Optical multisite monitoring of cell excitation phenomena in isolated cardiomyocytes

Human sodium current voltage-dependence at physiological temperature measured by coupling a patch-clamp experiment to a mathematical model

Voltage-gated Na+ channels are crucial to action potential propagation in excitable tissues. Because of the high amplitude and rapid activation of the Na+ current, voltage-clamp measurements are very challenging and are usually performed at room temperature. In this study, we measured Na+ current voltage-dependence in stem cell-derived cardiomyocytes at physiological temperature. While the apparent activation and inactivation curves, measured as the dependence of current amplitude on voltage, fall within the range reported in previous studies, we identified a systematic error in our measurements. This error is caused by the deviation of the membrane potential from the command potential of the amplifier. We demonstrate that it is possible to account for this artifact using computer simulation of the patch-clamp experiment. We obtained surprising results through patch-clamp model optimization: a half-activation of -11.5 mV and a half-inactivation of -87 mV. Although the half-activation deviates from previous research, we demonstrate that this estimate reproduces the conduction velocity dependence on extracellular potassium concentration. KEY POINTS: Voltage-gated Na+ currents play a crucial role in excitable tissues including neurons, cardiac and skeletal muscle. Measurement of Na+ current is challenging because of its high amplitude and rapid kinetics, especially at physiological temperature. We have used the patch-clamp technique to measure human Na+ current voltage-dependence in human induced pluripotent stem cell-derived cardiomyocytes. The patch-clamp data were processed by optimization of the model accounting for voltage-clamp experiment artifacts, revealing a large difference between apparent parameters of Na+ current and the results of the optimization. We conclude that actual Na+ current activation is extremely depolarized in comparison to previous studies. The new Na+ current model provides a better understanding of action potential propagation; we demonstrate that it explains propagation in hyperkalaemic conditions.

Ultrafast Imaging of Cardiomyocyte Contractions by Combining Scanning Ion Conductance Microscopy with a Microelectrode Array

Beating cardiomyocytes undergo fast morphodynamics during the contraction-relaxation cycle. However, imaging these morphodynamics with a high spatial and temporal resolution is difficult, owing to a lack of suitable techniques. Here, we combine scanning ion conductance microscopy (SICM) with a microelectrode array (MEA) to image the three-dimensional (3D) topography of cardiomyocytes during a contraction-relaxation cycle with 1 μm spatial and 1 ms time resolution. We record the vertical motion of cardiomyocytes at many locations across a cell by SICM and synchronize these data using the simultaneously recorded action potential by the MEA as a time reference. This allows us to reconstruct the time-resolved 3D morphology of cardiomyocytes during a full contraction-relaxation cycle with a raw data rate of 200 μs/frame and to generate spatially resolved images of contractile parameters (maximum displacement, time delay, asymmetry factor). We use the MEA-SICM setup to visualize the effect of blebbistatin, a myosin II inhibitor, on the morphodynamics of contractions. Further, we find an upper limit of 0.02% for cell volume changes during an action potential. The results show that MEA-SICM provides an ultrafast imaging platform for investigating the functional interplay of cardiomyocyte electrophysiology and mechanics.

Alternating bipolar field stimulation identifies muscle fibers with defective excitability but maintained local Ca(2+) signals and contraction

Background

Most cultured enzymatically dissociated adult myofibers exhibit spatially uniform (UNI) contractile responses and Ca²⁺ transients over the entire myofiber in response to electric field stimuli of either polarity applied via bipolar electrodes. However, some myofibers only exhibit contraction and Ca²⁺ transients at alternating (ALT) ends in response to alternating polarity field stimulation. Here, we present for the first time the methodology for identification of ALT myofibers in primary cultures and isolated muscles, as well as a study of their electrophysiological properties.

Results

We used high-speed confocal microscopic Ca²⁺ imaging, electric field stimulation, microelectrode recordings, immunostaining, and confocal microscopy to characterize the properties of action potential-induced Ca²⁺ transients, contractility, resting membrane potential, and staining of T-tubule voltage-gated Na⁺ channel distribution applied to cultured adult myofibers. Here, we show for the first time, with high temporal and spatial resolution, that normal control myofibers with UNI responses can be converted to ALT response myofibers by TTX addition or by removal of Na⁺ from the bathing medium, with reappearance of the UNI response on return of Na⁺. Our results suggest disrupted excitability as the cause of ALT behavior and indicate that the ALT response is due to local depolarization-induced Ca²⁺ release, whereas the UNI response is triggered by action potential propagation over the entire myofiber. Consistent with this interpretation, local depolarizing monopolar stimuli give uniform (propagated) responses in UNI myofibers, but only local responses at the electrode in ALT myofibers. The ALT responses in electrically inexcitable myofibers are consistent with expectations of current spread between bipolar stimulating electrodes, entering (hyperpolarizing) one end of a myofiber and leaving (depolarizing) the other end of the myofiber. ALT responses were also detected in some myofibers within intact isolated whole muscles from wild-type and MDX mice, demonstrating that ALT responses can be present before enzymatic dissociation.

Conclusions

We suggest that checking for ALT myofiber responsiveness by looking at the end of a myofiber during alternating polarity stimuli provides a test for compromised excitability of myofibers, and could be used to identify inexcitable, damaged or diseased myofibers by ALT behavior in healthy and diseased muscle.

Electronic supplementary material

The online version of this article (doi:10.1186/s13395-016-0076-8) contains supplementary material, which is available to authorized users.

Optical imaging of voltage and calcium in cardiac cells & tissues

Cardiac optical mapping has proven to be a powerful technology for studying cardiovascular function and disease. The development and scientific impact of this methodology are well-documented. Because of its relevance in cardiac research, this imaging technology advances at a rapid pace. Here, we review technological and scientific developments during the past several years and look toward the future. First, we explore key components of a modern optical mapping set-up, focusing on: (1) new camera technologies; (2) powerful light-emitting-diodes (from ultraviolet to red) for illumination; (3) improved optical filter technology; (4) new synthetic and optogenetic fluorescent probes; (5) optical mapping with motion and contraction; (6) new multiparametric optical mapping techniques; and (7) photon scattering effects in thick tissue preparations. We then look at recent optical mapping studies in single cells, cardiomyocyte monolayers, atria, and whole hearts. Finally, we briefly look into the possible future roles of optical mapping in the development of regenerative cardiac research, cardiac cell therapies, and molecular genetic advances.

