A PACAP-activated network for secretion requires coordination of calcium influx and calcium mobilization

Chromaffin cells of the adrenal medulla transduce sympathetic nerve activity into stress hormone secretion. The two neurotransmitters principally responsible for coupling cell stimulation to secretion are acetylcholine and pituitary adenylate activating polypeptide (PACAP). In contrast to acetylcholine, PACAP evokes a persistent secretory response from chromaffin cells. However, the mechanisms by which PACAP acts are poorly understood. Here, it is shown that PACAP induces sustained increases in cytosolic calcium which are disrupted when calcium influx through L-type channels is blocked or internal calcium stores are depleted. PACAP liberates stored calcium via inositol trisphosphate receptors (IP3Rs) on the endoplasmic reticulum (ER), thereby functionally coupling calcium mobilization to calcium influx and supporting calcium-induced calcium-release (CICR). These Ca2+ influx and mobilization pathways are unified by an absolute dependence on phospholipase C epsilon activity. Thus, the persistent secretory response that is a defining feature of PACAP activity, in situ, is regulated by a signaling network that promotes sustained elevations in intracellular calcium through multiple pathways.