Comparison of the rate-dependent properties of the class III antiarrhythmic agents azimilide (NE-10064) and E-4031: considerations on the mechanism of reverse rate-dependent action potential prolongation

INTRODUCTION: Reverse rate-dependence, a lessening in Class III antiarrhythmic agent action potential duration (APD) prolongation as heart rate is increased, has been proposed to be related to an incomplete deactivation of the slow component (IKs) of the delayed rectifier K+ current (IK). The rate-dependent properties of block of IK by azimilide were compared to E-4031, which selectively blocks the rapid component (IKr) of IK, in guinea pig ventricular muscle. METHODS AND RESULTS: Azimilide prolonged APD in isolated papillary muscles in a concentration-dependent manner and to a greater degree than E-4031. Both agents prolonged APD less at fast than slow rates, consistent with a similar reverse rate-dependent effect. Isolation of azimilide block of IKs by subtraction of APD during E-4031 plus azimilide from E-4031 alone revealed rate-independent prolongation of APD. In voltage clamp experiments on single ventricular myocytes, activation of IKs was similar following 30 seconds of conditioning pulses of physiological duration (125 to 200 msec) with either a fast (cycle length 250 msec) or slow (cycle length 2000 msec) rate. The block of IKs by azimilide 3 microM was greater after a fast conditioning pulse train. CONCLUSIONS: Selective block of IKs prolongs APD in a rate-independent manner. In voltage clamped myocytes, no evidence of a rate-dependent accumulation of IKs was observed. These findings support a mechanism of reverse rate-dependent APD prolongation by Class III antiarrhythmic agents that block IKr independent of IKs.     

Blood and tissue compatibility of modified polyester: thrombosis, inflammation, and healing

OBJECTIVE: The effects of BRL-32872, azimilide and a selective blocker of the delayed rectifier potassium current, E-4031, were measured at two different basic cycle lengths (BCL), 300 and 1000 ms. Calcium channel antagonists of sarcolemmal (verapamil and nitrendipine) and sarcoplasmic reticulum (ryanodine) membranes were used to investigate whether the inhibition of the calcium current or the calcium release from the sarcoplasmic reticulum could alter the reverse-rate dependence of E-4031 on action potential duration (APD). METHODS: Guinea pig isolated papillary muscles were superfused with a Tyrode solution maintained at 37 degrees C and stimulated at a BCL of 300 or 1000 ms. The standard microelectrode technique was used to record action potential parameters and to study the effects of azimilide, BRL-32872 and E-4031. E-4031 was superfused at increasing concentrations (0.01, 0.03, 0.1 and 0.3 microM) in the absence or in the presence of verapamil (0.3 microM), nitrendipine (0.03 microM) or ryanodine (0.1 microM). RESULTS: BRL-32872 and azimilide induced a self-limited concentration-dependent increase in APD. The effect of BRL-32872 was not dependent on the stimulation frequency whereas the effect of azimilide was significantly reduced at the shorter BCL. E-4031 induced a concentration-dependent increase in APD at both stimulation BCL. The increase in APD was significantly more pronounced in fibres stimulated at a BCL of 1000 ms than in fibres stimulated at a BCL of 300 ms, characterising the reverse-frequency dependent effect of class III antiarrhythmic agents. The reverse-frequency dependence in action potential prolongation induced by E-4031 was significantly reduced in the presence of a low concentration of verapamil (0.3 microM), nitrendipine (0.03 microM), or ryanodine (0.1 microM. CONCLUSION: The results show that BRL-32872, in contrast to azimilide, does not induce the reverse-rate dependency of action potential prolongation typically produced by class III antiarrhythmic agents such as E-4031. Our results also show that reverse-rate dependency induced by E-4031 can be reduced by the simultaneous administration of a low concentration of a calcium channel antagonist or an inhibitor of the release of calcium from the sarcoplasmic reticulum. It is thus suggested that compounds with a suitable balance of potassium and calcium antagonistic activities may have less adverse effects than purely selective potassium channel blockers.     

