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Circulation. 2000;101:86-93

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(Circulation. 2000;101:86.)
© 2000 American Heart Association, Inc.

Basic Science Reports

Electrophysiological Effects of Dofetilide in an In Vitro Model of "Border Zone" Between Normal and Ischemic/Reperfused Myocardium

René Rouet, PhD; Sandra Picard, PhD; Christian Libersa, MD, PhD; Mathieu Ghadanfar, MD; Colin Alabaster, PhD; Jean-Louis Gérard, MD, PhD

From Anesthésiologie Expérimentale et Physiologie Cellulaire, Université, Caen (R.R., S.P., J.-L.G.); Pharmacologie, Centre d’Investigations Cliniques, CH&U-INSERM, Hôpital Cardiologique, Lille (C.L.), France; and Pfizer Ltd, Central Research, Sandwich, Kent CT13 9NJ, UK (M.G., C.A.).

Correspondence to Sandra Picard, Anesthésiologie Expérimentale et Physiologie Cellulaire, Université, Campus I, Esplanade de la Paix, 14032 Caen Cedex, France.


*    Abstract
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 *Abstract
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Background—To evaluate both class III activity and antiarrhythmic action of dofetilide at the level of the "border zone," we investigated its electrophysiological effects on guinea pig ventricular strips submitted partly to normoxia (normal zone, NZ) and partly to simulated severe ischemia, then reperfusion (altered zone, AZ).

Methods and Results—Because of the differential class III effects of dofetilide in normal and ischemic regions, the dispersion of the action potential duration at 90% repolarization (APD90) between NZ and AZ was reduced by 5 nmol/L of drug during early ischemia (at 10 minutes, APD90 NZ/APD90 AZ was 1.68±0.22 versus 2.82±0.17 in control, P<0.05), whereas 50 nmol/L dofetilide worsened it during late ischemia (at 30 minutes, APD90 NZ/APD90 AZ was 4.62±0.76 versus 2.57±0.21 in control, P<0.05). Concomitantly, dofetilide at 5, 10, and 50 nmol/L abolished the early extrastimulus (ES)-induced arrhythmias, and at 10 and 50 nmol/L, it significantly enhanced the incidence of late spontaneous repetitive responses (in 86% and 75% of preparations treated with 10 and 50 nmol/L, respectively, versus 25% in control, P<0.05). During reperfusion, dofetilide at 5, 10, and 50 nmol/L exhibited concentration-dependent class III effects, as it did in the NZ, and did not modify the incidence of spontaneous arrhythmias.

Conclusions—Dofetilide 5 nmol/L decreased APD90 dispersion between NZ and AZ and reduced the early ES-induced arrhythmias. However, dofetilide 50 nmol/L increased APD90 dispersion, and at 10 and 50 nmol/L, it increased the late spontaneous arrhythmias.


Key Words: antiarrhythmia agents • ischemia • reperfusion • myocardium


*    Introduction
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Dofetilide (UK-68,798) is a potent class III antiarrhythmic agent that inhibits the rapid component of the delayed outward potassium current (IKr),1 inducing lengthening of action potential (AP) duration and effective refractory period without affecting conduction, as shown in vitro in various cardiac tissues2 3 4 5 and in vivo in anesthetized animals.6 7 Clinical studies also confirmed class III activities of dofetilide, showing significant prolongation of QT interval and ventricular effective refractory period in normal subjects8 9 and in patients with coronary artery disease.10 11 Dofetilide has been found to exert antiarrhythmic action on ventricular fibrillation in anesthetized animals7 12 13 14 and to show clinical efficacy for the termination of sustained atrial fibrillation and flutter15 and of ventricular tachycardias.16 17

