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Scardigli M, Pásek M, Santini L, Palandri C, Conti E, Crocini C, Campione M, Loew LM, de Vries AAF, Pijnappels DA, Pavone FS, Poggesi C, Cerbai E, Coppini R, Kohl P, Ferrantini C, Sacconi L. Optogenetic confirmation of transverse-tubular membrane excitability in intact cardiac myocytes. J Physiol 2024; 602:791-808. [PMID: 38348881 DOI: 10.1113/jp285202] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2023] [Accepted: 01/17/2024] [Indexed: 03/09/2024] Open
Abstract
T-tubules (TT) form a complex network of sarcolemmal membrane invaginations, essential for well-co-ordinated excitation-contraction coupling (ECC) and thus homogeneous mechanical activation of cardiomyocytes. ECC is initiated by rapid depolarization of the sarcolemmal membrane. Whether TT membrane depolarization is active (local generation of action potentials; AP) or passive (following depolarization of the outer cell surface sarcolemma; SS) has not been experimentally validated in cardiomyocytes. Based on the assessment of ion flux pathways needed for AP generation, we hypothesize that TT are excitable. We therefore explored TT excitability experimentally, using an all-optical approach to stimulate and record trans-membrane potential changes in TT that were structurally disconnected, and hence electrically insulated, from the SS membrane by transient osmotic shock. Our results establish that cardiomyocyte TT can generate AP. These AP show electrical features that differ substantially from those observed in SS, consistent with differences in the density of ion channels and transporters in the two different membrane domains. We propose that TT-generated AP represent a safety mechanism for TT AP propagation and ECC, which may be particularly relevant in pathophysiological settings where morpho-functional changes reduce the electrical connectivity between SS and TT membranes. KEY POINTS: Cardiomyocytes are characterized by a complex network of membrane invaginations (the T-tubular system) that propagate action potentials to the core of the cell, causing uniform excitation-contraction coupling across the cell. In the present study, we investigated whether the T-tubular system is able to generate action potentials autonomously, rather than following depolarization of the outer cell surface sarcolemma. For this purpose, we developed a fully optical platform to probe and manipulate the electrical dynamics of subcellular membrane domains. Our findings demonstrate that T-tubules are intrinsically excitable, revealing distinct characteristics of self-generated T-tubular action potentials. This active electrical capability would protect cells from voltage drops potentially occurring within the T-tubular network.
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Affiliation(s)
- Marina Scardigli
- Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy
| | - Michal Pásek
- Institute of Thermomechanics, Czech Academy of Science, Prague, Czech Republic
- Department of Physiology, Faculty of Medicine, Masaryk University, Brno, Czech Republic
| | - Lorenzo Santini
- Department of Neurology, Psychology, Drug Sciences and Child Health, University of Florence, Florence, Italy
| | - Chiara Palandri
- Department of Neurology, Psychology, Drug Sciences and Child Health, University of Florence, Florence, Italy
| | - Emilia Conti
- European Laboratory for Non-Linear Spectroscopy - LENS, Sesto Fiorentino, Italy
- Neuroscience Institute, National Research Council, Pisa, Italy
| | - Claudia Crocini
- DZHK (German Centre for Cardiovascular Research), Partner Site Berlin, Berlin, Germany
- Max Rubner Center for Cardiovascular Metabolic Renal Research (MRC), Deutsches Herzzentrum der Charité (DHZC), Charité-Universitätsmedizin Berlin, Berlin, Germany
| | - Marina Campione
- Institute of Neuroscience (IN-CNR) and Department of Biomedical Science, University of Padua, Padua, Italy
| | - Leslie M Loew
- Center for Cell Analysis and Modeling, University of Connecticut, Farmington, CT, USA
| | - Antoine A F de Vries
- Laboratory of Experimental Cardiology, Department of Cardiology, Leiden University Medical Center, Leiden, The Netherlands
| | - Daniël A Pijnappels
- Laboratory of Experimental Cardiology, Department of Cardiology, Leiden University Medical Center, Leiden, The Netherlands
| | - Francesco S Pavone
- European Laboratory for Non-Linear Spectroscopy - LENS, Sesto Fiorentino, Italy
| | - Corrado Poggesi
- Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy
| | - Elisabetta Cerbai
- Department of Neurology, Psychology, Drug Sciences and Child Health, University of Florence, Florence, Italy
- European Laboratory for Non-Linear Spectroscopy - LENS, Sesto Fiorentino, Italy
| | - Raffaele Coppini
