Prostaglandin synthase activity of fetal sheep cotyledons at 122 days of gestation and term: Expression of prostaglandin synthetic capacity in fetal cotyledonary tissue near labor is location-dependent

An activity assay is described for quantification of prostaglandin synthase (PGHS) in sheep placental cotyledon under initial velocity conditions through measurement of the stable product prostaglandin E₂ (PGE₂). The effects of temperature, time, and substrate concentration on initial reaction velocity, and lipoxygenase and PGHS product formation in cotyledonary tissue, were examined in detail. We used this activity assay to determine whether or not an increase of active PGHS by placental location within the uterus might contribute selected prostaglandins (PG) for the directed initiation of parturition. Sheep cotyledon tissue was collected (n = 6 animals) from the ventral aspect of the uterine body, mid-horn, and horn tip at 122 days of gestation (dga), and from the same locations in the ventral body and horn tip at 142-145 dga (in animals at term but not in labor; n = 4). At 122 dga, there was no increase in active PGHS in cotyledonary tissue from the horn tip, mid-horn, or uterine body. By 142-145 dga, the horn showed significantly (p < 0.01) more enzyme activity than the body. At the same time, production of PGE₂, expressed as a percentage of total eicosanoids, had not changed significantly. The development of an increase in PGHS toward the uterine tip implies that variations in regional PG production may contribute to the progression of labor.