Alterations in estrogen receptor isoforms in the mammary gland and uterus of the rat during differentiation

1. Estrogen receptors in lactating mammary glands and uteri of rats which were 10 and 19 days postpartum exhibited molecular heterogeneity based on their surface charge properties. 2. The polymorphism of estrogen receptors detected by high-performance ion exchange chromatography may be monitored in-line with a radioisotope detector. 3. Estrogen receptors from the mammary gland and uterus of rats at 10 days of lactation exhibited primarily two receptor isoforms eluting at 200-250 mM and 250-300 mM phosphate, whereas three ionic isoforms (eluting at 50-150, 200-250 and 325-375 mM phosphate) were found in the mammary glands of rats at 19 days of lactation. Similar changes in the profiles of estrogen receptor isoforms were observed in uterine cytosol preparations at each stage of postpartum differentiation. 4. The elution pattern of receptor-associated radioactivity was not altered by the addition of diisopropylphosphate, a potent inhibitor of trypsin-like proteases, either before, during or immediately after homogenization. This indicates that the differences observed in the receptor elution profile of 10 and 19 day postpartum lactating mammary glands were not due to artifactual proteolysis. 5. In summary, our data indicate that the differentiation stage of lactating mammary glands may dictate the final profile of receptor isoforms detected.