Presteady-state currents of the rabbit Na+/glucose cotransporter (SGLT1)

The rabbit Na+/glucose cotransporter (SGLT1) exhibits a presteady-state current after step changes in membrane voltage in the absence of sugar. These currents reflect voltage-dependent processes involved in cotransport, and provide insight on the partial reactions of the transport cycle. SGLT1 presteady-state currents were studied as a function of external Na+, membrane voltage V-m, phlorizin and temperature. Step changes in membrane voltage-from the holding V-h to test values, elicited transient currents that rose rapidly to a peak (at 3-4 msec), before decaying to the steady state, with time constants tau approximate to 4-20 msec, and were blocked by phlorizin (K-i approximate to 30 mu M). The total charge Q was equal for the application of the voltage pulse and the subsequent removal, and was a function of V-m. The Q-V curves obeyed the Boltzmann relation: the maximal charge Q(max) was 4-120 nC; V-0.5, the voltage for 50% Q(max) was -5 to +30 mV; and z, the apparent valence of the moveable charge, was 1. Q(max) and z were independent of V-h (between 0 and -100 mV) and temperature (20-30 degrees C), while increasing temperature shifted V-0.5 towards more negative values. Decreasing [Na+](o) decreased Q(max), and shifted V-0.5 to more negative voltages 9by-100 mV per 10-fold decrease in [Na+](o)). The time constant tau was voltage dependent: the tau-V relations were bell-shaped, with maximal tau(max) 8-20 msec. Decreasing [Na+](o) de creased tau(max), and shifted the tau-V curves towards more negative voltages. Increasing temperature also shifted the tau-V curves, but did not affect tau(max). The maximum temperature coefficient Q(10) for tau was 3-4, and corresponds to an activation energy of 25 kcal/mole. Simulations of a 6-state ordered kinetic model for rabbit Na+/glucose cotransport indicate that charge-movements are due to Na+-binding/dissociation and a conformational change of the empty transporter. The model predicts that (i) transient currents rise to a peak before decay to steady-state; (ii) the tau-V relations are bell-shaped, and shift towards more negative voltages as [Na+](o); is reduced; (iii) tau(max) is decreased with decreasing [Na+](o); and (iv) the Q-V relations are shifted towards negative voltages as [Na+](o) is reduced. In general, the kinetic properties of the presteady-state currents are qualitatively predicted by the model.