Age Increases Cardiac Gαi2 Expression, Resulting in Enhanced Coupling to G Protein-coupled Receptors

Cardiac G protein-coupled receptors that function through stimulatory G protein Gαs, such as β1- and β2-adrenergic receptors (β1ARs and β2ARs), play a key role in cardiac contractility. Recent data indicate that several Gαs-coupled receptors in heart also activate Gαi, including β2ARs (but not β1ARs). Coupling of cardiac β2ARs to Gαi inhibits adenylyl cyclase and opposes β1AR-mediated apoptosis. Dual coupling of β2AR to both Gαs and Gαi is likely to alter β2AR function in disease, such as congestive heart failure in which Gαi levels are increased. Indeed, heart failure is characterized by reduced responsiveness of βARs. Cardiac βAR-responsiveness is also decreased with aging. However, whether age increases cardiac Gαi has been controversial, with some studies reporting an increase and others reporting no change. The present study examines Gαi in left ventricular membranes from young and old Fisher 344 rats by employing a comprehensive battery of biochemical assays. Immunoblotting reveals significant increases with age in left ventricular Gαi2, but no changes in Gαi3, Gαo, Gαs, Gβ1, or Gβ2. Aging also increases ADP-ribosylation of pertussis toxin-sensitive G proteins. Consistent with these results, basal as well as receptor-mediated incorporation of photoaffinity label [32P]azidoanilido-GTP indicates higher amounts of Gαi2 in older left ventricular membranes. Moreover, both basal and receptor-mediated adenylyl cyclase activities are lower in left ventricular membranes from older rats, and disabling of Gαi with pertussis toxin increases both basal and receptor-stimulated adenylyl cyclase activity. Finally, age produces small but significant increases in muscarinic potency for the inhibition of both β1AR- and β2AR-stimulated adenylyl cyclase activity. The present study establishes that Gαi2 increases with age and provides data indicating that this increase dampens adenylyl cyclase activity.