Water Transport By The Sodium Glucose Cotransporter Sglt1

The small intestine reabsorbs about 8L of fluid daily by a yet unknown mechanism. In contrast to vectorial water transport in the kidney, intestinal water reabsorption is not mediated by aquaporins. The sodium-glucose cotransporter SGLT1 was implicated to play a crucial role because it (i) functions as a molecular water pump or (ii) provides a water channel. Here we tested both hypotheses by overexpressing SGLT1 in MDCK cell monolayers. Fluorescence correlation spectroscopy served to assess the unitary water channel permeability pf of SGLT1 in life cells by measuring (i) the dilution of an aqueous dye in the immediate monolayer vicinity and (ii) SGLT1 plasma membrane abundance1. pf agreed well the one obtained from deflation kinetics of proteoliposomes. We derived vesicle volume from the intensity of scattered light via our new adaptation of the Rayleigh-Gans-Debye equation2 and the number of the reconstituted SGLT1 molecules per vesicle via fluorescence correlation spectroscopy3. SGLT1u0027s pf is comparable to the pf of aquaporins rendering (i) an intracellular pathway plausible and (ii) water pumping against the osmotic gradient physiologically meaningless as the passive backflux would be orders of magnitude larger.Funding: This work was supported by grant P23574 of the Austrian Science Fund to P.P.1. Erokhova, L., Horner, A., Kugler, P., and Pohl, P. (2011). Monitoring single-channel water permeability in polarized cells. J. Biol. Chem. 286, 39926-39932.2. Hoomann, T., Jahnke, N., Horner, A., Keller, S., and Pohl, P. (2013). Filter gate closure inhibits ion but not water transport through potassium channels. PNAS 110, 10842-10847.3. Horner, A., Zocher, F., Preiner, J., Ollinger, N., Siligan, C., Akimov, S.A., and Pohl, P. (2015). The mobility of single-file water molecules is governed by the number of H-bonds they may form with channel-lining residues. Science Advances 1, e1400083.