Ovine trophoblast protein-one inhibits development of endometrial responsiveness to oxytocin in ewes

In experiment (Exp) 1, 12 cyclic ewes had catheters placed into each uterine horn on Day 7 (estrus = Day 0). On days 11-15, 6 ewes received twice-daily intrauterine infusions of 1.5 mg serum protein (SP) into each uterine horn and 6 ewes received infusions of 1.08 mg SP + 0.42 mg ovine conceptus secretory proteins (oCSP) containing 25 μg ovine trophoblast protein-one (oTP-1) as determined by radioimmunoassay (25-35% bioactive by antiviral assay). SP-infused and oCSP-infused ewes had similar plasma 13,14-dihydro-15-keto prostaglandin F(2α) (PGF(2α)) profiles in response to oxytocin on Day 11, but SP ewes became more responsive (p < 0.01) to oxytocin on Days 13 and 15 than oCSP ewes. SP ewes also had greater incorporation of [³H]inositol into inositol trisphosphate (IP₃) (+3 449%, p < 0.01) and total inositol phosphate (IP) (+760%, p < 0.08), in response to oxytocin, than did oCSP ewes (+553 and +168% + 138% for IP₃ and total IP, respectively) in endometrium collected at ovariectomy/hysterectomy on Day 16. Mean CL weights on Day 16 and mean concentrations of progesterone in plasma collected at 12-h intervals on Days 6-16 were not different for SP and oCSP ewes, but concentrations of progesterone were lower (p < 0.05) in SP ewes on Days 15-16 than for oCSP ewes. These results indicate that oTP-1 may prevent luteolysis by inhibiting development of endometrial responsiveness to oxytocin and, therefore, reduce oxytocin-induced synthesis of IP₃ and PGF(2α). In Exp 2, 3 cyclic and 3 pregnant ewes were hysterectomized on Day 16 and endometrium from each was incubated in a 3 x 2 x 2 factorial arrangement of 37°, 40°, or 43°C, with 0 or 100 nM oxytocin and with 0 or 500 nM oxytocin antagonist (L-365,209). Greater incorporation of [³H]inositol into IP₃ (p < 0.05) and total IP (p < 0.09) in response to oxytocin was detected for cyclic (+2 472 and +1 420%, respectively) than for pregnant ewes (+74 and +40%, respectively). The oxytocin antagonist reduced (p < 0.05) IP turnover primarily in endometrium from cyclic ewes and primarily inhibited (p < 0.01) oxytocin-stimulated total IP turnover in endometrium. Incorporation of [³H]inositol into IP₃ (p = 0.01) and total IP (p < 0.01) increased linearly with increased incubation temperatures, but elevated temperatures did not significantly enhance oxytocin-stimulated IP turnover in endometrium from pregnant and cyclic ewes. These results provide indirect evidence that oTP-1 does not reduce the number of available endometrial oxytocin receptors primarily by inhibiting oxytocin receptor recycling.