Water Transport By The Sodium Glucose Cotransporter

Abstract

In addition to transepithelial water flow along an osmotic gradient, isosmolal fluid absorption and water transport against the osmotic gradient were also observed. Two putative mechanisms of solute-solvent flux coupling may explain these findings: (i) local osmosis due to increased solute concentration in the poorly mixed water layers adjacent to the basolateral membrane; and (ii) “molecular water pumping” by secondary active cotransporters. The present work aims to identify the water transport mechanism of human sodium glucose cotransporter. Thus, we stably transfected MDCK cells with the hSGLT1-EGFP fusion protein and measured their osmotic water permeability by laser scanning reflection microscopy. We also assessed water flux through confluent cell monolayers, both in the presence and absence of an osmotic gradient by detecting tiny concentration changes with scanning fluorescence correlation spectroscopy (FCS). Fitting the solution of the differential equations for the osmotic drift and for back diffusion to the experimentally determined dye distribution adjacent to the epithelial monolayer allowed calculation of the osmotic water permeability. Assessment of the single transporter permeability coefficient pf required the simultaneous determination of hSGLT1 abundance in the plasma membrane by FCS. With 4.6 × 10−14 cm3/sec pf is close to the single channel permeability of aquaporin-1. Consequently, even small osmolyte concentration differences between the cytoplasm and the basolateral buffer solution are sufficient to drive a substantial water flux. Thus, the physiological importance of secondary active water transport is doubtful.