Cotransport of water by the Na+/glucose cotransporter.
Water is transported across epithelial membranes in the absence of any hydrostatic or osmotic gradients. A prime example is the small intestine, where 10 liters of water are absorbed each day. Although water absorption is secondary to active solute transport, the coupling mechanism between solute and water flow is not understood. We have tested the hypothesis that water transport is directly linked to solute transport by cotransport proteins such as the brush border Na+/glucose cotransporter. The Na+/glucose cotransporter was expressed in Xenopus oocytes, and the changes in cell volume were measured under sugar-transporting and nontransporting conditions. We demonstrate that 260 water molecules are directly coupled to each sugar molecule transported and estimate that in the human intestine this accounts for 5 liters of water absorption per day. Other animal and plant cotransporters such as the Na+/CI-/gamma-aminobutyric acid, Na+/iodide and H+/amino acid transporters are also able to transport water and this suggests that cotransporters play an important role in water homeostasis.
Loo
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Department of Physiology, University of California School of Medicine, Los Angeles 90095-1751, USA.
Zeuthen
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T
Chandy
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Wright
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EM
D000818:Animals; D001834:Body Water / Q000378:metabolism*; D002462:Cell Membrane / Q000502:physiology; D005260:Female; D005947:Glucose / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D007408:Intestinal Absorption; D007422:Intestines; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000096:biosynthesis / Q000502:physiology*; D009004:Monosaccharide Transport Proteins / Q000096:biosynthesis / Q000502:physiology*; D009865:Oocytes / Q000166:cytology / Q000502:physiology; D018075:RNA, Complementary; D011817:Rabbits; D051269:Sodium-Glucose Transporter 1; D013816:Thermodynamics; D014982:Xenopus laevis
D008562:Membrane Glycoproteins; D009004:Monosaccharide Transport Proteins; D018075:RNA, Complementary; C495674:SLC5A1 protein, human; D051269:Sodium-Glucose Transporter 1; D005947:Glucose