[Action of ethacizin on normal and anomalous forms of Purkinje fiber automaticity in the dog]

The effect of a new anti-arrhythmic drug etacysin was studied in various forms of dog Purkinje's fibres automativity: normal and abnormal arising at a late stage of experimental myocardial infarction (24 hrs after coronary artery ligation) and induced by ions Ba2+. An average 57% decrease in the frequency of automaticity of normal Purkinje's fibres due to steepening the slope of slow diastolic depolarization and almost complete depression of an abnormal one was observed for the drug concentrations 5 X 10(-7) and 10(-6) g/ml. Reduced automaticity was also related to decreased slow diastolic depolarization. Maximal values of diastolic potential were found unchanged.

Digoxin-induced toxicity and experimental atrioventricular block in dogs. Relation between ventricular arrhythmias and oscillatory afterpotentials

In anesthetized dogs, digoxin intoxication was performed after and before production of a definitive complete atrioventricular (A-V) block induced by formaldehyde injection in A-V node area, compared to intoxication in dogs in sinus or junctional rhythm. The results showed that digoxin toxicity was decreased in dogs with previous A-V block. This A-V block protective effect was abolished by resetting initial sinus-like frequency with ventricular pacing before digoxin administration. Ventricular arrhythmias were suppressed by production of complete A-V block. The A-V block protective effect can be explained by a lesser myocardial uptake of digoxin because of the low idioventricular frequency. However, ventricular arrhythmias in dogs with A-V block presented similarities both in occurrence and spreading with the development of oscillatory afterpotentials (OAPs) and rhythmical triggered activity demonstrated in isolated digitalis-poisoned Purkinje fibers and ventricular myocardium: the repetitive discharge of toxic foci masked normal idioventricular pacemakers and was interrupted by variable pauses followed by the resumption of either a very slow idioventricular rhythm or a toxic focus. It is suggested that because of the low idioventricular frequency, competition and/or superimposition of slow enhanced diastolic depolarization and OAPs can be electrocardiographically displayed. The terminal event was asystole in dogs with unpaced A-V block, and ventricular fibrillation in dogs with sinus, junctional and paced A-V block rhythms. The asystole (at least 30 seconds of electrical quiescence) may be explained partly as an intense depression of normal idioventricular pacemaker being overdriven by the discharge of toxic pacemakers, and partly as a consequence of the suppression of local autonomic influences in the A-V node area induced by the formaldehyde injection.

Mechanism of reversal of toxic effects of amitriptyline on cardiac Purkinje fibers by sodium bicarbonate

Alkalinization with NaHCO3 can effectively reverse ventricular arrhythmias caused by amitriptyline intoxication, but the mechanism is unclear. To test whether alkalinization per se is important or whether increases in extracellular Na concentration also contribute, we exposed Purkinje fibers to 500 ng/ml (1.8 microM) of amitriptyline and then superfused them with three different test solutions, viz. 1) high Na-Tyrode's, 2) high NaHCO3-Tyrode's and 3) high pH-low pCO2-Tyrode's. Amitriptyline significantly depressed action potential amplitude and Vmax without altering resting membrane potential and abbreviated action potential duration at all phases of repolarization. Effects on phase 0 were accompanied by a depression of conduction velocity. All three test solutions produced significant hyperpolarization and improvement in action potential amplitude and Vmax. However, the magnitude of improvement of phase 0 characteristics was significantly greater after high NaHCO3 and resulted in significant improvement of conduction velocity in fibers depressed by amitriptyline. The effects of amitriptyline on phase 0 were rate-dependent. Reversal of this effect by NaHCO3 was equally effective at all rates. Improvement of Vmax was partly related to a shift of the Vmax-membrane potential relationship in the depolarizing direction. NaHCO3 had minimal and variable effects on action potential duration. The results suggest that the beneficial effects of NaHCO3 are related to a reversal of drug effects on phase 0 characteristics and that this effect is due both to alkalinization and to increases in extracellular Na concentration.