However, clinical use of class III drugs has revealed risks in relation to their proarrhythmic effects,18 19 20 as recently demonstrated by the Survival With ORal D-sotalol (SWORD) study, which reported an increased mortality in patients treated with the pure class III agent d-sotalol21 and led to the discontinuation of clinical investigation of this drug. Using an in vitro model of partial ischemia/reperfusion mimicking the "border zone" existing between normal and ischemic/reperfused ventricular regions,22 we found that the proarrhythmic effects of d-sotalol might be related to its differential class III properties on adjacent normal and ischemic zones.23 Indeed, ischemic conditions are known to be able to impair the ability of class III agents to lengthen AP.24 The border zone between normal and ischemic myocardium has been suggested to be a site promoting the emergence of arrhythmias.25 26

In view of the class III potency of dofetilide and its reported antiarrhythmic actions, it was considered useful to determine its electrophysiological effects and antiarrhythmic efficacy in a model of myocardial border zone. For this purpose, we evaluated the effects of dofetilide at 5, 10, and 50 nmol/L on the AP parameters and the incidence of arrhythmias occurring around the border zone separating normal and ischemic/reperfused adjacent tissues of guinea pig right ventricular myocardium.


*    Methods
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Care of the animals conformed to the recommendations of the Helsinki Declaration, and the study was performed in accordance with the regulations of the official edict of the French Ministry of Agriculture.

Materials
Guinea pigs of either sex weighing 300 to 400 g were euthanized under anesthesia with ether. The hearts were quickly removed, and a thin longitudinal strip of the right ventricle was pinned, endocardial surface upward, in a special perfusion chamber.22 27 28 This chamber (5 mL) is bisected by a thin perforated latex membrane that allowed the preparation to be passed carefully through and divided into 2 zones, called the normal zone (NZ) and altered zone (AZ), respectively. The 2 compartments were independently superfused at the rate of 2 mL/min with Tyrode’s solution oxygenated with 95% O2 and 5% CO2 and maintained at 36.5±0.5°C (Polystat 5HP, Bioblock). The composition of the Tyrode’s solution is (in mmol/L): Na+ 135, K+ 4, Ca2+ 1.8, Mg2+ 1, H2PO4- 1.8, HCO3- 25, Cl- 117.8, and glucose 5.5. The pH was 7.35±0.05. At the end of each experiment, absence of leakage between the 2 compartments was tested by a dye injection (methylene blue) in 1 of the 2 zones.

Data Acquisition and Analysis
The preparations were stimulated at a frequency of 1 Hz via a bipolar Teflon-coated steel wire electrode positioned either in the NZ or in the AZ. Rectangular pulses of 2 ms in duration and twice diastolic threshold intensity were delivered by a programmable stimulator, SMP 310 (Biologic). During the protocol, stimulation was stopped whenever sustained spontaneous arrhythmias occurred. An extrastimulus (ES) was applied every 4 stimulations in an attempt to elicit ES-induced repetitive responses by a progressive increase in 5-ms steps of the time interval between the stimulus and the ES. Transmembrane potentials were recorded simultaneously in both myocardial regions by use of glass microelectrodes filled with KCl 3 mol/L (tip resistance 10 to 30 M{Omega}) and coupled to the input stages of a home-built high-impedance capacitance-neutralizing amplifier. The recordings were displayed on a memory dual-beam storage oscilloscope (Gould Instrument Systems Inc). The following AP characteristics were automatically stored and measured by a system of cardiac AP automatic acquisition and processing devices (DATAPAC, Biologic): resting membrane potential (RMP), AP amplitude (APA), AP duration at 50% of repolarization (APD50), AP duration at 90% of repolarization (APD90), and maximal upstroke velocity (Vmax). Whenever possible, the same impalement was maintained throughout the experiment; however, when it was lost, readjustment was attempted. If the readjusted parameters deviated <=10% from the previous ones, experiments were continued; otherwise, they were terminated.

Experimental Protocol
After a 120-minute equilibration period, simulated ischemia was induced for 30 minutes in 1 compartment (AZ) by superfusion with a modified Tyrode’s solution, while the other compartment remained in normal conditions (NZ) (Figure 1Down). The modified Tyrode’s solution differed from normal by elevated K+ concentration (from 4 to 12 mmol/L), decreased HCO3- concentration (from 25 to 9 mmol/L) leading to a decrease in pH (from 7.35±0.05 to 7.00±0.05), a decrease in PO2 replacement of 95% O2 and 5% CO2 by 95% N2 and 5% CO2, and withdrawal of glucose. As previously reported,22 27 29 the present modifications are similar to those reported by Morena et al,30 which reproduced in vitro the electrophysiological abnormalities induced in vivo by ischemia. The AZ then returned to superfusion with the normal Tyrode’s solution for 30 minutes (reperfusion period).



