The regulation of calcitonin in normal human plasma as assessed by immunoprecipitation and immunoextraction.

We have studied the secretion of calcitonin in normal adults with a new procedure of increased sensitivity for the measurement of the hormone in peripheral plasma. In this method, endogenous calcitonin is immunoprecipitated with specific antibodies from a 10-ml plasma sample. The calcitonin is then dissociated from the antibodies and extracted into alcohol. The alcohol is evaporated, and the calcitonin is recovered in assay diluent for subsequent radioimmunoassay. The procedure produces a fivefold increase in immunoassay sensitivity and eliminates plasma proteins which can produce spurious immunoassay effects. This procedure was used to measure calcitonin in normal adults. The mean (+/- SE) plasma calcitonin in 43 subjects was 10 (+/- 2) pg/ml. A+ the end of a 3-h calcium infusion (12 mg/kg), mean plasma calcitonin in 10 subjects had risen to 114 (+/- 21) pg/ml. In 11 subjects, a 10-min infusion of 150 mg of calcium caused calcitonin to rise to a mean concentration of 28 (+/- 7) pg/ml at 20 min. EDTA infusion (50 mg/kg 2 h) caused a slight decrease in plasma calcitonin. These results are consistent with our previous reports of the low concentrations of calcitonin in adult plasma. Our data may underestimate calcitonin levels since not all of the heterogenous species of hormone may be extracted by this method. In any case, this procedure has allowed us to determine that the low concentrations of plasma calcitonin in normal adults are responsive to perturbations of calcium homeostasis. The immunoextraction method may be applicable to other assays in which it is necessary to increase sensitivity or define specificity.