Growth hormone-releasing hormone facilitates hypoglycemia-induced release of cortisol

Early sleep in humans is characterized by a distinct suppression of pituitary-adrenal activity coinciding with enhanced activity of the somatotropic axis. Here, we tested in awake humans the hypothesis of an inhibiting influence of hypothalamic growth hormone-releasing hormone (GHRH) on pituitary-adrenal activity. For this purpose, pituitary-adrenal activity was stimulated in 10 men through a standard insulin-hypoglycemia-test (IHT) and in another 10 men through combined administration of CRH/vasopressin. Stimulation was performed in each man on three conditions following pretreatment with Placebo and GHRH administered intravenously (50 microg) or intranasally (300 microg) 1 h before. GH, ACTH and cortisol as well as blood pressure and heart rate were measured repeatedly. Contrary to expectations, pretreatment with GHRH did not suppress but enhanced secretion of cortisol upon insulin-induced hypoglycemia regardless of the route of GHRH pretreatment (p<0.05). In contrast, GHRH did not facilitate cortisol release after stimulation with CRH/vasopressin. Changes in ACTH remained inconsistent. Plasma levels of GH increased significantly after i.v. GHRH application, but remained unchanged after the intranasal administration. Blood pressure and heart rate were not influenced by the treatments. Results indicate facilitating effects of GHRH mediated at a suprapituitary (i.e. hypothalamic) level as suggested by restriction of the effect to the hypoglycemia-induced cortisol release with no effects after pituitary stimulation with CRH/vasopressin.