Mechanism of impaired beta-adrenoceptor responsiveness in atopic
sensitized airway smooth muscle .
Hakonarson, Hakon, David J. Herrick, and Michael M. Grunstein.
Division of Pulmonary Medicine, Joseph Stokes Jr. Research
Institute, The Children's Hospital of Philadelphia, University of
Pennsylvania School of Medicine, Philadelphia, PA 19104
APStracts 2:0137L, 1995.
Decreased airway relaxation to beta-adrenoceptor stimulation has been
hypothesized as a potential mechanism leading to enhanced
bronchoconstrictor responsiveness in asthma. In addressing potential
mechanisms underlying this phenomenon, the relative contributions of
beta-adrenoceptor - coupled transmembrane signaling mechanisms were
examined in isolated rabbit tracheal smooth muscle (TSM) passively
sensitized with serum from atopic asthmatic patients and TSM
comparably exposed to non-atopic (control) human serum. During half
-maximal isometric contraction of the tissues with acetylcholine,
relative to control TSM, the sensitized tissues exhibited significant
attenuation of both their maximal relaxation (Rmax) and sensitivity
(i.e., -log 50% Rmax) to cumulative administration of isoproterenol
(p<0.001) or PGE2 (p<0.001). In contrast, the relaxation
responses to forskolin, a diterpene which directly activates
adenylate cyclase, were similar in both tissue groups. Extended
studies demonstrated that the attenuated relaxation to isoproterenol
and PGE2 in sensitized TSM was: 1) ablated by pretreatment with the
muscarinic M2-receptor antagonists, methoctramine (10-6 M) or
gallamine (10-4 M); 2) also inhibited by pretreatment with pertussis
toxin (100 ng/ml), which ADP ribosylates the inhibitory G protein
(Gi) negatively coupled to adenylate cyclase activation; and 3)
associated with diminished cAMP accumulation in response to
isoproterenol administration. Moreover, based on Western immunoblot
analysis, we found that Gi protein expression was increased in
membranes fractions from sensitized TSM, related to enhanced
expression of the Gi[alpha]3 subunit. Collectively, these
observations provide new evidence that the impaired beta
-adrenoceptor-mediated relaxation in atopic sensitized airways is
associated with increased muscarinic M2 receptor/Gi protein-coupled
expression and function.
Received 13 March 1995; accepted in final form 20 June 1995.
APS Manuscript Number L80-5.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 14 August 1995.