The True Ionic Nature of the Na+/Glucose Cotransporter Leak Current

Abstract

Expression of the Na+/glucose cotransporter (SGLT1, SLC5A1) in Xenopus oocytes is characterized, by a phlorizin(Pz)-sensitive leak current (observed in the absence of glucose) which represents 5 to10% of the maximal Na+/glucose cotransport current. This current was considered to represent the glucose-independent steps of the cotranport mechanism and was originally called a Na-leak even if its reversal potential (Vr) was 80 mV more negative than ENa. Using a human SGLT1 mutant (C292A) displaying a large leak current (-290 nA at −155 mV), the leak Vr showed only a modest negative shift when external Na concentration ([Na+]e) was lowered and was insensitive to changes in external [Cl-]. When external pH (pHe) was decreased from 7.5 to 6.5 and 5.5, the leak current (at −155 mV) increased to −600 and −1500 nA and its Vr shifted by +15 and +40 mV, respectively. If protons appear to be the main charge carrier at low pHe's, other ions need to be involved at pHe 7.5 since rising pHe to 8.5 produced no further reduction of the leak current. Starting from a [Na+]e of 15 mM (pHe=7.5), adding 75 mM of either Na+, Li+, Cs+ or K+ generates a similar increase in the leak current amplitude. This is in sharp contrast with the cotransport activity which accepts only Na+ and, to a lesser extent, Li+ as driving cations. This demonstrates that a cationic pathway clearly distinct from the cotransport mechanism exists through SGLT1. This is reminiscent of the Pz-sensitive passive water permeability of SGLT1. Interestingly, breaking disulfide bridges in SGLT1 or the C292A mutant with dithiothreitol produces a decrease in both the leak current and the Pz-sensitive water permeability. This suggests that the cationic leak may share a common pathway with water across the cotransporter.