Na+ ions binding to the bradykinin B2 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 B2 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 B2 receptors activated the B2 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 B2 receptor was prevented by exchanging a conserved aspartate in transmembrane domain II for asparagine the B2 receptor was also constitutively-activated in the absence of agonist. Agonist-independence B2 receptor activation under decreased intracellular Na+ was similarly observed with primary human fibroblasts endogenously expressing human B2 receptors by a 2.5-fold increase in basal inositolphosphates. Activation of human B2 receptors in the absence of agonist under decreased intracellular Na+ was further evident by an increased basal phosphorylation of the B2 receptor protein. Thus our data suggest that the interaction of Na+ ions with the B2 receptor stabilizes or induces an inactive receptor conformation thereby providing a mechanism to suppress agonist-independent receptor activation in vivo.
Quitterer
U
U
Institute of Pharmacology and Toxicology, University of Würzburg, Germany.
AbdAlla
S
S
Jarnagin
K
K
Müller-Esterl
W
W
D000818:Animals; D001920:Bradykinin / Q000031:analogs & derivatives / Q000378:metabolism; D019556:COS Cells; D002460:Cell Line; D006801:Humans; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000378:metabolism; D007424:Intracellular Fluid / Q000378:metabolism; D008024:Ligands; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D017354:Point Mutation; D051381:Rats; D043782:Receptor, Bradykinin B2; D018002:Receptors, Bradykinin / Q000819:agonists / Q000235:genetics / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*
D007295:Inositol Phosphates; D008024:Ligands; D043782:Receptor, Bradykinin B2; D018002:Receptors, Bradykinin; C065679:icatibant; D012964:Sodium; D001920:Bradykinin