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1.
Moment closure for local control models of calcium-induced calcium release in cardiac myocytes
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In prior work, we introduced a probability density approach to modeling local control of Ca2+-induced Ca2+ release in cardiac myocytes, where we derived coupled advection-reaction equations for the time-dependent bivariate probability density of subsarcolemmal subspace and junctional sarcoplasmic reticulum (SR) [Ca2+] conditioned on Ca2+ release unit (CaRU) state. When coupled to ordinary differential equations (ODEs) for the bulk myoplasmic and network SR [Ca2+], a realistic but minimal model of cardiac excitation-contraction coupling was produced that avoids the computationally demanding task of resolving spatial aspects of global Ca2+ signaling, while accurately representing heterogeneous local Ca2+ signals in a population of diadic subspaces and junctional SR depletion domains. Here we introduce a computationally efficient method for simulating such whole cell models when the dynamics of subspace [Ca2+] are much faster than those of junctional SR [Ca2+]. The method begins with the derivation of a system of ODEs describing the time-evolution of the moments of the univariate probability density functions for junctional SR [Ca2+] jointly distributed with CaRU state. This open system of ODEs is then closed using an algebraic relationship that expresses the third moment of junctional SR [Ca2+] in terms of the first and second moments. In simulated voltage-clamp protocols using 12-state CaRUs that respond to the dynamics of both subspace and junctional SR [Ca2+], this moment-closure approach to simulating local control of excitation-contraction coupling produces high-gain Ca2+ release that is graded with changes in membrane potential, a phenomenon not exhibited by common pool models. Benchmark simulations indicate that the moment-closure approach is nearly 10,000-times more computationally efficient than corresponding Monte Carlo simulations while leading to nearly identical results. We conclude by applying the moment-closure approach to study the restitution of Ca2+-induced Ca2+ release during simulated two-pulse voltage-clamp protocols. 相似文献
2.
Of the many ongoing controversies regarding the workings of the sarcoplasmic reticulum (SR) in cardiac myocytes, two unresolved and interconnected topics are 1), mechanisms of calcium (Ca2+) wave propagation, and 2), speed of Ca2+ diffusion within the SR. Ca2+ waves are initiated when a spontaneous local SR Ca2+ release event triggers additional release from neighboring clusters of SR release channels (ryanodine receptors (RyRs)). A lack of consensus regarding the effective Ca2+ diffusion constant in the SR (DCa,SR) severely complicates our understanding of whether dynamic local changes in SR [Ca2+] can influence wave propagation. To address this problem, we have implemented a computational model of cytosolic and SR [Ca2+] during Ca2+ waves. Simulations have investigated how dynamic local changes in SR [Ca2+] are influenced by 1), DCa,SR; 2), the distance between RyR clusters; 3), partial inhibition or stimulation of SR Ca2+ pumps; 4), SR Ca2+ pump dependence on cytosolic [Ca2+]; and 5), the rate of transfer between network and junctional SR. Of these factors, DCa,SR is the primary determinant of how release from one RyR cluster alters SR [Ca2+] in nearby regions. Specifically, our results show that local increases in SR [Ca2+] ahead of the wave can potentially facilitate Ca2+ wave propagation, but only if SR diffusion is relatively slow. These simulations help to delineate what changes in [Ca2+] are possible during SR Ca2+release, and they broaden our understanding of the regulatory role played by dynamic changes in [Ca2+]SR. 相似文献
3.
Valeriy Lukyanenko Aristide Chikando W.J. Lederer 《The international journal of biochemistry & cell biology》2009,41(10):1957-1971
Ca2+ signaling is of vital importance to cardiac cell function and plays an important role in heart failure. It is based on sarcolemmal, sarcoplasmic reticulum and mitochondrial Ca2+ cycling. While the first two are well characterized, the latter remains unclear, controversial and technically challenging.In mammalian cardiac myocytes, Ca2+ influx through L-type calcium channels in the sarcolemmal membrane triggers Ca2+ release from the nearby junctional sarcoplasmic reticulum to produce Ca2+ sparks. When this triggering is synchronized by the cardiac action potential, a global [Ca2+]i transient arises from coordinated Ca2+ release events. The ends of intermyofibrillar mitochondria are located within 20 nm of the junctional sarcoplasmic reticulum and thereby experience a high local [Ca2+] during the Ca2+ release process. Both local and global Ca2+ signals may thus influence calcium signaling in mitochondria and, reciprocally, mitochondria may contribute to the local control of calcium signaling. In addition to the intermyofibrillar mitochondria, morphologically distinct mitochondria are also located in the perinuclear and subsarcolemmal regions of the cardiomyocyte and thus experience a different local [Ca2+].Here we review the literature in regard to several issues of broad interest: (1) the ultrastructural basis for mitochondrion – sarcoplasmic reticulum cross-signaling; (2) mechanisms of sarcoplasmic reticulum signaling; (3) mitochondrial calcium signaling; and (4) the possible interplay of calcium signaling between the sarcoplasmic reticulum and adjacent mitochondria.Finally, this review discusses experimental findings and mathematical models of cardiac calcium signaling between the sarcoplasmic reticulum and mitochondria, identifies weaknesses in these models, and suggests strategies and approaches for future investigations. 相似文献
4.
