The effect of bretylium on intracellular cardiac action potentials in relation to its anti-arrhythmic and local anaesthetic activity

Pharmacologic and pharmacokinetic profile of class III antiarrhythmic drugs

Cardiac arrhythmias frequently respond only to drugs that have as their predominant electrophysiologic effect the prolongation of repolarization and refractoriness. According to the Singh-Vaughan Williams classification, these drugs are known as class III agents. In the last few years, interest has increased in the development of class III antiarrhythmic drugs as alternatives to sodium channel blocking agents, which mainly affect cardiac conduction. Much of this interest results from a perceived danger of using drugs with sodium channel blocking properties, particularly in patients with ischemic heart disease, based on the results of the Cardiac Arrhythmia Suppression Trial (CAST) and several other trials. This article is a review of the pharmacology, including the pharmacokinetics and pharmacodynamics, of the most commonly used and investigated class III antiarrhythmic drugs. As will be seen from the discussion, each of these drugs has novel pharmacology that makes it applicable in specific clinical situations. Their putative effects on various arrhythmogenic mechanisms and their efficacy in treating specific target arrhythmias will be addressed.

Studies on the protective effect of azepexole on ouabain-induced cardiac arrhythmias and lethality in guinea-pig

Azepexole, an alpha 2-adrenoceptor agonist (125, 250 and 500 micrograms/kg i.v.), was examined for its effect on ouabain-induced ventricular premature beats, ventricular tachyarrhythmias and lethality in guinea-pigs. The doses of ouabain required to cause ventricular arrhythmias and lethality were significantly higher in azepexole-treated animals. However, it did not offer any protection in reserpinised guinea-pigs. Idazoxan, the alpha 2-adrenoceptor antagonist (100 micrograms/kg i.v.) inhibited the protective action of azepexole while corynanthine, the alpha 1-adrenoceptor antagonist (1 mg/kg i.v.), potentiated the effect. Azepexole inhibited the rate of the ouabain-induced rise in mean arterial blood pressure and the peak pressor response. In isolated paced left atria of guinea-pig, azepexole (2.76 x 10(-3) M) did not offer any protection against extrasystolic contractions induced by ouabain. Therefore the protective effect of azepexole may be mediated through the stimulation of alpha 2-adrenoceptors and the resultant suppression of the indirect neural components of ouabain toxicity.

Selective inhibition of K(+)-stimulation of Na,K-ATPase by bretylium

1. The effects of bretylium were investigated on purified Na,K-ATPase from guinea-pig heart and on the Na/K pump in trout erythrocytes, with a view to further identifying the mechanism(s) associated with its antiarrhythmic effects. 2. Na,K-ATPase activity of the thiocyanate-dispersed enzyme was determined by the measurement of inorganic phosphate produced by ATP hydrolysis. 3. When the concentrations of each of the Na,K-ATPase activating components were varied in turn, bretylium (1-5 mmol l-1) exhibited competitive-type effects against K+ with a Ki of 1.4 mmol l-1 and noncompetitive-type effects against Na+, Mg2+ and ATP. 4. In K+ influx studies in trout erythrocytes with 86Rb+ used as the marker, the inhibition of total influx observed with bretylium (5 and 10 mmol l-1) was attributable to the bretylium cation selectively inhibiting the Na/K pump-mediated influx with the associated tosylate anion inhibiting Na/K cotransport. 5. The observed inhibition kinetics indicated that the bretylium cation (2-15 mmol l-1) competitively inhibited K+ stimulation of the Na/K pump at 6 and 1.25 mmol l-1 external K+ with a mean K1 of 2.3 mmol l-1. 6. The effects demonstrated on the functioning Na/K pump in erythrocytes confirmed the Na,K-ATPase findings, with bretylium selectively inhibiting K+ stimulation of the pump mechanism in both cases. 7. It is suggested that Na,K-ATPase inhibition may contribute to the antiarrhythmic and positive inotropic effects of bretylium with the cardiac accumulation of bretylium also possibly being a further important factor.

