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* Department of Physiology, Medical School of Xi'an Jiaotong University, Xi'an 710061, China; Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224; and National Laboratory of Biomembrane and Membrane Biotechnology, College of Life Science, Peking University, Beijing, China
Correspondence: Address reprint requests to Heping Cheng, PhD, Laboratory of Cardiovascular Science, National Institute on Aging, NIH, 5600 Nathan Shock Dr., Baltimore, MD 21224. Tel.: 410-558-8634; Fax: 410-558-8150; E-mail: chengp{at}grc.nia.nih.gov.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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INTRODUCTION |
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; Cannell et al., 1995; Lipp and Niggli, 1994; Lopez-Lopez et al., 1995), brain (Haak et al., 2001), skeletal, and smooth muscle (Klein et al., 1996; Nelson et al., 1995; Tsugorka et al., 1995). Over the last decade, extensive studies have focused on morphometrics or anatomy of Ca2+ sparks (as visualized by confocal fluorescent microscopy), which are essential to elucidation of the exact nature of Ca2+ sparks and delineation of the spatial and temporal scope of spark-driven local Ca2+ signaling. Ironically, intrinsic properties of Ca2+ sparks continue to be a matter of debate. A major confounding factor is that Ca2+ sparks usually occur at random locations in relation to the focal volume under observation. As a result, those at out-of-focus positions will display reduced amplitude, broadened spatial spreading, and blunted kinetics, with the degree of distortion depending on the distance from the confocal volume (Pratusevich and Balke, 1996; Smith et al., 1998). Indeed, both confocal sampling theory and experiments aided with an automated spark detection algorithm have shown that the apparent spark amplitude always obeys a monotonically decaying distribution (modified by the detectability function), regardless of the true spark amplitude (Cheng et al., 1999).
Several approaches have been developed to curtail or correct for the effects of optical blurring on certain aspects of Ca2+ spark properties. In an effort to restore true population statistics of spark amplitude, Izu et al. and Rios et al. have attempted to deconvolve the optical blurring from the apparent spark amplitude distribution (Izu et al., 1998; Rios et al., 2001). We measured the release duration of Ca2+ sparks when it became relatively insensitive to optical blurring by limiting Ca2+ sparks in space and time with a nonfluorescent, slow Ca2+ buffer (Wang et al., 2002). Many investigators have exploited spark repeats from fixed release units that underwent an unusual, high-frequency spontaneous activity to analyze the variability in spark amplitude (Klein et al., 1999; Wang et al., 2002). Spark variability has also been analyzed using cardiac Ca2+ sparks that were evoked by repeated action potentials in the presence of a reduced external Ca2+ (0.5 mmol/L) at fixed T-tubule-SR junctions (Bridge et al., 1999). Notwithstanding limitations, information gleaned from these studies suggested that Ca2+ sparks are rather stereotypic by virtue of modal amplitude distribution or preferred release duration (Bridge et al., 1999; Izu et al., 1998; Klein et al., 1999; Rios et al., 2001; Wang et al., 2002). These impose important constraints on the possible mechanism underlying the genesis of Ca2+ sparks. For instance, it has been argued that the rise time characteristics are incompatible with the idea that a spark arises from a single-channel RyR with a reversible Markovian gating scheme. Rather, Ca2+ sparks either are a collective phenomenon of a group of interacting RyRs or originate from single RyRs that manifest a rare, thermodynamically irreversible gating (Shirokova et al., 1999; Wang et al., 2002).
Recently we have developed the loose-seal patch-clamp and confocal imaging technique by which one can visualize in-focus sparks triggered by single L-type Ca2+ channel (LCC) currents in intact cardiac myocytes (Wang et al., 2001), via the Ca2+-induced Ca2+ release (CICR) mechanism (Fabiato, 1985). Naturally, this approach eliminates the out-of-focus events; in-focus detection of sparks against a quiescent background further enhances the spark detectability. In contrast to the notion that sparks are stereotypical, a broad modal amplitude distribution was reported (Wang et al., 2001), hinting on polymorphism of Ca2+ sparks in appearance and origin.
