Do pharmacological methods for the quantification of agonists work when the ternary complex mechanism operates?

It is well established that many receptors couple to G-proteins in order to subserve their pharmacological or physiological effects. In those systems it is possible that a ternary complex mechanism operates in which initiation of an effect depends on the concentration of agonist-receptor-G-protein complex formed. Such systems may be considered to obey a receptor-transducer model (Black & Leff, 1983, Proc. R. Soc. B220, 141). A theoretical analysis of this model is presented which seeks to determine how the operation of the ternary complex mechanism affects the quantification of agonists by conventional pharmacological methods. Previous analyses have concluded that pharmacological models may or may not accommodate the ternary complex mechanism depending upon the relationship between the relative concentrations of receptor and transducer units, [R0], and [T0] respectively. The present study extends these in two ways. It considers the impact of the ternary complex mechanism on agonist quantification under a more complete range of conditions relating [R0] and [T0], and it does so with regard to the analysis of partial agonists (by the comparative method) as well as of full agonists (by the method of receptor inactivation). The following predictions are made: (i) reliable estimates of affinity and efficacy can be made using the comparative method under the conditions [R0] much greater than [T0] and [R0] much less than [T0] whereas the inactivation method only works under the former condition; (ii) errors occur in the estimation of affinity and efficacy by both methods when [R0] = [T0] although better estimates are produced by the comparative method; (iii) when errors occur in the absolute estimation of affinity and efficacy, the orders of affinity and efficacy determined by the comparative method will generally be correct but this is not the case for the inactivation method; (iv) in general, the comparative method for agonist quantification appears to produce more reliable information for the purposes of receptor classification and medicinal chemistry than does the receptor inactivation method.