Occurrence and prevention of contraction bands in Purkinje fibres,transitional cells and working myocardium during global ischaemia

Summary Contraction bands usually occur in the intramural working myocardium following post-ischaemic reperfusion. In the subendocardium, however, they are found during ischaemia. Thus, we ascertained the contraction states of Purkinje fibres, transitional cells, subendocardial and intramural parts of the working myocardium during 30 min global ischaemia at 25° C. The effects with and without myocardial protection were compared. At the onset of pure ischaemia contraction bands are completely lacking in all cell types. During pure ischaemia contraction bands are found in all subendocardial cell types but not in the intramural working myocardium. A peak of pathological contraction states is found in the intramural working myocardium at the onset (0 min), in the subendocardial working myocardium at 10 min, in the transitional cells and Purkinje fibres at 30 min of pure ischaemia. Histidine-, tryptophan-, ketoglutarate-enriched (HTK) cardioplegia prevents contraction bands completely at the onset of ischaemia and prevents both contraction bands and pathological contraction states during ischaemia almost completely. Striking differences in the physiological contraction states are seen only in the working myocardium: HTK cardioplegia brings about dominance of relaxation during ischaemia. These findings may be due mainly to the effects of global ischaemia on the one hand and to catecholamines, calcium and oxygen on the other.     

Acute subendocardial ischaemia leads to homogenous prolongation in ventricular repolarization

Acute transmural ischaemia often shortens ventricular repolarization and increases repolarization dispersion, leading to life threatening ventricular arrhythmias in animal models and human subjects. Experimental studies and clinical observations have shown that acute subendocardial ischaemia rarely causes serious ventricular arrhythmia. We hypothesized that the different arrhythmia outcomes between transmural and subendocardial ischaemia are largely due to the homogenous prolongation in ventricular repolarization after acute subendocardial ischaemia. Further experimental studies on a subendocardial model are required to assess the changes in ventricular repolarization and its spatial dispersion, and to investigate the role of these changes in the pathogenesis of ventricular arrhythmias. These studies will facilitate our understanding on the mechanisms of life-threatening ventricular arrhythmias during acute myocardial ischaemia.     

线粒体ATP敏感性钾通道参与超极化停搏的心肌保护作用

目的：探讨线粒体ATP敏感性钾通道（mitoKATP）开放在超极化停搏心肌保护中的作用机制。方法:将SD大鼠随机分为对照组（Control）、去极化停搏组（D）、超极化停搏组（H）、5-羟葵酸(5-HD) +去极化停搏组（5HD+D）、5-HD+超极化停搏组（5HD+H），每组8例。建立Langendorff灌注模型，平衡20 min，以不同方式停搏40 min，再灌注30 min，对比观察：(1)不同时间血流动力学变化；(2)再灌注末取心肌并分离、制备线粒体，电镜观察超微结构的变化。(3)平衡末、再灌注末线粒体活性氧的产生。结果: (1)各组再灌注末大鼠心脏功能明显低于平衡末, 心肌线粒体超微结构均遭受不同程度损伤，左室发展压（LVDP）、左室舒张末压（LVEDP）、率压双乘积（DP）、冠脉流量（CF）有显著差异（P<0.01）；（2）超极化停博组再灌注末心脏功能指标LVDP、LVEDP、DP、CF明显优于去极化停博组、5-HD+超极化停搏组、5-HD+去极化停搏组、对照组（P<0.01），电镜示：心肌、线粒体超微结构遭受的损伤较轻; (3)超极化停博组再灌注末心肌线粒体活性氧产生率低于对照组与其它3组（P<0.01）。结论:(1)超极化停搏能明显改善再灌注后心功能，保护心肌、线粒体超微结构，减少活性氧生成；(2)mitoKATP的早期开放参与超极化停搏，其作用可能通过保护再灌注后的线粒体呼吸功能，减轻线粒体的氧化损伤，为再灌注心肌提供较好的能量供应，从而使缺血再灌注后的心脏收缩功能得到一定恢复。     

