Hypothermic effects on action potential and force production of hedgehog and guinea pig papillary muscles

Action potentials and isometric force were recorded in papillary muscles from guinea pigs and summer hedgehogs at different temperatures between 37 and 0 degrees C. The action potential of the hedgehog was of a lower amplitude (mean 83 +/- 6 mV) than that of the guinea pig (mean 110 +/- 5 mV). The action potential duration at 50% repolarization was 22 +/- 2 msec in the hedgehog as compared to 105 +/- 11 msec in the guinea pig. Moreover, there was no distinct plateau phase of the hedgehog action potential. Lowering temperature prolonged the action potential duration in the two preparations by about the same percentage. However, the guinea pig preparation became progressively less excitable below 20 degrees C. Lowered temperature produced a positive inotropic effect in the guinea pig, whereas this effect was very slight in the hedgehog heart. Postextrasystolic potentiation was seen in the guinea pig but not in the hedgehog preparation. It is suggested that this difference between the preparations may be due to a greater relative amount of activator calcium in the hedgehog heart. The difference in cold tolerance between the preparations may reflect a difference in chemical composition of the sarcolemma.     

肌苷对缺氧心肌跨膜电位和收缩强度的影响

本工作在正常和缺氧情况下,观察肌苷对豚鼠心室乳头肌跨膜电位和收缩强度的影响。结果表明肌苷使正常心肌细胞动作电位时间(APD_(10)、APD_(50)延长。在缺氧心肌,肌苷使细胞静息电位增大,动作电位去极化幅度增高,零期最大去极化速度加快和动作电位时间延长。肌苷增加正常心肌收缩力,使缺氧心肌收缩的衰减显著缓和,亦即使收缩功能改善,且表现剂量-依从性。肌苷对心肌细胞跨膜电位的影响提示它很可能有抗心律失常作用,特别是在缺氧心脏。肌苷对离休乳头肌收缩的影响,证明其对心肌有直接的强心作用。     

Effect of an increase in coronary perfusion on transmural ventricular repolarization

The effect of increased coronary flow on transmural ventricular repolarization was investigated in six pentobabital-anesthetized sheep. Fresh blood at 10 ml/min was injected into the left circumflex coronary artery (LCX) in addition to the normal coronary flow. Unipolar electrocardiograms were simultaneously registered from epicardium, mid-myocardium and endocardium with fine plunge needles. Activation-recovery interval (ARI) was measured from the unipolar electrocardiograms and was used for estimating the ventricular repolarization duration. It was found that intracoronary blood injection (n=3) prolonged ARI in the epicardium, mid-myocardium and endocardium by an average of 34 +/- 16, 28 +/- 18 and 25 +/- 13 ms, respectively (p<0.01). Pretreatment with nitro-L-arginine (n=3), a nitric synthase inhibitor, diminished the flow-induced ARI prolongation across the ventricular wall. In conclusion, an increase in coronary flow lengthens the duration of transmural ventricular repolarization. These effects appear to be mediated by nitric oxide from the coronary endothelium.     

Effects of ectopic pacing on repolarization of the chicken left ventricle

Effects of ectopic pacing on left ventricular repolarization were studied in six anesthetized open-chest chickens. In each animal, unipolar electrograms were acquired from as many as 98 sites with 14 plunge needles (seven transmural locations between epicardium and endocardium in each needle). Activation-recovery intervals (ARIs), corrected to the cycle length, were used for estimating repolarization. At baseline, the nonuniform ARI distribution in the left ventricle resulted in the apicobasal differences being greater than the transmural gradient. Nonuniform ARI prolongation caused by ectopic pacing resulted in decreasing the transmural repolarization gradient and increasing the differences in the apex-to-base direction. The basal, but not apical transmural differences contributed to the total left ventricular transmural gradient. The total left ventricular apicobasal gradient was contributed by the apicobasal differences in mid-myocardial and subendocardial layers more than in subepicardial ones. Thus, in in situ chicken hearts, the transmural and apicobasal ARI gradients exist within the left ventricle with the shortest ARIs in the basal subepicardium and the longest ARIs in the subendocardium of the apical and middle parts of the left ventricle. Apicobasal compared to transmural heterogeneity of local repolarization properties contributes more to the total left ventricular repolarization gradient.     

