A NONCHOLINERGIC TRANSMITTER, PITUITARY ADENYLATE CYCLASE-ACTIVATING POLYPEPTIDE, UTILIZES A NOVEL MECHANISM TO EVOKE CATECHOLAMINE SECRETION IN RAT ADRENAL CHROMAFFIN CELLS

Pituitary adenylate cyclase-activating polypeptide (PACAP) is the most potent non-cholinergic neurotransmitter to stimulate catecholamine se cretion from rat chromaffin cells; however, the mechanism of action is not clear, We used amperometric detection of exocytosis and indo-1 mo nitoring of [Ca2+](i) to identify PACAP actions in cultured chromaffin cells, PACAP (100 nM) required external Ca2+ to evoke secretion, Howe ver, unlike nicotine and KCl which caused immediate and relatively bri ef secretion, PACAP had a latency of 6.8 +/- 0.96 s to the first secre tory response and secretion continued for up to 2 min, PACAP elevation of [Ca2+](i) showed similar latency and often remained above base lin e for several minutes following a brief exposure, ZnCl2 (100 ECM) sele ctively inhibited PACAP-stimulated secretion and [Ca2+](i) with little effect on nicotine-evoked responses, Nifedipine (10 mu M) had little effect on PACAP-evoked secretion but inhibited nicotine-evoked secreti on by more than 80%, while omega-conotoxin (100 nM) failed to affect e ither agonist, PACAP-stimulated cAMP levels required 5 s to significan tly increase, consistent with the latency of exocytotic and Ca2+ respo nses. Forskolin (10 mu M) caused responses similar to PACAP. PACAP-evo ked exocytosis was blocked by the protein kinase A inhibitor adenosine 3',5'-cyclic monophosphorothioate R(p)-diastereomer (R(p)-cAMPS). The se data show that PACAP stimulates exocytosis by a mechanism distinctl y different from cholinergic transmitters that appears to involve cAMP -mediated Ca2+ influx, Differences in receptor coupling mechanisms and pharmacology of Ca2+ entry stimulated by cholinergic and peptidergic agonists support the idea that the peptidergic system maintains catech olamine secretion under conditions where the cholinergic system desens itizes or otherwise fails.