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

Ca2+ Release Channels Join the 'Resolution Revolution'

Ryanodine receptors (RyRs) are calcium release channels expressed in the sarcoendoplasmic reticula of many cell types including cardiac and skeletal muscle cells. In recent years Ca2+ leak through RyRs has been implicated as a major contributor to the development of diseases including heart failure, muscle myopathies, Alzheimer's disease, and diabetes, making it an important therapeutic target. Recent mammalian RyR1 cryoelectron microscopy (cryo-EM) structures of multiple functional states have clarified longstanding questions including the architecture of the transmembrane (TM) pore and cytoplasmic domains, the location and architecture of the channel gate, ligand-binding sites, and the gating mechanism. As we advance toward complete models of RyRs this new information enables the determination of domain-domain interfaces and the location and structural effects of disease-causing RyR mutations.

Parvalbumin is overexpressed in the late phase of pharmacological preconditioning in skeletal muscle

Pharmacological preconditioning (PPC) with mitochondrial ATP-sensitive K(+) channel openers such as diazoxide, provides protection against ischemia in cardiac muscle, skeletal muscle, and other tissues. Effects on Ca(2+) homeostasis during the late phase of PPC have been described in cardiomyocytes, but no information is available regarding intracellular Ca(2+) changes in skeletal muscle fibers during late PPC. Intracellular Ca(2+) signals were measured in single fibers of adult mouse skeletal muscle, with fluorescent probes, 48 h after the administration of diazoxide. Parvalbumin levels in the myofibers were quantitated by Western blot. Diazoxide induction of late PPC was confirmed by partial protection of muscles from peroxide-induced damage. Late PPC was associated with a significant decrease in the duration of Ca(2+) signals during single twitches and tetanus with no changes in peak values. This effect was prevented by the reactive oxygen species (ROS) scavenger tiron. Late PPC was accompanied by a 30% increase in parvalbumin levels, and this effect was also blocked by tiron. Our data show, for the first time, a role of parvalbumin in late PPC in skeletal muscle.

Properties of Ca2+ sparks revealed by four-dimensional confocal imaging of cardiac muscle

Parameters (amplitude, width, kinetics) of Ca²⁺ sparks imaged confocally are affected by errors when the spark source is not in focus. To identify sparks that were in focus, we used fast scanning (LSM 5 LIVE; Carl Zeiss) combined with fast piezoelectric focusing to acquire x–y images in three planes at 1-µm separation (x-y-z-t mode). In 3,000 x–y scans in each of 34 membrane-permeabilized cat atrial cardiomyocytes, 6,906 sparks were detected. 767 sparks were in focus. They had greater amplitude, but their spatial width and rise time were similar compared with all sparks recorded. Their distribution of amplitudes had a mode at ΔF/F₀ = 0.7. The Ca²⁺ release current underlying in-focus sparks was 11 pA, requiring 20 to 30 open channels, a number at the high end of earlier estimates. Spark frequency was greater than in earlier imaging studies of permeabilized ventricular cells, suggesting a greater susceptibility to excitation, which could have functional relevance for atrial cells. Ca²⁺ release flux peaked earlier than the time of peak fluorescence and then decayed, consistent with significant sarcoplasmic reticulum (SR) depletion. The evolution of fluorescence and release flux were strikingly similar for in-focus sparks of different rise time (T). Spark termination involves both depletion of Ca²⁺ in the SR and channel closure, which may be synchronized by depletion. The observation of similar flux in sparks of different T requires either that channel closure and other termination processes be independent of the determinants of flux (including [Ca²⁺]_SR) or that different channel clusters respond to [Ca²⁺]_SR with different sensitivity.

Synthetic localized calcium transients directly probe signalling mechanisms in skeletal muscle

The contribution of Ca2+-induced Ca2+ release (CICR) to trigger muscle contraction is controversial. It was studied on isolated muscle fibres using synthetic localized increases in Ca2+ concentration, SLICs, generated by two-photon photorelease from nitrodibenzofuran (NDBF)-EGTA just outside the permeabilized plasma membrane. SLICs provided a way to increase cytosolic [Ca2+] rapidly and reversibly, up to 8 μM, levels similar to those reached during physiological activity. They improve over previous paradigms in rate of rise, locality and reproducibility. Use of NDBF-EGTA allowed for the separate modification of resting [Ca2+], trigger [Ca2+] and resting [Mg2+]. In frog muscle, SLICs elicited propagated responses that had the characteristics of CICR. The threshold [Ca2+] for triggering a response was 0.5 μM or less. As this value is much lower than concentrations prevailing near channels during normal activity, the result supports participation of CICR in the physiological control of contraction in amphibian muscle. As SLICs were applied outside cells, the primary stimulus was Ca2+, rather than the radiation or subproducts of photorelease. Therefore the responses qualify as ‘classic' CICR. By contrast, mouse muscle fibres did not respond unless channel-opening drugs were present at substantial concentrations, an observation contrary to the physiological involvement of CICR in mammalian excitation–contraction coupling. In mouse muscle, the propagating wave had a substantially lower release flux, which together with a much higher threshold justified the absence of response when drugs were not present. The differences in flux and threshold may be ascribed to the absence of ryanodine receptor 3 (RyR3) isoforms in adult mammalian muscle.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

