Block of delayed rectifier potassium current, IK, by flecainide and E-4031 in cat ventricular myocytes

Block of the delayed rectifier potassium current, IK, by the class IC antiarrhythmic agent, flecainide, and by the novel selective class III antiarrhythmic agent, E-4031, were compared in isolated cat ventricular myocytes using the single suction-pipette, voltage-clamp technique. Flecainide (10 microM) markedly reduced IK elicited on depolarization steps to plateau voltages (+10 mV) and nearly completely blocked the "tail currents" elicited on repolarization to -40 mV (93 +/- 4% block at +40 mV, n = 3). E-4031 (1 microM) produced similar effects (96 +/- 3% block at +40 mV, n = 3). Slow voltage ramps from -100 to +40 mV confirmed inward rectifying properties of IK and showed that flecainide and E-4031 have no effects on the background potassium current, IK1. Thus, the results demonstrate that block of IK is a common feature of flecainide and E-4031. IK block by E-4031 most likely underlies the drug's potent class III antiarrhythmic properties. On the other hand, flecainide block of IK during an action potential would tend to prolong repolarization, but this effect may be obscured by concomitant block of plateau Na+ channels to produce little or no change in action potential duration, consistent with its class IC classification.     

Extracellular acidosis modulates drug block of Kv4.3 currents by flecainide and quinidine

INTRODUCTION: As a molecular model of the effect of ischemia on drug block of the transient outward potassium current, the effect of acidosis on the blocking properties of flecainide and quinidine on Kv4.3 currents was studied. METHODS AND RESULTS: Kv4.3 channels were stably expressed in Chinese hamster ovary cells. Whole-cell, voltage clamp techniques were used to measure the effect of flecainide and quinidine on Kv4.3 currents in solutions of pH 7.4 and 6.0. Extracellular acidosis attenuated flecainide block of Kv4.3 currents, with the IC50 for flecainide (based on current-time integrals) increasing from 7.8 +/- 1.1 microM at pH 7.4 to 125.1 +/- 1.1 microM at pH 6.0. Similar effects were observed for quinidine (IC50 5.2 +/- 1.1 microM at pH 7.4 and 22.1 +/- 1.3 microM at pH 6.0). Following block by either drug, Kv4.3 channels showed a hyperpolarizing shift in the voltage sensitivity of inactivation and a slowing in the time to recover from inactivation/block that was unaffected by acidosis. In contrast, acidosis attenuated the effects on the time course of inactivation and the degree of tonic- and frequency-dependent block for both drugs. CONCLUSION: Extracellular acidosis significantly decreases the potency of blockade of Kv4.3 by both flecainide and quinidine. This change in potency may be due to allosteric changes in the channel, changes in the proportion of uncharged drug, and/or changes in the kinetics of drug binding or unbinding. These findings are in contrast to the effects of extracellular acidosis on block of the fast sodium channel by these agents and provide a molecular mechanism for divergent modulation of drug block potentially leading to ischemia-associated proarrhythmia.     

Recent antiarrhythmic drugs

Clinical failure of antiarrhythmic drugs often occurs in practice. Therefore, there is a need for new, effective and long-acting drugs with a wide therapeutic range and a low level of toxicity. Most new class I compounds block the fast sodium ion inward current of myocardial cells. According to their effects on the recovery kinetics of the sodium ion channel, these drugs are classified into 3 groups: IA (intermediate — cibenzoline, pirmenol, hydroxy-3-S-dihydroquinidine, quinacainol); IB (fast — tocainide, moricizine); IC (slow — flecainide, encainide, propafenone, lorcainide, indecainide, recainam and penticainide). Class IC drugs greatly depress intracardiac conduction and are the most potent antiarrhythmic compounds able to suppress ventricular premature beats. However, it is doubtful that longterm suppression of ventricular arrhythmias will improve survival of the patients. Some new drugs have been developed belonging to other classes: class II, esmolol, a new ultrashort-acting β blocker; class III, N-acetyl-procainamide and sotalol, which prolong duration of the action potential and increase ventricular refractoriness; class IV, the mixed sodium ion-eakaum ion-potassium ion antagonist, bepridil. The pharmacologic properties and the clinical effects of these new antiarrhythmic drugs are reviewed.

