News Physiol Sci 14: 98-100, 1999;
1548-9213/99 $5.00
News in Physiological Sciences, Vol. 14, No. 3, 98-100,
June 1999
© 1999 Int. Union Physiol. Sci./Am. Physiol. Soc.
Commentary on "Epithelial Fluid Transport—A Century of Investigation"
Donald D.F. Loo,
Ernest M. Wright,
Anne-Kristine Meinild,
Dan A. Klaerke and
Thomas Zeuthen
UCLA School of Medicine Los Angeles, California, 90095–1751, USA
The Panum Institute DK-2200 N, Copenhagen, Denmark
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Introduction
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Dr. Spring (6) has drawn attention to a question that has long intrigued physiologists, namely, how is water transported across epithelial cells in the absence of external osmotic gradients? Because it is now accepted that water transport is secondary to solute transport, the problem really is one of how solute and water transport are coupled. This dilemma was apparently resolved 30 years ago by the emergence of theories in which "local" osmotic gradients within the epithelium, generated by active Na transport, drive water absorption. Arguably, the most elegant "local osmosis" model was the standing-gradient hypothesis (1, 2). Over the past 30 years much effort has been made to test for local osmosis. Dr. Spring concludes from his review of this work that simple local osmosis models are adequate to explain fluid absorption and secretion.
Here we wish to comment on Dr. Spring's assertion that consideration of an alternative nonosmotic model for water transport, molecular water pumps, "seems fruitless and a potential distraction from efforts directed toward greater understanding of the sites and mechanism of coupling of solute and water movements in epithelia."
The concept of cotransporters as molecular water pumps has its origin in experimental studies of the choroid plexus, in which evidence was obtained for uphill water transport coupled to K-Cl cotransport (8). Compelling evidence supporting the concept of water cotransport was recently obtained in studies of a model system, the cloned Na-glucose cotransporter (SGLT-1), expressed in Xenopus laevis oocytes (3, 4, 10). In this model, more than 1 x 1011 cotransporters are expressed in a single large cell (1-mm diameter), where rapid (<1 s) and sensitive methods may be used to simultaneously measure Na-glucose and water transport after fast (t1/2 <1 s) changes in the composition of the extracellular fluid (7, 10). These experiments demonstrate that water transport is directly coupled to Na-glucose transport, with a fixed stoichiometry of 2 Na, 1 glucose, and 210 H2O for human SGLT-1. The stoichiometry was independent of external parameters such as Na and sugar concentrations, membrane potential, temperature, and osmotic gradients. Possible artifacts caused by unstirred layers, transport numbers, and electrode effects were eliminated by control experiments with ion channels (e.g., gramicidin, connexin 50). Similar results with different stoichiometries (50–500 H2O molecules per turnover) have been obtained with Na-iodide, H-amino acid, Na-succinate, and Na-Cl-GABA cotransporter clones and with the K-Cl and H-lactate cotransporters in intact tissues. We thus conclude that cotransporters have an inherent ability to transport water with a high coupling ratio.
Dr. Spring does not refute our conclusion in his review but only questions its physiological relevance. We believe that it is premature to dismiss cotransport of water as unimportant, for the following reasons.
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Osmotic mechanisms have failed to explain water transport in several important epithelia
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In the case of the small intestine, where ~10 liters of water are absorbed each day in humans, there is no experimental evidence, so far, to support osmotic coupling. No osmotic gradients have been found in the lateral intercellular spaces, and no water channels have been reported in the epithelial membranes. On the other hand, because 1 mole of glucose is absorbed by the gut each day, Na-glucose-water cotransport could account for 4 of the 10 liters of water uptake across the brush-border membrane. It is also expected that other cotransporters, e.g., Na-amino acid and K-Cl cotransporters, transport significant amounts of water across the plasma membranes of the epithelium.
Conceptual problems arise with osmotic mechanisms of fluid secretion across flat epithelia, in which the location of osmotic compartments is unclear. One example is cerebrospinal fluid secretion, where a tissue weighing 2 g in humans, the choroid plexus, secretes ~600 ml of fluid per day. Here it is difficult to explain where coupling occurs between solute and water transport by simple osmotic mechanisms. However, water is secreted across the apical membrane of the choroidal epithelium via K-Cl cotransporters with a stoichiometry of 1 K-1 Cl-500 H2O, and this can explain about two-thirds of the total secretion (8, 9).
