Evaluation of a Sensitive Radioimmunoassay of Plasma Corticotropin Using Commercial Reagents

A radioimmunological method for the sensitive estimation of plasma corticotropin immunoreactivity (ACTH) using commercially available reagents is presented. The method involves silica extraction of corticotropin from plasma, desorption with acid protein solution, neutralization and subsequent radiomimmunoassay (RIA). [125I]corticotropin is added as internal standard to the plasma sample. The amounts of corticotropin extractable with silica differed considerably between the plasma samples of individual subjects. There were marked differences in the affinity of five different corticotropin standards to the antiserum used. The detection limit of the method was found to be 1.47 pmol/l. Blanks arising in water and in charcoal stripped serum were lower than the detection limit. Precision and accuracy were within the range commonly achieved for RIA-methods. Morning levels of normal subjects ranged from 6.5-10.9-18.5 pmol/l. Hydrocortisone infusion suppressed plasma corticotropin from 12.6 +/- 6.4 (S.D.) to 4.4 +/- 3.1 (S.D.) pmol/l. Infusion of metyrapone increased corticotropin levels from 7.3 +/- 4.2 (S.D.) to 15.3 +/- 6.0 (S.D.) pmol/l.