However, future therapeutic trends will depend on the results of large multicenter clinical secondary prevention trials such as the Cardiac Arrhythmia Suppression Trial. New antiarrhythmic drugs with original electrophysiologic profiles and minimal adverse effects must prove their ability not only to suppress arrhythmias but also to reduce sudden cardiac death rate.     

Interactions of flecainide with guinea pig cardiac sodium channels. Importance of activation unblocking to the voltage dependence of recovery

Effects of flecainide, a potent antiarrhythmic agent, on sodium channel availability was investigated in guinea pig single cardiac cells by the whole-cell voltage-clamp technique. Sodium current (INa) experiments were performed at 17 degrees C, and maximum upstroke velocity (Vmax) experiments were performed at 37 degrees C. Flecainide (3 microM) caused little tonic block, but reduced sodium channel availability in a use-dependent manner. The latter effect was accentuated by depolarization and attenuated by hyperpolarization. Long (200-msec) and short (10-msec) depolarizations yielded similar use-dependent block. These results indicate that flecainide has a low affinity for rested (R) and inactivated (I) channels but a high affinity for activated ones (A). In each of these states, the channels can bind to drug to form the corresponding RD, ID, and AD states. Recovery from flecainide block consisted of two components. An initial fast component was strongly voltage dependent: with increasing hyperpolarization, recovery developed more quickly and to a larger extent. At 17 degrees C, the mean time constant shortened from 132 +/- 81.6 msec at -120 mV to 46.9 +/- 34.1 msec at +/- 160 mV (kinetics were too fast for accurate measurement at 37 degrees C). A later slow component was largely voltage independent: at 37 degrees C, the mean time constant was 9.8 +/- 3.2 seconds at -100 mV and 10.7 +/- 3.8 seconds at -75 mV. The slow component of recovery was similarly independent of voltage at 17 degrees C. In terms of the modulated-receptor theory, our results indicate that the fast recovery depends on availability for unblocking (RD) but occurs during activation (AD----A). Indeed, when the RD state is maximized by strong hyperpolarization, activation unblock was also maximized. However, during depolarization to -100 mV, availability for activation unblock declined with a time constant of 98 +/- 12 msec (RD----ID). Therefore, the voltage-dependent fast unblocking is mostly due to priming of the RD state (RD----ID), and the voltage-independent slow unblock reflects dissociation of flecainide from closed states (RD----R and ID----I). We conclude that flecainide interacts with sodium channels preferentially in the activated state, whereas unblocking occurs via two separate pathways: activated and closed states. Furthermore, drug association with channels shifts the voltage dependence of closed-state transitions (RD in equilibrium ID) and their kinetics toward more negative potentials.     

Common molecular determinants of local anesthetic, antiarrhythmic, and anticonvulsant block of voltage-gated Na+ channels.

