Variability in frequency and characteristics of Ca2+ sparks at different release sites in rat ventricular myocytes

High-speed optical imaging with sCMOS pixel reassignment

Fluorescence microscopy has undergone rapid advancements, offering unprecedented visualization of biological events and shedding light on the intricate mechanisms governing living organisms. However, the exploration of rapid biological dynamics still poses a significant challenge due to the limitations of current digital camera architectures and the inherent compromise between imaging speed and other capabilities. Here, we introduce sHAPR, a high-speed acquisition technique that leverages the operating principles of sCMOS cameras to capture fast cellular and subcellular processes. sHAPR harnesses custom fiber optics to convert microscopy images into one-dimensional recordings, enabling acquisition at the maximum camera readout rate, typically between 25 and 250 kHz. We have demonstrated the utility of sHAPR with a variety of phantom and dynamic systems, including high-throughput flow cytometry, cardiomyocyte contraction, and neuronal calcium waves, using a standard epi-fluorescence microscope. sHAPR is highly adaptable and can be integrated into existing microscopy systems without requiring extensive platform modifications. This method pushes the boundaries of current fluorescence imaging capabilities, opening up new avenues for investigating high-speed biological phenomena.

Balanced modulation of neuromuscular synaptic transmission via M1 and M2 muscarinic receptors during inhibition of cholinesterases

Organophosphorus (OP) compounds that inhibit acetylcholinesterase are a common cause of poisoning worldwide, resulting in several hundred thousand deaths each year. The pathways activated during OP compound poisoning via overstimulation of muscarinic acetylcholine receptors (mAChRs) play a decisive role in toxidrome. The antidotal therapy includes atropine, which is a nonspecific blocker of all mAChR subtypes. Atropine is efficient for mitigating depression in respiratory control centers but does not benefit patients with OP-induced skeletal muscle weakness. By using an ex vivo model of OP-induced muscle weakness, we studied the effects of the M1/M4 mAChR antagonist pirenzepine and the M2/M4 mAChR antagonist methoctramine on the force of mouse diaphragm muscle contraction. It was shown that weakness caused by the application of paraoxon can be significantly prevented by methoctramine (1 µM). However, neither pirenzepine (0.1 µM) nor atropine (1 µM) was able to prevent muscle weakness. Moreover, the application of pirenzepine significantly reduced the positive effect of methoctramine. Thus, balanced modulation of neuromuscular synaptic transmission via M1 and M2 mAChRs contributes to paraoxon-induced muscle weakness. It was shown that methoctramine (10 µmol/kg, i.p.) and atropine (50 µmol/kg, i.p.) were equieffective toward increasing the survival of mice poisoned with a 2xLD₅₀ dose of paraoxon.

Elementary calcium release events in the skeletal muscle cells of the honey bee Apis mellifera

Calcium sparks are involved in major physiological and pathological processes in vertebrate muscles but have never been characterized in invertebrates. Here, dynamic confocal imaging on intact skeletal muscle cells isolated enzymatically from the adult honey bee legs allowed the first spatio-temporal characterization of subcellular calcium release events (CREs) in an insect species. The frequency of CREs, measured in x–y time lapse series, was higher than frequencies usually described in vertebrates. Honey bee CREs had a larger spatial spread at half maximum than their vertebrate counterparts and a slightly ellipsoidal shape, two characteristics that may be related to ultrastructural features specific to invertebrate cells. In line-scan experiments, the histogram of CREs’ duration followed a bimodal distribution, supporting the existence of both sparks and embers. Unlike in vertebrates, embers and sparks had similar amplitudes, a difference that could be related to genomic differences and/or excitation–contraction coupling specificities in honey bee skeletal muscle fibres. The first characterization of CREs from an arthropod which shows strong genomic, ultrastructural and physiological differences with vertebrates may help in improving the research field of sparkology and more generally the knowledge in invertebrates cell Ca²⁺ homeostasis, eventually leading to a better understanding of their roles and regulations in muscles but also the myotoxicity of new insecticides targeting ryanodine receptors.

Spontaneous and action potential-evoked Ca2+ release from endoplasmic reticulum in neocortical synaptic boutons

Although the endoplasmic reticulum (ER) is present throughout axons, and IP3 and ryanodine receptors are widely expressed in nerve terminals, whether Ca2+ release from presynaptic stores contributes to action potential (AP)-evoked Ca2+ transients remains controversial. We investigated the release of Ca2+ from ER stores in boutons en passant of neocortical layer 5 pyramidal neurons. A hallmark of these stores is that they spontaneously release Ca2+ at a low frequency. Using a high-affinity Ca2+ indicator, we documented and characterised such spontaneous Ca2+ transients (sCaTs), which occurred at a rate of ~0.2 per min and raised the intracellular Ca2+ concentration ([Ca2+]i) by ~2 µM in the absence of exogenous buffers. Caffeine increased the average frequency of sCaTs by 90%, without affecting their amplitude and decay kinetics. Therefore, presynaptic ryanodine receptors were likely involved. To determine if presynaptic ER stores contribute to intracellular Ca2+ accumulation during repetitive stimulation, we measured [Ca2+]i during 2 s long trains of APs evoked at 10-50 Hz. We found that for frequencies <20 Hz, [Ca2+]i reached a steady state within ~500 ms after stimulation onset. However, for higher frequencies, [Ca2+]i continued to increase with AP number, suggesting that the rate of Ca2+ entry exceeded the rate of clearance. Comparison between measured and predicted values indicates supralinear summation of Ca2+. Block of the sarco/endoplasmic reticulum Ca2+-ATPase reduced the supralinearity of summation, without reducing the amplitude of a single AP-evoked Ca2+ transient. Together, our results implicate presynaptic ER stores as a source of Ca2+ during repetitive stimulation.

