Voltage-sensitive dye mapping in Langendorff-perfused rat hearts

Guidelines for assessment of cardiac electrophysiology and arrhythmias in small animals

Cardiac arrhythmias are a major cause of morbidity and mortality worldwide. Although recent advances in cell-based models, including human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM), are contributing to our understanding of electrophysiology and arrhythmia mechanisms, preclinical animal studies of cardiovascular disease remain a mainstay. Over the past several decades, animal models of cardiovascular disease have advanced our understanding of pathological remodeling, arrhythmia mechanisms, and drug effects and have led to major improvements in pacing and defibrillation therapies. There exist a variety of methodological approaches for the assessment of cardiac electrophysiology and a plethora of parameters may be assessed with each approach. This guidelines article will provide an overview of the strengths and limitations of several common techniques used to assess electrophysiology and arrhythmia mechanisms at the whole animal, whole heart, and tissue level with a focus on small animal models. We also define key electrophysiological parameters that should be assessed, along with their physiological underpinnings, and the best methods with which to assess these parameters.

Di-4-ANEPPS Modulates Electrical Activity and Progress of Myocardial Ischemia in Rabbit Isolated Heart

Aims

Although voltage-sensitive dye di-4-ANEPPS is a common tool for mapping cardiac electrical activity, reported effects on electrophysiological parameters are rather. The main goals of the study were to reveal effects of the dye on rabbit isolated heart and to verify, whether rabbit isolated heart stained with di-4-ANEPPS is a suitable tool for myocardial ischemia investigation.

Methods and Results

Study involved experiments on stained (n = 9) and non-stained (n = 11) Langendorff perfused rabbit isolated hearts. Electrophysiological effects of the dye were evaluated by analysis of various electrogram (EG) parameters using common paired and unpaired statistical tests. It was shown that staining the hearts with di-4-ANEPPS leads to only short-term sporadic prolongation of impulse conduction through atria and atrioventricular node. On the other hand, significant irreversible slowing of heart rate and ventricular conduction were found in stained hearts as compared to controls. In patch clamp experiments, significant inhibition of sodium current density was observed in differentiated NG108-15 cells stained by the dye. Although no significant differences in mean number of ventricular premature beats were found between the stained and the non-stained hearts in ischemia as well as in reperfusion, all abovementioned results indicate increased arrhythmogenicity. In isolated hearts during ischemia, prominent ischemic patterns appeared in the stained hearts with 3–4 min delay as compared to the non-stained ones. Moreover, the ischemic changes did not achieve the same magnitude as in controls even after 10 min of ischemia. It resulted in poor performance of ischemia detection by proposed EG parameters, as was quantified by receiver operating characteristics analysis.

Conclusion

Our results demonstrate significant direct irreversible effect of di-4-ANEPPS on spontaneous heart rate and ventricular impulse conduction in rabbit isolated heart model. Particularly, this should be considered when di-4-ANEPPS is used in ischemia studies in rabbit. Delayed attenuated response of such hearts to ischemia might lead to misinterpretation of obtained results.

Model Systems for Addressing Mechanism of Arrhythmogenesis in Cardiac Repair

PURPOSE OF REVIEW

Cardiac cell-based therapy represents a promising approach for cardiac repair. However, one of the main challenges is cardiac arrhythmias associated with stem cell transplantation. The current review summarizes the recent progress in model systems for addressing mechanisms of arrhythmogenesis in cardiac repair.

RECENT FINDINGS

Animal models have been extensively developed for mechanistic studies of cardiac arrhythmogenesis. Advances in human induced pluripotent stem cells (hiPSCs), patient-specific disease models, tissue engineering, and gene editing have greatly enhanced our ability to probe the mechanistic bases of cardiac arrhythmias. Additionally, recent development in multiscale computational studies and machine learning provides yet another powerful tool to quantitatively decipher the mechanisms of cardiac arrhythmias. Advancing efforts towards the integrations of experimental and computational studies are critical to gain insights into novel mitigation strategies for cardiac arrhythmias in cell-based therapy.

Two-photon excitation of FluoVolt allows improved interrogation of transmural electrophysiological function in the intact mouse heart

BACKGROUND & AIMS

Two-photon excitation of voltage sensitive dyes (VSDs) can measure rapidly changing electrophysiological signals deep within intact cardiac tissue with improved three-dimensional resolution along with reduced photobleaching and photo-toxicity compared to conventional confocal microscopy. Recently, a category of VSDs has emerged which records membrane potentials by photo-induced electron transfer. FluoVolt is a novel VSD in this category which promises fast response and a 25% fractional change in fluorescence per 100 mV, making it an attractive optical probe for action potential (AP) recordings within intact cardiac tissue. The purpose of this study was to characterize the fluorescent properties of FluoVolt as well as its utility for deep tissue imaging.

METHODS

Discrete tissue layers throughout the left ventricular wall of isolated perfused murine hearts loaded with FluoVolt or di-4-ANEPPS were sequentially excited with two-photon microscopy.

RESULTS

FluoVolt loaded hearts suffered significantly fewer episodes of atrio-ventricular block compared to di-4-ANEPPS loaded hearts, indicating comparatively low toxicity of FluoVolt in the intact heart. APs recorded with FluoVolt were characterized by a lower signal-to-noise ratio and a higher dynamic range compared to APs recorded with di-4-ANEPPS. Although both depolarization and repolarization parameters were similar in APs recorded with either dye, FluoVolt allowed deeper tissue excitation with improved three-dimensional resolution due to reduced out-of-focus fluorescence generation under two-photon excitation.

CONCLUSION

Our results demonstrate several advantages of two-photon excitation of FluoVolt in functional studies in intact heart preparations, including reduced toxicity and improved fluorescent properties.

Evidence for the impact of BAG3 on electrophysiological activity of primary culture of neonatal cardiomyocytes

Homeostasis of proteins involved in contractility of individual cardiomyocytes and those coupling adjacent cells is of critical importance as any abnormalities in cardiac electrical conduction may result in cardiac irregular activity and heart failure. Bcl2-associated athanogene 3 (BAG3) is a stress-induced protein whose role in stabilizing myofibril proteins as well as protein quality control pathways, especially in the cardiac tissue, has captured much attention. Mutations of BAG3 have been implicated in the pathogenesis of cardiac complications such as dilated cardiomyopathy. In this study, we have used an in vitro model of neonatal rat ventricular cardiomyocytes to investigate potential impacts of BAG3 on electrophysiological activity by employing the microelectrode array (MEA) technology. Our MEA data showed that BAG3 plays an important role in the cardiac signal generation as reduced levels of BAG3 led to lower signal frequency and amplitude. Our analysis also revealed that BAG3 is essential to the signal propagation throughout the myocardium, as the MEA data-based conduction velocity, connectivity degree, activation time, and synchrony were adversely affected by BAG3 knockdown. Moreover, BAG3 deficiency was demonstrated to be connected with the emergence of independently beating clusters of cardiomyocytes. On the other hand, BAG3 overexpression improved the activity of cardiomyocytes in terms of electrical signal amplitude and connectivity degree. Overall, by providing more in-depth analyses and characterization of electrophysiological parameters, this study reveals that BAG3 is of critical importance for electrical activity of neonatal cardiomyocytes.

Long-term contractile activity and thyroid hormone supplementation produce engineered rat myocardium with adult-like structure and function

The field of cardiac tissue engineering has developed rapidly, but structural and functional immaturity of engineered heart tissues hinder their widespread use. Here, we show that a combination of low-rate (0.2 Hz) contractile activity and thyroid hormone (T3) supplementation significantly promote structural and functional maturation of engineered rat cardiac tissues ("cardiobundles"). The progressive maturation of cardiobundles during first 2 weeks of culture resulted in cell cycle exit and loss of spontaneous activity, which in longer culture yielded decreased contractile function. Maintaining a low level of contractile activity by 0.2 Hz pacing between culture weeks 3 and 5, combined with T3 treatment, yielded significant growth of cardiobundle and myocyte cross-sectional areas (by 68% and 32%, respectively), increased nuclei numbers (by 22%), improved twitch force (by 39%), shortened action potential duration (by 32%), polarized N-cadherin distribution, and switch from immature (slow skeletal) to mature (fast) cardiac troponin I isoform expression. Along with advanced functional output (conduction velocity 53.7 ± 0.8 cm/s, specific force 70.1 ± 5.8 mN/mm²), quantitative ultrastructural analyses revealed similar metrics and abundance of sarcomeres, T-tubules, M-bands, and intercalated disks compared to native age-matched (5-week) and adult (3-month) ventricular myocytes. Unlike 0.2 Hz regime, chronic 1 Hz pacing resulted in significant cardiomyocyte loss and formation of necrotic core despite the use of dynamic culture. Overall, our results demonstrate remarkable ultrastructural and functional maturation of neonatal rat cardiomyocytes in 3D culture and reveal importance of combined biophysical and hormonal inputs for in vitro engineering of adult-like myocardium.