- Department of Neurology, Psychology, Drug Sciences and Child Health, University of Florence, Florence, Italy
| | - Peter Kohl
- Institute for Experimental Cardiovascular Medicine, University Heart Center and Medical Faculty, University of Freiburg, Freiburg, Germany
| | - Cecilia Ferrantini
- Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy
| | - Leonardo Sacconi
- European Laboratory for Non-Linear Spectroscopy - LENS, Sesto Fiorentino, Italy
- Institute for Experimental Cardiovascular Medicine, University Heart Center and Medical Faculty, University of Freiburg, Freiburg, Germany
- Institute of Clinical Physiology, National Research Council (IFC-CNR), Florence, Italy
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Pásek M, Šimurda J, Bébarová M, Christé G. Divergent estimates of the ratio between Na+-Ca2+ current densities in t-tubular and surface membranes of rat ventricular cardiomyocytes. J Cell Sci 2021; 134:jcs258228. [PMID: 34313306 DOI: 10.1242/jcs.258228] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2020] [Accepted: 05/24/2021] [Indexed: 11/20/2022] Open
Abstract
The ratio between Na+-Ca2+ exchange current densities in t-tubular and surface membranes of rat ventricular cardiomyocytes (JNaCa-ratio) estimated from electrophysiological data published to date yields strikingly different values between 1.7 and nearly 40. Possible reasons for such divergence were analysed by Monte Carlo simulations assuming both normal and log-normal distribution of the measured data. The confidence intervals CI95 of the mean JNaCa-ratios computed from the reported data showed an overlap of values between 1 and 3, and between 0.3 and 4.3 in the case of normal and log-normal distribution, respectively. Further analyses revealed that the published high values likely result from a large scatter of data due to transmural differences in JNaCa, dispersion of cell membrane capacitances and variability in incomplete detubulation. Taking into account the asymmetric distribution of the measured data, the reduction of mean current densities after detubulation and the substantially smaller CI95 of lower values of the mean JNaCa-ratio, the values between 1.6 and 3.2 may be considered as the most accurate estimates. This implies that 40 to 60% of Na+-Ca2+ exchanger is located at the t-tubular membrane of adult rat ventricular cardiomyocytes.
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Affiliation(s)
- Michal Pásek
- Institute of Thermomechanics, Czech Academy of Science, Dolejškova 5, 182 00, Prague, Czech Republic
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Jiří Šimurda
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Markéta Bébarová
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Georges Christé
- Laboratoire de Neurocardiologie, EA4612, Université Lyon 1, Lyon F-69003, France
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Synková I, Bébarová M, Andršová I, Chmelikova L, Švecová O, Hošek J, Pásek M, Vít P, Valášková I, Gaillyová R, Navrátil R, Novotný T. Long-QT founder variant T309I-Kv7.1 with dominant negative pattern may predispose delayed afterdepolarizations under β-adrenergic stimulation. Sci Rep 2021; 11:3573. [PMID: 33574382 PMCID: PMC7878757 DOI: 10.1038/s41598-021-81670-1] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2020] [Accepted: 12/30/2020] [Indexed: 11/30/2022] Open
Abstract
The variant c.926C > T (p.T309I) in KCNQ1 gene was identified in 10 putatively unrelated Czech families with long QT syndrome (LQTS). Mutation carriers (24 heterozygous individuals) were more symptomatic compared to their non-affected relatives (17 individuals). The carriers showed a mild LQTS phenotype including a longer QTc interval at rest (466 ± 24 ms vs. 418 ± 20 ms) and after exercise (508 ± 32 ms vs. 417 ± 24 ms), 4 syncopes and 2 aborted cardiac arrests. The same haplotype associated with the c.926C > T variant was identified in all probands. Using the whole cell patch clamp technique and confocal microscopy, a complete loss of channel function was revealed in the homozygous setting, caused by an impaired channel trafficking. Dominant negativity with preserved reactivity to β-adrenergic stimulation was apparent in the heterozygous setting. In simulations on a human ventricular cell model, the dysfunction resulted in delayed afterdepolarizations (DADs) and premature action potentials under β-adrenergic stimulation that could be prevented by a slight inhibition of calcium current. We conclude that the KCNQ1 variant c.926C > T is the first identified LQTS-related founder mutation in Central Europe. The dominant negative channel dysfunction may lead to DADs under β-adrenergic stimulation. Inhibition of calcium current could be possible therapeutic strategy in LQTS1 patients refractory to β-blocker therapy.
