Canine Heart Research Articles

Cardiovascular Disease Risk Varies by Birth Month in Canines

The canine heart is a robust physiological model for the human heart. Recently, birth month associations have been reported and replicated in humans using clinical health records. While animals respond readily to their environment in the wild, a systematic investigation of birth season dependencies among pets and specifically canines remains lacking. We obtained data from the Orthopedic Foundation of Animals on 129,778 canines representing 253 distinct breeds. Among canines that were not predisposed to cardiovascular disease, a clear birth season relationship is observed with peak risk occurring in June-August. Our findings indicate that acquired cardiovascular disease among canines, especially those that are not predisposed to cardiovascular disease, appears birth season dependent. The relative risk of cardiovascular disease for canines not predisposed to cardiovascular disease was as high as 1.47 among July pups. The overall adjusted odds ratio, when mixed breeds were excluded, for the birth season effect was 1.02 (95% CI: 1.002, 1.047, p = 0.032) after adjusting for breed and genetic cardiovascular predisposition effects. Studying birth season effects in model organisms can help to elucidate potential mechanisms behind the reported associations.

Microtubular remodeling and decreased expression of Nav1.5 with enhanced EHD4 in cells from the infarcted heart

Cardiac Na+ channel remodeling provides a critical substrate for generation of reentrant arrhythmias in border zones of the infarcted canine heart. Recent studies show that Nav1.5 cytoskeletal- and endosomal-based membrane trafficking and function are linked to tubulin, microtubular (MT) networks, and Eps15 homology domain containing proteins like EHD4. AimOur objective is to understand the relation of tubulin and EHD4 to Nav1.5 channel protein remodeling observed in border zone cells (IZs) when arrhythmias are known to occur; that is, 3-h, 48-h and 5-day post coronary occlusion. Materials methods findingsOur voltage clamp and immunostaining data show that INa density is decreased in the epicardial border zone cells of the 48 h infarcted heart (IZ48h). Immunostaining studies reveal that in post MI cells the cell surface staining of Nav1.5 was reduced and Nav1.5 distribution changed. However, intense co-staining of Nav1.5 and tubulin occurs in core planes and the perinuclear areas in post MI cells. At the same time, there were marked changes in the subcellular location of the EHD4 protein. EHD4 is co-localized with tubulin protein in discrete intracellular “highway” structures. SignificanceThe distribution and expression of the three proteins are altered dynamically in post MI cells. In sum, our work illustrates the spatiotemporal complexity of remodeling mechanisms in the post-infarct myocyte. It will be important in future experiments to further explore direct links between MT, EHD proteins, and cell proteins involved in forward trafficking.

Serum proteome profiling in canine idiopathic dilated cardiomyopathy using TMT-based quantitative proteomics approach

Idiopathic dilated cardiomyopathy (iDCM) is a primary myocardial disorder with an unknown aetiology, characterized by reduced contractility and ventricular dilation of the left or both ventricles. Naturally occurring canine iDCM was used herein to identify serum proteomic signature of the disease compared to the healthy state, providing an insight into underlying mechanisms and revealing proteins with biomarker potential. To achieve this, we used high-throughput label-based quantitative LC-MS/MS proteomics approach and bioinformatics analysis of the in silico inferred interactome protein network created from the initial list of differential proteins. To complement the proteomic analysis, serum biochemical parameters and levels of know biomarkers of cardiac function were measured. Several proteins with biomarker potential were identified, such as inter-alpha-trypsin inhibitor heavy chain H4, microfibril-associated glycoprotein 4 and apolipoprotein A-IV, which were validated using an independent method (Western blotting) and showed high specificity and sensitivity according to the receiver operating characteristic curve analysis. Bioinformatics analysis revealed involvement of different pathways in iDCM, such as complement cascade activation, lipoprotein particles dynamics, elastic fibre formation, GPCR signalling and respiratory electron transport chain. SignificanceIdiopathic dilated cardiomyopathy is a severe primary myocardial disease of unknown cause, affecting both humans and dogs. This study is a contribution to the canine heart disease research by means of proteomic and bioinformatic state of the art analyses, following similar approach in human iDCM research. Importantly, we used serum as non-invasive and easily accessible biological source of information and contributed to the scarce data on biofluid proteome research on this topic. Bioinformatics analysis revealed biological pathways modulated in canine iDCM with potential of further targeted research. Also, several proteins with biomarker potential have been identified and successfully validated.

