Spontaneous and TMS-related EEG changes as new biomarkers to measure anti-epileptic drug effects.

Robust biomarkers for anti-epileptic drugs (AEDs) activity in the human brain are essential to increase the probability of successful drug development. The frequency analysis of electroencephalographic (EEG) activity, either spontaneous or evoked by transcranial magnetic stimulation (TMS-EEG) can provide cortical readouts for AEDs. However, a systematic evaluation of the effect of AEDs on spontaneous oscillations and TMS-related spectral perturbation (TRSP) has not yet been provided. We studied the effects of Lamotrigine, Levetiracetam, and of a novel potassium channel opener (XEN1101) in two groups of healthy volunteers. Levetiracetam suppressed TRSP theta, alpha and beta power, whereas Lamotrigine decreased delta and theta but increased the alpha power. Finally, XEN1101 decreased TRSP delta, theta, alpha and beta power. Resting-state EEG showed a decrease of theta band power after Lamotrigine intake. Levetiracetam increased theta, beta and gamma power, while XEN1101 produced an increase of delta, theta, beta and gamma power. Spontaneous and TMS-related cortical oscillations represent a powerful tool to characterize the effect of AEDs on in vivo brain activity. Spectral fingerprints of specific AEDs should be further investigated to provide robust and objective biomarkers of biological effect in human clinical trials.

Pharmacological and toxicological activity of RSD921, a novel sodium channel blocker.

BACKGROUND: RSD921, the R,R enantiomer of the kappa (k) agonist PD117,302, lacks significant activity on opioid receptors. METHODS: The pharmacological and toxicological actions were studied with reference to cardiovascular, cardiac, antiarrhythmic, toxic and local anaesthetic activity. RESULTS: In rats, dogs and baboons, RSD921 dose-dependently reduced blood pressure and heart rate. In a manner consistent with sodium channel blockade it prolonged the PR and QRS intervals of the ECG. Furthermore, in rats and NHP, RSD921 increased the threshold currents for induction of extra-systoles and ventricular fibrillation (VFt), and prolonged effective refractory period (ERP). In rats, RSD921 was protective against arrhythmias induced by electrical stimulation and coronary artery occlusion. Application of RSD921 to voltage-clamped rat cardiac myocytes blocked sodium currents. RSD921 also blocked transient (ito) and sustained (IKsus) outward potassium currents, albeit with reduced potency relative to sodium current blockade. Sodium channel blockade due to RSD921 in myocytes and isolated hearts was enhanced under ischaemic conditions (low pH and high extracellular potassium concentration). When tested on the cardiac, neuronal and skeletal muscle forms of sodium channels expressed in Xenopus laevis oocytes, RSD921 produced equipotent tonic block of sodium currents, enhanced channel block at reduced pH (6.4) and marked use-dependent block of the cardiac isoform. RSD921 had limited but quantifiable effects in subacute toxicology studies in rats and dogs. Pharmacokinetic analyses were performed in baboons. Plasma concentrations producing cardiac actions in vivo after intravenous administration of RSD921 were similar to the concentrations effective in the in vitro assays utilized. CONCLUSIONS: RSD921 primarily blocks sodium currents, and possesses antiarrhythmic and local anaesthetic activity.

Kinetics of rate-dependent shortening of action potential duration in guinea-pig ventricle; effects of IK1 and IKr blockade.

1. The kinetics of shortening of action potential duration (APD) following an increase in pacing rate, from 2 to 3.3 Hz, was characterized in guinea-pig ventricular preparations. Terikalant (RP62719), an inhibitor of the inwardly rectifying K+ current (IK1), and dofetilide, a specific inhibitor of the rapidly activating delayed-rectifier current (IKr), were applied to determine the effect of inhibition of these ion currents on slow APD shortening. 2. Action potentials were recorded from isolated guinea-pig ventricular myocytes using the perforated-patch patch-clamp technique, and monophasic action potentials were recorded from Langendorff-perfused guinea-pig ventricles using a contact epicardial probe. 3. Under control conditions, after an increase in pacing rate, APD immediately decreased, and then shortened slowly with an exponential time course. In ventricular myocytes, the time constant of this exponential shortening was 28+/-4 s and the amount of slow shortening was 21.9+/-0.9 ms (n=8) for an increase in rate from 2 to 3.3 Hz. Similar values were observed in Langendorff-perfused ventricles. 4. Terikalant dose-dependently increased APD and the increase was enhanced by rapid pacing ('positive' rate-dependence). The drug dose-dependently decreased the time constant of shortening and amount of slow APD shortening. In contrast, dofetilide, an inhibitor of IKr, which shows 'reverse' rate-dependent APD widening, had no significant effect on the time constant or amount of slow shortening. 5. These observations suggest that IK1 plays a role in rate-dependent shortening of APD. The results appear to support the hypothesis that 'reverse' rate-dependent effects of IKr blockers are due to these drugs not affecting the ion current(s) mediating intrinsic rate-dependent slow shortening of APD.

