COMPARISON OF THYROTROPIN-INDUCED AND DIBUTYRYL CYCLIC AMP-INDUCED THYROGLOBULIN IODINATION IN CULTURED HUMAN THYROID-CELLS

The relative degree of 125-I labeling of thyroglobulin-bound monoiodotyrosine (MIT) and diiodotyrosine (DIT) in isolated, cultured human thyroid cells was compared following exposure of 125-I supplemented cells to 100 mU[milliunits]/ml of bovine thyrotropin (TSH) or 1.0 mM dibutyryl cyclic AMP (dBcAMP) for 96 h. Pronase digestion of the lysed cells and Sephadex G-10 fractionation of the digested lysates revealed a predominance of [125-I]MIT over [125-I]DIT in both sets of experimental cells and in controls. Levels of [125-I]DIT were only enhanced above control values in cells incubated with TSH. An increase in availability of intracellular iodide, following cellular exposure to TSH, may facilitate a preferential synthesis of DIT relative to that of MIT. This theory offers an explanation for the differential effects of TSH and dBcAMP on the levels of newly-synthesized T4 [Thyroxine] recovered from the cells used in this study and from the culture medium in a previous investigation.