, Volume 162, Issue 2, pp 101-106

Relationships Between Na+/Glucose Cotransporter (SGLT1) Currents and Fluxes

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The relationships between currents generated by the rabbit Na+/glucose cotransporter (SGLT1) and the fluxes of Na+ and sugar were investigated using Xenopus laevis oocytes expressing SGLT1. In individual voltage-clamped oocytes we measured: (i) the current evoked by 10 mmαMG and the 22Na+ uptake at 10 mm Na+; (ii) the currents evoked by 50 to 500 μm [14C]αMG and the [14C]αMG uptakes at 100 mm Na+; and (iii) phlorizin-sensitive leak currents in the absence of sugar and 22Na+ uptakes at 10 mm Na+. We demonstrate that the SGLT1 leak currents are Na+ currents, and that the sugar-evoked currents are directly proportional to both αMG and Na+ uptakes. The Na+/αMG coupling coefficients were estimated to be 1.6 at −70 mV and 1.9 at −110 mV. This suggests that the rabbit SGLT1 Na+/αMG stoichiometry for sugar uptake is 2 under fully saturating, zero-trans conditions. Coupling coefficients of less than 2 are expected under nonsaturating conditions due to uncoupled Na+ fluxes (slippage). The similarity between the Na+ Hill coefficients and the coupling coefficients suggests strong cooperativity between the two Na+ binding sites.

Received: 6 October 1997/Revised: 5 December 1997