Calcium content of peripheral and central mitochondria in the guinea pig myocardium: electron probe analysis

We quantitated subcellular elemental concentrations in stimulated and resting guinea pig myocardium to determine whether species-specific properties of guinea pigs or the subcellular localization of mitochondria accounted for reports of higher mitochondrial Ca in guinea pigs than in other species. Small papillary muscles or trabeculae isolated from guinea pig ventricles were stimulated to raise cytosolic [Ca(2+)](i) by two methods: (1). tetanizing by rapid pacing preparations in which Ca(2+) uptake by the sarcoplasmic reticulum was inhibited with cyclopiazonic acid or (2). freeze trapping paced muscles near-peak systole. Electron probe X-ray microanalysis showed no significant difference between the (low, approximately 0.4 mmol/kg dry weight) mitochondrial Ca content of stimulated guinea pig hearts, compared to mitochondria of other species, such as rat and hamsters, and the Ca contents of peripheral and central mitochondria were also not significantly different.     

Comparing cardiac action potentials recorded with metal and glass microelectrodes

Machine-pulled high-impedance glass capillary microelectrode is standard for transmembrane potential (TMP) recordings. However, it is fragile and difficult to impale, especially in beating myocardial tissues. We hypothesize that a high-impedance pure iridium metal electrode can be used as an alternative to the glass microelectrode for TMP recording. The TMPs were simultaneously recorded from isolated perfused swine right ventricles with a metal microelectrode and a standard glass microelectrode during pacing and during ventricular fibrillation. The basic morphology of TMP recorded with these electrodes was comparable. The action potential duration (APD) at 90% repolarization was 241 +/- 29 ms for the metal microelectrode and 236 +/- 31 ms for the glass microelectrode with a good correlation (r = 0.99, P < 0.0001). The maximum slope value of the APD restitution curves during pacing was also significantly correlated. One metal microelectrode and >20 glass microelectrodes were needed per study. We conclude that, in isolated perfused swine right ventricles, the TMP recorded by the metal microelectrode is comparable with that recorded by the glass microelectrode. Because the metal microelectrode is more durable than the glass microelectrode, it can serve as an alternative for APD recording and for restitution analyses.     

The electrophysiological and mechanical effects of atrial natriuretic peptide and acetylcholine on guinea pig ventricular papillary muscle

The direct effects of atrial natriuretic factor (ANF) and acetylcholine (ACh) on isolated guinea pig ventricular papillary muscle were studied. ANF (3 x 10(-9) - 3 x 10(-7) M), a cardiogenic hormone, had no significant electrical or mechanical effects on guinea pig papillary muscle driven at a frequency of 60 beats/min in normal (4 mM) and high [K]0 (27 mM) Tyrode solutions. On the other hand, ACh (3 x 10(-8) - 3 x 10(-7) M) caused a significant shortening of action potential duration and the contractile force showed no change or a slight decrease. At high concentration (5 microM), ACh reduced action potential durations at 50% and 90% repolarization (APD50 and APD90) by 10.5 +/- 2.1% and 12.4 +/- 1.8%, respectively, but the contractile force was slightly increased by 9.8 +/- 1.2%. In eleven of twenty-six preparations, spontaneous activity occurred and intermingled with driven activity. The ectopic rhythms were suppressed by ACh (1-5 microM). The changes in electrical but not mechanic activity induced by ACh were suppressed in the presence of five micromolar atropine. These results reveal that, in guinea pig papillary muscle, ANF had no direct chronotropic or inotropic effect. ACh may reduce APD and spontaneous discharges through an activation of muscarinic receptors but enhance twitch tension through other mechanisms.     

