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 similar to 300-500 nA, The apparent affinity constants for sugar (alpha-methyl-D-glucopyrano-side; alpha MDG) (K-0.5(alpha MDG)) sodium (K-0.5(Na)) at a membrane potential of -150 mV were 0.2 mM and 4 mM. The (Na)(0.5) increased continuously with depolarizing potentials reaching 40 mM at -30 mV, K-0.5(alpha MDG) 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 approximate to D-galactose > 3-O-Me-Glc much greater than beta-naphthyl-D-glucoside much greater than uridine, Phlorizin (Pt), the specific blocker of sugar transport, showed an extremely high affinity for the rat cotransporter with an inhibitor constant (K-i(Pz)) 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 similar to 1, a potential for 50% maximal charge transfer (V-0.5) of -43 mV, and a maximal charge (Q(max)) 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.