The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Donald M Bers - One of the best experts on this subject based on the ideXlab platform.
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increasing serca function promotes initiation of Calcium Sparks and breakup of Calcium waves
The Journal of Physiology, 2021Co-Authors: Daisuke Sato, Hitoshi Uchinoumi, Donald M BersAbstract:KEY POINTS Increasing sarcoplasmic/endoplasmic reticulum Calcium ATPase (SERCA) pump activity enhances sarcoplasmic reticulum Calcium (Ca) load, which increases both ryanodine receptor opening and driving force of Ca release flux. Both of these effects promote Ca spark formation and wave propagation. However, increasing SERCA activity also accelerates local cytosolic Ca decay as the wave front travels to the next cluster, which limits wave propagation. As a result, increasing SERCA pump activity has a biphasic effect on the propensity of arrhythmogenic Ca waves, but a monotonic effect to increase Ca spark frequency and amplitude. ABSTRACT Waves of sarcoplasmic reticulum (SR) Calcium (Ca) release can cause arrhythmogenic afterdepolarizations in cardiac myocytes. Ca waves propagate when Ca Sparks at one Ca release unit (CRU) recruit new Ca Sparks in neighbouring CRUs. Under normal conditions, Ca Sparks are too small to recruit neighbouring Ca Sparks where Ca sensitivity is also low. However, under pathological conditions such as a Ca overload or ryanodine receptor (RyR) sensitization, Ca Sparks can be larger and propagate more readily as macro-Sparks or full Ca waves. Increasing SERCA pump activity promotes SR Ca load, which promotes RyR opening and increases driving force of the Ca release flux from SR to cytosol, promoting Ca waves. However, high sarcoplasmic/endoplasmic reticulum Calcium ATPase (SERCA) activity can also decrease local cytosolic [Ca] as it approaches the next CRU, thereby reducing wave appearance and propagation. In this study, we use a physiologically detailed model of subcellular Ca cycling and experiments in phospholamban-knockout mice, to show how Ca waves are initiated and propagate and how different conditions contribute to the generation and propagation of Ca waves. We show that reducing diffusive coupling between Ca Sparks by increasing SERCA activity prevents Ca waves by reducing [Ca] at the next CRU, as do Ca buffers, low intra-SR Ca diffusion and distance between CRUs. Increasing SR Ca uptake rate has a biphasic effect on Ca wave propagation; initially it enhances Ca spark probability and amplitude and CRU coupling, thereby promoting arrhythmogenic Ca wave propagation, but at higher levels SR Ca uptake can abort those arrhythmogenic Ca waves.
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sarcoplasmic reticulum structure and functional properties that promote long lasting Calcium Sparks
Biophysical Journal, 2016Co-Authors: Daisuke Sato, Thomas R Shannon, Donald M BersAbstract:Calcium (Ca) Sparks are the fundamental sarcoplasmic reticulum (SR) Ca release events in cardiac myocytes, and they have a typical duration of 20-40 ms. However, when a fraction of ryanodine receptors (RyRs) are blocked by tetracaine or ruthenium red, Ca Sparks lasting hundreds of milliseconds have been observed experimentally. The fundamental mechanism underlying these extremely prolonged Ca Sparks is not understood. In this study, we use a physiologically detailed mathematical model of subcellular Ca cycling to examine how Ca spark duration is influenced by the number of functional RyRs in a junctional cluster (which is reduced by tetracaine or ruthenium red) and other SR Ca handling properties. One RyR cluster contains a few to several hundred RyRs, and we use a four-state Markov RyR gating model. Each RyR opens stochastically and is regulated by cytosolic and luminal Ca. We varied the number of functional RyRs in the single cluster, diffusion within the SR network, diffusion between network and junctional SR, cytosolic Ca diffusion, SERCA uptake activity, and RyR open probability. For long-lasting Ca release events, opening events within the cluster must occur continuously because the typical open time of the RyR is only a few milliseconds. We found the following: 1) if the number of RyRs is too small, it is difficult to maintain consecutive openings and stochastic attrition terminates the release; 2) if the number of RyRs is too large, the depletion of Ca from the junctional SR terminates the release; and 3) very long release events require relatively small-sized RyR clusters (reducing flux as seen experimentally with tetracaine) and sufficiently rapid intra-SR Ca diffusion, such that local junctional intra-SR [Ca] can be maintained by intra-SR diffusion and overall SR Ca reuptake.