EGTA induces prolonged summed depolarizations in Mytilus gill coupled ciliated epithelial cells: implications for the control of ciliary motility

Abfrontal ciliated cells of Mytilus edulis gill beat when mechanically stimulated, a consequence of a Ca++-based generator potential and regenerative response. In contrast, the lateral ciliated epithelial cells arrest when stimulated, a consequence of a Ca++-based generator potential and a Na+/Ca++-based regenerative response. Iontophoretic injection of EGTA in abfrontal cells, followed by mechanical stimulation, results in a large, prolonged depolarization that returns to the resting level stepwise. It has been hypothesized that this phenomenon is caused by successive Ca++-dependent repolarizations in coupled cells, first in adjacent cells and then in the injected cell, in accord with relative EGTA loading. We have now demonstrated this same stepwise repolarization phenomenon in the Na+/Ca++-dependent lateral ciliated cells. In this case, each repolarization step is often preceded by a small spike. With either cell type, using two-electrode recording techniques, we can detect the stepwise repolarization in distant cells, proportionately decremented when the second (KCl) electrode is some distance from the injection (EGTA) electrode and stimulus. When force is applied between the electrodes and nearest the KCl electrode, a greater initial response is recorded from this electrode, presumably resulting from depolarization of its impaled cell, prolonged by EGTA diffusion through the intervening cell junctions. The subsequent repolarization steps are of approximately the same size, suggesting repolarization of cells between the two electrodes. These observations are consistent with the cell coupling/EGTA loading hypothesis and indicate that both cell types mediate repolarization through Ca++ and propagate ciliary beat or arrest through intracellular coupling.     

Influence of glucose on the transmembrane action potential of papillary muscle. Effects of concentration, phlorizin and insulin, nonmetabolizable sugars, and stimulators of glycolysis

The action potential duration (APD) of isolated guinea pig papillary muscle is directly related to the medium glucose concentration regardless of the gas mixture with which it is in equilibrium. The APD can be maintained at control value for many hours by a glucose concentration of 50 mM in the complete absence of oxygen. Following reduction of the APD by incubation of the muscle in medium containing 5 mM glucose, adjustment of the glucose concentration to 50 mM will cause restoration of normal APD. Phlorizin has been shown to competitively interfere with the effect of glucose on the APD and insulin to prevent or reverse the effect of phlorizin. Nonmetabolizable sugars cannot produce glucose-like effects on the APD. Adrenaline, noradrenaline, and isopropylnoradrenaline increased the reduced APD of papillary muscles incubated in the absence of oxygen in a medium containing 5 mM glucose coincident with an increase in contractile force. The effect of isopropylnoradrenaline was blocked by acetylcholine and propranolol. In the presence of iodoacetate and 2-deoxyglucose, isopropylnoradrenaline increased contractile force but not the reduced APD. Aminophylline was found to produce changes in the reduced APD similar to those caused by the sympathomimetic amines. The findings clearly support the hypothesis that anaerobic metabolism utilizing either glycogen or exogenous glucose is capable of maintaining normal transmembrane electrical activity in guinea pig papillary muscle.     

Ventricular fibrillation in hibernators and nonhibernators

Previous studies have shown that there are differences between hibernators and nonhibernators in the susceptibility to ventricular fibrillation. In an attempt to clarify these differences ventricular fibrillation was induced in isolated hearts of the hibernator, the woodchuck, Marmota monax by cooling, warming, puncture, and by norepinephrine administration. It was shown that the hearts of the winter animals were completely resistant toward the ventricular fibrillation inducing agents, which was not the case for the hearts from summer, active animals. Further, the hearts of another hibernator, the hedgehog, Erinaceus europaeus, and guinea pig, Cavia porcellus, were studied electrophysiologically in anesthetized animals with open chests and with bipolar electrodes attached to the epicardium. During pacing it was shown that the hedgehog had a higher stimulus threshold and a lower maximal following frequency than the guinea pig. The summer hedgehogs showed resistance toward both ventricular premature beats and ventricular fibrillation. Sixty percent of the summer hedgehogs and 100 percent of the winter hedgehogs and guinea pigs developed ventricular fibrillation. The threshold for ventricular fibrillation was highest for summer hedgehogs. The effective refractory period of papillary muscle of summer hedgehogs was shorter than that of guinea pigs. The force frequency relationship of the isolated papillary muscle showed a greater degree of independence in the hedgehog than in the guinea pig. Consequently, the results show that the heart of the hibernator is more arrhythmia resistant than the heart of the nonhibernator, although there are seasonal differences.     

