Evidence for ultra-short loop autoregulation of adrenocorticotropin secretion in man

The existence of feedback inhibition of ACTH on its own secretion has been postulated. To investigate its existence in man, the effects of synthetic ACTH 1-24 on endogenous ACTH secretion were tested in 13 patients with Addison's disease. Plasma ACTH was measured using an immunoradiometric assay, specific for endogenous ACTH 1-39. Ten patients were given 50 micrograms ACTH 1-24 as a bolus iv dose followed by a 50-microgram infusion in 90 min. Blood samples for ACTH and cortisol assay were obtained at 0, 15, 30, 60, 90, and 120 min. As a control, a saline infusion was given 2 days earlier. Three other patients were given 100 micrograms ovine corticotropin-releasing hormone (oCRH) iv and ACTH 1-24 as described above. Blood samples for ACTH and cortisol assay were drawn every 15 min for 2 hours. A CRH test was performed during saline infusion as a control 2 days earlier. In all patients steroid replacement therapy was maintained during the studies. ACTH 1-24 caused a significant decrease (P less than 0.01) in endogenous plasma ACTH at 15 min compared to saline. oCHR administration markedly stimulated ACTH release in the three patients tested, and the ACTH response to oCRH was completely inhibited by the simultaneous administration of ACTH 1-24. These findings strongly support the presence of ACTH autoregulation in man. The complete inhibition of the ACTH response to oCRH by exogenous ACTH 1-24 provides evidence for ultra-short feed-back inhibition at the pituitary level.