Plasma β-Endorphin and β-Lipotropin in the Human Fetus at Delivery: Correlation with Arterial pH and pO2*

Abstract

β-Endorphin-like immunoactivity was measured in the umbilical cord plasma of 45 term human fetuses. Mean concentration was 91 ± 16 (SEM) pg/ml, significantly higher (P < 0.001) than the normal adult level of 30.7 ± 2.7 pg/ml. This immunoactivity was further characterized in 10 cases by Sephadex G-50 chromatography to separate β-endorphin from β- lipotropin (β-LPH). Mean β-endorphin and β-LPH concentrations were 57 ± 12.8 and 455 ± 101 pg/ml, respectively. Both were higher (P < 0.01) than the mean β-endorphin and β-LPH concentrations reported in the adult. The mean molar β-endorphin to β-LPH ratio was 0.35 in the fetus and 0.36 in the adult. In 17 fetuses whose umbilical arterial and venous concentrations were measured separately, mean β-endorphin-like immunoactivity was higher in the artery than in the vein. A highly significant negative correlation (r < −0.831; P < 0.001) was present between umbilical arterial pH and β-endorphin-like immunoactivity. A negative correlation (r <−0.611; P < 0.005) with arterial pO₂ was also noted. We conclude that high levels of β-endorphin-like immunoactivity, composed of both β-endorphin and β-LPH, circulate in the human fetus at term, and that hypoxia and secondary acidosis may be major stimuli to the release of these peptides.