Hydromineral and Metabolic Actions of Triiodothyronine during Hypoosmotic Challenge in Air-Breathing Fish (Anabas testudineus Bloch)

The effects of triiodothyronine (T₃) on hydromineral and metabolic regulations were examined in the air-breathing fish Anabas testudineus, kept in either distilled water (DW) or freshwater (FW). Administration of T₃ (40 ng g^-1) for 24 h increased the plasma T₃ level in both DW-challenged and FW fish. An activation of thyroid axis, as evident in the rise of plasma T₄, occurred in DW-challenged fish after T₃ injection whereas plasma T₄ in FW fish classically declined. Substantial reductions in plasma glucose and plasma urea contents occurred after T₃ administration in both FW and DW-challenged fish, and that indicates a metabolic role of T₃ in hypoosmotic acclimation. An increased Na⁺, K⁺-ATPase activity occurred after T₃ injection in the gills of FW fish, whereas the enzyme activity in the gills of DW-challenged fish failed to produce that effect. On the contrary, an increased Na⁺, K⁺-ATPase activity was found in the kidneys of DW-challenged fish but its activity failed to respond to T₃ in FW fish. T₃ administration had little effect on plasma Na⁺ and K⁺ contents in both FW and DW-challenged fish. Evidences are presented that thyroid has a direct role in hypoosmotic acclimation of climbing perch. Besides confirming the hydromineral and metabolic actions of T₃, the sensitivity of thyroid axis to hyposmotic media has also been demonstrated in this air-breathing fish.