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  		Figure 1. Effects of dofetilide at 5, 10, and 50 nmol/L on APD90 in normal and ischemic conditions. Data are expressed as mean±SEM. APD90 values were measured simultaneously on both NZ (top) and AZ (bottom) during 30 minutes of ischemic phase. Results of ANOVA are given for group and time factors among 4 groups: control (n=8) and dofetilide 5 (n=7), 10 (n=6), and 50 (n=8) nmol/L. Note that in normoxia (top), class III effects were observed with 10 and 50 nmol/L of dofetilide, whereas in simulated ischemic conditions (bottom), APD90 reduction was less rapid in presence of dofetilide, especially at 5 nmol/L.

Myocardial conduction disturbances and arrhythmias were recorded during both ischemia and reperfusion: (1) conduction blocks between the AZ and the NZ, (2) ES-induced repetitive responses defined as spontaneous extrasystoles induced by a single ES, and (3) spontaneous arrhythmias independent of the stimulation.

During the ischemia and reperfusion phases, dofetilide previously diluted in ethanol-HCl (0.05N) and in Tyrode’s solution at 5 (n=7), 10 (n=7), or 50 (n=8) nmol/L or Tyrode’s solution alone (control, n=12) was randomly superfused simultaneously in both zones. Thus, the electrophysiological effects of dofetilide were investigated (1) on AP parameters simultaneously in normal (NZ) and altered (AZ) conditions and (2) on the electrical disturbances occurring around the border zone between normal and ischemic/reperfused cardiac tissues.

Statistical Analysis
All results were expressed as mean±SEM. Student’s t test for paired data was performed for comparison from initial AP parameter values (measured before initiation of the ischemic period). ANOVA (2-factor analysis) was used to compare APD90 changes and APD90 NZ/APD90 AZ ratio between the 4 experimental groups, and Fisher’s exact test for comparison of nonparametric categorical data. Differences were considered significant when P<0.05.

Because of loss of microelectrode impalements, the AP parameters were analyzed for 29 preparations (8 control and 7, 6, and 8 for 5, 10, and 50 nmol/L dofetilide, respectively).


*    Results
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 *Results
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Effects of Dofetilide on the AP Parameters in Normoxic and Simulated Ischemic/Reperfused Conditions
As summarized in Table 1Down, in normoxic conditions (NZ), dofetilide did not significantly modify RMP, Vmax (except for the dofetilide 10 nmol/L group), or APA (except for the dofetilide 5 nmol/L group), whereas AP lengthening was obtained with dofetilide (P<0.05) in a concentration-dependent manner (after 60 minutes, APD50 was +18±3%, +36±3%, and +42±8% in the presence of 5, 10, and 50 nmol/L dofetilide, respectively, and APD90 was +22±5%, +36±4%, and +48±8% in the presence of 5, 10, and 50 nmol/L dofetilide, respectively).


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  		Table 1. Electrophysiological Effects of Dofetilide on the AP Parameters in Normoxic Conditions (NZ)

As shown in Table 2Down, simulated ischemia induced significant membrane depolarization and decreases of Vmax, APA, APD50, and APD90 (P<0.05 versus initial values). These AP alterations were similar for all groups; in particular, the AP shortening measured at the end of the ischemic period was not significantly modified by the class III agent (after 30 minutes of simulated ischemia, APD90 was reduced by 53±7%, 52±5%, and 67±7% in the presence of 5, 10, and 50 nmol/L of dofetilide, respectively, versus 61±4% for control). In all groups, reperfusion allowed recovery of AP parameters close to initial values for RMP, Vmax, and APA. APD50 and APD90 also returned to initial values in the control group, whereas they were decreased by dofetilide (P<0.05) in a concentration-dependent manner (after 30 minutes of reperfusion, APD50 was +8±3%, +25±5%, and +61±24% in the presence of 5, 10, and 50 nmol/L of dofetilide, respectively, and APD90 was +13±4%, +35±5%, and +64±13% in the presence of 5, 10, and 50 nmol/L dofetilide, respectively).


