Na+/K+-ATPase expression in gills of the euryhaline sailfin molly, Poecilia latipinna, is altered in response to salinity challenge

Sailfin molly (Poecilia latipinna) is an introduced species of euryhaline teleost mainly distributed in the lower reaches and river mouths over the southwestern part of Taiwan. Upon salinity challenge, the gill is the major organ responsible for ion-regulation, and the branchial Na+–K+-ATPase (NKA) is a primary driving force for the other ion transporters and channels. Hence we hypothesized that branchial NKA expression changed in response to salinity stress of sailfin molly so that they were able to survive in environments of different salinities. Before sampling, the fish were acclimated to fresh water (FW), brackish water (BW, 15‰), or seawater (SW, 35‰) for at least one month. The physiological (plasma osmolality), biochemical (activity and protein abundance of branchial NKA), cellular (number of NKA immunoreactive cells), and stress (plasma glucose levels and protein abundance of hepatic and branchial heat shock protein 90) indicators of osmoregulatory challenge in sailfin molly were significantly increased in the SW-acclimated group compared to the FW- or BW-acclimated group. Elevated levels of stress indicators revealed that SW was stressful than FW to the sailfin molly. Meanwhile, the elevated biochemical indicators showed that more active NKA expression was necessary to match the demand of ion secretion of SW-acclimated sailfin molly to survive in the environment with salinity challenge. The plasma osmolality of sailfin molly increased with environmental salinities within a tolerated range, while the muscle water content, another physiological indicator, was constant among different salinity groups. Taken together, through different ways of analyses, the sailfin molly was proved to be an efficient osmoregulator in environments of different salinities with their branchial NKA expression changing in response to salinity challenge to maintain homeostasis of ion and water.