Details

Autor(en) / Beteiligte

• Longpre, Jean-Philippe
• Gagnon, Dominique G
• Coady, Michael J
• Lapointe, Jean-Yves

Titel

The Actual Ionic Nature of the Leak Current through the Na super(+)/Glucose Cotransporter SGLT1

Ist Teil von

• Biophysical journal, 2010-01, Vol.98 (2), p.231-239

Erscheinungsjahr

2010

Quelle

EZB Electronic Journals Library

Beschreibungen/Notizen

• Expression of the Na super(+)/glucose cotransporter SGLT1 in Xenopus oocytes is characterized by a phlorizin-sensitive leak current (in the absence of glucose) that was originally called a "Na super(+) leak" and represents some 5-10% of the maximal Na super(+)/glucose cotransport current. We analyzed the ionic nature of the leak current using a human SGLT1 mutant (C292A) displaying a threefold larger leak current while keeping a reversal potential (V sub(R)) of [asymptotic to]-15 mV as observed for wt SGLT1. V sub(R) showed only a modest negative shift when extracellular Na super(+) concentration ([Na super(+)] sub(o)) was lowered and it was completely insensitive to changes in extracellular Cl super(-). When extracellular pH (pH sub(o)) was decreased from 7.5 to 6.5 and 5.5, V sub(R) shifted by +15 and +40 mV, respectively, indicating that protons may be the main charge carrier at low pH sub(o) but other ions must be involved at pH sub(o) 7.5. In the presence of 15 mM [Na super(+)] sub(o) (pH sub(o) = 7.5), addition of 75 mM of either Na super(+), Li super(+), Cs super(+), or K super(+) generated similar increases in the leak current amplitude. This observation, which was confirmed with wt SGLT1, indicates a separate pathway for the leak current with respect to the cotransport current. This means that, contrary to previous beliefs, the leak current cannot be accounted for by the translocation of the Na-loaded and glucose-free cotransporter. Using chemical modification and different SGLT1 mutants, a relationship was found between the cationic leak current and the passive water permeability suggesting that water and cations may share a common pathway through the cotransporter.