Voltage-gated Na+ channels are the molecular targets of local anesthetics, class I antiarrhythmic drugs, and some anticonvulsants. These chemically diverse drugs inhibit Na+ channels with complex voltage- and frequency-dependent properties that reflect preferential drug binding to open and inactivated channel states. The site-directed mutations F1764A and Y1771A in transmembrane segment IVS6 of type IIA Na+ channel alpha subunits dramatically reduce the affinity of inactivated channels for the local anesthetic etidocaine. In this study, we show that these mutations also greatly reduce the sensitivity of Na+ channels to state-dependent block by the class Ib antiarrhythmic drug lidocaine and the anticonvulsant phenytoin and, to a lesser extent, reduce the sensitivity to block by the class Ia and Ic antiarrhythmic drugs quinidine and flecainide. For lidocaine and phenytoin, which bind preferentially to inactivated Na+ channels, the mutation F1764A reduced the affinity for binding to the inactivated state 24.5-fold and 8.3-fold, respectively, while Y1771A had smaller effects. For quinidine and flecainide, which bind preferentially to the open Na+ channels, the mutations F1764A and Y1771A reduced the affinity for binding to the open state 2- to 3-fold. Thus, F1764 and Y1771 are common molecular determinants of state-dependent binding of diverse drugs including lidocaine, phenytoin, flecainide, and quinidine, suggesting that these drugs interact with a common receptor site. However, the different magnitude of the effects of these mutations on binding of the individual drugs indicates that they interact in an overlapping, but nonidentical, manner with a common receptor site. These results further define the contributions of F1764 and Y1771 to a complex drug receptor site in the pore of Na+ channels.     

Proarrhythmic response to sodium channel blockade. Theoretical model and numerical experiments.

BACKGROUND. The use of flecainide and encainide was terminated in the Cardiac Arrhythmia Suppression Trial because of an excess of sudden cardiac deaths in the active treatment group. Such events might arise from reentrant rhythms initiated by premature stimulation in the presence of anisotropic sodium channel availability. Drugs that bind to sodium channels increase the functional dispersion of refractoriness by slowing (a result of the drug-unbinding process) the transition from an inexcitable state to an excitable state. It is interesting that encainide and flecainide unbind slowly (15-20 seconds), whereas lidocaine and moricizine unbind rapidly (0.2-1.3 seconds). METHODS AND RESULTS. With a computer representation of a cable with Beeler-Reuter membrane properties, we found a small (6 msec) vulnerable window that occurred 338 msec after the last drive stimulus. Premature stimuli falling within the vulnerable window resulted in unidirectional block and reentrant activation. In the presence of a slowly unbinding drug, the window was delayed an additional 341 msec, and its duration was extended to 38 msec. The delay (antiarrhythmic effect) before the onset of the vulnerable window and its duration (proarrhythmic effect) were both dependent on the sodium channel availability and the recovery process. Both effects were also prolonged when sodium channel availability was reduced by membrane depolarization. Defining the proarrhythmic potential as the duration of the vulnerable window, we found that hypothetical use-dependent class I drugs have a greater proarrhythmic potential than non-use-dependent drugs. CONCLUSIONS. The antiarrhythmic and proarrhythmic properties of pure sodium channel antagonists are both dependent on sodium channel availability. Consequently, the price for increased antiarrhythmic efficacy (suppressed premature ventricular contractions) is an increased proarrhythmic vulnerability to unsuppressed premature ventricular contractions.     

Flecainide increases Kir2.1 currents by interacting with cysteine 311, decreasing the polyamine-induced rectification

Both increase and decrease of cardiac inward rectifier current (I_K1) are associated with severe cardiac arrhythmias. Flecainide, a widely used antiarrhythmic drug, exhibits ventricular proarrhythmic effects while effectively controlling ventricular arrhythmias associated with mutations in the gene encoding Kir2.1 channels that decrease I_K1 (Andersen syndrome). Here we characterize the electrophysiological and molecular basis of the flecainide-induced increase of the current generated by Kir2.1 channels (I_Kir2.1) and I_K1 recorded in ventricular myocytes. Flecainide increases outward I_Kir2.1 generated by homotetrameric Kir2.1 channels by decreasing their affinity for intracellular polyamines, which reduces the inward rectification of the current. Flecainide interacts with the HI loop of the cytoplasmic domain of the channel, Cys311 being critical for the effect. This explains why flecainide does not increase I_Kir2.2 and I_Kir2.3, because Kir2.2 and Kir2.3 channels do not exhibit a Cys residue at the equivalent position. We further show that incubation with flecainide increases expression of functional Kir2.1 channels in the membrane, an effect also determined by Cys311. Indeed, flecainide pharmacologically rescues R67W, but not R218W, channel mutations found in Andersen syndrome patients. Moreover, our findings provide noteworthy clues about the structural determinants of the C terminus cytoplasmic domain of Kir2.1 channels involved in the control of gating and rectification.     

