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Moreno C, Yano S, Bezanilla F, Latorre R, Holmgren M. Transient Electrical Currents Mediated by the Na +/K +-ATPase: A Tour from Basic Biophysics to Human Diseases. Biophys J 2020; 119:236-242. [PMID: 32579966 PMCID: PMC7376075 DOI: 10.1016/j.bpj.2020.06.006] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2020] [Revised: 04/06/2020] [Accepted: 06/03/2020] [Indexed: 01/14/2023] Open
Abstract
The Na+/K+-ATPase is a chemical molecular machine responsible for the movement of Na+ and K+ ions across the cell membrane. These ions are moved against their electrochemical gradients, so the protein uses the free energy of ATP hydrolysis to transport them. In fact, the Na+/K+-ATPase is the single largest consumer of energy in most cells. In each pump cycle, the protein sequentially exports 3Na+ out of the cell, then imports 2K+ into the cell at an approximate rate of 200 cycles/s. In each half cycle of the transport process, there is a state in which ions are stably trapped within the permeation pathway of the protein by internal and external gates in their closed states. These gates are required to open alternately; otherwise, passive ion diffusion would be a wasteful end of the cell's energy. Once one of these gates open, ions diffuse from their binding sites to the accessible milieu, which involves moving through part of the electrical field across the membrane. Consequently, ions generate transient electrical currents first discovered more than 30 years ago. They have been studied in a variety of preparations, including native and heterologous expression systems. Here, we review three decades' worth of work using these transient electrical signals to understand the kinetic transitions of the movement of Na+ and K+ ions through the Na+/K+-ATPase and propose the significance that this work might have to the understanding of the dysfunction of human pump orthologs responsible for some newly discovered neurological pathologies.
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Affiliation(s)
- Cristina Moreno
- Molecular Neurophysiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland
| | - Sho Yano
- Medical Genetics and Genomic Medicine Training Program, National Human Genome Research Institute, National Institutes of Health, Bethesda, Maryland
| | - Francisco Bezanilla
- Department of Biochemistry and Molecular Biology, University of Chicago, Gordon Center for Integrative Sciences, Chicago, Illinois
| | - Ramon Latorre
- Centro Interdisciplinario de Neurociencia de Valparaíso, Universidad de Valparaíso, Valparaíso, Chile
| | - Miguel Holmgren
- Molecular Neurophysiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland.
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2
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Zeng SL, Sudlow LC, Berezin MY. Using Xenopus oocytes in neurological disease drug discovery. Expert Opin Drug Discov 2019; 15:39-52. [PMID: 31674217 DOI: 10.1080/17460441.2020.1682993] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Introduction: Neurological diseases present a difficult challenge in drug discovery. Many of the current treatments have limited efficiency or result in a variety of debilitating side effects. The search of new therapies is of a paramount importance, since the number of patients that require a better treatment is growing rapidly. As an in vitro model, Xenopus oocytes provide the drug developer with many distinct advantages, including size, durability, and efficiency in exogenous protein expression. However, there is an increasing need to refine the recent breakthroughs.Areas covered: This review covers the usage and recent advancements of Xenopus oocytes for drug discovery in neurological diseases from expression and functional measurement techniques to current applications in Alzheimer's disease, painful neuropathies, and amyotrophic lateral sclerosis (ALS). The existing limitations of Xenopus oocytes in drug discovery are also discussed.Expert opinion: With the rise of aging population and neurological disorders, Xenopus oocytes, will continue to play an important role in understanding the mechanism of the disease, identification and validation of novel molecular targets, and drug screening, providing high-quality data despite the technical limitations. With further advances in oocytes-related techniques toward an accurate modeling of the disease, the diagnostics and treatment of neuropathologies will be becoming increasing personalized.
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Affiliation(s)
- Steven L Zeng
- Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA
| | - Leland C Sudlow
- Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA
| | - Mikhail Y Berezin
- Department of Radiology, Washington University School of Medicine, St. Louis, MO, USA
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3
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Kueh D, Barnett WH, Cymbalyuk GS, Calabrese RL. Na(+)/K(+) pump interacts with the h-current to control bursting activity in central pattern generator neurons of leeches. eLife 2016; 5. [PMID: 27588351 PMCID: PMC5010386 DOI: 10.7554/elife.19322] [Citation(s) in RCA: 34] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/01/2016] [Accepted: 08/08/2016] [Indexed: 01/12/2023] Open
Abstract
The dynamics of different ionic currents shape the bursting activity of neurons and networks that control motor output. Despite being ubiquitous in all animal cells, the contribution of the Na(+)/K(+) pump current to such bursting activity has not been well studied. We used monensin, a Na(+)/H(+) antiporter, to examine the role of the pump on the bursting activity of oscillator heart interneurons in leeches. When we stimulated the pump with monensin, the period of these neurons decreased significantly, an effect that was prevented or reversed when the h-current was blocked by Cs(+). The decreased period could also occur if the pump was inhibited with strophanthidin or K(+)-free saline. Our monensin results were reproduced in model, which explains the pump's contributions to bursting activity based on Na(+) dynamics. Our results indicate that a dynamically oscillating pump current that interacts with the h-current can regulate the bursting activity of neurons and networks.
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Affiliation(s)
- Daniel Kueh
- Department of Biology, Emory University, Atlanta, United States
| | - William H Barnett
- Neuroscience Institute, Georgia State University, Atlanta, United States
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Xu Y, Liu X, Schwarz S, Hu L, Guo D, Gu Q, Schwarz W. Inhibitory efficacy of bufadienolides on Na +,K +-pump activity versus cell proliferation. Biochem Biophys Rep 2016; 6:158-164. [PMID: 28955873 PMCID: PMC5600443 DOI: 10.1016/j.bbrep.2016.03.015] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2015] [Revised: 03/12/2016] [Accepted: 03/29/2016] [Indexed: 11/29/2022] Open
Abstract
Bufadienolides are cytotoxic drugs that may form the basis for anticancer agents. Due to structural and functional similarity to cardiotonic glycosides, application is restricted. We, therefore, investigated correlation of their putative anticancer effects with inhibition of Na+,K+pumps. The natural bufalin and three derivatives were tested. The anticancer effects of the drugs were checked by observing their inhibitory effects on proliferation of rat liver cancer cells using MTT assay. Inhibition of Na+,K+-pump was determined by measuring pump-mediated current of rat α1/β1 and α2/β1 Na+,K+pumps expressed in Xenopus oocytes. All tested bufadienolides inhibited cell proliferation and Na+,K+pump activity. An activity coefficient A=100xIC50Na,K pump/IC50proliferation was used to describe drug effectivity as anticancer drug. Natural bufalin exhibited lowest effectivity on cell proliferation, and also the A value for rat α1 isoform was the lowest (0.08), the α2 isoform was much less sensitive (A=1.00). The highest A values were obtained for the BF238 derivative with A=0.88 and 2.64 for the α1 and α2 isoforms, respectively. Therefore, we suggest that search for bufalin derivatives with high anticancer effect and low affinity for both Na+,K+pump isoforms may be a promising strategy for development of anticancer drugs. Effects of bufadienolides on Na pump are not correlated with their cytotoxicity. Bufadienolides with high anticancer effect but low side effect may exist. BF238 may form a basis for further anticancer drug research and development.
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Key Words
- BAP, Bufalin-3-MeON-arabinopyranoside
- BF238, Bufalin-3-Yl [3-(1h-imidazol-1-Yl)propyl]carbamate
- BF601, Bufalin-3-Yl [3-(methylamino)propyl]carbamate
- Bufadienolide
- Cell proliferation
- MTT, 3,[4,5-dimethylthiazol-2-Yl-] diphenyltetrazolium bromide
- Na+,K+-ATPase
- ORi, Oocyte Ringer's (solution)
- Rα1/β1, rat Na+,K+pump formed by Α1 and Β1 subunits
- Rα2/β1, rat Na+,K+pump formed by Α2 and Β1 subunits
- TEA‐Cl, Tetraethylammonium chloride
- Voltage clamp
- Xenopus oocyte
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Affiliation(s)
- Yinfang Xu
- Shanghai Research Center of Acupuncture and Meridians, Shanghai, China.,Shanghai Key Laboratory of Acupuncture Mechanism and Acupoint Function, Fudan-University, Shanghai, China
| | - Xuan Liu
- Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Silvia Schwarz
- Shanghai Research Center of Acupuncture and Meridians, Shanghai, China.,Shanghai Key Laboratory of Acupuncture Mechanism and Acupoint Function, Fudan-University, Shanghai, China
| | - Lihong Hu
- Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Dean Guo
- Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai, China
| | - Quanbao Gu
- Shanghai Research Center of Acupuncture and Meridians, Shanghai, China.,Shanghai Key Laboratory of Acupuncture Mechanism and Acupoint Function, Fudan-University, Shanghai, China
| | - Wolfgang Schwarz
- Shanghai Research Center of Acupuncture and Meridians, Shanghai, China.,Shanghai Key Laboratory of Acupuncture Mechanism and Acupoint Function, Fudan-University, Shanghai, China.,Institute for Biophysics, Goethe-University, Frankfurt am Main, Germany
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5
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Clausen MV, Nissen P, Poulsen H. The α4 isoform of the Na⁺,K⁺-ATPase is tuned for changing extracellular environments. FEBS J 2015; 283:282-93. [PMID: 26476261 DOI: 10.1111/febs.13567] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2015] [Revised: 09/20/2015] [Accepted: 10/15/2015] [Indexed: 11/30/2022]
Abstract
In their journey from the male to the female reproductive tract, spermatozoa are confronted with a constantly changing environment. To cope with the associated challenges, spermatozoa express a distinct set of transporters, channels and pumps. One of the membrane proteins unique to spermatozoa is the α4 isoform of the Na(+) ,K(+) -ATPase. In addition to α4, spermatozoa express the ubiquous α1 variant. To get a detailed understanding of how α1 and α4 differ, and why spermatozoa need an additional Na(+) ,K(+) -ATPase, we have conducted an electrophysiological comparison of the rodent isoforms (rat α4 versus mouse α1-3) using the Xenopus oocyte expression system. We demonstrate isoform-specific differences in the voltage sensitivity of steady-state turnover, with α2 being the more sensitive, and α1 and α2 having faster Na(+) release kinetics than α3 and α4. Our data further show that the α1 and α2 turnover rates are fast compared with those of α3 and α4. Finally, α4 is less influenced by changes in extracellular Na(+) and temperature than α1. Based on these findings, we discuss the possibility that evolution has selected robust activity rather than rapid turnover for α4.