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Affiliation(s)
- Iva Synková
- Department of Medical Genetics, University Hospital Brno and Faculty of Medicine, Masaryk University, Jihlavská 20, 625 00, Brno, Czech Republic.,Department of Experimental Biology, Faculty of Science, Masaryk University, Kotlářská 267/2, 611 37, Brno, Czech Republic
| | - Markéta Bébarová
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic.
| | - Irena Andršová
- Department of Internal Medicine and Cardiology, University Hospital Brno and Faculty of Medicine, Masaryk University, Jihlavská 20, 625 00, Brno, Czech Republic
| | - Larisa Chmelikova
- Department of Biomedical Engineering, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technická 10, 616 00, Brno, Czech Republic
| | - Olga Švecová
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Jan Hošek
- Division of Biologically Active Complexes and Molecular Magnets, Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University in Olomouc, Šlechtitelů 27, 783 71, Olomouc, Czech Republic
| | - Michal Pásek
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic.,Institute of Thermomechanics, Czech Academy of Sciences, Dolejškova 5, 182 00, Prague, Czech Republic
| | - Pavel Vít
- Department of Paediatrics, University Hospital Brno and Faculty of Medicine, Masaryk University, Černopolní 9, 613 00, Brno, Czech Republic
| | - Iveta Valášková
- Department of Medical Genetics, University Hospital Brno and Faculty of Medicine, Masaryk University, Jihlavská 20, 625 00, Brno, Czech Republic
| | - Renata Gaillyová
- Department of Medical Genetics, University Hospital Brno and Faculty of Medicine, Masaryk University, Jihlavská 20, 625 00, Brno, Czech Republic
| | - Rostislav Navrátil
- Repromeda, Clinic for Reproductive Medicine and Preimplantation Genetic Diagnosis, Biology Park, Studentská 812/6, 625 00, Brno, Czech Republic
| | - Tomáš Novotný
- Department of Internal Medicine and Cardiology, University Hospital Brno and Faculty of Medicine, Masaryk University, Jihlavská 20, 625 00, Brno, Czech Republic
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Kula R, Bébarová M, Matejovič P, Šimurda J, Pásek M. Current density as routine parameter for description of ionic membrane current: is it always the best option? Progress in Biophysics and Molecular Biology 2020; 157:24-32. [DOI: 10.1016/j.pbiomolbio.2019.11.011] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/10/2019] [Revised: 09/20/2019] [Accepted: 11/26/2019] [Indexed: 12/17/2022]
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Kula R, Bébarová M, Matejovič P, Šimurda J, Pásek M. Distribution of data in cellular electrophysiology: Is it always normal? Prog Biophys Mol Biol 2020; 157:11-17. [PMID: 32621819 DOI: 10.1016/j.pbiomolbio.2020.05.008] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/15/2019] [Revised: 05/11/2020] [Accepted: 05/14/2020] [Indexed: 12/16/2022]
Abstract
The distribution of data presented in many electrophysiological studies is presumed to be normal without any convincing evidence. To test this presumption, the cell membrane capacitance and magnitude of inward rectifier potassium currents were recorded by the whole-cell patch clamp technique in rat atrial myocytes. Statistical analysis of the data showed that these variables were not distributed normally. Instead, a positively skewed distribution appeared to be a better approximation of the real data distribution. Consequently, the arithmetic mean, used inappropriately in such data, may substantially overestimate the true mean value characterizing the central tendency of the data. Moreover, a large standard deviation describing the variance of positively skewed data allowed 95% confidence interval to include unrealistic negative values. We therefore conclude that the normality of the electrophysiological data should be tested in every experiment and, if rejected, the positively skewed data should be more accurately characterized by the median and interpercentile range or, if justified (namely in the case of log-normal and gamma data distribution), by the geometric mean and the geometric standard deviation.
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Affiliation(s)
- Roman Kula
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Markéta Bébarová
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Peter Matejovič
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Jiří Šimurda
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic
| | - Michal Pásek
- Department of Physiology, Faculty of Medicine, Masaryk University, Kamenice 5, 625 00, Brno, Czech Republic; Institute of Thermomechanics, Czech Academy of Sciences, Dolejškova 5, 182 00, Prague, Czech Republic.
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Vaverka J, Burša J, Šumbera J, Pásek M. Effect of Transmural Differences in Excitation-Contraction Delay and Contraction Velocity on Left Ventricle Isovolumic Contraction: A Simulation Study. Biomed Res Int 2018; 2018:4798512. [PMID: 29862273 PMCID: PMC5971307 DOI: 10.1155/2018/4798512] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 12/12/2017] [Revised: 03/01/2018] [Accepted: 03/13/2018] [Indexed: 12/14/2022]
Abstract
Recent studies have shown that left ventricle (LV) exhibits considerable transmural differences in active mechanical properties induced by transmural differences in electrical activity, excitation-contraction coupling, and contractile properties of individual myocytes. It was shown that the time between electrical and mechanical activation of myocytes (electromechanical delay: EMD) decreases from subendocardium to subepicardium and, on the contrary, the myocyte shortening velocity (MSV) increases in the same direction. To investigate the physiological importance of this inhomogeneity, we developed a new finite element model of LV incorporating the observed transmural gradients in EMD and MSV. Comparative simulations with the model showed that when EMD or MSV or both were set constant across the LV wall, the LV contractility during isovolumic contraction (IVC) decreased significantly ((dp/dt)max was reduced by 2 to 38% and IVC was prolonged by 18 to 73%). This was accompanied by an increase of transmural differences in wall stress. These results suggest that the transmural differences in EMD and MSV play an important role in physiological contractility of LV by synchronising the contraction of individual layers of ventricular wall during the systole. Reduction or enhancement of these differences may therefore impair the function of LV and contribute to heart failure.