Solubilization and partial characterization of a tumor-associated transplantation antigen of the guinea pig L2C leukemia.

A tumor-associated transplantation antigen (TATA) from guinea pig L2C leukemia cells was solubilized by different methods. It was found that the 3 M KCl extraction yielded the most immunogenic TATA of L2C cells. Immunization of normal strain 2 guinea pigs with this extract in complete Freund's adjuvant gave complete protection against a subsequent challenge with tumor cells. Further fractionation of the KCl extract of L2C cells by Sephadex G-200 chromatography suggested that the immunogenic activity was present in the fraction containing materials with estimated m.w. of less than 20,000 daltons.     

Diazepam reduces action potential duration in guinea-pig papillary muscle by a cAMP-dependent mechanism.

In this study, we have analyzed the role of cyclic AMP (cAMP) as the mediator of the decrease in action potential duration induced by diazepam. Diazepam (1-100 microM) reduced, in a dose-dependent manner, the duration of intracellular action potential recorded in the papillary muscle obtained from the right ventricle of the guinea pig heart. This effect was mimicked by the analog of cyclic AMP, 8-Br-cAMP (100 microM), but not by gamma-amino-butyric acid (GABA). Also, the selective antagonist of the benzodiazepine receptors, flumazenil did not modify the effect of diazepam. The diazepam-induced shortening of action potential duration was partially antagonized by the inhibitor of cAMP synthesis carbachol (1 microM) or the blocker of the cAMP-dependent protein kinase A, Rp-cAMP[S] (1 microM). These results indicate that cyclic AMP is involved in the diazepam-induced shortening of the action potential duration of the guinea pig papillary muscle.     

In vivo recording of monophasic action potentials in awake dogs

Assessment of cardiac repolarization in dogs in vivo is of interest in numerous experimental canine models. Previous studies have used monophasic action potentials (MAPs) to investigate repolarization in vitro and in vivo in anesthetized animal models. Therefore, an approach for recording MAPs in awake dogs without the interference of anesthesia is desirable. We describe an experimental technique to record MAPs in conscious dogs by means of conventional rubber introducers which are implanted into the internal jugular vein. Atrial as well as ventricular MAPs can be simultaneously measured without complications. Pacing thresholds are low and stable over time. Continuous MAP recordings of stable amplitude can be achieved from the same endocardial site for periods up to 1h. Antegrade and retrograde atrioventricular nodal conduction properties can be assessed. Programmed stimulation can be performed to simultaneously determine local refractory periods and MAP duration at cycle lengths from 500 to 200ms. In awake, unsedated dogs measuring MAPs via rubber introducers permits safe, long-term recording of MAPs. Such recordings may be useful for safety pharmacologic studies in evaluating cardiovascular and noncardiovascular drugs with respect to their effects on repolarization. In various canine in vivo models including in vivo models of long QT syndrome, heart failures or sudden cardiac death, the present technique permits electrophysiologic measurements without the interference of anesthesia.     