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Affiliation(s)
| | - Poul Nissen
- Department of Molecular Biology and Genetics, Aarhus University, Denmark
| | - Hanne Poulsen
- Department of Molecular Biology and Genetics, Aarhus University, Denmark
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6
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Choi S, Yu E, Kim DS, Sugimori M, Llinás RR. RNS60, a charge-stabilized nanostructure saline alters Xenopus Laevis oocyte biophysical membrane properties by enhancing mitochondrial ATP production. Physiol Rep 2015; 3:3/3/e12261. [PMID: 25742953 PMCID: PMC4393147 DOI: 10.14814/phy2.12261] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022] Open
Abstract
We have examined the effects of RNS60, a 0.9% saline containing charge-stabilized oxygen nanobubble-based structures. RNS60 is generated by subjecting normal saline to Taylor–Couette–Poiseuille (TCP) flow under elevated oxygen pressure. This study, implemented in Xenopus laevis oocytes, addresses both the electrophysiological membrane properties and parallel biological processes in the cytoplasm. Intracellular recordings from defolliculated X. laevis oocytes were implemented in: (1) air oxygenated standard Ringer's solution, (2) RNS60-based Ringer's solution, (3) RNS10.3 (TCP-modified saline without excess oxygen)-based Ringer's, and (4) ONS60 (saline containing high pressure oxygen without TCP modification)-based Ringer's. RNS60-based Ringer's solution induced membrane hyperpolarization from the resting membrane potential. This effect was prevented by: (1) ouabain (a blocker of the sodium/potassium ATPase), (2) rotenone (a mitochondrial electron transfer chain inhibitor preventing usable ATP synthesis), and (3) oligomycin A (an inhibitor of ATP synthase) indicating that RNS60 effects intracellular ATP levels. Increased intracellular ATP levels following RNS60 treatment were directly demonstrated using luciferin/luciferase photon emission. These results indicate that RNS60 alters intrinsic the electrophysiological properties of the X. laevis oocyte membrane by increasing mitochondrial-based ATP synthesis. Ultrastructural analysis of the oocyte cytoplasm demonstrated increased mitochondrial length in the presence of RNS60-based Ringer's solution. It is concluded that the biological properties of RNS60 relate to its ability to optimize ATP synthesis.
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Affiliation(s)
- Soonwook Choi
- Marine Biological Laboratory, Woods Hole, Massachusetts, USA Department of Neuroscience and Physiology, New York University School of Medicine, New York, New York, USA
| | - Eunah Yu
- Marine Biological Laboratory, Woods Hole, Massachusetts, USA Department of Neuroscience and Physiology, New York University School of Medicine, New York, New York, USA
| | - Duk-Soo Kim
- Marine Biological Laboratory, Woods Hole, Massachusetts, USA Department of Anatomy, College of Medicine, Soonchunhyang University, Cheonan-Si, Korea
| | - Mutsuyuki Sugimori
- Marine Biological Laboratory, Woods Hole, Massachusetts, USA Department of Neuroscience and Physiology, New York University School of Medicine, New York, New York, USA
| | - Rodolfo R Llinás
- Marine Biological Laboratory, Woods Hole, Massachusetts, USA Department of Neuroscience and Physiology, New York University School of Medicine, New York, New York, USA
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7
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Mares LJ, Garcia A, Rasmussen HH, Cornelius F, Mahmmoud YA, Berlin JR, Lev B, Allen TW, Clarke RJ. Identification of electric-field-dependent steps in the Na(+),K(+)-pump cycle. Biophys J 2015; 107:1352-63. [PMID: 25229143 DOI: 10.1016/j.bpj.2014.05.054] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2014] [Revised: 04/30/2014] [Accepted: 05/16/2014] [Indexed: 01/25/2023] Open
Abstract
The charge-transporting activity of the Na(+),K(+)-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme's reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na(+),K(+)-ATPase's transport sites in competition with Na(+) and K(+), but is not occluded within the protein. We find that only the occludable ions Na(+), K(+), Rb(+), and Cs(+) cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na(+) or K(+) binding in a high field access channel is a major electrogenic reaction of the Na(+),K(+)-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.
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Affiliation(s)
- Laura J Mares
- School of Chemistry, University of Sydney, Sydney, Australia
| | - Alvaro Garcia
- Department of Cardiology, Royal North Shore Hospital, Sydney, Australia; Kolling Institute, University of Sydney, Sydney, Australia
| | - Helge H Rasmussen
- Department of Cardiology, Royal North Shore Hospital, Sydney, Australia; Kolling Institute, University of Sydney, Sydney, Australia
| | | | | | - Joshua R Berlin
- Department of Pharmacology and Physiology, Rutgers University, Newark, New Jersey
| | - Bogdan Lev
- School of Applied Science and Health Innovations Research Institute, REMIT University, Melbourne, Australia
| | - Toby W Allen
- School of Applied Science and Health Innovations Research Institute, REMIT University, Melbourne, Australia
| | - Ronald J Clarke
- School of Chemistry, University of Sydney, Sydney, Australia.
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8
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Dürr KL, Tavraz NN, Spiller S, Friedrich T. Measuring cation transport by Na,K- and H,K-ATPase in Xenopus oocytes by atomic absorption spectrophotometry: an alternative to radioisotope assays. J Vis Exp 2013:e50201. [PMID: 23462593 DOI: 10.3791/50201] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/31/2022] Open
Abstract
Whereas cation transport by the electrogenic membrane transporter Na(+),K(+)-ATPase can be measured by electrophysiology, the electroneutrally operating gastric H(+),K(+)-ATPase is more difficult to investigate. Many transport assays utilize radioisotopes to achieve a sufficient signal-to-noise ratio, however, the necessary security measures impose severe restrictions regarding human exposure or assay design. Furthermore, ion transport across cell membranes is critically influenced by the membrane potential, which is not straightforwardly controlled in cell culture or in proteoliposome preparations. Here, we make use of the outstanding sensitivity of atomic absorption spectrophotometry (AAS) towards trace amounts of chemical elements to measure Rb(+) or Li(+) transport by Na(+),K(+)- or gastric H(+),K(+)-ATPase in single cells. Using Xenopus oocytes as expression system, we determine the amount of Rb(+) (Li(+)) transported into the cells by measuring samples of single-oocyte homogenates in an AAS device equipped with a transversely heated graphite atomizer (THGA) furnace, which is loaded from an autosampler. Since the background of unspecific Rb(+) uptake into control oocytes or during application of ATPase-specific inhibitors is very small, it is possible to implement complex kinetic assay schemes involving a large number of experimental conditions simultaneously, or to compare the transport capacity and kinetics of site-specifically mutated transporters with high precision. Furthermore, since cation uptake is determined on single cells, the flux experiments can be carried out in combination with two-electrode voltage-clamping (TEVC) to achieve accurate control of the membrane potential and current. This allowed e.g. to quantitatively determine the 3Na(+)/2K(+) transport stoichiometry of the Na(+),K(+)-ATPase and enabled for the first time to investigate the voltage dependence of cation transport by the electroneutrally operating gastric H(+),K(+)-ATPase. In principle, the assay is not limited to K(+)-transporting membrane proteins, but it may work equally well to address the activity of heavy or transition metal transporters, or uptake of chemical elements by endocytotic processes.
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9
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Inhibition of Activity of GABA Transporter GAT1 by δ-Opioid Receptor. EVIDENCE-BASED COMPLEMENTARY AND ALTERNATIVE MEDICINE 2012; 2012:818451. [PMID: 23365600 PMCID: PMC3543822 DOI: 10.1155/2012/818451] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/11/2012] [Revised: 11/04/2012] [Accepted: 11/04/2012] [Indexed: 01/16/2023]
Abstract
Analgesia is a well-documented effect of acupuncture. A critical role in pain sensation plays the nervous system, including the GABAergic system and opioid receptor (OR) activation. Here we investigated regulation of GABA transporter GAT1 by δOR in rats and in Xenopus oocytes. Synaptosomes of brain from rats chronically exposed to opiates exhibited reduced GABA uptake, indicating that GABA transport might be regulated by opioid receptors. For further investigation we have expressed GAT1 of mouse brain together with mouse δOR and μOR in Xenopus oocytes. The function of GAT1 was analyzed in terms of Na+-dependent [3H]GABA uptake as well as GAT1-mediated currents. Coexpression of δOR led to reduced number of fully functional GAT1 transporters, reduced substrate translocation, and GAT1-mediated current. Activation of δOR further reduced the rate of GABA uptake as well as GAT1-mediated current. Coexpression of μOR, as well as μOR activation, affected neither the number of transporters, nor rate of GABA uptake, nor GAT1-mediated current. Inhibition of GAT1-mediated current by activation of δOR was confirmed in whole-cell patch-clamp experiments on rat brain slices of periaqueductal gray. We conclude that inhibition of GAT1 function will strengthen the inhibitory action of the GABAergic system and hence may contribute to acupuncture-induced analgesia.
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10
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Dürr KL, Tavraz NN, Friedrich T. Control of gastric H,K-ATPase activity by cations, voltage and intracellular pH analyzed by voltage clamp fluorometry in Xenopus oocytes. PLoS One 2012; 7:e33645. [PMID: 22448261 PMCID: PMC3308979 DOI: 10.1371/journal.pone.0033645] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/09/2011] [Accepted: 02/14/2012] [Indexed: 11/24/2022] Open
Abstract
Whereas electrogenic partial reactions of the Na,K-ATPase have been studied in depth, much less is known about the influence of the membrane potential on the electroneutrally operating gastric H,K-ATPase. In this work, we investigated site-specifically fluorescence-labeled H,K-ATPase expressed in Xenopus oocytes by voltage clamp fluorometry to monitor the voltage-dependent distribution between E1P and E2P states and measured Rb+ uptake under various ionic and pH conditions. The steady-state E1P/E2P distribution, as indicated by the voltage-dependent fluorescence amplitudes and the Rb+ uptake activity were highly sensitive to small changes in intracellular pH, whereas even large extracellular pH changes affected neither the E1P/E2P distribution nor transport activity. Notably, intracellular acidification by approximately 0.5 pH units shifted V0.5, the voltage, at which the E1P/E2P ratio is 50∶50, by −100 mV. This was paralleled by an approximately two-fold acceleration of the forward rate constant of the E1P→E2P transition and a similar increase in the rate of steady-state cation transport. The temperature dependence of Rb+ uptake yielded an activation energy of ∼90 kJ/mol, suggesting that ion transport is rate-limited by a major conformational transition. The pronounced sensitivity towards intracellular pH suggests that proton uptake from the cytoplasmic side controls the level of phosphoenzyme entering the E1P→E2P conformational transition, thus limiting ion transport of the gastric H,K-ATPase. These findings highlight the significance of cellular mechanisms contributing to increased proton availability in the cytoplasm of gastric parietal cells. Furthermore, we show that extracellular Na+ profoundly alters the voltage-dependent E1P/E2P distribution indicating that Na+ ions can act as surrogates for protons regarding the E2P→E1P transition. The complexity of the intra- and extracellular cation effects can be rationalized by a kinetic model suggesting that cations reach the binding sites through a rather high-field intra- and a rather low-field extracellular access channel, with fractional electrical distances of ∼0.5 and ∼0.2, respectively.
