Effects of dexamethasone on G protein levels and adenylyl cyclase activity in rat vascular smooth muscle cells.
Dexamethasone administration in vitro has been shown to increase adenylyl cyclase activity in vascular smooth muscle cells (VSMC) from renal arteries and in non-vascular cell lines. To investigate whether G proteins are involved in this response, cultured VSMC from mesenteric arteries of Sprague-Dawley rats were incubated in the presence and absence of 10 nM dexamethasone for 24 and 48 h. Basal and stimulated adenylyl cyclase activities were increased by approximately 50% after treatment with dexamethasone. The changes were neither specifically associated with ligands which stimulate adenylyl cyclase catalytic unit via Gs (isoproterenol and prostaglandin E1) nor with guanylylimidodiphosphate (0.1 nM), which inhibits the catalytic unit via Gi. This suggests that dexamethasone enhances adenylyl cyclase activity through changes at the level of the catalytic unit, rather than through the G proteins which modulate its activity. No differences were seen in immunoblotting studies of the levels of Gi alpha 2, Gs alpha, Gi alpha 3 and beta subunits. Similarly, dexamethasone had no effect on the expression of mRNA for Gi alpha 2 and Gs alpha. The results indicate that glucocorticoid-induced increases of adenylyl cyclase activity are due to changes at the level of the adenylyl cyclase catalytic unit rather than alteration of the levels or turnover of Gs alpha, Gi alpha 2, Gi alpha 3 and beta subunits in the membranes of VSMC.