Interaction between autonomic tone and the negative chronotropic effect of adenosine in humans

Prior studies have demonstrated that sympathetic tone may influence the effects of adenosine on His-Purkinje automaticity, and that enhanced vagal tone may influence its effects on the sinus node. However, the interaction between autonomic tone and the effects of adenosine on the sinus node in humans remains unknown. Therefore, this study was designed to investigate the interaction between different states of autonomic tone and the bradycardiac response of the sinus node to adenosine. In 11 patients without structural heart disease who underwent a clinically indicated electrophysiology procedure, the sinus cycle length was measured before and after a 12-mg bolus of adenosine in the baseline state, during an infusion of 2 mcg/min of isoproterenol, after the administration of 0.2 mg/kg of propranolol, and again after the administration of 0.04 mg/kg of atropine. Adenosine significantly lengthened the sinus cycle length in the baseline state (760 +/- 165 vs 909 +/- 188 ms, P < 0.05), during isoproterenol infusion (516 +/- 67 vs 766 +/- 146 ms, P < 0.05), after propranolol (850 +/- 153 vs 914 +/- 143 ms, P < 0.05) and after the combination of propranolol and atropine (662 +/- 76 vs 801 +/- 121 ms, P < 0.05). The degree of lengthening in sinus cycle length was significantly greater (P < 0.05) during isoproterenol infusion (253 +/- 157 ms, or 51% +/- 40%) than in the baseline state (149 +/- 85 ms, or 20% +/- 12%), after propranolol (68 +/- 53 ms, or 8% +/- 8%), and after propranolol and atropine (140 +/- 110 ms, or 21% +/- 18%). The negative chronotropic effect of adenosine is influenced by autonomic tone. The effect of adenosine on the sinus node is accentuated by beta-adrenergic stimulation and unaffected by beta-adrenergic blockade or combined beta-adrenergic and cholinergic blockade.

Identification and properties of ATP-sensitive potassium channels in myocytes from rabbit Purkinje fibres

OBJECTIVE

Our goal was to identify the ATP-sensitive potassium (KATP) channels in cardiac Purkinje cells and to document the functional properties that might distinguish them from KATP channels in other parts of the heart.

METHODS

Single Purkinje cells and ventricular myocytes were isolated from rabbit heart. Standard patch-clamp techniques were used to record action potential waveforms. and whole-cell and single-channel currents.

RESULTS

The KATP channel opener levcromakalim (10 microM) caused marked shortening of the Purkinje cell action potential. Under whole-cell voltage-clamp, levcromakalim induced an outward current, which was blocked by glibenclamide (5 microM), in both Purkinje cells and ventricular myocytes. Metabolic poisoning of Purkinje cells with NaCN and 2-deoxyglucose caused a significant shortening of the action potential (control 376 +/- 51 ms; 6 min NaCN/2-deoxyglucose 153 +/- 21 ms). This effect was reversed with the application of glibenclamide. Inside-out membrane patches from Purkinje cells showed unitary current fluctuations which were inhibited by cytoplasmic ATP with an IC50 of 119 microM and a Hill coefficient of 2.1. This reflects approximately five-fold lower sensitivity to ATP inhibition than for KATP channels from ventricular myocytes under the same conditions. The slope conductance of Purkinje cell KATP channels, with symmetric, 140 mM K+, was 60.1 +/- 2.0 pS (mean +/- SEM). Single-channel fluctuations showed mean open and closed times of 3.6 +/- 1.5 ms and 0.41 +/- 0.2 ms, respectively, at -60 mV and approximately 21 degrees C. At positive potentials. KATP channels exhibited weak inward rectification that was dependent on the concentration of internal Mg2+. Computer simulations, based on the above results, predict significant shortening of the Purkinje cell action potential via activation of KATP channels in the range 1-5 mM cytoplasmic ATP.

CONCLUSIONS

Purkinje cell KATP channels may represent a molecular isoform distinct from that present in ventricular myocytes. The presence of KATP channels in the Purkinje network suggests that they may have an important influence on cardiac rhythm and conduction during periods of ischemia.

