Controlling cardiac arrhythmias by lengthening repolarization: rationale from experimental findings and clinical considerations

Antiarrhythmic and Proarrhythmic Properties of QT-Prolonging Antianginal Drugs

In recent years there has been a major reorientation of drug therapy for cardiac arrhythmias, its changing role, and above all, a radical change in the class of arrhythmia drugs because of their impact on mortality. The decline in the use of sodium-channel blockers has led to an expanding use of β-blockers and simple or complex class III agents for controlling cardiac arrhythmias. Success with these agents in the context of their side effects has spurred the development of compounds with simpler ion-channel blocking properties that have less complex adverse reactions. The resulting so-called pure class III agents, such as dofetilide or ibutilide, were found to have antifibrillatory effects in atrial fibrillation and flutter and in ventricular tachyarrhythmias. Such agents are effective and have diversity, but they have come into therapeutics with a price: the sometimes-fatal torsades de pointes. The drug amiodarone, a complex compound that was synthesized as an antianginal agent, has been an exception in this regard. Its therapeutic use is associated with a negligibly low incidence of torsades de pointes, even though the drug produces significant bradycardia and QT lengthening to 500 to 700 msec. Recent electrophysiologic studies suggest that this paradox is likely due to the differential block of ion channels in endocardium, epicardium, midmyocardial (M) cells, and Purkinje fibers in the ventricular myocardium. There is also clinical evidence suggesting that amiodarone reduces the “torsadogenic” effects of pure class III agents. Ranolazine was also synthesized for the development of antianginal properties that stem from a partial inhibition of fatty acid oxidation; it too has been found to have electrophysioloigic properties. These are somewhat similar to those of amiodarone on ion channels in endocardium, epicardium, M cells, and Purkinje fibers in the ventricular myocardium, but the drug does not prolong the QT interval to the same extent as amiodarone does. Thus, the drug produces modest increases in repolarization as judged by its effects on the action potential duration (APD) without the potential for the development of torsades de pointes. By virtue of its suppressant action on early afterdepolarizations and triggered activity in Purkinje fibers and M cells, the drug appears to have a powerful potential for reducing the torsadogenic proclivity of conventional class III antiarrhythmic compounds. The rationale for the therapeutic niche for amiodarone, and especially in the case of ranolazine, in the prevention of drug-induced torsades de pointes is discussed.

Mechanisms underlying the autonomic modulation of ventricular fibrillation initiation--tentative prophylactic properties of vagus nerve stimulation on malignant arrhythmias in heart failure

Classical physiology teaches that vagal post-ganglionic nerves modulate the heart via acetylcholine acting at muscarinic receptors, whilst it is accepted that vagus nerve stimulation (VNS) slows heart rate, atrioventricular conduction and decreases atrial contraction; there is continued controversy as to whether the vagus has any significant direct effect on ventricular performance. Despite this, there is a significant body of evidence from experimental and clinical studies, demonstrating that the vagus nerve has an anti-arrhythmic action, protecting against induced and spontaneously occurring ventricular arrhythmias. Over 100 years ago Einbrodt first demonstrated that direct cervical VNS significantly increased the threshold for experimentally induced ventricular fibrillation. A large body of evidence has subsequently been collected supporting the existence of an anti-arrhythmic effect of the vagus on the ventricle. The development of prognostic indicators of heart rate variability and baroreceptor reflex sensitivity—measures of parasympathetic tone and reflex activation respectively—and the more recent interest in chronic VNS therapy are a direct consequence of the earlier experimental studies. Despite this, mechanisms underlying the anti-arrhythmic actions of the vagus nerve have not been fully characterised and are not well understood. This review summarises historical and recently published data to highlight the importance of this powerful endogenous protective phenomenon.