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  		Table 2. Electrophysiological Effects of Dofetilide on the AP Parameters in Ischemic/Reperfused Conditions (AZ)

However, as illustrated in Figure 1Up (bottom), the kinetics of the ischemia-induced AP shortening (ANOVA for time, P<0.0001) were significantly different between control and treated groups (ANOVA for group, P=0.001). APD90 reduction occurred rapidly in the control group, namely, over the first 10 minutes of simulated ischemia, whereas it was significantly delayed in the presence of 5 nmol/L dofetilide (ANOVA, P<0.0001 versus control group). Conversely, in the NZ (top) during the 30 minutes of simulated ischemia, dofetilide exhibited class III effects (ANOVA for time, P<0.0001 and group, P<0.0001) at the 2 highest concentrations only (ANOVA versus control group, P<0.0001 for 50 nmol/L, P=0.0001 for 10 nmol/L).

The APD90 dispersion occurring between both normal and ischemic regions was affected by the presence of dofetilide, as illustrated in Figure 2Down, which shows examples of AP recorded simultaneously in the NZ and AZ during early (10 minutes) and late (30 minutes) ischemia (middle and right panels, respectively), in the absence (top) or in the presence of dofetilide at 5 nmol/L (middle) and 50 nmol/L (bottom). As summarized in Figure 3Down, the APD90 dispersion, measured as the ratio APD90 NZ/APD90 AZ, was significantly modified by 5 and 50 nmol/L dofetilide (P<0.05 and P<0.005, respectively, ANOVA versus control). Dofetilide at 5 nmol/L reduced the APD90 dispersion during the first 10 minutes of simulated ischemia (P<0.05). Conversely, the high concentration of the class III agent significantly worsened the APD90 dispersion during the late phase of ischemia (after 20 minutes, P<0.05). No significant variation was observed with the intermediate concentration (10 nmol/L) compared with control.



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  		Figure 2. Representative AP recordings obtained simultaneously in normal and simulated ischemic conditions in absence of drug (control, top) and in presence of dofetilide at 5 (middle) and 50 (bottom) nmol/L. Traces show examples of AP recorded in same cell in each myocardial zone (NZ and AZ) in initial conditions (before initiation of ischemia, left) and during early (at 10 minutes, middle) and late (at 30 minutes, right) ischemic phases. Dispersion of APD90 between NZ and AZ (r) is measured as ratio APD90 NZ/APD90 AZ. Note that during early simulated ischemia (at 10 minutes, middle traces), APs in AZ were shortened less in presence of dofetilide than in control, leading to a decreased dispersion of APD90 (low r value), especially with 5 nmol/L dofetilide, whereas during late ischemia (at 30 minutes, left traces), APs were shortened in AZ and lengthened in NZ, leading to an increase of APD90 dispersion with 50 nmol/L dofetilide (high r value).



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  		Figure 3. Effects of dofetilide at 5, 10, and 50 nmol/L on dispersion of APD90 between normal and ischemic myocardial zones. Data are expressed as mean±SEM. Dispersions of APD90 are represented by ratio APD90 NZ/APD90 AZ. For each group: control (n=8) and dofetilide 5 (n=7), 10 (n=6), and 50 (n=8) nmol/L, ratio values are given in initial conditions (0 minutes) and at 5, 10, 20, and 30 minutes of simulated ischemic phase. Results of ANOVA are given for group and time factors. *P<0.05, Student’s t test vs control. Note APD90 dispersion is reduced by 5 nmol/L dofetilide during early ischemic phase (at 5 and 10 minutes) and enhanced by 50 nmol/L dofetilide during late ischemic phase (at 20 and 30 minutes).

