Methylxanthine treatment of smooth muscle cells differentially modulates adenylate cyclase responsiveness

The DDT1 MF-2 smooth muscle cell line was used to study regulation of the A1 and A2 adenosine receptor (AR)-adenylate cyclase system by two different methylxanthines. 3-isobutyl 1-methylxanthine (IBMX) is both an AR antagonist and a phosphodiesterase inhibitor, while xanthine amine congener is an AR antagonist without phosphodiesterase activity. Incubation of cells for 18 hr with 100 microM IBMX produced a significant (P less than .05) decrease in the basal, isoproterenol- and sodium fluoride-stimulated adenylate cyclase activity. This generalized decrease in adenylate cyclase activity was associated with a significant decrease in the quantity of alpha s (Gs) as determined by Western blotting. In contrast, no alteration in alpha i (Gi) was observed in these same membranes. A significant increase in both the quantity of A1AR and the receptors' affinity for agonist occurred; however, no alteration in the ability of an A1AR selective agonist to inhibit adenylate cyclase activity was observed. Treatment for 18 hr with 50 nM xanthine amine congener, conversely, resulted in an increase in basal and isoproterenol stimulated adenylate cyclase activity with no change in membrane alpha s (Gs). With IBMX, there was an increase in agonist affinity for the A1AR without an associated change in the effect of adenosine agonists on adenylate cyclase activity. These data indicate that methylxanthine analogs which lack the ability to inhibit phosphodiesterases regulate receptor-mediated transmembrane signaling systems quite differently from those possessing such characteristics. The more prototypic methylxanthines regulate both receptors and G proteins in these smooth muscle cells.