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Asfaw TN, Tyan L, Glukhov AV, Bondarenko VE. A compartmentalized mathematical model of mouse atrial myocytes. Am J Physiol Heart Circ Physiol 2020; 318:H485-H507. [PMID: 31951471 DOI: 10.1152/ajpheart.00460.2019] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Various experimental mouse models are extensively used to research human diseases, including atrial fibrillation, the most common cardiac rhythm disorder. Despite this, there are no comprehensive mathematical models that describe the complex behavior of the action potential and [Ca2+]i transients in mouse atrial myocytes. Here, we develop a novel compartmentalized mathematical model of mouse atrial myocytes that combines the action potential, [Ca2+]i dynamics, and β-adrenergic signaling cascade for a subpopulation of right atrial myocytes with developed transverse-axial tubule system. The model consists of three compartments related to β-adrenergic signaling (caveolae, extracaveolae, and cytosol) and employs local control of Ca2+ release. It also simulates ionic mechanisms of action potential generation and describes atrial-specific Ca2+ handling as well as frequency dependences of the action potential and [Ca2+]i transients. The model showed that the T-type Ca2+ current significantly affects the later stage of the action potential, with little effect on [Ca2+]i transients. The block of the small-conductance Ca2+-activated K+ current leads to a prolongation of the action potential at high intracellular Ca2+. Simulation results obtained from the atrial model cells were compared with those from ventricular myocytes. The developed model represents a useful tool to study complex electrical properties in the mouse atria and could be applied to enhance the understanding of atrial physiology and arrhythmogenesis.NEW & NOTEWORTHY A new compartmentalized mathematical model of mouse right atrial myocytes was developed. The model simulated action potential and Ca2+ dynamics at baseline and after stimulation of the β-adrenergic signaling system. Simulations showed that the T-type Ca2+ current markedly prolonged the later stage of atrial action potential repolarization, with a minor effect on [Ca2+]i transients. The small-conductance Ca2+-activated K+ current block resulted in prolongation of the action potential only at the relatively high intracellular Ca2+.
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Affiliation(s)
- Tesfaye Negash Asfaw
- Department of Mathematics and Statistics, Georgia State University, Atlanta, Georgia
| | - Leonid Tyan
- Department of Medicine, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin
| | - Alexey V Glukhov
- Department of Medicine, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, Wisconsin
| | - Vladimir E Bondarenko
- Department of Mathematics and Statistics, Georgia State University, Atlanta, Georgia.,Neuroscience Institute, Georgia State University, Atlanta, Georgia
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2
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Cazade M, Bidaud I, Lory P, Chemin J. Activity-dependent regulation of T-type calcium channels by submembrane calcium ions. eLife 2017; 6. [PMID: 28109159 PMCID: PMC5308894 DOI: 10.7554/elife.22331] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2016] [Accepted: 01/20/2017] [Indexed: 12/19/2022] Open
Abstract
Voltage-gated Ca2+ channels are involved in numerous physiological functions and various mechanisms finely tune their activity, including the Ca2+ ion itself. This is well exemplified by the Ca2+-dependent inactivation of L-type Ca2+ channels, whose alteration contributes to the dramatic disease Timothy Syndrome. For T-type Ca2+ channels, a long-held view is that they are not regulated by intracellular Ca2+. Here we challenge this notion by using dedicated electrophysiological protocols on both native and expressed T-type Ca2+ channels. We demonstrate that a rise in submembrane Ca2+ induces a large decrease in T-type current amplitude due to a hyperpolarizing shift in the steady-state inactivation. Activation of most representative Ca2+-permeable ionotropic receptors similarly regulate T-type current properties. Altogether, our data clearly establish that Ca2+ entry exerts a feedback control on T-type channel activity, by modulating the channel availability, a mechanism that critically links cellular properties of T-type Ca2+ channels to their physiological roles. DOI:http://dx.doi.org/10.7554/eLife.22331.001 Neurons, muscle cells and many other types of cells use electrical signals to exchange information and coordinate their behavior. Proteins known as calcium channels sit in the membrane that surrounds the cell and can generate electrical signals by allowing calcium ions to cross the membrane and enter the cell during electrical activities. Although calcium ions are needed to generate these electrical signals, and for many other processes in cells, if the levels of calcium ions inside cells become too high they can be harmful and cause disease. Cells have a “feedback” mechanism that prevents calcium ion levels from becoming too high. This mechanism relies on the calcium ions that are already in the cell being able to close the calcium channels. This feedback mechanism has been extensively studied in two types of calcium channel, but it is not known whether a third group of channels – known as Cav3 channels – are also regulated in this way. Cav3 channels are important in electrical signaling in neurons and have been linked with epilepsy, chronic pain and various other conditions in humans. Cazade et al. investigated whether calcium ions can regulate the activity of human Cav3 channels. The experiments show that these channels are indeed regulated by calcium ions, but using a distinct mechanism to other types of calcium channels. For the Cav3 channels, calcium ions alter the gating properties of the channels so that they are less easily activated . As a result, fewer Cav3 channels are “available” to provide calcium ions with a route into the cell. The next steps following on from this work will be to identify the molecular mechanisms underlying this new feedback mechanism. Another challenge will be to find out what role this calcium ion-driven feedback plays in neurological disorders that are linked with altered Cav3 channel activity. DOI:http://dx.doi.org/10.7554/eLife.22331.002
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Affiliation(s)
- Magali Cazade
- IGF, CNRS, INSERM, University of Montpellier, Montpellier, France.,LabEx 'Ion Channel Science and Therapeutics', Montpellier, France
| | - Isabelle Bidaud
- IGF, CNRS, INSERM, University of Montpellier, Montpellier, France.,LabEx 'Ion Channel Science and Therapeutics', Montpellier, France
| | - Philippe Lory
- IGF, CNRS, INSERM, University of Montpellier, Montpellier, France.,LabEx 'Ion Channel Science and Therapeutics', Montpellier, France
| | - Jean Chemin
- IGF, CNRS, INSERM, University of Montpellier, Montpellier, France.,LabEx 'Ion Channel Science and Therapeutics', Montpellier, France
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3
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Forward suppression in the auditory cortex is caused by the Ca(v)3.1 calcium channel-mediated switch from bursting to tonic firing at thalamocortical projections. J Neurosci 2014; 33:18940-50. [PMID: 24285899 DOI: 10.1523/jneurosci.3335-13.2013] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Brief sounds produce a period of suppressed responsiveness in the auditory cortex (ACx). This forward suppression can last for hundreds of milliseconds and might contribute to mechanisms of temporal separation of sounds and stimulus-specific adaptation. However, the mechanisms of forward suppression remain unknown. We used in vivo recordings of sound-evoked responses in the mouse ACx and whole-cell recordings, two-photon calcium imaging in presynaptic terminals, and two-photon glutamate uncaging in dendritic spines performed in brain slices to show that synaptic depression at thalamocortical (TC) projections contributes to forward suppression in the ACx. Paired-pulse synaptic depression at TC projections lasts for hundreds of milliseconds and is attributable to a switch between firing modes in thalamic neurons. Thalamic neurons respond to a brief depolarizing pulse with a burst of action potentials; however, within hundreds of milliseconds, the same pulse repeated again produces only a single action potential. This switch between firing modes depends on Ca(v)3.1 T-type calcium channels enriched in thalamic relay neurons. Pharmacologic inhibition or knockdown of Ca(v)3.1 T-type calcium channels in the auditory thalamus substantially reduces synaptic depression at TC projections and forward suppression in the ACx. These data suggest that Ca(v)3.1-dependent synaptic depression at TC projections contributes to mechanisms of forward suppression in the ACx.
