Binding properties of a selective tritiated vasopressin V-2 receptor antagonist, [H-3]-SR 121463

Background. [H-3]-SR 121463 is the first radiolabeled selective nonpeptide vasopressin V-2 receptor antagonist ligand that has been reported to date. In the present work, we studied the binding properties of [H-3]-SR 121463 f or renal V-2 receptors from animal and human origins. Methods. Binding studies were performed with [H-3]-SR 121463 in Chinese ham ster ovary (CHO) cells transfected with the human V-2 receptor and in vario us kidney preparations expressing the native V-2 receptors (rat, rabbit, do g, pig, monkey, and human). Autoradiographies were performed in rat and hum an kidney sections. Results. [H-3]-SR 121463 binding to CHO cells stably transfected with the c loned human renal V-2 receptor was specific, highly stable, time dependent, saturable, and reversible. A single population of high-affinity binding si tes was identified (K-d = 0.94 +/- 0.34 nmol/L, B-max = 9876 +/- 317 fmol/m g protein). Of note, [H-3]-SR 121463 revealed a higher number (about 40%) o f V-3 sites than [H-3]-AVP in the same preparation. Displacement of [H-3]-S R 121463 binding by reference peptide and nonpeptide vasopressin/oxytocin c ompounds exhibited a typical AVP V-2 profile. [H-3]-SR 121463 also displaye d a high affinity for native V-2 receptors in several kidney preparations f rom rat, pig, dog, rabbit, bovine, monkey, and human. The autoradiographic experiments using rat and human kidney sections showed intense labeling in the medullopapillary region and lower intensity in the cortex, consistent w ith a main localization of V-2 receptors on collecting tubules. Conclusion. [H-3]-SR 121463 is a useful ligand for the specific labeling of animal and human V-2 receptors and could be a suitable probe for the searc h and in situ localization of V-2 sites.