Absence of detectable calcitonin synthesis in the pituitary using cloned complementary deoxyribonucleic acid probes

Recently, several investigators have detected calcitonin (CT)-like material in pituitary tissues by immunochemical techniques. In the present study we examined CT biosynthesis in rat pituitaries using specific and sensitive hybridization assays which used a cloned CT complementary DNA (cDNA) to detect CT mRNAs. Rat CT cDNA encoding the entire CT precursor, was radiolabeled to a high specific activity with 32P-labeled nucleotide and hybridized with mRNA extracted from pituitaries of both normal and genetically obese Zucker rats, in which elevated levels of immunoreactive CT-like material have been detected. Using these assays, we failed to detect mRNA with size and hybridization characteristics similar to those of thryoidal CT mRNA in either normal or Zucker rat pituitaries. We conclude that a mRNA homologous in sequence to that encoding rat thyroidal CT is not synthesized in the pituitary gland. In previous immunochemical studies a CT-like peptide may have been detected which is synthesized in the pituitary but lacks sufficient structural homology for the mRNA to react with our cDNA probe to thyroidal CT mRNA. Alternatively, in these prior studies thyroidal CT bound to pituitary receptors may have been detected.