Jose Millet Yuriana Aguilar-Sanchez Dmytro Kornyeyev Maedeh Bazmi Diego Fainstein Julio A. Copello Ariel L. Escobar 《The Journal of general physiology》2021,153(2)
Ca2+ alternans (Ca-Alts) are alternating beat-to-beat changes in the amplitude of Ca2+ transients that frequently occur during tachycardia, ischemia, or hypothermia that can lead to sudden cardiac death. Ca-Alts appear to result from a variation in the amount of Ca2+ released from the sarcoplasmic reticulum (SR) between two consecutive heartbeats. This variable Ca2+ release has been attributed to the alternation of the action potential duration, delay in the recovery from inactivation of RYR Ca2+ release channel (RYR2), or an incomplete Ca2+ refilling of the SR. In all three cases, the RYR2 mobilizes less Ca2+ from the SR in an alternating manner, thereby generating an alternating profile of the Ca2+ transients. We used a new experimental approach, fluorescence local field optical mapping (FLOM), to record at the epicardial layer of an intact heart with subcellular resolution. In conjunction with a local cold finger, a series of images were recorded within an area where the local cooling induced a temperature gradient. Ca-Alts were larger in colder regions and occurred without changes in action potential duration. Analysis of the change in the enthalpy and Q10 of several kinetic processes defining intracellular Ca2+ dynamics indicated that the effects of temperature change on the relaxation of intracellular Ca2+ transients involved both passive and active mechanisms. The steep temperature dependency of Ca-Alts during tachycardia suggests Ca-Alts are generated by insufficient SERCA-mediated Ca2+ uptake into the SR. We found that Ca-Alts are heavily dependent on intra-SR Ca2+ and can be promoted through partial pharmacologic inhibition of SERCA2a. Finally, the FLOM experimental approach has the potential to help us understand how arrhythmogenesis correlates with the spatial distribution of metabolically impaired myocytes along the myocardium. 相似文献
5.
Single channel properties of cardiac and fast-twitch skeletal muscle sarcoplasmic reticulum (SR) release channels were compared in a planar bilayer by fusing SR membranes in a Cs+-conducting medium. We found that the pharmacology, Cs+ conductance and selectivity to monovalent and divalent cations of the two channels were similar. The cardiac SR channel exhibited multiple kinetic states. The open and closed lifetimes were not altered from a range of 10–7 to 10–3 M Ca2+, but the proportion of closed and open states shifted to shorter closings and openings, respectively.However, while the single channel activity of the skeletal SR channel was activated and inactivated by micromolar and millimolar Ca2+, respectively, the cardiac SR channel remained activated in the presence of high [Ca2+]. In correlation to these studies, [3H]ryanodine binding by the receptors of the two channel receptors was inhibited by high [Ca2+] in skeletal but not in cardiac membranes in the presence of adenine nucleotides. There is, however, a minor inhibition of [3H]ryanodine binding of cardiac SR at millimolar Ca2+ in the absence of adenine nucleotides.When Ca2+-induced Ca2+ release was examined from preloaded native SR vesicles, the release rates followed a normal biphasic curve, with Ca2+-induced inactivation at high [Ca2+] for both cardiac and skeletal SR. Our data suggest that the molecular basis of regulation of the SR Ca2+ release channel in cardiac and skeletal muscle is different, and that the cardiac SR channel isoform lacks a Ca2+-inactivated site.This work was supported by research grants from the National Institutes of Health HL13870 and AR38970, and the Texas Affiliate of the American Heart Association, 91A-188. M. Fill was the recipient of an NIH fellowship AR01834. 相似文献
6.
