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provided by Elsevier - Publisher Connector 268 JACC Vol. 32, No. 1 July 1998:268–74

Effects of Mibefradil, a Novel Blocking Agent With T-Type Activity, in Acute Experimental Myocardial Ischemia Maintenance of Ventricular Fibrillation Threshold Without Inotropic Compromise

CECILIA A. MULLER, PHD, LIONEL H. OPIE, MD, PHD, FACC, JOY MCCARTHY, BSC, DIRK HOFMANN, MD, CARLOS A. PINEDA, PHD,* MAX PEISACH, DSC,PHD* Cape Town, South Africa

Objectives. We tested whether mibefradil, a selective T-type Results. During the ischemic period, the low (clinically rele- calcium channel blocking agent, could differentially inhibit experi- vant) dose of mibefradil prevented the fall of the ventricular mental ventricular arrhythmogenesis more than contractility dur- fibrillation threshold, without depressing the maximal rate of

ing acute regional ischemia and reperfusion compared with that pressure development of the left ventricle (LVmax dP/dt). This low during L-channel blockade by . dose increased left ventricular blood flow, whereas peripheral Background. T-type calcium channels are found in nodal and arterial pressure remained unchanged. The higher dose of both conduction tissue and in vascular smooth muscle, but in much mibefradil and verapamil was antiarrhythmic during ischemia, at lower density in contractile myocardium. The potential role of the cost of depressed contractile activity. During reperfusion, only mibefradil in ventricular arrhythmogenesis remains unclear. the higher dose of mibefradil and verapamil was antiarrhythmic Methods. Mibefradil (Ro 40-5967, 1 mg/kg body weight intra- but both depressed contractile activity. venously [IV]) was given as a bolus 30 min before anterior Conclusions. Mibefradil is antiarrhythmic, without inotropic descending coronary artery ligation, followed by 2 mg/kg per h IV compromise. Speculatively, both T-type and L-type calcium chan- during 20 min of ischemia and 25 min of reperfusion in open chest nel blockade are involved in these effects. pigs. In a second group, mibefradil was given in a dose twice as (J Am Coll Cardiol 1998;32:268–74) high. A third group received verapamil (0.3 mg/kg IV), followed by ©1998 by the American College of Cardiology an infusion of 0.6 mg/kg per h.

Despite the continuous advances in the therapy of acute point of view, there is no direct manner of modification of myocardial infarction, sudden cardiac death largely caused by cytosolic calcium in ischemic cells, but working from first ventricular arrhythmias remains important in the spectrum of principles, two indirect pharmacologic procedures are possi- mortality in coronary heart disease (1). Such deaths typically ble: 1) Beta-adrenergic blockade should, by virtue of inhibition occur out of the hospital, outside the reach of thrombolytic of the formation of the second-messenger cyclic adenosine therapy. Because the incidence of ischemic heart disease is monophosphate, lessen the probability of opening of the likely to rise very substantially in the developing world early in calcium channel and hence be antiarrhythmic (4). 2) Calcium the next millennium (2), the global issue of sudden cardiac channel blocking agents should achieve a similar effect by death will become even more important. directly binding to the calcium channel and lessening calcium Hypothetically, excess cytosolic calcium, as found in myo- ion influx, which might be important in the initiation and cardial ischemia, may play an important role in ischemic maintenance of ventricular fibrillation (7). These proposals ventricular arrhythmogenesis (3–6). From the therapeutic could have clinical relevance in postinfarction patients, as shown by the decrease in sudden cardiac death when beta- blockers are given prophylactically. Verapamil, a classic L-type From the Medical Research Council Heart Research Group, Cape Heart calcium , reduced mortality only in a predeter- Centre, University of Cape Town and *National Accelerator Centre, Cape mined subgroup without previous heart failure (8), suggesting Town, South Africa. This study was supported by the South African Medical that its protection could be limited by its negative inotropic Research Council, and the University of Cape Town. F. Hoffmann-La Roche Ltd., Basel, Switzerland provided additional support. action, as in the case of (9). Experimentally, vera- Manuscript received October 14, 1998; revised manuscript received March 5, pamil prevents ventricular fibrillation in variety of models 1998, accepted March 16, 1998. (10–14), but at the cost of depressed myocardial contractile Address for correspondence: Dr. Cecilia A. Muller, Heart Research Unit, University of Cape Town Medical School, Observatory 7925, Cape Town, South function (14). Africa. E-mail: [email protected]. Mibefradil is a structurally novel

©1998 by the American College of Cardiology 0735-1097/98/$19.00 Published by Elsevier Science Inc. PII S0735-1097(98)00182-X JACC Vol. 32, No. 1 MULLER ET AL. 269 July 1998:268–74 MIBEFRADIL VS. VERAPAMIL IN MYOCARDIAL ISCHEMIA

