β 2 -Adrenergic and Several Other G Protein–Coupled Receptors in Human Atrial Membranes Activate Both G s and G i

Abstract

—Cardiac G protein–coupled receptors that couple to Gα_s and stimulate cAMP formation (eg, β-adrenergic, histamine, serotonin, and glucagon receptors) play a key role in cardiac inotropy. Recent studies in rodent cardiac myocytes and transfected cells have revealed that one of these receptors, the β₂-adrenergic receptor (AR), also couples to the inhibitory G protein Gα_i (activation of which inhibits cAMP formation). If β₂ARs could be shown to couple to Gα_i in the human heart, it would have important ramifications, because levels of Gα_i increase with age and in failing human heart. Therefore, we investigated whether β₂ARs in the human heart activate Gα_i. By photoaffinity labeling human atrial membranes with [³²P]azidoanilido-GTP, followed by immunoprecipitation with antibodies specific for Gα_i, we found that Gα_i is activated by stimulation of β₂ARs but not of β₁ARs. In addition, we found that other Gα_s-coupled receptors also couple to Gα_i, including histamine, serotonin, and glucagon. When coupling of these receptors to Gα_i is disrupted by pertussis toxin, their ability to stimulate adenylyl cyclase is enhanced. These data provide the first evidence that β₂AR and many other Gα_s-coupled receptors in human atrium also couple to Gα_i and that abolishing the coupling of these receptors to Gα_i increases the receptor-mediated adenylyl cyclase activity.