Figure 1. Normalized Dose Response Curves of the Inhibition of Forskolin-Dependent Stimulation of Adenylyl Cyclase by Quinpirole.
Inhibition curves for the D2-like selective full agonist quinpirole were analyzed using a one site model with the TABLECURVE program, where the a) values for no inhibition (100% maximum stimulation) were constrained and b) values for maximal inhibition were not constrained. Individual values for the inhibition of cyclase activity are the mean ± S.E.M. with n ≥ 3. IC50 values were 1.3 nM and 0.9 nM for D2 (●, solid line)) and D3 (○, dashed line) receptors, respectively. The maximal inhibition observed was approximately 90% and 33% for D2 and D3 receptors, respectively. The Ki values for the low affinity state of the two D2-like receptors are also shown in the figure. Levels of expression of human D2 and D3 dopamine receptors in HEK cells are 57,941 ± 14,686 fmol/mg protein and 4,202 ± 1,516 fmol/mg protein, respectively.