Impact of subchronic administration of piperazine citrate on the electrocardiogram of the rat

The effect of piperazine citrate on the electrocardiogram of the rat after treatment with piperazine citrate at 30, 60, and 100 mg/kg body weight for 16 weeks was investigated. The results were compared with a control group. There was prolongation of P-R, Q-Tc, and J-T intervals, whereas the QRS interval remained virtually unchanged. The heart rate, on the other hand, decreased in the groups that received piperazine citrate. The average heart rate in the control group was 334 ± 17.20 beats/min. In the group of rats that received the three doses of piperazine, the average heart rate at the end of a 15-minute observation period was 308 ± 3.74 beats/min, 302 ± 16.55 beats/min, and 312 ± 13.93 beats/min, respectively, and none of the values was statistically significant compared with the control. The P-R interval showed statistically significant increases in the groups treated with the three doses of piperazine over the control group. In both the 30- and 60-mg/kg groups, the average P-R interval was 92.0 ± 0.5, which was statistically significant when compared with the control average of 80.0 ± 0.00 (P = 0.0427). For the 100-mg/kg group, the average P-R interval was 96.0 ± 0.4 ms. The difference between this value and the control average was equally statistically significant (P = 0.0043). Both the Q-Tc and J-T intervals also showed statistically significant increases in the piperazine-treated groups and the P values compared with the control group were very similar. Even at the very high dose of 100 mg/kg given two times daily for 16 weeks, piperazine citrate appeared quite safe to the rat heart because it did not provoke any cardiac dysrhythmic phenomenon on the surface electrocardiogram.

Computer-based analysis of dynamic QT changes: toward high precision and individual rate correction

BACKGROUND

New strategies are needed to improve the results of automatic measurement of the various parts of the ECG signal and their dynamic changes.

METHODS

The EClysis software processes digitally-recorded ECGs from up to 12 leads at 500 Hz, using strictly defined algorithms to detect the PQRSTU points and to measure ECG intervals and amplitudes. Calculations are made on the averaged curve of each sampling period (beat group) or as means +/- SD for beat groups, after being analyzed at the individual beat level in each lead. Resulting data sets can be exported for further statistical analyses. Using QT and R-R measured on beat level, an individual correction for the R-R dependence can be performed.

RESULTS

EClysis assigns PQRSTU points and intervals in a sensitive and highly reproducible manner, with coefficients of variation in ECG intervals corresponding to ca. 2 ms in the simulated ECG. In the normal ECG, the CVs are 2% for QRS, 0.8% for QT, and almost 6% for PQ intervals. EClysis highlights the increase in QT intervals and the decrease of T-wave amplitudes during almokalant infusion versus placebo. Using the observed linear or exponential relationships to adjust QT for R-R dependence in healthy subjects, one can eliminate this dependence almost completely by individualized correction.

CONCLUSIONS

The EClysis system provides a precise and reproducible method to analyze ECGs.

Differential effects of bupivacaine on cardiac K channels: role of channel inactivation and subunit composition in drug-channel interaction

INTRODUCTION

We examined the effects of a nonspecific ion channel blocker, bupivacaine, on K channels encoded by hERG, rKv1.4, rKv4.3, and hKvLQT1 along with hIsK. Their native counterparts in the heart are important for the function of I(Kr), I(to) and I(Ks) and, thus, play an important role in repolarization.

METHODS AND RESULTS

To elucidate the mechanisms and sites of bupivacaine's actions, we correlated the voltage and time dependencies of drug effects with those of channel gating. We also studied the effects of altering the C-type (hERG) or N-type (rKv1.4) inactivation process or the subunit composition (hKvLQT1 with or without hIsK) on bupivacaine's actions. The results suggest that, except for hKvLQT1 co-expressed with hIsK, bupivacaine binding occurred at depolarized voltages coinciding with channel activation. With hKvLQT1 co-expressed with hIsK, bupivacaine bound preferentially at negative voltages when channels were in the closed state, and unbound at depolarized voltages when channels opened. The C-type inactivation of hERG enhanced, whereas the N-type inactivation of rKv1.4 hindered, bupivacaine's effects.

CONCLUSION

We propose that bupivacaine's actions on these K channels can be described as a nonspecific pore blockade in the inner mouth region. However, the apparent binding affinity and voltage dependence of binding can be differentially influenced by the inactivation processes occurring at two ends of the pore (C-type inactivation at the outer end and N-type inactivation at the inner end), or by the interaction between hIsK and hKvLQT1 subunits.