Digitalis glycosides: a discussion of the similarities and differences in actions and existing controversies

The writing of this review was initiated to answer the question of whether differences in the actions of the various digitalis glycosides exist and to discuss current controversies in the research area of the digitalis glycosides. Data obtained in our laboratory indicated that the effect of digoxin on postganglionic cardiac sympathetic neural discharge in the minute prior to the occurrence of arrhythmia differed from that of ouabain. This raised the question of whether data published in other laboratories would support the contention that differences in glycosides do exist. To answer this question, a review of the literature was begun. Our survey of these studies are cited in the tables of this review. These tables summarize the actions of glycosides in vivo and in vitro in different animal models. The reader should bear in mind that the data included within the tables do not represent an inclusive summary of all studies in the literature. For detailed review articles, the reader is referred to the following references: Gillis et al; Gillis and Quest; Roberts et al; Lathers and Roberts; Farah and Alousi; Benthe; Levitt et al; Smith and Haber; Somberg; Lee and Klaus; Mason; Schwartz. Furthermore the summary of the results for each particular study cited in the table may not, in all cases, include each finding of the published data. Nevertheless, the tables do provide a summary of data obtained in various species with different glycosides in several different areas of research, and as such, represent an abridged compendium for the research working in the field of digitalis glycosides. This review has been organized firstly to consider glycoside-induced alterations in the autonomic nervous system and, secondly, to examine their direct actions on the heart.

A matrical approach to the basic and clinical pharmacology of antiarrhythmic drugs

In summary, the lethal cardiac arrhythmias remain a major public health problem and their treatment is a major challenge to the clinician. We possess rapidly increasing knowledge of the electrophysiologic events which underly arrhythmogenesis and the antiarrhythmic as well as the proarrhythmic actions of drugs. Much of this electrophysiologic knowledge is irrelevant to the practicing physician. While complex, we believe that the matrical approach provides the clinician with a useful intellectual framework within which to consider the actions of arrhythmogenic influences and antiarrhythmic drugs. The matrical approach is scientifically sound, reflects clinical realities, and serves as a rational guide to the treatment of cardiac arrhythmias. The traditional classifications of antiarrhythmic drugs have served a useful purpose, but they are clearly outmoded.

Interaction of calcium antagonists with beta-adrenoceptor blocking agents

Responsiveness to verapamil, the best studied calcium antagonist, was examined in cardiac preparations of rabbits pretreated with beta-adrenoceptor blockers (propranolol 2 mg/kg or oxprenolol 4 mg/kg s.c.) twice daily for either one or six weeks. Using this dose-regimen, the degree of cardiac beta-adrenoceptor blockade in conscious rabbits was substantial and similar for propranolol and oxprenolol. When administered for one week, neither propranolol nor oxprenolol affected to any marked extent the electrical and mechanical response to verapamil, diltiazem or fendiline in tissues isolated from various parts of the heart. In contrast, pretreatment with propranolol for six weeks resulted in a significant aggravation of the negative inotropic effect of verapamil in both atrial and ventricular muscle, and the verapamil-induced delay in atrio-ventricular and intra-ventricular conduction also became more pronounced. The same long-term administration of oxprenolol, one of the beta-blockers with "intrinsic" sympathomimetic activity, did not alter the atrial or ventricular contractile response to verapamil and did not significantly increase the lengthening of atrio-ventricular conduction time occurring in the presence of verapamil. It is concluded that from the point of view of adverse direct cardiac interactions with verapamil prolonged administration of oxprenolol appears to be less dangerous than chronic treatment with propranolol. It is also assumed that in those cases in which acute administration of verapamil may be necessary, concomitant chronic blockade of cardiac beta-adrenoceptors is less dangerous if drugs known to possess not only beta-adrenoceptor blocking properties, but also some "intrinsic" sympathomimetic activity are applied.

Interaction of bretylium tosylate with rat cardiac muscarinic receptors. Possible pharmacological relevance to antiarrhythmic action

The interaction of the antifibrillatory antiarrhythmic drug, bretylium tosylate, with the muscarinic receptor in tissue homogenates from regions of rat brain and heart was investigated. Competition-binding experiments were carried out with the highly specific tritiated antagonist N-methyl-4-piperidyl benzilate. Bretylium tosylate competitively displaced the labeled antagonist from the muscarinic receptor. The binding of the drug to the two brain preparations was found to be best fitted by a one-site model in each case. On the other hand, in the case of both heart preparations, a two-site model yielded a significantly better fit for the binding data than that given by a single-site model. The low affinity-binding constants in the atrium and the ventricle were similar (approximately 10 microM) to those in the brain regions examined, namely, the cortex and the medullapons. Sites with relatively higher affinity for the drug were detected in the heart only, with equilibrium-binding constants of 0.24 +/- 0.12 microM and 0.97 +/- 0.27 microM for the atrium and the ventricle, respectively. The drug also exerted anti-acetylcholine activity (K1 = 14 +/- 2 microM) measured physiologically in the guinea pig atrium, which correlated well with the concentration of the drug observed to be efficacious clinically (approximately 10 microM).