RSD1000: a novel antiarrhythmic agent with increased potency under acidic and high-potassium conditions.

This study reports the use of a novel agent, RSD1000 [(+/-)-trans-[2-(4-morpholinyl)cyclohexyl]naphthalene-1-acetate mono hydrochloride], to test the hypothesis that a drug with pKa close to the pH found in ischemic tissue may have selective antiarrhythmic actions against ischemia-induced arrhythmias. The antiarrhythmic ED50 for RSD1000 against ischemic arrhythmias was 2.5 +/- 0.1 micromol/kg/min in rats. This value was significantly lower than doses that suppressed electrically induced arrhythmias. In isolated rat hearts, RSD1000 was approximately 40 times more potent in producing ECG changes (i.e., P-R and QRS prolongation) in acid (pHo = 6.4) and high [K+]o (10.8 mM) buffer than in normal buffer (pHo = 7.4; [K+]o = 3.4 mM). In patch-clamped, whole-cell rat cardiac myocytes, inhibition of sodium (INa) currents by RSD1000 was pH- and use-dependent. The IC50 for INa blockade was lower (P <.05) in acid (0.8 +/- 0.1 microM) than in pH 7.3 (2.9 +/- 0.3 microM), respectively, whereas the IC50 for blockade of transient outward potassium current (ITO) at pH = 6.4 and 7.3 was 3.3 +/- 0.4 and 2.8 +/- 0.1 microM, respectively. Mixed ion channel block in ischemic myocardium with minimal effects on normal cardiac tissue, as governed by the low pKa of RSD1000, may account for its antiarrhythmic activity against ischemia-induced arrhythmias.

Dofetilide enhances shock-induced extension of refractoriness and lowers defibrillation threshold.

OBJECTIVE: To explore the hypothesis that class III antiarrhythmics reduce defibrillation threshold (DFT) by prolonging postshock refractoriness. ANIMALS AND METHODS: The effect of a new selective potassium channel blocker, dofetilide (DOFlow 2.5 micrograms/kg bolus plus 0.9 microgram/kg/h; DOFmed 10 micrograms/kg bolus plus 3.6 micrograms/kg/h; and DOFhigh 25 micrograms/kg bolus plus 9 micrograms/kg/h), on DFT was compared with that of placebo in anesthetized open chest dogs (n = 6 per group). The effects of dofetilide on refractory period extension (RPE) were assessed by using DFT strength shocks delivered at various stages of repolarization. RESULTS: DFT was significantly decreased in the DOFhigh group, whether expressed as shock peak voltage or energy (P < 0.05 compared with changes in placebo). At baseline, a shock timing of ventricular effective refractory period of 25 ms resulted in RPE of 100 +/- 24 ms, 80 +/- 11 ms, 91 +/- 14 ms and 90 +/- 20 ms in the placebo, DOFlow, DOFmed, and DOFhigh groups, respectively. After infusion, these RPE values were unchanged in the placebo group but tended to increase in the dofetilide treatment groups. DOFhigh significantly increased RPE by 20 +/- 18 ms (P < 0.05 compared with baseline values and changes in placebo). Dofetilide-induced changes in RPE and DFT were significantly correlated when expressed as voltage (r2 = 0.78, P < 0.01), current (r2 = 0.80, P < 0.01) and energy (r2 = 0.53, P < 0.01). CONCLUSIONS: These results show that dofetilide prolonged RPE at a plasma level that reduced DFT, thus providing support for the hypothesis that selective prolongation of refractoriness may synergize with shock-induced RPE to decrease the energy requirements for defibrillation.

Magnesium shifts voltage dependence of activation of delayed rectifier I(K) in guinea pig ventricular myocytes.

