Pituitary adenylyl cyclase-activating peptide activates multiple intracellular signaling pathways to regulate ion channels in PC12 cells

Pituitary adenylyl cyclase-activating peptide (PACAP) stimulates calcium tr ansients and catecholamine secretion in adrenal chromaffin and PC12 cells. The PACAP type 1 receptor in these cells couples to both adenylyl cyclase a nd phospolipase C pathways, but although phospolipase C has been implicated in the response to PACAP, the role of adenylyl cyclase is unclear. In this study, we show that PACAP38 stimulates Ca2+ influx in PC12 cells by activa ting a cation current that depends upon the dual activation of both the PLC and adenylyl cyclase signaling pathways but does not involve protein kinas e C. In activating the current, PACAP38 has to overcome an inhibitory effec t of Ras. Thus, in cells expressing a dominant negative form of Ras (PC12as n17-W7), PACAP38 induced larger, more rapidly activating currents. This eff ect of Ras could be overidden by intracellular guanosine-5'-O-3-(thio)triph osphate (GTP gamma S), suggesting that it was mediated by inhibition of dow n-stream G proteins. Ras may also inhibit the current through a G protein-i ndependent mechanism, because cAMP analogues activated the current in PC12a sn17-W7 cells, provided GTP gamma S was present, but not in PC12 cells expr essing wild type Ras. We conclude that coupling of PACAP to both adenylyl c yclase and phospholipase C is required to activate Ca2+ influx in PC12 cell s and that tonic inhibition by Ras delays and Limits the response.