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Mazhar F, Bartolucci C, Regazzoni F, Paci M, Dedè L, Quarteroni A, Corsi C, Severi S. A detailed mathematical model of the human atrial cardiomyocyte: integration of electrophysiology and cardiomechanics. J Physiol 2023. [PMID: 37641426 DOI: 10.1113/jp283974] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2022] [Accepted: 08/02/2023] [Indexed: 08/31/2023] Open
Abstract
Mechano-electric regulations (MER) play an important role in the maintenance of cardiac performance. Mechano-calcium and mechano-electric feedback (MCF and MEF) pathways adjust the cardiomyocyte contractile force according to mechanical perturbations and affects electro-mechanical coupling. MER integrates all these regulations in one unit resulting in a complex phenomenon. Computational modelling is a useful tool to accelerate the mechanistic understanding of complex experimental phenomena. We have developed a novel model that integrates the MER loop for human atrial cardiomyocytes with proper consideration of feedforward and feedback pathways. The model couples a modified version of the action potential (AP) Koivumäki model with the contraction model by Quarteroni group. The model simulates iso-sarcometric and isometric twitches and the feedback effects on AP and Ca2+ -handling. The model showed a biphasic response of Ca2+ transient (CaT) peak to increasing pacing rates and highlights the possible mechanisms involved. The model has shown a shift of the threshold for AP and CaT alternans from 4.6 to 4 Hz under post-operative atrial fibrillation, induced by depressed SERCA activity. The alternans incidence was dependent on a chain of mechanisms including RyRs availability time, MCF coupling, CaMKII phosphorylation, and the stretch levels. As a result, the model predicted a 10% slowdown of conduction velocity for a 20% stretch, suggesting a role of stretch in creation of substrate formation for atrial fibrillation. Overall, we conclude that the developed model provides a physiological CaT followed by a physiological twitch. This model can open pathways for the future studies of human atrial electromechanics. KEY POINTS: With the availability of human atrial cellular data, interest in atrial-specific model integration has been enhanced. We have developed a detailed mathematical model of human atrial cardiomyocytes including the mechano-electric regulatory loop. The model has gone through calibration and evaluation phases against a wide collection of available human in-vitro data. The usefulness of the model for analysing clinical problems has been preliminaryly tested by simulating the increased incidence of Ca2+ transient and action potential alternans at high rates in post-operative atrial fibrillation condition. The model determines the possible role of mechano-electric feedback in alternans incidence, which can increase vulnerability to atrial arrhythmias by varying stretch levels. We found that our physiologically accurate description of Ca2+ handling can reproduce many experimental phenomena and can help to gain insights into the underlying pathophysiological mechanisms.
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Affiliation(s)
- Fazeelat Mazhar
- Department of Electrical, Electronic and Information Engineering 'Guglielmo Marconi', University of Bologna, Cesena, Italy
| | - Chiara Bartolucci
- Department of Electrical, Electronic and Information Engineering 'Guglielmo Marconi', University of Bologna, Cesena, Italy
| | | | - Michelangelo Paci
- Department of Electrical, Electronic and Information Engineering 'Guglielmo Marconi', University of Bologna, Cesena, Italy
- BioMediTech, Faculty of Medicine and Health Technology, Tampere University, Tampere, Finland
| | - Luca Dedè
- MOX - Dipartimento di Matematica, Politecnico di Milano, Milan, Italy
| | - Alfio Quarteroni
- MOX - Dipartimento di Matematica, Politecnico di Milano, Milan, Italy
- Mathematics Institute, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
| | - Cristiana Corsi
- Department of Electrical, Electronic and Information Engineering 'Guglielmo Marconi', University of Bologna, Cesena, Italy
| | - Stefano Severi
- Department of Electrical, Electronic and Information Engineering 'Guglielmo Marconi', University of Bologna, Cesena, Italy
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With a grain of salt: Sodium elevation and metabolic remodelling in heart failure. J Mol Cell Cardiol 2021; 161:106-115. [PMID: 34371034 PMCID: PMC7611640 DOI: 10.1016/j.yjmcc.2021.08.003] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/05/2021] [Revised: 07/09/2021] [Accepted: 08/03/2021] [Indexed: 12/14/2022]
Abstract
Elevated intracellular Na (Nai) and metabolic impairment are interrelated pathophysiological features of the failing heart (HF). There have been a number of studies showing that myocardial sodium elevation subtly affects mitochondrial function. During contraction, mitochondrial calcium (Camito) stimulates a variety of TCA cycle enzymes, thereby providing reducing equivalents to maintain ATP supply. Nai elevation has been shown to impact Camito; however, whether metabolic remodelling in HF is caused by increased Nai has only been recently demonstrated. This novel insight may help to elucidate the contribution of metabolic remodelling in the pathophysiology of HF, the lack of efficacy of current HF therapies and a rationale for the development of future metabolism-targeting treatments. Here we review the relationship between Na pump inhibition, elevated Nai, and altered metabolic profile in the context of HF and their link to metabolic (in)flexibility and mitochondrial reprogramming.