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4
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Nguyen N, Biet M, Simard E, Béliveau E, Francoeur N, Guillemette G, Dumaine R, Grandbois M, Boulay G. STIM1 participates in the contractile rhythmicity of HL-1 cells by moderating T-type Ca(2+) channel activity. BIOCHIMICA ET BIOPHYSICA ACTA-MOLECULAR CELL RESEARCH 2013; 1833:1294-303. [PMID: 23458835 DOI: 10.1016/j.bbamcr.2013.02.027] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/05/2012] [Revised: 02/11/2013] [Accepted: 02/20/2013] [Indexed: 12/15/2022]
Abstract
STIM1 plays a crucial role in Ca(2+) homeostasis, particularly in replenishing the intracellular Ca(2+) store following its depletion. In cardiomyocytes, the Ca(2+) content of the sarcoplasmic reticulum must be tightly controlled to sustain contractile activity. The presence of STIM1 in cardiomyocytes suggests that it may play a role in regulating the contraction of cardiomyocytes. The aim of the present study was to determine how STIM1 participates in the regulation of cardiac contractility. Atomic force microscopy revealed that knocking down STIM1 disrupts the contractility of cardiomyocyte-derived HL-1 cells. Ca(2+) imaging also revealed that knocking down STIM1 causes irregular spontaneous Ca(2+) oscillations in HL-1 cells. Action potential recordings further showed that knocking down STIM1 induces early and delayed afterdepolarizations. Knocking down STIM1 increased the peak amplitude and current density of T-type voltage-dependent Ca(2+) channels (T-VDCC) and shifted the activation curve toward more negative membrane potentials in HL-1 cells. Biotinylation assays revealed that knocking down STIM1 increased T-VDCC surface expression and co-immunoprecipitation assays suggested that STIM1 directly regulates T-VDCC activity. Thus, STIM1 is a negative regulator of T-VDCC activity and maintains a constant cardiac rhythm by preventing a Ca(2+) overload that elicits arrhythmogenic events.
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Affiliation(s)
- Nathalie Nguyen
- Department of Pharmacology, Université de Sherbrooke, Sherbrooke, QC, Canada
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6
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Abstract
Although normally absent, spontaneous pacemaker activity can develop in human atrium to promote tachyarrhythmias. HL-1 cells are immortalized atrial cardiomyocytes that contract spontaneously in culture, providing a model system of atrial cell automaticity. Using electrophysiologic recordings and selective pharmacologic blockers, we investigated the ionic basis of automaticity in atrial HL-1 cells. Both the sarcoplasmic reticulum Ca release channel inhibitor ryanodine and the sarcoplasmic reticulum Ca ATPase inhibitor thapsigargin slowed automaticity, supporting a role for intracellular Ca release in pacemaker activity. Additional experiments were performed to examine the effects of ionic currents activating in the voltage range of diastolic depolarization. Inhibition of the hyperpolarization-activated pacemaker current, If, by ivabradine significantly suppressed diastolic depolarization, with modest slowing of automaticity. Block of inward Na currents also reduced automaticity, whereas inhibition of T- and L-type Ca currents caused milder effects to slow beat rate. The major outward current in HL-1 cells is the rapidly activating delayed rectifier, IKr. Inhibition of IKr using dofetilide caused marked prolongation of action potential duration and thus spontaneous cycle length. These results demonstrate a mutual role for both intracellular Ca release and sarcolemmal ionic currents in controlling automaticity in atrial HL-1 cells. Given that similar internal and membrane-based mechanisms also play a role in sinoatrial nodal cell pacemaker activity, our findings provide evidence for generalized conservation of pacemaker mechanisms among different types of cardiomyocytes.
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Affiliation(s)
- Zhenjiang Yang
- Department of Medicine and Clinical Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37232-6602, USA
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7
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Nelson MT, Milescu LS, Todorovic SM, Scroggs RS. A modeling study of T-type Ca2+ channel gating and modulation by L-cysteine in rat nociceptors. Biophys J 2010; 98:197-206. [PMID: 20338841 DOI: 10.1016/j.bpj.2009.10.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2009] [Revised: 09/02/2009] [Accepted: 10/05/2009] [Indexed: 11/29/2022] Open
Abstract
L-cysteine (L-cys) increases the amplitude of T-type Ca(2+) currents in rat T-rich nociceptor-like dorsal root ganglia neurons. The modulation of T-type Ca(2+) channel gating by L-cys was studied by fitting Markov state models to whole-cell currents recorded from T-rich neurons. The best fitting model tested included three resting states and inactivation from the second resting state and the open state. Inactivation and the final opening step were voltage-independent, whereas transitions between the resting states and deactivation were voltage-dependent. The transition rates between the first two resting states were an order of magnitude faster than those between the second and third resting states, and the voltage-dependency of forward transitions through resting states was two to three times greater than for analogous backward transitions. Analysis with the best fitting model suggested that L-cys increases current amplitude mainly by increasing the transition rate from resting to open and decreasing the transition rate from open to inactivated. An additional model was developed that could account for the bi-exponential time course of recovery from inactivation of the currents and the high frequency of blank sweeps in single channel recordings. This model detected basically the same effects of L-cys on channel gating as the best fitting model.