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Affiliation(s)
- J Vaverka
- Institute of Solid Mechanics, Mechatronics and Biomechanics, Faculty of Mechanical Engineering, University of Technology, Brno, Czech Republic
| | - J Burša
- Institute of Solid Mechanics, Mechatronics and Biomechanics, Faculty of Mechanical Engineering, University of Technology, Brno, Czech Republic
| | - J Šumbera
- Department of Cardiovascular Diseases, Faculty of Medicine, Masaryk University, Brno, Czech Republic
| | - M Pásek
- Department of Physiology, Faculty of Medicine, Masaryk University, Brno, Czech Republic
- Institute of Thermomechanics, Czech Academy of Science, Prague, Czech Republic
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Bébarová M, Matejovič P, Pásek M, Ohlídalová D, Jansová D, Simurdová M, Simurda J. Effect of ethanol on action potential and ionic membrane currents in rat ventricular myocytes. Acta Physiol (Oxf) 2010; 200:301-14. [PMID: 20618172 DOI: 10.1111/j.1748-1716.2010.02162.x] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
AIM Even though alcohol intoxication is often linked to arrhythmias, data describing ethanol effect on cardiac ionic channels are rare. In addition, ethanol is used as a solvent of hydrophobic compounds in experimental studies. We investigated changes of the action potential (AP) configuration and main ionic membrane currents in rat cardiomyocytes under 20-1500 m(M) ethanol. METHODS Experiments were performed on enzymatically isolated rat right ventricular myocytes using the whole cell patch-clamp technique at room temperature. RESULTS Ethanol reversibly decelerated the upstroke velocity and decreased AP amplitude and duration at 0.2 and 3 Hz. The fast sodium current I(Na) , l-type calcium current I(Ca) and transient outward potassium current I(to) were reversibly inhibited in a concentration-dependent manner (50% inhibition at 446 ± 12, 553 ± 49 and 1954 ± 234 m(M), respectively, with corresponding Hill coefficients 3.1 ± 0.3, 1.1 ± 0.2 and 0.9 ± 0.1). Suppression of I(Na) and I(Ca) magnitude was slightly voltage dependent. The effect on I(Ca) and I(to) was manifested mainly as an acceleration of their apparent inactivations with a decreased slow and fast time constant respectively. As a consequence of marked differences in n(H) , sensitivity of the currents to ethanol at 10% inhibition decreases in the following order: I(Ca) (75 mm, 3.5‰), I(to) (170 m(M), 7.8‰) and I(Na) (220 m(M), 10.1‰). CONCLUSION Our results suggest a slight inhibition of all the currents at ethanol concentrations relevant to deep alcohol intoxication. The concentration dependence measured over a wide range may serve as a guideline when using ethanol as a solvent.
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Affiliation(s)
- M Bébarová
- Department of Physiology, Masaryk University, Brno - Bohunice, Czech Republic.
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Bébarová M, Matejovič P, Pásek M, Jansová D, Šimurdová M, Nováková M, Šimurda J. Effect of antipsychotic drug perphenazine on fast sodium current and transient outward potassium current in rat ventricular myocytes. Naunyn Schmiedebergs Arch Pharmacol 2009; 380:125-33. [DOI: 10.1007/s00210-009-0420-1] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2008] [Accepted: 04/08/2009] [Indexed: 10/20/2022]
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Orchard CH, Pásek M, Brette F. The role of mammalian cardiac t-tubules in excitation-contraction coupling: experimental and computational approaches. Exp Physiol 2009; 94:509-19. [PMID: 19297389 DOI: 10.1113/expphysiol.2008.043984] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The sarcolemmal membrane of mammalian cardiac ventricular myocytes is characterized by the presence of invaginations called transverse tubules (t-tubules). Transverse tubules occur at the Z-line as transverse elements with longitudinal extensions. While the existence of t-tubules has been known for some time, recent experimental studies have suggested that their structure and function are more complex than previously believed. There are, however, aspects of t-tubule function that are not currently amenable to experimental investigation, but can be investigated using computational and mathematical approaches. Such studies have helped elucidate further the possible role of t-tubules in cell function. This review summarizes recent experimental and complementary computational studies which highlight the important role of t-tubules in cardiac excitation-contraction coupling.
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Affiliation(s)
- Clive H Orchard
- Department of Physiology & Pharmacology, School of Medical Sciences, University of Bristol, Bristol, UK.