Beta-estradiol acutely potentiates the depression of cardiac excitability by lidocaine and bupivacaine

Pregnancy is known to increase myocardial susceptibility to bupivacaine-induced cardiovascular collapse, and prolonged pretreatment of rabbits with high doses of progesterone potentiates bupivacaine's depression of the maximal rate of increase (Vmax) of the cardiac action potential. Short-term effects of progesterone are not detected in vitro, but other steroids elevated during pregnancy might be acutely active in this model. These experiments tested whether acute exposure to beta-estradiol potentiates local anesthetic/antiarrhythmic depression of Vmax and conduction velocity in rabbit cardiac tissue in vitro. Standard intracellular microelectrodes were used to measure electrophysiologic changes produced by beta-estradiol, local anesthetics, or both in dissected segments of heart containing the Purkinje fiber and ventricular muscle cells from ovariectomized rabbits. In tissues preincubated in beta-estradiol (3.3 nM), addition of bupivacaine (10.4 microM), or lidocaine (85.4 and 129 microM) decreased Vmax significantly more than in steroid-free Tyrode's (p<0.001). Alone, beta-estradiol had no effect on Vmax and depression of Vmax by the nonanesthetic Na+ channel blocker tetrodotoxin (TTX, 3 microM) was not potentiated by beta-estradiol. In preparations initially exposed to bupivacaine for 30 min, subsequent addition of beta-estradiol decreased Vmax further within 10 min (p<0.05). Bupivacaine's greater depression of Vmax at higher frequencies (3 Hz) was exaggerated by beta-estradiol. However, the rate-dependent slowing of conduction by bupivacaine was lessened or even reversed by beta-estradiol addition. Such rapid physiologic changes cannot be due to genomic actions by the hormone that take hours to manifest. Nor is the potentiation due to a general decrease in membrane excitability because the comparable inhibition by TTX is insensitive to estradiol. Because beta-estradiol potentiates the inhibition of myocardial excitability, but alleviates the slowing of impulse conduction between the Purkinje fiber and ventricular muscle produced by local anesthetics, the hormone must produce changes in more than one ionic conductance. Both pregnancy and conditions that abnormally alter levels of steroid hormones have ramifications for local anesthetic-induced cardiotoxicity and antiarrhythmic pharmacotherapeutics.

Electrophysiological characterization of BRL-32872 in canine Purkinje fiber and ventricular muscle. Effect on early after-depolarizations and repolarization dispersion

Amongst the different pharmacological approaches to the treatment of cardiac arrhythmias, compounds with multiple electrophysiological activities appear to exhibit a reduced adverse effect profile. BRL-32872 (N-(3,4-dimethoxyphenyl)-N-[3[[2-(3,4-dimethoxyphenyl) ethyl] propyl]-4-nitrobenzamide hydrochloride) is a typical example of an antiarrhythmic agent with combined K(+) and Ca(2+) blocking actions. In this study, we investigated the effects of BRL-32872 on early after-depolarizations and on dispersion of repolarization. Action potentials were recorded either in canine cardiac Purkinje fibers alone or in preparations containing both ventricular muscle and the attached Purkinje fibers. In Purkinje fibers, BRL-32872 (0. 3-10 microM) induced a bell-shaped concentration-dependent increase in action potential duration. At 90% of repolarization, the action potential was prolonged at concentrations up to 1 microM and was shortened when the concentration of BRL-32872 was further increased. In all 17 experiments, BRL-32872 did not cause early after-depolarizations in Purkinje fibers. On the contrary, BRL-32872 (3 microM) systematically suppressed early after-depolarizations induced by clofilium (4-chloro-N, N-diethyl-N-heptylbenzenebutanaminium tosylate, 1 microM), a selective inhibitor of the delayed rectifier K(+) current. A similar effect was observed once with 1 microM BRL-32872, a concentration able to prolong Purkinje fiber action potentials. Simultaneous recording of action potentials in ventricular and Purkinje preparations showed that increasing concentrations of BRL-32872 (0. 3-10 microM) induced a limited increase in the difference of repolarization time between the two tissues. The selective K(+) channel inhibitor E-4031 (N-(4-(1-[2-(6-methyl-2-pyridyl) ethyl]-4-piperidyl)-carbonyl] phenyl) methanesulfonamide dihydrochloride dihydrate) exhibited a significant concentration-dependent increase in dispersion of repolarization. We conclude from the present results that the Ca(2+) blocking activity of BRL-32872 (i) prevents the occurrence of early after-depolarizations associated with action potential prolongation and (ii) limits an excessive increase in action potential duration heterogeneity. These electrophysiological features might represent the basis for antiarrhythmic compounds with reduced proarrhythmic profile.

