1
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Gaburjakova J, Gaburjakova M. The Cardiac Ryanodine Receptor Provides a Suitable Pathway for the Rapid Transport of Zinc (Zn2+). Cells 2022; 11:cells11050868. [PMID: 35269490 PMCID: PMC8909583 DOI: 10.3390/cells11050868] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/02/2022] [Revised: 02/25/2022] [Accepted: 03/01/2022] [Indexed: 12/13/2022] Open
Abstract
The sarcoplasmic reticulum (SR) in cardiac muscle is suggested to act as a dynamic storage for Zn2+ release and reuptake, albeit it is primarily implicated in the Ca2+ signaling required for the cardiac cycle. A large Ca2+ release from the SR is mediated by the cardiac ryanodine receptor (RYR2), and while this has a prominent conductance for Ca2+ in vivo, it also conducts other divalent cations in vitro. Since Zn2+ and permeant Mg2+ have similar physical properties, we tested if the RYR2 channel also conducts Zn2+. Using the method of planar lipid membranes, we evidenced that the RYR2 channel is permeable to Zn2+ with a considerable conductance of 81.1 ± 2.4 pS, which was significantly lower than the values for Ca2+ (127.5 ± 1.8 pS) and Mg2+ (95.3 ± 1.4 pS), obtained under the same asymmetric conditions. Despite similar physical properties, the intrinsic Zn2+ permeability (PCa/PZn = 2.65 ± 0.19) was found to be ~2.3-fold lower than that of Mg2+ (PCa/PMg = 1.146 ± 0.071). Further, we assessed whether the channel itself could be a direct target of the Zn2+ current, having the Zn2+ finger extended into the cytosolic vestibular portion of the permeation pathway. We attempted to displace Zn2+ from the RYR2 Zn2+ finger to induce its structural defects, which are associated with RYR2 dysfunction. Zn2+ chelators were added to the channel cytosolic side or strongly competing cadmium cations (Cd2+) were allowed to permeate the RYR2 channel. Only the Cd2+ current was able to cause the decay of channel activity, presumably as a result of Zn2+ to Cd2+ replacement. Our findings suggest that the RYR2 channel can provide a suitable pathway for rapid Zn2+ escape from the cardiac SR; thus, the channel may play a role in local and/or global Zn2+ signaling in cardiomyocytes.
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2
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The function and regulation of calsequestrin-2: implications in calcium-mediated arrhythmias. Biophys Rev 2022; 14:329-352. [PMID: 35340602 PMCID: PMC8921388 DOI: 10.1007/s12551-021-00914-6] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2021] [Accepted: 11/14/2021] [Indexed: 01/09/2023] Open
Abstract
Cardiac arrhythmias are life-threatening events in which the heart develops an irregular rhythm. Mishandling of Ca2+ within the myocytes of the heart has been widely demonstrated to be an underlying mechanism of arrhythmogenesis. This includes altered function of the ryanodine receptor (RyR2)-the primary Ca2+ release channel located to the sarcoplasmic reticulum (SR). The spontaneous leak of SR Ca2+ via RyR2 is a well-established contributor in the development of arrhythmic contractions. This leak is associated with increased channel activity in response to changes in SR Ca2+ load. RyR2 activity can be regulated through several avenues, including interactions with numerous accessory proteins. One such protein is calsequestrin-2 (CSQ2), which is the primary Ca2+-buffering protein within the SR. The capacity of CSQ2 to buffer Ca2+ is tightly associated with the ability of the protein to polymerise in response to changing Ca2+ levels. CSQ2 can itself be regulated through phosphorylation and glycosylation modifications, which impact protein polymerisation and trafficking. Changes in CSQ2 modifications are implicated in cardiac pathologies, while mutations in CSQ2 have been identified in arrhythmic patients. Here, we review the role of CSQ2 in arrhythmogenesis including evidence for the indirect and direct regulation of RyR2 by CSQ2, and the consequences of a loss of functional CSQ2 in Ca2+ homeostasis and Ca2+-mediated arrhythmias. Supplementary Information The online version contains supplementary material available at 10.1007/s12551-021-00914-6.
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3
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Ottolini M, Sonkusare SK. The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells. Compr Physiol 2021; 11:1831-1869. [PMID: 33792900 PMCID: PMC10388069 DOI: 10.1002/cphy.c200030] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca2+ signals and Ca2+ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca2+ signal on arterial contractility depends on the type of Ca2+ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca2+ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca2+ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca2+ signals have already been confirmed, while further investigation is needed for other Ca2+ signals. This article focuses on endothelial and smooth muscle Ca2+ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca2+ entry pathways at the plasma membrane, Ca2+ release signals from the intracellular stores, the functional and physiological relevance of Ca2+ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca2+ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.
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Affiliation(s)
- Matteo Ottolini
- Department of Pharmacology, University of Virginia, Charlottesville, Virginia, USA
| | - Swapnil K Sonkusare
- Department of Pharmacology, University of Virginia, Charlottesville, Virginia, USA.,Department of Molecular Physiology & Biological Physics, University of Virginia, Charlottesville, Virginia, USA.,Robert M. Berne Cardiovascular Research Center, University of Virginia, Charlottesville, Virginia, USA
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4
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Palahniuk C, Mutawe M, Gilchrist JSC. Luminal Ca 2+ regulation of RyR1 Ca 2+ channel leak activation and inactivation in sarcoplasmic reticulum membrane vesicles. Can J Physiol Pharmacol 2020; 99:192-206. [PMID: 33161753 DOI: 10.1139/cjpp-2020-0409] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
In this study, we tested the hypothesis that the RyR1 Ca2+ channel closure is sensitive to outward trans-SR membrane Ca2+ gradients established by SERCA1 pumping. To perform these studies, we employed stopped-flow rapid-kinetic fluorescence methods to measure and assess how variation in trans-SR membrane Ca2+ distribution affects evolution of RyR1 Ca2+ leaks in RyR1/ CASQ1/SERCA1-rich membrane vesicles. Our studies showed that rapid filling of a Mag-Fura-2-sensitive free Ca2+ pool during SERCA1-mediated Ca2+ sequestration appears to be a crucial condition allowing RyR1 Ca2+ channels to close once reloading of luminal Ca2+ stores is complete. Disruption in the filling of this pool caused activation of Ruthenium Red inhibitable RyR1 Ca2+ leaks, suggesting that SERCA1 pump formation of outward Ca2+ gradients is an important aspect of Ca2+ flux control channel opening and closing. In addition, our observed ryanodine-induced shift in luminal Ca2+ from free to a CTC-Ca+-sensitive, CASQ1-associated bound compartment underscores the complex organization and regulation of SR luminal Ca2+. Our study provides strong evidence that RyR1 functional states directly and indirectly influence the compartmentation of luminal Ca2+. This, in turn, is influenced by the activity of SERCA1 pumps to fill luminal pools while synchronously reducing Ca2+ levels on the cytosolic face of RyR1 channels.
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Affiliation(s)
- C Palahniuk
- Department of Biology, St. Catherine University, 2004 Randolph Ave., St. Paul, MN 55105, USA
| | - M Mutawe
- Genome Analysis Core (GAC), 13-66 Stabile Building, MAYO Clinic, Rochester, MN 55905, USA
| | - J S C Gilchrist
- Department of Oral Biology, Rady Faculty of Health Sciences, University of Manitoba, MB R3E 0W2, Canada
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5
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Bovo E, Nikolaienko R, Bhayani S, Kahn D, Cao Q, Martin JL, Kuo IY, Robia SL, Zima AV. Novel approach for quantification of endoplasmic reticulum Ca 2+ transport. Am J Physiol Heart Circ Physiol 2019; 316:H1323-H1331. [PMID: 30901276 PMCID: PMC6620677 DOI: 10.1152/ajpheart.00031.2019] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/22/2019] [Revised: 03/01/2019] [Accepted: 03/04/2019] [Indexed: 01/14/2023]
Abstract
The type 2a sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA2a) plays a key role in Ca2+ regulation in the heart. However, available techniques to study SERCA function are either cell destructive or lack sensitivity. The goal of this study was to develop an approach to selectively measure SERCA2a function in the cellular environment. The genetically encoded Ca2+ sensor R-CEPIA1er was used to measure the concentration of Ca2+ in the lumen of the endoplasmic reticulum (ER) ([Ca2+]ER) in HEK293 cells expressing human SERCA2a. Coexpression of the ER Ca2+ release channel ryanodine receptor (RyR2) created a Ca2+ release/reuptake system that mimicked aspects of cardiac myocyte Ca2+ handling. SERCA2a function was quantified from the rate of [Ca2+]ER refilling after ER Ca2+ depletion; then, ER Ca2+ leak was measured after SERCA inhibition. ER Ca2+ uptake and leak were analyzed as a function of [Ca2+]ER to determine maximum ER Ca2+ uptake rate and maximum ER Ca2+ load. The sensitivity of this assay was validated by analyzing effects of SERCA inhibitors, [ATP]/[ADP], oxidative stress, phospholamban, and a loss-of-function SERCA2a mutation. In addition, the feasibility of using R-CEPIA1er to study SERCA2a in a native system was evaluated by using in vivo gene delivery to express R-CEPIA1er in mouse hearts. After ventricular myocyte isolation, the same methodology used in HEK293 cells was applied to study endogenous SERCA2a. In conclusion, this new approach can be used as a sensitive screening tool to study the effect of different drugs, posttranslational modifications, and mutations on SERCA function. NEW & NOTEWORTHY The aim of this study was to develop a sensitive approach to selectively measure sarco-/endoplasmic reticulum Ca2+-ATPase (SERCA) function in the cellular environment. The newly developed Ca2+ sensor R-CEPIA1er was used to successfully analyze Ca2+ uptake mediated by recombinant and native cardiac SERCA. These results demonstrate that this new approach can be used as a powerful tool to study new mechanisms of Ca2+ pump regulation.
