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  1. Article ; Online: Cotransport of water by Na⁺-K⁺-2Cl⁻ cotransporters expressed in Xenopus oocytes: NKCC1 versus NKCC2.

    Zeuthen, Thomas / Macaulay, Nanna

    The Journal of physiology

    2012  Volume 590, Issue 5, Page(s) 1139–1154

    Abstract: The NKCC1 and NKCC2 isoforms of the mammalian Na⁺–K⁺–2Cl⁻ cotransporter were expressed in Xenopus oocytes and the relation between external ion concentration and water fluxes determined.Water fluxes were determined from changes in the oocytes volume and ... ...

    Abstract The NKCC1 and NKCC2 isoforms of the mammalian Na⁺–K⁺–2Cl⁻ cotransporter were expressed in Xenopus oocytes and the relation between external ion concentration and water fluxes determined.Water fluxes were determined from changes in the oocytes volume and ion fluxes from 86Rb+ uptake. Isotonic increases in external K⁺ concentration elicited abrupt inward water fluxes in NKCC1; the K⁺ dependence obeyed one-site kinetics with a K₀.₅ of 7.5 mM. The water fluxes were blocked by bumetanide, had steep temperature dependence and could proceed uphill against an osmotic gradient of 20 mosmol l⁻¹. A comparison between ion and water fluxes indicates that 460 water molecules are cotransported for each turnover of the protein. In contrast, NKCC2 did not support water fluxes.Water transport in NKCC1 induced by increases in the external osmolarity had high activation energy and was blocked by bumetanide. The osmotic effects of NaCl were smaller than those of urea and mannitol. This supports the notion of interaction between ions and water in NKCC1 and allows for an estimate of around 600 water molecules transported per turnover of the protein. Osmotic gradients did not induce water transport in NKCC2. We conclude that NKCC1 plays a direct role for water balance in most cell types, while NKCC2 fulfils its role in the kidney of transporting ions but not water. The different behaviour of NKCC1 and NKCC2 is discussed on the basis of recent molecular models based on studies of structural and molecular dynamics.
    MeSH term(s) Animals ; Oocytes/physiology ; Sodium-Potassium-Chloride Symporters/physiology ; Solute Carrier Family 12, Member 1 ; Solute Carrier Family 12, Member 2 ; Water/physiology ; Xenopus
    Chemical Substances Sodium-Potassium-Chloride Symporters ; Solute Carrier Family 12, Member 1 ; Solute Carrier Family 12, Member 2 ; Water (059QF0KO0R)
    Language English
    Publishing date 2012-01-16
    Publishing country England
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ZDB-ID 3115-x
    ISSN 1469-7793 ; 0022-3751
    ISSN (online) 1469-7793
    ISSN 0022-3751
    DOI 10.1113/jphysiol.2011.226316
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  2. Article ; Online: Glial K⁺ clearance and cell swelling: key roles for cotransporters and pumps.

    Macaulay, Nanna / Zeuthen, Thomas

    Neurochemical research

    2012  Volume 37, Issue 11, Page(s) 2299–2309

    Abstract: An important feature of neuronal signalling is the increased concentration of K(+) in the extracellular space. The K(+) concentration is restored to its original basal level primarily by uptake into nearby glial cells. The molecular mechanisms by which K( ...

    Abstract An important feature of neuronal signalling is the increased concentration of K(+) in the extracellular space. The K(+) concentration is restored to its original basal level primarily by uptake into nearby glial cells. The molecular mechanisms by which K(+) is transferred from the extracellular space into the glial cell are debated. Although spatial buffer currents may occur, their quantitative contribution to K(+) clearance is uncertain. The concept of spatial buffering of K(+) precludes intracellular K(+) accumulation and is therefore (i) difficult to reconcile with the K(+) accumulation repeatedly observed in glial cells during K(+) clearance and (ii) incompatible with K(+)-dependent glial cell swelling. K(+) uptake into non-voltage clamped cultured glial cells is carried out by the Na(+)/K(+)-ATPase and the Na(+)/K(+)/Cl(-) cotransporter in combination. In brain slices and intact optic nerve, however, only the Na(+)/K(+)-ATPase has been demonstrated to be involved in stimulus-evoked K(+) clearance. The glial cell swelling associated with K(+) clearance is prevented under conditions that block the activity of the Na(+)/K(+)/Cl(-) cotransporter. The Na(+)/K(+)/Cl(-) cotransporter is activated by increased K(+) concentration and cotransports water along with its substrates. It thereby serves as a K(+)-dependent molecular water pump under conditions of increased extracellular K(+) load.
    MeSH term(s) Animals ; Homeostasis ; Humans ; Membrane Transport Proteins/physiology ; Neuroglia/metabolism ; Potassium/metabolism
    Chemical Substances Membrane Transport Proteins ; Potassium (RWP5GA015D)
    Language English
    Publishing date 2012-02-26
    Publishing country United States
    Document type Journal Article ; Research Support, Non-U.S. Gov't ; Review
    ZDB-ID 199335-5
    ISSN 1573-6903 ; 0364-3190
    ISSN (online) 1573-6903
    ISSN 0364-3190
    DOI 10.1007/s11064-012-0731-3
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  3. Article ; Online: Modulation of Kir4.1 and Kir4.1-Kir5.1 channels by small changes in cell volume.

