Published online before print July 17, 2008, doi: 10.1161/CIRCRESAHA.108.176677
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© 2008 American Heart Association, Inc.
Cellular Biology |
Burst Emergence of Intracellular Ca2+ Waves Evokes Arrhythmogenic Oscillatory Depolarization via the Na+–Ca2+ Exchanger
Simultaneous Confocal Recording of Membrane Potential and Intracellular Ca2+ in the Heart
From the Department of Pathology and Cell Regulation, Kyoto Prefectural University of Medicine Graduate School of Medical Science, Japan.
Correspondence to Hideo Tanaka, MD, PhD, Department of Pathology and Cell Regulation, Kyoto Prefectural University of Medicine Graduate School of Medical Science, Kawaramachi-Hirokoji, Kamigyo-Ku, Kyoto 602-8566, Japan. E-mail hideotan{at}koto.kpu-m.ac.jp
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Abstract |
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Intracellular Ca2+ waves (CaWs) of cardiomyocytes are spontaneous events of Ca2+ release from the sarcoplasmic reticulum that are regarded as an important substrate for triggered arrhythmias and delayed afterdepolarizations. However, little is known regarding whether or how CaWs within the heart actually produce arrhythmogenic membrane oscillation because of the lack of data confirming direct correlation between CaWs and membrane potentials (Vm) in the heart. On the hypothesis that CaWs evoke arrhythmogenic oscillatory depolarization when they emerge synchronously and intensively in the heart, we conducted simultaneous fluorescence recording of intracellular Ca2+ ([Ca2+]i) dynamics and Vm of ventricular myocytes on subepicardial surfaces of Langendorff-perfused rat hearts using in situ dual-view, rapid-scanning confocal microscopy. In intact hearts loaded with fluo4/acetoxymethyl ester and RH237 under perfusion with cytochalasin D at room temperature, individual myocytes exhibited Ca2+ transients and action potentials uniformly on ventricular excitation, whereas low-K+–perfused (2.4 mmol/L) hearts exhibited CaWs sporadically between Ca2+ transients without discernible membrane depolarization. Further [Ca2+]i loading of the heart, produced by rapid pacing and addition of isoproterenol, evoked triggered activity and subsequent oscillatory Vm, which are caused by burst emergence of CaWs in individual myocytes. Such arrhythmogenic membrane oscillation was abolished by ryanodine or the Na+–Ca2+ exchanger inhibitor SEA0400, indicating an essential role of CaWs and resultant Na+–Ca2+ exchanger–mediated depolarization in triggered activity. In summary, we demonstrate a mechanistic link between intracellular CaWs and arrhythmogenic oscillatory depolarizations in the heart. Our findings provide a cellular perspective on abnormal [Ca2+]i handling in the genesis of triggered arrhythmias in the heart.