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Pásek M, Brette F, Nelson A, Pearce C, Qaiser A, Christe G, Orchard CH. Quantification of t-tubule area and protein distribution in rat cardiac ventricular myocytes. Prog Biophys Mol Biol 2007; 96:244-57. [PMID: 17881039 DOI: 10.1016/j.pbiomolbio.2007.07.016] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
The transverse (t-) tubules of cardiac ventricular myocytes are invaginations of the surface membrane that form a complex network within the cell. Many of the key proteins involved in excitation-contraction coupling appear to be located predominantly at the t-tubule membrane. Despite their importance, the fraction of cell membrane within the t-tubules remains unclear: measurement of cell capacitance following detubulation suggests approximately 32%, whereas optical measurements suggest up to approximately 65%. We have, therefore, investigated the factors that may account for this discrepancy. Calculation of the combinations of t-tubule radius, length and density that produce t-tubular membrane fractions of 32% or 56% suggest that the true fraction is at the upper end of this range. Assessment of detubulation using confocal and electron microscopy suggests that incomplete detubulation can account for some, but not all of the difference. High cholesterol, and a consequent decrease in specific capacitance, in the t-tubule membrane, may also cause the t-tubule fraction calculated from the loss of capacitance following detubulation to be underestimated. Correcting for both of these factors results in an estimate that is still lower than that obtained from optical measurements suggesting either that optical methods overestimate the fraction of membrane in the t-tubules, or that other, unknown, factors, reduce the apparent fraction obtained by detubulation. A biophysically realistic computer model of a rat ventricular myocyte, incorporating a t-tubule network, is used to assess the effect of the altered estimates of t-tubular membrane fraction on the calculated distribution of ion flux pathways.
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Affiliation(s)
- M Pásek
- Institute of Thermomechanics, Czech Academy of Science-branch Brno; Department of Physiology, Faculty of Medicine, Masaryk University, Brno, Czech Republic
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Christé G, Chahine M, Chevalier P, Pásek M. Changes in action potentials and intracellular ionic homeostasis in a ventricular cell model related to a persistent sodium current in SCN5A mutations underlying LQT3. Prog Biophys Mol Biol 2007; 96:281-93. [PMID: 17892895 DOI: 10.1016/j.pbiomolbio.2007.07.023] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
In LQT3 patients, SCN5A mutations induce ultraslow inactivation of a small fraction of the hNav1.5 current, i.e. persistent Na+ current (IpNa). We explored the time course of effects of such a change on the intracellular ionic homeostasis in a model of guinea-pig cardiac ventricular cell [Pasek, M., Simurda, J., Orchard, C.H., Christé, G., 2007b. A model of the guinea-pig ventricular cardiomyocyte incorporating a transverse-axial tubular system. Prog. Biophys. Mol. Biol., this issue]. Sudden addition of IpNa prevented action potential (AP) repolarization when its conductance (gpNa) exceeded 0.12% of the maximal conductance of fast INa (gNa). With gpNa at 0.1% gNa, the AP duration at 90% repolarization (APD90) was initially lengthened to 2.6-fold that in control. Under regular stimulation at 1 Hz it shortened progressively to 1.37-fold control APD90, and intracellular [Na+]i increased by 6% with a time constant of 106 s. Further increasing gpNa to 0.2% gNa caused an immediate increase in APD90 to 5.7-fold that in control, which decreased to 2.2-fold that in control in 30s stimulation at 1 Hz. At this time diastolic [Na+]i and [Ca2+]i were, respectively, 34% and 52% higher than in control and spontaneous erratic SR Ca release occurred. In the presence of IpNa causing 46% lengthening of APD90, the model cell displayed arrhythmogenic behaviour when external [K+] was lowered to 5 mM from an initial value at 5.4 mM. By contrast, when K+ currents IKr and IKs were lowered in the model cell to produce the same lengthening of APD90, no proarrhythmic behaviour was observed, even when external [K+] was lowered to 2.5 mM.
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Abstract
The transverse-axial tubular system (TATS) of cardiac ventricular myocytes is a complex network of tubules that arises as invaginations of the surface membrane; it appears to form a specialised region of cell membrane that is particularly important for excitation-contraction coupling. However, much remains unknown about the structure and role of the TATS. In this brief review we use experimental data and computer modelling to address the following key questions: (i) What fraction of the cell membrane is within the TATS? (ii) Is the composition of the TATS membrane the same as the surface membrane? (iii) How good is electrical coupling between the surface and TATS membranes? (iv) What fraction of each current is within the TATS? (v) How important is the complex structure of the TATS network? (vi) What is the effect of current inhomogeneity on lumenal ion concentrations? (vii) Does the TATS contribute to the functional changes observed in heart failure? Although there are many areas in which experimental evidence is lacking, computer models provide a method to assess and predict the possible function of the TATS; such models suggest that although the surface and TATS membranes are electrically well coupled, concentration of ion flux pathways within the TATS, coupled to restricted diffusion, may result in the ionic composition in the TATS lumen being different from that in the bulk extracellular space, and varying with activity and in pathological conditions.