Basic understanding of the electrophysiologic actions of antiarrhythmic drugs. Sources, sinks, and matrices of information

The author creates an intellectual framework consisting of key electrophysiologic principles, basic mechanisms of arrhythmogenesis, and important drug reactions that will allow the rational use of antiarrhythmic drugs. Basic principles have been emphasized because current understanding requires it.

A classification of antiarrhythmic actions reassessed after a decade of new drugs

The past decade has seen the introduction of many new class 1 drugs, restricting fast inward current. Confirmative evidence has been obtained that the antiarrthymic action of lidocaine and diphenylhydantoin is indeed due to their effect as class 1 agents depressing conduction. The original class 3 drug, amiodarone, is increasingly in use as an antiarrhythmic of first choice for WPW and for arrhythmias associated with hypertrophic myopathy, and as a reserve drug in resistant arrhythmias of other types. Other compounds delaying repolarization have proved to be clinically effective as antiarrhythmics. In addition to their class 2 antiarrhythymic action exhibited acutely, on long-term treatment beta blockers have a class 3 action, which might be, at least in part, responsible for the protection of postinfarction patients against sudden death. Recent research suggests that inhibition of slow inward current may lead, as a secondary consequence of lowered [Ca]i, to improved cell-to-cell conduction. Finally, all but one of the new antiarrhythmic drugs, none of which existed in 1972, have turned out to possess one or more of the four classes of action originally described. This can hardly be a coincidence. The single exception, alinidine, a selective bradycardic agent, may restrict anionic currents, which would constitute a fifth class of action, but this is far from proved.

Antagonism between adenosine and bromobenzoyl-methyladamantylamine, a K+-channel blocker, in atrial myocardium of guinea pig

The effect of bromobenzoyl-methyladamantylamine (BMA) on the adenosine-induced changes in the electrical and mechanical activity of the atrial muscle, and the effect of adenosine on the slow action potentials induced by BMA in K+-depolarized atrial myocardium of guinea pig were studied. BMA was able to antagonize the adenosine-induced shortening of the action potential duration and negative inotropic effect. This action of BMA was potentiated by theophylline, and reduced by dipyridamole. Adenosine depressed the BMA-induced slow action potentials. The results suggest that there may be an antagonism between BMA and adenosine.

Action of dimethindene on the electrophysiological and mechanical properties of atrial and ventricular myocardium of guinea-pig

The effect of dimethindene (DMI) on transmembrane potentials and contractile force was studied in atrial and ventricular myocardium of guinea-pigs. DMI reduced the maximum rate of depolarization (Vmax) without changing the resting potential in both preparations. In ventricular myocardium, DMI shortened the action potential duration and exerted a negative inotropic effect. In atrial muscle, the drug prolonged the action potential duration and induced a positive inotropic effect which could be antagonized with neither the alpha-blocker phentolamine, nor the beta-blocker pindolol, H1-blocker mepyramine and H2-blocker cimetidine. DMI had no effect on the slow action potentials induced by caffeine in K+-depolarized myocardial preparations. The drug consistently shifted the sodium inactivation curve to more negative membrane potentials. The results suggest that DMI has a quinidine-like membrane-stabilizing property, which may be due to its fast Na+ channel blocking activity. The differential inotropic action of the drug in atrial and ventricular myocardium is discussed.

Electropharmacology of antiarrhythmic drugs

Cardiac arrhythmias may be caused by abnormalities of impulse initiation, impulse propagation, or a combination of the two. The specific mechanisms that may induce arrhythmias are reviewed, as are the means whereby antiarrhythmic drugs might be expected to modify arrhythmias. The cellular electrophysiologic effects of the following antiarrhythmic drugs are discussed: quinidine, procainamide, disopyramide, lidocaine, tocainide, mexiletine, phenytoin, beta-blocking and slow-channel-blocking drugs, aprindine, bretylium, ethmozin, and amiodarone. A knowledge of the similarities and differences of their actions on the determinants of conduction, on repolarization and refractoriness, on automatic mechanisms, and on afterdepolarizations, when considered in the context of the mechanism of clinically occurring tachyarrhythmias, may provide the correct framework for the choice of an appropriate agent for the control of an individual disorder of rhythm. However, it is emphasized that neither the precise mechanism of various dysrhythmias nor the fundamental basis for the salutary action of antiarrhythmic compounds is completely understood.

