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Dvinskikh L, Sparks H, MacLeod KT, Dunsby C. High-speed 2D light-sheet fluorescence microscopy enables quantification of spatially varying calcium dynamics in ventricular cardiomyocytes. Front Physiol 2023; 14:1079727. [PMID: 36866170 PMCID: PMC9971815 DOI: 10.3389/fphys.2023.1079727] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2022] [Accepted: 01/27/2023] [Indexed: 02/16/2023] Open
Abstract
Introduction: Reduced synchrony of calcium release and t-tubule structure organization in individual cardiomyocytes has been linked to loss of contractile strength and arrhythmia. Compared to confocal scanning techniques widely used for imaging calcium dynamics in cardiac muscle cells, light-sheet fluorescence microscopy enables fast acquisition of a 2D plane in the sample with low phototoxicity. Methods: A custom light-sheet fluorescence microscope was used to achieve dual-channel 2D timelapse imaging of calcium and the sarcolemma, enabling calcium sparks and transients in left and right ventricle cardiomyocytes to be correlated with the cell microstructure. Imaging electrically stimulated dual-labelled cardiomyocytes immobilized with para-nitroblebbistatin, a non-phototoxic, low fluorescence contraction uncoupler, with sub-micron resolution at 395 fps over a 38 μm × 170 µm FOV allowed characterization of calcium spark morphology and 2D mapping of the calcium transient time-to-half-maximum across the cell. Results: Blinded analysis of the data revealed sparks with greater amplitude in left ventricle myocytes. The time for the calcium transient to reach half-maximum amplitude in the central part of the cell was found to be, on average, 2 ms shorter than at the cell ends. Sparks co-localized with t-tubules were found to have significantly longer duration, larger area and spark mass than those further away from t-tubules. Conclusion: The high spatiotemporal resolution of the microscope and automated image-analysis enabled detailed 2D mapping and quantification of calcium dynamics of n = 60 myocytes, with the findings demonstrating multi-level spatial variation of calcium dynamics across the cell, supporting the dependence of synchrony and characteristics of calcium release on the underlying t-tubule structure.
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Affiliation(s)
- Liuba Dvinskikh
- Department of Physics, Imperial College London, London, United Kingdom
- National Heart and Lung Institute, Imperial College London, London, United Kingdom
- Department of Chemistry, Imperial College London, London, United Kingdom
| | - Hugh Sparks
- Department of Physics, Imperial College London, London, United Kingdom
| | - Kenneth T. MacLeod
- National Heart and Lung Institute, Imperial College London, London, United Kingdom
| | - Chris Dunsby
- Department of Physics, Imperial College London, London, United Kingdom
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Shkryl VM. The spatio-temporal properties of calcium transients in hippocampal pyramidal neurons in vitro. Front Cell Neurosci 2022; 16:1054950. [PMID: 36589284 PMCID: PMC9795003 DOI: 10.3389/fncel.2022.1054950] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/28/2022] [Accepted: 11/24/2022] [Indexed: 12/15/2022] Open
Abstract
The spatio-temporal properties of calcium signals were studied in cultured pyramidal neurons of the hippocampus using two-dimensional fluorescence microscopy and ratiometric dye Fura-2. Depolarization-induced Ca2+ transients revealed an asynchronous delayed increase in free Ca2+ concentration. We found that the level of free resting calcium in the cell nucleus is significantly lower compared to the soma, sub-membrane, and dendritic tree regions. Calcium release from the endoplasmic reticulum under the action of several stimuli (field stimulation, high K+ levels, and caffeine) occurs in all areas studied. Under depolarization, calcium signals developed faster in the dendrites than in other areas, while their amplitude was significantly lower since larger and slower responses inside the soma. The peak value of the calcium response to the application of 10 mM caffeine, ryanodine receptors (RyRs) agonist, does not differ in the sub-membrane zone, central region, and nucleus but significantly decreases in the dendrites. In the presence of caffeine, the delay of Ca2+ signals between various areas under depolarization significantly declined. Thirty percentage of the peak amplitude of Ca2+ transients at prolonged electric field stimulation corresponded to calcium release from the ER store by RyRs, while short-term stimulation did not depend on them. 20 μM dantrolene, RyRs inhibitor, significantly reduces Ca2+ transient under high K+ levels depolarization of the neuron. RyRs-mediated enhancement of the Ca2+ signal is more pronounced in the central part and nucleus compared to the sub-membrane or dendrites regions of the neuron. In summary, using the ratiometric imaging allowed us to obtain additional information about the involvement of RyRs in the intracellular dynamics of Ca2+ signals induced by depolarization or electrical stimulation train, with an underlying change in Ca2+ concentration in various regions of interest in hippocampal pyramidal neurons.
