Vitellogenin Association and Oocytic Accumulation of Thyroxine and 3,5,3′-Triiodothyronine in Gravid Fundulus heteroclitus

Abstract

The association of the thyroid hormone (TH) thyroxine (T4) and its metabolically active metabolite 3,5,3′-triiodothyronine (T3) with serum vitellogenin (VTG) in gravid female and estrogenized male (E2+) Fundulus heteroclitus was investigated. In in vivo, time-course experiments, sera from gravid female fish exposed to [125I]T4 showed time- and dose-dependent increases in total [125I]TH content. The [125I]T4:[125I]T3 ratio was also affected by dose and time. In analysis of sera from female fish, >80% of detected radioactivity was associated with VTG (approximately 35%) and a second chromatographic peak (approximately 45%), a lipoprotein fraction possibly consisting of high-density lipoproteins. In experiments comparing estrogenized versus control male fish, the presence of VTG significantly increased the overall quantity and altered the profile of serum protein-associated [125I]TH. When serum VTG was present in the very large quantities typical of male fish treated with high doses of E2, the majority (59–70%) of detected radioactivity was associated with VTG. Both [125I]T4 and [125I]T3 were detected in extracts from oocytes collected during the in vivo female study. The total TH content and [125I]T4:[125I]T3 ratios in these extracts presented an accumulation profile that mirrored, in a delayed manner, the profiles observed in sera data. Furthermore, this accumulation was related to oocyte maturational state (i.e., size) and, correspondingly, VTG uptake. Together, these data suggest an important role for VTG as a vector of maternal transfer for both T4 and T3.