Using this newly developed technique, here we intended to characterize systematically main aspects of spark morphometrics, including amplitude, rise time, spatial width, and rising rate, and to analyze their intrinsic variability as well as interrelationship, under physiological experimental conditions. Our results confirmed a rather synchronized spark rise time, demonstrated a stereotyped spatial width, but uncovered a substantial variability in spark amplitude and mean rising rate. The polymorphism of Ca2+ sparks suggests that spark genesis involves stochastic activation of variable numbers of RyRs. In addition, an inverse relationship between spark rise time and rising rate provides evidence for a negative feedback mechanism in the regulation of termination of Ca2+ sparks.
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METHODS |
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). Briefly, after anesthesia (sodium pentobarbitone, 100 mg kg-1 injected I.P.), the heart was removed from the chest and perfused retrogradely via the aorta using the Langendorff method and collagenase (Worthington type II, 1 mg ml-1). Single cells were shaken loose from the heart, minced after this perfusion procedure, in HEPES buffer solution containing (in mmol/L): 137 NaCl, 5.4 KCl, 1.2 MgCl2, 1.2 NaH2PO4, 1 CaCl2, 10 glucose, and 20 HEPES (pH 7.4 adjusted with NaOH).
Confocal Ca2+ imaging
Isolated single myocytes were first incubated with the Ca2+ indicator fluo-4-AM (15 µM) (Molecular Probes, Eugene, OR) for 5 min, followed by a 10-min rest allowing for de-esterfication of the indicator. The criteria for cell selection included rod shape, clear striation, crisp and clean cell surface, and lack of spontaneous contractions during a 1-min observation period. Confocal imaging was performed using a Zeiss LSM510 confocal microscope (Carl Zeiss Inc., Oberkochen, Germany) equipped with an argon laser (488 nm) and a 40x, 1.3 NA oil immersion objective, at axial and radial resolutions of 1.0 and 0.4 µm, respectively. An x-z section image across the pipette tip was first taken to guide the positioning of the focal plane such that half of the rim of the pipette tip (at 
45° angle to the horizontal plane) was discernible. Then, fast x-y imaging was performed at a rate of 16 ms per frame, or linescan (x-t) imaging was performed with space-time sampling rates of 0.77 ms per scan line and 0.045 µm per pixel.
Loose-seal patch clamp
Cell-attached patch clamping was established using axopatch 200B amplifier (Axon Instruments, Foster City, CA) in loose-seal configuration, as described previously (Wang et al., 2001). A glass pipette (5–7 M
, <1 µm at the tip) was gently pressed onto the cell surface to form a low-resistance seal (20–40 M). The patch membrane voltage was determined according to the equation VPM = RP - VcomRs/(Rs + Rp), where VPM refers to the patch membrane voltage, RP the resting potentials (-80.6 ± 7.0 mV, n = 8, measured in separate experiments), Vcom the command voltage applied, Rs and Rp the seal and pipette resistance, respectively. The extracellular and patch pipette filling solution contained (in mmol/L): 137 NaCl, 1 CaCl2, 4.9 KCl, 1 MgCl2, 1.2 NaH2PO4, 15 glucose, and 20 HEPES (pH 7.4 adjusted with NaOH). All experiments were done at room temperature (23–25°C), with the VPM at -10 mV or more negative.
Image data analysis
Ca2+ Spark detection algorithm was described previously (Cheng et al., 1999), with some minor modifications. Computer programs for the automated spark detection and measurement were coded with IDL software (Research Systems, Boulder, CO). The detectability and rate of false detection under current parameter settings and signal-to-noise characteristics were assessed with an averaged spark embedded in pixel-scrambled blank images (see text). Ca2+ spark amplitudes are usually measured as 
R = F/F0, where F0 refers to the background fluo-4 signal. The rise time was measured as the temporal interval between the takeoff (i.e., F first exceeds 2x standard deviation of the baseline) and the peak of the spark. The maximal rising rate was computed by differentiation of local Ca2+ transient using 3-point Langrangian interpolation after minimal smoothing. The mean rising rate was determined as the peak F/F0 divided by the corresponding rise time.
Statistical analysis
Data were shown as mean ± SD, if not otherwise specified. Nonparametric Kruskal-Wallis test was applied to appraise difference between means of spark parameters. A p-value less than 0.05 was considered statistically significant.