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Kempmann A, Gensch T, Offenhäusser A, Tihaa I, Maybeck V, Balfanz S, Baumann A. The Functional Characterization of GCaMP3.0 Variants Specifically Targeted to Subcellular Domains. Int J Mol Sci 2022; 23:ijms23126593. [PMID: 35743038 PMCID: PMC9223625 DOI: 10.3390/ijms23126593] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2022] [Revised: 05/31/2022] [Accepted: 06/06/2022] [Indexed: 11/16/2022] Open
Abstract
Calcium (Ca2+) ions play a pivotal role in physiology and cellular signaling. The intracellular Ca2+ concentration ([Ca2+]i) is about three orders of magnitude lower than the extracellular concentration, resulting in a steep transmembrane concentration gradient. Thus, the spatial and the temporal dynamics of [Ca2+]i are ideally suited to modulate Ca2+-mediated cellular responses to external signals. A variety of highly sophisticated methods have been developed to gain insight into cellular Ca2+ dynamics. In addition to electrophysiological measurements and the application of synthetic dyes that change their fluorescent properties upon interaction with Ca2+, the introduction and the ongoing development of genetically encoded Ca2+ indicators (GECI) opened a new era to study Ca2+-driven processes in living cells and organisms. Here, we have focused on one well-established GECI, i.e., GCaMP3.0. We have systematically modified the protein with sequence motifs, allowing localization of the sensor in the nucleus, in the mitochondrial matrix, at the mitochondrial outer membrane, and at the plasma membrane. The individual variants and a cytosolic version of GCaMP3.0 were overexpressed and purified from E. coli cells to study their biophysical properties in solution. All versions were examined to monitor Ca2+ signaling in stably transfected cell lines and in primary cortical neurons transduced with recombinant Adeno-associated viruses (rAAV). In this comparative study, we provide evidence for a robust approach to reliably trace Ca2+ signals at the (sub)-cellular level with pronounced temporal resolution.
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Affiliation(s)
- Annika Kempmann
- Institute of Biological Information Processing, IBI-1, Research Center Jülich, 52428 Jülich, Germany; (A.K.); (T.G.); (S.B.)
| | - Thomas Gensch
- Institute of Biological Information Processing, IBI-1, Research Center Jülich, 52428 Jülich, Germany; (A.K.); (T.G.); (S.B.)
| | - Andreas Offenhäusser
- Institute of Biological Information Processing, IBI-3, Research Center Jülich, 52428 Jülich, Germany; (A.O.); (I.T.); (V.M.)
| | - Irina Tihaa
- Institute of Biological Information Processing, IBI-3, Research Center Jülich, 52428 Jülich, Germany; (A.O.); (I.T.); (V.M.)
| | - Vanessa Maybeck
- Institute of Biological Information Processing, IBI-3, Research Center Jülich, 52428 Jülich, Germany; (A.O.); (I.T.); (V.M.)
| | - Sabine Balfanz
- Institute of Biological Information Processing, IBI-1, Research Center Jülich, 52428 Jülich, Germany; (A.K.); (T.G.); (S.B.)
| | - Arnd Baumann
- Institute of Biological Information Processing, IBI-1, Research Center Jülich, 52428 Jülich, Germany; (A.K.); (T.G.); (S.B.)
- Correspondence: ; Tel.: +49-2461-614014
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Shen X, van den Brink J, Hou Y, Colli D, Le C, Kolstad TR, MacQuaide N, Carlson CR, Kekenes‐Huskey PM, Edwards AG, Soeller C, Louch WE. 3D dSTORM imaging reveals novel detail of ryanodine receptor localization in rat cardiac myocytes. J Physiol 2019; 597:399-418. [PMID: 30412283 PMCID: PMC6332759 DOI: 10.1113/jp277360] [Citation(s) in RCA: 36] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2018] [Accepted: 11/08/2018] [Indexed: 11/08/2022] Open
Abstract
KEY POINTS Using 3D direct stochastic optical reconstruction microscopy (dSTORM), we developed novel approaches to quantitatively describe the nanoscale, 3D organization of ryanodine receptors (RyRs) in cardiomyocytes. Complex arrangements of RyR clusters were observed in 3D space, both at the cell surface and within the cell interior, with allocation to dyadic and non-dyadic pools. 3D imaging importantly allowed discernment of clusters overlapping in the z-axis, for which detection was obscured by conventional 2D imaging techniques. Thus, RyR clusters were found to be significantly smaller than previous 2D estimates. Ca2+ release units (CRUs), i.e. functional groupings of neighbouring RyR clusters, were similarly observed to be smaller than earlier reports. Internal CRUs contained more RyRs in more clusters than CRUs on the cell surface, and yielded longer duration Ca2+ sparks. ABSTRACT Cardiomyocyte contraction is dependent on Ca2+ release from ryanodine receptors (RyRs). However, the precise localization of RyRs remains unknown, due to shortcomings of imaging techniques which are diffraction limited or restricted to 2D. We aimed to determine the 3D nanoscale organization of RyRs in rat cardiomyocytes by employing direct stochastic optical reconstruction microscopy (dSTORM) with phase ramp technology. Initial observations at the cell surface showed an undulating organization of RyR clusters, resulting in their frequent overlap in the z-axis and obscured detection by 2D techniques. Non-overlapping clusters were imaged to create a calibration curve for estimating RyR number based on recorded fluorescence blinks. Employing this method at the cell surface and interior revealed smaller RyR clusters than 2D estimates, as erroneous merging of axially aligned RyRs was circumvented. Functional groupings of RyR clusters (Ca2+ release units, CRUs), contained an average of 18 and 23 RyRs at the surface and interior, respectively, although half of all CRUs contained only a single 'rogue' RyR. Internal CRUs were more tightly packed along z-lines than surface CRUs, contained larger and more numerous RyR clusters, and constituted ∼75% of the roughly 1 million RyRs present in an average cardiomyocyte. This complex internal 3D geometry was underscored by correlative imaging of RyRs and t-tubules, which enabled quantification of dyadic and non-dyadic RyR populations. Mirroring differences in CRU size and complexity, Ca2+ sparks originating from internal CRUs were of longer duration than those at the surface. These data provide novel, nanoscale insight into RyR organization and function across cardiomyocytes.