HTK液在大血管转位手术中的应用研究

目的探讨Switch手术中心肌保护的整体策略。方法回顾分析30例应用HTK液为心肌保护液,行大血管转位术(Arterial Switch Operation,Switch术)的病例。术中心肌保护液灌注,术后24小时内采血检测血清肌钙蛋白I(cardiac troponin I,cTnI),脑利钠肽(Brain Natriuretic Peptide,BNP)含量的变化。记录心脏自动复跳率,术后惊厥发生率,术后插管时间及监护室时间等指标,分析心肌保护效果及内环境变化检测。结果所有病例均于主动脉根部灌注1次,平均主动脉阻断时间91.33±24.90min,HTK心肌保护液平均用量51.73ml/Kg,停跳效果好,术中无需再行冠状动脉内灌注,自动复跳率100%,术毕Na+浓度处于正常范围,术后无惊厥发生,监护室恢复顺利。结论 HTK心肌保护液适用于未成熟心肌,心肌保护时间长、避免冠状动脉内灌注的特点更适用于Switch手术。     

Temperature preconditioning of isolated rat hearts – a potent cardioprotective mechanism involving a reduction in oxidative stress and inhibition of the mitochondrial permeability transition pore

We investigate whether temperature preconditioning (TP), induced by short-term hypothermic perfusion and rewarming, may protect hearts against ischaemic/reperfusion injury like ischaemic preconditioning ( IP ). Isolated rat hearts were perfused for 40 min, followed by 25 min global ischaemia and 60 min reperfusion (37°C). During pre-ischaemia, IP hearts underwent three cycles of 2 min global ischaemia and 3 min reperfusion at 37°C, whereas TP hearts received three cycles of 2 min hypothermic perfusion (26°C) interspersed by 3 min normothermic perfusion. Other hearts received a single 6 min hypothermic perfusion (SHP) before ischaemia. Both IP and TP protocols increased levels of high energy phosphates in the pre-ischaemic heart. During reperfusion, TP improved haemodynamic recovery, decreased arrhythmias and reduced necrotic damage (lactate dehydrogenase release) more than IP or SHP. Measurements of tissue NAD⁺ levels and calcium-induced swelling of mitochondria isolated at 3 min reperfusion were consistent with greater inhibition of the mitochondrial permeability transition at reperfusion by TP than IP; this correlated with decreased protein carbonylation, a surrogate marker for oxidative stress. TP increased protein kinase Cɛ (PKCɛ) translocation to the particulate fraction and pretreatment with chelerythrine (PKC inhibitor) blocked the protective effect of TP. TP also increased phosphorylation of AMP-activated protein kinase (AMPK) after 5 min index ischaemia, but not before ischaemia. Compound C (AMPK inhibitor) partially blocked cardioprotection by TP, suggesting that both PKC and AMPK may mediate the effects of TP. The presence of N -(2-mercaptopropionyl) glycine during TP also abolished cardioprotection, indicating an involvement of free radicals in the signalling mechanism.     

Propofol improves recovery of the isolated working hypertrophic heart from ischaemia–reperfusion

The general anaesthetic propofol shows promise in protecting normal hearts against various cardiac insults, but little is known about its cardioprotective potential in hypertrophic hearts. This study tested the hypothesis that propofol at a clinically relevant dose would enhance functional recovery in hypertrophic hearts following ischaemia. Hypertrophic hearts from spontaneously hypertensive rats and hearts from their normotensive controls, Wistar Kyoto Rats, were equilibrated in the working mode prior to global normothermic ischaemia. Reperfusion commenced with 10?min in Langendorff mode, followed by 30-min working reperfusion. Functional performance was measured throughout the working mode, whilst reperfusion damage was assessed from myocardial troponin I release during Langendorff reperfusion. Where used, 4?μg/ml propofol was added 10?min before ischaemia and was washed out 10?min into working reperfusion. An additional protocol investigated recovery of hearts protected by normothermic hyperkalaemic cardioplegic arrest. Following 20-min ischaemia, reperfusion damage was significantly worse in hypertrophic hearts compared to normal hearts, whilst addition of propofol to hypertrophic hearts significantly improved the aortic flow (31 ± 5.8 vs. 11.6 ± 2.0?ml/min, n?=?6–7 ± SE, p?<?0.05). Propofol also conferred significant protection following 30-min ischaemia where the recovery of cardiac output and stroke volume was similar to that for cardioplegia alone. Incubation with propofol improved the NADH/NAD⁺ ratio in freshly isolated cardiomyocytes from hypertrophic hearts, suggesting possible improvements in metabolic flux. These findings suggest that propofol at the clinically relevant dose of 4?μg/ml is as effective as cardioplegic arrest in protecting hypertrophic hearts against ischaemia–reperfusion.     

