Abstract

We show here that the rat vasopressin V_1b receptor simultaneously activates both the G_q/11-inositol phosphate (IP) and G_s-cAMP pathways when transiently expressed in Chinese hamster ovary, human embryonic kidney (HEK) 293, and COS-7 cells and stimulated with arginine-vasopressin. Higher concentrations of the hormone, however, were needed to trigger the cAMP pathway. The nonmammalian analog arginine-vasotocin and the selective V_1b agonist d[Cha⁴]vasopressin also activated the cAMP and IP pathways, although d[Cha⁴]-vasopressin elicited the two responses with equivalent potencies. We determined that the V_1b receptor is present as a homodimer at the plasma membrane. Treatment of V_1b-transfected HEK-293 cells with methyl-β-cyclodextrin, a drug known to dissociate cholesterol-rich domains of the plasma membrane, shifted the EC₅₀ of the vasopressin-induced cAMP accumulation to lower concentrations and, remarkably, increased the hormone efficacy related to the activation of this second messenger system. In parallel, the vasopressin-mediated activation of the IP pathway was slightly reduced without modification of its EC₅₀. These results suggest that, as with many other G protein-coupled receptors, when transfected in heterologous cell systems, the V_1b receptor forms dimers that signal differentially through the G_q/11 and G_s proteins depending on the nature of the ligand as well as on its localization within specialized compartments of the plasma membrane. The present study thus illustrates how signal transduction associated with the activation of a G protein-coupled receptor can be versatile and highly dependent on both the cell context and the chemical nature of the extracellular signaling messenger.