluated in 11 animals. Several distinct patterns of right atrial activation were seen, including single-activation wave fronts and multiple simultaneous-activation wave fronts. Mean correlation coefficient between virtual and contact electrograms during atrial fibrillation was 0.81+/-0.18. The accuracy of electrogram reconstruction was lower at sites >4.0 cm from the balloon center and at sites with a high spatial complexity of electrical activation. CONCLUSIONS: This novel noncontact mapping system can evaluate conduction patterns during sinus rhythm, demonstrate reentry during atrial flutter, and describe right atrial activation during atrial fibrillation. The accuracy of electrogram reconstruction was good at sites <4.0 cm from the balloon center, and thus the system has the ability to perform high-resolution multisite mapping of atrial tachyarrhythmias in vivo. (+info)