Methods
EAAT1 was subcloned into a vector optimized for oocyte expression and expressed in Xenopus laevis oocytes. The current from the electrogenic glutamate transport was recorded by two-electrode voltage clamp and the concurrent volume change of the oocyte was monitored by a sensitive optical system (Zeuthen et al 1997).

Results
In voltage clamped oocytes, L-glutamate elicited an abrupt inward current concomitant with a linear increase in oocyte volume. In solutions with low Cl^-concentrations (to avoid the conductive Cl^- fluxes inherent in the transporter), there was a linear relationship between water flux and the glutamate-induced current, 424 ± 9 water molecules per unit charge. The water transport was independent of external parameters, such as the osmolarity of the solutions and the membrane potential. In addition to the cotransport properties, the transporter had a passive water permeability. Water flow as a function of unstirred layer effects was ruled out from experiments using the cation-selective ionophore gramicidin.

Conclusions
The data show that Na⁺ coupled amino acid transporters can be included in the group of molecular water pumps. Specifically, the findings point to an important role of the Na⁺/glutamate transporter in cellular volume homeostasis in the nervous system.