Click here to close
Hello! We notice that you are using Internet Explorer, which is not supported by Xenbase and may cause the site to display incorrectly.
We suggest using a current version of Chrome,
FireFox, or Safari.
Passive water and ion transport by cotransporters.
Loo DD
,
Hirayama BA
,
Meinild AK
,
Chandy G
,
Zeuthen T
,
Wright EM
.
???displayArticle.abstract???
1. The rabbit Na+-glucose (SGLT1) and the human Na+-Cl--GABA (GAT1) cotransporters were expressed in Xenopus laevis oocytes, and passive Na+ and water transport were studied using electrical and optical techniques. Passive water permeabilities (Lp) of the cotransporters were determined from the changes in oocyte volume in response to osmotic gradients. The specific SGLT1 and GAT1 Lp values were obtained by measuring Lp in the presence and absence of blockers (phlorizin and SKF89976A). In the presence of the blockers, the Lp values of oocytes expressing SGLT1 and GAT1 were indistinguishable from the Lp of control oocytes. Passive Na+ transport (Na+ leak) was obtained from the blocker-sensitive Na+ currents in the absence of substrates (glucose and GABA). 2. Passive Na+ and water transport through SGLT1 were blocked by phlorizin with the same sensitivity (inhibitory constant (Ki), 3-5 microM). When Na+ was replaced with Li+, phlorizin also inhibited Li+ and water transport, but with a lower affinity (Ki, 100 microM). When Na+ was replaced by choline, which is not transported, the SGLT1 Lp was indistinguishable from that in Na+ or Li+, but in this case water transport was less sensitive to phlorizin. 3. The activation energies (Ea) for passive Na+ and water transport through SGLT1 were 21 and 5 kcal mol-1, respectively. The high Ea for Na+ transport is comparable to that of Na+-glucose cotransport and indicates that the process is dependent on conformational changes of the protein, while the low Ea for water transport is similar to that of water channels (aquaporins). 4. GAT1 also behaved as an SKF89976A-sensitive water channel. We did not observe passive Na+ transport through GAT1. 5. We conclude that passive water and Na+ transport through cotransporters depend on different mechanisms: Na+ transport occurs by a saturable uniport mechanism, and water permeation is through a low conductance water channel. In the case of SGLT1, we suggest that both the water channel and water cotransport could contribute to isotonic fluid transport across the intestinal brush border membrane.
Diamond,
Standing-gradient osmotic flow. A mechanism for coupling of water and solute transport in epithelia.
1967, Pubmed
Diamond,
Standing-gradient osmotic flow. A mechanism for coupling of water and solute transport in epithelia.
1967,
Pubmed
Diamond,
Wet transport proteins.
,
Pubmed
,
Xenbase
Fischbarg,
Glucose transporters serve as water channels.
1990,
Pubmed
,
Xenbase
Fischer,
Substrate specificity and expression profile of amino acid transporters (AAPs) in Arabidopsis.
1995,
Pubmed
Hasegawa,
A multifunctional aqueous channel formed by CFTR.
1992,
Pubmed
,
Xenbase
Hediger,
Expression cloning and cDNA sequencing of the Na+/glucose co-transporter.
,
Pubmed
,
Xenbase
Loike,
Sodium-glucose cotransporters display sodium- and phlorizin-dependent water permeability.
1996,
Pubmed
,
Xenbase
Loo,
Cotransport of water by the Na+/glucose cotransporter.
1996,
Pubmed
,
Xenbase
Loo,
Relaxation kinetics of the Na+/glucose cotransporter.
1993,
Pubmed
,
Xenbase
Mackenzie,
Relationships between Na+/glucose cotransporter (SGLT1) currents and fluxes.
1998,
Pubmed
,
Xenbase
Mager,
Steady states, charge movements, and rates for a cloned GABA transporter expressed in Xenopus oocytes.
1993,
Pubmed
,
Xenbase
Meinild,
The human Na+-glucose cotransporter is a molecular water pump.
1998,
Pubmed
,
Xenbase
Meinild,
Bidirectional water fluxes and specificity for small hydrophilic molecules in aquaporins 0-5.
1998,
Pubmed
,
Xenbase
Nelson,
Cloning of the human brain GABA transporter.
1990,
Pubmed
Pajor,
Sequence and functional characterization of a renal sodium/dicarboxylate cotransporter.
1995,
Pubmed
,
Xenbase
Panayotova-Heiermann,
Neutralization of conservative charged transmembrane residues in the Na+/glucose cotransporter SGLT1.
1998,
Pubmed
Panayotova-Heiermann,
Sodium/D-glucose cotransporter charge movements involve polar residues.
1994,
Pubmed
,
Xenbase
Panayotova-Heiermann,
Kinetics of steady-state currents and charge movements associated with the rat Na+/glucose cotransporter.
1995,
Pubmed
,
Xenbase
Parent,
Electrogenic properties of the cloned Na+/glucose cotransporter: II. A transport model under nonrapid equilibrium conditions.
1992,
Pubmed
Parent,
Electrogenic properties of the cloned Na+/glucose cotransporter: I. Voltage-clamp studies.
1992,
Pubmed
,
Xenbase
Preston,
Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein.
1992,
Pubmed
,
Xenbase
Schreiber,
Cystic fibrosis transmembrane conductance regulator activates water conductance in Xenopus oocytes.
1997,
Pubmed
,
Xenbase
Umbach,
Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic.
1990,
Pubmed
,
Xenbase
Wright,
Sodium cotransporters.
1996,
Pubmed
Wright,
Structure and function of the Na+/glucose cotransporter.
1998,
Pubmed
,
Xenbase
Zampighi,
A method for determining the unitary functional capacity of cloned channels and transporters expressed in Xenopus laevis oocytes.
1995,
Pubmed
,
Xenbase
Zeuthen,
Cotransport of H+, lactate and H2O by membrane proteins in retinal pigment epithelium of bullfrog.
1996,
Pubmed
Zeuthen,
Cotransport of K+, Cl- and H2O by membrane proteins from choroid plexus epithelium of Necturus maculosus.
1994,
Pubmed
Zeuthen,
Water transport by the Na+/glucose cotransporter under isotonic conditions.
1997,
Pubmed
,
Xenbase
Zeuthen,
Secondary active transport of water across ventricular cell membrane of choroid plexus epithelium of Necturus maculosus.
1991,
Pubmed
