Trafficking of L-triiodothyronine between ovarian fluid and oocytes of rainbow trout (Oncorhynchus mykiss)

The study examines the dynamics of thyroid hormone (TH) trafficking between rainbow trout (Oncorhynchus mykiss) oocytes and ovarian fluid (OF) to explore the processes involved in the transfer of hormone to the oocytes. We also examined the effects of enhancing oocyte T(3) content and subsequent embryo survival. Oocytes incubated in OF alone had significant losses of THs within 12 h, whereas the T(3) content of oocytes retained in T(3)-enriched OF (10 and 100 microg ml(-1)) was significantly elevated in a dose-dependant manner within 3 h. When transferred to non-supplemented OF, the T(3) content of the 10 micro ml(-1) treatment group decreased significantly within 24 h with a concomitant significant increase in OF T(3) concentration. Although there was no significant change in the 100 microg ml(-1) treatment group the significant increase in the OF T(3) concentration was evidence for marked T(3) efflux during this period. These findings provide evidence for the independent trafficking of T(3) based on concentration gradients across the oocyte cell membrane, and suggest that it is not vitellogenin-dependent. Fertilization of in ovo T(3)-supplemented oocytes resulted in a small, albeit significant, increase in mortality rate, but there was no significant effect of treatment on embryo growth rates up to the hatching stage of development.