Assessment of receptor internalization and recycling.

Internalization of G-protein-coupled receptors (GPCRs) occurs in response to agonist activation of the receptors and causes a redistribution of receptors away from the plasma membrane toward endosomes. Internalization of lower-affinity small molecule GPCRs such as muscarinic acetylcholine and adrenergic receptors has been measured using hydrophilic antagonist radioligands that are membrane impermeant. In contrast, internalization of peptide hormone receptors is assessed by measuring the internalization of a radiolabeled- or fluorescently labeled peptide hormone. More recently, the use of epitope-tagged receptors has allowed the measurement of changes in receptor subcellular distribution by the use of immunoassay and immunofluorescence confocal microscopy. This chapter describes each of these approaches to the measurement of receptor internalization and describes the advantages and disadvantages of each method.