PT - JOURNAL ARTICLE AU - Christiaan B. Brink AU - Susan M. Wade AU - Richard R. Neubig TI - Agonist-Directed Trafficking of Porcine α<sub>2A</sub>-Adrenergic Receptor Signaling in Chinese Hamster Ovary Cells: <em>l</em>-Isoproterenol Selectively Activates G<sub>s</sub> DP - 2000 Aug 01 TA - Journal of Pharmacology and Experimental Therapeutics PG - 539--547 VI - 294 IP - 2 4099 - http://jpet.aspetjournals.org/content/294/2/539.short 4100 - http://jpet.aspetjournals.org/content/294/2/539.full SO - J Pharmacol Exp Ther2000 Aug 01; 294 AB - In this study, we investigated the hypothesis of agonist-directed trafficking of receptor signaling for the α2A-adrenergic receptor (α2A-AR). α2A-ARs couple to both Gs and Gi to stimulate or inhibit adenylyl cyclase activity. Chinese hamster ovary-K1 cell lines expressing the porcine α2A-AR at high (α2A-H) and low (α2A-L) levels were used to estimate the relative efficacies (R.e.s) of a series of agonists for the Gs and Gi pathways. Gs-mediated responses were measured after pertussis toxin treatment to inactivate Gi in α2A-H, whereas Gi responses were measured in α2A-L, where Gs responses were absent. The full agonist UK-14,304 showed a large receptor reserve for Gi responses in α2A-H but little receptor reserve for Gs responses in α2A-H or for Gi responses in α2A-L. With the exception ofl-isoproterenol (ISO), all agonists showed similar R.e.s at the α2A-AR for Gs and Gi responses, with rank orders of R.e.s as follows: l-epinephrine =l-norepinephrine = UK-14,304 &gt;p-aminoclonidine ≥ BHT-920 ≥ BHT-933 &gt; clonidine = p-iodoclonidine ≥ xylazine ≥ guanabenz. Interestingly, ISO had the highest efficacy at the α2A-AR for activating Gs versus Gi (9-fold higher); however, it had low potency for both. By several criteria, the ISO response was mediated by the α2A-AR, supporting the hypothesis of agonist-directed trafficking of receptor signaling or agonist-specific G protein selectivity. In contrast, the apparent Gi pathway selectivity of oxymetazoline appears to be mediated by an endogenous serotonergic receptor. It is intriguing that a classic β-AR agonist that activates Gs through β2-ARs also appears to produce a Gs-selective conformation of the Gi-coupled α2A-AR. The American Society for Pharmacology and Experimental Therapeutics