Proarrhythmia with class III antiarrhythmic drugs: types, risks, and management

The nature of the proarrhythmic reactions induced by antiarrhythmic drugs is linked to the electrophysiologic effects of these agents. Torsades de pointes is the classic form of proarrhythmia observed during therapy with any drug that prolongs repolarization, for example, the class III agents. Its precise electrophysiologic mechanism is not fully elucidated, although the arrhythmia is generally considered to be due either to early afterdepolarization in the context of prolonged cardiac repolarization or to an increase in spatial or temporal dispersion of repolarization. Among the class III drugs the proarrhythmic risk appears to be lowest for amiodarone, probably due to its complex electrophysiologic profile that may create significant myocardial electrical homogeneity. In the case of d,l-sotalol, the incidence of torsades de pointes increases with dose and the baseline values of the QT interval. Where d-sotalol and other pure class III agents might fall into the varying spectrum of proarrhythmic potential remains unclear. That d-sotalol has been found to increase mortality in postinfarction patients with ventricular dysfunction (the Survival With Oral d-Sotalol [SWORD] trial) is a matter of considerable concern. It raises the possibility that such a phenomenon may be a common property of most, if not all, pure class III compounds. Accordingly, care must be taken to minimize the likelihood of proarrhythmia; in particular, therapy with a class III agent should only be initiated in the presence of a defined indication established on the basis of clinical trials. When class III antiarrhythmic drug-induced proarrhythmia occurs, immediate cessation of therapy with the responsible agent and correction of predisposing factors, such as electrolyte disorders or bradycardia, is mandatory. Intravenous administration of high-dose magnesium sulfate has been demonstrated to be effective in terminating and preventing new episodes of torsades de pointes. Temporary pacing may be necessary.

Adaptation of the QT interval to heart rate changes in isolated perfused guinea pig heart: influence of amiodarone and D-sotalol

The inadequacy of the QT interval to shorten following heart rate increase is a feature of the inherited long QT syndrome and may have a role in the genesis of the typical arrhythmias associated with this syndrome (torsade des pointes). The aim of our study was to evaluate whether drugs that prolong the QT interval, such as amiodarone and D-sotalol, may also impair the ability of the QT interval to adapt to sudden heart rate changes. Experiments were carried out on isolated perfused guinea pig hearts (Langendorff preparation). Driving frequency was changed, in steps, every two minutes (Hz: 2.5-3-2.5-3.75-2.5-5-2.5), while epicardial ECG was continuously recorded on magnetic tape. QT interval was automatically measured by means of a beat-by-beat analysis program. D-sotalol was added to the perfusion medium at a concentration of 4 micrograms ml-1, while amiodarone was administered, before in vitro evaluation, for seven days (50 mg kg-1 per day, intraperitoneally). In control experiments two phases of QT adaptation were identified: an abrupt QT shortening at the first beat after frequency change (QT1), followed by a gradual, exponential QT shortening that reached a new steady state in about 1 min (half life: 13 sec). The electrical restitution curve (the relation between QT1 and the corresponding diastolic interval) had a rate constant of 57 +/- 8 ms. Neither drug changed the slow component of QT adaptation. However, both drugs increased the ability of QT to shorten upon premature stimulation: D-sotalol by increasing the rate constant of the restitution curve and amiodarone by decreasing the y-intercept. Our results indicate that D-sotalol and amiodarone do not impair QT shortening during tachycardia but, on the contrary, they may favour QT adaptation, thus reducing the likelihood of the potentially lethal 'R on T phenomenon'. This may be an additional mechanism by which these drugs can exert their antifibrillatory action.

Efficacy and safety of repeated intravenous doses of ibutilide for rapid conversion of atrial flutter or fibrillation. Ibutilide Repeat Dose Study Investigators

BACKGROUND

Currently available antiarrhythmic drugs have limited efficacy for acute termination of atrial fibrillation and flutter, especially if the arrhythmia is not of recent onset. The purpose of this multicenter study was to determine the efficacy and safety of repeated doses of intravenous ibutilide, a class III antiarrhythmic drug, in terminating atrial fibrillation or flutter.