Dihydropyridine (DHPR) and ryanodine receptors (RyRs) are central to transduction of transverse (T) tubular membrane depolarisation initiated by surface action potentials into release of sarcoplasmic reticular (SR) Ca2+ in skeletal muscle excitation-contraction coupling. Electronmicroscopic methods demonstrate an orderly positioning of such tubular DHPRs relative to RyRs in the SR at triad junctions where their membranes come into close proximity. Biochemical and genetic studies associated expression of specific, DHPR and RyR, isoforms with the particular excitation-contraction coupling processes and related elementary Ca2+ release events found respectively in skeletal and cardiac muscle. Physiological studies of intramembrane charge movements potentially related to voltage triggering of Ca2+ release demonstrated a particular qγ charging species identifiable with DHPRs through its T-tubular localization, pharmacological properties, and steep voltage-dependence paralleling Ca2+ release. Its nonlinear kinetics implicated highly co-operative conformational events in its transitions in response to voltage change. The effects of DHPR and RyR agonists and antagonists upon this intramembrane charge in turn implicated reciprocal rather than merely unidirectional DHPR-RyR interactions in these complex reactions. Thus, following membrane potential depolarization, an orthograde qγ-DHPR-RyR signaling likely initiates conformational alterations in the RyR with which it makes contact. The latter changes could then retrogradely promote further qγ-DHPR transitions through reciprocal co-operative allosteric interactions between receptors. These would relieve the resting constraints on both further, delayed, nonlinear qγ-DHPR charge transfers and on RyR-mediated Ca2+ release. They would also explain the more rapid charging and recovery qγ transients following larger depolarizations and membrane potential repolarization to the resting level.

Intracellular Ca2+ transients in delta-sarcoglycan knockout mouse skeletal muscle

BACKGROUND

delta-Sarcoglycan (delta-SG) knockout (KO) mice develop skeletal muscle histopathological alterations similar to those in humans with limb muscular dystrophy. Membrane fragility and increased Ca(2+) permeability have been linked to muscle degeneration. However, little is known about the mechanisms by which genetic defects lead to disease.

METHODS

Isolated skeletal muscle fibers of wild-type and delta-SG KO mice were used to investigate whether the absence of delta-SG alters the increase in intracellular Ca(2+) during single twitches and tetani or during repeated stimulation. Immunolabeling, electrical field stimulation and Ca(2+) transient recording techniques with fluorescent indicators were used.

RESULTS

Ca(2+) transients during single twitches and tetani generated by muscle fibers of delta-SG KO mice are similar to those of wild-type mice, but their amplitude is greatly decreased during protracted stimulation in KO compared to wild-type fibers. This impairment is independent of extracellular Ca(2+) and is mimicked in wild-type fibers by blocking store-operated calcium channels with 2-aminoethoxydiphenyl borate (2-APB). Also, immunolabeling indicates the localization of a delta-SG isoform in the sarcoplasmic reticulum of the isolated skeletal muscle fibers of wild-type animals, which may be related to the functional differences between wild-type and KO muscles.

CONCLUSIONS

delta-SG has a role in calcium homeostasis in skeletal muscle fibers.

GENERAL SIGNIFICANCE

These results support a possible role of delta-SG on calcium homeostasis. The alterations caused by the absence of delta-SG may be related to the pathogenesis of muscular dystrophy.

Calcium-Induced Calcium Release in Skeletal Muscle

Calcium-induced calcium release (CICR) was first discovered in skeletal muscle. CICR is defined as Ca2+ release by the action of Ca2+ alone without the simultaneous action of other activating processes. CICR is biphasically dependent on Ca2+ concentration; is inhibited by Mg2+, procaine, and tetracaine; and is potentiated by ATP, other adenine compounds, and caffeine. With depolarization of the sarcoplasmic reticulum (SR), a potential change of the SR membrane in which the luminal side becomes more negative, CICR is activated for several seconds and is then inactivated. All three types of ryanodine receptors (RyRs) show CICR activity. At least one RyR, RyR1, also shows non-CICR Ca2+ release, such as that triggered by the t-tubule voltage sensor, by clofibric acid, and by SR depolarization. Maximum rates of CICR, at the optimal Ca2+ concentration in the presence of physiological levels of ATP and Mg2+ determined in skinned fibers and fragmented SR, are much lower than the rate of physiological Ca2+ release. The primary event of physiological Ca2+ release, the Ca2+ spark, is the simultaneous opening of multiple channels, the coordinating mechanism of which does not appear to be CICR because of the low probability of CICR opening under physiological conditions. The coordination may require Ca2+, but in that case, some other stimulus or stimuli must be provided simultaneously, which is not CICR by definition. Thus CICR does not appear to contribute significantly to physiological Ca2+ release. On the other hand, CICR appears to play a key role in caffeine contracture and malignant hyperthermia. The potentiation of voltage-activated Ca2+ release by caffeine, however, does not seem to occur through secondary CICR, although the site where caffeine potentiates voltage-activated Ca2+ release might be the same site where caffeine potentiates CICR.