Key Words: Ca2+ wave • delayed afterdepolarization • triggered activity • Na+-Ca2+ exchange • confocal microscopy
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Introduction |
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Intracellular Ca2+ waves (CaWs) in cardiomyocytes are wave-like propagation of spontaneous, local Ca2+-release events from the sarcoplasmic reticulum (SR) within the cells as a result of Ca2+ overload.1–5 Previous electrophysiological studies in isolated ventricular myocytes suggested that CaWs can evoke triggered arrhythmias by generating oscillatory depolarization via Ca2+-activated transient inward currents,5 which consist of the Na+–Ca2+ exchanger (NCX) current6,7 and other Ca2+-activated currents.8,9 When the depolarized membrane potential (Vm) reaches the threshold of firing, myocytes exhibit nondriven action potentials (APs) and subsequent oscillations called triggered activity and delayed afterdepolarizations (DADs), respectively.6,10–12 Although extensive studies have been conducted on CaWs for their electrogenic and proarrhythmic potentials, most studies were focused on isolated cardiomyocytes or small excised preparations for simultaneous microelectrode recording of electric activities and Ca2+-mediated contractile force.5,10,13 Indeed, these studies have established that CaWs evoke membrane depolarization, but it has not been addressed whether or how the spontaneously emerging intracellular CaWs in the heart contribute to generating arrhythmias. This is because it has been impossible to detect single-cell behaviors of Vms and precise intracellular Ca2+ ([Ca2+]i) dynamics simultaneously in multicellular tissues, although the macroscopic relationship between Ca2+ transients (CaTs) and membrane depolarization was demonstrated in perfused hearts.14–16 Considering the spontaneous nature of CaWs,1–5 one concern is that CaWs within multicellular tissues would not effectively produce depolarization in the heart: namely, the electric source in individual myocytes produced by randomly generating CaWs would be absorbed by the surrounding myocardium, generating insufficient depolarization for firing of the heart.6 Nevertheless, we previously found by using real-time confocal microscopy that CaWs in injured rat hearts emerge synchronously among individual myocytes immediately after high-frequency pacing but sporadically and asynchronously during the following unstimulated conditions.17,18 We further demonstrated that burst emergence of CaWs, ie, synchronous and intensive elevations of [Ca2+]i in the heart, evokes CaT with subsequent [Ca2+]i oscillations, a phenomenon reminiscent of triggered activity and DADs, respectively.18 Based on these observations, we postulated that simultaneous recording of CaWs and the corresponding Vm in individual myocytes would reveal a direct arrhythmogenic potential of intracellular CaWs in the heart.
The goals of our present study were: (1) to establish a confocal system for simultaneous detection of subcellular [Ca2+]i dynamics and Vm in individual myocytes of the heart; and (2) to assess the roles of CaWs in the genesis of oscillatory Vm and DADs as a precursor of triggered activity in the heart. The role of NCX in the triggered activity is also discussed.
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Materials and Methods |
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Experiments were conducted on Langendorff-perfused hearts of Wistar rats17–19 (n=36) loaded with both the fluo4/acetoxymethyl ester and RH237 under perfusion with cytochalasin D and at 24 to 25°C for precise measurement of [Ca2+]i dynamics because of the diminished fluorescence intensity (FI) for fluo4 at >30°C. A novel in situ dual-view, rapid-scanning confocal microscope that we developed was applied to the perfused hearts for simultaneous recording of Vm and subcellular [Ca2+]i dynamics of individual myocytes. In some experiments, enzymatically isolated ventricular myocytes were used. As shown in the schematic diagram (Figure 1A), an image-splitting module, Dual-view (Optical Insights), is attached to the confocal system composed of an upright microscope with a x40 objective lens and a spinning-disc confocal scanner (CSU-21; Yokogawa).17–19 Fluo4 and RH237 in the heart were excited by argon/krypton laser with peak wavelengths of 488 and 568 nm, respectively. Fluorescence signals are divided by a dichroic mirror (565 nm) in the image splitter into 2 components, the shorter of which is band-pass-filtered (535/20 nm; Ca2+ array) and the longer, long-pass-filtered (>665 nm; voltage array). The separated fluorescence signals from subepicardial myocardium (120x196 µm) of the left ventricular base are collected by a single charge-coupled device (CCD) camera (386x256 pixels, 14 bits) of a MiCaM02 image acquisition system (Brainvision) at 270 frames per second. The spatial resolutions of the optical system, 0.99 µm (lateral) and 1.18 µm (axial) with the objective lens,19 detected by the CCD camera with a pixel size of 1.67 µm/pixel, are precise enough for analyzing subcellular dynamics of CaWs.
