The response of rat and human uterus to oxytocin from different gestational stages in vitro.

Oxytocin (OT) and oxytocin receptors (OTRs) play essential roles in parturition and the effect of OT on uterine contractility is greatly influenced by the expression of OTRs in myometrium. We investigated the effect of OT on uterine strips isolated from non-pregnant, late-pregnant, term-pregnant, and labouring rats and from labouring and non-labouring women. Longitudinal uterine strips (from each gestational stage) were dissected and mounted vertically in an organ bath setup system and challenged with 5 nM OT and the effect was investigated on uterine contractility. In other experiments, phospholipase C (PLC), prostaglandin H synthase-2 (PGHS-2), and calcium-activated chloride channels (CaCCs) were blocked and the effect of OT was tested in labouring rats. OT stimulated the labouring uterus with greater force compared to other gestations in rats and also augmented the uterine force in labouring women compared to the non-labouring. However, blocking the PLC, PGHS-2, and CaCCs significantly reduced the OT-induced force increase in labouring rats. These data suggest that as labour approaches, the sensitivity of the uterine tissues to OT is greatly enhanced concomitant with the increased expression of OTR to ensure strong and adequate uterine contractions essential for the normal delivery and to prevent the postpartum haemorrhage.