Dynamic interreceptor coupling contributes to the consistent open duration of ryanodine receptors

Ca2+ spark is the elementary Ca2+ signaling event in muscle excitation-contraction coupling. The rise time of Ca2+ spark is rather stable under different conditions, suggesting consistent open duration of ryanodine receptors (RyRs) in vivo. It has been proposed that the array-based behavior of RyRs plays an important role in shaping Ca2+ spark dynamics, particularly in controlling the open duration of RyR clusters. Therefore, we investigated the possible role of inter-RyR coupling in stabilization of the open time of arrayed RyRs under several potential perturbations, for instance, array size, inter-RyR coupling noise, and up-regulation or down-regulation of the activity of partial RyRs in the array. We found that RyR arrays with dynamic coupling showed consistent open duration against the perturbations, whereas the RyR array with constant coupling did not. On the other hand, the open probability and amplitude of RyR arrays with dynamic interreceptor coupling were sensitive to the perturbations. These two points were consistent with experimental observations, indicating that the RyR array with dynamic coupling could recapitulate in vivo open properties of RyRs. Our findings support the idea that dynamic coupling is a feasible in vivo working mechanism of RyR arrays.

Control of cell volume in skeletal muscle

Regulation of cell volume is a fundamental property of all animal cells and is of particular importance in skeletal muscle where exercise is associated with a wide range of cellular changes that would be expected to influence cell volume. These complex electrical, metabolic and osmotic changes, however, make rigorous study of the consequences of individual factors on muscle volume difficult despite their likely importance during exercise. Recent charge-difference modelling of cell volume distinguishes three major aspects to processes underlying cell volume control: (i) determination by intracellular impermeant solute; (ii) maintenance by metabolically dependent processes directly balancing passive solute and water fluxes that would otherwise cause cell swelling under the influence of intracellular membrane-impermeant solutes; and (iii) volume regulation often involving reversible short-term transmembrane solute transport processes correcting cell volumes towards their normal baselines in response to imposed discrete perturbations. This review covers, in turn, the main predictions from such quantitative analysis and the experimental consequences of comparable alterations in extracellular pH, lactate concentration, membrane potential and extracellular tonicity. The effects of such alterations in the extracellular environment in resting amphibian muscles are then used to reproduce the intracellular changes that occur in each case in exercising muscle. The relative contributions of these various factors to the control of cell volume in resting and exercising skeletal muscle are thus described.

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.

Termination of cardiac Ca2+ sparks: role of intra-SR [Ca2+], release flux, and intra-SR Ca2+ diffusion

Ca(2+) release from cardiac sarcoplasmic reticulum (SR) via ryanodine receptors (RyRs) is regulated by dyadic cleft [Ca(2+)] and intra-SR free [Ca(2+)] ([Ca(2+)](SR)). Robust SR Ca(2+) release termination is important for stable excitation-contraction coupling, and partial [Ca(2+)](SR) depletion may contribute to release termination. Here, we investigated the regulation of SR Ca(2+) release termination of spontaneous local SR Ca(2+) release events (Ca(2+) sparks) by [Ca(2+)](SR), release flux, and intra-SR Ca(2+) diffusion. We simultaneously measured Ca(2+) sparks and Ca(2+) blinks (localized elementary [Ca(2+)](SR) depletions) in permeabilized ventricular cardiomyocytes over a wide range of SR Ca(2+) loads and release fluxes. Sparks terminated via a [Ca(2+)](SR)-dependent mechanism at a fixed [Ca(2+)](SR) depletion threshold independent of the initial [Ca(2+)](SR) and release flux. Ca(2+) blink recovery depended mainly on intra-SR Ca(2+) diffusion rather than SR Ca(2+) uptake. Therefore, the large variation in Ca(2+) blink recovery rates at different release sites occurred because of differences in the degree of release site interconnection within the SR network. When SR release flux was greatly reduced, long-lasting release events occurred from well-connected junctions. These junctions could sustain release because local SR Ca(2+) release and [Ca(2+)](SR) refilling reached a balance, preventing [Ca(2+)](SR) from depleting to the termination threshold. Prolonged release events eventually terminated at a steady [Ca(2+)](SR), indicative of a slower, [Ca(2+)](SR)-independent termination mechanism. These results demonstrate that there is high variability in local SR connectivity but that SR Ca(2+) release terminates at a fixed [Ca(2+)](SR) termination threshold. Thus, reliable SR Ca(2+) release termination depends on tight RyR regulation by [Ca(2+)](SR).

