Na+ ions binding to the bradykinin B-2 receptor suppress agonist-independent receptor activation

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 conformation 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 activation we chose the bradykinin B-2 receptor as a prototypic 7TM receptor, Decrease of the intracellular Na+ content from 40 mM to 10 mM of 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 activity of bradykinin to 1.2. In contrast, under increased intracellular 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 exchanging a conserved aspartate in transmembrane domain II for asparagine 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 fibroblasts endogenously expressing human B-2 receptors by a 2.5-fold increase in basal inositolphosphates. Activation of human B-2 receptors in the absence of agonist under decreased intracellular Na+ was further evident by an increased basal phosphorylation of the B-2 receptor protein, Thus our data suggest that the interaction of Na+ ions with the B-2 receptor stabilizes or induces an inactive receptor confirmation therapy providing a mechanism to suppress agonist-independent receptor activation in vivo. activation in vivo.