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depolarization of cardiac membrane potential synchronizes Calcium Sparks and waves in tissue
Biophysical Journal, 2014Co-Authors: Daisuke Sato, Daniel C Bartos, Kenneth S Ginsburg, Donald M BersAbstract:The diastolic membrane potential (Vm) can be hyperpolarized or depolarized by various factors such as hyperkalemia or hypokalemia in the long term, or by delayed afterdepolarizations in the short term. In this study, we investigate how Vm affects Ca Sparks and waves. We use a physiologically detailed mathematical model to investigate individual factors that affect Ca spark generation and wave propagation. We focus on the voltage range of −90 ∼ −70 mV, which is just below the Vm for sodium channel activation. We find that Vm depolarization promotes Ca wave propagation and hyperpolarization prevents it. This finding is directly validated in voltage clamp experiments with Ca waves using isolated rat ventricular myocytes. Ca transport by the sodium-Calcium exchanger (NCX) is determined by Vm as well as Na and Ca concentrations. Depolarized Vm reduces NCX-mediated efflux, elevating [Ca]i, and thus promoting Ca wave propagation. Moreover, depolarized Vm promotes spontaneous Ca releases that can cause initiation of multiple Ca waves. This indicates that during delayed afterdepolarizations, Ca release units (CRUs) interact with not just the immediately adjacent CRUs via Ca diffusion, but also further CRUs via fast (∼0.1 ms) changes in Vm mediated by the voltage and Ca-sensitive NCX. This may contribute significantly to synchronization of Ca waves among multiple cells in tissue.
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can the sodium Calcium exchanger initiate or suppress Calcium Sparks in cardiac myocytes
Biophysical Journal, 2012Co-Authors: Daisuke Sato, Sanda Despa, Donald M BersAbstract:Positive feedback of Calcium (Ca)-induced Ca release is the mechanism of Ca spark formation in cardiac myocytes. To initiate this process, a certain amount of Ca in the cleft space is necessary. When the membrane potential becomes higher during excitation-contraction coupling, Ca can enter through both Ca current (ICaL) and sodium-Calcium exchanger (NCX) and may activate ryanodine receptors to initiate a Ca spark. On the other hand, at the resting membrane potential (Vm ∼–80 mV), NCX removes Ca from the cell (forward mode). If Ca released from the sarcoplasmic reticulum is quickly removed via forward mode NCX before Ca-induced Ca release starts, the Ca release becomes nonspark Ca leak. This would also be influenced by the cleft/noncleft distribution of NCX, which is unknown. Using a physiologically detailed mathematical model of subcellular Ca cycling, we analyze how NCX strength and distribution alter Ca spark formation. During excitation-contraction coupling, most Ca Sparks are induced by ICaL with very few due to NCX current. At the resting membrane potential if most NCX is localized to the cleft, spontaneous Ca Sparks are significantly reduced.
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ca2 calmodulin dependent protein kinase ii phosphorylation of ryanodine receptor does affect Calcium Sparks in mouse ventricular myocytes
Circulation Research, 2006Co-Authors: Tao Guo, Tong Zhang, Ruben Mestril, Donald M BersAbstract:Previous studies in transgenic mice and with isolated ryanodine receptors (RyR) have indicated that Ca2+-calmodulin-dependent protein kinase II (CaMKII) can phosphorylate RyR and activate local diastolic sarcoplasmic reticulum (SR) Ca2+ release events (Ca2+ Sparks) and RyR channel opening. Here we use relatively controlled physiological conditions in saponin-permeabilized wild type (WT) and phospholamban knockout (PLB-KO) mouse ventricular myocytes to test whether exogenous preactivated CaMKII or endogenous CaMKII can enhance resting Ca2+ Sparks. PLB-KO mice were used to preclude ancillary effects of CaMKII mediated by phospholamban phosphorylation. In both WT and PLB-KO myocytes, Ca2+ spark frequency was increased by both preactivated exogenous CaMKII and endogenous CaMKII. This effect was abolished by CaMKII inhibitor peptides. In contrast, protein kinase A catalytic subunit also enhanced Ca2+ spark frequency in WT, but had no effect in PLB-KO. Both endogenous and exogenous CaMKII increased SR Ca2+ content in WT (presumably via PLB phosphorylation), but not in PLB-KO. Exogenous calmodulin decreased Ca2+ spark frequency in both WT and PLB-KO (K0.5 approximately 100 nmol/L). Endogenous CaMKII (at 500 nmol/L [Ca2+]) phosphorylated RyR as completely in <4 minutes as the maximum achieved by preactivated exogenous CaMKII. After CaMKII activation Ca2+ Sparks were longer in duration, and more frequent propagating SR Ca2+ release events were observed. We conclude that CaMKII-dependent phosphorylation of RyR by endogenous associated CaMKII (but not PKA-dependent phosphorylation) increases resting SR Ca2+ release or leak. Moreover, this may explain the enhanced SR diastolic Ca2+ leak and certain triggered arrhythmias seen in heart failure.