STATEMENT OF SIGNIFICANCE

Compared to human stem cell-derived cardiomyocytes, neonatal rat ventricular myocytes show advanced maturation state which makes them suitable for in vitro studies of postnatal cardiac development. Still, maturation process from a neonatal to an adult cardiomyocyte has not been recapitulated in rodent cell cultures. Here, we show that low-frequency pacing and thyroid hormone supplementation of 3D engineered neonatal rat cardiac tissues synergistically yield significant increase in cell and tissue volume, robust formation of T-tubules and M-lines, improved sarcomere organization, and faster and more forceful contractions. To the best of our knowledge, 5-week old engineered cardiac tissues described in this study are the first that exhibit both ultrastructural and functional characteristics approaching or matching those of adult ventricular myocardium.

Small-conductance Ca2+-activated K+ channel activation deteriorates hypoxic ventricular arrhythmias via CaMKII in cardiac hypertrophy

The molecular and electrophysiological mechanisms of acute ischemic ventricular arrhythmias in hypertrophied hearts are not well known. We hypothesized that small-conductance Ca2+-activated K+ (SK) channels are activated during hypoxia via the Ca2+/calmodulin-dependent protein kinase II (CaMKII)-dependent pathway. We used normotensive Wistar-Kyoto (WKY) rats and spontaneous hypertensive rats (SHRs) as a model of cardiac hypertrophy. The inhibitory effects of SK channels and ATP-sensitive K+ channels on electrophysiological changes and genesis of arrhythmias during simulated global hypoxia (GH) were evaluated. Hypoxia-induced abbreviation of action potential duration (APD) occurred earlier in ventricles from SHRs versus. WKY rats. Apamin, a SK channel blocker, prevented this abbreviation in SHRs in both the early and delayed phase of GH, whereas in WKY rats only the delayed phase was prevented. In contrast, SHRs were less sensitive to glibenclamide, a ATP-sensitive K+ channel blocker, which inhibited the APD abbreviation in both phases of GH in WKY rats. SK channel blockers (apamin and UCL-1684) reduced the incidence of hypoxia-induced sustained ventricular arrhythmias in SHRs but not in WKY rats. Among three SK channel isoforms, SK2 channels were directly coimmunoprecipitated with CaMKII phosphorylated at Thr286 (p-CaMKII). We conclude that activation of SK channels leads to the APD abbreviation and sustained ventricular arrhythmias during simulated hypoxia, especially in hypertrophied hearts. This mechanism may result from p-CaMKII-bound SK2 channels and reveal new molecular targets to prevent lethal ventricular arrhythmias during acute hypoxia in cardiac hypertrophy.

NEW & NOTEWORTHY We now show a new pathophysiological role of small-conductance Ca2+-activated K+ channels, which shorten the action potential duration and induce ventricular arrhythmias during hypoxia. We also demonstrate that small-conductance Ca2+-activated K+ channels interact with phosphorylated Ca2+/calmodulin-dependent protein kinase II at Thr286 in hypertrophied hearts.

Engineered cardiac tissue patch maintains structural and electrical properties after epicardial implantation

Functional cardiac tissue engineering holds promise as a candidate therapy for myocardial infarction and heart failure. Generation of "strong-contracting and fast-conducting" cardiac tissue patches capable of electromechanical coupling with host myocardium could allow efficient improvement of heart function without increased arrhythmogenic risks. Towards that goal, we engineered highly functional 1 cm × 1 cm cardiac tissue patches made of neonatal rat ventricular cells which after 2 weeks of culture exhibited force of contraction of 18.0 ± 1.4 mN, conduction velocity (CV) of 32.3 ± 1.8 cm/s, and sustained chronic activation when paced at rates as high as 8.7 ± 0.8 Hz. Patches transduced with genetically-encoded calcium indicator (GCaMP6) were implanted onto adult rat ventricles and after 4-6 weeks assessed for action potential conduction and electrical integration by two-camera optical mapping of GCaMP6-reported Ca²⁺ transients in the patch and RH237-reported action potentials in the recipient heart. Of the 13 implanted patches, 11 (85%) engrafted, maintained structural integrity, and conducted action potentials with average CVs and Ca²⁺ transient durations comparable to those before implantation. Despite preserved graft electrical properties, no anterograde or retrograde conduction could be induced between the patch and host cardiomyocytes, indicating lack of electrical integration. Electrical properties of the underlying myocardium were not changed by the engrafted patch. From immunostaining analyses, implanted patches were highly vascularized and expressed abundant electromechanical junctions, but remained separated from the epicardium by a non-myocyte layer. In summary, our studies demonstrate generation of highly functional cardiac tissue patches that can robustly engraft on the epicardial surface, vascularize, and maintain electrical function, but do not couple with host tissue. The lack of graft-host electrical integration is therefore a critical obstacle to development of efficient tissue engineering therapies for heart repair.

Striated muscle function, regeneration, and repair

As the only striated muscle tissues in the body, skeletal and cardiac muscle share numerous structural and functional characteristics, while exhibiting vastly different size and regenerative potential. Healthy skeletal muscle harbors a robust regenerative response that becomes inadequate after large muscle loss or in degenerative pathologies and aging. In contrast, the mammalian heart loses its regenerative capacity shortly after birth, leaving it susceptible to permanent damage by acute injury or chronic disease. In this review, we compare and contrast the physiology and regenerative potential of native skeletal and cardiac muscles, mechanisms underlying striated muscle dysfunction, and bioengineering strategies to treat muscle disorders. We focus on different sources for cellular therapy, biomaterials to augment the endogenous regenerative response, and progress in engineering and application of mature striated muscle tissues in vitro and in vivo. Finally, we discuss the challenges and perspectives in translating muscle bioengineering strategies to clinical practice.

Dynamic culture yields engineered myocardium with near-adult functional output

Engineered cardiac tissues hold promise for cell therapy and drug development, but exhibit inadequate function and maturity. In this study, we sought to significantly improve the function and maturation of rat and human engineered cardiac tissues. We developed dynamic, free-floating culture conditions for engineering "cardiobundles", 3-dimensional cylindrical tissues made from neonatal rat cardiomyocytes or human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) embedded in fibrin-based hydrogel. Compared to static culture, 2-week dynamic culture of neonatal rat cardiobundles significantly increased expression of sarcomeric proteins, cardiomyocyte size (∼2.1-fold), contractile force (∼3.5-fold), and conduction velocity of action potentials (∼1.4-fold). The average contractile force per cross-sectional area (59.7 mN/mm²) and conduction velocity (52.5 cm/s) matched or approached those of adult rat myocardium, respectively. The inferior function of statically cultured cardiobundles was rescued by transfer to dynamic conditions, which was accompanied by an increase in mTORC1 activity and decline in AMPK phosphorylation and was blocked by rapamycin. Furthermore, dynamic culture effects did not stimulate ERK1/2 pathway and were insensitive to blockers of mechanosensitive channels, suggesting increased nutrient availability rather than mechanical stimulation as the upstream activator of mTORC1. Direct comparison with phenylephrine treatment confirmed that dynamic culture promoted physiological cardiomyocyte growth rather than pathological hypertrophy. Optimized dynamic culture conditions also augmented function of human cardiobundles made reproducibly from cardiomyocytes derived from multiple hPSC lines, resulting in significantly increased contraction force (∼2.5-fold) and conduction velocity (∼1.4-fold). The average specific force of 23.2 mN/mm² and conduction velocity of 25.8 cm/s approached the functional metrics of adult human myocardium. In conclusion, we have developed a versatile methodology for engineering cardiac tissues with a near-adult functional output without the need for exogenous electrical or mechanical stimulation, and have identified mTOR signaling as an important mechanism for advancing tissue maturation and function in vitro.

The Zebrafish Heart as a Model of Mammalian Cardiac Function

Zebrafish (Danio rerio) are widely used as vertebrate model in developmental genetics and functional genomics as well as in cardiac structure-function studies. The zebrafish heart has been increasingly used as a model of human cardiac function, in part, due to the similarities in heart rate and action potential duration and morphology with respect to humans. The teleostian zebrafish is in many ways a compelling model of human cardiac function due to the clarity afforded by its ease of genetic manipulation, the wealth of developmental biological information, and inherent suitability to a variety of experimental techniques. However, in addition to the numerous advantages of the zebrafish system are also caveats related to gene duplication (resulting in paralogs not present in human or other mammals) and fundamental differences in how zebrafish hearts function. In this review, we discuss the use of zebrafish as a cardiac function model through the use of techniques such as echocardiography, optical mapping, electrocardiography, molecular investigations of excitation-contraction coupling, and their physiological implications relative to that of the human heart. While some of these techniques (e.g., echocardiography) are particularly challenging in the zebrafish because of diminutive size of the heart (~1.5 mm in diameter) critical information can be derived from these approaches and are discussed in detail in this article.

Remodeling of ECM patch into functional myocardium in an ovine model: A pilot study

BACKGROUND

Previous studies have demonstrated that surgical patches comprised of small intestinal submucosa-derived extracellular matrix (ECM) have biological remodeling potential. This pilot study investigated histological, mechanical, and bioelectrical properties of an ECM patch implanted in the ovine right-ventricular outflow tract (RVOT).

MATERIALS AND METHODS

ECM patches (2 × 2 cm² ) were implanted in four Western Range sheep (wether males, 37-49 kg, age <1 year) and explanted at 5 months (n = 2) and 8 months (n = 2). In vivo analysis included epicardial echocardiography and contact electrical mapping. Optical mapping was used to map electrical activity of two hearts on a Langendorff preparation. Mechanical testing quantified stiffness. Histological stains characterized structure, neovascularization, and calcification; immunohistochemistry (IHC) assessed cell phenotype.

