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Steffen Søndergaard Madsen, Lars Nørholm Jensen, Christian Kølbæk Tipsmark, Pia Kiilerich and Russell John Borski

Effects of salinity and hormones on cystic fibrosis transmembrane conductance regulator (CFTR) and α-subunit Na+,K+-ATPase (α-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 α-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, α-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). α-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.