Heping Cheng - One of the best experts on this subject based on the ideXlab platform.
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quarky Calcium Sparks in heart
Biophysical Journal, 2009Co-Authors: Didier X P Brochet, Heping Cheng, Dongmei Yang, Wen Jun Xie, W J LedererAbstract:Ca2+ Sparks are the stereotyped unit of Ca2+ release in the heart and Ca2+ blinks are the reciprocal Ca2+ depletion signal produced in the exquisitely small junctional sarcoplasmic reticulum (SR). With improved resolution and sensitivity that can be achieved by examining Ca2+ spark - blink pairs, we report here for the first time small, local but also sometimes spatially extensive Ca2+ releases (subSparks and Ca2+ “mist”) that co-exist with regular Ca2+ Sparks. Similar low-level Ca2+ releases also occur in the declining phase of regular Ca2+ Sparks and the abundance of these small Ca2+ releases dictates the kinetics of the spark-blink pair. We propose a model in which the Ca2+ release unit, consisting of a large array of type 2 ryanodine receptor (RyR2) Ca2+ release channels, underlies the initial high-flux release of a Ca2+ Sparks. In contrast, rogue unconstrained RyR2s, which may display higher Ca2+ sensitivity but smaller Ca2+ flux, produce the Ca2+ quark-like or “quarky” local releases. The existence of the additional release mechanism provides new fundamental mechanistic understanding of cardiac Ca2+ signaling in health and disease.
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uncontrolled Calcium Sparks act as a dystrophic signal for mammalian skeletal muscle
Nature Cell Biology, 2005Co-Authors: Xu Wang, Noah Weisleder, Jingsong Zhou, Claude Collet, Yi Chu, Yutaka Hirata, Xiaoli Zhao, Zui Pan, Marco Brotto, Heping ChengAbstract: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.
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a preferred amplitude of Calcium Sparks in skeletal muscle
Biophysical Journal, 2001Co-Authors: Eduardo Ríos, Heping Cheng, Natalia Shirokova, Michael D. Stern, Wolfgang G Kirsch, Gonzalo Pizarro, Adom GonzálezAbstract:In skeletal and cardiac muscle, Calcium release from the sarcoplasmic reticulum, leading to contraction, often results in Calcium Sparks. Because Sparks are recorded by confocal microscopy in line-scanning mode, their measured amplitude depends on their true amplitude and the position of the spark relative to the scanned line. We present a method to derive from measured amplitude histograms the actual distribution of spark amplitudes. The method worked well when tested on simulated distributions of experimental Sparks. Applied to massive numbers of Sparks imaged in frog skeletal muscle under voltage clamp in reference conditions, the method yielded either a decaying amplitude distribution (6 cells) or one with a central mode (5 cells). Caffeine at 0.5 or 1 mM reversibly enhanced this mode (5 cells) or induced its appearance (4 cells). The occurrence of a mode in the amplitude distribution was highly correlated with the presence of a mode in the distribution of spark rise times or in the joint distribution of rise times and spatial widths. If Sparks were produced by individual Markovian release channels evolving reversibly, they should not have a preferred rise time or amplitude. Channel groups, instead, could cooperate allosterically or through their Calcium sensitivity, and give rise to a stereotyped amplitude in their collective spark.
