Acute and Chronic Fluctuations of Immunoreactive and Biologically Active Plasma Calcitonin in the Rat*

Abstract

Using a sensitive calcitonin (CT) immunoassay and a newly developed bioassay capable of detecting 0.025 MRC mU [units] CT, acute and chronic plasma CT fluctuations in male and female rats were studied. Immunoassay of serial plasma samples revealed progressive increases in plasma CT concentrations during aging; female rats have higher CT concentrations than age-matched males. Acute periodic CT fluctuations were discovered by immunoassay of plasmas obtained at 3 h intervals; the greatest values occurred just before and during feeding. Fed rats have higher CT than starved rats. Immunoadsorbent chromatography was used to concentrate specifically CT moieties from large vol of plasma for concurrent immunoassay and bioassay measurements of circulating CT. The concurrent measurements of immunoextracted plasma CT demonstrate that for normal rats, the immunoassay measurements correspond to bioassay measurements. In 1 yr old rats on a regulated feeding schedule, biological (hypocalcemic) activity of CT recovered from 15 ml peripheral plasma ranged from less than 0.15 mU in starved males to 0.78 mU in feeding females. Biologically active CT circulates in normal rats and the blood concentration of biologically active CT progressively increases during somatosexual maturation, being highest in old females, and increases acutely just before and during feeding.