    Soe, Rikke / Macaulay, Nanna / Klaerke, Dan Arne

    Neuroscience letters

    2009  Volume 457, Issue 2, Page(s) 80–84

    Abstract: The K+ channels Kir4.1 and Kir4.1-Kir5.1 are expressed in the glial cells of the CNS and are involved in regulation of the K+ homeostasis. Several studies have shown that Kir4.1 channels are co-localized with aquaporins (AQP4) in the glial endfeet, and a ...

    Abstract The K+ channels Kir4.1 and Kir4.1-Kir5.1 are expressed in the glial cells of the CNS and are involved in regulation of the K+ homeostasis. Several studies have shown that Kir4.1 channels are co-localized with aquaporins (AQP4) in the glial endfeet, and a putative functional coupling between the Kir channels and aquaporins is therefore debated. To test a possible volume-sensitivity of the Kir channels, the Kir4.1 or Kir4.1-Kir5.1 channels were expressed in Xenopus oocytes with or without co-expression of aquaporins and subsequently exposed to cell volume alterations. Our results show an increase in Kir4.1 and Kir4.1-Kir5.1 currents upon swelling of the oocytes and a reduction in the current when the oocytes were shrunk. The volume-dependent changes in channel activity were not due to changes in the kinetics of the channels. These findings implicate a putative functional interaction between the Kir channels and aquaporins via small, fast cell volume changes in the glial cells.
    MeSH term(s) Animals ; Aquaporin 1/metabolism ; Aquaporin 4/metabolism ; Cell Size ; Humans ; Membrane Potentials/physiology ; Oocytes/cytology ; Oocytes/metabolism ; Patch-Clamp Techniques ; Potassium Channels, Inwardly Rectifying/metabolism ; Rats ; Xenopus laevis ; Kir5.1 Channel
    Chemical Substances Aquaporin 4 ; Kcnj10 (channel) ; Potassium Channels, Inwardly Rectifying ; Aquaporin 1 (146410-94-8)
    Language English
    Publishing date 2009-04-08
    Publishing country Ireland
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ZDB-ID 194929-9
    ISSN 1872-7972 ; 0304-3940
    ISSN (online) 1872-7972
    ISSN 0304-3940
    DOI 10.1016/j.neulet.2009.04.010
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  4. Article ; Online: Ammonium ion transport by the AMT/Rh homolog TaAMT1;1 is stimulated by acidic pH.

    Søgaard, Rikke / Alsterfjord, Magnus / Macaulay, Nanna / Zeuthen, Thomas

    Pflugers Archiv : European journal of physiology

    2009  Volume 458, Issue 4, Page(s) 733–743

    Abstract: It is unclear how ammonia is transported by proteins from the Amt/Mep/Rh superfamily. We investigated this for the ammonium transporter TaAMT1;1 from wheat expressed in Xenopus oocytes by two-electrode voltage clamp and radio-labeled uptakes. Inward ... ...