A combination of inactivated sodium channel blockers causes competitive interaction on dV/dtmax of single ventricular myocytes.

STUDY OBJECTIVE--The aim was to study an interaction between class I antiarrhythmic drugs on the cardiac sodium channels. DESIGN--The single pipette, whole cell clamp method was employed to control and record membrane potential. The maximum upstroke velocity (dV/dtmax) was measured as an index of sodium channel availability during treatment of the preparations with aprindine (5 microM) in combination with mexiletine (40 microM), and lignocaine (40 microM). EXPERIMENTAL MATERIAL--Single ventricular myocytes (n = 6-8 per experiment) isolated from guinea pig hearts were used. MEASUREMENTS AND MAIN RESULTS--Trains of depolarisation to 0 mV (0.2-2.0 Hz) were applied from the resting membrane potential (-85 mV) following a long quiescent period to evaluate "tonic" and "use dependent" decrease (block) of dV/dtmax. Additional application of mexiletine or lignocaine to aprindine resulted in an increase of tonic block and a decrease of use dependent block. Because of such counteracting action, the steady state dV/dtmax during the train of depolarisation was unaffected for mexiletine, and even increased for lignocaine. Dual exponential components of dV/dtmax recovery following a 1 s conditioning depolarisation after admixture of mexiletine or lignocaine to aprindine suggest their competitive interaction on cardiac sodium channels. CONCLUSION--A combination of class I antiarrhythmic drugs having high affinity for the inactivated state of sodium channels may cause a reductive effect on dV/dtmax through competitive displacement from common receptors.     

Auto-intoxication with flecainide and quinapril: ECG-changes, symptoms and treatment

Flecainide acetate is a sodium channel blocker and a class Ic antiarrhythmic agent with potential life-threatening proarrhythmic and cardioinhibitory properties when taken in overdose. Quinapril is an angiotensin-converting enzyme inhibitor (ACE-inhibitor) and overdose can lead to prolonged hypotension and, less frequently, transient renal impairment. We describe the first published case of intoxication with both drugs. The patient developed a broad-QRS-tachycardia and severe hypotension. Treatment with volume expansion, hypertonic sodium bicarbonate, inotropic support with norepinephrine and insertion of an intra-aortic balloon pump led to complete recovery after 72 hours. We assume that the clinical picture was mainly dictated by flecainide intoxication. Relevant literature data are discussed.     

ST segment elevation in the right precordial leads following administration of class Ic antiarrhythmic drugs

Electrocardiographic changes were evaluated retrospectively in five patients without previous episodes of syncope or ventricular fibrillation who developed abnormal ST segment elevation mimicking the Brugada syndrome in leads V1-V3 after the administration of class Ic antiarrhythmic drugs. Pilsicainide (four patients) or flecainide (one patient) were administered orally for the treatment of symptomatic paroxysmal atrial fibrillation or premature atrial contractions. The QRS duration, QTc, and JT intervals on 12 lead surface ECG before administration of these drugs were all within normal range. After administration of the drugs, coved-type ST segment elevation in the right precordial leads was observed with mild QRS prolongation, but there were no apparent changes in JT intervals. No serious arrhythmias were observed during the follow up periods. Since ST segment elevation with mild QRS prolongation was observed with both pilsicainide and flecainide, strong sodium channel blocking effects in the depolarisation may have been the main factors responsible for the ECG changes. As the relation between ST segment elevation and the incidence of serious arrhythmias has not yet been sufficiently clarified, electrocardiographic changes should be closely monitored whenever class Ic drugs are given.     