Calcium-dependent inactivation terminates calcium release in skeletal muscle of amphibians

In skeletal muscle of amphibians, the cell-wide cytosolic release of calcium that enables contraction in response to an action potential appears to be built of Ca²⁺ sparks. The mechanism that rapidly terminates this release was investigated by studying the termination of Ca²⁺ release underlying sparks. In groups of thousands of sparks occurring spontaneously in membrane-permeabilized frog muscle cells a complex relationship was found between amplitude a and rise time T, which in sparks corresponds to the active time of the underlying Ca²⁺ release. This relationship included a range of T where a paradoxically decreased with increasing T. Three different methods were used to estimate Ca²⁺ release flux in groups of sparks of different T. Using every method, it was found that T and flux were inversely correlated, roughly inversely proportional. A simple model in which release sources were inactivated by cytosolic Ca²⁺ was able to explain the relationship. The predictive value of the model, evaluated by analyzing the variance of spark amplitude, was found to be high when allowance was made for the out-of-focus error contribution to the total variance. This contribution was estimated using a theory of confocal scanning (Ríos, E., N. Shirokova, W.G. Kirsch, G. Pizarro, M.D. Stern, H. Cheng, and A. González. Biophys. J. 2001. 80:169–183), which was confirmed in the present work by simulated line scanning of simulated sparks. Considering these results and other available evidence it is concluded that Ca²⁺-dependent inactivation, or CDI, provides the crucial mechanism for termination of sparks and cell-wide Ca²⁺ release in amphibians. Given the similarities in kinetics of release termination observed in cell-averaged records of amphibian and mammalian muscle, and in spite of differences in activation mechanisms, CDI is likely to play a central role in mammals as well. Trivially, an inverse proportionality between release flux and duration, in sparks or in global release of skeletal muscle, maintains constancy of the amount of released Ca²⁺.

Ca2+ entry-independent effects of L-type Ca2+ channel modulators on Ca2+ sparks in ventricular myocytes

During the cardiac action potential, Ca(2+) entry through dyhidropyridine receptor L-type Ca(2+) channels (DHPRs) activates ryanodine receptors (RyRs) Ca(2+)-release channels, resulting in massive Ca(2+) mobilization from the sarcoplasmic reticulum (SR). This global Ca(2+) release arises from spatiotemporal summation of many localized elementary Ca(2+)-release events, Ca(2+) sparks. We tested whether DHPRs modulate Ca(2+)sparks in a Ca(2+) entry-independent manner. Negative modulation by DHPR of RyRs via physical interactions is accepted in resting skeletal muscle but remains controversial in the heart. Ca(2+) sparks were studied in cat cardiac myocytes permeabilized with saponin or internally perfused via a patch pipette. Bathing and pipette solutions contained low Ca(2+) (100 nM). Under these conditions, Ca(2+) sparks were detected with a stable frequency of 3-5 sparks.s(-1).100 microm(-1). The DHPR blockers nifedipine, nimodipine, FS-2, and calciseptine decreased spark frequency, whereas the DHPR agonists Bay-K8644 and FPL-64176 increased it. None of these agents altered the spatiotemporal characteristics of Ca(2+) sparks. The DHPR modulators were also without effect on SR Ca(2+) load (caffeine-induced Ca(2+) transients) or sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) activity (Ca(2+) loading rates of isolated SR microsomes) and did not change cardiac RyR channel gating (planar lipid bilayer experiments). In summary, DHPR modulators affected spark frequency in the absence of DHPR-mediated Ca(2+) entry. This action could not be attributed to a direct action of DHPR modulators on SERCA or RyRs. Our results suggest that the activity of RyR Ca(2+)-release units in ventricular myocytes is modulated by Ca(2+) entry-independent conformational changes in neighboring DHPRs.

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.

Effects of tetracaine on voltage-activated calcium sparks in frog intact skeletal muscle fibers

The properties of Ca²⁺ sparks in frog intact skeletal muscle fibers depolarized with 13 mM [K⁺] Ringer's are well described by a computational model with a Ca²⁺ source flux of amplitude 2.5 pA (units of current) and duration 4.6 ms (18 °C; Model 2 of Baylor et al., 2002). This result, in combination with the values of single-channel Ca²⁺ current reported for ryanodine receptors (RyRs) in bilayers under physiological ion conditions, 0.5 pA (Kettlun et al., 2003) to 2 pA (Tinker et al., 1993), suggests that 1–5 RyR Ca²⁺ release channels open during a voltage-activated Ca²⁺ spark in an intact fiber. To distinguish between one and greater than one channel per spark, sparks were measured in 8 mM [K⁺] Ringer's in the absence and presence of tetracaine, an inhibitor of RyR channel openings in bilayers. The most prominent effect of 75–100 μM tetracaine was an approximately sixfold reduction in spark frequency. The remaining sparks showed significant reductions in the mean values of peak amplitude, decay time constant, full duration at half maximum (FDHM), full width at half maximum (FWHM), and mass, but not in the mean value of rise time. Spark properties in tetracaine were simulated with an updated spark model that differed in minor ways from our previous model. The simulations show that (a) the properties of sparks in tetracaine are those expected if tetracaine reduces the number of active RyR Ca²⁺ channels per spark, and (b) the single-channel Ca²⁺ current of an RyR channel is ≤1.2 pA under physiological conditions. The results support the conclusion that some normal voltage-activated sparks (i.e., in the absence of tetracaine) are produced by two or more active RyR Ca²⁺ channels. The question of how the activation of multiple RyRs is coordinated is discussed.

