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Ferrero JM, Gonzalez-Ascaso A, Matas JFR. The mechanisms of potassium loss in acute myocardial ischemia: New insights from computational simulations. Front Physiol 2023; 14:1074160. [PMID: 36923288 PMCID: PMC10009276 DOI: 10.3389/fphys.2023.1074160] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2022] [Accepted: 02/13/2023] [Indexed: 03/03/2023] Open
Abstract
Acute myocardial ischemia induces hyperkalemia (accumulation of extracellular potassium), a major perpetrator of lethal reentrant ventricular arrhythmias. Despite considerable experimental efforts to explain this pathology in the last decades, the intimate mechanisms behind hyperkalemia remain partially unknown. In order to investigate these mechanisms, we developed a novel computational model of acute myocardial ischemia which couples a) an electrophysiologically detailed human cardiomyocyte model that incorporates modifications to account for ischemia-induced changes in transmembrane currents, with b) a model of cardiac tissue and extracellular K + transport. The resulting model is able to reproduce and explain the triphasic time course of extracellular K + concentration within the ischemic zone, with values of [ K + ] o close to 14 mmol/L in the central ischemic zone after 30 min. In addition, the formation of a [ K + ] o border zone of approximately 1.2 cm 15 min after the onset of ischemia is predicted by the model. Our results indicate that the primary rising phase of [ K + ] o is mainly due to the imbalance between K + efflux, that increases slightly, and K + influx, that follows a reduction of the NaK pump activity by more than 50%. The onset of the plateau phase is caused by the appearance of electrical alternans (a novel mechanism identified by the model), which cause an abrupt reduction in the K + efflux. After the plateau, the secondary rising phase of [ K + ] o is caused by a subsequent imbalance between the K + influx, which continues to decrease slowly, and the K + efflux, which remains almost constant. Further, the study shows that the modulation of these mechanisms by the electrotonic coupling is the main responsible for the formation of the ischemic border zone in tissue, with K + transport playing only a minor role. Finally, the results of the model indicate that the injury current established between the healthy and the altered tissue is not sufficient to depolarize non-ischemic cells within the healthy tissue.
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Affiliation(s)
- Jose M Ferrero
- Centro de Investigacion e Innovacion en Bioingenieria, Universitat Politecnica de Valencia, Valencia, Spain
| | - Ana Gonzalez-Ascaso
- Centro de Investigacion e Innovacion en Bioingenieria, Universitat Politecnica de Valencia, Valencia, Spain.,Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta", Politecnico di Milano, Milan, Italy
| | - Jose F Rodriguez Matas
- Dipartimento di Chimica, Materiali e Ingegneria Chimica "Giulio Natta", Politecnico di Milano, Milan, Italy
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2
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Analysis of vulnerability to reentry in acute myocardial ischemia using a realistic human heart model. Comput Biol Med 2021; 141:105038. [PMID: 34836624 DOI: 10.1016/j.compbiomed.2021.105038] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2021] [Revised: 10/25/2021] [Accepted: 11/12/2021] [Indexed: 11/21/2022]
Abstract
Electrophysiological alterations of the myocardium caused by acute ischemia constitute a pro-arrhythmic substrate for the generation of potentially lethal arrhythmias. Experimental evidence has shown that the main components of acute ischemia that induce these electrophysiological alterations are hyperkalemia, hypoxia (or anoxia in complete artery occlusion), and acidosis. However, the influence of each ischemic component on the likelihood of reentry is not completely established. Moreover, the role of the His-Purkinje system (HPS) in the initiation and maintenance of arrhythmias is not completely understood. In the present work, we investigate how the three components of ischemia affect the vulnerable window (VW) for reentry using computational simulations. In addition, we analyze the role of the HPS on arrhythmogenesis. A 3D biventricular/torso human model that includes a realistic geometry of the central and border ischemic zones with one of the most electrophysiologically detailed model of ischemia to date, as well as a realistic cardiac conduction system, were used to assess the VW for reentry. Four scenarios of ischemic severity corresponding to different minutes after coronary artery occlusion were simulated. Our results suggest that ischemic severity plays an important role in the generation of reentries. Indeed, this is the first 3D simulation study to show that ventricular arrhythmias could be generated under moderate ischemic conditions, but not in mild and severe ischemia. Moreover, our results show that anoxia is the ischemic component with the most significant effect on the width of the VW. Thus, a change in the level of anoxia from moderate to severe leads to a greater increment in the VW (40 ms), in comparison with the increment of 20 ms and 35 ms produced by the individual change in the level of hyperkalemia and acidosis, respectively. Finally, the HPS was a necessary element for the generation of approximately 17% of reentries obtained. The retrograde conduction from the myocardium to HPS in the ischemic region, conduction blocks in discrete sections of the HPS, and the degree of ischemia affecting Purkinje cells, are suggested as mechanisms that favor the generation of ventricular arrhythmias.
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Espinoza-Fonseca LM. The Ca 2+-ATPase pump facilitates bidirectional proton transport across the sarco/endoplasmic reticulum. MOLECULAR BIOSYSTEMS 2017; 13:633-637. [PMID: 28290590 DOI: 10.1039/c7mb00065k] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/11/2023]
Abstract
Ca2+ transport across the sarco/endoplasmic reticulum (SR) plays an essential role in intracellular Ca2+ homeostasis, signalling, cell differentiation and muscle contractility. During SR Ca2+ uptake and release, proton fluxes are required to balance the charge deficit generated by the exchange of Ca2+ and other ions across the SR. During Ca2+ uptake by the SR Ca2+-ATPase (SERCA), two protons are countertransported from the SR lumen to the cytosol, thus partially compensating for the charge moved by Ca2+ transport. Studies have shown that protons are also transported from the cytosol to the lumen during Ca2+ release, but a transporter that facilitates proton transport into the SR lumen has not been described. In this article we propose that SERCA forms pores that facilitate bidirectional proton transport across the SR. We describe the location and structure of water-filled pores in SERCA that form cytosolic and luminal pathways for protons to cross the SR membrane. Based on this structural information, we suggest mechanistic models for proton translocation to the cytosol during active Ca2+ transport, and into the SR lumen during SERCA inhibition by endogenous regulatory proteins. Finally, we discuss the physiological consequences of SERCA-mediated bidirectional proton transport across the SR membrane of muscle and non-muscle cells.