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Affiliation(s)
- Elisa Bovo
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Roman Nikolaienko
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Siddharth Bhayani
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Daniel Kahn
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Quan Cao
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Jody L Martin
- Department of Physiology and Biophysics, University of Illinois at Chicago , Chicago, Illinois
| | - Ivana Y Kuo
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Seth L Robia
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
| | - Aleksey V Zima
- Department of Cell and Molecular Physiology, Loyola University Chicago , Chicago, Illinois
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6
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Slabaugh JL, Brunello L, Elnakish MT, Milani-Nejad N, Gyorke S, Janssen PML. Synchronization of Intracellular Ca 2+ Release in Multicellular Cardiac Preparations. Front Physiol 2018; 9:968. [PMID: 30079034 PMCID: PMC6062622 DOI: 10.3389/fphys.2018.00968] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2018] [Accepted: 07/02/2018] [Indexed: 11/25/2022] Open
Abstract
In myocardial tissue, Ca2+ release from the sarcoplasmic reticulum (SR) that occurs via the ryanodine receptor (RyR2) channel complex. Ca2+ release through RyR2 can be either stimulated by an action potential (AP) or spontaneous. The latter is often associated with triggered afterdepolarizations, which in turn may lead to sustained arrhythmias. It is believed that some synchronization mechanism exists for afterdepolarizations and APs in neighboring myocytes, possibly a similarly timed recovery of RyR2 from refractoriness, which enables RyR2s to reach the threshold for spontaneous Ca2+ release simultaneously. To investigate this synchronization mechanism in absence of genetic factors that predispose arrhythmia, we examined the generation of triggered activity in multicellular cardiac preparations. In myocardial trabeculae from the rat, we demonstrated that in the presence of both isoproterenol and caffeine, neighboring myocytes within the cardiac trabeculae were able to synchronize their diastolic spontaneous SR Ca2+ release. Using confocal Ca2+ imaging, we could visualize Ca2+ waves in the multicellular preparation, while these waves were not always present in every myocyte within the trabeculae, we observed that, over time, the Ca2+ waves can synchronize in multiple myocytes. This synchronized activity was sufficiently strong that it could trigger a synchronized, propagated contraction in the whole trabecula encompassing even previously quiescent myocytes. The detection of Ca2+ dynamics in individual myocytes in their in situ setting at the multicellular level exposed a synchronization mechanism that could induce local triggered activity in the heart in the absence of global Ca2+ dysregulation.
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Affiliation(s)
- Jessica L Slabaugh
- Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, OH, United States.,Davis Heart Lung Research Institute, The Ohio State University, Columbus, OH, United States
| | - Lucia Brunello
- Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, OH, United States.,Davis Heart Lung Research Institute, The Ohio State University, Columbus, OH, United States
| | - Mohammad T Elnakish
- Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, OH, United States.,Davis Heart Lung Research Institute, The Ohio State University, Columbus, OH, United States.,Department of Pharmacology and Toxicology, Faculty of Pharmacy, Helwan University, Cairo, Egypt
| | - Nima Milani-Nejad
- Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, OH, United States.,Davis Heart Lung Research Institute, The Ohio State University, Columbus, OH, United States
| | - Sandor Gyorke
- Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, OH, United States.,Davis Heart Lung Research Institute, The Ohio State University, Columbus, OH, United States
| | - Paul M L Janssen
- Department of Physiology and Cell Biology, College of Medicine, The Ohio State University, Columbus, OH, United States.,Davis Heart Lung Research Institute, The Ohio State University, Columbus, OH, United States
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7
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Underlying mechanism of the contractile dysfunction in atrophied ventricular myocytes from a murine model of hypothyroidism. Cell Calcium 2018; 72:26-38. [DOI: 10.1016/j.ceca.2018.01.005] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2017] [Revised: 01/18/2018] [Accepted: 01/31/2018] [Indexed: 11/20/2022]
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8
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Sobie EA, Williams GSB, Lederer WJ. Ambiguous interactions between diastolic and SR Ca 2+ in the regulation of cardiac Ca 2+ release. J Gen Physiol 2017; 149:847-855. [PMID: 28798276 PMCID: PMC5583714 DOI: 10.1085/jgp.201711814] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2017] [Accepted: 07/21/2017] [Indexed: 01/20/2023] Open
Abstract
Sobie et al. highlight unresolved issues concerning the regulation of sarcoplasmic reticulum calcium release in cardiac myocytes.
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Affiliation(s)
- Eric A Sobie
- Department of Pharmacological Sciences, Graduate School of Biomedical Sciences, Icahn School of Medicine at Mount Sinai, New York, NY
| | - George S B Williams
- BioMET, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, MD
| | - W J Lederer
- BioMET, Center for Biomedical Engineering and Technology, University of Maryland, Baltimore, MD
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9
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Cannell MB, Kong CHT. Quenching the spark: Termination of CICR in the submicroscopic space of the dyad. J Gen Physiol 2017; 149:837-845. [PMID: 28798280 PMCID: PMC5583711 DOI: 10.1085/jgp.201711807] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/14/2017] [Accepted: 07/21/2017] [Indexed: 12/20/2022] Open
Abstract
Cannell and Kong discuss the different termination mechanisms proposed for CICR in cardiac myocytes.
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Affiliation(s)
- Mark B Cannell
- School of Physiology, Pharmacology, and Neuroscience, University of Bristol, Bristol, England, UK
| | - Cherrie H T Kong
- School of Physiology, Pharmacology, and Neuroscience, University of Bristol, Bristol, England, UK
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10
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Li J, Xie W, Chen X, Huo Y, Cheng H, Tan W. A novel stochastic reaction-diffusion model of Ca 2+ blink in cardiac myocytes. Sci Bull (Beijing) 2017; 62:5-8. [PMID: 36718071 DOI: 10.1016/j.scib.2016.12.001] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Affiliation(s)
- Jinghui Li
- State Key Laboratory of Turbulence and Complex Systems and Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China
| | - Wenjun Xie
- Institute of Molecular Medicine and National Laboratory of Biomembrane and Membrane Biotechnology, Peking University, Beijing 100871, China
| | - Xi Chen
- State Key Laboratory of Turbulence and Complex Systems and Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China
| | - Yunlong Huo
- State Key Laboratory of Turbulence and Complex Systems and Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China.
| | - Heping Cheng
- Institute of Molecular Medicine and National Laboratory of Biomembrane and Membrane Biotechnology, Peking University, Beijing 100871, China
| | - Wenchang Tan
- State Key Laboratory of Turbulence and Complex Systems and Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China; Shenzhen Graduate School, Peking University, Shenzhen 518055, China.
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11
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Salazar-Cantú A, Pérez-Treviño P, Montalvo-Parra D, Balderas-Villalobos J, Gómez-Víquez NL, García N, Altamirano J. Role of SERCA and the sarcoplasmic reticulum calcium content on calcium waves propagation in rat ventricular myocytes. Arch Biochem Biophys 2016; 604:11-9. [DOI: 10.1016/j.abb.2016.05.018] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2016] [Revised: 05/14/2016] [Accepted: 05/26/2016] [Indexed: 11/25/2022]
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12
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Abstract
Synchronized SR calcium (Ca) release is critical to normal cardiac myocyte excitation-contraction coupling, and ideally this release shuts off completely between heartbeats. However, other SR Ca release events are referred to collectively as SR Ca leak (which includes Ca sparks and waves as well as smaller events not detectable as Ca sparks). Much, but not all, of the SR Ca leak occurs via ryanodine receptors and can be exacerbated in pathological states such as heart failure. The extent of SR Ca leak is important because it can (a) reduce SR Ca available for release, causing systolic dysfunction; (b) elevate diastolic [Ca]i, contributing to diastolic dysfunction; (c) cause triggered arrhythmias; and (d) be energetically costly because of extra ATP used to repump Ca. This review addresses quantitative aspects and manifestations of SR Ca leak and its measurement, and how leak is modulated by Ca, associated proteins, and posttranslational modifications in health and disease.
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Affiliation(s)
- Donald M Bers
- Department of Pharmacology, University of California, Davis, California 95616;
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13
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Santiago DJ, Ríos E, Shannon TR. Isoproterenol increases the fraction of spark-dependent RyR-mediated leak in ventricular myocytes. Biophys J 2013; 104:976-85. [PMID: 23473480 DOI: 10.1016/j.bpj.2013.01.026] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2012] [Revised: 12/21/2012] [Accepted: 01/17/2013] [Indexed: 10/27/2022] Open
Abstract
Recent research suggests that the diastolic ryanodine-receptor-mediated release of Ca(2+) (J(leak)) from the sarcoplasmic reticulum of ventricular myocytes occurs in spark and nonspark forms. Further information about the role(s) of these release manifestations is scarce, however. This study addresses whether the fraction of spark-mediated J(leak) increases due to β-adrenergic stimulation. Confocal microscopy was used to simultaneously image Ca(2+) sparks and quantify J(leak) in intact rabbit myocytes, either in the absence or in the presence of 125 nM isoproterenol. It was found that isoproterenol treatment shifts the spark-frequency-J(leak) relationship toward an increased sensitivity to a [Ca(2+)] trigger. In agreement, a small but significant increase in spark width was found for cells with matched baseline [Ca(2+)] and total SR [Ca(2+)]. The reconstruction of release fluxes, when applied to the average sparks from those selected cells, yielded a wider release source in the isoproterenol event, indicating the recruitment of peripheral ryanodine receptors. Overall, the results presented here indicate that β-adrenergic stimulation increases the spark-dependent fraction of J(leak). Working together, the increased Ca(2+) sensitivity and the greater spark width found during isoproterenol treatment may increase the probability of Ca(2+) wave generation.
