Nbce1 and H⁺-atpase mRNA expression are stimulated in the mitochondria-rich cells of freshwater-acclimating Japanese eels (Anguilla japonica)

Mitochondria-rich cells (MRCs) of fish gills are well known for their roles in Na⁺ and Cl^- excretion in seawater environment. However, the level of understanding of their roles in fresh water is limited. In this study, we used the Percoll gradient technique to isolate gill MRCs from Japanese eels (Anguilla japonica Temminck and Schlegel, 1846) and measured the mRNA expression of two selected ion transporters, which were partially cloned in our laboratory: sodium bicarbonate co-transporter 1 (nbce1) and vacuolar proton pumps (H⁺-atpase). In addition, protein expressions of H⁺-ATPase and NBCe1 in whole gill were studied using Western blotting. The data indicated that both NBCe1 and H⁺-ATPase mRNA and protein expressions increased after seawater to freshwater transfer. This is the first report of changes in mRNA expression for these two ion transporters in MRCs of Japanese eels during seawater to freshwater transfer, confirming the roles of MRCs in osmoregulation in fresh water.