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) alpha 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 alpha isoforms (alpha 1a, alpha 1b, alpha 1c, alpha 2, alpha 3a and alpha 3b). Medaka acclimated long-term (> 30 days) to either FW or SW had similar gill expression of alpha 1c, alpha 2, alpha 3a and alpha 3b, while both alpha 1a and alpha 1b 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 alpha 1b and alpha 1a after 1 and 3 days. SW to FW acclimation reduced alpha 1b after 3 days with no other alpha 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 V (max) and lower apparent K (m) 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 alpha isoform switch and therefore suggests that Na+ affinity changes involve altered posttranslational modification or intermolecular interactions.