Size Matters: Ryanodine Receptor Cluster Size Heterogeneity Potentiates Calcium Waves

Ryanodine receptors (RyRs) mediate calcium (Ca)-induced Ca release and intracellular Ca homeostasis. In a cardiac myocyte, RyRs group into clusters of variable size from a few to several hundred RyRs, creating a spatially nonuniform intracellular distribution. It is unclear how heterogeneity of RyR cluster size alters spontaneous sarcoplasmic reticulum (SR) Ca releases (Ca sparks) and arrhythmogenic Ca waves. Here, we tested the impact of heterogeneous RyR cluster size on the initiation of Ca waves. Experimentally, we measured RyR cluster sizes at Ca spark sites in rat ventricular myocytes and further tested functional impacts using a physiologically detailed computational model with spatial and stochastic intracellular Ca dynamics. We found that the spark frequency and amplitude increase nonlinearly with the size of RyR clusters. Larger RyR clusters have lower SR Ca release threshold for local Ca spark initiation and exhibit steeper SR Ca release versus SR Ca load relationship. However, larger RyR clusters tend to lower SR Ca load because of the higher Ca leak rate. Conversely, smaller clusters have a higher threshold and a lower leak, which tends to increase SR Ca load. At the myocyte level, homogeneously large or small RyR clusters limit Ca waves (because of low load for large clusters but low excitability for small clusters). Mixtures of large and small RyR clusters potentiates Ca waves because the enhanced SR Ca load driven by smaller clusters enables Ca wave initiation and propagation from larger RyR clusters. Our study suggests that a spatially heterogeneous distribution of RyR cluster size under pathological conditions may potentiate Ca waves and thus afterdepolarizations and triggered arrhythmias.

Ryanodine receptor cluster fragmentation and redistribution in persistent atrial fibrillation enhance calcium release

AIMS

In atrial fibrillation (AF), abnormalities in Ca(2+) release contribute to arrhythmia generation and contractile dysfunction. We explore whether ryanodine receptor (RyR) cluster ultrastructure is altered and is associated with functional abnormalities in AF.

METHODS AND RESULTS

Using high-resolution confocal microscopy (STED), we examined RyR cluster morphology in fixed atrial myocytes from sheep with persistent AF (N = 6) and control (Ctrl; N = 6) animals. RyR clusters on average contained 15 contiguous RyRs; this did not differ between AF and Ctrl. However, the distance between clusters was significantly reduced in AF (288 ± 12 vs. 376 ± 17 nm). When RyR clusters were grouped into Ca(2+) release units (CRUs), i.e. clusters separated by <150 nm, CRUs in AF had more clusters (3.43 ± 0.10 vs. 2.95 ± 0.02 in Ctrl), which were more dispersed. Furthermore, in AF cells, more RyR clusters were found between Z lines. In parallel experiments, Ca(2+) sparks were monitored in live permeabilized myocytes. In AF, myocytes had >50% higher spark frequency with increased spark time to peak (TTP) and duration, and a higher incidence of macrosparks. A computational model of the CRU was used to simulate the morphological alterations observed in AF cells. Increasing cluster fragmentation to the level observed in AF cells caused the observed changes, i.e. higher spark frequency, increased TTP and duration; RyR clusters dispersed between Z-lines increased the occurrence of macrosparks.

CONCLUSION

In persistent AF, ultrastructural reorganization of RyR clusters within CRUs is associated with overactive Ca(2+) release, increasing the likelihood of propagating Ca(2+) release.

Mineralocorticoid modulation of cardiac ryanodine receptor activity is associated with downregulation of FK506-binding proteins

BACKGROUND

The mineralocorticoid pathway is involved in cardiac arrhythmias associated with heart failure through mechanisms that are incompletely understood. Defective regulation of the cardiac ryanodine receptor (RyR) is an important cause of the initiation of arrhythmias. Here, we examined whether the aldosterone pathway might modulate RyR function.

METHODS AND RESULTS

Using the whole-cell patch clamp method, we observed an increase in the occurrence of delayed afterdepolarizations during action potential recordings in isolated adult rat ventricular myocytes exposed for 48 hours to aldosterone 100 nmol/L, in freshly isolated myocytes from transgenic mice with human mineralocorticoid receptor expression in the heart, and in wild-type littermates treated with aldosterone. Sarcoplasmic reticulum Ca(2+) load and RyR expression were not altered; however, RyR activity, visualized in situ by confocal microscopy, was increased in all cells, as evidenced by an increased occurrence and redistribution to long-lasting and broader populations of spontaneous Ca(2+) sparks. These changes were associated with downregulation of FK506-binding proteins (FKBP12 and 12.6), regulatory proteins of the RyR macromolecular complex.

CONCLUSIONS

We suggest that in addition to modulation of Ca(2+) influx, overstimulation of the cardiac mineralocorticoid pathway in the heart might be a major upstream factor for aberrant Ca(2+) release during diastole, which contributes to cardiac arrhythmia in heart failure.

