Figure 3. Cotransport of water as a function of glutamate-induced clamp currents (I_s), high external Cl⁻.

A, the relation between I_s and clamp voltage obtained in high Cl⁻ and low Cl⁻ concentrations. The data obtained at low Cl⁻ concentrations (6 mm) are the averaged data from the oocytes presented in Fig. 2. The data obtained at high Cl⁻ concentrations (96 mm) are from an oocyte from the same batch as those used for the low Cl⁻ experiments. Curves are drawn by eye. B, water flux (J_H2O) versus glutamate-induced current (I_s) for an oocyte bathed in high external Cl⁻ concentrations (96 mm). Data are from the oocyte above, which is typical for ten oocytes. The curved line through the points is the J_H2Oversus I_s relation constructed from two assumptions: (1) the cotransport of water is linked to the Na⁺-glutamate transport and given by the relation measured in low Cl⁻ (Fig. 2) and (2) the conductive Cl⁻ current in the EAAT1 does not give rise to water transport. Compare, for example, the data obtained at a clamp voltage of -100 mV and at E_Cl, which is around -20 mV. The cotransport component of the current at -100 mV is about 100 nA larger than at E_Cl (Fig. 3A, ▪). Accordingly, J_H2O would be predicted to be 7 pl s⁻¹ larger at -100 mV, about 20 pl s⁻¹, see Fig. 2. The total current, however, at -100 mV is 400 nA. The dashed line through zero is the J_H2Oversus I_s relation in low Cl⁻ solutions (from Fig. 2).