Abstract

In this study, we investigated the hypothesis of agonist-directed trafficking of receptor signaling for the α_2A-adrenergic receptor (α_2A-AR). α_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 α_2A-AR at high (α_2A-H) and low (α_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 α_2A-H, whereas G_i responses were measured in α_2A-L, where G_s responses were absent. The full agonist UK-14,304 showed a large receptor reserve for G_i responses in α_2A-H but little receptor reserve for G_s responses in α_2A-H or for G_i responses in α_2A-L. With the exception ofl-isoproterenol (ISO), all agonists showed similar R.e.s at the α_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 ≥ BHT-920 ≥ BHT-933 > clonidine = p-iodoclonidine ≥ xylazine ≥ guanabenz. Interestingly, ISO had the highest efficacy at the α_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 α_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 β-AR agonist that activates G_s through β₂-ARs also appears to produce a G_s-selective conformation of the G_i-coupled α_2A-AR.