The Effects of Nicorandil on Electrophysiological Changes in Acute Myocardial Ischemia and Reperfusion

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1 Experimental Studies The Effects of Nicorandil on Electrophysiological Changes in Acute Myocardial Ischemia and Reperfusion Hisanori SHINOHARA, MD, Akiyoshi NISHIKADO, MD, Tetsuzo WAKATSUKI, MD, Koichi SAKABE, MD, and Susumu ITO, MD SUMMARY This study was undertaken to clarify the effects of nicorandil on electrophysiological changes during acute ischemia and following reperfusion. We prepared an acute ischemic heart model by ligating the left anterior descending coronary artery in 27 dogs. After 10 minutes, reperfusion was performed. The changes in ventricular effective refractory period (ERP) and intramyocardial conduction time (ICT) were compared between the nicorandil group (n=12) which received nicorandil intravenously before the coronary ligation and the control group (n=15). In the control group, the ERP was shortened during ischemia, and rapidly shortened immediately after reperfusion, but was slightly prolonged 10 minutes after reperfusion. The ICT was prolonged during ischemia, but returned to the pre-ischemia value after reperfusion. In the nicorandil group, the changes in ERP and ICT were significantly inhibited compared to those in the control group. The incidence of ventricular fibrillation (VF) during reperfusion was 42% in the control group. However, there was no VF during reperfusion in the nicorandil group. Therefore, nicorandil may correct both the delayed conduction and the uneven ventricular effective refractory period detected during acute ischemia and following reperfusion, inhibiting the development of ventricular arrhythmia during reperfusion. (Jpn Heart J 1998; 39: ) Key words: Nicorandil, Myocardial ischemia, Reperfusion arrhythmia, Electrophysiology N recent years, reperfusion therapy by thrombolysis or percutaneous transluminal coronary angioplasty (PTCA) is commonly performed for acute myocardial infarction (AMI). Many studies have reported that reperfusion therapy decreased mortality and improved cardiac function, demonstrating the effectiveness of this therapy.1-3) However, fatal arrhythmia such as ventricular fibrillation (VF) is known to From the Second Department of Internal Medicine, School of Medicine, The University of Tokushima, Tokushima, Japan. Address for correspondence: Hisanori Shinohara, MD, Division of Cardiology, Zentsuji National Hospital, Senyu-cho 2-1-1, Zentsuji, Kagawa , Japan. Received for publication December 15, Accepted March 2,

2 364 SHINOHARA ET AL Jpn Heart J May 1998 occur not only during acute ischemia but also during reperfusion, being an important cause of death in coronary disease.4) Currently, reperfusion therapy is commonly performed, and countermeasures for fatal arrhythmia during reperfusion are an important problem in clinical practice. Previous studies have experimentally examined the effectiveness of various agents on these arrhythmias. However, the clinical efficacy has not yet been sufficiently established. Nicorandil is commonly used as an antianginal drug. In addition, nicorandil has been shown to open K channels,5) and the protective actions of nicorandil on myocardium have recently been studied. With respect to the effects of nicorandil on the conduction system in humans, previous studies have indicated that there were no effects on the conduction system and that this agent did not change the effective refractory period or conduction time for ventricular muscle.6) None of the previous studies examined the effects of nicorandil on electrophysiological changes during acute ischemia and following reperfusion. This study investigated the effects of nicorandil on electrophysiological changes during acute ischemia and following reperfusion, and on reperfusion arrhythmia. METHODS Materials: An acute ischemic heart model was prepared using 27 mongrel adult dogs weighing 12 to 20kg. Dogs were divided into the nicorandil group (12 dogs) and the control group (15 dogs). The nicorandil group received nicorandil dissolved in physiological saline at a dose of 0.1 mg/kg administered intravenously at three minutes before the coronary ligation followed by an infusion of 1 Figure 1. Schematic illustration of the design of the experiment. LV=left ventricle; RV=right ventricle; LAD=the left anterior descending coronary artery; Epi=epicardium; End=endocardium; bipolar electrodes for stimulation; epicardial bipolar electrode; endocardial bipolar electrode.

