Synthesis and characterization of fluorescein- and rhodamine-labeled probes for vasotocin receptors

Fluorescent analogues of vasotocin, [1-(β-mercaptopropionic acid), 9-(p-aminofluoresceinylphenylalanine)]arginine vasotocin →MPA¹, (p-NH₂flu)Phe⁹]AVT← and [1-(β-mercaptopropionic acid), 9-(p-amino rhodaminylphenylalanine)]AVT →MPA¹, (p-NH₂rhod)-Phe⁹]AVT←, were synthesized by the solid-phase method. These compounds yielded half-maximal hydrosmotic responses (half-maximal values) in the toad urinary bladder at 2 x 10^-9 M. Epifluorescence microscopy showed punctate basal localization of analogues on the majority of bladder epithelial cells within 20 min. The cellular localization was prevented by vasotocin. These fluorescent analogues may serve as useful probes for vasotocin receptors in toad bladder and in tissues from other species that use vasotocin as their antidiuretic-pressor hormone.