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Affiliation(s)
- L Michel Espinoza-Fonseca
- Department of Biochemistry, Molecular Biology and Biophysics, University of Minnesota, Minneapolis, MN 55455, USA.
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4
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Saegusa N, Moorhouse E, Vaughan-Jones RD, Spitzer KW. Influence of pH on Ca²⁺ current and its control of electrical and Ca²⁺ signaling in ventricular myocytes. ACTA ACUST UNITED AC 2012; 138:537-59. [PMID: 22042988 PMCID: PMC3206307 DOI: 10.1085/jgp.201110658] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
Modulation of L-type Ca(2+) current (I(Ca,L)) by H(+) ions in cardiac myocytes is controversial, with widely discrepant responses reported. The pH sensitivity of I(Ca,L) was investigated (whole cell voltage clamp) while measuring intracellular Ca(2+) (Ca(2+)(i)) or pH(i) (epifluorescence microscopy) in rabbit and guinea pig ventricular myocytes. Selectively reducing extracellular or intracellular pH (pH(o) 6.5 and pH(i) 6.7) had opposite effects on I(Ca,L) gating, shifting the steady-state activation and inactivation curves to the right and left, respectively, along the voltage axis. At low pH(o), this decreased I(Ca,L), whereas at low pH(i), it increased I(Ca,L) at clamp potentials negative to 0 mV, although the current decreased at more positive potentials. When Ca(2+)(i) was buffered with BAPTA, the stimulatory effect of low pH(i) was even more marked, with essentially no inhibition. We conclude that extracellular H(+) ions inhibit whereas intracellular H(+) ions can stimulate I(Ca,L). Low pH(i) and pH(o) effects on I(Ca,L) were additive, tending to cancel when appropriately combined. They persisted after inhibition of calmodulin kinase II (with KN-93). Effects are consistent with H(+) ion screening of fixed negative charge at the sarcolemma, with additional channel block by H(+)(o) and Ca(2+)(i). Action potential duration (APD) was also strongly H(+) sensitive, being shortened by low pH(o), but lengthened by low pH(i), caused mainly by H(+)-induced changes in late Ca(2+) entry through the L-type Ca(2+) channel. Kinetic analyses of pH-sensitive channel gating, when combined with whole cell modeling, successfully predicted the APD changes, plus many of the accompanying changes in Ca(2+) signaling. We conclude that the pH(i)-versus-pH(o) control of I(Ca,L) will exert a major influence on electrical and Ca(2+)-dependent signaling during acid-base disturbances in the heart.
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Affiliation(s)
- Noriko Saegusa
- Department of Physiology, Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah, Salt Lake City, UT 84112, USA
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Swift F, Tovsrud N, Sjaastad I, Sejersted OM, Niggli E, Egger M. Functional coupling of α2-isoform Na+/K+-ATPase and Ca2+ extrusion through the Na+/Ca2+-exchanger in cardiomyocytes. Cell Calcium 2010; 48:54-60. [DOI: 10.1016/j.ceca.2010.06.006] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/17/2010] [Revised: 05/10/2010] [Accepted: 06/30/2010] [Indexed: 10/19/2022]
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Gusev K, Domenighetti AA, Delbridge LM, Pedrazzini T, Niggli E, Egger M. Angiotensin II–Mediated Adaptive and Maladaptive Remodeling of Cardiomyocyte Excitation–Contraction Coupling. Circ Res 2009; 105:42-50. [DOI: 10.1161/circresaha.108.189779] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Cardiac hypertrophy is associated with alterations in cardiomyocyte excitation–contraction coupling (ECC) and Ca
2+
handling. Chronic elevation of plasma angiotensin II (Ang II) is a major determinant in the pathogenesis of cardiac hypertrophy and congestive heart failure. However, the molecular mechanisms by which the direct actions of Ang II on cardiomyocytes contribute to ECC remodeling are not precisely known. This question was addressed using cardiac myocytes isolated from transgenic (TG1306/1R [TG]) mice exhibiting cardiac specific overexpression of angiotensinogen, which develop Ang II–mediated cardiac hypertrophy in the absence of hemodynamic overload. Electrophysiological techniques, photolysis of caged Ca
2+
and confocal Ca
2+
imaging were used to examine ECC remodeling at early (≈20 weeks of age) and late (≈60 weeks of age) time points during the development of cardiac dysfunction. In young TG mice, increased cardiac Ang II levels induced a hypertrophic response in cardiomyocyte, which was accompanied by an adaptive change of Ca
2+
signaling, specifically an upregulation of the Na
+
/Ca
2+
exchanger–mediated Ca
2+
transport. In contrast, maladaptation was evident in older TG mice, as suggested by reduced sarcoplasmic reticulum Ca
2+
content resulting from a shift in the ratio of plasmalemmal Ca
2+
removal and sarcoplasmic reticulum Ca
2+
uptake. This was associated with a conserved ECC gain, consistent with a state of hypersensitivity in Ca
2+
-induced Ca
2+
release. Together, our data suggest that chronic elevation of cardiac Ang II levels significantly alters cardiomyocyte ECC in the long term, and thereby contractility, independently of hemodynamic overload and arterial hypertension.
