Simultaneous optical mapping of transmembrane potential and intracellular calcium in myocyte cultures

Synchronous force and Ca2+ measurements for repeated characterization of excitation-contraction coupling in human myocardium

Dysfunctional Ca2+ signaling affects the myocardial systole and diastole, may trigger arrhythmia and cause transcriptomic and proteomic modifications in heart failure. Thus, synchronous real-time measurement of Ca2+ and force is essential to investigate the relationship between contractility and Ca2+ signaling and the alteration of excitation-contraction coupling (ECC) in human failing myocardium. Here, we present a method for synchronized acquisition of intracellular Ca2+ and contraction force in long-term cultivated slices of human failing myocardium. Synchronous time series of contraction force and intracellular Ca2+ were used to calculate force-calcium loops and to analyze the dynamic alterations of ECC in response to various pacing frequencies, post-pause potentiation, high mechanical preload and pharmacological interventions in human failing myocardium. We provide an approach to simultaneously and repeatedly investigate alterations of contractility and Ca2+ signals in long-term cultured myocardium, which will allow detecting the effects of electrophysiological or pharmacological interventions on human myocardial ECC.

Optical Mapping of Cardiomyocytes in Monolayer Derived from Induced Pluripotent Stem Cells

Optical mapping is a powerful imaging technique widely adopted to measure membrane potential changes and intracellular Ca2+ variations in excitable tissues using voltage-sensitive dyes and Ca2+ indicators, respectively. This powerful tool has rapidly become indispensable in the field of cardiac electrophysiology for studying depolarization wave propagation, estimating the conduction velocity of electrical impulses, and measuring Ca2+ dynamics in cardiac cells and tissues. In addition, mapping these electrophysiological parameters is important for understanding cardiac arrhythmia mechanisms. In this review, we delve into the fundamentals of cardiac optical mapping technology and its applications when applied to hiPSC-derived cardiomyocytes and discuss related advantages and challenges. We also provide a detailed description of the processing and analysis of optical mapping data, which is a crucial step in the study of cardiac diseases and arrhythmia mechanisms for extracting and comparing relevant electrophysiological parameters.

Methodology for Cross-Talk Elimination in Simultaneous Voltage and Calcium Optical Mapping Measurements With Semasbestic Wavelengths

Most cardiac arrhythmias at the whole heart level result from alteration of cell membrane ionic channels and intracellular calcium concentration ([Ca2+] i ) cycling with emerging spatiotemporal behavior through tissue-level coupling. For example, dynamically induced spatial dispersion of action potential duration, QT prolongation, and alternans are clinical markers for arrhythmia susceptibility in regular and heart-failure patients that originate due to changes of the transmembrane voltage (V m) and [Ca2+] i . We present an optical-mapping methodology that permits simultaneous measurements of the V m - [Ca2+] i signals using a single-camera without cross-talk, allowing quantitative characterization of favorable/adverse cell and tissue dynamical effects occurring from remodeling and/or drugs in heart failure. We demonstrate theoretically and experimentally in six different species the existence of a family of excitation wavelengths, we termed semasbestic, that give no change in signal for one dye, and thus can be used to record signals from another dye, guaranteeing zero cross-talk.

Lights, camera, path splitter: a new approach for truly simultaneous dual optical mapping of the heart with a single camera

Background

Optical mapping of transmembrane voltage and intracellular calcium is a powerful tool for investigating cardiac physiology and pathophysiology. However, simultaneous dual mapping of two fluorescent probes remains technically challenging. We introduce a novel, easy-to-use approach that requires a path splitter, single camera and excitation light to simultaneously acquire voltage and calcium signals from whole heart preparations, which can be applied to other physiological models – including neurons and isolated cardiomyocytes.

Results

Complementary probes were selected that could be excited with a single wavelength light source. Langendorff-perfused hearts (rat, swine) were stained and imaged using a sCMOS camera outfitted with an optical path splitter to simultaneously acquire two emission fields at high spatial and temporal resolution. Voltage (RH237) and calcium (Rhod2) signals were acquired concurrently on a single sensor, resulting in two 384 × 256 images at 814 frames per second. At this frame rate, the signal-to-noise ratio was 47 (RH237) and 85 (Rhod2). Imaging experiments were performed on small rodent hearts, as well as larger pig hearts with sufficient optical signals. In separate experiments, each dye was used independently to assess crosstalk and demonstrate signal specificity. Additionally, the effect of ryanodine on myocardial calcium transients was validated – with no measurable effect on the amplitude of optical action potentials. To demonstrate spatial resolution, ventricular tachycardia was induced –resulting in the novel finding that spatially discordant calcium alternans can be present in different regions of the heart, even when electrical alternans remain concordant. The described system excels in providing a wide field of view and high spatiotemporal resolution for a variety of cardiac preparations.

Conclusions

We report the first multiparametric mapping system that simultaneously acquires calcium and voltage signals from cardiac preparations, using a path splitter, single camera and excitation light. This approach eliminates the need for multiple cameras, excitation light patterning or frame interleaving. These features can aid in the adoption of dual mapping technology by the broader cardiovascular research community, and decrease the barrier of entry into panoramic heart imaging, as it reduces the number of required cameras.

Electronic supplementary material

The online version of this article (10.1186/s42490-019-0024-x) contains supplementary material, which is available to authorized users.

Multimodal on-axis platform for all-optical electrophysiology with near-infrared probes in human stem-cell-derived cardiomyocytes

Combined optogenetic stimulation and optical imaging permit scalable, contact-free high-throughput probing of cellular electrophysiology and optimization of stem-cell derived excitable cells, such as neurons and muscle cells. We report a new "on-axis" configuration (combined single optical path for stimulation and for multiparameter imaging) of OptoDyCE, our all-optical platform for studying human induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs) and other cell types, optically driven by Channelrhodopsin2 (ChR2). This solid-state system integrates optogenetic stimulation with temporally-multiplexed simultaneous recording of membrane voltage (V_m) and intracellular calcium ([Ca²⁺]_i) dynamics using a single photodetector. We demonstrate the capacity for combining multiple spectrally-compatible actuators and sensors, including newer high-performance near-infrared (NIR) voltage probes BeRST1 and Di-4-ANBDQBS, to record complex spatiotemporal responses of hiPSC-CMs to drugs in a high-throughput manner.

On identification of sinoatrial node in zebrafish heart based on functional time series from optical mapping

As a vertebrate cardiovascular model, the zebrafish heart has been used extensively in physiology research to study cardiac development and human cardiac disease. Optical mapping techniques provide an effective approach to record action potential propagation in the zebrafish heart. However, manual analysis of functional time series recorded from optical mapping can be laborious and time consuming. In this paper, a novel pipeline is proposed to assist physiologists in identifying the sinoatrial node (SAN) in zebrafish heart based on functional time series. First, the original optical mapping data are enhanced and averaged. Next, the heart is divided into small regions, and representative average time series are calculated. A 'discretization of derivative' (DoD) process is performed to model physiological similarity between signals. Finally, grouping is done on the DoD transformed representation, which is found to produce physiologically meaningful classification for SAN identification.

Optical Imaging of Cardiac Action Potential

This chapter reviews the major milestones and scientific achievements facilitated by optical imaging of the action potential in the heart over more than four decades since its introduction. We discuss the limitations of this technique, which sometimes are not fully recognized; the unresolved issues, such as motion artifacts, and the newest developments and future directions.