V. K. Sharma V. Ramesh C. Franzini-Armstrong S-S Sheu 《Journal of bioenergetics and biomembranes》2000,32(1):97-104
Studies with electron microscopy have shown that sarcoplasmic reticulum (SR) andmitochondria locate close to each other in cardiac muscle cells. We investigated the hypothesis thatthis proximity results in a transient exposure of mitochondrial Ca2+ uniporter (CaUP) to highconcentrations of Ca2+ following Ca2+ release from the SR and thus an influx of Ca2+into mitochondria. Single ventricular myocytes of rat were skinned by exposing them to aphysiological solution containing saponin (0.2 mg/ml). Cytosolic Ca2+ concentration ([Ca2+]c)and mitochondrial Ca2+ concentration ([Ca2+]m) were measured with fura-2 and rhod2,respectively. Application of caffeine (10 mM) induced a concomitant increase in[Ca2+]c and [Ca2+]m.Ruthenium red, at concentrations that block CaUP but not SR release, diminished thecaffeine-induced increase in [Ca2+]m but not[Ca2+]c. In the presence of 1 mM BAPTA, a Ca2+ chelator,the caffeine-induced increase in [Ca2+]m was reduced substantially less than [Ca2+]c. Moreover,inhibition of SR Ca2+ pump with two different concentrations of thapsigargin caused anincrease in [Ca2+]m, which was related to the rate of [Ca2+]c increase. Finally, electronmicroscopy showed that sites of junctions between SR and T tubules from which Ca2+ is released,or Ca2+ release units, CRUs, are preferentially located in close proximity to mitochondria.The distance between individual SR Ca2+ release channels (feet or ryanodine receptors) isvery short, ranging between approximately 37 and 270 nm. These results are consistent withthe idea that there is a preferential coupling of Ca2+ transport from SR to mitochondria incardiac muscle cells, because of their structural proximity. 相似文献
7.
《Channels (Austin, Tex.)》2013,7(5):391-392
Ca2+ signaling in skeletal and cardiac muscles is a bi-directional process that involves cross-talk between signaling molecules in the sarcolemmal membrane and Ca2+ release machinery in the intracellular organelles. Maintenance of a junctional membrane structure between the sarcolemmal membrane and the sarcoplasmic reticulum (SR) provides a framework for the conversion of action potential arrived at the sarcolemma into release of Ca2+ from the SR, leading to activation of a variety of physiological processes. Activity-dependent changes in Ca2+ storage inside the SR provides a retrograde signal for the activation of store-operated Ca2+ channel (SOC) on the sarcolemmal membrane, which plays important roles in the maintenance of Ca2+ homeostasis in physiology and pathophysiology. Research progress during the last 30 years had advanced our understanding of the cellular and molecular mechanisms for the control of Ca2+ signaling in muscle and cardiovascular physiology. Here we summarize the functions of three key molecules that are located in the junctional membrane complex of skeletal and cardiac muscle cells: junctophilin as a “glue” that physiologically links the SR membrane to the sarcolemmal membrane for formation of the junctional membrane framework, mitsugumin29 as a muscle-specific synaptophysin family protein that contributes to maintain the coordinated Ca2+ signaling in skeletal muscle, and TRIC as a novel cation-selective channel located on the SR membrane that provides counter-ion current during the rapid process of Ca2+ release from the SR. 相似文献
8.
Norma C. Perez-Rosas Norma L. Gomez-Viquez Adan Dagnino-Acosta Moises Santillan Agustín Guerrero-Hernandez 《PloS one》2015,10(9)
The process of Ca2+ release from sarcoplasmic reticulum (SR) comprises 4 phases in smooth muscle cells. Phase 1 is characterized by a large increase of the intracellular Ca2+ concentration ([Ca2+]i) with a minimal reduction of the free luminal SR [Ca2+] ([Ca2+]FSR). Importantly, active SR Ca2+ ATPases (SERCA pumps) are necessary for phase 1 to occur. This situation cannot be explained by the standard kinetics that involves a fixed amount of luminal Ca2+ binding sites. A new mathematical model was developed that assumes an increasing SR Ca2+ buffering capacity in response to an increase of the luminal SR [Ca2+] that is called Kinetics-on-Demand (KonD) model. This approach can explain both phase 1 and the refractory period associated with a recovered [Ca2+]FSR. Additionally, our data suggest that active SERCA pumps are a requisite for KonD to be functional; otherwise luminal SR Ca2+ binding proteins switch to standard kinetics. The importance of KonD Ca2+ binding properties is twofold: a more efficient Ca2+ release process and that [Ca2+]FSR and Ca2+-bound to SR proteins ([Ca2+]BSR) can be regulated separately allowing for Ca2+ release to occur (provided by Ca2+-bound to luminal Ca2+ binding proteins) without an initial reduction of the [Ca2+]FSR. 相似文献
9.