RESULTS

In vivo analysis showed that ECM patch tissue was contractile by M-mode and two-dimensional echocardiographic evaluation. In vivo electrical mapping, and optical mapping confirmed that ECM conducted an organized electrical signal. Mechanical testing of native and ECM patched RVOT tissue showed an elastic modulus of the implanted patch comparable to native tissue stiffness.

CONCLUSIONS

At 5 and 8 months, the ECM had undergone extracellular matrix remodeling and neovascularization without calcification. The ECM was populated with locally aligned muscle cells positive for sarcomeric alpha-actinin, CD45, and troponin I and T. In sheep, the ECM patch appears to have the potential of remodeling to resemble native, functional ventricular tissue as evidenced by histological, mechanical, and electrical properties. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1713-1720, 2016.

Biorealistic cardiac cell culture platforms with integrated monitoring of extracellular action potentials

Current platforms for in vitro drug development utilize confluent, unorganized monolayers of heart cells to study the effect on action potential propagation. However, standard cell cultures are of limited use in cardiac research, as they do not preserve important structural and functional properties of the myocardium. Here we present a method to integrate a scaffolding technology with multi-electrode arrays and deliver a compact, off-the-shelf monitoring platform for growing biomimetic cardiac tissue. Our approach produces anisotropic cultures with conduction velocity (CV) profiles that closer resemble native heart tissue; the fastest impulse propagation is along the long axis of the aligned cardiomyocytes (CVL) and the slowest propagation is perpendicular (CVT), in contrast to standard cultures where action potential propagates isotropically (CVL ≈ CVT). The corresponding anisotropy velocity ratios (CVL/CVT = 1.38 - 2.22) are comparable with values for healthy adult rat ventricles (1.98 - 3.63). The main advantages of this approach are that (i) it provides ultimate pattern control, (ii) it is compatible with automated manufacturing steps and (iii) it is utilized through standard cell culturing protocols. Our platform is compatible with existing read-out equipment and comprises a prompt method for more reliable CV studies.

Motion Artefact in Voltage-Sensitive Fluorescent Dye Emission During Repeated Ischemia of Isolated Heart

Motion artefact (MA) in voltage-sensitive fluorescent signals causes significant debasement of action potential. During ischemia and reperfusion in isolated rabbit heart, this artefact develops in a manner which may be described by the time of its onset, level, and shape. The MA during ischemia: (a) may become substantial with approximately two minutes delay after establishing global ischemia; (b) may be almost twice as high as the physiological action potential and decreases both with time and repetition of ischemia; (c) the MA shape is unpredictable and depends on individual rabbit.

Organ explant culture of neonatal rat ventricles: a new model to study gene and cell therapy

Testing cardiac gene and cell therapies in vitro requires a tissue substrate that survives for several days in culture while maintaining its physiological properties. The purpose of this study was to test whether culture of intact cardiac tissue of neonatal rat ventricles (organ explant culture) may be used as a model to study gene and cell therapy. We compared (immuno) histology and electrophysiology of organ explant cultures to both freshly isolated neonatal rat ventricular tissue and monolayers. (Immuno) histologic studies showed that organ explant cultures retained their fiber orientation, and that expression patterns of α-actinin, connexin-43, and α-smooth muscle actin did not change during culture. Intracellular voltage recordings showed that spontaneous beating was rare in organ explant cultures (20%) and freshly isolated tissue (17%), but common (82%) in monolayers. Accordingly, resting membrane potential was -83.9±4.4 mV in organ explant cultures, -80.5±3.5 mV in freshly isolated tissue, and -60.9±4.3 mV in monolayers. Conduction velocity, measured by optical mapping, was 18.2±1.0 cm/s in organ explant cultures, 18.0±1.2 cm/s in freshly isolated tissue, and 24.3±0.7 cm/s in monolayers. We found no differences in action potential duration (APD) between organ explant cultures and freshly isolated tissue, while APD of monolayers was prolonged (APD at 70% repolarization 88.8±7.8, 79.1±2.9, and 134.0±4.5 ms, respectively). Organ explant cultures and freshly isolated tissue could be paced up to frequencies within the normal range for neonatal rat (CL 150 ms), while monolayers could not. Successful lentiviral (LV) transduction was shown via Egfp gene transfer. Co-culture of organ explant cultures with spontaneously beating cardiomyocytes increased the occurrence of spontaneous beating activity of organ explant cultures to 86%. We conclude that organ explant cultures of neonatal rat ventricle are structurally and electrophysiologically similar to freshly isolated tissue and a suitable new model to study the effects of gene and cell therapy.

Absence of glucose transporter 4 diminishes electrical activity of mouse hearts during hypoxia

Insulin resistance, which characterizes type 2 diabetes, is associated with reduced translocation of glucose transporter 4 (GLUT4) to the plasma membrane following insulin stimulation, and diabetic patients with insulin resistance show a higher incidence of ischaemia, arrhythmias and sudden cardiac death. The aim of this study was to examine whether GLUT4 deficiency leads to more severe alterations in cardiac electrical activity during cardiac stress due to hypoxia. To fulfil this aim, we compared cardiac electrical activity from cardiac-selective GLUT4-ablated (G4H-/-) mouse hearts and corresponding control (CTL) littermates. A custom-made cylindrical 'cage' electrode array measured potentials (Ves) from the epicardium of isolated, perfused mouse hearts. The normalized average of the maximal downstroke of Ves ( (|d Ves/dt(min)|na), which we previously introduced as an index of electrical activity in normal, ischaemic and hypoxic hearts, was used to assess the effects of GLUT4 deficiency on electrical activity. The |d Ves/dt(min)|na of G4H −/− and CTL hearts decreased by 75 and 47%, respectively (P < 0.05), 30 min after the onset of hypoxia. Administration of insulin attenuated decreases in values of |d Ves/dt(min)|na in G4H −/− hearts as well as in CTL hearts, during hypoxia. In general, however, G4H −/− hearts showed a severe alteration of the propagation sequence and a prolonged total activation time. Results of this study demonstrate that reduced glucose availability associated with insulin resistance and a reduction in GLUT4-mediated glucose transport impairs electrical activity during hypoxia, and may contribute to cardiac vulnerability to arrhythmias in diabetic patients.

Toward on-chip, in-cell recordings from cultured cardiomyocytes by arrays of gold mushroom-shaped microelectrodes

Cardiological research greatly rely on the use of cultured primary cardiomyocytes (CMs). The prime methodology to assess CM network electrophysiology is based on the use of extracellular recordings by substrate-integrated planar Micro-Electrode Arrays (MEAs). Whereas this methodology permits simultaneous, long-term monitoring of the CM electrical activity, it limits the information to extracellular field potentials (FPs). The alternative method of intracellular action potentials (APs) recordings by sharp- or patch-microelectrodes is limited to a single cell at a time. Here, we began to merge the advantages of planar MEA and intracellular microelectrodes. To that end we cultured rat CM on micrometer size protruding gold mushroom-shaped microelectrode (gMμEs) arrays. Cultured CMs engulf the gMμE permitting FPs recordings from individual cells. Local electroporation of a CM converts the extracellular recording configuration to attenuated intracellular APs with shape and duration similar to those recorded intracellularly. The procedure enables to simultaneously record APs from an unlimited number of CMs. The electroporated membrane spontaneously recovers. This allows for repeated recordings from the same CM a number of times (>8) for over 10 days. The further development of CM-gMμE configuration opens up new venues for basic and applied biomedical research.

The voltage-sensitive dye di-4-ANEPPS slows conduction velocity in isolated guinea pig hearts

BACKGROUND

Voltage-sensitive dyes are important tools for mapping electrical activity in the heart. However, little is known about the effects of voltage-sensitive dyes on cardiac electrophysiology.

OBJECTIVE

To test the hypothesis that the voltage-sensitive dye di-4-ANEPPS modulates cardiac impulse propagation.

METHODS

Electrical and optical mapping experiments were performed in isolated Langendorff perfused guinea pig hearts. The effect of di-4-ANEPPS on conduction velocity and anisotropy of propagation was quantified. HeLa cells expressing connexin 43 were used to evaluate the effect of di-4-ANEPPS on gap junctional conductance.

RESULTS

In electrical mapping experiments, di-4-ANEPPS (7.5 μM) was found to decrease both longitudinal and transverse conduction velocities significantly compared with control. No change in the anisotropy of propagation was observed. Similar results were obtained in optical mapping experiments. In these experiments, the effect of di-4-ANEPPS was dose dependent. di-4-ANEPPS had no detectable effect on connexin 43-mediated gap junctional conductance in transfected HeLa cells.

CONCLUSION

Our results demonstrate that the voltage-sensitive dye di-4-ANEPPS directly and dose-dependently modulates cardiac impulse propagation. The effect is not likely mediated by connexin 43 inhibition. Our results highlight an important caveat that should be taken into account when interpreting data obtained using di-4-ANEPPS in cardiac preparations.