Measuring surface potential components necessary for transmembrane current computation using microfabricated arrays

This study was designed to test the feasibility of using microfabricated electrodes to record surface potentials with sufficiently fine spatial resolution to measure the potential gradients necessary for improved computation of transmembrane current density. To assess that feasibility, we recorded unipolar electrograms from perfused rabbit right ventricular free wall epicardium (n = 6) using electrode arrays that included 25-microm sensors fabricated onto a flexible substrate with 75-microm interelectrode spacing. Electrode spacing was therefore on the size scale of an individual myocyte. Signal conditioning adjacent to the sensors to control lead noise was achieved by routing traces from the electrodes to the back side of the substrate where buffer amplifiers were located. For comparison, recordings were also made using arrays built from chloridized silver wire electrodes of either 50-microm (fine wire) or 250-microm (coarse wire) diameters. Electrode separations were necessarily wider than with microfabricated arrays. Comparable signal-to-noise ratios (SNRs) of 21.2 +/- 2.2, 32.5 +/- 4.1, and 22.9 +/- 0.7 for electrograms recorded using microfabricated sensors (n = 78), fine wires (n = 78), and coarse wires (n = 78), respectively, were found. High SNRs were maintained in bipolar electrograms assembled using spatial combinations of the unipolar electrograms necessary for the potential gradient measurements and in second-difference electrograms assembled using spatial combinations of the bipolar electrograms necessary for surface Laplacian (SL) measurements. Simulations incorporating a bidomain representation of tissue structure and a two-dimensional network of guinea pig myocytes prescribed following the Luo and Rudy dynamic membrane equations were completed using 12.5-microm spatial resolution to assess contributions of electrode spacing to the potential gradient and SL measurements. In those simulations, increases in electrode separation from 12.5 to 75.0, 237.5, and 875.0 microm, which were separations comparable to the finest available with our microfabricated, fine wire, and coarse wire arrays, led to 10%, 42%, and 81% reductions in maximum potential gradients and 33%, 76%, and 96% reductions in peak-to-peak SLs. Maintenance of comparable SNRs for source electrograms was therefore important because microfabrication provides a highly attractive methods to achieve spatial resolutions necessary for improved computation of transmembrane current density.     

Effect of simulated I(to) on guinea pig and canine ventricular action potential morphology

The transient outward current (I(to)) is a major repolarizing current in the heart. Marked reduction of I(to) density occurs in heart failure and is accompanied by significant action potential duration (APD) prolongation. To understand the species-dependent role of I(to) in regulating the ventricular action potential morphology and duration, we introduced simulated I(to) conductance in guinea pig and canine endocardial ventricular myocytes using the dynamic clamp technique and perforated patch-clamp recordings. The effects of simulated I(to) in both types of cells were complex and biphasic, separated by a clear density threshold of approximately 40 pA/pF. Below this threshold, simulated I(to) resulted in a distinct phase 1 notch and had little effect on or moderately prolonged the APD. I(to) above the threshold resulted in all-or-none repolarization and precipitously reduced the APD. Qualitatively, these results agreed with our previous studies in canine ventricular cells using whole cell recordings. We conclude that 1) contrary to previous gene transfer studies involving the Kv4.3 current, the response of guinea pig ventricular myocytes to a fully inactivating I(to) is similar to that of canine ventricular cells and 2) in animals such as dogs that have a broad cardiac action potential, I(to) does not play a major role in setting the APD.     

Electrophysiological and pharmacological evidence that peripheral type benzodiazepine receptors are coupled to calcium channels in the heart

PK 11195, an antagonist of the peripheral type benzodiazepine receptor, does not affect either the duration of the action potential or the tension of the guinea pig papillary muscle. However, it antagonized the effects of the calcium channel blockers, nitrendipine, verapamil, diltiazem, and of BAY K8644, a calcium channel agonist in this heart preparation. On the other hand, PK 11195 does not change the increase in the action potential duration provoked by the potassium channel blocker tetraethylammonium. RO5-4864, an agonist of the peripheral type benzodiazepine receptor, decreased the tension of the guinea pig papillary muscle. The effect was reversed by increasing extracellular Ca2+ concentrations up to 4 mM. These results suggest that in the heart the peripheral type benzodiazepine receptors are coupled to calcium channels.     

