Sodium/iodide-symporter: distribution in different mammals and role in entero-thyroid circulation of iodide

The sodium (Na+ )/iodide (I- )-symporter (NIS) is abundantly expressed and accumulates iodide in thyroid follicular cells. The NIS is also found in extrathyroidal tissues, particularly gastric mucosa. Controversies exist on the localization of extrathyroidal NIS. We have studied the presence of both NIS peptide and NIS messenger RNA (mRNA) in the digestive tract and thyroid from different mammals. The role of gastric NIS is enigmatic and we aimed to unravel its possible involvement in iodide transport. Methods: Distribution and expression of NIS were studied using immunocytochemistry and in situ hybridization. Iodide transport in the gastrointestinal tract was measured after oral or intravenous (i.v.) administration of (125) I to rats with or without ligation of the pylorus. Results: All thyroid follicular cells in rat and mouse expressed NIS, whereas a patchy staining was noted in man, pig and guinea-pig. Gastric mucosa surface epithelium in all species and ductal cells of parotid gland in guinea-pig, rat and mouse expressed NIS. In parietal cells and in endocrine cells of intestines and pancreas NIS immunoreactivity but no NIS mRNA was found. Studies of (125) I uptake showed marked iodide transport from the circulation into the gastric lumen. Conclusions: The localization of NIS varies slightly among mammals. To establish expression of NIS in a particular cell type the need to correlate the presence of both NIS protein by immunocytochemistry and NIS mRNA by in situ hybridization is emphasized. An entero-thyroidal circulation of iodide mediated principally by gastric NIS, but possibly also by NIS in salivary glands is suggested.