A vasopressin analog that binds but does not activate V₁ or V₂ vasopressin receptors is not internalized into cells that express V₁ or V₂ receptors

To assess whether receptor binding is sufficient to initiate vasopressin receptor endocytosis in cells expressing the vasopressin V₁ or V₂ receptors, we synthesized a novel fluorescent-labeled vasopressin analog, [1-(β-mercapto-β,β-cyclopentamethylene propionic acid), 2-(O-ethyl)-D-tyrosine, 4-valine, 8-lysine-N⁶-carboxytetramethylrhodamine] vasopressin (R-CLVP), that binds to vasopressin receptors but does not activate intracellular events such as the mobilization of intracellular calcium or the activation of adenylate cyclase. We compared the manner in which this analog was endocytosed in cells expressing V₁ (A-10, rat smooth muscle cells) or V₂ (LLC-PK₁, porcine kidney cells) receptors with that of a full agonist, [1-(β-mercaptopropionic acid), 8-lysine-N⁶-carboxytetramethylrhodamine] vasopressin (R-MLVP) [Lutz et al. (1990) J. Biol. Chem. 265, 4657-4663; Lutz et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 6507-6511]. We showed that R-CLVP bound to both types of receptors with good affinity. It failed to increase cyclic AMP concentrations in LLC-PK₁ cells and did not increase the mobilization of intracellular calcium in A-10 cells. It bound to the surface of both these cell types in a diffuse manner and it did not undergo receptor endocytosis in either cell type. In contrast, R-MLVP, an agonist that bound to both receptor subtypes and elicited changes in intracellular cyclic AMP and calcium, bound to the surface of these cells in a diffuse manner at early times after exposure, and rapidly underwent endocytosis. We conclude that binding of vasopressin to its receptors alone is insufficient to cause receptor endocytosis, and other events distal to the receptor are required to initiate endocytosis. R-CLVP should be a useful analog in determining the factors responsible for initiating receptor endocytosis.