Physiology

Regulating Salt Intake

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Science  17 Jun 2005:
Vol. 308, Issue 5729, pp. 1715
DOI: 10.1126/science.308.5729.1715a

Euryhaline fish, such as Mozambique tilapia (Oreochromis mossambicus), are able to live in waters with a wide range of salinity. Mitochondrion-rich cells (MRCs, also known as ionocytes) are located in the gills of adult fish and are thought to regulate internal ionic composition, either by secreting excess salt in a seawater environment or by taking up needed ions during a freshwater stage of life.

Hiroi et al. have examined the adaptive deployment of ion transporters in MRCs, which are found in the yolk sac membrane in developing embryos, by transferring the embryos from fresh water to salt water or vice versa. The three primary molecular components are a Na/K-ATPase, a Na/K/2Cl cotransporter, and a chloride channel, and the authors used immunoflurorescence microscopy to group the MRCs into four types: (i) an immature type-I MRC that can give rise to any of the other three types; (ii) a salt-accumulating type-II cell for freshwater living; (iii) a dormant type-III cell held in reserve; and (iv) a seawater, salt-extrusion type-IV cell derived from and apparently convertible back into type-III cells. The remarkable changeover from absorption to secretion (fresh water to sea water) is achieved by moving the Na/K/2Cl cotransporter from the apical to the basolateral surface and by replacing it with the chloride channel. — GJC

J. Exp. Biol. 208, 2023 (2005).

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