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Affiliation(s)
| | | | - Thomas Friedrich
- Institute of Chemistry, Technical University of Berlin, Berlin, Germany
- * E-mail:
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11
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Mohanty BK, Gupta BL. A marked animal-vegetal polarity in the localization of Na(+),K(+) -ATPase activity and its down-regulation following progesterone-induced maturation. Mol Reprod Dev 2011; 79:138-60. [PMID: 22213374 DOI: 10.1002/mrd.22012] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2011] [Accepted: 11/22/2011] [Indexed: 01/01/2023]
Abstract
The stage-VI Xenopus oocyte has a very distinct animal-vegetal polarity with structural and functional asymmetry. In this study, we show the expression and distribution pattern of Na(+),K(+) -ATPase in stage-VI oocytes, and its changes following progesterone-induced maturation. Using enzyme-specific electron microscopy phosphatase histochemistry, [(3) H]-ouabain autoradiography, and immunofluorescence cytochemistry at light microscopic level, we find that Na(+),K(+) -ATPase activity is mainly confined to the animal hemisphere. Electron microscopy histochemical results also suggest that polarized distribution of Na(+),K(+) -ATPase activity persists following progesterone-induced maturation, and it becomes gradually more polarized towards the animal pole. The time course following progesterone-induced maturation suggests that there is an initial up-regulation and then gradual down-regulation of Na(+),K(+) -ATPase activity leading to germinal vesicle breakdown (GVBD). By GVBD, the Na(+),K(+) -ATPase activity is completely down-regulated due to endocytotic removal of pump molecules from the plasma membrane into the sub-cortical region of the oocyte. This study provides the first direct evidence for a marked asymmetric localization of Na(+),K(+) -ATPase activity in any vertebrate oocyte. Here, we propose that such asymmetry in Na(+),K(+) -ATPase activity in stage-VI oocytes, and their down-regulation following progesterone-induced maturation, is likely to have a role in the active state of the germinal vesicle in stage-VI oocytes and chromosomal condensation after GVBD.
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12
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Es-Salah-Lamoureux Z, Xiong PY, Goodchild SJ, Ahern CA, Fedida D. Blockade of permeation by potassium but normal gating of the G628S nonconducting hERG channel mutant. Biophys J 2011; 101:662-70. [PMID: 21806934 DOI: 10.1016/j.bpj.2011.06.028] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/09/2011] [Revised: 06/10/2011] [Accepted: 06/17/2011] [Indexed: 12/21/2022] Open
Abstract
G628S is a mutation in the signature sequence that forms the selectivity filter of the human ether-a-go-go-related gene (hERG) channel (GFG) and is associated with long-QT2 syndrome. G628S channels are known to have a dominant-negative effect on hERG currents, and the mutant is therefore thought to be nonfunctional. This study aims to assess the physiological mechanism that prevents the surface-expressing G628S channels from conducting ions. We used voltage-clamp fluorimetry along with two-microelectrode voltage clamping in Xenopus oocytes to confirm that the channels express well at the surface, and to show that they are actually functional, with activation kinetics comparable to that of wild-type, and that the mutation leads to a reduced selectivity to potassium. Although ionic currents are not detected in physiological solutions, removing extracellular K(+) results in the appearance of an inward Na(+)-dependent current. Using whole-cell patch clamp in mammalian transfected cells, we demonstrate that the G628S channels conduct Na(+), but that this can be blocked by both intracellular and higher-than-physiological extracellular K(+). Using solutions devoid of K(+) allows the appearance of nA-sized Na(+) currents with activation and inactivation gating analogous to wild-type channels. The G628S channels are functionally conducting but are normally blocked by intracellular K(+).
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Affiliation(s)
- Zeineb Es-Salah-Lamoureux
- Department of Anesthesiology, Pharmacology, and Therapeutics, University of British Columbia, Vancouver, British Columbia, Canada
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13
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Terhag J, Cavara NA, Hollmann M. Cave Canalem: How endogenous ion channels may interfere with heterologous expression in Xenopus oocytes. Methods 2010; 51:66-74. [DOI: 10.1016/j.ymeth.2010.01.034] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2009] [Revised: 01/26/2010] [Accepted: 01/27/2010] [Indexed: 10/19/2022] Open
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14
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Sobczak K, Bangel-Ruland N, Leier G, Weber WM. Endogenous transport systems in the Xenopus laevis oocyte plasma membrane. Methods 2009; 51:183-9. [PMID: 19963061 DOI: 10.1016/j.ymeth.2009.12.001] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2009] [Revised: 12/02/2009] [Accepted: 12/02/2009] [Indexed: 11/15/2022] Open
Abstract
Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.
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Affiliation(s)
- Katja Sobczak
- Institute of Animal Physiology, Westfalian Wilhelms-University, Hindenburgplatz 55, Muenster, Germany
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15
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Stewart AK, Vandorpe DH, Heneghan JF, Chebib F, Stolpe K, Akhavein A, Edelman EJ, Maksimova Y, Gallagher PG, Alper SL. The GPA-dependent, spherostomatocytosis mutant AE1 E758K induces GPA-independent, endogenous cation transport in amphibian oocytes. Am J Physiol Cell Physiol 2009; 298:C283-97. [PMID: 19907019 DOI: 10.1152/ajpcell.00444.2009] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The previously undescribed heterozygous missense mutation E758K was discovered in the human AE1/SLC4A1/band 3 gene in two unrelated patients with well-compensated hereditary spherostomatocytic anemia (HSt). Oocyte surface expression of AE1 E758K, in contrast to that of wild-type AE1, required coexpressed glycophorin A (GPA). The mutant polypeptide exhibited, in parallel, strong GPA dependence of DIDS-sensitive (36)Cl(-) influx, trans-anion-dependent (36)Cl(-) efflux, and Cl(-)/HCO(3)(-) exchange activities at near wild-type levels. AE1 E758K expression was also associated with GPA-dependent increases of DIDS-sensitive pH-independent SO(4)(2-) uptake and oxalate uptake with altered pH dependence. In marked contrast, the bumetanide- and ouabain-insensitive (86)Rb(+) influx associated with AE1 E758K expression was largely GPA-independent in Xenopus oocytes and completely GPA-independent in Ambystoma oocytes. AE1 E758K-associated currents in Xenopus oocytes also exhibited little or no GPA dependence. (86)Rb(+) influx was higher but inward cation current was lower in oocytes expressing AE1 E758K than previously reported in oocytes expressing the AE1 HSt mutants S731P and H734R. The pharmacological inhibition profile of AE1 E758K-associated (36)Cl(-) influx differed from that of AE1 E758K-associated (86)Rb(+) influx, as well as from that of wild-type AE1-mediated Cl(-) transport. Thus AE1 E758K-expressing oocytes displayed GPA-dependent surface polypeptide expression and anion transport, accompanied by substantially GPA-independent, pharmacologically distinct Rb(+) flux and by small, GPA-independent currents. The data strongly suggest that most of the increased cation transport associated with the novel HSt mutant AE1 E758K reflects activation of endogenous oocyte cation permeability pathways, rather than cation translocation through the mutant polypeptide.
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Affiliation(s)
- Andrew K Stewart
- Division of Nephrology, Beth Israel Deaconess Medical Center, Boston, Massachusetts 02215, USA
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16
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Hedrich R, Marten I. 30-year progress of membrane transport in plants. PLANTA 2006; 224:725-39. [PMID: 16835760 DOI: 10.1007/s00425-006-0341-x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/02/2006] [Accepted: 03/18/2006] [Indexed: 05/10/2023]
Abstract
In the past 30 years enormous progress was made in plant membrane biology and transport physiology, a fact reflected in the appearance of textbooks. The first book dedicated to 'Membrane Transport in Plants' was published on the occasion of the 'International Workshop on Membrane Transport in Plants' held at the Nuclear Research Center, Jülich, Germany [Zimmermann and Dainty (eds) 1974] and was followed in 1976 by a related volume 'Transport in plants II' in the 'Encyclopedia of plant physiology' [Lüttge and Pitman (eds) 1976]. A broad spectrum of topics including thermodynamics of transport processes, water relations, primary reactions of photosynthesis, as well as more conventional aspects of membrane transport was presented. The aim of the editors of the first book was to bring advanced thermodynamical concepts to the attention of biologists and to show physical chemists and biophysicist what the more complex biological systems were like. To bundle known data on membrane transport in plants and relevant fields for mutual understanding, interdisciplinary research and clarification of problems were considered highly important for further progress in this scientific area of plant physiology. The present review will critically evaluate the progress in research in membrane transport in plants that was achieved during the past. How did 'Membrane Transport in Plants' progress within the 30 years between the publication of the first book about this topic (Zimmermann and Dainty 1974), a recent one with the same title (Blatt 2004), and today?
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Affiliation(s)
- Rainer Hedrich
- Molecular Plant Physiology and Biophysics, Julius-von-Sachs Institute for Bioscience, University of Wuerzburg, Julius-von-Sachs-Platz 2, 97082 Wuerzburg, Germany.
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17
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Krause S, Schwarz W. Identification and selective inhibition of the channel mode of the neuronal GABA transporter 1. Mol Pharmacol 2005; 68:1728-35. [PMID: 16150932 DOI: 10.1124/mol.105.013870] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The function of GAT1, the transporter for the inhibitory neurotransmitter GABA, is characterized by expression in Xenopus laevis oocytes and measurements of GABA-induced uptake of [3H]GABA, 22Na+, and 36Cl-, and GABA-evoked currents under voltage-clamp conditions. N-[4,4-Diphenyl-3-butenyl]-nipecotic acid (SKF-89976-A), a specific inhibitor of GAT1, is used in our system as a pharmacological tool. The GABA-evoked current can be decomposed into a transport current, which is coupled to the GABA uptake, and a transmitter-gated current, which is uncoupled from the GABA uptake. The transport current results from a fixed stoichiometry of 1 GABA/2 Na+/1 Cl- transported during each cycle, as determined by radioactive tracer flux measurements. The transmitter-gated current is mediated by an Na+-conductance pathway. As a competitive inhibitor for GABA uptake, SKF-89976-A can separate the two current components. The GABA uptake is blocked with a K(I) value of approximately 7 microM, whereas the uncoupled transmitter-gated current is inhibited with a K(I) value of approximately 0.03 microM. Thus, the results of this study not only identify the transport mode and the channel mode of GAT1 but also raise the possibility of separating these components in a physiological environment.
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Affiliation(s)
- Stephan Krause
- Max-Planck-Institute for Biophysics, Frankfurt am Main, Germany.
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18
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Dempski RE, Friedrich T, Bamberg E. The beta subunit of the Na+/K+-ATPase follows the conformational state of the holoenzyme. ACTA ACUST UNITED AC 2005; 125:505-20. [PMID: 15851504 PMCID: PMC2217500 DOI: 10.1085/jgp.200409186] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The Na+/K+-ATPase is a ubiquitous plasma membrane ion pump that utilizes ATP hydrolysis to regulate the intracellular concentration of Na+ and K+. It is comprised of at least two subunits, a large catalytic alpha subunit that mediates ATP hydrolysis and ion transport, and an ancillary beta subunit that is required for proper trafficking of the holoenzyme. Although processes mediated by the alpha subunit have been extensively studied, little is known about the participation of the beta subunit in conformational changes of the enzyme. To elucidate the role of the beta subunit during ion transport, extracellular amino acids proximal to the transmembrane region of the sheep beta1 subunit were individually replaced for cysteines. This enabled sulfhydryl-specific labeling with the environmentally sensitive fluorescent dye tetramethylrhodamine-6-maleimide (TMRM) upon expression in Xenopus oocytes. Investigation by voltage-clamp fluorometry identified three reporter positions on the beta1 subunit that responded with fluorescence changes to alterations in ionic conditions and/or membrane potential. These experiments for the first time show real-time detection of conformational rearrangements of the Na+/K+-ATPase through a fluorophore-labeled beta subunit. Simultaneous recording of presteady-state or stationary currents together with fluorescence signals enabled correlation of the observed environmental changes of the beta subunit to certain reaction steps of the Na+/K+-ATPase, which involve changes in the occupancy of the two principle conformational states, E1P and E2P. From these experiments, evidence is provided that the beta1-S62C mutant can be directly used to monitor the conformational state of the enzyme, while the F64C mutant reveals a relaxation process that is triggered by sodium transport but evolves on a much slower time scale. Finally, shifts in voltage dependence and kinetics observed for mutant K65C show that this charged lysine residue, which is conserved in beta1 isoforms, directly influences the effective potential that determines voltage dependence of extracellular cation binding and release.