Mechanisms of defibrillation

Electrical shock has been the one effective treatment for ventricular fibrillation for several decades. With the advancement of electrical and optical mapping techniques, histology, and computer modeling, the mechanisms responsible for defibrillation are now coming to light. In this review, we discuss recent work that demonstrates the various mechanisms responsible for defibrillation. On the cellular level, membrane depolarization and electroporation affect defibrillation outcome. Cell bundles and collagenous septae are secondary sources and cause virtual electrodes at sites far from shocking electrodes. On the whole-heart level, shock field gradient and critical points determine whether a shock is successful or whether reentry causes initiation and continuation of fibrillation.

In vitro electrophysiological mapping of stem cells

The use of stem cells for cardiac regeneration is a revolutionary, emerging research area. For proper function as replacement tissue, stem cell-derived cardiomyocytes (SC-CMs) must electrically couple with the host cardiac tissue. Electrophysiological mapping techniques, including microelectrode array (MEA) and optical mapping, have been developed to study cardiomyocytes and cardiac cell monolayers, and these can be applied to study stem cells and SC-CMs. MEA recordings take extracellular measurements at numerous points across a small area of cell cultures and are used to assess electrical propagation during cell culture. Optical mapping uses fluorescent dyes to monitor electrophysiological changes in cells, most commonly transmembrane potential and intracellular calcium, and can be easily scaled to areas of different sizes. The materials and methods for MEA and optical mapping are presented here, together with detailed notes on their use, design, and fabrication. We also provide examples of voltage and calcium maps of mouse embryonic stem cell-derived cardiomyocytes (mESC-CMs), obtained in our laboratory using optical mapping techniques.

Uniform action potential repolarization within the sarcolemma of in situ ventricular cardiomyocytes

Previous studies have speculated, based on indirect evidence, that the action potential at the transverse (t)-tubules is longer than at the surface membrane in mammalian ventricular cardiomyocytes. To date, no technique has enabled recording of electrical activity selectively at the t-tubules to directly examine this hypothesis. We used confocal line-scan imaging in conjunction with the fast response voltage-sensitive dyes ANNINE-6 and ANNINE-6plus to resolve action potential-related changes in fractional dye fluorescence (DeltaF/F) at the t-tubule and surface membranes of in situ mouse ventricular cardiomyocytes. Peak DeltaF/F during action potential phase 0 depolarization averaged -21% for both dyes. The shape and time course of optical action potentials measured with the water-soluble ANNINE-6plus were indistinguishable from those of action potentials recorded with intracellular microelectrodes in the absence of the dye. In contrast, optical action potentials measured with the water-insoluble ANNINE-6 were significantly prolonged compared to the electrical recordings obtained from dye-free hearts, suggesting electrophysiological effects of ANNINE-6 and/or its solvents. With either dye, the kinetics of action potential-dependent changes in DeltaF/F during repolarization were found to be similar at the t-tubular and surface membranes. This study provides what to our knowledge are the first direct measurements of t-tubule electrical activity in ventricular cardiomyocytes, which support the concept that action potential duration is uniform throughout the sarcolemma of individual cells.

Effect of electroporation on cardiac electrophysiology

Defibrillation shocks are commonly used to terminate life-threatening arrhythmias. According to the excitation theory of defibrillation, such shocks are aimed at depolarizing the membranes of most cardiac cells, resulting in resynchronization of electrical activity in the heart. If shock-induced transmembrane potentials are large enough, they can cause transient tissue damage due to electroporation. In this review, evidence is presented that electroporation of the heart tissue can occur during clinically relevant intensities of the external electrical field and that electroporation can affect the outcome of defibrillation therapy, being both pro- and antiarrhythmic.Here, we present experimental evidence for electroporation in cardiac tissue, which occurs above a threshold of 25 V/cm as evident from propidium iodide uptake, transient diastolic depolarization, and reductions of action potential amplitude and its derivative. These electrophysiological changes can induce tachyarrhythmia, due to conduction block and possibly triggered activity; however, our findings provide the foundation for future design of effective methods to deliver genes and drugs to cardiac tissues, while avoiding possible side effects such as arrhythmia and mechanical stunning.

Quantification of shock-induced microscopic virtual electrodes assessed by subcellular resolution optical potential mapping in guinea pig papillary muscle

INTRODUCTION

The primary objective of this study was the quantitative description of shock-induced, locally occurring virtual electrodes in natural cardiac tissue.

METHODS AND RESULTS

Multiscale optical potential mapping using 10x, 20x, and 40x magnifying objectives, achieving resolutions of 0.13, 0.065, and 0.033 mm, was performed when applying uniform shocks (+/-10 V/cm, 5 ms) during diastole and action potential plateau. A procedure was developed to identify local potential deviations as depolarizing or hyperpolarizing peaks and to quantify their occurrence and characteristic amplitudes, lateral extents, and dynamics. At shock onset, peaks of either polarity developed significantly faster (tau = 0.92 +/- 0.65 ms, N = 64) than the average bulk polarization (tau = 2.25 +/- 0.96 ms, P < 0.001) and appeared locally fixed, changing their polarity at shock reversal. The mean peak magnitude (21.2 +/- 12 mV) and the amplitude distribution were essentially independent from the magnification. The peak density continuously increased with decreasing peak extent (taken at 70% of the amplitude), reaching a maximum of approximately 3 peaks/mm2 in the range of approximately 30-65 microm. There was no correlation between peak amplitude and size throughout. Potentially exciting peaks were found with a density of 0.04-0.2 peaks/mm2 corresponding to estimated 1-5 peaks/mm3.

CONCLUSIONS

Our results suggest that microscopic inhomogeneities form a substantial substrate for far-field excitation in natural cardiac tissue. Here, we effectively bridged the gap between the extensively studied myocyte cultures and larger heart preparations.