Effect of alpha adrenergic agonists and blockers on Purkinje fiber transmembrane potentials and automaticity in the dog

Previous studies have shown that alpha adrenergic stimulation can reduce the automaticity of canine Purkinje fibers. In the present study, our interest was to determine whether this is an alpha-1 or alpha-2 adrenergic action. We used standard microelectrode techniques to study the effects of the alpha agonists, phenylephrine and clonidine, and the alpha antagonists, prazosin and yohimbine, on phenylephrine-induced decreases in automaticity. Prazosin and yohimbine, 1 X 10(-5) M, alone, significantly reduced the amplitude and upstroke velocity (Vmax) of stimulated action potentials. In addition, yohimbine, 1 X 10(-5) M, accelerated and prazosin, 1 X 10(-5) M, attenuated repolarization. Prazosin, 1 X 10(-5) M, and yohimbine greater than or equal to 1 X 10(-7) M, alone, decreased automaticity. Phenylephrine, 5 X 10(-8) M, alone, significantly increased the cycle length of automatic Purkinje fibers. The effects of phenylephrine on automaticity were blocked by prazosin but not by yohimbine. Clonidine, alone, 1 X 10(-6) and 1 X 10(-5) M, significantly reduced automaticity, an effect that was not blocked by yohimbine or prazosin. Clonidine, 1 X 10(-6) and 1 X 10(-5) M, also significantly reduced Vmax and prolonged action potential duration of driven Purkinje fibers, an action consistent with direct membrane effects. These experiments suggest that the negative chronotropic effects of phenylephrine on Purkinje fiber automaticity are mediated by alpha-1 adrenergic receptors.

A pharmacological evaluation of electrical events in the myocardium

The pharmacologist is interested in both the therapeutic effect of the drug and its mode of action. This latter is mainly studied in the isolated heart in vitro. However, in vitro experiments are not satisfactory to predict antiarrhythmic activity in vivo because: they are mostly performed in preparations made from the normal myocardium; in vitro the autonomic and hormonal effects are absent; some drugs such as nitroglycerine or strophanthin do not produce antiarrhythmic electrophysiological changes in vitro but under appropriate conditions may have a clear-cut antiarrhythmic action in vivo; and arrhythmias mostly arise from the interaction of changes in several fundamental electrophysiological parameters which could be best studied in vivo. These facts are demonstrated by the example of the pathomechanism and pharmacotherapy of early postinfarction arrhythmias. In spite of these shortcomings, there is, however, some promising development in this field. Progress is being made in the in vivo recording of electric events in the heart (multiple KCl suction electrodes, in vivo recording of Purkinje activity). Prolonged drug treatment can evoke electrophysiological changes in vitro characteristic of the action of the drug in vivo. In the donor-perfused isolated heart preparation, the drug administered to the donor animal will evoke responses in concentrations producing the in vivo effect, and the possibility of drug metabolism and binding to plasma proteins is also present.

The effect of lidocaine on components of excitability in long mammalian cardiac Purkinje fibers

In contrast to the usual microelectrode techniques employing extracellular tissue stimulation, the double microelectrode technique of intracellular constant current application and intracellular transmembrane voltage recording permits quantitative definition of the components of cardiac excitability. This technique was employed to assess the effect of lidocaine, in a concentration equivalent to clinically effective antiarrhythmic plasma levels (5 mug/ml), on membrane characteristics, cable properties, strength-duration curves and change-duration curves in long sheep Purkinje fibers in normal Tyrode's solution at [K]0 = 4.0 mM. As determined by small hyperpolarizing pulses, lidocaine increased membrane conductance (GM) where GM approximates membrane potassium conductance (GM congruent to GK congruent to gK1) and decreased both the membrane length (lambdam) and time (taum) constants. Lidocaine shifted non-normalized strength-duration curves (threshold current, Ith, vs. current duration, t) and charge-duration curves (charge threshold th, vs. t) upward without altering either the resting transmembrane voltage (Vr) or threshold voltage (Vth). Normalized strength-duration curves and charge-duration curves, however, were superimposable during the control and lidocaine periods. This is best explained by lidocaine altering passive resistance-capacitance properties by increasing membrane potassium conductance without influencing active generator properties dependent on sodium conductance. Lidocaine did not alter the passive or active membrane properties relevant to conduction velocity. By increasing membrane potassium conductance, lidocaine decreases excitability in long Purkinje fibers by increasing Ith without altering Vr or Vth, by increasing Qth; by decreasing lambdam and by rendering local circuit currents less effectual in eliciting an action potential.

