LONG-TERM IODINATION OF THYROGLOBULIN BY PORCINE THYROID-CELLS CULTURED IN POROUS-BOTTOMED CULTURE CHAMBERS - REGULATION BY THYROTROPIN

Thyroid cells cultured as monolayers on the porous bottom of culture chambers have been shown to express some specific functions of thyroid follicles. This system, which allows independent access to apical and basal media, is suitable for the long-term study of polarized processes, as the cells maintain their polarized organization. Iodination of thyroglobulin has been investigated under different culture conditions in the presence or absence of TSH. Apical thyroglobulin accumulation, apical iodide concentration and thyroglobulin iodination have been followed simultaneously. Iodide (0.5 mu-mol/1) was added to basal medium at various stages: only once for 4-day incubations and at each medium change or daily for longer experiments. TSH increased the amount of thyroglobulin secreted into the apical medium by five- to sixfold, whereas high basal iodide concentrations (> 5 mu-mol/1) inhibited thyroglobulin secretion by TSH-stimulated cells. TSH increased iodide uptake giving an iodide concentration ratio between apical and basal media of about 5. Thyroglobulin iodination was dependent upon TSH. Thyroglobulin was iodinated only in the apical compartment. Secretion and iodination of thyroglobulin were polarized phenomena, but the polarity of iodination was total whereas the polarity of secretion was only partial (10% basal secretion). This functional asymmetry was maintained for up to 29 days. The maximal incorporation of iodine into thyroglobulin obtained was never higher than 3.5 atoms/mol. Apical iodide concentrations from 1 to 15 mu-mol/1, depending on culture conditions, did not increase this value. These results suggest that cells cultured in this culture system are able to reproduce several steps of thyroidal iodide metabolism although there may be unknown factors which could interfere and reduce the efficiency of thyroglobulin iodination.