The myocardial protection of HTK cardioplegic solution on the long-term ischemic period in pediatric heart surgery

Cardioplegic reperfusion during a long term ischemic period interrupts cardiac surgery and also increase cellular edema due to repeated administration. We reviewed the clinical experiences on myocardial protection of one single perfusion with histidine-ketoglutarate-tryptophan (HTK) for infants. This retrospective study included 118 infants who underwent open-heart surgery between January 2004 and December 2007. We divided the entire cohort into two groups: In group H (n = 63), myocardial protection was carried out with one single perfusion with HTK solution, and in group S (n = 55) with conventional St. Thomas crystalloid cardioplegia. The duration of cardiopulmonary bypass (CPB) did not differ between these two groups, but the duration of aortic cross-clamping time in group H was significantly shorter than that in group S (p < 0.05). During reperfusion, the spontaneous re-beating rate was higher in group H (p < 0.05). There were no differences in doses of inotropic agent and creatinekinase (CK) values on postoperative day 1 between these two groups, but the level of CK in group H was significantly less than that in group S on postoperative day 2 (p < 0.01). The mortality in group H was lower than in group S (p < 0.05). The HTK group had shorter cross-clamping time and more frequent spontaneous defibrillation than St. Thomas group. We propose that HTK is valid for some complicated cardiac surgeries with long term cross-clamping time.     

Comparison between blood and crystalloid cardioplegia in patients with left ventricular dysfunction undergoing coronary surgery

This study was done to compare the protective effect of blood and crystalloid cardioplegia in patients with left ventricular dysfunction undergoing coronary artery bypass grafting (CABG). Sixty consecutive patients with left ventricular ejection fraction < 35% scheduled for CABG with the use of cardiopulmonary bypass without additional procedures were randomly divided into two groups. In the first group we used cold blood cardioplegia, in the second group cold crystalloid cardioplegia, both delivered only ortogradely. We measured hemodynamic data in early hours after operation, enzyme release and we collected other clinical data which could be influenced by perioperative myocardial protection. There was no death in either group. We also didn't find any significant difference in incidence of perioperative myocardial infarction, arrhythmias and use of intraaortic balloon pumping between both groups. In an early hours after operation in the group with blood cardioplegia we found significantly better hemodynamic data (LVSWI, RVSWI) and significantly lower enzyme release. We conclude, that cold blood cardioplegia shows superior perioperative myocardial protection resulting in earlier restoration of myocardial function. This difference could be important in patients with high degree of left ventricular dysfunction.     

The development of the Purkinje fibre system in the bovine fetal heart

Summary In the bovine fetal heart, subendocardial bundles of cells could be distinguished from the main myocardial mass. Their morphological characteristics suggest that they represent bundles of Purkinje fibres. An intense fluorescence after incubation in antisera against the intermediate filament protein skeletin also supports this suggestion. Further, the bundles exhibited different histochemical reactions from the main myocardial mass. During development the histochemical pattern changed. Bundle cells in mitosis were observed. With increasing fetal age, binucleate cells were seen progressively more frequently.Our observations indicate that the Purkinje fibres differentiate along a line separate from the ordinary myocardial cells and that they acquire their adult characteristics gradually.This study was supported by grants from the Swedish Medical Research Council (12X-3934), the Faculty of Medicine, University of Umeå, and the Expressen Prenatal Fund     

Ventricular histamine concentrations and arrhythmias during acute myocardial ischaemia in rats

The relation between ventricular histamine concentrations and the occurrence of early ventricular arrhythmias during acute myocardial ischaemia was investigated in pentobarbitone-anaesthetized rats. There was significant decrease in the left, but not the right, ventricular histamine level at 5 min following acute left coronary artery ligation. Pretreatment with rhodanine caused remarkable reduction in ventricular histamine concentrations as well as significantly lower incidence and slower onset of ventricular tachycardia and fibrillation resulting from acute myocardial ischaemia. On the contrary, aminoguanidine pretreatment did not significantly alter ventricular histamine levels nor did it influence the occurrence of early ventricular arrhythmias induced by coronary artery ligation. The responses of blood pressure and heart rate to acute coronary artery ligation were not noticeably affected by rhodanine or aminoguanidine pretreatment. These findings support the hypothesis that histamine release from cardiac tissues may contribute to the genesis of early ventricular arrhythmias, but not to the changes in blood pressure and heart rate, during acute myocardial ischaemia.     