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Fluorescence signals for the voltage array, spatially averaged over the area of individual myocytes, were shown as plot profiles of FI. To clarify the changes in Vm, temporal filtering was performed on the RH237-FI with the first-order exponential filter as averaged by 2 consecutive-frame values with a weighted ratio of 4:1. With this procedure, APs of individual myocytes are clearly identified (Figure 1B), having a noise level of 10.8±3.1% (n=7) over the AP amplitude. Plot profiles for [Ca2+]i dynamics were measured by fluo4-FI from the whole area of each myocyte. Crosstalk between fluorescence signals from the 2 arrays was negligible (Figure 1Ca), and signals from Ca2+ and voltage arrays reflect changes in [Ca2+]i and Vm, respectively (Figure 1Cb). A macroscopic excitation-mapping study was conducted on the whole heart with di-4-ANEPPS by using a fluorescence vital microscope. The NCX inhibitor SEA0400 was provided by Taisho Pharmaceutical Co Ltd.20 Detailed experimental methods are provided in expanded Materials and Methods section in the online data supplement at http://circres.ahajournals.org.
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Results |
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Synchronization of APs and CaTs in Intact Heart
Figure 2a shows simultaneous fluorescence signals for [Ca2+]i and Vm of subepicardial myocytes in the left ventricle. The x-t images from 4 representative myocytes exhibited transient deflections of FI for fluo4 and RH237 on ventricular excitation. Normalized plot profiles corresponding to x-t images revealed that both the CaTs and APs arise nearly simultaneously to the QRS complex, showing regular sinus rhythm at 81 bpm, with almost superimposed waveforms (see also nonnormalized data in Figure I of the online data supplement), indicating spatial and temporal synchronization of [Ca2+]i dynamics and Vm among individual myocytes; APs precede CaTs with a delay of approximately 11 ms (Figure 2b), a value comparable to that for intact myocytes, reflecting a physiological process involved in the excitation–contraction coupling.21 Similar results were obtained in all 15 hearts showing regular sinus rhythm at 76±14 bpm with delay of onset for CaTs ranging from 10 to 26 ms (18.9±8.6 ms). Spatiotemporal synchronization of the APs and CaTs was preserved even at higher excitation rates (supplemental Figure II): both signals emerged synchronously with no discernible beat-to-beat alternans or failure of excitation up to 300 bpm. In addition, higher frequency of excitation shortened durations for APs and CaTs. Thus, our dual-view, rapid-scanning confocal system is validated in its spatiotemporal resolution for detecting both CaTs and APs of myocytes in the heart.
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Apparently Absent Depolarization of Sporadic CaWs in the Heart
Under [Ca2+]i overload, myocytes exhibit spontaneous CaWs asynchronously among individual myocytes in the heart.17,18,22,23 When the heart was [Ca2+]i overloaded by low K+ (2.4 mmol/L) solution, CaWs, propagating along the longitudinal axis of the myocytes, emerged sporadically in between CaTs and asynchronously among myocytes, as shown in the sequential x-y (Figure 3a) and x-t (arrowheads in Figure 3b) images. The corresponding plot profiles of CaWs revealed hump-like elevations of [Ca2+]i (green dots). However, no discernible hump was detected in Vm during occurrence of CaWs. The relative amplitudes for CaWs over CaTs measured 21.3±1.6% (n=10), whereas the amplitudes for Vm fluctuation (
Vm) relative to AP amplitudes were nominal (Figure 3c). Similar observations were made in all six hearts showing spontaneous CaWs with a mean propagation velocity (Vprop) of 87.6±14.3 µm/sec (n=30), values comparable to those previously identified in perfused rat hearts as sporadic CaWs17,22 and in isolated ventricular myocytes.2,3,10 In contrast, enzymatically isolated ventricular myocytes exhibited transient hump-like depolarizations that were accompanied by CaWs (supplemental Figure IIIA). Relative amplitudes of humps for [Ca2+]i and Vm over respective CaTs and APs were 60.4±8.3% and 13.4±4.1% (n=7), respectively (supplemental Figure IIIB), although values of Vprop (94.4±12.1 µm/sec) were similar to those in the whole heart (supplemental Figure IIIC). Thus, tiny but significant membrane depolarizations are accompanied by sporadic CaWs in myocytes under electrically isolated conditions but not within the heart.
