Combined effects of rate membrane potential, and drugs on maximum rate of rise (Vmax) of action potential upstroke of guinea pig papillary muscle

Antiarrhythmic drug effects on premature beats are partly determined by prior cardiac activation frequency in perfused guinea-pig heart

NEW FINDINGS

What is the central question of this study? Can antiarrhythmic drug effects on repolarization, conduction time and excitation wavelength in premature beats be determined by prior cardiac activation frequency? What is the main finding and its importance? In premature beats induced after a series of cardiac activations at a slow rate, antiarrhythmics prolong repolarization but evoke little or no conduction delay, thus increasing the excitation wavelength, which indicates an antiarrhythmic effect. Fast prior activation rate attenuates prolongation of repolarization, while amplifying the conduction delay induced by drugs, which translates into the reduced excitation wavelength, indicating proarrhythmia. These findings suggest that a sudden increase in heart rate can shape adverse pharmacological profiles in patients with ventricular ectopy.

ABSTRACT

Antiarrhythmic drugs used to treat atrial fibrillation can occasionally induce ventricular tachyarrhythmia, which is typically precipitated by a premature ectopic beat through a mechanism related, in part, to the shortening of the excitation wavelength (EW). The arrhythmia is likely to occur when a drug induces a reduction, rather than an increase, in the EW of ectopic beats. In this study, I examined whether the arrhythmic drug profile is shaped by the increased cardiac activation rate before ectopic excitation. Ventricular monophasic action potential durations, conduction times and EW values were assessed during programmed stimulations applied at long (S₁ -S₁ [basic drive cycle length] = 550 ms) and short (S₁ -S₁  = 200 ms) cycle lengths in perfused guinea-pig hearts. The premature activations were induced with extrastimulus application immediately upon termination of the refractory period. With dofetilide, a class III antiarrhythmic agent, a prolongation in action potential duration and the resulting increase in the EW obtained at S₁ -S₁  = 550 ms were significantly attenuated at S₁ -S₁  = 200 ms, in both the regular (S₁ ) and the premature (S₂ ) beats. With class I antiarrhythmic agents (quinidine, procainamide and flecainide), fast S₁ -S₁ pacing was found to attenuate the drug-induced increase in action potential duration, while amplifying drug-induced conduction slowing, in both S₁ and S₂ beats. As a result, although the EW was increased (quinidine and procainamide) or not changed (flecainide) at the long S₁ -S₁ intervals, it was invariably reduced by these agents at the short S₁ -S₁ intervals. These findings indicate that the increased heart rate before ectopic activation shapes the arrhythmic profiles by facilitating drug-induced EW reduction.

Molecular mechanisms of adverse drug reactions in cardiac tissue

The myocardium is the target of toxicity for a number of drugs. Based on pharmacological evidence, cellular targets for drugs that produce adverse reactions can be categorized into a number of sites that include the cell membrane-bound receptors, the second messenger system, ionic channels, ionic pumps, and intracellular organelles. Additionally, interference with the neuronal input to the heart can also present a global site where adverse drug effects can manifest themselves. Simply, a drug can interfere with the normal cardiac action by modifying an ion channel function at the plasma membrane level leading to abnormal repolarization and/or depolarization of the heart cells thus precipitating a disruption in the rhythm and causing dysfunction in contractions and/or relaxations of myocytes. It is now recognized that toxic actions of drugs against the myocardium are not exclusive to the antitumor or the so-called cardiac drugs, and many other drugs with diverse chemical structures, such as antimicrobial, antimalarial, antihistamines, psychiatric, and gastrointestinal medications, seem to be capable of severely compromising myocardium function. At present, great emphasis in terms of drug safety is being placed on the interaction of many classes of drugs with the hERG potassium channel in cardiac tissue. The interest in the latter channel stems from the simplified view that drugs that block the hERG potassium channel cause prolongation of the QT interval, and this can cause life-threatening cardiac arrhythmias. Based on the evidence in the current literature, this concept does not seem to always hold true.

Purkinje and ventricular contributions to endocardial activation sequence in perfused rabbit right ventricle

Interactions between peripheral conduction system and myocardial wave fronts control the ventricular endocardial activation sequence. To assess those interactions during sinus and paced ventricular beats, we recorded unipolar electrograms from 528 electrodes spaced 0.5 mm apart and placed over most of the perfused rabbit right ventricular free wall endocardium. Left ventricular contributions to electrograms were eliminated by cryoablating that tissue. Electrograms were systematically processed to identify fast (P) deflections separated by >2 ms from slow (V) deflections to measure P-V latencies. By using this criterion during sinus mapping (n = 5), we found P deflections in 22% of electrograms. They preceded V deflections at 91% of sites. Peripheral conduction system wave fronts preceded myocardial wave fronts by an overall P-V latency magnitude that measured 6.7 +/- 3.9 ms. During endocardial pacing (n = 8) at 500 ms cycle length, P deflections were identified on 15% of electrodes and preceded V deflections at only 38% of sites, and wave fronts were separated by a P-V latency magnitude of 5.6 +/- 2.3 ms. The findings were independent of apical, basal, or septal drive site. Modest changes in P-V latency accompanied cycle length accommodation to 125-ms pacing (6.8 +/- 2.6 ms), although more pronounced separation between wave fronts followed premature stimulation (11.7 +/- 10.4 ms). These results suggested peripheral conduction system and myocardial wave fronts became functionally more dissociated after premature stimulation. Furthermore, our analysis of the first ectopic beats that followed 12 of 24 premature stimuli revealed comparable separation between wave fronts (10.7 +/- 5.5 ms), suggesting the dissociation observed during the premature cycles persisted during the initiating cycles of the resulting arrhythmias.