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) to image SR Ca2+ release from a defined in-focus volume of cytoplasm. Specifically, a glass pipette (<1 µm at the tip) was gently pressed onto the cell surface to form a 20–40-M seal while retaining the integrity of the coupling between sarcolemmal LCCs and RyRs in the SR. This loose-seal configuration enables voltage control of the membrane delimited by the tip of the patch pipette and permits voltage-dependent activation of the LCCs therein, but precludes simultaneous recording of LCC single-channel currents due to excessive electrical noise (Wang et al., 2001). To visualize locally evoked SR Ca2+ release events, confocal imaging of the Ca2+-sensitive fluo-4 fluorescence was performed with the focal plane placed right beneath the patch membrane.
Fig. 1 A shows representative confocal x-y imaging of local Ca2+ dynamics underneath the loose-seal patch in an intact rat ventricular myocyte. The confocal plane was positioned such that half of the rim of the patch pipette (at 45° angle of the horizontal plane) was visible. Patch membrane depolarization to -25 mV for 300 ms elicited a solitary Ca2+ spark, which originated from a focal point beneath the patch membrane. Fast two-dimensional (x-y) imaging (16 ms per frame) revealed that, during its evolution, the evoked spark remained spatially confined, without igniting other spark-generating sites. This demonstrates that the current approach allows for investigation of Ca2+ sparks from a single in-focus release unit.
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0.12 pA lasting 1 ms at -25 mV) (Guia et al., 2001). To ensure single-release unit origin of sparks in individual patches, we further determined mass centers for events collected from the same patch (Fig. 2 A). Ca2+ sparks originated from different release sites should be projected onto different locations on the scan line, unless they are on the same vertical (z) axis. Those patches with spark mass centers wandering over a distance greater than 0.4 µm (4 out of 31 patches) were rejected without further analysis. Otherwise, all active patches (n = 27) bearing one or more evoked Ca2+ sparks were included for statistical analysis. Among them, a total of 166 Ca2+ sparks, excluding a few reactivating events, were activated in 215 voltage pulses beneath the pipette (i.e., in the image strips of 2 µm x 300 ms). The null event rate was 49 out of 215 pulses (69 null events in the first 200 ms), and the spark activation probability in a single unit during 300-ms depolarization to -25 mV was therefore 0.77 for 300-ms depolarization or 0.68 for 200-ms depolarization. The average latency for spark activation, defined as the time lapsed since the onset of depolarization, was 51 ± 46 ms (n = 147 sparks) for events in the first 200-ms period. When Ca2+ "spike" at a T-tubule-SR junction (Song et al., 2001) are mostly single-spark events (e.g., at low voltages under whole-cell voltage-clamp conditions), their activation latency should be the same as that of loose-seal triggered sparks. To this end, the latency reported for spike activation in 200 ms pulses is 70 or 45 ms at -30 or -20 mV, respectively (Song et al., 2001). Thus, the similarity in kinetics of spark and spike activation at around -25 mV provides a validation that local CICR remains largely intact under our experimental conditions. Moreover, since the rate of occurrence of spontaneous Ca2+ sparks is typically 1.0 event per second per 100-µm scan in intact cells (Cheng et al., 1996, 1993), contribution of spontaneous spark activity from adjacent sites to the defined volume of observation during depolarization would also be minimal (1%).
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). Evidently, both false positives and false negatives will not only distort the population statistics, but also affect the analysis of variability of spark morphometric parameters. In this study, in-focus activation of sparks against quiescent backgrounds should improve the detection of low-amplitude events. Moreover, owing to the superior signal-to-noise characteristics afforded by current generation of confocal microscope, we expected a low rate of false detection. To appraise quantitatively the detectability and false detection, we generated a spark test standard by averaging 11 unequivocal events (Fig. 2 B) and constructed noise backgrounds based on pixel scrambling of null images. By scaling and embedding the test spark in the noise background, we then fed test images through the same algorithm for spark detection. We found that the detectability was 7.5, 25, or 59% at 0.10, 0.125, or 0.15 in F/F0 unit, respectively, and reached nearly unity at 0.20 F/F0 units (Fig. 3, left), with the 50% cutoff level at around 0.15 F/F0 units. There were seven false positives detected in 100 blank images (512 x 512 pixels), so we estimated about one false positive in 215 image strips of 2 µm x 300 ms. Given that the smallest spark events detected by the loose-seal technique was of 0.27 F/F0 units (see Fig. 4 B), these indicate that virtually all local Ca2+ release events could be detected with little contamination of false positives under our experimental conditions, providing no subspark release events (Lipp and Niggli, 1996, 1998) substantially smaller than 0.15 F/F0 units. The high spark detectability and low rate of false detection were made possible, in part, because of the superior signal-to-noise characteristics of contemporary confocal microscopy. In blank traces the background fluctuation of fluo-4 signal was equivalent to merely 0.05 F/F0 units, and the peak noise level was 0.14 ± 0.03 F/F0 units (n = 21) (Fig. 3, right). Since spark detection used multiple cues rather than relying solely on peak amplitude, we were able to detect test events with amplitudes smaller than the peak noise (Fig. 3, left). Collectively, these improvements enabled us to depict true spark properties and their intrinsic variability with unprecedented accuracy under well-controlled experimental conditions.
