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Plasma concentrations of ACTH (by radioimmunoassay) and cortisol (by competitive protein binding) were measured in a normal man at 5-min intervals from 0100 to 0900 hr. During this time there were 18 episodes of ACTH secretion and 13 episodes of cortisol secretion. On occasion, 2 or more episodes of ACTH secretion in close temporal relationship were reflected in a single sustained elevation of cortisol concentration. Estimates were obtained of the amounts of ACTH produced per episode which ranged from 0.5 μg to 4.5 μg at an average rate over the whole study of 0.29 μg per min. The plasma concentrations achieved in most instances were more than sufficient to produce maximal adrenal secretion of cortisol. Cortisol secretion began about 10 min after initiation of ACTH secretion. Cortisol secretion ranged from 0.1 mg to 2.9 mg per episode at an average rate over the whole study of 105 (μg/min. It is concluded that any sampling period longer than 5 min gives an erroneous depiction of events and this was exemplified by a simulated 20-min sampling of the 5-min mode data.
Gallagher et al. (Fri,) studied this question.