101
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Xia L, Zhang Y, Zhang H, Wei Q, Liu F, Crozier S. Simulation of Brugada syndrome using cellular and three-dimensional whole-heart modeling approaches. Physiol Meas 2006; 27:1125-42. [PMID: 17028406 DOI: 10.1088/0967-3334/27/11/006] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Brugada syndrome (BS) is a genetic disease identified by an abnormal electrocardiogram (ECG) (mainly abnormal ECGs associated with right bundle branch block and ST-elevation in right precordial leads). BS can lead to increased risk of sudden cardiac death. Experimental studies on human ventricular myocardium with BS have been limited due to difficulties in obtaining data. Thus, the use of computer simulation is an important alternative. Most previous BS simulations were based on animal heart cell models. However, due to species differences, the use of human heart cell models, especially a model with three-dimensional whole-heart anatomical structure, is needed. In this study, we developed a model of the human ventricular action potential (AP) based on refining the ten Tusscher et al (2004 Am. J. Physiol. Heart Circ. Physiol. 286 H1573-89) model to incorporate newly available experimental data of some major ionic currents of human ventricular myocytes. These modified channels include the L-type calcium current (I(CaL)), fast sodium current (I(Na)), transient outward potassium current (I(to)), rapidly and slowly delayed rectifier potassium currents (I(Kr) and I(Ks)) and inward rectifier potassium current (I(Ki)). Transmural heterogeneity of APs for epicardial, endocardial and mid-myocardial (M) cells was simulated by varying the maximum conductance of I(Ks) and I(to). The modified AP models were then used to simulate the effects of BS on cellular AP and body surface potentials using a three-dimensional dynamic heart-torso model. Our main findings are as follows. (1) BS has little effect on the AP of endocardial or mid-myocardial cells, but has a large impact on the AP of epicardial cells. (2) A likely region of BS with abnormal cell AP is near the right ventricular outflow track, and the resulting ST-segment elevation is located in the median precordium area. These simulation results are consistent with experimental findings reported in the literature. The model can reproduce a variety of electrophysiological behaviors and provides a good basis for understanding the genesis of abnormal ECG under the condition of BS disease.
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Affiliation(s)
- Ling Xia
- Department of Biomedical Engineering, Zhejiang University, Hangzhou 310027, People's Republic of China.
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102
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Dong M, Sun X, Prinz AA, Wang HS. Effect of simulated Ito on guinea pig and canine ventricular action potential morphology. Am J Physiol Heart Circ Physiol 2006; 291:H631-7. [PMID: 16565319 DOI: 10.1152/ajpheart.00084.2006] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The transient outward current ( Ito) is a major repolarizing current in the heart. Marked reduction of Ito density occurs in heart failure and is accompanied by significant action potential duration (APD) prolongation. To understand the species-dependent role of Ito in regulating the ventricular action potential morphology and duration, we introduced simulated Ito conductance in guinea pig and canine endocardial ventricular myocytes using the dynamic clamp technique and perforated patch-clamp recordings. The effects of simulated Ito in both types of cells were complex and biphasic, separated by a clear density threshold of ∼40 pA/pF. Below this threshold, simulated Ito resulted in a distinct phase 1 notch and had little effect on or moderately prolonged the APD. Ito above the threshold resulted in all-or-none repolarization and precipitously reduced the APD. Qualitatively, these results agreed with our previous studies in canine ventricular cells using whole cell recordings. We conclude that 1) contrary to previous gene transfer studies involving the Kv4.3 current, the response of guinea pig ventricular myocytes to a fully inactivating Ito is similar to that of canine ventricular cells and 2) in animals such as dogs that have a broad cardiac action potential, Ito does not play a major role in setting the APD.
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Affiliation(s)
- Min Dong
- Dept. of Pharmacology and Cell Biophysics, Univ. of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0575, USA
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103
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Flaim SN, Giles WR, McCulloch AD. Contributions of sustained INa and IKv43 to transmural heterogeneity of early repolarization and arrhythmogenesis in canine left ventricular myocytes. Am J Physiol Heart Circ Physiol 2006; 291:H2617-29. [PMID: 16829642 DOI: 10.1152/ajpheart.00350.2006] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The roles of sustained components of I(Na) and I(Kv43) in shaping the action potentials (AP) of myocytes isolated from the canine left ventricle (LV) have not been studied in detail. Here we investigate the hypothesis that these two currents can contribute substantially to heterogeneity of early repolarization and arrhythmic risk. Quantitative data from voltage-clamp and expression profiling experiments were used to complete meaningful modifications to an existing "local control" model of canine midmyocardial myocyte excitation-contraction coupling for epicardial and endocardial cells. We include 1) heterogeneous I(Kv43), I(Ks), and I(SERCA) density; 2) modulation of I(Kv43) by Kv channel interacting protein type 2 (KChIP2) channel subunits; 3) a possible Ca(2+)-dependent open-state inactivation of I(Kv43); and 4) a sustained component of the inward Na(+) current, I(NaL). The resulting simulations illustrate ways in which KChIP2- and Ca(2+)-dependent control of I(Kv43) can result in a sustained outward current that can neutralize I(NaL) in a rate- and myocyte subtype-dependent manner. Both these currents appear to play significant roles in modulating AP duration and rate dependence in midmyocardial myocytes. Furthermore, an increased ratio of I(Kv43) to I(NaL) is capable of protecting epicardial myocytes from the early afterdepolarizations resulting from the SCN5A-I1768V mutation-induced increase in I(NaL). Experimentally observed transmural differences in Ca(2+) handling, including greater sarcoplasmic reticulum Ca(2+) content and faster Ca(2+) transient decay rates on the epicardium, were recapitulated in our simulations. By design, these models allow upward integration into organ models or may be used as a basis for further investigations into cellular heterogeneities.
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Affiliation(s)
- Sarah N Flaim
- University of California, San Diego, La Jolla, CA 92093-0412, USA
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104
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Bassingthwaighte JB, Chizeck HJ, Atlas LE. Strategies and Tactics in Multiscale Modeling of Cell-to-Organ Systems. PROCEEDINGS OF THE IEEE. INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS 2006; 94:819-830. [PMID: 20463841 PMCID: PMC2867355 DOI: 10.1109/jproc.2006.871775] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
Modeling is essential to integrating knowledge of human physiology. Comprehensive self-consistent descriptions expressed in quantitative mathematical form define working hypotheses in testable and reproducible form, and though such models are always "wrong" in the sense of being incomplete or partly incorrect, they provide a means of understanding a system and improving that understanding. Physiological systems, and models of them, encompass different levels of complexity. The lowest levels concern gene signaling and the regulation of transcription and translation, then biophysical and biochemical events at the protein level, and extend through the levels of cells, tissues and organs all the way to descriptions of integrated systems behavior. The highest levels of organization represent the dynamically varying interactions of billions of cells. Models of such systems are necessarily simplified to minimize computation and to emphasize the key factors defining system behavior; different model forms are thus often used to represent a system in different ways. Each simplification of lower level complicated function reduces the range of accurate operability at the higher level model, reducing robustness, the ability to respond correctly to dynamic changes in conditions. When conditions change so that the complexity reduction has resulted in the solution departing from the range of validity, detecting the deviation is critical, and requires special methods to enforce adapting the model formulation to alternative reduced-form modules or decomposing the reduced-form aggregates to the more detailed lower level modules to maintain appropriate behavior. The processes of error recognition, and of mapping between different levels of model complexity and shifting the levels of complexity of models in response to changing conditions, are essential for adaptive modeling and computer simulation of large-scale systems in reasonable time.
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105
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Bassani RA. Transient outward potassium current and Ca2+ homeostasis in the heart: beyond the action potential. Braz J Med Biol Res 2006; 39:393-403. [PMID: 16501819 DOI: 10.1590/s0100-879x2006000300010] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The present review deals with Ca2+-independent, K+-carried transient outward current (Ito), an important determinant of the early repolarization phase of the myocardial action potential. The density of total Ito and of its fast and slow components (I(to,f) and I(to,s), respectively), as well as the expression of their molecular correlates (pore-forming protein isoforms Kv4.3/4.2 and Kv1.4, respectively), vary during postnatal development and aging across species and regions of the heart. Changes in Ito may also occur in disease conditions, which may affect the profile of cardiac repolarization and vulnerability to arrhythmias, and also influence excitation-contraction coupling. Decreased Ito density, observed in immature and aging myocardium, as well as during several types of cardiomyopathy and heart failure, may be associated with action potential prolongation, which favors Ca2+ influx during membrane depolarization and limits voltage-dependent Ca2+ efflux via the Na+/Ca2+ exchanger. Both effects contribute to increasing sarcoplasmic reticulum (SR) Ca2+ content (the main source of contraction-activating Ca2+ in mammalian myocardium), which, in addition to the increased Ca2+ influx, should enhance the amount of Ca2+ released by the SR during systole. This change usually takes place under conditions in which SR function is depressed, and may be adaptive since it provides partial compensation for SR deficiency, although possibly at the cost of asynchronous SR Ca2+ release and greater propensity to triggered arrhythmias. Thus, Ito modulation appears to be an additional mechanism by which excitation-contraction coupling in myocardial cells is indirectly regulated.
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Affiliation(s)
- R A Bassani
- Centro de Engenharia Biomédica, Universidade Estadual de Campinas, Campinas, SP, Brazil.
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106
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Hopenfeld B. A mathematical analysis of the action potential plateau duration of a human ventricular myocyte. J Theor Biol 2005; 240:311-22. [PMID: 16303142 DOI: 10.1016/j.jtbi.2005.09.021] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/08/2005] [Revised: 07/13/2005] [Accepted: 09/21/2005] [Indexed: 11/24/2022]
Abstract
The plateau phase of a human ventricular myocyte is analysed. The plateau duration is a function of the time required for a myocyte's transmembrane voltage to decrease by a certain voltage, DeltaV. The timing of the plateau is shown to be controlled by two slowly changing gate variables, the inactivation gate that controls the inward/depolarizing L-type calcium current and the inactivation gate that controls the outward/repolarizing slow rectifier potassium current. The amount of current controlled by these variables is a function of the net conductivity of the corresponding sodium and potassium channels. An equation is derived that relates action potential duration to these net conductivities and the time dependence of the slowly moving variables. This equation is used to estimate plateau duration for a given value of DeltaV. The initial conditions of the slowly moving inactivation variables are shown to affect plateau duration. These initial conditions depend on the amount of time that has elapsed between a previous repolarization and a current depolarization (diastolic interval). The analysis thus helps to quantify the characteristics of action potential duration restitution.
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Affiliation(s)
- B Hopenfeld
- National Institutes of Health, National Heart, Lung and Blood Institute, 10 Center Dr., MSC 1061, Bethesda, MD 20892-1061, USA.
