Ligand-induced alpha2-adrenoceptor endocytosis: relationship to Gi protein activation.
Most G protein-coupled receptors are desensitized by a uniform two-step mechanism: phosphorylation followed by arrestin binding and internalization. In this study we explored the time-, ligand-, and concentration dependence of alpha2-adrenoceptor internalization in human embryonal kidney (HEK-293) cells expressing alpha2A- and alpha2B-adrenoceptors. We also explored the relationship between ligand-induced receptor internalization and agonist efficacy, determined with a [35S]GTPgammaS binding assay. The results showed rapid dose-dependent internalization of both alpha2A- and alpha2B-receptors; the extent of internalization was directly proportional to agonist efficacy. The agonist UK 14,304 had a subtype-specific high efficacy at alpha2A-AR and dexmedetomidine at alpha2B-AR. Agonist-induced [35S]GTPgammaS binding was totally blocked by pretreatment with pertussis toxin (PTX) for both receptor subtypes, while only about 50% of the internalization was blocked by PTX. The results indicate that the extent of internalization of alpha2A-AR and alpha2B-AR is proportional to agonist efficacy, but only partly dependent on Gi protein coupling.
Olli-Lähdesmäki
Tuire
T
Department of Pharmacology and Clinical Pharmacology, University of Turku, Turku, Finland.
Tiger
Mikael
M
Vainio
Minna
M
Scheinin
Mika
M
Kallio
Jaana
J
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000068438:Brimonidine Tartrate; D002460:Cell Line; D003000:Clonidine / Q000494:pharmacology; D020927:Dexmedetomidine / Q000494:pharmacology; D004705:Endocytosis / Q000502:physiology*; D004837:Epinephrine / Q000494:pharmacology; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000502:physiology*; D006801:Humans; D009638:Norepinephrine / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000502:physiology*
D000316:Adrenergic alpha-Agonists; D011810:Quinoxalines; D018341:Receptors, Adrenergic, alpha-2; D000068438:Brimonidine Tartrate; D020927:Dexmedetomidine; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D003000:Clonidine; D009638:Norepinephrine; D004837:Epinephrine