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Affiliation(s)
- Michael T Nelson
- Department of Anesthesiology, University of Virginia Health System, Charlottesville, Virginia, USA
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8
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Sun H, Varela D, Chartier D, Ruben PC, Nattel S, Zamponi GW, Leblanc N. Differential interactions of Na+ channel toxins with T-type Ca2+ channels. ACTA ACUST UNITED AC 2008; 132:101-13. [PMID: 18591418 PMCID: PMC2442173 DOI: 10.1085/jgp.200709883] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
Two types of voltage-dependent Ca2+ channels have been identified in heart: high (ICaL) and low (ICaT) voltage-activated Ca2+ channels. In guinea pig ventricular myocytes, low voltage–activated inward current consists of ICaT and a tetrodotoxin (TTX)-sensitive ICa component (ICa(TTX)). In this study, we reexamined the nature of low-threshold ICa in dog atrium, as well as whether it is affected by Na+ channel toxins. Ca2+ currents were recorded using the whole-cell patch clamp technique. In the absence of external Na+, a transient inward current activated near −50 mV, peaked at −30 mV, and reversed around +40 mV (HP = −90 mV). It was unaffected by 30 μM TTX or micromolar concentrations of external Na+, but was inhibited by 50 μM Ni2+ (by ∼90%) or 5 μM mibefradil (by ∼50%), consistent with the reported properties of ICaT. Addition of 30 μM TTX in the presence of Ni2+ increased the current approximately fourfold (41% of control), and shifted the dose–response curve of Ni2+ block to the right (IC50 from 7.6 to 30 μM). Saxitoxin (STX) at 1 μM abolished the current left in 50 μM Ni2+. In the absence of Ni2+, STX potently blocked ICaT (EC50 = 185 nM) and modestly reduced ICaL (EC50 = 1.6 μM). While TTX produced no direct effect on ICaT elicited by expression of hCaV3.1 and hCaV3.2 in HEK-293 cells, it significantly attenuated the block of this current by Ni2+ (IC50 increased to 550 μM Ni2+ for CaV3.1 and 15 μM Ni2+ for CaV3.2); in contrast, 30 μM TTX directly inhibited hCaV3.3-induced ICaT and the addition of 750 μM Ni2+ to the TTX-containing medium led to greater block of the current that was not significantly different than that produced by Ni2+ alone. 1 μM STX directly inhibited CaV3.1-, CaV3.2-, and CaV3.3-mediated ICaT but did not enhance the ability of Ni2+ to block these currents. These findings provide important new implications for our understanding of structure–function relationships of ICaT in heart, and further extend the hypothesis of a parallel evolution of Na+ and Ca2+ channels from an ancestor with common structural motifs.
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Affiliation(s)
- Hui Sun
- Excigen, Inc., Baltimore, MD 21224, USA
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9
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Abstract
The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.
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Affiliation(s)
- Matteo E Mangoni
- Institute of Functional Genomics, Department of Physiology, Centre National de la Recherche Scientifique UMR5203, INSERM U661, University of Montpellier I and II, Montpellier, France.
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10
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Schroder EA, Wei Y, Satin J. The developing cardiac myocyte: maturation of excitability and excitation-contraction coupling. Ann N Y Acad Sci 2007; 1080:63-75. [PMID: 17132775 DOI: 10.1196/annals.1380.006] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The study of cardiac myocyte (CM) differentiation, development, and maturation is of interest for several compelling reasons. First, mechanisms of development are of fundamental biological interest. Second, congenital malformation of the heart may be related to CM dysfunction during embryonic/fetal development. Third, adult myocardium in a variety of diseased states re-expresses a fetal-like gene program. Fourth, the mature heart cannot readily regenerate itself. Thus, cell replacement therapy is an emerging treatment paradigm. Among the obstacles for the realization of cell replacement therapy is our incomplete understanding of the function during CM maturation. This is crucial in the potential use of embryonic stem (ES) cell-derived CMs as a cell source. Although much progress has been realized with mouse ES-CMs, our understanding of human counterparts is scant. Here we discuss key molecular underpinnings of excitability and excitation-contraction coupling in developing mouse heart. We focus on the Ca channel multimeric complex and Ca handling. We compare mouse embryonic physiology to that previously described in mouse ES-CMs and draw parallels and highlight distinctions to human ES-CMs. During mouse embryonic and fetal maturation, the L-type Ca channel current (I(Ca,L)) predominates, but embryonic/fetal I(Ca,L) has distinct properties from mature I(Ca,L). In addition T-type Ca current (I(Ca,T)) present in the fetus is not present in the adult. It is neither ethical nor practical to experiment with live human embryonic/fetal CMs for I(Ca) and Ca handling studies, but we can draw inferences from human heart cell function based on studies of human ES-CMs, using the parallels noted between mouse embryonic heart cells and mouse ES-CMs.
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Affiliation(s)
- Elizabeth A Schroder
- Department of Physiology, MS-508, University of Kentucky College of Medicine, Lexington, KY 40536-0298, USA
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11
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Crump SM, Correll RN, Schroder EA, Lester WC, Finlin BS, Andres DA, Satin J. L-type calcium channel alpha-subunit and protein kinase inhibitors modulate Rem-mediated regulation of current. Am J Physiol Heart Circ Physiol 2006; 291:H1959-71. [PMID: 16648185 DOI: 10.1152/ajpheart.00956.2005] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Cardiac voltage-gated L-type Ca channels (Ca(V)) are multiprotein complexes, including accessory subunits such as Ca(V)beta2 that increase current expression. Recently, members of the Rad and Gem/Kir-related family of small GTPases have been shown to decrease current, although the mechanism remains poorly defined. In this study, we evaluated the contribution of the L-type Ca channel alpha-subunit (Ca(V)1.2) to Ca(V)beta2-Rem inhibition of Ca channel current. Specifically, we addressed whether protein kinase A (PKA) modulation of the Ca channel modifies Ca(V)beta2-Rem inhibition of Ca channel current. We first tested the effect of Rem on Ca(V)1.2 in human embryonic kidney 293 (HEK-293) cells using the whole cell patch-clamp configuration. Rem coexpression with Ca(V)1.2 reduces Ba current expression under basal conditions, and Ca(V)beta2a coexpression enhances Rem block of Ca(V)1.2 current. Surprisingly, PKA inhibition by 133 nM H-89 or 50 microM Rp-cAMP-S partially relieved the Rem-mediated inhibition of current activity both with and without Ca(V)beta2a. To test whether the H-89 action was a consequence of the phosphorylation status of Ca(V)1.2, we examined Rem regulation of the PKA-insensitive Ca(V)1.2 serine 1928 (S1928) to alanine mutation (Ca(V)1.2-S1928A). Ca(V)1.2-S1928A current was not inhibited by Rem and when coexpression with Ca(V)beta2a was not completely blocked by Rem coexpression, suggesting that the phosphorylation of S1928 contributes to Rem-mediated Ca channel modulation. As a model for native Ca channel complexes, we tested the ability of Rem overexpression in HIT-T15 cells and embryonic ventricular myocytes to interfere with native current. We find that native current is also sensitive to Rem block and that H-89 pretreatment relieves the ability of Rem to regulate Ca current. We conclude that Rem is capable of regulating L-type current, that release of Rem block is modulated by cellular kinase pathways, and that the Ca(V)1.2 COOH terminus contributes to Rem-dependent channel inhibition.
