Exendin-4, a GLP-1 receptor agonist, stimulates pituitary-adrenocortical axis in the rat: Investigations into the mechanism(s) underlying Ex4 effect

Exendin-4 (Ex4) is a potent and long-lasting agonist of glucagon-like peptide-1 (GLP-1), which has been previously found to stimulate pituitary-adrenal axis in the rat. Aim of the present study was to gain insight into the mechanism(s) involved in the Ex4-induced rise in the rat plasma concentrations of ACTH, aldosterone and corticosterone. Preliminary time- and dose-response studies showed that the maximum stimulating effect of Ex4 occurred within 1 or 2 h and at dose ranging from 0.5 to 2.0 nmol/100 g body weight. The GLP-1 receptor (GLP-1R) antagonist Ex(9-39) did not significantly affect ACTH, aldosterone and cortico-sterone responses to Ex4. Neither the administration of CRH and arginine vasopressin (AVP)-receptor antagonists nor adrenal demedullation prevented pituitary-adrenal axis responses to Ex4. The prolonged (4 or 6 days) suppression of the pituitary ACTH release by dexamethasone impaired corticosterone, but not aldosterone response to Ex4. The following conclusions are drawn: i) Ex4 stimulates rat pituitary-adrenal axis through receptors other than the classic GLP-1R; ii) neither CRH and AVP nor medullary catecholamines are involved in the Ex4-induced stimulation of ACTH release; iii) ACTH stimulation accounts for the rise in corticosterone plasma concentration; and iv) the aldosterone secretagogue effect of Ex4 occurs via a mechanism independent of the stimulation of either ACTH or medullary catecholamines.