(10 mg/kg body weight). Anesthesia was induced by Abbreviations and Acronyms intravenous thiopentone sodium (10 mg/kg). Approximately CAL ϭ coronary artery ligation 5 min later, a bolus dose of 55 mg/kg of alpha-chloralose was IV ϭ intravenously given, followed by a constant infusion of alpha-chloralose ϭ LVmax dP/dt maximal rate of pressure development of the left (60 mg/kg) over ϳ60 min. In addition, the neuromuscular ventricle LVSP ϭ left ventricular systolic pressure blocker alcuronium chloride (0.02 mg/kg per min) was given as MAP ϭ mean arterial pressure an intravenous infusion for the duration of the experiment. ϭ VF ventricular fibrillation Animals were ventilated with room air; pO2 was controlled VFT ϭ ventricular fibrillation threshold between 95 and 105 mm Hg and pH between 7.43 and 7.48. ϭ VT ventricular tachycardia Standard limb lead electrocardiograms and arterial pressure were monitored continuously. After a midsternal thoracotomy, the left anterior descending coronary artery was dissected free for ϳ5 mm at a point approximately one-half the distance from from a new class of benzimidazolyl-substituted tetraline deriv- its origin to its apical termination. It was abruptly ligated by atives (15,16). Mibefradil inhibits calcium influx through tightening thin polyvinyl tubing around a 2-cm length of rigid T-type low voltage–activated calcium channels (17) ten times tubing placed alongside the artery. more powerfully than influx through L-type channels (18). We measured the VFT by using the “train” method. An T-type channels are present in very low densities in adult, electrical stimulus consisting of a train of 10 square wave nondiseased ventricular tissue of various species (19,20), but pulses was triggered by the R wave and distributed over 210 ms there appear to be no data on humans (21). In contrast, during the T wave. The stimulus was passed between two T-channels are well expressed in pacemaker tissue, such as the platinum electrodes sutured onto the anterior wall of the left sinus node (22). A T-channel blocker such as mibefradil could and right ventricles, 2.5 cm apart. The anode was placed across therefore be expected to decrease heart rate, with a lesser the border of the expected ischemic zone in the left ventricle effect on myocardial contractility. In addition mibefradil has and the cathode within the nonischemic zone of the right relatively weak L-channel blocking activity (23). Mibefradil ventricle. Pulses were generated using a Grass S88 stimulator. increased survival in a rat model of heart failure (24). When The stimulus current was progressively increased in 2 mA steps given to patients with varying degrees of left ventricular starting at 4 mA until ventricular fibrillation (VF) occurred dysfunction, intravenous mibefradil is well tolerated. Ejection (28). The VFT was taken as the lowest current required to fraction increases, but a decrease in the maximal first deriva- produce VF. Care was taken not to use a longer train of pulses, tive of left ventricular pressure (LVmax dP/dt) occurs in some which may spill over into the refractory phase or late portion of patients with previous heart failure (25). Mibefradil has been the T wave (29). A single VFT measurement required ϳ1 min. reported (26) to stimulate the release of endothelial-derived Epicardial direct current shock was then applied usually within relaxing factor. Mibefradil may also be relatively selective, 4 s of the onset of VF and repeated if necessary. A return to causing coronary versus peripheral vasodilation (27). We sinus rhythm could almost always be achieved within 30 s. therefore hypothesized that T-channel blockade by mibefradil After 30 s, we alternated cardiac massage with defibrillation. would be able to inhibit ischemic ventricular arrhythmias However, if defibrillation could not be accomplished within without major depression of left ventricular contractile func- 90 s, the experiment was terminated. The VFT was measured tion. before CAL, 5 min after CAL and 10 min after reperfusion. In We used the open chest porcine model, subject to acute addition to measurement of VFT, spontaneously occurring coronary artery ligation (CAL), followed by reperfusion. The ventricular arrhythmias were also monitored. Ventricular tachy- vulnerability of the heart to ventricular arrhythmias was mon- cardia (VT) was defined as more than four consecutive uni- itored by repetitive measurements of the ventricular fibrillation form or multiform ventricular premature systoles. Runs of VF threshold (VFT). The coronary and myocardial effects of or VT were considered terminated when they were followed by mibefradil were assessed by measurements of regional myo- at least three normally conducted sinus beats. Twenty minutes cardial blood flow and left ventricular contractile activity. after ligation, the ligature was released to reperfuse.

