Monensin inhibition of corticotropin releasing factor mediated ACTH release

Monensin is a sodium selective carboxylic ionophore that has been helpful in studying the intracellular mechanisms of protein secretion by its ability to inhibit transport of secretory proteins, particularly through the Golgi apparatus, and by its capacity to block intracellular posttranslational processing events. We studied in rat anterior pituitary cell culture the effects of monensin on: CRF stimulated ACTH release; presynthesized (stored) ACTH release; and on forskolin- (activator of adenylate cyclase) and KCl- (a membrane depolarizer which does not stimulate ACTH synthesis) induced ACTH release. Monensin inhibited CRF stimulated ACTH release in a dose-dependent fashion. The ED₅₀ was 2.7×10^-8M and maximal inhibition was 52% at 1.5×10^-7M. Inhibition at 40 minutes of CRF incubation was similar to the percent inhibition noted at 1 hr 40 min and 2 hr 40 min. Monensin (1.5×10^-6M) decreased the amount of ACTH release from cells incubated with cycloheximide plus CRF by 32% (p<0.01). Monensin individually inhibited forskolin (2×10^-6M) and dibutyryl cyclic AMP (3×10^-3M) mediated ACTH release in a dose-dependent fashion. The inhibition of forskolin and dibutyryl cyclic AMP mediated ACTH release by 1.5×10^-6M monensin was 48% and 46% respectively. Monensin (1.5×10^-6M) also reduced KCl (50 mM) stimulated ACTH release by 48%. This study demonstrates that monensin inhibits CRF mediated ACTH release. This inhibitory action of monensin is: dose and time dependent; directed in part against presynthesized (stored) ACTH; not on or proximal to the activation of adenylate cyclase or generation of cyclic AMP; and present on cells that are depolarized with KCl, suggesting an inhibitory effect on the secretory granule.