Effects of short‐term 17β‐estradiol treatment on the properties of t3‐binding proteins in the plasma of immature rainbow trout, salmo gairdneri

Abstract

AbstractTo determine the effects of 17β‐estradiol (E2) on theproperties of the plasma proteins that bind 3,5,3′‐triiodo‐L‐thyronine (T3), immature rainbow trout, Salmo gairdneri, were intraperitoneally injected on days 0 and 3 with 0.5 mg E2‐3‐benzoate/100 g body weight, and plasma was sampled on days 4, 7, or 12. Control trout received peanut oil alone. E2 increased significantly the proportion of [125I]T3 bound to plasma‐binding sites. Separation of plasma proteins of control trout by agarose filtration on Bio‐Gel A‐1.5 m revealed that T3 bound to two main sites corresponding to molecular weights of 130 kDa (presumed prealbumin) and 55 kDa (presumed β‐globulin). Addition of a calculated amount of unlabeled T3 displaced [125I]T3 from the 55 kDa site, indicating its role as the T3 high‐affinity and low‐capacity site. E2 treatment initiated production of vitellogenin, which did not bind T3. Saturation analysis of [125I]T3 binding conducted on miniature G‐25 Sephadex columns indicated that E2 increased the capacity of the low‐affinity, high‐capacity T3‐binding site without effect on the high‐affinity, low‐capacity T3‐binding site. In conclusion, T3 binds to two main plasma sites and E2 causes an increase in the capacity of the low‐affinity site, resulting in a decrease in free T3 and in possible changes in T3 availability to tissues.