Electrogenic properties of the cloned Na+/glucose cotransporter: I. Voltage-clamp studies

The cloned rabbit intestinal Na+/glucose cotransporter was expressed in Xenopus laevis oocytes. Presteady-state and steady-state currents associated with cotransporter activity were measured with the two-electrode voltage-clamp technique. Steady-state sugar-dependent currents were measured between -150 and +90 mV as a function of external Na+ ([Na]o) and alpha-methyl-D-glucopyranoside concentrations ([alpha MDG]o). K alpha MDG0.5 was found to be dependent upon [Na]o and the membrane potential. At Vm = -50 mV, increasing [Na]o from 10 to 100 mM decreased K alpha MDG0.5 from 1.5 mM to 180 microM. Increasing membrane potential toward negative values decreased K alpha MDG0.5 at nonsaturating [Na]o. For instance, at 10 mM [Na]o, K alpha MDG0.5 decreased from 1.5 mM to 360 microM on increasing the membrane potential from -50 to -150 mV. The i alpha MDGmax was relatively insensitive to [Na]o between 10 and 100 mM and weakly voltage dependent (e-fold increase per 140 mV). KNa0.5 and iNamax were found to be dependent upon membrane potential and [sugar]o. In the presence of 1 mM [alpha MDG]o, KNa0.5 decreased from 50 to 5 mM between 0 and -150 mV and iNamax increased twofold between -30 and -200 mV. The voltage dependence of KNa0.5 is consistent with an effect of potential on Na+ binding (Na(+)-well effect), whereas the voltage dependence of iNamax is compatible with the translocation step being voltage dependent. It is concluded that voltage influences both Na+ binding and translocation. Presteady-state currents were observed for depolarization pulses in the presence of 100 mM [Na]o. The transient current relaxed with a half time of approximately 10 msec, and both the half time and magnitude of the transient varied with the holding potential and the size of depolarization pulse. Presteady-state currents were not observed after the addition of phlorizin or alpha MDG to the external Na+ solution and were not observed for water-injected control oocytes. We conclude that presteady-state currents are due to the activity of the carrier and that they may give a novel insight to the transport mechanism of the Na+/glucose cotransporter.