Tissue differences in the concentration of triiodothyronine nuclear binding sites in the rat: liver, kidney, pituitary, heart, brain, spleen, and testis.

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 ...