17β-Estradiol releases thyroxine from the thyroid follicles of a teleost, Channa gachua (Ham.)

Abstract

To observe a direct effect of 17β-estradiol (E₂) on thyroid activity, thyroid follicles were isolated from hypobranchial muscles of a freshwater murrel, Channa gachua. Thyroid follicles were incubated (5 × 10⁶ follicles/well) in vitro at 30ο for 2 hr without hormone and then 3 hr with E₂ or bovine thyroid-stimulating hormone (bTSH). Addition of 10 and 100 ng of E₂ to thyroid follicles resulted in 2- and 3-fold increases in thyroxine (T₄) release. When 100 ng of bTSH was added to check the isolated follicular function, it stimulated T₄ release to more than 3-fold. Increasing doses of E₂ from 1 to 100 ng caused a dose-dependent stimulation of T₄ release, while a 1000-ng dose reduced T₄ release compared to 100 ng. When thyroid follicles of this experiment were lysed by sonication and T₄ content was determined, it corroborated the profile of T₄ release in response to varied E₂ doses. E₂ was ineffective in increasing ¹²⁵I uptake by the follicles, while bTSH elevated it by 45% over the control. Incubation of varied concentrations of [³H]estradiol with cytosol and nuclear fractions of thyroid follicles in the presence of a 100-fold excess of diethylstilbestrol showed saturable specific binding of E₂.