Depletion "skraps" and dynamic buffering inside the cellular calcium store

Ca2+ signals, produced by Ca2+ release from cellular stores, switch metabolic responses inside cells. In muscle, Ca2+ sparks locally exhibit the rapid start and termination of the cell-wide signal. By imaging Ca2+ inside the store using shifted excitation and emission ratioing of fluorescence, a surprising observation was made: Depletion during sparks or voltage-induced cell-wide release occurs too late, continuing to progress even after the Ca2+ release channels have closed. This finding indicates that Ca2+ is released from a "proximate" compartment functionally in between store lumen and cytosol. The presence of a proximate compartment also explains a paradoxical surge in intrastore Ca2+, which was recorded upon stimulation of prolonged, cell-wide Ca2+ release. An intrastore surge upon induction of Ca2+ release was first reported in subcellular store fractions, where its source was traced to the store buffer, calsequestrin. The present results update the evolving concept, largely due to N. Ikemoto and C. Kang, of calsequestrin as a dynamic store. Given the strategic location and reduction of dimensionality of Ca2+-adsorbing linear polymers of calsequestrin, they could deliver Ca2+ to the open release channels more efficiently than the luminal store solution, thus constituting the proximate compartment. When store depletion becomes widespread, the polymers would collapse to increase store [Ca2+] and sustain the concentration gradient that drives release flux.

Ciliary neurotrophic factor promotes inactivation of muscle Ca2+ channels via PKC

The actions of the ciliary neurotrophic factor (CNTF) were assessed on adult mouse skeletal muscle L-type Ca2+ currents and on Ca2+ release from sarcoplasmic reticulum. Currents were measured with the whole cell patch clamp technique. Ca2+ signals in response to single action potentials were recorded with Fluo3-AM. CNTF (20 ng/ml) reversibly reduced the amplitude of Ca2+ channel currents by 50% within 15 min. In addition, CNTF greatly increased the rate of inactivation during depolarizing pulses and shifted the steady state inactivation curve by -12 mV. The effects of CNTF were mimicked by the PKC activator PMA and prevented by the PKC-inhibitor chelerythrine. In contrast to the effects on the Ca2+ conductance, charge movement and Ca2+ signals remained unaffected by CNTF. These results suggest that CNTF can rapidly decrease muscle Ca2+ channel currents by promoting inactivation, probably through an intracellular PKC-dependent mechanism.

Short-term regulation of excitation-contraction coupling by the beta1a subunit in adult mouse skeletal muscle

The beta1a subunit of the skeletal muscle voltage-gated Ca2+ channel plays a fundamental role in the targeting of the channel to the tubular system as well as in channel function. To determine whether this cytosolic auxiliary subunit is also a regulatory protein of Ca2+ release from the sarcoplasmic reticulum in vivo, we pressure-injected the beta1a subunit into intact adult mouse muscle fibers and recorded, with Fluo-3 AM, the intracellular Ca2+ signal induced by the action potential. We found that the beta1a subunit significantly increased, within minutes, the amplitude of Ca2+ release without major changes in its time course. beta1a subunits with the carboxy-terminus region deleted did not show an effect on Ca2+ release. The possibility that potentiation of Ca2+ release is due to a direct interaction between the beta1a subunit and the ryanodine receptor was ruled out by bilayer experiments of RyR1 single-channel currents and also by Ca2+ flux experiments. Our data suggest that the beta1a subunit is capable of regulating E-C coupling in the short term and that the integrity of the carboxy-terminus region is essential for its modulatory effect.

Ca2+ sparks and secretion in dorsal root ganglion neurons

Ca(2+) sparks as the elementary intracellular Ca(2+) release events are instrumental to local control of Ca(2+) signaling in many types of cells. Here, we visualized neural Ca(2+) sparks in dorsal root ganglion (DRG) sensory neurons and investigated possible role of DRG sparks in the regulation of secretion from the somata of the cell. DRG sparks arose mainly from type 3 ryanodine receptor Ca(2+) release channels on subsurface cisternae of the endoplasmic reticulum, rendering a striking subsurface localization. Caffeine- or 3,7-dimethyl-1-(2-propynyl)xanthine-induced store Ca(2+) release, in the form of Ca(2+) sparks, triggered exocytosis, independently of membrane depolarization and external Ca(2+). The spark-secretion coupling probability was estimated to be between 1 vesicle per 6.6 sparks and 1 vesicle per 11.4 sparks. During excitation, subsurface sparks were evoked by physiological Ca(2+) entry via the Ca(2+)-induced Ca(2+) release mechanism, and their synergistic interaction with Ca(2+) influx accounted for approximately 60% of the Ca(2+)-dependent exocytosis. Furthermore, inhibition of Ca(2+)-induced Ca(2+) release abolished endotoxin-induced secretion of pain-related neuropeptides. These findings underscore an important role for Ca(2+) sparks in the amplification of surface Ca(2+) influx and regulation of neural secretion.