Comparison of optical and electrical mapping of fibrillation

Optical recordings with transmembrane potential (Vm)-sensitive fluorescent dye, or extracellular potential (Ve) recordings are used to map spatiotemporal patterns of cardiac excitation during ventricular fibrillation (VF). While the optical and electrical methods are accepted, there has not been a test of whether they yield equivalent excitation times during VF. Times may differ since previous results indicate optical Vm interrogates deeper than Ve. We tested whether the steepest parts of the downward deflection of the Ve and upward deflection of optical Vm are synchronized during VF. We used simultaneous coepicentral optical and electrical mapping (32 spots, 4 kHz) with translucent indium tin oxide electrodes and a laser scanner on ventricular epicardium. VF was electrically induced in arterially-perfused rabbit hearts stained with di-4-ANEPPS. For both the optical and electrical deflections, maximum magnitudes of the slopes varied over a > 4 fold range, morphologies varied and spatiotemporal distributions were nonuniform. Time differences between the steepest parts of the optical and electrical deflections were typically a few ms. Standard deviations of time differences increased for the deflections that had the smaller slopes, which was only partly due to effects of recording noise as indicated by simulations. For deflections that had slopes ranging from the steepest found at each spot to 1/4 of the steepest, the optical deflections were on average 0.7-1 ms earlier than the Ve deflections. Thus, excitation times during VF measured optically and electrically differ. Considered together with our earlier results indicating that the optical Vm interrogates deeper than Ve, the results suggest that most fibrillatory excitations occur earlier in subsurface tissue than at the heart surface.

Pacing-induced heterogeneities in intracellular Ca2+ signaling, cardiac alternans, and ventricular arrhythmias in intact rat heart

Optical mapping studies have suggested that intracellular Ca2+ and T-wave alternans are linked through underlying alternations in Ca2+ cycling-inducing oscillations in action potential duration through Ca2+-sensitive conductances. However, these studies cannot measure single-cell behavior; therefore, the Ca2+ cycling heterogeneities within microscopic ventricular regions are unknown. The goal of this study was to measure cellular activity in intact myocardium during rapid pacing and arrhythmias. We used single-photon laser-scanning confocal microscopy to measure Ca2+ signaling in individual myocytes of intact rat myocardium during rapid pacing and during pacing-induced ventricular arrhythmias. At low rates, all myocytes demonstrate Ca2+ alternans that is synchronized but whose magnitude varies depending on recovery kinetics of Ca2+ cycling for each individual myocyte. As rate increases, some cells reverse alternans phase, giving a dyssynchronous activation pattern, even in adjoining myocytes. Increased pacing rate also induces subcellular alternans where Ca2+ alternates out of phase with different regions within the same cell. These forms of heterogeneous Ca2+ signaling also occurred during pacing-induced ventricular tachycardia. Our results demonstrate highly nonuniform Ca2+ signaling among and within individual myocytes in intact heart during rapid pacing and arrhythmias. Thus, certain pathophysiological conditions that alter Ca2+ cycling kinetics, such as heart failure, might promote ventricular arrhythmias by exaggerating these cellular heterogeneities in Ca2+ signaling.

Can optical recordings of membrane potential be used to screen for drug-induced action potential prolongation in single cardiac myocytes?

INTRODUCTION

Potential-sensitive dyes have primarily been used to optically record action potentials (APs) in whole heart tissue. Using these dyes to record drug-induced changes in AP morphology of isolated cardiac myocytes could provide an opportunity to develop medium throughout assays for the pharmaceutical industry. Ideally, this requires that the dye has a consistent and rapid response to membrane potential, is insensitive to movement, and does not itself affect AP morphology.

MATERIALS AND METHODS

We recorded the AP from isolated adult guinea-pig ventricular myocytes optically using di-8-ANEPPS in a single-excitation dual-emission ratiometric system, either separately in electrically field stimulated myocytes, or simultaneously with an electrical AP recorded with a patch electrode in the whole-cell bridge mode. The ratio of di-8-ANEPPS fluorescence signal was calibrated against membrane potential using a switch-clamp to voltage clamp the myocyte.

RESULTS

Our data show that the ratio of the optical signals emitted at 560/620 nm is linearly related to voltage over the voltage range of an AP, producing a change in ratio of 7.5% per 100 mV, is unaffected by cell movement and is identical to the AP recorded simultaneously with a patch electrode. However, the APD90 recorded optically in myocytes loaded with di-8-ANEPPS was significantly longer than in unloaded myocytes recorded with a patch electrode (355.6+/-13.5 vs. 296.2+/-16.2 ms; p<0.01). Despite this effect, the apparent IC50 for cisapride, which prolongs the AP by blocking IKr, was not significantly different whether determined optically or with a patch electrode (91+/-46 vs. 81+/-20 nM).

DISCUSSION

These data show that the optical AP recorded ratiometrically using di-8-ANEPPS from a single ventricular myocyte accurately follows the action potential morphology. This technique can be used to estimate the AP prolonging effects of a compound, although di-8-ANEPPS itself prolongs APD90. Optical dyes require less technical skills and are less invasive than conventional electrophysiological techniques and, when coupled to ventricular myocytes, decreases animal usage and facilitates higher throughput assays.

Electroporation of the heart

Defibrillation shocks are commonly used to terminate life-threatening arrhythmias. According to the excitation theory of defibrillation, such shocks are aimed at depolarizing the membranes of most cardiac cells resulting in resynchronization of electrical activity in the heart. If shock-induced changes in transmembrane potential are large enough, they can cause transient tissue damage due to electroporation. In this review evidence is presented that (a) electroporation of the heart tissue can occur during clinically relevant intensities of the external electrical field, and (b) electroporation can affect the outcome of defibrillation therapy; being both pro- and anti-arrhythmic.

