[Possible role of a neuropeptide PACAP (pituitary adenylate cyclase-activating polypeptide) on stimulus-secretion coupling in catecholamine neuron].

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide first isolated from ovine hypothalamic tissue. This peptide stimulates adenylate cyclase activation. However, few details were known of the function of this peptide on stimulus-secretion coupling in neuronal cells. The authors have investigated the role of PACAP on catecholamine biosynthesis and secretion using cultured bovine adrenal chromaffin cells as a model for catecholamine-containing neurons. PACAP38, the 38-amino acid form of PACAP, increased cAMP formation in bovine adrenal chromaffin cells. In addition, PACAP38 increased [Ca2+]i associated with PI turnover and Ca2+ influx into the cells. The synthesis of catecholamine and the phosphorylation of tyrosine hydroxylase, a rate-limiting enzyme of catecholamine biosynthesis, stimulated by the maximal effective concentration of dibutyryl cAMP or a high concentration (56 mM) of K+ were further enhanced by PACAP38. Thus PACAP38 stimulated the pathway of catecholamine biosynthesis mainly by both activation of cAMP- and Ca2(+)-dependent protein kinases. On catecholamine secretion from the cells, the effect of PACAP38 was markedly potentiated by addition of ouabain, an inhibitor of Na+/K+ ATPase. This markedly potentiated secretion was greatly reduced with Na+ omitted-sucrose medium. PACAP38 increased 22Na+ influx into the cells treated with ouabain. Thus PACAP38 with ouabain stimulated catecholamine secretion by accumulation of intracellular Na+, resulting in an increase in Ca2+ influx. These results indicate that the neuropeptide PACAP has an important role in stimulus-secretion coupling in adrenal chromaffin cells.