Calcium sparks in skeletal muscle fibers

Ca(2+) sparks monitor transient local releases of Ca(2+) from the sarcoplasmic reticulum (SR) into the myoplasm. The release takes place through ryanodine receptors (RYRs), the Ca(2+)-release channels of the SR. In intact fibers from frog skeletal muscle, the temporal and spatial properties of voltage-activated Ca(2+) sparks are well simulated by a model that assumes that the Ca(2+) flux underlying a spark is 2.5 pA (units of Ca(2+) current) for 4.6 ms (18 degrees C). This flux amplitude suggests that 1-5 active RYRs participate in the generation of a typical voltage-activated spark under physiological conditions. A major goal of future experiments is to estimate this number more precisely and, if it is two or more, to investigate the communication mechanism that allows multiple RYRs to be co-activated in a rapid but self-limited fashion.

Uncontrolled calcium sparks act as a dystrophic signal for mammalian skeletal muscle

Most excitable cells maintain tight control of intracellular Ca(2+) through coordinated interaction between plasma membrane and endoplasmic or sarcoplasmic reticulum. Quiescent sarcoplasmic reticulum Ca(2+) release machinery is essential for the survival and normal function of skeletal muscle. Here we show that subtle membrane deformations induce Ca(2+) sparks in intact mammalian skeletal muscle. Spontaneous Ca(2+) sparks can be reversibly induced by osmotic shock, and participate in a normal physiological response to exercise. In dystrophic muscle with fragile membrane integrity, stress-induced Ca(2+) sparks are essentially irreversible. Moreover, moderate exercise in mdx muscle alters the Ca(2+) spark response. Thus, membrane-deformation-induced Ca(2+) sparks have an important role in physiological and pathophysiological regulation of Ca(2+) signalling, and uncontrolled Ca(2+) spark activity in connection with chronic activation of store-operated Ca(2+) entry may function as a dystrophic signal in mammalian skeletal muscle.

Ca2+ blinks: rapid nanoscopic store calcium signaling

Luminal Ca(2+) in the endoplasmic and sarcoplasmic reticulum (ER/SR) plays an important role in regulating vital biological processes, including store-operated capacitative Ca(2+) entry, Ca(2+)-induced Ca(2+) release, and ER/SR stress-mediated cell death. We report rapid and substantial decreases in luminal [Ca(2+)], called "Ca(2+) blinks," within nanometer-sized stores (the junctional cisternae of the SR) during elementary Ca(2+) release events in heart cells. Blinks mirror small local increases in cytoplasmic Ca(2+),orCa(2+) sparks, but changes of [Ca(2+)] in the connected free SR network were below detection. Store microanatomy suggests that diffusional strictures may account for this paradox. Surprisingly, the nadir of the store depletion trails the peak of the spark by about 10 ms, and the refilling of local store occurs with a rate constant of 35 s(-1), which is approximately 6-fold faster than the recovery of local Ca(2+) release after a spark. These data suggest that both local store depletion and some time-dependent inhibitory mechanism contribute to spark termination and refractoriness. Visualization of local store Ca(2+) signaling thus broadens our understanding of cardiac store Ca(2+) regulation and function and opens the possibility for local regulation of diverse store-dependent functions.

Calcium sparks in muscle cells: interactive procedures for automatic detection and measurements on line-scan confocal images series

In muscle cells, force development is controlled by Ca2+ ions, which are rapidly released from the sarcoplasmic reticulum (SR) during sarcolemmal depolarization. In addition to this synchronized spatially homogeneous calcium signal, localized quantal Ca2+ release events (sparks) have been recorded using laser scanning confocal fluorescence microscopy. Previously, algorithms without user intervention have been developed to automatically detect and analyse sparks on confocal line-scan (space-time: 512 x 512 pixels) single images. Here we present a computer program that we called "HARVest of Elementary Events" (HARVELE) developed to both analyse events on series of confocal images and follow sparks morphology from one site during several seconds. HARVELE, coded in the image-processing language IDL 5.3., can be applied on series of n images (512 x 512 x n) obtained from the same scanning line. Computing simultaneously entire series of images allows to measure, for each release site, the frequency and the morphology of release with the conventional amplitude, size, time and duration parameters defined for sparks analysis. The use of these procedures rapidly provides much information about the properties of calcium release sites in muscle cells population and can be applied on any elementary calcium events whatever the type of cell.

The role of ryanodine receptors and consequences of their alterations during cardiac insufficiency

Congestive heart failure (CHF) is a leading cause of death. Although changes to other components contribute, it is generally agreed that much of the contractile deficit is due to reduced Ca(2+) homeostasis that includes alterations in Ca(2+) current and action potential characteristics, together with reduced Ca(2+) transient amplitude. CHF is also associated with progressive skeletal muscle dysfunction. In both cardiac and skeletal muscles, the global increase in myoplasmic Ca(2+) during depolarization, or Ca(2+) transient, appears to consist of the summation of large numbers of local, unitary Ca(2+) release events (ie, Ca(2+) sparks) resulting from the activity of a cluster of ryanodine receptors (RyRs) (ie, RyR1 or RyR2 in skeletal and cardiac muscles, respectively). RyR2 channels from failing hearts have been shown to be hyperphosphorylated by protein kinase A, leading to dissociation of FK506-binding protein 12.6 and altered RyR2 channel function. After reviewing the alterations occurring in cardiomyocytes, the present report summarizes the intrinsic alterations of Ca(2+) homeostasis in rat extensor digitorum longus skeletal muscle. They include a weaker and prolonged Ca(2+) transient that could be attributed to both a lower synchronization of the individual Ca(2+) sparks and a lower synchronization of these events triggered upon depolarization. As in cardiac muscle, these alterations in sarcoplasmic reticulum function are associated with protein kinase A-induced hyperphosphorylation of RyR1 and a concomitant reduction in FK506-binding protein 12. These specific alterations in RyR1-dependent Ca(2+) release could play a significant role in the specific force decrements in skeletal muscle as well as in the remodelling that occurs secondary to CHF.

