CALCITONIN-SECRETING CELLS OF THE THYROID EXPRESS AN EXTRACELLULAR CALCIUM RECEPTOR GENE

Calcitonin (CT) secretion by parafollicular cells of the thyroid (C ce lls) is regulated by small changes in the concentration of extracellul ar calcium ([Ca2+](e)). Elevation of [Ca2+](e) elicits a rise in the C cell cytoplasmic calcium concentration and stimulates CT release. The molecular entity through which C cells detect changes in [Ca2+](e) an d modulate hormone secretion is unknown. Recently, an extracellular ca lcium-sensing receptor (CaR) complementary DNA. was isolated from bovi ne parathyroid gland. To assess whether parathyroid cells and C cells use similar mechanisms to detect changes in ambient Ca2+, rat, human, and sheep C cells were examined for expression of the parathyroid CaR or a related receptor isoform. Reverse transcription-polymerase chain reaction analysis identified CaR transcripts in rat and human thyroid gland. Northern blot analysis demonstrated CaR messenger RNA. (mRNA) i n rat thyroid gland, a human medullary thyroid carcinoma (MTC) isolate , and a highly enriched preparation of sheep C cells. Rat MTC 44-2 cel ls, a cell line responsive to changes in [Ca2+](e), express abundant l evels of CaR mRNA. Human TT cells, a C cell line lacking the extracell ular calcium-sensing function, have undetectable levels of CaR mRNA by Northern blot analysis. Western blot analysis, using antiserum specif ic to the parathyroid CaR, detected CaR protein in rMTC 44-2, but not TT cells. Immunostaining of both dispersed sheep C cells and rat thyro id gland sections identified C cell-specific expression of the CaR pro tein, and in situ hybridization analysis confirmed the C cell-specific expression of CaR mRNA in the intact rat thyroid. The nucleotide sequ ence of the coding region of the rMTC 44-2 CaR transcript was found to encode the same CaR protein as that expressed in the parathyroid and kidney. The results demonstrate that C cells express the same extracel lular calcium-sensing receptor that is found in parathyroid and kidney , and the presence of this receptor protein in C cell lines correlates with the extracellular calcium-sensing function. This CaR is likely t o represent the primary molecular entity through which C cells detect changes in [Ca2+](e) and control CT release, suggesting that activatio n of the same receptor can either stimulate or inhibit hormone secreti on in different cell types.