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Affiliation(s)
- Robert E Dempski
- Department of Biophysical Chemistry, Max Planck Institute of Biophysics, Frankfurt am Main, Germany
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19
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Peluffo RD, Hara Y, Berlin JR. Quaternary organic amines inhibit Na,K pump current in a voltage-dependent manner: direct evidence of an extracellular access channel in the Na,K-ATPase. ACTA ACUST UNITED AC 2004; 123:249-63. [PMID: 14981136 PMCID: PMC2217448 DOI: 10.1085/jgp.200308872] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The effects of organic quaternary amines, tetraethylammonium (TEA) chloride and benzyltriethylammonium (BTEA) chloride, on Na,K pump current were examined in rat cardiac myocytes superfused in extracellular Na(+)-free solutions and whole-cell voltage-clamped with patch electrodes containing a high Na(+)-salt solution. Extracellular application of these quaternary amines competitively inhibited extracellular K(+) (K(+)(o)) activation of Na,K pump current; however, the concentration for half maximal inhibition of Na,K pump current at 0 mV (K(0)(Q)) by BTEA, 4.0 +/- 0.3 mM, was much lower than the K(0)(Q) for TEA, 26.6 +/- 0.7 mM. Even so, the fraction of the membrane electric field dissipated during K(+)(o) activation of Na,K pump current (lambda(K)), 39 +/- 1%, was similar to lambda(K) determined in the presence of TEA (37 +/- 2%) and BTEA (35 +/- 2%), an indication that the membrane potential (V(M)) dependence for K(+)(o) activation of the Na,K pump current was unaffected by TEA and BTEA. TEA was found to inhibit the Na,K pump current in a V(M)-independent manner, i.e., inhibition of current dissipated 4 +/- 2% of the membrane electric field. In contrast, BTEA dissipated 40 +/- 5% of the membrane electric field during inhibition of Na,K pump current. Thus, BTEA inhibition of the Na,K-ATPase is V(M)-dependent. The competitive nature of inhibition as well as the similar fractions of the membrane electric field dissipated during K(+)(o)-dependent activation and BTEA-dependent inhibition of Na,K pump current suggest that BTEA inhibits the Na,K-ATPase at or very near the enzyme's K(+)(o) binding site(s) located in the membrane electric field. Given previous findings that organic quaternary amines are not occluded by the Na,K-ATPase, these data clearly demonstrate that an ion channel-like structure provides access to K(+)(o) binding sites in the enzyme.
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Affiliation(s)
- R Daniel Peluffo
- Department of Pharmacology and Physiology, UMDNJ-New Jersey Medical School, 185 S. Orange Avenue. P.O. Box 1709, Newark, NJ 07101-1709, USA
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20
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Nessler S, Friedrich O, Bakouh N, Fink RHA, Sanchez CP, Planelles G, Lanzer M. Evidence for Activation of Endogenous Transporters in Xenopus laevis Oocytes Expressing the Plasmodium falciparum Chloroquine Resistance Transporter, PfCRT. J Biol Chem 2004; 279:39438-46. [PMID: 15258157 DOI: 10.1074/jbc.m404671200] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
A large body of genetic, reverse genetic, and epidemiological data has linked chloroquine-resistant malaria to polymorphisms within a gene termed pfcrt in the human malarial parasite Plasmodium falciparum. To investigate the biological function of the chloroquine resistance transporter, PfCRT, as well as its role in chloroquine resistance, we functionally expressed this protein in Xenopus laevis oocytes. Our data show that PfCRT-expressing oocytes exhibit a depolarized resting membrane potential and a higher intracellular pH compared with control oocytes. Pharmacological and electrophysiological studies link the higher intracellular pH to an enhanced amiloride-sensitive H(+) extrusion and the low membrane potential to an activated nonselective cation conductance. The finding that both properties are independent of each other, together with the fact that they are endogenously present in X. laevis oocytes, supports a model in which PfCRT activates transport systems. Our data suggest that PfCRT plays a role as a direct or indirect activator or modulator of other transporters.
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Affiliation(s)
- Susanne Nessler
- Hygiene Institut, Abteilung Parasitologie, Universitätsklinikum Heidelberg, Im Neuenheimer Feld 324, D-69120 Heidelberg, Germany
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21
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Hosoi K, Min KY, Iwagaki A, Murao H, Hanafusa T, Shimamoto C, Katsu KI, Kato M, Fujiwara S, Nakahari T. Delayed shrinkage triggered by the Na+-K+pump in terbutaline-stimulated rat alveolar type II cells. Exp Physiol 2004; 89:373-85. [PMID: 15123552 DOI: 10.1113/expphysiol.2003.026617] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Terbutaline (10 microm) induced a triphasic volume change in alveolar type II (AT-II) cells: an initial shrinkage (initial phase) followed by cell swelling (second phase) and a gradual shrinkage (third phase). The present study demonstrated that the initial and the third phases are evoked by the activation of K+ and Cl- channels and the second phase is evoked by the activation of Na+ and Cl- channels. Ouabain blocked the third phase, although it did not block the initial and second phases. This suggests that the third phase is triggered by the Na+-K+ pump. Tetraethylammonium (TEA, a K+ channel blocker) decreased the volume of AT-II cells and enhanced the terbutaline-stimulated third phase, although quinidine, another K+ channel blocker, increased the volume of AT-II cells. The TEA-induced cell shrinkage was inhibited by ouabain, suggesting that TEA increases Na+-K+ pump activity. Ba2+, 2,3-diaminopyridine and a high [K+]o (30 mm) similarly decreased the volume of AT-II cells. These findings suggest that depolarization induced by TEA increases Na+-K+ pump activity, which increases [K+]i. This [K+]i increase, in turn, hyperpolarizes membrane potential. Valinomycin (a K+ ionophore), which induces hyperpolarization, decreased the volume of AT-II cells and enhanced the third phase in these cells. In conclusion, in terbutaline-stimulated AT-II cells, an increase in Na+-K+ pump activity hyperpolarizes the membrane potential and triggers the third phase by switching net ion transport from NaCl entry to KCl release.
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Affiliation(s)
- Keita Hosoi
- Department of Physiology, Osaka Medical College, Takatsuki, 569-8686, Japan
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22
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Koenderink JB, Geibel S, Grabsch E, De Pont JJHHM, Bamberg E, Friedrich T. Electrophysiological analysis of the mutated Na,K-ATPase cation binding pocket. J Biol Chem 2003; 278:51213-22. [PMID: 14532287 DOI: 10.1074/jbc.m306384200] [Citation(s) in RCA: 52] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
Na,K-ATPase mediates net electrogenic transport by extruding three Na+ ions and importing two K+ ions across the plasma membrane during each reaction cycle. We mutated putative cation coordinating amino acids in transmembrane hairpin M5-M6 of rat Na,K-ATPase: Asp776 (Gln, Asp, Ala), Glu779 (Asp, Gln, Ala), Asp804 (Glu, Asn, Ala), and Asp808 (Glu, Asn, Ala). Electrogenic cation transport properties of these 12 mutants were analyzed in two-electrode voltage-clamp experiments on Xenopus laevis oocytes by measuring the voltage dependence of K+-stimulated stationary currents and pre-steady-state currents under electrogenic Na+/Na+ exchange conditions. Whereas mutants D804N, D804A, and D808A hardly showed any Na+/K+ pump currents, the other constructs could be classified according to the [K+] and voltage dependence of their stationary currents; mutants N776A and E779Q behaved similarly to the wild-type enzyme. Mutants E779D, E779A, D808E, and D808N had in common a decreased apparent affinity for extracellular K+. Mutants N776Q, N776D, and D804E showed large deviations from the wild-type behavior; the currents generated by mutant N776D showed weaker voltage dependence, and the current-voltage curves of mutants N776Q and D804E exhibited a negative slope. The apparent rate constants determined from transient Na+/Na+ exchange currents are rather voltage-independent and at potentials above -60 mV faster than the wild type. Thus, the characteristic voltage-dependent increase of the rate constants at hyperpolarizing potentials is almost absent in these mutants. Accordingly, dislocating the carboxamide or carboxyl group of Asn776 and Asp804, respectively, decreases the extracellular Na+ affinity.
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Affiliation(s)
- Jan B Koenderink
- Department of Biophysical Chemistry, Max-Planck-Institute of Biophysics, Marie-Curie-Strasse 15, D-60439 Frankfurt am Main, Germany.
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23
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Fueta Y, Vasilets LA, Takeda K, Kawamura M, Schwarz W. Down-regulation of GABA-transporter function by hippocampal translation products: its possible role in epilepsy. Neuroscience 2003; 118:371-8. [PMID: 12699773 DOI: 10.1016/s0306-4522(02)00924-7] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
The genetically epileptic mouse strain (El) is used as a model for human temporal lobe epilepsy. To address the question of whether altered function of the neuronal GABA transporter GAT1 is involved in the pathology of epilepsy of El mice, we expressed in Xenopus oocytes cloned GAT1 of mouse brain by injection of complementary ribonucleic acid (cRNA) and co-injected messenger ribonucleic acid (mRNA) isolated from the hippocampus of non-epileptic control mother strain (ddY) mice and from El mice. GABA transporter activity was investigated by measurements of [(3)H]-GABA uptake as well as by steady-state and transient current measurements under voltage clamp.Co-injection of hippocampal mRNA into oocytes reduced GAT1-mediated transport. This effect was more pronounced for mRNA from ddY mice than for that from El mice that never experienced seizures, El(-), and being absent for mRNA from El mice that have had high seizure experience, El(+). The pronounced inhibition of GABA transport after injection of mRNA from the ddY strain results from reduced expression of functional GAT1, but to about one third also from a reduced GABA translocation rate. The reduced translocation can be attributed to a reduced forward rate of a step associated with extracellular Na(+) binding. If the results can be applied to the mouse brain, we may hypothesise that in ddY mice some GAT down-regulating factor translated from hippocampal mRNA may be involved to keep GAT1 activity low, and hence GABA concentration in synaptic cleft high. In El(-) mice such regulatory mechanism may be reduced or counteracted by another unknown factor present in El(-) brain. The repeated seizure experience in El(+) mice enhances this compensatory effect.