Arrhythmogenic role of the border between two areas of cardiac cell alignment

The goal of this study is to develop experimental and computational models of the excitation transition between areas of cardiac tissue with different anatomical anisotropy. Alignment of seeded neonatal rat cardiomyocytes was achieved with the aid of guiding polymer (PMGI) nanofibers, and two areas with orthogonal alignment were placed into a contact. It was found that the excitation wave crossing border between the areas with different alignment direction experiences substantial perturbation, up to the complete conduction block. In addition to the experimental study, this effect was analyzed computationally using generic FitzHugh-Nagumo reaction-diffusion model. It was shown that the non-monotonous changes of the excitation wave velocity on this boundary may be explained by the source/sink mismatch. Thus, the border may play pro-arrhythmogenic role.

Automatic cycle averaging for denoising approximately periodic spatiotemporal signals

Optical mapping has become a common tool for cardiovascular research, providing high resolution spatiotemporal data of cardiac action potential propagation. However, noise in cardiac optical mapping (COM) data hampers quantitative measurements and analyses. Spatial and temporal filters have been used to increase the signal-to-noise ratio (SNR) of COM data. Although these help reduce noise and increase SNR, they also lower the spatial and temporal resolution of the data, which is not desirable. This paper utilizes the approximate periodicity of COM data to perform denoising by averaging cycles. We consider the entire approximately periodic spatiotemporal (APST) COM data as a concatenation of random samples generated from a deterministic single cycle spatiotemporal signal. The image difference signal (IDS) was defined and calculated to provide "global" periodicity information. Parameters for cycle segmentation, scaling and alignment were estimated based on the IDS. Finally, these parameters were used to segment, align and average cycles from each individual signal, which produces a clean and denoised single cycle spatiotemporal signal. The novel, fully automated pipeline was validated both qualitatively and quantitatively on zebrafish heart optical mapping data.

The role of dye affinity in optical measurements of Cai(2+) transients in cardiac muscle

Previous experiments in cultures of neonatal rat myocytes demonstrated that the shape of Cai(2+) transients measured using high-affinity Ca(2+)-sensitive dyes may be misrepresented. The purpose of this study was to examine the role of dye affinity in Cai(2+) measurements in intact adult cardiac tissue by comparing optical recordings obtained with high- and low-affinity dyes. Experiments were carried out in porcine left ventricular (LV) wedge preparations stained locally by intramural injection via microcapillaries (diameter = 150 μm) with a low-affinity Ca(2+)-sensitive dye Fluo-4FF or Fluo-2LA (nominal Kd, ~7-10 μmol/l), high-affinity dye Rhod-2 (Kd = 0.57 μmol/l), and Fluo-4 or Fluo-2MA (Kd, ~0.4 μmol/l); in addition, tissue was stained with transmembrane potential (Vm)-sensitive dye RH-237. Optical recordings of Vm and Cai(2+) were made using optical fibers (diameter = 325 μm) glued with the microcapillaries. The durations of Cai(2+) transients measured at 50% level of recovery (CaD50) using high-affinity Fluo-4/Fluo-2MA dyes were up to ~81% longer than those measured with low-affinity Fluo-4FF/Fluo-2LA at long pacing cycle lengths (CL). In Fluo-4/Fluo-2MA measurements at long CLs, Cai(2+) transients often (~50% of cases) exhibited slow upstroke rise and extended plateau. In Rhod-2 measurements, CaD50 was moderately longer (up to ~35%) than in Fluo-4FF recordings, but Cai(2+) transient shapes were similar. In all series of measurements, mean action potential duration values were not significantly different (P > 0.05). The delays between Vm and Cai(2+) upstrokes were comparable for low- and high-affinity dyes (P > 0.05). In conclusion, measurements of Cai(2+) transient in ventricular myocardium are strongly affected by the affinity of Ca(2+) dyes. The high-affinity dyes may overestimate the duration and alter the shape of Cai(2+) transients.

A century of optocardiography

In the past decade, optical mapping provided crucial mechanistic insight into electromechanical function and the mechanism of ventricular fibrillation. Therefore, to date, optical mapping dominates experimental cardiac electrophysiology. The first cardiac measurements involving optics were done in the early 1900s using the fast cinematograph that later evolved into methods for high-resolution activation and repolarization mapping and stimulation of specific cardiac cell types. The field of "optocardiography," therefore, emerged as the use of light for recording or interfering with cardiac physiology. In this review, we discuss how optocardiography developed into the dominant research technique in experimental cardiology. Furthermore, we envision how optocardiographic methods can be used in clinical cardiology.

Cardiac electrophysiological imaging systems scalable for high-throughput drug testing

Multi-parametric electrophysiological measurements using optical methods have become a highly valued standard in cardiac research. Most published optical mapping systems are expensive and complex. Although some applications demand high-cost components and complex designs, many can be tackled with simpler solutions. Here, we describe (1) a camera-based voltage and calcium imaging system using a single ‘economy’ electron-multiplying charge-coupled device camera and demonstrate the possibility of using a consumer camera for imaging calcium transients of the heart, and (2) a photodiode-based voltage and calcium high temporal resolution measurement system using single-element photodiodes and an optical fibre. High-throughput drug testing represents an application where system scalability is particularly attractive. Therefore, we tested our systems on tissue exposed to a well-characterized and clinically relevant calcium channel blocker, nifedipine, which has been used to treat angina and hypertension. As experimental models, we used the Langendorff-perfused whole-heart and thin ventricular tissue slices, a preparation gaining renewed interest by the cardiac research community. Using our simplified systems, we were able to monitor simultaneously the marked changes in the voltage and calcium transients that are responsible for the negative inotropic effect of the compound.

The design and use of an optical mapping system for the study of intracardiac electrical signaling

Fluorescent optical mapping of electrically active cardiac tissues provides a unique method to examine the excitation wave dynamics of underlying action potentials. Such mapping can be viewed as a bridge between cellular level and organ systems physiology, e.g., by facilitating the development of advanced theoretical concepts of arrhythmia. We present the design and use of a high-speed, high-resolution optical mapping system composed entirely of "off the shelf" components. The electrical design integrates a 256 element photodiode array with a 16 bit data acquisition system. Proper grounding and shielding at various stages of the design reduce electromagnetic interference. Our mechanical design provides flexibility in terms of mounting positions and applications (use for whole heart or tissue preparations), while maintaining precise alignment between all optical components. The system software incorporates a user friendly graphical user interface, e.g., spatially recorded action potentials can be represented as intensity graphs or in strip chart format. Thus, this system is capable of displaying cardiac action potentials with high spatiotemporal resolution. Results from cardiac action potential mapping with intact mouse hearts are provided. It should be noted that this system could be readily configured to study isolated myocardial biopsies (e.g., isolated ventricular trabeculae). We describe the details of a versatile, user-friendly system that could be employed for a magnitude of study protocols.

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.

Ionic mechanism of shock-induced arrhythmias: role of intracellular calcium

BACKGROUND

Strong electrical shocks can cause focal arrhythmias, the mechanism of which is not well known. Strong shocks have been shown to produce diastolic Ca(i)(2+) increase, which may initiate focal arrhythmias via spontaneous Ca(i)(2+) rise (SCR), activation of inward Na(+)/Ca(2+) exchange current (I(NCX)), and rise in membrane potential (V(m)). It can be hypothesized that this mechanism is responsible for generation of shock-induced arrhythmias.

OBJECTIVE

The purpose of this study was to examine the roles of SCRs and I(NCX) in shock-induced arrhythmias.

METHODS

The occurrence of SCRs during shock-induced arrhythmias was assessed in neonatal rat myocyte cultures.