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107
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Patel SP, Campbell DL. Transient outward potassium current, 'Ito', phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms. J Physiol 2005; 569:7-39. [PMID: 15831535 PMCID: PMC1464208 DOI: 10.1113/jphysiol.2005.086223] [Citation(s) in RCA: 147] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Revised: 03/07/2005] [Accepted: 04/13/2005] [Indexed: 11/08/2022] Open
Abstract
At least two functionally distinct transient outward K(+) current (I(to)) phenotypes can exist across the free wall of the left ventricle (LV). Based upon their voltage-dependent kinetics of recovery from inactivation, these two phenotypes are designated 'I(to,fast)' (recovery time constants on the order of tens of milliseconds) and 'I(to,slow)' (recovery time constants on the order of thousands of milliseconds). Depending upon species, either I(to,fast), I(to,slow) or both current phenotypes may be expressed in the LV free wall. The expression gradients of these two I(to) phenotypes across the LV free wall are typically heterogeneous and, depending upon species, may consist of functional phenotypic gradients of both I(to,fast) and I(to,slow) and/or density gradients of either phenotype. We review the present evidence (molecular, biophysical, electrophysiological and pharmacological) for Kv4.2/4.3 alpha subunits underlying LV I(to,fast) and Kv1.4 alpha subunits underlying LV I(to,slow) and speculate upon the potential roles of each of these currents in determining frequency-dependent action potential characteristics of LV subepicardial versus subendocardial myocytes in different species. We also review the possible functional implications of (i) ancillary subunits that regulate Kv1.4 and Kv4.2/4.3 (Kvbeta subunits, DPPs), (ii) KChIP2 isoforms, (iii) spider toxin-mediated block of Kv4.2/4.3 (Heteropoda toxins, phrixotoxins), and (iv) potential mechanisms of modulation of I(to,fast) and I(to,slow) by cellular redox state, [Ca(2)(+)](i) and kinase-mediated phosphorylation. I(to) phenotypic activation and state-dependent gating models and molecular structure-function relationships are also discussed.
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Affiliation(s)
- Sangita P Patel
- Department of Physiology and Biophysics, University at Buffalo, State University of New York, NY 14214-3078, USA.
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108
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Bassingthwaighte JB, Chizeck HJ, Atlas LE, Qian H. Multiscale modeling of cardiac cellular energetics. Ann N Y Acad Sci 2005; 1047:395-424. [PMID: 16093514 PMCID: PMC2864600 DOI: 10.1196/annals.1341.035] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Multiscale modeling is essential to integrating knowledge of human physiology starting from genomics, molecular biology, and the environment through the levels of cells, tissues, and organs all the way to integrated systems behavior. The lowest levels concern biophysical and biochemical events. The higher levels of organization in tissues, organs, and organism are complex, representing the dynamically varying behavior of billions of cells interacting together. Models integrating cellular events into tissue and organ behavior are forced to resort to simplifications to minimize computational complexity, thus reducing the model's ability to respond correctly to dynamic changes in external conditions. Adjustments at protein and gene regulatory levels shortchange the simplified higher-level representations. Our cell primitive is composed of a set of subcellular modules, each defining an intracellular function (action potential, tricarboxylic acid cycle, oxidative phosphorylation, glycolysis, calcium cycling, contraction, etc.), composing what we call the "eternal cell," which assumes that there is neither proteolysis nor protein synthesis. Within the modules are elements describing each particular component (i.e., enzymatic reactions of assorted types, transporters, ionic channels, binding sites, etc.). Cell subregions are stirred tanks, linked by diffusional or transporter-mediated exchange. The modeling uses ordinary differential equations rather than stochastic or partial differential equations. This basic model is regarded as a primitive upon which to build models encompassing gene regulation, signaling, and long-term adaptations in structure and function. During simulation, simpler forms of the model are used, when possible, to reduce computation. However, when this results in error, the more complex and detailed modules and elements need to be employed to improve model realism. The processes of error recognition and of mapping between different levels of model form complexity are challenging but are essential for successful modeling of large-scale systems in reasonable time. Currently there is to this end no established methodology from computational sciences.
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109
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Greenstein JL, Hinch R, Winslow RL. Mechanisms of excitation-contraction coupling in an integrative model of the cardiac ventricular myocyte. Biophys J 2005; 90:77-91. [PMID: 16214852 PMCID: PMC1367039 DOI: 10.1529/biophysj.105.065169] [Citation(s) in RCA: 112] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
It is now well established that characteristic properties of excitation-contraction (EC) coupling in cardiac myocytes, such as high gain and graded Ca(2+) release, arise from the interactions that occur between L-type Ca(2+) channels (LCCs) and nearby ryanodine-sensitive Ca(2+) release channels (RyRs) in localized microdomains. Descriptions of Ca(2+)-induced Ca(2+) release (CICR) that account for these local mechanisms are lacking from many previous models of the cardiac action potential, and those that do include local control of CICR are able to reconstruct properties of EC coupling, but require computationally demanding stochastic simulations of approximately 10(5) individual ion channels. In this study, we generalize a recently developed analytical approach for deriving simplified mechanistic models of CICR to formulate an integrative model of the canine cardiac myocyte which is computationally efficient. The resulting model faithfully reproduces experimentally measured properties of EC coupling and whole cell phenomena. The model is used to study the role of local redundancy in L-type Ca(2+) channel gating and the role of dyad configuration on EC coupling. Simulations suggest that the characteristic steep rise in EC coupling gain observed at hyperpolarized potentials is a result of increased functional coupling between LCCs and RyRs. We also demonstrate mechanisms by which alterations in the early repolarization phase of the action potential, resulting from reduction of the transient outward potassium current, alters properties of EC coupling.
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Affiliation(s)
- Joseph L Greenstein
- The Center for Cardiovascular Bioinformatics and Modeling and The Whitaker Biomedical Engineering Institute, The Johns Hopkins University Whiting School of Engineering and School of Medicine, Baltimore, Maryland, USA.
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110
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James AF, Choisy SCM, Hancox JC. Recent advances in understanding sex differences in cardiac repolarization. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2005; 94:265-319. [PMID: 15979693 DOI: 10.1016/j.pbiomolbio.2005.05.010] [Citation(s) in RCA: 119] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
A number of gender differences exist in the human electrocardiogram (ECG): the P-wave and P-R intervals are slightly longer in men than in women, whilst women have higher resting heart rates than do men, but a longer rate-corrected QT (QT(C)) interval. Women with the LQT1 and LQT2 variants of congenital long-QT syndrome (LQTS) are at greater risk of adverse cardiac events. Similarly, many drugs associated with acquired LQTS have a greater risk of inducing torsades de pointes (TdP) arrhythmia in women than in men. There are also male:female differences in Brugada syndrome, early repolarisation syndrome and sudden cardiac death. The differences in the ECG between men and women, and in particular those relating to the QT interval, have been explored experimentally and provide evidence of differences in the processes underlying ventricular repolarization. The data available from rabbit, canine, rat, mouse and guinea pig models are reviewed and highlight involvement of male:female differences in Ca and K currents, although the possible involvement of rapid and persistent Na current and Na-Ca exchange currents cannot yet be excluded. The mechanisms underlying observed differences remain to be elucidated fully, but are likely to involve the influence of gonadal steroids. With respect to the QT interval and risk of TdP, a range of evidence implicates a protective role of testosterone in male hearts, possibly by both genomic and non-genomic pathways. Evidence regarding oestrogen and progesterone is less unequivocal, although the interplay between these two hormones may influence both repolarization and pro-arrhythmic risk.
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Affiliation(s)
- Andrew F James
- Department of Physiology & Cardiovascular Research Laboratories, School of Medical Sciences, University of Bristol, Bristol, UK.
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111
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Rose J, Armoundas AA, Tian Y, DiSilvestre D, Burysek M, Halperin V, O'Rourke B, Kass DA, Marbán E, Tomaselli GF. Molecular correlates of altered expression of potassium currents in failing rabbit myocardium. Am J Physiol Heart Circ Physiol 2005; 288:H2077-87. [PMID: 15637125 PMCID: PMC2711868 DOI: 10.1152/ajpheart.00526.2003] [Citation(s) in RCA: 87] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Action potential (AP) prolongation is a hallmark of failing myocardium. Functional downregulation of K currents is a prominent feature of cells isolated from failing ventricles. The detailed changes in K current expression differ depending on the species, the region of the heart, and the mechanism of induction of heart failure. We used complementary approaches to study K current downregulation in pacing tachycardia-induced heart failure in the rabbit. The AP duration (APD) at 90% repolarization was significantly longer in cells isolated from failing hearts compared with controls (539 +/- 162 failing vs. 394 +/- 114 control, P < 0.05). The major K currents in the rabbit heart, inward rectifier potassium current (I(K1)), transient outward (I(to)), and delayed rectifier current (I(K)) were functionally downregulated in cells isolated from failing ventricles. The mRNA levels of Kv4.2, Kv1.4, KChIP2, and Kir2.1 were significantly downregulated, whereas the Kv4.3, Erg, KvLQT1, and minK were unaltered in the failing ventricles compared with the control left ventricles. Significant downregulation in the long splice variant of Kv4.3, but not in the total Kv4.3, Kv4.2, and KChIP2 immunoreactive protein, was observed in cells isolated from the failing ventricle with no change in Kv1.4, KvLQT1, and in Kir2.1 immunoreactive protein levels. Multiple cellular and molecular mechanisms underlie the downregulation of K currents in the failing rabbit ventricle.
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Affiliation(s)
- Jochen Rose
- Division of Cardiology, Johns Hopkins University, Baltimore, Maryland 21205, USA
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112
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Frank-Hansen R, Larsen LA, Andersen P, Jespersgaard C, Christiansen M. Mutations in the genes KCND2 and KCND3 encoding the ion channels Kv4.2 and Kv4.3, conducting the cardiac fast transient outward current (ITO,f), are not a frequent cause of long QT syndrome. Clin Chim Acta 2005; 351:95-100. [PMID: 15563876 DOI: 10.1016/j.cccn.2004.08.017] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2004] [Revised: 08/06/2004] [Accepted: 08/06/2004] [Indexed: 11/24/2022]
Abstract
BACKGROUND Long QT syndrome (LQTS) is a hereditary cardiac arrhythmogenic disorder characterized by prolongation of the QT interval in the electrocardiogram, torsades de pointes arrhythmia, and syncopes and sudden death. LQTS is caused by mutations in ion channel genes. However, only in half of the families is it possible to identify mutations in one of the seven known LQTS genes, why further genetic heterogeneity is expected. The genes KCND2 and KCND3, encoding the alpha-subunits of the voltage-gated potassium channels Kv4.2 and Kv4.3 conducting the fast transient outward current (I(TO,f)) of the cardiac action potential (AP) in the myocardium, have been associated with prolongation of AP duration and QT prolongation in murine models. METHODS KCND2 and KCND3 were examined for mutations using single-strand conformation polymorphism (SSCP) analysis in 43 unrelated LQTS patients, where mutations in the coding regions of known LQTS genes had been excluded. RESULTS Seven single nucleotide polymorphismsm (SNPs) were found, two exonic SNPs in KCND2 and three exonic and two intronic in KCND3. None of the five exonic SNPs had coding effect. All seven SNPs are considered normal variants. CONCLUSION The data suggest that mutations in KCND2 and KCND3 are not a frequent cause of long QT syndrome.