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Affiliation(s)
- Demetrio J Santiago
- Department of Molecular Biophysics & Physiology, Rush University, Chicago, Illinois, USA
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14
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Bers DM, Shannon TR. Calcium movements inside the sarcoplasmic reticulum of cardiac myocytes. J Mol Cell Cardiol 2013; 58:59-66. [PMID: 23321551 PMCID: PMC3628081 DOI: 10.1016/j.yjmcc.2013.01.002] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/15/2012] [Revised: 12/17/2012] [Accepted: 01/04/2013] [Indexed: 02/03/2023]
Abstract
Sarcoplasmic reticulum (SR) Ca content ([Ca]SRT) is critical to both normal cardiac function and electrophysiology, and changes associated with pathology contribute to systolic and diastolic dysfunction and arrhythmias. The intra-SR free [Ca] ([Ca]SR) dictates the [Ca]SRT, the driving force for Ca release and regulates release channel gating. We discuss measurement of [Ca]SR and [Ca]SRT, how [Ca]SR regulates activation and termination of release, and how Ca diffuses within the SR and influences SR Ca release during excitation-contraction coupling, Ca sparks and Cac waves. The entire SR network is connected and its lumen is also continuous with the nuclear envelope. Rapid Ca diffusion within the SR could stabilize and balance local [Ca]SR within the myocyte, but restrictions to diffusion can create spatial inhomogeneities. Experimental measurements and mathematical models of [Ca]SR to date have greatly enriched our understanding of these [Ca]SR dynamics, but controversies exist and may stimulate new measurements and analysis.
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Affiliation(s)
- Donald M Bers
- Department of Pharmacology, University of California, Davis, Davis, CA, USA.
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15
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Dulhunty AF, Beard NA, Hanna AD. Regulation and dysregulation of cardiac ryanodine receptor (RyR2) open probability during diastole in health and disease. ACTA ACUST UNITED AC 2012; 140:87-92. [PMID: 22851673 PMCID: PMC3409097 DOI: 10.1085/jgp.201210862] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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16
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Hou Z, Hu Z, Blackwell DJ, Miller TD, Thomas DD, Robia SL. 2-Color calcium pump reveals closure of the cytoplasmic headpiece with calcium binding. PLoS One 2012; 7:e40369. [PMID: 22808146 PMCID: PMC3394785 DOI: 10.1371/journal.pone.0040369] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2012] [Accepted: 06/07/2012] [Indexed: 11/19/2022] Open
Abstract
The sarco(endo)plasmic reticulum calcium ATPase (SERCA) undergoes conformational changes while transporting calcium, but the details of the domain motions are still unclear. The objective of the present study was to measure distances between the cytoplasmic domains of SERCA2a in order to reveal the magnitude and direction of conformational changes. Using fluorescence microscopy of live cells, we measured intramolecular fluorescence resonance energy transfer (FRET) from a donor fluorescent protein fused to the SERCA N-terminus to an acceptor fluorescent protein fused to either the N-, P-, or transmembrane domain. The "2-color" SERCA constructs were catalytically active as indicated by ATPase activity in vitro and Ca uptake in live cells. All constructs exhibited dynamic FRET changes in response to the pump ligands calcium and thapsigargin (Tg). These FRET changes were quantified as an index of SERCA conformational changes. Intramolecular FRET decreased with Tg for the two N-domain fusion sites (at residue 509 or 576), while the P- (residue 661) and TM-domain (C-terminus) fusions showed increased FRET with Tg. The magnitude of the Tg-dependent conformational change was not decreased by coexpression of phospholamban (PLB), nor did PLB slow the kinetics of Tg binding. FRET in ionophore-permeabilized cells was lower in EGTA than in saturating calcium for all constructs, indicating a decrease in domain separation distance with the structural transition from E2 (Ca-free) to E1 (Ca-bound). The data suggest closure of the cytoplasmic headpiece with Ca-binding. The present results provide insight into the structural dynamics of the Ca-ATPase. In addition, the 2-color SERCA constructs developed for this study may be useful for evaluating candidate small molecule regulators of Ca uptake activity.
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Affiliation(s)
- Zhanjia Hou
- Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois, United States of America
| | - Zhihong Hu
- Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois, United States of America
| | - Daniel J. Blackwell
- Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois, United States of America
| | - Tyler D. Miller
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota, United States of America
| | - David D. Thomas
- Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota, Minneapolis, Minnesota, United States of America
| | - Seth L. Robia
- Department of Cell and Molecular Physiology, Loyola University Chicago, Maywood, Illinois, United States of America
- * E-mail:
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17
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Cerra MC, Imbrogno S. Phospholamban and cardiac function: a comparative perspective in vertebrates. Acta Physiol (Oxf) 2012; 205:9-25. [PMID: 22463608 DOI: 10.1111/j.1748-1716.2012.02389.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Phospholamban (PLN) is a small phosphoprotein closely associated with the cardiac sarcoplasmic reticulum (SR). Dephosphorylated PLN tonically inhibits the SR Ca-ATPase (SERCA2a), while phosphorylation at Ser16 by PKA and Thr17 by Ca(2+) /calmodulin-dependent protein kinase (CaMKII) relieves the inhibition, and this increases SR Ca(2+) uptake. For this reason, PLN is one of the major determinants of cardiac contractility and relaxation. In this review, we attempted to highlight the functional significance of PLN in vertebrate cardiac physiology. We will refer to the huge literature on mammals in order to describe the molecular characteristics of this protein, its interaction with SERCA2a and its role in the regulation of the mechanic and the electric performance of the heart under basal conditions, in the presence of chemical and physical stresses, such as β-adrenergic stimulation, response to stretch, force-frequency relationship and intracellular acidosis. Our aim is to provide the basis to discuss the role of PLN also on the cardiac function of nonmammalian vertebrates, because so far this aspect has been almost neglected. Accordingly, when possible, the literature on PLN will be analysed taking into account the nonuniform cardiac structural and functional characteristics encountered in ectothermic vertebrates, such as the peculiar and variable organization of the SR, the large spectrum of response to stresses and the disaptive absence of crucial proteins (i.e. haemoglobinless and myoglobinless species).
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Affiliation(s)
| | - S. Imbrogno
- Department of Cell Biology; University of Calabria; Arcavacata di Rende (CS); Italy
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Hake J, Edwards AG, Yu Z, Kekenes-Huskey PM, Michailova AP, McCammon JA, Holst MJ, Hoshijima M, McCulloch AD. Modelling cardiac calcium sparks in a three-dimensional reconstruction of a calcium release unit. J Physiol 2012; 590:4403-22. [PMID: 22495592 DOI: 10.1113/jphysiol.2012.227926] [Citation(s) in RCA: 74] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022] Open
Abstract
Triggered release of Ca2+ from an individual sarcoplasmic reticulum (SR) Ca(2+) release unit (CRU) is the fundamental event of cardiac excitation–contraction coupling, and spontaneous release events (sparks) are the major contributor to diastolic Ca(2+) leak in cardiomyocytes. Previous model studies have predicted that the duration and magnitude of the spark is determined by the local CRU geometry, as well as the localization and density of Ca(2+) handling proteins. We have created a detailed computational model of a CRU, and developed novel tools to generate the computational geometry from electron tomographic images. Ca(2+) diffusion was modelled within the SR and the cytosol to examine the effects of localization and density of the Na(+)/Ca(2+) exchanger, sarco/endoplasmic reticulum Ca(2+)-ATPase 2 (SERCA), and calsequestrin on spark dynamics. We reconcile previous model predictions of approximately 90% local Ca(2+) depletion in junctional SR, with experimental reports of about 40%. This analysis supports the hypothesis that dye kinetics and optical averaging effects can have a significant impact on measures of spark dynamics. Our model also predicts that distributing calsequestrin within non-junctional Z-disc SR compartments, in addition to the junctional compartment, prolongs spark release time as reported by Fluo5. By pumping Ca(2+) back into the SR during a release, SERCA is able to prolong a Ca(2+) spark, and this may contribute to SERCA-dependent changes in Ca(2+) wave speed. Finally, we show that including the Na(+)/Ca(2+) exchanger inside the dyadic cleft does not alter local [Ca(2+)] during a spark.
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Affiliation(s)
- Johan Hake
- Department of Bioengineering, University of California San Diego, CA, USA.
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Cerra MC, Imbrogno S. Phospholamban and cardiac function: a comparative perspective in vertebrates. Acta Physiol (Oxf) 2012. [DOI: 10.1111/j.1748-1716.2011.02389.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
| | - S. Imbrogno
- Department of Cell Biology; University of Calabria; Arcavacata di Rende (CS); Italy
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Altschafl BA, Arvanitis DA, Fuentes O, Yuan Q, Kranias EG, Valdivia HH. Dual role of junctin in the regulation of ryanodine receptors and calcium release in cardiac ventricular myocytes. J Physiol 2011; 589:6063-80. [PMID: 22025663 DOI: 10.1113/jphysiol.2011.215988] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
Junctin, a 26 kDa intra-sarcoplasmic reticulum (SR) protein, forms a quaternary complex with triadin, calsequestrin and the ryanodine receptor (RyR) at the junctional SR membrane. The physiological role for junctin in the luminal regulation of RyR Ca(2+) release remains unresolved, but it appears to be essential for proper cardiac function since ablation of junctin results in increased ventricular automaticity. Given that the junctin levels are severely reduced in human failing hearts, we performed an in-depth study of the mechanisms affecting intracellular Ca(2+) homeostasis in junctin-deficient cardiomyocytes. In concurrence with sparks, JCN-KO cardiomyocytes display increased Ca(2+) transient amplitude, resulting from increased SR [Ca(2+)] ([Ca(2+)](SR)). Junctin ablation appears to affect how RyRs 'sense' SR Ca(2+) load, resulting in decreased diastolic SR Ca(2+) leak despite an elevated [Ca(2+)](SR). Surprisingly, the β-adrenergic enhancement of [Ca(2+)](SR) reverses the decrease in RyR activity and leads to spontaneous Ca(2+) release, evidenced by the development of spontaneous aftercontractions. Single channel recordings of RyRs from WT and JCN-KO cardiac SR indicate that the absence of junctin produces a dual effect on the normally linear response of RyRs to luminal [Ca(2+)]: at low luminal [Ca(2+)] (<1 mmol l(-1)), junctin-devoid RyR channels are less responsive to luminal [Ca(2+)]; conversely, high luminal [Ca(2+)] turns them hypersensitive to this form of channel modulation. Thus, junctin produces complex effects on Ca(2+) sparks, transients, and leak, but the luminal [Ca(2+)]-dependent dual response of junctin-devoid RyRs demonstrates that junctin normally acts as an activator of RyR channels at low luminal [Ca(2+)], and as an inhibitor at high luminal [Ca(2+)]. Because the crossover occurs at a [Ca(2+)](SR) that is close to that present in resting cells, it is possible that the activator-inhibitor role of junctin may be exerted under periods of prevalent parasympathetic and sympathetic activity, respectively.