Effects of l-penbutolol (Hoe 893d) on the transmembrane potentials of canine purkinje fibers and ventricular muscle fibers

Intrinsic membrane effects (non-specific effects) and beta-adrenergic blocking action (specific effects) of l-penbutolol were studied using intracellular microelectrode techniques. l-Penbutolol in concentrations above 0.1 mg/L reduced the maximum rate of depolarization at phase O of both Purkinje fibers and ventricular muscle fibers without affecting their maximum diastolic potential. The action potential duration of Purkinje fibers were also shortened. The shortening of action potential duration and enhancement of automaticity in Purkinje fibers, normally induced by isoproterenol (0.2 mg/L), were antagonized by l-penbutolol in much lower concentraions (0.1--1.0 microgram/L) than those at which nonspecific effects are observed. The difference between these two concentration ranges is sufficient to suggest that l-penbutolol might be a highly specific beta-adrenergic blocking agent.

[Comparative study, in the anesthetized dog, of the dromotropic effects of N-propyl ajmaline bitartrate, ajmaline hydrochloride and ajmaline monochloroacetate, by recording of His electrogram]

A-V and intraventricular conduction disturbances induced by 3 different salts of ajmaline: N-propyl ajmaline bitartrate (NPAB), hydrochloride (CHA) and mono chloro-acetate (MCAA), were studied by recording endocavitary His bundle activity in pentobarbital anesthetized dogs. Cumulative dose-response curves were obtained with 3 doses of each compound. The results demonstrate that: a) NPAB exerts a significant depressor effect (9 to 10 times more potent than CHA) on the following three conduction times: auriculo-Hisian, His-Pinkinje and Purkinje-ventricular; b) on His-Purkinje and intra ventricular conduction, MCAA exerts a weaker depressor effect than that of Nab. The lack of parallelism of dose-response curves prevents further comparative quantification; c) on atrio-hisian conduction, MCAA presents a delayed dose-related depressor effect suggesting the presence of an active metabolite. It is concluded that among ajmaline derivatives studied, Nab appears to be the most depressor on A-V and intraventricular conduction in the pentobarbital anesthetized dog.

Digoxin-induced delayed afterdepolarizations: biphasic effects of digoxin on action potential duration and the Q-T interval in cardiac Purkinje fibers

Few reports exist of digoxin-induced delayed afterdepolarizations (DADs) and triggered activity recorded in cardiac fibers, and the electrophysiological characteristics of digoxin-induced DADs and triggered activity have not been reported in detail. We studied the electrophysiological properties of digoxin-induced DADs and triggered activity is sheep cardiac Purkinje fibers. Transmembrane voltage was recorded using conventional microelectrodes and extracellular electrograms were recorded using a high-gain, signal averaging method. DADs were induced by digoxin (1.25 microM, n = 9 fibers). After exposure to the drug for 20.8 +/- 2.0 min at the pacing cycle lengths of 990, 690, and 490 msec, the DAD amplitudes were 3.7 +/- 0.3, 5.7 +/- 0.6, 6.4 +/- 0.8 mV, respectively. The coupling intervals of DADs to the previous action potential at the same cycle lengths were 845.8 +/- 37.6, 581.3 +/- 23.1, 434.6 +/- 7.0 msec, respectively. Thus, digoxin-induced DADs show typical frequency dependence. Digoxin-induced DADs also occasionally caused triggered action potentials. DADs also were recorded simultaneously using an extracellular signal averaging technique. DADs were easily detected an most of the DAD characteristics measured intracellular could be confirmed in the extracellular electrograms. Digoxin induced a biphasic effect on the action potential duration (measured at 50% of repolarization (APD50) and on the Q-T interval measured from the extracellular electrograms, and in an additional group of fibers (n = 5) this was studied in detail. Digoxin initially lengthened the APD50 and the Q-T interval within the first 10 min of drug exposure, at a time when DADs had not yet developed.(ABSTRACT TRUNCATED AT 250 WORDS)

[Origin of Luciani periods during the effects of acetylcholine and eserine on the automation of Purkinje fibers]

Experiments were conducted on the isolated rabbit hearts with a complete atrio-ventricular block. It was shown that under the effect of acetylcholine (1.10(-7)-2.10(-6)g/ml) or eserine (1.10(-7)-2.10(-6)g/ml) there can occur Luciani periods which disappeared under the action of atropine (1.10(-6)g/ml). With the aid of microelectrodes it was revealed that Luciani periods appearing under the effect of acetylcholine or eserine were caused by periodic depression of automation of the Purkinje's fibers. This automation depression is regarded to be the result of a relative insufficiency of the active ionic transport processes.