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Affiliation(s)
- Konstantin Gusev
- From the Department of Physiology (K.G., E.N., M.E.), University of Bern, Switzerland; Department of Medicine (A.A.D., T.P.), University of Lausanne, Centre Hospitalier Universitaire Vaudois, Switzerland; and Department of Physiology (L.M.D.D.), University of Melbourne, Australia. Present address for A.A.D.: Department of Medicine, University of California at San Diego, La Jolla
| | - Andrea A. Domenighetti
- From the Department of Physiology (K.G., E.N., M.E.), University of Bern, Switzerland; Department of Medicine (A.A.D., T.P.), University of Lausanne, Centre Hospitalier Universitaire Vaudois, Switzerland; and Department of Physiology (L.M.D.D.), University of Melbourne, Australia. Present address for A.A.D.: Department of Medicine, University of California at San Diego, La Jolla
| | - Lea M.D. Delbridge
- From the Department of Physiology (K.G., E.N., M.E.), University of Bern, Switzerland; Department of Medicine (A.A.D., T.P.), University of Lausanne, Centre Hospitalier Universitaire Vaudois, Switzerland; and Department of Physiology (L.M.D.D.), University of Melbourne, Australia. Present address for A.A.D.: Department of Medicine, University of California at San Diego, La Jolla
| | - Thierry Pedrazzini
- From the Department of Physiology (K.G., E.N., M.E.), University of Bern, Switzerland; Department of Medicine (A.A.D., T.P.), University of Lausanne, Centre Hospitalier Universitaire Vaudois, Switzerland; and Department of Physiology (L.M.D.D.), University of Melbourne, Australia. Present address for A.A.D.: Department of Medicine, University of California at San Diego, La Jolla
| | - Ernst Niggli
- From the Department of Physiology (K.G., E.N., M.E.), University of Bern, Switzerland; Department of Medicine (A.A.D., T.P.), University of Lausanne, Centre Hospitalier Universitaire Vaudois, Switzerland; and Department of Physiology (L.M.D.D.), University of Melbourne, Australia. Present address for A.A.D.: Department of Medicine, University of California at San Diego, La Jolla
| | - Marcel Egger
- From the Department of Physiology (K.G., E.N., M.E.), University of Bern, Switzerland; Department of Medicine (A.A.D., T.P.), University of Lausanne, Centre Hospitalier Universitaire Vaudois, Switzerland; and Department of Physiology (L.M.D.D.), University of Melbourne, Australia. Present address for A.A.D.: Department of Medicine, University of California at San Diego, La Jolla
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7
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Qin Y, Thomas D, Fontaine CP, Colvin RA. Mechanisms of Zn2+efflux in cultured cortical neurons. J Neurochem 2008; 107:1304-13. [DOI: 10.1111/j.1471-4159.2008.05700.x] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
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8
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Abstract
Calcium (Ca) is a universal intracellular second messenger. In muscle, Ca is best known for its role in contractile activation. However, in recent years the critical role of Ca in other myocyte processes has become increasingly clear. This review focuses on Ca signaling in cardiac myocytes as pertaining to electrophysiology (including action potentials and arrhythmias), excitation-contraction coupling, modulation of contractile function, energy supply-demand balance (including mitochondrial function), cell death, and transcription regulation. Importantly, although such diverse Ca-dependent regulations occur simultaneously in a cell, the cell can distinguish distinct signals by local Ca or protein complexes and differential Ca signal integration.
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Affiliation(s)
- Donald M Bers
- Department of Physiology and Cardiovascular Institute, Stritch School of Medicine, Loyola University Chicago, Maywood, IL 60153, USA.
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9
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The role of the Na+/Ca2+ exchangers in Ca2+ dynamics in ventricular myocytes. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2008; 96:377-98. [DOI: 10.1016/j.pbiomolbio.2007.07.018] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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10
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Török TL. Electrogenic Na+/Ca2+-exchange of nerve and muscle cells. Prog Neurobiol 2007; 82:287-347. [PMID: 17673353 DOI: 10.1016/j.pneurobio.2007.06.003] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/15/2006] [Revised: 04/12/2007] [Accepted: 06/12/2007] [Indexed: 12/19/2022]
Abstract
The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.
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Affiliation(s)
- Tamás L Török
- Department of Pharmacodynamics, Semmelweis University, P.O. Box 370, VIII. Nagyvárad-tér 4, H-1445 Budapest, Hungary.
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11
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Posada V, Beaugé L, Berberián G. Maximal Ca2+i stimulation of cardiac Na+/Ca2+ exchange requires simultaneous alkalinization and binding of PtdIns-4,5-P2 to the exchanger. Biol Chem 2007; 388:281-8. [PMID: 17338635 DOI: 10.1515/bc.2007.031] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
Using bovine heart sarcolemma vesicles we studied the effects of protons and phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2) on the affinity of the mammalian Na(+)/Ca(2+) exchanger (NCX1) for intracellular Ca(2+). By following the effects of extravesicular ligands in inside-out vesicles, their interactions with sites of NCX1 facing the intracellular medium were investigated. Two Na(+)-gradient-dependent fluxes were studied: Ca(2+) uptake and Ca(2+) release. PtdIns-4,5-P2 binding to NCX1 was investigated in parallel. Without MgATP (no 'de novo' synthesis of PtdIns-4,5-P2), alkalinization increased the affinity for Ca(2+) and the PtdIns-4,5-P2 bound to NCX1. Vesicles depleted of phosphoinositides were insensitive to alkalinization, but became responsive following addition of exogenous PtdIns-4,5-P2 or PtdIns plus MgATP. Acidification reduced the affinity for Ca(2+)(ev); this was only partially reversed by MgATP, despite the increase in bound PtdIns-4,5-P2 to levels observed with alkalinization. Inhibition of Ca(2+) uptake by increasing extravesicular [Na(+)] indicates that it is related to H(+)(i) and Na(+)(i) synergistic inhibition of the Ca(2+)(i) regulatory site. Therefore, the affinity of the NCX1 Ca(2+)(i) regulatory site for Ca(2+) was maximal when both intracellular alkalinization and an increase in PtdIns-4,5-P2 bound to NCX1 (not just of the total membrane PtdIns-4,5-P2) occurred simultaneously. In addition, protons influenced the distribution, or the exposure, of PtdIns-4,5-P2 molecules in the surroundings and/or on the exchanger protein.