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Affiliation(s)
- Y Fueta
- Max-Planck Institut für Biophysik, Marie-Curie Str 15, 60439 Frankfurt am Main, Germany
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24
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Geibel S, Kaplan JH, Bamberg E, Friedrich T. Conformational dynamics of the Na+/K+-ATPase probed by voltage clamp fluorometry. Proc Natl Acad Sci U S A 2003; 100:964-9. [PMID: 12552111 PMCID: PMC298709 DOI: 10.1073/pnas.0337336100] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
The method of voltage clamp fluorometry combined with site-directed fluorescence labeling was used to detect local protein motions of the fully active Na(+)K(+)-ATPase in real time under physiological conditions. Because helix M5 extends from the cytoplasmic site of ATP hydrolysis into the cation binding region, we chose the extracellular M5-M6 loop of the sheep alpha(1)-subunit for the insertion of cysteine residues to identify reporter positions for conformational rearrangements during the catalytic cycle. After expression of the single cysteine mutants in Xenopus oocytes and covalent attachment of tetramethylrhodamine-6-maleimide, only mutant N790C reported molecular rearrangements of the M5-M6 loop by showing large, ouabain-sensitive fluorescence changes ( approximately 5%) on addition of extracellular K(+). When the enzyme was subjected to voltage jumps under Na(+)Na(+)-exchange conditions, we observed fluorescence changes that directly correlated to transient charge movements originating from the E(1)P-E(2)P transition of the transport cycle. The voltage jump-induced fluorescence changes and transient currents were abolished after replacement of Na(+) by tetraethylammonium or on addition of ouabain, showing that conformational flexibility is impaired under these conditions. Voltage-dependent fluorescence changes could also be observed in the presence of subsaturating K(+) concentrations. This allowed to monitor the time course of voltage-dependent relaxations into a new stationary distribution of states under turnover conditions, showing the acceleration of relaxation kinetics with increasing K(+) concentrations. As a result, the stationary distribution between E(1) and E(2) states and voltage-dependent relaxation times can be determined at any time and membrane potential under Na(+)Na(+) exchange as well as Na(+)K(+) turnover conditions.
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Affiliation(s)
- Sven Geibel
- Max Planck Institute of Biophysics, Kennedyallee 70, D-60596 Frankfurt, Germany
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25
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Abstract
Ion pumps and exchangers are considered to be different from ion channels for two principal reasons. Ion pumps move ions against, whereas ion channels allow ions to move with, the electrochemical potential gradient, and pumps transport ions relatively slowly, approximately 10(2) s(-1), whereas channels conduct ions rapidly, approximately 10(7) s(-1). However, the latter high rate refers only to the open pore, and yet all ion channels contain at least one gate. Not surprisingly, the conformational changes associated with channel gating occur with kinetics similar to those of ion pumping. Indeed, ion pumps may be viewed as ion channels with two gates, one external to, and the other internal to, the ion binding cavity. The simple operational rule for such a pump is that the two gates should never be open simultaneously; otherwise, the pump would become a channel and conduct dissipative fluxes several orders of magnitude larger than, and in the opposite direction to, the active transport fluxes. Analyses of Na(+) ion movements mediated by the Na(+)/K(+) pump under various conditions have suggested that in at least one, short-lived, conformation of the pump, an ion-channel-like structure, closed at its intracellular end, connects the extracellular solution with the ion binding sites deep in the protein core. Here we use the marine toxin, palytoxin, to act on Na(+)/K(+) pumps in outside-out patches excised from cardiac myocytes and so transform the pumps into nonselective cation channels which we study using macroscopic, and single-channel, recording. We find that gating of the palytoxin-induced channels is regulated by the pump's natural ligands. Thus, external K(+) congeners tend to close, and external Na(+) tends to open, an extracellular gate, whereas ATP acts from the cytoplasmic solution to open an intracellular gate. These gating influences echo the normal ion occlusion and deocclusion reactions that first entrap two extracellular K(+) ions within the interior of the pump (between the two gates) and then release them to the cytoplasmic side in a step accelerated by ATP. These results offer the promise of being able to examine ion occlusion and deocclusion steps at the microscopic level in single Na(+)/K(+) pump molecules.
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Affiliation(s)
- Pablo Artigas
- Laboratory of Cardiac/Membrane Physiology, The Rockefeller University, New York, New York 10021, USA
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26
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Bamberg E, Clarke RJ, Fendler K. Electrogenic properties of the Na+,K+-ATPase probed by presteady state and relaxation studies. J Bioenerg Biomembr 2001; 33:401-5. [PMID: 11762915 DOI: 10.1023/a:1010667407003] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Electrical measurements on planar lipid bilayers, patch/voltage clamp experiments, and spectroscopic investigations involving a potential sensitive dye are reviewed. These experiments were performed to analyze the kinetics of charge translocation of the Na+,K+-ATPase. High time resolution was achieved by applying caged ATP, voltage-jump, and stopped-flow techniques, respectively. Kinetic parameters and the electrogenicity of the relevant transitions in the Na+,K+-ATPase reaction cycle are discussed.
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Affiliation(s)
- E Bamberg
- Max-Planck-Institut für Biophysik, Frankfurt am Main, Germany.
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27
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Abstract
Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.
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Affiliation(s)
- H G Glitsch
- Arbeitsgruppe Muskelphysiologie, Fakultät für Biologie, Ruhr-Universität Bochum, Bochum, Germany
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28
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Krishna S, Woodrow C, Webb R, Penny J, Takeyasu K, Kimura M, East JM. Expression and functional characterization of a Plasmodium falciparum Ca2+-ATPase (PfATP4) belonging to a subclass unique to apicomplexan organisms. J Biol Chem 2001; 276:10782-7. [PMID: 11145964 DOI: 10.1074/jbc.m010554200] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We have obtained a full-length P type ATPase sequence (PfATP4) encoded by Plasmodium falciparum and expressed PfATP4 in Xenopus laevis oocytes to study its function. Comparison of the hitherto incomplete open reading frame with other Ca(2+)-ATPase sequences reveals that PfATP4 differs significantly from previously defined categories. The Ca(2+)-dependent ATPase activity of PfATP4 is stimulated by a much broader range of [Ca(2+)](free) (3.2-320 micrometer) than are an avian SERCA1 pump or rabbit SERCA 1a (maximal activity < 10 micrometer). The activity of PfATP4 is resistant to inhibition by ouabain (200 micrometer) or thapsigargin (0.8 micrometer) but is inhibited by vanadate (1 mM) or cyclopiazonic acid (1 microM). We used a quantitative polymerase chain reaction to assay expression of mRNA encoding PfATP4 relative to that for beta-tubulin in synchronized asexual stages and found variable expression throughout the life cycle with a maximal 5-fold increase in meronts compared with ring stages. This analysis suggests that PfATP4 defines a novel subclass of Ca(2+)-ATPases unique to apicomplexan organisms and therefore offers potential as a drug target.
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Affiliation(s)
- S Krishna
- Department of Infectious Diseases, St. George's Hospital Medical School, Cranmer Terrace, London SW17 ORE, United Kingdom.
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29
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De Weer P, Gadsby DC, Rakowski RF. Voltage dependence of the apparent affinity for external Na(+) of the backward-running sodium pump. J Gen Physiol 2001; 117:315-28. [PMID: 11279252 PMCID: PMC2217255 DOI: 10.1085/jgp.117.4.315] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
The steady-state voltage and [Na(+)](o) dependence of the electrogenic sodium pump was investigated in voltage-clamped internally dialyzed giant axons of the squid, Loligo pealei, under conditions that promote the backward-running mode (K(+)-free seawater; ATP- and Na(+)-free internal solution containing ADP and orthophosphate). The ratio of pump-mediated (42)K(+) efflux to reverse pump current, I(pump) (both defined by sensitivity to dihydrodigitoxigenin, H(2)DTG), scaled by Faraday's constant, was -1.5 +/- 0.4 (n = 5; expected ratio for 2 K(+)/3 Na(+) stoichiometry is -2.0). Steady-state reverse pump current-voltage (I(pump)-V) relationships were obtained either from the shifts in holding current after repeated exposures of an axon clamped at various V(m) to H(2)DTG or from the difference between membrane I-V relationships obtained by imposing V(m) staircases in the presence or absence of H(2)DTG. With the second method, we also investigated the influence of [Na(+)](o) (up to 800 mM, for which hypertonic solutions were used) on the steady-state reverse I(pump)-V relationship. The reverse I(pump)-V relationship is sigmoid, I(pump) saturating at large negative V(m), and each doubling of [Na(+)](o) causes a fixed (29 mV) rightward parallel shift along the voltage axis of this Boltzmann partition function (apparent valence z = 0.80). These characteristics mirror those of steady-state (22)Na(+) efflux during electroneutral Na(+)/Na(+) exchange, and follow without additional postulates from the same simple high field access channel model (Gadsby, D.C., R.F. Rakowski, and P. De Weer, 1993. Science. 260:100-103). This model predicts valence z = nlambda, where n (1.33 +/- 0.05) is the Hill coefficient of Na binding, and lambda (0.61 +/- 0.03) is the fraction of the membrane electric field traversed by Na ions reaching their binding site. More elaborate alternative models can accommodate all the steady-state features of the reverse pumping and electroneutral Na(+)/Na(+) exchange modes only with additional assumptions that render them less likely.
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Affiliation(s)
- P De Weer
- Marine Biological Laboratory, Woods Hole, Massachusetts 02543, USA.
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Salonikidis PS, Kirichenko SN, Tatjanenko LV, Schwarz W, Vasilets LA. Extracellular pH modulates kinetics of the Na(+),K(+)-ATPase. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1509:496-504. [PMID: 11118558 DOI: 10.1016/s0005-2736(00)00356-4] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
To investigate effects of pH on the Na(+),K(+)-ATPase, we used the Xenopus oocytes to measure transient charge movements in the absence of extracellular K(+), and steady-state currents mediated by the pump as well as ATPase activity. The activity of purified Na(+), K(+)-ATPase strongly depends on pH, which has been attributed to protonation of intracellular sites. The steady-state current reflects pump activity, the transient charge movement voltage-dependent interaction of external Na(+) ions with the pump molecule and/or conformational changes during Na(+)/Na(+) exchange. The steady-state current exhibits a characteristic voltage dependence with maximum at about 0 mV at low external K(+) (< or =2 mM) and with 50 Na(+). This dependency is not significantly affected by changes in external pH in the range from pH 9 to pH 6. Only below pH 6, the voltage dependence of pump current becomes less steep, and may be attributed to a pH-dependent inhibition of the forward pump cycle by external Na(+). External stimulation of the pump by K(+) in the absence of Na(+) can be described by a voltage-dependent K(m) value with an apparent valency z(K). At higher external pH the z(K) value is reduced. The transient current signal in the absence of external K(+) can be described by the sum of three exponentials with voltage-dependent time constants of about 50 ms, 700 micros and less than 100 micros during pulses to 0 mV. The charge distribution was calculated by integration of the transient current signals. The slowest component and the associated charge distributions do not significantly depend on external pH changes. The intermediate component of the transients is represented by a voltage-dependent rate constant which shows a minimum at about -120 mV and increases with decreasing pH. Nevertheless, the contribution to the charge movement is not altered by pH changes due to a simultaneous increase of the amplitude of this component. We conclude that reduction of external pH counteracts external K(+) and Na(+) binding.
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Li J, Fei J, Huang F, Guo LH, Schwarz W. Functional significance of N- and C-terminus of the amino acid transporters EAAC1 and ASCT1: characterization of chimeric transporters. BIOCHIMICA ET BIOPHYSICA ACTA 2000; 1467:338-46. [PMID: 11030592 DOI: 10.1016/s0005-2736(00)00232-7] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
To localize functionally significant domains in the amino acid transporters of mouse brain mEAAC1 and mASCT1, cRNA encoding for wild-type and chimeric transporters was injected into Xenopus oocytes. Activity of expressed transporters was investigated by measurements of uptake of 3H-labeled glutamate and serine and of glutamate- and serine-induced currents under voltage clamp. Though all transporters accept glutamate and serine as substrate, the central part of the protein (Ala94-Met418 of mEAAC1 and Ala119-Ile393 of mASCT1) determines substrate selectivity. The C-terminus rectifies the interaction with the respective substrate. A channel mode of the glutamate transporter can be activated by glutamate and serine, and the N- and C-termini of the mEAAC1 seem to be essential for the channel formation.