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Even in the renal tubule water cotransport plays a physiological role
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In the studies on aquaporin-1 (AQP-1) knockout mice cited by Dr. Spring (5), the basic observations are that knocking out the expression of the water channel reduced the transepithelial osmotic water permeability (Pf or Lp) by 78% and reduced fluid absorption by ~50%. We agree that these experiments indicate that the high Lp of the proximal tubule is caused by the expression of AQP-1 in the apical and basolateral membranes, but a central question to be resolved is why fluid absorption continues at 50% of the normal rate in the absence of water channels. It is reasonable to suggest that nonosmotic water transport could play a role. Clearly, Na-glucose cotransporters alone will not be sufficient to explain fluid absorption in the proximal tubule, and so we suggest that other transporters are involved (see above). Even so, the amount of water transported by proximal tubule SGLT per se will be important. In humans, each day, ~120 liters of fluid and 1 mole of glucose are reabsorbed from the glomerular filtrate in the proximal tubule. Assuming that the stoichiometry of the major renal SGLT (SGLT-2) is similar to that for SGLT-1, cotransport of water across the apical membrane of the proximal tubule by SGLT-2 accounts for the absorption of 4 liters of fluid [total glucose reabsorbed (1 mole) x glucose-water coupling coefficient (210) = 210 moles of water], which is about four times the volume of urine excretion per day. Consistent with this link between glucose and water absorption is the finding that patients with defects in renal glucose reabsorption (renal glycosuria) have increased urine excretion (polyuria).
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Conclusion
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Given the direct experimental evidence for the direct coupling of water transport to solute transport through cotransporters in epithelial membranes and model systems, the quantitative importance of water cotransport in the intestine, choroid plexus, and kidney, and the failure of local osmosis theories to account for fluid secretion and absorption, we suggest that one should keep an open mind about alternative mechanisms for water transport.
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Footnotes
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Editor's Note
As a new feature of this journal, the Editor will invite commentaries on papers dealing with controversial subjects. These commentaries will be read and approved by the author(s) of the original article. Commentaries on articles that have already appeared in NIPS will not be accepted.
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References
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Diamond, J. M., and W. H. Bossert. Standing-gradient osmotic flow. A mechanism for coupling of water and solute transport in epithelia. J. Gen. Physiol. 50: 2061–2083, 1967..[Abstract/Free Full Text]
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Diamond, J. M., and W. H. Bossert. Functional consequences of ultrastructural geometry in "backwards" fluid-transporting epithelia. J. Cell Biol. 37: 694–702, 1968.[Medline]
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Loo, D. D., T. Zeuthen, G. Chandy, and E. M. Wright. Cotransport of water by the Na+/glucose cotransporter. Proc. Natl. Acad. Sci. USA 93: 13367–13370, 1996.[Abstract/Free Full Text]
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Meinild, A., D. A. Klaerke, D. D. Loo, E. M. Wright, and T. Zeuthen. The human Na+/glucose cotransporter is a molecular water pump. J. Physiol. (Lond.) 508:15–21, 1998.[Abstract/Free Full Text]
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Schnermann, J., C. L. Chou, T. Ma, T. Traynor, M. A. Knepper, and A. S. Verkman. Defective proximal tubular fluid reabsorption in transgenic aquaporin-1 null mice. Proc. Natl. Acad. Sci. USA 95: 9660–9664, 1998.[Abstract/Free Full Text]
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Spring, K. R. Epithelial fluid transport—a century of investigation. News Physiol. Sci. 14: 98–100, 1999.[Free Full Text]
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Wright, E. M., D. D. Loo, M. Panayotova-Heiermann, B. A. Hirayama, E. Turk, S. Eskandari, and J. T. Lam. Structure and function of the Na+/glucose cotransporter. Acta. Physiol. Scand. Suppl. 643: 257–264, 1998.[Medline]
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Zeuthen, T. Secondary active transport of water across ventricular cell membrane of choroid plexus epithelium of Necturus maculosus. J. Physiol. (Lond.) 444: 153–173, 1991.[Abstract/Free Full Text]
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Zeuthen, T. Cotransport of K+, Cl- and H2O by membrane proteins from choroid plexus epithelium of Necturus maculosus. J. Physiol. (Lond). 478: 203–219, 1994.[Medline]
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Zeuthen, T., A. K. Meinild, D. A. Klaerke, D. D. Loo, E. M. Wright, B. Belhage, and T. Litman. Water transport by the Na+/glucose cotransporter under isotonic conditions. Biol. Cell. 89: 307–312, 1997.[Medline]
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Introduction
Osmotic mechanisms have failed...