RESULTS

Simultaneous V(m)-Ca(i)(2+) optical mapping at arrhythmia source demonstrated that V(m) upstrokes always preceded Ca(i)(2+) transients, and V(m)-Ca(i)(2+) delays were not different between arrhythmic and paced beats (5.5 ± 0.9 and 5.7 ± 0.4 ms, respectively, P = .5). Shocks caused gradual rise of diastolic Ca(i)(2+) consistent with membrane electroporation but no significant Ca(i)(2+) rises immediately before V(m) upstrokes. Application of the Ca(i)(2+) chelator BAPTA-AM (10 μmol/L) decreased the duration of shock-induced arrhythmias whereas application of the I(NCX) inhibitor KB-R7943 (2 μmol/L) increased it, indicating that, despite the absence of SCRs, changes in Ca(i)(2+) affected arrhythmias. It is hypothesized that this effect is mediated by Ca(i)(2+) inhibition of outward I(K1) current and destabilization of resting V(m). The possible role of I(K1) was supported by application of the I(K1) inhibitor BaCl(2) (0.2 mmol/L), which increased the arrhythmia duration.

CONCLUSION

Shock-induced arrhythmias in neonatal rat myocyte monolayers are not caused by SCRs and inward I(NCX). However, these arrhythmias depend on Ca(i)(2+) changes, possibly via Ca(i)(2+)-dependent modulation of outward I(K1) current.

Single-detector simultaneous optical mapping of V(m) and [Ca(2+)](i) in cardiac monolayers

Simultaneous mapping of transmembrane voltage (V(m)) and intracellular Ca(2+) concentration (Ca(i)) has been used for studies of normal and abnormal impulse propagation in cardiac tissues. Existing dual mapping systems typically utilize one excitation and two emission bandwidths, requiring two photodetectors with precise pixel registration. In this study we describe a novel, single-detector mapping system that utilizes two excitation and one emission band for the simultaneous recording of action potentials and calcium transients in monolayers of neonatal rat cardiomyocytes. Cells stained with the Ca(2+)-sensitive dye X-Rhod-1 and the voltage-sensitive dye Di-4-ANEPPS were illuminated by a programmable, multicolor LED matrix. Blue and green LED pulses were flashed 180° out of phase at a rate of 488.3 Hz using a custom-built dual bandpass excitation filter that transmitted blue (482 ± 6 nm) and green (577 ± 31 nm) light. A long-pass emission filter (>605 nm) and a 504-channel photodiode array were used to record combined signals from cardiomyocytes. Green excitation yielded Ca(i) transients without significant crosstalk from V(m). Crosstalk present in V(m) signals obtained with blue excitation was removed by subtracting an appropriately scaled version of the Ca(i) transient. This method was applied to study delay between onsets of action potentials and Ca(i) transients in anisotropic cardiac monolayers.

Multiparametric optical mapping of the Langendorff-perfused rabbit heart

Optical imaging and fluorescent probes have significantly advanced research methodology in the field of cardiac electrophysiology in ways that could not have been accomplished by other approaches¹. With the use of the calcium- and voltage-sensitive dyes, optical mapping allows measurement of transmembrane action potentials and calcium transients with high spatial resolution without the physical contact with the tissue. This makes measurements of the cardiac electrical activity possible under many conditions where the use of electrodes is inconvenient or impossible¹. For example, optical recordings provide accurate morphological changes of membrane potential during and immediately after stimulation and defibrillation, while conventional electrode techniques suffer from stimulus-induced artifacts during and after stimuli due to electrode polarization¹.

The Langendorff-perfused rabbit heart is one of the most studied models of human heart physiology and pathophysiology. Many types of arrhythmias observed clinically could be recapitulated in the rabbit heart model. It was shown that wave patterns in the rabbit heart during ventricular arrhythmias, determined by effective size of the heart and the wavelength of reentry, are very similar to that in the human heart². It was also shown that critical aspects of excitation-contraction (EC) coupling in rabbit myocardium, such as the relative contribution of sarcoplasmic reticulum (SR), is very similar to human EC coupling³. Here we present the basic procedures of optical mapping experiments in Langendorff-perfused rabbit hearts, including the Langendorff perfusion system setup, the optical mapping systems setup, the isolation and cannulation of the heart, perfusion and dye-staining of the heart, excitation-contraction uncoupling, and collection of optical signals. These methods could be also applied to the heart from species other than rabbit with adjustments to flow rates, optics, solutions, etc.

Two optical mapping systems are described. The panoramic mapping system is used to map the entire epicardium of the rabbit heart^4-7. This system provides a global view of the evolution of reentrant circuits during arrhythmogenesis and defibrillation, and has been used to study the mechanisms of arrhythmias and antiarrhythmia therapy^8,9. The dual mapping system is used to map the action potential (AP) and calcium transient (CaT) simultaneously from the same field of view^10-13. This approach has enhanced our understanding of the important role of calcium in the electrical alternans and the induction of arrhythmia^14-16.

Single-sensor system for spatially resolved, continuous, and multiparametric optical mapping of cardiac tissue

Background

Simultaneous optical mapping of multiple electrophysiologically relevant parameters in living myocardium is desirable for integrative exploration of mechanisms underlying heart rhythm generation under normal and pathophysiologic conditions. Current multiparametric methods are technically challenging, usually involving multiple sensors and moving parts, which contributes to high logistic and economic thresholds that prevent easy application of the technique.

Objective

The purpose of this study was to develop a simple, affordable, and effective method for spatially resolved, continuous, simultaneous, and multiparametric optical mapping of the heart, using a single camera.

Methods

We present a new method to simultaneously monitor multiple parameters using inexpensive off-the-shelf electronic components and no moving parts. The system comprises a single camera, commercially available optical filters, and light-emitting diodes (LEDs), integrated via microcontroller-based electronics for frame-accurate illumination of the tissue. For proof of principle, we illustrate measurement of four parameters, suitable for ratiometric mapping of membrane potential (di-4-ANBDQPQ) and intracellular free calcium (fura-2), in an isolated Langendorff-perfused rat heart during sinus rhythm and ectopy, induced by local electrical or mechanical stimulation.

Results

The pilot application demonstrates suitability of this imaging approach for heart rhythm research in the isolated heart. In addition, locally induced excitation, whether stimulated electrically or mechanically, gives rise to similar ventricular propagation patterns.

Conclusion

Combining an affordable camera with suitable optical filters and microprocessor-controlled LEDs, single-sensor multiparametric optical mapping can be practically implemented in a simple yet powerful configuration and applied to heart rhythm research. The moderate system complexity and component cost is destined to lower the threshold to broader application of functional imaging and to ease implementation of more complex optical mapping approaches, such as multiparametric panoramic imaging. A proof-of-principle application confirmed that although electrically and mechanically induced excitation occur by different mechanisms, their electrophysiologic consequences downstream from the point of activation are not dissimilar.

Simultaneous optical mapping of intracellular free calcium and action potentials from Langendorff perfused hearts

The cardiac action potential (AP) controls the rise and fall of intracellular free Ca2+ (Ca(i)), and thus the amplitude and kinetics of force generation. Besides excitation-contraction coupling, the reverse process where Ca(i) influences the AP through Ca(i)-dependent ionic currents has been implicated as the mechanism underlying QT alternans and cardiac arrhythmias in heart failure, ischemia/reperfusion, cardiac myopathy, myocardial infarction, congenital and drug-induced long QT syndrome, and ventricular fibrillation. The development of dual optical mapping at high spatial and temporal resolution provides a powerful tool to investigate the role of Ca(i) anomalies in eliciting cardiac arrhythmias. This unit describes experimental protocols to map APs and Ca(i) transients from perfused hearts by labeling the heart with two fluorescent dyes, one to measure transmembrane potential (Vm), the other Ca(i) transients. High spatial and temporal resolution is achieved by selecting Vm and Ca(i) probes with the same excitation but different emission wavelengths, to avoid cross-talk and mechanical components.