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Zhang H, Zhang S, Wang W, Wang K, Shen W. A Mathematical Model of the Mouse Atrial Myocyte With Inter-Atrial Electrophysiological Heterogeneity. Front Physiol 2020; 11:972. [PMID: 32848887 PMCID: PMC7425199 DOI: 10.3389/fphys.2020.00972] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2020] [Accepted: 07/16/2020] [Indexed: 12/20/2022] Open
Abstract
Biophysically detailed mathematical models of cardiac electrophysiology provide an alternative to experimental approaches for investigating possible ionic mechanisms underlying the genesis of electrical action potentials and their propagation through the heart. The aim of this study was to develop a biophysically detailed mathematical model of the action potentials of mouse atrial myocytes, a popular experimental model for elucidating molecular and cellular mechanisms of arrhythmogenesis. Based on experimental data from isolated mouse atrial cardiomyocytes, a set of mathematical equations for describing the biophysical properties of membrane ion channel currents, intracellular Ca2+ handling, and Ca2+-calmodulin activated protein kinase II and β-adrenergic signaling pathways were developed. Wherever possible, membrane ion channel currents were modeled using Markov chain formalisms, allowing detailed representation of channel kinetics. The model also considered heterogeneous electrophysiological properties between the left and the right atrial cardiomyocytes. The developed model was validated by its ability to reproduce the characteristics of action potentials and Ca2+ transients, matching quantitatively to experimental data. Using the model, the functional roles of four K+ channel currents in atrial action potential were evaluated by channel block simulations, results of which were quantitatively in agreement with existent experimental data. To conclude, this newly developed model of mouse atrial cardiomyocytes provides a powerful tool for investigating possible ion channel mechanisms of atrial electrical activity at the cellular level and can be further used to investigate mechanisms underlying atrial arrhythmogenesis.
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Affiliation(s)
- Henggui Zhang
- Department of Physics and Astronomy, Biological Physics Group, School of Physics & Astronomy, The University of Manchester, Manchester, United Kingdom.,Peng Cheng Laboratory, Shenzhen, China
| | - Shanzhuo Zhang
- Department of Physics and Astronomy, Biological Physics Group, School of Physics & Astronomy, The University of Manchester, Manchester, United Kingdom.,School of Computer Science and Technology, Harbin Institute of Technology, Harbin, China
| | - Wei Wang
- Department of Physics and Astronomy, Biological Physics Group, School of Physics & Astronomy, The University of Manchester, Manchester, United Kingdom.,Peng Cheng Laboratory, Shenzhen, China.,Shenzhen Key Laboratory of Visual Object Detection and Recognition, Harbin Institute of Technology, Shenzhen, China
| | - Kuanquan Wang
- School of Computer Science and Technology, Harbin Institute of Technology, Harbin, China
| | - Weijian Shen
- Department of Physics and Astronomy, Biological Physics Group, School of Physics & Astronomy, The University of Manchester, Manchester, United Kingdom
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Is there a causal link between intracellular Na elevation and metabolic remodelling in cardiac hypertrophy? Biochem Soc Trans 2018; 46:817-827. [PMID: 29970448 PMCID: PMC6103460 DOI: 10.1042/bst20170508] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/11/2018] [Revised: 05/23/2018] [Accepted: 05/24/2018] [Indexed: 01/23/2023]
Abstract
