Oxytocin-stimulated inositol phosphate turnover in endometrium of ewes is influenced by stage of the estrous cycle, pregnancy, and intrauterine infusion of ovine conceptus secretory proteins

Three experiments (Exp) assessed the influence of stage of the estrous cycle, pregnancy, and intrauterine infusion of ovine conceptus secretory proteins (oCSP) on turnover of inositol trisphosphate (the putative second-messenger for oxytocin-stimulated secretion of prostaglandin F(2α)) in ovine endometrium during luteolysis and maternal recognition of pregnancy. In Exp 1, endometrium was collected from 5 cyclic (Cy) and 6 pregnant (P) ewes on Day 16 after onset of estrus. In Exp 2, endometrium was collected from Day 12 (n = 5), Day 12 P (n = 3), Day 16 Cy (n = 4), and Day 16 P (n = 3) ewes. In Exp 3, 12 Cy ewes were allotted randomly, in a 2x 2 factorial arrangement, to receive serum protein (SP), or oCSP and estradiol-17β (E₂), or vehicle treatments. Ewes were injected i.v. with 0.5 mg E₂ or vehicle on Day 12 and received twice-daily infusions of 1.5 mg SP or oCSP (containing 25 μg ovine trophoblast protein-1 by radioimmunoassay [RIA] + SP (1.5 mg total protein) into each uterine horn on Days 12, 13, and 14. Blood samples for RIA of plasma progesterone were collected on Days 10-15 (before treatment on each day) and endometrium was collected on Day 15. For each Exp, 100 mg endometrium was incubated, in duplicate, for 2 h with 10 μCi [³H]inositol and treated with 0 or 100 nM oxytocin (OT) for 20 min, then [³H]inositol mono-, bis-, and trisphosphates (IP₁, IP₂, and IP₃, respectively) were quantified. In Exp 1, OT increased IP₃ (p < 0.01) and total IP (p < 0.07) in Cy, but not P, ewes on Day 16. In Exp 2, OT increased IP₃ (p < 0.05) and total IP (p < 0.05) in Cy ewes on Day 16, but not on Day 12, suggesting that a conceptus-dependent block to OT-stimulated IP turnover is initiated between Days 12 and 16 of pregnancy. In Exp. 3, oCSP inhibited OT stimulation of IP₃ (p < .05) and total IP p < 0.09). Results indicate that OT-stimulated endometrial turnover of IP is initiated between Days 12 and 16 of the estrous cycle and blocked by pregnancy and intrauterine infusion of oCSP.