Fluorescent localization of the β-adrenergic receptor on DDT-1 cells. Down-regulation by adrenergic agonists

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Abstract
Continuous incubation of cultured cells with .beta.-adrenergic agonists results in the desensitization of adrenergic responsiveness accompanied by the down-regulation of cell surface .beta.-adrenergic receptors (.beta.AR). Previous studies have relied on measurements of ligand binding activity for the detection of the .beta.AR in the cell. In the present study, we have raised a monoclonal antibody to a synthetic peptide corresponding to amino acid numbers 226-239 of the hamster .beta.2AR. This antibody was used to localize the .beta.AR in hamster smooth-muscle DDT-1 cells by immunofluorescence, without regard for the ability of the receptor to bind ligands. The .beta.AR was found to be localized primarily at the plasma membrane of these cells, with a nonhomogenous pattern of distribution. A rapid loss of .beta.AR-specific immunofluorescence, which paralleled receptor down-regulation as measured by ligand-binding activity, was seen with .beta.-adrenergic agonists, but not with antagonists. In addition, a transient increase in fluorescence was observed after short times of exposure of the cells to agonists. This fluorescence increase may reflect a ligand-induced conformational change in the receptor.

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