Effect of T-3 treatment and food ration on hepatic deiodination and conjugation of thyroid hormones in rainbow trout, Oncorhynchus mykiss

We studied the 7-day effects of 3,5,3'-triiodothyronine (T-3) hyperthyroidi sm (induced by 12 ppm T-3 in food) and food ration (0, 0.5, or 2% body weig ht/day) on in vitro hepatic glucuronidation, sulfation, and deiodination of thyroxine (T-4), T-3, and 3,3',5'-triiodothyronine (rT(3)). T-3 treatment doubled plasma T-3 with no change in plasma T-4, depressed hepatic low-K-m (1 mM) outer-ring deiodination (ORD) of T-4, induced low-K-m (1 nM) inner-r ing deiodination (IRD) of both T-4 and T-3 but did not alter high-K-m (1 mu M) rT(3)ORD, glucuronidation, or sulfation of T-4, T-3, or rT(3). Plasma T -4 let els were greater for 0 and 2% rations than for a 0.5% ration. Fastin g decreased low-K-m T4ORD activity and increased high-K-m rT(3)ORD activity but did not alter T4IRD or T3IRD activities. T-4, T-3, and rT(3) glucuroni dation were greater for 0 and 0.5% rations than for a 2% ration. T-3 glucur onidation was greater for a 0.5% ration than for a 0% ration. T-3 and rT(3) Sulfation were greater for a 2% ration than for a 0 or a 0.5% ration; rati on did not change T-4 sulfation. We conclude that (i) modest experimental T -3 hyperthyroidism induces T-3 autoregulation by adjusting hepatic low-K-m ORD and IRD activities but not high-K-m rT(3)ORD or conjugation activities; (ii) in contrast, ration level changes both deiodination and conjugation p athways, suggesting that the response to ration does not solely reflect alt ered T-3 production; (iii) deiodination and conjugation appear complementar y in regulating thyroidal status in response to ration; and (iv) high-K-m r T(3)ORD in trout differs from rat type I deiodination in that it does not r espond to T-3 hyperthyroidism and it increases, rather than decreases, its activity during fasting. (C) 1999 Academic Press.