Relationship of nocturnal plasma bioactive and immunoactive ACTH concentrations to cortisol secretion in normal men

Plasma concentration of ACTH and cortisol were measured at 15-min intervals from 22.30 to 07.00 h in 8 normal male volunteers. ACTH was measured both by a modified and sensitized isolated adrenal cell bioassay and by radioimmunoassay. Within-subject correlations between bioactive ACTH and cortisol concentrations ranged between 0.86 and 0.95, with an average correlation of 0.93 (86% shared variance). In contrast, within-subject correlations between immunoassayable ACTH and cortisol ranged between 0.03 and 0.92, with an average correlation of 0.69 (48% shared variance). These results suggest that plasma ACTH concentrations determined by RIA might not accurately reflect the capacity of plasma to regulate the secretion of adrenal glucocorticoids.