THE 24-HOUR RHYTHM OF OXYTOCIN AND BETA-ENDORPHIN SECRETION IN HUMAN-PREGNANCY

OBJECTIVE Animal work suggests that maternal oxytocin secretion is inf luenced by the secretion of endogenous opioids in pregnancy. Spontaneo us labour and pre-labour uterine activity follow a 24-hour rhythm the origin of which has not been explained but may be related to diurnal c hanges in oxytocin secretion. This study was performed to document the changes over a 24-hour period in maternal oxytocin and beta-endorphin secretion. DESIGN A 4-hourly blood profile was undertaken for a 24-ho ur period. PATIENTS Sixteen women with singleton pregnancies of more t han 36 weeks gestation and 10 women with pregnancies in the mid trimes ter were studied. MEASUREMENTS Blood was sampled 4-hourly for 24 hours beginning at 1200 h. Oxytocin was measured in all patients and beta-e ndorphin-like immunoreactivity was measured in 15 patients. RESULTS A simple index was defined for comparing night-time levels to daytime le vels for both oxytocin and beta-endorphin. In all cases more than 36 w eeks gestation the index was positive for oxytocin (night-time levels were higher) and in all cases the index was negative for beta-endorphi n (night-time levels were lower). In the mid trimester women ail value s of the index for oxytocin were positive but in the beta-endorphin gr oup equal numbers demonstrated a positive or a negative index. CONCLUS IONS Reciprocal 24-hour rhythms were demonstrated between oxytocin and beta-endorphin; however, it is not clear whether this relationship is causal.