The dyadic organization of ventricular myocytes ensures synchronized activation of sarcoplasmic reticulum (SR) Ca2+ release during systole. However, it remains obscure how the dyadic organization affects SR Ca2+ handling during diastole. By measuring intraluminal SR Ca2+ ([Ca2+]SR) decline during rest in rabbit ventricular myocytes, we found that ∼76% of leaked SR Ca2+ is extruded from the cytosol and only ∼24% is pumped back into the SR. Thus, the majority of Ca2+ that leaks from the SR is removed from the cytosol before it can be sequestered back into the SR by the SR Ca2+-ATPase (SERCA). Detubulation decreased [Ca2+]SR decline during rest, thus making the leaked SR Ca2+ more accessible for SERCA. These results suggest that Ca2+ extrusion systems are localized in T-tubules. Inhibition of Na+-Ca2+ exchanger (NCX) slowed [Ca2+]SR decline during rest by threefold, however did not prevent it. Depolarization of mitochondrial membrane potential during NCX inhibition completely prevented the rest-dependent [Ca2+]SR decline. Despite a significant SR Ca2+ leak, Ca2+ sparks were very rare events in control conditions. NCX inhibition or detubulation increased Ca2+ spark activity independent of SR Ca2+ load. Overall, these results indicate that during rest NCX effectively competes with SERCA for cytosolic Ca2+ that leaks from the SR. This can be explained if the majority of SR Ca2+ leak occurs through ryanodine receptors in the junctional SR that are located closely to NCX in the dyadic cleft. Such control of the dyadic [Ca2+] by NCX play a critical role in suppressing Ca2+ sparks during rest. 相似文献
10.
The dyadic organization of ventricular myocytes ensures synchronized activation of sarcoplasmic reticulum (SR) Ca2+ release during systole. However, it remains obscure how the dyadic organization affects SR Ca2+ handling during diastole. By measuring intraluminal SR Ca2+ ([Ca2+]SR) decline during rest in rabbit ventricular myocytes, we found that ∼76% of leaked SR Ca2+ is extruded from the cytosol and only ∼24% is pumped back into the SR. Thus, the majority of Ca2+ that leaks from the SR is removed from the cytosol before it can be sequestered back into the SR by the SR Ca2+-ATPase (SERCA). Detubulation decreased [Ca2+]SR decline during rest, thus making the leaked SR Ca2+ more accessible for SERCA. These results suggest that Ca2+ extrusion systems are localized in T-tubules. Inhibition of Na+-Ca2+ exchanger (NCX) slowed [Ca2+]SR decline during rest by threefold, however did not prevent it. Depolarization of mitochondrial membrane potential during NCX inhibition completely prevented the rest-dependent [Ca2+]SR decline. Despite a significant SR Ca2+ leak, Ca2+ sparks were very rare events in control conditions. NCX inhibition or detubulation increased Ca2+ spark activity independent of SR Ca2+ load. Overall, these results indicate that during rest NCX effectively competes with SERCA for cytosolic Ca2+ that leaks from the SR. This can be explained if the majority of SR Ca2+ leak occurs through ryanodine receptors in the junctional SR that are located closely to NCX in the dyadic cleft. Such control of the dyadic [Ca2+] by NCX play a critical role in suppressing Ca2+ sparks during rest. 相似文献
11.
The factors responsible for the regulation of regenerative calcium-induced calcium release (CICR) during Ca2+ spark evolution remain unclear. Cardiac ryanodine receptor (RyR) gating in rats and sheep was recorded at physiological Ca2+, Mg2+, and ATP levels and incorporated into a 3D model of the cardiac dyad, which reproduced the time course of Ca2+ sparks, Ca2+ blinks, and Ca2+ spark restitution. The termination of CICR by induction decay in the model principally arose from the steep Ca2+ dependence of RyR closed time, with the measured sarcoplasmic reticulum (SR) lumen Ca2+ dependence of RyR gating making almost no contribution. The start of CICR termination was strongly dependent on the extent of local depletion of junctional SR Ca2+, as well as the time course of local Ca2+ gradients within the junctional space. Reducing the dimensions of the dyad junction reduced Ca2+ spark amplitude by reducing the strength of regenerative feedback within CICR. A refractory period for Ca2+ spark initiation and subsequent Ca2+ spark amplitude restitution arose from 1), the extent to which the regenerative phase of CICR can be supported by the partially depleted junctional SR, and 2), the availability of releasable Ca2+ in the junctional SR. The physical organization of RyRs within the junctional space had minimal effects on Ca2+ spark amplitude when more than nine RyRs were present. Spark amplitude had a nonlinear dependence on RyR single-channel Ca2+ flux, and was approximately halved by reducing the flux from 0.6 to 0.2 pA. Although rat and sheep RyRs had quite different Ca2+ sensitivities, Ca2+ spark amplitude was hardly affected. This suggests that moderate changes in RyR gating by second-messenger systems will principally alter the spatiotemporal properties of SR release, with smaller effects on the amount released. 相似文献
12.