Comparisons were made with verapamil, using a dose known Left ventricular pressures and LVmax dP/dt were measured to have antiarrhythmic effects in this model (14). using a Cardiomax computerized system (Columbus Instru- ments). This system gives real-time displays of digital and analog signals. Each variable is measured over at least three Methods consecutive cardiac cycles at a time when no arrhythmias are The ethics and research committee of the University of evident. Each data point represents the mean of two readings. Cape Town approved this study. Animals were handled in Left ventricular blood flow was measured in the placebo accordance with the “Principles of Laboratory Animal Care” group, the low dose mibefradil group and the verapamil group of the National Society for Medical Research. but not in the higher dose mibefradil group. Microspheres Study protocol. Male pigs (Large White crossed with Land- were given before administration and CAL, 15 min after race) weighing 27 to 30 kg were sedated by intramuscular CAL as well as 15 min after reperfusion using the sample 270 MULLER ET AL. JACC Vol. 32, No. 1 MIBEFRADIL VS. VERAPAMIL IN MYOCARDIAL ISCHEMIA July 1998:268–74 reference method. The radionuclides utilized were Table 1. Occurrence of Spontaneous Tachyarrhythmias and gadolinium-153, strontium-85 and tin-113 (New England Nu- Ventricular Fibrillation Threshold During Ischemia and Reperfusion clear). The microspheres (mean size 15 ␮m) were prepared in VFT (mA) a 0.9% saline solution containing Tween-80 0.01% and admin- VT VF (mean Ϯ SEM) istered according to previously described details and precau- Ischemia tions (30,31). Samples for gamma-spectrometry were dissected Placebo 1/13 0/13 9.8 Ϯ 0.7 from the midischemic and remote nonischemic zones of the (n ϭ 13) Ϯ left ventricle. Mibefradil (1-mg/kg bolus followed 0/15 0/15 17.6 1.0* by 2-mg/kg per h infusion) (n ϭ 15) Size of underperfused zone. At the end of the experiment, Mibefradil (2-mg/kg bolus followed 1/9 0/9 17.6 Ϯ 1.9† the coronary artery was again ligated at the same site as before, by 4-mg/kg per h infusion) (n ϭ 9) and the underperfused zone was delineated by injection of 5 ml Verapamil (0.3-mg/kg bolus followed 0/9 0/9 16.2 Ϯ 0.8* of patent blue 7.5% solution (May and Baker, UK) in saline by 0.6-mg/kg per h infusion) (n ϭ 8) into the left atrium. The heart was then excised and arrested. Reperfusion Placebo 9/13 10/13 6.0 Ϯ 1.0 The size of this zone was expressed as a percent of total left ϭ Ͻ (n 11) ventricular mass. When the underperfused zone was 26% or Ϯ Ͼ Mibefradil (1-mg/kg bolus followed 15/15 8/15 16.9 4.0 34%, the results were not processed. This predetermined by 2-mg/kg per h infusion) (n ϭ 15) exclusion criterion was used because of links between eventual Mibefradil (2-mg/kg bolus followed 0/9† 1/9† 19.6 Ϯ 2.3* infarct size and arrhythmias (32,33). by 4-mg/kg per h infusion) (n ϭ 9) Study groups. The prime aim of the present study was to Verapamil (0.3-mg/kg bolus followed 1/9‡ 1/9† 15.1 Ϯ 2.6† ϭ compare the effects of antiarrhythmic doses of mibefradil and by 0.6-mg/kg per h infusion) (n 9) verapamil on contractile function. *p Ͻ 0.001, †p Ͻ 0.02, ‡p Ͻ 0.05, versus placebo. VF ϭ ventricular ϭ ϭ Selection of doses. In a pilot study, mibefradil, 1 mg/kg fibrillation; VFT ventricular fibrillation threshold; VT ventricular tachy- cardia. bolus followed by 2 mg/kg per h, was the lowest dose that showed antiarrhythmic activity during ischemia. In a previous study, verapamil at a clinically relevant dose (0.2 mg/kg) was not antiarrhythmic in our model. A supraclinical dose of tion was applied to compensate for multiple comparisons (34). 