Water transport in the brain: Role of cotransporters

• Published in: Neuroscience 2004, Vol.129 (4), p.1029-1042
• Main Authors: MacAulay, N., Hamann, S., Zeuthen, T.
• Format: Article
• Language: English
• Subjects: Amino Acid Transport System X-AG - metabolism; Animals; astrocytes; Astrocytes - metabolism; Biological Transport - physiology; Brain - physiology; cotransport; Excitatory Amino Acid Transporter 1; extracellular space; Extracellular Space - metabolism; glutamate; Glutamate Plasma Membrane Transport Proteins; Humans; Ions - metabolism; Neurotransmitter Agents - metabolism; NKCC1; Symporters - metabolism; water; Water - metabolism; Water-Electrolyte Balance - physiology; Xenopus
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/j.neuroscience.2004.06.045

It is generally accepted that cotransporters transport water in addition to their normal substrates, although the precise mechanism is debated; both active and passive modes of transport have been suggested. The magnitude of the water flux mediated by cotransporters may well be significant: both the number of cotransporters per cell and the unit water permeability are high. For example, the Na +-glutamate cotransporter (EAAT1) has a unit water permeability one tenth of that of aquaporin (AQP) 1. Cotransporters are widely distributed in the brain and participate in several vital functions: inorganic ions are transported by K +–Cl − and Na +–K +–Cl − cotransporters, neurotransmitters are reabsorbed from the synaptic cleft by Na +-dependent cotransporters located on glial cells and neurons, and metabolites such as lactate are removed from the extracellular space by means of H +-lactate cotransporters. We have previously determined water transport capacities for these cotransporters in model systems ( Xenopus oocytes, cell cultures, and in vitro preparations), and will discuss their role in water homeostasis of the astroglial cell under both normo- and pathophysiologal situations. Astroglia is a polarized cell with EAAT localized at the end facing the neuropil while the end abutting the circulation is rich in AQP4. The water transport properties of EAAT suggest a new model for volume homeostasis of the extracellular space during neural activity.