Modulation of ion transporter expression in gill mitochondrion-rich cells of eels acclimated to low-Na⁺ or-Cl^- freshwater

In this study, we aimed to establish an experimental model to study the role of the gill mitochondrion-rich cells (MRCs) of freshwater fish in Na⁺ uptake and to examine the effect of adjusting external Na⁺ and Cl^- ions on selected ion transporters in gill MRCs. Japanese eels (Anguilla japonica) acclimated to deionized (DI) water for 2 weeks were transferred directly to (a) ion-supplemented artificial freshwater (AF), (b) Na⁺-deficient AF, or (c) Cl^--deficient AF for 2 days. The effects of the transfer on the expression levels of ion transporters in isolated gill cells were investigated. Our data demonstrated that the 2-day acclimation in ion-supplemented AF, Na⁺-deficient AF, or Cl^--deficient AF led to a significant increase in serum osmolarity attributed mainly to an increase in serum Na⁺ and/or Cl^- levels when compared with DI-acclimated eel. Significant inductions of V-type H⁺-ATPase (V-H⁺-ATPase) and cotransporter (NBC1) mRNA expression in gill MRCs were detected in AF-acclimated fish. In fish acclimated to Na⁺-deficient AF, mRNA expression levels of V-H⁺-ATPase, NBC1, and Na⁺/H⁺-exchanger-3 (NHE3) were significantly increased in MRCs. Fish acclimated to Cl^--deficient AF showed no observable change in expression levels of ion transporters in gill MRCs. In addition, expression levels of ion transporters in pavement cells were stable throughout the 2-day experiments. These data indicate that the level of Na⁺ in freshwater is important for altering the mRNA expression of ion transporters in gill MRCs, which supports the notion that gill MRCs play important roles in freshwater Na⁺ uptake.