Plasma beta-endorphin and beta-lipotropin in the human fetus at delivery: correlation with arterial pH and pO2.

Beta-endorphin-like immunoactivity was measured in the umbilical cord plasma of 45 term human fetuses. Mean concentration was 91 +/- 16 (SEM) pg/ml,an 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 beta-endorphin from beta-lipotropin (beta-LPH). Mean beta-endorphin and beta-LPH concentrations were 57 +/- 12.8 and 455 +/- 101 pg/ml, respectively. Both were higher (P less than 0.01) than the mean beta-endorphin and beta-LPH concentrations reported in the adult. The mean molar beta-endorphin to beta-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 beta-endorphin-like immunoactivity was higher in the artery than in the vein. A highly significant negative correlation (r = -0.831; P less than 0.001) was present between umbilical arteiral pH and beta-endorphin-like immunoactivity. A negative correlation (r = -0.611; P less than 0.005) with arterial pO2 was also noted. We conclude that high levels of beta-endorphin-like immunoactivity, composed of both beta-endorphin and beta-LPH, circulate in the human fetus at term, and that hypoxia and secondary acidosis may be major stimuli to the release of these peptides.