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Studies of the binding of thyroid hormones and their short loop feedback effects in bovine thyroid tissue.

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 uniform...