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Affiliation(s)
- M Pásek
- Institute of Thermomechanics, Czech Academy of Science, Branch Brno, Brno, Czech Republic
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Pásek M, Simurda J, Orchard CH, Christé G. A model of the guinea-pig ventricular cardiac myocyte incorporating a transverse-axial tubular system. Prog Biophys Mol Biol 2007; 96:258-80. [PMID: 17888503 DOI: 10.1016/j.pbiomolbio.2007.07.022] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/23/2022]
Abstract
A model of the guinea-pig cardiac ventricular myocyte has been developed that includes a representation of the transverse-axial tubular system (TATS), including heterogeneous distribution of ion flux pathways between the surface and tubular membranes. The model reproduces frequency-dependent changes of action potential shape and intracellular ion concentrations and can replicate experimental data showing ion diffusion between the tubular lumen and external solution in guinea-pig myocytes. The model is stable at rest and during activity and returns to rested state after perturbation. Theoretical analysis and model simulations show that, due to tight electrical coupling, tubular and surface membranes behave as a homogeneous whole during voltage and current clamp (maximum difference 0.9 mV at peak tubular INa of -38 nA). However, during action potentials, restricted diffusion and ionic currents in TATS cause depletion of tubular Ca2+ and accumulation of tubular K+ (up to -19.8% and +3.4%, respectively, of bulk extracellular values, at 6 Hz). These changes, in turn, decrease ion fluxes across the TATS membrane and decrease sarcoplasmic reticulum (SR) Ca2+ load. Thus, the TATS plays a potentially important role in modulating the function of guinea-pig ventricular myocyte in physiological conditions.
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Affiliation(s)
- Michal Pásek
- Institute of Thermomechanics, Czech Academy of Science-branch Brno, Czech Republic
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Bébarová M, Matejovic P, Pásek M, Nováková M. Effect of haloperidol on transient outward potassium current in rat ventricular myocytes. Eur J Pharmacol 2006; 550:15-23. [PMID: 17045259 DOI: 10.1016/j.ejphar.2006.08.046] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2006] [Revised: 08/25/2006] [Accepted: 08/28/2006] [Indexed: 11/15/2022]
Abstract
Although sigma ligand haloperidol is known to affect repolarization in heart, its effect on potassium currents in cardiomyocytes has not yet been studied. We analyzed the effect of 1 micromol/l haloperidol on transient outward K(+) current (I(to)) in enzymatically isolated rat right ventricular cardiomyocytes using the whole-cell patch-clamp technique at room temperature. Haloperidol induced a decrease of amplitude and an acceleration of apparent inactivation of I(to), both in a voltage-independent manner. The averaged inhibition of I(to), evaluated as a change of its time integral, was 23.0+/-3.2% at stimulation frequency of 0.1 Hz. As a consequence of slow recovery of I(to) from the haloperidol-induced block (time constant 1482+/-783 ms), a cumulation of the block up to about 40% appeared at 3.3 Hz. We conclude that haloperidol causes a voltage-independent block of I(to) that cumulates at higher stimulation frequencies. Based on the computer reconstruction of experimental data, a block of I(to)-channels in both open and open-inactivated states appears to be likely mechanism of haloperidol-induced inhibition of I(to).
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Affiliation(s)
- Markéta Bébarová
- Department of Physiology, Faculty of Medicine, Masaryk University, Komenského nám. 2, 662 43 Brno, Czech Republic.
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Nováková M, Bébarová M, Tarabová B, Pásek M, Matejoviè P, Lacinová L. Effects of sigma receptor ligand haloperidol on ionic currents in rat cardiomyocyte. J Mol Cell Cardiol 2006. [DOI: 10.1016/j.yjmcc.2006.03.181] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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Pásek M, Simurda J, Christé G. The functional role of cardiac T-tubules explored in a model of rat ventricular myocytes. Philos Trans A Math Phys Eng Sci 2006; 364:1187-206. [PMID: 16608703 DOI: 10.1098/rsta.2006.1764] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
The morphology of the cardiac transverse-axial tubular system (TATS) has been known for decades, but its function has received little attention. To explore the possible role of this system in the physiological modulation of electrical and contractile activity, we have developed a mathematical model of rat ventricular cardiomyocytes in which the TATS is described as a single compartment. The geometrical characteristics of the TATS, the biophysical characteristics of ion transporters and their distribution between surface and tubular membranes were based on available experimental data. Biophysically realistic values of mean access resistance to the tubular lumen and time constants for ion exchange with the bulk extracellular solution were included. The fraction of membrane in the TATS was set to 56%. The action potentials initiated in current-clamp mode are accompanied by transient K+ accumulation and transient Ca2+ depletion in the TATS lumen. The amplitude of these changes relative to external ion concentrations was studied at steady-state stimulation frequencies of 1-5 Hz. Ca2+ depletion increased from 7 to 13.1% with stimulation frequency, while K+ accumulation decreased from 4.1 to 2.7%. These ionic changes (particularly Ca2+ depletion) implicated significant decrease of intracellular Ca2+ load at frequencies natural for rat heart.