兔LQT2模型心室肌复极化的性别差异

本研究旨在探讨长QT综合征（long QT syndromes,LQTS）室性心律失常发生的性别差异及其电生理机制,初步观察了不同性别兔LQT2模型左心事原已存在的电生理异质性和心事复极动力学的特征。实验分为3组,上下常组以标准台氏液灌流;LQT2模型组给予含100gmol／L dl-sotalol的台式液灌流;LQT2模型＋低钾组给了含3．0mmol／LKCl、100μmol／L dl-sotalol的台式液灌流。采用冠状动脉旋支灌注兔左室心肌楔形组织块标本,应用浮置玻璃微电极记录技术进行记录。给予基础刺激周长（basic cycle length,BCL）为500、l000和2000ms的S1刺激,同步记录心事肌内膜侧、外膜侧细胞动作电位,并记录跨壁心电图：在BCL为500和1000ms时加用S2程序刺激以记录动作电位时程（action potential duration,APD）恢复曲线。研究发现：在不同刺激频率时,3组实验雌兔心肌细胞的跨壁复极化离散（transmural dispersion of repolarization,TDR）、APD恢复曲线斜率均大于雄兔,有显著性差异（P〈0．05）,并呈频率依赖性;LQT2模型组及LQT2模型＋低钾组雌雄兔TDR、APD恢复曲线斜率较正常组明显增人（P〈0．01）。BCL为1000ms时,LQT2模型组雌兔7例中1例发生尖端扭转性窀性心动过速（torsade de pointes,TdP）;LQT2模型＋低钾组雌兔7例中5例诱发TdP,雄兔7例中2例诱发TdP,有显著性差异（P〈0．05）。结果提示：LQT2模型心肌原已存在的电生理异质性和动态异质性均有明显的性别差异,并≯频率依赖性。存LQT2模型中,TDR以及APD恢复曲线斜率的增大可能是雌性动物较雄性更易发生尖端扭转性心律失常的原因。     

Mechanism of hypoxic preconditionin in guinea pig papillary muscles

Brief ischemia or hypoxia has been found to protect the heart against susbsequent long-lasting ischemia and to improve contractile dysfunction as well to reduce cell necrosis and the incidence of lethal arrhythmias. This phenomenon, termed preconditioning (PC) has been demonstrated in different species. However, little is known about PC in guinea pigs. Moreover, electrophysiological changes underlying protection have not been studied so far in conjuntion with force recovery in a setting of PC. The aim of the study was to study PC in a guinea pig papillary muscle, using recovery of contractility after long hypoxic challenge as the main end-point of protection, and to investigate concominant electrophysiological alterations. In guinea pig papillary muscle preparations contracting isometrically (paced at 2 Hz), transmembrane action potentials (AP) and developed force (DF) were recorded by conventional microelectrode technique and a force tranducer. In addition, effective refractory periods (ERP) were determined. Hypoxia was induced by superfusion with 100% N₂ (pO₂ < 5 kPa) and pacing at 3,3 Hz. In the control group, long hypoxia lasted for 45 min and was followed by 30 min reoxygenation. In the PC group, muscles were subjected to 5 min hypoxia followed by 10 min recovery prior to sustained hypoxia/reoxygenation. Results: Long hypoxia induced a similar depression of DF in both, PC and control groups. However, a loss of contractile activity occured earlier in the PC group. AP duration and ERP decreased faster and were significantly shorter after PC. Upon reoxygenation, preconditioned muscles showed significantly better recovery of function (DF 86% of prehypoxic value vs. 36% in controls; p < 0,05). AP and ERP were completely restored in both, PC and control groups. Guinea pig papillary muscle can be preconditioned with a brief hypoxic challenge against contractile dysfunction upon long-lasting hypoxia/reoxygenation. Shortening of AP and loss of contractility occured more quickly during hypoxia and may participate in the protective effect of preconditioning. Possible mechanisms might involve facilitated opening of K_ATP-dependent channels.     

