Functional studies either directly observe the effects of ligands on receptor systems when those ligands possess efficacy (agonists, inverse agonists) or measure the effects of ligands on tracer agonists by observation of the modification of tracer response. It should be noted that ligand-mediated agonist response results from the activation of a receptor to produce a stimulus and the subsequent processing of that stimulus through complicated biochemical cascade reactions to yield an observed response. Therefore, it is not possible to directly equate the amount of response with the degree of receptor occupancy to gauge the 'power' of a ligand to induce that response. Instead, indirect approaches, which compare equieffective agonist concentrations based on the null method, are used to estimate relative affinity and efficacy in functional studies.
With the ability to transfect surrogate cells with GPCRs has come the ability to create functional assays in reporter, melanophore, second messenger, and yeast formats (Chapter 10). This technology therefore allows the measurement of agonism for virtually any receptor known to couple to G proteins. The tools to quantify this agonism (and antagonism of agonist response) historically have been developed in isolated tissue systems but can directly be applied to recombinant functional assays.
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