Reverse T3 in rainbow trout, Salmo gairdneri.

Plasma levels, peripheral metabolism and extrathyroidal in vivo production of reverse T3 (rT3 = 3,3',5'-triiodo-L-thyronine) were studied in rainbow trout at 12 degrees C. Plasma rT3 levels (less than 40 pg/ml) corresponded to the detection limit of the radioimmunoassay. By in vitro analysis, a high proportion (0.95%) of rT3 added to plasma existed in the free (dialyzable) form. Injected [125I]rT3 was cleared more rapidly from plasma (minimum MCR = 7.7 ml/hr/100 g) than T3. No phenolic (outer ring) rT3 deiodination was observed. rT3 rapidly entered the liver and up to 70% of the injected [125I]rT3 was lost via the biliary route, mainly as unidentified derivatives; about 10% of the biliary-excreted 125I label was identified as rT3 or its glucuronide conjugate. Using Sephadex column chromatography combined with specific antibody separations, it was not possible to demonstrate in vivo [125I]rT3 production from [125I]T4. It is concluded that in laboratory trout, in contrast to the situation in mammals, the T4 to rT3 pathway is not prominent and that iodothyronine deiodination is restricted to T3 formation. These findings may relate to differences in extrathyroidal iodine metabolism between trout and mammals.