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Affiliation(s)
- Shawn M Crump
- Dept. of Physiology, MS-508, Univ. of Kentucky College of Medicine, 800 Rose St. Lexington, KY 40536-0298, USA
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12
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Mangoni ME, Couette B, Marger L, Bourinet E, Striessnig J, Nargeot J. Voltage-dependent calcium channels and cardiac pacemaker activity: from ionic currents to genes. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2005; 90:38-63. [PMID: 15979127 DOI: 10.1016/j.pbiomolbio.2005.05.003] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
The spontaneous activity of pacemaker cells in the sino-atrial node controls the heart rhythm and rate under physiological conditions. Compared to working myocardial cells, pacemaker cells express a specific array of ionic channels. The functional importance of different ionic channels in the generation and regulation of cardiac automaticity is currently subject of an extensive research effort and has long been controversial. Among families of ionic channels, Ca(2+) channels have been proposed to substantially contribute to pacemaking. Indeed, Ca(2+) channels are robustly expressed in pacemaker cells, and influence the cell beating rate. Furthermore, they are regulated by the activity of the autonomic nervous system in both a positive and negative way. In this manuscript, we will first discuss how the concept of the involvement of Ca(2+) channels in cardiac pacemaking has been proposed and then subsequently developed by the recent advent in the domain of cardiac physiology of gene-targeting techniques. Secondly, we will indicate how the specific profile of Ca(2+) channels expression in pacemaker tissue can help design drugs which selectively regulate the heart rhythm in the absence of concomitant negative inotropism. Finally, we will indicate how the new possibility to assign a specific gene activity to a given ionic channel involved in cardiac pacemaking could implement the current postgenomic research effort in the construction of the cardiac Physiome.
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Affiliation(s)
- Matteo E Mangoni
- Departement de Physiologie, Institut de Génomique Fonctionnelle, University of Montpellier I, CNRS UMR 5203, Montpellier F-34094, France.
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Brueggemann LI, Martin BL, Barakat J, Byron KL, Cribbs LL. Low voltage-activated calcium channels in vascular smooth muscle: T-type channels and AVP-stimulated calcium spiking. Am J Physiol Heart Circ Physiol 2004; 288:H923-35. [PMID: 15498818 DOI: 10.1152/ajpheart.01126.2003] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
An important path of extracellular calcium influx in vascular smooth muscle (VSM) cells is through voltage-activated Ca2+ channels of the plasma membrane. Both high (HVA)- and low (LVA)-voltage-activated Ca2+ currents are present in VSM cells, yet little is known about the relevance of the LVA T-type channels. In this report, we provide molecular evidence for T-type Ca2+ channels in rat arterial VSM and characterize endogenous LVA Ca2+ currents in the aortic smooth muscle-derived cell line A7r5. AVP is a vasoconstrictor hormone that, at physiological concentrations, stimulates Ca2+ oscillations (spiking) in monolayer cultures of A7r5 cells. The present study investigated the role of T-type Ca2+ channels in this response with a combination of pharmacological and molecular approaches. We demonstrate that AVP-stimulated Ca2+ spiking can be abolished by mibefradil at low concentrations (<1 microM) that should not inhibit L-type currents. Infection of A7r5 cells with an adenovirus containing the Cav3.2 T-type channel resulted in robust LVA Ca2+ currents but did not alter the AVP-stimulated Ca2+ spiking response. Together these data suggest that T-type Ca2+ channels are necessary for the onset of AVP-stimulated calcium oscillations; however, LVA Ca2+ entry through these channels is not limiting for repetitive Ca2+ spiking observed in A7r5 cells.
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Affiliation(s)
- Lioubov I Brueggemann
- Department of Medicine, Cardiovascular Institute, Loyola University Medical Center, Maywood, Illinois 60153, USA
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14
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Niwa N, Yasui K, Opthof T, Takemura H, Shimizu A, Horiba M, Lee JK, Honjo H, Kamiya K, Kodama I. Cav3.2 subunit underlies the functional T-type Ca2+ channel in murine hearts during the embryonic period. Am J Physiol Heart Circ Physiol 2004; 286:H2257-63. [PMID: 14988077 DOI: 10.1152/ajpheart.01043.2003] [Citation(s) in RCA: 78] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
T-type Ca2+ channels are implicated in cardiac automaticity, cell growth, and cardiovascular remodeling. Two voltage-gated Ca2+ subtypes (Ca(v)3.1 and Ca(v)3.2) have been cloned for the pore-forming alpha(1)-subunit of the T-type Ca2+ channel in cardiac muscle, but their differential roles remain to be clarified. The aim of this study was to elucidate the relative contribution of the two subtypes in the normal development of mouse hearts. A whole cell patch clamp was used to record ionic currents from ventricular myocytes isolated from mice of early (E9.5) and late embryonic days (E18) and from adult 10-wk-old mice. Large T-type Ca2+ current (I(Ca,T)) was observed at both E9.5 and E18, displaying similar voltage-dependence and kinetics of activation and inactivation. The current was inhibited by Ni2+ at relatively low concentrations (IC(50) 26-31 microM). I(Ca,T) was undetectable in adult myocytes. Quantitative PCR analysis revealed that Ca(v)3.2 mRNA is the predominant subtype encoding T-type Ca2+ channels at both E9.5 and E18. Ca(v)3.1 mRNA increased from E9.5 to E18, but remained low compared with Ca(v)3.2 mRNA during the whole embryonic period. In the adulthood, in contrast, Ca(v)3.1 mRNA is greater than Ca(v)3.2 mRNA. These results indicate that Ca(v)3.2 underlies the functional T-type Ca2+ channels in the embryonic murine heart, and there is a subtype switching of transcripts from Ca(v)3.2 to Ca(v)3.1 in the perinatal period.
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Affiliation(s)
- Noriko Niwa
- Department of Circulation, Division of Regulation of Organ Function, Research Institute of Environmental Medicine, Nagoya University, Nagoya 464-8601, Japan
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15
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Lee SC, Hayashida Y, Ishida AT. Availability of low-threshold Ca2+ current in retinal ganglion cells. J Neurophysiol 2004; 90:3888-901. [PMID: 14665686 PMCID: PMC3237121 DOI: 10.1152/jn.00477.2003] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Spiking in central neurons depends on the availability of inward and outward currents activated by depolarization and on the activation and priming of currents by hyperpolarization. Of these processes, priming by hyperpolarization is the least described. In the case of T-type Ca2+ current availability, the interplay of hyperpolarization and depolarization has been studied most completely in expression systems, in part because of the difficulty of pharmacologically separating the Ca2+ currents of native neurons. To facilitate understanding of this current under physiological conditions, we measured T-type current of isolated goldfish retinal ganglion cells with perforated-patch voltage-clamp methods in solutions containing a normal extracellular Ca2+ concentration. The voltage sensitivities and rates of current activation, inactivation, deactivation, and recovery from inactivation were similar to those of expressed alpha1G (CaV3.1) Ca2+ channel clones, except that the rate of deactivation was significantly faster. We reproduced the amplitude and kinetics of measured T currents with a numerical simulation based on a kinetic model developed for an alpha1G Ca2+ channel. Finally, we show that this model predicts the increase of T-type current made available between resting potential and spike threshold by repetitive hyperpolarizations presented at rates that are within the bandwidth of signals processed in situ by these neurons.
