RESUMO
OBJECTIVE: The acute decrease in iodide organification in the thyroid in response to excess iodide is termed the acute Wolff-Chaikoff effect and normal organification resumes in spite of continued high plasma iodide concentrations (escape from the acute Wolff-Chaikoff effect). We have recently reported that large doses of iodide given to rats chronically decrease the sodium/iodide symporter (NIS) mRNA and protein, suggesting that escape is due to a decrease in NIS and subsequent iodide transport. We have now studied the effect of excess iodide on NIS in FRTL-5 cells to further explore the mechanisms whereby excess iodide decreases NIS. DESIGN: FRTL-5 cells were employed and were incubated in the presence or absence of various concentrations of iodide. NIS mRNA and protein and the turnover of NIS were assessed. METHODS: NIS mRNA was measured by Northern analysis, NIS protein by Western analysis and NIS turnover by pulse-chase labeling experiments. RESULTS: Iodide (10(-) mol/l) had no effect on NIS mRNA in FRTL-5 cells at 24 and 48 h compared with cells cultured in the absence of iodide. However, excess iodide decreased NIS protein by 50% of control values at 24 h and by 70% at 48 h. This effect of iodide was dose dependent. Pulse-chase experiments demonstrated that there was no effect of iodide on new NIS protein synthesis and that the turnover of NIS protein in the presence of iodide was 27% faster than in the absence of added iodide. CONCLUSIONS: Excess iodide does not decrease NIS mRNA in FRTL-5 cells but does decrease NIS protein, suggesting that in this in vitro thyroid cell model iodide modulates NIS, at least in part, at a post-transcriptional level. This iodide-induced decrease in NIS protein appears to be due, at least partially, to an increase in NIS protein turnover.
