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-agonist binding and
Gs
protein expression in airway smooth
muscle
Pediatric Pulmonary Division, University of Connecticut School of Medicine, Farmington, Connecticut 06030
Corticosteroids enhance 
-adrenergic
responses by actions at both -adrenoceptor (-AR) and post--AR
sites. The present study investigated the effects of dexamethasone on
-AR density, high-affinity -agonist binding, Gs
and Gi protein expression, and cAMP responses in bovine
tracheal smooth muscle (bTSM). Dexamethasone treatment of cultured bTSM
cells increased total -AR density 1.6- to 1.9-fold as assessed by
the saturation binding of [3H]CGP-12177 and by
displacement of radioligand binding with isoproterenol. Isoproterenol
bound to the -AR at two sites, a high-affinity site with a density
of 5.9 ± 1.2 fmol/mg protein and a low-affinity site with a density
of 16.9 ± 1.0 fmol/mg protein. Dexamethasone increased both high- and
low-affinity isoproterenol binding sites to 11.1 ± 2.2 and 25.9 ± 2.1 fmol/mg protein, respectively, without influencing agonist binding
affinities. Dexamethasone also selectively increased Gs
protein levels from 0.99 ± 0.14 to 1.46 ± 0.17 µg/mg protein
without affecting Gi levels. The net effect of these changes was a 1.8-fold increase in maximal isoproterenol-induced cAMP
generation in dexamethasone-treated bTSM cells. These findings provide
new insights into the corticosteroid regulation of -adrenergic signaling pathways in airway smooth muscle.
-adrenergic receptors; bovine; CGP-12177; corticosteroid; G
proteins; isoproterenol
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