Hearts during ischemia with or without HTK-protection analysed by dielectric spectroscopy

Objective: We investigated canine hearts during ischemia after aortic cross clamping (UI, n = 20) and after HTK-cardioplegia (HTK, n = 24) at 35 °C, 25 °C, 15 °C, and 5 °C with the aim to compare tissue changes caused by the activity of anaerobic metabolism(AAM), cell membrane destruction(CD), and gap junction uncoupling(GJU). Approach: We measured continuously the complex dielectric spectrum(DS), ATP- and lactate content, extracellular pH, and rigor contracture. To identify changes in DS caused by AAM, CD, and GJU we performed additional experiments on the gap junction-free skeletal muscle. We used heart model simulations to calculate the effect of temperature. Main results: AAM affected the DS at 10 MHz and we found a strong correlation between DS and the proton concentration with a maximum of DS at 10 mmol g−1 dry weight in ATP-concentration. The time of GJU was detected by a characteristic increase in DS and CD by a characteristic decrease at 13 kHz. In comparison to UI, GJU, AAM and CD were delayed by HTK and by hypothermia, indicating a minimization of energy consumption and an improved preservation of tissue structure. Significance: The novel findings were that in UI at 5 °C GJU occurred earlier and AAM remained constant, indicating a less effective preservation in UI by deep hypothermia in contrast to HTK.

WITHDRAWN: Beneficial effect of endo-CRT might be mediated by more improvement of SERCA2A in chronic dyssynchronous canine heart failure models

// Hongyun Chen 1 , Shuang Liu 2 , Zhihong Zong 3 , Cuiting Zhao 2 , Guangyuan Li 2 , Yonghuai Wang 2 , Guoxian Qi 1 , Jun Yang 2 and Chunyan Ma 2 1 Department of Geriatric Cardiology, First Hospital of China Medical University, Shenyang, Liaoning, China 2 Department of Cardiovascular Ultrasound, First Hospital of China Medical University, Shenyang, Liaoning, China 3 China Medical University, Shenyang, Liaoning, China Correspondence to: Chunyan Ma, email: cmu1h_mcy@126.com Keywords: cardiac resynchronization therapy (CRT); SERCA2A; endo-CRT; epi-CRT; dyssynchronous heart failure Received: March 29, 2017 Accepted: December 29, 2017 Published: January 02, 2018 ABSTRACT The aims of the present study were to compare the effects of endocardial cardiac resynchronization therapy (endo-CRT) and epicardial CRT (epi-CRT) on cardiac contractile function and to delineate the underlying molecular mechanisms. Twenty-four adult mongrel canines were divided into four groups: sham-operation control group (CT, n = 6), dyssynchronous heart failure (DHF) group ( n = 6), epi-CRT group ( n = 6), and endo-CRT group ( n = 6). The global and local left ventricle (LV) systolic function, dyssynchrony and myocardium calcium handling molecular mRNA and protein levels were measured. Compared to CT group, we found the global and local LV function, dyssynchrony and SERCA2A decreased in DHF group and increased in CRT groups, particularly in endo-CRT group at LV lateral wall (pacing site). Therefore, endo-CRT appears to be an effective procedure in the treatment of DHF, which might be mediated by upregulating the mRNA and protein expression of SERCA2A in the LV at the pacing and remote site.

Methods for the Preparation of an Excised, Cross-Circulated Rat Heart.

The Emax-Pressure-Volume Area (PVA)-VO2 framework proposed by Dr. Suga for canine hearts has dramatically advanced the field of cardiac mechanical work and energetics, i.e., mechanoenergetics. He and his collaborators investigated mechanoenergetics in the left ventricle (LV) of excised, cross-circulated canine heart preparations. We instituted the excised cross-circulated rat whole heart preparations and found a curvilinear end-systolic pressure-volume relation (ESPVR) in the rat LV, in contrast to the linear ESPVR in canine, rabbit, and human LVs. Although Emax, the slope of the linear ESPVR, could be used as an index of LV contractility, it was not applicable for evaluating LV contractility in the rat LV. Thus, we proposed a new index of contractility, equivalent Emax (eEmax) in the rat LV. We also found a linear VO2-PVA relationship in the rat LV. Here, we introduce the methods for the preparation of excised, cross-circulated rat whole hearts and the eEmax-PVA-VO2 framework in the rat LV. Using this method, we can obtain accurate LV volume and myocardial O2 consumption in real time for estimating cardiac mechanoenergetics, which is very challenging in in vivo experiments.

