Cells were plated in 10-cm plates as previously described. Cells were transfected with plasmids encoding cDNA for the Glosensor reporter (Promega, Madison, WI), receptor, and Gα-subunit at a ratio of 2:1:1 (2 μg: 1 μg: 1μg). The next day, cells were plated in black, clear-bottom, 384-well white plates. After aspiration of the medium on the day of the assay, cells were incubated for 60 minutes at 37°C with 20 μL of 5 mM luciferin substrate (GoldBio, St. Louis, MO) freshly prepared in assay buffer. For Gαs activity, 10 μL of drugs were added using the FLIPR Tetra® liquid-handling robot and read after 15 minutes in a Spectramax luminescence plate reader (Molecular Devices, San Jose, CA) with a 0.5 second signal integration time. For Gαi activity, 10 μL of drugs were added for a 15-minute incubation period. Subsequently, 10 μL of isoproterenol (final concentration of 200 nM) was added and incubated for an additional 15-minute period before reading.