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Affiliation(s)
- Beth A Altschafl
- Department of Physiology, University of Wisconsin Medical School, Madison, WI 53711, USA
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Ramirez RJ, Sah R, Liu J, Rose RA, Backx PH. Intracellular [Na(+)] modulates synergy between Na(+)/Ca (2+) exchanger and L-type Ca (2+) current in cardiac excitation-contraction coupling during action potentials. Basic Res Cardiol 2011; 106:967-77. [PMID: 21779914 DOI: 10.1007/s00395-011-0202-z] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/11/2011] [Revised: 06/22/2011] [Accepted: 07/04/2011] [Indexed: 02/03/2023]
Abstract
Excitation-contraction coupling (ECC) in cardiac myocytes involves triggering of Ca(2+) release from the sarcoplasmic reticulum (SR) by L-type Ca channels, whose activity is strongly influenced by action potential (AP) profile. The contribution of Ca(2+) entry via the Na(+)/Ca(2+) exchanger (NCX) to trigger SR Ca(2+) release during ECC in response to an AP remains uncertain. To isolate the contribution of NCX to SR Ca(2+) release, independent of effects on SR Ca(2+) load, Ca(2+) release was determined by recording Ca(2+) spikes using confocal microscopy on patch-clamped rat ventricular myocytes with [Ca(2+)](i) fixed at 150 nmol/L. In response to AP clamps, normalized Ca(2+) spike amplitudes (ΔF/F (0)) increased sigmoidally and doubled as [Na(+)](i) was elevated from 0 to 20 mmol/L with an EC(50) of ~10 mmol/L. This [Na(+)](i)-dependence was independent of I (Na) as well as SR Ca(2+) load, which was unchanged under our experimental conditions. However, NCX inhibition using either KB-R7943 or XIP reduced ΔF/F (0) amplitude in myocytes with 20 mmol/L [Na(+)](i), but not with 5 mmol/L [Na(+)](i). SR Ca(2+) release was complete before the membrane repolarized to -15 mV, indicating Ca(2+) entry into the dyad (not reduced extrusion) underlies [Na(+)](i)-dependent enhancement of ECC. Because I (Ca,L) inhibition with 50 mmol/L Cd(2+) abolished Ca(2+) spikes, our results demonstrate that during cardiac APs, NCX enhances SR Ca(2+) release by synergistically increasing the efficiency of I (Ca,L)-mediated ECC.
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Affiliation(s)
- Rafael J Ramirez
- Department of Physiology, University of Toronto, Heart and Stroke/Richard Lewar Centre of Excellence, Fitzgerald Building, 150 College Street, Room 68, Toronto, ON M5S 3E2, Canada
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Zhou L, Aon MA, Liu T, O'Rourke B. Dynamic modulation of Ca2+ sparks by mitochondrial oscillations in isolated guinea pig cardiomyocytes under oxidative stress. J Mol Cell Cardiol 2011; 51:632-9. [PMID: 21645518 DOI: 10.1016/j.yjmcc.2011.05.007] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/12/2011] [Accepted: 05/11/2011] [Indexed: 01/10/2023]
Abstract
Local control of Ca(2+)-induced Ca(2+) release (CICR) depends on the spatial organization of L-type Ca(2+) channels and ryanodine receptors (RyR) in the dyad. Analogously, Ca(2+) uptake by mitochondria is facilitated by their close proximity to the Ca(2+) release sites, a process required for stimulating oxidative phosphorylation during changes in work. Mitochondrial feedback on CICR is less well understood. Since mitochondria are a primary source of reactive oxygen species (ROS), they could potentially influence the cytosolic redox state, in turn altering RyR open probability. We have shown that self-sustained oscillations in mitochondrial inner membrane potential (ΔΨ(m)), NADH, ROS, and reduced glutathione (GSH) can be triggered by a laser flash in cardiomyocytes. Here, we employ this method to directly examine how acute changes in energy state dynamically influence resting Ca(2+) spark occurrence and properties. Two-photon laser scanning microscopy was used to monitor cytosolic Ca(2+) (or ROS), ΔΨ(m), and NADH (or GSH) simultaneously in isolated guinea pig cardiomyocytes. Resting Ca(2+) spark frequency increased with each ΔΨ(m) depolarization and decreased with ΔΨ(m) repolarization without affecting Ca(2+) spark amplitude or time-to-peak. Stabilization of mitochondrial energetics by pretreatment with the superoxide scavenger TMPyP, or by acute addition of 4'-chlorodiazepam, a mitochondrial benzodiazepine receptor antagonist that blocks the inner membrane anion channel, prevented or reversed, respectively, the increased spark frequency. Cyclosporine A did not block the ΔΨ(m) oscillations or prevent Ca(2+) spark modulation by ΔΨ(m). The results support the hypothesis that mitochondria exert an influential role on the redox environment of the Ca(2+) handling subsystem, with mechanistic implications for the pathophysiology of cardiac disease.
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Affiliation(s)
- Lufang Zhou
- Department of Medicine, Division of Cardiology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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23
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Capes EM, Loaiza R, Valdivia HH. Ryanodine receptors. Skelet Muscle 2011; 1:18. [PMID: 21798098 PMCID: PMC3156641 DOI: 10.1186/2044-5040-1-18] [Citation(s) in RCA: 55] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2010] [Accepted: 05/04/2011] [Indexed: 12/31/2022] Open
Abstract
Excitation-contraction coupling involves the faithful conversion of electrical stimuli to mechanical shortening in striated muscle cells, enabled by the ubiquitous second messenger, calcium. Crucial to this process are ryanodine receptors (RyRs), the sentinels of massive intracellular calcium stores contained within the sarcoplasmic reticulum. In response to sarcolemmal depolarization, RyRs release calcium into the cytosol, facilitating mobilization of the myofilaments and enabling cell contraction. In order for the cells to relax, calcium must be rapidly resequestered or extruded from the cytosol. The sustainability of this cycle is crucially dependent upon precise regulation of RyRs by numerous cytosolic metabolites and by proteins within the lumen of the sarcoplasmic reticulum and those directly associated with the receptors in a macromolecular complex. In addition to providing the majority of the calcium necessary for contraction of cardiac and skeletal muscle, RyRs act as molecular switchboards that integrate a multitude of cytosolic signals such as dynamic and steady calcium fluctuations, β-adrenergic stimulation (phosphorylation), nitrosylation and metabolic states, and transduce these signals to the channel pore to release appropriate amounts of calcium. Indeed, dysregulation of calcium release via RyRs is associated with life-threatening diseases in both skeletal and cardiac muscle. In this paper, we briefly review some of the most outstanding structural and functional attributes of RyRs and their mechanism of regulation. Further, we address pathogenic RyR dysfunction implicated in cardiovascular disease and skeletal myopathies.
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Affiliation(s)
- E Michelle Capes
- Department of Cellular and Regenerative Biology, University of Wisconsin Medical School, Madison, WI 53711, USA.
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Ginet P, Montagne K, Akiyama S, Rajabpour A, Taniguchi A, Fujii T, Sakai Y, Kim B, Fourmy D, Volz S. Towards single cell heat shock response by accurate control on thermal confinement with an on-chip microwire electrode. LAB ON A CHIP 2011; 11:1513-1520. [PMID: 21394336 DOI: 10.1039/c0lc00701c] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/30/2023]
Abstract
Metal electrodes with micron scale width enable the heating of less than a dozen cells in a confluent layer at predictable temperatures up to 85 °C with an accuracy of ±2 °C. Those performances were obtained by a preliminary robust temperature calibration based on biotin-rhodamine fluorescence and by controlling the temperature map on the substrate through thermal modeling. The temperature accuracy was proved by inducing the expression of heat shock proteins (HSP) in a few NIH-3T3 cells through a confined and precise temperature rise. Our device is therefore effective to locally induce a heat shock response with almost single-cell resolution. Furthermore, we show that cells heated at a higher temperature than the one of heat shock remain alive without producing HSP. Electrode deposition being one of the most common engineering processes, the fabrication of electrode arrays with a simple control circuit is clearly within reach for parallel testing. This should enable the study of several key mechanisms such as cell heat shock, death or signaling. In nanomedicine, controlled drug release by external stimuli such as for example temperature has attracted much attention. Our device could allow fast and efficient testing of thermoactivable drug delivery systems.
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Affiliation(s)
- Patrick Ginet
- Laboratory of Integrated Micro and Mechatronics Systems/IIS UMI CNRS 2820, Institute of Industrial Sciences, The University of Tokyo, Tokyo, Japan
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25
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Murphy RM, Mollica JP, Beard NA, Knollmann BC, Lamb GD. Quantification of calsequestrin 2 (CSQ2) in sheep cardiac muscle and Ca2+-binding protein changes in CSQ2 knockout mice. Am J Physiol Heart Circ Physiol 2010; 300:H595-604. [PMID: 21131479 DOI: 10.1152/ajpheart.00902.2010] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Calsequestrin 2 (CSQ2) is generally regarded as the primary Ca2+-buffering molecule present inside the sarcoplasmic reticulum (SR) in cardiac cells, but findings from CSQ2 knockout experiments raise major questions about its role and necessity. This study determined the absolute amount of CSQ2 present in cardiac ventricular muscle to gauge its likely influence on SR free Ca2+ concentration ([Ca2+]) and maximal Ca2+ capacity. Ventricular tissue from hearts of freshly killed sheep was examined by SDS-PAGE without any fractionation, and CSQ2 was detected by Western blotting; this method avoided the >90% loss of CSQ2 occurring with usual fractionation procedures. Band intensities were compared against those for purified CSQ2 run on the same blots. Fidelity of quantification was verified by demonstrating that CSQ2 added to homogenates was detected with equal efficacy as purified CSQ2 alone. Ventricular tissue from sheep (n=8) contained 24±2 μmol CSQ2/kg wet wt. Total Ca2+ content of the ventricular tissue, measured by atomic absorption spectroscopy, was 430±20 μmol/kg (with SR Ca2+ likely<250 μmol/kg) and displayed a linear correlation with CSQ2 content, with gradient of ∼10 Ca2+ per CSQ2. The large amount of CSQ2 bestows the SR with a high theoretical maximal Ca2+-binding capacity (∼1 mmol Ca2+/kg ventricular tissue, assuming a maximum of ∼40 Ca2+ per CSQ2) and would keep free [Ca2+] within the SR relatively low, energetically favoring Ca2+ uptake and reducing SR leak. In mice with CSQ2 ablated, histidine-rich Ca2+-binding protein was upregulated ∼35% in ventricular tissue, possibly in compensation.