Physiologic cardiovascular strain and intrinsic wave imaging

Cardiovascular disease remains the primary killer worldwide. The heart, essentially an electrically driven mechanical pump, alters its mechanical and electrical properties to compensate for loss of normal mechanical and electrical function. The same adjustment also is performed in the vessels, which constantly adapt their properties to accommodate mechanical and geometrical changes related to aging or disease. Real-time, quantitative assessment of cardiac contractility, conduction, and vascular function before the specialist can visually detect it could be feasible. This new physiologic data could open up interactive therapy regimens that are currently not considered. The eventual goal of this technology is to provide a specific method for estimating the position and severity of contraction defects in cardiac infarcts or angina. This would improve care and outcomes as well as detect stiffness changes and overcome the current global measurement limitations in the progression of vascular disease, at little more cost or risk than that of a clinical ultrasound.

Application of scale-space descriptors for the reliable detection of keypoints for image registration in optical mapping studies in whole heart preparations

Data acquired using Optical Mapping (OM) studies are affected by motion artifacts due to the inherent contraction of the heart. Those artifacts can be reduced by registering the images obtained by the OM system or by the combination of approaches like physical restraint of the heart or ratiometry with image registration. Due to the lack of high contrast features most registration methods are not suitable for this application. This paper is focused on the utilization of scale space theory and local descriptors to enhance the detection of local features in OM images and to describe the movement of keypoints. This information can be used to determine a suitable set of transformations to perform the registration process.

Electrophysiological mapping of embryonic mouse hearts: mechanisms for developmental pacemaker switch and internodal conduction pathway

INTRODUCTION

Understanding sinoatrial node (SAN) development could help in developing therapies for SAN dysfunction. However, electrophysiological investigation of SAN development remains difficult because mutant mice with SAN dysfunctions are frequently embryonically lethal. Most research on SAN development is therefore limited to immunocytochemical observations without comparable functional studies.

METHODS AND RESULTS

We applied a multielectrode array (MEA) recording system to study SAN development in mouse hearts acutely isolated at embryonic ages (E) 8.5-12.5 days. Physiological heart rates were routinely restored, enabling accurate functional assessment of SAN development. We found that dominant pacemaking activity originated from the left inflow tract (LIFT) region at E8.5, but switched to the right SAN by E12.5. Combining MEA recordings and pharmacological agents, we show that intracellular calcium (Ca(2+))-mediated automaticity develops early and is the major mechanism of pulse generation in the LIFT of E8.5 hearts. Later in development at E12.5, sarcolemmal ion channels develop in the SAN at a time when pacemaker channels are down-regulated in the LIFT, leading to a switch in the dominant pacemaker location. Additionally, low micromolar concentrations of tetrodotoxin (TTX), a sodium channel blocker, minimally affect pacemaker rhythm at E8.5-E12.5, but suppress atrial activation and reveal a TTX-resistant SAN-atrioventricular node (internodal) pathway that mediates internodal conduction in E12.5 hearts.

CONCLUSIONS

Using a physiological mapping method, we demonstrate that differential mechanistic development of automaticity between the left and right inflow tract regions confers the pacemaker location switch. Moreover, a TTX-resistant pathway mediates preferential internodal conduction in E12.5 mouse hearts. 

Electrical activation of sinus venosus myocardium and expression patterns of RhoA and Isl-1 in the chick embryo

Electrical Activity and RhoA in the Embryo.

INTRODUCTION

Myocardium at the venous pole (sinus venosus) of the heart has gained clinical interest as arrhythmias can be initiated from this area. During development, sinus venosus myocardium is incorporated to the primary heart tube and expresses different markers than primary myocardium. We aimed to elucidate the development of sinus venosus myocardium, including the sinoatrial node (SAN), by studying expression patterns of RhoA in relation to other markers, and by studying electrical activation patterns of the developing sinus venosus myocardium.

METHODS AND RESULTS

Expression of RhoA, myocardial markers cTnI and Nkx2.5, transcription factors Isl-1 and Tbx18, and cation channel HCN4 were examined in sequential stages in chick embryos. Electrical activation patterns were studied using microelectrodes and optical mapping. Embryonic sinus venosus myocardium is cTnI and HCN4 positive, Nkx2.5 negative, complemented by distinct patterns of Isl-1 and Tbx18. During development, initial myocardium-wide expression of RhoA becomes restricted to right-sided sinus venosus myocardium, comprising the SAN. Electrophysiological measurements revealed initial capacity of both atria to show electrical activity that in time shifts to a right-sided dominance, coinciding with persistence of RhoA, Tbx18, and HCN4 and absence of Nkx2.5 expression in the definitive SAN.

CONCLUSION

Results show an initially bilateral electrical potential of sinus venosus myocardium evolving into a right-sided activation pattern during development, and suggest a role for RhoA in conduction system development. We hypothesize an initial sinus venosus-wide capacity to generate pacemaker signals, becoming confined to the definitive SAN. Lack of differentiation toward a chamber phenotype would explain ectopic pacemaker foci.

Reduced conduction reserve in the diabetic rat heart: role of iPLA2 activation in the response to ischemia

Hearts from streptozotocin (STZ)-induced diabetic rats have previously been shown to have impaired intercellular electrical coupling, due to reorganization (lateralization) of connexin43 proteins. Due to the resulting reduction in conduction reserve, conduction velocity in diabetic hearts is more sensitive to conditions that reduce cellular excitability or intercellular electrical coupling. Diabetes is a known risk factor for cardiac ischemia, a condition associated with both reduced cellular excitability and reduced intercellular coupling. Activation of Ca(2+)-independent phospholipase A(2) (iPLA(2)) is known to be part of the response to acute ischemia and may contribute to the intercellular uncoupling by causing increased levels of arachidonic acid and lysophosphatidyl choline. Normally perfused diabetic hearts are known to exhibit increased iPLA(2) activity and may thus be particularly sensitive to further activation of these enzymes. In this study, we used voltage-sensitive dye mapping to assess changes in conduction velocity in response to acute global ischemia in Langendorff-perfused STZ-induced diabetic hearts. Conduction slowing in response to ischemia was significantly larger in STZ-induced diabetic hearts compared with healthy controls. Similarly, slowing of conduction velocity in response to acidosis was also more pronounced in STZ-induced diabetic hearts. Inhibition of iPLA(2) activity using bromoenol lactone (BEL; 10 μM) had no effect on the response to ischemia in healthy control hearts. However, in STZ-induced diabetic hearts, BEL significantly reduced the amount of conduction slowing observed beginning 5 min after the onset of ischemia. BEL treatment also significantly increased the time to onset of sustained arrhythmias in STZ-induced diabetic hearts but had no effect on the time to arrhythmia in healthy control hearts. Thus, our results suggest that iPLA(2) activation in response to acute ischemia in STZ-induced diabetic hearts is more pronounced than in control hearts and that this response is a significant contributor to arrhythmogenic conduction slowing.

Noninvasive electromechanical wave imaging and conduction-relevant velocity estimation in vivo

Electromechanical wave imaging is a novel technique for the noninvasive mapping of conduction waves in the left ventricle through the combination of ECG gating, high frame rate ultrasound imaging and radio-frequency (RF)-based displacement estimation techniques. In this paper, we describe this new technique and characterize the origin and velocity of the wave under distinct pacing schemes. First, in vivo imaging (30 MHz) was performed on anesthetized, wild-type mice (n=12) at high frame rates in order to take advantage of the transient electromechanical coupling occurring in the myocardium. The RF signal acquisition in a long-axis echocardiographic view was gated between consecutive R-wave peaks of the mouse electrocardiogram (ECG) and yielded an ultra-high RF frame rate of 8000 frames/s (fps). The ultrasound RF signals in each frame were digitized at 160 MHz. Axial, frame-to-frame displacements were estimated using 1D cross-correlation (window size of 240 microm, overlap of 90%). Three pacing protocols were sequentially applied in each mouse: (1) sinus rhythm (SR), (2) right-atrial (RA) pacing and (3) right-ventricular (RV) pacing. Pacing was performed using an eight-electrode catheter placed into the right side of the heart with the capability of pacing from any adjacent bipole. During a cardiac cycle, several waves were depicted on the electromechanical wave images that propagated transmurally and/or from base to apex, or apex to base, depending on the type of pacing and the cardiac phase. Through comparison between the ciné-loops and their corresponding ECG obtained at different pacing protocols, we were able to identify and separate the electrically induced, or contraction, waves from the hemodynamic (or, blood-wall coupling) waves. In all cases, the contraction wave was best observed along the posterior wall starting at the S-wave of the ECG, which occurs after Purkinje fiber, and during myocardial, activation. The contraction wave was identified based on the fact that it changed direction only when the pacing origin changed, i.e., it propagated from the apex to the base at SR and RA pacing and from base to apex at RV pacing. This reversal in the wave propagation direction was found to be consistent in all mice scanned and the wave velocity values fell within the previously reported conduction wave range with statistically significant differences between SR/RA pacing (0.85+/-0.22 m/s and 0.84+/-0.20 m/s, respectively) and RV pacing (-0.52+/-0.31 m/s; p<0.0001). This study thus shows that imaging the electromechanical function of the heart noninvasively is feasible. It may therefore constitute a unique noninvasive method for conduction wave mapping of the entire left ventricle. Such a technology can be extended to 3D mapping and/or used for early detection of dyssynchrony, arrhythmias, left-bundle branch block, or other conduction abnormalities as well as diagnosis and treatment thereof.

