Effect of oxytocin on estrogen-induced uterine luminal fluid accumulation in ovariectomized rats and the role of prostaglandins

Abstract

In estrogen-treated ovariectomized rats, uterine luminal fluid accumulation has been used as an indirect measure of cervical tone, with loss of luminal fluid being interpreted as indicating reduced tone. Since the recently reported ability of analogues of prostaglandins E2 and F2α to cause loss of luminal fluid might have resulted, not from reduced cervical tone, but from uterine contractions forcing fluid through constricted cervices, the effects of oxytocin, also a stimulator of myometrial activity, on luminal fluid accumulation were investigated. When ovariectomized rats, implanted 72 h previously with a Silastic capsule containing 17β-estradiol, were given a single injection of oxytocin and killed 1–2 h later, oxytocin, in a dose-dependent fashion, reduced uterine luminal fluid accumulation. For a supramaximal dose of oxytocin, this effect was seen within 15 min. In rats in which one uterine horn was ligated at the cervical end, fluid accumulation in the ligated horns did not differ significantly in control and treated rats, whereas in nonligated horns, oxytocin reduced the fluid accumulation, suggesting that loss of fluid occurred through the cervix rather than by reabsorption. Given twice daily for 3 days commencing immediately after the insertion of the estrogen-containing capsule, oxytocin had no effect on luminal fluid accumulation in horns ligated at the cervical end, indicating that oxytocin did not inhibit luminal fluid formation. Prostaglandin biosynthesis did not appear to be essential for the acute effect of oxytocin since treatment of rats with indomethacin, although reducing uterine and cervical prostaglandin levels, did not inhibit the effect of oxytocin. However, loss of fluid occurred slightly sooner following oxytocin administration in controls than in indomethacin-treated animals. In addition, for animals autopsied 10 min after oxytocin injection, the dose–response relationship between oxytocin and uterine luminal fluid was affected by indomethacin treatment. These data suggest that prostaglandins act synergistically with oxytocin to bring about loss of uterine luminal fluid accumulation. However, the validity of the use of luminal fluid accumulation as an indicator of cervical tone remains to be resolved.