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Affiliation(s)
- Robyn M Murphy
- Department of Zoology, La Trobe University, Melbourne, Victoria, 3086, Australia
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Santiago DJ, Curran JW, Bers DM, Lederer WJ, Stern MD, Ríos E, Shannon TR. Ca sparks do not explain all ryanodine receptor-mediated SR Ca leak in mouse ventricular myocytes. Biophys J 2010; 98:2111-20. [PMID: 20483318 DOI: 10.1016/j.bpj.2010.01.042] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2009] [Revised: 01/04/2010] [Accepted: 01/19/2010] [Indexed: 11/17/2022] Open
Abstract
Diastolic Ca leak from the sarcoplasmic reticulum (SR) of ventricular myocytes reduces the SR Ca content, stabilizing the activity of the SR Ca release channel ryanodine receptor for the next beat. SR Ca leak has been visualized globally using whole-cell fluorescence, or locally using confocal microscopy, but never both ways. When using confocal microscopy, leak is imaged as "Ca sparks," which are fluorescent objects generated by the local reaction-diffusion of released Ca and cytosolic indicator. Here, we used confocal microscopy and simultaneously measured the global ryanodine-receptor-mediated leak rate (J(leak)) and Ca sparks in intact mouse ventricular myocytes. We found that spark frequency and J(leak) are correlated, as expected if both are manifestations of a common phenomenon. However, we also found that sparks explain approximately half of J(leak). Our strategy unmasks the presence of a subresolution (i.e., nonspark) release of potential physiological relevance.
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Affiliation(s)
- Demetrio J Santiago
- Department of Molecular Biophysics & Physiology, Rush University, Chicago, Illinois, USA
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Li L, Niederer SA, Idigo W, Zhang YH, Swietach P, Casadei B, Smith NP. A mathematical model of the murine ventricular myocyte: a data-driven biophysically based approach applied to mice overexpressing the canine NCX isoform. Am J Physiol Heart Circ Physiol 2010; 299:H1045-63. [PMID: 20656884 DOI: 10.1152/ajpheart.00219.2010] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mathematical modeling of Ca(2+) dynamics in the heart has the potential to provide an integrated understanding of Ca(2+)-handling mechanisms. However, many previous published models used heterogeneous experimental data sources from a variety of animals and temperatures to characterize model parameters and motivate model equations. This methodology limits the direct comparison of these models with any particular experimental data set. To directly address this issue, in this study, we present a biophysically based model of Ca(2+) dynamics directly fitted to experimental data collected in left ventricular myocytes isolated from the C57BL/6 mouse, the most commonly used genetic background for genetically modified mice in studies of heart diseases. This Ca(2+) dynamics model was then integrated into an existing mouse cardiac electrophysiology model, which was reparameterized using experimental data recorded at consistent and physiological temperatures. The model was validated against the experimentally observed frequency response of Ca(2+) dynamics, action potential shape, dependence of action potential duration on cycle length, and electrical restitution. Using this framework, the implications of cardiac Na(+)/Ca(2+) exchanger (NCX) overexpression in transgenic mice were investigated. These simulations showed that heterozygous overexpression of the canine cardiac NCX increases intracellular Ca(2+) concentration transient magnitude and sarcoplasmic reticulum Ca(2+) loading, in agreement with experimental observations, whereas acute overexpression of the murine cardiac NCX results in a significant loss of Ca(2+) from the cell and, hence, depressed sarcoplasmic reticulum Ca(2+) load and intracellular Ca(2+) concentration transient magnitude. From this analysis, we conclude that these differences are primarily due to the presence of allosteric regulation in the canine cardiac NCX, which has not been observed experimentally in the wild-type mouse heart.
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Affiliation(s)
- L Li
- Computing Laboratory, University of Oxford and John Radcliffe Hospital, Oxford, United Kingdom
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Curran J, Brown KH, Santiago DJ, Pogwizd S, Bers DM, Shannon TR. Spontaneous Ca waves in ventricular myocytes from failing hearts depend on Ca(2+)-calmodulin-dependent protein kinase II. J Mol Cell Cardiol 2010; 49:25-32. [PMID: 20353795 DOI: 10.1016/j.yjmcc.2010.03.013] [Citation(s) in RCA: 106] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/04/2009] [Revised: 03/12/2010] [Accepted: 03/15/2010] [Indexed: 10/19/2022]
Abstract
Increased cardiac ryanodine receptor (RyR)-dependent diastolic SR Ca leak is present in heart failure and in conditions when adrenergic tone is high. Increasing Ca leak from the SR could result in spontaneous Ca wave (SCaW) formation. SCaWs activate the inward Na/Ca exchanger (NCX) current causing a delayed afterdepolarization (DAD), potentially leading to arrhythmia. Here we examine SCaWs in ventricular myocytes isolated from failing and healthy rabbit hearts. Myocytes from healthy hearts did not exhibit SCaWs under baseline conditions versus 43% of those exposed to isoproterenol (ISO). This ISO-induced increase in activity was reversed by inhibition of Ca-calmodulin-dependent protein kinase II (CaMKII) by KN93. Inhibition of cAMP-dependent protein kinase (PKA) by H89 had no observed effect. Of myocytes treated with forskolin 50% showed SCaW activity, attributable to a large increase in SR Ca load ([Ca](SRT)) versus control. At similar [Ca](SRT) (121muM) myocytes treated with ISO plus KN93 had significantly fewer SCaWs versus those treated with ISO or ISO plus H89 (0.2+/-0.28 vs. 1.1+/-0.28 and 1.29+/-0.39 SCaWs cell(-)(1), respectively). In myocytes isolated from failing hearts ISO induced an increase in the percentage of cells generating SCaWs vs. baseline (74% vs. 11%) with no increase in [Ca](SRT). Inhibiting CaMKII reversed this effect (14%). At similar [Ca](SRT) (71microM) myocytes treated with ISO or ISO plus H89 had significantly more SCaWs per cell vs. untreated (2.5+/-0.5; 1.6+/-0.7 vs. 0.36+/-0.3, respectively). Treatment with ISO plus KN93 completely abolished this effect. The evidence suggests the ISO-dependent increase in SCaW activity in both healthy and failing myocytes is CaMKII-dependent, implicating CaMKII in arrhythmogenesis.
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Korhonen T, Rapila R, Ronkainen VP, Koivumäki JT, Tavi P. Local Ca2+ releases enable rapid heart rates in developing cardiomyocytes. J Physiol 2010; 588:1407-17. [PMID: 20211983 DOI: 10.1113/jphysiol.2009.185173] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The ability to generate homogeneous intracellular Ca(2+) oscillations at high frequency is the basis of the rhythmic contractions of mammalian cardiac myocytes. While the specific mechanisms and structures enabling homogeneous high-frequency Ca(2+) signals in adult cardiomyocytes are well characterized, it is not known how these kind of Ca(2+) signals are produced in developing cardiomyocytes. Here we investigated the mechanisms reducing spatial and temporal heterogeneity of cytosolic Ca(2+) signals in mouse embryonic ventricular cardiomyocytes. We show that in developing cardiomyocytes the propagating Ca(2+) signals are amplified in cytosol by local Ca(2+) releases. Local releases are based on regular 3-D sarcoplasmic reticulum (SR) structures containing SR Ca(2+) uptake ATPases (SERCA) and Ca(2+) release channels (ryanodine receptors, RyRs) at regular intervals throughout the cytosol. By evoking [Ca(2+)](i)-induced Ca(2+) sparks, the local release sites promote a 3-fold increase in the cytosolic Ca(2+) propagation speed. We further demonstrate by mathematical modelling that without these local release sites the developing cardiomyocytes lose their ability to generate homogeneous global Ca(2+) signals at a sufficiently high frequency. The mechanism described here is robust and indispensable for normal mammalian cardiomyocyte function from the first heartbeats during the early embryonic phase till terminal differentiation after birth. These results suggest that local cytosolic Ca(2+) releases are indispensable for normal cardiomyocyte development and function of developing heart.
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Affiliation(s)
- Topi Korhonen
- University of Eastern Finland, A.I. Virtanen Institute for Molecular Sciences, Department of Biotechnology and Molecular Medicine, PO Box 1627, FIN-70211 Kuopio, Finland.
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31
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Korajoki H, Vornanen M. Expression of calsequestrin in atrial and ventricular muscle of thermally acclimated rainbow trout. ACTA ACUST UNITED AC 2010; 212:3403-14. [PMID: 19837881 DOI: 10.1242/jeb.031617] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Calsequestrin (CASQ) is the main Ca(2+) binding protein within the sarcoplasmic reticulum (SR) of the vertebrate heart. The contribution of SR Ca(2+) stores to contractile activation is larger in atrial than ventricular muscle, and in ectothermic fish hearts acclimation to low temperatures increases the use of SR Ca(2+) in excitation-contraction coupling. The hypotheses that chamber-specific and temperature-induced differences in SR function are due to the increased SR CASQ content were tested in rainbow trout (Oncorhynchus mykiss) acclimated at either 4 degrees C (cold acclimation, CA) or 18 degrees C (warm acclimation, WA). To this end, the trout cardiac CASQ (omCASQ2) was cloned and sequenced. The omCASQ2 consists of 1275 nucleotides encoding a predicted protein of 425 amino acids (54 kDa in molecular mass, MM) with a high (75-87%) sequence similarity to other vertebrate cardiac CASQs. The transcript levels of the omCASQ2 were 1.5-2 times higher in CA than WA fish and about 2.5 times higher in the atrium than ventricle (P<0.001). The omCASQ2 protein was measured from western blots using a polyclonal antibody against the amino acid sequence 174-315 of the omCASQ2. Unlike the omCASQ2 transcripts, no differences were found in the abundance of the omCASQ2 protein between CA and WA fish, nor between the atrium and ventricle (P>0.05). However, a prominent qualitative difference appeared between the acclimation groups: two CASQ isoforms with apparent MMs of 54 and 59 kDa, respectively, were present in atrial and ventricular muscle of the WA trout whereas only the 54 kDa protein was clearly expressed in the CA heart. The 59 kDA isoform was a minor CASQ component representing 22% and 13% of the total CASQ proteins in the atrium and ventricle of the WA fish, respectively. In CA hearts, the 59 kDa protein was present in trace amounts (1.5-2.4%). Collectively, these findings indicate that temperature-related and chamber-specific differences in trout cardiac SR function are not related to the abundance of luminal Ca(2+) buffering by cardiac CASQ.