The sensitivity of the delayed rectifier K+ current (I(K)) to intracellular Mg2+ was investigated in guinea pig ventricular myocytes using the whole cell patch-clamp technique. An increase in free intracellular Mg2+ concentration ([Mg2+]i) led to a dose-dependent decrease in I(K) with a half-maximal effect of approximately 20 nM. Activation of I(K) was shifted toward more positive voltages on increasing [Mg2+]i, but little effect was observed on activation and deactivation kinetics. Isoproterenol increased I(K) and was partially reversible in both control and 100 nM [Mg2+]i. The antiarrhythmic drug dofetilide was used to separate I(K) into its two components, rapidly activating (I(Kr)) and slowly activating (I(Ks)). The magnitude of both components decreased to a similar extent with an increase in [Mg2+]i. As [Mg2+]i was reduced, however, the number of experiments in which the dofetilide-sensitive current I(Kr) displayed inward rectification was reduced. In contrast to results previously reported for frog myocytes, it is unlikely that Mg2+ effects on guinea pig I(K) are mediated by a protein phosphatase.

Development and evaluation of a fully automated monophasic action potential analysis program.

A fully automated program has been developed for analysing digitised cardiac monophasic action potentials (MAPs) of several animal species. Under diverse conditions, the program's performance is evaluated by comparison with high-resolution manual analysis of action potential duration, MAP detection, baseline and plateau determination. For each variable, the differences between both forms of analysis are categorised according to species (rabbit, dog or baboon), intervention (control, sematilide or LY-190147), rhythm source (sinus or paced), and rates. They are then examined using ANOVA (null hypothesis: no difference between methods for any category). Intra-group differences are significant for baseline selection (p < 0.05), and treatment effects are significant (p < 0.05) for one species (baboon). However, the differences found are negligible when compared with expected measurement errors. Thus the program reliably detects and accurately analyses MAPs, independent of species, drug or pacing. The program's strength is its use of simple heuristic algorithms to identify and profile MAP signals while rejecting spurious transients. These algorithms focus on the procedural aspects of automatic MAP detection and validation, with minimum use of complex mathematics. The program's estimated throughput exceeds 8200 MAPs min-1 on a 50 MHz 1486 DX machine. With available data format conversion packages, signals from almost any data-acquisition source can be analysed.

Rate-dependent effects of sematilide on ventricular monophasic action potential duration and delayed rectifier K+ current in rabbits.

Our objective was to define the actions of sematilide in rabbits and to assess the contribution of the delayed rectifier (IK) to rate dependence of action potential duration (APD) in rabbit ventricular myocardium. In studies in vivo, New Zealand White rabbits were used to obtain dose/response curves of the effects of sematilide on APD from contact monophasic action potentials (MAP), ventricular effective refractory period (VERP), and ECG. Sematilide or placebo was administered as an i.v. bolus followed by a 45-min infusion in the following cumulative manner: infusion 1 (1 mg/kg bolus + 8 micrograms/kg/min); infusion 2 (2 mg/kg + 20 micrograms/kg/min); and infusion 3 (7 mg/kg + 68 micrograms/kg/min). At each infusion level, VERP and APD at 75% repolarization (APD75) were measured during cardiac pacing between 200- and 400-ms cycle length (CL). Serum sematilide levels were analyzed by high-performance liquid chromatography (HPLC). In studies in vitro, sematilide's effects on the delayed rectifier were assessed in isolated rabbit ventricular myocytes by using patch-clamp techniques. Sematilide infusion in vivo resulted in stable serum levels of 1.3 +/- 0.5, 3.7 +/- 1.4, and 13.4 +/- 1.8 micrograms/ml during infusions 1, 2, and 3, respectively. Maximal effects occurred at infusion 2, such that at 400 ms CL, sematilide widened predrug APD75 (145 +/- 5 ms) by 27 +/- 4% (p < 0.001 vs. placebo), and at 200-ms CL, sematilide prolonged predrug APD75 (115 +/- 10 ms) by only 18 +/- 4% (p < 0.001 vs. placebo; p < 0.05 vs. 400-ms CL). Similar effects were observed in VERP. Sematilide enhanced rate dependence of APD and produced the same degree of APD prolongation at a given CL, during accommodation to and recovery from rapid pacing. Rabbit ventricular myocytes appeared to have at least two types of delayed rectifier. Sematilide selectively blocked IKr, and block was not relieved by repetitive stimulation. In conclusion, the APD-widening effect of sematilide was independent of previous pacing history. Sematilide had little influence on background processes likely responsible for shortening APD because of rapid repetitive stimulation.

