Does Japanese medaka (Oryzias latipes) exhibit a gill Na+/K+-ATPase isoform switch during salinity change?
Some euryhaline teleosts exhibit a switch in gill Na+/K+-ATPase (Nka) α isoform when moving between fresh water (FW) and seawater (SW). The present study tested the hypothesis that a similar mechanism is present in Japanese medaka and whether salinity affects ouabain, Mg2+, Na+ and K+ affinity of the gill enzyme. Phylogenetic analysis classified six separate medaka Nka α isoforms (α1a, α1b, α1c, α2, α3a and α3b). Medaka acclimated long-term (>30 days) to either FW or SW had similar gill expression of α1c, α2, α3a and α3b, while both α1a and α1b were elevated in SW. Since a potential isoform shift may rely on early changes in transcript abundance, we conducted two short-term (1–3 days) salinity transfer experiments. FW to SW acclimation induced an elevation of α1b and α1a after 1 and 3 days. SW to FW acclimation reduced α1b after 3 days with no other α isoforms affected. To verify that the responses were typical, additional transport proteins were examined. Gill ncc and nhe3 expression were elevated in FW, while cftr and nkcc1a were up-regulated in SW. This is in accordance with putative roles in ion-uptake and secretion. SW-acclimated medaka had higher gill Nka Vmax and lower apparent Km for Na+ compared to FW fish, while apparent affinities for K+, Mg2+ and ouabain were unchanged. The present study showed that the Japanese medaka does not exhibit a salinity-induced α isoform switch and therefore suggests that Na+ affinity changes involve altered posttranslational modification or intermolecular interactions.