Effects of Dofetilide on the Incidence of Electrical Disturbances During Simulated Ischemia/Reperfusion
The different types of electrical disturbances occurring in this in vitro model of border zone are illustrated in Figures 4Down and 5Down, which show APs recorded simultaneously in the NZ and AZ during simulated ischemia and reperfusion. We recorded (1) myocardial conduction blocks, either unidirectional, for example from the AZ toward the NZ (Figure 4ADown), or bidirectional between the 2 ventricular regions (Figure 4BDown); (2) repetitive responses induced by an ES (Figure 4CDown); and (3) spontaneous repetitive responses independent of stimulation (Figure 5Down). We subdivided the severity of the spontaneous repetitive responses into 1, 2, or 3 spontaneous extrasystoles (Figure 5ADown, 1Up spontaneous AP), salvos (Figure 5BDown, 7 spontaneous extrasystoles), and sustained activities (Figure 5CDown, >10 spontaneous APs).



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  		Figure 4. Representative recordings of myocardial electrical disturbances, such as conduction blocks, either unidirectional from AZ to NZ (A) or bidirectional between regions (B) and extrastimulus-induced repetitive responses (C). Traces show APs recorded simultaneously on both NZ and AZ during simulated ischemia. Stimulation was applied to ventricular preparation either in NZ (open arrows) or in AZ (solid arrows). Note in C that spontaneous extrasystoles ({blacktriangledown}) were elicited in each zone by a single extrastimulus.



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  		Figure 5. Representative recordings of spontaneous repetitive responses ({blacktriangledown}) occurring during simulated ischemia or reperfusion, namely 1 spontaneous extrasystole (A), salvos (4 to 10 spontaneous APs, B), and sustained activity (>10 spontaneous APs, C). Traces show APs recorded simultaneously in both NZ and AZ, and stimulation was applied either in NZ (open arrows) or in AZ (solid arrows). Note that stimulation was stopped just after onset of arrhythmia.

As summarized in Table 3Down, dofetilide at 5 nmol/L significantly decreased the incidence of ischemia-induced conduction blocks (P<0.05) and delayed their occurrence (at 27.5±1.7 minutes of the ischemic phase versus 17.1±1.7 minutes with no drug, P<0.05), whereas at 10 and 50 nmol/L, no significant effect was observed either on the incidence of blocks or on their occurrence time (at 19.9±4.0 minutes and 19.5±1.4 minutes in the presence of dofetilide at 10 and 50 nmol/L, respectively, versus 17.1±1.7 minutes in control). During ischemia, dofetilide at all concentrations completely abolished the occurrence of ES-induced repetitive responses, whereas the incidence of spontaneous arrhythmias was significantly enhanced by 10 and 50 nmol/L of drug (P<0.05 each; P=0.07 for the dofetilide 5 nmol/L group). The percentage of preparations with severe spontaneous arrhythmias (sustained type) appeared to be higher in treated groups than in control, although this was not significant. The spontaneous repetitive responses occurred significantly later during simulated ischemia than those of ES-induced type (at 17.4±3.0 versus 9.8±2.1 minutes in control group, P<0.05), and their occurrence time was similar in all groups (at 15.9±2.7, 14.4±1.8, and 17.9±2.3 minutes in the presence of 5, 10, and 50 nmol/L dofetilide, respectively, versus 17.4±3.0 minutes during ischemia alone).


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  		Table 3. Effects of Dofetilide on the Incidence of Conduction Blocks and Arrhythmias During Simulated Ischemia and Reperfusion

During reperfusion, all myocardial conduction blocks disappeared with a similar delay for all groups (at 1.5±0.3, 1.6±0.5, and 1.6±0.2 minutes after the reperfusion onset in the presence of 5, 10, and 50 nmol/L dofetilide, respectively, and 2.0±1.0 minutes in control). Dofetilide at 50 nmol/L prevented the occurrence of ES-induced repetitive responses, although it was already low in control. The incidence of spontaneous arrhythmias remained high for all treated groups, as did the percentage of preparations exhibiting severe spontaneous arrhythmic events. The occurrence time of spontaneous repetitive responses also was similar among groups (at 11.7±2.2 minutes of the reperfusion phase in control and 14.8±1.8, 15.9±1.2, and 14.9±2.1 minutes in the presence of dofetilide at 5, 10, and 50 nmol/L, respectively, P=NS).


