ATPase Expression in the Branchial Epithelium of Brown Trout ( Salmo trutta ) and Atlantic Salmon ( Salmo salar )

The dynamics of branchial Naþ,Kþ,2Cl cotransporter (NKCC) and Naþ,KþATPase (NKA) expression were investigated in brown trout and Atlantic salmon during salinity shifts and the parr-smolt transformation, respectively. In the brown trout, Western blotting revealed that NKCC and NKA abundance increased gradually and in parallel (30and ten-fold, respectively) after transfer to seawater (SW). The NKA hydrolytic activity increased ten-fold after SW-transfer. Following back-transfer to fresh water (FW), the levels of both proteins and NKA activity decreased. The NKCC immunostaining in the gill of SW-acclimated trout was strong, and mainly localized in large cells in the filament and around the bases of the lamellae. In FW-acclimated trout, immunostaining was less intense and more diffuse. Partial cDNAs of the secretory NKCC1 isoform were cloned and sequenced from both brown trout and Atlantic salmon gills. Two differently sized transcripts were detected by Northern blotting in the gill but not in other osmoregulatory tissues (kidney, pyloric caeca, intestine). The abundance in the gill of these transcripts and of the associated NKCC protein increased fourand 30-fold, respectively, during parr-smolt transformation. The abundance of NKA a-subunit protein also increased in the gill during parr-smolt transformation though to a lesser extent than enzymatic activity (2.5and eight-fold, respectively). In separate series of in vitro experiments, cortisol directly stimulated the expression of NKCC mRNA in gill tissue of both salmonids. The study demonstrates the coordinated regulation of NKCC and NKA proteins in the gill during salinity shifts and parr-smolt transformation of salmonids. J. Exp. Zool. 293:106–118, 2002. r 2002 Wiley-Liss, Inc. Anadromous salmonids are modestly euryhaline throughout their life, which means that they tolerate transfers between freshwater (FW) and seawater (SW) of moderate salinity. However, the euryhaline capacity is greatly enhanced in most species during a narrow period in the spring of their first or second year. This occurs through a process termed the parr-smolt transformation, which normally precedes or concurs with the onset of downstream migration. The transformation involves several morphological, physiological, and biochemical changes which have been the subject of numerous studies (Boeuf, ’93). A major focus has been put on the preparative osmoregulatory changes associated with transition from FW to SW, and it is well known that the gill is transformed from a hyperinto a hypo-osmoregulatory organ. A key enzyme in salt-transport in the two operative modes of the gill is the Naþ, Kþ-ATPase (NKA), the protein and mRNA Grant sponsor: Novo Nordisk Foundation; Grant sponsor: National Science Foundation. *Correspondence to: Steffen S. Madsen, Institute of Biology, University of Southern Denmark-Main Campus: Odense University, Campusvej 55, DK-5230 Odense M, Denmark. E-mail: steffen@biology.sdu. dk Received 20 November 2001; Accepted 20 March 2002 Published online in Wiley InterScience (www.interscience.wiley. com). DOI: 10.1002/jez.10118 r 2002 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 293:106–118 (2002)