Assessment of atrioventricular nodal physiology in the mouse

Transgenic mice are increasingly being utilized for understanding cardiac electrophysiologic abnormalities. However, little is known about the normal atrioventricular nodal and infraHisian physiology in the mouse because of the prior inability to record a His-bundle deflection. We present the first comprehensive examination of the murine atrioventricular nodal and His-Purkinje systems employing His-bundle recordings. Normal, healthy, male C57BL/6J mice (n = 48) underwent an in vivo electrophysiology study using a 2 F octapolar electrode catheter. Effective refractory periods were determined during premature atrial and ventricular stimulation. The PR interval measured 44 +/- 6 ms with a mean sinus cycle length of 185 +/- 42 ms. Baseline AH intervals were 36 +/- 5 ms and HV intervals were 10 +/- 2 ms. At a pacing cycle length of 140 ms the atrioventricular nodal effective refractory period (AVNERP) and atrial effective refractory period (AERP) were 86 +/- 19 ms and 57 +/- 17 ms, respectively. The mean AV Wenckebach and 2:1 paced cycle length were 103 +/- 14 ms and 84 +/- 13 ms, respectively. Premature atrial stimulation curves were asymptotic without discontinuity. A subset of nine mice was studied after administration of isoproterenol. The sinus cycle length, AVNERP and AERP decreased significantly from baseline measurements. This method establishes a practical and feasible technique to record in vivo His-bundle electrograms in the mouse to assess atrioventricular nodal and infraHisian physiology. Use of this model will allow for the examination of abnormalities of atrioventricular nodal and infraHisian conduction in transgenic murine models.

Differences in the electrophysiologic response of four canine ventricular cell types to alpha 1-adrenergic agonists

OBJECTIVE

The present study was designed to examine regional differences in the response of alpha 1 adrenoceptor stimulation in the canine ventricle.

METHODS

Standard microelectrode techniques were used to record transmembrane action potentials from epicardial, M cell, and endocardial as well as Purkinje fiber preparations isolated from the canine left ventricle.

RESULTS

Phenylephrine (0.1-10 microM+ propranolol 0.2 microM) and methoxamine (1-10 microM) produced dose- and rate-dependent prolongation of action potential duration (APD90) in Purkinje fibers (P < 0.05, at 0.1-10 microM, BCL = 0.5-2 s), but an abbreviation of APD90 in tissues from the M region (P < 0.05, at 10 microM, BCL = 0.5-2 s). At slow pacing rates (> or = 2 s), phenylephrine (1 microM) exerted a small, significant (P < 0.05) prolongation of APD90 in epicardium and endocardium which returned to control values when the concentration was increased to 10 microM. The amplitude of phase 1 of the action potential in M and epicardial cells was significantly increased by phenylephrine at concentrations of 10 microM (P < 0.05). Prazosin (1 microM), a nonspecific alpha 1 antagonist, reversed these effects of phenylephrine (10 microM) and methoxamine (10 microM) on APD90 and the action potential notch. The alpha 1b-antagonist, chloroethylclonidine (0.1-1.0 microM), but not the alpha 1a-antagonist, WB-4101 (0.1-1.0 microM), reversed the APD-abbreviating effect of methoxamine in the M cell.

CONCLUSION

Our results demonstrate striking regional differences in the electrophysiological response of the four canine ventricular cell types to alpha 1 adrenergic agonists. Our data provide support for the hypothesis that different adrenoceptor subtypes underlie the opposite response of M cells (alpha 1b-APD abbreviation) and Purkinje fibers (alpha 1a-APD prolongation) to alpha 1-adrenoceptor activation.

Potassium channel openers antagonize the effects of class III antiarrhythmic agents in canine Purkinje fiber action potentials. Implications for prevention of proarrhythmia induced by class III agents

We studied the effects of potassium channel openers (PCOs) on frequency dependent prolongations of action potential duration (APD), triggered activities and oscillatory action potentials (OSC) induced by E-4031 and dofetilide. The action potentials of canine Purkinje fibers were recorded by a glass microelectrode technique. The effects of E-4031 (10(-6)M) as well as that of additional nicorandil (2 x 10(-5) M) on the APD were examined. When abnormal automaticity was observed under perfusion of E-4031 (10(-5) M) or dofetilide (10(-5) M), action potentials were recorded continuously to estimate the sequential effects of additional perfusion of nicorandil (6 x 10(-5) M) or Y-26763 (10(-5) M) on triggered activities and OSC. APD prolongation by E-4031 at slower stimulation rates (cycle lengths > or = 1,000 msec) was suppressed by nicorandil in a dose dependent manner. Both nicorandil and Y-26763 abolished the train of early afterdepolarization (EAD) due to E-4031 or dofetilide with a shifting of the resting membrane potential to a more negative level. PCOs also normalized dofetilide induced abnormal automaticities (EAD, OSC). The antagonistic actions of PCOs on changes in action potential induced by class III antiarrhythmic agents may prevent the development of proarrhythmias produced by these agents.