Novel micropatterned cardiac cell cultures with realistic ventricular microstructure

Systematic studies of cardiac structure-function relationships to date have been hindered by the intrinsic complexity and variability of in vivo and ex vivo model systems. Thus, we set out to develop a reproducible cell culture system that can accurately replicate the realistic microstructure of native cardiac tissues. Using cell micropatterning techniques, we aligned cultured cardiomyocytes at micro- and macroscopic spatial scales to follow local directions of cardiac fibers in murine ventricular cross sections, as measured by high-resolution diffusion tensor magnetic resonance imaging. To elucidate the roles of ventricular tissue microstructure in macroscopic impulse conduction, we optically mapped membrane potentials in micropatterned cardiac cultures with realistic tissue boundaries and natural cell orientation, cardiac cultures with realistic tissue boundaries but random cell orientation, and standard isotropic monolayers. At 2 Hz pacing, both microscopic changes in cell orientation and ventricular tissue boundaries independently and synergistically increased the spatial dispersion of conduction velocity, but not the action potential duration. The realistic variations in intramural microstructure created unique spatial signatures in micro- and macroscopic impulse propagation within ventricular cross-section cultures. This novel in vitro model system is expected to help bridge the existing gap between experimental structure-function studies in standard cardiac monolayers and intact heart tissues.

Superiority of biphasic over monophasic defibrillation shocks is attributable to less intracellular calcium transient heterogeneity

OBJECTIVES

The purpose of this study was to test the hypothesis that superiority of biphasic waveform (BW) over monophasic waveform (MW) defibrillation shocks is attributable to less intracellular calcium (Ca(i)) transient heterogeneity.

BACKGROUND

The mechanism by which BW shocks have a higher defibrillation efficacy than MW shocks remains unclear.

METHODS

We simultaneously mapped epicardial membrane potential (Vm) and Ca(i) during 6-ms MW and 3-ms/3-ms BW shocks in 19 Langendorff-perfused rabbit ventricles. After shock, the percentage of depolarized area was plotted over time. The maximum (peak) post-shock values (VmP and Ca(i)P, respectively) were used to measure heterogeneity. Higher VmP and Ca(i)P imply less heterogeneity.

RESULTS

The defibrillation thresholds for BW and MW shocks were 288 +/- 99 V and 399 +/- 155 V, respectively (p = 0.0005). Successful BW shocks had higher VmP (88 +/- 9%) and Ca(i)P (70 +/- 13%) than unsuccessful MW shocks (VmP 76 +/- 10%, p < 0.001; Ca(i)P 57 +/- 8%, p < 0.001) of the same shock strength. In contrast, for unsuccessful BW and MW shocks of the same shock strengths, the VmP and Ca(i)P were not significantly different. The MW shocks more frequently created regions of low Ca(i) surrounded by regions of high Ca(i) (post-shock Ca(i) sinkholes). The defibrillation threshold for MW and BW shocks became similar after disabling the sarcoplasmic reticulum (SR) with thapsigargin and ryanodine.

CONCLUSIONS

The greater efficacy of BW shocks is directly related to their less heterogeneous effects on shock-induced SR Ca release and Ca(i) transients. Less heterogeneous Ca(i) transients reduces the probability of Ca(i) sinkhole formation, thereby preventing the post-shock reinitiation of ventricular fibrillation.

Virtual electrodes and the induction of fibrillation in Langendorff-perfused rabbit ventricles: the role of intracellular calcium

A strong premature electrical stimulus (S(2)) induces both virtual anodes and virtual cathodes. The effects of virtual electrodes on intracellular Ca(2+) concentration ([Ca(2+)](i)) transients and ventricular fibrillation thresholds (VFTs) are unclear. We studied 16 isolated, Langendorff-perfused rabbit hearts with simultaneous voltage and [Ca(2+)](i) optical mapping and for vulnerable window determination. After baseline pacing (S(1)), a monophasic (10 ms anodal or cathodal) or biphasic (5 ms-5 ms) S(2) was applied to the left ventricular epicardium. Virtual electrode polarizations and [Ca(2+)](i) varied depending on the S(2) polarity. Relative to the level of [Ca(2+)](i) during the S(1) beat, the [Ca(2+)](i) level 40 ms after the onset of monophasic S(2) increased by 36+/-8% at virtual anodes and 20+/-5% at virtual cathodes (P<0.01), compared with 25+/-5% at both virtual cathode-anode and anode-cathode sites for biphasic S(2). The VFT was significantly higher and the vulnerable window significantly narrower for biphasic S(2) than for either anodal or cathodal S(2) (n=7, P<0.01). Treatment with thapsigargin and ryanodine (n=6) significantly prolonged the action potential duration compared with control (255+/-22 vs. 189+/-6 ms, P<0.05) and eliminated the difference in VFT between monophasic and biphasic S(2), although VFT was lower for both cases. We conclude that virtual anodes caused a greater increase in [Ca(2+)](i) than virtual cathodes. Monophasic S(2) is associated with lower VFT than biphasic S(2), but this difference was eliminated by the inhibition of the sarcoplasmic reticulum function and the prolongation of the action potential duration. However, the inhibition of the sarcoplasmic reticulum function also reduced VFT, indicating that the [Ca(2+)](i) dynamics modulate, but are not essential, to ventricular vulnerability.

Optical measurements of intramural action potentials in isolated porcine hearts using optrodes

BACKGROUND

Measurements of intramural membrane potential (Vm) would greatly increase knowledge of cardiac arrhythmias and defibrillation. Optrodes offer the possibility for three-dimensional Vm mapping, but their signal quality has been inadequate.

OBJECTIVE

The purpose of this work was to improve optrode signal quality and use optrodes to measure intramural distribution of action potentials and shock-induced Vm changes in porcine hearts.

METHODS

Optrodes were made from seven optical fibers 225 or 325 microm in diameter. Fiber ends were polished at a 45 degrees angle, which improved light collection and allowed their insertion without a needle. Fluorescent measurements were performed in isolated porcine hearts perfused with Tyrode's solution or blood using Vm-sensitive dye RH-237 and a 200-W Hg/Xe lamp.

RESULTS

The signal-to-noise ratio for 325-microm fibers was 44 +/- 23 in blood-perfused hearts (n = 5) and 106 +/- 45 in Tyrode's-perfused hearts (n = 3), which represents an approximately four-fold improvement over previously reported data. There was close correspondence between optical and electrical measurements of activation times and action potential duration (APD). No significant intramural APD gradients were observed at cycle lengths up to 4 s and in the presence of dofetilide or d-sotalol. Application of shocks (5-50 V/cm) produced large intramural Vm changes (up to approximately 200% action potential amplitude), possibly reflecting a combined effect of tissue discontinuities and optrode geometry.

CONCLUSIONS

A substantial improvement of optrode signal quality was achieved. Optical measurements of APD and activation times matched electrical measurements. Optrode measurements revealed no significant intramural APD gradients. Application of shocks caused large intramural Vm changes that could be influenced by the optrode geometry.

Interaction between spiral and paced waves in cardiac tissue

For prevention of lethal arrhythmias, patients at risk receive implantable cardioverter-defibrillators, which use high-frequency antitachycardia pacing (ATP) to convert tachycardias to a normal rhythm. One of the suggested ATP mechanisms involves paced-induced drift of rotating waves followed by their collision with the boundary of excitable tissue. This study provides direct experimental evidence of this mechanism. In monolayers of neonatal rat cardiomyocytes in which rotating waves of activity were initiated by premature stimuli, we used the Ca(2+)-sensitive indicator fluo 4 to observe propagating wave patterns. The interaction of the spiral tip with a paced wave was then monitored at a high spatial resolution. In the course of the experiments, we observed spiral wave pinning to local heterogeneities within the myocyte layer. High-frequency pacing led, in a majority of cases, to successful termination of spiral activity. Our data show that 1) stable spiral waves in cardiac monolayers tend to be pinned to local heterogeneities or areas of altered conduction, 2) overdrive pacing can shift a rotating wave from its original site, and 3) the wave break, formed as a result of interaction between the spiral tip and a paced wave front, moves by a paced-induced drift mechanism to an area where it may become unstable or collide with a boundary. The data were complemented by numerical simulations, which was used to further analyze experimentally observed behavior.

