Characterization of plasma beta-endorphin immunoactivity in the fetal lamb: effects of gestational age and hypoxia.

To characterize the immunological forms of beta-endorphin (beta-EP) in the fetal circulation, total beta-EP immunoactivity [beta-EPi] and N-acetyl beta-EPi were measured in the plasma of chronically catheterized fetal lambs in undisturbed conditions and before, during, and after periods of controlled hypoxia. Measurements of the peptide concentrations in each plasma sample were made by RIA using an antiserum to the midportion of beta-endorphin which cross-reacts with both acetylated and unacetylated forms of the peptide as well as with beta-lipotropin, and a second antiserum which reacts only with acetylated forms of beta-EP. In 25 plasma samples from 12 fetal animals at 113-142 days gestation, total beta-EPi was 87.0 +/- 10.9 pg/ml, while N-acetyl beta-EPi was 90.8 +/- 7.7 pg/ml (mean +/- SE). When a plasma pool obtained from 3 fetuses in the basal state was extracted and chromatographed on Sephadex G-50, most of the N-acetyl beta-EPi eluted in the same position as the synthetic N-acetyl beta-EP standard. Thus, most of the beta-EPi in the plasma of the unstressed fetus could be accounted for by N-acetylated forms of the peptide. These are the major forms of beta-EP produced by the intermediate lobe of the pituitary. To examine the effects of acute hypoxia on fetal plasma peptide levels, pregnant ewes were exposed to 10% O2 in N2 for 30 min. In 15 studies at 113-142 days gestation, mean fetal PO2 decreased from 21.7 +/- 0.6 to 11.0 +/- 0.7 mm Hg (P less than 0.001). Total beta-EPi increased significantly from 93.0 +/- 17.7 to 527 +/- 146 pg/ml during hypoxia and returned toward basal values after 30 min of recovery to 372 +/- 116 pg/ml (P less than 0.02). Over the same intervals, N-acetyl beta-EPi did not change significantly, with mean levels of 88.5 +/- 10.7, 123 +/- 16.3, and 130 +/- 16.8 pg/ml. This shows that the increase in total beta-EPi with hypoxia could not be accounted for by an increase in N-acetyl beta-EPi. Our finding that most of the total beta-EPi in the circulation of the undisturbed fetus is N-acetyl beta-EPi favors an intermediate lobe origin. Since beta-EP is inactivated by N-acetylation, these data suggest that this immunoactivity has little or no biological activity. Enhanced release of total beta-EPi during hypoxia, which could not be accounted for by acetylated forms, suggests that this type of stress activates the anterior pituitary lobe and results in increased plasma concentrations of the biologically active peptide.