Purkinje fibers of sheep papillary muscle: Occurrence of discontinuous fibers

Mapping of the peripheral specialized conducting system of the heart could contribute to an understanding of normal and abnormal impulse propagation. In this study, a sheep right anterior papillary muscle was sectioned serially and stained for Purkinje fibers with the periodic acid Schiff and hematoxylin procedure. The subendocardial Purkinje fibers entered the papillary muscle through its base and extended nearly to its apex. Except for one “mixed” fiber group, the subendocardial fibers did not connect to the deep Purkinje fibers, and both groups contained discontinuous Purkinje fibers. The fibers of the moderator band radiated in such a complex fashion that they could not be reconstructed exactly, but they generally disappeared in the ventricular wall through the base of the papillary muscle and false tendons. No direct connections between the subendocardial Purkinje fibers and those of the moderator band could be found within the boundaries of the papillary muscle, and this may form the basis for the relatively late activation of the papillary muscle as compared to the surrounding myocardial wall. The discontinuous fibers we observed are compatible with a current concept regarding the embryology of the ventricular specialized conducting system, and are not in conflict with normal impulse propagation.     

牦牛心室蒲肯野纤维的立体分布和结构特点

目的 探讨牦牛心室普肯耶纤维的分布和结构特点,为高原哺乳动物心室传导系统的研究积累形态学资料。 方法 采用墨汁灌注、ABS树脂灌注铸型,石蜡切片HE染色、Masson染色和免疫组织化学染色技术,观察60只成年牦牛的心室普肯耶纤维分布及结构特征。结果 牦牛心室普肯耶纤维束周围包绕有结缔组织鞘。左束支在室间隔内膜下层有2条或3条分支;右束支经隔缘肉柱在右前乳头肌根部心肌层中有3条或4条分支。心室普肯耶纤维在心内膜下层呈多边形网状分布,并在心肌层中发出大量分支,左、右心室乳头肌顶端内膜下层未观察到普肯耶纤维分布。普肯耶纤维呈卷轴状、蜂窝状、漏斗状或脊状构型。Cx43在普肯耶纤维呈膜阳性。结论ABS树脂灌注铸型技术能用于心室普肯耶纤维分布的研究。牦牛心脏左束支和右束支呈不对称分布,左束支较发达。     

Preconditioning does not attenuate cardiac dysfunction after global ischaemia in the guinea-pig

Ischaemic preconditioning reduces infarct size, but the effects on cardiac function after global ischaemia are more controversial. Additionally, species differences may exist. The present study investigates the effects of preconditioning on cardiac performance in the globally ischaemic, Langendorff-perfused guinea-pig heart. Hearts were stabilized for 25 min, and divided into the following groups: (1) (n = 8) control perfusion for 16 min before 30-min global ischaemia and 30-min reperfusion, (2) (n = 7) two episodes of 3-min ischaemia and 5-min reperfusion before global ischaemia, (3) (n = 7) 5-min ischaemia and 10-min reperfusion before ischaemia, (4) (n = 8) control perfusion before 40-min ischaemia and 30-min reperfusion, (5) (n = 8) Preconditioning as group 2 before ischaemia as group 4, (6) (n = 9) Control perfusion before 50-min ischaemia and 30-min reperfusion, (7) (n = 10) Preconditioning as group 2 before ischaemia as group 6. A dose-dependent reduction of left ventricular systolic pressure, and increase of end-diastolic pressure was observed during reperfusion after 30-, 40- and 50-min ischaemia. Preconditioning did not influence these changes, nor did it attenuate the incidence of severe reperfusion arrhythmias or reduction of coronary flow. In conclusion, ischaemic preconditioning does not improve cardiac function during reperfusion of the globally ischaemic, isolated guinea-pig heart.     

Structural basis of ventricular arrhythmias in human myocardial infarction: A hypothesis

The present study was undertaken using light and electron microscopic techniques to determine whether Purkinje fibers survive in the subendocardial region of anteroseptal infarcts in humans. Tissue was obtained for this purpose from 11 patients with 12 documented infarctions at the time of autopsy; six patients died within 72 hours of the infarction and five had healed infarcts. Seven of the 11 patients had ventricular arrhythmias.Light microscopic study indicated that intact cells with a normal appearance remained on the subendocardial surface, although the underlying ventricular muscle either was necrotic or was replaced by fibrous tissue. Electron microscopy demonstrated that these intact surviving cells over the surface of the infarct had few randomly oriented myofibrils, abundant glycogen, and other characteristics of Purkinje fibers. These cells could be readily distinguished from normal or infarcted ventricular muscle cells. Purkinje fibers, the most peripheral part of the conduction system, survive in extensive anteroseptal infarcts and may be the site of origin of ventricular arrhythmias.     