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Burst Emergence of CaWs Generates Oscillatory Membrane Depolarization
When the heart was electrically overdriven, the following [Ca2+]i exhibited oscillations. After 30-second pacing at 2.5 Hz under 2.4 mmol/L K+ perfusion, individual myocytes showed oscillations of both the CaW-mediated [Ca2+]i humps and Vm synchronously with successive decline in magnitudes (Figure 4Aa). Such gradual [Ca2+]i reduction would be attributable to progressive extrusion of released Ca2+ from the cell and resulting reduction in Ca2+ loading into the SR. On close observation, spontaneous [Ca2+]i rises emerged nearly simultaneously to hump-like rises of Vm, indicating that the transient depolarization closely relates to CaWs. The relative amplitudes of oscillatory [Ca2+]i and Vm over the respective CaTs and APs decreased sequentially (from the first CaWs to the third ones) after pacing (Figure 4Ab). Conversely, no discernible Vm fluctuation was noted on sporadic CaWs under near-steady-state conditions after an idioventricular excitation (Figure 4Ab and 4Ac). Properties of CaWs were determined by frequency of prior stimulation. With increasing frequency, initial CaWs propagated faster (higher Vprop; Figure 4Ba) and earlier (shorter latency; Figure 4Bb) after stimulation. The amplitudes of fluctuating Vm and [Ca2+]i were also higher after higher-frequency pacing (Figure 4Bc). In sum, oscillatory Vm is strongly influenced by Vprop and latency of CaWs, as well as by amplitude of [Ca2+]i.
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Triggered Activity Is Mediated by CaWs
[Ca2+]i oscillation-mediated Vm fluctuation is known to evoke triggered activity,5,11–13 especially under β-adrenergic stimulation.24 We reasoned that CaWs can evoke excitation in the heart when they emerge intensively and synchronously among individual myocytes. During 2.4 mmol/L K+ perfusion plus isoproterenol (3 nmol/L), myocytes exhibited CaWs frequently in between intrinsic CaTs at 55.5±29.3 bpm with Vprop of 114.2±17.1 µm/sec (n=30), values higher than those for sporadic CaWs.17 After pacing of the heart at 2 Hz and higher, myocytes exhibited a single event of nondriven APs (star symbols) accompanied by a QRS complex in ECG, ie, triggered activity (Figure 5A). The AP triggered after 655-ms latency was followed by oscillatory Vm, ie, DADs. On detailed observation, the slow diastolic depolarization immediately before the AP upstroke appeared to arise nearly simultaneously to [Ca2+]i elevation (Figure 5Ab). In addition, incidence and Vprop of CaWs were dependent on the time after the preceding excitation (Figure 5Ac): after cessation of pacing, CaWs emerged nearly simultaneously among individual myocytes and propagated rapidly, but they gradually emerged less synchronously with slower propagation, resulting in a progressive reduction of [Ca2+]i. These sequential declines were also observed under nontriggered conditions, in which CaWs emerged less intensively with significantly slower Vprop than those under the triggered conditions (x symbols in Figure 5Ac; P<0.05). Comparison of integrated values of [Ca2+]i among 5 different CaWs, ie, the CaWs accompanying triggered activity (TA), 3 consecutive CaWs after pacing (first to third CaW), and sporadic CaWs, clearly indicated that the Ca2+-mediated depolarization is closely related to the amount of the Ca2+ release from the SR (Figure 5B). Furthermore, the latency of triggered beat was shorter after higher-frequency stimulation (Figure 5C). Thus, as CaWs emerge more synchronously and more intensively in the heart, [Ca2+]i increases more quickly and more intensely, leading to arrhythmogenic depolarizations.