Alterations in excitation–contraction coupling and elevated intracellular sodium (Nai) are hallmarks of pathological cardiac remodelling that underline contractile dysfunction. In addition, changes in cardiac metabolism are observed in cardiac hypertrophy and heart failure (HF) that lead to a mismatch in ATP supply and demand, contributing to poor prognosis. A link between Nai and altered metabolism has been proposed but is not well understood. Many mitochondrial enzymes are stimulated by mitochondrial calcium (Camito) during contraction, thereby sustaining production of reducing equivalents to maintain ATP supply. This stimulation is thought to be perturbed when cytosolic Nai is high due to increased Camito efflux, potentially compromising ATPmito production and leading to metabolic dysregulation. Increased Nai has been previously shown to affect Camito; however, whether Nai elevation plays a causative role in energetic mismatching in the hypertrophied and failing heart remains unknown. In this review, we discuss the relationship between elevated Nai, NaK ATPase dysregulation and the metabolic phenotype in the contexts of pathological hypertrophy and HF and their link to metabolic flexibility, capacity (reserve) and efficiency that are governed by intracellular ion homeostasis. The development of non-invasive analytical techniques using nuclear magnetic resonance able to probe metabolism in situ in the functioning heart will enable a better understanding of the underlying mechanisms of Nai overload in cardiac pathophysiology. They will lead to novel insights that help to explain the metabolic contribution towards these diseases, the incomplete rescue observed with current therapies and a rationale for future energy-targeted therapies.
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Abstract
Intracellular Na+-concentration, [Na+]i modulates excitation-contraction coupling of cardiac myocytes via the Na+/Ca2+ exchanger (NCX). In cardiomyocytes from rainbow trout ( Oncorhyncus mykiss), whole cell patch-clamp studies have shown that Ca2+ influx via reverse-mode NCX contributes significantly to contraction when [Na+]i is 16 mM but not 10 mM. However, physiological [Na+]i has never been measured. We recorded [Na+]i using the fluorescent indicator sodium-binding benzofuran isophthalate in freshly isolated atrial and ventricular myocytes from rainbow trout. We examined [Na+]i at rest and during increases in contraction frequency across three temperatures that span those trout experience in nature (7, 14, and 21°C). Surprisingly, we found that [Na+]i was not different between atrial and ventricular cells. Furthermore, acute temperature changes did not affect [Na+]i in resting cells. Thus, we report a resting in vivo [Na+]i of 13.4 mM for rainbow trout cardiomyocytes. [Na+]i increased from rest with increases in contraction frequency by 3.2, 4.7, and 6.5% at 0.2, 0.5, and 0.8 Hz, respectively. This corresponds to an increase of 0.4, 0.6, and 0.9 mM at 0.2, 0.5, and 0.8 Hz, respectively. Acute temperature change did not significantly affect the contraction-induced increase in [Na+]i. Our results provide the first measurement of [Na+]i in rainbow trout cardiomyocytes. This surprisingly high [Na+]i is likely to result in physiologically significant Ca2+ influx via reverse-mode NCX during excitation-contraction coupling. We calculate that this Ca2+-source will decrease with the action potential duration as temperature and contraction frequency increases.
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Affiliation(s)
- Rikke Birkedal
- Faculty of Life Sciences, The University of Manchester, Core Technology Facility, Second Floor, 46 Grafton St., Manchester M13 9NT, United Kingdom.
