A comparison of thyrotropin- and dibutyryl cyclic AMP- induced thyroglobulin iodination in cultured human thyroid cells.

The relative degree of 125-I labelling of thyroglobulin-- bound mono-iodotyrosine (MIT) and di-iodotyrosine (DIT) in isolated, cultured human thyroid cells has been compared following exposure of 125-I supplemented cells to 100 mU/ml of bovine thyrotropin (TSH) or 1.0 mM dibutyryl cyclic AMP (dBcAMP) for 96 hours. 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 as well as in controls. Levels of [125-I]DIT, however, were only enhanced above control values in cells incubated with TSH. These findings suggest that 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 dibutyryl cyclic AMP on the levels of newly--synthesised T4 recovered from the cells used in this study, and from the culture medium in a previous investigation.