Purkinje-mediated effects in the response of quiescent ventricles to defibrillation shocks

In normal cardiac function, orderly activation of the heart is facilitated by the Purkinje system (PS), a specialized network of fast-conducting fibers that lines the ventricles. Its role during ventricular defibrillation remains unelucidated. Physical characteristics of the PS make it a poor candidate for direct electrical observation using contemporary experimental techniques. This study uses a computer modeling approach to assess contributions by the PS to the response to electrical stimulation. Normal sinus rhythm was simulated and epicardial breakthrough sites were distributed in a manner consistent with experimental results. Defibrillation shocks of several strengths and orientations were applied to quiescent ventricles, with and without PS, and electrical activation was analyzed. All shocks induced local polarizations in PS branches parallel to the field, which led to the rapid spread of excitation through the network. This produced early activations at myocardial sites where tissue was unexcited by the shock and coupled to the PS. Shocks along the apico-basal axis of the heart resulted in a significant abbreviation of activation time when the PS was present; these shocks are of particular interest because the fields generated by internal cardioverter defibrillators tend to have a strong component in the same direction. The extent of PS-induced changes, both temporal and spatial, was constrained by the amount of shock-activated myocardium. Increasing field strength decreased the transmission delay between PS and ventricular tissue at Purkinje-myocardial junctions (PMJs), but this did not have a major effect on the organ-level response. Weaker shocks directly affect a smaller volume of myocardial tissue but easily excite the PS, which makes the PS contribution to far field excitation more substantial than for stronger shocks.

Experimental and theoretical ventricular electrograms and their relation to electrophysiological gradients in the adult rat heart

The electrical activity of adult mouse and rat hearts has been analyzed extensively, often as a prerequisite for genetic engineering studies or for the development of rodent models of human diseases. Some aspects of the initiation and conduction of the cardiac action potential in rodents closely resemble those in large mammals. However, rodents have a much higher heart rate and their ventricular action potential is triangular and very short. As a consequence, an interpretation of the electrocardiogram in the mouse and rat remains difficult and controversial. In this study, optical mapping techniques have been applied to an in vitro left ventricular adult rat preparation to obtain patterns of conduction and action potential duration measurements from the epicardial surface. This information has been combined with previously published mathematical models of the rat ventricular myocyte to develop a bidomain model for action potential propagation and electrogram formation in the rat left ventricle. Important insights into the basis for the repolarization waveform in the ventricular electrogram of the adult rat have been obtained. Notably, our model demonstrated that the biphasic shape of the rat ventricular repolarization wave can be explained in terms of the transmural and apex-to-base gradients in action potential duration that exist in the rat left ventricle.

Shilianhua extract inhibits GSK-3beta and promotes glucose metabolism

The extract of plant Shilianhua (SLH; Sinocrassula indica Berge) is a component in a commercial product for control of blood glucose. However, it remains to be investigated whether the SLH extract enhances insulin sensitivity in a model of type 2 diabetes. To address this question, the SLH crude extract was fractionated into four parts on the basis of polarity, and bioactivities of each part were tested in cells. One of the fractions, F100, exhibited a strong activity in the stimulation of glucose consumption in vitro. Glucose consumption was induced significantly by F100 in 3T3-L1 adipocytes, L6 myotubes, and H4IIE hepatocytes in the absence of insulin. F100 also increased insulin-stimulated glucose consumption in L6 myotubes and H4IIE hepatocytes. It increased insulin-independent glucose uptake in 3T3-L1 adipocytes and insulin-dependent glucose uptake in L6 cells. The glucose transporter-1 (GLUT1) protein was induced in 3T3-L1 cells, and the GLUT4 protein was induced in L6 cells by F100. Mechanism study indicated that F100 induced GSK-3beta phosphorylation, which was comparable with that induced by insulin. Additionally, the transcriptional activity of NF-kappaB was inhibited by F100. In RAW 264.7 macrophages, mRNA expression of NF-kappaB target genes (TNFalpha and MCP-1) was suppressed by F100. In KK.Cg-A(y)/+ mice, F100 decreased fasting insulin and blood glucose and improved insulin tolerance significantly. We conclude that the F100 may be a bioactive component in the SLH plant. It promotes glucose metabolism in vitro and in vivo. Inhibition of GSK-3beta and NF-kappaB may be the potential mechanism.

Sex-dependent impairment of cardiac action potential conduction in type 1 diabetic rats

The incidence of diabetes mellitus is increasing. Cardiac dysfunction often develops, resulting in diverse arrhythmias. These arise from ion channel remodeling or from altered speed and pattern of impulse propagation. Few studies have investigated impulse propagation in the diabetic heart. We previously showed a reduced conduction reserve in the diabetic heart, with associated changes in intercellular gap junctions. The present study investigated whether these effects are sex specific. Hearts from control and streptozotocin-diabetic male and female rats were used. Optical mapping was performed with the voltage-sensitive dye di-4-ANEPPS, using Langendorff-perfused hearts. Isolated ventricular cells and tissue sections were used for immunofluorescent labeling of the gap junction protein connexin43 (Cx43). The gap junction uncoupler heptanol (0.75 mM) or elevated K+ (9 mM, to reduce cell excitability) produced significantly greater slowing of propagation in diabetic males than females. In ovariectomized diabetic females, 9 mM K+ slowed conduction significantly more than in nonovariectomized females. The subcellular redistribution (lateralization) of the gap junction protein Cx43 was smaller in diabetic females. Pretreatment of diabetic males with the angiotensin-converting enzyme inhibitor quinapril reduced Cx43 lateralization and the effects of 9 mM K+ on propagation. In conclusion, the slowing of cardiac impulse propagation in type 1 diabetes is smaller in female rats, partly due to the presence of female sex hormones. This difference is (partly) mediated by sex differences in activation of the cardiac renin-angiotensin system.

Upregulation of the heme oxygenase system ameliorates postprandial and fasting hyperglycemia in type 2 diabetes

In type 2 diabetes (T2D), postprandial and fasting hyperglycemia are important predictors of cardiovascular diseases; however, few drugs are currently available to simultaneously suppress these conditions. Here, we report an enduring antidiabetic effect of the heme oxygenase (HO) inducer hemin on Goto-Kakizaki rats (GK), a nonobese insulin-resistant T2D model. HO breaks down the heme-moiety-generating antioxidants (biliverdin/bilirubin and ferritin) and carbon monoxide, which stimulate insulin secretion. Hemin induces HO-1 to potentiate HO activity and the HO-derived products. Chronically applied hemin (30 mg/kg ip) for a month reduced and maintained fasting glucose at physiological levels for 3 mo. Before therapy, glucose levels were 9.3 +/- 0.3 mmol/l (n = 14). At 1, 2, and 3 mo posttherapy, we recorded 6.7 +/- 0.13, 5.9 +/- 0.2, and 7.2 +/- 0.2 mmol/l, respectively. Hemin was also effective against postprandial hyperglycemia (14.6 +/- 1.1 vs. 7.5 +/- 0.4 mmol/l; n = 14; P < 0.01), and the effect remained sustained for 3 mo after therapy. The reduction of hyperglycemia was accompanied by enhanced HO-1, HO activity, and cGMP of the soleus muscle, alongside increased plasma bilirubin, ferritin, SOD, total antioxidant capacity, and insulin levels, whereas markers/mediators of oxidative stress like urinary-8-isoprostane and soleus muscle nitrotyrosine, NF-kappaB, and activator protein-1 and -2 were abated. Furthermore, inhibitors of insulin signaling including soleus muscle glycogen synthase kinase-3 and JNK were reduced, while the insulin-sensitizing adipokine, adiponectin, alongside AMPK were increased. Correspondingly, hemin improved glucose tolerance, suppressed insulin intolerance, reduced insulin resistance, and overturned the inability of insulin to enhance glucose transporter 4, a protein required for glucose uptake. Hemin also upregulated HO-1/HO activity and cGMP and lowered glucose in euglycemic Sprague-Dawley control rats albeit less intensely, suggesting greater selectivity of the HO system in diabetic conditions. In conclusion, reduced oxidative stress alongside the concomitant and paradoxical enhancement of insulin secretion and insulin-sensitizing pathways may account for the 3-mo-enduring antidiabetic effect. The synergistic interaction among HO, adiponectin, and GLUT4 may be explored against insulin-resistant diabetes.

Cardiac action potential wavefront tracking using optical mapping

A wealth of knowledge is available about the effect of diabetes on the heart but very little has been done to quantify the conduction velocity of the diabetic heart. This study intends to develop a technique for tracking the cardiac wavefront across the heart in order to achieve the total activation time as well as the conduction velocity of the heart at any point during its activation, to compare the newly determined activation times with previously determined activation times, and to also compute the average conduction velocity of the heart from diabetic and control rats. The technique developed for tracking the action potential wavefront across the heart extracts the wavefront and provides the activation time as well as the conduction velocity - both instantaneous and average - successfully. The method reproduces previously measured activation times well, with a correlation of R(2) = 0.875, which suggests that this technique is reliable and that its determination of conduction velocity will allow for the examination of healthy and diseased hearts using a new criterion. In addition, the method for determining the conduction velocity of the heart allows direct comparison of the baseline conduction velocity in control and diabetic hearts. The results of this comparison indicated that conduction velocity in the diabetic hearts is slower (0.47 +/- 0.02 m/s) than in control hearts (0.55 +/- 0.02 m/s) (p = 0.001).

