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

Abstract

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 Vm, phlorizin and temperature. Step changes in membrane voltage-from the holding Vh 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 approximately 4-20 msec, and were blocked by phlorizin (Ki approximately 30 microm). The total charge Q was equal for the application of the voltage pulse and the subsequent removal, and was a function of Vm. The Q-V curves obeyed the Boltzmann relation: the maximal charge Qmax was 4-120 nC; V0.5, the voltage for 50% Qmax was -5 to +30 mV; and z, the apparent valence of the moveable charge, was 1. Qmax and z were independent of Vh (between 0 and -100 mV) and temperature (20-30 degrees C), while increasing temperature shifted V0.5 towards more negative values. Decreasing [Na+]o decreased Qmax, and shifted V0.5 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 taumax 8-20 msec. Decreasing [Na+]o decreased taumax, and shifted the tau-V curves towards more negative voltages. Increasing temperature also shifted the tau-V curves, but did not affect taumax. The maximum temperature coefficient Q10 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) taumax 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.