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Affiliation(s)
- Velia Posada
- Laboratorio de Biofísica, Instituto de Investigación Médica Mercedes y Martín Ferreyra, Casilla de Correo 389, 5000 Córdoba, Argentina
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Bers DM, Ginsburg KS. Na:Ca Stoichiometry and Cytosolic Ca-Dependent Activation of NCX in Intact Cardiomyocytes. Ann N Y Acad Sci 2007; 1099:326-38. [PMID: 17303827 DOI: 10.1196/annals.1387.060] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
We are studying both Na:Ca exchange stoichiometry and cytosolic [Ca] ([Ca]i)-dependent regulation of Na-Ca exchange (NCX) in intact rabbit ventricular myocytes. Analysis of NCX fluxes in subcellular systems strongly supports a dominant 3Na:1Ca exchange, and our measurements in intact cells confirm this. However, in intact native cells, local ion gradients and other factors complicate the process of inferring stoichiometry. From a functional viewpoint, NCX stoichiometry is near 3:1 but is affected by ion accumulation/depletion as well as non-NCX fluxes. We and others have viewed [Ca]i-dependent NCX regulation as a static process (dependent on instantaneous local [Ca]i). However, evidence from subcellular and expression systems shows the process to be dynamic, and our observations confirm this to be the case in intact cardiac cells as well.
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Affiliation(s)
- Donald M Bers
- Department of Physiology, Loyola University Chicago, Stritch School of Medicine, 2160 South First Avenue, Maywood, IL 60153, USA.
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13
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Du YM, Nathan RD. Ionic basis of ischemia-induced bradycardia in the rabbit sinoatrial node. J Mol Cell Cardiol 2007; 42:315-25. [PMID: 17101146 DOI: 10.1016/j.yjmcc.2006.10.004] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/11/2006] [Revised: 10/03/2006] [Accepted: 10/04/2006] [Indexed: 10/23/2022]
Abstract
To investigate the basis of ischemia-induced bradycardia (<60 beats/min), we isolated pacemaker cells from the rabbit sinoatrial node and exposed them to ischemic-like conditions, including omission of glucose, pH 6.6, and either 5.4 or 10 mM KCl to evaluate the role of increased serum [K]. A perforated-patch technique was employed to test the hypothesis that the arrhythmia is caused by attenuation of inward currents that contribute to the diastolic depolarization. After exposure to "ischemic" Tyrode containing 5.4 mM KCl, the pacemaker cells exhibited 13% slower beat rates and action potentials with 6-mV greater overshoots and 44% longer durations. In contrast, after exposure to "ischemic" Tyrode containing 10 mM KCl, the pacemaker cells exhibited a 7-mV depolarization of the maximum diastolic potential but no significant change in the overshoot. Beat rates were slowed by 43%, and the action potentials were prolonged by 46%. "Ischemic" Tyrode containing 5.4 mM KCl increased L-type Ca current, decreased T-type Ca current and reduced Ni-sensitive inward current tails (presumably Na-Ca exchange current), even after treatment with 40 muM ryanodine to block Ca release from the sarcoplasmic reticulum. "Ischemic" Tyrode containing 10 mM KCl increased hyperpolarization-activated inward current at diastolic potentials and reduced the slowly activating component, but not the rapidly activating component, of delayed rectifier K current. Our results suggest that reductions of inward Na-Ca exchange current and T-type Ca current contribute to "ischemia"-induced "bradycardia" in sinoatrial node pacemaker cells.
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Affiliation(s)
- Yi-Mei Du
- Department of Physiology, Texas Tech University Health Sciences Center, 3601 Fourth Street, Lubbock, TX 79430, USA
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14
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Maltsev VA, Lakatta EG. Cardiac pacemaker cell failure with preserved I(f), I(CaL), and I(Kr): a lesson about pacemaker function learned from ischemia-induced bradycardia. J Mol Cell Cardiol 2006; 42:289-94. [PMID: 17188292 PMCID: PMC1868668 DOI: 10.1016/j.yjmcc.2006.11.009] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/17/2006] [Accepted: 11/17/2006] [Indexed: 10/23/2022]
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15
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DiPolo R, Beaugé L. Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions. Physiol Rev 2006; 86:155-203. [PMID: 16371597 DOI: 10.1152/physrev.00018.2005] [Citation(s) in RCA: 166] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The Na(+)/Ca(2+) exchanger's family of membrane transporters is widely distributed in cells and tissues of the animal kingdom and constitutes one of the most important mechanisms for extruding Ca(2+) from the cell. Two basic properties characterize them. 1) Their activity is not predicted by thermodynamic parameters of classical electrogenic countertransporters (dependence on ionic gradients and membrane potential), but is markedly regulated by transported (Na(+) and Ca(2+)) and nontransported ionic species (protons and other monovalent cations). These modulations take place at specific sites in the exchanger protein located at extra-, intra-, and transmembrane protein domains. 2) Exchange activity is also regulated by the metabolic state of the cell. The mammalian and invertebrate preparations share MgATP in that role; the squid has an additional compound, phosphoarginine. This review emphasizes the interrelationships between ionic and metabolic modulations of Na(+)/Ca(2+) exchange, focusing mainly in two preparations where most of the studies have been carried out: the mammalian heart and the squid giant axon. A surprising fact that emerges when comparing the MgATP-related pathways in these two systems is that although they are different (phosphatidylinositol bisphosphate in the cardiac and a soluble cytosolic regulatory protein in the squid), their final target effects are essentially similar: Na(+)-Ca(2+)-H(+) interactions with the exchanger. A model integrating both ionic and metabolic interactions in the regulation of the exchanger is discussed in detail as well as its relevance in cellular Ca(i)(2+) homeostasis.
