In vivo activity of the potent oxytocin antagonist on uterine activity in the rat

Oxytocin antagonist (OTA), TT-235, was developed by our group and shown to inhibit either spontaneous or oxytocin-induced uterine contractions in primates. The purpose of the present study was to confirm the duration of TT-235 to block oxytocin-induced uterine contractions in estrous rats. In Experiment 1, the time-response of the three OTAs on uterine contractility was examined. The rats were anesthetized and cannulas were placed in the jugular vein for infusing vehicle (sterile saline), Antag I, Antag II and TT-235. The uterine activity was monitored through a water-filled balloon-tipped cannula placed in the uterine horn. The uterine contractile activity was determined as the integrated area for 10 minutes. Each OTA was administered as a single bolus injection of 5 microg, followed by 100 mU of oxytocin 5 minutes later, also done as a single bolus. Oxytocin injection of the same dosage was repeated every hour for 5 hours. Experiment 2 determined the effect of the three OTAs on uterine oxytocin receptor number (Rn) and binding affinity (Kd). Rats treated with either OTA or vehicle were sacrificed at 0.5 and 4 hours for receptor assay. In Experiment 1, Antag I, Antag II and TT-235 inhibited the integrated uterine response to oxytocin at 5 minutes by 76%, 77% and 80%, respectively, compared to controls (p<0.05). Two hours after injecting Antag I, inhibition of uterine contractility was 55% lower than controls (p<0.05). At 3 hours, uterine contractility was no longer affected in rats treated with Antag I compared with controls. The suppressive uterine activity with Antag II continued up to 3 hours. However, uterine contractility remained lower (53%) in rats treated with TT-235 5 hours later. In Experiment 2, TT-235 induced a significant decrease (p<0.05) in oxytocin receptor number and binding affinity at both 0.5 and 4 hours compared with controls. Antag I and Antag II did not alter oxytocin receptor number or binding affinity significantly at each timepoint studied compared with controls. In conclusion, TT-235 may inhibit the uterine response to oxytocin by decreasing oxytocin receptor numbers and oxytocin binding affinity, which might explain the prolonged oxytocin antagonist activity of TT-235.