Time-course changes in the expression of Na+,K+-ATPase in gills and pyloric caeca of brown trout (Salmo trutta) during acclimation to seawater

Changes in protein and mRNA expression of Na+,K+-ATPase in gills and pylori c caeca of brown trout were investigated on a detailed time course after tr ansfer from freshwater to 25 ppt seawater (SW). A transient deflection in p lasma osmolality and muscle water content lasting from 4 h until day 3 was followed by restoration of hydromineral balance from day 5 onward. Gills an d pyloric caeca responded to SW transfer by increasing Na+,K+-ATPase activi ty from days 5 and 3, respectively, onward. In both tissues, this response was preceded by an increase in alpha -subunit Na+,K+-ATPase mRNA as early a s 12 h posttransfer. The similarity of the response in these two organs sug gests that they both play significant physiological roles in restoring hydr omineral balance after abrupt increase in salinity. Further, SW transfer in duced a slight, though significant, increase in primary gill filament Na+,K +-ATPase immunoreactive (NKIR) cell abundance. This was paralleled by a mar ked (50%) decrease in secondary lamellar NKIR cell abundance after less tha n 1 d in SW. Thus, SW acclimation in brown trout is characterised by a last ing decrease in overall NKIR cell abundance in the gill. We propose that SW transfer stimulates Na+,K+-ATPase enzymatic activity within individual chl oride cells long before (<1 d) it becomes apparent in measurements of whole -gill homogenate enzymatic activity. This is supported by the early stabili sation (12 h) of hydromineral balance.