0.6 mg/kg did show antiarrhythmic activity (14). We therefore A p value of Ͻ0.05 was considered significant. used a comparable supraclinical dose of verapamil in the present study. Placebo group (n ϭ 13). Each pig received 8 ml of saline Results 0.9% starting 30 min before CAL, followed by an infusion of A total of 50 pigs were used in the main study. Two pigs in saline 0.9% for the remainder of the experiment. the mibefradil low dose group and two in the verapamil group Mibefradil, 1-mg/kg bolus followed by 2-mg/kg per h infusion were excluded because the size of the underperfused zone was ϭ over 90 min (n 17): “low dose.” Each pig received mibefradil Ͻ26% of total left ventricular mass. For various practical (Hoffman-La Roche, Basel, Switzerland), 1 mg/kg intrave- reasons, all measurements could not always be made in all pigs. nously (IV) (powder dissolved in 8 ml of saline 0.9%), as a Furthermore, certain procedures, for example, blood flow bolus dose over 5 min starting at 30 min before CAL, followed measurements by costly radioactive microspheres, were con- by 2 mg/kg per h IV for the remainder of the experiment. fined to a limited number of experiments. These two factors Mibefradil, 2-mg/kg bolus followed by 4-mg/kg per h infusion explain the differing number of observations within each ϭ over 90 min (n 9): “higher dose.” Each pig received mibe- group. fradil (Hoffman-La Roche), 2-mg/kg powder dissolved in 8 ml Spontaneous ventricular arrhythmias and VFT during of saline 0.9%), as a bolus dose over 5 min starting at 30 min ischemia and reperfusion (Table 1). Ischemia. VFT before before CAL, followed by 4 mg/kg per h IV for the remainder CAL was 17.5 Ϯ 2.7 mA (placebo group), 21.7 Ϯ 2.5 mA of the experiment. (mibefradil low dose group), 19.8 Ϯ 2.0 (mibefradil higher Verapamil, 0.3-mg/kg bolus followed by 0.6-mg/kg per h dose group) and 18 Ϯ 2.1 mA (verapamil group). In the infusion over 90 min (n ϭ 11). Verapamil was obtained as a placebo group, CAL caused a marked fall in VFT. In both 5-mg/2-ml solution (Lennon, South Africa). Each pig received mibefradil groups as well as in the verapamil group, VFT was verapamil (0.3-mg/kg bolus dose over 5 min) starting 30 min maintained at higher levels than in the placebo group, indicat- before CAL, followed by an infusion of 0.6 mg/kg per hour IV ing antiarrhythmic activity. The incidence of spontaneous for the remainder of the experiment. ventricular tachyarrhythmias was low in all groups. Statistical procedures. Results are expressed as mean Reperfusion. All runs of VT and VF occurred within the value Ϯ SEM. One-way analyses of variance and the Student t first 10 min of reperfusion. At the time of the VFT measure- test (two-tailed) were applied using paired analyses where ment, no rhythm disturbances were evident. Only the higher appropriate. The Fisher exact test was used for comparing the dose of mibefradil and verapamil decreased the incidence of incidence of spontaneous arrhythmias. The Bonferroni correc- VT and VF. Similarly, mibefradil (higher dose) and verapamil JACC Vol. 32, No. 1 MULLER ET AL. 271 July 1998:268–74 MIBEFRADIL VS. VERAPAMIL IN MYOCARDIAL ISCHEMIA

