title: Beta-adrenergic receptor desensitization of wild-type but not cyc lymphoma cells unmasked by submillimolar Mg2+.
abstract: Treatment with low physiological concentrations of epinephrine (5-50 nM) rapidly desensitizes beta-adrenergic stimulation of cAMP formation in S49 wild-type (WT) lymphoma cells. Previous attempts to detect this early phase of desensitization in cell-free assays of adenylate cyclase (EC 4.6.1.1) after intact cell treatment were unsuccessful. We have now found that reducing the Mg2+ concentrations in the adenylate cyclase assays to less than 1.0 mM unmasked this rapid phase of desensitization of the WT cells, and that high Mg2+ concentrations (5-10 mM) largely obscured the desensitization. Submillimolar Mg2+ conditions also revealed a two- to threefold decrease in the affinity of epinephrine binding to the beta-adrenergic receptor after desensitization with 20 nM epinephrine. Detection of 4 beta-phorbol 12-myristate 13-acetate (PMA) desensitization of the WT beta-adrenergic receptor was also dependent on low Mg2+ as measured either by the decrease in epinephrine stimulation of adenylate cyclase or by the reduction in the affinity of epinephrine binding. Unexpectedly, when cyc- cells were pretreated with 50 nM epinephrine, the beta-adrenergic stimulation of reconstituted adenylate cyclase was not desensitized. The characteristics of the Mg2+ effect on epinephrine- and PMA-induced desensitizations suggest a similar mechanism of action with the most likely events being phosphorylations of the beta-adrenergic receptors. Our data indicate that cAMP-dependent protein kinase (EC 2.7.1.37) may play a role in the desensitization caused by low epinephrine concentrations inasmuch as this phase of desensitization did not occur in the cyc-. For the PMA-induced desensitization, the phosphorylation may be mediated by protein kinase C (EC 2.7.1.37).
authors.affiliation: Graduate School of Biomedical Sciences, University of Texas Health Science Center at Houston 77225.
authors.forename: R B
authors.initials: RB
authors.lastname: Clark
authors.forename: J
authors.initials: J
authors.lastname: Friedman
authors.forename: J A
authors.initials: JA
authors.lastname: Johnson
authors.forename: M W
authors.initials: MW
authors.lastname: Kunkel
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002460:Cell Line; D004837:Epinephrine / Q000494:pharmacology; D008223:Lymphoma / Q000378:metabolism; D008274:Magnesium / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D013347:Subcellular Fractions / Q000378:metabolism; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured / Q000378:metabolism
chemical_list: D011943:Receptors, Adrenergic, beta; D000262:Adenylyl Cyclases; D008274:Magnesium; D013755:Tetradecanoylphorbol Acetate; D004837:Epinephrine