Evidence for oxytocin receptors in cultured bovine luteal cells

Specific receptors for oxytocin (OT) on intact luteal cells are demonstrated. Cultured cells from bovine corpora lutea (CL) at different stages (Days 3-5, 8-12, and 15-18 of the estrous cycle) were examined for OT receptors by a radioreceptor assay using the ¹²⁵I-labeled OT antagonist [d(CH₂)₅,Tyr(Me)², Thr⁴,Tyr-NH₂⁹]-vasotocin. Binding specificity was demonstrated in displacement studies with various related peptides. Scatchard analysis revealed the presence of a binding site with an association constant of K(a) = 2.6 x 10⁹ M^-1 and a capacity of 5.9 fmol/μg DNA. Additionally, in 50% of the experiments (n = 6) two different binding sites were observed. The K(a) of the high-affinity site was 2.6 x 10¹⁰ M^-1; its capacity was 0.73 fmol/μg DNA. The low-affinity site had an apparent K(a) of 4.9 x 10⁸ M^-1 and a capacity of 8.8 fmol/μg DNA. Observation of one versus two binding sites related neither to the assay conditions nor to the state of the individual CL used for the cell culture and therefore appeared to reflect individual variation within the OT receptor population. Significant binding of OT was observed at all luteal stages. OT binding was maximal at the mid- luteal stage (Days 8-12). We conclude that a direct action of OT on the bovine CL is mediated by the OT receptor, supporting the hypothesis that luteal OT plays an important physiological role in the regulation of progesterone release and/or other luteal functions in a paracrine or autocrine fashion.