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Affiliation(s)
- Rune Frank-Hansen
- Department of Clinical Biochemistry, Statens Serum Institut, Artillerivej 5, DK-2300 Copenhagen, Denmark
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113
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Jerng HH, Pfaffinger PJ, Covarrubias M. Molecular physiology and modulation of somatodendritic A-type potassium channels. Mol Cell Neurosci 2005; 27:343-69. [PMID: 15555915 DOI: 10.1016/j.mcn.2004.06.011] [Citation(s) in RCA: 239] [Impact Index Per Article: 12.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/18/2004] [Revised: 05/22/2004] [Accepted: 06/08/2004] [Indexed: 11/23/2022] Open
Abstract
The somatodendritic subthreshold A-type K+ current (ISA) in nerve cells is a critical component of the ensemble of voltage-gated ionic currents that determine somatodendritic signal integration. The underlying K+ channel belongs to the Shal subfamily of voltage-gated K+ channels. Most Shal channels across the animal kingdom share a high degree of structural conservation, operate in the subthreshold range of membrane potentials, and exhibit relatively fast inactivation and recovery from inactivation. Mammalian Shal K+ channels (Kv4) undergo preferential closed-state inactivation with features that are generally inconsistent with the classical mechanisms of inactivation typical of Shaker K+ channels. Here, we review (1) the physiological and genetic properties of ISA, 2 the molecular mechanisms of Kv4 inactivation and its remodeling by a family of soluble calcium-binding proteins (KChIPs) and a membrane-bound dipeptidase-like protein (DPPX), and (3) the modulation of Kv4 channels by protein phosphorylation.
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Affiliation(s)
- Henry H Jerng
- Division of Neuroscience, Baylor College of Medicine, Houston, TX 77030, USA.
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114
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Aiba T, Shimizu W, Inagaki M, Noda T, Miyoshi S, Ding WG, Zankov DP, Toyoda F, Matsuura H, Horie M, Sunagawa K. Cellular and ionic mechanism for drug-induced long QT syndrome and effectiveness of verapamil. J Am Coll Cardiol 2005; 45:300-7. [PMID: 15653031 DOI: 10.1016/j.jacc.2004.09.069] [Citation(s) in RCA: 90] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/24/2004] [Accepted: 09/28/2004] [Indexed: 11/17/2022]
Abstract
OBJECTIVES We examined the cellular and ionic mechanism for QT prolongation and subsequent Torsade de Pointes (TdP) and the effect of verapamil under conditions mimicking KCNQ1 (I(Ks) gene) defect linked to acquired long QT syndrome (LQTS). BACKGROUND Agents with an I(Kr)-blocking effect often induce marked QT prolongation in patients with acquired LQTS. Previous reports demonstrated a relationship between subclinical mutations in cardiac K+ channel genes and a risk of drug-induced TdP. METHODS Transmembrane action potentials from epicardial (EPI), midmyocardial (M), and endocardial (ENDO) cells were simultaneously recorded, together with a transmural electrocardiogram, at a basic cycle length of 2,000 ms in arterially perfused feline left ventricular preparations. RESULTS The I(Kr) block (E-4031: 1 micromol/l) under control conditions (n = 5) prolonged the QT interval but neither increased transmural dispersion of repolarization (TDR) nor induced arrhythmias. However, the I(Kr) blocker under conditions with I(Ks) suppression by chromanol 293B 10 micromol/l mimicking the KCNQ1 defect (n = 10) preferentially prolonged action potential duration (APD) in EPI rather than M or ENDO, thereby dramatically increasing the QT interval and TDR. Spontaneous or epinephrine-induced early afterdepolarizations (EADs) were observed in EPI, and subsequent TdP occurred only under both I(Ks) and I(Kr) suppression. Verapamil (0.1 to 5.0 micromol/l) dose-dependently abbreviated APD in EPI more than in M and ENDO, thereby significantly decreasing the QT interval, TDR, and suppressing EADs and TdP. CONCLUSIONS Subclinical I(Ks) dysfunction could be a risk of drug-induced TdP. Verapamil is effective in decreasing the QT interval and TDR and in suppressing EADs, thus preventing TdP in the model of acquired LQTS.
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Affiliation(s)
- Takeshi Aiba
- Department of Cardiovascular Dynamics, Research Institute, National Cardiovascular Center, 5-7-1 Fujishiro-dai, Suita, Osaka 565-8565, Japan
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115
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Winslow RL, Cortassa S, Greenstein JL. Using models of the myocyte for functional interpretation of cardiac proteomic data. J Physiol 2005; 563:73-81. [PMID: 15611013 PMCID: PMC1665560 DOI: 10.1113/jphysiol.2004.080457] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2004] [Accepted: 12/15/2004] [Indexed: 01/03/2023] Open
Abstract
There has been significant progress towards the development of highly integrative computational models of the cardiac myocyte over the past decade. Models now incorporate descriptions of voltage-gated ionic currents and membrane transporters, mechanisms of calcium-induced calcium release and intracellular calcium cycling, mitochondrial ATP production and its coupling to energy-requiring membrane transport processes and mechanisms of force generation. There is an extensive literature documenting both the reconstructive and predictive abilities of these models and there is no question that an interplay between quantitative modelling and experimental investigation has become a central component of modern cardiovascular research. As data regarding the cardiovascular proteome in both health and disease emerge, integrative models of the myocyte are becoming useful tools for interpreting the functional significance of changes in protein expression and post-translational modifications (PTMs). Data of particular importance include information on: (a) changes of expressed protein level, (b) changes of protein PTMs, (c) protein localization, and (d) protein-protein interactions, as it is often possible to incorporate and interpret the functional significance of such findings using computational models. We provide two examples of how models may be used in this fashion. In the first example, we show how information on altered expression of the sarcoplasmic reticulum Ca2+-ATPase, when interpreted through the use of a computational model, has provided key insights into fundamental mechanisms regulating cardiac action potential duration. In the second example, we show how information on the effects of phosphorylation of L-type Ca2+ channels, when interpreted through the use of a model, provides insights on how this post-translational modification alters the properties of excitation-contraction coupling and risk for arrhythmia.
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Affiliation(s)
- Raimond L Winslow
- Center for Cardiovascular Bioinformatics and Modelling, The Johns Hopkins University School of Medicine and Whiting School of Engineering, Baltimore, MD, USA.
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Sun X, Wang HS. Role of the transient outward current (Ito) in shaping canine ventricular action potential--a dynamic clamp study. J Physiol 2005; 564:411-9. [PMID: 15649977 PMCID: PMC1464429 DOI: 10.1113/jphysiol.2004.077263] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Transmural electrical dispersion determines the repolarization sequence across the ventricular wall, and plays an important role in the development of arrhythmias under pathological conditions. While it is clear that the transmural gradient of the transient outward current (I(to)) underlies the dramatic difference in phase 1 repolarization across the ventricle, its contribution to the transmural action potential duration (APD) dispersion is not clear. We investigated this problem using the dynamic clamp technique in canine ventricular myocytes. The dynamic clamp allows quantitative 'insertion' of simulated conductances in real, biological cells, bridging pure computer modelling and experimental electrophysiology. 'Insertion' of an epicardial level of I(to) in endocardial cells produced a prominent phase 1 repolarization and a 'spike-and-dome' action potential morphology, but did not significantly affect the APD. Increasingly larger I(to) densities prolonged, and then dramatically shortened the endocardial APD. We also used the dynamic clamp to subtract, or 'block' the native I(to) in epicardial cells. Such 'blockade' eliminated the epicardial action potential notch, but had no significant effect on the APD. We conclude that I(to), while being a key regulator of phase 1 repolarization, does not significantly affect the APD of canine ventricular myocytes, and that the I(to) gradient is not a significant contributor to the transmural APD dispersion in the canine ventricle. By allowing computer simulation on a biological background, the dynamic clamp is a new and effective tool to study the ionic basis of the electrical properties of cardiac cells.
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Affiliation(s)
- Xiaoyin Sun
- Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0575, USA
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117
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Nishida K, Fujiki A, Mizumaki K, Sakabe M, Sugao M, Tsuneda T, Inoue H. Canine model of Brugada syndrome using regional epicardial cooling of the right ventricular outflow tract. J Cardiovasc Electrophysiol 2004; 15:936-41. [PMID: 15333091 DOI: 10.1046/j.1540-8167.2004.04041.x] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
INTRODUCTION Myocardial cooling can induce J point elevation (Osborn wave) as seen on ECG of the Brugada syndrome by activating transient outward current (Ito) and causing a spike-and-dome configuration of the monophasic action potential (MAP) in the ventricular epicardium in isolated canine ventricular wedge preparations. We determined the effect of regional epicardial cooling of the right ventricular outflow tract (RVOT) on surface ECG and ventricular vulnerability in the dog. METHODS AND RESULTS In 12 dogs, a cooling device (20-mm diameter) was attached to the RVOT epicardium, and surface ECG, epicardial MAP, and endocardial MAP were recorded. Regional cooling (29.7 degrees C +/- 2.2 degrees C) elevated the J point from 0.05 +/- 0.06 mV to 0.12 +/- 0.11 mV and induced T wave inversion (from 0.02 +/- 0.12 mV to -0.27 +/- 0.20 mV) in lead V1 in association with "spike-and-dome" configuration of the epicardial MAP. Cooling prolonged MAP duration in the RVOT epicardium from 172 +/- 27 ms to 213 +/- 30 ms (P < 0.01) but not in the RV endocardium and increased transmural dispersion of MAP duration from 9 +/- 8 ms to 44 +/- 21 ms (P < 0.01). Cooling also prolonged the QT interval in lead V1 from 191 +/- 19 ms to 212 +/- 23 ms (P < 0.05), but not in lead V5, and increased spatial dispersion of QT interval from 7 +/- 5 ms to 20 +/- 10 ms (P < 0.01). QT interval in lead V1 correlated positively with MAP duration in the RVOT epicardium (r = 0.75). T wave amplitude in lead V1 correlated inversely with transmural dispersion of MAP duration in the RVOT (r =-0.74). Vagal nerve stimulation accentuated the cooling-induced changes. During cooling, ventricular fibrillation was induced by a single extrastimulus in 2 of 4 dogs, and additional vagal nerve stimulation during isoproterenol administration induced spontaneous ventricular fibrillation in one dog. CONCLUSION Localized epicardial cooling of the RVOT could be an in vivo experimental model of Brugada syndrome.