Electrophysiological effects of LU111995 on canine hearts: in vivo and in vitro studies

We studied the electrophysiological effects of LU111995 (1-15 mg/kg p.o.) in conscious dogs with chronic atrioventricular block and ventricular pacing at 50 to 130 beats/min. LU111995 had no effects on idioventricular rhythm, QRS duration, and ventricular conduction time. It significantly prolonged Q-T interval (by 5-8%) and effective refractory period (ERP) (by 5-12%) with the maximal effect at 4 h after a 10 mg/kg dose. At 10 and 15 mg/kg, it increased the ERP/Q-T ratio. In vitro, the effects of LU111995 (1 x 10(-7) to 1 x 10(-5) M) on action potentials of Purkinje fibers (PFs) and M cells were studied at cycle lengths (CL) of 300 to 2000 ms. It had no effects on maximum diastolic potential and action potential amplitude in either tissue. High concentrations induced a moderate, rate-independent decrease of Vmax in M cells. In PFs and M cells, it produced reverse use-dependent lengthening of action potential duration (APD). In PFs at long CL, the drug exhibited a biphasic concentration-dependent effect on APD: maximum prolongation (by 26% at a CL of 2000 ms) was attained at 1 x 10(-6) M, and a decrease of APD occurred at higher concentrations. In M cells, the maximum effect on APD occurred at 3 x 10(-6) M. Early afterdepolarizations were seen in 50% of M cell preparations but only at CL of 2000 ms. Triggered activity did not occur. In summary, LU111995 prolongs the Q-T interval to a limited degree and is not arrhythmogenic over the physiological range of CLs.

Muscarinic activation causes biphasic inotropic response and decreases cellular Na+ activity in canine cardiac Purkinje fibers

In this study, the effects of carbachol (CCh) on twitch tension, intracellular Na+ activity (aiNa), and action potential were simultaneously measured in canine cardiac Purkinje fibers in order to examine the regulation of inotropy through muscarinic receptors and its relation to aiNa. In fibers driven at 1 Hz, CCh (10 microM) initially and transiently decreased and then increased the twitch tension by 36 +/- 8%. The action potential showed a significant elevation of the plateau and a significant shortening of the duration at 90% repolarization (APD90), from 403 +/- 7 to 389 +/- 7 ms. The aiNa decreased from 7.4 +/- 0.4 to 6.7 +/- 0.3 mM (n = 23, p < 0.05). Atropine (1 microM) decreased the twitch tension by 21 +/- 6% (n = 7, p < 0.05) without significant effects on the action potential and aiNa, and inhibited the effects of CCh. Cs+ (20 mM) increased the plateau height and APD90, enhanced the twitch tension by 66 +/- 24%, but decreased aiNa from 7.3 +/- 0.3 to 6.3 +/- 0.4 mM (n = 6, p < 0.05). In the presence of 20 mM Cs+, some fibers generated slow responses. The addition of 10 microM CCh further increased the twitch tension and APD90, and decreased aiNa from 6.3 +/- 0.4 to 5.3 +/- 0.3 mM. Ouabain (0.3 microM) increased the twitch tension and aiNa, and inhibited the CCh-induced decrease of aiNa. In the presence of ouabain, 20 mM Cs+ depolarized the fiber and generated slow responses with a decreased aiNa. The addition of 10 microM CCh enhanced the slow action potential, and increased aiNa although there was a transient decrease during early exposure. These results suggest that activation of muscarinic receptors in canine Purkinje fibers results in an enhancement of the Na+-K+ pump activity and a biphasic inotropic response, probably via different receptor subtypes. The inhibitory effect, most likely through M2 receptors, is associated with the activation of K+ channels. The stimulatory effect, on the other hand, is probably due to the action on the M1 receptors, resulting in increases in Ca2+ currents.

Abnormal cardiac repolarization and impulse initiation in German shepherd dogs with inherited ventricular arrhythmias and sudden death

OBJECTIVE

We tested the hypothesis that delayed afterdepolarization (DAD)-associated rhythms in German shepherd dogs with reduced anteroseptal left ventricular (LV) sympathetic innervation derive from abnormal beta-adrenergic receptor effector coupling.

METHODS AND RESULTS

In anteroseptal LV midmyocardium of afflicted dogs, beta-receptor density was greater than that in normal dogs (P < .05), with affinity being equal in both groups. Basal and maximum isoproterenol (ISO) stimulated adenylyl cyclase activity of anteroseptal LV of afflicted dogs was greater than that in normal dogs (P < .05). Isolated anteroseptal M cell preparations of afflicted dogs studied with microelectrodes showed abnormal lengthening, rather than shortening of action potential duration in response to ISO, as well as a 61% incidence of 10(-7) mol/l ISO-induced triggered activity as compared to 12% in normals (P < .05). In contrast, there was no difference between afflicted and control dogs in triggered activity, beta-receptors or adenylyl cyclase activity in a normally innervated region of the ventricles.

CONCLUSION

In this model there is an increase in beta-receptor density and beta-adrenergic stimulation of adenylyl cyclase and of triggered activity in anteroseptal myocardium but not in a normally innervated region of the heart. Hence, abnormal beta-adrenergic signal transduction appears associated with the neural abnormality identified in dogs with inherited VT.

