Dexamethasone potentiates high-affinity β-agonist binding and Gsα protein expression in airway smooth muscle

Abstract

Corticosteroids enhance β-adrenergic responses by actions at both β-adrenoceptor (β-AR) and post-β-AR sites. The present study investigated the effects of dexamethasone on β-AR density, high-affinity β-agonist binding, Gsα and Giα protein expression, and cAMP responses in bovine tracheal smooth muscle (bTSM). Dexamethasone treatment of cultured bTSM cells increased total β-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 β-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 Gsα protein levels from 0.99 ± 0.14 to 1.46 ± 0.17 μg/mg protein without affecting Giα 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 β-adrenergic signaling pathways in airway smooth muscle.