We have developed a quantitative model for the creation of cytoplasmic Ca2+ gradients near the inner surface of the plasma membrane (PM). In particular we simulated the refilling of the sarcoplasmic reticulum (SR) via PM–SR junctions during asynchronous [Ca2+]i oscillations in smooth muscle cells of the rabbit inferior vena cava. We have combined confocal microscopy data on the [Ca2+]i oscillations, force transduction data from cell contraction studies and electron microscopic images to build a basis for computational simulations that model the transport of calcium ions from Na+/Ca2+ exchangers (NCX) on the PM to sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) pumps on the SR as a three-dimensional random walk through the PM–SR junctional cytoplasmic spaces. Electron microscopic ultrastructural images of the smooth muscle cells were elaborated with software algorithms to produce a very clear and dimensionally accurate picture of the PM–SR junctions. From this study, we conclude that it is plausible and possible for enough Ca2+ to pass through the PM–SR junctions to replete the SR during the regenerative Ca2+ release, which underlies agonist induced asynchronous Ca2+ oscillations in vascular smooth muscle. 相似文献
13.
In this study, Ca2+ release due to spontaneous Ca2+ waves was measured both from inside the sarcoplasmic reticulum (SR) and from the cytosol of rabbit cardiomyocytes. These measurements utilized Fluo5N-AM for intra-SR Ca2+ from intact cells and Fluo5F in the cytosol of permeabilized cells. Restricted subcellular volumes were resolved with the use of laser-scanning confocal microscopy. Local Ca2+ signals during spontaneous Ca2+ release were compared with those induced by rapid caffeine application. The free cytoplasmic [Ca2+] increase during a Ca2+ wave was 98.1% ± 0.3% of that observed during caffeine application. Conversion to total Ca2+ release suggested that Ca2+ release from a Ca2+ wave was not significantly different from that released during caffeine application (104% ± 6%). In contrast, the maximum decrease in intra-SR Fluo-5N fluorescence during a Ca2+ wave was 82.5% ± 2.6% of that observed during caffeine application. Assuming a maximum free [Ca2+] of 1.1 mM, this translates to a 96.2% ± 0.8% change in intra-SR free [Ca2+] and a 91.7% ± 1.6% depletion of the total Ca2+. This equates to a minimum intra-SR free Ca2+ of 46 ± 7 μM during a Ca2+ wave. Reduction of RyR2 Ca2+ sensitivity by tetracaine (50 μM) reduced the spontaneous Ca2+ release frequency while increasing the Ca2+ wave amplitude. This did not significantly change the total depletion of the SR (94.5% ± 1.1%). The calculated minimum [Ca2+] during these Ca2+ waves (87 ± 19 μM) was significantly higher than control (p < 0.05). A computational model incorporating this level of Ca2+ depletion during a Ca2+ wave mimicked the transient and sustained effects of tetracaine on spontaneous Ca2+ release. In conclusion, spontaneous Ca2+ release results in substantial but not complete local Ca2+ depletion of the SR. Furthermore, measurements suggest that Ca2+ release terminates when luminal [Ca2+] reaches ∼50 μM. 相似文献
14.
Michele Scorzeto Marta Giacomello Luana Toniolo Marta Canato Bert Blaauw Cecilia Paolini Feliciano Protasi Carlo Reggiani Ger J. M. Stienen 《PloS one》2013,8(10)
Mitochondrial calcium handling and its relation with calcium released from sarcoplasmic reticulum (SR) in muscle tissue are subject of lively debate. In this study we aimed to clarify how the SR determines mitochondrial calcium handling using dCASQ-null mice which lack both isoforms of the major Ca2+-binding protein inside SR, calsequestrin. Mitochondrial free Ca2+-concentration ([Ca2+]mito) was determined by means of a genetically targeted ratiometric FRET-based probe. Electron microscopy revealed a highly significant increase in intermyofibrillar mitochondria (+55%) and augmented coupling (+12%) between Ca2+ release units of the SR and mitochondria in dCASQ-null vs. WT fibers. Significant differences in the baseline [Ca2+]mito were observed between quiescent WT and dCASQ-null fibers, but not in the resting cytosolic Ca2+ concentration. The rise in [Ca2+]mito during electrical stimulation occurred in 20−30 ms, while the decline during and after stimulation was governed by 4 rate constants of approximately 40, 1.6, 0.2 and 0.03 s−1. Accordingly, frequency-dependent increase in [Ca2+]mito occurred during sustained contractions. In dCASQ-null fibers the increases in [Ca2+]mito were less pronounced than in WT fibers and even lower when extracellular calcium was removed. The amplitude and duration of [Ca2+]mito transients were increased by inhibition of mitochondrial Na+/Ca2+ exchanger (mNCX). These results provide direct evidence for fast Ca2+ accumulation inside the mitochondria, involvement of the mNCX in mitochondrial Ca2+-handling and a dependence of mitochondrial Ca2+-handling on intracellular (SR) and external Ca2+ stores in fast skeletal muscle fibers. dCASQ-null mice represent a model for malignant hyperthermia. The differences in structure and in mitochondrial function observed relative to WT may represent compensatory mechanisms for the disease-related reduction of calcium storage capacity of the SR and/or SR Ca2+-leakage. 相似文献
15.
