Localization of β-adrenergic receptors in A431 cells in situ. Effects of chronic exposure to agonist

The status of β-adrenergic receptors was investigated in A431 cells exposed to chronic stimulation by the β-adrenergic agonist, (-)-isoproterenol. Specific binding of β-adrenergic antagonist (-)-[¹²⁵I]iodocyanopindolol declined to 60-80% below control values within 12 h of agonist treatment. This decline in ligand binding was also observed in high-speed membrane fractions prepared from agonist-treated cells. Immunoblots probed with anti-receptor antibodies revealed both that β-adrenergic receptors from untreated and treated cells migrated as 65000-M(r) peptides and that the cellular complement of receptor was unchanged. Indirect immunofluorescence localization of β-adrenergic receptors was comparable in control (untreated) cells and cells challenged with (-)-isoproterenol for 1, 12, or 24 h. Thus receptor complement, migration on SDS/polyacrylamide-gel electrophoresis, and localization in situ are largely unaffected by agonist stimulation. Receptor binding of antagonist radioligands, in contrast, is markedly down-regulated in cells stimulated chronically with β-adrenergic agonists. These data argue in favour of agonist-induced alteration(s) in the conformation of the receptor that preclude radioligand binding rather than agonist-induced receptor sequestration and/or degradation.