METHODS AND RESULTS

Two hundred sixty-six patients with sustained atrial fibrillation (n = 133) or flutter (n = 133), with an arrhythmia duration of 3 hours to 45 days, were randomized to receive up to two 10-minute infusions, separated by 10 minutes, of ibutilide (1.0 and 0.5 mg or 1.0 and 1.0 mg) or placebo. The conversion rate was 47% after ibutilide and 2% after placebo (P < .0001). The two ibutilide dosing regimens did not differ in conversion efficacy (44% versus 49%). Efficacy was higher in atrial flutter than fibrillation (63% versus 31%, P < .0001). In atrial fibrillation but not flutter, conversion rates were higher in patients with a shorter arrhythmia duration or a normal left atrial size. Arrhythmia termination occurred a mean of 27 minutes after start of the infusion. Of 180 ibutilide-treated patients, 15 (8.3%) developed polymorphic ventricular tachycardia during or soon after the infusion. The arrhythmia required cardioversion in 3 patients (1.7%) and was nonsustained in 12 patients (6.7%).

CONCLUSIONS

Intravenous ibutilide given in repeated doses is effective in rapidly terminating atrial fibrillation and flutter and under monitored conditions is an alternative to current cardioversion options.

Effects of class III antiarrhythmic drugs on the Na(+)-activated K+ channels in guinea-pig ventricular cells

1. Class III antiarrhythmic drugs are known to block the outward currents through voltage-gated K+ channels. However, effects of class III antiarrhythmic drugs on the ligand-gated K+ channels have not been thoroughly examined. In this study effects of amiodarone and newer class III antiarrhythmic drugs, E-4031 and MS-551, on the Na+-activated K+ (KNa) current were examined in inside-out membrane patches and in whole cells isolated from guinea-pig ventricle. 2. The KNa channel current was activated by increasing [Na+]i from 0 mM to 30-100 mM with 150 mM [K+]o in inside-out membrane patches of ventricular myocytes. The channel current showed a larger slope conductance (210 pS), inward-going rectification and subconductance levels of various amplitudes. 3. E-4031 and MS-551 at high concentrations (300 microM) inhibited the K+ current by decreasing the open time (flickering block). On the other hand, amiodarone at relatively low concentrations (0.1-10 microM) inhibited the KNa channel current by decreasing the open probability rather than by decreasing the open time. The IC50 value of amiodarone for inhibiting the KNa channel current was 1.0 microM. 4. These drugs also inhibited the whole-cell outward current activated by intracellular loading of 50 mM [Na+]i and extracellular application of 10 microM ouabain. 5. These results indicate that class III antiarrhythmic drugs inhibit the KNa channel current in cardiac cells. However, there are sharp differences in the effective concentrations and the mode of inhibition between amiodarone and the newer class III antiarrhythmic drugs.

Electrophysiological, rate dependent, and autonomic effects of the class III antiarrhythmic almokalant after myocardial infarction in the pig

Ventricular arrhythmias remain a major problem, in particular in patients with left ventricular dysfunction or heart failure. In this group of patients, Class I drugs were shown to be ineffective, and they even increased mortality during chronic treatment. New antiarrhythmic agents should preferably not only have pure antiarrhythmic effects, but should also be free from adverse autonomic properties. In the present study, the electrophysiological, rate dependent and autonomic effects of intravenously administered almokalant, a new Class III antiarrhythmic drug, were investigated in nine pigs surviving a myocardial infarction. The ventricular effective refractory period (VERP) increased after almokalant (loading dose: 0.05 mumol.kg-1.min-1, continuous infusion: 0.0025 mumol.kg-1.min-1) from 292 +/- 25 to 308 +/- 13 ms (pacing cycle length [PCL] 500 ms + 1 extrasystole [ES]), from 249 +/- 19 to 261 +/- 16 ms (PCL 400 ms +1ES), and from 209 +/- 18 to 219 +/- 18 ms (PCL 300 ms +1ES). The VERPs increased most after three ES at PCL 400 ms: from 167 +/- 27 to 186 +/- 29 ms (P < 0.05) and at PCL 300 ms: from 150 +/- 29 to 174 +/- 27 ms (P < 0.05). The ventricular monophasic action potential durations (MAPD) were similarly prolonged and the ratio VERP/MAPD did not change. Prolongation of MAPD after almokalant remained present at short pacing cycle lengths. Before almokalant infusion, sustained monomorphic ventricular tachycardia (VT) was inducible in two pigs, and nonsustained VT in a third animal. After almokalant, only one pig remained inducible. Two weeks after myocardial infarction, heart rate variability and baroreflex sensitivity were reduced. Furthermore, subsequent electrophysiological testing transiently reduced these parameters of autonomic activity. During almokalant however, no changes in autonomic functions were observed after programmed stimulation. Heart rate variability decreased after myocardial infarction from 6.3 +/- 2.5 ms to 5.4 +/- 4.2 ms (P = NS). After programmed stimulation, it further decreased to 2.8 +/- 2.0 ms (P = 0.028). Almokalant infusion prevented autonomic deterioration: 3.3 +/- 2.2 ms before stimulation and 3.3 +/- 1.3 after stimulation (P = NS). In postinfarct pigs, almokalant prolongs VERP and MAPD at shorter pacing cycle lengths. The results indicate absence of reverse rate dependence and of adverse autonomic changes.