Novel Synthesized Radical-Containing Nanoparticles Limit Infarct Size Following Ischemia and Reperfusion in Canine Hearts.

Although nitroxyl radicals such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) scavenge free radicals, their short half-life and considerable side effects such as systemic hypotension and bradycardia have limited their clinical application. Since a radical-containing nanoparticle (RNP) delivers nitroxyl radicals with a prolonged half-life specific to ischemic hearts, we investigated whether RNPs reduce infarct size without the occurrence of substantial side effects and whether nitric oxide (NO) contributes to the cardioprotective effects of RNPs. The left anterior descending coronary arteries of dogs were occluded for 90min, followed by reperfusion for 6h. Either RNPs, micelles (not containing TEMPO) (control), or 4-hydroxy-TEMPO (TEMPOL) was injected into a systemic vein for 5min before reperfusion. We evaluated the infarct size, myocardial apoptosis, plasma NO levels in coronary venous blood, and the RNP spectra using an electron paramagnetic resonance assay. RNPs reduced infarct size compared with the control group and TEMPOL group (19.5±3.3 vs. 42.2±3.7 vs. 30.2±3.4%). RNPs also reduced myocardial apoptosis compared with the control and TEMPOL group. Coronary venous NO levels increased in the RNP group. In conclusion, the administration of 2,2,6,6-tetramethylpiperidine-1-oxyl as a RNP exerted cardioprotective effects against ischemia and reperfusion injury in canine hearts without exerting unfavorable hemodynamic effects. RNPs may represent a promising new therapy for patients with acute myocardial infarction.

GW28-e0744 Effects of low-level vagus nerve stimulation on ventricular electrophysiological properties in normal canine heart

GW28-e0744 Effects of low-level vagus nerve stimulation on ventricular electrophysiological properties in normal canine heart

GW28-e0744 Effects of low-level vagus nerve stimulation on ventricular electrophysiological properties in normal canine heart

GW28-e0744 Effects of low-level vagus nerve stimulation on ventricular electrophysiological properties in normal canine heart

Catheterized Fiber-Optics Confocal Microscopy of the Beating Heart In Situ.

The beating heart can be imaged using several modalities including magnetic resonance imaging, fluoroscopy, computed tomography, and ultrasound.1,2 These modalities yield images and image sequences at millimeter resolutions. Here, we present, for the first time, real-time microscopic imaging of the beating heart in situ acquired in a clinical setting using catheterized fiber-optics confocal microscopy (FCM). We demonstrate that catheterized FCM provides information on tissue microstructure in normal and fibrotic canine hearts in situ at micrometer resolution. The catheter-based approach is based on methods that we developed in previous work for imaging the arrested heart during open heart surgery.3 The study protocol was approved by the Institutional Animal Care and Use Committee, University of Utah, and performed according to National Institutes of Health guidelines. Mongrel dogs weighing 25 to 35 kg were anesthetized with sodium pentobarbital (30 mg/kg) and mechanically ventilated. Venous access was obtained in the right femoral vein under ultrasound guidance (9L4 Transducer, ACUSON SC2000; Siemens Medical Solutions, Malvern, PA). Under ultrasound (ACUSON AcuNav, Siemens) and fluoroscopic (syngo DynaCT, Siemens) guidance, a transseptal puncture was performed followed by …

Triggered intracellular calcium waves in dog and human left atrial myocytes from normal and failing hearts.

