Properties of the cytoplasmic binding sites of the rabbit Na+/glucose cotransporter, SGLT1, expressed in Xenopus oocytes were investigated using the giant excised patch clamp technique. Voltage and substrate dependence of the outward cotransport were studied using α-methyl D-glucopyranoside (αMDG) as a substrate. The apparent affinity for αMDG depends on the cytoplasmic Na+ concentration and voltage. At 0 mV the K M for αMDG is 7 mM at 110 mM Na+ and 31 mM at 10 mM Na+. The apparent affinity for αMDG and Na+ is voltage dependent and increases at positive potentials. At 0 mV holding potential the outward current is half-maximal at about 70 mM. The results show that SGLT1 can mediate sugar transport out of the cell under appropriate concentration and voltage conditions, but under physiological conditions this transport is highly improbable due to the low affinity for sugar.
