Reverse T 3 in rainbow trout, Salmo gairdneri

Abstract

Plasma levels, peripheral metabolism and extrathyroidal in vivo production of reverse T 3 (rT 3 = 3,3′,5′-triiodo- l-thyronine) were studied in rainbow trout at 12°C. Plasma rT 3 levels (< 40 pg/ml) corresponded to the detection limit of the radioimmunoassay. By in vitro analysis, a high proportion (0.95%) of rT 3 added to plasma existed in the free (dialyzable) form. Injected [ 125I]rT 3 was cleared more rapidly from plasma (minimum MCR = 7.7 ml/hr/100 g) than T 3. No phenolic (outer ring) rT 3 deiodination was observed. rT 3 rapidly entered the liver and up to 70% of the injected [ 115I]rT 3 was lost via the biliary route, mainly as unidentified derivatives; about 10% of the biliary-excreted 125I label was identified as rT 3 or its glucuronide conjugate. Using Sephadex column chromatography combined with specific antibody separations, it was not possible to demonstrate in vivo [ 1231]rT 3 production from [ 125I]T 4. It is concluded that in laboratory trout, in contrast to the situation in mammals, the T 4 to rT 3 pathway is not prominent and that iodothyronine deiodination is restricted to T 3 formation. These findings may relate to differences in extrathyroidal iodine metabolism between trout and mammals.