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Wang HY, Huang RC. Diurnal modulation of the Na+/K+-ATPase and spontaneous firing in the rat retinorecipient clock neurons. J Neurophysiol 2004; 92:2295-301. [PMID: 15381747 DOI: 10.1152/jn.00061.2004] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The ventral "core" suprachiasmatic nucleus (vSCN) neurons are the retinorecipient neurons in the mammalian circadian clock and maintain a diurnal firing rhythm in reduced preparations. We tested the possibility that daily changes in Na+/K+-ATPase accompany diurnal variation in spontaneous electrical activity. In control, bath application of 9 microM strophanthidin increased the spontaneous firing both at day and night but to different extents. In the presence of 1 mM Ni2+ to block spontaneous firing, addition of 9 microM strophanthidin, but not higher concentrations (6.5-20 mM) of external K+, induced the silenced cells to fire action potentials in a diurnal rhythm, suggesting a diurnal change in Na+/K+-ATPase activity. Consistently, voltage-clamp recordings demonstrated that the pump current blocked by 9 microM strophanthidin was approximately three times larger in daytime than nighttime and was little affected by the presence of 1 mM Ni2+. Experiments with various concentrations of strophanthidin further suggests day-night differences in maximum Na+/K+-ATPase activity, amounting to 6 pA of pump current at day and down to 3.5 pA at night, and in its half-block concentrations, changing from a daytime value of 4 microM to a nighttime value of 8 microM. Our results indicate that the vSCN neurons exhibit a diurnal rhythm in the Na+/K+-ATPase the activity of which is higher during the day when the firing rate is also higher. Mechanistically, the modulation could be accounted for in terms of changes in the maximum activity of Na+/K+-ATPase and its ability to block by strophanthidin.
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Affiliation(s)
- Han-Ying Wang
- Dept. of Physiology, Chang Gung University School of Medicine, 259 Wen-Hwa 1st Rd., Kwei-San, Tao-Yuan, Taiwan
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Pieske B, Maier LS, Piacentino V, Weisser J, Hasenfuss G, Houser S. Rate dependence of [Na+]i and contractility in nonfailing and failing human myocardium. Circulation 2002; 106:447-53. [PMID: 12135944 DOI: 10.1161/01.cir.0000023042.50192.f4] [Citation(s) in RCA: 223] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
BACKGROUND In the failing human heart, altered Ca2+ homeostasis causes contractile dysfunction. Because Ca2+ and Na+ homeostasis are intimately linked through the Na+/Ca2+ exchanger, we compared the regulation of [Na+]i in nonfailing (NF) and failing human myocardium. METHODS AND RESULTS [Na+]i was measured in SBFI-loaded muscle strips. At slow pacing rates (0.25 Hz, 37 degrees C), isometric force was similar in NF (n=6) and failing (n=12) myocardium (6.4+/-1.2 versus 7.2+/-1.9 mN/mm2), but [Na+]i and diastolic force were greater in failing (22.1+/-2.6 mmol/L and 15.6+/-3.2 mN/mm2) than in NF (15.9+/-3.1 mmol/L and 3.50+/-0.55 mN/mm2; P<0.05) myocardium. In NF hearts, increasing stimulation rates resulted in a parallel increase in force and [Na+]i without changes in diastolic tension. At 2.0 Hz, force increased to 136+/-17% of the basal value (P<0.05), and [Na+]i to 20.5+/-4.2 mmol/L (P<0.05). In contrast, in failing myocardium, force declined to 45+/-3%, whereas [Na+]i increased to 27.4+/-3.2 mmol/L (both P<0.05), in association with significant elevations in diastolic tension. [Na+]i was higher in failing than in NF myocardium at every stimulation rate. [Na+]i predicted in myocytes from Na+ (pipette)-contraction relations was 8.0 mmol/L in NF (n=9) and 12.1 mmol/L in failing (n=57; P<0.05) myocardium at 0.25 Hz. Reverse-mode Na+/Ca2+ exchange induced significant Ca2+ influx in failing but not NF myocytes, compatible with higher [Na+]i in failing myocytes. CONCLUSIONS Na+i homeostasis is altered in failing human myocardium. At slow heart rates, the higher [Na+]i in failing myocardium appears to enhance Ca2+ influx through Na+/Ca2+ exchange and maintain sarcoplasmic reticulum Ca(2+) load and force development. At faster rates, failing myocytes with high [Na+]i cannot further increase sarcoplasmic reticulum Ca2+ load and are prone to diastolic Ca2+ overload.
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Affiliation(s)
- Burkert Pieske
- Abteilung Kardiologie und Pneumologie, Georg-August-Universität Göttingen, Germany.
