[beta]2-adrenoceptor activation augments acetylcholine release from tracheal
parasympathetic nerves.
Zhang, Xiang-Yang, Michal A. Olszewski, and N. Edward Robinson.
Department of Large Animal Clinical Sciences and Department of Physiology,
Michigan State University, East Lansing, MI 48824
APStracts 2:0008L, 1995.
We determined the effects of isoproterenol (ISO) on parasympathetic
neurotransmission in isolated equine trachealis strips by comparing the
effects of ISO on the contractile response to electrical field stimulation
(EFS) and acetylcholine (ACh), as well as by measuring EFS-induced ACh
release. The interaction of ISO with muscarinic receptors and endogenous
nitric oxide was also investigated. Acetylcholine release was measured by
high-performance liquid chromatography with electrochemical detection.
Isoproterenol (10-7 M or greater) caused significantly more inhibition of
EFS- than of ACh-induced contraction, an observation usually interpreted as
evidence of prejunctional inhibition of ACh release. However, the latter
conclusion was not supported by measurement of ACh release. Isoproterenol
concentration-dependently augmented ACh release, which was reversed by the
[beta]2-adrenoceptor antagonist ICI 118,551, but not by the [beta]1-
adrenoceptor antagonist CGP 20712A. Our results indicate that activation of
[beta]2 -adrenoceptors augments ACh release. Moreover, the comparison of
inhibitory effects on EFS- and ACh-induced contraction does not provide
correct information about the prejuctional actions of [beta]-agonists.
Acetylcholine release was increased more by atropine (10-7 M) than ISO (10-6
M), indicating the predominance of prejunctional inhibitory muscarinic
autoreceptors over excitatory [beta]2-adrenoceptors. Additionally, we found
that inhibition of nitric oxide synthase by NG-nitro-L-arginine did not affect
either the cholinergic contractile response or ACh release in both the absence
and presence of ISO.
Received 3 October 1994; accepted in final form 17 January 1995.
APS Manuscript Number L290-4.
Article publication pending Am. J. Physiol. (Lung Cell. Mol. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 24 February 1995.