Theoretically, efficacy is the power of an agonist to impart stimulus to a system for a given receptor occupancy. Thus, if one agonist produces 50 per cent maximal system response through occupation of 10 per cent of the existing receptor population while another requires 40 per cent receptor occupation, then the former can be thought to have four times the 'efficac/ of the latter in terms of producing tissue response. The method of Furchgott (1966) does this through comparison of equiactive responses on log occupancy-response curves ( response expressed as a function of the logarithm of receptor occupancy). However, a pre-requisite to the correct use of this method is an unbiased estimate of agonist affinity (needed to calculate receptor occupancy). As noted previously, the very fact that receptor isomerization of the receptor by agonists corrupts estimates of affinity (see equation (7)) makes the use of this method circular and flawed.
An alternative method is to compare the relative maximal response of agonists as an estimate of relative efficacy. Thus, comparisons are made at saturating concentrations of agonist (with respect to receptor occupancy) and affinity ceases to be an issue. The relative maxima of two agonists can be a useful surrogate measurement of relative efficacy providing that the system maximal response is not obtained. Often because of the amplifying effects of cellular stimulus-response mechanisms, receptor stimulus exceeds the system capability to register response and a 'tissue' maximal response for a number of agonists is observed, that is, all are full agonists. This does not mean that the agonists are of identical efficacy, but rather that the efficacy of the agonists exceeds the capability of the system to differentiate them. Generally, the further away from the receptor the response is measured, in terms of biochemical reactions that take place after receptor activation, the more amplification occurs. Thus, the best opportunity to measure relative maxima as a reflection of true agonist efficacy is at the earliest step in the stimulus-response chain. This is activation of the G protein. Thus, differences in the capability of agonists to stimulate exchange of GDP to GTP on the G protein can be a useful measure of relative efficacy (i.e. Jasper etal. 1998; Umland etal. 2001).
In terms of the models of GPCR systems, the relative maxima of two agonists denoted A and B are given as:
It can be seen that this experimentally derived parameter depends upon efficacy terms and does not involve affinity.
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