DOES A RADIOLABELED LIGAND BIND TO A HOMOGENEOUS POPULATION OF NONINTERACTING RECEPTOR-SITES

Receptor subtypes of pharmacological interest are often characterized by studies in which a particular radiolabelled ligand is competitively displaced from its binding sites, either by its unlabelled analogue ( homologous displacement), or by different, unlabelled, ligands (hetero logous displacement). In this article, Stephane Swillens, Magali Waelb roeck and Philippe Champeil emphasize the fact that a single homologou s displacement curve is generally unable to reveal that the radioligan d binds to a heterogeneous receptor population. Overlooking this heter ogeneity may, in turn, dramatically affect interpretation of heterolog ous displacement curves displaying more than one receptor type far the unlabelled ligand: neither the binding affinities of the ligands for the different receptors, nor the proportion of these receptors, can be estimated reliably.