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Affiliation(s)
- Kunihiro Nishida
- Second Department of Internal Medicine, Toyama Medical and Pharmaceutical University, Toyama, Japan
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118
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Baltaev R, Strutz-Seebohm N, Korniychuk G, Myssina S, Lang F, Seebohm G. Regulation of cardiac shal-related potassium channel Kv 4.3 by serum- and glucocorticoid-inducible kinase isoforms in Xenopus oocytes. Pflugers Arch 2004; 450:26-33. [PMID: 15578212 DOI: 10.1007/s00424-004-1369-z] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2004] [Accepted: 10/27/2004] [Indexed: 12/28/2022]
Abstract
The human cardiac transient outward potassium current I(to) is formed by co-assembly of voltage-dependent K(+) channel (Kv 4.3) pore-forming alpha-subunits with differently spliced K channel interacting protein (KChIP) accessory proteins. I(to) is of considerable importance for the normal course of the cardiac ventricular action potential. The present study was performed to determine whether isoforms of the serum- and glucocorticoid-inducible kinase (SGK) family influence Kv 4.3/KChIP2b channel activity in the Xenopus laevis heterologous expression system. Co-expression of SGK1, but not of SGK2 or SGK3, increased Kv 4.3/KChIP2b channel currents. The up-regulation of the current was not due to changes in the activation curve or changes of channel inactivation. The currents in oocytes expressing Kv 4.3 alone were smaller than those in Kv 4.3/KChIP2b expressing oocytes, but were still stimulated by SGK1. The effect of wild-type SGK1 was mimicked by constitutively active SGK1 (SGK1 S422D) but not by an inactive mutant (SGK1 K127N). The current amplitude increase mediated by SGK1 was not dependent on NEDD4.2 or RAB5, nor did it reflect increased cell surface expression. In conclusion, SGK1 stimulates Kv 4.3 potassium channels expressed in Xenopus oocytes by a novel mechanism distinct from the known NEDD4.2-dependent pathway.
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Affiliation(s)
- Ravshan Baltaev
- Department of Physiology I, University of Tübingen, Tübingen, Germany
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119
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Hund TJ, Rudy Y. Rate dependence and regulation of action potential and calcium transient in a canine cardiac ventricular cell model. Circulation 2004; 110:3168-74. [PMID: 15505083 PMCID: PMC1851913 DOI: 10.1161/01.cir.0000147231.69595.d3] [Citation(s) in RCA: 253] [Impact Index Per Article: 12.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
BACKGROUND Computational biology is a powerful tool for elucidating arrhythmogenic mechanisms at the cellular level, where complex interactions between ionic processes determine behavior. A novel theoretical model of the canine ventricular epicardial action potential and calcium cycling was developed and used to investigate ionic mechanisms underlying Ca2+ transient (CaT) and action potential duration (APD) rate dependence. METHODS AND RESULTS The Ca2+/calmodulin-dependent protein kinase (CaMKII) regulatory pathway was integrated into the model, which included a novel Ca2+-release formulation, Ca2+ subspace, dynamic chloride handling, and formulations for major ion currents based on canine ventricular data. Decreasing pacing cycle length from 8000 to 300 ms shortened APD primarily because of I(Ca(L)) reduction, with additional contributions from I(to1), I(NaK), and late I(Na). CaT amplitude increased as cycle length decreased from 8000 to 500 ms. This positive rate-dependent property depended on CaMKII activity. CONCLUSIONS CaMKII is an important determinant of the rate dependence of CaT but not of APD, which depends on ion-channel kinetics. The model of CaMKII regulation may serve as a paradigm for modeling effects of other regulatory pathways on cell function.
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Affiliation(s)
- Thomas J Hund
- Department of Biomedical Engineering, Washington University, St. Louis, MO 63130-4899, USA
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120
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Tanskanen AJ, Greenstein JL, O'Rourke B, Winslow RL. The role of stochastic and modal gating of cardiac L-type Ca2+ channels on early after-depolarizations. Biophys J 2004; 88:85-95. [PMID: 15501946 PMCID: PMC1201453 DOI: 10.1529/biophysj.104.051508] [Citation(s) in RCA: 124] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Certain signaling events that promote L-type Ca2+ channel (LCC) phosphorylation, such as beta-adrenergic stimulation or an increased expression of Ca(2+)/calmodulin-dependent protein kinase II, promote mode 2 gating of LCCs. Experimental data suggest the hypothesis that these events increase the likelihood of early after-depolarizations (EADs). We test this hypothesis using an ionic model of the canine ventricular myocyte incorporating stochastic gating of LCCs and ryanodine-sensitive calcium release channels. The model is extended to describe myocyte responses to the beta-adrenergic agonist isoproterenol. Results demonstrate that in the presence of isoproterenol the random opening of a small number of LCCs gating in mode 2 during the plateau phase of the action potential (AP) can trigger EADs. EADs occur randomly, where the likelihood of these events increases as a function of the fraction of LCCs gating in mode 2. Fluctuations of the L-type Ca2+ current during the AP plateau lead to variability in AP duration. Consequently, prolonged APs are occasionally observed and exhibit an increased likelihood of EAD formation. These results suggest a novel stochastic mechanism, whereby phosphorylation-induced changes in LCC gating properties contribute to EAD generation.
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Affiliation(s)
- Antti J Tanskanen
- Center for Cardiovascular Bioinformatics and Modeling, The Johns Hopkins University School of Medicine and Whiting School of Engineering, Baltimore, Maryland 21218, USA.
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121
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Pennec JP, Talarmin H, Droguet M, Giroux-Metgès MA, Gioux M, Dorange G. Characterization of the voltage-activated currents in cultured atrial myocytes isolated from the heart of the common oysterCrassostrea gigas. J Exp Biol 2004; 207:3935-44. [PMID: 15472024 DOI: 10.1242/jeb.01221] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
SUMMARYUsing the macro-patch clamp technique, we show that cardiac myocytes isolated from the heart of the oyster Crassostrea gigas possess several types of voltage-activated ionic currents. (1) A classical non-inactivating potassium current of the IK type that is inhibited by tetraethyl ammonium and shows an outward rectification and a slow activation. (2) A potassium current of the IA type that shows rapid activation and inactivation, and is blocked by 4-amino pyridine or preliminary depolarisation. (3) A potassium calcium-dependent current that is inhibited by charybdotoxin, activated by strong depolarisations and shows a large conductance. (4) A calcium inward current of the L-type that is inhibited by verapamil, cobalt and high concentrations of cadmium. This current is identified in most cells, but a T-type calcium current and classical fast sodium current are only identified in few cells, and only after a strong hyperpolarizing pulse. This suggests that these channels are normally inactivated in cultured cells and are not involved in the spontaneous activity of these cells. When they exist, the fast sodium channel is blocked by tetrodotoxin. The L-type calcium conductance is increased by serotonin. The identification in cultured oyster atrial cells of classical ionic currents,which are observed in most vertebrate species but only in a few species of molluscs, demonstrates that these cells are an interesting model. Moreover the viability and the electrophysiological properties of these cells are not significantly modified by freezing and thawing, thus increasing their usefulness in various bioassays.
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Affiliation(s)
- Jean-Pierre Pennec
- EA 3879, Unité de Physiologie Comparée et Intégrative, Institut de Synergie des Sciences et de la Santé, 22 avenue Camille Desmoulins, CS93837, 29238, Brest-cedex 3, France.
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122
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Berecki G, Zegers JG, Verkerk AO, Bhuiyan ZA, de Jonge B, Veldkamp MW, Wilders R, van Ginneken ACG. HERG channel (dys)function revealed by dynamic action potential clamp technique. Biophys J 2004; 88:566-78. [PMID: 15475579 PMCID: PMC1305034 DOI: 10.1529/biophysj.104.047290] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The human ether-a-go-go-related gene (HERG) encodes the rapid component of the cardiac delayed rectifier potassium current (I(Kr)). Per-Arnt-Sim domain mutations of the HERG channel are linked to type 2 long-QT syndrome. We studied wild-type and/or type 2 long-QT syndrome-associated mutant (R56Q) HERG current (I(HERG)) in HEK-293 cells, at both 23 and 36 degrees C. Conventional voltage-clamp analysis revealed mutation-induced changes in channel kinetics. To assess functional implication(s) of the mutation, we introduce the dynamic action potential clamp technique. In this study, we effectively replace the native I(Kr) of a ventricular cell (either a human model cell or an isolated rabbit myocyte) with I(HERG) generated in a HEK-293 cell that is voltage-clamped by the free-running action potential of the ventricular cell. Action potential characteristics of the ventricular cells were effectively reproduced with wild-type I(HERG), whereas the R56Q mutation caused a frequency-dependent increase of the action potential duration in accordance with the clinical phenotype. The dynamic action potential clamp approach also revealed a frequency-dependent transient wild-type I(HERG) component, which is absent with R56Q channels. This novel electrophysiological technique allows rapid and unambiguous determination of the effects of an ion channel mutation on the ventricular action potential and can serve as a new tool for investigating cardiac channelopathies.
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Affiliation(s)
- Géza Berecki
- Experimental and Molecular Cardiology Group and the Department of Physiology, Academic Medical Center, University of Amsterdam, The Netherlands.
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123
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Iyer V, Mazhari R, Winslow RL. A computational model of the human left-ventricular epicardial myocyte. Biophys J 2004; 87:1507-25. [PMID: 15345532 PMCID: PMC1304558 DOI: 10.1529/biophysj.104.043299] [Citation(s) in RCA: 219] [Impact Index Per Article: 11.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2004] [Accepted: 05/24/2004] [Indexed: 11/18/2022] Open
Abstract
A computational model of the human left-ventricular epicardial myocyte is presented. Models of each of the major ionic currents present in these cells are formulated and validated using experimental data obtained from studies of recombinant human ion channels and/or whole-cell recording from single myocytes isolated from human left-ventricular subepicardium. Continuous-time Markov chain models for the gating of the fast Na(+) current, transient outward current, rapid component of the delayed rectifier current, and the L-type calcium current are modified to represent human data at physiological temperature. A new model for the gating of the slow component of the delayed rectifier current is formulated and validated against experimental data. Properties of calcium handling and exchanger currents are altered to appropriately represent the dynamics of intracellular ion concentrations. The model is able to both reproduce and predict a wide range of behaviors observed experimentally including action potential morphology, ionic currents, intracellular calcium transients, frequency dependence of action-potential duration, Ca(2+)-frequency relations, and extrasystolic restitution/post-extrasystolic potentiation. The model therefore serves as a useful tool for investigating mechanisms of arrhythmia and consequences of drug-channel interactions in the human left-ventricular myocyte.