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). The overall amplitude histogram was empirically fitted to a bimodal Gaussian function, with a prominent mode at 0.67 (F/F0 unit) and a second minor mode at 1.45. Of the 166-event data set, the dimmest spark was 0.27, well above our detection threshold (Fig. 3, left), and the brightest one was 2.75. The mean and median spark amplitudes were 0.98 and 0.88, respectively, similar to those reported previously for randomly sampled events (Cannell et al., 1995; Cheng et al., 1993; Lipp and Niggli, 1994; Lopez-Lopez et al., 1995). The similarity of in-focus and randomly sampled sparks in terms of their average amplitudes is somewhat surprising, because one might expect in-focus sparks be brighter than randomly sampled ones. However, this can be explained as the net result of two opposing factors, the absence of out-of-focus events (which tends to increase the average amplitude) and the enhanced detectability for low-amplitude events (which tends to decrease the average amplitude) in this study. Similar to the case with spark amplitude, a large variation was observed for the mean rising rate (99.1 ± 62.8 s-1, n = 166 events). The variations in the amount of Ca2+ released and the releasing flux indicate that Ca2+ sparks are polymorphic, rather than a stereotyped or all-or-none phenomenon (Bridge et al., 1999; Cannell et al., 1995; Cheng et al., 1993; Lipp and Niggli, 1994; Lopez-Lopez et al., 1995).
As shown in Fig. 4 B, middle, spark rise time histogram rose steeply at 6 ms (eight scan lines) and then decayed precipitately at 14 ms, with the vast majority (90%) distributed over this narrow time window. Thus, the rise time distribution of in-focus sparks deviates from exponential distributions expected for Markovian channels or channel groups that gate reversibly (Colquhoun and Hawkes, 1995; Wang et al., 2002). This underpins the notion that Ca2+ release duration in a spark or the rate of spark termination is tightly regulated by some unknown, thermodynamically irreversible mechanism.
Ca2+ sparks sampled at random locations exhibit a broad FWHM distribution with an average value of 2.0 µm (Cannell et al., 1995; Cheng et al., 1993; Lipp and Niggli, 1994; Lopez-Lopez et al., 1995). Yet, models of spark formation often predicted a FWHM around 1.0 µm even after confocal blurring effects were accounted for (Colquhoun and Hawkes, 1995; Pratusevich and Balke, 1996; Smith et al., 1998). Deceptively trivial, this twofold discrepancy in FWHM would translate into an eightfold discrepancy in spatial volume, and it has been difficult to reconcile this "spark-width paradox" (Smith et al., 1998), unless a large Ca2+ current (20–40 pA) was assumed, which results in gross saturation of the Ca2+ indicator and flat-top ("platykurtic") Ca2+ sparks (Izu et al., 2001). The experimental setting permitted us to revisit this important issue by measuring spatial characteristics of in-focus Ca2+ sparks. Fig. 4 A shows that, whether big or small, in-focus sparks displayed a sharply peaked spatial profile that rapidly decayed at increasing radius, giving no sign of local indicator saturation (at the optical resolution). In the absence of out-of-focus events, the FWHM distribution showed a prominent mode at 1.25 µm, with a greatly reduced dispersion compared to randomly sampled events (Fig. 4 B, right). The average and median FWHW for in-focus sparks were 1.41 and 1.35 µm, respectively, which is considerably smaller than the reported values. The downwardly revised FWHM, together with the moderate amplitude, would greatly reduce the Ca2+ current needed to account for the genesis of Ca2+ sparks. The remaining discrepancy between theoretical and observed FWHM, if any, has to be accounted for by reasons other than gross indicator saturation. For instance, regenerative recruitment of adjacent release units (Parker et al., 1996) might explain the platykurtic subpopulation of sparks under some experimental conditions (Izu et al., 2001).
