Immunoreactive calcitonin in the rat anterior pituitary gland and its localization in thyrotrophs

The distribution of those cells in the anterior pituitary gland of the rat which stain immunohistochemically with rabbit anti-human calcitonin serum has been examined. Immunoreactive cells were confined primarily to the ventral surface of the gland and possessed both a distribution and morphology distinct from the corticotrophs. Staining of serial thin sections with rabbit anti-rat TSH-subunit serum resulted in an immunoreaction in those cells that stained for calcitonin. However, not all the thyrotrophs gave a positive immunoreaction for calcitonin. It is concluded from this study that it is inappropriate to attribute calcitonin as being part of the 31K-dalton precursor for adrenocorticotropin, a hypothesis that was proposed earlier. In the immunohistochemical reaction with anti-calcitonin serum, it was found that relatively high concentrations of antigen (500 micrograms/ml) were required in absorption experiments in order to inhibit staining. Furthermore, the staining of thyrotrophs with the anti-calcitonin serum was inhibited after preadsorption of the primary antiserum with excess rat beta-TSH (1000 micrograms/ml). Because of these immunochemical characteristics, it is questionable whether the calcitonin-like material observed in the rat pituitary gland is chemically identical to that of thyroidal calcitonin.