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.

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.

Phospholipase C-ε defines a PACAP-stimulated pathway for secretion in the chromaffin cell.

Adrenomedullary chromaffin cells respond to splanchnic (sympathetic) nerve stimulation by releasing stress hormones into the circulation. The signal for hormone secretion is encoded in the neurotransmitters - especially acetylcholine (ACh) and pituitary adenylate cyclase activating polypeptide (PACAP) - that are released into the splanchnic-chromaffin cell synapse. However, functional differences in the effects of ACh and PACAP on the chromaffin cell secretory response are not well defined. Here, selective agonists of PACAP receptors or nicotinic and muscarinic acetylcholine receptors were applied to chromaffin cells. The major differences in the effects of these agents were not on exocytosis, per se, but rather on the steps upstream of exocytosis. In almost every respect, the properties of individual fusion events triggered by PACAP and cholinergic agonists were similar. On the other hand, the properties of the Ca2+ transients evoked by PACAP differed in several ways from those evoked by muscarinic and nicotinic receptor stimulation. A defining feature of the PACAP-stimulated secretory pathway was its dependence on signaling through exchange protein directly activated by cAMP (Epac) and PLCε. However, the absence of PLCε did not disrupt Ca2+ transients evoked by cholinergic agonists. Accordingly, inhibition of Epac activity did not disrupt secretion triggered by acetylcholine or specific agonists of muscarinic and nicotinic receptors. Thus, PACAP and acetylcholine stimulate chromaffin cell secretion via separate and independent pathways. This feature of stimulus-secretion coupling may be important for sustaining hormone release from the adrenal medulla under conditions associated with the sympathetic stress response.

PACAP and acetylcholine cause distinct Ca2+ signals and secretory responses in chromaffin cells.

The adrenomedullary chromaffin cell transduces chemical messages into outputs that regulate end organ function throughout the periphery. At least two important neurotransmitters are released by innervating preganglionic neurons to stimulate exocytosis in the chromaffin cell-acetylcholine (ACh) and pituitary adenylate cyclase activating polypeptide (PACAP). Although PACAP is widely acknowledged as an important secretagogue in this system, the pathway coupling PACAP stimulation to chromaffin cell secretion is poorly understood. The goal of this study is to address this knowledge gap. Here, it is shown that PACAP activates a Gαs-coupled pathway that must signal through phospholipase C ε (PLCε) to drive Ca2+ entry and exocytosis. PACAP stimulation causes a complex pattern of Ca2+ signals in chromaffin cells, leading to a sustained secretory response that is kinetically distinct from the form stimulated by ACh. Exocytosis caused by PACAP is associated with slower release of peptide cargo than exocytosis stimulated by ACh. Importantly, only the secretory response to PACAP, not ACh, is eliminated in cells lacking PLCε expression. The data show that ACh and PACAP, acting through distinct signaling pathways, enable nuanced and variable secretory outputs from chromaffin cells.