Interpretation of relative potencies, relative efficacies and apparent affinity constants of agonist drugs estimated from concentration-response curves.

Differences in the relative potencies of agonists have been used successfully in the past to classify receptors. Such use of agonists can be justified on the basis of ideas and equations developed using the occupancy model of drug action. However the occupancy model makes no allowance for possible complications which may arise when the drug-receptor complex interacts with a transducer-effector system. For some receptor-effector systems use of an equilibrium ternary complex model may be better than use of the occupancy model but the former still does not take into account the possible effect of guanosine-5'-triphosphate on the system. A steady-state version of the ternary complex model has therefore been analysed to explore possible interpretations of relative potencies, relative efficacies and apparent affinity constants estimated from concentration-response curves. It is concluded that for agonists which act on receptors which function through G-proteins these pharmacological parameters may depend on the concentration of the relevant G-protein in the cell membranes and on the intracellular concentrations of guanosine-5'-triphosphate and guanosine-5'-diphosphate. If these concentrations vary appreciably between tissues then the parameters are also likely to vary, even for a single receptor-transducer system. It follows that the use of such agonist parameters to classify receptors or receptor-transducer systems is not likely to be totally dependable. It is also possible that agonists which interact with only one receptor-transducer system may show selectivity between tissues with different concentrations of G-proteins and of guanine nucleotides.