TRIIODOTHYRONINE IS REQUIRED FOR THE ACCLIMATION TO SEAWATER OF THE BROWN TROUT (SALMO-TRUTTA) AND RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

Brown and rainbow trout, held in freshwater at 13 +/- 1-degrees, were injected, every 3 days, with iopanoic acid (IOP: 5 mg/100 g body wt), an inhibitor of deiodination of thyroxine (T4) to triiodothyronine (T3). One group of IOP-treated rainbow, trout was immersed in T3 (20-mu-g/l water). In IOP trout, plasma T3 fell to very low levels by day 7, while changes in T4 levels were less marked. In IOP + T3 trout plasma T3 increased fivefold, plasma T4 being unchanged. No mortality occurred and plasma osmolarity (OP) was not altered by, any treatment. After direct transfer to seawater (30 parts per thousand), IOP trout were unable to acclimate to salinity: all died within 2 or 3 days, while the survival at day 3 was 100% in control brown trout and 45 and 74% in control and IOP + T3 rainbow trout respectively. OP increased more in IOP and less in IOP + T3 than in controls. There was a significant inverse correlation between T3, but not T4, plasma level, at the time of transfer and the OP 1 day later. In conclusion, although T3 does not play a significant role in osmoregulation in freshwater, T3 and therefore the deiodination of T4 into T3, were required for the development of hypoosmoregulatory capacity involved in acclimation of trout to seawater.