Impaired sarcolemmal membrane permeability in reperfused ischemic myocardium

Summary Sarcolemmal membrane permeability to intravenously injected horseradish peroxidase HRP (MW=40,000) was examined in 8 Wistar rats which had temporary ischaemia produced by left coronary artery ligation. HRP reaction product was identified following 6 min of circulation time by light and electron microscopy. Controls included 4 uninjected animals with coronary ligation, 2 uninjected animals without myocardial ischaemia and 2 injected non operated rats.In normal myocardium, the tracer permeated endothelial plasmalemmal vesicles, intercellular spaces and intracellular vesicles of the T-tubule system, but never permeated the cytoplasm of myocardium cells.As early as 15 min after coronary artery ligation followed by 6 min of reperfusion with circulation of the tracer, HRP product could be seen in the cytoplasm of muscle cells randomly distributed in the subendocardial area. The quantity of permeated cells increased when the ischaemic myocardium is reperfused during 10 min before injecting the tracer.These data indicate that sarcolemmal membrane alteration is an early event in myocardial ischaemic injury and precede the irreversible cellular degenerative changes.This work was supported by research grant from INSERM (ATP 78-95)     

二氮嗪后处理对离体大鼠心功能及线粒体心磷脂的影响

目的:建立离体大鼠心肌缺血/再灌注损伤模型,观察二氮嗪(diazoxide,D)后处理对缺血/再灌注损伤离体大鼠心功能及线粒体心磷脂的影响,并探讨ATP敏感性钾通道在二氮嗪后处理心肌保护中的作用。方法:采用Langendorff装置建立离体大鼠心肌缺血/再灌注损伤模型,将SD大鼠随机分为对照组(control)、缺血再灌注模型组(I/R)、二氮嗪后处理组(I/R+D)、5-羟葵酸拮抗二氮嗪后处理组(I/R+5-HD+D),每组8只,均先灌注平衡20 min。Control组:灌注平衡后续灌70 min;I/R组:缺血前灌注4℃ST.Thomas停跳液,全心缺血40 min,再灌30 min;I/R+D组:全心缺血40 min,缺血后给予含二氮嗪(50μmol/L)的K-H液灌注5 min后,再灌25 min;I/R+5-HD+D组:二氮嗪后处理前给予含5-羟葵酸(100μmol/L)的K-H液灌注5 min,再灌20 min。观察各组续(再)灌注末心率、冠脉流出液量、心功能、心肌酶学及心肌线粒体心磷脂的变化。结果:各组续(再)灌注末比较,I/R组较control组及I/R+D组心率减慢、冠脉流出液量降低,心功能明显受损,心肌酶增加,心磷酯含量减少,但与I/R+5-HD+D无明显差异。结论:二氮嗪后处理通过增加线粒体心磷脂含量,减少心肌酶的释放,改善心脏功能,减轻心肌的再灌注损伤,产生心肌保护作用。5-羟葵酸能够完全阻断二氮嗪的心肌保护作用。     

Preservation of cardiac myocytes subjected to different preconditions: A comparative morphometric study of beating,fibrillating, and cardioplegically arrested canine hearts

This study compares the ultrastructure of beating canine hearts with that of hearts subjected to different clinically common forms of cardiac arrest. The contraction state per test field was ascertained according to a specially developed classification. The volume density of myofibrils and the surface to volume ratio of mitochondria were used as parameters for cellular and mitochondrial swelling. Contraction bands were not found in any of the differently pretreated hearts. Following immersion fixation, contractions as well as over- and hypercontractions in beating, fibrillating, and St. Thomas-arrested hearts are significantly more pronounced than in HTK-arrested hearts. Cellular and mitochondrial volumes were similar in beating and fibrillating hearts. St. Thomas-perfusion significantly decreased cellular and mitochondrial volume compared to beating hearts, but these values were in the same range as in fibrillating hearts. Only HTK-solution actually led to a strong reduction of these compartments. Compared to immersion, perfusion fixation after coronary perfusion with cardioplegic solutions led to comparable cellular volumes, but significantly elevated the percentage of relaxed sarcomeres and significantly reduced mitochondrial swelling. The best structural preservation of myocytes was found after HTK-perfusion and perfusion fixation. Such ultrastructural quantitative and morphometrical parameters are powerful tools since results confirm that the degree of myocardial preservation depends on the method of cardiac arrest. This forms the basis for the choice of preconditions for subsequent ischemia. Furthermore, significant alterations of myocardial ultrastructure depend on a combination of the functional state of the heart, the method of cardioplegia, and the technique of fixation. © 1993 Wiley-Liss, Inc.     