Polymorphism of Ca2+ sparks within and among individual release units
There are competing hypotheses as to whether a single RyR (Cheng et al., 1993; Shirokova et al., 1999), the entire RyR release unit (Bers and Fill, 1998; Sobie et al., 2002), or a fraction of RyRs in a unit are activated in a spark (Bridge et al., 1999; Cannell et al., 1995; Cheng et al., 1993; Lipp and Niggli, 1994; Lopez-Lopez et al., 1995; Shirokova et al., 1999; Wang et al., 2001). To this end, analysis of variability of Ca2+ sparks evoked from single in-focus release units should be informative. Among 15 events observed in patch [1] in Fig. 5, morphometric measurement revealed a 6.6-fold difference between the brightest and the dimmest spark amplitudes (Fig. 5 A, left), accompanying a 5.6-fold difference in mean rising rate (Fig. 5 A, right). Similar results were obtained from three other patches illustrated in Figs. 5 A, and diaries of mean and standard deviation in all 20 patches displaying four or more events are shown in Fig. 5 B. It is noteworthy that there was a significant patch-to-patch variation in spark characteristics. For instance, the average spark amplitudes in patch [2] and [3] of Fig. 5 A were 0.61 ± 0.16 (n = 17) and 0.86 ± 0.26 (n = 14, p < 0.01), the mean rising rates were 59.6 ± 22.3 (n = 17) and 79.4 ± 25.7 s-1 (n = 14, p < 0.05), respectively. The patch-to-patch variation suggests that cardiac release units are somewhat heterogeneous with respect to organization or operation.
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) as the ratio of standard deviation (SD) and the corresponding mean value. For the pooled population of sparks, the was 0.51 for amplitude or 0.63 for mean rising rate; the -value within release units (1) was, on average, 0.34 or 0.41 (n = 20), and the -value among release units (2) was 0.38 or 0.46, for spark amplitude or mean rising rate, respectively. In either case, these -values satisfied the relationship 1 
2 and 2 
+ 
. Thus, both intra- and interrelease unit variability contributed equally to the overall spark variability. Since the noise in blank traces was equivalent to 3–5 nM Ca2+ fluctuation (at the resting Ca2+ level) (Fig. 3, right), photon collection noise should account for only a small portion of the observed amplitude variability.
Interrelationship between spark parameters
As shown in Fig. 4 A, spark activation was scattered over almost the entire depolarization pulse at -25 mV. This provided an opportunity to investigate whether early sparks differ from those activated late into the pulse. Fig. 6, A and B, show scatter plot of spark amplitude and maximal rising rate as a function of the latency of activation, with overlay of the linear regression lines, respectively. Our data show clearly that spark parameters are stationary during the pulse, independently of the latency of spark production and the time-dependent changes in L-type channel activation.
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Furthermore, we uncovered a substantial negative correlation between the release duration and flux (rise time and mean rising rate) (r = -0.43), as if a stronger release current terminated the spark sooner. This observation is consistent with previous reports that the rate of termination of spark release flux is directly related to the magnitude of the flux (Lukyanenko et al., 1998; Soeller and Cannell, 2002). Since the opposite is predicted for local CICR among RyRs in a cluster, some intraunit negative feedback mechanism must be at work for spark termination. Taken together, our morphometric data support a model in which cohorts of variable number of RyRs are activated in the genesis of Ca2+ sparks, and the ensuing negative feedback, which strength depends on the degree of activation, overrides the positive feedback by CICR to extinguish the ongoing Ca2+ spark.
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DISCUSSION |
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; Izu et al., 1998; Klein et al., 1999; Rios et al., 2001; Shirokova et al., 1999; Wang et al., 2002), the unique advantages of the current approach include the absence of out-of-focus events, the suppression of positive and negative false detection, and the ability to activate repetitive Ca2+ sparks from the same well-defined, in-focus release units. In addition, all experiments were performed with intact cardiac myocytes with normal physiological saline. Under the experimental conditions, we have demonstrated that, despite relatively stereotyped spark rise time and spatial width, Ca2+ sparks exhibit large intrinsic variability in rising rate and amplitude, even when they are evoked from the same release units.
