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First published online March 22, 2004
Journal of Experimental Biology 207, 1439-1452 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00907

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The mechanism of sodium chloride uptake in hyperregulating aquatic animals

Leonard B. Kirschner

School of Biological Sciences, Washington State University, Pullman, WA 99164-4236, USA

View larger version (18K): [in a new window]   Fig. 1. An MR cell and associated principal cell. The system will absorb Na+ and eliminate H+ even in Cl–-free media. It is unable to absorb Cl–. Solid lines indicate transport; broken lines indicate diffusion.

View larger version (18K): [in a new window]   Fig. 2. The MR cell. With this configuration of transport elements, both Na+ and Cl– can be absorbed from ambient NaCl, and Cl– can be taken up from Na+-free solution. As pictured, it lacks a basolateral mechanism for eliminating HCO3– and hence would be unable to absorb Na+ from Cl–-free media. Whether this is, in fact, the case remains to be determined. The apical Cl– channel is not shown, since it would be closed in FW.

View larger version (11K): [in a new window]   Fig. 3. A tentative model for Na+ and Cl– cotransport by Carcinus gill. Note that transcellular Na+ influx is only half that of Cl–. To maintain electrical neutrality, paracellular Na+ influx must equal transcellular Na+ influx. The model requires higher ambient Na+ and K+ than is usually found in freshwater in order to operate the cotransporter, but it would be effective in brackish water. The apical membrane potential difference (–80 mV) is an estimate, supposing that it is a K+ diffusion potential with [K+]2 mmol l–1 in 20% seawater. The transepithelial potential difference (–8 mV) is based on many published values between –5 mV and –10 mV.