Fig. A.1.

When the full model is initialized from a steady state with 0 NE, and 1 μM NE is abruptly added, $G_{s,\alpha }^{\mathit{\text{GTP}}}$ adapts to its quasi-steady state value much more rapidly than do cAMP and β₁AR concentrations. This justifies the simplification that $G_{s,\alpha }^{\mathit{\text{GTP}}}$ reaches quasi-steady state instantaneously, reducing the system of three variables to a two-dimensional system.