Class III antiarrhythmic effects of LY-190147 on defibrillation threshold.

Defibrillation strength shocks delivered within an action potential (AP) delay repolarization. Shock-induced AP duration extension (APDE) may prolong refractoriness and terminate or prevent reinitiation of reentry, favoring defibrillation. This study examined LY-190147 (LY) effects on defibrillation threshold (DFT) in 11 dogs. Ventricular effective refractory period (VERP) and epicardial monophasic AP duration at 75% repolarization (APD75) were recorded at 300-, 400-, 500-, and 600-ms pacing cycle length (CL). APDE was measured as the time to 50% repolarization after a DFT strength shock delivered at 50, 25, and 0 ms before or 25 ms after VERP during pacing at 300 ms CL in 4 of the dogs. We made all recordings before drug administration and after infusions of 0.03, 0.3, and 3.0 mg/kg LY, using 1.5-h dosing intervals. LY lowered DFT in a saturating dose-response manner whether expressed as shock peak voltage (V) or energy. LY decreased DFT-V from 357 +/- 77 V before drug to 331 +/- 60 V (-6 +/- 12%), 290 +/- 43 V (-17 +/- 13%, p < 0.001), and 312 +/- 45 V (-11 +/- 12%, p < 0.05) at 0.03, 0.3, and 3.0 mg/kg, respectively. Similarly, LY treatment decreased defibrillation energy requirements from 6.9 +/- 2.7 J before drug by 7 +/- 25%, 26 +/- 24%, and 12 +/- 25% at the same doses. At 300-600 ms CL, LY prolonged APD75 by an average of 10 +/- 8% at 0.03 mg/kg, 17 +/- 6% at 0.3 mg/kg, and 24 +/- 9% at 3 mg/kg. At these CL, LY prolonged VERP by an average of 4 +/- 6% at 0.03 mg/kg, 15 +/- 10% at 0.3 mg/kg, and 11 +/- 9% at 3 mg/kg. APDE was increased from 62 +/- 9 ms before to 68 +/- 14, 80 +/- 16 (p < 0.001) and 72 +/- 13 ms (p < 0.05) at 0.03, 0.3, and 3.0 mg/kg LY, respectively. Therefore, LY prolonged VERP and APDE and affected DFT in the same saturating dose-response manner. LY may facilitate defibrillation by increasing the duration of postshock refractoriness.

Effects of optical enantiomers CK-4000(S) and CK-4001(R) on defibrillation and enhancement of shock-induced extension of action potential duration.

INTRODUCTION: Class III antiarrhythmics have been reported to lower defibrillation threshold (DFT); however, the mechanism(s) of this effect is unknown. Recent evidence suggests that DFT strength DC shocks may terminate reentrant arrhythmias through prolongation of action potential duration and refractory periods. Since Class III antiarrhythmic drugs prolong repolarization, we examined the hypothesis that these drugs enhance shock-induced action potential duration extension (APDE), which might contribute to lowering of DFT. METHODS AND RESULTS: In order to investigate the specificity of drug effects on action potential repolarization following a shock, an optical enantiomer with mixed beta-blocking and Class III effects (CK-4000) and its enantiomer with "pure" Class III antiarrhythmic effects (CK-4001) were compared against placebo in a canine defibrillation model (n = 8 per group). At the 3 mg/kg dose, the enantiomers nonstereoselectively lowered DFT voltage by 19 +/- 15% (CK-4000, P < 0.05 compared to placebo) and 25 +/- 12% (CK-4001, P < 0.05 compared to placebo), indicating that Class III antiarrhythmic actions alone were sufficient for this effect. Similarly, CK-4000 and CK-4001 at 3 mg/kg enhanced APDE (P < 0.01 compared to placebo) by 20 +/- 11% and 24 +/- 17%, respectively. APDE prolongation significantly correlated with reduction in DFT voltage for both CK-4000 (r = -0.55, P < 0.03) and CK4001 (r = -0.63, P < 0.01). At 3 mg/kg, the enantiomers stereoselectively prolonged action potential duration (APD75) by an average of 37 +/- 14% (CK-4000, P < 0.001) and 23 +/- 14% (CK-4001, P < 0.001), and ventricular effective refractory period (VERP) by 38 +/- 15% (CK-4000, P < 0.01) and 27 +/- 13% (CK-4001, P < 0.05). Prolongations of APD75 and VERP did not correlate with reductions of DFT in individual dogs. CONCLUSIONS: These results show that Class III antiarrhythmics and DFT strength shocks additively delay repolarization, which suggests that drug enhancement of APDE may contribute to their effects on DFT.

