PT - JOURNAL ARTICLE AU - Bronwyn A. Evans AU - Natalie Broxton AU - Jon Merlin AU - Masaaki Sato AU - Dana S. Hutchinson AU - Arthur Christopoulos AU - Roger J. Summers TI - Quantification of Functional Selectivity at the Human α<sub>1A</sub>-Adrenoceptor AID - 10.1124/mol.110.067454 DP - 2011 Feb 01 TA - Molecular Pharmacology PG - 298--307 VI - 79 IP - 2 4099 - http://molpharm.aspetjournals.org/content/79/2/298.short 4100 - http://molpharm.aspetjournals.org/content/79/2/298.full SO - Mol Pharmacol2011 Feb 01; 79 AB - Although G protein-coupled receptors are often categorized in terms of their primary coupling to a given type of Gα protein subunit, it is now well established that many show promiscuous coupling and activate multiple signaling pathways. Furthermore, some agonists selectively activate signaling pathways by promoting interaction between distinct receptor conformational states and particular Gα subunits or alternative signaling proteins. We have tested the capacity of agonists to stimulate Ca2+ release, cAMP accumulation, and changes in extracellular acidification rate (ECAR) at the human α1A-adrenoceptor. Signaling bias factors were determined by novel application of an operational model of agonism and compared with the reference endogenous agonist norepinephrine; values significantly different from 1.0 indicated an agonist that promoted receptor conformations distinct from that favored by norepinephrine. Oxymetazoline was a full agonist for ECAR and a partial agonist for Ca2+ release (bias factor 8.2) but failed to stimulate cAMP production. Phenylephrine showed substantial bias toward ECAR versus Ca2+ release or cAMP accumulation (bias factors 21 and 33, respectively) but did not display bias between Ca2+ and cAMP pathways. Cirazoline and N-[5-(4,5-dihydro-1H-imidazol-2-yl)-2-hydroxy-5,6,7,8-tetrahydronaphthalen-1-yl]methanesulfonamide (A61603) displayed bias toward cAMP relative to Ca2+ release (bias factors of 7.4 and 8.6). It is noteworthy that epinephrine, a second endogenous adrenoceptor agonist, did not display bias relative to norepinephrine. Our finding that phenylephrine displayed significant signaling bias, despite being highly similar in structure to epinephrine, indicates that subtle differences in agonist-receptor interaction can affect conformational changes in cytoplasmic domains and thereby modulate the repertoire of effector proteins that are activated.