Water transport by the human Na+-coupled glutamate cotransporter expressedin Xenopus oocytes

1. The water transport properties of the human Na(+)coupled glutamate cotra nsporter (EAAT1) were investigated. The protein was expressed in Xenopus la evis oocytes and electrogenic glutamate transport was recorded by two-elect rode voltage clamp, while the concurrent water transport. was monitored as oocyte volume changes. 2. Water transport by EAAT1 was bimodal. Water was cotransported along with glutamate and Na+ by a mechanism within the protein. The transporter also sustained passive water transport in response to osmotic challenges. The tw o modes could be separated and could proceed in parallel. 3. The cotransport modality was characterized in solutions of low Cl- conce ntration. Addition of glutamate promptly initiated an influx of 438 +/- 55 water molecules per unit charge, irrespective of the clamp potential. 4. The cotransport of water occurred in the presence of adverse osmotic gra dients. In accordance with the Gibbs equation, energy was transferred withi n the protein primarily from the downhill fluxes of Na+ to the uphill fluxe s of water. 5. Experiments using the cation-selective ionophore gramicidin showed no un stirred layer effects. Na+ currents in the ionophore did not lead to any si gnificant initial water movements. 6. In the absence of glutamate, EAAT1 contributed a passive water permeabil ity (L-p) of (11.3 +/- 2.0) x 10(-6) m s(-1) (osmol l(-1))(-1). In the pres ence of glutamate, L-p was about 50% higher for both high and low Cl- conce ntrations. 7. The physiological role of EAAT1 as a molecular water pump is discussed i n relation to cellular volume homeostasis in the nervous system.