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Affiliation(s)
- Vivek Iyer
- The Center for Cardiovascular Bioinformatics and Modeling and the Whitaker Biomedical Engineering Institute, The Johns Hopkins University School of Medicine and Whiting School of Engineering, Baltimore, Maryland 21093, USA
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124
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GREENSTEIN JOSEPHL, TANSKANEN ANTTIJ, WINSLOW RAIMONDL. Modeling the actions of beta-adrenergic signaling on excitation--contraction coupling processes. Ann N Y Acad Sci 2004; 1015:16-27. [PMID: 15201146 PMCID: PMC1201510 DOI: 10.1196/annals.1302.002] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Activation of the beta-adrenergic (beta-AR) signaling pathway enhances cardiac function through protein kinase A (PKA)-mediated phosphorylation of target proteins involved in the process of excitation-contraction (EC) coupling. Experimental studies of the effects of beta-AR stimulation on EC coupling have yielded complex results, including increased, decreased, or unchanged EC coupling gain. In this study, we extend a previously developed model of the canine ventricular myocyte describing local control of sarcoplasmic reticulum (SR) calcium (Ca(2+)) release to include the effects of beta-AR stimulation. Incorporation of phosphorylation-dependent effects on model membrane currents and Ca(2+)-cycling proteins yields changes of action potential (AP) and Ca(2+) transients in agreement with those measured experimentally in response to the nonspecific beta-AR agonist isoproterenol (ISO). The model reproduces experimentally observed alterations in EC coupling gain in response to beta-AR agonists and predicts the specific roles of L-type Ca(2+) channel (LCC) and SR Ca(2+) release channel phosphorylation in altering the amplitude and shape of the EC coupling gain function. The model also indicates that factors that promote mode 2 gating of LCCs, such as beta-AR stimulation or activation of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), may increase the probability of occurrence of early after-depolarizations (EADs), due to the random, long-duration opening of LCC gating in mode 2.
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Affiliation(s)
| | | | - RAIMOND L. WINSLOW
- Address for correspondence: Prof. Raimond L. Winslow, Ph.D., Center for Cardiovascular Bioinformatics and Modeling, Dept. of Biomedical Engineering, The Johns Hopkins University School of Medicine, and Whiting School of Engineering, Clark Hall, 3400 N. Charles St., Baltimore, MD 21218. Fax: 410-516-5294.
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125
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Puglisi JL, Wang F, Bers DM. Modeling the isolated cardiac myocyte. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2004; 85:163-78. [PMID: 15142742 DOI: 10.1016/j.pbiomolbio.2003.12.003] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Computer modeling of cardiac myocytes has flourished in recent years. Models have evolved from mathematical descriptions of ionic channels alone to more sophisticated formulations that include calcium transport mechanisms, ATP production and metabolic pathways. The increased complexity is fueled by the new data available in the field. The continuous production of experimental data has led to the evolution of increasingly refined descriptions of the phenomena by modelers. Integrating the numerous systems involved in cardiac myocyte homeostasis makes the use of computer models necessary due to the unreliability of intuitive approaches. However the complexity of the model should not imply a cumbersome operation of the program. As with any tool, computer models have to be easy to operate or their strength will be diminished and potential users will not benefit fully from them. The contribution of the computer modeler to their respective biological fields will be more successful and enduring if modelers devote sufficient time to implement their equations into a model with user-friendly characteristics.
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Affiliation(s)
- Jose L Puglisi
- Department of Physiology, Loyola University Chicago, 2160 S. First Avenue, Maywood, IL 60153, USA.
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126
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Bassani RA, Altamirano J, Puglisi JL, Bers DM. Action potential duration determines sarcoplasmic reticulum Ca2+ reloading in mammalian ventricular myocytes. J Physiol 2004; 559:593-609. [PMID: 15243136 PMCID: PMC1665117 DOI: 10.1113/jphysiol.2004.067959] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
After sarcoplasmic reticulum (SR) Ca2+ depletion in intact ventricular myocytes, electrical activity promotes SR Ca2+ reloading and recovery of twitch amplitude. In ferret, recovery of twitch and caffeine-induced contracture required fewer twitches than in rabbit or rat. In rat, there was no difference in action potential duration at 90% repolarization (APD90) at steady state (SS) versus at the first post-depletion (PD) twitch. The SS APD90 was similar in ferret and rabbit (but longer than in rat). However, compared to SS, the PD APD90 was lengthened in ferret, but shortened in rabbit. When rabbit myocytes were subjected to AP-clamp patterns during SR Ca2+ reloading (ferret- or rabbit-type APs), reloading was much faster using the ferret AP templates. We conclude that the faster SR Ca2+ refilling in ferret is due to the increased Ca2+ influx during the longer PD AP. The PD versus SS APD90 difference was suppressed by thapsigargin in ferret (indicating Ca2+ dependence). In rabbit, the PD AP shortening depended on the preceding diastolic interval (rather than Ca2+), because rest produced the same AP shortening, and SS APD90 increased as a function of frequency (in contrast to ferret). Transient outward current (Ito) was larger and recovered from inactivation much faster in ferret than in rabbit. Moreover, slow Ito recovery (tau approximately 3 s) in rabbit was a much larger fraction of Ito. Our data and a computational model (including two Ito components) suggest that in rabbit the slowly recovering Ito is responsible for short post-rest and PD APs, for the unusual frequency dependence of APD90, and ultimately for the slower post-depletion SR Ca2+ reloading.
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Affiliation(s)
- Rosana A Bassani
- Centro de Engenharia Biomédica, Universidade Estadual de Campinas, 13084-971 Campinas, SP, Brazil.
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127
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Clayton RH, Holden AV. Propagation of normal beats and re-entry in a computational model of ventricular cardiac tissue with regional differences in action potential shape and duration. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2004; 85:473-99. [PMID: 15142758 DOI: 10.1016/j.pbiomolbio.2003.12.002] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
There is substantial experimental evidence from studies using both intact tissue and isolated single cells to support the existence of different cell types within the ventricular wall of the heart, each possessing different electrical properties. However other studies have failed to find these differences, and instead support the idea that electrical coupling in vivo between regions with different cell types smoothes out differences in action potential shape and duration. In this study we have used a computational model of electrical activation in heterogenous 2D and 3D cardiac tissue to investigate the propagation of both normal beats and arrhythmias. We used the Luo-Rudy dynamic model for guinea pig ventricular cells, with simplified Ca2+ handling and transmural heterogeneity in IKs and Ito. With normal cell-to-cell coupling, a layer of M cells was not necessary for the formation of an upright T wave in the simulated electrocardiogram, and the amplitude and configuration of the T wave was not greatly affected by the thickness and configuration of the M cell layer. Transmural gradients in repolarisation pushed re-entrant waves with an intramural filament towards either the base or the apex of the ventricles, and caused transient break up of re-entry with a transmural filament.
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Affiliation(s)
- Richard H Clayton
- Department of Computer Science, University of Sheffield and School of Biomedical Sciences, Regent Court, 211 Portobello Street, Sheffield S1 4DP, UK.
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128
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Bassingthwaighte JB, Vinnakota KC. The computational integrated myocyte: a view into the virtual heart. Ann N Y Acad Sci 2004; 1015:391-404. [PMID: 15201177 PMCID: PMC2864609 DOI: 10.1196/annals.1302.034] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The presentation outlines an integrative approach for developing a computational model of cardiomyocytes. A modular approach is proposed, and strategies of linking the modules (intermediary metabolism, electrophysiology, and mechanics) of the model are presented. A strong recommendation is given toward an integrated system approach backed by experimental validation.
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Affiliation(s)
- James B Bassingthwaighte
- Department of Bioengineering, University of Washington, Box 357962, Seattle, WA 98195-7962, USA.
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129
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Decher N, Barth AS, Gonzalez T, Steinmeyer K, Sanguinetti MC. Novel KChIP2 isoforms increase functional diversity of transient outward potassium currents. J Physiol 2004; 557:761-72. [PMID: 15107477 PMCID: PMC1665146 DOI: 10.1113/jphysiol.2004.066720] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Kv4.3 channels conduct transient outward K(+) currents in the human heart and brain where they mediate the early phase of action potential repolarization. KChIP2 proteins are members of a new class of calcium sensors that modulate the surface expression and biophysical properties of Kv4 K(+) channels. Here we describe three novel isoforms of KChIP2 with an alternatively spliced C-terminus (KChIP2e, KChIP2f) or N-terminus (KChIP2g). KChIP2e and KChIP2f are expressed in the human atrium, whereas KChIP2g is predominantly expressed in the brain. The KChIP2 isoforms were coexpressed with Kv4.3 channels in Xenopus oocytes and currents recorded with two-microelectrode voltage-clamp techniques. KChIP2e caused a reduction in current amplitude, an acceleration of inactivation and a slowing of the recovery from inactivation of Kv4.3 currents. KChIP2f increased the current amplitude and slowed the rate of inactivation, but did not alter the recovery from inactivation or the voltage of half-maximal inactivation of Kv4.3 channels. KChIP2g increased current amplitudes, slowed the rate of inactivation and shifted the voltage of half-maximal inactivation to more negative potentials. The biophysical changes induced by these alternatively spliced KChIP2 proteins differ markedly from previously described KChIP2 proteins and would be expected to increase the diversity of native transient outward K(+) currents.
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Affiliation(s)
- Niels Decher
- Department of Physiology, Nora Eccles Harrison Cardiovascular Research and Training Institute, University of Utah, 95 N 2000 E, Salt Lake City, UT 84112, USA.
