Triiodothyronine Receptors in Rat Lung*

Specific triiodothyronine receptor activity has been detected in rat lung nuclei isolated by Triton X-100, which was sensitive to pH and exhibited temperature-dependent association kinetics. This activity represented a single order of high affinity binding sites, with an apparent dissociation constant for 3,3',5-triiodo-L-thyronine (L-T3) of 0.254 +/- 0.027 nM and an apparent capacity of 249.4 +/- 15.5 fmol 3,3',5-triiodo-L-thyronine/mg DNA. Its specificity was such that triiodothyronine derivatives were bound at least 10 times more avidly than were the tetraiodothyronines. The presence of specific triiodothyronine receptors in lung nuclei suggests that the effect of thyroid hormones on pulmonary surfactant metabolism may be mediated through these binding sites.