Imaging microdomain Ca2+ in muscle cells

Ca2+ ions passing through a single or a cluster of Ca2+-permeable channels create microscopic, short-lived Ca2+ gradients that constitute the building blocks of cellular Ca2+ signaling. Over the last decade, imaging microdomain Ca2+ in muscle cells has unveiled the exquisite spatial and temporal architecture of intracellular Ca2+ dynamics and has reshaped our understanding of Ca2+ signaling mechanisms. Major advances include the visualization of "Ca2+ sparks" as the elementary events of Ca2+ release from the sarcoplasmic reticulum (SR), "Ca2+ sparklets" produced by openings of single Ca2+-permeable channels, miniature Ca2+ transients in single mitochondria ("marks"), and SR luminal Ca2+ depletion transients ("scraps"). As a model system, a cardiac myocyte contains a 3-dimensional grid of 104 spark ignition sites, stochastic activation of which summates into global Ca2+ transients. Tracking intermolecular coupling between single L-type Ca2+ channels and Ca2+ sparks has provided direct evidence validating the local control theory of Ca2+-induced Ca2+ release in the heart. In vascular smooth muscle myocytes, Ca2+ can paradoxically signal both vessel constriction (by global Ca2+ transients) and relaxation (by subsurface Ca2+ sparks). These findings shed new light on the origin of Ca2+ signaling efficiency, specificity, and versatility. In addition, microdomain Ca2+ imaging offers a novel modality that complements electrophysiological approaches in characterizing Ca2+ channels in intact cells.

Defects in ryanodine receptor calcium release in skeletal muscle from post-myocardial infarct rats

Defective calcium (Ca2+) signaling and impaired contractile function have been observed in skeletal muscle secondary to impaired myocardial function. However, the molecular basis for these muscle defects have not been identified. In this study, we evaluated the alterations of the ryanodine-sensitive Ca2+ release channels (RyR1) by analyzing global and local Ca2+ signaling in a rat postmyocardial infarction (PMI) model of myocardial overload. Ca2+ transients, measured with multiphoton imaging in individual fibers within a whole extensor digitorum longus (EDL) muscle, exhibited significantly reduced amplitude and a prolonged time course in PMI. Spatio-temporal properties of spontaneous Ca2+ sparks in fibers isolated from PMI EDL muscles were also significantly altered. In addition, RyR1 from PMI skeletal muscles were PKA-hyperphosphorylated and depleted of the FK506 binding protein (FKBP12). These data show that PMI skeletal muscles exhibit altered local Ca2+ signaling, associated with hyperphosphorylation of RyR1. The observed changes in Ca2+ signaling may contribute to defective excitation-contraction coupling in muscle that can contribute to the reduced exercise capacity in PMI, out of proportion to the degree of cardiac dysfunction.

Intracellular Ca(2+) release as irreversible Markov process

In striated muscles, intracellular Ca(2+) release is tightly controlled by the membrane voltage sensor. Ca(2+) ions are necessary mediators of this control in cardiac but not in skeletal muscle, where their role is ill-understood. An intrinsic gating oscillation of Ca(2+) release-not involving the voltage sensor-is demonstrated in frog skeletal muscle fibers under voltage clamp. A Markov model of the Ca(2+) release units is shown to reproduce the oscillations, and it is demonstrated that for Markov processes to have oscillatory transients, its transition rates must violate thermodynamic reversibility. Such irreversibility results in permanent cycling of the units through a ring of states, which requires a source of free energy. Inhibition of the oscillation by 20 to 40 mM EGTA or partial depletion of Ca(2+) in the sarcoplasmic reticulum (SR) identifies the SR [Ca(2+)] gradient as the energy source, and indicates a location of the critical Ca(2+)-sensing site at distances greater than 35 nm from the open channel. These results, which are consistent with a recent demonstration of irreversibility in gating of cardiac Ca(2+) sparks, (Wang, S.-Q., L.-S. Song, L. Xu, G. Meissner, E. G. Lakatta, E. Ríos, M. D. Stern, and H. Cheng. 2002. Biophys. J. 83:242-251) exemplify a cell-wide oscillation caused by coupling between ion permeation and channel gating.