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involvement of multiple intracellular release channels in Calcium Sparks of skeletal muscle
Proceedings of the National Academy of Sciences of the United States of America, 2000Co-Authors: Adom González, Heping Cheng, Natalia Shirokova, Michael D. Stern, Wolfgang G Kirsch, Gonzalo Pizarro, Gustavo Brum, Isaac N Pessah, Eduardo RíosAbstract:In many types of muscle, intracellular Ca2+ release for contraction consists of brief Ca2+ Sparks. Whether these result from the opening of one or many channels in the sarcoplasmic reticulum is not known. Examining massive numbers of Sparks from frog skeletal muscle and evaluating their Ca2+ release current, we provide evidence that they are generated by multiple channels. A mode is demonstrated in the distribution of spark rise times in the presence of the channel activator caffeine. This finding contradicts expectations for single channels evolving reversibly, but not for channels in a group, which collectively could give rise to a stereotyped spark. The release channel agonists imperatoxin A, ryanodine, and bastadin 10 elicit fluorescence events that start with a spark, then decay to steady levels roughly proportional to the unitary conductances of 35%, 50%, and 100% that the agonists, respectively, promote in bilayer experiments. This correspondence indicates that the steady phase is produced by one open channel. Calculated Ca2+ release current decays 10- to 20-fold from spark to steady phase, which requires that six or more channels be open during the spark.
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Calcium Sparks release packets of uncertain origin and fundamental role
The Journal of General Physiology, 1999Co-Authors: Natalia Shirokova, Adom González, Eduardo Ríos, Michael D. Stern, Wolfgang G Kirsch, Gonzalo Pizarro, Heping ChengAbstract:Ca2+ Sparks have intrigued researchers since their discovery (Cheng et al., 1993). No one anticipated their existence; therefore, they must be telling us something we did not know. Also appealing are the esthetics of a sharp rise in local Ca2+, extremely limited in space and time but large compared with the recording noise.
W J Lederer - One of the best experts on this subject based on the ideXlab platform.
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dynamics of Calcium Sparks and sr Calcium leak during excitation contraction coupling in mouse heart cells
Biophysical Journal, 2014Co-Authors: George S B Williams, W J Lederer, Andrew P Wescott, Saleet M JafriAbstract:Stable Calcium-induced Calcium release (CICR) is critical for maintaining normal cell contraction during cardiac excitation-contraction (EC) coupling in heart cells. The fundamental element of CICR in heart is the Calcium (Ca2+) spark which arises from the regenerative release of Ca2+ from a cluster of ryanodine receptors (RyR2) in the junctional sarcoplasmic reticulum membrane. We have shown how stochastic gating of RyR2s could produce an SR Ca2+ leak capable of balancing SR Ca2+-ATPase (SERCA2a) activity under quiescent conditions (Williams et al. BJ 2011). This investigation suggested the surprising finding that a single, or even multiple, RyR2 openings could fail probabilistically to trigger a Ca2+ spark. To further investigate this effect, we expanded upon that formulation to create a detailed, local control model of EC coupling in mouse heart. A number of features were added or modified, including the addition of a novel seven-state Markov chain model of the sarcolemmal L-type Ca2+ channel (LCC). This model features dynamic action-potential (AP) generation, robust Ca2+ spark initiation and termination, realistic [Ca2+]i transients, and true SR Ca2+ pump/leak balance. The model suggests that numerous LCC openings are often required to trigger a single Ca2+ spark. This is consistent with our prior work where multiple RyR2 openings were needed to trigger a spontaneous Ca2+ spark during quiescent conditions. We also investigated how RyR2 mutations associated with catecholaminergic polymorphic ventricular tachycardia (CPVT) influence the dynamics of Ca2+ Sparks, “invisible” non-spark Ca2+ leak, [Ca2+]i transients, and SR Ca2+ content. We observe that CPVT mutations can lead to unstable Ca2+ spark dynamics, altering SR Ca2+ content and promoting [Ca2+]i signaling instability. Our new model provides significant insights into the dynamics of local control of CICR under physiological and pathological conditions.
