Fig. 1.

Structures of β receptor ligands: (A) noradrenaline; (B) isoprenaline; (C) carazolol; (D) ICI118551; (E) cyanopindolol; and (F) 7-methylcyanopindolol. Portions of the structures in blue are regions analogous to those found in noradrenaline. The oxymethylene spacer between the ethanolamine backbone and ligand head group that prevents contraction of the ligand-binding pocket in the antagonists is shown in red. Noradrenaline and isoprenaline are regarded as full agonists (Baker, 2010b), carazolol and cyanopindolol are regarded as weak partial agonists (Baker, 2010b), and ICI18551 is regarded as an inverse agonist (Bond et al., 1995; Azzi et al., 2001).