Origin and quantification of cytoplasmic estradiol receptor in resting target cells

The exhaustive extraction of microsomal estradiol receptor by Surfynol/dithiothreitol/trypsin in low ionic strength buffer was employed for distribution studies on non-stimulated porcine endometrium. It was found that more than half of the cytoplasmic receptor contents were of microsomal origin. "Empty" structures did not interfere with receptor analysis by agargel electrophoresis. The combined yields from homogenate fractions corresponded to those obtained from unfractionated homogenates. Freeze-fracturing of endometrium had a moderate receptor-solubilizing effect.