Evidence of specific nuclear binding sites for T3 in the mouse cultured fibroblast

Mouse L 929 cultured fibroblasts revealed saturable and high affinity nuclear receptors for triiodothyronine. 125I-T3 bound rapidly to intact cells, with a steady state achieved between 1-2 h at 37 C. By Scatchard estimation, the maximal binding capacity averaged 18 fentomoles of T3 bound per 100 microgram of DNA, which approximated to 2,000 sites per cell nucleus; the apparent equilibrium association constant Ka averaged 4.90 x 109 LM-1. The relative affinity of T3 analogs tested correlated with their respective thyromimetic potency. Limited capacity, high affinity and specificity of T3 nuclear receptors in these fibroblasts were found to have properties similar to those previously observed in other cell lines or tissues. Such cultured fibroblasts may provide a useful experimental model to investigate the intracellular biological effects of thyroid hormones.