Proarrhythmic effects of antidepressants and neuroleptic drugs on isolated, spontaneously beating guinea-pig Purkinje fibers

Antidepressants and neuroleptic drugs are sometimes the reason for the occurrence of the polymorphic ventricular arrhythmia torsades de pointes in patients. Therefore, it was of interest to study the actions of some of these drugs such as imipramine, amitriptyline, doxepin, chlorpromazine, trifluoperazine and thioridazine in isolated, spontaneously beating Purkinje fibers of guinea-pig hearts using the intracellular microelectrode technique because experimentally induced early afterdepolarizations (EADs) may be associated with this special type of arrhythmia. If the extracellular K+ concentration was 2.7mM none of these drugs could elicit EADs. For that reason the K+ concentration was lowered to 1. 35mM and EADs were evoked by imipramine (2 and 5 microM). Amitriptyline (2 and 5 microM) and doxepin (2 microM) did not induce EADs. Only a concentration of 5 microM doxepin elicited EADs. Among the neuroleptic drugs, chlorpromazine at a concentration of 2 and 5 microM was responsible for the occurrence of EADs as well as thioridazine in the same concentrations. When trifluoperazine (2 and 5 microM) was applied no EADs could be observed. Tetrodotoxin (0. 2 microMl-1) abolished thioridazine-induced EADs. Several membrane depolarizing currents may participate in the initiation of these EADs. Our results demonstrate that in guinea-pig Purkinje fibers some tricyclic antidepressants and some neuroleptic drugs are responsible for the rare occurrence of EADs under hypokalemic conditions.

Electrophysiological effects of newly synthesized 1,4-pyridothiazepines on various guinea pig heart muscle preparations

Slow channel blockers play a major role in the treatment of cardiovascular disease. The intention of this study was to investigate the electrophysiological properties of MM 4 (1-[N-[2-(3,4-dimethoxy-phenyl)ethyl]-N-methylaminoacetyl]-1,2,3,4 -tetrahydropyrido[2,3-b][1,4]thiazepine fumarate) and MM 6 (1-[N-[2-(3,4-dimethoxy-phenyl)ethyl]-N-methylaminopropionyl]-1,2, 3,4-tetrahydropyrido[2,3-b][1,4]thiazepine fumarate), two newly synthesized compounds structurally related to KT-362 (5-[3-[[2-(3,4-dimethoxy-phenyl)ethyl]-amino]-1-oxopropyl]-2,3,4,5-tetra -hydro-1,5-benzothiazepine fumarate), by means of the conventional intracellular microelectrode technique. In various guinea pig heart muscle preparations, MM 4 and MM 6 exerted very similar effects though the action of MM 6 was more pronounced. In a concentration range from 3 to 100 micromol/l the compounds did not produce any significant change in transmembrane action potential parameters of papillary muscle and left atria, whereas the action potential duration at 20% and 50% time to repolarization in spontaneously beating Purkinje fibers was significantly shortened. In sinoatrial nodes action potential amplitude, Vmax, rate of activity and slope of slow diastolic depolarization were decreased, whereas the time to 50% and 90% repolarization was significantly prolonged. A decrease in the slow calcium inward current may account for the observed effects. In contrast to KT-362, MM 4 and MM 6 do not seem to affect the fast sodium inward current. It was concluded that replacement of the 1,5-benzothiazepine nucleus by a 1,4-pyridothiazepine structure and/or methylation of the side chain may weaken or even eliminate sodium channel blocking ability while calcium antagonistic characteristics are preserved. Shortening of the side chain might result in a general loss of activity.

[Electrophysiologic effects and arrhythmogenic potential of diphemanil methylsulfate on rabbit purkinje fibers. Correlations with clinical observations of QT prolongation in pediatrics]

Serious undesirable cardiac side effects have been reported with treatment with diphemanil methylsulfate (Prantal) in premature babies or neonates. To understand the origin of this problem, the authors undertook an electrophysiological study of the effects of this product in vitro on rabbit Purkinje fibres. In three separate series (N = 5 to N = 8), the effects of increasing concentrations (0.1 microM-30 microM) of diphemanil methylsulfate, different frequencies of stimulation (0.2 Hz, 1 Hz, 2 Hz) and duration of exposition (60 min followed by 120 min washout) were observed on the properties of the action potential. The results show a clearcut antiarrhythmic Class III type action characterised by a concentration-dependent prolongation of the action potential duration with an inverse frequency dependency without significant changes of the other parameters. During stimulation at 0.2 Hz, early post-depolarizations and induced activity were observed in 3/8 of the fibres exposed to 10 microM and 8/8 fibres exposed to 30 microM. The effect did not attain a steady state after 60 min of exposition. It was not reversed by 120 min of washout of the preparation. These results were compatible with the reported cardiac arrhythmic effects of prolongation of the QT interval and torsades de pointe.