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130
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Libbus I, Wan X, Rosenbaum DS. Electrotonic load triggers remodeling of repolarizing current Ito in ventricle. Am J Physiol Heart Circ Physiol 2004; 286:H1901-9. [PMID: 14715504 DOI: 10.1152/ajpheart.00581.2003] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
A change in activation sequence electrically remodels ventricular myocardium, causing persistent changes in repolarizing currents (T-wave memory). However, the underlying mechanism for triggering activation sequence-dependent remodeling is unknown. Optical action potentials were mapped with high resolution from the epicardial surface of the arterially perfused canine wedge preparation (n = 23) during 30 min of baseline endocardial stimulation, followed by 40 min of epicardial stimulation, and, finally, restoration of endocardial stimulation. Immediately after the change from endocardial to epicardial stimulation, phase 1 notch amplitude of epicardial cells was attenuated by 74 +/- 8% (P < 0.001) compared with baseline and continued to diminish during the period of epicardial pacing, suggesting progressive remodeling of the transient outward current (Ito). When endocardial pacing was restored, notch amplitude did not immediately recover but remained attenuated by 23 +/- 10% (P < 0.001), also consistent with a remodeling effect. Peak Ito current measured from isolated epicardial myocytes changed by 12 +/- 4% (P < 0.025), providing direct evidence for Ito remodeling occurring on a surprisingly short time scale. The mechanism for triggering remodeling of Ito was a significant reduction (by 14 +/- 4%, P < 0.001) of upstroke amplitude in epicardial cells during epicardial stimulation. Reduction in upstroke amplitude during epicardial pacing was explained by electrotonic load on epicardial cells by fully repolarized downstream endocardial cells. These data suggest a novel mechanism for triggering electrical remodeling in the ventricle. Electrotonic load imposed by a change in activation sequence reduces upstroke amplitude, which, in turn, attenuates Ito according to its known voltage-dependent properties, triggering downregulation of current.
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Affiliation(s)
- Imad Libbus
- The Heart and Vascular Research Center and Departments of Medicine and Biomedical Engineering, MetroHealth Campus, Case Western Reserve University, Cleveland, Ohio 44109-1998, USA
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131
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ten Tusscher KHWJ, Noble D, Noble PJ, Panfilov AV. A model for human ventricular tissue. Am J Physiol Heart Circ Physiol 2003; 286:H1573-89. [PMID: 14656705 DOI: 10.1152/ajpheart.00794.2003] [Citation(s) in RCA: 686] [Impact Index Per Article: 32.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The experimental and clinical possibilities for studying cardiac arrhythmias in human ventricular myocardium are very limited. Therefore, the use of alternative methods such as computer simulations is of great importance. In this article we introduce a mathematical model of the action potential of human ventricular cells that, while including a high level of electrophysiological detail, is computationally cost-effective enough to be applied in large-scale spatial simulations for the study of reentrant arrhythmias. The model is based on recent experimental data on most of the major ionic currents: the fast sodium, L-type calcium, transient outward, rapid and slow delayed rectifier, and inward rectifier currents. The model includes a basic calcium dynamics, allowing for the realistic modeling of calcium transients, calcium current inactivation, and the contraction staircase. We are able to reproduce human epicardial, endocardial, and M cell action potentials and show that differences can be explained by differences in the transient outward and slow delayed rectifier currents. Our model reproduces the experimentally observed data on action potential duration restitution, which is an important characteristic for reentrant arrhythmias. The conduction velocity restitution of our model is broader than in other models and agrees better with available data. Finally, we model the dynamics of spiral wave rotation in a two-dimensional sheet of human ventricular tissue and show that the spiral wave follows a complex meandering pattern and has a period of 265 ms. We conclude that the proposed model reproduces a variety of electrophysiological behaviors and provides a basis for studies of reentrant arrhythmias in human ventricular tissue.
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Affiliation(s)
- K H W J ten Tusscher
- Department of Theoretical Biology, Utrecht University, 3584 CH Utrecht, The Netherlands.
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132
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Akar FG, Wu RC, Deschenes I, Armoundas AA, Piacentino V, Houser SR, Tomaselli GF. Phenotypic differences in transient outward K+ current of human and canine ventricular myocytes: insights into molecular composition of ventricular Ito. Am J Physiol Heart Circ Physiol 2003; 286:H602-9. [PMID: 14527940 DOI: 10.1152/ajpheart.00673.2003] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The Ca(2+)-independent transient outward K(+) current (I(to)) plays an important electrophysiological role in normal and diseased hearts. However, its contribution to ventricular repolarization remains controversial because of differences in its phenotypic expression and function across species. The dog, a frequently used model of human cardiac disease, exhibits altered functional expression of I(to). To better understand the relevance of electrical remodeling in dogs to humans, we studied the phenotypic differences in ventricular I(to) of both species with electrophysiological, pharmacological, and protein-chemical techniques. Several notable distinctions were elucidated, including slower current decay, more rapid recovery from inactivation, and a depolarizing shift of steady-state inactivation in human vs. canine I(to). Whereas recovery from inactivation of human I(to) followed a monoexponential time course, canine I(to) recovered with biexponential kinetics. Pharmacological sensitivity to flecainide was markedly greater in human than canine I(to), and exposure to oxidative stress did not alter the inactivation kinetics of I(to) in either species. Western blot analysis revealed immunoreactive bands specific for Kv4.3, Kv1.4, and Kv channel-interacting protein (KChIP)2 in dog and human, but with notable differences in band sizes across species. We report for the first time major variations in phenotypic properties of human and canine ventricular I(to) despite the presence of the same subunit proteins in both species. These data suggest that differences in electrophysiological and pharmacological properties of I(to) between humans and dogs are not caused by differential expression of the K channel subunit genes thought to encode I(to), but rather may arise from differences in molecular structure and/or posttranslational modification of these subunits.
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Affiliation(s)
- Fadi G Akar
- Institute of Molecular Cardiobiology, Division of Cardiology, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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133
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Kobayashi T, Yamada Y, Nagashima M, Seki S, Tsutsuura M, Ito Y, Sakuma I, Hamada H, Abe T, Tohse N. Contribution of KChIP2 to the developmental increase in transient outward current of rat cardiomyocytes. J Mol Cell Cardiol 2003; 35:1073-82. [PMID: 12967630 DOI: 10.1016/s0022-2828(03)00199-8] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
The Ca(2+)-independent, voltage-gated transient outward current (I(to)) displays a marked increase during development of cardiomyocytes. However, the molecular mechanism remained unclear. In rat adult ventricular myocytes, I(to) can be divided into a fast (I(to,f)) and a slow (I(to,s)) component by recovery process from inactivation. Voltage-gated K(+) channel-interacting proteins 2 (KChIP2) has recently been shown to modify membrane expressions and current densities of I(to,f). Here we examined the developmental change of I(to) and the putative molecular correlates of I(to,f) (Kv4.2 and Kv4.3) and KChIP2 in rat ventricular myocytes. Even in rat embryonic day 12 (E12) myocytes, we detected I(to). However, I(to) in E12 was solely composed of I(to,s). In postnatal day 10 (P10), we recorded much increased I(to) composed of two components (I(to,f) and I(to,s)), and I(to,f) was dominant. Thus, the developmental increase of I(to) from E12 to P10 can be explained by the dramatic appearance of I(to,f). Real-time RT-PCR revealed that Kv4.2 and Kv4.3 mRNA levels were slightly changed. By contrast, KChIP2 mRNA level increased from E12 to P10 by 731-fold. Therefore, the huge increase of KChIP2 expression was likely to be the cause of the great increase of I(to,f). In order to confirm that KChIP2 is crucial to induce I(to,f), we used adenoviral gene transfer technique. When KChIP2 was over-expressed in E12 myocytes, a great amplitude of I(to,f) appeared. Immunocytochemical experiments also demonstrated that KChIP2 enhanced the trafficking of Kv4.2 channels to cell surface. These results indicate that KChIP2 plays an important role in the generation of functional I(to,f) channels during development.
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Affiliation(s)
- Takeshi Kobayashi
- Department of Cellular Physiology and Signal Transduction, Sapporo Medical University School of Medicine, South 1 West 17, Chuo-ku, Sapporo 060 8556, Japan.
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134
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Armoundas AA, Hobai IA, Tomaselli GF, Winslow RL, O’Rourke B. Role of sodium-calcium exchanger in modulating the action potential of ventricular myocytes from normal and failing hearts. Circ Res 2003; 93:46-53. [PMID: 12805237 PMCID: PMC1237121 DOI: 10.1161/01.res.0000080932.98903.d8] [Citation(s) in RCA: 140] [Impact Index Per Article: 6.7] [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
Increased Na+-Ca2+ exchange (NCX) activity in heart failure and hypertrophy may compensate for depressed sarcoplasmic reticular Ca2+ uptake, provide inotropic support through reverse-mode Ca2+ entry, and/or deplete intracellular Ca2+ stores. NCX is electrogenic and depends on Na+ and Ca2+ transmembrane gradients, making it difficult to predict its effect on the action potential (AP). Here, we examine the effect of [Na+]i on the AP in myocytes from normal and pacing-induced failing canine hearts and estimate the direction of the NCX driving force using simultaneously recorded APs and Ca2+ transients. AP duration shortened with increasing [Na+]i and was correlated with a shift in the reversal point of the NCX driving force. At [Na+]i > or =10 mmol/L, outward NCX current during the plateau facilitated repolarization, whereas at 5 mmol/L [Na+]i, NCX had a depolarizing effect, confirmed by partially inhibiting NCX with exchange inhibitory peptide. Exchange inhibitory peptide shortened the AP duration at 5 mmol/L [Na+]i and prolonged it at [Na+]i > or =10 mmol/L. With K+ currents blocked, total membrane current was outward during the late plateau of an AP clamp at 10 mmol/L [Na+]i and became inward close to the predicted reversal point for the NCX driving force. The results were reproduced using a computer model. These results indicate that NCX plays an important role in shaping the AP of the canine myocyte, helping it to repolarize at high [Na+]i, especially in the failing heart, but contributing a depolarizing, potentially arrhythmogenic, influence at low [Na+]i.
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Affiliation(s)
| | | | | | | | - Brian O’Rourke
- Correspondence to Brian O’Rourke, PhD, The Johns Hopkins University, School of Medicine, Institute of Molecular Cardiobiology, 844 Ross Bldg, 720 Rutland Ave, Baltimore, MD 21205-2195. E-mail
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135
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Abstract
This article reviews recent advances in genomics and informatics relevant to cardiovascular research. In particular, we review the status of (1) whole genome sequencing efforts in human, mouse, rat, zebrafish, and dog; (2) the development of data mining and analysis tools; (3) the launching of the National Heart, Lung, and Blood Institute Programs for Genomics Applications and Proteomics Initiative; (4) efforts to characterize the cardiac transcriptome and proteome; and (5) the current status of computational modeling of the cardiac myocyte. In each instance, we provide links to relevant sources of information on the World Wide Web and critical appraisals of the promises and the challenges of an expanding and diverse information landscape.
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Affiliation(s)
- Raimond L Winslow
- The Whitaker Biomedical Engineering Institute and Center for Cardiovascular Bioinformatics and Modeling, The Johns Hopkins University School of Medicine, Baltimore, Md 21218, USA.