Comparative efficacy of intravenous ibutilide versus procainamide for enhancing termination of atrial flutter by atrial overdrive pacing

This study compares the influence of intravenous ibutilide, a class III antiarrhythmic agent, with procainamide, a class IA antiarrhythmic agent, and with placebo on its ability to terminate atrial flutter using rapid atrial pacing. Fifty-nine episodes of atrial flutter in 54 patients who failed to terminate with an intravenous infusion of ibutilide, procainamide, or placebo alone underwent attempts at pacing termination using a standard protocol of burst atrial overdrive pacing. Atrial flutter cycle length and atrial monophasic action potential duration recorded from the right atrium during atrial flutter were measured at baseline and following infusion of ibutilide, procainamide, or placebo. Both ibutilide and procainamide significantly enhanced (p <0.001) pacing-induced termination of atrial flutter compared with placebo. Pacing converted 2 of 11 patients (18%) who received placebo, 13 of 15 patients (87%) who received ibutilide, and 29 of 33 patients (88%) who received procainamide to sinus rhythm. Ibutilide and procainamide compared with placebo markedly reduced (p <0.001) the incidence of pacing-induced atrial fibrillation. The atrial flutter cycle length was prolonged significantly less (p <0.001), and the atrial monophasic action potential duration was increased significantly more (p <0.001) by ibutilide than by procainamide. Although the electrophysiologic changes induced by these antiarrhythmic agents contributed to facilitating pacing-induced termination, neither tachycardia cycle length nor action potential duration were useful predictors of the ability of pacing to terminate atrial flutter. In conclusion, despite differing electrophysiologic effects, the use of intravenous ibutilide or procainamide enhances the termination of atrial flutter by atrial overdrive pacing.

Hemodynamic effects of intravenous sematilide in patients with congestive heart failure: a class III antiarrhythmic agent without cardiodepressant effects

OBJECTIVES

This study sought to evaluate the hemodynamic effects of intravenous sematilide hydrochloride, a selective class III antiarrhythmic agent, in patients with heart failure and left ventricular systolic dysfunction.

BACKGROUND

Class I antiarrhythmic agents, which primarily slow conduction, can depress ventricular function, particularly in patients with heart failure. In contrast, pure class III agents, which selectively prolong repolarization, do not adversely affect hemodynamic variables in animal models, but there are no data evaluating their hemodynamic effects in humans.

METHODS

In 39 patients with congestive heart failure and a left ventricular ejection fraction < 40%, hemodynamic and electrocardiographic measurements were obtained at baseline, after a loading dose and during a maintenance infusion of intravenous sematilide using either a low (0.75 then 0.3 mg/min) or high dose (1.5 then 0.6 mg/min) regimen. The study had an 80% power to detect clinically meaningful differences in hemodynamic variables.

RESULTS

Both low (n = 20) and high (n = 19) dose sematilide infusions produced dose-dependent increases in QT interval (5 +/- 8% [mean +/- SD] and 18 +/- 10%, respectively) and corrected QT interval (4 +/- 8% and 14 +/- 10%), and high dose sematilide decreased heart rate by 7 +/- 10% (all p < 0.025 vs. baseline). Neither dose regimen had a statistically significant effect on any other hemodynamic variable, including mean arterial, right atrial, pulmonary artery and pulmonary capillary wedge pressures; cardiac index, stroke volume, systemic and pulmonary vascular resistances; and left ventricular stroke work index. Sematilide showed no adverse hemodynamic effects in patients with left ventricular ejection fraction < or = 25% or > 25% and in patients with cardiac index < 2 or > or = 2 liters/min per m2. Sustained polymorphic ventricular tachycardia (n = 1) and excessive QT prolongation (n = 4) were seen during the high dose.