Electrophysiologic effects of civamide (zucapsaicin) on canine cardiac tissue in vivo and in vitro

The cardiac electrophysiologic effects of civamide (zucapsaicin), the cis-isomer of the alkyl vanillylamide, capsaicin, were evaluated in intact dogs and isolated Purkinje fibers. In anesthetized dogs, the mechanism of ventricular tachycardia inducible from 1 to 3 h after coronary artery occlusion was determined by activation mapping. Of 16 dogs studied, nine had ventricular tachycardia of focal endocardial origin; four, a reentrant mechanism; and three had no inducible arrhythmia. Civamide (50 microg/kg) was administered to 10 of 13 dogs that were inducible, but three dogs were used as time controls. Transmural activation times were unaltered by civamide, but mean arterial pressure decreased from 76 +/- 10 to 66 +/- 10 mm Hg (p < 0.05), and muscle refractory periods shortened from 138 +/- 3 to 132 +/- 4 ms (p < 0.05). Civamide altered inducibility in five of six dogs with ventricular tachycardia of focal endocardial origin, but those with epicardial reentrant mechanisms were not affected in three of four dogs. With microelectrode techniques in vitro, civamide (10(-5) M) shortened the action-potential duration at 50% repolarization (APD50) from 193 +/- 13 to 177 +/- 12 ms (p < 0.01) and APD90 from 260 +/- 15 to 248 +/- 13 ms (p < 0.01) in isolated Purkinje fibers (n = 10). Nifedipine prevented the effects of civamide in vitro. These results show that civamide may alter inducibility of ventricular tachycardia with focal endocardial origin and shorten APD of Purkinje fibers in vitro. The effects of civamide in vitro are prevented by preexposure of the Purkinje fibers to nifedipine, suggesting that the electrophysiologic effects of civamide may be mediated through blockade of calcium channels.

Pharmacological stress testing of the His-Purkinje system in patients with bifascicular block

This literature review, based mainly on the English-language literature, focuses on pharmacological stress testing of the His-Purkinje system as part of an invasive electrophysiological study. The main target group for this investigation is patients with bifascicular block and syncope in which intermittent high grade AV block is suspected. Several drugs have been used for this purpose, mainly Class I antiarrhythmic agents such as ajmaline, procainamide, disopyramide, and flecainide. Most studies, unfortunately, suffer from limited patient numbers, lack of adequate control groups, and/or adequate follow-up. The sensitivity of the disopyramide stress test has been shown to be 75%-100% for prediction of impending high grade AV block. The specificity was > 90%. Studies on procainamide have shown a sensitivity of 60% but the specificity has not been assessed. There are no studies allowing a strict comparison of the diagnostic value of pharmacological provocation with different drugs. Based on the similarities of the electrophysiological effects on the His-Purkinje system of the above Class I agents, it is reasonable to assume that all of them might be of diagnostic value in the present clinical context, provided atrial and ventricular stimulation after drug is included in the protocol.

Comparison of the cellular electrophysiological characteristics of canine left ventricular epicardium, M cells, endocardium and Purkinje fibres

Electrophysiological differences among M cells, epicardium, endocardium and Purkinje fibres of the canine ventricle were studied over a wide range of stimulation cycle lengths, and the pharmacological response of these cell types to the sodium channel blocker tetrodotoxin, calcium channel blocker nifedipine and ATP-sensitive potassium channel activator pinacidil was compared. The experiments were carried out by applying standard intracellular microelectrode technique in isolated dog left ventricular preparations. The results confirmed the existence of M cells in the canine ventricle, in addition, the distribution of the rate of rise of the action potential upstroke and action potential amplitude values reflecting probably the inhomogeneity of the fast sodium current in these cells was revealed. It was also demonstrated that M cells differ from Purkinje fibres in some aspects which were not expected from previous investigations: (1) The early portion of the action potential duration restitution curve in M cells is more similar to that of endocardial and epicardial cells than to Purkinje fibres. (2) The plateau phase of the action potentials in Purkinje fibres developed at a more negative potential range than that in the other cell types studied. (3) The pharmacological response to tetrodotoxin and pinacidil in M cells resembles to that in the endocardial and epicardial cells more than in the Purkinje fibres. Our results provide further evidence in support of the existence of M cells but also indicate that there are important electrophysiological as well as pharmacological differences between M cells and Purkinje fibres.

Acceleration-induced action potential prolongation and early afterdepolarizations

INTRODUCTION

Precipitation of torsades de pointes (TdP) has been shown to be associated with acceleration of heart rate in both experimental and clinical studies. To gain insight into the cellular mechanism(s) responsible for the initiation of acceleration-induced TdP, we studied the effect of acceleration of pacing rate in canine left ventricular epicardial, M region, endocardial, and Purkinje fiber preparations pretreated with E-4031, an IKr blocker known to induce the long QT syndrome and TdP.