Effects of gonadal steroids on ventricular repolarization and on the response to E4031

Gonadal steroids are thought to be important determinants of gender-related differences in electrophysiology, such as the longer rate-corrected QTc intervals in women and the incidences of some clinical arrhythmias. We studied the chronic effects of gonadal steroids on cardiac action potentials (APs) using standard electrophysiological techniques. Papillary muscles were removed from the hearts of oophorectomized rabbits that had been treated with placebo, estradiol or dihydrotestosterone (DHT). The electrocardiograms of the three groups did not differ. Papillary muscle APs were studied during drive at cycle lengths of 330 to 5000 msec. The APD30 of the DHT group was significantly shorter than that of the others at cycle lengths of >500 msec. The APD90 of the estradiol group was significantly longer than that of the DHT group at cycle lengths of >1000 msec. The APD90 of the placebo group tended to be intermediate. The effects of the antiarrhythmic drug E4031 (10(-8)-10(-6) M) also were examined. E4031-induced prolongation of APD90 and magnitude of early afterdepolarizations was significantly greater in the estradiol-treated than the DHT-treated and placebo groups. In conclusion, in rabbit heart, gonadal steroids are important determinants of base-line electrophysiological properties and the proarrhythmic response to E4031.     

Enhanced susceptibility for acquired torsade de pointes arrhythmias in the dog with chronic, complete AV block is related to cardiac hypertrophy and electrical remodeling

BACKGROUND: Chronic, complete AV block (CAVB) in the dog leads to ventricular hypertrophy, which has been described as an independent risk factor for arrhythmias. In this model, we examined (1) whether the short- and long-term electrical adaptations predispose to acquired torsade de pointes arrhythmias (TdP) and (2) the nature of the structural and functional adaptations involved. METHODS AND RESULTS: We determined (1) endocardial right (RV) and left (LV) ventricular APD, DeltaAPD (LV APD-RV APD), presence of EADs at 0 weeks (acute: AAVB), and CAVB (6 weeks) and inducibility of TdP by pacing and d-sotalol (n=10); (2) steady-state and dynamic LV hemodynamics at 0 and 6 weeks (n=6); (3) plasma neurohumoral levels in time (n=7); (4) structural parameters of the LV and RV of CAVB dogs (n=6) compared with sinus rhythm (SR) dogs (n=6); and (5) expression of ventricular mRNA atrial natriuretic factor (ANF) in CAVB (n=4) and SR (n=4) dogs. Compared with AAVB, CAVB led to nonhomogeneous prolongation of LV and RV APD and different sensitivity for d-sotalol, leading to EADs (4 of 14 versus 9 of 18, P<0.05), increased DeltaAPD (45+/-30 versus 125+/-60 ms, P<0.05), and induction of TdP in most dogs (0% versus 60%, P<0.05). CAVB led to biventricular hypertrophy, whereas LV function was similar in AAVB and CAVB. The neurohumoral levels were transiently elevated. The LV and RV collagen and the capillary/fiber ratio remained normal, whereas ventricular ANF mRNA was not detectable. CONCLUSIONS: The electrical remodeling occurring after CAVB predisposes the heart to acquired TdP, whereas the structural changes (hypertrophy) are successfully aimed at maintaining cardiac function.     

The ABC maltose transporter

OBJECTIVE: To define the electrophysiologic mechanism(s) by which MCI-154, a putative Ca2+ sensitizer, produces a positive inotropic response without a positive chronotropic response, we examined effects of MCI-154 on the action potential of atrial preparations and the membrane currents of atrial myocytes. METHODS: The action potentias were recorded from left atrial and sinoatrial node preparations of guinea pigs by the use of standard microelectrode techniques. The whole-cell membrane currents were recorded from enzymatically-dissociated guinea pig atrial myocytes using conventional patch clamp techniques. RESULTS: In isolated left atria, MCI-154 increased the developed tension in a concentration-dependent manner. MCI-154 at concentrations of 10 and 100 microM increased the action potential duration (APD) in left atria stimulated at 0.5 Hz. In sinoatrial node preparations MCI-154 at a concentration of 100 microM produced a negative chronotropic response and prolonged APD. In single right atrial myocytes, MCI-154 at concentrations of 10 and 100 microM failed to increase the inward L-type Ca2+ current, but decreased the delayed rectifier K+ current (IK) in a concentration-dependent manner. MCI-154 decreased IK elicited by short depolarizing pulses more markedly than that induced by long depolarizing pulses. In addition, MCI-154 produced only a little inhibition of IK in the presence of E-4031, a specific blocker of rapidly activating component of IK (IKr). CONCLUSIONS: MCI-154 preferentially blocks IKr and the inhibitory action on IKr may be partly involved in the negative chronotropic and positive inotropic responses in atrial preparations.     

