Oxytocin receptors in the porcine endometrium during the estrous cycle and early pregnancy

Oxytocin (OT) receptors in the porcine endometrium were investigated at four stages of the estrous cycle (Days (D) 0, 5, 10 and 15, n = 3), and at two stages of early pregnancy (D5 and D15 after mating, n = 3) by a radioreceptor assay using ¹²⁵I-labeled OT antagonist [d(CH₂)₅,Tyr(Me)²,Thr⁴,Tyr-NH ₂⁹]-vasotocin. Binding specificity was demonstrated by displacement with four peptides related to oxytocin ([Arg⁷]-vasopressin, [Thr⁴,Gly⁷]-OT, OVT, OT) and two peptides unrelated to oxytocin (luteinizing hormone-releasing hormone, [Ile³]-pressinoic acid (tocinoic acid)). The dissociation constant (K_d) of endometrial OT receptors on D0 (0.59 ± 0.10 nM) was similar to those on D10 and D15 (D10, 0.75 ± 0.21; D15, 0.60 ± 0.14 nM; mean ± SEM). In the early luteal stage (D5), K_d (2.41 ± 0.24 nM) was higher than on D0, D₁₀ and D15 (P < 0.01). In early pregnancy, K_d values were 3.25 ± 0.29 nM on D5 and 2.44 ± 0.44 nM on D15. Binding site concentration (B_max) on D0 (910.0 ± 25.1 fmol mg^-1 protein) was significantly higher than on D5 and D10 (D5, 322.5 ± 71.7; D10, 147.5 ± 25.8 fmol mg^-1 protein; P < 0.01) of the estrous cycle and D5 and D15 (D5, 302.5 ± 82.6; D15, 315.0 ± 20.1 fmol mg^-1 protein; P < 0.01) of early pregnancy. In the two stages of early pregnancy, B_max values were constant and similar to that on D5 of the early luteal stage. Our results reveal the existence of specific OT binding sites in the porcine endometrium during the estrous cycle and early pregnancy. Furthermore, the fluctuation in the binding of OT to the endometrium during the different stages of the estrous cycle suggests that OT plays an important role in regulating the estrous cycle of the pig as seen in other animals.