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We observed various patterns of triggered activity, from single triggered beat to sustained ventricular tachycardia (VT) showing runs of fixed QRS complexes (Figure 6A). Higher-frequency pacing resulted in higher-amplitude [Ca2+]i fluctuations, leading to generation of triggered activities (b) and VT at 
120 bpm (c). Of 22 hearts examined, 6 hearts showed monomorphic VT with runs of 5 beats or more at 178±87 bpm; 10 hearts, short runs of triggered activities having less than 5 successive beats (n=16); and 6 hearts, no triggered activity. No ventricular fibrillation was induced. Pooled data for CaWs with and without triggered activity or VT (Figure 6B) clearly indicated that both the amplitude of [Ca2+]i hump (relative FI) and Vprop were higher in triggered than in nontriggered cases. Moreover, Vprop in VT was higher than that in short runs of triggered beats. The latency for the first 3 CaWs, reflecting incidence of CaWs, was shorter in triggered than in nontriggered cases (n=6).
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Roles of SR Ca2+ Release and NCX in Triggered Activity and DADs
The CaW-mediated mechanism for triggered activity was also evidenced by pharmacological interventions. Ryanodine (10 µmol/L) abolished triggered activity and subsequent oscillatory Vm, in accordance with abolition of [Ca2+]i oscillation (Figure 7Aa). Similar results were consistently obtained in 5 hearts. Triggered activity and DADs were diminished also by the NCX inhibitor SEA040020 at 3 µmol/L, whereas [Ca2+]i oscillation was preserved or rather augmented in Figure 7Ab (confirmed in all 6 hearts). Of note, even on nontriggered beats, [Ca2+]i oscillation was augmented after SEA0400 application (Figure 7B) in spite of abolition of DADs; sequential decline in [Ca2+]i was relatively inconspicuous as compared with that before SEA application. Pooled data for 6 myocytes revealed that SEA increased Vprop of the initial CaW (Figure 7Bb) and shortened the time for the first 3 consecutive CaWs after stimulation (Figure 7Bc) in spite of the abolition of oscillatory Vm (Figure 7Bd).
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Discussion |
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Recent advancement in cardiac activation mapping technology has enabled simultaneous optical measurements of Vm and [Ca2+]i in arrhythmic whole hearts, where intimate relationship between these 2 signals was elucidated at the macroscopic level.14–16 Although some of these mapping studies suggested a culprit role of Ca2+ in triggered activity,14,16 not yet established was the mechanistic link between the intracellular behaviors of CaWs and concomitant arrhythmogenic Vm oscillation. The present study demonstrated both the [Ca2+]i and Vm of individual ventricular myocytes on the subepicardial surface of the rat heart by using novel dual-view, rapid-scanning confocal microscopy, having high spatial resolution for analyzing subcellular properties of CaWs, comparable to those attained in previous studies by us17–19 and other groups.22,23,25 With this inventive microscopic system, we demonstrated that CaWs propagating within individual myocytes contribute to generating Vm oscillation and triggered activity when they emerge synchronously and intensively in the heart. Furthermore, we confirmed that the CaW-evoked membrane oscillation and triggered activity are mediated by NCX.
Absence of Membrane Depolarization by Sporadic CaWs
The first concern of the present study was whether sporadic CaWs emerging asynchronously among individual myocytes17 effectively depolarize the heart. As expected, CaWs of this type accompanied no discernible depolarization in the heart. The apparently quiescent Vm in sporadic CaWs would be explained by 2 possibilities. First, the slowly propagating, less synchronous CaWs would be insufficient to produce detectable depolarization because the depolarizing currents of the individual myocytes are trivial. This is based on the previous study by Capogrossi et al,10 who measured 5-mV depolarization by microelectrode in isolated rat cardiomyocytes in association with sporadic contractile waves. Although we also detected small hump-like depolarizations in isolated myocytes (supplemental Figure III), such tiny voltage changes could be obscured by the background fluorescence signals of RH237 (ie, low signal-to-noise ratio). Secondly, an ample current source, even if generated by CaWs in individual myocytes, would be diminished by surrounding cells acting electrotonically as current sinks to prevent sufficient depolarization. This would be important for protecting against the arrhythmogenicity of intracellular CaWs in the multicellular tissue.
