Selective enhancement of beta-adrenergic receptor signaling by overexpression of adenylyl cyclase type 6: Colocalization of receptor and adenylyl cyclase in caveolae of cardiac myocytes

We investigated the effect of adenovirally mediated overexpression of adeny lyl cyclase type 6 (AC6), a major form of AC expressed in mammalian heart, on G protein-coupled receptor regulation of cAMP production in neonatal rat ventricular myocytes. Following gene transfer of AC6, isoproterenol- and f orskolin-stimulated increases in cAMP were markedly enhanced, whereas basal levels of cAMP and responses to several other agonists that stimulate cAMP formation, e.g., prostaglandin E-2 (PGE(2)), H-2 agonist, glucagon, and A( 2) agonist were not increased. Studies to test whether the selective enhanc ement in beta-adrenergic receptor (AR) response might result from inhibitio n of AC6 by G alpha(i) and G beta gamma indicated that pertussis toxin-sens itive inhibition by the muscarinic cholinergic agonist carbachol was unalte red in myocytes overexpressing AC6. Pertussis toxin treatment failed to rev eal an enhancement by AC6 overexpression of basal or PGE(2)-stimulated cAMP . Immunoblot analysis of membrane fractions indicated that beta(1)-AR and A C6 are expressed in fractions enriched in caveolin-3 and morphologic caveol ae. The data suggest that loss of G(i)-mediated inhibition is not the mecha nism for enhancement of beta-AR-stimulated cAMP formation and that key comp onents of beta-AR-mediated activation of AC exist in caveolae of cardiac my ocytes, providing a means by which beta-AR response is selectively enhanced by increasing AC6 expression.