KN-93, an inhibitor of multifunctional Ca++/calmodulin-dependent protein kinase, decreases early afterdepolarizations in rabbit heart

The multifunctional Ca++/calmodulin-dependent protein kinase II (CaM kinase) mediates Ca++-induced augmentation of L-type Ca++ current (ICa); therefore it may act as a proarrhythmic signaling molecule during early afterdepolarizations (EADs) due to ICa. To investigate the hypothesis that ICa-dependent EADs are favored by CaM kinase activation EADs were induced with clofilium in isolated rabbit hearts. All EADs were rapidly terminated with ICa antagonists. Hearts were pretreated with the CaM kinase inhibitor KN-93 or the inactive analog KN-92 (0.5 microM) for 10 min before clofilium exposure. EADs were significantly suppressed by KN-93 (EADs present in 4/10 hearts) compared to KN-92 (EADs present in 10/11 hearts) (P =.024). There were no significant differences in parameters favoring EADs such as monophasic action potential duration or heart rate in KN-93- or KN-92-treated hearts. CaM kinase activity in situ increased 37% in hearts with EADs compared to hearts without EADs (P =.015). This increase in CaM kinase activity was prevented by pretreatment with KN-93. In vitro, KN-93 potently inhibited rabbit myocardial CaM kinase activity (calculated Ki 100 microM). The actions of KN-93 and KN-92 on ICa and other repolarizing K+ currents did not explain preferential EAD suppression by KN-93. These data show a novel association between CaM kinase activation and EADs and are consistent with the hypothesis that the ICa and CaM kinase activation both contribute to EADs in this model.     

Comparison of direct negative chronotropic and positive inotropic effects of sematilide to those of E-4031 and MS-551 and the reverse frequency-dependent prolongation of cardiac refractoriness of sematilide

Direct cardiac effects of sematilide, a new class III antiarrhythmic drug, were compared with those of E-4031 and MS-551 in canine isolated blood-perfused heart preparations. Doses of sematilide, E-4031, and MS-551 causing a 10% decrease in the spontaneous sinoatrial beating rate were 58 +/- 15, 9 +/- 5, and 84 +/- 10 micrograms (n = 5); those causing a 10% increase in developed tension of the papillary muscle were 485 +/- 49, 17 +/- 2, and 267 +/- 50 micrograms (n = 6); and those causing a 10% prolongation of effective refractory period (ERP) of the atrioventricular node were 68 +/- 10, 11 +/- 2, and 53 +/- 15 micrograms (n = 5), respectively. There were few effects on atrio-His or His-ventricular intervals. Also, in in situ open-chest dog hearts, the percent increases in ERP of the atrioventricular conduction system caused by 1 mg/kg of sematilide were 21 +/- 3, 16 +/- 2 and 9 +/- 1% at cycle lengths of 800, 600, and 400 ms, respectively (p < 0.01; n = 8). These results indicate that (a) sematilide, as well as E-4031 and MS-551, has direct negative chronotropic and positive inotropic effects and prolongs cardiac refractoriness without affecting conduction velocities; (b) quantitatively, the cardiac effects of sematilide were almost identical to those of MS-551 and five to ten times less potent than those of E-4031; (c) and prolongation of ERP of the atrioventricular conduction system by sematilide occurred in a reverse frequency-dependent manner.     

Azimilide causes reverse rate-dependent block while reducing both components of delayed-rectifier current in canine ventricular myocytes

Most class III antiarrhythmic drugs reduce the rapidly activating component of delayed-rectifier current (IKr) without affecting the slowly activating component (IKs). Recently the novel antiarrhythmic agent azimilide (NE-10064) was reported to enhance IKs at low (nanomolar) concentrations and to block both IKr and IKs at higher (micromolar) concentrations. Further to understand the electrophysiologic effects of azimilide, we compared its effects on IKr and IKs (by using whole cell clamp techniques) and action potentials (microelectrode and perforated-patch techniques) on canine ventricular myocytes. A lower azimilide concentration (50 nM) did not enhance IKs. In contrast, a therapeutic azimilide concentration (2 microM) was equieffective in reducing IKr (300-ms isochrones) and IKs (3-s isochrones) by approximately 40% during depolarizing test pulses, as well as reducing IKr (38% decrease) and IKs (33% decrease) tail currents on repolarization. Block of IKs was independent of voltage at positive test potentials. In action-potential studies, 50 nM azimilide had no effect on the action-potential duration (APD), whereas 2 microM azimilide delayed repolarization and caused reverse rate-dependent effects on the APD. Whereas the extent of APD prolongation by azimilide was not correlated with the drug-free APD, azimilide preferentially exaggerated the APD-rate relationship of myocytes displaying the steepest APD-rate relationship under drug-free conditions. In conclusion, therapeutic concentrations of azimilide that cause comparable reduction of canine ventricular IKr and IKs exert reverse rate-dependent effects, which are dependent on the steepness of the APD-rate relationship.     