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Affiliation(s)
- Sherwin C Lee
- Section of Neurobiology, Physiology, and Behavior, University of California, Davis, California 95616-8519, USA
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16
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Hering J, Feltz A, Lambert RC. Slow inactivation of the Ca(V)3.1 isotype of T-type calcium channels. J Physiol 2003; 555:331-44. [PMID: 14694144 PMCID: PMC1664842 DOI: 10.1113/jphysiol.2003.054361] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
T-type calcium channels (the Ca(V)3 channel family) are involved in defining the resting membrane potential and in neuronal activities such as oscillations and rebound depolarization. Their physiological roles depend upon the channel activation and inactivation kinetics. A fast inactivation that stops the ionic flux of calcium in tens of milliseconds has already been described in both native and heterologously expressed channels. Here, using HEK 293 cells expressing the rat Ca(V)3.1 channel and whole-cell voltage clamp, we investigate an additional inactivation process, which can be distinguished from the previously described fast inactivation by its slow time course of recovery from inactivation (tau= 1 s) and by its sensitivity to external calcium. Steady-state slow inactivation is voltage dependent around the resting membrane potential (the potential of half-inactivation (V(0.5)) =-70 mV, slope factor = 7.4 mV) and can reduce the calcium current by up to 50%. Near resting potential, the slow inactivation displays a half-time of induction of tens of seconds. The slow inactivation therefore modulates the availability of T-type calcium channels depending upon recent cell history, providing a mechanism to store information in a time scale of seconds.
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Affiliation(s)
- Julien Hering
- Laboratoire de neurobiologie, Ecole Normale Supérieure, 46 rue d'Ulm, 75005 Paris, France
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17
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Finlin BS, Crump SM, Satin J, Andres DA. Regulation of voltage-gated calcium channel activity by the Rem and Rad GTPases. Proc Natl Acad Sci U S A 2003; 100:14469-74. [PMID: 14623965 PMCID: PMC283615 DOI: 10.1073/pnas.2437756100] [Citation(s) in RCA: 163] [Impact Index Per Article: 7.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Rem, Rem2, Rad, and Gem/Kir (RGK) represent a distinct GTPase family with largely unknown physiological functions. We report here that both Rem and Rad bind directly to Ca2+ channel beta-subunits (CaV beta) in vivo. No calcium currents are recorded from human embryonic kidney 293 cells coexpressing the L type Ca2+ channel subunits CaV1.2, CaV beta 2a, and Rem or Rad, but CaV1.2 and CaV beta 2a transfected cells elicit Ca2+ channel currents in the absence of these small G proteins. Importantly, CaV3 (T type) Ca2+ channels, which do not require accessory subunits for ionic current expression, are not inhibited by expression of Rem. Rem is expressed in primary skeletal myoblasts and, when overexpressed in C2C12 myoblasts, wild-type Rem inhibits L type Ca2+ channel activity. Deletion analysis demonstrates a critical role for the Rem C terminus in both regulation of functional Ca2+ channel expression and beta-subunit association. These results suggest that all members of the RGK GTPase family, via direct interaction with auxiliary beta-subunits, serve as regulators of L type Ca2+ channel activity. Thus, the RGK GTPase family may provide a mechanism for achieving cross talk between Ras-related GTPases and electrical signaling pathways.
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Affiliation(s)
- Brian S Finlin
- Department of Molecular and Cellular Biochemistry, University of Kentucky College of Medicine, Lexington, KY 40536-0298, USA
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18
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Abstract
T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.
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Affiliation(s)
- Edward Perez-Reyes
- Department of Pharmacology, University of Virginia, Charlottesville, Virginia 22908-0735, USA.
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19
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Gomora JC, Murbartián J, Arias JM, Lee JH, Perez-Reyes E. Cloning and expression of the human T-type channel Ca(v)3.3: insights into prepulse facilitation. Biophys J 2002; 83:229-41. [PMID: 12080115 PMCID: PMC1302142 DOI: 10.1016/s0006-3495(02)75164-3] [Citation(s) in RCA: 70] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The full-length human Ca(v)3.3 (alpha(1I)) T-type channel was cloned, and found to be longer than previously reported. Comparison of the cDNA sequence to the human genomic sequence indicates the presence of an additional 4-kb exon that adds 214 amino acids to the carboxyl terminus and encodes the 3' untranslated region. The electrophysiological properties of the full-length channel were studied after transient transfection into 293 human embryonic kidney cells using 5 mM Ca(2+) as charge carrier. From a holding potential of -100 mV, step depolarizations elicited inward currents with an apparent threshold of -70 mV, a peak of -30 mV, and reversed at +40 mV. The kinetics of channel activation, inactivation, deactivation, and recovery from inactivation were very similar to those reported previously for rat Ca(v)3.3. Similar voltage-dependent gating and kinetics were found for truncated versions of human Ca(v)3.3, which lack either 118 or 288 of the 490 amino acids that compose the carboxyl terminus. A major difference between these constructs was that the full-length isoform generated twofold more current. These results suggest that sequences in the distal portion of Ca(v)3.3 play a role in channel expression. Studies on the voltage-dependence of activation revealed that a fraction of channels did not gate as low voltage-activated channels, requiring stronger depolarizations to open. A strong depolarizing prepulse (+100 mV, 200 ms) increased the fraction of channels that gated at low voltages. In contrast, human Ca(v)3.3 isoforms with shorter carboxyl termini were less affected by a prepulse. Therefore, Ca(v)3.3 is similar to high voltage-activated Ca(2+) channels in that depolarizing prepulses can regulate their activity, and their carboxy termini play a role in modulating channel activity.
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Affiliation(s)
- Juan Carlos Gomora
- Department of Pharmacology, University of Virginia, Charlottesville, Virginia 22908, USA
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20
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Abstract
The normal electrophysiologic behavior of the heart is determined by ordered propagation of excitatory stimuli that result in rapid depolarization and slow repolarization, thereby generating action potentials in individual myocytes. Abnormalities of impulse generation, propagation, or the duration and configuration of individual cardiac action potentials form the basis of disorders of cardiac rhythm, a continuing major public health problem for which available drugs are incompletetly effective and often dangerous. The integrated activity of specific ionic currents generates action potentials, and the genes whose expression results in the molecular components underlying individual ion currents in heart have been cloned. This review discusses these new tools and how their application to the problem of arrhythmias is generating new mechanistic insights to identify patients at risk for this condition and developing improved antiarrhythmic therapies.
