PACAP regulates immediate catecholamine release from adrenal chromaffin cells in an activity-dependent manner through a protein kinase C-dependent pathway.

Adrenal medullary chromaffin cells are a major peripheral output of the sympathetic nervous system. Catecholamine release from these cells is driven by synaptic excitation from the innervating splanchnic nerve. Acetylcholine has long been shown to be the primary transmitter at the splanchnic-chromaffin synapse, acting through ionotropic nicotinic acetylcholine receptors to elicit action potential-dependent secretion from the chromaffin cells. This cholinergic stimulation has been shown to desensitize under sustained stimulation, yet catecholamine release persists under this same condition. Recent evidence supports synaptic chromaffin cell stimulation through alternate transmitters. One candidate is pituitary adenylate cyclase activating peptide (PACAP), a peptide transmitter present in the adrenal medulla shown to have an excitatory effect on chromaffin cell secretion. In this study we utilize native neuronal stimulation of adrenal chromaffin cells in situ and amperometric catecholamine detection to demonstrate that PACAP specifically elicits catecholamine release under elevated splanchnic firing. Further data reveal that the immediate PACAP-evoked stimulation involves a phospholipase C and protein kinase C-dependent pathway to facilitate calcium influx through a Ni2+ and mibefradil-sensitive calcium conductance that results in catecholamine release. These data demonstrate that PACAP acts as a primary secretagogue at the sympatho-adrenal synapse under the stress response.

Increased secretory capacity of mouse adrenal chromaffin cells by chronic intermittent hypoxia: involvement of protein kinase C.

Previous studies have shown that catecholamine secretion from the adrenal medulla plays a critical role in chronic intermittent hypoxia (CIH)-induced alterations in cardiovascular function. In the present study we examined the cellular mechanisms associated with the effects of CIH on adrenal chromaffin cell catecholamine secretion. Experiments were performed on adult male mice (C57/BL6) that were exposed to 1-4 days of CIH or to normoxia. Perforated patch electrical capacitance recordings were performed on freshly prepared adrenal medullary slices that permit separating the chromaffin cell secretion from sympathetic input. CIH resulted in a significant increase in the readily releasable pool (RRP) of secretory granules, and decreased stimulus-evoked Ca(2+) influx. Continuous hypoxia (CH) either for 2.5 h (equivalent to hypoxic duration accumulated over 4 days of CIH) or for 4 days were ineffective in evoking changes in the RRP and Ca(2+) influx. CIH activated PKC in adrenal medullae as evidenced by increased phosphorylation of PKC at Thr(514) and PKC inhibitors prevented CIH-induced increases in the RRP and restored stimulus-evoked attenuation of Ca(2+) influx. CIH resulted in elevated thio-barbituric acid reactive substances (TBARSs, an index of oxidized proteins) and an antioxidant prevented CIH-induced changes in the RRP, suggesting the involvement of reactive oxygen species (ROS). These results demonstrate that CIH increases the RRP in adrenal chromaffin cells via ROS-mediated activation of PKC and suggest that CIH can directly affect the secretory capacity of chromaffin cells and contribute, in part, to elevated catecholamine levels.