Plasma and red blood cell β-endorphin immunoreactivity in normal and complicated pregnancies: Gestational age variation

Recent observations suggest that there may be two pools of β-endorphin-like immunoreactivity in mammalian circulation. One of these pools is present in plasma and the other is detected in association with erythrocytes. Elucidation of an erythrocyte-associated pool may explain some of the wide variability of plasma β-endorphin levels reported in the literature. We measured β-endorphin immunoreactivity levels in 85 normal and 33 complicated pregnancies to delineate a possible correlation between gestational age and β-endorphin immunoreactivity levels in plasma and in erythrocytes. Our results indicate that β-endorphin immunoreactivity levels in both plasma and erythrocytes vary systematically throughout the gestational period, reaching a peak at 31 to 32 weeks of gestation. Amniotic fluids at midgestation were also analyzed and no correlation was observed between the levels of β-endorphin immunoreactivity and fetal sex. Compared to normal patients, diabetic patients had significantly lower levels of β-endorphin immunoreactivity in plasma and higher levels in erythrocytes although the total β-endorphin immunoreactivity was not statistically different from that in normal subjects. We conclude that (1) the total β-endorphin immunoreactivity level in whole blood is much higher than that reported in plasma, (2) both plasma- and erythrocyte-associated β-endorphin immunoreactivity levels vary with gestational age, with a peak level at 24 to 32 weeks of gestation, (3) amniotic fluid β-endorphin immunoreactivity levels are unrelated to fetal sex, and (4) diabetic patients may have a different distribution of β-endorphin immunoreactivity pools than normal individuals.