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Affiliation(s)
- Dan M Roden
- Departments of Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee 37232, USA.
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21
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Ferron L, Capuano V, Deroubaix E, Coulombe A, Renaud JF. Functional and molecular characterization of a T-type Ca(2+) channel during fetal and postnatal rat heart development. J Mol Cell Cardiol 2002; 34:533-46. [PMID: 12056857 DOI: 10.1006/jmcc.2002.1535] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
T-type calcium current (I(CaT)) is distributed among a large variety of species and tissues. The main functions of I(CaT) are thought to be related to pacemaker activity and to the cell cycle. Using the whole-cell patch-clamp configuration, we showed that fetal rat ventricular cells exhibit an I(CaT) with electrophysiological and pharmacological characteristics similar to those already described for this current. We investigated I(CaT) density and found that this current was mainly expressed in fetal cells and remained stable until birth (3.1+/-0.3 pA/pF for 18-day-old fetus, n=9). I(CaT) density decreased soon after birth (2.0+/-0.3 pA/pF, n=6, 1.1+/-0.2 pA/pF, n=5, for 1- and 5-day-old rats, respectively) and was no longer detected in 21-day-old rats. The rat ventricular cells express an alpha 1H isoform in addition to a homologous alpha 1G variant. Interestingly, the Ni(2+) sensitivity of I(CaT) indicates that in newborn myocytes, I(CaT) is only generated by alpha 1G subunits, whereas both alpha 1G and alpha 1H subunits participate in the fetal I(CaT). Moreover, the relative contribution of each subunit varies during fetal developmental stages, with a major contribution of alpha 1H in 16-day-old fetuses. Through quantitative RT-PCR we showed that the amount of both alpha 1G and alpha 1H transcripts are developmentally regulated. In fetuses of less than 18 days and in newborn rats after 1 day old, the transcriptional levels of alpha 1G and alpha 1H subunits clearly mismatch the functional contribution of these subunits to I(CaT). However, in perinatal period, the amount of alpha 1G mRNA seems to be in accordance to alpha 1G-related I(CaT) density. In conclusion, we showed that I(CaT) is mainly expressed during fetal stages, that alpha 1G and alpha 1H differentially participate to I(CaT) and that alpha 1G and alpha 1H isoforms are regulated by both transcriptional and post-transcriptional mechanisms.
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Affiliation(s)
- Laurent Ferron
- CNRS ESA 8078, Laboratoire de Physiologie Cardiovasculaire et Thymique, Hôpital Marie Lannelongue, Le Plessis Robinson, France.
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22
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Burgess DE, Crawford O, Delisle BP, Satin J. Mechanism of inactivation gating of human T-type (low-voltage activated) calcium channels. Biophys J 2002; 82:1894-906. [PMID: 11916848 PMCID: PMC1301986 DOI: 10.1016/s0006-3495(02)75539-2] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Recovery from inactivation of T-type Ca channels is slow and saturates at moderate hyperpolarizing voltage steps compared with Na channels. To explore this unique kinetic pattern we measured gating and ionic currents in two closely related isoforms of T-type Ca channels. Gating current recovers from inactivation much faster than ionic current, and recovery from inactivation is much more voltage dependent for gating current than for ionic current. There is a lag in the onset of gating current recovery at -80 mV, but no lag is discernible at -120 mV. The delay in recovery from inactivation of ionic current is much more evident at all voltages. The time constant for the decay of off gating current is very similar to the time constant of deactivation of open channels (ionic tail current), and both are strongly voltage dependent over a wide voltage range. Apparently, the development of inactivation has little influence on the initial deactivation step. These results suggest that movement of gating charge occurs for inactivated states very quickly. In contrast, the transitions from inactivated to available states are orders of magnitude slower, not voltage dependent, and are rate limiting for ionic recovery. These findings support a deactivation-first path for T-type Ca channel recovery from inactivation. We have integrated these concepts into an eight-state kinetic model, which can account for the major characteristics of T-type Ca channel inactivation.
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Affiliation(s)
- Don E Burgess
- Department of Physiology, University of Kentucky College of Medicine, Lexington, Kentucky 40536, USA
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23
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Chemin J, Monteil A, Perez-Reyes E, Bourinet E, Nargeot J, Lory P. Specific contribution of human T-type calcium channel isotypes (alpha(1G), alpha(1H) and alpha(1I)) to neuronal excitability. J Physiol 2002; 540:3-14. [PMID: 11927664 PMCID: PMC2290209 DOI: 10.1113/jphysiol.2001.013269] [Citation(s) in RCA: 166] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022] Open
Abstract
In several types of neurons, firing is an intrinsic property produced by specific classes of ion channels. Low-voltage-activated T-type calcium channels (T-channels), which activate with small membrane depolarizations, can generate burst firing and pacemaker activity. Here we have investigated the specific contribution to neuronal excitability of cloned human T-channel subunits. Using HEK-293 cells transiently transfected with the human alpha(1G) (Ca(V)3.1), alpha(1H) (Ca(V)3.2) and alpha(1I) (Ca(V)3.3) subunits, we describe significant differences among these isotypes in their biophysical properties, which are highlighted in action potential clamp studies. Firing activities occurring in cerebellar Purkinje neurons and in thalamocortical relay neurons used as voltage clamp waveforms revealed that alpha(1G) channels and, to a lesser extent, alpha(1H) channels produced large and transient currents, while currents related to alpha(1I) channels exhibited facilitation and produced a sustained calcium entry associated with the depolarizing after-potential interval. Using simulations of reticular and relay thalamic neuron activities, we show that alpha(1I) currents contributed to sustained electrical activities, while alpha(1G) and alpha(1H) currents generated short burst firing. Modelling experiments with the NEURON model further revealed that the alpha(1G) channel and alpha(1I) channel parameters best accounted for T-channel activities described in thalamocortical relay neurons and in reticular neurons, respectively. Altogether, the data provide evidence for a role of alpha(1I) channel in pacemaker activity and further demonstrate that each T-channel pore-forming subunit displays specific gating properties that account for its unique contribution to neuronal firing.