Optical Action Potential Upstroke Morphology Reveals Near-Surface Transmural Propagation Direction

The analysis of surface-activation patterns and measurements of conduction velocity in ventricular myocardium is complicated by the fact that the electrical wavefront has a complex 3D shape and can approach the heart surface at various angles. Recent theoretical studies suggest that the optical upstroke is sensitive to the subsurface orientation of the wavefront. Our goal here was to (1) establish the quantitative relationship between optical upstroke morphology and subsurface wavefront orientation using computer modeling and (2) test theoretical predictions experimentally in isolated coronary-perfused swine right ventricular preparations. We show in numerical simulations that by suitable placement of linear epicardial stimulating electrodes, the angle φ of wavefronts with respect to the heart surface can be controlled. Using this method, we developed theoretical predictions of the optical upstroke shape dependence on φ. We determined that the level V F * at which the rate of rise of the optical upstroke reaches the maximum linearly depends on φ. A similar relationship was found in simulations with epicardial point stimulation. The optical mapping data were in good agreement with theory. Plane waves propagating parallel to myocardial fibers produced upstrokes with V F *<0.5, consistent with theoretical predictions for φ>0. Similarly, we obtained good agreement with theory for plane waves propagating in a direction perpendicular to fibers ( V F *>0.5 when φ<0). Finally, during epicardial point stimulation, we discovered characteristic saddle-shaped V F * maps that were in excellent agreement with theoretically predicted changes in φ during wavefront expansion. Our findings should allow for improved interpretation of the results of optical mapping of intact heart preparations.

Artifactual contractions triggered by field stimulation of cardiomyocytes

Although cell shortening in patch-clamped cells (current-clamp mode) is triggered by an ordinary action potential, the trigger mechanism in field-stimulated cells is not so obvious. The contraction characteristics of the two methods differ, and we, therefore, examined the triggering sequence in field-stimulated cells. Isolated rat cardiomyocytes were plated on laminin-coated coverslips that were mounted on an inverted light microscope and superfused with HEPES-Tyrode buffer (pH 7.4; 37°C). The cells were stimulated to contract either by a 0.5-ms current injection (CC cells) through high-resistance electrodes or a 5-ms biphasic field-stimulation pulse (FS cells), and drugs were added to block sarcolemmal proteins involved in excitation-contraction coupling. Time to peak contraction (TTP) was significantly longer in FS cells and was not affected by the polarity or the length of the stimulus pulse. Tetrodotoxin (TTX; 20 μM) blocked cell shortening in CC cells but not in FS cells. Ni2+ (5 mM) blocked cell shortening in FS cells, whereas KB-R7943 (KB; 5 μM) had no effect either on cell shortening or TTP. In FS cells, nifedipine (Nif; 100 μM) and Cd2+ (300 μM) reduced fractional shortening by 34 and 63%, respectively, but only Cd2+ affected TTP (reduced by 48%). A combination of Nif and KB reduced cell shortening by 50%, whereas a combination of Cd2+ and KB almost abolished cell shortening. We conclude that field stimulation per se prolongs TTP and that cell shortening in FS cells is not dependent on Na+ current but is triggered by a combination of L-type Ca2+ current and reverse mode Na+/Ca2+ exchange.

Ionic currents involved in shock-induced nonlinear changes in transmembrane potential responses of single cardiac cells

An exhaustive characterization of how an isolated cardiac cell responds to applied electric fields could serve as an important groundwork for understanding responses of more complex higher order systems. Field stimulation of single cardiac cells during the early plateau of the action potential results in a nonuniform change in transmembrane potential (Vm) across the cell length that is more heavily weighted in the negative direction. These negatively shifted Vm responses are not replicated theoretically using present day membrane models. The goal of this study was to explore the membrane currents involved in the field responses during the plateau by selectively blocking various ion channels. Enzymatically isolated single guinea pig cells were stimulated with uniform field S1-S2 pulses, and the transmembrane potential responses were optically recorded from several sites along the cell length to assess the drug effect. We used nine different pharmacological agents to manipulate the conductance of major cardiac ion channels of which only barium (Ba2+) altered the transmembrane potential responses. At 50 microM Ba2+, which specifically blocks inwardly rectifying current I(K1), the negative shift in Vm responses was accentuated. At 1 mM Ba2+ , which blocks both I(K1) and sustained plateau current I(Kp), the negative shift diminished. However, 1 mM Ba2+ also depolarized the cells, and depressed or completely eliminated the action potential. Based on these results we conclude that I(K1) contributes to field-induced responses during the plateau stimulation by passing a net inward current, which when blocked accentuates the negative shift in the Vm responses. A conclusive role of I(Kp) could not be demonstrated because of confounding changes in membrane potential. However, from our results it remains as the most viable candidate for the elusive current that contributes a net outward current to produce negatively weighted Vm responses during plateau stimulation and warrants further investigation.

Characteristics of a charged-coupled-device-based optical mapping system for the study of cardiac arrhythmias

We develop an optical fluorescent mapping system that is able to record the action potential wavefront propagation within cardiac tissue samples with high spatial and temporal resolutions. The system's main component, the fluorescence acquisition device (customized CCD camera), offers a high spatial resolution of 128 x 128 pixels, with 12-bit digitization and a frame rate of 490 frames/s. The system is designed and implemented to image an area of approximately 20 x 20 mm at its minimum object distance of 140 mm, corresponding to a spatial resolution of approximately 3 line pairs/mm. Experiments using this system with di-4-ANEPPS-stained canine cardiac tissues with stimulated action potentials through external electrodes result in successful mappings of the distribution and propagation of the action potential wavefronts, showing the system's sensitivity to the change in fluorescence intensity in regions of action potentials. These data demonstrate this optical mapping system as a powerful device in the study of cardiac arrhythmia mechanisms.

Imaging neuronal seal resistance on silicon chip using fluorescent voltage-sensitive dye

The electrical sheet resistance between living cells grown on planar electronic contacts of semiconductors or metals is a crucial parameter for bioelectronic devices. It determines the strength of electrical signal transduction from cells to chips and from chips to cells. We measured the sheet resistance by applying AC voltage to oxidized silicon chips and by imaging the voltage change across the attached cell membrane with a fluorescent voltage-sensitive dye. The phase map of voltage change was fitted with a planar core-coat conductor model using the sheet resistance as a free parameter. For nerve cells from rat brain on polylysine as well as for HEK293 cells and MDCK cells on fibronectin we find a similar sheet resistance of 10 MOmega. Taking into account the independently measured distance of 50 nm between chip and membrane for these cells, we obtain a specific resistance of 50 Omegacm that is indistinguishable from bulk electrolyte. On the other hand, the sheet resistance for erythrocytes on polylysine is far higher, at approximately 1.5 GOmega. Considering the distance of 10 nm, the specific resistance in the narrow cleft is enhanced to 1500 Omegacm. We find this novel optical method to be a convenient tool to optimize the interface between cells and chips for bioelectronic devices.