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Affiliation(s)
- J Li
- Max-Planck-Institute for Biophysics, Frankfurt/Main, Germany
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Rettinger J, Schmalzing G, Damer S, Müller G, Nickel P, Lambrecht G. The suramin analogue NF279 is a novel and potent antagonist selective for the P2X(1) receptor. Neuropharmacology 2000; 39:2044-53. [PMID: 10963748 DOI: 10.1016/s0028-3908(00)00022-8] [Citation(s) in RCA: 69] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
The suramin analogue 8,8'-(carbonylbis(imino-4, 1-phenylenecarbonylimino-4,1-phenylenecarbonylimino)) bis(1,3,5-naphthalenetrisul fonic acid) (NF279) was analysed with respect to its potency and P2X receptor subtype selectivity. Two-electrode voltage-clamp measurements were performed with Xenopus laevis oocytes expressing homomultimeric rat P2X(1), P2X(2), P2X(3) and human P2X(4) receptors. For the fast desensitising P2X(1) and P2X(3) receptors, IC(50) values strongly depended on whether oocytes were pre-incubated with NF279 prior to ATP superfusion or exposed to NF279 simultaneously with ATP. With a 10 s pre-incubation period of NF279, IC(50) values of 19 nM and 1.62 microM were obtained for rat P2X(1) and P2X(3), respectively. Without pre-incubation, IC(50) values amounted to 2 microM and 85.5 microM for P2X(1) and P2X(3), respectively. For the non-desensitising rat P2X(2) receptor NF279 appeared to act as a competitive antagonist with an IC(50) value of 0.76 microM and a K(B) value of 0.36 microM, as derived from Schild analysis. P2X(4) receptors were the least sensitive subtypes for NF279 (IC(50)>300 microM). The antagonism was fully reversible at all P2X subtypes analysed. Our results indicate that NF279 is a potent P2X(1) receptor-selective and reversible antagonist.
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Affiliation(s)
- J Rettinger
- Department of Pharmacology, Biocentre Niederursel, University of Frankfurt, Marie-Curie-Strasse 9, D-60439 Frankfurt/M, Germany.
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Weber W. Ion currents of Xenopus laevis oocytes: state of the art. BIOCHIMICA ET BIOPHYSICA ACTA 1999; 1421:213-33. [PMID: 10518693 DOI: 10.1016/s0005-2736(99)00135-2] [Citation(s) in RCA: 100] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- W Weber
- Laboratory of Physiology, K.U. Leuven, Campus Gasthuisberg, B-3000, Leuven, Belgium.
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Eckstein-Ludwig U, Fei J, Schwarz W. Inhibition of uptake, steady-state currents, and transient charge movements generated by the neuronal GABA transporter by various anticonvulsant drugs. Br J Pharmacol 1999; 128:92-102. [PMID: 10498839 PMCID: PMC1571623 DOI: 10.1038/sj.bjp.0702794] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
1. We have expressed the GABA transporter (GAT1) of mouse brain in Xenopus oocytes and have investigated the effects of four antiepileptic drugs, tiagabine (TGB), vigabatrin (VGB), gabapentin (GBP) and valproate (VAL), on GAT1 transporter function by measurements of 3H-labelled GABA uptake and GAT1-mediated currents. 2. Not only TGB, a well-known inhibitor of GAT1-mediated transport, but also the other drugs efficiently inhibit the uptake of [3H]-GABA by GAT1. Inhibition at 50% is obtained for VGB, TGB, GBP, and VAL at concentrations of about 1 nM, 1 microM, 50 microM and 100 microM, respectively. 3. However, GAT1-mediated steady-state and transient currents are nearly unaffected by VGB, GBP, and VAL at even five times higher concentrations. Only TGB blocks the uptake and steady-state and transient currents at micromolar concentrations. 4. VGB exhibits a complex interaction with GAT1; at concentrations about 1 nM, the inhibition of uptake is released, but at millimolar concentrations the uptake is inhibited again, and also the GAT1-mediated current is finally inhibited at these concentrations with a KI value of 0.5 mM. The concentration dependency of inhibition of uptake can be explained by two interaction sites with different affinities, a blocking site and a transport site. 5. The differences in effects of VAL, GBP, and VGB on uptake and currents can be attributed to the fact that GAT1 has the capability to operate in an electrogenic mode without uptake of GABA. We suggest that inhibition occurs only when GAT1 operates in the GABA-uptake mode. 6. The inhibition of GABA uptake by these four drugs will result in an elevation of the GABA concentration in the synaptic cleft, which will enhance synaptic inhibition and thereby contribute to their antiepileptic effects.
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Affiliation(s)
| | - Jian Fei
- Max-Planck Institut für Biophysik, Kennedyallee 70, D-60596 Frankfurt/Main, Germany
| | - Wolfgang Schwarz
- Max-Planck Institut für Biophysik, Kennedyallee 70, D-60596 Frankfurt/Main, Germany
- Author for correspondence:
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Barmashenko G, Kockskämper J, Glitsch HG. Depolarization increases the apparent affinity of the Na+-K+ pump to cytoplasmic Na+ in isolated guinea-pig ventricular myocytes. J Physiol 1999; 517 ( Pt 3):691-8. [PMID: 10358110 PMCID: PMC2269372 DOI: 10.1111/j.1469-7793.1999.0691s.x] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
Abstract
1. In order to investigate the possible effect of membrane potential on cytoplasmic Na+ binding to the Na+-K+ pump, we studied Na+-K+ pump current-voltage relationships in single guinea-pig ventricular myocytes whole-cell voltage clamped with pipette solutions containing various concentrations of Na+ ([Na+]pip) and either tetraethylammonium (TEA+) or N-methyl-D-glucamine (NMDG+) as the main cation. The experiments were conducted at 30 C under conditions designed to abolish the known voltage dependence of other steps in the pump cycle, i.e. in Na+-free external media containing 20 mM Cs+. 2. Na+-K+ pump current (Ip) was absent in cells dialysed with Na+-free pipette solutions and was almost voltage independent at 50 mM Na+pip (potential range: -100 to +40 mV). By contrast, the activation of Ip by 0.5-5 mM Na+pip was clearly voltage sensitive and increased with depolarization, independently of the main intracellular cation species. 3. The apparent affinity of the Na+-K+ pump for cytoplasmic Na+ increased monotonically with depolarization. The [Na+]pip required for half-maximal Ip activation (K0.5 value) amounted to 5.6 mM at -100 mV and to 2.2 mM at +40 mV. 4. The results suggest that cytoplasmic Na+ binding and/or a subsequent partial reaction in the pump cycle prior to Na+ release is voltage dependent. From the voltage dependence of the K0.5 values the dielectric coefficient for intracellular Na+ binding/translocation was calculated to be approximately 0.08. The voltage-dependent mechanism might add to the activation of the cardiac Na+-K+ pump during cardiac excitation.
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Affiliation(s)
- G Barmashenko
- Arbeitsgruppe Muskelphysiologie, Ruhr-Universitat, D-44780 Bochum, Germany
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Liu Y, Eckstein-Ludwig U, Fei J, Schwarz W. Effect of mutation of glycosylation sites on the Na+ dependence of steady-state and transient currents generated by the neuronal GABA transporter. BIOCHIMICA ET BIOPHYSICA ACTA 1998; 1415:246-54. [PMID: 9858742 DOI: 10.1016/s0005-2736(98)00200-4] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
Abstract
The GABA (gamma-aminobutyric acid) transporter (GAT1) belongs to a superfamily of secondary active uptake systems for neurotransmitters that depend on the electrochemical gradients for Na+ and Cl-. In the GAT1, two Na+ ions and one Cl- ion are co-transported with one GABA molecule. Steady-state transport activity and transient charge movements during partial reactions of the transport cycle of the GAT1 of mouse brain expressed in Xenopus oocytes were investigated by two-electrode voltage clamp. Functional expression was demonstrated by Na+-dependent [3H]GABA uptake. Effects of mutation of two out of three N-glycosylation sites located in the extracellular loop between transmembrane domains 3 and 4 (Asn176, Asn181, Asn184) were analysed. Simultaneous substitution of two Asn by Asp leaves the transport system intact but leads to a reduction in turnover and complex changes in the interaction of external Na+ with the transport protein. If Asn176 is mutated to Asp and simultaneously Asn181 to Gly, no transport and no charge movements can be detected. In conclusion, mutations of the glycosylation sites result in altered transport, and the local conformation at Asn181 is critical for expression of transport activity.
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Affiliation(s)
- Y Liu
- Max-Planck Institut für Biophysik, Kennedyallee 70, D-60596 Frankfurt/Main, Germany
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37
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Eckstein-Ludwig U, Rettinger J, Vasilets LA, Schwarz W. Voltage-dependent inhibition of the Na+,K+ pump by tetraethylammonium. BIOCHIMICA ET BIOPHYSICA ACTA 1998; 1372:289-300. [PMID: 9675315 DOI: 10.1016/s0005-2736(98)00066-2] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
Tetraethylammonium (TEA+) is an effective inhibitor of a variety of K+ channels, and has been widely used to reduce K+-sensitive background conductances in electrophysiological investigations of the Na+,K+-ATPase. Here we demonstrate by combination of two-electrode voltage clamp (TEVC) and giant patch clamp of Xenopus oocytes, and measurements of the activity of purified ATPase of pig kidney that TEA+ directly inhibits the Na+,K+-ATPase from the outside. The KI value in TEVC experiments at 0 mV is about 10 mM increasing with more negative potentials. A similar voltage-dependent inhibition by TEA+ was observed in the excised membrane patches except that the apparent KI value at 0 mV is about 100 mM, a value nearly identical to that found for inhibition of purified kidney ATPase. The voltage-dependent inhibition can be described by an effective valency of 0.39 and is attributed to an interference with the voltage-dependent binding of K+ at an external access channel. The apparent dielectric length of the access channel for K+ is not affected by TEA+.
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Affiliation(s)
- U Eckstein-Ludwig
- Max-Planck Institut für Biophysik, Kennedyallee 70, D-60596 Frankfurt/Main, Germany
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Cougnon M, Bouyer P, Planelles G, Jaisser F. Does the colonic H,K-ATPase also act as an Na,K-ATPase? Proc Natl Acad Sci U S A 1998; 95:6516-20. [PMID: 9600998 PMCID: PMC27839 DOI: 10.1073/pnas.95.11.6516] [Citation(s) in RCA: 69] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
We previously have demonstrated that the colonic P-ATPase alpha subunit cDNA encodes an H,K-ATPase when expressed in Xenopus laevis oocytes. Besides its high level of amino acid homology (75%) with the Na,K-ATPase, the colonic H,K-ATPase also shares a common pharmacological profile with Na,K-ATPase, because both are ouabain-sensitive and Sch 28080-insensitive. These features raise the possibility that an unrecognized property of the colonic H, K-ATPase would be Na+ translocation. To test this hypothesis, ion-selective microelectrodes were used to measure the intracellular Na+ activity of X. laevis oocytes expressing various combinations of P-ATPase subunits. The results show that expression in oocytes of the colonic H,K-ATPase affects intracellular Na+ homeostasis in a way similar to the expression of the Bufo marinus Na,K-ATPase; intracellular Na+ activity is lower in oocytes expressing the colonic H,K-ATPase or the B. marinus Na,K-ATPase than in oocytes expressing the gastric H,K-ATPase or a beta subunit alone. In oocytes expressing the colonic H,K-ATPase, the decrease in intracellular Na+ activity persists when diffusive Na+ influx is enhanced by functional expression of the amiloride-sensitive epithelial Na+ channel, suggesting that the decrease is related to increased active Na+ efflux. The Na+ decrease depends on the presence of K+ in the external medium and is inhibited by 2 mM ouabain, a concentration that inhibits the colonic H,K-ATPase. These data are consistent with the hypothesis that the colonic H,K-ATPase may transport Na+, acting as an (Na,H),K-ATPase. Despite its molecular and functional characterization, the physiological role of the colonic (Na,H),K-ATPase in colonic and renal ion homeostasis remains to be elucidated.