Mechanism of alpha-2 adrenergic modulation of canine cardiac Purkinje action potential

We reported recently that stimulation of postjunctional alpha-2 adrenergic receptors prolongs the action potential durations (APD) of isolated canine Purkinje fibers. With standard microelectrode techniques, we examined the ionic mechanism through which alpha-2 adrenergic stimulation prolonged Purkinje APD, by measuring the effects of inhibitors of the various plateau currents on the alpha-2-mediated prolongation of APD. The alpha-2-specific agonist UK 14,304 (0.1 microM) prolonged the Purkinje APD at 50% repolarization and the APD at 90% repolarization, and these effects were inhibited by yohimbine (0.1 microM). The Purkinje APD at 50% repolarization and the APD at 90% repolarization were prolonged significantly with the transient outward potassium current inhibitor 4-aminopyridine (1 mM), the rapid component of delayed rectifier potassium current inhibitor d-sotalol (10 microM), the slow component of delayed rectifier potassium current inhibitor indapamide (0.1 microM) and the chloride current inhibitor mefenamic acid (10 nM) and were shortened significantly with the calcium current inhibitor nifedipine (0.3 microM). Prolongation of Purkinje APD at 50% repolarization and APD at 90% repolarization by UK 14,304 remained intact in the presence of d-sotalol, indapamide, mefenamic acid and nifedipine. All of these UK 14,304 effects were significantly reversed by yohimbine. Only in the presence of 4-aminopyridine did UK 14,304 fail to prolong Purkinje APD. The phase 1 magnitudes of Purkinje action potentials were also significantly inhibited by UK 14,304. This effect was completely abolished only in the presence of 4-aminopyridine. These results suggest that inhibition of the 4-aminopyridine-sensitive transient outward potassium current is the major ionic mechanism by which alpha-2 adrenergic stimulation prolongs Purkinje APD.     

Differential class III and glibenclamide effects on action potential duration in guinea-pig papillary muscle during normoxia and hypoxia/ischaemia

1. Microelectrode recording techniques were used to study the effects of several potassium channel blockers which are considered to be Class III antiarrhythmic compounds. The effects of (+)-sotalol, UK-66,914, UK-68,798 and E-4031 on action potential duration (APD) were determined in guinea-pig isolated papillary muscles. The compounds were evaluated under normoxic or hypoxic/ischaemic conditions at 36.5 degrees C and compared to glibenclamide, which is considered to be a blocker of ATP-dependent potassium channels. Prolongation of action potential duration at 90% repolarization (APD90) was taken as an indirect measure of potassium channel blockade. 2. Under normoxic conditions, the Class III compounds prolonged APD in a concentration-dependent manner. According to EC15 values, the order of potency of the Class III compounds was found to be UK-68,798 > E-4031 > UK-66,914 > (+)-sotalol. Glibenclamide did not significantly prolong APD90 under normoxic conditions. 3. Perfusion with an experimental hypoxic or ischaemic bathing solution produced qualitatively similar effects on action potentials. Over a period of 20-25 min in either of the experimental solutions, there was a small decrease in action potential amplitude (APA) and a prominent shortening of APD. The ischaemic solution also depolarized the resting membrane potential by about 15 mV. 4. (+)-Sotalol and UK-66,914 did not reverse the shortening of APD induced by perfusion with hypoxic Krebs solution. High concentrations of glibenclamide (10 microM) and UK-68,798 (30 and 60 microM) partially reversed the hypoxia-shortened APD. Glibenclamide was more potent and exhibited a greater time-dependent action than UK-68,798. 5. During experimental ischaemia, the Class III compound E-4031 (10 microM, n = 7) produced small, but significant, increases in the APD90 (11 +/-3 ms after 20 min) which were not clearly time-dependent(14 +/- 4 ms after 30 min). UK-68,798 (10 microM) also produced a small, but insignificant, increase in APD90(12 =/-6 ms at 20 min, n = 4). Higher concentrations of UK-68,798 (30 and 60 microM, n = 4) did not produce a consistently significant increase in APD90 during ischaemia: significance was only attained after 20 min in the presence of 60 microM UK-68,798 (24 +/- 12 ms). However, in marked contrast to the effects of the Class III compounds, glibenclamide (10 microM) produced large time-dependent increases in ischaemic APD90 (34 +/- 11 ms at 7 min, n = 9) which were significant 15 min or more after drug addition(52 +/- 12 ms at 20 min, n = 7; 74 +/- 5 ms at 30 min, n = 6).6. The present microelectrode data suggest that blockers of ATP-dependent potassium channels, such as glibenclamide, might prove to be more effective than Class III compounds against ischaemia-induced shortening of cardiac action potentials.     

