INFUSION WITH AN OXYTOCIN RECEPTOR ANTAGONIST DELAYS PARTURITION IN AMARSUPIAL

The oxytocin receptor antagonist [1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn] -oxytocin (Atosiban) is a specific antagonist of both mesotocin- and o xytocin-induced myometrial contractions in late pregnant tammars in vi tro. Continuous intravenous infusion of Atosiban (1 mg kg(-1) day(-1)) for 3 or 7 days from day 24 of the 26.5 day gestation significantly d elayed births. In both the 3 day and 7 day infusion groups, all 15 con trol animals were pregnant and gave birth within the normal time (day 26.75 +/- 0.20, mean +/- SEM), during the infusion of saline. The neon ates weighed 387 +/- 8 mg. Deliveries were observed in 15 Atosiban-tre ated animals significantly (P < 0.05) later than in the controls (day 27.85 +/- 0.19; neonate weight 413 +/- 9 mg). All pouch young were suc cessfully suckled, even in the continued presence of Atosiban. Baselin e plasma concentrations of the prostaglandin F metabolite (PGFM) in pr egnant tammars were < 200 pg ml(-1). A surge in plasma PGFM occurred a t birth (811 +/- 116 pg ml(-1)), followed by a rapid fall to baseline concentrations within 1 h after birth. This was observed both in salin e- and in Atosiban-treated animals that gave birth during the observat ion period, and did not differ significantly between the treatment gro ups. Plasma progesterone concentrations in the control and the Atosiba n-treated animals showed the normal pattern of luteolysis immediately after birth. Thus, infusion of an oxytocin receptor antagonist at the end of gestation delays birth, the peripartum surge in prostaglandin r elease, and the fall in progesterone, suggesting that mesotocin is an important part of the hormonal cascade associated with delivery in thi s marsupial.