The spatio-temporal properties of Ca2+ transients during excitation-contraction coupling and elementary Ca2+ release events (Ca2+ sparks) were studied in atrial and ventricular myocytes with ultra-fast confocal microscopy using a Zeiss LSM 5 LIVE system that allows sampling rates of up to 60 kHz. Ca2+ sparks which originated from subsarcolemmal junctional sarcoplasmic reticulum (j-SR) release sites in atrial myocytes were anisotropic and elongated in the longitudinal direction of the cell. Ca2+ sparks in atrial cells originating from non-junctional SR and in ventricular myocytes were symmetrical. Ca2+ spark recording in line scan mode at 40,000 lines/s uncovered step-like increases of [Ca2+]i. 2-D imaging of Ca2+ transients revealed an asynchronous activation of release sites and allowed the sequential recording of Ca2+ entry through surface membrane Ca2+ channels and subsequent activation of Ca2+-induced Ca2+ release. With a latency of 2.5 ms after application of an electrical stimulus, Ca2+ entry could be detected that was followed by SR Ca2+ release after an additional 3 ms delay. Maximum Ca2+ release was observed 4 ms after the beginning of release. The timing of Ca2+ entry and release was confirmed by simultaneous [Ca2+]i and membrane current measurements using the whole cell voltage-clamp technique. In atrial cells activation of discrete individual release sites of the j-SR led to spatially restricted Ca2+ release events that fused into a peripheral ring of elevated [Ca2+]i that subsequently propagated in a wave-like fashion towards the center of the cell. In ventricular myocytes asynchronous Ca2+ release signals from discrete sites with no preferential subcellular location preceded the whole-cell Ca2+ transient. In summary, ultra-fast confocal imaging allows investigation of Ca2+ signals with a time resolution similar to patch clamp technique, however in a less invasive fashion. 相似文献
16.
This study investigated the mechanisms underlying the propagation of cytoplasmic calcium waves and the genesis of systolic Ca2+ alternans in cardiac myocytes lacking transverse tubules (t-tubules). These correspond to atrial cells of either small mammals or large mammals that have lost their t-tubules due to disease-induced structural remodeling (e.g., atrial fibrillation). A mathematical model was developed for a cluster of ryanodine receptors distributed on the cross section of a cell that was divided into 13 elements with a spatial resolution of 2 μm. Due to the absence of t-tubules, L-type Ca2+ channels were only located in the peripheral elements close to the cell-membrane surface and produced Ca2+ signals that propagated toward central elements by triggering successive Ca2+-induced Ca2+ release (CICR) via Ca2+ diffusion between adjacent elements. Under control conditions, the Ca2+ signals did not fully propagate to the central region of the cell. However, with modulation of several factors responsible for Ca2+ handling, such as the L-type Ca2+ channels (Ca2+ influx), SERCA pumps (sarcoplasmic reticulum (SR) Ca2+ uptake), and ryanodine receptors (SR Ca2+ release), Ca2+ wave propagation to the center of the cell could occur. These simulation results are consistent with previous experimental data from atrial cells of small mammals. The model further reveals that spatially functional heterogeneity in Ca2+ diffusion within the cell produced a steep relationship between the SR Ca2+ content and the cytoplasmic Ca2+ concentration. This played an important role in the genesis of Ca2+ alternans that were more obvious in central than in peripheral elements. Possible association between the occurrence of Ca2+ alternans and the model parameters of Ca2+ handling was comprehensively explored in a wide range of one- and two-parameter spaces. In addition, the model revealed a spontaneous second Ca2+ release in response to a single voltage stimulus pulse with SR Ca2+ overloading and augmented Ca2+ influx. This study provides what to our knowledge are new insights into the genesis of Ca2+ alternans and spontaneous second Ca2+ release in cardiac myocytes that lack t-tubules. 相似文献
17.