Remote vs. local ischaemic preconditioning in the rat heart: infarct limitation,suppression of ischaemic arrhythmia and the role of reactive oxygen species

The unmet clinical need for myocardial salvage during ischaemia–reperfusion injury requires the development of new techniques for myocardial protection. In this study the protective effect of different local ischaemic preconditioning (LIPC) and remote ischaemic preconditioning (RIPC) protocols was compared in the rat model of myocardial ischaemia–reperfusion, using infarct size and ischaemic tachyarrhythmias as end‐points. In addition, the hypothesis that there is involvement of reactive oxygen species (ROS) in the protective signalling by RIPC was tested, again in comparison with LIPC. The animals were subjected to 30‐min coronary occlusion and 90‐min reperfusion. RIPC protocol included either transient infrarenal aortic occlusion (for 5, 15 and 30 min followed by 15‐min reperfusion) or 15‐min mesenteric artery occlusion with 15‐min reperfusion. Ventricular tachyarrhythmias during test ischaemia were quantified according to Lambeth Conventions. It was found that the infarct‐limiting effect of RIPC critically depends on the duration of a single episode of remote ischaemia, which fails to protect the heart from infarction when it is too short or, instead, too prolonged. It was also shown that RIPC is ineffective in reducing the incidence and severity of ischaemia‐induced ventricular tachyarrhythmias. According to our data, the infarct‐limiting effect of LIPC could be partially eliminated by the administration of ROS scavenger N‐2‐mercaptopropionylglycine (90 mg/kg), whereas the same effect of RIPC seems to be independent of ROS signalling.     

Histochemical and ultrastructural characterisation of an arrhythmogenic substrate in ischemic pig heart

The aim of the present study was to reveal by enzyme histochemistry and ultrastructural examination the possible anatomic substrate that may be the cause of high susceptibility of the pig heart to ischemia and/or reperfusion-induced severe arrhythmias. The heart of landrace pigs was subjected to 90 min of left coronary occlusion followed by 30 min reperfusion, whereby both conditions elicited arrhythmias and often even ventricular fibrillation. We found for the first time, besides common contractile cardiomyocytes, Purkinje fibers, and "transitional cells" in mid-myocardium. Transitional cells likely correspond to the recently described M cells. Importantly, these cells and Purkinje fibers exhibited reversible ischemia-related subcellular alterations, whereas the majority of contractile cardiomyocytes were irreversibly injured in the area of infarction. In correlation with these findings, glycogen-dependent phosphorylase activity was abolished, whereas it was still persistent in Purkinje fibers and small islands of contractile cardiomyocytes. Moreover, a distinct heterogeneity in the activity of all enzymes selected and subcellular alterations within a border zone were observed. These results suggest that particularly the preserved viability of specialized conducting cells spanning the ventricular wall may account for electrical disturbances that consequently contribute to increased susceptibility of the pig heart to ischemia- and reperfusion-induced severe arrhythmias.     

Time course of infarct growth toward the endocardium after coronary occlusion.

Transmembrane potentials and ultrastructure of subendocardial Purkinje and ventricular muscle fibers, isolated 1, 3, 5, 6, 14, and 24 h after coronary occlusion were investigated. Action potentials were recorded from progressively fewer layers of muscle cells as the age of the infarct increased. At 14 h little viable muscle remained. The decrease in the number of electrophysiologically viable muscle fibers correlated with structural evidence that the infarct moved with time toward the endocardial surface until only viable Purkinje fibers remained. Purkinje and surviving ventricular muscle fibers demonstrated a progressive decrease in resting potential, action potential amplitude, and Vmax and a progressive increase in action potential duration. Spontaneous diastolic depolarizations were found in Purkinje fibers only in 24-h infarcts and occasionally in cells deep to the endocardial surface, which may have been muscle cells. We hypothesize that during the first 24 h after coronary occlusion arrhythmias originate near the interface of infarcted and ischemic myocardium. As this interface moves toward the endocardium, this site of origin of arrhythmias moves with it until the Purkinje network is reached.