Precautions should be taken in the acquisition, analysis, and interpretations of the loose-seal spark data. Since the pipette can only access the LCCs residing on the surface of the cell, the spark data reflect primarily the peripheral excitation-contraction (EC) coupling. To this end, we compared spark activation to T-tubule spike generation (under whole-cell patch-clamp conditions). Our results revealed similar coupling kinetics in either case, suggesting that the peripheral EC coupling is representative of those at the T-tubule sites. Besides, even in the loose-seal configuration, local CICR may sometimes be altered by spontaneous -shaped deformation. We therefore from time to time monitor the seal resistance and membrane boundary to ensure minimal perturbation on the local CICR. Further, we limited the imaging scan time at a given release site such that statistical spark properties did not drift within the sequence of collection (data not shown). These efforts help to minimize possible limitations of the loose-seal method and ensure that the obtained spark data are physiologically relevant.
The polymorphism of Ca2+ sparks within and among cardiac release units explains our previous report on a broad modal amplitude distribution for in-focus Ca2+ sparks (Wang et al., 2001), but is in apparent contrast with the conclusion by Bridge et al. (1999). Although both studies reported on a modal (rather than monotonic decreasing) spark amplitude distribution, they attributed a large portion of the apparent variability among sparks evoked by action potentials at fixed T-tubule-SR sites to the photon collection noise. In light of the finding that sparks can be both dim and bright, a biased rejection of small-amplitude events amidst noise could have compromised their conclusion. In their histogram analysis of local [Ca2+] at a fixed time after the action potential, the identity of the intermediates (i.e., those between the peaks for baseline and sparks) was uncertain, making it difficult to appraise the true spark variability. Even though sparks of varying amplitude were commonly seen at many T-tubules in their study, it was unclear whether this apparent variability was intrinsic to the spark genesis, because activation of multiple out-of-focus sites could also render large apparent same-site variation. Thus, we believe our observations are consistent with data in the previous study. Other circumventive evidence for polymorphism of Ca2+ sparks includes the observation that recurrent sparks at Imperatoxin A-modified sites are highly variable (Terentyev et al., 2002).
The origin of polymorphism has not yet been determined experimentally. In principle, the interrelease unit variability should reflect heterogeneity in the organization or operation of different release units (Franzini-Armstrong et al., 1999). Within individual release units, spark variability may arise from genuine moment-to-moment variation in the recruitment of RyRs. Also, the stochastic nature of the coupling between single LCCs and the abutting RyR array may add variability to sequence of the same-patch events (triggered by different LCCs). Theoretical analysis of a model array of RyRs has suggested a possible spark origin as intraunit mesoscopic Ca2+ wave driven by CICR (Stern et al., 1999). To this end, an intriguing possibility might be that the variable spark rising rate may due to simultaneous ignition of more than one focus within a unit: those of fast rising rate and large release current could reflect events of multiple intraunit foci. If this were the case, spark properties might be expected to vary with the probability of LCC and spark activation. Under our experimental conditions, spark activation at -25 mV dispersed widely with varying probability during 300-ms depolarization. However, spark properties including the rising rate do not vary with the latency of spark production. This indicates that polymorphism of Ca2+ sparks does not reflect time-dependent changes in LCC and spark activation under current experimental conditions. Future investigation is warranted to delineate the molecular and cellular origin of spark polymorphism.
