Positive rate-sensitive corticosteroid feedback mechanism of ACTH secretion in Cushing's disease.

To define the nature of the disturbance of the corticosteroid feedback mechanism in Cushing's disease, the dynamic aspects of the ACTH response to corticosteroid administration have been studied in patients with Cushing's disease after total adrenalectomy (C.d. post adx.). The results were compared with those obtained in patients with Addison's disease (control group). Different experimental designs for administration of cortisol were chosen to provide extreme variations in the input signal. The response of the system was evaluated by measuring plasma ACTH concentrations (radioimmunoassay) at short time intervals. Infusion of cortisol at constant rate (50 mg/h for 2 h) resulted in a transient, paradoxical rise in ACTH levels with a maximum at 15 min. (315+/-65%, mean+/-SEM). In contrast, in the control group there was an immediate and rapid decrease in ACTH levels with a significant inhibition after 15 min (80+/-6%, mean+/-SEM). Infusion of 50 mg cortisol for 5 and 15 min, respectively, produced an increase in ACTH levels, which was confined to the time when cortisol levels were rising (maximum: 137+/-30% and 139+/-10% at 5 and 15 min, respectively, mean+/-SEM). This increase corresponded in time to the first decrease in ACTH levels in the Addisonian patients. With bolus injections of 25 mg cortisol, ACTH levels remained unchanged during the first 15 min. The time-course in the patients with C.d. post adx. was essentially the same as in the Addisonian patients. From these results it is concluded that in the patients with C.d. post adx. the rapid, rate-sensitive feedback mechanism was converted into a positive one, whereas the delayed, dose-sensitive mechanism was completely undisturbed. The capacity of dexamethasone to activate rate-sensitive feedback elements was markedly diminished. Accordingly, there were only minor positive feedback effects upon ACTH secretion in the patients with C.d. post adx.