Immunoreactive Pit-1 protein in hyperplastic pars intermedia induced by calcitonin of the rat pituitary gland

To elucidate the effects of synthetic salmon calcitonin (sCT) on the cells in the rat pituitary gland, we histopathologically and immunohistochemicall y examined the early changes after 4 or 13 weeks treatment with sCT 120 IU/ kg. Focal proliferative lesions of the anterior pituitary glands were consi stently found after treatment with sCT for 13 weeks. Histologically, the ce lls with the focal proliferative lesions were classified into the following three groups: 1) enlarged basophilic cell focus, 2) vacuolated cell focus and 3) chromophobe cell focus. These focal proliferative lesions had positi ve staining only for the alpha-subunit and failed to show Pit-1 protein imm unoreactivity. The sCT treatment also increased the thickness of the pars i ntermedia. Hypertrophy of the pars intermediate cells was characteristicall y seen. Furthermore, Pit-1 protein immunoreactivity was clearly detected in the nuclei of the hyperplastic pars intermediate cells. All pars intermedi ate cells were equally stained by alpha- or beta-MSH and beta-endorphin in both vehicle- and sCT-treatment. No difference was seen. These findings str ongly suggest a very close relationship between Pit-1 protein immunoreactiv ity and cellular proliferation induced by sCT.