Mexiletine's antifibrillatory actions are limited by the occurrence of convulsions in conscious animals.

The antiarrhythmic actions of the racemate and enantiomers of mexiletine were studied in conscious and anaesthetised rats. Racemate or enantiomers, at 20 mg/kg i.v., had little effect on ischaemia-induced ventricular fibrillation in conscious or anaesthetised rats. In conscious rats 20 mg/kg caused convulsions in 78-89% of rats when the plasma concentration of racemate was 20 +/- 2 microM. In anaesthetized animals a higher dose (40 mg/kg) of racemate could be given; this completely prevented ischaemia-induced fibrillation when the plasma concentration was 26 +/- 2 microM. Racemate and enantiomers accumulated in the heart and brain of conscious animals to give tissue: plasma ratios of 7.5 and 23, respectively. With electrical stimulation, both racemate and enantiomers dose dependently (4-32 mg/kg) increased threshold currents for induction of ventricular fibrillation, increased refractory period and minimally changed the ECG; findings expected with a Class Ib antiarrhythmic. The above studies failed to show major differences between racemate or enantiomers except for consistently lower (20-30%) plasma concentrations of R(-) at all dose levels. In conclusion, mexiletine prevented ischaemia-induced ventricular fibrillation in anaesthetised animals but only when given at doses producing convulsions in conscious animals.

Tissue distribution of mexiletine enantiomers in rats.

1. The kinetics of distribution of the enantiomers of mexiletine were studied in various tissue (heart, brain, lungs, liver, kidneys and fat) in male Sprague-Dawley rats after administration of a single i.v. dose (10 mg/kg) of racemic mexiletine. 2. The pharmacokinetic parameters calculated from the serum data showed a 32% greater systemic clearance (162 ml/min per kg vs 123 ml/min per kg) and a 22% greater steady-state volume of distribution (9.0 l/kg vs 7.4 l/kg) for R(-)-mexiletine relative to the S(+)-enantiomer. However, the terminal elimination half-lives of the enantiomers (1.4 and 1.3 h for R(-)- and S(+)-mexiletine, respectively) did not exhibit stereoselectivity. 3. Maximum tissue concentrations of the enantiomers were observed at 5 min after dosage in all tissues studied. Stereoselective uptake was evident only in the liver tissue and was 2.4-fold greater for S(+)-mexiletine. High tissue/serum ratios (greater than 20 for both enantiomers) were observed in lungs, brain and kidneys. The cardiac concentrations of R(-)- and S(+)-mexiletine were 8- and 7-fold those of serum, respectively. 4. The results demonstrate that the uptake of mexiletine enantiomers into the target tissue (heart) is not stereoselective. However, the relatively high brain accumulation of the enantiomers may be related to the CNS side-effects commonly associated with mexiletine therapy.

Antiarrhythmic properties of tedisamil (KC8857), a putative transient outward K+ current blocker.

1. Rats were used to evaluate the antiarrhythmic properties of tedisamil, a novel agent with the electrophysiological properties of a Class III antiarrhythmic drug. Tedisamil was tested against coronary artery occlusion-induced arrhythmias in conscious animals. 2. The actions of tedisamil on the ECG, as well as responses to electrical stimulation, were compared with those on the configuration of epicardial intracellular action potentials recorded in vivo. 3. Tedisamil (1-4 mg kg-1, i.v.) caused bradycardia, elevated blood pressure and dose-dependently reduced ventricular fibrillation (VF) induced by occlusion of the left anterior descending coronary artery. Other ischaemia-associated arrhythmias were not so well suppressed. Antiarrhythmic activity was greatest when the tedisamil-induced bradycardia was prevented by electrically-pacing the left ventricle. 4. Tedisamil dose-dependently lengthened the effective refractory period and prevented electrically-induced VF. In vivo, tedisamil (0.5-4 mg kg-1, i.v.) prolonged the duration of epicardial intracellular action potentials by up to 400%. 5. Results showed that tedisamil possessed antifibrillatory actions in rats that were related to Class III electrophysiological actions as revealed by electrical stimulation and electrophysiological analyses.

