PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE EFFECTS PITUITARY-CELLS - MODULATION BY GONADOTROPIN-RELEASING-HORMONE IN ALPHA-T3-1 CELLS

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts via ty pe I receptors in the pituitary to stimulate cAMP production. Gonadotr opes are likely target cells for PACAP action, and Ne have recently sh own alpha T3-1 cells, a clonal gonadotrope-derived cell line, to be PA CAP responsive. Here we have explored the influence of GnRH on PACAP a ction in alpha T3-1 cells and show that PACAP38-stimulated cAMP produc tion is inhibited by GnRH in both the presence and the absence of a ph osphodiesterase inhibitor. This effect appears not to be Ca++ mediated but is mimicked by protein kinase C activation with phorbol 12-myrist ate 13-acetate. However, GnRH and phorbol 12 myristate 18-acetate do n ot inhibit binding of [I-125]PACAP27 to intact alpha T3-1 cells, nor d o they inhibit forskolin- or cholera toxin-stimulated cAMP accumulatio n, implying that the inhibitory effects are exerted at early stages in the PACAP receptor signaling pathway but distal to receptor occupancy . When cells were preincubated with PACAP38, extensive washing failed to prevent the stimulatory effect of the polypeptide presumably becaus e of the slow rate of receptor-ligand dissociation. However, when the time course of PACAP38-stimulated effects on intracellular cAMP was as sessed, the stimulatory effect of PACAP38 could be rapidly reversed by GnRH addition, and the inhibitory effect of GnRH was rapidly be rever sed by a GnRH receptor antagonist. The data provide the first demonstr ation of cross-talk between phospholipase C and adenylate cyclase-acti vating peptides in gonadotrope-derived cells and establish the potenti al for hormonal modulation of PACAP action. We suggest that this inhib itory effect of GnRH might enable the releasing hormone to control the kinetics of cAMP signaling in gonadotropes in. vivo.