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Zushi I, Shimura M, Tamai M, Kakazu Y, Akaike N. Characterization of the electrogenic Na+ -K+ pump in bipolar cells isolated from carp retina. Neuropharmacology 1998; 37:1053-61. [PMID: 9833634 DOI: 10.1016/s0028-3908(98)00084-7] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
The electrogenic Na+ -K+ pump current (Ip) in carp bipolar cells was investigated under voltage-clamp conditions. The Ip was activated in a concentration-dependent manner by adding external K+ (Ko+) and was completely suppressed with 10(-4) M ouabain (EC50=1.23 mM; Hill coefficient=1.36). The Ip was suppressed in a concentration-dependent manner by ouabain (IC50=1.90 mM; Hill coefficient=0.93). The Ip did not show a distinct voltage dependency either with or without Na(o)+. A large outward shift of the holding current was observed by completely removing Na(o)+. In the presence of Na(o)+, a steady Ip was observed even in the absence of internal Na+ (Na(i)+). These results suggest that continuous Na+ influxes exist across the membrane. When external and internal Na+ was removed, a transient Ip was observed (half decay time (t1/2) was 5.0+/-0.6 s), thus indicating that the transient Ip was activated by the residual Na(i)+. In the absence of Na(o)+, the transient Ip was also observed with lower than 8 mM Na(i)+. The t1/2 depended on Na(i)+. However, a steady Ip was observed with 10 mM Na(i)+ or more. The functional properties of the Ip are discussed.
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Affiliation(s)
- I Zushi
- Department of Physiology, Faculty of Medicine, Kyushu University, Fukuoka, Japan
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Müller-Ehmsen J, Brixius K, Schwinger RH. Positive inotropic effects of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium. J Cardiovasc Pharmacol 1998; 31:684-9. [PMID: 9593067 DOI: 10.1097/00005344-199805000-00006] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The aim of this study was to investigate the inotropic properties of the novel Na+-channel modulator BDF 9198 in human nonfailing and failing myocardium. For comparison the Na+-channel modulator BDF 9148, the beta-adrenoceptor-agonist isoprenaline, and calcium were studied. Concentration-response curves for BDF 9198 (0.01-30 microM), BDF 9148 (0.01-30 microM), isoprenaline (0.001-1 microM), and calcium (1.8-15 mM) were obtained in electrically driven left ventricular human papillary muscle strips (1 Hz, 37 degrees C; dilated cardiomyopathy, NYHA IV, heart transplantation; nonfailing, donor hearts). Whereas isoprenaline was significantly less effective and less potent in increasing the force of contraction in failing human myocardium than in nonfailing myocardium (p < 0.01), BDF 9198 and BDF 9148 were (in NYHA IV) as effective as in nonfailing human tissue. In both tissues, BDF 9198 and BDF 9148 exerted similar positive inotropic effects as calcium, with the novel Na+-channel modulator BDF 9198 being more potent in increasing force of contraction than was the preceding agent BDF 9148. The potencies of both Na+-channel modulators, BDF 9198 and BDF 9148, were enhanced in human failing myocardium when compared with nonfailing myocardium. In summary, the novel Na+-channel modulator BDF 9198 increases force of contraction to the same extent as calcium and with a higher potency than BDF 9148. The sensitivity of failing human myocardium to Na+-channel modulators is increased when compared with nonfailing myocardium, which might be the result of an altered Na+ homeostasis in human heart failure.
