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

Objective: 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.

Study Design: 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.

Results: 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).

Conclusion: 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).