Muscarinic inhibition of adenylyl cyclase regulates intracellular
calcium in single airway smooth muscle cells.
Madison, J. Mark, and Hiroshi Yamaguchi.
Departments of Medicine and Physiology, University of Massachusetts
Medical School, Worcester, MA 01655
APStracts 2:0139L, 1995.
To determine whether muscarinic agonists attenuated isoproterenol
-stimulated decreases in intracellular calcium ([Ca2+]i), changes in
[Ca2+]i were measured in single airway smooth muscle cells using
ratiometric analysis of fura-2 fluorescence. Isoproterenol (10-5 M)
and 8-bromo-cAMP decreased [Ca2+]i by 24+/-3% (p<0.05, n=28) and
17+/-1% (p<0.05, n=6), respectively. The decreased [Ca2+]i in
response to isoproterenol was inhibited by propranolol (10-6 M) and
Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS) (10-40
[mu]M). In subsequent experiments assessing the effects of muscarinic
agonists, isoproterenol did not decrease [Ca2+]i in the presence of
carbachol (5 x10-8 M)(6+/-8% increase, N.S., n=8). To determine the
mechanism underlying this inhibitory effect of carbachol, cells were
loaded with caged cAMP. For cells loaded with 20 [mu]M caged cAMP,
photolysis of caged cAMP decreased basal [Ca2+]i by 28+/-3%
(p<0.05, n=12). In the presence of carbachol (5 x 10-8 M),
photolysis of caged cAMP still induced a 27+/-4% decrease in [Ca+2]i
(p<0.05, n=12). We concluded that a low concentration of
carbachol did attenuate isoproterenol-stimulated decreases in
[Ca+2]i. Because low concentrations of carbachol attenuated the
decreases in [Ca+2]i stimulated by isoproterenol but not the
comparable decreases stimulated by cAMP directly, we concluded that
the inhibition of adenylyl cyclase activity by muscarinic agonists
contributed to the regulation of [Ca2+]i in airway smooth muscle
cells. The findings suggested that physiologic levels of cholinergic
stimulation inhibit adenylyl cyclase, thereby attenuating the effects
that beta-adrenergic agonists have on intracelluar [Ca2+]i.
Received 6 July 1994; accepted in final form 12 July 1995.
APS Manuscript Number L194-4.
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.