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Affiliation(s)
- J Müller-Ehmsen
- Laboratory of Muscle Research and Molecular Cardiology, Klinik III für Innere Medizin, der Universität zu Köln, Germany
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Minajeva A, Kaasik A, Paju K, Seppet E, Lompré AM, Veksler V, Ventura-Clapier R. Sarcoplasmic reticulum function in determining atrioventricular contractile differences in rat heart. THE AMERICAN JOURNAL OF PHYSIOLOGY 1997; 273:H2498-507. [PMID: 9374790 DOI: 10.1152/ajpheart.1997.273.5.h2498] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The relationships between the contractile characteristics and the sarcoplasmic reticulum (SR) function of rat atrial and ventricular trabeculae were compared. The isometric developed tension (DT) and the rates of contraction (+ dT/dt) and relaxation (-dT/dt) normalized to cross-sectional area were 3.7, 2.2, and 1.8 times lower, respectively, in intact atrial strips compared with ventricular strips, whereas + dT/dt and -dT/dt (normalized to DT) were 2.3 and 2.8 times higher, respectively, in atria. Atria exhibited a maximal potentiation of DT after shorter rest periods than ventricles and a lower reversal for prolonged rest periods. Caffeine-induced tension transients in saponin-permeabilized fibers suggested that the Ca2+ concentration released in atrial myofibrils reached a lower maximum and decayed more slowly than in ventricular preparations. However, the tension-time integrals indicated an equivalent capacity of sequestrable Ca2+ in SR from both tissues. In atrial, as in ventricular myocardium, the SR Ca2+ uptake was more efficiently supported by ATP produced by the SR-bound MM form of creatine kinase (CK; MM-CK) than by externally added ATP, suggesting a tight functional coupling between the SR Ca2+ adenosinetriphosphatase (ATPase) and MM-CK. The maximal rate of oxalate-supported Ca2+ uptake was two times higher in atrial than in ventricular tissue homogenates. The SR Ca(2+)-ATPase 2a mRNA content normalized to 18S RNA was 38% higher in atria than in ventricles, whereas the amount of mRNA encoding the alpha-myosin heavy chain, calsequestrin, and the ryanodine receptor was similar in both tissues. Thus a lower amount of readily releasable Ca2+ together with a faster uptake rate may partly account for the shorter time course and lower tension development in intact atrial myocardium compared with ventricular myocardium.
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Affiliation(s)
- A Minajeva
- Department of Pathological Physiology, Medical Faculty, University of Tartu, Estonia
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11
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Senatorov VV, Mooney D, Hu B. The electrogenic effects of Na(+)-K(+)-ATPase in rat auditory thalamus. J Physiol 1997; 502 ( Pt 2):375-85. [PMID: 9263917 PMCID: PMC1159556 DOI: 10.1111/j.1469-7793.1997.375bk.x] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
1. The electrogenic effects of the Na(+)-K(+)-ATPase in thalamic neurones were investigated by means of intracellular and whole-cell patch-clamp recording techniques in rat medial geniculate body (MGB) maintained in vitro. 2. In twenty-six out of thirty-one neurones recorded intracellularly, application of the Na(+)-K+ pump inhibitor strophanthidin induced two different types of membrane depolarization: a small, reversible depolarization with a peak amplitude of 4 +/- 2.6 mV or a prolonged depolarization of large amplitude (48.6 +/- 9.0 mV) with or without a decrease in apparent membrane resistance. Blockade of glutamate receptors with kynurenic acid or 6-cyano-7-nitroquinoxaline-2,3-dione and (+/-)-2-amino-5-phosphonopentanoic acid did not prevent either type of pump response, but the large depolarization was not seen when the medium contained the sodium channel blocker TTX. 3. Whole-cell patch-clamp recording showed that the small membrane depolarization is mediated by an inward membrane current (39.00 +/- 5.70 pA) that exhibited a weak voltage dependence. An inward current of similar amplitude was also induced in MGB cells when the pipette solution contained nominally zero Na+ or when K+ was temporarily omitted from the extracellular medium. The large membrane depolarization or the corresponding membrane current was not observed in whole-cell conditions. 6. Transient inhibition of the electrogenic Na(+)-K(+)-ATPase consistently led to a change in the mode of synaptic transmission in MGB cells, during which the synaptically evoked burst response was either blocked or converted into a single spike discharge. 7. Taken together, these data suggest that blockade of the electrogenic pump produces a dual membrane effect in mammalian thalamic neurones: a small electrogenic membrane depolarization and a large depolarization response that can be prevented by artificially maintaining the transmembrane ionic gradients. The electrogenic activity of the Na(+)-K(+)-ATPase may play an important role in setting the mode of synaptic transmission in sensory thalamus.