Blockade of 2,4-dinitrophenol induced ATP sensitive potassium current in guinea pig ventricular myocytes by class I antiarrhythmic drugs.

OBJECTIVE: The aim was to assess the effects of various antiarrhythmic drugs on 2,4-dinitrophenol (DNP) induced outward current (IDNP), presumably the ATP sensitive K+ current (IK,ATP) of isolated cardiac cells and to discuss mechanisms involved in the hypoglycaemia which occurs in patients on these drugs. METHODS: The quasi-steady state current-voltage relationship from the isolated guinea pig ventricular cells was measured using whole cell voltage clamp techniques with a ramp pulse programme. The effects of seven different antiarrhythmic drugs on IDNP were examined. Action potentials were elicited at a rate of 0.2 Hz by an intracellular current injection. RESULTS: DNP (50 mumol.litre-1) increased the quasi-steady state outward current at potentials positive to about -60 mV. This current (IDNP) was completely inhibited by the subsequent application of glibenclamide (1 mumol.litre-1), thereby suggesting that the IDNP is probably IK,ATP. Cibenzoline (10 mumol.litre-1, class Ia), disopyramide (30 mumol.litre-1, class Ia), and procainamide (100 mumol.litre-1, class Ia) significantly inhibited the IDNP by 95.5(SD 11.3)%, 77.8(21.2)%, and 76.4(23.9)% respectively. Flecainide (class 1c) inhibited the IDNP by 66.9(23.9)% at 10 mumol.litre-1 but not at 2 mumol.litre-1. Mexiletine (30 mumol.litre-1, class Ib), pilsicainide (50 mumol.litre-1, class Ic), and E4031 (10 mumol.litre-1, class III) at concentrations as high as approximately fivefold the clinically effective blood levels, did not suppress IDNP. Except for 10 mumol.litre-1 flecainide, all the concentrations listed above which blocked IDNP were within twofold of the clinical blood concentrations documented to be effective for suppression of arrhythmias. Cibenzoline, disopyramide, and procainamide, but not flecainide, belong to class Ia antiarrhythmic drugs. All these class Ia antiarrhythmic drugs "shortened" the action potential duration of guinea pig ventricular cells, an opposite change to that noted for multicellular preparations, eg, guinea pig papillary muscles. CONCLUSIONS: Class Ia antiarrhythmic drugs (cibenzoline, disopyramide, and procainamide) inhibit IDNP (presumably IK,ATP) in guinea pig ventricular cells within a range of therapeutic concentrations. This inhibitory effect of IK,ATP can probably explain the hypoglycaemia which occurs in some patients receiving these drugs, and the prolongation of the action potential duration alleged to occur in "superfused" papillary muscles.     

Combined class I antiarrhythmic agents have differential effects on tonic and use dependent block of maximum rate of depolarisation of action potentials in guinea pig cardiac muscle.

OBJECTIVE: The aim was to study the difference between tonic and use dependent block of the cardiac sodium channel produced by the combined application of the same subclass of antiarrhythmic agents (class Ia or Ib). METHODS: Conventional glass microelectrode technique was used to record the maximum rate of depolarisation (dV/dtmax) of action potentials reflecting sodium channel availability, before and after the combined application of quinidine plus disopyramide, aprindine plus lignocaine, aprindine plus mexiletine, and lignocaine plus mexiletine. Guinea pig papillary muscles (n = 4-8 per experiment) were used for the study. RESULTS: All combinations increased tonic block additively compared to use of a single drug. On the other hand, use dependent block was increased by the combination of quinidine 10 microM plus disopyramide 30 microM compared to a single drug, and was not changed by lignocaine 50 microM plus mexiletine 20 microM, whereas it was decreased by aprindine 3 microM plus lignocaine 50 microM or mexiletine 20 microM. When concentrations of mexiletine and lignocaine were increased, both tonic and use dependent block in a single drug were increased dose dependently, whereas the combination produced an additive increase in tonic block but no change in use dependent block compared to a single drug. CONCLUSIONS: The results suggested that the binding and unbinding process of the drug to produce tonic block was different from that to produce use dependent block, and that combination of different drugs produced diverse effects on use dependent block even though state dependent affinity of individual drugs seemed similar. These two factors must be born in mind in evaluating the combination therapy.     