CONCLUSIONS

Sematilide, in the doses administered, prolonged repolarization but did not alter hemodynamic variables in patients with heart failure. These data suggest that class III antiarrhythmic agents, which selectively prolong repolarization, are not cardiodepressant but may be proarrhythmic in humans, especially at high doses.

Proarrhythmia with class III antiarrhythmic drugs: definition, electrophysiologic mechanisms, incidence, predisposing factors, and clinical implications

Antiarrhythmic drugs can and do induce unexpected and sometimes fatal reactions by either producing new symptomatic arrhythmias or by aggravating existing arrhythmias. The definition of proarrhythmia has changed since controlled clinical studies showed a dichotomy between arrhythmia suppression and mortality. The nature of proarrhythmic reactions is linked to the electrophysiologic effects of various antiarrhythmic drugs. Whereas Class I agents without accompanying effects on repolarization generally produce ventricular tachycardia (often incessant) or fibrillation, Class III agents typically produce torsades de pointes that may deteriorate into ventricular fibrillation. The precise mechanism of torsades de pointes is not fully elucidated, although early after-depolarization and increases in spatial or temporal dispersion of repolarization are likely possibilities. Proarrhythmic risk is lowest for amiodarone and is probably related to the drug's complex electrophysiologic profile. The incidence of torsades with sotalol increases with dose and the baseline values of the QT interval; the incidence with d-sotalol and other pure Class III agents remains unclear. Prospective, randomized, placebo-controlled studies to evaluate this are under way. The fact that d-sotalol increases mortality in postinfarction patients suggests that it may possibly be a common property of most, if not all, pure Class III compounds. The ongoing clinical trials with various Class III agents are likely to provide the critical information on this important therapeutic issue.

Basic concepts in cellular cardiac electrophysiology: Part II: Block of ion channels by antiarrhythmic drugs

Antiarrhythmic drugs have relative specificity for blocking each of the major classes of ion channels that control the action potential. The kinetics of block is determined by the state of the channel. Those channel states occupied at depolarized potentials generally have greater affinity for the blocking drugs. The kinetics of the drug-channel interaction is important in determining the blocking profile observed clinically. The increased mortality resulting from drug treatment in CAST and several atrial fibrillation trials has resulted in a shift in antiarrhythmic drug development from the Na+ channel blocking (Class I) drugs to the K+ channel blocking (Class III) drugs. While both Classes of drugs have a proarrhythmic potential, this may be less for the Class III agents. Their lack of negative inotropy also make them more attractive. It is important that the potential advantages of these agents be evaluated in controlled clinical trials. In several laboratories, the techniques of molecular biology and biophysics are being combined to determine the block site of available drugs. This information will aid in the future development of agents with greater specificity, and hopefully greater efficacy and safety than those currently in clinical use.

Characterization of the sensitivity of cardiac outwardly-rectifying K+ channels to class III antiarrhythmics: the influence of inhibitory sulfonamide derivatives

Elementary K+ currents through cardiac 66 pS outwardly-rectifying K+ channels isolated from cultured neonatal rat cardiocytes were recorded in the inside-out patch configuration. By analyzing the influence of inhibitory sulfonamide derivatives, the block phenomenology evoked by these class III antiarrhythmic drugs was studied. After isolation from their cellular environment, K+(outw.-rect.) channels became usually upregulated so that open probability increased with time to reach, within 3 min or longer, a several-fold enhanced steady state level. Nevertheless, the novel sulfonamide derivative HE93 (10-100 mumol/l) depressed NPo significantly within some hundred milliseconds on cytosolic administration with a calculated IC50 value of 38 mumol/l. Drug-induced channel blockade mainly emerged from an increased life time of the prolonged C2-state; tau closed (2) rose (at 100 mumol/l) to 269 +/- 20%. A C1-C2 reaction scheme can adequately describe closed time kinetics in the presence of HE93 but the occurrence of a specific, drug-evoked ultralong (< or = 60 ms) C-state and mainly underlying the NPo depression cannot be excluded. Sotalol (100 mumol/l) caused the same block phenomenology although a 2.6-fold larger IC50 value (half maximal inhibitory concentration) suggests a smaller potency to depress channel activity. Despite a close structural relationship with the both compounds HE93 and sotalol, glibenclamide (100 mumol/l) exerted no significant inhibitory influence (IC50 = 530 mumol/l) on K+ channel activity. Instead, this sulfonylurea interfered with open K+ channels with an association rate constant of 8.2 +/- 3.8 x 10(6) mol-1 s-1 to shorten their 0-state, as a sign of open channel blockade.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of amiodarone on membrane properties, refractoriness, and conduction in guinea pig papillary muscles