Activity-dependent changes in the electrical behaviour of sheep cardiac Purkinje fibres

Rate-dependent changes in the electrical activity of sheep Purkinje fibres maintained at 37 °C have been investigated. The duration of the action potential is maximal at a frequency of about 60 min -1 . When the rate is increased above 60 min -1 there is a substantial shortening of the action potential; this occurs abruptly in the first beat at the higher rate although subsequently there can be further changes in duration and these can result in a small prolongation, no change, or a small further shortening of the action potential and can take up to 10 min to reach a steady-state. When the rate is reduced from 60 min -1 there is also a shortening of the action potential but it occurs gradually over several hundred seconds. Action potential duration reaches a minimum value at a rate of about 6 min -1 . 70% of preparations studied showed an increase in duration again at rates below 6 min -1 but duration is always constant at frequencies below about 0.1 min -1 . The maximum diastolic potential is more negative and the pacemaker potential is larger at higher rates of stimulation. When the frequency is raised these variables increase over a time course lasting several hundred seconds. At rates below 60 min -1 the slow changes in action potential duration, maximum diastolic potential and pacemaker potential, after a change in the stimulus frequency, all have similar monoexponential time courses (Ƭ ≈ 3 min) and are accompanied by slow changes in tension production over a similar time course. In Purkinje fibres that exhibit spontaneous activity, rapid stimulation results in overdrive excitation: an acceleration of spontaneous activity when stimulation is ceased.

How can we facilitate spontaneous termination of ventricular fibrillation and prevent sudden cardiac death? A working hypothesis

Ventricular fibrillation (VF) is one of the most life-threatening arrhythmias encountered in daily clinical practice. Its occurrence cannot be completely prevented by currently used antiarrhythmic drugs, and, in most instances, VF is sustained and leads to the patient's death unless a successful DC defibrillation is applied. However, spontaneous reversion of VF to sinus rhythm has been observed in various animals and occasionally even in man. Hence, facilitation of self-ventricular defibrillation must be explored as an alternative therapeutic approach. In experimental studies using several mammalian species, we have found that self ventricular defibrillation requires a good intercellular coupling and well synchronized electrical activity in the ventricles, which, in untreated animals, depend on their myocardial catecholamine content. It can then be hypothesized that any agent that elevates the catecholamine level during VF would facilitate spontaneous ventricular defibrillation, and drugs inhibiting extraneuronal catecholamine reuptake have indeed been shown to possess this ability. It is suggested that their effects are mediated by an increase in the intracellular cAMP level, and any compounds sharing this property could well prove efficacious in making VF transient and in reducing sudden cardiac death.

The role of catecholamines on intercellular coupling, myocardial cell synchronization and self ventricular defibrillation

Ventricular fibrillation (VF) is one of the most life threatening events. Although in humans VF is generally sustained (SVF) requiring artificial defibrillation, in various mammals and in some cases in humans VF terminates by itself, reverting spontaneously into sinus rhythm. Since VF is one of the main causes of sudden death, one of the important clinical problems today is if and how we can transform the fatal SVF into a self limited transient one (TVF). From electrophysiological studies carried out on anaesthetized open chest animals, we have found that TVF requires a high degree of intercellular coupling and synchronization. Cardiac myocytes are electrically coupled with adjacent cells. The intercellular coupling is a focus of low electrical resistance which allows rapid transmission of electrical impulses between cells. Any decrease in intercellular coupling decreases the ability of the heart for self defibrillation. The cell-to-cell coupling decreases with age, ischemia, VF and variations in physiological conditions probably due to an increase in intercellular resistance (Ri), widening in the internexal gaps, decrease in electrotonic space constant (lambda) etc. All of these factors are known to be affected by intracellular concentration of free Ca++ ([Ca++]). On the basis of studies carried out on various mammals at different ages, we hypothesized that the ability of the heart to defibrillate depends on the cardiac catecholamine level [CA], during VF. This hypothesis is supported by the facts, known from the literature, that increase in [CA] decreases intracellular free Ca++ concentration, decreases Ri and increases lambda.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiarrhythmic drug classifications and the clinician: a gambit in the land of chaos

Several classifications of antiarrhythmic drugs have appeared and all suffer considerable limitations. Recently a Task Force of the Working Group on Arrhythmias of the European Society of Cardiology proposed a novel classification of antiarrhythmic drugs (the so-called Sicilian Gambit) based on their action on the most vulnerable parameter of an arrhythmogenic mechanism. The present article attempts a critical reappraisal of the antiarrhythmic drug actions and the relationship of vulnerable parameters with cellular mechanisms such as ion channels. The clinical applicability of these concepts, the implications of the new classification in the pharmacologic therapy of arrhythmias, and its potential limitations are discussed.

Effect of rate on changes in conduction velocity and extracellular potassium concentration during acute ischemia in the in situ pig heart

INTRODUCTION

The purpose of our study was to determine if the slowing of longitudinal intraventricular conduction in the in situ porcine heart during acute regional no-flow ischemia was rate dependent. Further, we investigated whether any rate dependence could be correlated to a rate-dependent component of the ischemia-induced rise in extracellular potassium concentration, [K+]e.

METHODS AND RESULTS

We studied in situ hearts in nine anesthetized open chest pigs in which acute no-flow ischemia was induced by occlusion of the left anterior descending coronary artery. To determine the effects of steady-state rate on the slowing of conduction and rise in [K+]e during ischemia, we varied the rate of stimulation during sequential occlusions from 90 to 150 beats/min. Longitudinal conduction velocity was determined by unipolar electrodes embedded in a plaque that was sutured to the epicardial surface in the center of the ischemic zone. Myocardial [K+]e was determined simultaneously by potassium-sensitive electrodes placed at or within 1 to 2 mm of the epicardium in close proximity to the activation recording electrodes. Conduction velocity decreased more rapidly at the more rapid rates of stimulation although the reduction in conduction velocity occurring prior to the onset of conduction block was similar at both rates. The potassium change was not rate dependent and rose at the same rate regardless of the rate of stimulation.

