Kinetics of Steady-state Currents and Charge Movements Associated with the Rat Na+/Glucose Cotransporter

The rat Na+/glucose cotransporter (SGLT1) was expressed in Xenopus oocytes and steady-state and transient currents were measured using a two-electrode voltage clamp. The maximal glucose induced Na(+)-dependent inward current was approximately 300-500 nA. The apparent affinity constants for sugar (alpha-methyl-D-glucopyranoside; alpha MDG) (K alpha MDG 0.5) and sodium (KNa0.5) at a membrane potential of -150 mV were 0.2 mM and 4 mM. The KNa0.5 increased continuously with depolarizing potentials reaching 40 mM at -30 mV, K alpha MDG 0.5 was steeply voltage dependent, 0.46 mM at -30 mV and 1 mM at -10 mV. From all tested monovalent cations only Li+ could substitute for Na+, but with lower affinity. The relative substrate specificity was D-glucose > alpha MDG approximately D-galactose > 3-O-Me-Glc >> beta-naphthyl-D-glucoside >> uridine. Phlorizin (Pz), the specific blocker of sugar transport, showed an extremely high affinity for the rat cotransporter with an inhibitor constant (KPzi) of 12 nM. SGLT1 charge movements in the absence of sugar were fitted by the Boltzmann equation with an apparent valence of the movable charge of approximately 1, a potential for 50% maximal charge transfer (V0.5) of -43 mV, and a maximal charge (Qmax) of 9 nanocoulombs. The apparent turnover number for the rat SGLT1 was 30 s-1. Model simulations showed that the kinetics of the rat SGLT1 are described by a six-state ordered nonrapid equilibrium model, and comparison of the kinetics of the rat, rabbit and human cotransporters indicate that they differ mainly in their presteady-state kinetic parameters.