Paradoxical loss of excitation with high intensity pulses during electric field stimulation of single cardiac cells

Transmembrane potential responses of single cardiac cells stimulated at rest were studied with uniform rectangular field pulses having durations of 0.5-10 ms. Cells were enzymatically isolated from guinea pig ventricles, stained with voltage sensitive dye di-8-ANEPPS, and stimulated along their long axes. Fluorescence signals were recorded with spatial resolution of 17 microm for up to 11 sites along the cell. With 5 and 10 ms pulses, all cells (n = 10) fired an action potential over a broad range of field amplitudes (approximately 3-65 V/cm). With 0.5 and 1 ms pulses, all cells (n = 7) fired an action potential for field amplitudes ranging from the threshold value (approximately 4-8 V/cm) to 50-60 V/cm. However, when the field amplitude was further increased, five of seven cells failed to fire an action potential. We postulated that this paradoxical loss of excitation for higher amplitude field pulses is the result of nonuniform polarization of the cell membrane under conditions of electric field stimulation, and a counterbalancing interplay between sodium current and inwardly rectifying potassium current with increasing field strength. This hypothesis was verified using computer simulations of a field-stimulated guinea pig ventricular cell. In conclusion, we show that for stimulation with short-duration pulses, cells can be excited for fields ranging between a low amplitude excitation threshold and a high amplitude threshold above which the excitation is suppressed. These results can have implications for the mechanistic understanding of defibrillation outcome, especially in the setting of diseased myocardium.

Optical imaging of the heart

Optical techniques have revolutionized the investigation of cardiac cellular physiology and advanced our understanding of basic mechanisms of electrical activity, calcium homeostasis, and metabolism. Although optical methods are widely accepted and have been at the forefront of scientific discoveries, they have been primarily applied at cellular and subcellular levels and considerably less to whole heart organ physiology. Numerous technical difficulties had to be overcome to dynamically map physiological processes in intact hearts by optical methods. Problems of contraction artifacts, cellular heterogeneities, spatial and temporal resolution, limitations of surface images, depth-of-field, and need for large fields of view (ranging from 2x2 mm2 to 3x3 cm2) have all led to the development of new devices and optical probes to monitor physiological parameters in intact hearts. This review aims to provide a critical overview of current approaches, their contributions to the field of cardiac electrophysiology, and future directions of various optical imaging modalities as applied to cardiac physiology at organ and tissue levels.

Effects of electroporation on optically recorded transmembrane potential responses to high-intensity electrical shocks

The outcome of defibrillation shocks is determined by the nonlinear transmembrane potential (Δ Vm) response induced by a strong external electrical field in cardiac cells. We investigated the contribution of electroporation to Δ Vm transients during high-intensity shocks using optical mapping. Rectangular and ramp stimuli (10–20 ms) of different polarities and intensities were applied to the rabbit heart epicardium during the plateau phase of the action potential (AP). Δ Vm were optically recorded under a custom 6-mm-diameter electrode using a voltage-sensitive dye. A gradual increase of cathodal and well as anodal stimulus strength was associated with 1) saturation and subsequent reduction of Δ Vm; 2) postshock diastolic resting potential (RP) elevation; and 3) postshock AP amplitude (APA) reduction. Weak stimuli induced a monotonic Δ Vm response and did not affect the RP level. Strong shocks produced a nonmonotonic Δ Vm response and caused RP elevation and a reduction of postshock APA. The maximum positive and maximum negative Δ Vm were recorded at 170 ± 20 mA/cm2 for cathodal stimuli and at 240 ± 30 mA/cm2 for anodal stimuli, respectively (means ± SE, n = 8, P = 0.003). RP elevation reached 10% of APA at a stimulus strength of 320 ± 40 mA/cm2 for both polarities. Strong ramp stimuli (20 ms, 600 mA/cm2) induced a nonmonotonic Δ Vm response, reaching the same largest positive and negative values as for rectangular shocks. The transition from monotonic to nonmonotonic morphology correlates with RP elevation and APA reduction, which is consistent with cell membrane electroporation. Strong shocks resulted in propidium iodide uptake, suggesting sarcolemma electroporation. In conclusion, electroporation is a likely explanation of the saturation and nonmonotonic nature of cellular responses reported for strong electric stimuli.

Decomposition of field-induced transmembrane potential responses of single cardiac cells

In this study, we used a multi-site optical mapping system to record field-induced responses of single cells isolated from guinea pig hearts. The cells were stained with voltage sensitive dye di-8-ANEPPS and stimulated with two uniform field (S1-S2) pulses along their longitudinal axes. The first pulse (S1 = 5 ms, <10 V/cm) was applied during rest and elicited an action potential. The second pulse (S2 = 10 ms, 4-50 V/cm) was applied 15 ms after the break of the S1 pulse (during the action potential plateau). The transmembrane potential responses, Vm(F)s, were optically recorded from up to 12 sites along the cell length using a fiber optic based optical mapping system at a resolution of 17 or 25 microm. The field-induced Vm(F)s had a complex spatio-temporal pattern. We show that these responses can be decomposed into simpler components. The first component, termed the differential-mode component (Vmd(F)), is like the response of a passive cell. The second component, termed the common-mode component (Vmc(F)), is identical all along the cell and adds a constant offset to the differential mode response of various sites along the cell length, to produce the total Vm(F) responses of the cell.

Artifacts, assumptions, and ambiguity: Pitfalls in comparing experimental results to numerical simulations when studying electrical stimulation of the heart

Insidious experimental artifacts and invalid theoretical assumptions complicate the comparison of numerical predictions and observed data. Such difficulties are particularly troublesome when studying electrical stimulation of the heart. During unipolar stimulation of cardiac tissue, the artifacts include nonlinearity of membrane dyes, optical signals blocked by the stimulating electrode, averaging of optical signals with depth, lateral averaging of optical signals, limitations of the current source, and the use of excitation-contraction uncouplers. The assumptions involve electroporation, membrane models, electrode size, the perfusing bath, incorrect model parameters, the applicability of a continuum model, and tissue damage. Comparisons of theory and experiment during far-field stimulation are limited by many of these same factors, plus artifacts from plunge and epicardial recording electrodes and assumptions about the fiber angle at an insulating boundary. These pitfalls must be overcome in order to understand quantitatively how the heart responds to an electrical stimulus. (c) 2002 American Institute of Physics.