Calcium sparks

The calcium ion (Ca(2+)) is the simplest and most versatile intracellular messenger known. The discovery of Ca(2+) sparks and a related family of elementary Ca(2+) signaling events has revealed fundamental principles of the Ca(2+) signaling system. A newly appreciated "digital" subsystem consisting of brief, high Ca(2+) concentration over short distances (nanometers to microns) comingles with an "analog" global Ca(2+) signaling subsystem. Over the past 15 years, much has been learned about the theoretical and practical aspects of spark formation and detection. The quest for the spark mechanisms [the activation, coordination, and termination of Ca(2+) release units (CRUs)] has met unexpected challenges, however, and raised vexing questions about CRU operation in situ. Ample evidence shows that Ca(2+) sparks catalyze many high-threshold Ca(2+) processes involved in cardiac and skeletal muscle excitation-contraction coupling, vascular tone regulation, membrane excitability, and neuronal secretion. Investigation of Ca(2+) sparks in diseases has also begun to provide novel insights into hypertension, cardiac arrhythmias, heart failure, and muscular dystrophy. An emerging view is that spatially and temporally patterned activation of the digital subsystem confers on intracellular Ca(2+) signaling an exquisite architecture in space, time, and intensity, which underpins signaling efficiency, stability, specificity, and diversity. These recent advances in "sparkology" thus promise to unify the simplicity and complexity of Ca(2+) signaling in biology.

Effects of aconitine on [Ca2+] oscillation in cultured myocytes of neonatal rats

In order to investigate the effects of aconitine on [Ca2+] oscillation patterns in cultured myocytes of neonatal rats, fluorescent Ca2+ indicator Fluo-4 NW and laser scanning confocal microscope (LSCM) were used to detect the real-time changes of [Ca2+] oscillation patterns in the cultured myocytes before and after aconitine (1.0 micromol/L) incubation or antiarrhythmic peptide (AAP) and aconitine co-incubation. The results showed under control conditions, [Ca2+] oscillations were irregular but relatively stable, occasionally accompanied by small calcium sparks. After incubation of the cultures with aconitine, high frequency [Ca2+] oscillations emerged in both nuclear and cytoplasmic regions, whereas typical calcium sparks disappeared and the average [Ca2+] in the cytoplasm of the cardiomyocyte did not change significantly. In AAP-treated cultures, intracellular [Ca2+] oscillation also changed, with periodic frequency, increased amplitudes and prolonged duration of calcium sparks. These patterns were not altered significantly by subsequent aconitine incubation. The basal value of [Ca2+] in nuclear region was higher than that in the cytoplasmic region. In the presence or absence of drugs, the [Ca2+] oscillated synchronously in both the nuclear and cytoplasmic regions of the same cardiomyocyte. It was concluded that although oscillating strenuously at high frequency, the average [Ca2+] in the cytoplasm of cardiomyocyte did not change significantly after aconitine incubation, compared to the controls. The observations indicate that aconitine induces the changes in [Ca2+] oscillation frequency other than the Ca2+ overload.

Multidimensional detection and analysis of Ca2+ sparks in cardiac myocytes

Examining calcium spark morphology and its relationship to the structure of the cardiac myocyte offers a direct means of understanding excitation-contraction coupling mechanisms. Traditional confocal line scanning achieves excellent temporal spark resolution but at the cost of spatial information in the perpendicular dimension. To address this, we developed a methodology to identify and analyze sparks obtained via two-dimensional confocal or charge-coupled device microscopy. The technique consists of nonlinearly subtracting the background fluorescence, thresholding the data on the basis of noise level, and then localizing the spark peaks via a generalized extrema test, while taking care to detect and separate adjacent peaks. In this article, we describe the algorithm, compare its performance to a previously validated spark detection algorithm, and demonstrate it by applying it to both a synthetic replica and an experimental preparation of a two-dimensional isotropic myocyte monolayer exhibiting sparks during a calcium transient. We find that our multidimensional algorithm provides better sensitivity than the conventional method under conditions of temporally heterogeneous background fluorescence, and the inclusion of peak segmentation reduces false negative rates when spark density is high. Our algorithm is robust and can be effectively used with different imaging modalities and allows spark identification and quantification in subcellular, cellular, and tissue preparations.

Calcium signalling during excitation-contraction coupling in mammalian atrial myocytes

Atrial cardiomyocytes make an important contribution to the refilling of ventricles with blood, which enhances the subsequent ejection of blood from the heart. The dependence of cardiac function on the contribution of atria becomes increasingly important with age and exercise. We know much less about the calcium signals that link electrical depolarisation to contraction within atrial myocytes in comparison with ventricular myocytes. Nevertheless, recent work has shed new light on calcium signalling in atrial cells. At an ultrastructural level, atrial and ventricular myocytes have many similarities. However, a few key structural differences, in particular the lack of transverse tubules (;T-tubules') in atrial myocytes, make these two cell types display vastly different calcium patterns in response to depolarisation. The lack of T-tubules in atrial myocytes means that depolarisation provokes calcium signals that largely originate around the periphery of the cells. To engage the contractile machinery, the calcium signal must propagate centripetally deeper into the cells. This inward movement of calcium is ultimately controlled by hormones that can promote or decrease calcium release within the myocytes. Enhanced centripetal movement of calcium in atrial myocytes leads to increased contraction and a more substantial contribution to blood pumping. The calcium signalling paradigm within atrial cells applies to other cardiac cell types that also do not express T-tubules, such as neonatal ventricular myocytes, and Purkinje cells that aid in the spread of electrical depolarisation. Furthermore, during heart failure ventricular myocytes progressively lose their regular T-tubule expression, and their pattern of response resembles that of atrial cells.