3 Vol 39 No 3 NICORANDIL AND ISCHEMIC MYOCARDIUM 365 g/kg/min. The control group received only normal saline. Preparation of an experimental dog acute ischemic heart model: Mongrel adult dogs were anesthetized with intravenous pentobarbital sodium (20-25mg/ kg). They were intubated and ventilated with a volume-cycled respirator. Thoracotomy was performed at the 5th left intercostal space. After the heart was suspended in a pericardial cradle, the left anterior descending coronary artery (LAD) was isolated at the origin of the 1st diagonal branch, and an occluder for coronary artery ligation was applied to the intermediate region of LAD behind the 1st diagonal branch (Figure 1). Acute myocardial ischemia was induced by completely obstructing the occluder placed in LAD. The coronary artery was ligated for ten minutes and then immediately reopened. Recording methods: Myocardial electric potentials were amplified using a physiologic data amplifier (MCM-8000, Fukuda Denshi Co., Tokyo, Japan) and recorded with an ink jet-type recorder (CR-800, Fukuda Denshi). A programmable cardiac electrostimulator (SEC-2102, Nihon-Koden Co., Tokyo) was used to deliver electric stimuli to the heart. Measurement of the ventricular effective refractory period and intramyocardial conduction time: Bipolar electrodes with an interelectrode distance of 1-2mm were inserted subendocardially and subepicardially in the ischemic zone below the ligation site of LAD and the nonischemic zone in the left circumflex coronary artery to record endocardial and epicardial electric potentials. Myocardial electric potential was filtered at 53 to 500Hz. Furthermore, the bipolar electrodes used for stimulation were inserted in the middle myocardium on the septal side about 1.5cm from the epicardial electrode at the ischemic and nonischemic zones. Body surface electrocardiogram and the endocardial and epicardial myocardial electric potentials in each zone were recorded at a chart speed of mm/sec. The ventricular effective refractory period (ERP) was determined by applying premature stimuli after 8-beat pacing at 300msec to the ventricular myocardium at the ischemic and nonischemic zones with a programmable cardiac electrostimulator. This premature stimulation was applied with the coupling interval shortened by 5msec steps. The ERP was defined as the longest coupling interval without ventricular capture. The time between the peak of the first sharp QRS wave of the endocardial electric potential and the peak of the final sharp QRS wave of the epicardial electric potential at own beat was regarded as intramyocardial conduction time (ICT). The ERP and ICT at the ischemic and nonischemic zones for 10 minutes during coronary occlusion and within 10 minutes after reperfusion were measured. The changes in ERP and ICT were expressed as a percentage of the value obtained before the nicorandil administration in the nicorandil group

4 366 SHINOHARA ET AL Jpn Heart J May 1998 (baseline: 100) The changes were expressed as a percentage of the value obtained before coronary ligation in the control group (baseline: 100). Comparison between the two groups: The changes in ERP and ICT during ischemia and following reperfusion were compared between the nicorandil and control groups. The incidence of VF was also compared between the two groups. Statistical analysis: All values are expressed as the mean }standard deviation. The changes in ERP and ICT were compared using the Student's t test. The incidence of VF was compared using the chi-square test. A value of p<0.05 was considered statistically significant. RESULTS Electrophysiological changes in the control group: The changes in ERP in the control group are shown in Figure 2. The ERP at the ischemic zone was gradually shortened after LAD ligation, and significantly shortened to 93 }6% five minutes after ischemia compared to the baseline (p<0.05). The value was further shortened to 88 }9% eight minutes after ischemia, being minimal during ischemia. The ERP was further shortened to 77 }12% immediately after reperfusion, but was gradually prolonged thereafter and returned to 96 }8%, which was similar to the baseline, three minutes after reperfusion. Thereafter, ERP was further prolonged to 111 }6% seven minutes after reperfusion. There were significant increases compared to the baseline (p<0.01). At the nonischemic zone, there were no significant changes in ERP during ischemia and following reperfusion compared to the baseline. The changes in ICT in the control group are shown in Figure 3. The ICT at the ischemic zone was prolonged immediately after LAD ligation, and significantly prolonged to 138 }18% two minutes after ischemia compared to the baseline (p<0.01). Thereafter, the value reached a peak, 154 }38%, seven minutes after ischemia. The ICT was rapidly shortened to 119 }10% immediately after reperfusion. Thereafter, the ICT further shortened to 106 }6%, which was similar to the baseline, two minutes after reperfusion. At the nonischemic zone, there were no significant changes in ICT during ischemia and following reperfusion compared to the baseline. Electrophysiological changes in the nicorandil group: Heart rate in the nicorandil group was not significantly different between baseline and administration of nicorandil (145 }17 and 150 }19/min). The changes in ERP in the nicorandil group are shown in Figure 2. During ischemia, there were no significant changes in ERP at the ischemic zone compared to the baseline. The ERP was 101 }10% immediately after reperfusion, and there was no shortening observed in the control group. The ERP was slightly and insignificantly prolonged