Spatial heterogeneity of transmembrane potential responses of single guinea-pig cardiac cells during electric field stimulation

Changes in transmembrane voltage (V(m)) of cardiac cells during electric field stimulation have a complex spatial- and time-dependent behaviour that differs significantly from electrical stimulation of space-clamped membranes by current pulses. A multisite optical mapping system was used to obtain 17 or 25 microm resolution maps of V(m) along the long axis of guinea-pig ventricular cells (n = 57) stained with voltage-sensitive dye (di-8-ANEPPS) and stimulated longitudinally with uniform electric field (2, 5 or 10 ms, 3-62 V cm(-1)) pulses (n = 201). The initial polarizations of V(m) responses (V(mr)) varied linearly along the cell length and reversed symmetrically upon field reversal. The remainder of the V(m) responses had parallel time courses among the recording sites, revealing a common time-varying signal component (V(ms)). V(ms) was depolarizing for pulses during rest and hyperpolarizing for pulses during the early plateau phase. V(ms) varied in amplitude and time course with increasing pulse amplitude. Four types of plateau response were observed, with transition points between the different responses occurring when the maximum polarization at the ends of the cell reached values estimated as 60, 110 and 220 mV. Among the cells that had a polarization change of > 200 mV at their ends (for fields > 45 V cm(-1)), some (n = 17/25) had non-parallel time courses among V(m) recordings of the various sites. This implied development of an intracellular field (E(i)) that was found to increase exponentially with time (tau = 7.2 +/- 3.2 ms). Theoretical considerations suggest that V(ms) represents the intracellular potential (phi(i)) as well as the average polarization of the cell, and that V(mr) is the manifestation of the extracellular potential gradient resulting from the field stimulus. For cells undergoing field stimulation, phi(i) acts as the cellular physiological state variable and substitutes for V(m), which is the customary variable for space-clamped membranes.

Effects of uniform electric fields on intracellular calcium transients in single cardiac cells

Although intracellular calcium ([Ca(2+)](i)) transients in cardiac cells have been well studied in the uniformly polarized cell membrane, how these transients are modified during field stimulation when the cell membrane is nonuniformly polarized has not been investigated. In this study we characterized the effects of uniform field stimuli on [Ca(2+)](i) transients in isolated guinea pig cardiac cells. Single guinea pig cells were enzymatically isolated, loaded with the [Ca(2+)](i) fluorescent indicator fluo-3, and stimulated along their longitudinal axes with S1 or S1-S2 (S1-S2 = 50 ms) pulses. The fluorescence signals were recorded simultaneously from up to 12 sites along the cell length using a multisite mapping system. S1 pulse, applied during the resting phase of the action potential, induced [Ca(2+)](i) transients that had an earlier onset at the anodal-facing end, suggesting that [Ca(2+)](i) gradients (nabla[Ca(2+)](i)) develop during the rising phase of the [Ca(2+)](i) transients. With the assumption that the peak change in [Ca(2+)](i) is 980 nM, nabla[Ca(2+)](i) was estimated to be approximately 3.4 nM/microm in the anodal half of the cell for a nominal 10 V/cm field and negligible in the cathodal half. The S2 pulse that was applied during the plateau of the action potential also perturbed the [Ca(2+)](i) transients and produced [Ca(2+)](i) gradients directed from the center to either end of the cell. Mean nabla[Ca(2+)](i) in the anodal half of the cell (approximately 4.2 nM/microm) was found to be statistically higher than in the cathodal half (approximately 2.8 nM/microm).

Null mutation of connexin43 causes slow propagation of ventricular activation in the late stages of mouse embryonic development

Connexin43 (Cx43) is the principal connexin isoform in the mouse ventricle, where it is thought to provide electrical coupling between cells. Knocking out this gene results in anatomic malformations that nevertheless allow for survival through early neonatal life. We examined electrical wave propagation in the left (LV) and right (RV) ventricles of isolated Cx43 null mutated (Cx43(-/-)), heterozygous (Cx43(+/)(-)), and wild-type (WT) embryos using high-resolution mapping of voltage-sensitive dye fluorescence. Consistent with the compensating presence of the other connexins, no reduction in propagation velocity was seen in Cx43(-/-) ventricles at postcoital day (dpc) 12.5 compared with WT or Cx43(+/)(-) ventricles. A gross reduction in conduction velocity was seen in the RV at 15.5 dpc (in cm/second, mean [1 SE confidence interval], WT 9.9 [8.7 to 11.2], Cx43(+/)(-) 9.9 [9.0 to 10.9], and Cx43(-/-) 2.2 [1.8 to 2.7; P<0.005]) and in both ventricles at 17.5 dpc (in RV, WT 8.4 [7.6 to 9.3], Cx43(+/)(-) 8.7 [8.1 to 9.3], and Cx43(-/-) 1.1 [0.1 to 1.3; P<0.005]; in LV, WT 10.1 [9.4 to 10.7], Cx43(+/)(-) 8.3 [7.8 to 8.9], and Cx43(-/-) 1.7 [1.3 to 2.1; P<0.005]) corresponding with the downregulation of Cx40. Cx40 and Cx45 mRNAs were detectable in ventricular homogenates even at 17.5 dpc, probably accounting for the residual conduction function. Neonatal knockout hearts were arrhythmic in vivo as well as ex vivo. This study demonstrates the contribution of Cx43 to the electrical function of the developing mouse heart and the essential role of this gene in maintaining heart rhythm in postnatal life.

Virtual electrodes and deexcitation: new insights into fibrillation induction and defibrillation

Previous models of fibrillation induction and defibrillation stressed the contribution of depolarization during the response of the heart to a shock. This article reviews recent evidence suggesting that comprehending the role of negative polarization (hyperpolarization) also is crucial for understanding the response to a shock. Negative polarization can "deexcite" cardiac cells, creating regions of excitable tissue through which wavefronts can propagate. These wavefronts can result in new reentrant circuits, inducing fibrillation or causing defibrillation to fail. In addition, deexcitation can lead to rapid propagation through newly excitable regions, resulting in the elimination of excitable gaps soon after the shock and causing defibrillation to succeed.