CONCLUSION

Our study demonstrates that the steady-state rate-dependent component of the slowing of intraventricular conduction induced by acute ischemia in the in situ porcine heart occurs in the absence of a rate-dependent component in the rise of [K+]e. Between rates of 90 and 150 beats/min, the rate dependence of the conduction slowing may be attributed to one or more potassium-independent factors such as the rate-dependent changes in resting membrane potential, in Vmax of the action potential upstroke, and in cell-to-cell uncoupling, which have been observed in other models of acute ischemia.

A self-protecting servo-model for explanation of the mechanism involved in spontaneous ventricular defibrillation

Ventricular fibrillation (VF) is the most life-threatening arrhythmia. It has been suggested that VF in humans is always sustained. Recent publications indicated that VF can be either sustained (SVF) or transient (TVF), reverting spontaneously into sinus rhythm. In previous studies we have hypothesized that TVF requires, during VF, a high cardiac catecholamine level ([CA]). Since during VF sympathetic activity is enhanced, the question arises of why VF is sustained in the majority of cases. Looking on the living body as a self-protecting servo-mechanism, we propose a servo-model that on the one hand describes the mechanism involved in TVF and on the other proposes a therapeutic procedure which can help the heart in its effort to transform VF into TVF. Our model has been examined by various experimental studies. The results obtained strongly support our hypothesis.

Fast and slow blockade of sodium channels by flecainide in rabbit cardiac Purkinje fibres

The electrophysiological effects of flecainide were tested using the two-microelectrode voltage-clamp technique and Vmax-measurements in isolated rabbit cardiac Purkinje fibres. Flecainide predominantly unfolds its sodium-channel blocking action during the upstroke phase of the cardiac action potential, because its Vmax-depressant effects are independent of the duration of the depolarizing interval. Very long lasting depolarizations caused a second, very slow blocking activity. Starting from a steady-state block, recovery from block was tested and yielded a time constant of 7.3 s for a membrane potential of -105 mV. The strong blockade of sodium-channels combined with a delayed recovery behaviour of the drug-associated channels gives reasons for the observation of a marked use-dependent block. This block increased when the cycle length was shortened or the holding potential was less negative. Additional application of lidocaine in several concentrations did not significantly increase or attenuate the phasic block caused by flecainide alone. Under special conditions we investigated flecainide's depression and shift of the Vmax/Vm-relation and we observed that the concentration dependence of both parameters could be described by simple 1:1 binding reaction. The effects of flecainide are largely reversible often greater than or equal to 15 min. Flecainide could be characterized as an open channel blocker with a very slow inactivated channel blocking activity. For the qualitative description of the sodium-channel block by flecainide we used the "modulated-receptor hypothesis", whereas for reconstructions of the use-dependent action we applied the "guarded-receptor hypothesis", which enables computations of phasic block with the knowledge of only one forward and one reverse rate constant.

Characterization of concentration- and use-dependent effects of quinidine from conduction delay and declining conduction velocity in canine Purkinje fibers

The dynamic response of squared conduction velocity, theta 2, to repetitive stimulation in canine Purkinje fibers with quinidine was studied using a double-microelectrode technique. With stimulation, a frequency-dependent monoexponential increase in conduction delay (CD) and a decline in theta 2 were observed. The exponential rates and changes in steady-state CD and theta 2 were frequency- and concentration-dependent. The overall drug uptake rates describing blockade and the interpulse recovery interval were linearly related and steady-state values of theta 2 were linearly related to an exponential function of the stimulus intervals. Based on first-order binding, the frequency- and concentration-dependent properties of quinidine were characterized by the apparent binding and unbinding rates of 14.2 +/- 5.7 X 10(6) mol-1.s-1 and 63 +/- 12 s-1 for activated and 14.8 +/- 1.0 X 10(2) mol-1.s-1 and 0.16 +/- 0.03 s-1 for resting states. The recovery time constant extracted from the pulse train interpulse interval was 5.8 +/- 1.5 s compared with 5.1 +/- 0.6 s determined from a posttrain test pulse protocol. This study demonstrates that the kinetics of drug action can be derived from measures of impulse propagation. This provides a basis for characterizing frequency-dependent properties of antiarrhythmic agents in vivo and suggests the plausibility of a quantitative assessment of drug binding and recovery rates in man.

Electrophysiological effects of E-3753, a new antiarrhythmic drug, in guinea-pig ventricular muscle

1. The electrophysiological effects of E-3753, a new antiarrhythmic drug, were studied in guinea-pig papillary muscles. 2. E-3753 (10(-7) - 10(-4) M) produced a concentration-dependent decrease in the action potential amplitude and Vmax of the upstroke, shortened the action potential duration (APD) and shifted the resting membrane potential to less negative values. E-3753 also shortened the effective refractory period (ERP), lengthening the ERP relative to APD. 3. E-3753 (10(-5) M) shifted the membrane responsiveness curve along the voltage axis to more negative potentials. 4. In the presence of E-3753 (10(-5) M) trains of stimuli at rates between 0.5 and 3 Hz led to an exponential decline in Vmax (onset rate at 3 Hz, 0.05 +/- 0.009 per action potential), to a new steady-state level. This use-dependent Vmax block was augmented at higher rates of stimulation. The time constant for the recovery of Vmax from the use-dependent block was 41.1 +/- 4.8 s. 5. E-3753 (10(-5) - 10(-4) M) had no effect on the characteristics of the slow action potentials elicited by isoprenaline in ventricular fibres depolarized by 27 mM KCl. 6. The slow onset of use-dependent block during repetitive activity and the slow offset kinetics of use-dependent Vmax block suggest that E-3753 exhibits class Ic antiarrhythmic actions in ventricular muscle fibres but does not exhibit class IV (Ca antagonist) antiarrhythmic actions.