    Abstract It is unclear how ammonia is transported by proteins from the Amt/Mep/Rh superfamily. We investigated this for the ammonium transporter TaAMT1;1 from wheat expressed in Xenopus oocytes by two-electrode voltage clamp and radio-labeled uptakes. Inward currents were activated by NH (4) (+) or methylammonium ions (MeA(+)). Importantly, currents increased fivefold when the external pH was decreased from 7.4 to 5.5; this type of pH dependence is unique and is a strong indication of NH (4) (+) or MeA(+) transport. This was confirmed by the close correlation between the uptake of radio-labeled MeA(+) and MeA(+)-induced currents. Homology models of members of the Amt/Mep/Rh superfamily exhibited major divergences in their cytoplasmic regions. A point mutation in this region of TaAMT1;1 abolished the pH sensitivity and decreased the apparent affinities for NH (4) (+) and MeA(+). We suggest a model where NH (4) (+) is transported as NH(3) and H(+) via separate pathways but the latter two recombine before leaving the protein.
    MeSH term(s) Animals ; Biological Transport, Active/physiology ; Cation Transport Proteins/chemistry ; Cation Transport Proteins/metabolism ; Cell Membrane/chemistry ; Cell Membrane/physiology ; Cells, Cultured ; Hydrogen-Ion Concentration ; Ion Channel Gating/physiology ; Membrane Potentials/physiology ; Oocytes/physiology ; Plant Proteins/chemistry ; Plant Proteins/metabolism ; Quaternary Ammonium Compounds/chemistry ; Quaternary Ammonium Compounds/metabolism ; Triticum/genetics ; Triticum/metabolism ; Xenopus laevis
    Chemical Substances Cation Transport Proteins ; Plant Proteins ; Quaternary Ammonium Compounds ; ammonium transporters, plant
    Language English
    Publishing date 2009-04-02
    Publishing country Germany
    Document type Journal Article ; Research Support, Non-U.S. Gov't
    ZDB-ID 6380-0
    ISSN 1432-2013 ; 0031-6768
    ISSN (online) 1432-2013
    ISSN 0031-6768
    DOI 10.1007/s00424-009-0665-z
    Database MEDical Literature Analysis and Retrieval System OnLINE

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  5. Article: Water transport by GLUT2 expressed in Xenopus laevis oocytes.

    Zeuthen, Thomas / Zeuthen, Emil / Macaulay, Nanna

    The Journal of physiology

    2007  Volume 579, Issue Pt 2, Page(s) 345–361

    Abstract: The glucose transporter GLUT2 has been shown to also transport water. In the present paper we investigated the relation between sugar and water transport in human GLUT2 expressed in Xenopus oocytes. Sugar transport was determined from uptakes of non- ... ...

    Abstract The glucose transporter GLUT2 has been shown to also transport water. In the present paper we investigated the relation between sugar and water transport in human GLUT2 expressed in Xenopus oocytes. Sugar transport was determined from uptakes of non-metabolizable glucose analogues, primarily 3-O-methyl-D-glucopyranoside; key experimental results were confirmed using D(+)-glucose. Water transport was derived from changes in oocyte volume monitored at a high resolution (20 pl, 1 s). Expression of GLUT2 induced a sugar permeability, P(S), of about 5 x 10(-6) cm s(-1) and a passive water permeability, L(p), of 5.5 x 10(-5) cm s(-1). Accordingly, the passive water permeability of a GLUT2 protein is about 10 times higher than its sugar permeability. Both permeabilities were abolished by phloretin. Isosmotic addition of sugar to the bathing solution (replacing mannitol) induced two parallel components of water influx in GLUT2, one by osmosis and one by cotransport. The osmotic driving force arose from sugar accumulation at the intracellular side of the membrane and was given by an intracellular diffusion coefficient for sugar of 10(-6) cm(2) s(-1), one-fifth of the free solution value. The diffusion coefficient was determined in oocytes coexpressing GLUT2 and the water channel AQP1 where water transport was predominantly osmotic. By the cotransport mechanism about 35 water molecules were transported for each sugar molecule by a mechanism within the GLUT2. These water molecules could be transported uphill, against an osmotic gradient, energized by the flux of sugar. This capacity for cotransport is 10 times smaller than that of the Na(+)-coupled glucose transporters (SGLT1). The physiological role of GLUT2 for intestinal transport under conditions of high luminal sugar concentrations is discussed.
    MeSH term(s) Animals ; Aquaporin 1/physiology ; Biological Transport/physiology ; Carbohydrate Metabolism/physiology ; Cell Membrane Permeability/physiology ; Female ; Gene Expression Regulation ; Glucose Transporter Type 2/genetics ; Glucose Transporter Type 2/physiology ; Oocytes/metabolism ; Osmolar Concentration ; Sodium-Glucose Transporter 1/physiology ; Water/metabolism ; Xenopus laevis
    Chemical Substances Glucose Transporter Type 2 ; Sodium-Glucose Transporter 1 ; Water (059QF0KO0R) ; Aquaporin 1 (146410-94-8)
    Language English
    Publishing date 2007-03-01
    Publishing country England
    Document type Comparative Study ; Journal Article ; Research Support, Non-U.S. Gov't
    ZDB-ID 3115-x
    ISSN 1469-7793 ; 0022-3751
    ISSN (online) 1469-7793
    ISSN 0022-3751
    DOI 10.1113/jphysiol.2006.123380
    Database MEDical Literature Analysis and Retrieval System OnLINE

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