Characterization of conduction in the ventricles of normal and heterozygous Cx43 knockout mice using optical mapping

INTRODUCTION

Gap junction channels are important determinants of conduction in the heart and may play a central role in the development of lethal cardiac arrhythmias. The recent development of a Cx43-deficient mouse has raised fundamental questions about the role of specific connexin isoforms in intercellular communication in the heart. Although a homozygous null mutation of the Cx43 gene (Cx43-/-) is lethal, the heterozygous (Cx43+/-) animals survive to adulthood. Reports on the cardiac electrophysiologic phenotype of the Cx43+/- mice are contradictory. Thus, the effects of a null mutation of a single Cx43 allele require reevaluation.

METHODS AND RESULTS

High-resolution video mapping techniques were used to study propagation in hearts from Cx43+/- and littermate control (Cx43+/+) mice. Local conduction velocities (CVs) and conduction patterns were quantitatively measured by determining conduction vectors. We undertook the characterization of ECG parameters and epicardial CVs of normal and Cx43+/- mouse hearts. ECG measurements obtained from 12 Cx43+/+ and 6 Cx43+/- age matched mice did not show differences in any parameter, including QRS duration (14.5 +/- 0.9 and 15.7 +/- 2.3 msec for Cx43+/+ and Cx43+/-, respectively). In addition, using a sensitive method of detecting changes in local CV, video images of epicardial wave propagation revealed similar activation patterns and velocities in both groups of mice.

CONCLUSION

A sensitive method that accurately measures local CVs throughout the ventricles revealed no changes in Cx43+/- mice, which is consistent with the demonstration that ECG parameter values in the heterozygous mice are the same as those in wild-type mice.

Nonuniform responses of transmembrane potential during electric field stimulation of single cardiac cells

The response of cellular transmembrane potentials (V(m)) to applied electric fields is a critical factor during electrical pacing, cardioversion, and defibrillation, yet the coupling relationship of the cellular response to field intensity and polarity is not well documented. Isolated guinea pig ventricular myocytes were stained with a voltage-sensitive fluorescent dye, di-8-ANEPPS (10 microM). A green helium-neon laser was used to excite the fluorescent dye with a 15-micrometers-diameter focused spot, and subcellular V(m) were recorded optically during field stimulation directed along the long axis of the cell. The membrane response was measured at the cell end with the use of a 30-ms S1-S2 coupling interval and a 10-ms S2 pulse with strength of up to approximately 500-mV half-cell length potential (field strength x one-half the cell length). The general trends show that 1) the response of V(m) at the cell end occurs in two stages, the first being very rapid (<1 ms) and the second much slower in time scale, 2) the rapid response consists of hyperpolarization when the cell end faces the anode and depolarization when the cell end faces the cathode, 3) the rapid response varies nonlinearly with field strengths and polarity, being relatively larger for the hyperpolarizing responses, and 4) the slower, time-dependent response has a time course that varies in slope with field strength. Furthermore, the linearity of the dye response was confirmed over a voltage range of -280 to +140 mV by simultaneous measurements of optically and electrically recorded V(m). These experimental findings could not be reproduced by the updated, Luo-Rudy dynamic model but could be explained with the addition of two currents that activate outside the physiological range of voltages: a hypothetical outward current that activates strongly at positive potentials and a second current that represents electroporation of the cell membrane.

Field stimulation of isolated chick heart cells: comparison of experimental and theoretical activation thresholds

INTRODUCTION

This study examines the accuracy of using membrane models to predict activation thresholds for chick heart cells during field stimulation.

METHODS AND RESULTS

Activation thresholds were measured experimentally in ten embryonic chick heart cells at 37 degrees C for stimulus durations 0.2 to 40 msec. Activation was assessed by observing the mechanical twitch of the cell. The heart cells ranged in diameter from 15.0 to 26.7 microm. Since the electric field required for activation depends on diameter, the thresholds were expressed as the maximum field-induced transmembrane potential, Vth = 1.5 a Eth, where a is the cell radius and Eth is the strength of the electric field at threshold. A cell model was created using a singular perturbation method and membrane models describing the ionic currents of a heart cell. The study used membrane models of Ebihara and Johnson (1980), Luo and Rudy (1991), Shrier and Clay (1994), and their combinations. The results show that for stimuli longer than 1 msec, theoretical activation thresholds were within one standard deviation of experimental thresholds. For shorter stimuli, the models failed to predict thresholds because of a premature deactivation of the sodium current. The modification of the m gates dynamics, so that they closed with a time constant of 1.4 msec, allowed to predict thresholds for all durations. The root mean square error between experimental and theoretical thresholds was 6.14%.

CONCLUSIONS

The existing membrane models can predict thresholds for field stimulation only for stimuli longer than 1 msec. For shorter stimuli, the models need a more accurate representation of the sodium tail current.

Postshock potential gradients and dispersion of repolarization in cells stimulated with monophasic and biphasic waveforms

INTRODUCTION

Even though the clinical advantage of biphasic defibrillation waveforms is well documented, the mechanisms that underlie this greater efficacy remain incompletely understood. It is established, though, that the response of relatively refractory cells to the shock is important in determining defibrillation success or failure. We used two computer models of an isolated ventricular cell to test the hypothesis that biphasic stimuli cause a more uniform response than the equivalent monophasic shocks, decreasing the likelihood that fibrillation will be reinduced.

METHODS AND RESULTS

Models of reciprocally polarized and uniformly polarized cells were used. Rapid pacing and elevated [K]o were simulated, and either 10-msec rectangular monophasic or 5-msec/5-msec symmetric biphasic stimuli were delivered in the relative refractory period. The effects of stimulus intensity and coupling interval on response duration and postshock transmembrane potential (Vm) were quantified for each waveform. With reciprocal polarization, biphasic stimuli caused a more uniform response than monophasic stimuli, resulting in fewer large gradients of Vm (only for shock strengths < or = 1.25x threshold vs < or = 2.125x threshold) and a smaller dispersion of repolarization (1611 msec2 vs 1835 msec2). The reverse was observed with uniform polarization: monophasic pulses caused a more uniform response than did biphasic stimuli.

