ABSTRACT
Pulsatile administration of oxytocin was compared with continuous infusion of oxytocin for induction of labor in pregnant rats. The dosages consisted of intravenous injections of 0, 2.5, and 5 mU oxytocin every 10 minutes and intravenous infusion of 1 mU/minute of oxytocin in 0.9% sodium chloride. These doses are within the range of endogenously secreted pulses. All treatments began on day 22 at 2 p.m. and continued for 8 hours. Pulsatile administration resulted in a marked reduction in the dose of oxytocin required to induce labor. Using 5 mU pulses, birth was induced with 18.4%, and using 2.5 mU pulses, with 24% of the dose needed using continuous infusion. Parturition was advanced by 12 hours on the average by oxytocin treatment, but no significant differences were observed between the various oxytocin dosage regimens in this regard or in regard to gestation length, induction-delivery interval, duration of delivery, or the proportion of living or dead pups. Significantly more uterine activity was induced with each mU of oxytocin using pulsatile administration than using continuous infusion. There was no evidence for down-regulation of oxytocin receptors during a continuous infusion of oxytocin. We postulate that the greater efficacy of oxytocin pulses to induce uterine activity and delivery in comparison to continuous infusions is due to a more effective stimulation of prostaglandin F2α release from the decidua. The amount of oxytocin needed for induction of labor with 2.5 mU pulses was similar to the decrease in neurohypophyseal oxytocin content during the first stage of spontaneous labor, and uterine activity elicited was also similar to that observed during spontaneous labor. The secretion of oxytocin during normal labor may hence occur in pulses of this magnitude.
1 Presented in part at the annual meeting of the Society of Gynecologic Investigation, Toronto, March 19-22, 1986. This work was supported in part by a grant from the Warner Lambert Foundation.