Optical mapping of V(m) and Ca(i)(2+) in a model of arrhythmias induced by local catecholamine application in patterned cell cultures

Catecholamines are known to provoke cardiac arrhythmias, but important aspects such as localization of the arrhythmia source in multicellular tissue and exact ionic mechanisms are not well-known. In this work, a multicellular model of arrhythmias caused by local epinephrine application was developed; V (m) and Ca(i)(2+) changes at the arrhythmia source were measured using fluorescent dyes and high-resolution optical mapping. Cultured strands of neonatal rat myocytes (width approximately 0.4 mm) were produced by patterned growth. Epinephrine (1 micromol/l) was applied over an area of 0.3-0.6 mm via two micropipettes, and strands were stimulated by burst pacing. Local epinephrine application caused triggered arrhythmias with cycle lengths of 202-379 ms and duration of >10 s in 9 out of 16 preparations. Optical V(m) mapping demonstrated that in 78% of cases, the source of arrhythmia was located at the boundary of the locally perfused area. Staining with Ca(i)(2+)-sensitive dye Fluo-4 prevented arrhythmia induction in most cases (85%) likely due to Ca(2+) buffering by the dye. Optical Ca(i)(2+) mapping revealed non-propagated Ca(i)(2+) oscillations at the boundary of the locally perfused area in 45% cases. In conclusion, we developed a new model of catecholamine-dependent arrhythmias allowing mapping of V(m) and Ca(i)(2+) at the arrhythmia source with microscopic resolution. The arrhythmias typically originated from the boundary of the epinephrine-perfused area. The location of the arrhythmia source correlated with localized Ca(i)(2+) oscillations suggesting that arrhythmias were caused by Ca(i)(2+) overload at these locations.

Simultaneous optical imaging of membrane potential and intracellular calcium

Imaging of membrane potential (Vm) and intracellular calcium (Cai2+) is important for studying mechanisms of cardiac excitation, arrhythmias and defibrillation. We have developed an optical technique for simultaneous mapping of Vm and Cai2+ in cultured cell monolayers using fluorescent Vm- and Ca2+-sensitive dyes. Cultures of neonatal rat myocytes were double-stained with dyes RH-237 (Vm) and an analog of Fluo-3 or Rhod-2 dyes (Ca2+). These dyes have overlapping excitation spectra, allowing simultaneous excitation at the same wavelength range, and separate emission spectra allowing division of the fluorescent light into components sensitive to Vm and Cai2+, which were measured with two 16x16 photodiode arrays. It was found that Cai2+ measurements were strongly dependent on the properties of Ca2+-sensitive dyes. Thus, high-affinity Ca2+ dyes such as Fluo-4 and Rhod-2 reported Cai2+ transients approximately twice as long as those reported by low-affinity dyes Fluo-4FF and Rhod-FF. In addition, dyes with different affinities resulted in different measurements of Cai2+ responses to electrical shocks. When shocks were applied during the early plateau phase of the action potential, low-affinity dyes reported transient Cai2+ decreases at sites of both negative and positive Vm changes. In contrast, high-affinity Ca2+ dyes reported only a negligible change of plateau Cai2+ and a large elevation of diastolic Cai2+. These discrepancies between high- and low-affinity dyes were explained by a model of dye-ion interaction, which indicated that apparently long Cai2+ transients and Cai2+ responses to electrical shocks measured with high-affinity dyes were due to their non-linear response. These results indicate that optical measurements of Cai2+ transient duration and shock-induced Cai2+ changes require the use of low-affinity Cai2+ dyes.

New near-infrared optical probes of cardiac electrical activity

Styryl voltage-sensitive dyes (e.g., di-4-ANEPPS) have been widely and successfully used as probes for mapping membrane potential changes in cardiac cells and tissues. However, their utility has been somewhat limited because their excitation wavelengths have been restricted to the 450- to 550-nm range. Longer excitation/emission wavelength probes can minimize interference from endogenous chromophores and, because of decreased light scattering and lower absorption by endogenous chromophores, improve recording from deeper tissue layers. In this article, we report efforts to develop new potentiometric styryl dyes that have excitation wavelengths ranging above 700 nm and emission spectra extending to 900 nm. Three dyes for cardiac optical mapping were investigated in depth from several hundred dyes containing 47 variants of the styryl chromophores. Absorbance and emission spectra in ethanol and multilamellar vesicles, as well as voltage-dependent spectral changes in a model lipid bilayer, have been recorded for these dyes. Optical action potentials were recorded in typical cardiac tissues (rat, guinea pig, pig) and compared with those of di-4-ANEPPS. The voltage sensitivities of the fluorescence of these new potentiometric indicators are as good as those of the widely used ANEP series of probes. In addition, because of molecular engineering of the chromophore, the new dyes provide a wide range of dye loading and washout time constants. These dyes will enable a series of new experiments requiring the optical probing of thick and/or blood-perfused cardiac tissues.

Macroscopic optical mapping of excitation in cardiac cell networks with ultra-high spatiotemporal resolution

Optical mapping of cardiac excitation using voltage- and calcium-sensitive dyes has allowed a unique view into excitation wave dynamics, and facilitated scientific discovery in the cardiovascular field. At the same time, the structural complexity of the native heart has prompted the design of simplified experimental models of cardiac tissue using cultured cell networks. Such reduced experimental models form a natural bridge between single cells and tissue/organ level experimental systems to validate and advance theoretical concepts of cardiac propagation and arrhythmias. Macroscopic mapping (over >1cm(2) areas) of transmembrane potentials and intracellular calcium in these cultured cardiomyocyte networks is a relatively new development and lags behind whole heart imaging due to technical challenges. In this paper, we review the state-of-the-art technology in the field, examine specific aspects of such measurements and outline a rational system design approach. Particular attention is given to recent developments of sensitive detectors allowing mapping with ultra-high spatiotemporal resolution (>5 megapixels/s). Their interfacing with computer platforms to match the high data throughput, unique for this new generation of detectors, is discussed here. This critical review is intended to guide basic science researchers in assembling optical mapping systems for optimized macroscopic imaging with high resolution in a cultured cell setting. The tools and analysis are not limited to cardiac preparations, but are applicable for dynamic fluorescence imaging in networks of any excitable media.

Stimulation of Isolated Ventricular Myocytes Within an Open Architecture Microarray

This paper is concerned with the physiological responses of single heart cells within microfluidic chambers, in response to stimulation by integrated microelectrodes. To enable these investigations, which included the measurement of action potential duration, intracellular Ca2+ and cell shortening, a series of microfluidic chambers (50 microm wide, 180 microm long, 400 microm high, 500 microm pitch) and connecting channels (200 microm wide, 5000 microm long, 50 microm high, 500 microm pitch) were replica-moulded into the silicone elastomer, polydimethylsiloxane (PDMS). The structures were formed against a master of posts and lines, photolithograhically patterned into the high aspect ratio photoresist SU-8. The chambers within the slab of PDMS were aligned against pairs of stimulating gold microelectrodes (50 microm long, 20 microm wide, 0.1-10 microm thick, 180 microm apart) patterned on a microscope coverslip base, thus defining cavities of approximately 4 nL volume. The assembly was filled with physiological saline and single isolated rabbit ventricular myocytes were introduced by micropipetting, thus creating limited volumes of saline above individual myocytes that could be varied between 4 nL and > or = 4 microL. The application of transient current pulses to the cells via the electrodes caused transient contractions with constant amplitude (recorded as changes in sarcomere length), confirming that excitation contraction coupling (EC coupling) remained functional in these limited volumes. Continuous monitoring of the intracellular Ca2+ (using calcium sensitive dyes) showed, that in the absence of bath perfusion, the amplitude of the transients remained constant for approximately 3 min in the 4-nL volume and approximately 20 min for the 4 microL volume. Beyond this time, the cells became unexcitable until the bath was renewed. The action potential duration (APD) was recorded at stimulation frequencies of 1 Hz and 0.5 Hz using potential sensitive dyes and was prolonged at the higher pacing rate. These studies show the prolonged electrical stimulation of isolated adult cardiac myocytes in microchambers with unimpaired EC coupling as verified on optical records of the action potential, Ca2+ transients and cell shortening. The open architecture provided free (pipetting) access for drug dispensation without cross talk between neighboring microwells, and multiplexed optical detection can be realized to study EC coupling on arrays of cells under both control and experimental conditions.

