Investigating the conformational states of the rabbit Na+/glucose cotransporter.

The Na+ and voltage-dependence of transient rabbit Na+/glucose cotransporter (rSGLT1) kinetics was studied with the two-electrode voltage-clamp technique and Xenopus laevis oocytes. Using step changes in membrane potential, in the absence of glucose but with 100 or 10mM Na+, transient currents were measured corresponding to binding/debinding of Na+ and conformational changes of the protein. Previously, only a single time constant has been published for rSGLT1. We, however, observed three decay components; a fast (τf, 0.5–1ms) voltage- and Na+-independent decay, and medium (τm, 0.5–4ms) and slow (τs, 8–50ms) voltage- and Na+-dependent decays. Transient currents were simulated and fit using a four-state model to obtain kinetic parameters for the system. The four-state model was able to reconstitute an assortment of experimental data.