TY - JOUR

T1 - Passive water and urea permeability of a human Na(+)-glutamate cotransporter expressed in Xenopus oocytes

AU - Macaulay, Nanna

AU - Gether, Ulrik

AU - Klærke, Dan Arne

AU - Zeuthen, Thomas

PY - 2002

Y1 - 2002

N2 - The human Na(+)-glutamate transporter (EAAT1) was expressed in Xenopus laevis oocytes. The passive water permeability, L(p), was derived from volume changes of the oocyte induced by changes in the external osmolarity. Oocytes were subjected to two-electrode voltage clamp. In the presence of Na(+), the EAAT1-specific (defined in Discussion) L(p) increased linearly with positive clamp potentials, the L(p) being around 23 % larger at +50 mV than at -50 mV. L-Glutamate increased the EAAT1-specific L(p) by up to 40 %. The K(0.5) for the glutamate-dependent increase was 20 +/- 6 microM, which is similar to the K(0.5) value for glutamate activation of transport. The specific inhibitor DL-threo-beta-benzyloxyaspartate (TBOA) reduced the EAAT1-specific L(p) to 72 %. EAAT1 supported passive fluxes of [(14)C]urea and [(14)C]glycerol. The [(14)C]urea flux was increased in the presence of glutamate. The data suggest that the permeability depends on the conformational equilibrium of the EAAT1. At positive potentials and in the presence of Na(+) and glutamate, the pore is enlarged and water and urea penetrate more readily. The L(p) was larger when measured with urea or glycerol as osmolytes as compared with mannitol. Apparently, the properties of the pore are not uniform along its length. The outer section may accommodate urea and glycerol in an osmotically active form, giving rise to larger water fluxes. The physiological role of EAAT1 for water homeostasis in the central nervous system is discussed.

AB - The human Na(+)-glutamate transporter (EAAT1) was expressed in Xenopus laevis oocytes. The passive water permeability, L(p), was derived from volume changes of the oocyte induced by changes in the external osmolarity. Oocytes were subjected to two-electrode voltage clamp. In the presence of Na(+), the EAAT1-specific (defined in Discussion) L(p) increased linearly with positive clamp potentials, the L(p) being around 23 % larger at +50 mV than at -50 mV. L-Glutamate increased the EAAT1-specific L(p) by up to 40 %. The K(0.5) for the glutamate-dependent increase was 20 +/- 6 microM, which is similar to the K(0.5) value for glutamate activation of transport. The specific inhibitor DL-threo-beta-benzyloxyaspartate (TBOA) reduced the EAAT1-specific L(p) to 72 %. EAAT1 supported passive fluxes of [(14)C]urea and [(14)C]glycerol. The [(14)C]urea flux was increased in the presence of glutamate. The data suggest that the permeability depends on the conformational equilibrium of the EAAT1. At positive potentials and in the presence of Na(+) and glutamate, the pore is enlarged and water and urea penetrate more readily. The L(p) was larger when measured with urea or glycerol as osmolytes as compared with mannitol. Apparently, the properties of the pore are not uniform along its length. The outer section may accommodate urea and glycerol in an osmotically active form, giving rise to larger water fluxes. The physiological role of EAAT1 for water homeostasis in the central nervous system is discussed.

KW - Animals

KW - Aspartic Acid

KW - Excitatory Amino Acid Transporter 1

KW - Female

KW - Glutamic Acid

KW - Glycerol

KW - Humans

KW - Membrane Potentials

KW - Oocytes

KW - Patch-Clamp Techniques

KW - Permeability

KW - Sodium

KW - Temperature

KW - Urea

KW - Water

KW - Xenopus laevis

U2 - 10.1113/jphysiol.2002.020586

DO - 10.1113/jphysiol.2002.020586

M3 - Journal article

C2 - 12154181

VL - 542

SP - 817

EP - 828

JO - The Journal of Physiology

JF - The Journal of Physiology

SN - 0022-3751

IS - 3

ER -