CONCLUSION

These results show that the response of relatively refractory cardiac cells to biphasic stimuli is less dependent on the coupling interval and stimulus strength than the response to monophasic stimuli under conditions of reciprocal polarization. Because this may lead to fewer and smaller spatial gradients in Vm, these data support the hypothesis that biphasic defibrillation waveforms will be less likely to reinduce fibrillation. Further, published experimental results correlate to a greater degree with conditions of reciprocal polarization than of uniform polarization, providing indirect evidence that interactions between depolarized and hyperpolarized regions play a role in determining the effects of defibrillation shocks on cardiac tissue.

Deexcitation of cardiac cells

Excitation and deexcitation are fundamental phenomena in the electrophysiology of excitable cells. Both of them can be induced by stimulating a cell with intracellularly injected currents. With extracellular stimulation, deexcitation was never observed; only cell excitation was found. Why? A generic model with two variables (FitzHugh) predicts that an extracellular stimulus can both excite the cell and terminate the action potential (AP). Our experiments with single mouse myocytes have shown that short (2-5 ms) extracellular pulses never terminated the AP. This result agrees with our numerical experiments with the Beeler-Reuter model. To analyze the problem, we exploit the separation of time scales to derive simplified models with fewer equations. Our analysis has shown that the very specific form of the current-voltage (I-V) characteristics of the time-independent potassium current (almost no dependence on voltage for positive membrane potentials) is responsible here. When the shape of the I-V characteristics of potassium currents was modified to resemble that in ischemic tissues, or when the external potassium concentration (K0) is increased, the AP was terminated by extracellular pulses. These results may be important for understanding the mechanisms of defibrillation.

Models of defibrillation of cardiac tissue

Heterogeneities, such as gap junctions, defects in periodical cellular lattices, intercellular clefts and fiber curvature allow one to understand the effect of an electric field in cardiac tissue. They induce membrane potential variations even in the bulk of the myocardium, with a characteristic sawtooth shape. The sawtooth potential, induced by heterogeneities at large scales (tissue strands) can be more easily observed, and lead to stronger effects than the one induced at the cellular level. In the generic model of propagation in cardiac tissue (FitzHugh), 4 mechanisms of defibrillation were found, two mechanisms based on excitation (E(A),E(M)), and two-on de-excitation (D(A),D(M)). The lowest electric field is required by an E(M) mechanism. In the Beeler-Reuter ionic model, mechanism D(M) is impossible. We critically review the experimental basis of the theory and propose new experiments. (c) 1998 American Institute of Physics.

Activation of cardiac tissue by extracellular electrical shocks: formation of 'secondary sources' at intercellular clefts in monolayers of cultured myocytes

This study investigated the activation of cardiac tissue by "secondary sources," which are localized changes of the transmembrane potential (Vm) during the application of strong extracellular electrical shocks far from the shock electrodes, in cultures of neonatal rat myocytes. Cell monolayers with small intercellular clefts (length, 45 to 270 microm; width, 20 to 70 microm [mean+/-SD, 54+/-13 microm]; n = 46) were produced using a technique of directed cell growth. Changes in Vm relative to the action potential amplitude (deltaVm/APA) were measured using a fluorescent voltage-sensitive dye and a 10 x 10 photodiode array. Shocks with voltage gradients of 4 to 18 V/cm were applied across the clefts during either the action potential (AP) plateau or diastole. During the AP plateau, shocks induced secondary sources in the form of localized hyperpolarizations and depolarizations in the regions immediately adjacent to opposite sides of the clefts. The strength of the secondary sources, defined as the difference of deltaVm/APA across a cleft, increased with increasing cleft length or increasing electrical field gradient. For shocks with a gradient of 8.5 V/cm, the estimated critical cleft length necessary to reach a Vm level corresponding to the diastolic threshold of excitation was 171+/-7 microm. Accordingly, shocks with average strength of 8.2 V/cm applied during diastole produced secondary sources that directly excited cells adjacent to the clefts when the cleft length was 196+/-53 microm (n = 14) and that failed when the cleft length was 84+/-23 microm (n = 9, P<.001). The area of earliest excitation in such cases coincided with the area of maximal depolarization induced during the plateau phase. These data suggest that small inexcitable obstacles may contribute to the Vm changes during the application of strong extracellular electrical shocks in vivo.

Spatial changes in transmembrane potential during extracellular electrical shocks in cultured monolayers of neonatal rat ventricular myocytes

This study investigated the role of different types of discontinuities in tissue architecture on the spatial distribution of the transmembrane potential. Specifically, we tested the occurrence of so-called "secondary sources," ie, localized hyperpolarizations and depolarizations during the application of extracellular electrical shocks (EESs). Changes in transmembrane potential relative to action potential amplitude (delta Vm/APA) were measured in patterned cultures of neonatal rat myocytes, stained with voltage-sensitive dye (RH-237), by optical mapping (96-channel photodiode array, 6- to 30-micron resolution) during the application of EES (field strength, 8 to 22 V/cm; duration, 6 ms). Across narrow cell strands (width, 218 +/- 59 [mean +/- SD] microns), EES applied during the relative refractory period produced a linear and symmetrical profile of delta Vm/APA (-65 +/- 23% maximal hyperpolarization versus +64 +/- 15% maximal depolarization). In contrast, the profile of delta Vm/APA was asymmetrical when EESs were applied during the action potential plateau (-95 +/- 32% versus +37 +/- 14%). At high magnification, no secondary sources were observed at the borders between cells. In dense isotropic cell monolayers or in monolayers and strands showing intercellular clefts, secondary sources were frequently observed. Intercellular clefts of the size of one to several myocytes were sufficient to produce secondary sources of the same magnitude as those that elicited action potentials in dense cell strands. There was a close correlation between the location of secondary sources during EES and localized conduction slowing during propagation. Thus, densely packed cultured cell strands behave as an electrical continuum with no secondary sources occurring at cell borders. Small intercellular clefts can create secondary sources of sufficient magnitude to exert a stimulatory effect.