title: Agonist-directed trafficking of porcine alpha(2A)-adrenergic receptor signaling in Chinese hamster ovary cells: l-isoproterenol selectively activates G(s).
abstract: In this study, we investigated the hypothesis of agonist-directed trafficking of receptor signaling for the alpha(2A)-adrenergic receptor (alpha(2A)-AR). alpha(2A)-ARs couple to both G(s) and G(i) to stimulate or inhibit adenylyl cyclase activity. Chinese hamster ovary-K1 cell lines expressing the porcine alpha(2A)-AR at high (alpha(2A)-H) and low (alpha(2A)-L) levels were used to estimate the relative efficacies (R.e.s) of a series of agonists for the G(s) and G(i) pathways. G(s)-mediated responses were measured after pertussis toxin treatment to inactivate G(i) in alpha(2A)-H, whereas G(i) responses were measured in alpha(2A)-L, where G(s) responses were absent. The full agonist UK-14,304 showed a large receptor reserve for G(i) responses in alpha(2A)-H but little receptor reserve for G(s) responses in alpha(2A)-H or for G(i) responses in alpha(2A)-L. With the exception of l-isoproterenol (ISO), all agonists showed similar R.e.s at the alpha(2A)-AR for G(s) and G(i) responses, with rank orders of R.e.s as follows: l-epinephrine = l-norepinephrine = UK-14,304 > p-aminoclonidine > or = BHT-920 > or = BHT-933 > clonidine = p-iodoclonidine > or = xylazine > or = guanabenz. Interestingly, ISO had the highest efficacy at the alpha(2A)-AR for activating G(s) versus G(i) (9-fold higher); however, it had low potency for both. By several criteria, the ISO response was mediated by the alpha(2A)-AR, supporting the hypothesis of agonist-directed trafficking of receptor signaling or agonist-specific G protein selectivity. In contrast, the apparent G(i) pathway selectivity of oxymetazoline appears to be mediated by an endogenous serotonergic receptor. It is intriguing that a classic beta-AR agonist that activates G(s) through beta(2)-ARs also appears to produce a G(s)-selective conformation of the G(i)-coupled alpha(2A)-AR.
authors.affiliation: Department of Pharmacology, University of Michigan, Ann Arbor 48109- 0632, USA.
authors.forename: C B
authors.initials: CB
authors.lastname: Brink
authors.forename: S M
authors.initials: SM
authors.lastname: Wade
authors.forename: R R
authors.initials: RR
authors.lastname: Neubig
mesh_terms: D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D001665:Binding Sites; D000068438:Brimonidine Tartrate; D016466:CHO Cells; D006224:Cricetinae; D004347:Drug Interactions; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000819:agonists / Q000502:physiology; D019205:GTP-Binding Protein alpha Subunits, Gs / Q000819:agonists / Q000502:physiology*; D007545:Isoproterenol / Q000494:pharmacology*; D007700:Kinetics; D010109:Oxymetazoline / Q000037:antagonists & inhibitors / Q000494:pharmacology; D010869:Pindolol / Q000031:analogs & derivatives / Q000494:pharmacology; D011485:Protein Binding; D011810:Quinoxalines / Q000494:pharmacology; D018341:Receptors, Adrenergic, alpha-2 / Q000378:metabolism / Q000502:physiology; D012702:Serotonin Antagonists / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D013552:Swine; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D000318:Adrenergic beta-Agonists; D011810:Quinoxalines; D018341:Receptors, Adrenergic, alpha-2; D012702:Serotonin Antagonists; D015016:Yohimbine; C047975:cyanopindolol; D000068438:Brimonidine Tartrate; D010109:Oxymetazoline; D010869:Pindolol; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D019205:GTP-Binding Protein alpha Subunits, Gs; D007545:Isoproterenol