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

Abstract

We examined the relationship between five stages of ovarian growth and development and hepatic microsomal thyroxine (T4) deiodinating activity in rainbow trout, Oncorhynchus mykiss, held in the laboratory at 11.5°C. Thyroxine outer-ring deiodination (T4ORD) activity, which forms biologically active 3,5,3'-triiodothyronine (T3), was low when the gonadosomatic index (GSI = (ovary mass X 100) / body mass) was below 0.2 (stage I, previtellogenesis), but rose sharply between GSI values of 0.2 and 0.3 (stage II), when oocyte Balbiani activity, RNA synthesis, and endogenous vitellogenesis were maximal. Hepatic T4ORD activity was again low during exogenous vitellogenesis at GSI values of 0.3-12, which correspond to stages III-V. Hepatic T4 inner-ring deiodination (T4IRD), which forms biologically inactive 3,3',5'-triiodothyronine, was detected only at stage II. Plasma T3 concentrations were significantly higher at stage II than at stage IV (primary yolk globule formation). Our data indicate changes in systemic thyroidal status at stage II and are consistent with a role for T3 in early oogenesis in trout. Enhanced hepatic T3 production at the late Balbiani stage may be required for oocyte development.