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Affiliation(s)
- Xin Shen
- Institute for Experimental Medical ResearchOslo University Hospital and University of OsloNO‐0424OsloNorway
| | | | - Yufeng Hou
- Institute for Experimental Medical ResearchOslo University Hospital and University of OsloNO‐0424OsloNorway
| | - Dylan Colli
- Department of ChemistryUniversity of KentuckyLexingtonKYUSA
| | - Christopher Le
- Institute for Experimental Medical ResearchOslo University Hospital and University of OsloNO‐0424OsloNorway
| | - Terje R. Kolstad
- Institute for Experimental Medical ResearchOslo University Hospital and University of OsloNO‐0424OsloNorway
| | - Niall MacQuaide
- Institute of Cardiovascular SciencesUniversity of GlasgowGlasgowUK
| | - Cathrine R. Carlson
- Institute for Experimental Medical ResearchOslo University Hospital and University of OsloNO‐0424OsloNorway
| | | | | | | | - William E. Louch
- Institute for Experimental Medical ResearchOslo University Hospital and University of OsloNO‐0424OsloNorway
- KG Jebsen Center for Cardiac ResearchUniversity of OsloOsloNorway
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Shah SI, Smith M, Swaminathan D, Parker I, Ullah G, Demuro A. CellSpecks: A Software for Automated Detection and Analysis of Calcium Channels in Live Cells. Biophys J 2018; 115:2141-2151. [PMID: 30447989 DOI: 10.1016/j.bpj.2018.10.015] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2018] [Revised: 09/27/2018] [Accepted: 10/22/2018] [Indexed: 02/01/2023] Open
Abstract
To couple the fidelity of patch-clamp recording with a more high-throughput screening capability, we pioneered a, to our knowledge, novel approach to single-channel recording that we named "optical patch clamp." By using highly sensitive fluorescent Ca2+ indicator dyes in conjunction with total internal fluorescence microscopy techniques, we monitor Ca2+ flux through individual Ca2+-permeable channels. This approach provides information about channel gating analogous to patch-clamp recording at a time resolution of ∼2 ms with the additional advantage of being massively parallel, providing simultaneous and independent recording from thousands of channels in the native environment. However, manual analysis of the data generated by this technique presents severe challenges because a video recording can include many thousands of frames. To overcome this bottleneck, we developed an image processing and analysis framework called CellSpecks capable of detecting and fully analyzing the kinetics of ion channels within a video sequence. By using randomly generated synthetic data, we tested the ability of CellSpecks to rapidly and efficiently detect and analyze the activity of thousands of ion channels, including openings for a few milliseconds. Here, we report the use of CellSpecks for the analysis of experimental data acquired by imaging muscle nicotinic acetylcholine receptors and the Alzheimer's disease-associated amyloid β pores with multiconductance levels in the plasma membrane of Xenopus laevis oocytes. We show that CellSpecks can accurately and efficiently generate location maps and create raw and processed fluorescence time traces; histograms of mean open times, mean close times, open probabilities, durations, and maximal amplitudes; and a "channel chip" showing the activity of all channels as a function of time. Although we specifically illustrate the application of CellSpecks for analyzing data from Ca2+ channels, it can be easily customized to analyze other spatially and temporally localized signals.
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Affiliation(s)
| | | | - Divya Swaminathan
- Department of Neurobiology and Behavior,University of California Irvine, Irvine, California
| | - Ian Parker
- Department of Neurobiology and Behavior,University of California Irvine, Irvine, California; Department of Physiology and Biophysics, University of California Irvine, Irvine, California
| | - Ghanim Ullah
- Department of Physics, University of South Florida, Tampa, Florida.
| | - Angelo Demuro
- Department of Neurobiology and Behavior,University of California Irvine, Irvine, California.
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Kolstad TR, van den Brink J, MacQuaide N, Lunde PK, Frisk M, Aronsen JM, Norden ES, Cataliotti A, Sjaastad I, Sejersted OM, Edwards AG, Lines GT, Louch WE. Ryanodine receptor dispersion disrupts Ca 2+ release in failing cardiac myocytes. eLife 2018; 7:39427. [PMID: 30375974 PMCID: PMC6245731 DOI: 10.7554/elife.39427] [Citation(s) in RCA: 73] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/21/2018] [Accepted: 10/24/2018] [Indexed: 12/22/2022] Open
Abstract
Reduced cardiac contractility during heart failure (HF) is linked to impaired Ca2+ release from Ryanodine Receptors (RyRs). We investigated whether this deficit can be traced to nanoscale RyR reorganization. Using super-resolution imaging, we observed dispersion of RyR clusters in cardiomyocytes from post-infarction HF rats, resulting in more numerous, smaller clusters. Functional groupings of RyR clusters which produce Ca2+ sparks (Ca2+ release units, CRUs) also became less solid. An increased fraction of small CRUs in HF was linked to augmented ‘silent’ Ca2+ leak, not visible as sparks. Larger multi-cluster CRUs common in HF also exhibited low fidelity spark generation. When successfully triggered, sparks in failing cells displayed slow kinetics as Ca2+ spread across dispersed CRUs. During the action potential, these slow sparks protracted and desynchronized the overall Ca2+ transient. Thus, nanoscale RyR reorganization during HF augments Ca2+ leak and slows Ca2+ release kinetics, leading to weakened contraction in this disease. The muscle cells of the heart coordinate how they contract and relax in order to produce the heartbeat. During heart failure, these cells become less able to contract. As a result the heart becomes inefficient, pumping less blood around the body. For the cardiac muscle cells to contract, the levels of calcium ions in the cells needs to rapidly increase. In failing hearts, these increases in calcium ion levels are smaller, slower and less well coordinated. It was not known what causes these changes, making it difficult to treat heart failure. Calcium ions are released in cardiac muscle cells through protein channels called ryanodine receptors. These receptors form clusters that allow them to synchronize when they open and close. Could the reorganization of ryanodine receptors account for the problems seen in failing hearts? To investigate, Kolstad et al. examined rat hearts using a technique called super-resolution microscopy. This showed that the clusters of ryanodine receptors break apart during heart failure to form smaller clusters. Further experiments showed that calcium ions ‘leak’ from these smaller clusters, reducing the amount of calcium that can be released into cardiac muscle cells during each heartbeat. Released calcium also spreads between the dispersed clusters, resulting in a slower rise of the calcium levels in the cells. Both changes contribute to weakened contractions of cells in failing hearts. Therefore, heart failure can be traced back to very small rearrangements of the ryanodine receptors. This understanding will help researchers as they investigate new ways to treat heart failure.