A novel missense mutation in the SCN5A gene associated with Brugada syndrome bidirectionally affecting blocking actions of antiarrhythmic drugs

Brugada syndrome is an inherited cardiac disorder caused by mutations in the SCN5A gene encoding the cardiac sodium channel alpha subunit, which can lead ventricular fibrillation and sudden death. Inattentive use of antiarrhythmic drugs potentially triggers fatal cardiac arrhythmias through further reduction of sodium current (I(Na)). We studied the molecular mechanism underlying a case of Brugada syndrome that showed no response to a class Ic antiarrhythmic drug. Molecular genetic studies of a patient with Brugada syndrome identified a novel mutation in SCN5A, which causes substitution of serine for asparagine (N406S) in S6 of domain I (IS6). The provocation test with pilsicainide, a class Ic antiarrhythmic drug, failed to exacerbate ST-segment elevation in this case. Electrophysiological analyses of the N406S-mutant channel expressed together with the beta1 subunit in HEK293 cells showed that the voltage dependence of activation was positively shifted by 16 mV and that intermediate inactivation was enhanced. Whereas tonic block by pilsicainide was not changed in the N406S channel, use-dependent block by pilsicainide was almost completely abolished, consistent with the clinical findings of the negative provocation test. In contrast, the N406S channel showed stronger use-dependent block by quinidine than the wild-type channel. We demonstrate a novel Brugada mutation N406S, which is associated with the discordant effects on blocking actions of antiarrhythmic drugs as well as the multiple channel gating defects. We emphasis that an antiarrhythmic drug may exert unpredicted effects in patients with channel mutations.     

Norpropoxyphene-induced cardiotoxicity is associated with changes in ion-selectivity and gating of HERG currents

OBJECTIVE: Norpropoxyphene (NP) is a major metabolite of propoxyphene (P), a relatively weak mu-opioid receptor agonist. Toxic blood concentrations ranging from 3 to 180 mumol/l have been reported and the accumulation of NP in cardiac tissue leads to naloxone-insensitive cardiotoxicity. Since several lines of evidence suggest that not only block of INa but also IK block may contribute to the non-opioid cardiotoxic effects of P and NP, we investigated the effects of P and NP on HERG channels. HERG presumably encodes IKr, the rapidly-activating delayed rectifier K+ current, which is known to have an important role in initiating repolarization of action potentials in cardiac myocytes. METHODS: Using the 2-microelectrode voltage clamp technique we investigated the interaction of P and NP with HERG channels, expressed in Xenopus oocytes. RESULTS: Our experiments show that low drug concentrations (5 mumol/l) facilitate HERG currents, while higher drug concentrations block HERG currents (IC50-values of approx. 40 mumol/l) and dramatically shift the reversal potential to a more positive value because of a 30-fold increased Na(+)-permeability. P and NP also alter gating of HERG channels by slowing down channel activation and accelerating channel deactivation kinetics. The mutant S631C nullifies the effect of P and NP on the channel's K(+)-selectivity. CONCLUSION: P and NP show a complex and unique drug-channel interaction, which includes altering ion-selectivity and gating. Site-directed mutagenesis suggests that an interaction with S631 contributes to the drug-induced disruption of K(+)-selectivity. No specific role of the minK subunit in the HERG block mechanism could be determined.     

