Extraction of a substance with calcitonin-like immunoreactivity from pituitary glands of intact and thyroidectomized rats.

We have previously demonstrated the presence of calcitonin (CT)-like immunoreactivity in the pituitary glands of a number of vertebrate species. Subsequent work by ourselves and others has confirmed this finding by immunohistology and radioimmunoassay (RIA) of pituitary extracts. However, no data have been reported on the site of biosynthesis of the CT-like pituitary peptide, or the relationship of pituitary levels of the substance to experimental alterations of systemic CT levels. In the present study, we measured the content of CT-like peptide in rat pituitaries by using gel filtration chromatography and an RIA, which recognizes rat and human calcitonin. The CT-like peptide has an elution position on gel filtration similar to that of rat and human calcitonin. Its RIA binding appears indistinguishable from that of rat thyroidal CT. Various conditions for extraction were evaluated, and homogenization in acetic acid at 100 degrees C gave the best results. The CT-like content of rat pituitaries was generally 75-125 pg/gland. Presence of CT-like peptide in rat pituitaries was not abolished by prior thyroidectomy. We conclude that the CT-like pituitary peptide is not synthesized in the thyroid, but is similar to thyroidal CT in the characteristics studied.