Ambient salinity modifies the action of triiodothyronine in the air-breathing fish Anabas testudineus Bloch: Effects on mitochondria-rich cell distribution, osmotic and metabolic regulations

The hydromineral and metabolic actions of thyroid hormone on osmotic acclimation in fish is less understood. We, therefore, studied the short-term action of triiodothyronine (T3), the potent thyroid hormone, on the distribution and the function of gill mitochondria-rich (MR) cells and on the whole body hydromineral and metabolic regulations of air-breathing fish (Anabas testudineus) adapted to either freshwater (FW) or acclimated to seawater (SA; 30 g L−1). As expected, 24 h T3 injection (100 ng g−1) elevated (P < 0.05) plasma T3 but classically reduced (P < 0.05) plasma T4. The higher Na+, K+-ATPase immunoreactivity and the varied distribution pattern of MR cells in the gills of T3-treated FW and SA fish, suggest an action of T3 on gill MR cell migration, though the density of these cells remained unchanged after T3 treatment. The ouabain–sensitive Na+, K+-ATPase activity, a measure of hydromineral competence, showed increases (P < 0.05) in the gills of both FW and SA fish after T3 administration, but inhibited (P < 0.05) in the kidney of the FW fish and not in the SA fish. Exogenous T3 reduced glucose (P < 0.05) and urea (P < 0.05) in the plasma of FW fish, whereas these metabolites were elevated (P < 0.05) in the SA fish, suggesting a modulatory effect of ambient salinity on the T3-driven metabolic actions. Our data identify gill MR cell as a target for T3 action as it promotes the spatial distribution and the osmotic function of these cells in both fresh water and in seawater. The results besides confirming the metabolic and osmotic actions of T3 in fish support the hypothesis that the differential actions of T3 may be due to the direct influence of ambient salinity, a major environmental determinant that alters the osmotic and metabolic strategies of fish.

Research highlights
► Gill mitochondria-rich (MR) cells are the target cells for triiodothyronine (T3).
► T3 promotes spatial distribution and osmotic function of MR cells in fish kept at hypo- or hyperosmotic medium.
► T3 also directs metabolic and osmotic functions of fish.
► The differential actions of T3 appears to be due to the influence of ambient salinity.