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Affiliation(s)
- Michal Pásek
- Institute of Thermomechanics, Czech Academy of Science, Branch Brno, Technická, Brno, Czech Republic
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Bébarová M, Matejovic P, Pásek M, Simurdová M, Simurda J. Effect of ajmaline on action potential and ionic currents in rat ventricular myocytes. Gen Physiol Biophys 2005; 24:311-25. [PMID: 16308426] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
The effect of ajmaline on action potential (AP) and ionic current components has been investigated in right ventricular myocytes of rat at room temperature using the whole cell patch clamp technique. Ajmaline decreased the upstroke velocity ((dV/dt)max) of AP and the AP amplitude, increased the AP duration measured at 50 and 90% repolarization, and reversibly inhibited most components of membrane ionic current in a concentration-dependent manner. The following values of IC50 and of the Hill coefficient (nH) resulted from approximation of the measured data by the Hill formula: for fast sodium current (INa) IC50=27.8+/-1.14 micromol/l and nH=1.27+/-0.25 at holding potential -75 mV, IC50=47.2+/-1.16 micromol/l and nH=1.16+/-0.21 at holding potential -120 mV; for L-type calcium current (ICa-L) IC50=70.8+/-0.09 micromol/l and n(H)=0.99+/-0.09; for transient outward potassium current (Ito) IC50=25.9+/-2.91 micromol/l and nH=1.07+/-0.15; for ATP-sensitive potassium current (IK(ATP)) IC50=13.3+/-1.1 micromol/l and nH=1.16+/-0.15. The current measured at the end of 300 ms depolarizing impulse was composed of an ajmaline-insensitive component and a component inhibited with IC50=61.0+/-1.1 micromol/l and nH=0.91+/-0.08. At hyperpolarizing voltages, ajmaline at high concentration of 300 micromol/l reduced the inward moiety of time-independent potassium current (IK1) by 36%. The results indicate that the inhibition of INa causes both the decreased rate of rise of depolarizing phase and the lowered amplitude of AP. The inhibition of Ito is responsible for the ajmaline-induced AP prolongation.
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Affiliation(s)
- M Bébarová
- Department of Physiology, Faculty of Medicine, Masaryk University, Komenského nám. 2, 662 43 Brno, Czech Republic.
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Bébarová M, Matejovic P, Pásek M, Simurdová M, Simurda J. Effect of ajmaline on transient outward current in rat ventricular myocytes. Gen Physiol Biophys 2005; 24:27-45. [PMID: 15900085] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/02/2023]
Abstract
The mechanism of ajmaline-induced inhibition of the transient outward current (I(to)) has been investigated in right ventricular myocytes of rat using the whole cell patch clamp technique. Ajmaline decreased the amplitude and the time integral of I(to) in a concentration-dependent, but frequency- and use-independent manner. In contrast to the single exponential time course of I(to)-inactivation in control conditions (tau(i) = 37.1 +/- 2.7 ms), the apparent inactivation was fitted by a sum of two exponentials under the effect of ajmaline with concentration-dependent fast and slow components (tau(f) = 11.7 +/- 0.8 ms, tau(s) = 57.6 +/- 2.7 ms at 10 micromol/l) suggesting block development primarily in the open channel state. An improved expression enabling to calculate the association and dissociation rate constants from the concentration dependence of tau(f) and tau(s) was derived and resulted in k(on) = 4.57 x 10(6) +/- 0.32 x 10(6) mol(-1).l.s(-1) and k(off) = 20.12 +/- 5.99 s(-1). The value of K(d) = 4.4 micromol/l calculated as k(off) / k(on) was considerably lower than IC(50) = 25.9 +/- 2.9 micromol/l evaluated from the concentration dependence of the integrals of I(to). Simulations on a simple model combining Hodgkin-Huxley type gating kinetics and drug-channel interaction entirely in open channel state agreed well with the experimental data including the difference between the K(d) and IC(50). According to the model, the fraction of blocked channels increases upon depolarization and declines if depolarization is prolonged. The repolarizing step induces recovery from block with time constant of 52 ms. We conclude that in the rat right ventricular myocytes, ajmaline is an open channel blocker with fast recovery from the block at resting voltage.
