NA-2 RECEPTOR SUPPRESS AGONIST-INDEPENDENT RECEPTOR ACTIVATION( IONS BINDING TO THE BRADYKININ B)

Control of the balance between receptor activation and inactivation is a prerequisite for seven transmembrane domain (7TM) receptor function , We asked for a mechanism to stabilize the inactive receptor conforma tion which prevents agonist-independent receptor activation. Na+ ions have reciprocal effects on agonist versus antagonist interaction with various 7TM receptors, To investigate the Na+ dependence of receptor a ctivation we chose the bradykinin B-2 receptor as a prototypic 7TM rec eptor, Decrease of the intracellular Na+ content from 40 mM to 10 mM o f COS-1 cells transiently expressing rat B-2 receptors activated the B -2 receptor in the absence of agonist as shown by a 3-fold increase in the basal release of inositolphosphates and increased the intrinsic a ctivity of bradykinin to 1.2. In contrast, under increased intracellul ar Na+ (148 mM) the intrinsic activity of bradykinin decreased to 0.72 . When the interaction of Na+ with the B-2 receptor was prevented by e xchanging a conserved aspartate in transmembrane domain II for asparag ine the B-2 receptor was also constitutively-activated in the absence of agonist, Agonist-independent B receptor activation under decreased intracellular Na+ was similarly observed with primary human fibroblast s endogenously expressing human B-2 receptors by a 2.5-fold increase i n basal inositolphosphates. Activation of human B-2 receptors in the a bsence of agonist under decreased intracellular Na+ was further eviden t by an increased basal phosphorylation of the B-2 receptor protein, T hus our data suggest that the interaction of Na+ ions with the B-2 rec eptor stabilizes or induces an inactive receptor confirmation therapy providing a mechanism to suppress agonist-independent receptor activat ion in vivo. activation in vivo.