Cardiac cellular calcium (Ca2+) handling is the well-investigated mediator of excitation–contraction coupling, the process that translates cardiac electrical activation into mechanical events. The reverse—effects of mechanical stimulation on cardiomyocyte Ca2+ handling—are much less well understood, in particular during the inter-beat period, called ‘diastole’. We have investigated the effects of diastolic length changes, applied axially using a pair of carbon fibres attached to opposite ends of Guinea pig isolated ventricular myocytes, on the availability of Ca2+ in the main cellular stores (the sarcoplasmic reticulum; SR), by studying the rest-decay of SR Ca2+ content [Ca2+]SR, and the reloading of the SR after prior depletion of Ca2+ from the cell.Cells were loaded with Fura-2 AM (an indicator of the cytosolic ‘free’ Ca2+ concentration, [Ca2+]i), and pre-conditioned by field-stimulation (2 Hz) at 37 °C, while [Ca2+]i transients and sarcomere length (SL) were recorded simultaneously. After reaching a steady state in the behaviour of observed parameters, stimulation was interrupted for between 5 and 60 s, while cells were either held at resting length, or stretched (controlled to cause a 10% increase in SL, to aid inter-individual comparison). Thereafter, each cell was returned to its original resting length, followed by swift administration of 10 mM of caffeine (in Na+/Ca2+-free solution), which causes the release of Ca2+ from the SR (caffeine), but largely prevents extrusion of Ca2+ from the cytosol to the cell exterior (Na+/Ca2+-free solution). By comparing the [Ca2+]i in cells exposed/not exposed to diastolic stretch of different duration, we assessed the rest-decay dynamics of [Ca2+]SR. To assess SR reloading after initial Ca2+ depletion, the same stretch protocol was implemented after prior emptying of the cell by application of 10 mM of caffeine in normal Tyrode solution (which causes Ca2+ to be released from the SR and extruded from the cell via the Na+/Ca2+ exchanger; NCX).Axial stretch enhanced the rate of both rest-decay and reloading of [Ca2+]SR. Application of 40 μM streptomycin, a blocker of stretch-activated ion channels, did not affect the stretch-induced increase in SR reloading. This behaviour was reproduced in a computer simulation study, using a modified version of the 2006 Iribe–Kohl–Noble model of single cardiac myocyte Ca2+ handling, suggesting that stretch increases both Ca2+ leak from the SR and Ca2+ influx via the sarcolemma. This may have important implications for the mobilisation of Ca2+ in stretched cells, and could contribute to the regional ‘matching’ of individual cardiomyocyte contractility to dynamic, and regionally varying, changes in mechanical loads, such as diastolic pre-load, of cardiac tissue. 相似文献
18.
Imaizumi Yuji Ohi Yoshiaki Yamamura Hisao Morimura Kozo Muraki Katsuhiko 《Neurophysiology》2003,35(3-4):169-174
The contribution of the Ca2+-induced Ca2+ release (CICR) mechanism in excitation-contraction (E-C) coupling and the tightness of the coupling between Ca2+ influx and Ca2+ release are still controversial in smooth muscle cells (SMC). In SMC isolated from the guinea-pig vas deferens or urinary bladder, a depolarizing stimulus initially induced spot-like increases in the intracellular Ca2+ concentration ([Ca2+] i ), called “Ca2+ hot spots,” at several superficial areas in the cell. When a weak stimulus (a small or a short depolarizing step) was applied, only a few Ca2+ hot spots appeared transiently in the superficial area but did not spread into other regions, to trigger global [Ca2+] i rise. Such depolarization-evoked local Ca2+ transients were distinctive from spontaneous Ca2+ sparks, since the former were susceptible to Ca2+ blockers, ryanodine, and inhibitors of the Ca2+ pump in the sarcoplasmic reticulum (SR), suggesting pivotal roles of Ca2+ influx through voltage-dependent Ca2+ channels (VDCC) and Ca2+ release from the SR through ryanodine receptors (RyR) for the activation of Ca2+ spots. Frequently discharging Ca2+ spark sites (FDS) under resting conditions were located exactly in the same areas as Ca2+ hot spots evoked by depolarization, indicating the existence of distinct local junction sites for tight coupling between VDCC in the plasmalemma and RyR in the SR. Co-localization of clusters of RyR and large-conductance Ca2+-activated K+ (BK) channels was also suggested. The fast and tight coupling for CICR in these junctional sites was triggered also by an action potential, whereas a slower spread of Ca2+ wave to the whole-cell areas suggests the loose coupling in propagating CICR to other cell areas. It can therefore be postulated that CICR may occur in two steps upon depolarization; the initial CICR in distinct junctional sites shows tight coupling between Ca2+ influx and release, and the following CICR may propagate slow Ca2+ waves to other areas. Ryanodine receptors form a multiprotein complex with molecules such as calsequestrin, junctin, triadin, junctophilins, and FK506-binding proteins, which directly or indirectly regulate the RyR activity and the tight coupling. Moreover, an evoked Ca2+ spot may enhance Ca2+ uptake by neighboring mitochondria and their ATP production to increase energy supply to the Ca2+ pump of the SR in the microdomain. 相似文献
19.