Ventricular activation is impaired in aged rat hearts

Ventricular arrhythmias are frequently observed in the elderly population secondary to alterations of electrophysiological properties that occur with the normal aging process of the heart. However, the underlying mechanisms remain poorly understood. The aim of the present study was to determine specific age-related changes in electrophysiological properties and myocardial structure in the ventricles that can be related to a structural-functional arrhythmogenic substrate. Multiple unipolar electrograms were recorded in vivo on the anterior ventricular surface of four control and seven aged rats during normal sinus rhythm and ventricular pacing. Electrical data were related to morphometric and immunohistochemical parameters of the underlying ventricular myocardium. In aged hearts total ventricular activation time was significantly delayed (QRS duration: +69%), while ventricular conduction velocity did not change significantly compared with control hearts. Moreover, ventricular activation patterns displayed variable numbers of epicardial breakthrough points whose appearance could change with time. Morphological analysis in aged rats revealed that heart weight and myocyte transverse diameter increased significantly, scattered microfoci of interstitial fibrosis were mostly present in the ventricular subendocardium, and gap junction connexin expression decreased significantly in ventricular myocardium compared with control rats. Our results show that in aged hearts delayed total ventricular activation time and abnormal activation patterns are not due to delayed myocardial conduction and suggest the occurrence of impaired impulse propagation through the conduction system leading to uncoordinated myocardial excitation. Impaired interaction between the conduction system and ventricular myocardium might create a potential reentry substrate, contributing to a higher incidence of ventricular arrhythmias in the elderly population.

A GSK-3/TSC2/mTOR pathway regulates glucose uptake and GLUT1 glucose transporter expression

Glucose transport is a highly regulated process and is dependent on a variety of signaling events. Glycogen synthase kinase-3 (GSK-3) has been implicated in various aspects of the regulation of glucose transport, but the mechanisms by which GSK-3 activity affects glucose uptake have not been well defined. We report that basal glycogen synthase kinase-3 (GSK-3) activity regulates glucose transport in several cell types. Chronic inhibition of basal GSK-3 activity (8-24 h) in several cell types, including vascular smooth muscle cells, resulted in an approximately twofold increase in glucose uptake due to a similar increase in protein expression of the facilitative glucose transporter 1 (GLUT1). Conversely, expression of a constitutively active form of GSK-3beta resulted in at least a twofold decrease in GLUT1 expression and glucose uptake. Since GSK-3 can inhibit mammalian target of rapamycin (mTOR) signaling via phosphorylation of the tuberous sclerosis complex subunit 2 (TSC2) tumor suppressor, we investigated whether chronic GSK-3 effects on glucose uptake and GLUT1 expression depended on TSC2 phosphorylation and TSC inhibition of mTOR. We found that absence of functional TSC2 resulted in a 1.5-to 3-fold increase in glucose uptake and GLUT1 expression in multiple cell types. These increases in glucose uptake and GLUT1 levels were prevented by inhibition of mTOR with rapamycin. GSK-3 inhibition had no effect on glucose uptake or GLUT1 expression in TSC2 mutant cells, indicating that GSK-3 effects on GLUT1 and glucose uptake were mediated by a TSC2/mTOR-dependent pathway. The effect of GSK-3 inhibition on GLUT1 expression and glucose uptake was restored in TSC2 mutant cells by transfection of a wild-type TSC2 vector, but not by a TSC2 construct with mutated GSK-3 phosphorylation sites. Thus, TSC2 and rapamycin-sensitive mTOR function downstream of GSK-3 to modulate effects of GSK-3 on glucose uptake and GLUT1 expression. GSK-3 therefore suppresses glucose uptake via TSC2 and mTOR and may serve to match energy substrate utilization to cellular growth.

Analyzing the electrophysiological effects of local epicardial temperature in experimental studies with isolated hearts

As a result of their modulating effects upon myocardial electrophysiology, both hypo- and hyperthermia can be used to study the mechanisms that generate or sustain cardiac arrhythmias. The present study describes an original electrode developed with thick-film technology and capable of controlling regional temperature variations in the epicardium while simultaneously registering its electrical activity. In this way, it is possible to measure electrophysiological parameters of the heart at different temperatures. The results obtained with this device in a study with isolated and perfused rabbit hearts are reported. An exploration has been made of the effects of local temperature changes upon the electrophysiological parameters implicated in myocardial conduction. Likewise, an analysis has been made of the influence of local temperature upon ventricular fibrillation activation frequency. It is concluded that both regional hypo- and hyperthermia exert reversible and opposite effects upon myocardial refractoriness and conduction velocity in the altered zone. The ventricular activation wavelength determined during constant pacing at 250 ms cycles is not significantly modified, however. During ventricular fibrillation, the changes in the fibrillatory frequency do not seem to be transmitted to normal temperature zones.

The high-fat-fed lean Zucker rat: a spontaneous isocaloric model of fat-induced insulin resistance associated with muscle GSK-3 overactivity

High-fat feeding (HFF) is a well-accepted model for nutritionally-induced insulin resistance. The purpose of this investigation was to assess the metabolic responses of female lean Zucker rats provided regular chow (4% fat) or a high-fat chow (50% fat) for 15 wk. HFF rats spontaneously adjusted food intake so that daily caloric intake matched that of chow-fed (CF) controls. HFF animals consumed more ( P < 0.05) calories from fat (31.9 ± 1.2 vs. 2.4 ± 0.2 kcal/day) and had significantly greater final body weights (280 ± 10 vs. 250 ± 5 g) and total visceral fat (24 ± 3 vs. 10 ± 1 g). Fasting plasma insulin was 2.3-fold elevated in HFF rats. Glucose tolerance (58%) and whole body insulin sensitivity (75%) were markedly impaired in HFF animals. In HFF plantaris muscle, in vivo insulin receptor β-subunit (IR-β) and insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation and phosphorylation of Akt Ser473 and glycogen synthase kinase-3β (GSK-3β) Ser9, relative to circulating insulin levels, were decreased by 40–59%. In vitro insulin-stimulated glucose transport in HFF soleus was decreased by 54%, as were IRS-1 tyrosine phosphorylation (26%) and phosphorylation of Akt Ser473 (38%) and GSK-3β Ser9 (25%), the latter indicative of GSK-3 overactivity. GSK-3 inhibition in HFF soleus using CT98014 increased insulin-stimulated glucose transport (28%), IRS-1 tyrosine phosphorylation (28%) and phosphorylation of Akt Ser473 (38%) and GSK-3β Ser9 (48%). In summary, the female lean Zucker rat fed a high-fat diet represents an isocaloric model of nutritionally-induced insulin resistance associated with moderate visceral fat gain, hyperinsulinemia, and impairments of skeletal muscle insulin-signaling functionality, including GSK-3β overactivity.

Coordinated phosphorylation of insulin receptor substrate-1 by glycogen synthase kinase-3 and protein kinase C betaII in the diabetic fat tissue

Serine/threonine phosphorylation of insulin receptor substrate-1 (IRS-1) is an important negative modulator of insulin signaling. Previously, we showed that glycogen synthase kinase-3 (GSK-3) phosphorylates IRS-1 at Ser(332). However, the fact that GSK-3 requires prephosphorylation of its substrates suggested that Ser(336) on IRS-1 was the "priming" site phosphorylated by an as yet unknown protein kinase. Here, we sought to identify this "priming kinase" and to examine the phosphorylation of IRS-1 at Ser(336) and Ser(332) in physiologically relevant animal models. Of several stimulators, only the PKC activator phorbol ester PMA enhanced IRS-1 phosphorylation at Ser(336). Treatment with selective PKC inhibitors prevented this PMA effect and suggested that a conventional PKC was the priming kinase. Overexpression of PKCalpha or PKCbetaII isoforms in cells enhanced IRS-1 phosphorylation at Ser(336) and Ser(332), and in vitro kinase assays verified that these two kinases directly phosphorylated IRS-1 at Ser(336). The expression level and activation state of PKCbetaII, but not PKCalpha, were remarkably elevated in the fat tissues of diabetic ob/ob mice and in high-fat diet-fed mice compared with that from lean animals. Elevated levels of PKCbetaII were also associated with enhanced phosphorylation of IRS-1 at Ser(336/332) and elevated activity of GSK-3beta. Finally, adenoviral mediated expression of PKCbetaII in adipocytes enhancedphosphorylation of IRS-1 at Ser(336). Taken together, our results suggest that IRS-1 is sequentially phosphorylated by PKCbetaII and GSK-3 at Ser(336) and Ser(332). Furthermore, these data provide evidence for the physiological relevance of these phosphorylation events in the pathogenesis of insulin resistance in fat tissue.

