The expression of potassium-related ion transporters/channels in the gills of euryhaline Asian sea bass (Lates calcarifer) under salinity change.

Ion regulation in fish has been intensively studied for decades, primarily focused on the regulation of Na⁺ and Cl^-, the major electrolytes. However, the regulation mechanisms of K⁺ in teleosts, which is critical for maintaining cell function and the functions of important transporters such as Na⁺/K⁺ ATPase (NKA) and Na⁺-K⁺-2Cl^- cotransporter (NKCC), remained poorly understood. Inwardly rectifying K⁺ channel 1.1 (Kir1.1) was recently found to be responsible for potassium excretion in tilapia and medaka but has not been broadly investigated in other species. This study aimed to investigate the branchial expression and localization of Kir1.1 in the gills of the euryhaline Asian sea bass acclimated to fresh water (FW), 10‰ and 20‰ brackish water (BW10 and BW20) and seawater (SW). The gene expression of NKAα1a was the lowest in BW10, whereas that of NKCC1a remained stable in various salinities. The gene expression of Kir1.1a was elevated in BW20 and SW. Using immunofluorescent staining, the basolateral NKCC and apical Na⁺-Cl^- cotransporter (NCC) were identified in the SW ionocytes and the FW/BW ionocytes, respectively. Kir1.1 was visible only in FW and was localized in basolateral membranes. This research not only demonstrated the expression and localization of Kir1.1, NKCC and NKA in Asian sea bass but also discovered a FW-dependent basolateral-localized Kir1.1 expression that might indicate a distinct function from previous research in other teleost species.