VOLTAGE-CLAMP STUDIES OF THE NA+/GLUCOSE COTRANSPORTER CLONED FROM RABBIT SMALL-INTESTINE

Inward Na+ currents associated with the cloned intestinal Na+/glucose cotransporter expressed in Xenopus oocytes have been studied using the two-microeletrode voltage-clamp method. The steady-state current/voltage relations showed voltage-dependent (V(m) from +20 to -75 mV) and relatively voltage-independent (V(m) from -75 to -150 mV) regions. The apparent I(max) for Na+ and glucose increased with negative membrane potentials, and the apparent K0.5 for glucose (K0.5Glc) depended on V(m) and [Na]o. Increasing [Na]o from 7 to 110 mmol/l had the same effect in decreasing K0.5Glc from 0.44 to 0.03 mmol/l as increasing the V(m) from -40 to -150 mV. The I/V curves under saturating conditions (20 mmol/l external sugars and 110 mmol/l [Na]o) were identical for D-glucose, D-galactose, alpha-methyl D-glucopyranoside and 3-O-methyl D-glucoside. The specificity of the cotransporter for sugars was: D-glucose, D-galactose, alpha-methyl D-glucopyranoside > 3-O-methyl D-glucoside >> D-xylose > D-allose >> D-mannose. K(i) for phlorizin (almost-equal-to 10-mu-mol/l) was independent of V(m) at saturating [Na]o. We conclude that a variety of sugars are transported by the cloned Na+/glucose cotransporter at the same maximal rate and that membrane potential affects both the maximal current and the apparent K0.5 of the cotransporter for Na+ and glucose.