Oxidative stress-induced insulin resistance in rat skeletal muscle: role of glycogen synthase kinase-3

Oxidative stress can contribute to the multifactorial etiology of whole body and skeletal muscle insulin resistance. No investigation has directly assessed the effect of an in vitro oxidant stress on insulin action in intact mammalian skeletal muscle. Therefore, the purpose of the present study was to characterize the molecular actions of a low-grade oxidant stress (H(2)O(2)) on insulin signaling and glucose transport in isolated skeletal muscle of lean Zucker rats. Soleus strips were incubated in 8 mM glucose for 2 h in the absence or presence of 100 mU/ml glucose oxidase, which produces H(2)O(2) at approximately 90 microM. By itself, H(2)O(2) significantly (P < 0.05) activated basal glucose transport activity, net glycogen synthesis, and glycogen synthase activity and increased phosphorylation of insulin receptor (Tyr), Akt (Ser(473)), and GSK-3beta (Ser(9)). In contrast, this oxidant stress significantly inhibited the expected insulin-mediated enhancements in glucose transport, glycogen synthesis, and these signaling factors and allowed GSK-3beta to retain a more active form. In the presence of CT-98014, a selective GSK-3 inhibitor, the ability of insulin to stimulate glucose transport and glycogen synthesis during exposure to this oxidant stress was enhanced by 20% and 39% (P < 0.05), respectively, and insulin stimulation of the phosphorylation of insulin receptor, Akt, and GSK-3 was significantly increased by 36-58% (P < 0.05). These results indicate that an oxidant stress can directly and rapidly induce substantial insulin resistance of skeletal muscle insulin signaling, glucose transport, and glycogen synthesis. Moreover, a small, but significant, portion of this oxidative stress-induced insulin resistance is associated with a reduced insulin-mediated suppression of the active form of GSK-3beta.

Depth-resolved optical imaging of transmural electrical propagation in perfused heart

We present a study of the 3-dimensional (3D) propagation of electrical waves in the heart wall using Laminar Optical Tomography (LOT). Optical imaging contrast is provided by a voltage sensitive dye whose fluorescence reports changes in membrane potential. We examined the transmural propagation dynamics of electrical waves in the right ventricle of Langendorf perfused rat hearts, initiated either by endo-cardial or epi-cardial pacing. 3D images were acquired at an effective frame rate of 667Hz. We compare our experimental results to a mathematical model of electrical transmural propagation. We demonstrate that LOT can clearly resolve the direction of propagation of electrical waves within the cardiac wall, and that the dynamics observed agree well with the model of electrical propagation in rat ventricular tissue.

Near-infrared voltage-sensitive fluorescent dyes optimized for optical mapping in blood-perfused myocardium

BACKGROUND

Styryl voltage-sensitive dyes (e.g., di-4-ANEPPS) have been used successfully for optical mapping in cardiac cells and tissues. However, their utility for probing electrical activity deep inside the myocardial wall and in blood-perfused myocardium has been limited because of light scattering and high absorption by endogenous chromophores and hemoglobin at blue-green excitation wavelengths.

OBJECTIVE

The purpose of this study was to characterize two new styryl dyes--di-4-ANBDQPQ (JPW-6003) and di-4-ANBDQBS (JPW-6033)--optimized for blood-perfused tissue and intramural optical mapping.

METHODS

Voltage-dependent spectra were recorded in a model lipid bilayer. Optical mapping experiments were conducted in four species (mouse, rat, guinea pig, and pig). Hearts were Langendorff perfused using Tyrode's solution and blood (pig). Dyes were loaded via bolus injection into perfusate. Transillumination experiments were conducted in isolated coronary-perfused pig right ventricular wall preparations.

RESULTS

The optimal excitation wavelength in cardiac tissues (650 nm) was >70 nm beyond the absorption maximum of hemoglobin. Voltage sensitivity of both dyes was approximately 10% to 20%. Signal decay half-life due to dye internalization was 80 to 210 minutes, which is 5 to 7 times slower than for di-4-ANEPPS. In transillumination mode, DeltaF/F was as high as 20%. In blood-perfused tissues, DeltaF/F reached 5.5% (1.8 times higher than for di-4-ANEPPS).

CONCLUSION

We have synthesized and characterized two new near-infrared dyes with excitation/emission wavelengths shifted >100 nm to the red. They provide both high voltage sensitivity and 5 to 7 times slower internalization rate compared to conventional dyes. The dyes are optimized for deeper tissue probing and optical mapping of blood-perfused tissue, but they also can be used for conventional applications.

Postpartum glycemic homeostasis in early lactating rats is accompanied by transient and specific increase of soleus insulin response through IRS2/AKT pathway

It is known that at the moment of delivery immediate lost of conceptus (main site of glucose disposal in late pregnancy) is not able to disturb glucose homeostasis in early lactating mothers. However, the mechanism by which this adaptation takes place in early lactation is still unknown. Most studies concerning insulin sensitivity in lactating rats were carried out at 11–13 days postpartum and did not describe functional changes in insulin response in early lactation. Here we show that lactation hypersensitivity to insulin is observed as early as 3 days after delivery (L3). We show that the oxidative soleus muscle displays a transient increased maximal insulin-induced glucose uptake and CO2production, which is temporally limited to L3. Response of soleus muscle was accompanied by an increase in glucose transporter 4 (GLUT4) content at L3. This adaptive response was not detected in the glycolytic plantaris muscle, which displayed lower content of GLUT4. We also found that soleus muscle from early lactating rats have higher insulin receptor expression and tyrosine phosphorylation. Downstream steps of insulin signaling pathway; e.g., insulin receptor substrate 2 tyrosine phosphorylation and its association with phosphatidylinositol 3-kinase were also upregulated in soleus muscle. In parallel, protein tyrosine phosphatase 1B expression, a negative regulator of insulin signal, was reduced. Importantly, all of these molecular alterations were time limited to L3 and were not observed in plantaris muscle. These results suggest that improved insulin action in oxidative, but not in glycolytic muscle might contribute to achievement of glucose homeostasis postpartum.

Propagation of the cardiac impulse in the diabetic rat heart: reduced conduction reserve

Diabetes mellitus is a growing epidemic with severe cardiovascular complications. Although much is known about mechanical and electrical cardiac dysfunction in diabetes, few studies have investigated propagation of the electrical signal in the diabetic heart and the associated changes in intercellular gap junctions. This study was designed to investigate these issues, using hearts from control and diabetic rats. Diabetic conditions were induced by streptozotocin (STZ), given i.v. 7-14 days before experiments. Optical mapping with the voltage-sensitive dye di-4-ANEPPS, using hearts perfused on a Langendorff apparatus, showed little change in baseline conduction velocity in diabetic hearts, reflecting the large reserve of function. However, both the gap junction uncoupler heptanol (0.5-1 mM) and elevated potassium (9 mM, to reduce cell excitability) produced a significantly greater slowing of impulse propagation in diabetic hearts than in controls. The maximal action potential upstroke velocity (an index of the sodium current) and resting potential was similar in single ventricular myocytes from control and diabetic rats, suggesting similar electrical excitability. Immunoblotting of connexin 43 (Cx43), a major gap junction component, showed no change in total expression. However, immunofluorescence labelling of Cx43 showed a significant redistribution, apparent as enhanced Cx43 lateralization. This was quantified and found to be significantly larger than in control myocytes. Labelling of two other gap junction proteins, N-cadherin and beta-catenin, showed a (partial) loss of co-localization with Cx43, indicating that enhancement of lateralized Cx43 is associated with non-functional gap junctions. In conclusion, conduction reserve is smaller in the diabetic heart, priming it for impaired conduction upon further challenges. This can desynchronize contraction and contribute to arrhythmogenesis.

Optical mapping system for recording action potential durations in adult mouse left and right atrium

The increasing availability of murine models of the cardiovascular system has created a need for instrumentation and methods for assessing murine cardiovascular function. We have adapted an existing optical mapping system based on voltage-sensitive dyes to record from an isolated mouse atrial preparation. Initial results indicate that our approach is capable of recording action potentials from isolated mouse atria with sufficient signal quality to determine action potential duration (APD). Preliminary observations suggest that gradients in APD exist in the mouse atria and are similar to those observed in the atria of larger mammals. Future work with this technique will provide important information about mouse atrial electrophysiology and how it relates to that of larger mammals.

Tissue-specific expression and regulation of GSK-3 in human skeletal muscle and adipose tissue

Glycogen synthase kinase-3 (GSK-3) is a ubiquitous kinase implicated in both insulin action and adipogenesis. To determine how these multiple roles may relate to insulin resistance, we studied the regulation of GSK-3 protein expression and phosphorylation in skeletal muscle and isolated adipocytes from nonobese healthy control (HC), obese control (OC), and obese type 2 diabetic (OT2D) subjects. At baseline there were no differences in the GSK-3 protein expression in adipocytes. OC subjects underwent a 6-mo caloric restriction resulting in a 7% decrease in body mass index (BMI) and a 21% improvement in insulin-stimulated whole body glucose disposal rate (GDR). GSK-3alpha and GSK-3beta expression decreased in adipocytes (P < 0.05), whereas GSK-3alpha protein expression increased in skeletal muscle (P < 0.05). OT2D subjects were treated with troglitazone or metformin for 3-4 mo. After troglitazone treatment GDR improved (P < 0.05) despite an increase in BMI (P < 0.05), whereas metformin had no significant effect on GDR. There was no significant change in GSK-3 expression in adipocytes following troglitazone, whereas both GSK-3alpha and -beta were decreased in skeletal muscle (P < 0.05). Metformin treatment had no significant impact on GSK-3 protein expression in either adipocytes or skeletal muscle. Neither treatment influenced GSK-3 serine phosphorylation in skeletal muscle or adipocytes. These results suggest that there is tissue specificity for the regulation of GSK-3 in humans. In skeletal muscle GSK-3 plays a role in control of metabolism and insulin action, whereas the function in adipose tissue is less clear.

