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Biophys J 2004 Jan 01;861 Pt 1:125-33.
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Glucose accumulation can account for the initial water flux triggered by Na+/glucose cotransport.

Gagnon MP , Bissonnette P , Deslandes LM , Wallendorff B , Lapointe JY .

Over the last decade, several cotransport studies have led to the proposal of secondary active transport of water, challenging the dogma that all water transport is passive. The major observation leading to this interpretation was that a Na+ influx failed to reproduce the large and rapid cell swelling induced by Na+/solute cotransport. We have investigated this phenomenon by comparing a Na+/glucose (hSGLT1) induced water flux to water fluxes triggered either by a cationic inward current (using ROMK2 K+ channels) or by a glucose influx (using GLUT2, a passive glucose transporter). These proteins were overexpressed in Xenopus oocytes and assayed through volumetric measurements combined with double-electrode electrophysiology or radioactive uptake measurements. The osmotic gradients driving the observed water fluxes were estimated by comparison with the swelling induced by osmotic shocks of known amplitude. We found that, for equivalent cation or glucose uptakes, the combination of substrate accumulations observed with ROMK2 and GLUT2 are sufficient to provide the osmotic gradient necessary to account for a passive water flux through SGLT1. Despite the fact that the Na+/glucose stoichiometry of SGLT1 is 2:1, glucose accumulation accounts for two-thirds of the osmotic gradient responsible for the water flux observed at t = 30 s. It is concluded that the different accumulation processes for neutral versus charged solutes can quantitatively account for the fast water flux associated with Na+/glucose cotransport activation without having to propose the presence of secondary active water transport.

PubMed ID: 14695256
PMC ID: PMC1303776
Article link: Biophys J

Species referenced: Xenopus laevis
Genes referenced: slc2a2 slc5a1 slc5a1.2

References [+] :
Bissonnette, Functional expression of tagged human Na+-glucose cotransporter in Xenopus laevis oocytes. 1999, Pubmed, Xenbase