Abnormal intracellular Ca2+ cycling contributes to triggered activity and arrhythmias in the heart. We investigated the properties and underlying mechanisms for systolic triggered Ca2+ waves in left atria from normal and failing dog hearts. Intracellular Ca2+ cycling was studied using confocal microscopy during rapid pacing of atrial myocytes (36 °C) isolated from normal and failing canine hearts (ventricular tachypacing model). In normal atrial myocytes (NAMs), Ca2+ waves developed during rapid pacing at rates ≥ 3.3 Hz and immediately disappeared upon cessation of pacing despite high sarcoplasmic reticulum (SR) load. In heart failure atrial myocytes (HFAMs), triggered Ca2+ waves (TCWs) developed at a higher incidence at slower rates. Because of their timing, TCW development relies upon action potential (AP)-evoked Ca2+ entry. The distribution of Ca2+ wave latencies indicated two populations of waves, with early events representing TCWs and late events representing conventional spontaneous Ca2+ waves. Latency analysis also demonstrated that TCWs arise after junctional Ca2+ release has occurred and spread to non-junctional (cell core) SR. TCWs also occurred in intact dog atrium and in myocytes from humans and pigs. β-adrenergic stimulation increased Ca2+ release and abolished TCWs in NAMs but was ineffective in HFAMs making this a potentially effective adaptive mechanism in normals but potentially arrhythmogenic in HF. Block of Ca-calmodulin kinase II also abolished TCWs, suggesting a role in TCW formation. Pharmacological manoeuvres that increased Ca2+ release suppressed TCWs as did interventions that decreased Ca2+ release but these also severely reduced excitation-contraction coupling. TCWs develop during the atrial AP and thus could affect AP duration, producing repolarization gradients and creating a substrate for reentry, particularly in HF where they develop at slower rates and a higher incidence. TCWs may represent a mechanism for the initiation of atrial fibrillation particularly in HF.

P3630Association of complex fractionated electrograms with atrial myocardial thickness and fibrosis: Insights from pathologic analyses of canine hearts

P3630Association of complex fractionated electrograms with atrial myocardial thickness and fibrosis: Insights from pathologic analyses of canine hearts

Technical Note: A 3-D rendering algorithm for electromechanical wave imaging of a beating heart.

Arrhythmias can be treated by ablating the heart tissue in the regions of abnormal contraction. The current clinical standard provides electroanatomic 3-D maps to visualize the electrical activation and locate the arrhythmogenic sources. However, the procedure is time-consuming and invasive. Electromechanical wave imaging is an ultrasound-based noninvasive technique that can provide 2-D maps of the electromechanical activation of the heart. In order to fully visualize the complex 3-D pattern of activation, several 2-D views are acquired and processed separately. They are then manually registered with a 3-D rendering software to generate a pseudo-3-D map. However, this last step is operator-dependent and time-consuming. This paper presents a method to generate a full 3-D map of the electromechanical activation using multiple 2-D images. Two canine models were considered to illustrate the method: one in normal sinus rhythm and one paced from the lateral region of the heart. Four standard echographic views of each canine heart were acquired. Electromechanical wave imaging was applied to generate four 2-D activation maps of the left ventricle. The radial positions and activation timings of the walls were automatically extracted from those maps. In each slice, from apex to base, these values were interpolated around the circumference to generate a full 3-D map. In both cases, a 3-D activation map and a cine-loop of the propagation of the electromechanical wave were automatically generated. The 3-D map showing the electromechanical activation timings overlaid on realistic anatomy assists with the visualization of the sources of earlier activation (which are potential arrhythmogenic sources). The earliest sources of activation corresponded to the expected ones: septum for the normal rhythm and lateral for the pacing case. The proposed technique provides, automatically, a 3-D electromechanical activation map with a realistic anatomy. This represents a step towards a noninvasive tool to efficiently localize arrhythmias in 3-D.

Prediction of optimal cardiac resynchronization by vectors extracted from electrograms in dyssynchronous canine hearts.

Proper optimization of atrioventricular (AV) and interventricular (VV) intervals can improve the response to cardiac resynchronization therapy (CRT). It has been demonstrated that the area of the QRS complex (QRSarea) extracted from the vectorcardiogram can be used as a predictor of optimal CRT-device settings. We explored the possibility of extracting vectors from the electrograms (EGMs) obtained from pacing electrodes and of using these EGM-based vectors (EGMVs) to individually optimize acute hemodynamic CRT response. Biventricular pacing was performed in 13 dogs with left bundle branch block (LBBB) of which five also had myocardial infarction (MI), using 100 randomized AV- and VV-settings. Settings providing an acute increase in LV dP/dtmax ≥ 90% of the highest achieved value were defined as optimal. The prediction capability of QRSarea derived from the EGMV (EGMV-QRSarea) was compared with that of QRS duration. EGMV-QRSarea strongly correlated to the change in LV dP/dtmax (R = -0.73 ± 0.19 [LBBB] and -0.66 ± 0.14 [LBBB + MI]), while QRS duration was more poorly related to LV dP/dtmax changes (R = -0.33 ± 0.25 [LBBB] and -0.47 ± 0.39 [LBBB + MI]). This resulted in a better prediction of optimal CRT-device settings by EGMV-QRSarea than by QRS duration (LBBB: AUC = 0.89 [0.86-0.93] vs. 0.76 [0.69-0.83], P < 0.01; LBBB + MI: AUC = 0.91 [0.84-0.99] vs. 0.82 [0.59-1.00], P = 0.20, respectively). In canine hearts with chronic LBBB with or without MI, the EGMV-QRSarea predicts acute hemodynamic CRT response and identifies optimal AV and VV settings accurately. These data support the potency of EGM-based vectors as a noninvasive, easy and patient-tailored tool to optimize CRT-device settings.