Table 2. Left Ventricular Pressures, Mean Arterial Pressure and Heart Rate Before Drug Before CAL During Ischemia During Reperfusion (mean Ϯ SEM) (mean Ϯ SEM) (mean Ϯ SEM) (mean Ϯ SEM) Placebo (n ϭ 11–12) LVSP — 105 Ϯ 586Ϯ 5* 85 Ϯ 5* Ϯ Ϯ Ϯ LVmax dP/dt — 1,876 130 1,544 92† 1,310 55† MAP — 93 Ϯ 375Ϯ 4* 76 Ϯ 4* HR — 117 Ϯ 5 125 Ϯ 5 125 Ϯ 6 Mibefradil (1-mg/kg bolus followed by 2-mg/kg per h infusion) (n ϭ 11–12) LVSP 100 Ϯ 5 105 Ϯ 395Ϯ 485Ϯ 6* Ϯ Ϯ Ϯ Ϯ LVmax dP/dt 2,003 68 1,884 85 1,645 51† 1,332 122† MAP 93 Ϯ 386Ϯ 383Ϯ 4† 73 Ϯ 5† HR 122 Ϯ 4 111 Ϯ 6 113 Ϯ 8 124 Ϯ 8 Mibefradil (2-mg/kg bolus followed by 4-mg/kg per h infusion) (n ϭ 6–9) LVSP 111 Ϯ 880Ϯ 7†‡§ 61 Ϯ 6†‡§ 62 Ϯ 6†‡§ Ϯ Ϯ ࿣ Ϯ ࿣ Ϯ LVmax dP/dt 1,844 96 1,429 102† § 766 86† ¶ 758 67†‡§ MAP 87 Ϯ 659Ϯ 5†࿣¶45Ϯ 4†࿣¶44Ϯ 6†‡¶ HR 110 Ϯ 699Ϯ 890Ϯ 7*§ 86 Ϯ 9‡§ Verapamil (0.3-mg/kg bolus-followed by 0.6-mg/kg per h infusion) (n ϭ 6–9) LVSP 97 Ϯ 587Ϯ 5*‡§ 83 Ϯ 4† 83 Ϯ 4† Ϯ Ϯ Ϯ Ϯ LVmax dP/dt 1,831 106 1,215 109†‡§ 1,195 100†‡§ 910 87†‡§ MAP 90 Ϯ 373Ϯ 3†‡§ 65 Ϯ 2†‡§ 69 Ϯ 2† HR 116 Ϯ 4 107 Ϯ 6 113 Ϯ 5 106 Ϯ 6§ *p Ͻ 0.05, †p Ͻ 0.02 versus before drug (or coronary artery ligation [CAL] in placebo group). ‡p Ͻ 0.05, ࿣p Ͻ 0.001 versus mibefradil low dose. §p Ͻ 0.05, ¶p Ͻ ϭ ϭ ϭ 0.001 versus placebo. HR heart rate (beats/min); LVmax dP/dt maximal rate of left ventricular pressure development (mm Hg/s); LVSP left ventricular systolic pressure (mm Hg); MAP ϭ mean arterial pressure (mm Hg). maintained VFT at higher levels than in the placebo group, was higher than that in the placebo group (129 Ϯ 14% vs. 93 Ϯ whereas the low dose of mibefradil did not. 5%, p Ͻ 0.05). Blood flow in the verapamil group did not differ Left ventricular pressures, arterial pressure and heart rate from that in the placebo group. (Table 2). CAL in the placebo group resulted in a decrease in Blood flow during reperfusion period. Midischemic zone:

LVmax dP/dt, mean arterial pressure (MAP) and left ventric- There was a marked increase in blood flow in the mibefradil ular systolic pressure (LVSP). All three of these variables low dose group (175 Ϯ 19% vs. 125 Ϯ 9% in the placebo group, remained low throughout the periods of ischemia and reper- fusion. The administration of low dose mibefradil did not change left ventricular or arterial pressures before CAL. All Figure 1. Regional left ventricular blood flow. During ischemia, low dose mibefradil increased blood flow in the nonischemic (N I) zone. values of LVmax dP/dt, LVSP and MAP during ischemia and reperfusion in this group were comparable to those in the During reperfusion, low dose mibefradil caused increases in blood flow in both the ischemic (ISCH) and nonischemic zones, whereas vera- placebo group. The higher dose of mibefradil as well as pamil increased blood flow only in the nonischemic zone (blood flow verapamil markedly decreased LVmax dP/dt, LVSP and MAP was not measured with higher dose of mibefradil). *p Ͻ 0.05, †p Ͻ before CAL. These lower values were maintained during 0.01, both versus placebo. ‡p Ͻ 0.01 versus verapamil. ischemia and reperfusion. Heart rate was decreased during ischemia and reperfusion in the higher dose mibefradil group and during reperfusion in the verapamil group. Regional left ventricular blood flow (Fig. 1). Left ventric- ular blood flow before CAL in the placebo group was 185 Ϯ 18.6 ml/100 g per min, which did not differ from the predrug and pre-CAL levels in the low dose mibefradil and verapamil groups. Blood flow in each tissue sample during ischemia and reperfusion was expressed as a percent of the predrug and pre-CAL values for that specific sample. Blood flow was not measured in the mibefradil higher dose group. Blood flow during ischemic period. Midischemic zone:Inall three groups, blood flow fell to Ͻ10% of pre-CAL values. Nonischemic zone: Blood flow in the