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Affiliation(s)
- Reinaldo DiPolo
- Laboratorio de Permebilidad Ionica, Centro de Biofísica y Bioquímica, Instituío Venezolano de Investigaciones Científicas, Caracas 1020A, Venezuela.
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Crampin EJ, Smith NP. A dynamic model of excitation-contraction coupling during acidosis in cardiac ventricular myocytes. Biophys J 2006; 90:3074-90. [PMID: 16473911 PMCID: PMC1432112 DOI: 10.1529/biophysj.105.070557] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Acidosis in cardiac myocytes is a major factor in the reduced inotropy that occurs in the ischemic heart. During acidosis, diastolic calcium concentration and the amplitude of the calcium transient increase, while the strength of contraction decreases. This has been attributed to the inhibition by protons of calcium uptake and release by the sarcoplasmic reticulum, to a rise of intracellular sodium caused by activation of sodium-hydrogen exchange, decreased calcium binding affinity to Troponin-C, and direct effects on the contractile machinery. The relative contributions and concerted action of these effects are, however, difficult to establish experimentally. We have developed a mathematical model to examine altered calcium-handling mechanisms during acidosis. Each of the alterations was incorporated into a dynamical model of pH regulation and excitation-contraction coupling to predict the time courses of key ionic species during acidosis, in particular intracellular pH, sodium and the calcium transient, and contraction. This modeling study suggests that the most significant effects are elevated sodium, inhibition of sodium-calcium exchange, and the direct interaction of protons with the contractile machinery; and shows how the experimental data on these contributions can be reconciled to understand the overall effects of acidosis in the beating heart.
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Affiliation(s)
- Edmund J Crampin
- Bioengineering Institute and Department of Engineering Science, University of Auckland, Auckland, New Zealand.
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Laskay G, Kálmán K, Van Kerkhove E, Steels P, Ameloot M. Store-operated Ca2+-channels are sensitive to changes in extracellular pH. Biochem Biophys Res Commun 2005; 337:571-9. [PMID: 16198307 DOI: 10.1016/j.bbrc.2005.09.086] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2005] [Accepted: 09/09/2005] [Indexed: 10/25/2022]
Abstract
The sensitivity of store-operated Ca(2+)-entry to changes in the extra- and intracellular pH (pH(o) and pH(i), respectively) was investigated in SH-SY5Y human neuroblastoma cells. The intracellular Ca(2+)-stores were depleted either with 1 mM carbachol (CCH) or with 2 microM thapsigargin (TG). Extracellular acidification suppressed both the CCH- and TG-mediated Ca(2+)-entry while external alkalinization augmented both the CCH- and the TG-induced Ca(2+)-influx. Mn(2+)-quenching experiments revealed that the rates of Ca(2+)-entry at the thapsigargin- or carbachol-induced plateau were both accelerated at pH(o) 8.2 and slowed down at pH(o) 6.8 with respect to the control at pH(o) 7.4. Alteration of pH(o) between 6.8 and 8.2 did not have any significant prompt effect on pH(i) and changes in pH(i) left the CCH-induced Ca(2+)-entry unaffected. These findings demonstrate that physiologically relevant changes in pH(o) affect the store-operated Ca(2+)-entry in SH-SY5Y cells and suggest that endogenous pH(o) shifts may regulate cell activity in situ via modulating the store-operated Ca(2+)-entry.
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Affiliation(s)
- G Laskay
- Department of Botany, University of Szeged, Hungary.
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18
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Egger M, Porzig H, Niggli E, Schwaller B. Rapid turnover of the "functional" Na(+)-Ca2+ exchanger in cardiac myocytes revealed by an antisense oligodeoxynucleotide approach. Cell Calcium 2005; 37:233-43. [PMID: 15670870 DOI: 10.1016/j.ceca.2004.10.006] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2004] [Revised: 10/07/2004] [Accepted: 10/12/2004] [Indexed: 11/28/2022]
Abstract
Antisense oligodeoxynucleotides (AS-ODNs) were used in combination with transient functional expression of the cardiac Na(+)-Ca2+ exchanger (NCX1) to correlate suppression of the Na(+)-Ca2+ exchange function with down-regulation of NCX1 protein expression. In a de-novo expression system (Sf9 cells), a decrease in both, NCX1 mRNA and protein after AS-ODN application was paralleled by diminished NCX1 activity, a typical hallmark of a true "antisense effect". Although AS-ODN uptake was also efficient in rat neonatal cardiac myocytes, in whole-cell extracts of these cells treated with AS-ODNs, the amount of NCX1 protein determined in a quantitative binding assay remained almost unchanged, despite a prompt loss of NCX1 function. Immunocytochemical staining of myocytes revealed that most of the immunoreactivity was not localized in the plasma membrane, but in intracellular compartments and was barely affected by AS-ODN treatment. These results indicate that the "functional half-life" of the NCX1 protein in the plasma membrane of neonatal cardiac myocytes is surprisingly short, much shorter than reported half-lifes of about 30 h for other membrane proteins.
