Response of rainbow trout gill (Na++K+)-ATPase and chloride cells to T3 and NaCl administration

With the aim of comparing the effects of oral T₃ and NaCl administration on trout hypoosmoregulatory mechanisms, three groups of rainbow trout (Oncorhynchus mykiss Walbaum) held in freshwater (FW) were fed a basal diet (C), the same diet containing 8.83 ppm of 3,5,3-triiodo-L-thyronine (T₃) (T) or 10% (w/w) NaCl (N) respectively for 30 d. They were then transferred to brackish water (BW) for 22 d and fed on diet C. Gill (Na⁺+K⁺)-ATPase activity and its dependence on ATP, Na⁺ and pH, number of gill chloride cells (CC), serum T₃ level as well as fish growth, condition factor (K) and mortality were evaluated. During the FW phase, as compared to C trout, T trout showed a two fold higher serum T₃ level, had unchanged gill (Na⁺+K⁺)-ATPase activity and increased CC number, whereas N trout showed higher gill (Na⁺+K⁺)-ATPase activity and CC number. At the end of the experiment the enzyme activity was in the order T>N>C groups and all groups showed similar CC number. Both treatments changed the enzyme activation kinetics by ATP and Na⁺. A transient increase in K value occurred in N group during the period of salt administration. In BW, T and N groups had higher and lower survival than C group respectively. Other parameters were unaffected by the treatments. This trial suggests that T₃ administration promotes the development of hypoosmoregulatory mechanisms of trout but it leaves the (Na⁺+K⁺)-ATPase activity unaltered till the transfer to a hyperosmotic environment.