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ros dependent modulation of Calcium Sparks in cardiomyocytes
Biophysical Journal, 2012Co-Authors: Aristide C Chikando, Liron Boyman, Ramzi J Khairallah, Christopher W Ward, Godfrey L Smith, Joseph P Y Kao, W J LedererAbstract:Mitochondrial regulation of cytosolic Calcium ([Ca2+]i) is thought to depend on the mitochondrial inner membrane potential (ΔΨm). ΔΨm arises from activity of the electron transport chain and is thought to play a critical role in all ion movements across the inner membrane. With ΔΨmito ≈ −150 mV to −200 mV, there is clearly a strong electrochemical potential for the movement of Ca2+ from the cytosol (about 100 nM) into the matrix (around 100 nM). If significant rapid Ca2+ influx occurs, then ΔΨmito per se should influence the time-course, frequency, magnitude and other characteristics of Ca2+ Sparks and the [Ca2+]i transients. Tetramethylrhodamine methyl ester (TMRM) was used to monitor ΔΨmito in freshly isolated rat cardiomyocytes and photon stress was used to depolarize the mitochondria. In addition to the changes in electrochemical potential for Ca2+ entry, depolarization of mitochondria was associated with an increase in cellular reactive oxygen species (ROS) as measured by DCF (as has been previously reported by several laboratories). Quantitative analysis of these findings permits us to separate the influence of each on the changes in Ca2+ signaling observed. Consistent with findings by Zhou et al., 2011, we report a role for local [ROS] in altering Ca2+ signaling.ReferencesZhou, L., Aon., M.A., Lui, T., O'Rourke, B. Dynamic modulation of Ca2+ Sparks by mitochondrial oscillations in isolated guinea pig cardiomyocytes under oxidative stress. JMCC. 2011. 51(5):632–9.
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dynamics of Calcium Sparks and Calcium leak in the heart
Biophysical Journal, 2011Co-Authors: W J Lederer, Aristide C Chikando, Saleet M Jafri, George S B Williams, Hoangtrong M Tuan, Eric A SobieAbstract:We present what we believe to be a new mathematical model of Ca2+ leak from the sarcoplasmic reticulum (SR) in the heart. To our knowledge, it is the first to incorporate a realistic number of Ca2+-release units, each containing a cluster of stochastically gating Ca2+ channels (RyRs), whose biophysical properties (e.g., Ca2+ sensitivity and allosteric interactions) are informed by the latest molecular investigations. This realistic model allows for the detailed characterization of RyR Ca2+-release properties, and shows how this balances reuptake by the SR Ca2+ pump. Simulations reveal that SR Ca2+ leak consists of brief but frequent single RyR openings (∼3000 cell−1 s−1) that are likely to be experimentally undetectable, and are, therefore, “invisible”. We also observe that these single RyR openings can recruit additional RyRs to open, due to elevated local (Ca2+), and occasionally lead to the generation of Ca2+ Sparks (∼130 cell−1 s−1). Furthermore, this physiological formulation of “invisible” leak allows for the removal of the ad hoc, non-RyR mediated Ca2+ leak terms present in prior models. Finally, our model shows how Ca2+ Sparks can be robustly triggered and terminated under both normal and pathological conditions. Together, these discoveries profoundly influence how we interpret and understand diverse experimental and clinical results from both normal and diseased hearts.
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quarky Calcium Sparks in heart
Biophysical Journal, 2009Co-Authors: Didier X P Brochet, Heping Cheng, Dongmei Yang, Wen Jun Xie, W J LedererAbstract:Ca2+ Sparks are the stereotyped unit of Ca2+ release in the heart and Ca2+ blinks are the reciprocal Ca2+ depletion signal produced in the exquisitely small junctional sarcoplasmic reticulum (SR). With improved resolution and sensitivity that can be achieved by examining Ca2+ spark - blink pairs, we report here for the first time small, local but also sometimes spatially extensive Ca2+ releases (subSparks and Ca2+ “mist”) that co-exist with regular Ca2+ Sparks. Similar low-level Ca2+ releases also occur in the declining phase of regular Ca2+ Sparks and the abundance of these small Ca2+ releases dictates the kinetics of the spark-blink pair. We propose a model in which the Ca2+ release unit, consisting of a large array of type 2 ryanodine receptor (RyR2) Ca2+ release channels, underlies the initial high-flux release of a Ca2+ Sparks. In contrast, rogue unconstrained RyR2s, which may display higher Ca2+ sensitivity but smaller Ca2+ flux, produce the Ca2+ quark-like or “quarky” local releases. The existence of the additional release mechanism provides new fundamental mechanistic understanding of cardiac Ca2+ signaling in health and disease.