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Affiliation(s)
- Marcel Egger
- University of Bern, Department of Physiology, Bühlplatz 5, CH-3012 Bern, Switzerland.
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19
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Keller M, Pignier C, Niggli E, Egger M. Mechanisms of Na+-Ca2+ exchange inhibition by amphiphiles in cardiac myocytes: importance of transbilayer movement. J Membr Biol 2005; 198:159-75. [PMID: 15216417 DOI: 10.1007/s00232-004-0668-9] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2003] [Revised: 03/01/2004] [Indexed: 10/26/2022]
Abstract
The membrane lipid environment and lipid signaling pathways are potentially involved in the modulation of the activity of the cardiac Na(+)-Ca(2+) exchanger (NCX). In the present study biophysical mechanisms of interactions of amphiphiles with the NCX and the functional consequences were examined. For this purpose, intracellular Ca(2+) concentration jumps were generated by laser-flash photolysis of caged Ca(2+) in guinea-pig ventricular myocytes and Na(+)-Ca(2+) exchange currents ( I(Na/Ca)) were recorded in the whole-cell configuration of the patch-clamp technique. The inhibitory effect of amphiphiles increased with the length of the aliphatic chain between C(7) and C(10) and was more potent with cationic or anionic head groups than with uncharged head groups. Long-chain cationic amines (C(12)) exhibited a cut-off in their efficacy in I(Na/Ca) inhibition. Analysis of the time-course, comparison with the Ni(2+)-induced I(Na/Ca) block and confocal laser scanning microscopy experiments with fluorescent lipid analogs (C(6)- and C(12)-NBD-labeled analogs) suggested that amphiphiles need to be incorporated into the membrane. Furthermore, NCX block appears to require transbilayer movement of the amphiphile to the inner leaflet ("flip"). We conclude that both, hydrophobic and electrostatic interactions between the lipids and the NCX may be important factors for the modulation by lipids and could be relevant in cardiac diseases where the lipid metabolism is altered.
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Affiliation(s)
- M Keller
- Department of Physiology, University of Bern, CH-3012, Bern, Switzerland
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20
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Hobai IA, O'Rourke B. The potential of Na+/Ca2+ exchange blockers in the treatment of cardiac disease. Expert Opin Investig Drugs 2004; 13:653-64. [PMID: 15174951 DOI: 10.1517/13543784.13.6.653] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
The Na(+)/Ca(2+) exchanger (NCX), a surface membrane antiporter, is the primary pathway for Ca(2+) efflux from the cardiac cell and a determinant of both the electrical and contractile state of the heart. Enhanced expression of NCX has recently been recognised as one of the molecular mechanisms that contributes to reduced Ca(2+) release, impaired contractility and an increased risk of arrhythmias during the development of cardiac hypertrophy and failure. The NCX has also been implicated in the mechanism of arrhythmias and cellular injury associated with ischaemia and reperfusion. Hence, NCX blockade represents a potential therapeutic strategy for treating cardiac disease, however, its reversibility and electrogenic properties must be taken into consideration when predicting the outcome. NCX inhibition has been demonstrated to be protective against ischaemic injury and to have a positive inotropic and antiarrhythmic effect in failing heart cells. However, progress has been impaired by the absence of clinically useful agents. Two drugs, KB-R7943 and SEA-0400, have been developed as NCX blockers but both lack specificity. Selective peptide inhibitors have been well characterised but are active only when delivered to the intracellular space. Gene therapy strategies may circumvent the latter problem in the future. This review discusses the effects of NCX blockade, supporting its potential as a new cardiovascular therapeutic strategy.
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Affiliation(s)
- Ion A Hobai
- Department of Medicine and Institute of Cardiobiology, Johns Hopkins University, Baltimore, MD 21205, USA
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21
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Abstract
Pancreatic beta-cells maintain glucose homeostasis by their regulated Ca(2+)-dependent secretion of insulin. Several cellular mechanisms control intracellular Ca(2+) levels, but their relative significance in mouse beta-cells is not fully known. We used photometry to measure the dynamics of cytosolic Ca(2+) ([Ca(2+)](i)) clearance after brief, depolarization-induced Ca(2+) entry. Treatment with thapsigargin or cyclopiazonic acid, inhibitors of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) pumps, nearly doubled the peak and slowed the decay of the depolarization-induced Ca(2+) transients. The remaining thapsigargin-insensitive decay was slowed further by inhibition of the plasma membrane Ca(2+)-ATPase (PMCA) and plasma membrane Na(+)/Ca(2+) exchanger (NCX) via alkalization of the bath solution, by adding lanthanum, or by substitution of Na(+) with Li(+). Mitochondrial Ca(2+) uptake contributed little to clearance in thapsigargin-pretreated cells. Together, the SERCA, PMCA, and NCX transport mechanisms accounted for 89 to 97% of clearance in normal solutions. We developed a quantitative model for the dynamic role of removal mechanisms over a wide range of [Ca(2+)](i). According to our model, 50 to 64% of initial Ca(2+) removal is via the SERCA pump, whereas the NCX contributes 21-30% of the extrusion at high [Ca(2+)](i), and the PMCA contributes 21-27% at low [Ca(2+)](i).
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Affiliation(s)
- Liangyi Chen
- Department of Physiology & Biophysics, School of Medicine, University of Washington, Seattle 98195-7290, USA.
