BINDING OF [H-3] SR-49059, A POTENT NONPEPTIDE VASOPRESSIN V-1A ANTAGONIST, TO RAT AND HUMAN LIVER MEMBRANES

The new potent and selective nonpeptide vasopressin V-1a antagonist, SR 49059, was tritiated and used for the characterization of rat and human liver AVP V-1a receptors. Binding of [H-3] SR 49059 was time-dependent, reversible and saturable. A single class of high affinity binding sites was identified with K-d values of 0.63 +/- 0.13 and 2.95 +/- 0.64 nM, in rat and human liver membranes, respectively. The maximal binding capacity (B-MAX) was about 7 times higher in rat than in human liver preparations. The relative potencies of several AVP/oxytocin agonists or antagonists to inhibit [H-3] SR 49059 binding confirmed that this ligand labeled a homogenous population of sites with the expected AVP V-1a profile. Furthermore, [H-3] SR 49059 or unlabeled SR 49059 displayed only slight species differences between rat and human V-1a receptors, whereas OPC-21268, another nonpeptide V-1a antagonist, exhibited a high species-related potency with more than 500 fold higher affinity for rat than for human liver V-1a receptors. Thus, [H-3] SR 49059 is the first nonpeptide AVP V-1a ligand reported having highly specific activity, stability, specificity and affinity. This makes it a suitable probe for labeling AVP V-1a receptors in rat and also in human tissues. (C) 1994 Academic Press, Inc.