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Affiliation(s)
- Terje R Kolstad
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway.,KG Jebsen Center for Cardiac Research, University of Oslo, Oslo, Norway
| | | | - Niall MacQuaide
- Institute of Cardiovascular Sciences, University of Glasgow, Glasgow, United Kingdom
| | - Per Kristian Lunde
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway
| | - Michael Frisk
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway.,KG Jebsen Center for Cardiac Research, University of Oslo, Oslo, Norway
| | - Jan Magnus Aronsen
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway.,Bjørknes College, Oslo, Norway
| | - Einar S Norden
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway.,Bjørknes College, Oslo, Norway
| | - Alessandro Cataliotti
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway.,KG Jebsen Center for Cardiac Research, University of Oslo, Oslo, Norway
| | - Ivar Sjaastad
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway.,KG Jebsen Center for Cardiac Research, University of Oslo, Oslo, Norway
| | - Ole M Sejersted
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway
| | | | | | - William E Louch
- Institute for Experimental Medical Research, Oslo University Hospital and University of Oslo, Oslo, Norway.,KG Jebsen Center for Cardiac Research, University of Oslo, Oslo, Norway
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8
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Sikkel MB, Francis DP, Howard J, Gordon F, Rowlands C, Peters NS, Lyon AR, Harding SE, MacLeod KT. Hierarchical statistical techniques are necessary to draw reliable conclusions from analysis of isolated cardiomyocyte studies. Cardiovasc Res 2017; 113:1743-1752. [PMID: 29016722 PMCID: PMC5852514 DOI: 10.1093/cvr/cvx151] [Citation(s) in RCA: 91] [Impact Index Per Article: 13.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/17/2017] [Revised: 06/27/2017] [Accepted: 08/29/2017] [Indexed: 02/01/2023] Open
Abstract
AIMS It is generally accepted that post-MI heart failure (HF) changes a variety of aspects of sarcoplasmic reticular Ca2+ fluxes but for some aspects there is disagreement over whether there is an increase or decrease. The commonest statistical approach is to treat data collected from each cell as independent, even though they are really clustered with multiple likely similar cells from each heart. In this study, we test whether this statistical assumption of independence can lead the investigator to draw conclusions that would be considered erroneous if the analysis handled clustering with specific statistical techniques (hierarchical tests). METHODS AND RESULTS Ca2+ transients were recorded in cells loaded with Fura-2AM and sparks were recorded in cells loaded with Fluo-4AM. Data were analysed twice, once with the common statistical approach (assumption of independence) and once with hierarchical statistical methodologies designed to allow for any clustering. The statistical tests found that there was significant hierarchical clustering. This caused the common statistical approach to underestimate the standard error and report artificially small P values. For example, this would have led to the erroneous conclusion that time to 50% peak transient amplitude was significantly prolonged in HF. Spark analysis showed clustering, both within each cell and also within each rat, for morphological variables. This means that a three-level hierarchical model is sometimes required for such measures. Standard statistical methodologies, if used instead, erroneously suggest that spark amplitude is significantly greater in HF and spark duration is reduced in HF. CONCLUSION Ca2+ fluxes in isolated cardiomyocytes show so much clustering that the common statistical approach that assumes independence of each data point will frequently give the false appearance of statistically significant changes. Hierarchical statistical methodologies need a little more effort, but are necessary for reliable conclusions. We present cost-free simple tools for performing these analyses.
