Selective Enhancement of β-Adrenergic Receptor Signaling by Overexpression of Adenylyl Cyclase Type 6: Colocalization of Receptor and Adenylyl Cyclase in Caveolae of Cardiac Myocytes

Abstract

We investigated the effect of adenovirally mediated overexpression of adenylyl 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 forskolin-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 E₂ (PGE₂), H₂agonist, glucagon, and A₂ agonist were not increased. Studies to test whether the selective enhancement in β-adrenergic receptor (AR) response might result from inhibition of AC6 by Gα_i and Gβγ indicated that pertussis toxin-sensitive inhibition by the muscarinic cholinergic agonist carbachol was unaltered in myocytes overexpressing AC6. Pertussis toxin treatment failed to reveal an enhancement by AC6 overexpression of basal or PGE₂-stimulated cAMP. Immunoblot analysis of membrane fractions indicated that β₁-AR and AC6 are expressed in fractions enriched in caveolin-3 and morphologic caveolae. The data suggest that loss of G_i-mediated inhibition is not the mechanism for enhancement of β-AR-stimulated cAMP formation and that key components of β-AR-mediated activation of AC exist in caveolae of cardiac myocytes, providing a means by which β-AR response is selectively enhanced by increasing AC6 expression.