PCO(2) in the large intestine of mice, rats, guinea pigs, and dogs and effects of the dietary substrate.

PCO(2) in the lumen and serosa of cecum and colon was measured in rats, guinea pigs, and dogs to examine the relationship between serosal PCO(2) and the incidence of intestinal necrotic lesions after administration of gas-carrier contrast agents in rodents. The effects of the dietary substrate were tested in a group of mice maintained on a diet based on glucose as the only carbohydrate source. The anesthetic used was a fentanyl-fluanison-midazolam mixture (rodents) and pentobarbital (dogs). PCO(2) was measured in vivo and postmortem, and the kinetics of the postmortem serosal PCO(2) [transmural CO(2) flux (J(CO(2)))] was calculated. PCO(2) in the cecal serosa and lumen, respectively, was 64 +/- 4 and 392 +/- 18 Torr in rats, 67 +/- 3 and 276 +/- 17 Torr in guinea pigs, and 73 +/- 6 and 137 +/- 7 Torr in mice on glucose-based diet. In the colon serosa and lumen of dogs, PCO(2) was 30 +/- 6 and 523 +/- 67 Torr, respectively. Serosal PCO(2) increased rapidly after death in rats and slower in guinea pigs and mice, and the slowest change was observed in dogs. Compared with dogs, serosal PCO(2) and J(CO(2)) of rats and guinea pigs were significantly higher. Serosal PCO(2) of guinea pigs was similar to that of rats, whereas the J(CO(2)) of guinea pigs was significantly lower. These data suggest a causal relationship between the ability of the cecal and colonic wall to act as a barrier to CO(2) diffusion and the presence of characteristic gas-carrier contrast agent-induced intestinal lesions in mice and rats and their absence in guinea pigs, dogs, and other species.     

Altered connexin43 expression produces arrhythmia substrate in heart failure

Recently, we found that repolarization heterogeneities between subepicardial and midmyocardial cells can form a substrate for reentrant ventricular arrhythmias in failing myocardium. We hypothesized that the mechanism responsible for maintaining transmural action potential duration heterogeneities in heart failure is related to intercellular uncoupling from downregulation of cardiac gap junction protein connexin43 (Cx43). With the use of the canine model of pacing-induced heart failure, left ventricles were sectioned to expose the transmural surface (n = 5). To determine whether heterogeneous Cx43 expression influenced electrophysiological function, high-resolution transmural optical mapping of the arterially perfused canine wedge preparation was used to measure conduction velocity (theta(TM)), effective transmural space constant (lambda(TM)), and transmural gradients of action potential duration (APD). Absolute Cx43 expression in failing myocardium, quantified by confocal immunofluorescence, was uniformly reduced (by 40 +/- 3%, P < 0.01) compared with control. Relative Cx43 expression was heterogeneously distributed and lower (by 32 +/- 18%, P < 0.05) in the subepicardium compared with deeper layers. Reduced Cx43 expression in heart failure was associated with significant reductions in intercellular coupling between transmural muscle layers, as evidenced by reduced theta(TM) (by 18.9 +/- 4.9%) and lambda(TM) (by 17.2 +/- 1.4%; P < 0.01) compared with control. Heterogeneous transmural distribution of Cx43 in failing myocardium was associated with lower subepicardial theta(TM) (by 12 +/- 10%) and lambda(TM) (by 13 +/- 7%), compared with deeper transmural layers (P < 0.05). APD dispersion was greatest in failing myocardium, and the largest transmural APD gradients were consistently found in regions exhibiting lowest relative Cx43 expression. These data demonstrate that reduced Cx43 expression produces uncoupling between transmural muscle layers leading to slowed conduction and marked dispersion of repolarization between epicardial and deeper myocardial layers. Therefore, Cx43 expression patterns can potentially contribute to an arrhythmic substrate in failing myocardium.