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Affiliation(s)
- Markus B Sikkel
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
- Department of Electrophysiology, Imperial College Healthcare NHS Trust, Hammersmith Hospital, London, UK
| | - Darrel P Francis
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
| | - James Howard
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
| | - Fabiana Gordon
- Statistics Advisory Service, Imperial College London, London, UK
| | - Christina Rowlands
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
| | - Nicholas S Peters
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
- Department of Electrophysiology, Imperial College Healthcare NHS Trust, Hammersmith Hospital, London, UK
| | - Alexander R Lyon
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
- Department of Cardiology, Royal Brompton Hospital, London, UK
| | - Sian E Harding
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
| | - Kenneth T MacLeod
- Myocardial Function Section, Fourth Floor, Imperial Centre for Translational and Experimental Medicine, National Heart and Lung Institute, Imperial College London, Hammersmith Campus, Du Cane Road, London, UK
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Sikkel MB, Kumar S, Maioli V, Rowlands C, Gordon F, Harding SE, Lyon AR, MacLeod KT, Dunsby C. High speed sCMOS-based oblique plane microscopy applied to the study of calcium dynamics in cardiac myocytes. JOURNAL OF BIOPHOTONICS 2016; 9:311-23. [PMID: 26488431 PMCID: PMC4874460 DOI: 10.1002/jbio.201500193] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/10/2015] [Revised: 08/28/2015] [Accepted: 09/16/2015] [Indexed: 05/18/2023]
Abstract
Oblique plane microscopy (OPM) is a form of light sheet microscopy that uses a single high numerical aperture microscope objective for both fluorescence excitation and collection. In this paper, measurements of the relative collection efficiency of OPM are presented. An OPM system incorporating two sCMOS cameras is then introduced that enables single isolated cardiac myocytes to be studied continuously for 22 seconds in two dimensions at 667 frames per second with 960 × 200 pixels and for 30 seconds with 960 × 200 × 20 voxels at 25 volumes per second. In both cases OPM is able to record in two spectral channels, enabling intracellular calcium to be studied via the probe Fluo-4 AM simultaneously with the sarcolemma and transverse tubule network via the membrane dye Cellmask Orange. The OPM system was then applied to determine the spatial origin of spontaneous calcium waves for the first time and to measure the cell transverse tubule structure at their point of origin. Further results are presented to demonstrate that the OPM system can also be used to study calcium spark parameters depending on their relationship to the transverse tubule structure.
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Affiliation(s)
- Markus B Sikkel
- Myocardial Function Section, National Heart and Lung Institute, Imperial College, London, United Kingdom
| | - Sunil Kumar
- Photonics Group, Department of Physics, Imperial College London, London, United Kingdom
| | - Vincent Maioli
- Photonics Group, Department of Physics, Imperial College London, London, United Kingdom.
| | - Christina Rowlands
- Myocardial Function Section, National Heart and Lung Institute, Imperial College, London, United Kingdom
| | - Fabiana Gordon
- Statistics Advisory Service, Imperial College London, London, United Kingdom
| | - Sian E Harding
- Myocardial Function Section, National Heart and Lung Institute, Imperial College, London, United Kingdom
| | - Alexander R Lyon
- Myocardial Function Section, National Heart and Lung Institute, Imperial College, London, United Kingdom
- NIHR Cardiovascular Biomedical Research Unit, Royal Brompton Hospital, London, United Kingdom
| | - Kenneth T MacLeod
- Myocardial Function Section, National Heart and Lung Institute, Imperial College, London, United Kingdom
| | - Chris Dunsby
- Photonics Group, Department of Physics, Imperial College London, London, United Kingdom
- Centre for Pathology, Department of Medicine, Imperial College London, United Kingdom
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Macquaide N, Tuan HTM, Hotta JI, Sempels W, Lenaerts I, Holemans P, Hofkens J, Jafri MS, Willems R, Sipido KR. Ryanodine receptor cluster fragmentation and redistribution in persistent atrial fibrillation enhance calcium release. Cardiovasc Res 2015; 108:387-98. [PMID: 26490742 PMCID: PMC4648199 DOI: 10.1093/cvr/cvv231] [Citation(s) in RCA: 72] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/06/2015] [Accepted: 10/06/2015] [Indexed: 12/19/2022] Open
Abstract
AIMS In atrial fibrillation (AF), abnormalities in Ca(2+) release contribute to arrhythmia generation and contractile dysfunction. We explore whether ryanodine receptor (RyR) cluster ultrastructure is altered and is associated with functional abnormalities in AF. METHODS AND RESULTS Using high-resolution confocal microscopy (STED), we examined RyR cluster morphology in fixed atrial myocytes from sheep with persistent AF (N = 6) and control (Ctrl; N = 6) animals. RyR clusters on average contained 15 contiguous RyRs; this did not differ between AF and Ctrl. However, the distance between clusters was significantly reduced in AF (288 ± 12 vs. 376 ± 17 nm). When RyR clusters were grouped into Ca(2+) release units (CRUs), i.e. clusters separated by <150 nm, CRUs in AF had more clusters (3.43 ± 0.10 vs. 2.95 ± 0.02 in Ctrl), which were more dispersed. Furthermore, in AF cells, more RyR clusters were found between Z lines. In parallel experiments, Ca(2+) sparks were monitored in live permeabilized myocytes. In AF, myocytes had >50% higher spark frequency with increased spark time to peak (TTP) and duration, and a higher incidence of macrosparks. A computational model of the CRU was used to simulate the morphological alterations observed in AF cells. Increasing cluster fragmentation to the level observed in AF cells caused the observed changes, i.e. higher spark frequency, increased TTP and duration; RyR clusters dispersed between Z-lines increased the occurrence of macrosparks. CONCLUSION In persistent AF, ultrastructural reorganization of RyR clusters within CRUs is associated with overactive Ca(2+) release, increasing the likelihood of propagating Ca(2+) release.