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Affiliation(s)
- Jean Chemin
- Institut de Génétique Humaine, CNRS UPR 1142, 141 rue de la Cardonille, F-34396 Montpellier cedex 05, France
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24
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Kline R, Jiang T, Xu X, Rybin VO, Steinberg SF. Abnormal calcium and protein kinase C-epsilon signaling in hypertrophied atrial tumor myocytes (AT-1 cells). Am J Physiol Heart Circ Physiol 2001; 280:H2761-9. [PMID: 11356634 DOI: 10.1152/ajpheart.2001.280.6.h2761] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Cardiac hypertrophy leads to contractile dysfunction and altered hormone responsiveness through incompletely understood mechanisms. Atrial tumor (AT-1) myocytes (AT-1 cells) are a cardiomyocyte lineage that proliferates but hypertrophies when proliferation is prevented with mitomycin C. Because both states maintain a highly differentiated phenotype, AT-1 cells were used to explore the signaling pathways that accompany and/or contribute to hypertrophic cardiomyocyte growth. Mitomycin C-induced AT-1 cell enlargement is associated with a pronounced increase in the amplitude and the duration of both electrically stimulated calcium transients and endothelin receptor-dependent calcium responses. Studies with caffeine indicate that the intracellular pool of releasable calcium is similar in control and hypertrophied AT-1 cells. This agrees with the results of Northern analyses that show similar steady-state levels of transcripts encoding the sarcoplasmic reticulum Ca-ATPase (and higher levels of transcripts encoding the Na+/Ca2+ exchanger) in hypertrophied AT-1 cells, relative to proliferating control cultures. However, immunoblot analyses reveal a marked increase in the expression of protein kinase C (PKC)-epsilon (a critical intermediate in the signaling pathway for endothelin receptor-dependent modulation of intracellular calcium) during AT-1 cell hypertrophy; the abundance of other PKC isoforms is not changed. Collectively, these results identify reciprocal regulation between calcium/PKC signaling and hypertrophic growth. The evidence that AT-1 cell hypertrophy leads to abnormalities in calcium regulation and specific changes in PKC-epsilon expression that alter endothelin receptor responsiveness supports the notion that pathophysiological changes in PKC-epsilon abundance lead to functionally important changes in hormonal modulation of cardiomyocyte function.
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Affiliation(s)
- R Kline
- Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, New York 10032, USA
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25
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Abstract
We studied the gating kinetics, especially the kinetics of recovery from inactivation, of T-type Ca(2+) channels (T-channels) in thalamic neurons. The recovery course is associated with no discernible Ca(2+) current and is characterized by an initial delay, as well as a subsequent exponential phase. These findings are qualitatively similar to previous observations on neuronal Na(+) channels and suggest that T-channels also must deactivate to recover from inactivation. In contrast to Na(+) channels in which both the delay and the time constant of the exponential phase are shortened with increasing hyperpolarization, in T-channels the time constant of the exponential recovery phase remains unchanged between -100 and -200 mV, although the initial delay is still shortened e-fold per 43 mV hyperpolarization over the same voltage range. The deactivating kinetics of tail T-currents also show a similar voltage dependence between -90 and -170 mV. According to the hinged-lid model of fast inactivation, these findings suggest that the affinity difference between inactivating peptide binding to the activated channel and binding to the fully deactivated channel is much smaller in T-channels than in Na(+) channels. Moreover, the inactivating peptide in T-channels seems to have much slower binding and unbinding kinetics, and the unbinding rates probably remain unchanged once the inactivated T-channel has gone through the initial steps of deactivation and "closes" the pore (with the activation gate). T-channels thus might have a more rigid hinge and a more abrupt conformational change in the inactivation machinery associated with opening and closing of the pore.
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26
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Weber CR, Ginsburg KS, Philipson KD, Shannon TR, Bers DM. Allosteric regulation of Na/Ca exchange current by cytosolic Ca in intact cardiac myocytes. J Gen Physiol 2001; 117:119-31. [PMID: 11158165 PMCID: PMC2217247 DOI: 10.1085/jgp.117.2.119] [Citation(s) in RCA: 129] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The cardiac sarcolemmal Na-Ca exchanger (NCX) is allosterically regulated by [Ca](i) such that when [Ca](i) is low, NCX current (I(NCX)) deactivates. In this study, we used membrane potential (E(m)) and I(NCX) to control Ca entry into and Ca efflux from intact cardiac myocytes to investigate whether this allosteric regulation (Ca activation) occurs with [Ca](i) in the physiological range. In the absence of Ca activation, the electrochemical effect of increasing [Ca](i) would be to increase inward I(NCX) (Ca efflux) and to decrease outward I(NCX). On the other hand, Ca activation would increase I(NCX) in both directions. Thus, we attributed [Ca](i)-dependent increases in outward I(NCX) to allosteric regulation. Ca activation of I(NCX) was observed in ferret myocytes but not in wild-type mouse myocytes, suggesting that Ca regulation of NCX may be species dependent. We also studied transgenic mouse myocytes overexpressing either normal canine NCX or this same canine NCX lacking Ca regulation (Delta680-685). Animals with the normal canine NCX transgene showed Ca activation, whereas animals with the mutant transgene did not, confirming the role of this region in the process. In native ferret cells and in mice with expressed canine NCX, allosteric regulation by Ca occurs under physiological conditions (K(mCaAct) = 125 +/- 16 nM SEM approximately resting [Ca](i)). This, along with the observation that no delay was observed between measured [Ca](i) and activation of I(NCX) under our conditions, suggests that beat to beat changes in NCX function can occur in vivo. These changes in the I(NCX) activation state may influence SR Ca load and resting [Ca](i), helping to fine tune Ca influx and efflux from cells under both normal and pathophysiological conditions. Our failure to observe Ca activation in mouse myocytes may be due to either the extent of Ca regulation or to a difference in K(mCaAct) from other species. Model predictions for Ca activation, on which our estimates of K(mCaAct) are based, confirm that Ca activation strongly influences outward I(NCX), explaining why it increases rather than declines with increasing [Ca](i).
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Affiliation(s)
- Christopher R. Weber
- Department of Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois 60153
| | - Kenneth S. Ginsburg
- Department of Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois 60153
| | - Kenneth D. Philipson
- Cardiovascular Research Lab, University of California Los Angeles School of Medicine, Los Angeles, California 90095
| | - Thomas R. Shannon
- Department of Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois 60153
| | - Donald M. Bers
- Department of Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois 60153
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27
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Seisenberger C, Specht V, Welling A, Platzer J, Pfeifer A, Kühbandner S, Striessnig J, Klugbauer N, Feil R, Hofmann F. Functional embryonic cardiomyocytes after disruption of the L-type alpha1C (Cav1.2) calcium channel gene in the mouse. J Biol Chem 2000; 275:39193-9. [PMID: 10973973 DOI: 10.1074/jbc.m006467200] [Citation(s) in RCA: 198] [Impact Index Per Article: 8.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The L-type alpha(1C) (Ca(v)1.2) calcium channel is the major calcium entry pathway in cardiac and smooth muscle. We inactivated the Ca(v)1.2 gene in two independent mouse lines that had indistinguishable phenotypes. Homozygous knockout embryos (Ca(v)1. 2-/-) died before day 14.5 postcoitum (p.c.). At day 12.5 p.c., the embryonic heart contracted with identical frequency in wild type (+/+), heterozygous (+/-), and homozygous (-/-) Ca(v)1.2 embryos. Beating of isolated embryonic cardiomyocytes depended on extracellular calcium and was blocked by 1 microm nisoldipine. In (+/+), (+/-), and (-/-) cardiomyocytes, an L-type Ba(2+) inward current (I(Ba)) was present that was stimulated by Bay K 8644 in all genotypes. At a holding potential of -80 mV, nisoldipine blocked I(Ba) of day 12.5 p.c. (+/+) and (+/-) cells with two IC(50) values of approximately 0.1 and approximately 1 microm. Inhibition of I(Ba) of (-/-) cardiomyocytes was monophasic with an IC(50) of approximately 1 microm. The low affinity I(Ba) was also present in cardiomyocytes of homozygous alpha(1D) (Ca(v)1.3) knockout embryos at day 12.5 p.c. These results indicate that, up to day 14 p.c., contraction of murine embryonic hearts requires an unidentified, low affinity L-type like calcium channel.