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136
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Bodi I, Muth JN, Hahn HS, Petrashevskaya NN, Rubio M, Koch SE, Varadi G, Schwartz A. Electrical remodeling in hearts from a calcium-dependent mouse model of hypertrophy and failure: complex nature of K+ current changes and action potential duration. J Am Coll Cardiol 2003; 41:1611-22. [PMID: 12742305 DOI: 10.1016/s0735-1097(03)00244-4] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
OBJECTIVES This study was designed to identify possible electrical remodeling (ER) in transgenic (Tg) mice with over-expressed L-type Ca(2+) channels. Transient outward K(+) current (I(to)) and action potential duration (APD) were studied in 2-, 4-, 8-, and 9- to 12-month-old mice to determine linkage to ventricular remodeling (VR), ER, and heart failure (HF). BACKGROUND Prolongation of APD and reduction in current density of I(to) are thought to be hallmarks of VR and HF. Mechanisms are not understood. METHODS Patch-clamp, perfused hearts, echocardiography, and Western blots were employed using 2-, 4-, 8-, and 9- to 12-month-old Tg mice. RESULTS Transgenic mice developed slow VR statistically manifesting at four months and continuing through death at 12 to 14 months, despite a slight up-regulation of I(to). A slight decrease or no change in APD was observed up to eight months; however, at 9 to 12 months, a small increase in APD was detected. Early afterdepolarizations were observed after application of 4-aminopyridine in Tg mice. No change was detected in protein of Kv4.3 and Kv4.2 up to eight months. At 9 to 12 months, Tg mice showed a slight decrease (41.4 +/- 6.9%, p < 0.05) in Kv4.2, consistent with a decrease in I(to). Surprisingly, Kv1.4 (the "fetal" K(+)-channel form) was up-regulated, and restitution of I(to) was slowed. Echocardiography revealed cardiac enlargement with impaired chamber function in hearts that were taken from the older animals. CONCLUSIONS Contrary to accepted dogma, APD and I(to) in a mouse model of hypertrophy and HF are not hallmarks of pathophysiology. We suggest that [Ca(2+)](i) (i.e., [Ca(2+)] concentration) is the primary factor in triggering cardiac enlargement and arrhythmogenesis.
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Affiliation(s)
- Ilona Bodi
- Institute of Molecular Pharmacology and Biophysics, Department of Surgery, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267-0828, USA
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137
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Zhang Y, MacLean JN, An WF, Lanning CC, Harris-Warrick RM. KChIP1 and frequenin modify shal-evoked potassium currents in pyloric neurons in the lobster stomatogastric ganglion. J Neurophysiol 2003; 89:1902-9. [PMID: 12612050 DOI: 10.1152/jn.00837.2002] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The transient potassium current (I(A)) plays an important role in shaping the firing properties of pyloric neurons in the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus. The shal gene encodes I(A) in pyloric neurons. However, when we over-expressed the lobster Shal protein by shal RNA injection into the pyloric dilator (PD) neuron, the increased I(A) had somewhat different properties from the endogenous I(A). The recently cloned K-channel interacting proteins (KChIPs) can modify vertebrate Kv4 channels in cloned cell lines. When we co-expressed hKChIP1 with lobster shal in Xenopus oocytes or lobster PD neurons, they produced A-currents resembling the endogenous I(A) in PD neurons; compared with currents evoked by shal alone, their voltage for half inactivation was depolarized, their kinetics of inactivation were slowed, and their recovery from inactivation was accelerated. We also co-expressed shal in PD neurons with lobster frequenin, which encodes a protein belonging to the same EF-hand family of Ca(2+) sensing proteins as hKChIP. Frequenin also restored most of properties of the shal-evoked currents to those of the endogenous A-currents, but the time course of recovery from inactivation was not corrected. These results suggest that lobster shal proteins normally interact with proteins in the KChIP/frequenin family to produce the transient potassium current in pyloric neurons.
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Affiliation(s)
- Y Zhang
- Department of Neurobiology and Behavior, Cornell University, Ithaca, New York 14853, USA.
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138
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Abstract
INTRODUCTION Although it is well established that alterations in heart rate or activation sequence induce electrical remodeling in the atria, electrical remodeling in the ventricle is poorly understood. METHODS AND RESULTS To determine the changes in cellular repolarization that underlie ventricular electrical remodeling caused separately by altered heart rate and activation sequence, optical action potentials were recorded simultaneously from 256 sites spanning the transmural wall of the arterially perfused canine wedge preparation (n = 15). Action potentials were compared from the same sites under identical conditions [endocardial pacing, cycle length (CL) = 1,000 msec], before and after an intervening 20- to 60-minute period of remodeling induced by (1) rapid pacing (CL = 300 msec) with no change in activation sequence; (2) altered activation sequence (epicardial pacing) with no change in rate; or (3) no change in rate or activation sequence (control). Action potential duration (APD) shortened by 24.8 +/- 4.8 msec following a period of rapid heart rate (P < 0.05) but prolonged (by 12.7 +/- 1.8 msec) following a period of altered activation sequence (P < 0.05). Hence, even after restoration of baseline heart rate and activation sequence, there were persistent changes in APD from baseline, indicative of electrical remodeling. Moreover, the orientation of the maximum APD gradient across the transmural wall changed more significantly following heart rate remodeling (by 27.7 degrees +/- 4.9 degrees, P < 0.05) than following activation sequence remodeling (by 12.3 degrees +/- 2.4 degrees, P < 0.05). CONCLUSION Persistent changes in ventricular repolarization can be induced by surprisingly short periods of altered rate or activation sequence. In contrast to atrial remodeling, electrical remodeling in the ventricle can result in prolonged APD (with altered activation sequence) or reversal of APD gradient orientation (with rapid rate), suggesting that the nature of ventricular electrical remodeling induced by these two perturbations is different.
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Affiliation(s)
- Imad Libbus
- The Heart and Vascular Research Center, Department of Medicine, MetroHealth Campus, Case Western Reserve University, Cleveland, Ohio 44109-1998, USA
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139
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Czarnecki A, Dufy-Barbe L, Huet S, Odessa MF, Bresson-Bepoldin L. Potassium channel expression level is dependent on the proliferation state in the GH3 pituitary cell line. Am J Physiol Cell Physiol 2003; 284:C1054-64. [PMID: 12620897 DOI: 10.1152/ajpcell.00446.2002] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Previously, we showed that the peak density of the transient outward K(+) current (I(to)) expressed in GH3 cells was different in the S phase than in other phases of the cell cycle. Using cell synchronization, we show here that I(to) drops precisely at the quiescent (G(0) phase)/proliferating transition. This change is not due to a modification in the voltage dependence of I(to), but rather to a modification in its inactivation kinetics. Molecular determination of K(+) channel subunits showed that I(to) required the expression of Kv1.4, Kv4.1, and Kv4.3. We found that the increase in I(to) density during the quiescent state was accompanied by an increase in Kv1.4 protein expression, whereas Kv4.3 expression remained unchanged. We further demonstrate that the link between I(to) expression and cell proliferation is not mediated by variations in cell excitability. These results provide new evidence for the cell cycle dependence of I(to) expression, which could be relevant in understanding the mechanisms leading to pituitary adenomas.
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Affiliation(s)
- Antonny Czarnecki
- Laboratoire de Physiologie et Physiopathologie de la Signalisation Cellulaire, CNRS UMR 5543, Université de Bordeaux 2, 33076 Cedex Bordeaux, France
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140
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Sah R, Ramirez RJ, Oudit GY, Gidrewicz D, Trivieri MG, Zobel C, Backx PH. Regulation of cardiac excitation-contraction coupling by action potential repolarization: role of the transient outward potassium current (I(to)). J Physiol 2003; 546:5-18. [PMID: 12509475 PMCID: PMC2342473 DOI: 10.1113/jphysiol.2002.026468] [Citation(s) in RCA: 204] [Impact Index Per Article: 9.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
The cardiac action potential (AP) is critical for initiating and coordinating myocyte contraction. In particular, the early repolarization period of the AP (phase 1) strongly influences the time course and magnitude of the whole-cell intracellular Ca(2+) transient by modulating trans-sarcolemmal Ca(2+) influx through L-type Ca(2+) channels (I(Ca,L)) and Na-Ca exchangers (I(Ca,NCX)). The transient outward potassium current (I(to)) has kinetic properties that make it especially effective in modulating the trajectory of phase 1 repolarization and thereby cardiac excitation-contraction coupling (ECC). The magnitude of I(to) varies greatly during cardiac development, between different regions of the heart, and is invariably reduced as a result of heart disease, leading to corresponding variations in ECC. In this article, we review evidence supporting a modulatory role of I(to) in ECC through its influence on I(Ca,L), and possibly I(Ca,NCX). We also discuss differential effects of I(to) on ECC between different species, between different regions of the heart and in heart disease.
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Affiliation(s)
- Rajan Sah
- Department of Physiology, University of Toronto, Heart & Stroke/Richard Lewar Centre, Room 68, Fitzgerald Building, 150 College Street, Toronto, Ontario, M5S 3E2, Canada
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141
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Greenstein JL, Winslow RL. An integrative model of the cardiac ventricular myocyte incorporating local control of Ca2+ release. Biophys J 2002; 83:2918-45. [PMID: 12496068 PMCID: PMC1201479 DOI: 10.1016/s0006-3495(02)75301-0] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
The local control theory of excitation-contraction (EC) coupling in cardiac muscle asserts that L-type Ca(2+) current tightly controls Ca(2+) release from the sarcoplasmic reticulum (SR) via local interaction of closely apposed L-type Ca(2+) channels (LCCs) and ryanodine receptors (RyRs). These local interactions give rise to smoothly graded Ca(2+)-induced Ca(2+) release (CICR), which exhibits high gain. In this study we present a biophysically detailed model of the normal canine ventricular myocyte that conforms to local control theory. The model formulation incorporates details of microscopic EC coupling properties in the form of Ca(2+) release units (CaRUs) in which individual sarcolemmal LCCs interact in a stochastic manner with nearby RyRs in localized regions where junctional SR membrane and transverse-tubular membrane are in close proximity. The CaRUs are embedded within and interact with the global systems of the myocyte describing ionic and membrane pump/exchanger currents, SR Ca(2+) uptake, and time-varying cytosolic ion concentrations to form a model of the cardiac action potential (AP). The model can reproduce both the detailed properties of EC coupling, such as variable gain and graded SR Ca(2+) release, and whole-cell phenomena, such as modulation of AP duration by SR Ca(2+) release. Simulations indicate that the local control paradigm predicts stable APs when the L-type Ca(2+) current is adjusted in accord with the balance between voltage- and Ca(2+)-dependent inactivation processes as measured experimentally, a scenario where common pool models become unstable. The local control myocyte model provides a means for studying the interrelationship between microscopic and macroscopic behaviors in a manner that would not be possible in experiments.