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Affiliation(s)
- V V Senatorov
- Loeb Medical Research Institute, Ottawa Civic Hospital, University of Ottawa, Ontario, Canada
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12
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Senatorov VV, Hu B. Differential Na(+)-K(+)-ATPase activity in rat lemniscal and non-lemniscal auditory thalami. J Physiol 1997; 502 ( Pt 2):387-95. [PMID: 9263918 PMCID: PMC1159557 DOI: 10.1111/j.1469-7793.1997.387bk.x] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
1. Using whole-cell recording and confocal immunofluorescent microscopy, we have investigated the differential electrogenic activity, subunit expression and subcellular distribution of the Na(+)-K(+)-ATPase in the lemniscal (ventral) and non-lemniscal (dorsal) pathways of the rat medial geniculate body (MGB) in vitro. 2. Bath application of Na(+)-K(+)-ATPase inhibitors strophanthidin or dihydro-ouabain produced a transient, dose-dependent inward current or membrane depolarization which were significantly larger in dorsal MGB neurones than in ventral cells (45.9 +/- 6.45 vs. 24.3 +/- 4.1 pA; P < 0.05). Electrophysiological and morphometric measurements showed that the dorsal MGB neurones had a significantly lower input conductance and a smaller somata than their ventral counterparts. The level of the resting membrane potential also differed by about 6 mV between the two cell populations, with the dorsal cells being more hyperpolarized (-74.2 +/- 0.6 vs. -67.7 +/- 1.3 mV; P < 0.001). 3. Incubation of enzymatically dissociated MGB neurones with fluorescent monoclonal antibodies against alpha 1-alpha 3 isoforms of Na(+)-K(+)-ATPase showed that both dorsal and ventral cells expressed primarily alpha 3 subunits. Confocal laser scanning revealed, however, that the mean pixel density of alpha 3 fluorescent antibodies in the plasma membrane domain, but not in the cytoplasmic compartment, was about 40% higher in dorsal neurones than in the ventral cells (29.7 +/- 4.7 vs. 16.9 +/- 2.3 grey shadow per pixel; P < 0.05). 4. The above results suggest that the electrogenic activity of the Na(+)-K(+)-ATPase is differentially regulated between lemniscal and non-lemniscal auditory thalami through a mechanism that probably involves differential pump densities in the cell membrane.
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Affiliation(s)
- V V Senatorov
- Loeb Medical Research Institute, Ottawa Civic Hospital, University of Ottawa, Ontario, Canada
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13
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Wang J, Schwinger RH, Frank K, Müller-Ehmsen J, Martin-Vasallo P, Pressley TA, Xiang A, Erdmann E, McDonough AA. Regional expression of sodium pump subunits isoforms and Na+-Ca++ exchanger in the human heart. J Clin Invest 1996; 98:1650-8. [PMID: 8833915 PMCID: PMC507599 DOI: 10.1172/jci118960] [Citation(s) in RCA: 76] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Cardiac glycosides exert a positive inotropic effect by inhibiting sodium pump (Na,K-ATPase) activity, decreasing the driving force for Na+-Ca++ exchange, and increasing cellular content and release of Ca++ during depolarization. Since the inotropic response will be a function of the level of expression of sodium pumps, which are alpha(beta) heterodimers, and of Na+-Ca++ exchangers, this study aimed to determine the regional pattern of expression of these transporters in the heart. Immunoblot assays of homogenate from atria, ventricles, and septa of 14 nonfailing human hearts established expression of Na,K-ATPase alpha1, alpha2, alpha3, beta1, and Na+-Ca++ exchangers in all regions. Na,K-ATPase beta2 expression is negligible, indicating that the human cardiac glycoside receptors are alpha1beta1, alpha2beta1, and alpha3beta1. alpha3, beta1, sodium pump activity, and Na+-Ca++ exchanger levels were 30-50% lower in atria compared to ventricles and/or septum; differences between ventricles and septum were insignificant. Functionally, the EC50 of the sodium channel activator BDF 9148 to increase force of contraction was lower in atria than ventricle muscle strips (0.36 vs. 1.54 microM). These results define the distribution of the cardiac glycoside receptor isoforms in the human heart and they demonstrate that atria have fewer sodium pumps, fewer Na+-Ca++ exchangers, and enhanced sensitivity to inotropic stimulation compared to ventricles.
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Affiliation(s)
- J Wang
- Department of Physiology and Biophysics, University of Southern California School of Medicine, Los Angeles 90033, USA
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