An Alpha(1a)-Adrenoreceptor Variant With Differential Response To Agonists Reveals A Key Site In The Third Intracellular Loop For Agonist-Specific Regulation Of Receptor Function

Agonist occupied α1A-adrenorceptor (α1A-AR) engages several signaling pathways including phosphoinositide hydrolysis, calcium mobilization, arachidonic acid release, MAP kinase activation and cAMP accumulation. The agonists A-61603 and norepinephrine elicit maximal responses for calcium release and IP3 accumulation in cell lines stably expressing the human α1A-AR. Here we show that the actions of these two compounds differed in their activation of a variant of this receptor bearing a six amino acid insertion placed in the third intracellular loop (ICL3): A-61603 was only a weak partial agonist while norepinephrine still displayed full agonist properties in a transient Ca+2 release assay. The EC50 values for both compounds were comparable to those observed with wild-type α1A-AR. The differences in maximal response observed for these agonists at the ICL3 receptor variant may result from changes in agonist regulation of receptor coupling to the G-protein complement. We hypothesize that the ICL3 mutation of α1A-AR limits the accessible active state conformations in a manner that discriminates the agonist properties of norepinephrine and A-61603. Signaling and binding profiles for A-61603 and norepinephrine at the wild-type and ICL-3 variant α1A-AR will be presented in support of this interpretation.