Effect of 48-hour infusion of the synthetic oxytocin antagonist, [1-beta-mercapto(beta-(CH2)5)1(OMe)Tyr2,Orn8]-oxytocin, on myometrial activity of pregnant sheep at 139-140 days of gestation.

Currently there is considerable interest in the actions of oxytocin antagonists on the pregnant myometrium. Few studies have been conducted involving long-term infusions of oxytocin antagonists to late-pregnant experimental animals. We set out to determine whether continuous infusion of an oxytocin antagonist ([1-beta-mercapto(beta-(CH2)5)1(OMe)Tyr2,Orn8]-oxytocin) would influence basal levels of myometrial activity of the contracture type and maternal prostaglandins in pregnant sheep. The antagonist was infused into a uterine vein at 80 micrograms.h-1 for 48 h starting at 139 days of gestational age. The antagonist significantly reduced total myometrial electromyogram activity and the frequency of contractures but did not change contracture duration. Antagonist infusion did not produce any significant alterations in maternal carotid or uterine vein 13,14-dihydro-15 keto prostaglandin F2 alpha concentrations. Contractures probably represent an intrinsic instability of the resting membrane potential of uterine muscle and these results suggest that oxytocin may play a role in regulating their frequency in sheep during the last third of gestation.