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22
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Bers DM. Regulation of Cellular Calcium in Cardiac Myocytes. Compr Physiol 2002. [DOI: 10.1002/cphy.cp020109] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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23
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Abstract
Here, we address three issues in intact ventricular myocytes that specifically relate to the role of Na/Ca exchange (NCX) current under physiological conditions. First, we revisit the issue of NCX stoichiometry in light of some recent findings that the stoichiometry of the NCX may not be fixed at 3Na: 1Ca. We discuss some data that strongly favor the 3:1 stoichiometry, at least under physiological conditions. Second, we address the controversy over the role of allosteric Ca regulation in intact myocytes. We show that outward and inward I(NCX) can be activated dynamically by changing [Ca](i) over the physiological range and that outward I(NCX) can be activated quite rapidly with sarcoplasmic reticulum Ca release. These data are well described using an instantaneous equation for NCX current that includes an allosteric activation factor with K(mCaAct) = 125 nM. Finally, we consider the effect on NCX current of submembrane elevations in [Ca](i) (that are far greater than are measured in the bulk cytoplasm). Taken together with a NCX stoichiometry of 3, these findings have allowed us to make some predictions of the role of I(NCX) during an AP. Our simulations suggest that NCX current is outward for less than approximately 10 ms at the beginning of the action potential.
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Affiliation(s)
- Donald M Bers
- Department of Physiology, Loyola University Chicago, Maywood, Illinois 60153, USA.
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24
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Ginsburg KS, Weber CR, Despa S, Bers DM. Simultaneous measurement of [Na]i, [Ca]i, and I(NCX) in intact cardiac myocytes. Ann N Y Acad Sci 2002; 976:157-8. [PMID: 12502556 DOI: 10.1111/j.1749-6632.2002.tb04736.x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- Kenneth S Ginsburg
- Department of Physiology, Loyola University Chicago, Maywood, Illinois 60153, USA
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25
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Gómez AM, Schwaller B, Porzig H, Vassort G, Niggli E, Egger M. Increased exchange current but normal Ca2+ transport via Na+-Ca2+ exchange during cardiac hypertrophy after myocardial infarction. Circ Res 2002; 91:323-30. [PMID: 12193465 DOI: 10.1161/01.res.0000031384.55006.db] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Hypertrophied and failing cardiac myocytes generally show alterations in intracellular Ca2+ handling associated with changes in the contractile function and arrhythmogenicity. The cardiac Na+-Ca2+ exchange (NCX) is an important mechanism for Ca2+ extrusion and cell relaxation. Its possible involvement in changes of excitation-contraction coupling (EC-coupling) with disease remains uncertain. We analyzed the NCX function in rat ventricular myocytes 5 to 6 months after experimental myocardial infarction (PMI) produced by left coronary artery ligation and from sham-operated (SO) hearts. Caged Ca2+ was dialyzed into the cytoplasm via a patch-clamp pipette and Ca2+ was released by flash photolysis to activate NCX and measure the associated currents (I(NaCa)), whereas [Ca2+]i changes were simultaneously recorded with a confocal microscope. I(NaCa) density normalized to the [Ca2+]i jumps was 2.6-fold higher in myocytes from PMI rats. The level of total NCX protein expression in PMI myocytes was also increased. Interestingly, although the I(NaCa) density in PMI cells was larger, PMI and SO myocytes presented virtually identical Ca2+ transport via the NCX. This discrepancy was explained by a reduced surface/volume ratio (34.8%) observed in PMI cells. We conclude that the increase in NCX density may be a mechanism to maintain the required Ca2+ extrusion from a larger cell to allow adequate relaxation.
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Affiliation(s)
- Ana Maria Gómez
- Department of Physiology, University of Bern, Bern, Switzerland
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26
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Dong H, Dunn J, Lytton J. Stoichiometry of the Cardiac Na+/Ca2+ exchanger NCX1.1 measured in transfected HEK cells. Biophys J 2002; 82:1943-52. [PMID: 11916852 PMCID: PMC1301990 DOI: 10.1016/s0006-3495(02)75543-4] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The stoichiometry with which the Na+/Ca2+ exchanger, NCX1, binds and transports Na+ and Ca2+ has dramatic consequences for ionic homeostasis and cellular function of heart mycocytes and brain neurons, where the exchanger is highly expressed. Previous studies have examined this question using native NCX1 in its endogenous environment. We describe here whole-cell voltage clamp studies using recombinant rat heart NCX1.1 expressed heterologously in HEK-293 cells. This system provides the advantages of a high level of NCX1 protein expression, very low background ion transport levels, and excellent control over clamped voltage and ionic composition. Using ionic conditions that allowed bi-directional currents, voltage ramps were employed to determine the reversal potential for NCX1.1-mediated currents. Analysis of the relation between reversal potential and external [Na+] or [Ca2+], under a variety of intracellular conditions, yielded coupling ratios for Na+ of 1.9-2.3 ions per net charge and for Ca2+ of 0.45 +/- 0.03 ions per net charge. These data are consistent with a stoichiometry for the NCX1.1 protein of 4 Na+ to 1 Ca2+ to 2 charges moved per transport cycle.