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Affiliation(s)
- M Bébarová
- Department of Physiology, Faculty of Medicine, Masaryk University, Komenského nám 2, 662 43 Brno, Czech Republic.
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Abstract
A mathematical model of the interaction of propafenone with cardiac sodium channels is based on experimental data that demonstrate use-dependent effects of the drug. The Clancy-Rudy model is applied to describe Na-channels in absence of the drug. The values of rate constants of the drug-receptor reaction are fitted to experimental data by iterative computer simulations using a genetic algorithm. The model suggests the following interpretation of available experimental results: First, drug molecules have access to the binding sites predominantly in the inactivated states. Secondly, the biphasic development of the block during depolarisation is consistent with a rapid increase due to drug binding in the fast inactivated state (rate constants k(on) = 645 micromol(-1) l s(-1), k(off) = 16.21 s(-1)) and a slow increase due to binding in the intermediate inactivated state (rate constants approximately 100-fold lower), followed by transition to the drug-occupied slow inactivated state (rate constants 0.784 and 0.921 s(-1)). Thirdly, the observed biphasic time course of recovery of I(Na) from block following restoration of the resting voltage results from simultaneous relief of block from the channels residing in the intermediate and slow inactivated states. Fourthly, the accumulation of blocked channels in the slow inactivated state is responsible for the observed use-dependent effects. Fifthly, when incorporated into a quantitative description of the electrical activity of a ventricular cell, the model predicts that propafenone (0.2 micromol l(-1)) effectively suppresses premature excitations, leaving the regular action potentials nearly unaffected.
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Affiliation(s)
- M Pásek
- Institute of Thermomechanics, Czech Academy of Science, Branch Brno, Czech Republic.
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Pásek M, Christé G, Simurda J. A quantitative model of the cardiac ventricular cell incorporating the transverse-axial tubular system. Gen Physiol Biophys 2003; 22:355-68. [PMID: 14986886] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 04/29/2023]
Abstract
The role of the transverse-axial tubular system (TATS) in electrical activity of cardiac cells has not been investigated quantitatively. In this study a mathematical model including the TATS and differential distribution of ionic transfer mechanisms in peripheral and tubular membranes was described. A model of ventricular cardiac cell described by Jafri et al. (1998) was adopted and slightly modified to describe ionic currents and Ca2+ handling. Changes of concentrations in the lumen of the TATS were computed from the total of transmembrane ionic fluxes and ionic exchanges with the pericellular medium. Long-term stability of the model was attained at rest and under regular stimulation. Depletion of Ca2+ by 12.8% and accumulation of K+ by 4.7% occurred in the TATS-lumen at physiological conditions and at a stimulation frequency of 1 Hz. The changes were transient and subsided on repolarization within 800 ms (Ca2+) and 300 ms (K+). Nevertheless, the course of action potentials remained virtually unaltered. Simulations of voltage clamp experiments demonstrated that variations in tubular ionic concentrations were detectable as modulation of the recorded membrane currents.
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Affiliation(s)
- M Pásek
- Institute of Thermomechanics, Czech Academy of Sciences, Branch Brno, Czech Republic.
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Abstract
Electrical restitution (ER) of cardiac cells is an aggregate of events that rhythmically restore the initial conditions of electric signal (action potential) generation. Its analysis represents an important insight into cardiac arrhythmogenesis. The aim of this work is to theoretically substantiate and verify a novel approach allowing for the quantification of the individual ionic current components of ER. A method of analysis of the primary, initial conditions-setting restitution processes (apart from the secondary, test pulse-affected ones) is proposed. Both processes are described as sums of their measurable constituents. It is demonstrated that the optimum parameter of ER is the electric charge that is transferred through ionic channels and carriers during the test impulse. The theory was tested by using voltage-clamped canine ventricular preparations and by computer simulations. The experimental ER curve of canine ventricular muscle was constructed using action potential (AP) plateau voltage and half-repolarization time as parameters. At 30 degrees C and 0.5 Hz stimulation, the ER curve peaked, on average, after 400 ms with a 10% overshoot. Of this plateau elevation, 50% was due to 4-aminopyridine-sensitive transient outward current and 44% was due to verapamil-sensitive current. The delayed outward current antagonized the overshoot by about 6%. It was found that the initial conditions (i.e. the primary restitution processes) tend to strongly alter the plateau voltage of the premature AP. However, the final deviation is by about one order less. It is concluded that the voltage-dependent secondary processes counteract the effect of the primary processes, thereby suggesting strong negative feedback control of natural APs.
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Affiliation(s)
- J Simurda
- Department of Physiology, Faculty of Medicine, Masaryk University, Brno, Czech Republic.
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