METHODS AND RESULTS

Standard microelectrode techniques were used. E-4031 (1 to 2 microM) induced early after depolarization (EAD) activity in 31 of 36 M cell, 0 of 10 epicardial, 0 of 10 endocardial, and 9 of 12 Purkinje fiber preparations at basic cycle lengths (BCLs) > or = 800 msec. In 30 of 36 M cells, sudden acceleration from a BCL range of 900 to 4,000 msec to a range of 500 to 1,500 msec induced transient EAD activity if none existed before or increased the amplitude of EADs if already present. Acceleration-induced augmentation of EAD activity was far less impressive and less readily demonstrable in Purkinje fibers (4/12). In M cells, appearance of EAD activity during acceleration usually was accompanied by an abbreviation of action potential duration (APD). Within discrete ranges of rates in the physiologic range, acceleration caused a transient prolongation of APD in 38% of M cells, whether or not a distinct EAD was generated. Acceleration produced still more dramatic APD prolongation and EADs in M cells after the BCL was returned to the original slow rate. Epicardium and endocardium APD showed little change immediately after acceleration. A decrease of BCL as small as 10% and, in some cases, a single premature beat could promote EAD activity and APD prolongation in some M cells. Ryanodine (1 microM, 10/10), flunarizine (10 microM, 3/6), and low Na (97 vs 129 mM, 5/5) abolished the acceleration-induced EAD activity and APD prolongation as well as the EAD activity observed at slow rates in M cells pretreated with E-4031.

CONCLUSION

Our results suggest that acceleration from an initially slow rate or a single premature beat can induce or facilitate transient EAD activity and APD prolongation in canine ventricular M cell preparations pretreated with an IKr blocker via a mechanism linked to intracellular calcium loading. Our data provide evidence in support of an important contribution of electrogenic Na/Ca exchange current to this process. These acceleration-induced changes can result in the development of triggered activity as well as a marked dispersion of repolarization in ventricular myocardium and, thus, may contribute to the precipitation of TdP in patients with the congenital (HERG defect) and acquired (drug-induced) long QT syndrome.

Action-potential duration and contractility in canine cardiac tissues: action of inotropic drugs

1. Inotropic and electrophysiologic effects of veratrine, vesnarinone, d-sotalol and tetraethylammonium (TEA) were compared. Action-potential duration (APD) and contractility were measured in isolated canine Purkinje fiber and ventricular trabecular muscle preparations by using standard microelectrode techniques. Each drug significantly increased APD and force development in either tissue. 2. Drug-induced increases in force development were normalized to increases in APD. The order of efficacy was found to be vesnarinone>veratrine>TEA in ventricular myocardium, whereas it was veratrine>>vesnarinone=d-sotalol=TEA in Purkinje fibers. 3. The force-APD relation was linear for all drugs in the concentrations used. 4. Simultaneous measurements of APD, force development and intracellular sodium ion activity (a(i)Na) in the presence of either veratrine or lidocaine indicated a linear relation between force development and changes in a(i)Na. 5. The relation between APD and force development was different in ventricular and Purkinje fiber preparations. Differences in the veratrine sensitivity of the force-APD relation observed between Purkinje and ventricular preparations suggest that a(i)Na-dependent changes in Na+/Ca2+ exchange may play a more important role in regulation of force generation in Purkinje fibers than in ventricular myocardium.

Mid-diastolic potential is related to the reentrant circuit in a patient with verapamil-sensitive idiopathic left ventricular tachycardia

We report a case of verapamil-sensitive idiopathic ventricular tachycardia in which a mid-diastolic potential (MDP) 45 msec preceding the Purkinje potential (P potential) was recorded. Pacing during the tachycardia caused concealed entrainment, and the stimulus-QRS interval was equal to the P potential-QRS interval. The interval between the last pacing stimulus and the next P potential (postpacing interval) was longer than the ventricular tachycardia cycle length, but the MDP was orthodromically activated. These findings suggest that the MDP was on the reentrant circuit and the P potential was not on the reentrant circuit, but a bystander.

