Differences in the effects of vasopressin and oxytocin on rabbit myometrial activity and a possible mediation of prostaglandins

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Abstract
Uterine responses to vasopressin and oxytocin were monitored in nonpregnant and 3- or 6-8-day-pregnant rabbits by recording the intrauterine pressure. Oxytocin stimulated uterine activity in all groups, but the effect of vasopressin was stimulatory in non-pregnant animals, inhibitory in those 3 days post coitum and weakly stimulatory in those later in pregnancy. Inhibition of prostaglandin (PG) synthesis by the administration of indomethacin reduced the spontaneous uterine activity as well as the responses to oxytocin and vasopressin in the non-pregnant rabbits but had little effect in the pregnant animals. During infusion of PGF2.alpha., PGE-1 or PGE 2 in 6-8 day-pregnant rabbits, the stimulatory response to vasopressin, although slight before the infusion, was inhibited whereas e stimulatory response to oxytocin remained virtually unchanged. Vasopressin and oxytocin, under certain hormonal conditions apparently are able to activate the uterine contractions by mechanisms in which the involvement of PG is not obligatory.