Effect of adenosine on sinoatrial and ventricular automaticity of the guinea pig

In isolated spontaneously beating right ventricular strips and right atrial preparations of guinea pigs adenosine was found to exert a concentration-dependent suppressing effect on the pacemaker activity. Responsiveness to adenosine was approximately 30-fold higher in ventricular than in atrial preparations. A decrease in the rate of slow diastolic (phase 4) depolarization of Purkinje and sinoatrial nodal fibers proved to be a major determinant of the adenosine-induced alteration in pacemaker activity. It is suggested that adenosine might exert its depressant effect on ventricular automaticity via direct excitation of purine receptors located in the specialized pacemaker fibres of the ventricular tissue.

The effects on cardiac muscle and nerve of a fluorinated decahydroquinoline derivative, L7810, rapidly absorbed after oral administration

1. L7810 (4-carbamoyloxy-1-(4-(4-fluorophenyl)-4-oxobutyl) decahydroquinoline, has a local anaesthetic action on frog nerve 1.75 times that of procaine.2. L7810 protected anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation and increased the lethal dose of ouabain.3. L7810 reduced the rate of rise of intracellularly recorded action potentials in rabbit isolated atria; the resting potential was not affected, but the duration of the action potential was prolonged.4. Unlike most drugs with local anaesthetic properties L7810 did not depress contractions in isolated atria but increased them.5. L7810 reduced the spontaneous frequency, maximum follow frequency and conduction velocity of rabbit isolated atria.6. L7810 had no blocking action on the chronotropic or positive inotropic actions of isoprenaline on isolated atrial muscle.7. In anaesthetized dogs L7810 caused a small dose-related bradycardia, and a large dose-related decrease in peripheral vascular resistance. There was no blockade of the effects of isoprenaline on heart rate or peripheral blood flow.

Deleterious effects of bretylium in cats with digitalis-induced ventricular tachycardia

The effect of bretylium on ventricular tachycardia induced by digitalis was studied in Dialurethane-anesthetized cats with continuous monitoring of the electrocardiogram and arterial blood pressure. Ventricular tachycardia was produced by repeated injections of deslanoside (25 µg/kg) at 15 min intervals. Only one of eight animals showed conversion of ventricular tachycardia to sinus rhythm with bretylium administration (2-100 mg/kg i.v.). In each animal, bretylium increased the ventricular rate. The effect of bretylium (30 mg/kg i.v.) on duration of ventricular tachycardia induced by deslanoside (0.75 µg/kg/min) was also studied. Duration was shortened from 65 ± 8.0 min to 7 ± 1.0 min, but the reduction was due to the intervention of either ventricular fibrillation or severe hypotension caused by bretylium. Pretreatment with propranolol (0.5 mg/kg i.v.) prevented these deleterious effects and unmasked an antiarrhythmic effect of bretylium since duration of ventricular tachycardia was significantly shorter (40 ± 6.0 min) than the controls (65 ± 8.0 min). In addition, an antiarrhythmic effect of bretylium was noted when it was administered as a dose of 30 mg/kg ip to animals 24 and 4 hours prior to deslanoside intoxication; 231 µg/kg deslanoside were now required for inducing ventricular tachycardia as compared to 175 µg/kg deslanoside in controls. Thus, bretylium given alone during a digitalis-induced ventricular tachycardia increases both the rate of ventricular tachycardia and the likelihood of ventricular fibrillation. These deterious effects appear to be related to a norepinephrine-releasing action and may be avoided by either pretreatment with propranolol or administration of bretylium several hours prior to digitalis intoxication.

