Oxytocin Pretreatment of Pregnant Rat Myometrium Reduces the Efficacy of Oxytocin but Not of Ergonovine Maleate or Prostaglandin F2α

Oxytocin receptors (OTRs) in both human and rat myometrial cells are desensitized by exposure to oxytocin, thereby reducing the ability of the cells to respond to the subsequent administration of oxytocin. However, it is unclear if this desensitization phenomenon is confined to oxytocin, or extends to other uterotonic agents such as ergonovine or prostaglandin F-2 alpha (PGF(2 alpha)). We compared the in vitro contractile responses of myometrial samples from pregnant Wistar rats at 20 to 22 days of gestation. Longitudinal myometrial strips isolated from each animal were pretreated with either oxytocin 10(-8) mol/L (experimental groups) or physiological salt solution (control groups) for 1 hour in organ bath chambers, and then subjected to a dose-response study with oxytocin (n = 28), ergonovine (n = 16), or PGF(2 alpha) (n = 16), with cumulative increases in the organ bath concentrations from 10(- 10) to 10(-5) mol/L. The amplitude and frequency of the contractions during the dose-response period were analyzed using mixed linear modeling and compared between the groups. Oxytocin pretreatment significantly suppressed the mean amplitude of the myometrial contractions as compared to the controls when the strips were further subjected to oxytocin (1.02 vs 1.74 g; P < .0001), but not with further exposure to ergonovine (0.77 vs. 0.58 g; P = .11) or PGF(2 alpha) (0.83 vs 0.94 g; P = .4). However, oxytocin produced superior contractions in the control and oxytocin-pretreated myometrium compared to either ergonovine or PGF(2 alpha). Our study shows that the uterotonic effects of ergonovine and PGF(2 alpha) are not affected by the phenomenon of oxytocin desensitization in pregnant rat myometrium. However, oxytocin, despite the phenomenon of desensitization, provides superior uterine contractions when compared to ergonovine or PGF(2 alpha).