Glucagon Inhibits ACTH-Stimulated Cortisol Secretion from Dispersed Human Adrenocortical Cells by Activating Unidentified Receptors Negatively Coupled with the Adenylate Cyclase Cascade

We have investigated the direct effect of glucagon on collagenase-dispersed adrenocortical cells obtained from consenting patients undergoing unilateral adrenalectomy and nephrectomy for renal cancer. Dispersed cells, actually a mixture of zona glomerulosa and zona fasciculata-reticularis (ZF/R) cells, were incubated with glucagon (from 10(-10) to 10(-6) M) alone or in the presence of 10(-9) M angiotensin-II, 10(-10) M ACTH or 10(-5) M forskolin, and the effects on aldosterone, cortisol and cyclic-AMP (cAMP) production were measured by radioimmune assay. Glucagon concentration-dependently inhibited ACTH-stimulated cortisol production and ACTH- or forskolin-enhanced cAMP release, minimal and maximal effective concentrations being 10(-9) and 10(-7) M. The effects of glucagon were suppressed by 10(-5) M Des-His1-[Glu9]glucagon amide, an antagonist of glucagon receptors (glucagon-A). Reverse transcription-polymerase chain reaction did not reveal the presence of specific glucagon-receptor mRNA in the human adrenal cortex. However, autoradiography demonstrated the presence of [125I]glucagon binding sites in the ZF/R, which were displaced by glucagon but not by ACTH. Taken together, these findings suggest that glucagon, through the activation of unidentified receptors located on ZF/R cells, inhibits adenylate cyclase, thereby dampening glucocorticoid response to ACTH.