Antiarrhythmic properties of tetrodotoxin against occlusion-induced arrhythmias in the rat: a novel approach to the study of the antiarrhythmic effects of ventricular sodium channel blockade.

Blockade of ventricular sodium conductance (gNa) is believed to play an important role in the beneficial antiarrhythmic effects of class I antiarrhythmic agents. The present study was undertaken to examine the importance of ventricular gNa blockade by assessing the antiarrhythmic profile of tetrodotoxin (TTX), a selective sodium channel blocker. Experiments were performed in pentobarbital-anesthetized and artificially ventilated rats. Two doses of TTX were tested for antiarrhythmic action: a low dose (low TTX, 10 micrograms/kg of bolus + infusion of 10 micrograms/kg/hr) which blocked only neuronal activity, and a high dose (TTXh, 50 micrograms/kg of bolus + infusion of 50 micrograms/kg/hr) which also produced signs of ventricular gNa blockade in normal hearts. To control for the decreases in blood pressure and heart rate caused by TTX, hexamethonium, nitroprusside and propranolol were also used. Only TTXh possessed antiarrhythmic activity in rats subjected to myocardial ischemia (produced by ligation of the left anterior descending coronary artery). Arrhythmia scores (mean, n = 9) were: saline, 3.8; hexamethonium, 3.8; nitroprusside, 3.2; nitroprusside + propranolol, 4.3; low TTX, 3.9; and TTXh, 0.9. Only TTXh reduced dV/dt max. of the action potential (recorded in vivo by means of 3 M KCl filled microelectrodes) as well as action potential height, and concomitantly prolonged the P-R and QRS intervals of normal hearts. In conclusion, our study demonstrated that drugs which produced hypotension, bradycardia and loss of autonomic function were not antiarrhythmic. On the other hand, the marked antiarrhythmic activity of TTXh appeared to depend upon ventricular gNa blockade. Thus, TTX provides a useful tool for examining the antiarrhythmic properties of ventricular gNa blockade.

Mechanisms underlying the antiarrhythmic properties of beta-adrenoceptor blockade against ischaemia-induced arrhythmias in acutely prepared rats.

1. The mechanism underlying the limited antiarrhythmic effects of beta-adrenoceptor blocking agents against occlusion-induced arrhythmias in acutely prepared, pentobarbitone-anaesthetized rats has been investigated. 2. Atenolol, ICI 111,581 and propranolol were given at low, medium and high doses calculated to shift dose-response curves to exogenous agonists by factors of 10-30, 100-300 and 1000-3000, respectively. 3. Arrhythmias, blood pressure, heart rate, ECG changes and serum K+ were measured. 4. Antiarrhythmic activity was seen with beta-blocker treatment. This was minimal with atenolol (0.1, 1 and 10 mg kg-1) and only statistically significant with the highest dose of ICI 111,581 (5 mg kg-1), and propranolol (10 mg kg-1). 5. Treatment with beta-adrenoceptor blockers elevated serum potassium concentrations, as compared with saline controls, especially when measured at 30 min post-occlusion. 6. Only ICI 111,581 (5 mg kg-1) and propranolol (1 and 10 mg kg-1) prolonged P-R interval. 7. In order to evaluate possible mechanisms of antiarrhythmic action, attempts were made to correlate antiarrhythmic activity with beta-blockade, serum potassium concentrations, and/or with changes in the P-R interval of the ECG. 8. Reductions in arrhythmias did not correlate well with presumed beta-blockade. Better correlation was obtained with elevations of serum potassium concentration, and with prolongation of P-R interval (a presumed Class I antiarrhythmic action). 9. These results suggested that antiarrhythmic effects of adrenoceptor blocking agents in acutely-prepared anaesthetized rats, subjected to occlusion of a coronary artery, are unrelated to cardiac beta-blockade. The limited antiarrhythmic effects which were observed could be attributed to elevations in serum potassium concentration (due to peripheral beta-blockade) and/or possible Class I antiarrhythmic actions.