Sodium-glucose cotransporters display sodium- and phlorizin-dependent water permeability.
Expression of Na(+)-glucose cotransporters of the SGLT-1 type by Xenopus laevis oocytes increased the osmotic water permeability (Pf) of oocytes by a factor of 1.9-2.8, in the presence and in the absence of 5 mM extracellular glucose. The Pf increase was correlated with the amount of SGLT-1 cRNA injected. In oocytes expressing SGLT-1, either addition of phlorizin to the medium or the replacement of Na+ by choline inhibited the uptake of methyl-alpha-D-glucopyranoside, a specific substrate for SGLT-1, and returned oocyte Pf to its level in uninjected oocytes. Phlorizin inhibited the SGLT-1-attributable increase in Pf with an inhibition constant (Ki) of 6.1 microM, a value analogous to the Ki for phlorizin inhibition of sugar uptake. However, neither the presence of phlorizin nor the absence of extracellular Na+ significantly affected the increase in Pf elicited in oocytes expressing GLUT-1, a facilitative glucose transporter. These findings suggest that SGLT-1 forms a pore that allows the transmembrane passage of water and that water and glucose traverse the protein through this pore. The finding that removal of extracellular Na+ abolishes the increase in Pf attributable to SGLT-1 suggests that extracellular Na+ is required to maintain patency of this transporter's water-permeable transmembrane pore.
Loike
J D
JD
Rover Laboratory, Department of Physiology and Cellular Biophysics, Columbia University, New York 10032, USA.
Hickman
S
S
Kuang
K
K
Xu
M
M
Cao
L
L
Vera
J C
JC
Silverstein
S C
SC
Fischbarg
J
J
D000818:Animals; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002463:Cell Membrane Permeability / Q000187:drug effects; D002794:Choline / Q000494:pharmacology; D005260:Female; D051272:Glucose Transporter Type 1; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000096:biosynthesis / Q000187:drug effects / Q000502:physiology*; D008954:Models, Biological; D009004:Monosaccharide Transport Proteins / Q000096:biosynthesis / Q000187:drug effects / Q000502:physiology*; D009865:Oocytes / Q000502:physiology; D010695:Phlorhizin / Q000494:pharmacology*; D018075:RNA, Complementary; D011817:Rabbits; D011994:Recombinant Proteins / Q000378:metabolism; D012964:Sodium / Q000494:pharmacology*; D051269:Sodium-Glucose Transporter 1; D014867:Water / Q000378:metabolism; D014982:Xenopus laevis
D051272:Glucose Transporter Type 1; D008562:Membrane Glycoproteins; D009004:Monosaccharide Transport Proteins; D018075:RNA, Complementary; D011994:Recombinant Proteins; D051269:Sodium-Glucose Transporter 1; D014867:Water; D012964:Sodium; D010695:Phlorhizin; D002794:Choline