Adaptation of Cardiac Action Potential Durations to Stimulation History with Random Diastolic Intervals

INTRODUCTION

The restitution hypothesis proposes that adaptation of cardiac action potential duration (APD) to rate changes is a predictor of ventricular fibrillation (VF). Conventional restitution kinetics plots the APD of a premature beat as a function of the previous diastolic interval (DI), and VF vulnerability is related to how rapidly APD shortens with decreasing DI. However, APD depends not only on the previous DI but also on the history of previous APDs and DIs. For a comprehensive understanding of APD restitution, we developed a random stimulation protocol and curve fitted each APD with the previous DIs and APDs using multiple autoregressive analyses.

METHODS AND RESULTS

Guinea pig hearts (n = 5) were perfused and stained with di-4 ANEPPS to record optical APs from 252 sites. Activation and repolarization times were detected in real time from one pixel and hearts were stimulated at random DIs (range 0-50 or 0-100 ms). We found that the first, second, and third previous APDs and DIs are required to obtain the best curve fit, which provides the most significant feedback control to APD and up to six previous beats contributed to curve fits (R > 0.8). The coefficients relating the previous DI to APD increased systematically in going from apex to base reflecting the intrinsic gradient of APD across the epicardium.

CONCLUSION

Random restitution is more comprehensive than steady-state restitution, being based on random and dynamic DIs, and makes possible characterization of restitution in only 32 seconds to track changes in restitution during time-varying conditions such as ischemia/reperfusion.

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 Electrical Shocks on Ca i 2+ and V m in Myocyte Cultures

Changes in intracellular calcium concentration (ΔCa i 2+ ) induced by electrical shocks may play an important role in defibrillation, but high-resolution ΔCa i 2+ measurements in a multicellular cardiac tissue and their relationship to corresponding V m changes (ΔV m ) are lacking. Here, we measured shock-induced ΔCa i 2+ and ΔV m in geometrically defined myocyte cultures. Cell strands (width=0.8 mm) were double-stained with V m -sensitive dye RH-237 and a low-affinity Ca i 2+ -sensitive dye Fluo-4FF. Shocks (E≈5 to 40 V/cm) were applied during the action potential plateau. Shocks caused transient Ca i 2+ decrease at sites of both negative and positive ΔV m . Similar Ca i 2+ changes were observed in an ionic model of adult rat myocytes. Simulations showed that the Ca i 2+ decrease at sites of ΔV + m was caused by the outward flow of I CaL and troponin binding; at sites of ΔV − m it was caused by inactivation of I CaL combined with extrusion by Na–Ca exchanger and troponin binding. The important role of I CaL was supported by experiments in which application of nifedipine eliminated Ca i 2+ decrease at ΔV + m sites. Largest ΔCa i 2+ were observed during shocks of ≈10 V/cm causing simple monophasic ΔV m . Shocks stronger than ≈20 V/cm caused smaller ΔCa i 2+ and postshock elevation of diastolic Ca i 2+ . This was paralleled with occurrence of biphasic negative ΔV m that indicated membrane electroporation. Thus, these data indicate that shocks transiently decrease Ca i 2+ at sites of both ΔV − m and ΔV + m . Outward flow of I CaL plays an important role in Ca i 2+ decrease in the ΔV + m areas. Very strong shocks caused smaller negative ΔCa i 2+ and postshock elevation of diastolic Ca i 2+ , likely caused by membrane electroporation.

Development of an optrode for intramural multisite optical recordings of Vm in the heart

INTRODUCTION

Optical mapping of transmembrane potential (Vm) is an important tool in the investigation of impulse propagation in the heart. It provides valuable information about spatiotemporal changes of Vm that cannot be obtained by other techniques, but it presently is limited to measurements from the heart surfaces. Therefore, the goal of this work was to develop a technique for intramural multisite optical measurements of Vm using fiberoptic technology.

METHODS AND RESULTS

An optrode, a bundle of thin optical fibers, was developed for measuring intramural optical signals at multiple sites in the heart. The optrode consisted of seven fibers with diameter of 225 microm arranged in a hexagonal pattern that were used to deliver excitation light to the myocardium, to collect the emitted fluorescence, and to project the light onto a 16 x 16 array of photodiode detectors. Rabbit hearts were stained with the Vm-sensitive dye RH-237. Fluorescence was excited using a 100-W Hg lamp. Intramural action potentials were recorded at multiple sites separated by 2 mm inside the left ventricle. Signal-to-noise (RMS) ratio was 21.2 +/- 12 (n = 7) without averaging or ratiometry and with negligible cross-talk (<1.9%) between the neighboring photodiodes. The size of the recording area for an individual fiber was estimated at approximately 0.8 mm.

CONCLUSION

These data demonstrate feasibility of multisite transmural measurements of Vm without signal averaging and ratiometry. This technique might become useful in studies of transmural impulse conduction during arrhythmias and defibrillation.

Optical mapping of transmural activation induced by electrical shocks in isolated left ventricular wall wedge preparations

INTRODUCTION

It is believed that electrical shocks interrupt fibrillation by directly stimulating the bulk of ventricular myocardium in excitable states, but how shocks activate intramural tissue layers is not known. In this study, Vm responses and transmural activation patterns induced by shocks during diastole were measured in isolated coronary perfused preparations of porcine left ventricle.

METHODS AND RESULTS

Rectangular shocks (duration = 10 ms; field strength, E = 1-44 V/cm) were applied across preparations (thickness = 14.9 +/- 2.5 mm, n = 9) via large mesh electrodes during diastole or action potential (AP) plateau. Vm responses at the transmural surface were measured using optical mapping technique (resolution = 1.2 mm). Depending on shock strength, three types of Vm responses were observed. (1) Weak shocks (E approximately 1-4 V/cm) applied in diastole induced APs with simple monophasic upstrokes. The latency and time of transmural activation (TTA) rapidly decreased with increasing shock strength. Earliest activation occurred predominantly at the cathodal side of preparations in the areas that exhibited maximal DeltaVm during AP plateau. (2) Intermediate shocks (E approximately 4-23 V/cm) induced monophasic and biphasic upstrokes that were paralleled with predominantly negative plateau DeltaVm. Activation was initiated at multiple transmural sites and rapidly spread across the myocardial wall (TTA = 0.6 +/- 0.2 ms). (3) Very strong shocks (E approximately 23-44 V/cm) could cause triphasic upstrokes, likely reflecting occurrence of membrane electroporation, and delayed activation (TTA = 6.7 +/- 3.8 ms) at sites of largest negative plateau DeltaVm.

CONCLUSION

Shocks applied during diastole cause direct and rapid (within 1 ms) activation of ventricular bulk over a wide range of shock strengths, supporting the excitatory hypothesis of defibrillation. Very strong shocks can cause multiphasic Vm responses and delayed activation.

