Characterization and localization of calcitonin messenger ribonucleic acid in rat thyroid

DNA/RNA hybridization assays have been used to examine calcitonin (CT) RNA production in normal rat thyroids. A cloned CT cDNA which codes for the entire rat CT precursor was radiolabeled to a high specific activity and used in hybridization assays to explore 1) the sizes and relative quantities of CT RNA extractable from thyroids obtained from rats of differing ages; 2) the effect of calcium on the in vitro production of CT RNA in rat thyroid tissue slices; and 3) the localization, by hybridization histochemistry, of C cells in rat thyroid that contain CT RNA. The relative concentrations of CT RNA, per microgram of total thyroid RNA, increased remarkably with age, with 14-month-old rats having approximately 14-fold elevated concentrations of thyroidal CT RNA compared to 19-day-old rat fetuses. Of interest was the finding that a second larger species of CT RNA is only evident in thyroids obtained from 14-month-old animals. The effect of calcium on the in vitro production of CT RNA in rat thyroid tissues was studied over 3- and 6-h periods. Although previous investigations have shown that calcium causes an immediate and linear increase in CT secretion from the thyroid gland, no net increase vs. controls in the amount of CT RNA extractable from calcium-stimulated thyroid slices was observed. Finally, hybridization histochemistry, a technique that identifies in fixed tissue sections those areas that contain a specific mRNA population, was used to localize C cells in the thyroid containing CT RNA. Specific areas of rat thyroid hybridized with the CT cDNA probe and autoradiography revealed these areas to be parafollicular cells located only in the central portion of the thyroid lobes, mRNA quantities detected by hybridization histochemistry showed little variation over the central area of the thyroid, indicating the C cells in this region of the thyroid are accumulating CT RNA at approximately the same rate.