The possibility of heterologous expression of ion-dependent cotransporters has revealed that they characteristically display two kinds of membrane current: a pre-steady-state (I_pre) current in the absence of organic substrate, and a transport-associated (I_tr) current in its presence (Bossi et al. 1999; Forlani et al. 2001). A thorough comparison of these two currents elicited by the neuronal GABA transporter rGAT1 expressed in Xenopus oocytes leads to a number of unforeseen qualitative and quantitative consistencies between them. In saturating amounts of GABA, the following relation holds: I_tr = Q_inr, where Q_in is the quantity of charge in the inner side of the transporter, obtained in the absence of GABA from integration of I_pre, and r is the relaxation rate of I_pre at the same membrane potential (see Fig. 1).

The relation remains valid when [Na⁺]_o and [Cl^–]_o are changed. At non-saturating GABA, decreases in the amplitude of I_tr are compensated by complementary variations in Q_in. Complementarity of magnitude, superimposable kinetic properties and dependence on V_m and [Na⁺]_o, point to the unicity of the charge carrier for both processes. The partition of the molecule between the transporting and non-transporting forms is in agreement with the apparent affinity for the organic substrate, previously estimated from the GABA concentration eliciting half-maximal I_tr at each potential. The system may be simulated by a simple three-state kinetic scheme in which GABA must bind after Na⁺. On the whole, these observations suggest that transport and charge migration in rGAT1 arise from the same molecular mechanism.

All experiments were carried out according to the institutional and national ethical guidelines; frogs were anaesthetized in MS222 (tricaine methansulfonate) 0.10 % (w/v) solution in tap water before oocyte dissection and humanely killed at the end of the experiments.

We are grateful to Professor H.A. Lester and C. Labarca for the generous gift of rGAT cDNA. This work was supported by a PRIN grant from the Italian Ministry for University and Research to A. Peres.