Possible role of T4 binding to cytoplasmic fraction of pituitary cells on the control of the TSH response to TRH in the rat

Abstract

It has previously been shown that thyroxine (T4) can inhibit the thyrotropin (TSH) response to TSH-releasing hormone (TRH) before a significant intrapituitary conversion of T4 to triiodothyronic (T3) had occurred in eutryroid rats. The present study has investigated the in vivo binding of T4 and T3 to cytoplasm and nucleus of pituitary cells of hypothyroid rats. The results indicate that 125I-T4 was specifically bound to cytoplasms and non-specifically-bound to nuclei. On the other hand, nuclear binding of 125I-T3 was specific whereas cytoplasmic binding was non-specific. The data suggest that the inhibition of TRH stimulation of THS by T4 previously reported may involve the binding of T4 to extranuclear cell components, perphaps those located in the cytoplasmic fractions.