A PACAP-activated network for secretion requires coordination of Ca ²⁺ influx and Ca ²⁺ 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 Ca ²⁺ which are disrupted when Ca ²⁺ influx through L-type channels is blocked or internal Ca ²⁺ stores are depleted. PACAP liberates stored Ca ²⁺ via inositol trisphosphate receptors (IP3Rs) on the endoplasmic reticulum (ER), thereby functionally coupling Ca ²⁺ mobilization to Ca ²⁺ influx and supporting Ca ²⁺ -induced Ca ²⁺ -release. These Ca ²⁺ influx and mobilization pathways are unified by an absolute dependence on phospholipase C epsilon (PLCε) 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 Ca ²⁺ through multiple pathways.