Does prostaglandin F(2α) released from the uterus by oxytocin mediate the oxytocic action of oxytocin?

Abstract

The role of prostaglandin F2α (PGF2α) in the oxytocic response to oxytocin (OT) was investigated by infusing OT directly into the uterine artery of the sheep and recording changes in both the pattern of uterine motility and the rate of secretion of PGF2α at different stages of the estrous cycle. On Day 3 (estrus = Day 0) OT infused from 0.1 to 5.0 mU/min produced a dose related increase in the rate of secretion of PGF2α but increased intrauterine pressure (IUP) and the frequency of contractions only at intermediate (1 mU/min) or high (5 mU/min) doses. Although exogenous PGF2α infused at 15 μg/h mimicked OT, the myometrium responded normally to OT when the production of endogenous PGF2α was suppressed with indomethacin. On Day 8, an intermediate dose of OT changed neither the pattern of uterine motility nor the rate of secretion of PGF2α. On Day 14, OT infused at rates as low as 0.025 mU/min increased the secretion of PGF2α 4 fold but induced no change in IUP or frequency of contraction. At 0.1 mU/min, OT increased the rate of secretion of PGF2α nearly 10 fold within 3 min but changed IUP only slightly and after a delay of 5 min. Even at higher doses, OT invariably elevated PGF2α secretion but did not consistently raise IUP. The authors conclude that while increased contractile activity is not a sine qua non for OT induced synthesis of PGF2α, neither is increased synthesis of PGF2α an essential intermediate step in the activation of the myometrium by OT.