Effect of encainide and flecainide on chronic ectopic atrial tachycardia

In the treatment of chronic ectopic atrial tachycardia, standard antiarrhythmic therapy has been shown to be ineffective in the majority of patients. The intravenous and oral effects of two class IC antiarrhythmic drugs, encainide and flecainide, in five patients with chronic ectopic atrial tachycardia were studied using exercise testing, 24 hour long-term electrocardiography and programmed electrical stimulation. All patients had been treated unsuccessfully with at least four antiarrhythmic drugs. In two patients tachycardia was persistent, and in three patients tachycardia occurred intermittently for more than 12 hours/day. Intravenous encainide and flecainide at doses ranging from 0.3 to 2.0 mg/kg and from 0.5 to 1.5 mg/kg body weight, respectively, terminated atrial ectopic tachycardia in all patients. Oral encainide, 150 to 225 mg/day, completely suppressed ectopic atrial activity in four patients during a mean follow-up period of 8 +/- 3 months. In the remaining patient encainide markedly reduced the number of episodes of tachycardia. In three patients encainide had to be withdrawn because of intolerable side effects. These patients were well controlled with oral flecainide, 200 to 300 mg/day, without side effects. On the basis of these results, the efficacy of encainide and flecainide in the treatment of chronic ectopic atrial tachycardia appears to be not drug-specific but rather a general class IC property.     

Overview of the clinical pharmacology of antiarrhythmic drugs

Antiarrhythmic drugs have been recognized to possess 1 or more classes of antiarrhythmic action. This classification scheme is useful, but has major limitations because the available drugs and their metabolites have multiple actions. This report presents an overview of the distinguishing features of the most frequently used agents having class I or III actions. Agents with class I actions are local anesthetic agents that depress the fast inward depolarizing sodium current and thereby slow the rate of the rise of the action potential (phase 0). This category is further divided into classes IA, IB, and IC according to the degree of potency as sodium channel inhibitors, and the individual effects of the drug on action potential, conduction velocity and repolarization. Included in the spectrum of agents with class I action are quinidine, procainamide, disopyramide, lidocaine, tocainide, mexiletine, flecainide, amiodarone, encainide and lorcainide. The antiarrhythmic drugs that exert class III action lengthen repolarization and refractoriness; included in this category are amiodarone, quinidine, bretylium and sotalol. Because of the broad range of effects that antiarrhythmic agents may exert, safe and effective therapy requires a thorough familiarity with the pharmacologic profile of each drug administered and a careful evaluation of the presenting condition and the patient history. In some cases, a multiple drug regimen may be most appropriate. Various combinations such as class IA and IB agents, have been shown to slow conduction synergistically and increase refractoriness while keeping adverse effects to a minimum.     

Comparative studies of ATP sensitive potassium channels in heart and pancreatic beta cells using Vaughan-Williams class Ia antiarrhythmics.

OBJECTIVE: Actions of cibenzoline and disopyramide, agents with Vaughan-Williams class Ia antiarrhythmic action, on ATP sensitive K+ (KATP) channels were examined in heart and pancreatic beta cells. METHODS: Single ventricular myocytes and beta cells were prepared enzymatically from adult Wistar rat hearts and pancreatic islets. Using patch clamp techniques, KATP channel activities were recorded in whole cell and single channel modes. In whole cell experiments, myocytes were bathed with Tyrode's medium (34 degrees C); inside out patches were bathed with internal solutions (22-24 degrees C) containing 1 microM ATP and varying concentrations of cibenzoline or disopyramide. Myocytes were voltage clamped at -40 mV and glibenclamide blockade conductance was produced by cromakalim. RESULTS: Micromolar concentrations of both cibenzoline and disopyramide suppressed cromakalim induced conductance. When applied to the cytosolic surface of the cell membrane in inside out configuration, both drugs reversibly inhibited single KATP channel activities. Neither unitary conductance nor intraburst fast kinetics was affected by the compounds. At a holding potential of -40 mV under symmetrical approximately 150 mM K+ conditions, half maximum doses (IC50) were 0.9 microM [Hill coefficient (h) = 1.3] for cibenzoline induced block of cardiac KATP channels and 1.8 microM (h = 1.0) for disopyramide block. At +40 mV, IC50 for cibenzoline block was 1.4 microM (h = 0.9). Thus there was little voltage dependence in cibenzoline induced channel block. A similar IC50 value of 2.5 microM (h = 1.2 at -60 mV under symmetrical approximately 150 mM K+) was observed for cibenzoline induced block of KATP channels. CONCLUSIONS: Near therapeutic concentrations of cibenzoline and disopyramide inhibit KATP channel activities in both heart and pancreatic beta cells. This may be causally related to the fasting hypoglycaemia which is sometimes reported in patients receiving the drugs. These antiarrhythmic agents may also modulate myocardial electrical properties during hypoxia or ischaemia.     