Effect of altering potassium concentration on the action of lidocaine and diphenylhydantoin on rabbit atrial and ventricular muscle

All previously studied antiarrhythmic drugs which also have local anaesthetic properties reduce the maximum rate of depolarization (MRD) of cardiac muscle. Recent evidence suggested that lidocaine and diphenylhydantoin (DPH) might have a different mode of action, because they did not reduce MRD except at high concentration. The latter evidence, however, was obtained from tissues nourished by solutions containing 2.7-3 m M potassium. In the present experiments, the effects of lidocaine and DPH were studied in solutions with 5.6 and 3 mM KCl. In the former, both drugs reduced MRD at concentrations similar to those found in the blood of treated patients, but in low [KCl] higher concentrations were necessary. It was concluded that there was no reason to suppose that the mode of action of lidocaine and DPH on the cardiac membrane is fundamentally different from that of quinidine, procaine, procainamide and other compounds which interfere directly with depolarization.

The effect of amiodarone, a new anti-anginal drug, on cardiac muscle

1. Amiodarone (2-butyl, 3-(4-diethylaminoethoxy, 3,5-diiodo, benzoyl) benzofuran hydrochloride), an anti-anginal drug which causes coronary dilatation and depresses myocardial oxygen consumption, was found to protect anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation.2. A 5% (73.4 mM) solution of amiodarone had no local anaesthetic action on guinea-pig skin.3. Amiodarone, 20 mg/kg (29.4 mumol/kg) given daily for 6 weeks intraperitoneally, had no effect on the resting potential or action potential height, and only a small effect on the maximum rate of depolarization, of isolated rabbit atrial or ventricular muscle fibres as shown by intracellular recording. It caused a considerable prolongation of the action potential in both tissues.4. Simultaneous administration of thyroxine (5 mug; 6.26 nmol), given daily for 3 weeks intraperitoneally, prevented the prolongation by amiodarone of the duration of the action potential.5. Treatment of rabbits with 20 mg/kg of amiodarone daily intraperitoneally for 6 weeks had no effect on the weight of the thyroid gland, but was associated with a reduction in body growth rate.6. Treatment of rabbits with 10 mg/kg (60.3 mumol/kg) of potassium iodide (equal in its iodine content to that of 20 mg/kg of amiodarone), given daily for 6 weeks intraperitoneally, had no effect on body growth rate or the duration of cardiac action potentials.7. It was concluded that amiodarone had effects on cardiac action potentials similar to those which occur after thyroidectomy.

A third class of anti-arrhythmic action. Effects on atrial and ventricular intracellular potentials, and other pharmacological actions on cardiac muscle, of MJ 1999 and AH 3474

1. Both MJ 1999 and AH 3474 protected guinea-pigs anaesthetized with urethane against ouabain-induced ventricular fibrillation.2. MJ 1999 had 1/90, and AH 3474 1/30, of the activity of procaine in reducing the height of the action potential of frog sciatic nerve.3. MJ 1999 and AH 3474 reduced the rate of rise of intracellularly recorded action potentials at concentrations in excess of 160 x 10(-6)M (50 mg/l.). It was concluded that direct depression of depolarization could have contributed little to the protection against ouabain-induced fibrillation.4. MJ 1999, but not AH 3474, greatly prolonged the duration of the action potential in acute experiments on isolated atrial and ventricular muscle, and prolonged the Q-Tc interval of the electrocardiogram in anaesthetized guinea-pigs. It is suggested that this effect contributes to anti-arrhythmic activity.5. At concentrations up to 80 x 10(-6)M AH 3474 had positive chronotropic and inotropic effects on isolated rabbit atrial muscle, but at higher concentrations these were superseded by negative effects. MJ 1999 was depressant at all concentrations studied, the threshold concentrations being 19 x 10(-6)M for chronotropic, and 162 x 10(-6)M for inotropic effects.

A comparison of the anti-arrhythmic actions of I.C.I. 50172 and (--)-propranolol and their effects on intracellular cardiac action potentials and other features of cardiac function

1. I.C.I. 50172 had marked quinidine-like effects on intracellular cardiac action potentials at concentrations above 20 mg/l. (6.61 x 10(-5)M). The rate of rise and overshoot of the action potential, conduction velocity and contractions were decreased. (-)-Propranolol had similar effects at less than 1/30 this concentration.2. I.C.I. 50172 had 1/100 the activity of (-)-propranolol as a local anaesthetic. Since this is also the ratio of their in vitro beta-receptor blocking activities, I.C.I. 50172 provides no net increase in specificity of beta-receptor blockade.3. In contrast, the in vivo activity of I.C.I. 50172 in protecting anaesthetized guinea-pigs against ouabain-induced ventricular fibrillation was 40% that of (-)-propranolol.4. Structure-activity relations of beta-receptor blocking drugs are discussed.