Figure 7.

Agonist dependence of β-arrestin/β₂AR interactions assessed by BRET. BRET ratio measured in cells coexpressing β₂AR-Rluc and β₂-arrestin-YFP in the presence of increasing concentrations of the agonist isoproterenol (■). The open square represents the value observed in the absence of isoproterenol, and the open circle represents the value observed in cells expressing β₂AR-Rluc alone. The data represent the mean ± SEM of four independent readings that were analyzed by using a four-parameter logistic equation (GraphPad prism 2.01) and fixing the Hill coefficient to 1 (EC₅₀ = 0.45 nM). The data also were analyzed allowing for variable Hill coefficients. In this case, the fitted Hill coefficient was found to be 0.74 and the EC₅₀ was 0.43 nM.