Characterization of estrogen, progesterone and glucocorticoid receptors in rabbit mammary glands and their measurement during pregnancy and lactation.

Specific cytosolic receptors for estrogen, progesterone and glucocorticoids have been identified in mammary glands of virgin, pregnant and lactating rabbits. Receptors for these three steroids hormones show similar specificity, stability, dissociation, sedimentation characteristics on sucrose gradient and binding constants to steroid hormone receptors which have been characterized in a number of other target tissues. Receptors for estrogen and progesterone are highest at day zero of pregnancy, with values declining by 80-90% during the first week. Estrogen and progesterone receptor levels remain low but measurable throughout pregnancy, with progesterone receptor levels becoming undetectable late in lactation. Glucocorticoid receptors, which are 52.4 +/- 14.5 fmol/mg protein on day 0, decline slightly during the first week of pregnancy and then increase on day 14 to 81.4 +/- 18.9 fmol/mg protein, after which the values decline continuously until parturition. During lactation, values are slightly elevated on day 2 and somewhat lower on day 10. A difference in the receptor binding capacity for estrogens, progesterone and glucocorticoids was observed for mammary glands removed from virgin females (day 0 of pregnancy) which had either accepted or refused the male. Binding capacities were 2.4-, 3.4- and 1.7-fold higher for estrogen, progesterone and glucocorticoid receptors, respectively, in mammary glands of virgins accepting the male than in those females not accepting the male.