Progesterone regulation of catecholamine secretion from chromaffin cells

Stress stimulates the adrenal medulla to rapidly secrete catecholamines (CAs), and the adrenal cortex to release progesterone (PROG), which may locally regulate stress-induced CA release. We used bovine chromaffin cells to investigate the effects of PROG on CA secretion. PROG dose-dependently inhibited CA secretion induced by nicotinic acetylcholine receptor (nAChR) agonist 1,1-dimethyl-4-phenlypiperazinium iodide (DMPP) up to 77%. Pre-incubation with PROG up to 1 h increased this inhibition. 3α,5α- Tetrahydroprogesterone (3α,5α-THP) and dexamethasone were less potent inhibitors. Patch-clamp techniques revealed that PROG co-applied with DMPP inhibited peak DMPP-induced current up to 68% and with 3 min pre-incubation inhibited both peak and integrated current up to ∼95%. Monitoring of FURA-2 showed that PROG similarly inhibited parallel changes in intracellular-free Ca⁺⁺ concentration. PROG also inhibited CA secretion elicited by elevated K⁺ (38%), and, in single cells, suppressed Ca⁺⁺ current evoked by step depolarization, inhibiting amplitude by 15%, and reducing the time constant of current decay during depolarization by 57%. In contrast to the immediate inhibition of nicotinic current, inhibition of Ca⁺⁺ current became statistically significant only after 1 min exposure to PROG. PROG did not inhibit secretion stimulated by high Ca⁺⁺ perfusion of permeabilized cells. These data suggest that PROG inhibits CA secretion from chromaffin cells predominantly by rapidly inhibiting nAChRs, and by gradually enhancing the inactivation of voltage-dependent Ca⁺⁺ channels (VDCCs), but not by affecting secretory processes downstream of Ca⁺⁺ influx. This study supports a role for adrenocortical PROG in the regulation of CA secretion during stress.