The relationship between early afterdepolarization and the occurrence of torsades de pointes--an in vivo canine model study

The relationship between early afterdepolarization (EAD) and the occurrence of torsades de pointes (TdP) was studied in a canine model. Twelve dogs of both sexes, weighing 9.9-16 Kg, were studied. After reducing the concentration of serum potassium to 3.0-4.0 mEq/l, by administration of calcium polystyrene sulfonate at 15-20 g/day for 1 or 2 weeks, a 6F electrode catheter was introduced via the femoral vein and positioned at the atrioventricular (AV) junction. Complete AV block was produced by catheter ablation using a high frequency current. A Franz 6F catheter was introduced into the right ventricle to record monophasic action potentials (MAPs) using the contact electrode technique. After a stable recording of the MAPs was achieved, cesium chloride (CsCl; 1 mM/Kg) was administered as an intravenous bolus over 15 sec. The MAPs and electrocardiogram (ECG) changes were simultaneously recorded for 30 min after the administration of CsCl. The administration was repeated several times at intervals 30 min. Sustained or non-sustained ventricular tachycardia was produced in all dogs. EAD appeared in 8 of 12 dogs. When EAD developed sufficiently high amplitude, ventricular premature beats occurred near the peak of EAD and TdP was induced in 3 of 8 EAD-positive dogs. TdP was not induced in EAD-negative dogs. Although TdP was comparatively difficult to induce, EAD-triggered activity was suggested to be one of the necessary conditions for TdP, because TdP occurred only when EAD reached a sufficiently high amplitude to produce ventricular premature beats.     

Accentuated antagonism in canine subendocardium is not altered by chronic exercise

Acetylcholine often affects cardiac action potential repolarization only during augmented adrenergic tone, i.e., the phenomenon of accentuated antagonism. Since chronic exercise involves repeated changes in autonomic outflow, we determined whether it also influenced adrenergic/cholinergic interactions in isolated canine cardiac tissue. Using standard micro-electrode techniques in thin ventricular subendocardial slices isolated from exercised (EX: 8-10 wk daily exercise) and sedentary (SED): 8-10 wk cage rest) dogs, we examined transmembrane potential responses to isoproterenol (ISO: 10(-8), 10(-7), 10(-6) M) and to ISO in the presence of ACH (10(-5) M). Control transmembrane characteristics at BCL = 500 ms were similar for EX (N = 8 dogs) and SED (N = 9 dogs). ISO (10(-6) M) decreased action potential duration at 50% repolarization (APD50): EX = -29 +/- 15 ms; SED = 11 ms and at 90% repolarization (APD90): EX = -37 +/- 17 ms; and SED = -24 +/- 14 ms (P > 0.05, EX vs SED). ACH alone did not alter APD. With ACH (10(-5) M), delta APD50 with ISO (10(-6) M) was -5 +/- ms and 0 +/- 5 ms for EX and SED, respectively; delta APD90 was -8 +/- 4 ms and -8 +/- 7 ms for EX and SED, respectively (P > 0.05, EX vs SED). Thus, ACH antagonized ISO-mediated acceleration of repolarization equally in both groups. Chronic daily exercise does not influence adrenergic/cholinergic interactions at the cellular level.     

