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Institute of Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
¹ Department of Zoology, North Carolina State University, Raleigh, North Carolina 27695-7617, USA

(Requests for offprints should be addressed to S S Madsen; Email: steffen{at}biology.sdu.dk)

Effects of salinity and hormones on cystic fibrosis transmembrane conductance regulator (CFTR) and -subunit Na⁺,K⁺-ATPase (-NKA) mRNA (analysed by semi-quantitative PCR) and protein expression (analysed by western blotting and immunocytochemistry) were investigated in gills of striped bass. Freshwater (FW) to seawater (SW) transfer induced a disturbance in serum [Na⁺]. Gill CFTR protein, mRNA level and Na⁺,K⁺-ATPase activity were unaffected by SW transfer, whereas -NKA mRNA increased after transfer. CFTR immunoreactivity was observed in large cells in FW and SW gill filaments at equal intensity. Cortisol decreased serum [Na⁺] in FW fish, but had no effect on gill Na⁺,K⁺-ATPase activity, -NKA and CFTR mRNA levels. Incubation of gill tissue with cortisol (24 h, >0.01 µg/ml) and epidermal growth factor (EGF 10 µg/ml) decreased CFTR mRNA levels relative to pre-incubation and control levels. CFTR expression was unaffected by IGF-I (10 µg/ml). -NKA mRNA levels decreased by 50% after 24 h control incubation; it was slightly stimulated by cortisol and unaffected by IGF-I and EGF. In isolated gill cells, phosphorylation of extracellular-regulated kinase (ERK) 1/2 was stimulated by EGF but not affected by IGF-I. This study is the first to report a branchial EGF response and to demonstrate a functional ERK 1/2 pathway in the teleost gill. In conclusion, CFTR and Na⁺,K⁺-ATPase are differentially regulated by salinity and hormones in gills of striped bass, despite the putative involvement of both in salt excretion.

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