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MESH Headings
- 3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester/pharmacology
- Animals
- Barium/metabolism
- Calcium Channel Agonists/pharmacology
- Calcium Channel Blockers/pharmacology
- Calcium Channels, L-Type/genetics
- Calcium Channels, L-Type/physiology
- Cell Line
- DNA, Complementary/metabolism
- Dose-Response Relationship, Drug
- Electrophysiology
- Exons
- Genetic Vectors
- Genotype
- Heart/embryology
- Homozygote
- Inhibitory Concentration 50
- Ions
- Kinetics
- Mice
- Mice, Knockout
- Models, Genetic
- Myocardium/cytology
- Nisoldipine/pharmacology
- Phenotype
- RNA/metabolism
- Time Factors
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Affiliation(s)
- C Seisenberger
- Institut für Pharmakologie und Toxikologie, TU München, Biedersteiner Strasse 29, D-80802 München, Germany
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28
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Leuranguer V, Monteil A, Bourinet E, Dayanithi G, Nargeot J. T-type calcium currents in rat cardiomyocytes during postnatal development: contribution to hormone secretion. Am J Physiol Heart Circ Physiol 2000; 279:H2540-8. [PMID: 11045992 DOI: 10.1152/ajpheart.2000.279.5.h2540] [Citation(s) in RCA: 66] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
T-type Ca(2+) channels have been suggested to play a role in cardiac automaticity, cell growth, and cardiovascular remodeling. Although three genes encoding for a T-type Ca(2+) channel have been identified, the nature of the isoform(s) supporting the cardiac T-type Ca(2+) current (I(Ca,T)) has not yet been determined. We describe the postnatal evolution of I(Ca,T) density in freshly dissociated rat atrial and ventricular myocytes and its functional properties at peak current density in young atrial myocytes. I(Ca,T) displays a classical low activation threshold, rapid inactivation kinetics, negative steady-state inactivation, slow deactivation, and the presence of a window current. Interestingly, I(Ca,T) is poorly sensitive to Ni(2+) and insensitive to R-type current toxin SNX-482. RT-PCR experiments and comparison of functional properties with recombinant Ca(2+) channel subtypes suggest that neonatal I(Ca,T) is related to the alpha(1G)-subunit. Atrial natriuretic factor (ANF) secretion was measured using peptide radioimmunoassays in atrial tissue. Pharmacological dissection of ANF secretion indicates an important contribution of I(Ca,T) to Ca(2+) signaling during the neonatal period.
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Affiliation(s)
- V Leuranguer
- Physiopathologie des Canaux Ioniques, Institut de Génétique Humaine-Centre National de la Recherche Scientifique (CNRS) UPR 1142, 34396 Montpellier cedex 05, France
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29
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Bohn G, Moosmang S, Conrad H, Ludwig A, Hofmann F, Klugbauer N. Expression of T- and L-type calcium channel mRNA in murine sinoatrial node. FEBS Lett 2000; 481:73-6. [PMID: 10984618 DOI: 10.1016/s0014-5793(00)01979-7] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
At the cellular level, cardiac pacemaking which sets the rate and rhythm of the heartbeat is produced by the slow diastolic depolarization. Several ion channels contribute to this pacemaker depolarization, including T-type and L-type calcium currents. To evaluate the molecular basis of the currents involved, we investigated the cellular distribution of various low voltage activated (LVA) and high voltage activated (HVA) calcium channel mRNAs in the murine sinoatrial (SA) node by in-situ hybridization. The most prominently expressed LVA calcium channel in the SA node is Ca(v)3.1, whereas Ca(v)3.2 is present at moderate levels. The dominant HVA calcium channel transcript is Ca(v)1.2; only traces of Ca(v)1.3 mRNA are detectable in SA myocytes of mice.
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Affiliation(s)
- G Bohn
- Institut für Pharmakologie und Toxikologie, Technische Universität München, Biedersteiner Strasse 29, 80802 Munich, Germany
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30
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Abstract
External pH (pH(o)) modifies T-type calcium channel gating and permeation properties. The mechanisms of T-type channel modulation by pH remain unclear because native currents are small and are contaminated with L-type calcium currents. Heterologous expression of the human cloned T-type channel, alpha1H, enables us to determine the effect of changing pH on isolated T-type calcium currents. External acidification from pH(o) 8.2 to pH(o) 5.5 shifts the midpoint potential (V(1/2)) for steady-state inactivation by 11 mV, shifts the V(1/2) for maximal activation by 40 mV, and reduces the voltage dependence of channel activation. The alpha1H reversal potential (E(rev)) shifts from +49 mV at pH(o) 8.2 to +36 mV at pH(o) 5.5. The maximal macroscopic conductance (G(max)) of alpha1H increases at pH(o) 5.5 compared to pH(o) 8.2. The E(rev) and G(max) data taken together suggest that external protons decrease calcium/monovalent ion relative permeability. In response to a sustained depolarization alpha1H currents inactivate with a single exponential function. The macroscopic inactivation time constant is a steep function of voltage for potentials < -30 mV at pH(o) 8.2. At pH(o) 5.5 the voltage dependence of tau(inact) shifts more depolarized, and is also a more gradual function of voltage. The macroscopic deactivation time constant (tau(deact)) is a function of voltage at the potentials tested. At pH(o) 5.5 the voltage dependence of tau(deact) is simply transposed by approximately 40 mV, without a concomitant change in the voltage dependence. Similarly, the delay in recovery from inactivation at V(rec) of -80 mV in pH(o) 5.5 is similar to that with a V(rec) of -120 mV at pH(o) 8.2. We conclude that alpha1H is uniquely modified by pH(o) compared to other calcium channels. Protons do not block alpha1H current. Rather, a proton-induced change in activation gating accounts for most of the change in current magnitude with acidification.
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Affiliation(s)
- B P Delisle
- Department of Physiology, University of Kentucky College of Medicine, Lexington, Kentucky 40536-0298, USA.
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