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Affiliation(s)
- Hanna Korajoki
- University of Joensuu, Faculty of Biosciences, Joensuu, Finland.
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Hove-Madsen L, Baudet S, Bers D. Making and Using Calcium-Selective Mini- and Microelectrodes. Methods Cell Biol 2010; 99:67-89. [DOI: 10.1016/b978-0-12-374841-6.00003-7] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/20/2023]
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Liu Y, Porta M, Qin J, Ramos J, Nani A, Shannon TR, Fill M. Flux regulation of cardiac ryanodine receptor channels. ACTA ACUST UNITED AC 2009; 135:15-27. [PMID: 20008518 PMCID: PMC2806413 DOI: 10.1085/jgp.200910273] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The cardiac type 2 ryanodine receptor (RYR2) is activated by Ca2+-induced Ca2+ release (CICR). The inherent positive feedback of CICR is well controlled in cells, but the nature of this control is debated. Here, we explore how the Ca2+ flux (lumen-to-cytosol) carried by an open RYR2 channel influences its own cytosolic Ca2+ regulatory sites as well as those on a neighboring channel. Both flux-dependent activation and inhibition of single channels were detected when there were super-physiological Ca2+ fluxes (>3 pA). Single-channel results indicate a pore inhibition site distance of 1.2 ± 0.16 nm and that the activation site on an open channel is shielded/protected from its own flux. Our results indicate that the Ca2+ flux mediated by an open RYR2 channel in cells (∼0.5 pA) is too small to substantially regulate (activate or inhibit) the channel carrying it, even though it is sufficient to activate a neighboring RYR2 channel.
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Affiliation(s)
- Yiwei Liu
- Department of Molecular Physiology and Biophysics, Rush University Medical Center, Chicago, IL 60612, USA
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Haverinen J, Vornanen M. Comparison of sarcoplasmic reticulum calcium content in atrial and ventricular myocytes of three fish species. Am J Physiol Regul Integr Comp Physiol 2009; 297:R1180-7. [PMID: 19692664 DOI: 10.1152/ajpregu.00022.2009] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Ryanodine (Ry) sensitivity of cardiac contraction differs between teleost species, between atrium and ventricle, and according to the thermal history of the fish. The hypothesis that variability in Ry sensitivity of contraction is due to species-specific, chamber-specific, and temperature-related differences in the sarcoplasmic reticulum (SR) Ca(2+) content, was tested by comparing steady-state (SS) and maximal (Max) Ca(2+) loads of the SR in three teleost fish, rainbow trout (Oncorhynchus mykiss), burbot (Lota lota), and crucian carp (Carassius carassius), which differ in the extent of SR contribution to excitation-contraction coupling. Fish were acclimated at 4 degrees C (cold-acclimation, CA) or 18 degrees C (warm-acclimation, WA), and SR Ca(2+) content was released by a rapid application of 10 mM caffeine to single cardiac myocytes; its amount was determined from the Na(+)-Ca(2+) exchange current at 18 degrees C. SS Ca(2+) load was larger in atrial (304-915 micromol/l) than ventricular (224-540 micromol/l) myocytes in all fish species (P < 0.05), and the same was true for Max SR Ca(2+) content: 550-1,522 micromol/l and 438-840 micromol/l for atrial and ventricular myocytes, respectively (P < 0.05). Consistent with the hypothesis, acclimation to cold increased Ca(2+) load of the cardiac SR in the burbot heart, but contrary to the hypothesis, temperature acclimation did not affect SR Ca(2+) content in rainbow trout and crucian carp hearts. Furthermore, there was an inverse relation between SR Ca(2+) content and Ry sensitivity of contraction force: the species with the smallest SR Ca(2+) content (burbot) is most sensitive to Ry. Collectively, these findings show that SR Ca(2+) content of fish cardiac myocytes is several times larger than that in mammalian cardiac SR.
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Affiliation(s)
- Jaakko Haverinen
- University of Joensuu, Faculty of Biosciences, Joensuu, Finland.
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Assessment of sarcoplasmic reticulum Ca2+ depletion during spontaneous Ca2+ waves in isolated permeabilized rabbit ventricular cardiomyocytes. Biophys J 2009; 96:2744-54. [PMID: 19348757 DOI: 10.1016/j.bpj.2008.12.3944] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/31/2008] [Revised: 12/22/2008] [Accepted: 12/24/2008] [Indexed: 11/21/2022] Open
Abstract
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 microM during a Ca2+ wave. Reduction of RyR2 Ca2+ sensitivity by tetracaine (50 microM) 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 microM) 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 approximately 50 microM.
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Koivumäki JT, Takalo J, Korhonen T, Tavi P, Weckström M. Modelling sarcoplasmic reticulum calcium ATPase and its regulation in cardiac myocytes. PHILOSOPHICAL TRANSACTIONS. SERIES A, MATHEMATICAL, PHYSICAL, AND ENGINEERING SCIENCES 2009; 367:2181-2202. [PMID: 19414452 DOI: 10.1098/rsta.2008.0304] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/27/2023]
Abstract
When developing large-scale mathematical models of physiology, some reduction in complexity is necessarily required to maintain computational efficiency. A prime example of such an intricate cell is the cardiac myocyte. For the predictive power of the cardiomyocyte models, it is vital to accurately describe the calcium transport mechanisms, since they essentially link the electrical activation to contractility. The removal of calcium from the cytoplasm takes place mainly by the Na(+)/Ca(2+) exchanger, and the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). In the present study, we review the properties of SERCA, its frequency-dependent and beta-adrenergic regulation, and the approaches of mathematical modelling that have been used to investigate its function. Furthermore, we present novel theoretical considerations that might prove useful for the elucidation of the role of SERCA in cardiac function, achieving a reduction in model complexity, but at the same time retaining the central aspects of its function. Our results indicate that to faithfully predict the physiological properties of SERCA, we should take into account the calcium-buffering effect and reversible function of the pump. This 'uncomplicated' modelling approach could be useful to other similar transport mechanisms as well.
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Affiliation(s)
- Jussi T Koivumäki
- Division of Biophysics, Department of Physical Sciences, University of Oulu, 90014 Oulu, Finland
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Abstract
Intracellular calcium transient alternans (CTA) has a recognized role in arrhythmogenesis, but its origin is not yet fully understood. Recent models of CTA are based on a steep relationship between calcium release from the sarcoplasmic reticulum (SR) and its calcium load before release. This mechanism alone, however, does not explain recent observations of CTA without diastolic SR calcium content alternations. In addition, nanoscopic imaging of calcium dynamics has revealed that the elementary calcium release units of the SR can become refractory independently of their local calcium content. Here we show using a new physiologically detailed mathematical model of calcium cycling that luminal gating of the calcium release channels (RyRs) mediated by the luminal buffer calsequestrin (CSQN) can cause CTA independently of the steepness of the release-load relationship. In this complementary mechanism, CTA is caused by a beat-to-beat alternation in the number of refractory RyR channels and can occur with or without diastolic SR calcium content alternans depending on pacing conditions and uptake dynamics. The model has unique features, in that it treats a realistic number of spatially distributed and diffusively coupled dyads, each one with a realistic number of RyR channels, and that luminal CSQN buffering and gating is incorporated based on experimental data that characterizes the effect of the conformational state of CSQN on its buffering properties. In addition to reproducing observed features of CTA, this multiscale model is able to describe recent experiments in which CSQN expression levels were genetically altered as well as to reproduce nanoscopic measurements of spark restitution properties. The ability to link microscopic properties of the calcium release units to whole cell behavior makes this model a powerful tool to investigate the arrhythmogenic role of abnormal calcium handling in many pathological settings.
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Györke S, Carnes C. Dysregulated sarcoplasmic reticulum calcium release: potential pharmacological target in cardiac disease. Pharmacol Ther 2008; 119:340-54. [PMID: 18675300 DOI: 10.1016/j.pharmthera.2008.06.002] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/14/2008] [Accepted: 06/17/2008] [Indexed: 12/15/2022]
Abstract
In the heart, Ca(2+) released from the intracellular Ca(2+) storage site, the sarcoplasmic reticulum (SR), is the principal determinant of cardiac contractility. SR Ca(2+) release is controlled by dedicated molecular machinery, composed of the cardiac ryanodine receptor (RyR2) and a number of accessory proteins, including FKBP12.6, calsequestrin (CASQ2), triadin (TRD) and junctin (JN). Acquired and genetic defects in the components of the release channel complex result in a spectrum of abnormal Ca(2+) release phenotypes ranging from arrhythmogenic spontaneous Ca(2+) releases and Ca(2+) alternans to the uniformly diminished systolic Ca(2+) release characteristic of heart failure. In this article, we will present an overview of the structure and molecular components of the SR and Ca(2+) release machinery and its modulation by different intracellular factors, such as Ca(2+) levels inside the SR as well as phosphorylation and redox modification of RyR2s. We will also discuss the relationships between abnormal SR Ca(2+) release and various cardiac disease phenotypes, including, arrhythmias and heart failure, and consider SR Ca(2+) release as a potential therapeutic target.
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Affiliation(s)
- Sandor Györke
- Department of Physiology and Cell Biology, The Ohio State University, Columbus, OH, United States.
