Muscarinic receptor reserve for inhibition of cyclic amp
accumulation in bovine trachealis cells.
Ethier, Michael F., Oren P. Schaefer, Ninad Samant, Hiroshi Yamaguchi,
and J. Mark Madison.
Departments of Medicine and Physiology, University of Massachusetts
Medical School, Worcester, MA 01655
APStracts 2:0196L, 1995.
In airway smooth muscle, muscarinic agonists inhibit synthesis of
cAMP. The goal was to characterize the relationship between agonist
occupancy of muscarinic receptors and regulation of cAMP for bovine
trachealis cells. For intact cells dispersed by enzyme, carbachol
maximally inhibited 58+/-4% (mean+/-SEM, n=5) of isoproterenol
-stimulated cAMP accumulation at low concentrations (log EC50 =
-8.34+/-0.16). In radioligand binding experiments, carbachol competed
for 3H-quinuclidinyl benzilate (n=7) and 3H-N-methylscopolamine (n=3)
binding sites on intact cells with both low (log= -4.26+/-0.06 and
-4.50+/-0.20, respectively) and high affinities (log= -5.91+/-0.24 and
-6.39+/-0.19, respectively). In separate experiments, a fraction of
the muscarinic receptors on intact cells were inactivated with either
phenoxybenzamine (POB) or propylbenzylcholine mustard (PBCM). By
comparing equally effective concentrations of carbachol before and
after partial inactivation of receptors, the calculated equilibrium
dissociation constants (log KA= -4.36+/-0.42 to -3.20+/-0.40 for POB;
log KA = -4.27+/-0.45 for PBCM) were much greater than the half
maximally effective concentration of carbachol in control cells (log
EC50 = -8.34+/-0.16). Based on these equilibrium dissociation
constants, we calculated that maximum responses (EC95) to carbachol
were obtained by occupancy of 0.8% of the receptors coupled to cAMP
regulation. We concluded that muscarinic inhibition of cAMP
accumulation is characterized by a muscarinic receptor reserve.
Received 6 July 1994; accepted in final form 20 October 1995.
APS Manuscript Number L193-4.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95