| Summary: | The Na,K‐ATPase is the major driving force for ion secretion in teleost gill chloride cells. Accordingly, gill enzyme abundance increases and isoform composition changes in response to increased salinity and during the salmon parr‐smolt transformation (PST) prior to seaward migration in the spring. We identified 3 isoforms of the FXYD protein family (FXYD5, FXYD9 and FXYD11) in the salmon gill, which may assist in regulating the catalytic rate of the Na,K‐ATPase in these situations. FXYD proteins are auxiliary subunits which may associate with the alpha‐subunit of the Na,K‐ATPase. Expression in the gill of the 3 isoforms was investigated during PST and acclimation of freshwater (FW) salmon to sea water (SW). During the PST, FXYD5 mRNA levels were unaltered, whereas the expression of FXYD9 and FXYD11 peaked in May, coinciding with optimal SW‐tolerance and peak levels of Na,K‐ATPase of the FW dwelling salmon. Abrupt SW‐transfer induced an increase in Na,K‐ATPase abundance and FXYD5 expression but no overall changes in FXYD9 and FXYD11. The parallel increase in FXYD9 and FXYD11 levels with Na,K‐ATPase abundance during PST but not during SW‐acclimation suggests that these auxillary proteins play a particular role in controlling catalytic rate in situ when the FW fish prepares for SW entry. The research was supported by the Carlsberg Foundation (2005‐1‐311) and the Danish Natural Research Council (272‐06‐0526).
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