Fluorescence imaging of electrical activity in cardiac cells using an all-solid-state system

Tracking spatial and temporal determinants of cardiac arrhythmogenesis at the cellular level presents challenges to the optical mapping techniques employed. In this paper, we describe a compact system combining two nontraditional low-cost solutions for excitation light sources and emission filters in fluorescence measurements of transmembrane potentials, Vm, or intracellular calcium, [Ca2+]i in cardiac cell networks. This is the first reported use of high-power blue and green light emitting diodes (LEDs), to excite cell monolayers stained with Vm - (di-8-ANEPPS) or [Ca2+]i - (Fluo-3) sensitive dyes. In addition, we use simple techniques for fabrication of suitable thin emission filters with uniform properties, no auto-fluorescence, high durability and good flexibility for imaging Vm or [Ca2+]i. The battery-operated LEDs and the fabricated emission filters, integrated with a fiber-optic system for contact fluorescence imaging, were used as tools to characterize conduction velocity restitution at the macro-scale. The versatility of the LEDs for illumination is further emphasized through 1) demonstration of their usage for epi-illumination recordings at the single-cell level, and 2) demonstration of their unique high-frequency light modulation ability. The LEDs showed excellent stability as excitation light sources for fluorescence measurements; acceptable signal-to-noise ratio and negligible cell photodamage and indicator dye photobleaching were observed.

Combined phase singularity and wavefront analysis for optical maps of ventricular fibrillation

Much of the research into the mechanisms of ventricular fibrillation (VF) employs high-resolution mapping of electrical activation and recovery patterns. We previously developed a method for analyzing electrically mapped VF patterns that was based on identifying individual VF wavefronts. We now introduce a related method designed to take into account the information on repolarization that is present in optically mapped VF data. The new method first converts raw fluorescence data to an angular variable that tracks the phase of the mapped tissue through the depolarization-repolarization cycle. We define wavefronts in this context as isolines of phase that terminate either at boundaries or at singular points within the phase field. These singularities are the pivots of functional reentry and are important determinants of VF patterns. We parameterize VF by constructing data structures that describe wavefronts and singularities and also maintain wavefront-wavefront, wavefront-singularity, and singularity-singularity relationships. We describe one important application of this parameterization, which is to identify, localize, and characterize the importance of occurrences of propagation block during VF.

Nonlinear changes of transmembrane potential during electrical shocks: role of membrane electroporation

Defibrillation shocks induce nonlinear changes of transmembrane potential (DeltaVm) that determine the outcome of defibrillation. As shown earlier, strong shocks applied during action potential plateau cause nonmonotonic negative DeltaVm, where an initial hyperpolarization is followed by Vm shift to a more positive level. The biphasic negative DeltaVm can be attributable to (1) an inward ionic current or (2) membrane electroporation. These hypotheses were tested in cell cultures by measuring the effects of ionic channel blockers on DeltaVm and measuring uptake of membrane-impermeable dye. Experiments were performed in cell strands (width approximately 0.8 mm) produced using a technique of patterned cell growth. Uniform-field shocks were applied during the action potential plateau, and DeltaVm was measured by optical mapping. Shock-induced negative DeltaVm exhibited a biphasic shape starting at a shock strength of approximately 15 V/cm when estimated peak DeltaV-m was approximately -180 mV; positive DeltaVm remained monophasic. Application of a series of shocks with a strength of 23+/-1 V/cm resulted in uptake of membrane-impermeable dye propidium iodide. Dye uptake was restricted to the anodal side of strands with the largest negative DeltaVm, indicating the occurrence of membrane electroporation at these locations. The occurrence of biphasic negative DeltaVm was also paralleled with after-shock elevation of diastolic Vm. Inhibition of I(f) and I(K1) currents that are active at large negative potentials by CsCl and BaCl2, respectively, did not affect DeltaVm, indicating that these currents were not responsible for biphasic DeltaVm. These results provide evidence that the biphasic shape of DeltaVm at sites of shock-induced hyperpolarization is caused by membrane electroporation.

Spontaneous Initiation and Termination of Complex Rhythms in Cardiac Cell Culture

INTRODUCTION

Complex cardiac arrhythmias often start and stop spontaneously. These poorly understood behaviors frequently are associated with pathologic modification of the structural heterogeneity and functional connectivity of the myocardium. To evaluate underlying mechanisms, we modify heterogeneity by varying the confluence of embryonic chick monolayer cultures that display complex bursting behaviors. A simple mathematical model was developed that reproduces the experimental behaviors and reveals possible generic mechanisms for bursting dynamics in heterogeneous excitable systems.

METHODS AND RESULTS

Wave propagation was mapped in embryonic chick myocytes monolayers using calcium-sensitive dyes. Monolayer confluence was varied by plating cultures with different cell densities and by varying times in culture. At high plating densities, waves propagate without breaks, whereas monolayers plated at low densities display spirals with frequent breaks and irregular activation fronts. Monolayers at intermediate densities display bursting rhythms in which there is paroxysmal starting and stopping of spiral waves of activity. Similar spatiotemporal patterns of activity were also observed as a function of the time in culture; irregular activity dominates the first 30 hours, followed by repetitive bursting dynamics until 54 hours, after which periodic target patterns or stable spirals prevail. In some quiescent cultures derived from older embryos, it was possible to trigger pacemaker activity following a single activation. We are able to reproduce all of these behaviors by introducing spatial heterogeneity and varying neighborhood size, equivalent to cell connectivity, in a spontaneous cellular automaton model containing a rate-dependent fatigue term.

CONCLUSION

We observe transitions from irregular propagating waves, to spiral waves that spontaneously start and stop, to target waves originating from localized pacemakers in cell culture and a simple theoretical model of heterogeneous excitable media. The results show how physiologic properties of spontaneous activity, heterogeneity, and fatigue can give rise to a wide range of different complex dynamic behaviors similar to clinically observed cardiac arrhythmias.

Reentrant waves in a ring of embryonic chick ventricular cells imaged with a Ca2+ sensitive dye

According to the classic model initially formulated by Mines, reentrant cardiac arrhythmias may be associated with waves circulating in a ring geometry. This study was designed to study the dynamics of reentry in a ring geometry of cardiac tissue culture. Reentrant calcium waves in rings of cultured embryonic chick cardiac myocytes were imaged using a macroscope to monitor the fluorescence of intracellular Calcium Green-1 dye. The rings displayed a variety of stable rhythms including pacemaker activity and spontaneous reentry. Waves originating from a localized pacemaker could lead to reentry as a consequence of unidirectional block. In addition, more complex patterns were observed due to the interactions between reentrant and pacemaker rhythms. These rhythms included instances in which pacemakers accelerated the reentrant rhythm, and instances in which the excitation was blocked in the vicinity of pacemakers. During reentrant activity an appropriately timed electrical stimulus could induce resetting of activity or cause complete annihilation of the propagating waves. This experimental preparation reveals many spontaneously occuring complex rhythms. These complex rhythms are hypothesized to reflect interactions between spontaneous pacemakers, wave propagation, refractory period, and overdrive suppression. This preparation may serve as a useful model system to further investigate complex dynamics arising during reentrant rhythms in cardiac tissue.

Emission ratiometry for simultaneous calcium and action potential measurements with coloaded dyes in rabbit hearts: reduction of motion and drift

INTRODUCTION

Optical measurements of the cardiac calcium transient (Ca) and transmembrane action potential (AP) may be performed simultaneously with emission ratiometry to lessen motion artifacts and photobleaching effects. We examined changes in emission spectrum in perfused rabbit hearts coloaded with Rh237 and a green-emitting Ca dye (Fluo-4 or Oregon Green BAPTA 1) to determine wavelength bands for emission ratiometry and to test whether ratiometry reduces motion artifacts and drift.