Ca2+ sparks in skeletal muscle

The study of Ca2+ sparks has led to extensive new information regarding the gating of the Ca2+ release channels underlying these events in skeletal, cardiac and smooth muscle cells, as well as the possible roles of these local Ca2+ release events in muscle function. Here we review basic procedures for studying Ca2+ sparks in skeletal muscle, primarily from frog, as well as the basic results concerning the properties of these events, their pattern and frequency of occurrence during fiber depolarization and the mechanisms underlying their termination. Finally, we also consider the contribution of different ryanodine receptor (RyR) isoforms to Ca2+ sparks and the number of RyR Ca2+ release channels that may contribute to the generation of a Ca2+ spark. Over the decade since their discovery, Ca2+ sparks have provided a wealth of information concerning the function of Ca2+ release channels within their intracellular environment.

The effect of residual on the stochastic gating of -regulated channel models

Single-channel models of intracellular Ca(2+) channels such as the inositol 1,4,5-trisphosphate receptor and ryanodine receptor often assume that Ca(2+)-dependent transitions are mediated by a constant background [Ca(2+)] as opposed to a dynamic [Ca(2+)] representing the formation and collapse of a localized Ca(2+) domain. This assumption neglects the fact that Ca(2+) released by open intracellular Ca(2+) channels may influence subsequent gating through the processes of Ca(2+)-activation or -inactivation. We study the effect of such "residual Ca(2+)" from previous channel opening on the stochastic gating of minimal and realistic single-channel models coupled to a restricted cytoplasmic compartment. Using Monte Carlo simulation as well as analytical and numerical solution of a system of advection-reaction equations for the probability density of the domain [Ca(2+)] conditioned on the state of the channel, we determine how the steady-state open probability (p(open)) of single-channel models of Ca(2+)-regulated Ca(2+) channels depends on the time constant for Ca(2+) domain formation and collapse. As expected, p(open) for a minimal model including Ca(2+) activation increases as the domain time constant becomes large compared to the open and closed dwell times of the channel, that is, on average the channel is activated by residual Ca(2+) from previous openings. Interestingly, p(open) for a channel model that is inactivated by Ca(2+) also increases as a function of the domain time constant when the maximum domain [Ca(2+)] is fixed, because slow formation of the Ca(2+) domain attenuates Ca(2+)-mediated inactivation. Conversely, when the source amplitude of the channel is fixed, increasing the domain time constant leads to elevated domain [Ca(2+)] and decreased open probability. Consistent with these observations, a realistic De Young-Keizer-like IP(3)R model responds to residual Ca(2+) with a steady-state open probability that is a monotonic function of the domain time constant, though minimal models that include both Ca(2+)-activation and -inactivation show more complex behavior. We show how the probability density approach described here can be generalized for arbitrarily complex channel models and for any value of the domain time constant. In addition, we present a comparatively simple numerical procedure for estimating p(open) for models of Ca(2+)-regulated Ca(2+) channels in the limit of a very fast or very slow Ca(2+) domain. When the ordinary differential equation for the [Ca(2+)] in a restricted cytoplasmic compartment is replaced by a partial differential equation for the buffered diffusion of intracellular Ca(2+) in a homogeneous isotropic cytosol, we find the dependence of p(open) on the buffer time constant is qualitatively similar to the above-mentioned results.

Thermodynamically irreversible gating of ryanodine receptors in situ revealed by stereotyped duration of release in Ca(2+) sparks

For a single or a group of Markov channels gating reversibly, distributions of open and closed times should be the sum of positively weighted decaying exponentials. Violation of this microscopic reversibility has been demonstrated previously on a number of occasions at the single channel level, and has been attributed to possible channel coupling to external sources of free energy. Here we show that distribution of durations of Ca(2+) release underlying Ca(2+) sparks in intact cardiac myocytes exhibits a prominent mode at approximately 8 ms. Analysis of the cycle time for repetitive sparks at hyperactive sites revealed no intervals briefer than approximately 35 ms and a mode at approximately 90 ms. These results indicate that, regardless of whether Ca(2+) sparks are single-channel or multi-channel in origin, they are generated by thermodynamically irreversible stochastic processes. In contrast, data from planar lipid bilayer experiments were consistent with reversible gating of RyR under asymmetric cis (4 microM) and trans Ca(2+) (10 mM), suggesting that the irreversibility for Ca(2+) spark genesis may reside at a supramolecular level. Modeling suggests that Ca(2+)-induced Ca(2+) release among adjacent RyRs may couple the external energy derived from Ca(2+) gradients across the SR to RyR gating in situ, and drive the irreversible generation of Ca(2+) sparks.

Estimation of the sarcoplasmic reticulum Ca2+ release flux underlying Ca2+ sparks

Using a combination of experimental and numerical approaches, we have tested two different approaches to calculating the sarcoplasmic reticulum (SR) Ca2+ release flux, which gives rise to cardiac muscle Ca2+ sparks. By using two-photon excited spot photolysis of DM-Nitrophen, known Ca2+ release flux time courses were generated to provide the first experimental validation of spark flux reconstruction algorithms. These artificial Ca2+ sparks show that it is possible to calculate the SR Ca2+ release waveform with reasonable accuracy, provided the flux equations reasonably reflect the properties of the experimental system. Within cardiac muscle cells, we show that Ca2+ flux reconstruction is complicated by the substantial dye binding to proteins, a factor that has not been adequately addressed in previous flux reconstruction algorithms. Furthermore, our numerical experiments suggest that the calculated time course of release flux inactivation based on conventional flux reconstruction algorithms is likely to be in error. We therefore developed novel algorithms based on an explicit dye binding scheme. When these algorithm were applied to evoked Ca2+ sparks in rat cardiac ventricular myocytes, the reconstructed Ca2+ release waveform peaked in ~5 ms and decayed with a halftime of approximately 5 ms. The peak flux magnitude was 7-12 pA, suggesting that sparks must arise from clusters of >15 ryanodine receptors.

