A Non-cholinergic Transmitter, Pituitary Adenylate Cyclase-activating Polypeptide, Utilizes a Novel Mechanism to Evoke Catecholamine Secretion in Rat Adrenal Chromaffin Cells (*)
- From the Departments of Pharmacology and Pediatrics, Wayne State University School of Medicine, Detroit, Michigan 48201
- ↵§ To whom correspondence should be addressed: Dept. of Pharmacology, Wayne State University School of Medicine, 540 East Canfield Ave., Detroit, MI 48201.
Abstract
Pituitary adenylate cyclase-activating polypeptide (PACAP) is the most potent non-cholinergic neurotransmitter to stimulate
catecholamine secretion from rat chromaffin cells; however, the mechanism of action is not clear. We used amperometric detection
of exocytosis and indo-1 monitoring of [Ca
]
to identify PACAP actions in cultured chromaffin cells. PACAP (100 nM) required external Ca
to evoke secretion. However, unlike nicotine and KCl which caused immediate and relatively brief secretion, PACAP had a latency
of 6.8 ± 0.96 s to the first secretory response and secretion continued for up to 2 min. PACAP elevation of [Ca
]
showed similar latency and often remained above base line for several minutes following a brief exposure. ZnCl
(100 μM) selectively inhibited PACAP-stimulated secretion and [Ca
]
with little effect on nicotine-evoked responses. Nifedipine (10 μM) had little effect on PACAP-evoked secretion but inhibited
nicotine-evoked secretion by more than 80%, while 
-conotoxin (100 nM) failed to affect either agonist. PACAP-stimulated cAMP levels required 5 s to significantly increase,
consistent with the latency of exocytotic and Ca
responses. Forskolin (10 μM) caused responses similar to PACAP. PACAP-evoked exocytosis was blocked by the protein kinase
A inhibitor adenosine 3′,5′-cyclic monophosphorothioate R
-diastereomer (R
-cAMPS). These data show that PACAP stimulates exocytosis by a mechanism distinctly different from cholinergic transmitters
that appears to involve cAMP-mediated Ca
influx. Differences in receptor coupling mechanisms and pharmacology of Ca
entry stimulated by cholinergic and peptidergic agonists support the idea that the peptidergic system maintains catecholamine
secretion under conditions where the cholinergic system desensitizes or otherwise fails.
Footnotes
-
↵* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Received September 18, 1995.
- Revision received February 23, 1996.
- © 1996 by The American Society for Biochemistry and Molecular Biology, Inc.
