Using a previously reported in vivo isotopic displacement technique, we have demonstrated limited nuclear binding sites for L-triiodothyronine (T3) in the following rat tissues: liver, kidney, heart, anterior pituitary, brain, spleen and testis. The concentration of these sites expressed both per mg DNA and per g tissue (wet wt) varied widely. If liver is normalized to 1, the relative binding capacity per mg DNA is: pituitary, 1.3; liver, 1.0; kidney, 0.87; heart, 0.65; brain, 0.44; spleen, 0.03; testis, 0.004. The relative binding capacity per g tissue is: pituitary, 3.7; kidney, 1.5; liver, 1.0; heart, 0.45; brain, 0.24; spleen, 0.18; and testis, 0.01. Approximately 40–50% of available sites are saturated at endogenous levels of circulating T3 and the calculated association constants of nuclear binding appear similar in the various tissues studied. The relatively low concentration of binding sites in brain, spleen, and testis is of interest since these tissues do not respond to thyroid hormone with the expected increase in oxygen consumption and in the level of mitochondrial alphaglycerophosphate dehydrogenase. A high proportion (53%) of cellular T3 in the anterior pituitary is associated with specific nuclear sites and accounts for our previous demonstration of limited capacity sites in unfractionated pituitary.
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