Slow antegrade excitation and delayed retrograde activation through an "inexcitable zone": A basis for arrhythmia formation in infarcted myocardium ex vivo.

The electrophysiologic mechanism for rate-dependent PVBs associated with double potentials (DPs) was investigated in infarcted canine hearts using bipolar and intracellular microelectrode recordings. Dogs exhibiting rate-related ventricular ectopic beats (coupling interval, 390 ± 54 milliseconds) during sinus rhythm or atrial pacing were studied 4-5 days (N = 63) or 25 days (N = 16) following anterior descending coronary artery ligation. Sites of DP and rate-dependent arrhythmia formation were identified in vivo using bipolar recordings for subsequent ex vivo studies. Rate-dependent conduction delays with increasing duration isoelectric intervals representing very slow conduction were observed at sites of DP formation, frequently provoking both manifest and concealed reentry (non-stimulated beats) over a narrow range of paced cycle lengths. Both slow antegrade and retrograde activation across an inexcitable gap (reflection) were integral components of extrasystole formation. Retrograde reflection to a region of very slow conduction (mid-potential) during antegrade activation was routinely observed at 4-5 days (42 of 63 preparations, 67%) and 25 days (22 of 26 preparations, 85%) postcoronary artery ligation. Reflection and premature re-activation of the proximal site was then observed in 6 of 63 (9%), and 3 of 26 preparations (12%). The present experiments demonstrate DP formation and rate-dependent constant-coupled late epicardial premature beats in infarcted dog hearts. Microelectrode recordings at DP sites demonstrating prolonged isoelectric intervals display very slow conduction preceding distal activation and "reentrant" re-activation of more proximal sites, representing reflection as an arrhythmia mechanism in ischemically injured epicardium during subacute myocardial infarction.

The role of spatial organization of Ca2+ release sites in the generation of arrhythmogenic diastolic Ca2+ release in myocytes from failing hearts.

In heart failure (HF), dysregulated cardiac ryanodine receptors (RyR2) contribute to the generation of diastolic Ca2+ waves (DCWs), thereby predisposing adrenergically stressed failing hearts to life-threatening arrhythmias. However, the specific cellular, subcellular, and molecular defects that account for cardiac arrhythmia in HF remain to be elucidated. Patch-clamp techniques and confocal Ca2+ imaging were applied to study spatially defined Ca2+ handling in ventricular myocytes isolated from normal (control) and failing canine hearts. Based on their activation time upon electrical stimulation, Ca2+ release sites were categorized as coupled, located in close proximity to the sarcolemmal Ca2+ channels, and uncoupled, the Ca2+ channel-free non-junctional Ca2+ release units. In control myocytes, stimulation of β-adrenergic receptors with isoproterenol (Iso) resulted in a preferential increase in Ca2+ spark rate at uncoupled sites. This site-specific effect of Iso was eliminated by the phosphatase inhibitor okadaic acid, which caused similar facilitation of Ca2+ sparks at coupled and uncoupled sites. Iso-challenged HF myocytes exhibited increased predisposition to DCWs compared to control myocytes. In addition, the overall frequency of Ca2+ sparks was increased in HF cells due to preferential stimulation of coupled sites. Furthermore, coupled sites exhibited accelerated recovery from functional refractoriness in HF myocytes compared to control myocytes. Spatially resolved subcellular Ca2+ mapping revealed that DCWs predominantly originated from coupled sites. Inhibition of CaMKII suppressed DCWs and prevented preferential stimulation of coupled sites in Iso-challenged HF myocytes. These results suggest that CaMKII- (and phosphatase)-dependent dysregulation of junctional Ca2+ release sites contributes to Ca2+-dependent arrhythmogenesis in HF.

