Pregnancy-associated changes in ovarian immunoreactive β-endorphin in rats

Abstract

ABSTRACT Ovaries from pregnant and postpartum Sprague–Dawley rats were examined for content of immunoreactive β-endorphin by radioimmunoassay, and for its localization by the peroxidase-antiperoxidase technique. In addition, the molecular forms of β-endorphin immunoreactivity were separated by gel chromatography and reverse-phase high performance liquid chromatography (RP-HPLC). Ovaries from rats early in pregnancy showed intense granular cytoplasmic staining of luteal cells, with an even distribution of granular material throughout the cytoplasm. By middle to late pregnancy the staining pattern was changed, with immunoreactive material showing a less granular and unevenly distributed staining pattern and with some areas of the cytoplasm totally devoid of immunoreactive material. The concentrations of immunoreactive β-endorphin measured during pregnancy were significantly lower than levels in mature non-pregnant rat ovary. The ovarian concentration of immunoreactive β-endorphin fell progressively during pregnancy and early lactation, returning to normal cyclic rat levels at 20 days post partum. The ovarian concentration of β-endorphin-like material was lowest at 6 days post partum (0·53 ± 0·08 ng/g wet weight; mean ± s.e.m.), representing approximately 10% of the concentration found in pooled ovaries from randomly cyclic adult rats. Gel chromatography revealed only a single peak of immunoreactive β-endorphin, co-eluting with 3·5 kD molecular weight ovine β-endorphin(1–31). This contrasts with gel profiles of adult cyclic rat ovary, where large molecular weight species pro-opiomelanocortin (31 kD) and β-lipotrophin (11·5 kD) are also present. On RP-HPLC the predominant species of low molecular weight immunoreactive material co-eluted with β-endorphin(1–31). These data show that pregnancy in the rat is associated with marked changes in the levels, cellular localization and chromatographic profiles of ovarian β-endorphin. The aetiology and physiological significance of these changes remains to be established. J. Endocr. (1986) 108, 343–350