Glucose transport by human renal Na+/d-glucose cotransporters SGLT1 and SGLT2

Abstract

The human Na+/d-glucose cotransporter 2 (hSGLT2) is believed to be responsible for the bulk of glucose reabsorption in the kidney proximal convoluted tubule. Since blocking reabsorption increases urinary glucose excretion, hSGLT2 has become a novel drug target for Type 2 diabetes treatment. Glucose transport by hSGLT2 was studied at 37°C in human embryonic kidney 293T cells using whole cell patch-clamp electrophysiology. We compared hSGLT2 with hSGLT1, the transporter in the straight proximal tubule (S3 segment). hSGLT2 transports with surprisingly similar glucose affinity and lower concentrative power than hSGLT1: Na+/d-glucose cotransport by hSGLT2 was electrogenic with apparent glucose and Na+ affinities of 5 and 25 mM, and a Na+:glucose coupling ratio of 1; hSGLT1 affinities were 2 and 70 mM and coupling ratio of 2. Both proteins showed voltage-dependent steady-state transport; however, unlike hSGLT1, hSGLT2 did not exhibit detectable pre-steady-state currents in response to rapid jumps in membrane voltage. d-Galactose was transported by both proteins, but with very low affinity by hSGLT2 (≥100 vs. 6 mM). β-d-Glucopyranosides were either substrates or blockers. Phlorizin exhibited higher affinity with hSGLT2 ( Ki 11 vs. 140 nM) and a lower Off-rate (0.03 vs. 0.2 s−1) compared with hSGLT1. These studies indicate that, in the early proximal tubule, hSGLT2 works at 50% capacity and becomes saturated only when glucose is ≥35 mM. Furthermore, results on hSGLT1 suggest it may play a significant role in the reabsorption of filtered glucose in the late proximal tubule. Our electrophysiological study provides groundwork for a molecular understanding of how hSGLT inhibitors affect renal glucose reabsorption.