Glucocorticoids increase GTP-dependent adenylate cyclase activity in cultured fibroblasts

Glucocorticoid treatment of cultured fibroblasts increases intracellular cyclic AMP accumulation induced by isoproterenol or cholera toxin. This increase in agonist activity is not a direct action of glucocorticoids on cyclic AMP metabolism since about 2 days are necessary for maximal effect. Basal cyclic AMP levels are not changed. In membrane preparations, GTP-dependent adenylate cyclase activity is increased, basal adenylate cyclase activity is unchanged, and NaF-stimulated activity is decreased. The number of beta-adrenergic receptors is unchanged, but the affinity of receptor for the antagonist dihydroalprenolol is increased about 3-fold. This change in affinity is probably not responsible for the increased response to isoproterenol since the augmented response is noted at 0.1 mM isoproterenol, a concentration much larger than the apparent KD (about 5 nM). The results suggest that an alteration in some component in the GTP-dependent regulatory complex is responsible for the increase in agonist response.