Some recent concepts concerning the mechanisms of action of antiarrhythmic drugs

The administration of antiarrhythmic drugs is determined largely on the basis of empiricism, and the experience of individual physicians and the results of clinical studies are probably the two major factors determining the approach to treatment. Although much effort has been expended in learning the mechanisms of action of antiarrhythmic drugs, the applicability to clinical treatment of the knowledge attained has been limited. Nonetheless, recent advances in our understanding of the biology of the cardiac cell, of the factors that predispose to arrhythmias and of drug-receptor interactions, have not only provided new insights into the mechanisms whereby specific drugs exert their effects, but promise to provide means for designing and testing compounds whose actions will be more specific and more predictable than is presently the case. This paper will review some of the advances that have been made and will consider some of their implications.

Use dependence of amiodarone during the sinus tachycardia of exercise in coronary artery disease

The QRS duration at rest and during exercise was studied in 19 patients with coronary artery disease before and after oral amiodarone therapy to determine if this drug produces detectable rate-dependent conduction slowing during physiologic increases in heart rate. QRS duration did not change significantly during exercise in the absence of the drug. However, after amiodarone, QRS duration at rest increased from 99 to 114 ms (p less than 0.001), and increased further from 114 to 127 ms (p less than 0.001) during the 45 beats/min mean increase in heart rate produced by exercise. The magnitude of this effect was related to the resting QRS duration. After amiodarone therapy, the QRS increased during exercise by only 6% in 8 patients with QRS less than 110 ms, while in 12 patients with QRS greater than or equal to 110 ms, the QRS increased by 15% (p less than 0.05). Rate-dependent conduction slowing occurs during the sinus tachycardia of exercise in patients treated with amiodarone, presumbably due to use-dependent sodium channel blockade. This result is most pronounced in patients with abnormal ventricular conduction at rest.

Electrophysiological effects of labetalol on canine atrial, cardiac Purkinje fibres and ventricular muscle

1. Using conventional microelectrode techniques for the intracellular recordings of the membrane potential, the effects of labetalol were studied on cardiac Purkinje, atrial and ventricular muscle fibres of the dog. 2. Labetalol (1-10 microM) reduced, in a concentration-dependent manner, the action potential amplitude (APA) and the maximum rate of rise of the action potential (Vmax) in Purkinje fibres. 3. The action potential duration (APD) was decreased in Purkinje fibres but significantly increased in ventricular fibres after small concentrations of labetalol (1-3 microM). The atrial fibres were not very sensitive to labetalol. 4. Depolarization of the cardiac Purkinje fibres by increasing the external potassium concentration (8-12 mM), potentiated the labetalol effects on APA and Vmax but blocked its effects on the APD. 5. The effects of labetalol on Vmax of Purkinje fibres were dependent on the frequency of stimulation. 6. The ratio of the effective refractory period to the APD was increased both in normally polarized and depolarized Purkinje fibres after treatment with labetalol (10 microM). 7. Labetalol (10 microM) shifted the membrane responsiveness curve of Purkinje fibres by about 10 mV in the hyperpolarizing direction. 8. The slow response obtained in K-depolarized, Ba-treated Purkinje fibres was not significantly affected by labetalol (10-100 microM). 9. It is suggested that labetalol can exert Class I and Class III antiarrhythmic actions in cardiac muscle of the dog in vitro.

Arrhythmogenic actions of antiarrhythmic drugs

Cardiac arrhythmias may result from abnormalities of impulse propagation or abnormalities of impulse initiation. When arrhythmias are initiated by antiarrhythmic drugs, the most common mechanisms appear to be conduction block or reentry, and abnormal impulse initiation, which may be triggered by afterdepolarizations. The effects of drugs on conduction may result from their actions on the fast sodium channel, the slow calcium channel or their ability to prolong repolarization. The extent to which a drug that depresses fast sodium or slow calcium entry will exert its toxic effects depends in large part on its binding characteristics to its channel receptor site. Such toxicity represents a continuum for the therapeutic effects of these drugs. The factors that control drug access to binding sites, including lipid solubility, molecular size and extent of ionization, are reviewed, as are the contributions to conduction abnormalities of drug-induced changes in repolarization. The mechanisms whereby drugs induce abnormalities of impulse initiation are still a matter of conjecture. Apparently, drugs that increase inward plateau currents or decrease repolarizing potassium ion currents carry increased risk. Moreover, there is evidence for the role of early after depolarizations occurring secondary to prolonged repolarization as a possible cause of arrhythmias, including torsades de pointes. The mechanisms whereby antiarrhythmic drugs may contribute to this type of tachyarrhythmia are reviewed.

Electrophysiological interactions between quinidine-lidocaine and quinidine-phenytoin in guinea-pig papillary muscle

The interactions between quinidine and lidocaine or phenytoin at the sodium channel level have been studied in the present work. The maximum upstroke velocity (Vmax) of the guinea-pig papillary muscle action potential has been used as a measure of the sodium current. Lidocaine interfered with the use-dependent blocking effects on Vmax of quinidine, by decreasing the fraction of sodium channels blocked by quinidine during the conditioning action potential, in an apparently competitive way. These results strongly suggest that quinidine and lidocaine bind to a common receptor site. Alternatively, it has been suggested that lidocaine and quinidine bind to different but related receptor sites, since lidocaine may induce allosteric changes in quinidine's receptor. Phenytoin increased the use-dependent blocking effects on Vmax of quinidine by slowing the time course of the slow component of reactivation of Vmax induced by quinidine. Phenytoin did not change the fraction of sodium channels blocked by quinidine during the conditioning action potential. These results suggest that phenytoin binds to a different receptor site than quinidine.