Antiarrhythmic effects of the class 1c antiarrhythmic drug, flecainide, on canine ventricular arrhythmia models

The properties of a class 1c antiarrhythmic drug, flecainide, were examined using 3 canine ventricular arrhythmia models: (1) digitalis-, (2) adrenaline- and (3) two-stage coronary ligation-induced arrhythmias. The minimum effective plasma concentration of flecainide was determined for each model. Flecainide suppressed the arrhythmias. The minimum effective plasma concentrations for arrhythmias induced by digitalis, adrenaline and 24 hr coronary ligation, were 0.9 +/- 0.2, 1.4 +/- 0.6, 1.5 +/- 0.4 mcg/ml, respectively (mean +/- SD, n = 6). These minimum effective plasma concentrations of flecainide were almost equal to the reported in vitro concentration that suppress the Na channels of isolated canine ventricular tissues. Thus, flecainide may suppress these arrhythmias by inhibiting the Na channels of cardiac cells.     

Safety of antiarrhythmics during pregnancy: case report and review of the literature

A young woman is reported with intractable sustained ventricular tachycardia thought to originate in the right ventricle, which was treated successfully with encainide after failure to respond to beta-blockers and several class IA antiarrhythmic agents. She became pregnant twice while on encainide and gave birth to two healthy children. This is the first report of pregnancy during treatment with encainide. A literature review showed no other reported case of encainide taken during pregnancy, but several reports of the safe use of flecainide, a similar class IC drug, during pregnancy. Other antiarrhythmics are also reviewed.     

Mechanism of block and identification of the verapamil binding domain to HERG potassium channels.

Calcium channel antagonists have diverse effects on cardiac electrophysiology. We studied the effects of verapamil, diltiazem, and nifedipine on HERG K+ channels that encode IKr in native heart cells. In our experiments, verapamil caused high-affinity block of HERG current (IC50=143.0 nmol/L), a value close to those reported for verapamil block of L-type Ca2+ channels, whereas diltiazem weakly blocked HERG current (IC50=17.3 micromol/L), and nifedipine did not block HERG current. Verapamil block of HERG channels was use and frequency dependent, and verapamil unbound from HERG channels at voltages near the normal cardiac cell resting potential or with drug washout. Block of HERG current by verapamil was reduced by lowering pHO, which decreases the proportion of drug in the membrane-permeable neutral form. N-methyl-verapamil, a membrane-impermeable, permanently charged verapamil analogue, blocked HERG channels only when applied intracellularly. Verapamil antagonized dofetilide block of HERG channels, which suggests that they may share a common binding site. The C-type inactivation-deficient mutations, Ser620Thr and Ser631Ala, reduced verapamil block, which is consistent with a role for C-type inactivation in high-affinity drug block, although the Ser620Thr mutation decreased verapamil block 20-fold more than the Ser631Ala mutation. Our findings suggest that verapamil enters the cell membrane in the neutral form to act at a site within the pore accessible from the intracellular side of the cell membrane, possibly involving the serine at position 620. Thus, verapamil shares high-affinity HERG channel blocking properties with other class III antiarrhythmic drugs, and this may contribute to its antiarrhythmic mechanism.