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In sheep, prostaglandin (PG) F-2\g=a\concentrations rise in uterine venous plasma and in myometrium and maternal cotyledons before dexamethasone-induced parturition (Liggins & Grieves, 1971). As in many other species these high PGF-2\g=a\concentrations are thought to play a role in stimulating uterine contractions (Thorburn, Nicol, Bassett, Shutt & Cox, 1972). The experimental evidence to date does not support an acute effect of PGF-2\g=a\in the pregnant ewe (Liggins, Grieves, Kendall & Knox, 1972; Oakes, Mofid, Brinkman & Assali, 1973 ; Keirse et al., 1973). However, a positive effect of chronic PGF-2\g=a\administration on uterine contractility has been reported (Liggins, Fairclough, Grieves, Kendall & Knox, 1973); in this case the myometrial sensitivity to oxytocin was greatly increased after PGF infusion for 12 hr and spontaneous uterine contractions were observed after 24 hr of treatment. This report describes experiments designed to determine whether PGF-2\g=a\can exert an acute effect on uterine activity during parturition in sheep. Aortic, amniotic, fetal and utero-ovarian venous catheters were established in 5 ewes between Days 128 and 137 of gestation. All animals gave birth to live lambs. Intra-fetal dexamethasone administration was used to induce parturition in 4 of these animals; in such ewes the endocrine changes are similar to those associated with spontaneous parturition at term (Liggins et al., 1972; Thorburn et ai, 1972). Dexamethasone was infused at a rate of 1-3 mg/24 hr into the fetal vena cava through a catheter inserted via a tarsal vein. For these 4 animals the mean (+S.D.) time from induction to parturition was 41 ±9 hr. The fifth animal gave birth spontaneously on Day 144 of gesta¬ tion. The concentrations of progesterone, total unconjugated oestrogens and PGF were measured by radioimmunoassays as previously described (Flint, Anderson, Patten & Turnbull, 1974; Mitchell, Flint & Turnbull, 1975). Serial infusions of PGF-2a were performed via the maternal aorta, with PGF-2a (THAM salt) being infused at a rate which resulted in uterine venous plasma PGF levels similar to those found previously at term (Table 1).

Stimulation of uterine activity by administration of prostaglandin F-2 during parturition in sheep