The following receptors were studied: wild type (●), S277A (○), I274A (■), H278A (□); untransfected COS-7 (×). Transfected COS-7 cells were incubated for 30 min at 37 °C (for details, see “Experimental Procedures”) with increasing concentrations of CGS 21680. Data are presented as fold increase in cyclic AMP above basal levels (400–600 cpm) in the absence of CGS 21680. Each curve represents the fold-stimulation average of two or three independent experiments, each carried out in duplicate. Alternately, the conversion factor (f) was calculated using the following equation, f = cAMP(cpm)/(cAMP(cpm) + ATP(cpm)). The ATP fraction gave very similar values in all samples (598,000 ± 4000 cpm). The maximum conversion factor was 0.7.