5 Vol 39 No 3 NICORANDIL AND ISCHEMIC MYOCARDIUM 367 Figure 2. Changes in the effective refractory period (ERP) during acute myocardial ischemia and following reperfusion in the control () and nicorandil () groups. (A) the ischemic zone, (B) the nonischemic zone. Values are mean }SD. Values are expressed as percent change from the baseline. Cont.=control group; Nico.=nicorandil group. *p<0.05, **p<0.01 between the control and nicorandil groups. õp<0.05, öp<0.01 vs baseline. to 107 }11% after reperfusion. At the nonischemic zone, there were no significant changes in ERP during ischemia and following reperfusion compared to the baseline. The changes in ICT in the nicorandil group are shown in Figure 3. The ICT at the ischemic zone was significantly prolonged immediately after LAD ligation. The value reached a peak, 114 }12%, six minutes after ischemia. The value was shortened to 106 }10% immediately after reperfusion, and returned to the baseline two minutes after reperfusion. At the nonischemic zone, the ICT was gradually prolonged, and significantly prolonged only 10 minutes after ischemia compared to the baseline (p<0.05). The peak value was 108 }12%. There were

6 368 SHINOHARA ET AL Jpn Heart J May 1998 Figure 3. Changes in the intramyocardial conduction time (ICT) during acute myocardial ischemia and following reperfusion in the control () and nicorandil () groups. (A) the ischemic zone, (B) the nonischemic zone. Values are mean }SD. Values are expressed as percent change from the baseline. Cont.=control group; Nico.=nicorandil group. *p<0.05, **p<0.01 between the control and nicorandil groups. p<0.05, p<0.01 vs baseline. no significant changes after reperfusion compared to the baseline. Comparison between the nicorandil and control group: The changes in ERP at the ischemic zones in the nicorandil and control groups are shown in Figure 2A. In the nicorandil group, nicorandil significantly inhibited the shortening in ERP as observed in the control group between 6 minutes after ischemia and 2 minutes after reperfusion. The changes in ERP at the nonischemic zones in the two groups are shown in Figure 2B. There were no significant differences in ERP between the two groups during ischemia and following reperfusion. The changes in ICT at the ischemic zones in the two groups are shown in Figure 3A. In the nicorandil group, nicorandil significantly inhibited the prolongation in ICT as observed in the control group between 2 minutes after ischemia and immediately after reperfusion. The changes in ICT at the nonischemic zones

7 Vol 39 No 3 NICORANDIL AND ISCHEMIC MYOCARDIUM 369 Figure 4. Incidence of VF in the control and nicorandil groups. (A) during ischemia, (B) after reperfusion. VF=ventricular fibrillation; NS=not significant. in the two groups are shown in Figure 3B. The ICT was significantly prolonged in the nicorandil group between 6 minutes and 9 minutes after ischemia. Incidence of ventricular fibrillation: The incidences of VF are shown in Figure 4. In the control group, VF occurred in 3 of 15 dogs (20%) during ischemia and in 5 of 12 dogs (42%) after reperfusion. In the nicorandil group, VF occurred in 2 of 12 dogs (17%) during ischemia. However, there was no VF in 10 dogs after reperfusion. The incidence of VF during ischemia did not significantly differ between the two groups. However, the incidence of VF after reperfusion in the nicorandil group was significantly lower than that in the control group (p<0.05). DISCUSSION In recent years, reperfusion therapy for AMI is commonly performed in an increasing number of cases, and has already become an established treatment. This treatment is considered to reduce infarction size by reperfusion of an occluded coronary artery during the early stage, thus decreasing the acute-phase mortality and improving chronic-phase cardiac function. Many studies have demonstrated the efficacy of reperfusion therapy.1-3) However, among patients with AMI, there have been many patients in whom reperfusion of occluded coronary artery may have caused progression of the clinical course. Therefore, the concept of reperfusion myocardial injury has arisen. Currently, reperfusion injury includes 1) reperfusion arrhythmia, 2) micro-vascular damage following reperfusion and no-reflow phenomenon, 3) stunned myocardium and 4) lethal myocyte cell injury following reperfusion.7) Among these reperfusion injuries, various studies have described reperfusion arrhythmia. Several experiments demonstrated that a high incidence of VF occurred after reperfusion in ischemic myocardium.8-10) It is also known

