Agonist-induced internalization of the β adrenergic receptor in a smooth muscle cell line

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The mechanism of agonist-induced desensitization of the β adrenergic receptor coupled adenylate cyclase has been studied in a smooth muscle cell line, BC3H-1, which expresses both α and β adrenergic receptors and nicotinic receptors. β receptors have been investigated in intact cells using as radioligand 3HCGP-12177, an hydrophilic compound which labels only surface receptors. The treatment of BC3H-1 cells with the agonist Isoproterenol, at 37° but not at 4°, induced a dose dependent internalization of the β adrenergic receptor. Agonist-induced internalization was very rapid, in the order of few minutes. β adrenergic receptor internalization was very specific: the α adrenergic agonist Phenylefrine had almost no effect on βreceptor levels, while Isoproterenol treatment had no effect on the number of α adrenergic or nicotinic receptors expressed at the cell surface of these cells. β adrenergic receptor internalization is probably the major mechanism responsible for catecholamine desensitization in smooth muscle cells.