mibefradil low dose group 272 MULLER ET AL. JACC Vol. 32, No. 1 MIBEFRADIL VS. VERAPAMIL IN MYOCARDIAL ISCHEMIA July 1998:268–74 p Ͻ 0.05). Blood flow in the verapamil group was lower than fradil is ischemia selective through various effects mediated by that in the mibefradil group and did not differ from that in the T-type calcium channel blockade: 1) Blockade of T-channels in placebo group. Nonischemic zone: Both the mibefradil low the coronary vasculature (41) may result in an increased blood dose and verapamil increased blood flow (126 Ϯ 14% [p Ͻ flow in the ischemic border zone, where reentrant arrhythmias 0.05] and 135 Ϯ 12% [p Ͻ 0.01], respectively, vs. 89 Ϯ 5% in are believed to originate (39). During the ischemic period in the placebo group). the present study, blood flow in the nonischemic zone was Size of underperfused zone. The zone sizes were 31 Ϯ 2% increased by mibefradil, whereas that in the midischemic zone (placebo group), 28 Ϯ 1% (mibefradil low dose group), 29 Ϯ remained unchanged. We did not measure blood flow in the Ϯ 2% (mibefradil higher dose group), and 30 2% (verapamil ischemic border zone. An increased blood flow in this zone group). may have contributed to the antiarrhythmic activity of mibe- fradil. 2) Blockade of T-type channels in Purkinje fibers (42) by Discussion mibefradil might also help to inhibit arrhythmogenesis. 3) Blockade of T-type calcium channels in cardiac pacemaker We tested the hypothesis that mibefradil, a proposed cells may decrease heart rate and thereby limit calcium- selective T-type calcium channel blocker, could have antiar- induced arrhythmias (5,43). However, the lower antiarrhyth- rhythmic activity without depressing contractile activity. Dur- mic dose of mibefradil used did not decrease heart rate in this ing ischemia, the lower dose of mibefradil decreased the model. 4) Because T-type calcium channels play a small or no vulnerability of the heart to VF, whereas changes in contractile role in excitation–contraction coupling (44), a relative absence activity were comparable to those in the placebo group. Ͼ of a negative inotropic effect could be expected, as seen in the Because of the excellent of mibefradil ( 90%) present study. (23), intravenous doses are comparable to oral doses. Thus, the Mibefradil and reperfusion arrhythmias. Cytosolic cal- lower intravenous dose used in our study is clinically relevant cium ion overload has been directly implicated in reperfusion because it more or less equals the oral antianginal dose of ϳ2 arrhythmias. Various L-type calcium channel blockers de- mg/kg in patients (35). creased reperfusion arrhythmias (40,45). In the present study The finding that mibefradil reduced experimental arrhyth- the L-type channel blocker verapamil reduced reperfusion mias at a clinically relevant dose without depressing contractile arrhythmias. Mibefradil, a weak L-type calcium channel activity may render the agent useful in the setting of acute blocker, also decreased reperfusion arrhythmias but only at a myocardial infarction. In contrast, no dose of verapamil in our higher supraclinical dose. The failure of the low dose of model offered antiarrhythmic activity during ischemia without a concomitant depression of left ventricular contractile activity. mibefradil to prevent reperfusion arrhythmias may in addition In a previous study (14), a clinically relevant dose of verapamil be related to its effects on coronary blood flow. Mibefradil at (0.2 mg/kg) was not antiarrhythmic. A higher (supraclinical) the low dose markedly