Rate-dependent effects of procainamide on His-Purkinje conduction in man

Microelectrode studies in isolated cardiac tissues have shown that the depressant effect of several antiarrhythmic drugs on the maximal upstroke velocity of the cardiac action potential is rate-dependent. To determine whether this effect of antiarrhythmic drugs is seen in humans, 14 patients undergoing atrial pacing at several rates were prospectively studied before and after the infusion of procainamide (15 mg/kg). The HV interval (His-Purkinje conduction rate) and the QRS duration (intraventricular conduction rate) were measured. Before procainamide infusion, atrial pacing did not significantly prolong the maximal HV interval (from 54 +/- 15 to 58 +/- 13 ms). After procainamide infusion (mean serum level 10.0 +/- 3 micrograms/ml) atrial pacing at an average of 5 pacing rates significantly prolonged the HV interval (from 67 +/- 18 to 80 +/- 20 ms, p less than 0.001). The extent of HV prolongation with atrial pacing after procainamide infusion was independent of the HV interval at rest before procainamide. The duration of the QRS complex also tended to prolong with atrial pacing after procainamide infusion, but this prolongation was not statistically significant. Thus, procainamide produces a rate-dependent depressant effect on His-Purkinje and intraventricular conduction, confirming observations made in isolated tissue preparations.

The cellular mechanisms of cardiac antiarrhythmic drug action

In the preceding pages we have considered a number of mechanisms whereby drugs may modify arrhythmias and we also have demonstrated that certain types of action are specific to individual drugs. In addition, through the description of the use-dependence of drug effects, we have shown that the specificity of a drug's action often is attributable to its interactions with specific channels. As we learn more about the specificity of individual drug effects we should be better able not only to understand the actions of presently available drugs, but also to discover compounds that promise to be of use in the future.

Electrophysiological profile of the antiarrhythmic compound asocainol studied on perfused guinea-pig hearts and on isolated cardiac preparations

The studies deal with electrophysiological effects of asocainol [(+/-)-6,7,8,9-tetrahydro-2,12-dimethoxy-7-methyl-6-phenetyl-5 H-dibenz(d,f)azonine-1-ol] on isolated perfused guinea-pig hearts (Langendorff-preparation), on right ventricular papillary muscles, on Purkinje fibres from the guinea pig, and on isolated sinus nodes from the rabbit. In the perfused heart (n = 5) the lowest effective concentration of asocainol is about 0.2 mumol/l. At a concentration of 2 mumol/l the cardiac electrogram shows in spontaneously beating hearts a mean decrease in frequency of 15%, in electrically driven hearts (150/min at 32 degrees C) prolongation of PQ (+31%), of QRS (+24%) and of QT (+5%). In papillary muscles (32 degrees C; K+e 5.9 mmol/l; stimulation rate 0.5 Hz) asocainol (3-30 mumol/l) exerts the following effects: no change of the resting potential, concentration-dependent reduction of the maximum rate of rise (Vmax) of the action potential (AP) (-16 to -67%) as well as of the AP-amplitude (-4 to -16%), and shortening of the AP-duration at 50% repolarisation (-18 to -43%). The steady-state dependence of Vmax on the resting potential (RP) determined by variation of K+e (5.9-15 mmol/l) is shifted by asocainol to more negative potentials. The percentage deviation from controls of the Vmax-RP relationship is more pronounced at lower membrane potentials. The influence of asocainol on the recovery from inactivation of Vmax shows marked time-dependence. Slow response (Ca2+-mediated) APs elicited by strong stimuli in a K+e-rich solution (K+e 20-24 mmol/l) respond to asocainol (3-10 mumol/l) with a marked reduction in amplitude, Vmax and duration.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of bepridil on the electrophysiological properties of guinea-pig ventricular muscles

Effects of bepridil, a new antianginal and potential antiarrhythmic agent, on transmembrane action potentials of ventricular muscles were examined in isolated right ventricular papillary muscles of guinea-pig. Bepridil at concentrations above 5 X 10(-6)M caused a dose-dependent decrease in both the maximum upstroke velocity (Vmax) and the action potential duration from the upstroke to 30% repolarization ( APD30 ). On the other hand, the resting potential (RP), the amplitude of action potential (AMP), and the action potential duration from the upstroke to 90% repolarization ( APD90 ) were not affected even at the highest concentration applied (10(-5)M). The curves relating membrane potential and Vmax were shifted by bepridil at 5 X 10(-6)M along the voltage axis in the direction of more negative potentials. The recovery kinetics of Vmax assessed by premature stimuli were definitely slowed by bepridil at above 10(-6)M. This effect was more pronounced with higher [K+]o (10 mM) than normal [K+]o (5 mM). Bepridil at 5 X 10(-6)M caused a rate-dependent decrease of Vmax (use-dependent block) with rapid onset and offset, as did lidocaine. Slow responses, which had been induced by isoprenaline (5 X 10(-6)M) in K+-depolarized preparations, were suppressed significantly by additional application of bepridil at 10(-5)M. These findings suggest that bepridil has electrophysiological characteristics similar to those both of Class Ib and Class IV antiarrhythmic drugs.

