Binding of triiodothyronine by fully differentiated rat enterocytes.

The cellular binding characteristics of triiodothyronine (T3) have been measured in a preparation of isolated fully differentiated enterocytes prepared from thyroidectomized rats. Specific binding of [125I]T3, defined as that portion displaced by the presence of 20 microM-non-radioactive hormone, was shown to become maximal after 100 min incubation with cells at a temperature of 15 degrees C. The cellular release of lactic dehydrogenase measured under these conditions was found to be 18%. Thin layer chromatography of radioactive compounds recovered from enterocytes and from incubation medium showed less than 2% radioactivity to be as free iodide. The amount of [125I]T3 specifically bound to enterocytes showed a curvilinear dependence on the concentration of T3 present in the incubation medium. Scatchard plot analysis of this data revealed the presence of both high (KD 1.2 +/- 1.1 X 10(-9) M) and low (KD 1.9 +/- 0.8 X 10(-7) M) affinity binding sites in these cells. Previous injection of non-radioactive T3 into thyroidectomized rats, at a time chosen to ensure negligible levels of circulating T3 at the time of the experiment, caused a 3-fold reduction in the maximal binding capacity of the low-affinity site (83 compared with 28 pmol/mg protein). Neither the affinity nor the number of high-affinity binding sites for T3 were significantly affected by this treatment. These results are compared with those obtained using other types of cell responsive to T3. The possible physiological importance of these findings is discussed.