Calcium signaling mechanisms in dedifferentiated cardiac myocytes: comparison with neonatal and adult cardiomyocytes

Our studies focused on calcium sparking and calcium transients in cultured adult rat cardiomyocytes and compared these findings to those in cultured neonatal and freshly isolated adult cardiomyocytes. Using deconvolution fluorescence microscopy and spec trophotometric image capture, sequence acquisitions were examined for calcium spark intensities, calcium concentrations and whether sparks gave rise to cell contraction events. Observations showed that the preparation of dedifferentiated cardiomyocytes resulted in stellate, neonatal-like cells that exhibited some aspects of calcium transient origination and proliferation similar to events seen in both neonatal and adult myocytes. Ryanodine treatment in freshly isolated adult myocytes blocked the calcium waves, indicating that calcium release at the level of the sarcoplasmic reticulum and t-tubule complex was the initiating factor, and this effect of ryanodine treatment was also seen in cultured-dedifferentiated adult myocytes. However, experiments revealed that in both neonatal and cultured adult myocytes, the inositol triphosphate pathway (IP3) was a major mechanism in the control of intracellular calcium concentrations. In neonatal myocytes, the nucleus and regions adjacent to the plasma membrane we re major sites of calcium release and flux. We conclude: (1) culturing of adult cardiomyocytes leads them to develop mechanisms of calcium homeostasis similar in some aspects to those seen in neonatal cardiomyocytes; (2) neonatal myocytes rely on both extracellular and nuclear calcium for contractile function; and (3) freshly isolated adult myocytes use sarcoplasmic reticulum calcium stores for the initiation of contractile function.

Characteristics of calcium sparks in cardiomyocytes derived from embryonic stem cells

In embryonic stem (ES) cell-derived cardiomyocytes, spontaneous Ca(2+) sparks representing Ca(2+) release through ryanodine receptor (RyR) channels were characterized and correlated to the expression of RyRs as well as the Ca(2+) load of the sarcoplasmic reticulum (SR). In very early developmental stage (VEDS) cardiac precursor cells, global intracellular Ca(2+) concentration ([Ca(2+)](i)) fluctuations occurred, whereas Ca(2+) sparks and contractions were absent. In early developmental stages (EDS), contractions as well as Ca(2+) sparks were obvious. During the further differentiation to late developmental stage (LDS) cardiomyocytes, a marked increase in the frequency of global [Ca(2+)](i) transients, the amplitude and the frequency of Ca(2+) sparks, as well as the expression of RyRs and the volume of RyR-positive SR, was observed. Furthermore, the caffeine-releasable SR Ca(2+) load was elevated in LDS compared with EDS cardiomyocytes. A high-Ca(2+) solution raised spark frequency as well as amplitude in EDS cardiomyocytes to the levels of LDS cardiomyocytes. The characteristics of Ca(2+) sparks occurring in cardiomyocytes differentiated from ES cells may be governed by the Ca(2+) load of the SR and/or the density of RyRs.

Predetermined recruitment of calcium release sites underlies excitation-contraction coupling in rat atrial myocytes

Excitation-contraction coupling (E-C coupling) was studied in isolated fluo-3-loaded rat atrial myocytes at 22 and 37 degrees C using rapid confocal microscopy. Within a few milliseconds of electrical excitation, spatially discrete subsarcolemmal Ca2+ signals were initiated. Twenty to forty milliseconds after stimulation the spatial overlap of these Ca2+ signals gave a 'ring' of elevated Ca2+ around the periphery of the cells. However, this ring was not continuous and substantial Ca2+ gradients were observed. The discrete subsarcolemmal Ca2+-release sites, which responded in a reproducible sequence to repetitive depolarisations and displayed the highest frequencies of spontaneous Ca2+ sparks in resting cells, were denoted 'eager sites'. Immunostaining atrial myocytes for type II ryanodine receptors (RyRs) revealed both subsarcolemmal 'junctional' RyRs, and also 'non-junctional' RyRs in the central bulk of the cells. A subset of the junctional RyRs comprises the eager sites. For cells paced in the presence of 1 mM extracellular Ca2+, the response was largely restricted to a subsarcolemmal 'ring', while the central bulk of the cell displayed a approximately 5-fold lower Ca2+ signal. Under these conditions the non-junctional RyRs were only weakly activated during E-C coupling. However, these channels are functional and the Ca2+ stores were at least partially loaded, since substantial homogeneous Ca2+ signals could be stimulated in the central regions of atrial myocytes by application of 2.5 mM caffeine. Neither the location nor activation order of the eager sites was affected by increasing the trigger Ca2+ current (by increasing extracellular Ca2+ to 10 mM) or the sarcoplasmic reticulum (SR) Ca2+ load (following 1 min incubation in 10 mM extracellular Ca2+), although with increased SR Ca2+ load, but not greater Ca2+ influx, the delay between the sequential activation of eager sites was reduced. In addition, increasing the trigger Ca2+ current or the SR Ca2+ load changed the spatial pattern of the Ca2+ response, in that the Ca2+ signal propagated more reliably from the subsarcolemmal initiation sites into the centre of the cell. Due to the greater spatial spread of the Ca2+ signals, the averaged global Ca2+ transients increased by approximately 500 %. We conclude that rat atrial myocytes display a predetermined spatiotemporal pattern of Ca2+ signalling during early E-C coupling. A consistent set of eager Ca2+ release sites with a fixed location and activation order on the junctional SR serve to initiate the cellular response. The short latency for activation of these eager sites suggests that they reflect clusters of RyRs closely coupled to voltage-operated Ca2+ channels in the sarcolemma. Furthermore, their propensity to show spontaneous Ca2+ sparks is consistent with an intrinsically higher sensitivity to Ca2+-induced Ca2+ release. While the subsarcolemmal Ca2+ response can be considered as stereotypic, the central bulk of the cell grades its response in direct proportion to cellular Ca2+ load and Ca2+ influx.