P1530Prediction of optimal cardiac resynchronization by 2-dimensional vectorloops in dyssynchronous canine hearts

P1530Prediction of optimal cardiac resynchronization by 2-dimensional vectorloops in dyssynchronous canine hearts

Synchronization as a mechanism for low-energy anti-fibrillation pacing

Low-energy anti-fibrillation pacing (LEAP) has been suggested as an alternative treatment in symptomatic fibrillation patients. It significantly lowers the energy required compared with standard 1-shock defibrillation. In this study, we investigated the mechanism of arrhythmia termination by LEAP and systematically analyzed the influence of shock period and timing on the success rate of LEAP. We induced atrial and ventricular fibrillation in isolated canine hearts and applied LEAP and standard 1-shock defibrillation to terminate the arrhythmia. We simulated the arrhythmia and LEAP using a 2-dimensional bidomain human atrial model. The exvivo experiments showed successful termination of atrial fibrillation and ventricular fibrillation using LEAP, with an average 88% and 81% energy reduction, respectively, and both experiments and simulations verified that synchronization from virtual electrodes is the key mechanism for termination of arrhythmia by LEAP using modified Kuramoto phase plots and fraction of tissue excited (FTE) plots. We also observed in simulations that LEAP is more effective when the shock period is close to the dominant period and the first shock is delivered when FTE is decreasing. Our results support synchronization as the mechanism for arrhythmia termination by LEAP, and its effectiveness can be improved by adjusting shock period and timing.

Age-dependent down-regulation of hyperpolarization-activated cyclic nucleotide-gated channel 4 causes deterioration of canine sinoatrial node function.

The activity of pacemaker cells in the sinoatrial node (SAN) is an indicator of normal sinus rhythm. Clinical studies have revealed that the dysfunction of the SAN progressively increases with aging. In this study, we determined the changes in hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) expression and the relationship between aging and canine SAN dysfunction. The results of cardiac electrophysiological determination revealed that the intrinsic heart rate decreased from 168 ± 11 beats min-1 in young canines to 120 ± 9 beats min-1 in adults and to 88 ± 9 beats min-1 in aged canines. The sinus node recovery time (SNRT) increased from 412 ± 32 ms in young canines to 620 ± 56 ms in adults and to 838 ± 120 ms in aged canines. Corrected SNRT (CSNRT) increased from 55 ± 12 ms in young canines to 117 ± 27 ms in adults and to 171 ± 37 ms in aged canines. These results indicated that SAN function deteriorated with aging in the canine heart. However, histological staining illustrated that fibrosis was not significantly increased with aging in canine SAN. Real-time polymerase chain reaction indicated that the expression of HCN4 mRNA was downregulated in the elderly canine SAN. Similarly, we also verified that HCN4 protein expression within the SAN declined with aging via immunofluorescence staining and western blot analysis. Taken together, our data show that electrical remodeling, related to the down-regulation of HCN4, is responsible for the gradually increased incidence of SAN dysfunction with aging. Our results provide further evidence for explaining the mechanisms of age-related deterioration in the SAN.

Local transmural action potential gradients are absent in the isolated, intact dog heart but present in the corresponding coronary-perfused wedge.

The left ventricular (LV) coronary‐perfused canine wedge preparation is a model commonly used for studying cardiac repolarization. In wedge studies, transmembrane potentials typically are recorded; whereas, extracellular electrical recordings are commonly used in intact hearts. We compared electrically measured activation recovery interval (ARI) patterns in the intact heart with those recorded at the same location in the LV wedge preparation. We also compared electrically recorded and optically obtained ARIs in the LV wedge preparation. Five Langendorff‐perfused canine hearts were paced from the right atrium. Local activation and repolarization times were measured with eight transmural needle electrodes. Subsequently, left ventricular coronary‐perfused wedge preparations were prepared from these hearts while the electrodes remained in place. Three electrodes remained at identical positions as in the intact heart. Both electrograms and optical action potentials were recorded (pacing cycle length 400–4000 msec) and activation and repolarization patterns were analyzed. ARIs found in the subepicardium were shorter than in the subendocardium in the LV wedge preparation but not in the intact heart. The transmural ARI gradient recorded at the cut surface of the wedge was not different from that recorded internally. ARIs recorded internally and at the cut surface in the LV wedge preparation, both correlated with optically recorded action potentials. ARI and RT gradients in the LV wedge preparation differed from those in the intact canine heart, implying that those observations in human LV wedge preparations also should be extrapolated to the intact human heart with caution.