A PACAP-activated network for secretion requires coordination of Ca²⁺ influx and Ca²⁺ 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 Ca²⁺ which are disrupted when Ca²⁺ influx through L-type channels is blocked or internal Ca²⁺ stores are depleted. PACAP liberates stored Ca²⁺ via inositol trisphosphate receptors (IP3Rs) on the endoplasmic reticulum (ER), thereby functionally coupling Ca²⁺ mobilization to Ca²⁺ influx and supporting Ca²⁺-induced Ca²⁺-release. These Ca²⁺ 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 Ca²⁺ through multiple pathways.