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mutant ryanodine receptor dependent Calcium leak ryr2 open probability Calcium Sparks and cardiac arrhythmogenesis
Biophysical Journal, 2009Co-Authors: Stephan E Lehnart, W J Lederer, Christopher W Ward, Eric A SobieAbstract:Ca2+ leak from the sarcoplasmic reticulum (SR) represents a major mechanism underlying arrhythmic disease in the heart. Nevertheless the links between cardiac ryanodine receptor (RyR2) single channel behavior, Ca2+ Sparks, and Ca2+ waves remains surprisingly enigmatic. Here we investigate the relationship between a known missense mutation in RyR2 (R2474S) and Ca2+ Sparks. R2474S has been shown to cause catecholaminergic polymorphic ventricular tachycardia (CPVT) in humans and knockin mice under conditions of stress and catecholamine exposure. Heterozygous CPVT-mutant RyR2-R2474S/WT (RS/WT) or wild-type (WT) channels were isolated from the hearts of 1) resting, control mice or 2) mice which underwent exercise stress testing (‘stressed’). Using the lipid bilayer method (cytosolic [Ca2+]cis = 150 nM to approximate diastolic concentrations), RyR2-RS/WT from ‘stressed’ mice showed a significant gain-of-function defect as evidenced by increased open probability (Po = 0.134±0.008, n=7) versus WT (0.018±0.008, n=7; P<0.05). When isolated cardiac myocytes were pre-treated with isoproterenol (1 μM) to mimic catecholaminergic stress and following 1 Hz field pacing, fluo-4-AM loaded RS/WT cells showed a significantly increased spark rate (341.8 ±115.7% vs. 108.1±44.6%) when compared to WT cells. These findings suggest that changes in RyR2 Po, Ca2+ Sparks and arrhythmogenesis are linked mechanistically. How the ensemble of findings are interrelated dynamically, however, is model-dependent and this modeling will be presented. These findings suggest that our approach to the investigation of Ca2+ dependent arrhythmogenesis broadens understanding of molecular cardiac defects in disease and lays the foundation for the development and testing of novel therapeutic agents.
Stephen M Baylor - One of the best experts on this subject based on the ideXlab platform.
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effects of tetracaine on voltage activated Calcium Sparks in frog intact skeletal muscle fibers
The Journal of General Physiology, 2006Co-Authors: Stephen Hollingworth, Knox W Chandler, Stephen M BaylorAbstract: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.
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Calcium Sparks in skeletal muscle fibers
Cell Calcium, 2005Co-Authors: Stephen M BaylorAbstract:Abstract 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 °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.
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simulation of Calcium Sparks in cut skeletal muscle fibers of the frog
The Journal of General Physiology, 2003Co-Authors: W. K. Chandler, Stephen Hollingworth, Stephen M BaylorAbstract:Spark mass, the volume integral of ΔF/F, was investigated theoretically and with simulations. These studies show that the amount of Ca2+ bound to fluo-3 is proportional to mass times the total concentration of fluo-3 ([fluo-3T]); the proportionality constant depends on resting Ca2+ concentration ([Ca2+]R). In the simulation of a Ca2+ spark in an intact frog fiber with [fluo-3T] = 100 μM, fluo-3 captures approximately one-fourth of the Ca2+ released from the sarcoplasmic reticulum (SR). Since mass in cut fibers is several times that in intact fibers, both with similar values of [fluo-3T] and [Ca2+]R, it seems likely that SR Ca2+ release is larger in cut fiber Sparks or that fluo-3 is able to capture a larger fraction of the released Ca2+ in cut fibers, perhaps because of reduced intrinsic Ca2+ buffering. Computer simulations were used to identify these and other factors that may underlie the differences in mass and other properties of Sparks in intact and cut fibers. Our spark model, which successfully simulates Calcium Sparks in intact fibers, was modified to reflect the conditions of cut fiber measurements. The results show that, if the protein Ca2+-buffering power of myoplasm is the same as that in intact fibers, the Ca2+ source flux underlying a spark in cut fibers is 5–10 times that in intact fibers. Smaller source fluxes are required for less buffer. In the extreme case in which Ca2+ binding to troponin is zero, the source flux needs to be 3–5 times that in intact fibers. An increased Ca2+ source flux could arise from an increase in Ca2+ flux through one ryanodine receptor (RYR) or an increase in the number of active RYRs per spark, or both. These results indicate that the gating of RYRs, or their apparent single channel Ca2+ flux, is different in frog cut fibers—and, perhaps, in other disrupted preparations—than in intact fibers.
