Uterine oxytocin receptors in an Australian marsupial, the brushtail possum, Trichosurus vulpecula.

1. Oxytocin receptors in the uterus of the brushtail possum (T. vulpecula) were characterized by radioreceptor assay and compared with those of the sheep and rat uterus. 2. A single oxytocin binding site was found with an affinity (Kd) and receptor concentration (Ro) of 3.0 +/- 0.8 nmol/l and 200 +/- 60 fmol/mg protein, respectively (SEM; n = 5). The receptor was stable at -20 degrees C; divalent ions were required for optimum binding. 3. Competitive displacement curves with related peptides showed the following order of specificity: vasotocin greater than oxytocin greater than mesotocin = arginine-vasopressin = [Thr4, Gly7]-oxytocin greater than lysine-vasopressin = isotocin much greater than [d(CH2)5, D-Phe2, Ile4, Ala9-NH2]-AVP. 4. It was concluded that oxytocin receptors in the possum have similar characteristics to those of placental mammals.