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Affiliation(s)
- M Cougnon
- Institut National de la Santé et de la Recherche Médicale, U. 467, Faculté de Médecine Necker, Université Paris V, F-75015 Paris, France
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Yoshimura SH, Vasilets LA, Ishii T, Takeyasu K, Schwarz W. The Na+,K+-ATPase carrying the carboxy-terminal Ca2+/calmodulin binding domain of the Ca2+ pump has 2Na+,2K+ stoichiometry and lost charge movement in Na+/Na+ exchange. FEBS Lett 1998; 425:71-4. [PMID: 9541009 DOI: 10.1016/s0014-5793(98)00202-6] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
An altered ion-transport stoichiometry from 3Na+,2K+ to 2Na+,2K+ is observed in a chimeric Na+,K+ATPase, which carries the Ca2+/calmodulin binding domain (CBD) of the plasma membrane Ca2+-ATPase at its carboxy-terminus [Zhao et al., FEBS Lett. 408 (1997) 271-2751. The ouabain-resistant mutant of this chimera (ORalpha1-CBD) was constructed to further investigate the effect of the CBD on ion-transport properties. The ORalpha1-CBD still shows the 2Na+,2K+ stoichiometry. The loss of electrogenicity is accompanied by the disappearance of transient charge movements in the Na+/Na+ exchange mode. We conclude that the binding of the third Na+ ion, but not of the two others, in 3Na+,2K+ transport mode apparently senses the electric field, and that the voltage-dependent Na+ binding is likely to be lost in the chimera with CBD.
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Affiliation(s)
- S H Yoshimura
- Max-Planck-Institut für Biophysik, Frankfurt, Germany
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40
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Gropp T, Cornelius F, Fendler K. K+-dependence of electrogenic transport by the NaK-ATPase. BIOCHIMICA ET BIOPHYSICA ACTA 1998; 1368:184-200. [PMID: 9459597 DOI: 10.1016/s0005-2736(97)00162-4] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Charge translocation by the NaK-ATPase from shark rectal gland was measured by adsorption of proteoliposomes to a planar lipid membrane. The proteoliposomes were prepared by reconstitution of purified NaK-ATPase into liposomes consisting of E. coli lipids. The protein was activated by applying an ATP concentration jump produced by photolysis of a protected derivative of ATP, caged ATP. K+ titrations were used to study the effect of K+ on the charge translocation kinetics of the protein. The time-dependent currents obtained after activation of the enzyme with caged ATP were analyzed with a simplified Albers-Post model (E1 (k1)-->E1ATP (k2)-->E2P (k3)-->E1) taking into account the capacitive coupling of the protein to the measuring system. The results of the K+ titrations show a strong dependence of the rate constant k3 on the K+ concentration at the extracellular side of the protein, indicating the K+ activated dephosphorylation reaction. In contrast, k1 and k2 remained constant. The K+ dependence of the rate k3 could be well described with a K+ binding model with two equivalent binding sites (E2P + 2K+ <==> E2P(K) + K+ <==> E2 P(2K)) followed by a rate limiting reaction (E2P(2K) --> E1(2K)). The half saturating K+ concentration K3,0.5 and the microscopic dissociation constant K3 for the K+ dependence of k3 were 4.5mM and 1.9mM respectively. At saturating K+ concentration the rate constant k3 was approximately 100 s(-1). The relative amount of net charge transported during the Na+ and the K+ dependent reactions could be determined from the experiments. Our results suggest electroneutral K+ translocation and do not support electrogenic K+ binding in an extracellular access channel. This is compatible with a model where 2 negative charges are cotransported with 3Na+ and 2K+ ions. Error analysis gives an upper limit of 20% charge transported during K+ translocation or during electrogenic K+ binding in a presumptive access channel compared to Na+ translocation.
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Affiliation(s)
- T Gropp
- Max-Plank-Institut für Biophysik, Frankfurt, Germany
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41
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Jennings ML, Milanick MA. Membrane Transport in Single Cells. Compr Physiol 1997. [DOI: 10.1002/cphy.cp140107] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
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Hansen PS, Gray DF, Buhagiar KA, Rasmussen HH. Voltage-dependent inhibition of the Na(+)-K+ pump by intracellular potassium in rabbit ventricular myocytes. Ann N Y Acad Sci 1997; 834:347-9. [PMID: 9405822 DOI: 10.1111/j.1749-6632.1997.tb52267.x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- P S Hansen
- Department of Cardiology, Royal North Shore Hospital, Sydney, Australia
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43
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Friedrich T, Nagel G. Comparison of Na+/K(+)-ATPase pump currents activated by ATP concentration or voltage jumps. Biophys J 1997; 73:186-94. [PMID: 9199783 PMCID: PMC1180920 DOI: 10.1016/s0006-3495(97)78059-7] [Citation(s) in RCA: 24] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Using the giant patch technique, we combined two fast relaxation methods on excised patches from guinea pig cardiomyocytes to compare the rate constants of the involved reaction steps. Experiments were done in the absence of intra- or extracellular K+. Fast ATP concentration jumps were generated by photolysis of caged ATP at pH 6.3 with laser flash irradiation at a wavelength of 308 nm and 10 ns duration, as described previously. Transient outward currents with a fast rising phase, followed by a slower decay and a small stationary current, were obtained. Voltage pulses were applied to the same patch in the presence or absence of intracellular ATP. Subtraction of the voltage jump-induced currents in the absence of ATP from those taken in the presence of ATP yielded monoexponential transient current signals, which were dependent on external Na+ but did not differ between intracellular pH (pHi) values 6.3 or 7.4. Rate constants showed a characteristic voltage dependence, i.e., saturating at positive potentials (approximately 200 s-1, 24 degrees C) and exponentially rising with increasing negative potentials. Rate constants of the fast component from transient currents obtained after an ATP concentration jump agree well with rate constants from currents obtained after a voltage jump to zero or positive potentials (pHi 6.3), and the two exhibit the same activation energy of approximately 80 kJ.mol-1. For a given membrane patch, the amount of charge that is moved across the plasma membrane is roughly the same for each of the two relaxation techniques.
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Affiliation(s)
- T Friedrich
- Johann Wolfgang Goethe-Universität Frankfurt am Main, Germany
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Gao J, Mathias RT, Cohen IS, Shi J, Baldo GJ. The effects of beta-stimulation on the Na(+)-K+ pump current-voltage relationship in guinea-pig ventricular myocytes. J Physiol 1996; 494 ( Pt 3):697-708. [PMID: 8865067 PMCID: PMC1160670 DOI: 10.1113/jphysiol.1996.sp021525] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
1. The whole cell patch clamp technique was used to study effects of the beta agonist isoprenaline (Iso) on the current-voltage (I-V) relationship of the Na(+)-K+ pump current (Ip) in acutely isolated guinea-pig ventricular myocytes. 2. The effect of Iso on Ip at high [Ca2+]i (1.4 microM) was voltage dependent. The I-V relationship of Ip in Iso shifted by approximately 30 mV in the negative direction on the voltage axis, increasing Ip at negative voltages but leaving Ip unchanged at positive voltages. 3. Intracellular application of the calmodulin antagonist, calmodulin-dependent protein kinase II fragment 290-309, did not eliminate or reduce the Iso-induced voltage shift, suggesting calmodulin-dependent protein kinase II was not involved. 4. The Iso inhibition of Ip at low [Ca2+]i (15 nM) was not voltage dependent. Ip was reduced by 20 to 30% in the presence of Iso at each holding potential. 5. When the voltage dependence of Ip was largely reduced by substitution of N-methyl-D-glucamine+ for external Na+, the magnitude of the low [Ca2+]i, Iso-induced inhibition of Ip was progressively eliminated by increasing the [Ca2+]i. At a [Ca2+]i of 1.4 microM, this inhibition disappeared. 6. At intermediate values of [Ca2+]i, the I-V curves in Na(+)-containing solution in the presence and the absence of Iso crossed over. The higher the [Ca2+]i, the more positive the voltage at which the two I-V curves intersected. 7. During beta-adrenergic activation our results suggest intracellular Ca2+ has two effects: (a) It prevents protein kinase A (PKA) phosphorylation-induced inhibition of Ip. (b) It causes a PKA phosphorylation-induced shift of the pump I-V relationship in the negative direction on the voltage axis. These effects may have important physiological significance in the regulation of heart rate and cardiac contractility.
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Affiliation(s)
- J Gao
- Department of Physiology and Biophysics, SUNY, Stony Brook, 11794-8661, USA
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Abstract
Glial cell Na,K-ATPase is suggested to participate in extracellular K+ concentration ([K+]o) control by being activated when [K+]o rises in the brain. The extent of that activation directly depends on the Na/K pump affinity to [K+]o, intracellular Na+ ([Na+]i) and, indirectly on pump cycle regulation by membrane potential (Vm). In the present investigation, these Na/K pump properties were studied with the whole-cell patch-clamp technique in cultured mouse oligodendrocytes (N20.1 cell line). N20.1 cells possess ouabain-sensitive Na/K pump current (Ip) with a maximal density of 0.5-0.6 pA/pF (estimated for conditions of Na/K pump stimulation by saturating [Na+]i, [ATP]i, [K+]o and at positive Vm). This current was half-inhibited at 83 +/- 31 microM ouabain, and half-activated by [Na+]i of 9.6 +/- 1.1 mM, by [K+]o of 2.0 +/- 0.1 mM and by membrane potential at about -65 mV. High levels of nervous activity may increase [K+]o from 3 to 12 mM which would only increase Na/K pump current by 40% due to the direct effect of [K+]o. However, elevated [K+]o would also depolarize the glial cell membrane which would indirectly activate Ip and together with the direct effect of [K+]o would increase Ip as much as 2-2.5-fold. These data suggest that glial cell Na/K pump regulation by Vm may be an important factor in determining the participation of the Na/K pump in [K+]o homeostasis in the nervous system.
