Presteady-State Currents of the Rabbit Na + /Glucose Cotransporter (SGLT1)

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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 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 τ≈4–20 msec, and were blocked by phlorizin (K i ≈30 μm). 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°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 τ was voltage dependent: the τ-V relations were bell-shaped, with maximal τmax 8–20 msec. Decreasing [Na+] o decreased τmax, and shifted the τ-V curves towards more negative voltages. Increasing temperature also shifted the τ-V curves, but did not affect τmax. The maximum temperature coefficient Q 10 for τ 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 τ-V relations are bell-shaped, and shift towards more negative voltages as [Na+] o is reduced; (iii) τmax 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.

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