Cotransport of water by the Na+-K+-2Cl cotransporter NKCC1 in mammalian epithelial cells

Nordic Centre for Water Imbalance Related Disorders, Institute of Cellular and Molecular Medicine, The Panum Institute, University of Copenhagen, Copenhagen, Denmark.
The Journal of Physiology (Impact Factor: 5.04). 11/2010; 588(Pt 21):4089-101. DOI: 10.1113/jphysiol.2010.194738
Source: PubMed


Water transport by the Na+-K+-2Cl(-) cotransporter (NKCC1) was studied in confluent cultures of pigmented epithelial (PE) cells from the ciliary body of the fetal human eye. Interdependence among water, Na+ and Cl(-) fluxes mediated by NKCC1 was inferred from changes in cell water volume, monitored by intracellular self-quenching of the fluorescent dye calcein. Isosmotic removal of external Cl(-) or Na+ caused a rapid efflux of water from the cells, which was inhibited by bumetanide (10 μm). When returned to the control solution there was a rapid water influx that required the simultaneous presence of external Na+ and Cl(-). The water influx could proceed uphill, against a transmembrane osmotic gradient, suggesting that energy contained in the ion fluxes can be transferred to the water flux. The influx of water induced by changes in external [Cl(-)] saturated in a sigmoidal fashion with a Km of 60 mm, while that induced by changes in external [Na+] followed first order kinetics with a Km of about 40 mm. These parameters are consistent with ion transport mediated by NKCC1. Our findings support a previous investigation, in which we showed water transport by NKCC1 to be a result of a balance between ionic and osmotic gradients. The coupling between salt and water transport in NKCC1 represents a novel aspect of cellular water homeostasis where cells can change their volume independently of the direction of an osmotic gradient across the membrane. This has relevance for both epithelial and symmetrical cells.

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Available from: Thomas Zeuthen
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    • "Of note, NKCC1 is also known to promote water transport across cell membranes [51]. Studies performed on cultured pigmented epithelial cells from the ciliary body of the fetal human eye transfected with NKCC1 reveal that NKCC1 works both as a water channel allowing passive water flux and as a water pump that transports water across apical membranes regardless of osmotic gradients [63]. Recent studies have demonstrated that NKCC1 transports 500 water molecules for each cycle of cation-chloride transport [64], which is comparable to that of aquaporins. "
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    • "At non-elevated extracellular K+ concentration β1-adrenergic stimulation enhances regulatory volume increase in mouse astrocytes during hypertonic stimulation, again via a glycogenolytic effect (Xu, Song, Du, Yan, Hertz and Peng, unpublished results). The β1-adrenergic stimulation of volume increase relies on Na+,K+-ATPase-dependent stimulation of a combined uptake of H2O, Na+, K+ and 2Cl−, mediated by the co-transporter NKCC1 (Hamann et al., 2010). In brain slices extracellular hypertonicity reduces excitatory activity (Huang and Somjen, 1997), an effect similar to that of slow neuronal afterhyperpolarization (sAHP). "
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    • "There is evidence that water can be transported even against osmotic gradients (Zeuthen, 2010) by coupling its movements to the transport of a substrate that energizes its uphill transport. Likely molecules for such a mechanism are the Na + /K + /2Cl – cotransporter NKCC1 (Hamann et al., 2010) and some K + /Cl – cotransporters (Zeuthen, 2010). The activity of the basolaterally localized NKCC1 of alveolar epithelial cells could mediate the uptake of water into the epithelial cells, a hypothesis that needs to be tested. "
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