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Affiliation(s)
- M Dobretsov
- Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, Little Rock 72205, USA
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46
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Chen XZ, Coady MJ, Jackson F, Berteloot A, Lapointe JY. Thermodynamic determination of the Na+: glucose coupling ratio for the human SGLT1 cotransporter. Biophys J 1995; 69:2405-14. [PMID: 8599647 PMCID: PMC1236478 DOI: 10.1016/s0006-3495(95)80110-4] [Citation(s) in RCA: 72] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
Phlorizin-sensitive currents mediated by a Na-glucose cotransporter were measured using intact or internally perfused Xenopus laevis oocytes expressing human SGLT1 cDNA. Using a two-microelectrode voltage clamp technique, measured reversal potentials (Vr) at high external alpha-methylglucose (alpha MG) concentrations were linearly related to In[alpha MG]o, and the observed slope of 26.1 +/- 0.8 mV/decade indicated a coupling ratio of 2.25 +/- 0.07 Na ions per alpha MG molecule. As [alpha MG]o decreased below 0.1 mM, Vr was no longer a linear function of In[alpha MG]o, in accordance with the suggested capacity of SGLT1 to carry Na in the absence of sugar (the "Na leak"). A generalized kinetic model for SGLT1 transport introduces a new parameter, Kc, which corresponds to the [alpha MG]o at which the Na leak is equal in magnitude to the coupled Na-alpha MG flux. Using this kinetic model, the curve of Vr as a function of In[alpha MG]o could be fitted over the entire range of [alpha MG]o if Kc is adjusted to 40 +/- 12 microM. Experiments using internally perfused oocytes revealed a number of previously unknown facets of SGLT1 transport. In the bilateral absence of alpha MG, the phlorizin-sensitive Na leak demonstrated a strong inward rectification. The affinity of alpha MG for its internal site was low; the Km was estimated to be between 25 and 50 mM, an order of magnitude higher than that found for the extracellular site. Furthermore, Vr determinations at varying alpha MG concentrations indicate a transport stoichiometry of 2 Na ions per alpha MG molecule: the slope of Vr versus In[alpha MG]o averaged 30.0 +/- 0.7 mV/decade (corresponding to a stoichiometry of 1.96 +/- 0.04 Na ions per alpha MG molecule) whenever [alpha MG]o was higher than 0.1 mM. These direct observations firmly establish that Na ions can utilize the SGLT1 protein to cross the membrane either alone or in a coupled manner with a stoichiometry of 2 Na ions per sugar, molecule.
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Affiliation(s)
- X Z Chen
- Département de Physique, Université de Montréal, Québec, Canada
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47
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Weber WM, Liebold KM, Reifarth FW, Clauss W. The Ca(2+)-induced leak current in Xenopus oocytes is indeed mediated through a Cl- channel. J Membr Biol 1995; 148:263-75. [PMID: 8747558 DOI: 10.1007/bf00235044] [Citation(s) in RCA: 26] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Defolliculated oocytes of Xenopus laevis responded to removal of external divalent cations with large depolarizations and, when voltage clamped, with huge currents. Single channel analysis revealed a Cl- channel with a slope conductance of about 90 pS at positive membrane potentials with at least four substates. Single channel amplitudes and mean channel currents had a reversal potential of approximately -15 mV as predicted by the Nernst equation for a channel perfectly selective for Cl-. Readdition of Ca2+ immediately inactivated the channel and restored the former membrane potential or clamp current. The inward currents were mediated by a Ca2+ inactivated Cl- channel (CaIC). The inhibitory potency of Ca2+ was a function of the external Ca2+ concentration with a half maximal blocker concentration of about 20 microM. These channels were inhibited by the Cl- channel blockers flufenamic acid, niflumic acid and diphenylamine-2-carboxylate (DPC). In contrast, 4,4'-acetamido-4'-isothiocyanatostilbene-2, 2'-disulfonicacid (SITS), another Cl- channel blocker, led to activation of this Cl- channel. Like other Cl- channels, the CaIC was activated by cytosolic cAMP. Extracellular ATP inhibited the channel while ADP was without any effect. Injection of phorbol 12-myristate 13-acetate (PMA), a protein kinase C activating phorbol ester, stimulated the Cl- current. Cytochalasin D, an actin filament disrupting compound, reversibly decreased the clamp current demonstrating an influence of the cytoskeleton. The results indicate that removal of divalent cations activates Cl- channels in Xenopus oocytes which share several features with Cl- channels of the CLC family. The former so-called leak current of oocytes under divalent cation-free conditions is nothing else than an activation of Cl- channels.
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Affiliation(s)
- W M Weber
- Institut für Tierphysiologie, Jusois-Liebig-Universităt, Giessen, FRG
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48
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Porschke D, Grell E. Electric parameters of Na+/K(+)-ATPase by measurements of the fluorescence-detected electric dichroism. BIOCHIMICA ET BIOPHYSICA ACTA 1995; 1231:181-8. [PMID: 7662697 DOI: 10.1016/0005-2728(95)00082-t] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The electric parameters of Na+/K(+)-ATPase labeled by FITC have been characterized by measurements of the fluorescence-detected electric dichroism. The fluorescence emission was measured with polarizers at the magic angle and the light for excitation was usually polarized parallel to the field vector. The FITC-Na+/K(+)-ATPase preparations exhibit a negative electric dichroism at field strengths up to about 600 V/cm and a positive dichroism at higher field strengths. Pulse reversal experiments reveal a dominant permanent electric moment at low electric field strengths and an increasing contribution from an induced electric moment at higher field strengths. The dichroism rise curves and the transients upon pulse reversal show two relaxation processes with opposite amplitudes, whereas the dichroism decay curves in most cases can be represented by single exponentials at a reasonable accuracy. The amplitude A2 associated with the slower of the rise processes is dominant at low field strengths and also approaches saturation already at low field strengths. The dependence of A2 on the electric field strength is consistent with the orientation function for permanent dipoles and cannot be represented by the orientation function for induced dipoles. The fitted permanent dipole moment is in the range of 3.5 x 10(-24) Cm [1 x 10(6) D] and shows only a relatively small decrease with increasing ionic strength. The stationary values of the electric dichroism up to field strengths E < or = 800 V/cm can be represented with high accuracy by an orientation function for disk-shaped particles with a permanent moment along the particle symmetry axis and an induced moment along the semi-major axis. The permanent electric moment determined according to this function is consistent with the one obtained from the amplitudes A2. In summary, our measurements indicate that Na+/K(+)-ATPase is associated with a large permanent electric moment directed perpendicular to the membrane plane. The dipole moment per ATPase monomer unit is estimated to be 1.4 x 10(-27) Cm [430 +/- 50 D].
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Affiliation(s)
- D Porschke
- Max Planck Institut für biophysikalische Chemie, Göttingen, Germany
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49
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Glitsch HG, Tappe A. Change of Na+ pump current reversal potential in sheep cardiac Purkinje cells with varying free energy of ATP hydrolysis. J Physiol 1995; 484 ( Pt 3):605-16. [PMID: 7623279 PMCID: PMC1157947 DOI: 10.1113/jphysiol.1995.sp020690] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
1. The Na(+)-K+ pump current, Ip, of cardioballs from isolated sheep cardiac Purkinje cells was measured at 30-34 degrees C by means of whole-cell recording. 2. Under physiological conditions Ip is an outward current. Experimental conditions which cause a less negative free energy of intracellular ATP hydrolysis (delta GATP) and steeper sarcolemmal gradients for the pumped Na+ and Cs+ ions evoked an Ip in the inward direction over a wide range of membrane potentials. The reversal of the Ip direction was reversible. 3. The inwardly directed Ip increased with increasingly negative membrane potentials and amounted to -0.13 +/- 0.03 microA cm-2 (mean +/- S.E.M.; n = 6) at -95 mV. 4. The reversal potential (Erev) of Ip was studied as a function of delta GATP at constant sarcolemmal gradients of the pumped cations. 5. In order to vary delta GATP the cell interior was dialysed with patch pipette solutions containing 10 mM ATP and different concentrations of ADP and inorganic phosphate. The media were composed to produce delta GATP levels of about -58, -49 and -39 kJ mol-1. 6. A less negative delta GATP shifted Erev to more positive membrane potentials. From measurements of Ip as a function of membrane potential Erev was estimated to be -195, -115 and -60 mV at delta GATP levels of approximately -58, -49 and -39 kJ mol-1, respectively. The calculated Erev amounted to -224 mV at delta GATP approximately -58 kJ mol-1, -126 mV at delta GATP approximately 49 kJ mol-1 and -24 mV at delta GATP approximately -39 kJ mol-1. 7. Possible reasons for the discrepancy between estimated and calculated Erev values are discussed. 8. Shifting delta GATP to less negative values not only altered Erev but also diminished Ip at each membrane potential tested. The maximal Ip (Ip,max), which can be activated by external Cs+ (Cs+o), decreased under these conditions, whereas [Cs+]o causing half-maximal Ip activation remained unchanged. Similarly, the voltage dependence of Ip activation by Cs+o was unaffected. 9. It is concluded that Erev of Ip varies with delta GATP at constant sarcolemmal gradients of the pumped cations. This agrees with thermodynamic considerations.
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Affiliation(s)
- H G Glitsch
- Department of Cell Physiology, Ruhr-University, Bochum, Germany
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50
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Sagar A, Rakowski RF. Access channel model for the voltage dependence of the forward-running Na+/K+ pump. J Gen Physiol 1994; 103:869-93. [PMID: 8035166 PMCID: PMC2219222 DOI: 10.1085/jgp.103.5.869] [Citation(s) in RCA: 78] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
The voltage dependence of steady state current produced by the forward mode of operation of the endogenous electrogenic Na+/K+ pump in Na(+)-loaded Xenopus oocytes has been examined using a two-microelectrode voltage clamp technique. Four experimental cases (in a total of 18 different experimental conditions) were explored: variation of external [Na+] ([Na]o) at saturating (10 mM) external [K+] ([K]o), and activation of pump current by various [K]o at 0, 15, and 120 mM [Na]o (tetramethylammonium replacement). Ionic current through K+ channels was blocked by Ba2+ (5 mM) and tetraethylammonium (20 mM), thereby allowing pump-mediated current to be measured by addition or removal of external K+. Control measurements and corrections were made for pump current run-down and holding current drift. Additional controls were done to estimate the magnitude of the inwardly directed pump-mediated current that was present in K(+)-free solution and the residual K(+)-channel current. A pseudo two-state access channel model is described in the Appendix in which only the pseudo first-order rate coefficients for binding of external Na+ and K+ are assumed to be voltage dependent and all transitions between states in the Na+/K+ pump cycle are assumed to be voltage independent. Any three-state or higher order model with only two oppositely directed voltage-dependent rate coefficients can be reduced to an equivalent pseudo two-state model. The steady state current-voltage (I-V) equations derived from the model for each case were simultaneously fit to the I-V data for all four experimental cases and yielded least-squares estimates of the model parameters. The apparent fractional depth of the external access channel for Na+ is 0.486 +/- 0.010; for K+ it is 0.256 +/- 0.009. The Hill coefficient for Na+ is 2.18 +/- 0.06, and the Hill coefficient for K+ (which is dependent on [Na]o) ranges from 0.581 +/- 0.019 to 1.35 +/- 0.034 for 0 and 120 mM [Na]o, respectively. The model provides a reasonable fit to the data and supports the hypothesis that under conditions of saturating internal [Na+], the principal voltage dependence of the Na+/K+ pump cycle is a consequence of the existence of an external high-field access channel in the pump molecule through which Na+ and K+ ions must pass in order to reach their binding sites.
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Affiliation(s)
- A Sagar
- Department of Physiology and Biophysics, University of Health Sciences/Chicago Medical School, Illinois 60064
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