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Comparison of Simulated and Measured Calcium Sparks in Intact Skeletal Muscle Fibers of the Frog
The Journal of general physiology, 2002Co-Authors: Stephen M Baylor, Stephen Hollingworth, W. K. ChandlerAbstract:Calcium Sparks in frog intact skeletal muscle fibers were modeled as stereotypical events that arise from a constant efflux of Ca2+ from a point source for a fixed period of time (e.g., 2.5 pA of Ca2+ current for 4.6 ms; 18°C). The model calculates the local changes in the concentrations of free Ca2+ and of Ca2+ bound to the major intrinsic myoplasmic Ca2+ buffers (troponin, ATP, parvalbumin, and the SR Ca2+ pump) and to the Ca2+ 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 Ca2+-reaction kinetics, its apparent diffusion constant, and hence the morphology of Sparks. ΔF/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 Ca2+ 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 Ca2+ to troponin, produced reasonable agreement with the measurements. Under the assumption that the single channel Ca2+ current of a ryanodine receptor (RYR) is 0.5–2 pA, the results suggest that 1–5 active RYRs generate an average Ca2+ spark in a frog intact muscle fiber.
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Calcium Sparks in intact skeletal muscle fibers of the frog
The Journal of General Physiology, 2001Co-Authors: Stephen Hollingworth, W. K. Chandler, J Peet, Stephen M BaylorAbstract:Calcium Sparks were studied in frog intact skeletal muscle fibers using a home-built confocal scanner whose point-spread function was estimated to be � 0.21 � m in x and y and � 0.51 � m in z. Observations were made at 17-20 � 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, � 4 ms; peak amplitude, � 1 � F/F (change in fluorescence divided by resting fluorescence); decay time constant, � 5 ms; full duration at half maximum (FDHM), � 6 ms; late offset, � 0.01 � F/F; full width at half maximum (FWHM), � 1.0 � m; mass (calculated as amplitude � 1.206 � FWHM 3 ), 1.3-1.9 � m 3 . Although the rise time is similar to that measured pre- viously in frog cut fibers (5-6 ms; 17-23 � C), cut fiber Sparks have a longer duration (FDHM, 9-15 ms), a wider ex- tent (FWHM, 1.3-2.3 � m), 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.
Michael D. Stern - One of the best experts on this subject based on the ideXlab platform.
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filling the gap between Calcium Sparks and waves automatic detection and classification of local Calcium releases in cardiac pacemaker cells
Biophysical Journal, 2015Co-Authors: Alexander V Maltsev, Michael D. SternAbstract:Local Calcium releases (LCRs) observed in cardiac pacemaker cells have a complex spatiotemporal structure that has never been studied. We developed a computer algorithm for automatic detection and classification of LCRs in simulations of rabbit sinoatrial-node cells (using our recent 3D-model) to get new insights into pacemaker cell operation, specifically, the role of sarcoplasmic reticulum Calcium pumping rate (Pup).Identified release events that share a common intensity level are categorized as a release cluster, i.e. a complex release with multiple intensity peaks. These complex LCRs tend to live longer and propagate farther via Calcium-induced-Calcium release, thus occupying larger areas. Release events that don’t share any intensity level with other events are Calcium Sparks that do not live for a long time and do not propagate. Collisions and splits of LCRs are handled as follows. When an LCR separates into different parts, all parts are still considered part of the LCR. On the other hand, when an LCR collides with another, the one with the weaker signal mass is considered dead and the one with the larger signal mass takes its signal mass as its own. An LCR may also die by stochastic attrition when all its components fade out.Under voltage clamp, LCR areas and signal masses were paradoxically smaller at larger Pup, likely reflecting uptake of cytosolic Calcium before it can propagate. Under spontaneous beating conditions, however, higher Pup greatly increased diastolic LCR signal mass and beating rate as predicted by the coupled-clock theory. Interestingly, the total integral of all LCRs during diastolic depolarization in both cases remained almost the same as longer integration time with smaller events is comparable to shorter time with larger events.