8 370 SHINOHARA ET AL Jpn Heart J May 1998 that fatal arrhythmia such as VF occurs not only during acute myocardial ischemia but also after reperfusion therapy and spontaneous recanalization for AMI in clinical practice. Hager et al.11) reported that the incidence of VF or sustained ventricular tachycardia immediately after reperfusion confirmed on angiography was approximately 6%, and indicated that the interval between onset and reperfusion was slightly shorter in these patients. Many studies have investigated the pathogenesis and prevention of reperfusion arrhythmia. The effects of various antiarrhythmic agents, ƒ -blocker, ƒà-blockers and Ca antagonists on reperfusion arrhythmia have been examined, but beneficial effects have not yet been demonstrated. Nicorandil is a nitrate compound developed in Japan in Since nicorandil dilates coronary artery and inhibits coronary artery spasm, this agent is commonly used to treat angina pectoris. Furthermore, nicorandil has recently been shown to open ATP-sensitive K channels.5) In 1983, ATP-sensitive K channels were discovered by Noma12) as an ion channel that opened with decreased intracellular ATP concentrations. ATP-sensitive K channels have been studied as an ion channel activated during hypoxia or metabolic disorders. When myocardium causes ischemia, tissue ATP levels are decreased, opening ATP-sensitive K channels, thus promoting extracellular outflow of intracellular K. As a result, hyperpolarization of the membrane potential is induced and potential-dependent Ca channels are inhibited, decreasing the intracellular influx of Ca, thus relieving ischemic cellular injury. It has also been indicated that intracellular influx of a large number of Ca ions is involved in the development of myocardium injury during reperfusion.13) Nicorandil is considered to inhibit intracellular influx of Ca by opening K channels and to enhance myocardial resistance to ischemia, thus inhibiting progression of ischemic injury.14-16) Auchampach et al.14) performed reperfusion after the coronary artery of dogs was ligated for 15 minutes, and examined serial improvement in myocardial contractility at the ischemic site. They reported that myocardial contractility in the group pretreated with nicorandil improved by approximately 80%, while that in the control group improved by approximately 20%. In addition, they indicated that the effects of nicorandil disappeared after administration of a K channelblocker, glibenclamide, suggesting the involvement of K channels. Furthermore, Pieper et al.15) and Gross et al.16) reported the protective effects of nicorandil on myocardium during ischemia. With respect to the effects of nicorandil on the conduction system in humans, Tsuchioka et al.6) reported that nicorandil did not influence the conduction system and did not change the ERP or conduction time for ventricular muscle. However, none of the previous studies investigated the effects of nicorandil on arrhythmia, especially on reperfusion arrhythmia.

9 Vol 39 NICORANDIL AND ISCHEMIC MYOCARDIUM 371 No 3 Among previous studies investigating electrophysiological changes in myocardium during acute ischemia and following reperfusion, Levites et al.17) reported that the ERP was shortened during ischemia, but prolonged after reperfusion. Naimi et al.18) indicated that the ERP was rapidly shortened immediately after reperfusion. Furthermore, Elharrar et al.19) reported that ischemia caused delayed conduction. Generally, reduced coronary blood flow decreases ATP production in ischemic myocardium. In addition, it is known that intracellular Ca ion levels are increased, enhancing intercellular electric resistance.20) In this study, the ERP in the control group was shortened after ischemia, and rapidly shortened immediately after reperfusion. Reperfusion arrhythmia was noted during this period. The ICT was prolonged after ischemia, but returned to the baseline value after reperfusion. These findings were similar to those described in previous studies. The shortened ERP during ischemia, especially rapid changes immediately after reperfusion, and markedly delayed conduction during ischemia markedly increased differences in refractory period and conduction between the ischemic and nonischemic zones. This may inhibit the electrophysiological stability of myocardium, causing fatal arrhythmia during ischemia and following reperfusion. In the nicorandil group, there were no significant changes in ERP during ischemia or immediately after reperfusion. Although the ICT was prolonged during ischemia, the changes were significantly inhibited compared to those in the control group. The mechanism by which nicorandil inhibited delayed conduction during ischemia and following reperfusion in this study may involve 1) improvement in myocardial ischemia by vasodilator actions, 2) inhibition of excessive Ca influx by opening K channels and relief of intercellular electric resistance, and 3) protective actions on myocardium. Nicorandil may have inhibited differences in refractory period and conduction between the ischemic and nonischemic zones compared to those in the control group, improving the electrophysiological instability of myocardium. Ischemic ventricular arrhythmia which occurs early after coronary artery occlusion is mainly associated with intrawall reentry of ventricular muscle based on delayed conduction and block in ischemic myocardium. However, it is speculated that mechanisms other than reentry partially contribute to this condition.21) With respect to the pathogenesis of reperfusion arrhythmia, Kaplinsky et al.10 performed reperfusion after occluding LAD of dogs for 30 minutes, and reported two kinds of ventricular arrhythmia which were observed 0 to 1 minute after reperfusion and 2 to 7 minutes after reperfusion, respectively. They indicated that the former was reentry arrhythmia associated with electric unevenness during the recovery process of ischemia, causing a high incidence of VF. However, the latter was mainly associated with enhanced automaticity, causing a low inci-

