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. Author manuscript; available in PMC: 2013 Feb 11.
Published in final edited form as: Methods Mol Biol. 2011;756:133–148. doi: 10.1007/978-1-61779-160-4_6

Fig. 3.

Fig. 3

Direct recording of intrinsic efficacy at a GPCR. (a) Chemical structure of norepinephrine (NE), clonidine (Clo), and yohimbine (Yoh) as examples of full, partial and inverse α2A-AR agonists, respectively. (b) Example of FRET signals seen during sequential application of ligands of distinct efficacies to a single HEK-293 cell expressing α2AAR YFP/CFP. The left trace represents the FRET signals mediated by NE, and yohimbine. The right trace represents the action of saturating concentrations of the NE or clonidine added alone or together. Note that the simultaneous application of NE and clonidine restored the partial response seen with clonidine alone. This corresponds to the predicted properties of a high-affinity partial agonist. (c) The correlation between the rate constant (k−1) of receptor activation, and respective extent of FRET amplitude seen with ligands of different efficacies. Adapted from (11,12).