The amount and sources of T3 associated with high affinity, low capacity cellular nuclear receptors in brown adipose tissue (BAT) have been estimated by in vivo pulse-labeling techniques. Maximal binding capacity was measured by in vivo saturation analysis. Nuclear receptor occupancy at endogenous levels of T3 and T4 in euthyroid rats was estimated from the equilibrium nuclear to serum ratio of tracer T3, and the locally generated nuclear T3 to serum T4 ratio after injecting tracer T3 and T4. These ratios were multiplied, respectively, by the endogenous concentrations of T3 and T4 as measured by RIA. The maximal binding capacity was 0.65 ng T3/mg DNA, and saturation was 71%. Fifty-five percent of the nuclear T3 was generated locally, and 45% was derived from circulating T3. BAT is, hence, comparable to the liver in number of receptors (~5000/cell) and to the pituitary with regard to saturation and relative contributions of locally generated t3 and plasma T3 to nuclear T3. These results suggest that BAT may be an important target for thyroid hormones and, along with other data, that alterations in the activity of the type II 5'-deiodinase of this tissue may influence the saturation of nuclear T3 receptors.
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