Inhibition of neuronal calcium channels by a novel peptide spider toxin, DW13.3

Peptide toxins have proved to be useful agents, both in discriminating between different components of native calcium channel currents and in the molecular isolation and designation of their cloned channel counterparts. Here, we describe the isolation and characterization of the biochemical and physiological properties of a novel 74-amino acid peptide toxin (DW13.3) extracted from the venom of the spider Filistata hibernalis. The subtype specificity of DW13.3 was investigated using calcium channel currents recorded from two separate expression systems and several different cultured mammalian cell preparations. Overall, DW13.3 potently blocked all native calcium channel currents studied, with the exception of T-type currents recorded from GH3 cells. Examination of transiently expressed calcium channels in oocytes showed that DW13.3 had the highest affinity for alpha1A, followed by alpha1B > alpha1C > alpha1E. The affinity of DW13.3 for alpha1B N-type currents varied by 10-fold between expressed channels and native currents. Although block occurred in a similar 1:1 manner for all subtypes, DW13.3 produced a partial block of both alpha1A currents and P-type currents in cerebellar Purkinje cells. Selective occlusion of the P/Q-type channel ligand omega-conotoxin MVIIC (but not omega-agatoxin IVA) from its binding site in Purkinje neurons suggests that DW13.3 binds to a site close to the pore of the channel. The inhibition of different subtypes of calcium channels by DW13.3 reflects a common "macro" binding site present on all calcium channels except T-type.

Differential effects of d-sotalol on subendocardial Purkinje myocytes isolated from normal or 10 to 14 days postinfarction canine hearts: role of extracellular potassium concentration

Electrophysiologic properties of surviving Purkinje cardiomyocytes in the late postmyocardial-infarction phase are not well established. By using standard microelectrode techniques, we evaluated the effects of the class III agent d-sotalol on action potential parameters of single Purkinje cardiomyocytes isolated from normal canine hearts or those 10-14 days after infarction. Measurements were obtained at 2.5, 3.5, and 6 mM extracellular potassium concentrations. Action-potential parameters recorded at baseline did not differ significantly between normal and infarct-surviving Purkinje cardiomyocytes. At 3.5 and 6 mM extracellular potassium concentrations, surviving Purkinje cells appeared to be more sensitive to the effects of d-sotalol than normal Purkinje cells. In contrast, at 2.5 mM extracellular potassium concentration, the differential responses of normal and infarct-surviving Purkinje cells to d-sotalol was abolished. Reverse rate dependence was more prominent in normal than in postinfarction Purkinje cells, independent of the extracellular potassium concentration studied. The previously described enhanced sensitivity of subacutely infarcted tissue to class III agents seems to persist on a cellular level 10-14 days after myocardial infarction, even after full normalization of baseline action-potential parameters. Differential membrane-regulation mechanisms, dependent on the extracellular potassium concentrations, may account for the increased susceptibility to antiarrhythmia agents in the late postinfarction phase.

Ionic effects of capsinolol, a calcitonin gene-related peptide releasing beta-adrenoceptor blocker, on isolated cardiac muscles

1. Capsinolol (1.0-30.0 microM) in a cumulating manner decreased the maximum upstroke velocity (Vmax), the action potential amplitude and twitch tension in isolated guinea-pig atria and papillary muscle, rabbit papillary muscle, dog Purkinje fibers and human ventricle tissues. 2. In the isolated guinea-pig atrium, perfusing with capsinolol at 3 microM for 3 min temporarily increased the twitch force and decreased spontaneous cycle length; however, the results were reversed after longer exposure of the tissue. 3. Capsinolol prolonged the duration of action potential in the guinea-pig atrium and rabbit papillary muscles. The maximum diastolic potential was shifted to a less-negative level in dog Purkinje fibers and human ventricular muscles.

Conduction block in Purkinje fibers by homogeneous versus localized decrease of the gap junction conductance

Gap junction channels provide the pathway for the cell-to-cell propagation of cardiac action potential. Impairment of junctional conductance decreases conduction velocity and can cause block, two conditions that favor ventricular arrhythmias and fibrillation by re-entrant excitation. These experiments were designed to examine the effects of homogeneous versus localized decrease of the gap junction conductance on propagation of action potential in Purkinje fibers from sheep hearts. The fibers were mounted in a three-compartment chamber, and cell-to-cell conductance was progressively reduced by applying heptanol either over a central 2-mm segment or over the whole fiber length. The internal resistivities (Ri) at which conduction of the action potential became blocked were determined in both cases. With 3.5 mM heptanol in the central compartment, conduction failed when Ri was increased by only 3-4.6 times the control values. In contrast, when the same concentration of heptanol was added simultaneously to all three compartments, Ri had to rise by a factor of 7.5-9.4 before conduction became decremental and was blocked. In both situations, dV/dt(max) at the time of conduction block was similarly decreased to about 50% of the control values. Other parameters being equal, a moderate decrease of the gap junction conductance and of the fast sodium current, insufficient to block propagation of the action potential when they are homogeneously distributed, become sufficient to interrupt conduction if the action potential merges abruptly into a portion of fiber with normal internal conductivity at the outlet of the area of increased resistance. This greater sensitivity to block is accounted for by the increase in electrical load at the discontinuity in the core conductor between the region of increased internal resistance and the normal part of fiber that follows. Areas of steep transition from high to low input resistances of the core conductor, such as may develop in localized ischemia, therefore appear particularly susceptible to conduction failure.