Comparison of simulated and measured calcium sparks in intact skeletal muscle fibers of the frog

Calcium sparks in frog intact skeletal muscle fibers were modeled as stereotypical events that arise from a constant efflux of Ca(2+) from a point source for a fixed period of time (e.g., 2.5 pA of Ca(2+) current for 4.6 ms; 18 degrees C). The model calculates the local changes in the concentrations of free Ca(2+) and of Ca(2+) bound to the major intrinsic myoplasmic Ca(2+) buffers (troponin, ATP, parvalbumin, and the SR Ca(2+) pump) and to the Ca(2+) indicator (fluo-3). A distinctive feature of the model is the inclusion of a binding reaction between fluo-3 and myoplasmic proteins, a process that strongly affects fluo-3's Ca(2+)-reaction kinetics, its apparent diffusion constant, and hence the morphology of sparks. DeltaF/F (the change in fluo-3's fluorescence divided by its resting fluorescence) was estimated from the calculated changes in fluo-3 convolved with the microscope point-spread function. To facilitate comparisons with measured sparks, noise and other sources of variability were included in a random repetitive fashion to generate a large number of simulated sparks that could be analyzed in the same way as the measured sparks. In the initial simulations, the binding of Ca(2+) to the two regulatory sites on troponin was assumed to follow identical and independent binding reactions. These simulations failed to accurately predict the falling phase of the measured sparks. A second set of simulations, which incorporated the idea of positive cooperativity in the binding of Ca(2+) to troponin, produced reasonable agreement with the measurements. Under the assumption that the single channel Ca(2+) current of a ryanodine receptor (RYR) is 0.5-2 pA, the results suggest that 1-5 active RYRs generate an average Ca(2+) spark in a frog intact muscle fiber.

Calcium sparks in intact skeletal muscle fibers of the frog

Calcium sparks were studied in frog intact skeletal muscle fibers using a home-built confocal scanner whose point-spread function was estimated to be approximately 0.21 microm in x and y and approximately 0.51 microm in z. Observations were made at 17-20 degrees C on fibers from Rana pipiens and Rana temporaria. Fibers were studied in two external solutions: normal Ringer's ([K(+)] = 2.5 mM; estimated membrane potential, -80 to -90 mV) and elevated [K(+)] Ringer's (most frequently, [K(+)] = 13 mM; estimated membrane potential, -60 to -65 mV). The frequency of sparks was 0.04-0.05 sarcomere(-1) s(-1) in normal Ringer's; the frequency increased approximately tenfold in 13 mM [K(+)] Ringer's. Spark properties in each solution were similar for the two species; they were also similar when scanned in the x and the y directions. From fits of standard functional forms to the temporal and spatial profiles of the sparks, the following mean values were estimated for the morphological parameters: rise time, approximately 4 ms; peak amplitude, approximately 1 DeltaF/F (change in fluorescence divided by resting fluorescence); decay time constant, approximately 5 ms; full duration at half maximum (FDHM), approximately 6 ms; late offset, approximately 0.01 DeltaF/F; full width at half maximum (FWHM), approximately 1.0 microm; mass (calculated as amplitude x 1.206 x FWHM(3)), 1.3-1.9 microm(3). Although the rise time is similar to that measured previously in frog cut fibers (5-6 ms; 17-23 degrees C), cut fiber sparks have a longer duration (FDHM, 9-15 ms), a wider extent (FWHM, 1.3-2.3 microm), and a strikingly larger mass (by 3-10-fold). Possible explanations for the increase in mass in cut fibers are a reduction in the Ca(2+) buffering power of myoplasm in cut fibers and an increase in the flux of Ca(2+) during release.

ATP-dependent adenophostin activation of inositol 1,4,5-trisphosphate receptor channel gating: kinetic implications for the durations of calcium puffs in cells

The inositol 1,4,5-trisphosphate (InsP(3)) receptor (InsP(3)R) is a ligand-gated intracellular Ca(2+) release channel that plays a central role in modulating cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)). The fungal metabolite adenophostin A (AdA) is a potent agonist of the InsP(3)R that is structurally different from InsP(3) and elicits distinct calcium signals in cells. We have investigated the effects of AdA and its analogues on single-channel activities of the InsP(3)R in the outer membrane of isolated Xenopus laevis oocyte nuclei. InsP(3)R activated by either AdA or InsP(3) have identical channel conductance properties. Furthermore, AdA, like InsP(3), activates the channel by tuning Ca(2+) inhibition of gating. However, gating of the AdA-liganded InsP(3)R has a critical dependence on cytoplasmic ATP free acid concentration not observed for InsP(3)-liganded channels. Channel gating activated by AdA is indistinguishable from that elicited by InsP(3) in the presence of 0.5 mM ATP, although the functional affinity of the channel is 60-fold higher for AdA. However, in the absence of ATP, gating kinetics of AdA-liganded InsP(3)R were very different. Channel open time was reduced by 50%, resulting in substantially lower maximum open probability than channels activated by AdA in the presence of ATP, or by InsP(3) in the presence or absence of ATP. Also, the higher functional affinity of InsP(3)R for AdA than for InsP(3) is nearly abolished in the absence of ATP. Low affinity AdA analogues furanophostin and ribophostin activated InsP(3)R channels with gating properties similar to those of AdA. These results provide novel insights for interpretations of observed effects of AdA on calcium signaling, including the mechanisms that determine the durations of elementary Ca(2+) release events in cells. Comparisons of single-channel gating kinetics of the InsP(3)R activated by InsP(3), AdA, and its analogues also identify molecular elements in InsP(3)R ligands that contribute to binding and activation of channel gating.