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-mRNA and -protein
levels
Laboratoire de Physiologie des Poissons, Institut National de la Recherche Agronomique, Campus de Beaulieu, 35042 Rennes Cedex, France
Several parameters were
analyzed to determine the mechanisms responsible for the enhancement of
the gill
Na+-K+-ATPase
activity of Atlantic salmon smolts. A major 
-subunit transcript of
3.7 kb was revealed by Northern blot in both parr and smolt gills when
hybridized with two distinct cDNA probes. The -mRNA abundance
demonstrated an increase to maximal levels in smolts at an early stage
of the parr-smolt transformation. This was followed by a gradual rise
in -protein levels, revealed by Western blots with specific
antibodies and by an increase in gill
Na+-K+-ATPase
hydrolytic activity, both only reaching maximum levels a month later,
at the peak of the transformation process. Parr fish experienced a
decrease in -mRNA abundance and had basal levels of -protein and
enzyme activity. Measurement of the binding of
[3H]ouabain to
Na+-K+-ATPase
was characterized in smolts and parr gill membranes showing more than a
twofold elevation in smolts and was of high affinity in both groups
(dissociation constant = 20-23 nM). Modulation of the enzyme
due to increased salinity was also observed in seawater-transferred smolts, as demonstrated by an increase in -mRNA levels after 24 h
with a rise in
Na+-K+-ATPase
activity occurring only after 11 days. No qualitative change in
-expression was revealed at either the mRNA or protein level.
Immunological identification of the -protein was performed with
polyclonal antibodies directed against the rat -specific isoforms,
revealing that parr, freshwater, and seawater smolts have an
3-like isoform. This study
shows that the increase in Na+-K+-ATPase
activity in smolt gills depends first on an increase in the -mRNA
expression and is followed by a slower rise in -protein abundance
that eventually leads to a higher synthesis of
Na+-K+ pumps.
sodium pump; salmonids; smoltification; [3H]ouabain binding; transcript
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