increased blood flow in the reperfused dose of verapamil (0.6 mg/kg) was antiarrhythmic but at the zone compared with the placebo and verapamil groups. Hypo- cost of depressing contractile activity (14). A comparable dose thetically, a more rapid reintroduction of oxygen supply could of verapamil (0.3-mg/kg bolus followed by 0.6 mg/kg per h) in have enhanced the rate of generation of free radicals, resulting the present study again depressed contractile activity. in increased calcium oscillations (46) and early calcium- Antiarrhythmic mechanisms of mibefradil during ische- dependent reperfusion ventricular tachyarrhythmias (45). We mia. A recent report (36) proposed that when intracellular did not measure blood flow in the group that received the coupling is reduced, L-type calcium currents could cause higher dose of mibefradil. microreentry, thereby predisposing to arrhythmias in condi- How relevant is VFT? In an earlier study (14), we reported tions such as ischemia. L-type calcium current blockade should on a high incidence of spontaneous arrhythmias during isch- therefore be particularly effective as antiarrhythmic agents emia in our pig-model. For reasons unknown, the incidence of under these conditions. Moreover, mibefradil is a more effec- spontaneous arrhythmias in our model has decreased over tive blocker of L-type calcium channels in depolarized myo- several years (47). However, VFT is a good index of the cardial cells than verapamil (37,38). Mibefradil may therefore vulnerability of the heart to arrhythmias, as we have shown be particularly active in blocking L-channels in myocardial cells (48). We are therefore confident that results of the present that are depolarized by ischemia (39), with a greater effect as study offer a reliable basis for assessment of the antiarrhythmic cells become more depolarized. It is in such depolarized cells activity of verapamil versus mibefradil. With regard to reper- that calcium overload may lead to severe ventricular arrhyth- fusion arrhythmias, there was good correlation in the present mias (3,40). We therefore speculate that mibefradil is “ische- study between the incidence of spontaneous ventricular tachy- mia selective,” by virtue of these effects on L-channels of arrhythmias and the changes in VFT. depolarized cells. However, in the absence of the appropriate Measurement of VFT by the train method as an index of electrophysiologic studies on isolated ventricular myocytes the vulnerability of the heart to ventricular arrhythmias re- subject to simulated ischemia, this proposal remains hypothet- quires special precautions. The train of stimuli, which is meant ical. to coincide with the vulnerable period of the cardiac cycle, may In addition to L-channel effects, we speculate that mibe- spill over into the adjacent refractory zone and thereby fail to JACC Vol. 32, No. 1 MULLER ET AL. 273 July 1998:268–74 MIBEFRADIL VS. VERAPAMIL IN MYOCARDIAL ISCHEMIA provoke VF (49). We took suitable precautions, as described in dial infarction (the Danish Verapamil Infarction Trial-II). Am J Cardiol Methods (29). 1990;66:779–85. 9. MDPIT Study (Multicentre Diltiazem Postinfarction Trial Research Group). Mibefradil and left ventricular contractile activity. An The effect of diltiazem on mortality and reinfarction after myocardial important observation in the present study is that the lower infarction. N Engl J Med 1988;319:385–92. (clinically relevant) dose of mibefradil, given during ischemia, 10. Thandroyen FT. 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