Triiodothyronine Nuclear Binding in Fetal and Adult Rabbit Lung and Cultured Lung Cells*

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Abstract
To investigate the possible mechanism of thyroid hormone action in the lung, fetal and adult rabbit lung, and cell lines derived from lung, were examined for specific nuclear binding sites for T3 [triiodothyronine]. Using incubation of isolated nuclei with L-[125I]T3 at 37.degree. C, approximately 2400 specific binding sites/cell were found in fetal lung and 1120 sites/cell in adult lung, with a similar dissociation constant ($500 pM) for both tissues. The L-2 [rat lung clone] and A549 [human alveolar cell carcinoma] cell lines, which may have originated from pulmonary type II alveolar cells, contained 2280 and 1580 nuclear sites/cell, and the dissociation constants were 280 and 200 pM respectively. In fetal lung, the ability of various analogs to compete for L-T3 (100%) binding was: 3,5-diiodo-3''-isopropylthyronine, 81%; D-T3, 73%; L-T4, 6.7%; 3,3''-diiodothyronine, 0.19%; 3,5-dimethyl-3''-isopropyl-L-thyronine, 0.15%; and r[reverse]T3, 0.08%. Both fetal and adult lung and cultured lung cells contain specific nuclear binding sites for T3, suggesting that these tissues are their type II alveolar cells may be directly influenced by thyroid hormones.