Relationships between Na+/glucose cotransporter (SGLT1) currents and fluxes

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 mM alpha MG and the (22)Na(+) uptake at 10 mM Na(+); (ii) the currents evoked by 50 to 500 mu M [(14)C]alpha MG and the [(14)C]alpha MG uptakes at 100 mM Na(+); and (iii) phlorizin-sensitive leak currents in the absence of sugar and (22)Na(+) 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 alpha MG and Na(+) uptakes. The Na(+)/alpha MG coupling coefficients were estimated to be 1.6 at -70 mV and 1.9 at -110 mV, This suggests that the rabbit SGLT1 Na(+)/alpha MG 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.