Karine Fénelon Cédric R.H. Lamboley Nicole Carrier Paul C. Pape 《The Journal of general physiology》2012,140(4):403-419
Experiments were performed to characterize the properties of the intrinsic Ca2+ buffers in the sarcoplasmic reticulum (SR) of cut fibers from frog twitch muscle. The concentrations of total and free calcium ions within the SR ([CaT]SR and [Ca2+]SR) were measured, respectively, with the EGTA/phenol red method and tetramethylmurexide (a low affinity Ca2+ indicator). Results indicate SR Ca2+ buffering was consistent with a single cooperative-binding component or a combination of a cooperative-binding component and a linear binding component accounting for 20% or less of the bound Ca2+. Under the assumption of a single cooperative-binding component, the most likely resting values of [Ca2+]SR and [CaT]SR are 0.67 and 17.1 mM, respectively, and the dissociation constant, Hill coefficient, and concentration of the Ca-binding sites are 0.78 mM, 3.0, and 44 mM, respectively. This information can be used to calculate a variable proportional to the Ca2+ permeability of the SR, namely d[CaT]SR/dt ÷ [Ca2+]SR (denoted release permeability), in experiments in which only [CaT]SR or [Ca2+]SR is measured. In response to a voltage-clamp step to −20 mV at 15°C, the release permeability reaches an early peak followed by a rapid decline to a quasi-steady level that lasts ∼50 ms, followed by a slower decline during which the release permeability decreases by at least threefold. During the quasi-steady level of release, the release amplitude is 3.3-fold greater than expected from voltage activation alone, a result consistent with the recruitment by Ca-induced Ca2+ release of 2.3 SR Ca2+ release channels neighboring each channel activated by its associated voltage sensor. Release permeability at −60 mV increases as [CaT]SR decreases from its resting physiological level to ∼0.1 of this level. This result argues against a release termination mechanism proposed in mammalian muscle fibers in which a luminal sensor of [Ca2+]SR inhibits release when [CaT]SR declines to a low level. 相似文献
20.
Vijay Rajagopal Gregory Bass Cameron G. Walker David J. Crossman Amorita Petzer Anthony Hickey Ivo Siekmann Masahiko Hoshijima Mark H. Ellisman Edmund J. Crampin Christian Soeller 《PLoS computational biology》2015,11(9)
Spatio-temporal dynamics of intracellular calcium, [Ca2+]i, regulate the contractile function of cardiac muscle cells. Measuring [Ca2+]i flux is central to the study of mechanisms that underlie both normal cardiac function and calcium-dependent etiologies in heart disease. However, current imaging techniques are limited in the spatial resolution to which changes in [Ca2+]i can be detected. Using spatial point process statistics techniques we developed a novel method to simulate the spatial distribution of RyR clusters, which act as the major mediators of contractile Ca2+ release, upon a physiologically-realistic cellular landscape composed of tightly-packed mitochondria and myofibrils. We applied this method to computationally combine confocal-scale (~ 200 nm) data of RyR clusters with 3D electron microscopy data (~ 30 nm) of myofibrils and mitochondria, both collected from adult rat left ventricular myocytes. Using this hybrid-scale spatial model, we simulated reaction-diffusion of [Ca2+]i during the rising phase of the transient (first 30 ms after initiation). At 30 ms, the average peak of the simulated [Ca2+]i transient and of the simulated fluorescence intensity signal, F/F0, reached values similar to that found in the literature ([Ca2+]i ≈1 μM; F/F0≈5.5). However, our model predicted the variation in [Ca2+]i to be between 0.3 and 12.7 μM (~3 to 100 fold from resting value of 0.1 μM) and the corresponding F/F0 signal ranging from 3 to 9.5. We demonstrate in this study that: (i) heterogeneities in the [Ca2+]i transient are due not only to heterogeneous distribution and clustering of mitochondria; (ii) but also to heterogeneous local densities of RyR clusters. Further, we show that: (iii) these structure-induced heterogeneities in [Ca2+]i can appear in line scan data. Finally, using our unique method for generating RyR cluster distributions, we demonstrate the robustness in the [Ca2+]i transient to differences in RyR cluster distributions measured between rat and human cardiomyocytes. 相似文献