Abstract
Abstract Milkfish (Chanos chanos) is an euryhaline teleost which is able to survive in environments with a broad range of salinities. Since previous studies revealed that upon salinity challenge milkfish exhibited adaptive changes in branchial NKA activity with different Na+ and K+ affinity, it was hypothesized that alteration in activity and ion-affinity was derived from changes of different isoforms of NKA α-subunit (i.e., the catalytic subunit). It is thus intriguing to compare the expression patterns of three major isoforms of NKA α-subunit (i.e., α1, α2, and α3) in gills of milkfish following salinity changes to realize their roles. In this study, milkfish were reared in seawater (SW, 35‰), fresh water (FW), or hypersaline water (HSW, 60‰) and the protein abundances of three NKA α-isoforms were analysed by immunoblotting. In acclimation experiments the milkfish were raised in SW, FW, or HSW for more than two weeks. The SW group exhibited significantly higher levels of NKA α1- and α3-subunit than the FW or HSW group. In addition, the time-course experiments for which milkfish were transferred from SW to FW or HSW and then sampled at 12, 24, 48, 96, and 168 h showed that after transfer from SW to HSW, NKA α1-isoform levels in gills declined gradually before 48 h and achieved a significant 4-folds decrease at 96 and 168 h, while NKA α3-isoform decreased significantly since 12 h post-transfer and no significant difference was found in NKA α2-isoform abundance. When fish were transferred from SW to FW, NKA α1-, α2-, α3-isoform levels decreased at 12 h post-transfer; α1-isoform increased gradually and peaked at 48 h while α2-isoform increased significantly at 24 h post-transfer. Then NKA α1- and α3-isoform levels achieved a significant decrease at 96 and 168 h following salinity transfer. Taken together, upregulation of gill NKA α1- and α3-isoforms was essential for the euryhaline milkfish to inhabit in SW, their primary natural habitat, and the changes of gill NKA α1- and α3-isoforms may lead to different Na+ or K+ affinities and fulfill some of the requirements for the subtly altered enzyme behaviors in gills of the marine euryhaline milkfish upon salinity challenge.
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