Role of anion-cation interactions on the pre-steady-state currents of the rat Na+-Cl--dependent GABA cotransporter rGAT1

The effects of sodium and chloride on the properties of the sodium-dependent component of the 'pre-steady-state' currents of rGAT1, a GABA cotransporter of the Na+-Cl--dependent family, were studied using heterologous oocyte expression and voltage clamp. Reductions in either extracellular sodium or chloride shifted the charge-voltage (Q-V) and time constant-voltage ((T)-V) characteristics of the process towards more negative potentials. The shift induced by sodium (TMA(1), tetramethylammonium substitution) was stronger than that induced by chloride (acetate substitution), and the shift of (T) was accompanied by a decrease in its maximum value. Increasing extracellular Ca2+ did not produce significant shifts in Q-Vand (T)-V curves. The negative shift of the Q-V curve upon chloride reduction and the decrease in the value of the relaxation time constant, (T), when either sodium or chloride were lowered, contrasted with the prediction of the Hill-Boltzmann interpretation of the process. Analysis of the unidirectional rate constants under different conditions revealed that both sodium and chloride shifted the outward rate more than the inward rate; furthermore, the shifts induced by sodium were larger than those induced by chloride. These observations are qualitatively compatible with the existence of a selective vestibule at the mouth of the transporters, acting similarly to a Dorman system.