Combination of sotalol and quinidine in a canine model of torsades de pointes: no increase in the QT-related proarrhythmic action of sotalol

INTRODUCTION: Clinical treatment with a combination of Class IA and III antiarrhythmic drugs is not recommended, as they both favor bradycardia-dependent proarrhythmic events such as torsades de pointes (TdP). However, this theoretical additive effect on ventricular repolarization has never been demonstrated and could be questioned as other Class I drugs, such as mexiletine, a Class IB drug, limit the number of sotalol-induced TdP in dogs with AV block, suggesting the possibility of an antagonistic action of Class I properties against Class III effects. METHODS AND RESULTS: We compared the electrophysiologic and proarrhythmic effects of sotalol (Class III) alone and combined with quinidine (Class IA) in a canine model of acquired long QT syndrome. Seven hypokalemic (K+: 3 +/- 0.1 mEq/L) dogs with chronic AV block had a demand pacemaker implanted and set at a rate of 25 beats/min. They were submitted to two (sotalol-alone and sotalol-plus-quinidine) experiments 48 hours apart using a randomized cross-over protocol. They were pretreated with quinidine (10 mg/kg + 1.8 mg/kg per hour) or saline infused throughout the experiment, and given sotalol (4.5 mg/kg + 1.5 mg/kg per hour) for 2 hours, 30 minutes after the beginning of the pretreatment infusion during both experiments. Ventricular and atrial cycle lengths were similarly increased by sotalol after quinidine or saline. The sotalol-induced prolongation of the QT interval was significantly shorter in quinidine-pretreated dogs (24 +/- 7 msec after quinidine vs 40 +/- 8 msec after saline). Fewer dogs developed TdP: significantly during the first hour of infusion (1/7 sotalol-plus-quinidine vs 6/7 sotalol-alone dogs, P < 0.05) but nonsignificantly during the second hour (3/7 vs 6/7). CONCLUSION: In this model, the sotalol-plus-quinidine combination is at least no more arrhythmogenic than either of the drugs given alone.     

Carbohydrate-deficient transferrin and alcohol dependency: variation in response to alcohol intake among different groups of patients

1. The effects of rilmakalim, a potassium channel opener, were studied on rabbit cardiac Purkinje, ventricular muscle and atrial fibers, with the use of conventional microelectrode techniques. 2. Rilmakalim (0.24-7.2 microM) shortened, in a concentration-dependent manner, the action potential duration (APD) in Purkinje and ventricular muscle without affecting other parameters of the action potential. Pinacidil (30-300 microM) also decreased the APD of Purkinje fibers. 3. Rilmakalim (2.4 microM) and cromakalim (100 microM) hyperpolarized and abolished abnormal automaticity of cardiac Purkinje fibers pretreated with barium (0.2-0.3 mM). Glibenclamide (5 microM) blocked the hyperpolarizing effect. 4. Stable early afterdepolarizations induced in Purkinje fibers by berberine (100 microM) were reversibly blocked by rilmakalim (2.4 microM), which also suppressed late afterdepolarizations induced in Purkinje fibers treated with ouabain (0.3-0.5 microM). 5. The rate of spontaneous discharges of the rabbit sinoatrial node was not affected by rilmakalim (7.2 microM) or by pinacidil (100 microM). Both agents were also unable to affect the APD of atrial muscle fibers. 6. In cardiac Purkinje fibers, tetraethylammonium (TEA; 20 mM) significantly reduced the effects of rilmakalim (2. 4 microM) on the APD. However, neither TEA nor glibenclamide (100 microM) reduced the shortening of the APD induced by dinitrophenol (30 microM) or by salicylate (1 mM).     

Effects of the novel antiarrhythmic agent azimilide on experimental atrial fibrillation and atrial electrophysiologic properties

