Studies of the Binding of Thyroid Hormones and Their Short Loop Feedback Effects in Bovine Thyroid Tissue*

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Abstract
In vitro studies in bovine thyroid tissue have been conducted to explore the postulated short loop negative feedback of thyroid hormones on several thyroid functions and their responses to TSH. Initial experiments in thyroid plasma membranes provided no evidence that T4 or T3 in concentrations as high as 5 × 105 M and 4 × 105 M, respectively, could displace [125I]iodobovine TSH (bTSH) from its specific binding sites within the membranes. In addition, as judged from the lack of variation in the percentage binding of [125I]T3 and [125I]T4 to membranes over a wide range of T3 and T4 concentrations, no evidence of specific or saturable binding sites for either of the two hormones could be demonstrated up to concentrations of 2 X 105 M. Preincubation of thyroid slices for 3 h with T3 at concentrations of 105 and 10“4 M failed to alter basal cAMP concentrations or the pronounced stimulation of cAMP concentrationsproduced by incubation with bTSH. Basal and bTSHstimulated rates of generation of 14CO2 from uniformly labeled [l4C]glucose were similarly unaffected by preincubation of thyroid slices with 105 or 10“” M T3 for 2 h. Neither basal nor bTSH-stimulated colloid droplet counts was altered by preincubation of slices for 1 h with 105 M T3, but the response of colloid droplet counts to bTSH was significantly decreased by T3 at a concentration of 104 M

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