Fig. 2.
Relationship between the stimulus and efficacy (ε), intrinsic inactivity (εi), the observed affinity constant (Kobs), and the affinity constants of the active (Kb) and inactive (Ka) states of the receptor. The theoretical plots show the amount of agonist bound to the active state of the receptor (DR*) for two agonists, D and D′. The curves are based on the model shown in Scheme 1 and were derived using eqs. 5 to 7. The maximal amounts of the active and inactive states of the agonist-receptor complexes are denoted ε and εi, respectively. RAi is a relative measure of the product of affinity and efficacy of one agonist (D) expressed relative to that of another (D′), whereas RIi is a relative measure of the product of affinity and intrinsic inactivity. The mathematics described by eqs. 24 and 11 show that RAi and RIi are relative measures of the microscopic affinity constants of the active (Kb/Kb′) and inactive (Ka/Ka′) states of the receptor, respectively.