Electrophysiological effects of cesium and tetraethylammonium in canine cardiac Purkinje fibers

Cesium (Cs) and tetraethylammonium (TEA) have been shown to increase action potential duration. However, action potential duration is known to be influenced by the rate of stimulation. In this study, the effect of stimulation rate on action potential characteristics was studied in Cs-treated and TEA-loaded canine Purkinje fiber preparations. Action potentials of Purkinje fibers from Cs-treated and TEA-loaded preparations had longer durations than action potentials of Purkinje fibers from normal preparations. Greater prolongation of action potential duration was observed when the rate of stimulation was reduced in Purkinje fibers from Cs-treated and TEA-loaded preparations than those from normal preparations. Whereas the increase in action potential duration of Purkinje fibers from Cs-treated preparations was accompanied by a significant membrane depolarization, no change in membrane potential was observed in Purkinje fibers from TEA-loaded preparations. In some Cs-treated and TEA-loaded preparations, the prolonged duration observed at slow stimulation rates was associated with the appearance of early afterdepolarizations. Lidocaine and cromakalim, agents known to reduce action potential duration in normal Purkinje fibers, also shortened action potential duration in Purkinje fibers from both Cs-treated and TEA-loaded preparations. However, lidocaine and cromakalim caused a significant membrane depolarization in Cs-treated Purkinje fibers but not in TEA-loaded Purkinje fibers. Our results suggested that although Cs and TEA are capable of producing rate-dependent prolongation of action potential duration and the occurrence of bradycardia-dependent early afterdepolarization, differences exist in Cs-treated Purkinje fibers in terms of the appearance of membrane depolarization at reduced stimulation rate and in the presence of lidocaine and cromakalim.

Reduction of Vmax by QX-314 and benzocaine in neonatal and adult canine cardiac Purkinje fibers

The authors have previously shown that the use-dependent action of lidocaine on the Vmax of canine Purkinje fibers and on intraventricular conduction in the in situ heart undergoes significant developmental changes. In this study, they use standard microelectrode techniques to test whether these age-related differences are due to the charged, more hydrophilic form or to the uncharged, more lipophilic form of a local anesthetic. QX-314, a permanently charged lidocaine derivative, depressed Vmax to a significantly greater extent in adult than in neonatal Purkinje fibers. This difference was due to its use-dependent blocking action and not to its tonic blocking action. The kinetic time constant (tau on) for the development of use dependence was shorter in adults (90 +/- 9 vs. 134 +/- 15 beats; P less than .05), whereas the time constant for recovery from use dependence (tau off) was shorter in neonates (53 +/- 4 vs. 106 +/- 10 sec; P less than .05). QX-314 (3 X 10(-5) M) shifted the curve of Vmax vs. activation voltage in a hyperpolarizing direction by 16.2 +/- 2.4 mV in adults and 5.1 +/- 1.1 mV in neonates (P less than .05). In contrast, the uncharged tertiary amine benzocaine (1 X 10(-5)-5 X 10(-4) M) showed no developmental differences in its effects on Vmax. Adult and neonatal fibers showed comparable tonic block and no use-dependent block. These results extend those of the authors' previous studies and suggest that developmental differences in the action of local anesthetics depend primarily on the use-dependent action of the charged molecular form.

Electrophysiologic, antiarrhythmic and hemodynamic effects of transcainide

Transcainide was selected as an antiarrhythmic drug with potential clinical application. In isolated dog, sheep and rabbit Purkinje fibres, in dog and guinea-pig trabecular preparations and in the guinea-pig right auricle, transcainide decreases the rate of rise of the transmembrane action potential, with no effect on normal spontaneous activity and calcium-mediated action potentials; it inhibits early after-depolarizations. The effect on the rate of rise is very slow in onset. In vivo a prolongation of QRS duration is observed. In dogs, the drug is effective against post-infarction and ouabain-induced ventricular arrhythmias, and abolishes acetylcholine and aconitine-induced atrial fibrillation; it elevates the threshold of electrically induced ventricular fibrillation. Hemodynamic studies in anaesthetized and unaesthetized dogs indicate that transcainide moderately decreases contractility, while slightly increasing frequency. No major side effects are seen. Preliminary data on the pharmacokinetics suggest that in the dog the observed effects after i.v. infusion are related to the parent drug. Transcainide is an antiarrhythmic of the local anaesthetic type, with very slow kinetics. It is characterized by a good oral absorption and a long duration of action.

