Radioimmunoassay of calcitonin in plasma, normal thyroid, and medullary thyroid carcinoma of the rat.

We have developed a sensitive immunoassay for murine calcitonin which can measure the basal concentration of the hormone in peripheral plasma of rats as well as the calcitonin, extracted from rat thyroid tissue. Human calcitonin was used for tracer, standard, and antibody production. Assay methods were devised to minimize artifacts which have been shown to spuriously influence immunoassay performance. The basal plasma concentration of calcitonin in rats was 6 to 75 pg. per milliliter. Infusions of calcium, pentagastrin, and PGE2 produced a 2- to 7-fold increase in plasma calcitonin. Elevated concentrations of calcitonin were demonstrated in tumor specimens obtained from the second and fourth generations of a transplanted rat medullary thyroid carcinoma. Our results demonstrate that calcitonin circulates in the peripheral plasma of rats in the basal state and that hormone concentrations change during functional tests of secretion. This assay will permit studies of calcitonin secretion in physiological and pathophysiological states in the rat.