Triiodothyronine is necessary for the action of growth hormone in acclimation to seawater of brown (Salmo trutta) and rainbow trout (Oncorhynchus mykiss).

Brown (BT) and rainbow trout (RT) in freshwater (FW) were treated with ovine growth hormone (GH), GH + iopanoic acid (IOP), and GH + IOP plus triiodothyronine (T3) for RT only. After 1 week of treatment, trout were transferred to 30 o/oo SW and treatment continued. In FW, GH treatment increased significantly plasma T3 level (BT) and T3/T4 ratio (BT and RT) by stimulating T4 to T3 deiodination. In the GH + IOP group, the plasma T3 levels and T3/T4 ratio fell significantly as T4 to T3 deiodination was inhibited. In GH + IOP + T3-treated RT, plasma T3 and T3/T4 ratios increased significantly relative to other groups. No mortality occurred and plasma osmolarity (PO) was not altered by any treatment in FW. After transfer to SW, all IOP + GH trout died within 2 (BT) or 3 days (RT). All GH-treated or control BT survived to the end of the experiment (6 days). RT survival rates tended to be improved in GH and GH + IOP + T3 groups relative to controls. Correlatively on day 1 the PO increase was significantly higher in IOP + GH groups (BT and RT) than in the other groups and significantly lower in GH and GH + IOP + T3 treated RT than in controls from days 1 to 6. These data confirm the requirement of T3 and deiodination of T4 to T3 for the development of hypoosmoregulatory mechanisms in SW as previously shown (Lebel and Leloup 1992). Furthermore, the suppression of the hypoosmoregulatory effect of GH, when conversion of T4 to T3 was inhibited by IOP and the reversal when T3 was added to IOP + GH treatment suggests that GH osmoregulatory action in SW acts via the simulation of T4-5' monodeiodination which increases T3 production.