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Kinetics of the Reverse Mode of the Na+/Glucose Cotransporter

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Abstract

This study investigates the reverse mode of the Na+/glucose cotransporter (SGLT1). In giant excised inside-out membrane patches from Xenopus laevis oocytes expressing rabbit SGLT1, application of α-methyl-D-glucopyranoside (αMDG) to the cytoplasmic solution induced an outward current from cytosolic to external membrane surface. The outward current was Na+- and sugar-dependent, and was blocked by phlorizin, a specific inhibitor of SGLT1. The current-voltage relationship saturated at positive membrane voltages (30–50 mV), and approached zero at −150 mV. The half-maximal concentration for αMDG-evoked outward current (K αMDG0.5 ) was 35 mM (at 0 mV). In comparison, K αMDG0.5 for forward sugar transport was 0.15 mM (at 0 mV). K Na0.5 was similar for forward and reverse transport (≈35 mM at 0 mV). Specificity of SGLT1 for reverse transport was: αMDG (1.0) > D-galactose (0.84) > 3-O-methyl-glucose (0.55) > D-glucose (0.38), whereas for forward transport, specificity was: αMDG ≈ D-glucose ≈ D-galactose > 3-O-methyl-glucose. Thus there is an asymmetry in sugar kinetics and specificity between forward and reverse modes. Computer simulations showed that a 6-state kinetic model for SGLT1 can account for Na+/sugar cotransport and its voltage dependence in both the forward and reverse modes at saturating sodium concentrations. Our data indicate that under physiological conditions, the transporter is poised to accumulate sugar efficiently in the enterocyte.

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Acknowledgements

We thank Ms. Manuela Contreras for her assistance with the oocytes, and Drs. Andrea Doering and Ken Phillipson for guidance in establishing the giant-patch method. This work was supported by NIH grant DK-19567.

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  1. S. Eskandari

    Present address: Biological Sciences Department, California State Polytechnic University, 3801 West Temple Avenue, Pomona, CA, 91768-4032, USA

Authors and Affiliations

  1. Department of Physiology, David Geffen School of Medicine at UCLA, Los Angeles, CA, 90095-1751, USA

    S. Eskandari, E.M. Wright & D.D.F. Loo

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  1. S. Eskandari
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  3. D.D.F. Loo
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Correspondence to D.D.F. Loo.

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Eskandari, S., Wright, E. & Loo, D. Kinetics of the Reverse Mode of the Na+/Glucose Cotransporter. J Membrane Biol 204, 23–32 (2005). https://doi.org/10.1007/s00232-005-0743-x

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  • DOI: https://doi.org/10.1007/s00232-005-0743-x

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