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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Changes in gene expression in gills of the euryhaline killifish Fundulus heteroclitus after abrupt salinity transfer

¹Department of Zoology, University of British Columbia, Vancouver, Canada V6T 1Z4; ²Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510; ³Mount Desert Island Biological Laboratory, Salsbury Cove, Maine 04672; and ⁴Groupe de Recherche en Néphrologie, Department of Medicine, Faculty of Medicine, Laval University, Quebec, Canada G1R 2J6

Submitted 27 January 2004 ; accepted in final form 20 March 2004

Maintenance of ion balance requires that ionoregulatory epithelia modulate ion flux in response to internal or environmental osmotic challenges. We have explored the basis of this functional plasticity in the gills of the euryhaline killifish Fundulus heteroclitus. The expression patterns of several genes encoding ion transport proteins were quantified after transfer from near-isosmotic brackish water [10 parts/thousand (ppt)] to either freshwater (FW) or seawater (SW). Many changes in response to SW transfer were transient. Increased mRNA expression occurred 1 day after transfer for Na⁺-K⁺-ATPase-_1a (3-fold), Na⁺-K⁺-2Cl^–-cotransporter 1 (NKCC1) (3-fold), and glucocorticoid receptor (1.3-fold) and was paralleled by elevated Na⁺-K⁺-ATPase activity (2-fold). The transient increase in NKCC1 mRNA expression was followed by a later 2-fold rise in NKCC protein abundance. In contrast to the other genes studied in the present work, mRNA expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl^– channel generally remained elevated (2-fold) in SW. No change in protein abundance was detected, however, suggesting posttranscriptional regulation. The responses to FW transfer were quite different from those to SW transfer. In particular, FW transfer increased Na⁺-K⁺-ATPase-_1a mRNA expression and Na⁺-K⁺-ATPase activity to a greater extent than did SW transfer but had no effect on V-type H⁺-ATPase expression, supporting the current suggestion that killifish gills transport Na⁺ via Na⁺/H⁺ exchange. These findings demonstrate unique patterns of ion transporter expression in killifish gills after salinity transfer and illustrate important mechanisms of functional plasticity in ion-transporting epithelia.

Na⁺-K⁺-ATPase; Na⁺-K⁺-2Cl^– cotransporter; cystic fibrosis transmembrane conductance regulator; glucocorticoid receptor; H⁺-ATPase

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