Receptor binding techniques: principles

Receptor binding studies are possible because of the high affinity that some agonists and antagonists have for their receptor. Consequently, at low concentrations of drug, a high proportion is bound to the receptor compared to the proportion which binds to non-receptor sites.

Although only minute amounts of receptor are present in most tissues (typically less than 1pmol/mg protein), the amount of drug bound can be measured by radiolabelling it and measuring the amount of radioactivity bound to the tissue. It is essential to separate the bound drug from that which is free in solution, and this can be achieved by filtration (drug bound to the tissue is retained on the filter paper, but unbound drug passes through) or by centrifugation (in which the bound drug is trapped in the tissue pellet). The former is generally the method of choice provided that the drug does not dissociate significantly from the membranes during the process of filtration or does not bind to the filter.

Saturation analysis
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