Large currents generate cardiac Ca2+ sparks

Previous models of cardiac Ca2+ sparks have assumed that Ca2+ currents through the Ca2+ release units (CRUs) were approximately 1-2 pA, producing sparks with peak fluorescence ratio (F/F(0)) of approximately 2.0 and a full-width at half maximum (FWHM) of approximately 1 microm. Here, we present actual Ca2+ sparks with peak F/F(0) of >6 and a FWHM of approximately 2 microm, and a mathematical model of such sparks, the main feature of which is a much larger underlying Ca2+ current. Assuming infinite reaction rates and no endogenous buffers, we obtain a lower bound of approximately 11 pA needed to generate a Ca2+ spark with FWHM of 2 microm. Under realistic conditions, the CRU current must be approximately 20 pA to generate a 2- microm Ca2+)spark. For currents > or =5 pA, the computed spark amplitudes (F/F(0)) are large (approximately 6-12 depending on buffer model). We considered several factors that might produce sparks with FWHM approximately 2 microm without using large currents. Possible protein-dye interactions increased the FWHM slightly. Hypothetical Ca2+ "quarks" had little effect, as did blurring of sparks by the confocal microscope. A clusters of CRUs, each producing 10 pA simultaneously, can produce sparks with FWHM approximately 2 microm. We conclude that cardiac Ca2+ sparks are significantly larger in peak amplitude than previously thought, that such large Ca2+ sparks are consistent with the measured FWHM of approximately 2 microm, and that the underlying Ca2+ current is in the range of 10-20 pA.

Immunogold-labeled L-type calcium channels are clustered in the surface plasma membrane overlying junctional sarcoplasmic reticulum in guinea-pig myocytes-implications for excitation-contraction coupling in cardiac muscle

Ca(2+) release through ryanodine receptors, located in the membrane of the junctional sarcoplasmic reticulum (SR), initiates contraction of cardiac muscle. Ca(2+)influx through plasma membrane L-type Ca(2+)channels is thought to be an important trigger for opening ryanodine receptors ("Ca(2+)-induced Ca(2+)-release"). Optimal transmission of the transmembrane Ca(2+)influx signal to SR release is predicted to involve spatial juxtaposition of L-type Ca(2+)channels to the ryanodine receptors of the junctional SR. Although such spatial coupling has often been implicitly assumed, and data from immunofluorescence microscopy are consistent with its existence, the definitive demonstration of such a structural organization in mammalian tissue is lacking at the electron-microscopic level. To determine the spatial distribution of plasma membrane L-type Ca(2+)channels and their location in relation to underlying junctional SR, we applied two high-resolution immunogold-labeling techniques, label-fracture and cryothin-sectioning, combined with quantitative analysis, to guinea-pig ventricular myocytes. Label-fracture enabled visualization of colloidal gold-labeled L-type Ca(2+)channels in planar freeze-fracture electron-microscopic views of the plasma membrane. Mathematical analysis of the gold label distribution (by nearest-neighbor distance distribution and the radial distribution function) demonstrated genuine clustering of the labeled channels. Gold-labeled cryosections showed that labeled L-type Ca(2+)channels quantitatively predominated in domains of the plasma membrane overlying junctional SR. These findings provide an ultrastructural basis for functional coupling between L-type Ca(2+)channels and junctional SR and for excitation-contraction coupling in guinea-pig cardiac muscle.

Two mechanisms for termination of individual Ca2+ sparks in skeletal muscle

Ca(2+) sparks are brief, localized elevations of myoplasmic [Ca(2+)] caused by release of increments of Ca(2+) via sarcoplasmic reticulum Ca(2+) release channels in muscle. The properties of individual sparks provide information regarding the opening of sarcoplasmic reticulum Ca(2+) channels within functioning cells. Here we use high-speed confocal microscopy to show that individual Ca(2+) sparks activated by membrane depolarization in single frog skeletal muscle fibers can be terminated prematurely by repolarization. Thus, either voltage sensor deactivation on repolarization or release channel inactivation during continued depolarization can terminate the Ca(2+) release channel activity underlying voltage-activated Ca(2+) sparks in skeletal muscle.