OBJECTIVES: This study was designed to evaluate how the atrial electrophysiological and antiarrhythmic effects of azimilide compare with those of the specific rapid delayed rectifier (IKr) blocker dofetilide. BACKGROUND: Azimilide, a new class III drug, was initially believed to be a highly selective blocker of the slow delayed rectifier (IKs), but recent studies suggest that azimilide potently blocks IKr. Thus, it has been suggested that azimilide's in vivo effects may simply be due to IKr blockade. METHODS: Dose regimens producing stable effects over time were developed, and two dose levels of azimilide (10 and then 20 mg/kg) or dofetilide (0.08 and then 0.16 mg/kg) were administered to morphine/chloralose-anesthetized dogs during sustained vagal atrial fibrillation (AF). Epicardial mapping was used to measure conduction velocity and AF cycle length. RESULTS: Azimilide terminated AF in 13/14 dogs (93%), while dofetilide terminated AF in 6/12 (50%, P < 0.05). While dofetilide had strong reverse use-dependent effects on atrial ERP (e.g. at lower doses, dofetilide increased ERP by 51 +/- 3% at a basic cycle length, BCL, of 400 ms and by 17 +/- 3% at a BCL of 200 ms), azimilide's effects on ERP were rate-independent (ERP increased at lower dose by 38 +/- 6%, BCL 400 ms; 35 +/- 10%, BCL 200 ms). Neither drug affected conduction. CONCLUSIONS: Azimilide is effective against experimental AF, and increases ERP with a frequency dependence different from the IKr blocker dofetilide, suggesting that azimilide's actions on atrial tissue cannot be attributed exclusively to IKr block, and that effects on other currents (such as IKs) are likely to be important.     

Comparison of the effects of class I anti-arrhythmic drugs, cibenzoline, mexiletine and flecainide, on the delayed rectifier K+ current of guinea-pig ventricular myocytes

The effect of class I anti-arrhythmic drugs, cibenzoline, mexiletine and flecainide, on the delayed rectifier potassium current (IK) in guinea-pig ventricular myocytes was studied using whole cell voltage clamp techniques and under blockade of the L-type calcium current by 5 microM nitrendipine. IK consisted of two different current systems, as reported by Sanguinetti and Jurkiewicz (1990), i.e. an E4031-sensitive rapidly activating component (IKr) with a strong inward-going rectification property and an E4031-insensitive slowly activating component (IKs) with little rectification. Cibenzoline (30 microM) decreased both IKr and IKs while flecainide (10 and 30 microM) decreased the IKr exclusively. Mexiletine (30 microM), in contrast, affected neither IKr nor IKs. Since the inhibition of IK(r) and/or IKs prolongs duration of action potentials and refractory periods, class I drugs which also possess the class III effect may have additional effects in treating certain re-entrant arrhythmias.     

Ionic mechanisms of regional action potential heterogeneity in the canine right atrium

Atrial action potential heterogeneity is a major determinant of atrial reentrant arrhythmias, but the underlying ionic mechanisms are poorly understood. To evaluate the basis of spatial heterogeneity in canine right atrial repolarization, we isolated cells from 4 regions: the crista terminalis (CT), appendage (APG), atrioventricular ring (AVR) area, and pectinate muscles. Systematic action potential (AP) differences were noted: CT cells had a "spike-and-dome" morphology and the longest AP duration (APD; value to 95% repolarization at 1 Hz, 270+/-10 ms [mean+/-SEM]); APG and pectinate muscle cells had intermediate APDs (180+/-3 and 190+/-3 ms, respectively; P<0.001 versus CT for each), with APG cells having a small phase 1; and AVR cells had the shortest APD (160+/-4 ms, P<0.001 versus other regions). The inward rectifier and the slow and ultrarapid delayed rectifier currents were similar in all regions. The transient outward K+ current was significantly smaller in APG cells, explaining their small phase 1 and high plateau. L-type Ca2+ current was greatest in CT cells and least in AVR cells, contributing to their longer and shorter APD, respectively. The E-4031-sensitive rapid delayed rectifier K+ current was larger in AVR cells compared with other regions. Voltage- and time-dependent current properties were constant across regions. We conclude that myocytes from different right atrial regions of the dog show systematic variations in AP properties and ionic currents and that the spatial variation in ionic current density may explain AP differences. Regional variation in atrial ionic currents may play an important role in atrial arrhythmia generation and may present opportunities for improving antiarrhythmic drug therapy.     

Effects of 5-HT4-receptor agonists, cisapride, mosapride citrate, and zacopride, on cardiac action potentials in guinea pig isolated papillary muscles