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Affiliation(s)
- Hui Dong
- Department of Biochemistry and Molecular Biology, University of Calgary, Calgary T2N 4N1, Canada
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27
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Seki S, Taniguchi M, Takeda H, Nagai M, Taniguchi I, Mochizuki S. Inhibition by KB-r7943 of the reverse mode of the Na+/Ca2+ exchanger reduces Ca2+ overload in ischemic-reperfused rat hearts. Circ J 2002; 66:390-6. [PMID: 11954956 DOI: 10.1253/circj.66.390] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
Ca2+ influx via the Na+/Ca2+ exchanger (NCX) may lead to Ca2+ overload and myocardial injury in ischemia-reperfusion. Direct evidence that increased cytoplasmic Ca2+ concentration ([Ca2+]i) is mediated by the reverse mode of the NCX is limited, so in the present study the [Ca2+]i dynamics and left ventricular pressure were monitored in perfused beating hearts. The effects of KB-R7943 (KBR), a selective inhibitor of the NCX in the reverse mode, were analyzed during low-Na+ exposure and ischemia-reperfusion. Hearts from Sprague-Dawley rats were retrogradely perfused and loaded with 4 micromol/L fura-2 to measure the fluorescence ratio as an index of [Ca2+]i. To evaluate KBR effects on the reverse mode exchanger, the increase in [Ca2+]i induced by low-Na+ exposure (Na+: 30 mmol/L, 10 mmol/L caffeine pre-treatment) was measured with and without 10 micromol/L KBR (n=5). In another series, the hearts were subjected to 10 min of low-flow ischemia with pacing, followed by reperfusion in the absence (n=6) or in the presence of 10 micromol/L KBR (n=6). Background autofluorescence was subtracted to estimate the ratio in the ischemia-reperfusion protocol. KBR significantly suppressed the increase in [Ca2+]i induced by low-Na+ (40.2 +/- 11.2% of control condition, p=0.014), as well as on increase in diastolic [Ca2+]i during ischemia (% increase from pre-ischemia in [Ca2+]i at 10 min: KBR, 17.9 +/- 6.4%; no KBR, 44.4 +/- 7.7%; p=0.024). After reperfusion, diastolic [Ca2+]i normalized more rapidly in KBR-treated hearts (% increase at 1 min: KBR, 4.5 +/- 7.0%; no KBR, 39.8 +/- 12.2%; p=0.03). Treatment with KBR also accelerated recovery of the rate-pressure product on reperfusion (1 min: KBR, 8,944 +/- 1,554 min(-1) mmHg; no KBR, 4,970 +/- 1,325; p<0.05). Thus, inhibition of the reverse mode exchanger by KBR reduced ischemic Ca2+ overload and possibly improved functional myocardial recovery during reperfusion in a whole heart model.
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Affiliation(s)
- Shingo Seki
- Department of Internal Medicine, Aoto Hospital, The Jikei University, School of Medicine, Tokyo, Japan.
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28
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Abstract
Of the ions involved in the intricate workings of the heart, calcium is considered perhaps the most important. It is crucial to the very process that enables the chambers of the heart to contract and relax, a process called excitation-contraction coupling. It is important to understand in quantitative detail exactly how calcium is moved around the various organelles of the myocyte in order to bring about excitation-contraction coupling if we are to understand the basic physiology of heart function. Furthermore, spatial microdomains within the cell are important in localizing the molecular players that orchestrate cardiac function.
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Affiliation(s)
- Donald M Bers
- Department of Physiology, Stritch School of Medicine, Loyola Unversity Chicago, IL 60153, USA.
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29
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Abstract
Abstract
—The Na
+
-Ca
2+
exchanger (NCX) is one of the essential regulators of Ca
2+
homeostasis in cardiomyocytes and thus an important modulator of the cardiac contractile function. The purpose of this review is to survey recent advances in cardiac NCX research, with particular emphasis on molecular and pharmacological aspects. The NCX function is thought to be regulated by a variety of cellular factors. However, data obtained by use of different experimental systems often appear to be in conflict. Where possible, we endeavor to provide a rational interpretation of such data. We also provide a summary of current work relating to the structure and function of the cardiac NCX. Recent molecular studies of the NCX protein are beginning to shed light on structural features of the ion translocation pathway in the NCX membrane domain, which seems likely to be formed, at least partly, by the phylogenetically conserved α-1 and α-2 repeat structures and their neighboring membrane-spanning segments. Finally, we discuss new classes of NCX inhibitors with improved selectivity. One of these, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate (KB-R7943), appears to exhibit unique selectivity for Ca
2+
-influx–mode NCX activity. Data obtained with these inhibitors should provide a basis for designing more selective and clinically useful drugs targeting NCX.
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Affiliation(s)
- M Shigekawa
- Department of Molecular Physiology, National Cardiovascular Center Research Institute, Suita, Osaka, Japan.
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30
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Abstract
Defective excitation-contraction coupling in heart failure is generally associated with both a reduction in sarcoplasmic reticulum (SR) Ca(2+) uptake and a greater dependence on transsarcolemmal Na(+)-Ca(2+) exchange (NCX) for Ca(2+) removal. Although a relative increase in NCX is expected when SR function is impaired, few and contradictory studies have addressed whether there is an absolute increase in NCX activity. The present study examines in detail NCX density and function in left ventricular midmyocardial myocytes isolated from normal or tachycardic pacing-induced failing canine hearts. No change of NCX current density was evident in myocytes from failing hearts when intracellular Ca(2+) ([Ca(2+)](i)) was buffered to 200 nmol/L. However, when [Ca(2+)](i) was minimally buffered with 50 micromol/L indo-1, Ca(2+) extrusion via NCX during caffeine application was doubled in failing versus normal cells. In other voltage-clamp experiments in which SR uptake was blocked with thapsigargin, both reverse-mode and forward-mode NCX currents and Ca(2+) transport were increased >2-fold in failing cells. These results suggest that, in addition to a relative increase in NCX function as a consequence of defective SR Ca(2+) uptake, there is an absolute increase in NCX function that depends on [Ca(2+)](i) in the failing heart.
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Affiliation(s)
- I A Hobai
- Institute of Molecular Cardiobiology, Division of Cardiology, Department of Medicine, The Johns Hopkins University, Baltimore, MD 21205, USA
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