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Lee YS, Keener JP. A calcium-induced calcium release mechanism mediated by calsequestrin. J Theor Biol 2008; 253:668-79. [PMID: 18538346 DOI: 10.1016/j.jtbi.2008.04.027] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2007] [Revised: 02/27/2008] [Accepted: 04/04/2008] [Indexed: 10/22/2022]
Abstract
Calcium (Ca(2+))-induced Ca(2+) release (CICR) is widely accepted as the principal mechanism linking electrical excitation and mechanical contraction in cardiac cells. The CICR mechanism has been understood mainly based on binding of cytosolic Ca(2+) with ryanodine receptors (RyRs) and inducing Ca(2+) release from the sarcoplasmic reticulum (SR). However, recent experiments suggest that SR lumenal Ca(2+) may also participate in regulating RyR gating through calsequestrin (CSQ), the SR lumenal Ca(2+) buffer. We investigate how SR Ca(2+) release via RyR is regulated by Ca(2+) and calsequestrin (CSQ). First, a mathematical model of RyR kinetics is derived based on experimental evidence. We assume that the RyR has three binding sites, two cytosolic sites for Ca(2+) activation and inactivation, and one SR lumenal site for CSQ binding. The open probability (P(o)) of the RyR is found by simulation under controlled cytosolic and SR lumenal Ca(2+). Both peak and steady-state P(o) effectively increase as SR lumenal Ca(2+) increases. Second, we incorporate the RyR model into a CICR model that has both a diadic space and the junctional SR (jSR). At low jSR Ca(2+) loads, CSQs are more likely to bind with the RyR and act to inhibit jSR Ca(2+) release, while at high SR loads CSQs are more likely to detach from the RyR, thereby increasing jSR Ca(2+) release. Furthermore, this CICR model produces a nonlinear relationship between fractional jSR Ca(2+) release and jSR load. These findings agree with experimental observations in lipid bilayers and cardiac myocytes.
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Affiliation(s)
- Young-Seon Lee
- Department of Biomedical Sciences, Cornell University, Ithaca, NY 14853, USA
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Abstract
The sarcoplasmic reticulum (SR) in ventricular myocytes contains releasable Ca(2+) for activating cellular contraction. Recent measurements of intra-SR (luminal) Ca(2+) suggest a high diffusive Ca(2+)-mobility constant (D(CaSR)). This could help spatially to unify SR Ca(2+)-content ([Ca(2+)](SRT)) and standardize Ca(2+)-release throughout the cell. But measurements of localized depletions of luminal Ca(2+) (Ca(2+)-blinks), associated with local Ca(2+)-release (Ca(2+)-sparks), suggest D(CaSR) may actually be low. Here we describe a novel method for measuring D(CaSR). Using a cytoplasmic Ca(2+)-fluorophore, we estimate regional [Ca(2+)](SRT) from localized, caffeine-induced SR Ca(2+)-release. Caffeine microperfusion of one end of a guinea pig or rat myocyte diffusively empties the whole SR at a rate indicating D(CaSR) is 8-9 microm(2)/s, up to tenfold lower than previous estimates. Ignoring background SR Ca(2+)-leakage in our measurement protocol produces an artifactually high D(CaSR) (>40 microm(2)/s), which may also explain the previous high values. Diffusion-reaction modeling suggests that a low D(CaSR) would be sufficient to support local SR Ca(2+)-signaling within sarcomeres during excitation-contraction coupling. Low D(CaSR) also implies that [Ca(2+)](SRT) may readily become spatially nonuniform, particularly under pathological conditions of spatially nonuniform Ca(2+)-release. Local control of luminal Ca(2+), imposed by low D(CaSR), may complement the well-established local control of SR Ca(2+)-release by Ca(2+)-channel/ryanodine receptor couplons.
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Smith G. Matters of the heart: the physiology of cardiac function and failure. Exp Physiol 2007. [DOI: 10.1113/expphysiol.2006.034314] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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MacQuaide N, Dempster J, Smith GL. Measurement and modeling of Ca2+ waves in isolated rabbit ventricular cardiomyocytes. Biophys J 2007; 93:2581-95. [PMID: 17545234 PMCID: PMC1965444 DOI: 10.1529/biophysj.106.102293] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The time course and magnitude of the Ca(2+) fluxes underlying spontaneous Ca(2+) waves in single permeabilized ventricular cardiomyocytes were derived from confocal Fluo-5F fluorescence signals. Peak flux rates via the sarcoplasmic reticulum (SR) release channel (RyR2) and the SR Ca(2+) ATPase (SERCA) were not constant across a range of cellular [Ca(2+)] values. The Ca(2+) affinity (K(mf)) and maximum turnover rate (V(max)) of SERCA and the peak permeability of the RyR2-mediated Ca(2+) release pathway increased at higher cellular [Ca(2+)] loads. This information was used to create a computational model of the Ca(2+) wave, which predicted the time course and frequency dependence of Ca(2+) waves over a range of cellular Ca(2+) loads. Incubation of cardiomyocytes with the Ca(2+) calmodulin (CaM) kinase inhibitor autocamtide-2-related inhibitory peptide (300 nM, 30 mins) significantly reduced the frequency of the Ca(2+) waves at high Ca(2+) loads. Analysis of the Ca(2+) fluxes suggests that inhibition of CaM kinase prevented the increases in SERCA V(max) and peak RyR2 release flux observed at high cellular [Ca(2+)]. These data support the view that modification of activity of SERCA and RyR2 via a CaM kinase sensitive process occurs at higher cellular Ca(2+) loads to increase the maximum frequency of spontaneous Ca(2+) waves.
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Affiliation(s)
- N MacQuaide
- Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, United Kingdom
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Hiranandani N, Raman S, Kalyanasundaram A, Periasamy M, Janssen PML. Frequency-dependent contractile strength in mice over- and underexpressing the sarco(endo)plasmic reticulum calcium-ATPase. Am J Physiol Regul Integr Comp Physiol 2007; 293:R30-6. [PMID: 17255213 DOI: 10.1152/ajpregu.00508.2006] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
One of the prominent markers of end-stage heart failure at the molecular level is a decrease in function and/or expression of the sarcoplasmic reticulum ATPase protein [sarco(endo)plasmic reticulum calcium-ATPase, SERCA]. It has been often postulated that a decrease in SERCA pump activity can contribute in a major way to decreased cardiac function. To establish a functional relationship, we assessed how alterations in SERCA activity level affect basic contractile function in healthy myocardium devoid of other significant molecular changes. We investigated baseline contractile function, frequency-dependent activation, and beta-adrenergic response in ultrathin trabeculae isolated from hearts of mice overexpressing SERCA (transgenic, TG), underexpressing SERCA2a (heterozygous knockout, Het), and their respective wild-type (WT) littermates. At physiological temperature and frequency, compared with their respective WT littermates, SERCA1a mice displayed increased developed force at frequencies of 4-8 Hz ( approximately 90% increase at 4 Hz) and force equal to WT mice at 10-14 Hz. Force development at 4 Hz in presence of 1 muM isoproterenol was similar in TG and WT mice. In Het mice, developed force was nearly identical at the lower end of the frequency range (4-8 Hz) but slightly depressed at higher frequency (P < 0.05 at 14 Hz). In presence of 1 muM isoproterenol, developed force at 4 Hz was equal to that in WT mice. Compared with normal levels, increased SERCA activity enhanced force development only at subphysiological frequencies. A reduction in SERCA activity only showed a depression of force at the higher frequency range. Thus generalizations regarding the correlation between SERCA activity and contractility can be highly ambiguous, because this relationship is critically dependent on other factors including stimulation frequency.
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Affiliation(s)
- Nitisha Hiranandani
- Department of Physiology and Cell Biology, Ohio State University, 1645 Neil Avenue, Columbus, OH 43210-1218, USA
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Suzuki M, Tseeb V, Oyama K, Ishiwata S. Microscopic detection of thermogenesis in a single HeLa cell. Biophys J 2007; 92:L46-8. [PMID: 17237208 PMCID: PMC1861787 DOI: 10.1529/biophysj.106.098673] [Citation(s) in RCA: 90] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We report here the technique for detection and measurement of the temperature changes in single cells using a recently devised microthermometer (a glass micropipette filled with the thermosensitive fluorescent dye Europium (III) thenoyltrifluoroacetonate trihydrate). We found that the heat production in a single HeLa cell occurred with some time delay after the ionomycin-induced Ca(2+) influx from the extracellular space. The time delay inversely depended on extracellular [Ca(2+)], and the increase in temperature was suppressed when Ca(2+)-ATPases were blocked by thapsigargin. These observations strongly suggest that the enzymatic activity of Ca(2+)-ATPases in endoplasmic reticulum leads to the heat production. This study has therefore paved the way for studying the thermogenesis at the single-cell level.
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Ríos E, Launikonis BS, Royer L, Brum G, Zhou J. The elusive role of store depletion in the control of intracellular calcium release. J Muscle Res Cell Motil 2006; 27:337-50. [PMID: 16933025 DOI: 10.1007/s10974-006-9082-5] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/22/2006] [Accepted: 06/26/2006] [Indexed: 10/24/2022]
Abstract
The contractile cycle of striated muscles, skeletal and cardiac, is controlled by a cytosolic [Ca2+] transient that requires rapid movements of the ion through channels in the sarcoplasmic reticulum (SR). A functional signature of these channels is their closure after a stereotyped time lapse of Ca2+ release. In cardiac muscle there is abundant evidence that termination of release is mediated by depletion of the Ca2+ store, even if the linkage mechanism remains unknown. By contrast, in skeletal muscle the mechanisms of release termination are not understood. This article reviews measurements of store depletion, the experimental evidence for dependence of Ca2+ release on the [Ca2+] level inside the SR, as well as tests of the molecular nature of putative intra-store Ca2+ sensors. Because Ca2+ sparks exhibit the basic release termination mechanism, much attention is dedicated to the studies of store depletion caused by sparks and its relationship with termination of sparks. The review notes the striking differences in volume, content and buffering power of the stores in cardiac vs. skeletal muscle, differences that explain why functional depletion is much greater for cardiac than skeletal muscle stores. Because in skeletal muscle store depletion is minimal and reduction in store [Ca2+] does not appear to greatly inhibit Ca2+ release, it is concluded that decrease in free SR [Ca2+] does not mediate physiological termination of Ca2+ release in this type of muscle. In spite of the apparent absence of store depletion and its putative channel closing effect, termination of Ca2+ sparks is faster and more robust in skeletal than cardiac muscle. A gating role of a hypothetical "proximate store" constituted by polymers of calsequestrin and associated proteins is invoked in an attempt to preserve a role for store depletion and unify mechanisms in both types of striated muscle.