Irrespective of its origin, polymorphism of Ca2+ sparks has multifaceted implications. The large intrinsic variation in Ca2+ spark mean rising rate would be difficult to reconcile with any model in which sparks are generated by the entire release unit (Bers and Fill, 1998; Sobie et al., 2002), or a single RyR (Cheng et al., 1993; Shirokova et al., 1999), or any fixed number of RyRs. Instead, it is in favor of the idea that variable cohorts of RyRs in the same unit can be activated in a stochastic, rather than deterministic, manner. If the coupled gating whereby multiple RyRs operate in unison (Marx et al., 2001, 1998) does exist in vivo, our data suggest that the mechanical coupling of RyRs operates over a limited range, and undergoes dynamic reorganization within a unit. Finally, the polymorphism of Ca2+ sparks adds an interesting twist to the debate on the molecular nature of Ca2+ sparks: a subpopulation of sparks may be genuinely single-channel events. To this end, the presence of very weak in-focus sparks and the existence of a small population of long-lasting, slow-rising events may represent single RyR "quarks" (Lipp and Niggli, 1998) or RyR "sparklets" (Wang et al., 2001) triggered under physiological conditions. It should be noted that since the number of RyRs participating in an average cardiac spark is estimated to be 4–6 (Wang et al., 2001), the small-amplitude Gaussian component shown in Fig. 4 B may not correspond to single-RyR events.
With the current 50% event detection level at 0.15 F/F0 units, the scarcity of events smaller than 0.30 F/F0 units suggests that, during microscopic EC coupling, SR Ca2+ is released in discrete packets that are resolvable with our current Ca2+ imaging capability. Hence, our data support the notion that, albeit polymorphic, Ca2+ sparks constitute the elementary events of cardiac EC coupling, and summation of discrete sparks accounts for the totality of SR Ca2+ release (Cannell et al., 1995; Cheng et al., 1993; Lipp and Niggli, 1994; Lopez-Lopez et al., 1995). Should subspark events or eventless releases (Lipp and Niggli, 1996, 1998) exist in heart cells, their unitary amplitudes must be below the current detection limit (0.10 F/F0 units).
Characterization of spark morphometrics also sheds some new light on possible mechanism that is responsible for the termination of Ca2+ sparks. The narrow rise time distribution, in defying of the stochastic law that governs channel gating (Colquhoun and Hawkes, 1995; Wang et al., 2002), strongly suggests that spark termination is under rigorous regulation. The negative correlation between spark rise time and the mean rising time suggests that this regulation is a negative feedback in nature, the strength of which is proportional to the ongoing release flux or the number of activated RyRs. Mechanistically, this could either be a Ca2+-dependent inactivation overriding local CICR (Fabiato, 1985; Sham et al., 1998), or inhibitory regulation of RyRs through depletion of local SR lumenal Ca2+ (Terentyev et al., 2002). The manyfold variation in the amplitude (thereby the amount of Ca2+ released) of Ca2+ sparks from individual release sites argues against local SR Ca2+ depletion as the sole or primary determinant of termination of Ca2+ sparks, although a modulatory role for SR lumenal Ca2+ cannot be excluded (Terentyev et al., 2002). Finally, the inverse relationship between spark rising rate and rise time is in contrast to the observation that, in planar lipid bilayers, coupled RyRs have their open duration prolonged by orders of magnitudes compared to RyRs acting solo (Marx et al., 2001).
In summary, characterization of in-focus Ca2+ sparks has revealed both stereotyped and polymorphic spark properties. The newly quantified spark amplitude, rise time, mean rising rate, and full width at half maximum are, on average, 1.0 F/F0 unit, 10 ms, 100 s-1, and 1.4 µm, respectively, which are independent of the latency of spark production during depolarization. The spark morphometrics, their population statistics and intrinsic variability support a model in which spark activation involves stochastic recruitment of variable number of RyRs in a release unit. With Ca2+ release duration being the most tightly regulated spark parameter, termination of Ca2+ spark apparently entails a strong negative feedback mechanism. Furthermore, the identification of interrelease unit variability provides a means to appraise heterogeneity among Ca2+ release units. The novel approach developed in this and recent studies (Wang et al., 2001) should prove to be crucial in mechanistic studies of microscopic EC coupling in heart and local Ca2+ signaling in many types of cells.
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ACKNOWLEDGEMENTS |
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This work was supported by Predoctoral Fellowship of the National Institutes of Health Graduate Partnership Program (JXS), National Institute on Aging (Scientific Director's Award in Research), awards by China Ministry of Education (SQW), National Institutes of Health intramural research programs, National Natural Science Foundation of China (TH, HC), and Major State Basic Research Development Program of China (HC).
Submitted on July 10, 2003; accepted for publication September 11, 2003.
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REFERENCES |
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and 
(B) Space-time characteristics of a spark averaged from 11 unequivocal events. Note that the pipette rim artifact in 