Effects of procaine amide, quinidine and ethmozin on delayed afterdepolarizations

We studied the effects of three chemically different antiarrhythmic drugs on ouabain-induced delayed afterdepolarizations (DAD) in canine Purkinje fibers. The three drugs, ethmozin, 4.6 X 10(-6) M; procaine amide, 1.1 X 10(-4) M; and quinidine, 1.13 X 10(-6) M reduced DAD amplitude equivalently at drive cycle lengths less than 500 ms. Quinidine and procaine amide in these concentrations had no effect on the action potential characteristics except for a prolongation of action potential duration (APD) induced by procaine amide. Ethmozin reduced action potential amplitude, maximum upstroke velocity of phase 0 (Vmax), and APD measured to 50% and full repolarization (APD50 and APD100). Rate dependent changes in Vmax and maximum diastolic potential (MDP) were not exaggerated by quinidine in the ouabain intoxicated Purkinje fibers. The DAD coupling interval was increased as DAD amplitude decreased with all three drugs. Although ethmozin, procaine amide and quinidine similarly reduced DAD amplitude; procaine amide and quinidine exerted these effects in the absence of other transmembrane potential effects, whereas ethmozin did so only in concentrations that depressed the action potential as well.

Computer analysis of prolongation of cardiac action potential duration

Using Beeler and Reuter's mathematical model of action potential in myocardial fibers, a similar extent of prolongation of action potential duration at -60 mV (APD -60) was produced at the infinite interval of stimulation by altering parameter values for slow inward current (is) time-independent (ik1) and time-dependent (ix1) outward currents. Among five types of prolongation of APD -60 thus produced, the plateau height is high in two types at this interval, but in three at a shorter interval. The degree of prolongation varies between these five types at this shorter interval. The stimulus interval-action potential duration (interval-duration) relation curves reflect changes in time-dependent parameters of these currents. Thus various types of APD prolongation may be distinguished from each other by the different behavior of action potentials at low and high frequencies of stimulation. ix1 increases by 40-fold at the moment of the maximum rate of depolarization at an interval of 330 msec, but is still negligibly small as compared with sodium current.

Lidocaine block of cardiac sodium channels

Lidocaine block of cardiac sodium channels was studied in voltage-clamped rabbit purkinje fibers at drug concentrations ranging from 1 mM down to effective antiarrhythmic doses (5-20 muM). Dose-response curves indicated that lidocaine blocks the channel by binding one-to-one, with a voltage-dependent K(d). The half-blocking concentration varied from more than 300 muM, at a negative holding potential where inactivation was completely removed, to approximately 10 muM, at a depolarized holding potential where inactivation was nearly complete. Lidocaine block showed prominent use dependence with trains of depolarizing pulses from a negative holding potential. During the interval between pulses, repriming of I (Na) displayed two exponential components, a normally recovering component (tauless than 0.2 s), and a lidocaine-induced, slowly recovering fraction (tau approximately 1-2 s at pH 7.0). Raising the lidocaine concentration magnified the slowly recovering fraction without changing its time course; after a long depolarization, this fraction was one-half at approximately 10 muM lidocaine, just as expected if it corresponded to drug-bound, inactivated channels. At less than or equal to 20 muM lidocaine, the slowly recovering fraction grew exponentially to a steady level as the preceding depolarization was prolonged; the time course was the same for strong or weak depolarizations, that is, with or without significant activation of I(Na). This argues that use dependence at therapeutic levels reflects block of inactivated channels, rather than block of open channels. Overall, these results provide direct evidence for the "modulated-receptor hypothesis" of Hille (1977) and Hondeghem and Katzung (1977). Unlike tetrodotoxin, lidocaine shows similar interactions with Na channels of heart, nerve, and skeletal muscle.

Perspectives and future trends in cellular electrophysiology: implications for the clinician

As a result of single fiber electrophysiologic studies, the clinical approach to the electrical behavior of the heart has improved. Three areas are examined: 1) the electrocardiographic waveform, 2) normal and abnormal cardiac rhythms, and 3) the mechanism of action of antiarrhythmic drugs. In each area, the results of single fiber studies have provided a conceptual framework for diagnostic and therapeutic decisions. These studies have also enabled investigators to test hypotheses formulated from clinical observations. It may be only a slight exaggeration to attribute many of our recent advances in each of the three areas to the development and use of the microelectrode.

Voltage- and time-dependent depression of maximum rate of depolarization of guinea-pig ventricular action potentials by two steroidal antiarrhythmic drugs, CCI 22277 and ORG 6001

1 The voltage- and time-dependence of the depression of the maximum rate of depolarization (Vmax) by two steroidal anti-arrhythmic drugs, CCI22277 and Org 6001 were studied in guinea-pig ventricle. 2 At normal resting potentials CCI22277 (2 microM and 4 microM) produced very little depression of Vmax at very low driving rates (resting block) but trains of stimuli at interstimulus intervals (ISI) of less than 10,000 ms led to an exponential decline in Vmax to a new plateau over 100-200 beats. 3 This 'rate-dependent block' (RDB) increased with rate over the range ISI=4800 to ISI=200 ms. 4 Org 6001 30 microM and 60 microM produced a similar degree of RDB over the same range of frequencies but the new plateau level of Vmax was reached much more rapidly (20-30 beats) and there was a moderate degree of depression of Vmax in the resting tissue. 5 Recovery from RDB in the presence of both drugs was an exponential process with time constants (tau re) of 80.4 +/- 7.4 s for CCI22277 and 4.6 +/- 0.5 s for Org 6001. 6 Both drugs shifted the steady-state inactivation curve, relating Vmax to resting membrane potential, in the hyperpolarizing direction, implying selective depression of depolarized cells.