Amiodarone has potent and complex antiarrhythmic effects associated with a rare incidence of proarrhythmia. For a comprehensive understanding of its antiarrhythmic mechanisms in the same preparations, amiodarone (50 microM) was employed as it would be in the clinical setting and applied to guinea pig papillary muscles impaled by microelectrodes, paced at different rates, and superfused with various concentrations of potassium ([K]e). Amiodarone exerted complex actions, as follows: (1) The maximum rate of rise (Vmax) of the fast action potential (i.e., [K]e = 5.4-9.0 mM) as well as that of the slow action potential (i.e., [K]e = 15.0 mM in the presence of 1.0 microM isoproterenol) was suppressed in a rate-dependent manner. (2) Amiodarone exhibited a rate- and [K]e-dependent increase in the ratio of effective refractory period vs action potential duration at 90% repolarization (ERP/APD90), disclosing post-repolarization refractoriness. (3) Amiodarone had no effect on passive cable factors, such as threshold current and tissue resistance, during propagation. These versatile electrophysiological effects of amiodarone may contribute to its unique antiarrhythmic effects, as well as the low incidence of proarrhythmia with this drug.

Effects of the potassium channel blocking agent ambasilide on ventricular arrhythmias induced by acute myocardial ischemia and sympathetic activation

The ineffectiveness of traditional antiarrhythmic agents in preventing sudden cardiac death has increased the interest in drugs that prolong refractoriness. Ambasilide is a new potassium channel blocking agent that appears to prolong refractoriness at short and long cycle lengths. We assessed the effects of ambasilide, 5 mg/kg intravenous (i.v.) bolus plus 5 mg.kg-1.hr-1 i.v. infusion, in 16 anesthetized cats in which ventricular arrhythmias could be induced reproducibly by the combination of acute myocardial ischemia and increased sympathetic activity. Ambasilide decreased heart rate and blood pressure and prolonged QRS duration (26%, p < 0.05), QTc (17%, p < 0.0001), and JTc (16%, p < 0.005). Ambasilide also shifted the strength-interval curve for ventricular refractoriness by 17 to 22 msec to the right (p < 0.001). Ventricular fibrillation was observed in 7 animals and never occurred after ambasilide (p < 0.001); however, 4 (57%) of these cats had sustained ventricular tachycardia requiring cardiac massage. Ambasilide prevented nonsustained ventricular tachycardia in 2 (40%) of 5 animals. The antiarrhythmic effect of ambasilide persisted when heart rate was kept constant by atrial pacing. In no case was proarrhythmia observed. Ambasilide had a significant electrophysiologic effect at the ventricular level in the cat because it did prolong QTc and ventricular refractoriness. Therefore ambasilide showed an antifibrillatory effect but provided only a partial protection against lethal arrhythmias induced by acute myocardial ischemia and sympathetic activation.

Controlling cardiac arrhythmias by lengthening repolarization: historical overview

An increasing confluence of experimental and clinical data on the gravity of proarrhythmic effects of class I agents has led to a shift to other electrophysiologic classes of agents for treating cardiac arrhythmias. The fact that beta blockers reduce mortality in a variety of subsets of patients has suggested a wider role for this class of agents. Recent investigations have focused on the potential role of sotalol and amiodarone, 2 agents that not only block sympathetic antagonism, but also prolong cardiac repolarization. They have been test drugs in an increasing number of controlled and uncontrolled trials in patients at risk for arrhythmic deaths. Their properties have formed the basis for the hypothesis that arrhythmias may be effectively controlled independently of changes in conduction; they are prototypes of compounds that may favorably influence electrical instability in the myocardium. Amiodarone and sotalol are, however, complex molecules with attendant side effects; therefore, attention is also focusing on compounds that act simply by selective prolongation of cardiac repolarization. These agents have been termed "pure" class III agents. The properties of sotalol, the prototype of class III agents, are of particular interest because it is a racemic mixture with the levo-isomer having 50 times the beta-blocking potency of the dextro-isomer; both have equipotent class III actions. Studies of the antiarrhythmic properties of beta blockers, D- and D,L-sotalol, and amiodarone may provide insights into the nature of class III actions. There is clinical evidence indicating that class III drugs exert a varying spectrum of antifibrillatory and proarrhythmic (characterized by torsades de pointes) actions for a given degree of prolongation of repolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiologic basis for the antiarrhythmic actions of sotalol and comparison with other agents