The purpose of this study was to examine the effects of 5-HT4-receptor agonists cisapride, mosapride citrate (mosapride), and zacopride on action potentials (APs) in guinea pig isolated papillary muscles. Cisapride (0.1-3 microM) concentration-relatedly prolonged the duration of APs (APD) without affecting the other AP parameters. Mosapride and its main metabolite M1 (des-4-fluoro-benzyl-mosapride) did not affect APs at 10 microM. Zacopride at 10 microM shortened APD, and the APD-shortening effect was not affect by GR113808 (10 microM), a 5-HT4-receptor antagonist. The cisapride (1 microM)-induced prolongation of APD was not affected by GR113808 (10 microM), ritanserin (10 microM), a 5-HT2A/2C-receptor antagonist, or prazosin (10 microM), an alpha 1-receptor antagonist. The same concentrations of GR113808, ritanserin, and prazosin did not affect APD. Clofilium, a class III antiarrhythmic agent, prolonged APD; the effect was more pronounced at a stimulus frequency of 0.3 Hz than at 2.0 Hz. Cisapride did not exert such reverse use dependence, suggesting that its mechanism of action is different from that of clofilium. These results suggest that cisapride prolongs APD without involvement of 5-HT2, 5-HT4, or alpha 1 receptors. Mosapride is unlikely to induce the prolongation of electrocardiographic QT intervals correlated with the prolongation of APD in isolated ventricular muscles.     

Children''s understanding of extended identity

OBJECTIVE: To compare the properties of single myocytes isolated from different layers of the basal region of the left ventricle and to test the hypothesis that differences in the delayed rectifier current (IK) contribute to regional differences in action potential duration. METHODS: Myocytes were isolated from basal sub-endocardial, mid-myocardial and sub-epicardial layers of the guinea-pig left ventricle. Membrane voltage and current were measured using the switch-clamp technique. RESULTS: Mean action potential duration measured at 90% repolarisation (APD90) was longer in sub-endocardial myocytes than in mid-myocardial and sub-epicardial myocytes [APD90 ms at 0.2 Hz: sub-endocardial 292 +/- 12 (n = 40), mid-myocardial 243 +/- 8 (n = 42) and sub-epicardial 227 +/- 9 (n = 36), P < 0.001, analysis of variance (ANOVA)]. The APD-rate relationship (stimulation frequencies 2, 1, 0.2 and 0.017 Hz) was steeper in sub-endocardial than in mid-myocardial or sub-epicardial myocytes (P < 0.001, ANOVA). The density of IK was greater in mid-myocardial (4.05 +/- 0.09 pA pF-1) and sub-epicardial (3.90 +/- 0.41 pA pF-1) than in sub-endocardial myocytes (2.74 +/- 0.27 pA pF-1, P < 0.01 ANOVA). The rapidly-activating (IKr) and slowly-activating (IKs) components of IK were significantly smaller in sub-endocardial than in mid-myocardial or sub-epicardial myocytes. D,L-Sotalol-induced prolongation of APD90 was similar in the three regions studied. CONCLUSIONS: There are significant transmural gradients in the electrophysiological properties of myocytes isolated from the base of the left ventricular free wall in guinea-pig. Sub-endocardial myocytes had a longer APD90 attributable in part to a significantly smaller IK density. We have been unable to identify M cells in the guinea-pig left ventricular free wall.     

Human brain activity related to speed discrimination tasks

This study examined the ionic mechanism of ibutilide, a class III antiarrhythmic in clinical use, on freshly isolated human atrial cells. Cells had resting potentials of -71.4 +/- 2.4 mV, action potentials with overshoot of 36.8 +/- 1.8 mV, duration of 265 +/- 89 msec at 90% repolarization and slow repolarization (n = 16). Ibutilide, at 10(-7) M, markedly increased action potential duration. Four types of outward currents were detected: Ito, Iso, a delayed rectifier and IK1. Ibutilide had no inhibitory effect on these outward currents at 10(-7) M (n = 28). In K(+)-free solutions and -40 mV holding potential, mean peak inward current at 20 mV was -1478 +/- 103 pA (n = 12). Ibutilide increased this current to -2347 +/- 75 pA at 10(-7) M, with half maximal effect (Kd) of 0.1 to 0.9 nM between -10 and +40 mV (n = 21). At similar concentrations, the drug increased APD, with Kd of 0.7 and 0.23 nM at 70 and 90% repolarization, respectively (n = 8). Ibutilide shifted the mid-point of the steady-state inactivation curve from -21 to -12.2 mV (n = 6), and reduced current decline during repetitive depolarization (n = 5). The drug induced inward current was carried by Na+o through a nifedipine inhibited inward channel because Na+o removal eliminated the effect, and nifedipine abolished the inward current and the drug induced APD prolongation. We propose that a Na+ current through the L-type Ca++ channel mediates ibutilide's potent clinical class III antiarrhythmic action.