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Affiliation(s)
- E Ríos
- Section of Cellular Signaling, Department of Molecular Biophysics and Physiology, Rush University School of Medicine, Chicago, IL 60612, USA.
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Kurata Y, Hisatome I, Matsuda H, Shibamoto T. Dynamical mechanisms of pacemaker generation in IK1-downregulated human ventricular myocytes: insights from bifurcation analyses of a mathematical model. Biophys J 2005; 89:2865-87. [PMID: 16040746 PMCID: PMC1366784 DOI: 10.1529/biophysj.105.060830] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2005] [Accepted: 07/15/2005] [Indexed: 11/18/2022] Open
Abstract
Dynamical mechanisms of the biological pacemaker (BP) generation in human ventricular myocytes were investigated by bifurcation analyses of a mathematical model. Equilibrium points (EPs), periodic orbits, stability of EPs, and bifurcation points were determined as functions of bifurcation parameters, such as the maximum conductance of inward-rectifier K+ current (I(K1)), for constructing bifurcation diagrams. Stable limit cycles (BP activity) abruptly appeared around an unstable EP via a saddle-node bifurcation when I(K1) was suppressed by 84.6%. After the bifurcation at which a stable EP disappears, the I(K1)-reduced system has an unstable EP only, which is essentially important for stable pacemaking. To elucidate how individual sarcolemmal currents contribute to EP instability and BP generation, we further explored the bifurcation structures of the system during changes in L-type Ca2+ channel current (I(Ca,L)), delayed-rectifier K+ currents (I(K)), or Na(+)/Ca2+ exchanger current (I(NaCa)). Our results suggest that 1), I(Ca,L) is, but I(K) or I(NaCa) is not, responsible for EP instability as a requisite to stable BP generation; 2), I(K) is indispensable for robust pacemaking with large amplitude, high upstroke velocity, and stable frequency; and 3), I(NaCa) is the dominant pacemaker current but is not necessarily required for the generation of spontaneous oscillations.
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Affiliation(s)
- Yasutaka Kurata
- Department of Physiology, Kanazawa Medical University, Ishikawa 920-0293, Japan.
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Shannon TR, Wang F, Bers DM. Regulation of cardiac sarcoplasmic reticulum Ca release by luminal [Ca] and altered gating assessed with a mathematical model. Biophys J 2005; 89:4096-110. [PMID: 16169970 PMCID: PMC1366975 DOI: 10.1529/biophysj.105.068734] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cardiac excitation-contraction coupling is initialized by the release of Ca from the sarcoplasmic reticulum (SR) in response to a sudden increase in local cytosolic [Ca] ([Ca]i) within the junctional cleft. We have tested the hypothesis that functional ryanodine receptor (RyR) regulation plays a major role in the regulation of myocyte Ca. A mathematical model with unique characteristics was used to simulate Ca homeostasis. Specifically, the model was designed to accurately represent the SR [Ca]-dependence of release from a variety of experimentally produced data sets. The simulated data for altered RyR Ca sensitivity demonstrated a regulatory feedback loop that resulted in the same release at lower [Ca]SR. This suggests that the primary role of myocyte RyR regulation may be to decrease SR [Ca] without decreasing the size of the [Ca]i transient. The model results suggest that this action moderates the increased SR [Ca] observed with adrenergic stimulation and may keep the [Ca]SR below the threshold for delayed afterdepolarizations and arrhythmia. However, increased Ca affinity of the RyR increased the probability of delayed afterdepolarizations when heart failure was simulated. We conclude that RyR regulation may play a role in preventing arrhythmias in healthy myocytes but that the same regulation may have the opposite effect in chronic heart failure.
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Affiliation(s)
- Thomas R Shannon
- Department of Molecular Biophysics and Physiology, Rush University, Chicago, Illinois 60612, USA.
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Teucher N, Prestle J, Seidler T, Currie S, Elliott EB, Reynolds DF, Schott P, Wagner S, Kogler H, Inesi G, Bers DM, Hasenfuss G, Smith GL. Excessive sarcoplasmic/endoplasmic reticulum Ca2+-ATPase expression causes increased sarcoplasmic reticulum Ca2+ uptake but decreases myocyte shortening. Circulation 2004; 110:3553-9. [PMID: 15505097 DOI: 10.1161/01.cir.0000145161.48545.b3] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
BACKGROUND Increasing sarcoplasmic/endoplasmic reticulum (SR) Ca2+-ATPase (SERCA) uptake activity is a promising therapeutic approach for heart failure. We investigated the effects of different levels of SERCA1a expression on contractility and Ca2+ cycling. We tested whether increased SERCA1a expression levels enhance myocyte contractility in a gene-dose-dependent manner. METHODS AND RESULTS Rabbit isolated cardiomyocytes were transfected at different multiplicities of infection (MOIs) with adenoviruses encoding SERCA1a (or beta-galactosidase as control). Myocyte relaxation half-time was decreased by 10% (P=0.052) at SERCA1a MOI 10 and by 28% at MOI 50 (P<0.05). Myocyte fractional shortening was increased by 12% at MOI 10 (P<0.05) but surprisingly decreased at MOI 50 (-22%, P<0.05) versus control. SR Ca2+ uptake (in permeabilized myocytes) demonstrated a gene-dose-dependent decrease in K(m) by 29% and 46% and an increase in Vmax by 37% and 72% at MOI 10 and MOI 50, respectively (all P<0.05 versus control). Ca2+ transient amplitude was increased in Ad-SERCA1a-infected myocytes at MOI 10 (by 121%, P<0.05), but at MOI 50, the Ca2+ transient amplitude was not significantly changed. Caffeine-induced Ca2+ transients indicated significantly increased SR Ca2+ content in Ad-SERCA1a-infected cells, by 72% at MOI 10 and by 87% at MOI 50. Mathematical simulations demonstrate that the functional increase in SR Ca2+-ATPase uptake activity at MOI 50 (and increased cytosolic Ca2+ buffering) is sufficient to curtail the Ca2+ transient amplitude and explain the reduced contraction. CONCLUSIONS Moderate SERCA1a gene transfer and expression improve contractility and Ca(2+) cycling. However, higher SERCA1a expression levels can impair myocyte shortening because of higher SERCA activity and Ca2+ buffering.
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Affiliation(s)
- Nils Teucher
- Department of Cardiology and Pneumology, University of Goettingen, Goettingen, Germany
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Zeeb V, Suzuki M, Ishiwata S. A novel method of thermal activation and temperature measurement in the microscopic region around single living cells. J Neurosci Methods 2004; 139:69-77. [PMID: 15351523 DOI: 10.1016/j.jneumeth.2004.04.010] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/11/2004] [Revised: 04/13/2004] [Accepted: 04/16/2004] [Indexed: 11/30/2022]
Abstract
We present a simple approach to bring fast and reversible temperature steps of a wide range of amplitudes from the temperature of the experimental chamber up to the boiling point of water in a desired position, with rise and fall times of around 10 ms in a microvolume of microm in size, such as in a single cell. For this purpose, we applied a technique for illuminating a metal aggregate (1-2 microm in diameter) placed at the tip of a glass micropipette with a focused infrared (1064 nm) laser beam under an optical microscope. Stable temperature gradients were created around the metal aggregate using an appropriate neutral density filter set for the laser output. To monitor the local temperature, we devised a new microthermometer composed of the tip of a micropipette filled with thermosensitive fluorescent dye Europium-TTA possessing steep temperature-dependent phosphorescence upon 365 nm excitation. The microm size of the tip of this pipette was able to measure the local temperature with 0.1 degrees C precision and microm spatial resolution. This new approach is compatible with standard electrophysiological and imaging techniques.
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Affiliation(s)
- Vadim Zeeb
- Department of Physics, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
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Shannon TR, Wang F, Puglisi J, Weber C, Bers DM. A mathematical treatment of integrated Ca dynamics within the ventricular myocyte. Biophys J 2004; 87:3351-71. [PMID: 15347581 PMCID: PMC1304803 DOI: 10.1529/biophysj.104.047449] [Citation(s) in RCA: 410] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We have developed a detailed mathematical model for Ca2+ handling and ionic currents in the rabbit ventricular myocyte. The objective was to develop a model that: 1), accurately reflects Ca-dependent Ca release; 2), uses realistic parameters, particularly those that concern Ca transport from the cytosol; 3), comes to steady state; 4), simulates basic excitation-contraction coupling phenomena; and 5), runs on a normal desktop computer. The model includes the following novel features: 1), the addition of a subsarcolemmal compartment to the other two commonly formulated cytosolic compartments (junctional and bulk) because ion channels in the membrane sense ion concentrations that differ from bulk; 2), the use of realistic cytosolic Ca buffering parameters; 3), a reversible sarcoplasmic reticulum (SR) Ca pump; 4), a scheme for Na-Ca exchange transport that is [Na]i dependent and allosterically regulated by [Ca]i; and 5), a practical model of SR Ca release including both inactivation/adaptation and SR Ca load dependence. The data describe normal electrical activity and Ca handling characteristics of the cardiac myocyte and the SR Ca load dependence of these processes. The model includes a realistic balance of Ca removal mechanisms (e.g., SR Ca pump versus Na-Ca exchange), and the phenomena of rest decay and frequency-dependent inotropy. A particular emphasis is placed upon reproducing the nonlinear dependence of gain and fractional SR Ca release upon SR Ca load. We conclude that this model is more robust than many previously existing models and reproduces many experimental results using parameters based largely on experimental measurements in myocytes.
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Affiliation(s)
- Thomas R Shannon
- Department of Molecular Biophysics and Physiology, Rush University, Chicago, Illinois, USA
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