LOCALIZATION AND QUANTIFICATION OF NONAPEPTIDE BINDING-SITES IN THE KIDNEY OF XENOPUS-LAEVIS - EVIDENCE FOR THE EXISTENCE OF 2 DIFFERENT NONAPEPTIDE RECEPTORS

The distribution and properties of nonapeptide binding sites in the kidney of the anuran Xenopus laevis were investigated using quantitative in vitro autoradiography. The binding studies were performed with [3H]arginine vasopressin (AVP) as ligand because [125I] arginine vasotocin (AVT) lacks biological activity. Specific binding sites for [3H]AVP are located in the glomeruli of the kidney. [3H]AVP binding results in a steady state of association and dissociation between ligand and binding sites. Scatchard and Hill analyses of saturation experiments showed that [3H]AVP binds to a single class of binding sites with a dissociation constant (Kd) of 430 ± 109 pM and a maximum binding capacity (Bmax) of 5.306 ± 1.379 fmol/mm2 (n = 8). Displacement studies demonstrated the same affinity of these [3H]AVP binding sites to [3H]AVP, unlabeled AVP, and AVT, whereas mesotocin possesses only weak affinity. Further nonapeptides like oxytocin and isotocin or the mammalian-specific V1 receptor antagonist ((1-β-mercapto-β,β-cyclopentamethylene propionic acid)-2-(O-methyl)-tyrosine)-AVP or the V2 receptor agonist (1-deamino-8-d-arginine)-vasopressin or unrelated peptides did not alter the binding of [3H]AVP. The localization of nonapeptide binding sites in the glomeruli with the same affinity to AVP as to AVT agrees with the finding that AVT causes antidiuresis in Xenopus laevis. An earlier study demonstrated Xenopus laevis interrenal tissue to possess a higher sensitivity for AVT than AVP which points to a nonapeptide receptor with a higher affinity for AVT than AVP. There is now evidence that two different nonapeptide receptors may exist in Xenopus laevis: one interrenal (affinity to AVT ≫ AVP) and another glomeruli (affinity to AVT = AVP). These studies confirm that each type of nonapeptide receptor is amphibian specific because neither the mammalian V1 antagonist nor the V2 agonist gave specific effects. © 1992.