Effects of disopyramide on transmembrane action potentials in guinea-pig papillary muscles

The effects of disopyramide on the transmembrane action potential in guinea-pig papillary muscles were studied with special reference to the dependence of the effects on the external concentrations of K+ ([K+]0), stimulation rates, and diastolic intervals. Disopyramide, 2 micrometer, prolonged the 90% action potential duration (ADP90) without changing the maximum rate of phase 0 depolarization (Vmax) (class 3 action), whereas disopyramide, 20 micrometer, reduced Vmax, (class 1 action). The class 3 action was prominent in 2.7 and 5.4 mM [K+]0, at 0.25--1 Hz, and so was the class 1 action in 8.1 mM [K+]0, at 2--5 Hz. Disopyramide, 20 micrometer, enhanced the rate-dependent reduction in Vmax only in 8.1 mM [K+]0. Neither concentration of disopyramide altered the recovery kinetics of APD90 in the premature responses in three [K+]0, but slowed that of Vmax only in 8.1 mM [K+]0. These results indicate that the manifestation of class 3 action of disopyramide is favored with low concentrations of the drug, low [K+]0, and low stimulation rates, whereas the reverse is true in the class of the class 1 action.

Classification of cardioactive drugs in vivo by using programmed electrical stimulation in combination with monophasic action potential recordings at different pacing rates

The effect of antiarrhythmic drugs on myocardial refractoriness may be due to changes either in Vmax of phase 0 or the phase of repolarization of the AP or both. By using programmed electrical stimulation in combination with MAP recordings at different pacing rates in the intact dog heart, it was possible to classify and to a certain extent to elucidate the mode of action of various cardioactive drugs in vivo.

Effects of mexiletine on transmembrane action potentials as affected by external potassium concentration and by rate of stimulation in guinea-pig papillary muscles

1. The effects of mexiletine and quinidine were compared on transmembrane potentials in guinea-pig papillary muscles, using conventional microelectrode techniques. 2. Mexiletine (23.1 mumol/l) decreased the maximum rate of rise of the action potential (Vmax) and increased the ratio of the effective refractory period to the action potential duration at 90% repolarization level (ERP/APD90); these effects were prominent with elevation of the external potassium concentration ([K]o) from 2.7 to 5.4, 8.1 and 10.0 mmol/l. 3. The percentage decrease in Vmax induced by 5 and 10 mumol/l of quinidine was approximately constant in 2.7, 5.4 and 10.0 mmol/l [K]o solutions. 4. The decrease in Vmax produced by mexiletine was progressively increased as the driving rate was raised from 0.25 to 5Hz. This rate-dependent change was pronounced when the concentration was raised from 23.1 to 46.2 and 92.4 mumol/l. 5. Mexiletine in concentrations of 23.1 and 92.4 mumol/l delayed the recovery of Vmax in a premature action potential to the level of Vmax in the conditioning action potentials at the driving rate of 0.25 Hz. 6. It appears that mexiletine exerts its anti-arrhythmic action by a selective depressant effect on depolarized cells (high [K]0) and cells with high frequency discharges, as is the case with lignocaine.

Effects of tocainide and lidocaine on the transmembrane action potentials as related to external potassium and calcium concentrations in guinea-pig papillary muscles

Effects of lidocaine and tocainide on transmembrane potentials were studied in isolated guinea-pig papillary muscles, superfused with modified Tyrode's solution containing either 5.4, 2.7, 10.0 or 8.1 mmol/l potassium concentration, [K]0. The last solution applied contained either 1.8 (normal [Ca]0) or 7.2 mmol/l [Ca]0 (high [Ca]0. The concentrations of lidocaine and tocainide used were 18.5, 36.9 and 73.9 mumol/l and 43.7, 87.5 and 174.9 mumol/l in 5.4 mmol/l [K]0 solution and 36.9 and 87.5 mumol/l in the other solutions, respectively. At the driving rate of 1 Hz in 5.4 mmol/l "K]0 solution, both drugs produced dose-dependently a reduction of maximum rate of rise of action potential (Vmax), together with a prolongation of the relative refractory period. Vmax decreased progressively as the driving rate was increased from 1 Hz (for lidocaine) and from 0.25 Hz (for tocainide) to 5 Hz. This action was accentuated dose-dependently. A slow component (time constant tau = 232 ms for lidocaine, 281--303 ms for tocainide) and slower component (tau = 2.1--3.8 s for tocainide) of the recovery (reactivation) of Vmax were observed in premature responses at 0.25 Hz and in the first response after interruption of the basic driving rate at 1 Hz. All these effects were accentuated with rising [K]0 and attenuated in the high [Ca]0 solution. Both drugs abbreviated the action potential duration at 50% (APD50) and 90% (APD90) levels at 5.4, 8.1 and 10.0 mmol/l [K]0 but not at 2.7 mmol/l [K]0 nor a high [Ca]0 at 1 Hz. These [K]0-dependent effects of lidocaine on Vmax were successfully simulated by the model proposed by Hondeghem and Katzung (1977), with a slight change in parameter values. The mode of interaction of lidocaine with sodium channels in the open, closed and rested states was deduced from these results.