Expression of ryanodine receptor RyR3 produces Ca2+ sparks in dyspedic myotubes

Discrete, localized elevations of myoplasmic [Ca2+], Ca2+ 'sparks', were readily detected using the fluorescent Ca2+ indicator fluo-3 and laser scanning confocal microscopy in 'dyspedic' 1B5 myotubes, i.e. myotubes which do not express ryanodine receptors (RyRs), transduced with virions containing cDNA for RyR type 3 that were saponin permeabilized to allow dye entry. Ca2+ sparks were never observed in non-transduced RyR null myotubes. The spatial locations of sparks observed in permeabilized myotubes roughly corresponded to regions of RyR protein expression in the same myotube as detected after subsequent fixation and antibody staining. Permeabilized RyR3-transduced myotubes exhibited similar punctate peripheral RyR3 protein immunohistochemical patterns as myotubes fixed before permeabilization indicating that permeabilization did not affect the structural organization of the triad. Ca2+ sparks, recorded in line scan mode, in permeabilized myotubes expressing RyR3 exhibited mean amplitudes (change in fluorescence/mean fluorescence, DeltaF/F: 1.20 +/- 0.04) and temporal rise times (10-90%; 6.31 +/- 0.12 ms) similar to those of sparks recorded in permeabilized frog skeletal muscle fibres (0.98 +/- 0.01; 6.11 +/- 0.07, respectively) using the same confocal system. Spatial extent and temporal duration of the Ca2+ sparks were approximately 40% larger in the RyR3-expressing myotube cultures than in frog fibres. Ca2+ sparks recorded in line scan mode often occurred repetitively at the same spatial location in RyR3-expressing myotubes. Such repetitive events were highly reproducible in amplitude and spatio-temporal properties, as previously observed for repetitive mode sparks in frog skeletal muscle. Ca2+ sparks recorded in xy mode were frequently compressed in the y (slower scan) direction compared to the x direction. This asymmetry was reproduced assuming spatially symmetric events having the time course of Ca2+ sparks recorded in line scan (xt) mode. These expression studies demonstrate that the presence of RyR3 is sufficient for the production of Ca2+ sparks in a skeletal muscle system lacking the expression of any other RyR isoform.

Imaging Ca(2+) entering the cytoplasm through a single opening of a plasma membrane cation channel

Discrete localized fluorescence transients due to openings of a single plasma membrane Ca(2+) permeable cation channel were recorded using wide-field digital imaging microscopy with fluo-3 as the Ca(2+) indicator. These transients were obtained while simultaneously recording the unitary channel currents using the whole-cell current-recording configuration of the patch-clamp technique. This cation channel in smooth muscle cells is opened by caffeine (Guerrero, A., F.S. Fay, and J.J. Singer. 1994. J. Gen. Physiol. 104:375-394). The localized fluorescence transients appeared to occur at random locations on the cell membrane, with the duration of the rising phase matching the duration of the channel opening. Moreover, these transients were only observed in the presence of sufficient extracellular Ca(2+), suggesting that they are due to Ca(2+) influx from the bathing solution. The fluorescence transient is characterized by an initial fast rising phase when the channel opens, followed by a slower rising phase during prolonged openings. When the channel closes there is an immediate fast falling phase followed by a slower falling phase. Computer simulations of the underlying events were used to interpret the time course of the transients. The rapid phases are mainly due to the establishment or removal of Ca(2+) and Ca(2+)-bound fluo-3 gradients near the channel when the channel opens or closes, while the slow phases are due to the diffusion of Ca(2+) and Ca(2+)-bound fluo-3 into the cytoplasm. Transients due to short channel openings have a "Ca(2+) spark-like" appearance, suggesting that the rising and early falling components of sparks (due to openings of ryanodine receptors) reflect the fast phases of the fluorescence change. The results presented here suggest methods to determine the relationship between the fluorescence transient and the underlying Ca(2+) current, to study intracellular localized Ca(2+) handling as might occur from single Ca(2+) channel openings, and to localize Ca(2+) permeable ion channels on the plasma membrane.

Shape, size, and distribution of Ca(2+) release units and couplons in skeletal and cardiac muscles

Excitation contraction (e-c) coupling in skeletal and cardiac muscles involves an interaction between specialized junctional domains of the sarcoplasmic reticulum (SR) and of exterior membranes (either surface membrane or transverse (T) tubules). This interaction occurs at special structures named calcium release units (CRUs). CRUs contain two proteins essential to e-c coupling: dihydropyridine receptors (DHPRs), L-type Ca(2+) channels of exterior membranes; and ryanodine receptors (RyRs), the Ca(2+) release channels of the SR. Special CRUs in cardiac muscle are constituted by SR domains bearing RyRs that are not associated with exterior membranes (the corbular and extended junctional SR or EjSR). Functional groupings of RyRs and DHPRs within calcium release units have been named couplons, and the term is also loosely applied to the EjSR of cardiac muscle. Knowledge of the structure, geometry, and disposition of couplons is essential to understand the mechanism of Ca(2+) release during muscle activation. This paper presents a compilation of quantitative data on couplons in a variety of skeletal and cardiac muscles, which is useful in modeling calcium release events, both macroscopic and microscopic ("sparks").

Ca2+ spark as a regulator of ion channel activity

Ca2+ spark is a local and transient Ca2+ release from sarcoplasmic reticulum (SR) through the ryanodine receptor Ca2+-releasing channel (RyR). In cardiac myocytes, Ca2+ spark is an elementary unit of Ca2+-induced Ca2+ release (CICR) by opening of RyR(s) in junctional SR (jSR), which is triggered by Ca2+-influx through L-type Ca2+ channels to the narrow space between a transverse tubule and jSR. Ca2+ spark has, therefore, been described as the evidence for "the local control of excitation-contraction coupling". In contrast, Ca2+ sparks in smooth muscle have been reported in relation to Ca2+-dependent K+ (K(Ca)) channel activation and muscle relaxation. A spontaneous Ca2+ spark in a superficial area activates 10-100 K(Ca) channels nearby and induces membrane hyperpolarization, which reduces Ca2+ channel activity. In several types of smooth muscle cells, which have relatively high membrane excitability, an action potential (AP) elicits 5-20 Ca2+ hot spots (evoked sparks with long life) in the early stage via CICR in discrete superficial SR elements and activates K(Ca)-channel current highly responsible for AP repolarization and afterhyperpolarization. CICR available for contraction may occur more slowly by the propagation of CICR from superficial SR to deeper ones. The regulatory mechanism of ion channel activity on plasma membrane by superficial SR via Ca2+ spark generation in smooth muscle cells may be analogously common in several types of cells including neurons.