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Affiliation(s)
- Niall Macquaide
- Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium Department of Molecular Neuroscience, George Mason University, Fairfax, VA, USA Institute of Cardiovascular Sciences, University of Glasgow, Glasgow, UK
| | | | - Jun-Ichi Hotta
- Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, KU Leuven, Leuven, Belgium
| | - Wouter Sempels
- Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, KU Leuven, Leuven, Belgium
| | - Ilse Lenaerts
- Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium
| | - Patricia Holemans
- Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium
| | - Johan Hofkens
- Laboratory of Photochemistry and Spectroscopy, Department of Chemistry, KU Leuven, Leuven, Belgium
| | - M Saleet Jafri
- Department of Cardiology, University Hospitals Leuven, Leuven, Belgium
| | - Rik Willems
- Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium Department of Molecular Neuroscience, George Mason University, Fairfax, VA, USA Institute of Cardiovascular Sciences, University of Glasgow, Glasgow, UK
| | - Karin R Sipido
- Department of Cardiovascular Sciences, Experimental Cardiology, KU Leuven, Campus Gasthuisberg O/N 7th Floor, Herestraat 49, B-3000 Leuven, Belgium
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11
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Ca2+-activation kinetics modulate successive puff/spark amplitude, duration and inter-event-interval correlations in a Langevin model of stochastic Ca2+ release. Math Biosci 2015; 264:101-7. [DOI: 10.1016/j.mbs.2015.03.012] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2014] [Revised: 03/16/2015] [Accepted: 03/27/2015] [Indexed: 01/12/2023]
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12
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Jaafari N, De Waard M, Canepari M. Imaging fast calcium currents beyond the limitations of electrode techniques. Biophys J 2015; 107:1280-8. [PMID: 25229136 DOI: 10.1016/j.bpj.2014.07.059] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2014] [Revised: 06/06/2014] [Accepted: 07/30/2014] [Indexed: 10/24/2022] Open
Abstract
The current understanding of Ca(2+) channel function is derived from the use of the patch-clamp technique. In particular, the measurement of fast cellular Ca(2+) currents is routinely achieved using whole-cell voltage-clamp recordings. However, this experimental approach is not applicable to the study of local native Ca(2+) channels during physiological changes of membrane potential in complex cells, since the voltage-clamp configuration constrains the membrane potential to a given value. Here, we report for the first time to our knowledge that Ca(2+) currents from individual cells can be quantitatively measured beyond the limitations of the voltage-clamp approach using fast Ca(2+) imaging with low-affinity indicators. The optical measurement of the Ca(2+) current was correlated with the membrane potential, simultaneously measured with a voltage-sensitive dye to investigate the activation of Ca(2+) channels along the apical dendrite of the CA1 hippocampal pyramidal neuron during the back-propagation of an action potential. To validate the method, we analyzed the voltage dependence of high- and low-voltage-gated Ca(2+) channels. In particular, we measured the Ca(2+) current component mediated by T-type channels, and we investigated the mechanisms of recovery from inactivation of these channels. This method is expected to become a reference approach to investigate Ca(2+) channels in their native physiological environment.
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Affiliation(s)
- Nadia Jaafari
- Institut national de la santé et de la recherche médicale, Grenoble Institute of Neuroscience, Grenoble, France; Université Joseph Fourier, Laboratoire Interdisciplinare de Physique (Centre National de la Recherche Scientifique UMR 5588), France; Laboratories of Excellence, Ion Channel Science and Therapeutics, France
| | - Michel De Waard
- Institut national de la santé et de la recherche médicale, Grenoble Institute of Neuroscience, Grenoble, France; Laboratories of Excellence, Ion Channel Science and Therapeutics, France
| | - Marco Canepari
- Institut national de la santé et de la recherche médicale, Grenoble Institute of Neuroscience, Grenoble, France; Université Joseph Fourier, Laboratoire Interdisciplinare de Physique (Centre National de la Recherche Scientifique UMR 5588), France; Laboratories of Excellence, Ion Channel Science and Therapeutics, France.
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13
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Ellefsen KL, Settle B, Parker I, Smith IF. An algorithm for automated detection, localization and measurement of local calcium signals from camera-based imaging. Cell Calcium 2014; 56:147-56. [PMID: 25047761 DOI: 10.1016/j.ceca.2014.06.003] [Citation(s) in RCA: 54] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2014] [Revised: 06/10/2014] [Accepted: 06/15/2014] [Indexed: 10/25/2022]
Abstract
Local Ca(2+) transients such as puffs and sparks form the building blocks of cellular Ca(2+) signaling in numerous cell types. They have traditionally been studied by linescan confocal microscopy, but advances in TIRF microscopy together with improved electron-multiplied CCD (EMCCD) cameras now enable rapid (>500 frames s(-1)) imaging of subcellular Ca(2+) signals with high spatial resolution in two dimensions. This approach yields vastly more information (ca. 1 Gb min(-1)) than linescan imaging, rendering visual identification and analysis of local events imaged both laborious and subject to user bias. Here we describe a routine to rapidly automate identification and analysis of local Ca(2+) events. This features an intuitive graphical user-interfaces and runs under Matlab and the open-source Python software. The underlying algorithm features spatial and temporal noise filtering to reliably detect even small events in the presence of noisy and fluctuating baselines; localizes sites of Ca(2+) release with sub-pixel resolution; facilitates user review and editing of data; and outputs time-sequences of fluorescence ratio signals for identified event sites along with Excel-compatible tables listing amplitudes and kinetics of events.
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Affiliation(s)
- Kyle L Ellefsen
- Department of Neurobiology & Behavior, University of California, Irvine, CA 92697, USA
| | - Brett Settle
- Department of Neurobiology & Behavior, University of California, Irvine, CA 92697, USA
| | - Ian Parker
- Department of Neurobiology & Behavior, University of California, Irvine, CA 92697, USA; Department of Physiology & Biophysics, University of California, Irvine, CA 92697, USA
| | - Ian F Smith
- Department of Neurobiology & Behavior, University of California, Irvine, CA 92697, USA.
