Nuclear triiodothyronine binding sites in rat adipocytes

Most effects of thyroid hormones appear to be mediated by the binding of triiodothyronine (T3) to nuclear triiodothyronine binding sites (NT3BS). Although thyroid hormones influence adipocyte metabolism, NT3BS have not been described in mature adipocytes yet. This report describes T3 nuclear binding in isolated nuclei from rat epididymal fat pad adipocytes. Nuclei were isolated by exposing collagenase-dispersed adipocytes to STM (sucrose, 0.25 mol/L; TRIS, 20 mmol/L; MgCl2, 1.1 mmol/L, pH 7.85) containing 0.5% (vol/vol) Triton X-100. Incubation of nuclei suspended in STM/EDTA (2 mmol/L)/DTT (5 mmol/L) with 125I-T3 and varying concentrations of unlabeled T3 at 37 degrees C for one hour revealed the presence of high-affinity, low-capacity NT3BS. Their MBC was 0.39 +/- 0.04 (SD) ng of T3 per milligram of DNA and their Kd was 1.4 +/- 0.5 (SD) X 10(-10) mol/L T3. Specific binding reached a plateau between 30 minutes and two hours of incubation. The addition of 5 X 10(-7) mol/L T3 to nuclei incubated for one hour with 2 X 10(-11) mol/L T3 completely displaced the specifically bound 125I-T3 within 30 minutes. Thyroxine (T4) and 3, 3', 5'-triiodothyronine (rT3) could displace 125I-T3 from the NT3BS but were less than 10% and 1% as effective, respectively, as T3. Rat epididymal fat pad adipocytes contain NT3BS, the binding characteristics of which are similar to those of rat hepatic NT3BS.