*    Discussion
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 *Discussion
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The main findings of this study may be summarized as follows: (1) Dofetilide at 5 nmol/L prevented early ischemia-induced AP shortening, whereas at 10 and 50 nmol/L, it exerted class III effects in normal conditions; (2) as a result of its differential class III efficacy in the NZ and AZ, dofetilide at 5 nmol/L prevented the early APD90 dispersion (10 minutes) between zones, whereas at 10 and 50 nmol/L it worsened it during the late ischemic phase (20 to 30 minutes); (3) dofetilide at all concentrations inhibited the early ES-induced arrhythmias, but at 10 and 50 nmol/L it increased the spontaneous repetitive responses occurring later during the ischemic phase; and (4) dofetilide did not significantly affect the incidence of arrhythmias induced by reperfusion.

In this in vitro model, dofetilide did not antagonize the AP shortening that developed after 20 minutes of exposure to simulated ischemic conditions. This inability to prevent the late APD90 decrease is most likely related to the high K+ concentration in the ischemia-mimicking solution (12 mmol/L). It has been clearly established that hyperkalemia is able to impair the class III efficacy of dofetilide3 31 and that the IKr block induced by this agent in AT-1 cells is strikingly reduced by elevated extracellular K+ content.32 In addition, the ischemia-induced AP shortening has been attributed mainly to the activation of an ATP-dependent potassium conductance (IK-ATP).33 These potassium channels are not affected by dofetilide,34 and the IK-ATP-induced changes in AP shape observed during the late ischemic phase may override any effect of IKr blockade. More interestingly, this study demonstrated that dofetilide delayed the decrease in APD90 during the early ischemic period (until 15 minutes, Figure 1BUp), suggesting that the class III efficacy of dofetilide might be different and more beneficial under less severe ischemic conditions, as might occur in patients with chronic coronary artery disease. It is unclear, however, why this protective effect of dofetilide was obtained only with the low concentration (5 nmol/L). Further investigations would be needed to clarify this point.

The present study also showed that the dispersion of APD90 around the border zone was affected differently depending on the concentration of dofetilide, which prevented it at 5 nmol/L during the early ischemic phase, worsened it at 50 nmol/L during the late ischemic phase, and had no significant effect at 10 nmol/L. The dispersion of repolarization is implicated in the generation of arrhythmia, as suggested by a previous study using simultaneous monophasic AP recordings from 2 sites of the right ventricle in human heart that has clearly proposed a link between the dispersion of repolarization and the inducibility of monomorphic ventricular tachycardia.35 Injury currents with the border zone, as established in isolated porcine and canine hearts,36 are thought to be a possible mechanism responsible for some arrhythmias such as automatic activities, focal reexcitation, reentry arrhythmias,37 or triggered activities.38