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a preferred amplitude of Calcium Sparks in skeletal muscle
Biophysical Journal, 2001Co-Authors: Eduardo Ríos, Heping Cheng, Natalia Shirokova, Michael D. Stern, Wolfgang G Kirsch, Gonzalo Pizarro, Adom GonzálezAbstract:In skeletal and cardiac muscle, Calcium release from the sarcoplasmic reticulum, leading to contraction, often results in Calcium Sparks. Because Sparks are recorded by confocal microscopy in line-scanning mode, their measured amplitude depends on their true amplitude and the position of the spark relative to the scanned line. We present a method to derive from measured amplitude histograms the actual distribution of spark amplitudes. The method worked well when tested on simulated distributions of experimental Sparks. Applied to massive numbers of Sparks imaged in frog skeletal muscle under voltage clamp in reference conditions, the method yielded either a decaying amplitude distribution (6 cells) or one with a central mode (5 cells). Caffeine at 0.5 or 1 mM reversibly enhanced this mode (5 cells) or induced its appearance (4 cells). The occurrence of a mode in the amplitude distribution was highly correlated with the presence of a mode in the distribution of spark rise times or in the joint distribution of rise times and spatial widths. If Sparks were produced by individual Markovian release channels evolving reversibly, they should not have a preferred rise time or amplitude. Channel groups, instead, could cooperate allosterically or through their Calcium sensitivity, and give rise to a stereotyped amplitude in their collective spark.
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involvement of multiple intracellular release channels in Calcium Sparks of skeletal muscle
Proceedings of the National Academy of Sciences of the United States of America, 2000Co-Authors: Adom González, Heping Cheng, Natalia Shirokova, Michael D. Stern, Wolfgang G Kirsch, Gonzalo Pizarro, Gustavo Brum, Isaac N Pessah, Eduardo RíosAbstract:In many types of muscle, intracellular Ca2+ release for contraction consists of brief Ca2+ Sparks. Whether these result from the opening of one or many channels in the sarcoplasmic reticulum is not known. Examining massive numbers of Sparks from frog skeletal muscle and evaluating their Ca2+ release current, we provide evidence that they are generated by multiple channels. A mode is demonstrated in the distribution of spark rise times in the presence of the channel activator caffeine. This finding contradicts expectations for single channels evolving reversibly, but not for channels in a group, which collectively could give rise to a stereotyped spark. The release channel agonists imperatoxin A, ryanodine, and bastadin 10 elicit fluorescence events that start with a spark, then decay to steady levels roughly proportional to the unitary conductances of 35%, 50%, and 100% that the agonists, respectively, promote in bilayer experiments. This correspondence indicates that the steady phase is produced by one open channel. Calculated Ca2+ release current decays 10- to 20-fold from spark to steady phase, which requires that six or more channels be open during the spark.
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Calcium Sparks release packets of uncertain origin and fundamental role
The Journal of General Physiology, 1999Co-Authors: Natalia Shirokova, Adom González, Eduardo Ríos, Michael D. Stern, Wolfgang G Kirsch, Gonzalo Pizarro, Heping ChengAbstract:Ca2+ Sparks have intrigued researchers since their discovery (Cheng et al., 1993). No one anticipated their existence; therefore, they must be telling us something we did not know. Also appealing are the esthetics of a sharp rise in local Ca2+, extremely limited in space and time but large compared with the recording noise.
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Amplitude Distribution of Calcium Sparks in Confocal Images: Theory and Studies with an Automatic Detection Method
Biophysical journal, 1999Co-Authors: Heping Cheng, Long-sheng Song, Natalia Shirokova, Adom González, Edward G. Lakatta, Eduardo Ríos, Michael D. SternAbstract: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.
