An α 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 α 1A ‐adrenorceptor (α 1A ‐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 IP 3 accumulation in cell lines stably expressing the human α 1A ‐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 EC 50 values for both compounds were comparable to those observed with wild‐type α 1A ‐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 α 1A ‐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 α 1A ‐AR will be presented in support of this interpretation.