[Clinico-electrophysiologic studies on the action of the anti-arrhythmic substance 3-carbethoxyamino-5-dimethylamino-acetyl-10,11-dihydro-5H- dibenz[b,f]azepine hydrochloride (Bonnecor, AWD 19-166, GS 015)]

The effects of the new antiarrhythmic drug 3-carbethoxy-amino-5-dimethyl-amino-acetyl-iminodibenzyl-hydroc hlorid (Bonnecor) (B.) were investigated by means of clinical-electrophysiologic methods (His-bundle electrography, programmed electrical stimulation) in 11 patients with normal cardiac output and paroxysmal supraventricular tachycardias. In a maximal dosage of 0.24 mg/kg body mass. B. affects several compartments of the impulse initiation and conduction. B. has positive chronotropic actions on the sinus node automaticity, negative dromotropic effects on the sinu-atrial, intraatrial, AV nodal and intraventricular conduction and finally negative bathmotropic actions on the myocardium of the atria and ventricles. B. suppress the artificial induction of paroxysmal supraventricular tachycardias in patients with AV nodal reentry. These results help to recognize indications (supraventricular and ventricular premature beats or tachycardias) and contraindications (sick sinus syndrome, atrio-ventricular block of higher degreé and bifascicular blocks).

In vitro electrophysiologic properties of amrinone in mammalian cardiac tissue

We studied the actions of amrinone on transmembrane electrical activity in isolated normal and physiologically compromised mammalian cardiac Purkinje and ventricular tissues. No arrhythmogenic effects of amrinone were identified in canine Purkinje tissue superfused with concentrations of 18.7 and 187 micrograms/ml (10(-4) and 10(-3) M) or in feline papillary tissue at concentrations of 18.7 and 56.1 micrograms/ml (10(-4) and 3 X 10(-4)M). In K+-depolarized canine Purkinje tissue, amrinone at 100 micrograms/ml (5.3 X 10(-4) M) failed to restore excitability; however, in norepinephrine-activated, K+-depolarized Purkinje tissue, amrinone significantly increased slow potential upstroke velocity and action potential amplitude and duration at doses ranging from 10 to 300 micrograms/ml. In K+-depolarized canine trabecula carneae, amrinone initiated slow potentials in the absence of norepinephrine and produced dose-dependent increases in slow potential upstroke velocity and in action potential amplitude and duration over a dose range of 30-300 micrograms/ml. Amrinone at 100 micrograms/ml did not influence transmembrane electrical activity of Purkinje tissue with tetracaine-depressed Na+-channel function. Amrinone did not produce oscillatory afterpotentials, influence ouabain-induced afterpotentials, or alter overdrive-induced depolarization. Amrinone caused no distinctly arrhythmia-producing effects in a heterogeneous population of depolarized canine Purkinje tissues removed from canine myocardial infarction, although small changes were observed in some tissues exhibiting slow channel action potentials. These experiments show that amrinone did not influence either Na+-dependent inward currents or Na+-K+ exchange mechanisms. Instead, amrinone facilitated slow channel action potentials in cardiac tissue and thereby could alter cardiac contractility, as well as conduction within reentry circuits in hearts which possess appropriate pathophysiologic substrates.

Electrophysiological properties of butoprozine studied by multiple intracardiac recordings on the anaesthetized dog

Electrophysiological effects of butoprozine (L 9394 Labaz) were investigated on the anaesthetized dog by programmed electrical stimulation of the heart and simultaneous recording of the His bundle electrogram and monophasic action potentials from the auricular and ventricular endocardial wall. Butoprozine injected intravenously depressed sino-atrial node function, lengthened A-V nodal conduction time and the A-V nodal refractory period, and prolonged the atrial refractory period. Thus butoprozine acted preferentially on parts of the myocardial tissue where the slow inward current seems to be particularly involved. In this respect, butoprozine was more active than amiodarone, but in contrast to this drug, butoprozine did neither prolong the ventricular monophasic action potential duration nor the ventricular refractory period. In the anaesthetized dog both drugs have practically analogous actions except at the ventricular level, which suggests that their effect on ionic currents at this level may be different.