Relationship between hepatic deiodination of thyroxine and early oocyte development in rainbow trout, Oncorhynchus mykiss

We examined the relationship between five stages of ovarian growth and deve lopment and hepatic microsomal thyroxine (T-4) deiodinating activity in rai nbow trout, Oncorhynchus mykiss, held in the laboratory at 11.5 degrees C. Thyroxine outer-ring deiodination (T4ORD) activity, which forms biologicall y active 3,5,3'-triiodothyronine (T-3), was low when the gonadosomatic inde x (GSI = (ovary mass x 100) / body mass) was below 0.2 (stage I, previtello genesis), but rose sharply between GSI values of 0.2 and 0.3 (stage II), wh en oocyte Balbiani activity, RNA synthesis, and endogenous vitellogenesis w ere maximal. Hepatic T4ORD activity was again low during exogenous vitellog enesis at GSI values of 0.3-12, which correspond to stages III-V. Hepatic T -4 inner-ring deiodination (T4IRD), which forms biologically inactive 3,3:5 '-triiodothyronine, was detected only at stage II. Plasma T-3 concentration s were significantly higher at stage II than at stage IV (primary yolk glob ule formation). Our data indicate changes in systemic thyroidal status at s tage II and are consistent with a role for T-3 in early oogenesis in trout. Enhanced hepatic T-3 production at the late Balbiani stage may be required for oocyte development.