Application of a probabilistic method for the determination of drug-induced QT prolongation in telemetered cynomolgus monkeys: effects of moxifloxacin

INTRODUCTION

Moxifloxacin is the most widely used positive reference agent in clinical cardiac repolarization safety studies, but it has not been characterized in the cynomolgus monkey. This important experimental animal species exhibits pronounced heart rate variability, complicating the temporal evaluation of QT interval data.

METHODS

Digitized epicardial ECGs and aortic blood pressures were collected for 20 h in telemetered cynomolgus monkeys (n=6) following the administration of either vehicle or moxifloxacin (10 or 50 mg/kg, p.o.). Moxifloxacin plasma concentrations were determined 4 h postdose. ECG intervals were analyzed by computerized algorithms. Individual probabilistic QT rate-corrections (QTc) were derived from the slopes of predose log-transformed QT-RR data where each QT value was the mean of >250 beats/RR increment. The resulting QTc was used to determine the repolarization effects of moxifloxacin, expressed as the placebo-adjusted change in QTc (DeltaQTc), and as the integrated response from 0 to 12 h (AUC(0-->12)) postdose.

RESULTS

No DeltaQTc effect was produced by 10 mg/kg moxifloxacin. However, moxifloxacin (50 mg/kg; 5.86+/-0.5 microg/mL C(max)) significantly prolonged the RR interval by 50 to 112 ms from 3.5 to 7.5 h postdose and DeltaQTc by >or=7.2 ms from 1.83 to 9.17 h, with a maximal DeltaQTc effect of +26.4 ms. No notable effects on either systemic blood pressure or body temperature occurred with either dose.

DISCUSSION

Probabilistic QT rate-corrections appear to have eliminated the confounding effects of heart rate, provided for a stable QTc baseline, and enabled the demonstration of an exposure-dependent QTc prolongation by moxifloxacin. The duration and magnitude of the QTc effect paralleled moxifloxacin pharmacokinetics, and C(max) values were similar to those achieved clinically in thorough QT/QTc studies. Thus, novel probabilistic QT rate-corrections may offer highly robust assessments of repolarization risk in both nonclinical and clinical investigations.

Fatty acid transport and FAT/CD36 are increased in red but not in white skeletal muscle of ZDF rats

An increased rate of fatty acid transport into skeletal muscle has been has been linked to the accumulation of intramuscular lipids and insulin resistance, and red muscles are more susceptible than white muscles in developing fatty acid-mediated insulin resistance. Therefore, we examined in Zucker diabetic fatty (ZDF) rats, relative to lean rats, 1) whether rates of fatty acid transport and transporters (FAT/CD36 and FABPpm) were upregulated in skeletal muscle during the transition from insulin resistance (week 6) to type 2 diabetes (weeks 12 and 24), 2) whether such changes occurred primarily in red skeletal muscle, and 3) whether changes in FAT/CD36 and GLUT4 were correlated. In red muscles of ZDF compared with lean rats, the rates of fatty acid transport were upregulated (+66%) early in life (week 6). Compared with the increase in fatty acid transport in lean red muscle from weeks 12-24 (+57%), the increase in fatty acid transport rate in ZDF red muscle was 50% greater during this same period. In contrast, no differences in fatty acid transport rates were observed in the white muscles of lean and ZDF rats at any time (weeks 6-24). In red muscle only, there was an inverse relationship between FAT/CD36 and GLUT4 protein expression as well as their plasmalemmal content. These studies have shown that, 1) before the onset of diabetes, as well as during diabetes, fatty acid transport and FAT/CD36 expression and plasmalemmal content are upregulated in ZDF rats, but importantly, 2) these changes occurred only in red, not white, muscles of ZDF rats.

Cardiac conduction through engineered tissue

In children, interruption of cardiac atrioventricular (AV) electrical conduction can result from congenital defects, surgical interventions, and maternal autoimmune diseases during pregnancy. Complete AV conduction block is typically treated by implanting an electronic pacemaker device, although long-term pacing therapy in pediatric patients has significant complications. As a first step toward developing a substitute treatment, we implanted engineered tissue constructs in rat hearts to create an alternative AV conduction pathway. We found that skeletal muscle-derived cells in the constructs exhibited sustained electrical coupling through persistent expression and function of gap junction proteins. Using fluorescence in situ hybridization and polymerase chain reaction analyses, myogenic cells in the constructs were shown to survive in the AV groove of implanted hearts for the duration of the animal's natural life. Perfusion of hearts with fluorescently labeled lec-tin demonstrated that implanted tissues became vascularized and immunostaining verified the presence of proteins important in electromechanical integration of myogenic cells with surrounding re-cipient rat cardiomyocytes. Finally, using optical mapping and electrophysiological analyses, we provide evidence of permanent AV conduction through the implant in one-third of recipient animals. Our experiments provide a proof-of-principle that engineered tissue constructs can function as an electrical conduit and, ultimately, may offer a substitute treatment to conventional pacing therapy.

Chronic selective glycogen synthase kinase-3 inhibition enhances glucose disposal and muscle insulin action in prediabetic obese Zucker rats

Increasing evidence supports a negative role of glycogen synthase kinase-3 (GSK-3) in regulation of skeletal muscle glucose transport. We assessed the effects of chronic treatment of insulin-resistant, prediabetic obese Zucker (fa/fa) rats with a highly selective GSK-3 inhibitor (CT118637) on glucose tolerance, whole body insulin sensitivity, plasma lipids, skeletal muscle insulin signaling, and in vitro skeletal muscle glucose transport activity. Obese Zucker rats were treated with either vehicle or CT118637 (30 mg/kg body wt) twice per day for 10 days. Fasting plasma insulin and free fatty acid levels were reduced by 14 and 23% (P < 0.05), respectively, in GSK-3 inhibitor-treated animals compared with vehicle-treated controls. The glucose response during an oral glucose tolerance test was reduced by 18% (P < 0.05), and whole body insulin sensitivity was increased by 28% (P < 0.05). In vivo insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation (50%) and IRS-1-associated phosphatidylinositol-3' kinase (79%) relative to fasting plasma insulin levels were significantly elevated (P < 0.05) in plantaris muscles of GSK-3 inhibitor-treated animals. Whereas basal glucose transport in isolated soleus and epitrochlearis muscles was unaffected by chronic GSK-3 treatments, insulin stimulation of glucose transport above basal was significantly enhanced (32-60%, P < 0.05). In summary, chronic treatment of insulin-resistant, prediabetic obese Zucker rats with a specific GSK-3 inhibitor enhances oral glucose tolerance and whole body insulin sensitivity and is associated with an amelioration of dyslipidemia and an improvement in IRS-1-dependent insulin signaling in skeletal muscle. These results provide further evidence that selective targeting of GSK-3 in muscle may be an effective intervention for the treatment of obesity-associated insulin resistance.

Novel probabilistic method for precisely correcting the QT interval for heart rate in telemetered dogs and cynomolgus monkeys

INTRODUCTION

QT intervals are not regulated on a beat-to-beat cadence, but are strongly influenced by the preceding heart rate history (hysteresis). ECG sampling, when performed over sufficiently long periods, results in the detection of ranges of different QT values for each discrete RR interval. Given the potential impact of QT hysteresis in QT interval rate-correction procedures, we hypothesized that, physiologically, the QT interval exists as a probabilistic variable where the exact value corresponding to any RR interval is precisely estimated from the associated QT population.

METHODS

Digital ECGs were collected for 18-21 h in telemetered dogs (n=7) and cynomolgus monkeys (n=7) employing epicardial ECG leads for accurate T(end) detection, and analyzed by computerized algorithms. Descriptive statistics were calculated for raw QT values in 10 ms RR increments. Individual rate-corrected QT (QTc) formulae were derived from the slopes of log-transformed QT-RR data where each QT point was the mean of >250 beats/RR increment. The aptness of this QTc model was assessed by residual analysis.

RESULTS

Beat-to-beat ECG analysis demonstrated that for all discrete cycle lengths, the associated raw QT intervals were normally distributed populations, spanning approximately 30-40 and 45-100 ms in the dog and cynomolgus monkey, respectively. In both species, QTc was stable (< or =5 ms variation) over all physiological RR intervals.

DISCUSSION

The probabilistic treatment of raw QT interval populations natively associated to any RR interval provides hysteresis-free raw QT estimates which can be accurately modeled, allowing the derivation of a precise QTc value. Previous unawareness of the probabilistic nature of the QT interval explains the historical failure of numerous QT rate-correction formulae to correctly solve this scientific issue. Importantly, QT distribution analysis has the potential to provide, for the first time, a universal and sensitive method for QT heart rate-correction, providing a robust method for nonclinical and clinical cardiac safety investigations of repolarization delay.