Intracellular effect of imidazoline receptor on alpha(2A)-noradrenergic receptor.

Imidazoline-1 receptors (I(1)R) and alpha(2)-noradrenergic receptors (alpha(2)AR) are known to coexist in many cell types and bind many of the same imidazoline ligands. Herein, the possibility of an interaction between these receptors was explored using a cloned cDNA that encodes a protein with I(1)R-like binding properties, designated imidazoline receptor antisera-selected (IRAS). Chinese hamster ovary (CHO) sublines permanently expressing the human subtype alpha(2A)AR cDNA were transiently cotransfected with the human IRAS cDNA (pIRAS). Saturation radioligand binding experiments on membranes isolated from the various sublines allowed distinction between I(1)R and alpha(2A)AR. Transfection of pIRAS into either subline led to a rise in membrane I(1)R-binding sites. Immunoblotting revealed that IRAS was enriched in membranes more than in cytosolic fractions. Transfection of pIRAS in CHO cells harboring the alpha(2A)AR cDNA resulted in a twofold increase in alpha(2A)AR binding sites with no change in alpha(2A)AR binding affinity, compared with controls. Immunoblotting also revealed increased expression of membranous alpha(2A)AR by IRAS. Thus, pIRAS transfection led to I(1) binding sites and to an increase in alpha(2A)AR binding sites in CHO cells expressing the human alpha(2A)AR. Although the mechanism is unclear, this increase in binding sites may explain previous imidazoline drug effects suggestive of interactions between these two receptors.