A repetitive mode of activation of discrete Ca2+ release events (Ca2+ sparks) in frog skeletal muscle fibres

1. Ca2+ release events (Ca2+ 'sparks'), which are believed to arise from the opening of a sarcoplasmic reticulum (SR) Ca2+ release channel or a small cluster of such channels that act as a release unit, have been measured in single, frog (Rana pipiens) skeletal muscle fibres. 2. Under conditions of extremely low rates of occurrence of Ca2+ sparks we observed, within individual identified triads, repetitive Ca2+ release events which occurred at a frequency more than 100-fold greater than the prevailing average event rate. Repetitive sparks were recorded during voltage-clamp test depolarizations after a brief (0.3-2 s) repriming interval in fibres held at 0 mV and in chronically depolarized, 'notched' fibres. 3. These repetitive events are likely to arise from the re-opening of the same SR Ca2+ release channel or release unit operating in a repetitive gating mode ('rep-mode'), rather than from the random activation of multiple, independent channels or release units within a triad. A train of rep-mode events thus represents a series of Ca2+ sparks arising from a single location within the fibre. Rep-mode events are activated among different triads in a random manner after brief repriming. The frequency of repetitive events among all identified events during voltage-clamp depolarization to 0 mV after brief repriming was 3.9 +/- 1.3 %. The occurrence of repetitive events was not related to exposure of the fibre to laser illumination. 4. The events observed within a rep-mode train exhibited a relatively uniform amplitude. Analysis of intervals between identified events in triads exhibiting rep-mode trains indicated similar variations of fluorescence as in neighbouring, quiescent triads, suggesting there was not a significant number of small, unidentified events at the triads exhibiting rep-mode activity. 5. The distribution of rep-mode interspark intervals exhibited a paucity of events at short intervals, consistent with the need for recovery from inactivation before activation of the next event in a repetitive train. The mean interspark interval of repetitive sparks during voltage-clamp depolarizations was 88 +/- 5 ms, and was independent of membrane potential. 6. The individual Ca2+ sparks within a rep-mode train were similar in average amplitude and spatiotemporal extent to singly occurring sparks, suggesting a common mechanism for termination of the channel opening(s) underlying both types of events. The average properties of the sparks did not vary during a train. The relative amplitude of a spark within a rep-mode was not correlated with its rise time. 7. Repetitive Ca2+ release events represent a mode of gating of SR Ca2+ release channels which may be significant during long depolarizations and which may be influenced by the biochemical state of the SR ryanodine receptor Ca2+ release channels.

Amplitude distribution of calcium sparks in confocal images: theory and studies with an automatic detection method

Determination of the calcium spark amplitude distribution is of critical importance for understanding the nature of elementary calcium release events in striated muscle. In the present study we show, on general theoretical grounds, that calcium sparks, as observed in confocal line scan images, should have a nonmodal, monotonic decreasing amplitude distribution, regardless of whether the underlying events are stereotyped. To test this prediction we developed, implemented, and verified an automated computer algorithm for objective detection and measurement of calcium sparks in raw image data. When the sensitivity and reliability of the algorithm were set appropriately, we observed highly left-skewed or monotonic decreasing amplitude distributions in skeletal muscle cells and cardiomyocytes, confirming the theoretical predictions. The previously reported modal or Gaussian distributions of sparks detected by eye must therefore be the result of subjective detection bias against small amplitude events. In addition, we discuss possible situations when a modal distribution might be observed.

Hormone-evoked elementary Ca2+ signals are not stereotypic, but reflect activation of different size channel clusters and variable recruitment of channels within a cluster

Previous studies of (InsP3)-evoked elementary Ca2+ events suggested a hierarchy of signals; fundamental events ("Ca2+ blips") arising from single InsP3 receptors (InsP3Rs), and intermediate events ("Ca2+ puffs") reflecting the coordinated opening of a cluster of InsP3Rs. The characteristics of such elementary Ca2+ release signals provide insights into the functional interaction and distribution of InsP3Rs in living cells. Therefore we investigated whether elementary Ca2+ signaling is truly represented by such stereotypic release events. A histogram of >900 events revealed a wide spread of signal amplitudes (20-600 nM; mean 216 +/- 4 nM; n = 206 cells), which cannot be explained by stochastic variation of a stereotypic Ca2+ release site. We identified elementary Ca2+ release sites with consistent amplitudes (<20% difference) and locations with variable amplitudes (approximately 500% difference). Importantly, within single cells, distinct sites displayed events with significantly different mean amplitudes. Additional determinants affecting the magnitude of elementary Ca2+ release were identified to be (i) hormone concentration, (ii) day-to-day variability, and (iii) a progressively decreasing Ca2+ release during prolonged stimulation. We therefore suggest that elementary Ca2+ events are not stereotypic, instead a continuum of signals can be achieved by either recruitment of entire clusters with different numbers of InsP3Rs or by a graded recruitment of InsP3Rs within a cluster.