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

Abstract

To observe a direct effect of 17β-estradiol (E 2) on thyroid activity, thyroid follicles were isolated from hypobranchial muscles of a freshwater murrel, Channa gachua. Thyroid follicles were incubated (5 × 10 6 follicles/well) in vitro at 30° for 2 hr without hormone and then 3 hr with E 2 or bovine thyroid-stimulating hormone (bTSH). Addition of 10 and 100 ng of E 2 to thyroid follicles resulted in 2- and 3-fold increases in thyroxine (T 4) release. When 100 ng of bTSH was added to check the isolated follicular function, it stimulated T 4 release to more than 3-fold. Increasing doses of E 2 from 1 to 100 ng caused a dose-dependent stimulation of T 4 release, while a 1000-ng dose reduced T 4 release compared to 100 ng. When thyroid follicles of this experiment were lysed by sonication and T 4 content was determined, it corroborated the profile of T 4 release in response to varied E 2 doses. E 2 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 E 2.