Multiplicity of expression of Na+,K+-ATPase α-subunit isoforms in the gill of Atlantic salmon (Salmo salar): Cellular localisation and absolute quantification in response to salinity change

Abstract

The ability to reverse the net direction of gill ion transport in response to a salinity change is critical for euryhaline teleosts and involves a complex cellular and molecular remodelling of the gill epithelium. The present study aimed to clarify the cellular localisation and exact quantitative inter-relationship of Na+,K+-ATPase α- and β-subunit transcripts in Atlantic salmon gill during salinity change. The combined expression level of all α-isoforms in the gill increased by 100% after freshwater (FW) to seawater (SW) transfer. The α 1a and α1b isoforms were both in the range 1-6 amol 20 ng-1 total RNA; α1a decreased and α1b increased after SW-transfer, their ratio changing from 5:1 in FW to 0.26:1 in SW. The α1c and α3 levels were 10- and 100-fold lower, respectively. The β1-subunit mRNA level was 0.1-0.3 amol 20 ng-1 total RNA, thus much lower than the sum of α-subunits. Even though increasing 3-fold after SW-transfer, β-subunit availability may still limit functional pump synthesis. The mRNAs of the predominant α1a and α1b isoforms were localised by in situ hybridisation in specific gill cells of both FW and SW salmon. Labelling occurred mainly in presumed chloride cells and cells deep in the filament but occasionally also on lamellae. Overall, the salinity-induced variation in labelling pattern and intensity matched the quantification data. In conclusion, the predominant switching of Na+,K+-ATPase α-subunit isoform mRNA during salinity acclimation reflects a marked remodelling of mitochondrion-rich cells (MRCs) in the gill and probably tuning of the pump performance to accomplish a net reversal of gill ion transport in hypo- and hypertonic environments.