Human placental acetylcholine content and release at parturition.

1. Measurements were made of human placental acetylcholine (ACh) content and release into maternal and fetal circulations from placenta obtained before, during and after labours of both spontaneous and induced onsets. ACh content was determined in ex vivo placental biopsies using a radio-enzymatic assay. ACh release was determined by bioassay of the effluent from placental lobules perfused with amniotic fluid-like Kreb's containing physostigmine (2.4 mumol/l). 2. ACh content of placentae obtained after labour (spontaneous onset of labour, normal vaginal delivery) and during labour (spontaneous onset of labour, Caesarean section delivery) was significantly less than before labour (no labour onset, Caesarean section delivery). 3. Mean ACh output into maternal vessels 1.5-4.0 h after commencement of perfusion of placentae obtained after labour (spontaneous onset of labour, normal vaginal delivery) was significantly less than from those obtained before labour (no labour onset, Caesarean section delivery). No differences were found in ACh output into fetal vessels of placentae obtained before, during or after labour. 4. These results suggest a role for placental ACh in the events of human labour. The decrease in ACh content and maternal vascular release in placenta obtained after labour is consistent with a depletion of placental ACh during labour which may indicate ACh release at this time. The lack of any significant change in fetal vascular release of ACh in placenta obtained before, during or after labour, raises the possibility of different roles for ACh released into fetal and maternal vasculature.