METHODS AND RESULTS

A 488-nm laser illuminated hearts while a spectrofluorometer collected fluorescence from 489 to 838 nm at 1 kHz. Ratiometry with the Ca- and AP-insensitive emission band 663 to 685 nm (IS) as denominator and the Ca-sensitive band 510 to 532 nm as numerator lessened motion artifacts, which was quantified as a 1.4-fold increase in relative amplitude of the Ca (P < 0.05). Ratiometry with the AP-sensitive band 772 to 794 nm as denominator and the IS as numerator produced a 1.7-fold increase in relative amplitude of the AP (P < 0.05). The ratiometry decreased photobleaching-dependent drift by a factor of 0.6 (P < 0.05) for Ca and 0.45 (P < 0.05) for AP.

CONCLUSION

Simultaneous Ca and AP emission ratiometry reduces motion artifacts and drift in hearts with coloaded dyes.

Intramural virtual electrodes during defibrillation shocks in left ventricular wall assessed by optical mapping of membrane potential

BACKGROUND

It is believed that defibrillation is due to shock-induced changes of transmembrane potential (DeltaV(m)) in the bulk of ventricular myocardium (so-called virtual electrodes), but experimental proof of this hypothesis is absent. Here, intramural shock-induced DeltaV(m) were measured for the first time in isolated preparations of left ventricle (LV) by an optical mapping technique.

METHODS AND RESULTS

LV preparations were excised from porcine hearts (n=9) and perfused through a coronary artery. Rectangular shocks (duration 10 ms, field strength E approximately 2 to 50 V/cm) were applied across the wall during the action potential plateau by 2 large electrodes. Shock-induced DeltaV(m) were measured on the transmural wall surface with a 16x16 photodiode array (resolution 1.2 mm/diode). Whereas weak shocks (E approximately 2 V/cm) induced negligible DeltaV(m) in the wall middle, stronger shocks produced intramural DeltaV(m) of 2 types. (1) Shocks with E>4 V/cm produced both positive and negative intramural DeltaV(m) that changed their sign on changing shock polarity, possibly reflecting large-scale nonuniformities in the tissue structure; the DeltaV(m) patterns were asymmetrical, with DeltaV-(m)>DeltaV+(m). (2) Shocks with E>34 V/cm produced predominantly negative DeltaV(m) across the whole transmural surface, independent of the shock polarity. These relatively uniform polarizations could be a result of microscopic discontinuities in tissue structure.

CONCLUSIONS

Strong defibrillation shocks induce DeltaV(m) in the intramural layers of LV. During action potential plateau, intramural DeltaV(m) are typically asymmetrical (DeltaV-(m)>DeltaV+(m)) and become globally negative during very strong shocks.

Initiation and propagation of ectopic waves: insights from an in vitro model of ischemia-reperfusion injury

The objective of the present study was to directly visualize ectopic activity associated with ischemia-reperfusion and its progression to arrhythmia. To accomplish this goal, we employed a two-dimensional network of neonatal rat cardiomyocytes and a recently developed model of localized ischemia-reperfusion. Washout of the ischemia-like solution resulted in tachyarrhythmic episodes lasting 15-200 s. These episodes were preceded by the appearance of multiple ectopic sources and propagation of ectopic activity along the border of the former ischemic zone. The ectopic sources exhibited a slow rise in diastolic calcium, which disappeared upon return to the original pacing pattern. Border zone propagation of ectopic activity was followed by its escape into the surrounding control network, generating arrhythmias. Together, these observations suggest that upon reperfusion, a distinct layer, which consists of ectopically active, poorly coupled cells, is formed transiently over an injured area. Despite being neighbored by a conductive and excitable tissue, this transient functional layer is capable of sustaining autonomous waves and serving as a special conductive medium through which ectopic activity can propagate before spreading into the surrounding healthy tissue.

Optical mapping of arrhythmias induced by strong electrical shocks in myocyte cultures

Strong electrical shocks can induce arrhythmias, which might explain why shocks fail to defibrillate. In this work, the localization of arrhythmia source and the relationship with local changes of transmembrane potential (V(m)) were determined in geometrically defined cell cultures using optical mapping technique. Uniform-field shocks with strength (E) of 10 to 50 V/cm were applied across cell strands with width of 0.2 and 0.8 mm. The threshold for arrhythmia induction was dependent on the strand width: in the 0.8- and 0.2-mm strands, arrhythmias were induced at E>/=20.6+/-1.8 V/cm (n=8) and E>/=30.3+/-1.8 V/cm (n=8), respectively. At the same shock strength, the arrhythmia rate and duration were larger in the wider strands. During shocks that induced arrhythmias, the V(m) waveforms on the anodal side revealed a positive V(m) shift that followed the initial large hyperpolarization and postshock elevation of the diastolic V(m). These V(m) changes were absent during failed shocks. To determine the localization of the arrhythmia source, arrhythmias were induced in narrow cell strands containing regions of local expansion. Optical mapping of the first extrabeat with a coupling interval of 315+/-60 ms revealed that in the majority of cases (9 out of 13) the source of arrhythmias was localized in the areas of shock-induced hyperpolarization. Thus, (1) induction of postshock arrhythmias, their rate, and their duration strongly depend on the tissue structure; (2) arrhythmia induction coincides with the appearance of a positive V(m) shift in the areas of hyperpolarization; and (3) the source of postshock arrhythmias is located in the areas of shock-induced hyperpolarization.

Mapping action potentials and calcium transients simultaneously from the intact heart

Intracellular calcium handling plays an important role in cardiac electrophysiology. Using two fluorescent indicators, we developed an optical mapping system that is capable of measuring calcium transients and action potentials at 256 recording sites simultaneously from the intact guinea pig heart. On the basis of in vitro measurements of dye excitation and emission spectra, excitation and emission filters at 515 +/- 5 and >695 nm, respectively, were used to measure action potentials with di-4-ANEPPS, and excitation and emission filters at 365 +/- 25 and 485 +/- 5 nm, respectively, were used to measure calcium transients with indo 1. The percent error due to spectral overlap was small when action potentials were measured (1.7 +/- 1.0%, n = 3) and negligible when calcium transients were measured (0%, n = 3). Recordings of calcium transients, action potentials, and isochrone maps of depolarization time and the time of calcium transient onset indicated negligible error due to fluorescence emission overlap. These data demonstrate that the error due to spectral overlap of indo 1 and di-4-ANEPPS is sufficiently small, such that optical mapping techniques can be used to measure calcium transients and action potentials simultaneously in the intact heart.

Localized injury in cardiomyocyte network: a new experimental model of ischemia-reperfusion arrhythmias

We developed a new experimental approach to study the effects of local injury in a multicellular preparation and tested the ability of the method to induce reperfusion arrhythmias in cardiomyocyte monolayers. A small region of injury was created using geometrically defined flows of control and ischemia-like solutions. Calcium transients were acquired simultaneously from injured, control, and border zone cells using fluo 4. Superfusion with the injury solution rapidly diminished the amplitude of calcium transients within the injury zone, followed by cessation of cell beating. Reperfusion caused an immediate tachyarrhythmic response in approximately 17% of experiments, with a wave front propagating from a single cell or small cell cluster within the former injury zone. Inclusion of a gap junction uncoupler (1 mM heptanol) in the injury solution narrowed the functional border and sharply increased the number of ectopic foci and the incidence of reperfusion arrhythmias. The model holds a potential to reveal both micro- and macroscopic features of propagation, conduction, and cell coupling in the normal and diseased myocardium and to serve as a new tool to test antiarrhythmic protocols in vitro.