Our findings show that dofetilide exerted antiarrhythmic and proarrhythmic effects around the border zone during ischemia depending on the type of arrhythmia, reducing the ES-induced arrhythmic events and enhancing the incidence of those of the spontaneous type at all concentrations, although significantly only at 10 and 50 nmol/L. Antiarrhythmic effects of the class III agents have been reported, especially in ventricular fibrillation models.7 13 14 Chen et al39 demonstrated in dogs that dofetilide exerted no benefit on arrhythmias linked to abnormal automaticity but suppressed the reentry arrhythmias induced by programmed electrical stimulation. Our results might be compared with these latter findings, although a difference in mechanism responsible for the arrhythmias in the 2 different models may exist. Indeed, as previously discussed,22 40 the ES-induced arrhythmias recorded in our model are probably due to reentry between normal and ischemic myocardial zones. Briefly, the increase of myocardial conduction times and the occurrence of conduction blocks between the two regions would favor the emergence of reentry movements. In addition, it is known that single or multiple premature impulses, such as extrastimuli, are able to either provoke or inhibit reentrant circuits by altering refractory periods of the tissue involved. The decrease of the incidence of myocardial conduction blocks around the border zone observed with 5 nmol/L dofetilide might explain its preventive action on the ES-induced repetitive responses. Our findings also suggest that the antiarrhythmic efficacy of 5 nmol/L dofetilide might be related to the lessened APD90 dispersion between the two regions during the early ischemic phase. The antiarrhythmic action of 10 and 50 nmol/L dofetilide on ES-induced arrhythmias is more likely related to its significant class III effects in NZ during the early ischemic phase, thus terminating reentry circuits that traverse the normal tissue. In contrast, the border zone spontaneous arrhythmias are unlikely to be related to reentry movements, because they are independent of the stimulation. Early and delayed afterdepolarizations or abnormal automaticity, induced by injury currents originating from the border zone, cannot be excluded. Whatever the mechanisms involved, the significant increase of the incidence of spontaneous arrhythmias obtained with 10 and 50 nmol/L dofetilide might be related to its potent class III action in the normal region, unlike in the ischemic tissue.

During reperfusion, dofetilide did not affect the border zone spontaneous arrhythmias. These findings are consistent with the absence of protective effects of dofetilide against ventricular fibrillation in dogs39 and in minipigs,14 although it is difficult to compare data obtained in vivo during coronary blood flow restoration with data obtained in vitro using Tyrode’s solution simulating ischemia/reperfusion. We recently demonstrated, in this model of myocardial border zone, that another class III antiarrhythmic drug, d-sotalol, was proarrhythmic during reperfusion, whereas dl-sotalol and propranolol, both of which exert ß-blocking activities, prevented the occurrence of spontaneous repetitive responses.23 These findings clearly suggested that an adrenergic stimulation by catecholamines might be responsible, at least in part, for the reperfusion-related spontaneous arrhythmias and might explain the lack of antiarrhythmic effect of dofetilide, like d-sotalol, around the border zone. Alternatively, the present in vitro model may have limitations with respect to the relevance to border zones occurring during pathophysiological conditions. Indeed, the partition between normal and ischemic/reperfused ventricular tissues was narrow and regular, whereas this border zone may be larger and more patchworked in vivo in disease states such as chronic infarction and fibrosis than in the present model. However, this does not modify the implications of our findings for the understanding of the differential antiarrhythmic and proarrhythmic effects of dofetilide, depending on the concentration used and the arrhythmia type, around the potentially crucial myocardial border zone. The validity and relevance of this in vitro model was previously discussed and recognized.22

In conclusion, the present work in an in vitro model of border zone provides evidence for benefits of a low concentration of dofetilide (5 nmol/L) in preventing both dispersion of repolarization between normal and ischemic tissues and the occurrence of ES-induced arrhythmias during myocardial acute ischemia with no significant proarrhythmic effect on either spontaneous arrhythmic events or reperfusion-induced arrhythmias. In contrast, high concentrations of dofetilide may exert proarrhythmic effects on ischemia-induced spontaneous arrhythmias around the border zone, in relation to its differential class III efficacy in normal and ischemic tissues. Interestingly, the concentration of 5 nmol/L relates closely to human plasma concentrations detected after doses of dofetilide that prevent inducible sustained ventricular tachyarrhythmia.16 41 The higher concentrations of dofetilide (10 and 50 nmol/L) used in the present study are consistent with the observations of torsades de pointes observed in patients after 15 mg/kg.41 Although the 50 nmol/L concentration of drug is beyond what is attainable with clinically recommended doses, the present data clearly indicate the need for close attention to dosage to optimize the benefit relative to the risk in patients treated with dofetilide.


*    Acknowledgments
 
This work was supported by a grant from Pfizer Central Research, Sandwich, UK.

Received September 18, 1998; revision received July 1, 1999; accepted July 14, 1999.


*    References
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 up arrowAbstract
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 *References
 

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