10 372 SHINOHARA ET AL Jpn Heart J May 1998 dence of VF. They speculated that reperfusion arrhythmia is not associated with a single mechanism. In this study, there was no VF following reperfusion in the nicorandil group, and nicorandil significantly inhibited VF after reperfusion. Nicorandil may have inhibited reperfusion arrhythmia by correcting both unevenness in the refractory period for ventricular muscle and delayed conduction during myocardial ischemia and following reperfusion. In particular, this agent appeared to correct unstable myocardial conduction by a rapid reduction in the refractory period and a rapid improvement of delayed conduction immediately after reperfusion, thus significantly inhibiting VF after reperfusion. But, the incidence of VF during ischemia in the nicorandil group did not significantly differ from that in the control group. Though nicorandil corrected unevenness in the refractory period during ischemia, delayed conduction was not adequately corrected, thus nicorandil did not sufficiently inhibit VF during ischemia. Whether the effects of nicorandil on electrophysiological changes and reperfusion arrhythmia were due to improvement in myocardial ischemia by vasodilator actions or due to protective actions on ischemic myocardium by opening ATP-sensitive K channels, it was not clarified in this study. To clarify this problem, we further need to experiment in the same protocol with another vasodilator drug without opening ATP-sensitive K channels. These findings suggest that nicorandil administration prior to reperfusion after AMI inhibits fatal arrhythmia after reperfusion. REFERENCES 1. GISSI. Effectiveness of intravenous thrombolytic treatment in acute myocardial infarction. Lancet 1986; i: The GUSTO Investigators. The effects of tissue plasminogen activator, streptokinase, or both on coronary-artery patency, ventricular function, and survival after acute myocardial infarction. N Engl J Med 1993; 329: O'Keefe JH, Bailey WL, Rutherford BD, Hartzler GO. Primary angioplasty for acute myocardial infarction in 1000 consecutive patients: results in an unselected population and high-risk subgroups. Am J Cardiol 1993; 72: 107G-15G. 4. Fulton M, Julian DG, Oliver MF. Sudden death and myocardial infarction. Circulation 1969; 40(Suppl IV): IV Hiraoka M, Fan Z. Activation of ATP-sensitive outward K+ current by nicorandil (2- nicotinamidoethyl nitrate) in isolated ventricular myocytes. J Pharmacol Exp Ther 1989; 250: Tsuchioka Y, Yamagata T, Amioka H, et al. Clinical electrophysiological effects of nicorandil on the conduction system in humans. Kokyu to Junkan 1990; 38: Kloner RA. Does reperfusion injury exist in humans? J Am Coll Cardiol 1993; 21: Corbalan R, Verrier RL, Lown B. Differing mechanisms for ventricular vulnerability during coronary artery occlusion and release. Am Heart J 1976; 92: Balke CW, Kaplinsky E, Michelson EL, Naito M, Dreifus LS. Reperfusion ventricular tachyarrhythmias: correlation with antecedent coronary artery occlusion tachyarrhythmias and duration of myocardial ischemia. Am Heart J 1981; 101:

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