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Affiliation(s)
- Joseph L Greenstein
- Department of Biomedical Engineering, Center for Computational Medicine and Biology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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142
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Clancy CE, Rudy Y. Na(+) channel mutation that causes both Brugada and long-QT syndrome phenotypes: a simulation study of mechanism. Circulation 2002; 105:1208-13. [PMID: 11889015 PMCID: PMC1997279 DOI: 10.1161/hc1002.105183] [Citation(s) in RCA: 234] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
BACKGROUND Complex physiological interactions determine the functional consequences of gene abnormalities and make mechanistic interpretation of phenotypes extremely difficult. A recent example is a single mutation in the C terminus of the cardiac Na(+) channel, 1795insD. The mutation causes two distinct clinical syndromes, long QT (LQT) and Brugada, leading to life-threatening cardiac arrhythmias. Coexistence of these syndromes is seemingly paradoxical; LQT is associated with enhanced Na(+) channel function, and Brugada with reduced function. METHODS AND RESULTS Using a computational approach, we demonstrate that the 1795insD mutation exerts variable effects depending on the myocardial substrate. We develop Markov models of the wild-type and 1795insD cardiac Na(+) channels. By incorporating the models into a virtual transgenic cell, we elucidate the mechanism by which 1795insD differentially disrupts cellular electrical behavior in epicardial and midmyocardial cell types. We provide a cellular mechanistic basis for the ECG abnormalities observed in patients carrying the 1795insD gene mutation. CONCLUSIONS We demonstrate that the 1795insD mutation can cause both LQT and Brugada syndromes through interaction with the heterogeneous myocardium in a rate-dependent manner. The results highlight the complexity and multiplicity of genotype-phenotype relationships, and the usefulness of computational approaches in establishing a mechanistic link between genetic defects and functional abnormalities.
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Affiliation(s)
- Colleen E Clancy
- Cardiac Bioelectricity Research and Training Center, Department of Biomedical Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7207, USA
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143
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Beck EJ, Bowlby M, An WF, Rhodes KJ, Covarrubias M. Remodelling inactivation gating of Kv4 channels by KChIP1, a small-molecular-weight calcium-binding protein. J Physiol 2002; 538:691-706. [PMID: 11826158 PMCID: PMC2290090 DOI: 10.1113/jphysiol.2001.013127] [Citation(s) in RCA: 111] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Calcium-binding proteins dubbed KChIPs favour surface expression and modulate inactivation gating of neuronal and cardiac A-type Kv4 channels. To investigate their mechanism of action, Kv4.1 or Kv4.3 were expressed in Xenopus laevis oocytes, either alone or together with KChIP1, and the K+ currents were recorded using the whole-oocyte voltage-clamp and patch-clamp methods. KChIP1 similarly remodels gating of both channels. At positive voltages, KChIP1 slows the early phase of the development of macroscopic inactivation. By contrast, the late phase is accelerated, which allows complete inactivation in < 500 ms. Thus, superimposed traces from control and KChIP1-remodelled currents crossover. KChIP1 also accelerates closed-state inactivation and recovery from inactivation (3- to 5-fold change). The latter effect is dominating and, consequently, the prepulse inactivation curves exhibit depolarizing shifts (DeltaV = 4-12 mV). More favourable closed-state inactivation may also contribute to the overall faster inactivation at positive voltages because Kv4 channels significantly inactivate from the preopen closed state. KChIP1 favours this pathway further by accelerating channel closing. The peak G-V curves are modestly leftward shifted in the presence of KChIP1, but the apparent 'threshold' voltage of current activation remains unaltered. Single Kv4.1 channels exhibited multiple conductance levels that ranged between 1.8 and 5.6 pS in the absence of KChIP1 and between 1.9 and 5.3 pS in its presence. Thus, changes in unitary conductance do not contribute to current upregulation by KChIP1. An allosteric kinetic model explains the kinetic changes by assuming that KChIP1 mainly impairs open-state inactivation, favours channel closing and lowers the energy barrier of closed-state inactivation.
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Affiliation(s)
- Edward J Beck
- Jefferson Medical College at Thomas Jefferson University, Philadelphia, PA 19107, USA.
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144
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Bénitah JP, Perrier E, Gómez AM, Vassort G. Effects of aldosterone on transient outward K+ current density in rat ventricular myocytes. J Physiol 2001; 537:151-60. [PMID: 11711569 PMCID: PMC2278939 DOI: 10.1111/j.1469-7793.2001.0151k.x] [Citation(s) in RCA: 80] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022] Open
Abstract
1. Aldosterone, a major ionic homeostasis regulator, might also regulate cardiac ion currents. Using the whole-cell patch-clamp technique, we investigated whether aldosterone affects the 4-aminopyridine-sensitive transient outward K+ current (I(to1)). 2. Exposure to 100 nM aldosterone for 48 h at 37 degrees C produced a 1.6-fold decrease in the I(to1) density compared to control myocytes incubated without aldosterone. Neither the time- nor voltage-dependent properties of the current were significantly altered after aldosterone treatment. RU28318 (1 microM), a specific mineralocorticoid receptor antagonist, prevented the aldosterone-induced decrease in I(to1) density. 3. When myocytes were incubated for 24 h with aldosterone, concentrations up to 1 microM did not change I(to1) density, whereas L-type Ca(2+) current (I(Ca,L)) density increased. After 48 h, aldosterone caused a further increase in I(Ca,L). The delay in the I(to1) response to aldosterone might indicate that it occurs secondary to an increase in I(Ca,L). 4. After 24 h of aldosterone pretreatment, further co-incubation for 24 h either with an I(Ca,L) antagonist (100 nM nifedipine) or with a permeant Ca(2+) chelator (10 microM BAPTA-AM) prevented a decrease in I(to1) density. 5. After 48 h of aldosterone treatment, we observed a 2.5-fold increase in the occurrence of spontaneous Ca(2+) sparks, which was blunted by co-treatment with nifedipine. 6. We conclude that aldosterone decreases I(to1) density. We suggest that this decrease is secondary to the modulation of intracellular Ca(2+) signalling, which probably arises from the aldosterone-induced increase in I(Ca,L). These results provide new insights into how cardiac ionic currents are modulated by hormones.
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Affiliation(s)
- J P Bénitah
- Laboratoire de Physiopathologie Cardiovasculaire, INSERM U-390, IFR3, CHU Arnaud de Villeneuve, 34295 Montpellier, France.
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145
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Bähring R, Boland LM, Varghese A, Gebauer M, Pongs O. Kinetic analysis of open- and closed-state inactivation transitions in human Kv4.2 A-type potassium channels. J Physiol 2001; 535:65-81. [PMID: 11507158 PMCID: PMC2278757 DOI: 10.1111/j.1469-7793.2001.00065.x] [Citation(s) in RCA: 100] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
Abstract
1. We studied the gating kinetics of Kv4.2 channels, the molecular substrate of neuronal somatodendritic A-type currents. For this purpose wild-type and mutant channels were transiently expressed in the human embryonic kidney (HEK) 293 cell line and currents were measured in the whole-cell patch-clamp configuration. 2. Kv4.2 channels inactivated from pre-open closed state(s) with a mean time constant of 959 ms at -50 mV. This closed-state inactivation was not affected by a deletion of the Kv4.2 N-terminus (Delta2-40). 3. Kv4.2 currents at +40 mV inactivated with triple-exponential kinetics. A fast component (tau = 11 ms) accounted for 73 %, an intermediate component (tau = 50 ms) for 23 % and a slow component (tau = 668 ms) for 4 % of the total decay. 4. Both the fast and the intermediate components of inactivation were slowed by a deletion of the Kv4.2 N-terminus (tau = 35 and 111 ms) and accounted for 33 and 56 %, respectively, of the total decay. The slow component was moderately accelerated by the truncation (tau = 346 ms) and accounted for 11 % of the total Kv4.2 current inactivation. 5. Recovery from open-state inactivation and recovery from closed-state inactivation occurred with similar kinetics in a strongly voltage-dependent manner. Neither recovery reaction was affected by the N-terminal truncation. 6. Kv4.2 Delta2-40 channels displayed slowed deactivation kinetics, suggesting that the N-terminal truncation leads to a stabilization of the open state. 7. Simulations with an allosteric model of inactivation, supported by the experimental data, suggested that, in response to membrane depolarization, Kv4.2 channels accumulate in the closed-inactivated state(s), from which they directly recover, bypassing the open state.
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Affiliation(s)
- R Bähring
- Institut für Neurale Signalverarbeitung, Zentrum für Molekulare Neurobiologie der Universität Hamburg, 20246 Hamburg, Germany.
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146
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Oudit GY, Kassiri Z, Sah R, Ramirez RJ, Zobel C, Backx PH. The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium. J Mol Cell Cardiol 2001; 33:851-72. [PMID: 11343410 DOI: 10.1006/jmcc.2001.1376] [Citation(s) in RCA: 131] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
G. Y. Oudit, Z. Kassiri, R. Sah, R. J. Ramirez, C. Zobel and P. H. Backx. The Molecular Physiology of the Cardiac Transient Outward Potassium Current (I(to)) in Normal and Diseased Myocardium. Journal of Molecular and Cellular Cardiology (2001) 33, 851-872. The Ca(2+)-independent transient outward potassium current (I(to)) plays an important role in early repolarization of the cardiac action potential. I(to)has been clearly demonstrated in myocytes from different cardiac regions and species. Two kinetic variants of cardiac I(to)have been identified: fast I(to), called I(to,f), and slow I(to), called I(to,s). Recent findings suggest that I(to,f)is formed by assembly of K(v4.2)and/or K(v4.3)alpha pore-forming voltage-gated subunits while I(to,s)is comprised of K(v1.4)and possibly K(v1.7)subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac I(to)have been identified. Experimental findings and data from computer modeling of cardiac action potentials have conclusively established an important physiological role of I(to)in rodents, with its role in large mammals being less well defined due to complex interplay between a multitude of cardiac ionic currents. A central and consistent electrophysiological change in cardiac disease is the reduction in I(to)density with a loss of heterogeneity of I(to)expression and associated action potential prolongation. Alterations of I(to)in rodent cardiac disease have been linked to repolarization abnormalities and alterations in intracellular Ca(2+)homeostasis, while in larger mammals the link with functional changes is far less certain. We review the current literature on the molecular basis for cardiac I(to)and the functional consequences of changes in I(to)that occur in cardiovascular disease.
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Affiliation(s)
- G Y Oudit
- Department of Medicine and Physiology, Toronto General Hospital, 101 College Street, Toronto, M5G 2C4, Canada
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