Burst Emergence of CaWs Evokes Triggered Activity and DADs
Previously, we demonstrated that in locally [Ca2+]i-overloaded myocardium produced by cryoablation, synchronous, intensive CaWs produce oscillatory changes in [Ca2+]i.18 Our second concern was whether such CaWs do generate membrane depolarization leading to triggered arrhythmias. The present study demonstrated that burst emergence of CaWs can be electrogenic in the heart in contrast to sporadic CaWs that arise during prolonged diastolic periods. By rapid pacing under low-K+ perfusion, CaWs emerged in individual myocytes showing synchronous, rapid propagation, with conspicuous oscillatory changes in both [Ca2+]i and Vm. Such oscillations were dependent on the frequency of prior pacing (Figure 4B): higher-frequency pacing exhibited more rapid propagation (ie, higher Vprop) and earlier emergence (ie, shorter latency) of CaWs, both of which indicate greater Ca2+ loading in the cytosol and SR according to the [Ca2+]i load dependence of Vprop and frequency of CaWs.17,26 In addition, higher [Ca2+]i resulted in more marked depolarization concomitant to larger [Ca2+]i oscillation.
Role of SR Ca2+ Release and NCX in Triggered Activity and DADs
We demonstrated that in the heart burst emergence of CaWs often accompanied triggered APs (Figure 5). Such nondriven beats would be triggered by the CaW-mediated membrane depolarization because the [Ca2+]i humps preceding the triggered beats appeared to emerge nearly simultaneously or before the slow diastolic depolarization (Figure 5Ab). This is in good agreement with the direct measurements of Vm and [Ca2+]i by Boyden et al,13 who demonstrated in enzymatically dispersed Purkinje cells that nondriven APs were preceded by CaW-mediated diastolic depolarization. The pacing frequency-dependent shortening of latency for the triggered beat (Figure 5C) also indicates the importance of augmented [Ca2+]i cycling (ie, rapid repriming of Ca2+ released from the SR)27 in triggered activity. Such intensive increase in [Ca2+]i would be attributable to the pacing-induced synchronization of Ca2+ cycling among individual myocytes, not to propagation of CaWs to the surrounding cells: intercellular propagation of CaWs was barely observed. Intensiveness of Ca2+ release estimated by integration of [Ca2+]i also strongly suggests the importance of burst emergence of CaWs in the arrhythmogenic depolarization (Figure 5B). Analogously, 3 indicators for [Ca2+]i loading, ie, Vprop, incidence, and amplitudes of CaWs,17,18,26 were all statistically higher in the presence of accompanying triggered activity than without it (Figures 5Ac and 6B), indicating a definite cause-effect relationship between CaWs and triggered activity.