Although synthesized as a nonselective beta-adrenergic blocking compound, sotalol has emerged as the prototype of the so-called class III antiarrhythmic compounds. It delays cardiac repolarization by inhibiting the delayed rectifier potassium current, having a lesser effect on the inward rectifying potassium current with little or no effect on the inward calcium or sodium currents. This property of prolonging repolarization with an accompanying increase in the effective refractory period is not due to blockade of the beta-adrenergic receptors. The major electrophysiologic profile of sotalol constitutes the summed effects of beta blockade and prolonged repolarization. Sotalol exerts a potent antifibrillatory action modulated by its antiadrenergic effects. It suppresses premature ventricular contractions and nonsustained ventricular tachycardia while preventing inducible ventricular tachycardia and fibrillation in patients with advanced structural heart disease. The compound is therefore likely to exert a broad spectrum of antiarrhythmic actions in ventricular arrhythmias.

Pharmacological profiles of muscarinic receptors in the longitudinal smooth muscle of guinea pig ileum

We have examined the pharmacological subtypes of muscarinic receptors mediating phosphoinositide hydrolysis and contraction in the longitudinal smooth muscle of guinea pig ileum with the use of muscarinic antagonists. Carbachol increased the formation of 3H-inositol phosphates (IPs) in a dose-dependent manner in both ileal smooth muscle and the frontal cortex of rats. The rank order of muscarinic antagonists for inhibition of IP formation induced by carbachol was 4-DAMP = atropine > pirenzepine > AF-DX 116 in guinea pig ileal smooth muscle. In ileal smooth muscle, the inhibition by the M1 antagonist pirenzepine was about 15 times less than that by atropine. However, in the rat frontal cortex, the inhibition by pirenzepine was only about 3 times less than that by atropine. The inhibitory effect of the M2 antagonist AF-DX 116 was weak in both ileal muscle and the frontal cortex. The M3 antagonist 4-DAMP strongly inhibited carbachol-induced IP formation in ileal smooth muscle. The rank order of muscarinic antagonists for inhibition of contraction induced by 10(-7) M carbachol was atropine > or = 4-DAMP > pirenzepine > AF-DX 116. These results suggest that both IP formation and the contractile response induced by muscarinic agonists are mediated through the muscarinic M3 subtype in guinea pig ileum.

Effects of MS-551, a new class III antiarrhythmic drug, on action potential and membrane currents in rabbit ventricular myocytes

1. Electrophysiological effects of MS-551, a new class III antiarrhythmic drug, were examined and compared with those of (+)-sotalol in rabbit ventricular cells. 2. In rabbit ventricular muscles stimulated at 1.0 Hz, MS-551 (0.1-10 microM) and (+)-sotalol (3-100 microM) prolonged action potential duration (APD) and effective refractory period without affecting the maximum upstroke velocity of phase 0 depolarization (Vmax). The class III effect of MS-551 was approximately 30 times more potent than that of (+)-sotalol. 3. Class III effects of MS-551 and (+)-sotalol showed reverse use-dependence, i.e., a greater prolongation of APD at a longer cycle length. 4. In rabbit isolated ventricular cells, 3 microM MS-551 and 100 microM sotalol inhibited the delayed rectifier potassium current (IK) which was activated at more positive potentials than -50 mV and saturated around +20 mV. 5. MS-551 at a higher concentration of 10 microM decreased the transient outward current (Ito) and the inward rectifier potassium current (IK1) although 100 microM sotalol failed to inhibit these currents. 6. MS-551 is a non-specific class III drug which can inhibit three voltage-gated K+ channels in rabbit ventricular cells.