Effect of experimental 3,5,3 '-triiodothyronine hyperthyroidism on thyroidhormone deiodination in brain regions and liver of rainbow trout, Oncorhynchus mykiss

We studied the effect of 3,5,3'-triiodothyronine (T-3) hyperthyroidism, ind uced by 12 ppm T-3 in food for 10 days, on the low-K-m activities of thyrox ine (T-4) outer-ring deiodination (ORD) to form T-3, T-4 inner-ring deiodin ation (IRD) to form 3,3',5'-triiodothyronine (reverse T-3 (rT(3))), T3ORD t o form 3,5-diiodothyronine (3,5-T-2), and T3IRD to form 3,3'-diiodothyronin e (3,3'- T-2) in six brain regions and in liver of immature rainbow trout ( Oncorhynchus mykiss) at 12C. Throughout the brain, T4ORD activity of contro l trout was uniformly low and T3ORD activity was negligible. T4IRD and T3IR D activities were about 5-fold and 50-fold greater, respectively, than T4OR D activity and were higher in the optic lobes, hypothalamus, and telencepha lon/olfactory bulbs than in the medulla or cerebellum. T-3 treatment double d the plasma T-3 level with no change in plasma T-4 level andreduced T4ORD and T4IRD activities in all brain regions but did not alter T3IRD activity or the negligible T(3)ORDactivity. Relative to controls, T-3 treatment redu ced liver T4ORD activity 6-fold, increased T4IRD activity 8-fold, and incre ased T3IRD activity 12-fold. We conclude that (i) there are regional differ ences in trout brain T4IRD and T3IRD activitiesbut not in T4ORD activity, i ndicating spatial variation in brain T-4 and T-3 catabolism, (ii) in respon se to a mild T-3 challenge the brain deiodination pathways do not undergo t he same autoregulatory adjustments as those in liver, and (iii) a T-3 chall enge reduces brain T4IRD activity with no change in T3IRD activity, which s uggests that the two IRDs may be controlled by separate deiodinases.