Abolition by ryanodine of triggered activity and DADs also indicates mediation of CaW in these changes because this agent inhibits Ca2+ release from the SR without diminution of APs.28 Furthermore, blockade of NCX by SEA0400 also attenuated triggered activity and subsequent DADs, whereas the CaW-mediated [Ca2+]i oscillation preserved or augmented its frequency and amplitude (Figure 7B). The preservation of oscillatory [Ca2+]i can be attributable to uncoupling of NCX, ie, inhibition of the trans-sarcolemmal extrusion of Ca2+ and concomitant suppression of Na+ influx. This also indicates that the successive decline of [Ca2+]i is caused by Ca2+ extrusion via NCX. Thus, our results support the notion that NCX current contributes to triggered activity and subsequent DADs.6,7,24,29
Study Limitations and Evaluation of the Experimental Conditions
Among the study limitations, the lower-temperature perfusion and application of cytochalasin D would obviously render the heart nonphysiological. The former intervention would increase the resting [Ca2+]i30 and, in turn, decrease frequency and Vprop of CaWs.31 The latter would attenuate the Ca2+ release from the contractile protein, eg, troponin C, during the relaxation phase,32 although this chemical reportedly increases diastolic [Ca2+]i.33 In addition, low-K+ perfusion would further increase the resting [Ca2+]i because of inhibition of Na+–K+ pump function and enhance the inward NCX current34 by hyperpolarizing shift in the resting Vm (10 mV).35 Isoproterenol would augment Ca2+ cycling, ie, amount of Ca2+ released from and undergoing reuptake into the SR, promoting Ca2+-activated transient inward currents.24,36 Most of these conditions would augment the arrhythmogenic [Ca2+]i oscillations and resultant membrane depolarizations. Although we have no absolute value for [Ca2+]i, we speculate that the observed [Ca2+]i would not be far above the physiological level because (1) myocytes exhibited spatiotemporally uniform CaTs (Figure 2), by which CaWs were annihilated; and (2) high-frequency "agonal" waves17,18 that emerge under highly [Ca2+]i overloaded conditions were barely observed. Besides the [Ca2+]i-loading conditions, low-K+ perfusion would reduce the threshold of triggered activity via reduction of the inward rectifying K+ current (IK1).36 Despite these limitations, our experimental results would not nullify the conclusion that burst emergence of CaWs is responsible for triggered activity in the heart.
Because our experimental conditions rendered the myocytes [Ca2+]i overloaded throughout the heart, CaWs would emerge prevalently in the heart. However, excitation-mapping studies of the whole heart with di-4-ANEPPS revealed that the triggered beat originated not throughout the heart but preferentially from a basal portion of the left ventricle, distinct from the pacing site (supplemental Figure IV). Therefore, it is reasonable to assume that triggered activity is initiated by CaW-mediated regional depolarization and is subsequently propagated throughout the heart. The exact origin of triggered activity is, however, undetermined because we have no information for [Ca2+]i dynamics on the myocardium other than the subepicardial region. Although detailed analysis for the regional difference in [Ca2+]i is beyond the focus of the present study, we speculate that it is initiated inside the ventricular wall. In this respect, Katra and Laurita37 demonstrated that the endocardial surface preferentially initiates triggered activity in transventricular wedge preparations of canine hearts. Such a preferential region for initiating triggered activity may arise from the regionally different expression of NCX and SR,38 as well as IK1,39 in the heart. Although we detected no remarkable cell-to-cell differences in CaT configurations on subepicardial myocytes (Figure 2), we expect that there should be regional differences in [Ca2+]i dynamics in the heart, eg, between subepicardial and subendocardial regions, leading to generating the preferential region of triggered activity.
In conclusion, the present study demonstrates the culprit role of CaWs in the genesis of triggered arrhythmias with mediation of NCX in the heart. Our findings point to an important mechanistic link between the cellular events of abnormal [Ca2+]i handlings of the heart and arrhythmogenesis.
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Acknowledgments |
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We thank Dr Shien-Fong Lin (Krannert Institute of Cardiology, Indiana University School of Medicine) for critical advice on the optical setup and Dr J. Andrew Wasserstrom (Division of Cardiology, Northwestern University, Feinberg School of Medicine) for critical advice on experimental procedures. Dr Hitoshi Yaku (Department of Cardiovascular Surgery, Kyoto Prefectural University of Medicine) is gratefully acknowledged for providing K.F. with the opportunity to work in the laboratory of T.T., and Dr Masaaki Yamagishi is gratefully acknowledged for providing encouragement and advice during the course of the research performed by K.F.
Sources of Funding
This work was supported by grants-in-aid from the Japan Society for the Promotion of Science.
Disclosures
None.
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Footnotes |
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*Both authors contributed equally to this work.
Original received August 1, 2007; resubmission received April 1, 2008; revised resubmission received June 30, 2008; accepted July 8, 2008.
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References |
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