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14
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Abstract
Calcium sparks in cardiac myocytes are brief, localized calcium releases from the sarcoplasmic reticulum (SR) believed to be caused by locally regenerative calcium-induced calcium release (CICR) via couplons, clusters of ryanodine receptors (RyRs). How such regeneration is terminated is uncertain. We performed numerical simulations of an idealized stochastic model of spark production, assuming a RyR gating scheme with only two states (open and closed). Local depletion of calcium in the SR was inevitable during a spark, and this could terminate sparks by interrupting CICR, with or without assumed modulation of RyR gating by SR lumenal calcium. Spark termination by local SR depletion was not robust: under some conditions, sparks could be greatly and variably prolonged, terminating by stochastic attrition-a phenomenon we dub "spark metastability." Spark fluorescence rise time was not a good surrogate for the duration of calcium release. Using a highly simplified, deterministic model of the dynamics of a couplon, we show that spark metastability depends on the kinetic relationship of RyR gating and junctional SR refilling rates. The conditions for spark metastability resemble those produced by known mutations of RyR2 and CASQ2 that cause life-threatening triggered arrhythmias, and spark metastability may be mitigated by altering the kinetics of the RyR in a manner similar to the effects of drugs known to prevent those arrhythmias. The model was unable to explain the distributions of spark amplitudes and rise times seen in chemically skinned cat atrial myocytes, suggesting that such sparks may be more complex events involving heterogeneity of couplons or local propagation among sub-clusters of RyRs.
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Affiliation(s)
- Michael D Stern
- Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, National Institutes of Health, Baltimore, MD 21224, USA.
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15
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Steele EM, Steele DS. Automated detection and analysis of Ca(2+) sparks in x-y image stacks using a thresholding algorithm implemented within the open-source image analysis platform ImageJ. Biophys J 2014; 106:566-76. [PMID: 24507597 PMCID: PMC3944640 DOI: 10.1016/j.bpj.2013.12.040] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2013] [Revised: 11/28/2013] [Accepted: 12/23/2013] [Indexed: 11/25/2022] Open
Abstract
Previous studies have used analysis of Ca(2+) sparks extensively to investigate both normal and pathological Ca(2+) regulation in cardiac myocytes. The great majority of these studies used line-scan confocal imaging. In part, this is because the development of open-source software for automatic detection of Ca(2+) sparks in line-scan images has greatly simplified data analysis. A disadvantage of line-scan imaging is that data are collected from a single row of pixels, representing only a small fraction of the cell, and in many instances x-y confocal imaging is preferable. However, the limited availability of software for Ca(2+) spark analysis in two-dimensional x-y image stacks presents an obstacle to its wider application. This study describes the development and characterization of software to enable automatic detection and analysis of Ca(2+) sparks within x-y image stacks, implemented as a plugin within the open-source image analysis platform ImageJ. The program includes methods to enable precise identification of cells within confocal fluorescence images, compensation for changes in background fluorescence, and options that allow exclusion of events based on spatial characteristics.
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Affiliation(s)
- Elliot M Steele
- School of Biomedical Sciences, University of Leeds, Leeds, England
| | - Derek S Steele
- School of Biomedical Sciences, University of Leeds, Leeds, England.
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16
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Abstract
Ca(2+) waves were probably first observed in the early 1940s. Since then Ca(2+) waves have captured the attention of an eclectic mixture of mathematicians, neuroscientists, muscle physiologists, developmental biologists, and clinical cardiologists. This review discusses the current state of mathematical models of Ca(2+) waves, the normal physiological functions Ca(2+) waves might serve in cardiac cells, as well as how the spatial arrangement of Ca(2+) release channels shape Ca(2+) waves, and we introduce the idea of Ca(2+) phase waves that might provide a useful framework for understanding triggered arrhythmias.
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Affiliation(s)
- Leighton T Izu
- Department of Pharmacology, University of California, Davis, USA.
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17
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Shkryl VM, Blatter LA. Ca(2+) release events in cardiac myocytes up close: insights from fast confocal imaging. PLoS One 2013; 8:e61525. [PMID: 23637847 PMCID: PMC3630194 DOI: 10.1371/journal.pone.0061525] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/06/2012] [Accepted: 03/11/2013] [Indexed: 02/02/2023] Open
Abstract
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.
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Affiliation(s)
- Vyacheslav M. Shkryl
- Deptartment of General Physiology of the Nervous System, A. A. Bogomoletz Institute of Physiology, Kiev, Ukraine
| | - Lothar A. Blatter
- Department of Molecular Biophysics and Physiology, Rush University Medical Center, Chicago, Illinois, United States of America
- * E-mail:
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18
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Using two dyes with the same fluorophore to monitor cellular calcium concentration in an extended range. PLoS One 2013; 8:e55778. [PMID: 23409040 PMCID: PMC3567097 DOI: 10.1371/journal.pone.0055778] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2012] [Accepted: 01/04/2013] [Indexed: 11/19/2022] Open
Abstract
We extend the sensitivity of quantitative concentration imaging to an approximately 1000-fold range of concentrations by a method that uses two fluorescent dyes with the same fluorophore, having different affinity for the monitored species. While the formulation and illustration refer to a monitor of calcium concentration, the method is applicable to any species that binds to multiple indicators with the same spectral properties. The use of a common fluorophore has the virtue of leaving vast regions of the electromagnetic spectrum available for other applications. We provide the exact analytic expression relating measured fluorescence to [Ca2+] at equilibrium and an approximate analytic expression that does not require the equilibrium assumption. The sensitivity of the method is calculated numerically for two useful dye pairs. As illustrative application of the enhanced measurement, we use fluo-4 and fluo-4FF to image the calcium wave produced by a cardiac myocyte in response to a small artificial calcium spark.
