Uterine contractile response to the graded infusion of oxytocin in guinea pigs near term.

Abstract

Myometrial oxytocin binding characteristics do not change near term in guinea pigs. We tested the hypothesis that the uterine contractile response to oxytocin does not change near term. Chronically instrumented guinea pigs were given graded infusions of oxytocin (n = 19 animals) or saline solution (n = 4 animals) on days 58, 60, and 62 and then daily until delivery (term, 68 days). Uterine contractile response was assessed by the area of the quasi-integrated electromyogram. Concentrations of 13,14-dihydro-15-keto-prostaglandin F(2)(alpha) were measured in maternal plasma. The uterine contractile response to oxytocin increased with advancing gestation (P =.007, random coefficients model) and labor (P =.005). Plasma concentrations of 13,14-dihydro-15-keto-prostaglandin F(2)(alpha) increased during oxytocin infusion (P <.022 vs saline solution infusion). The uterine contractile response to oxytocin increases during the 1 to 2 weeks before term in guinea pigs, despite stable myometrial oxytocin receptor density. Oxytocin stimulates the production of prostaglandin F(2)(alpha).