Electrophysiological effects of encainide on isolated cardiac muscle and Purkinje fibers and on the Langendorff-perfused guinea-pig heart

The effect of encainide (0.1, 0.3 and 1.0 mg/l) on transmembrane electrical activity was studied in left auricle and papillary muscles of the guinea pig and in Purkinje fibers of cow and sheep hearts. Conduction times and refractoriness of the AV node and the His-Purkinje system were measured in the Langendorff-perfused guinea-pig heart by His-bundle recording. Encainide had no effect on the maximum diastolic potential. Action potential duration (100% repolarization) remained constant or slightly prolonged in guinea-pig auricle and ventricle preparations but was shortened in cow and sheep Purkinje fibers. Action potential amplitude and Vmax decreased in a dose-dependent manner, the effect being more pronounced at higher frequencies. The frequency effect was not related to a change in the recovery of Vmax following the repolarization. The drug reduced Vmax at all membrane potential values, the relative effect being slightly more pronounced at low membrane potentials. In three different tests for pacemaker activity encainide had no effect on the rate of diastolic depolarization at high levels of membrane potential, but clearly reduced the spontaneous oscillations at the plateau level and the transient depolarizations characteristic for triggered pacemaker activity induced by ouabain. Ca-mediated action potentials were not affected even at concentrations of encainide up to 8 mg/l. In the Langendorff-perfused guinea-pig heart, conduction through the His-Purkinje system was slowly more than conduction through the AV node. All effects were quickly reversible by drug-free perfusion. In comparison with lidocaine, the effect of encainide on electrical parameters showed the following differences: the decrease in Vmax in the presence of encainide was more frequency-dependent, and less potential-dependent, the effect of encainide on pacemaker activity was more selective.

Effect of procainamide on transmembrane action potentials in guinea-pig papillary muscles as affected by external potassium concentration

Effects of procainamide (PA), 0.18, 0.37 and 0.74 mmol/l, on the transmembrane potential were studied in isolated guinea-pig papillary muscles, superfused with modified Tyrode's solution (external K concentration, [K]0 = 5.4 mmol/l) at the basic driving rate of 1 Hz. PA, at 0.37 mmol/l, significantly reduced the maximum rate of rise of action potential (Vmax) with no change in the resting potential. When 2.7 mmol/l [K]0 of the superfusate was exchanged for 15 mmol/l [K]0 solution a decrease in Vmax induced by 0.37 mmol/l PA became more prominent with decrease in resting potential. The reduction of Vmax at steady state was less at lower driving rates (0.25 and 0.5 Hz) and more at higher driving rates (2-5 Hz) than at 1 Hz in 2.7, 5.4 and 10.0 mmol/l [K]0 solution. Such changes were enhanced concentration-dependently by PA at 5.4 mmol/l [K]0. Also, the changes became more significant with an increase in [K]0 from 2.7 mmol/l to 5.4 mmol/l and then to 10.0 mmol/l. The recovery process of Vmax proceeded with two components. The time course of the slow component seen in the Vmax of the first response after interruption of basic driving stimulation at 1 Hz, followed an approximate monoexponential function. The time constants were 6.3, 4.4 and 5.8 s in the presence of 0.18, 0.37 and 0.74 mmol/l PA at 5.4 mmol/l [K]0 and 3.4 and 3.7 s both in the presence of 0.37 mmol/l PA at 2.7 and 10.0 mmol/l [K]0. Vmax values after 30 or 60 s interruption of stimulation were 80-92% of the predrug Vmax value at 1 Hz. The time constants of the first component, estimated by the peeling-off methods at the driving rate of 0.1 Hz, were 11, 31 and 5-22 ms in the presence of 0.37 mmol/l at 5.4, 10.0 and 2.7 mmol/l [K]0 and did not differ significantly from the time constants in control preparations. The results were found to be consistent, to a certain extent, with the model proposed by Hondeghem and Katzung (1977).

Effect of phentolamine, alprenolol and prenylamine on maximum rate of rise of action potential in guinea-pig papillary muscles

Effects of phentolamine (13.3, 26.5 and 53.0 micron), alprenolol (3.5, 7.0 and 17.5 micron) and prenylamine (2.4, 4.8 and 11.9 micron) on the transmembrane potential were studied in isolated guinea-pig papillary muscles, superfused with Tyrode's solution. 1. Phentolamine, alprenolol and prenylamine reduced the maximum rate of rise of action potential (.Vmax) dose-dependently. Higher concentrations of phentolamine and prenylamine caused a loss of plateau in a majority of the preparations. Resting potential was not altered by any of the drugs. Readmittance of drug-free Tyrode's solution reversed these changes induced by 13.3 micron of phentolamine and all conconcentrations of alprenolol almost completely but those induced by higher concentrations of phentolamine and all concentrations of prenylamine only slightly. 2. .Vmax at steady state was increased with decreasing driving frequencies (0.5 and 0.25 Hz) and was decreased with increasing ones (2--5 Hz) in comparison with that at 1 Hz. Such changes were all exaggerated by the above drugs, particularly by prenylamine. 3. Prenylamine and, to a lesser degree, phentolamine and alprenolol delayed dose-dependently the recovery process of .Vmax in premature responses. 4. .Vmax in the first response after interruption of stimulation recovered toward the predrug value in the presence of the above three drugs. The time constants of recovery process ranged between 10.5 and 15.0s for phentolamine, between 4.5 and 15.5s for alprenolol. The time constant of the main component was estimated to be approximately 2s for the recovery process with prenylamine. 5. On the basis of the model recently proposed by Hondeghem and Katzung (1977), it is suggested that the drug molecules associate with the open sodium channels and dissociated slowly from the closed channels and that the inactivation parameter in the drug-associated channels is shifted in the hyperpolarizing direction.

Studies on electrical and mechanical activity in hypoxic papillary muscles of the guinea-pig

The hypoxia-induced changes in transmembrane potentials and in force of contraction of isolated papillary muscles of the guinea-pig were studied. With different glucose concentrations in the hypoxic medium both the extent and the time course of the reduction in force of contraction and action potential duration could be modified. A time lag of 10 minutes was observed in the onset of action potential shortening. The membrane potential decreased only at advanced stages of hypoxia. Lack of glucose was tolerated for a considerable longer period of time when the preparations were mainly quiescent during hypoxia, although membrane depolarization could not be prevented. Memembrane depolarization was basent only if the mucles were completely unloaded when exposed to hypoxia. The dissociation of events during hypoxia is interpredted as evidence for compartmentalization of energy supply within the cells.