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19
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Kim JJ, Němec J, Papp R, Strongin R, Abramson JJ, Salama G. Bradycardia alters Ca(2+) dynamics enhancing dispersion of repolarization and arrhythmia risk. Am J Physiol Heart Circ Physiol 2013; 304:H848-60. [PMID: 23316064 DOI: 10.1152/ajpheart.00787.2012] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Bradycardia prolongs action potential (AP) durations (APD adaptation), enhances dispersion of repolarization (DOR), and promotes tachyarrhythmias. Yet, the mechanisms responsible for enhanced DOR and tachyarrhythmias remain largely unexplored. Ca(2+) transients and APs were measured optically from Langendorff rabbit hearts at high (150 × 150 μm(2)) or low (1.5 × 1.5 cm(2)) magnification while pacing at a physiological (120 beats/min) or a slow heart rate (SHR = 50 beats/min). Western blots and pharmacological interventions were used to elucidate the regional effects of bradycardia. As a result, bradycardia (SHR 50 beats/min) increased APDs gradually (time constant τf→s = 48 ± 9.2 s) and caused a secondary Ca(2+) release (SCR) from the sarcoplasmic reticulum during AP plateaus, occurring at the base on average of 184.4 ± 9.7 ms after the Ca(2+) transient upstroke. In subcellular imaging, SCRs were temporally synchronous and spatially homogeneous within myocytes. In diastole, SHR elicited variable asynchronous sarcoplasmic reticulum Ca(2+) release events leading to subcellular Ca(2+) waves, detectable only at high magnification. SCR was regionally heterogeneous, correlated with APD prolongation (P < 0.01, n = 5), enhanced DOR (r = 0.9277 ± 0.03, n = 7), and was gradually reversed by pacing at 120 beats/min along with APD shortening (P < 0.05, n = 5). A stabilizer of leaky ryanodine receptors (RyR2), 3-(4-benzylcyclohexyl)-1-(7-methoxy-2,3-dihydrobenzo[f][1,4]thiazepin-4(5H)-yl)propan-1-one (K201; 1 μM), suppressed SCR and reduced APD at the base, thereby reducing DOR (P < 0.02, n = 5). Ventricular ectopy induced by bradycardia (n = 5/15) was suppressed by K201. Western blot analysis revealed spatial differences of voltage-gated L-type Ca(2+) channel protein (Cav1.2α), Na(+)-Ca(2+) exchange (NCX1), voltage-gated Na(+) channel (Nav1.5), and rabbit ether-a-go-go-related (rERG) protein [but not RyR2 or sarcoplasmic reticulum Ca(2+) ATPase 2a] that correlate with the SCR distribution and explain the molecular basis for SCR heterogeneities. In conclusion, acute bradycardia elicits synchronized subcellular SCRs of sufficient magnitude to overcome the source-sink mismatch and to promote afterdepolarizations.
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Affiliation(s)
- Jong J Kim
- Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA 15261, USA
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20
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Zahradníková A, Zahradník I. Construction of calcium release sites in cardiac myocytes. Front Physiol 2012; 3:322. [PMID: 22934071 PMCID: PMC3429091 DOI: 10.3389/fphys.2012.00322] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2012] [Accepted: 07/23/2012] [Indexed: 11/13/2022] Open
Abstract
Local character of calcium release in cardiac myocytes, as defined by confocal recordings of calcium sparks, implies independent activation of individual calcium release sites based on ryanodine receptor (RyR) channel recruitment. We constructed virtual calcium release sites (vCRSs) composed of a variable number of RyR channels distributed in clusters in accordance with the experimentally observed cluster size distribution. The vCRSs consisted either of a single virtual calcium release unit (vCRU), in which all clusters shared a common dyadic space, or of multiple virtual calcium release units (CRUs) containing one cluster each and having separate dyadic spaces. We explored the stochastic behavior of vCRSs to understand the activation and recruitment of RyRs during calcium sparks. RyRs were represented by the published allosteric gating model that included regulation by cytosolic Ca(2+) and Mg(2+). The interaction of Mg(2+) with the RyR Ca(2+)-binding sites and the refractory period of vCRSs were optimized to accord with the experimentally observed calcium dependence of calcium spark frequency. The Mg(2+)-binding parameters of RyRs that provided the best description of spark frequency depended on the number of RyRs assembled in the vCRSs. Adequate inhibitory effect of Mg(2+) on the calcium dependence of RyR open probability was achieved if the vCRSs contained at least three clusters. For the distribution of the number of open RyRs in evoked calcium sparks to correspond to the experimentally observed distribution of spark calcium release fluxes, at least three clusters had to share a common virtual CRU, in which ∼3 RyRs open to form an average spark. These results reconcile the small cluster size and stochastic placement of RyRs in the release sites with the estimates of the amount of RyR protein, volume density of calcium release sites, and the size of calcium release sites in rat cardiac myocytes.
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Affiliation(s)
- Alexandra Zahradníková
- Department of Muscle Cell Research, Institute of Molecular Physiology and Genetics, Slovak Academy of Sciences Bratislava, Slovakia
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